WO2015106211A1

WO2015106211A1 - Methods for separating mixtures of fatty acids

Info

Publication number: WO2015106211A1
Application number: PCT/US2015/011049
Authority: WO
Inventors: Ned B. Bowden; Abhinaba Gupta
Original assignee: University Of Iowa Research Foundation
Priority date: 2014-01-13
Filing date: 2015-01-12
Publication date: 2015-07-16

Abstract

Provided herein are methods for separating mixtures of two or more fatty acids and compositions comprising fatty acids and counterions.

Description

METHODS FOR SEPARATING MIXTURES OF FATTY ACIDS

Related Application

This application claims the benefit of priority of U.S. Provisional Application Serial No. 61/926,752 filed on January 13, 2014, which application is herein incorporated by reference.

Government Funding

This invention was made with government support under CHE-1213325 awarded by the National Science Foundation. The government has certain rights in the invention.

Background of the Invention

Over 140 million tons of vegetable oils are produced in the world each year and approximately 96% of the production of these oils are used for food for humans, feed for animals, and biodiesel. This number is expected to greatly increase if biodiesels from algal are produced in significant quantities. These oils are triesters of glycerol

(HOCH₂CHOHCH₂OH) and three fatty acids; each fatty acid typically contains 16, 18, 20, or 22 carbons and zero, one, or more carbon-carbon double bonds (Figure 1). Vegetable oils are found in over 20,000 different foods, and they are the source of 95% of the trans-fatty acids within the human diet. The consumption of fatty acids from dietary sources has both positive and negative implications for health. Consumption of trans-fatty acids have a direct correlation with various health problems including thrombogenesis which leads to increased risk for coronary heart and cardiovascular diseases, increased levels of low-density

lipoproteins, and decreased levels of high-density lipoproteins. Significant consumption of trans-fatty acids also leads to an increased risk of diabetes from a rise in blood insulin levels when these fats are consumed. In 2011 the United States government banned the sale of food with more than 2 grams of trans-fatty acid per 100 grams of oil or fat. In the US, food with trans-fatty acid levels of less than 0.5 grams per serving can be labeled as "Trans Fat Free", but critics of the this plan have expressed concern that the 0.5 gram per serving threshold is too high to refer to a food as free of trans fat. A person eating many servings of a product or eating multiple products over the course of the day, may still consume significant amounts of trans-fatty acids. In contrast to trans-fatty acids, the consumption of cw-fatty acids are often associated with positive health benefits.

Although partially hydrogenated vegetable oils account for 95% of the trans-fatty acids that are consumed each year, virgin vegetable oils do not possess any trans-fatty acids prior to hydrogenation. For example, over 35 million tons of soybean oil are produced each year, and it has an approximate composition of 10% palmitic acid, 4% stearic acid, 18% oleic acid, 55% linoleic acid, and 13% linolenic acid (palmitic oil is a 16 carbon saturated fatty acid, see Figure 1 for the structures of certain fatty acids). One major limitation with soybean oil as a food source is that it has a significant fraction of polyunsaturated fats that are problematic for applications in food and animal feed. Polyunsaturated fatty acids are prone to oxidation which leads to rancidity and off flavors. Hydrogenation is the most common method to lower the levels of polyunsaturated fatty acids (over 10 million tons of partially hydrogenated soybean oil is produced each year, primarily for human consumption), but this process leads to the formation of significant amounts of trans-fatty acids.

In addition to their uses in food, vegetable oils are the most important renewable feedstock for the chemical industry and have grown by 5% a year since 2000. Despite the large scale production of vegetable oils and the fact that they are a critical biorenewable source of starting materials, both the oils and their fatty acids are used only in small quantities in industrial applications. Over 96% of vegetable oils are "burned" by humans or animals after being consumed or in engines when used as biodiesel.

A critical reason for the lack of applications of fatty acids as a starting material for industrial applications is that it is not possible to inexpensively separate a mixture of fatty acids into individual components on a scale of millions of tons per year. For example, fatty acids isolated from vegetable oils are a mixture of five or more different fatty acids with different reactivities and that will yield different products after a reaction. Thus, when a mixture of five fatty acids derived from soybean oil are used as starting materials in an industrial process, many different products are obtained. The challenge of utilizing a mixture of fatty acids as starting materials limits their broader transformations into more valuable commercial products.

Methods to purify fatty acids include selective precipitation, liquid chromatography or selective hydrolysis of fatty acids from glycerol. Although each method has found applications, they each are limited in the amount of fatty acids that can be quickly and inexpensively purified and on the final purity of the fatty acids. Accordingly, there is a need for better methods to separate mixtures of two or more different fatty acids. In particular, there is a need for better methods to separate a mixture of two or more different fatty acids to provide an enriched mixture of two or more fatty acids or to provide an individual fatty acid that has been enriched.

Summary of the Invention

Membranes are commonly used in industry to remove impurities from a mixture of molecules. Separations using membranes are a preferred method for large industrial applications because it is one of the simplest and least energy intensive methods of purification. Membranes have been used to remove impurities (i.e. proteins and glycerols) from fatty acids, however, they have not been used to separate a mixture of two or more fatty acids to provide an enriched mixture of fatty acids or individual fatty acids. One reason it is difficult to separate fatty acids using membranes is that fatty acids are similar in size and polarity. Although cis and trans double bonds confer differences in overall shape to fatty acids, the ease of rotation about the numerous carbon-carbon sigma bonds leads to a large number of energetically assessable conformations for each fatty acid which increases the complexity of separating them with membranes.

Applicant has discovered that when fatty acids are associated with counterions (e.g., a fatty acid salt) their critical size is increased and the resulting fatty acid salts can be separated via a membrane.

Accordingly, one embodiment provides a method for separating a mixture of two or more fatty acids associated with counterions comprising contacting a membrane with a first mixture comprising two or more different fatty acids associated with counterions, so that the mixture is fractionated into a permeate comprising one or more different fatty acids associated with counterions and a retentate comprising one or more different fatty acids associated with counterions, wherein at least one of the permeate or retentate is enriched in one or more different fatty acids associated with counterions wherein the counterion is:

(a) a heteroaryl counterion or bicyclic heterocycle counterion;

(b) dipropylamine, diisobutyl amine, dibutylamine, N-ethylpiperidine,

triethanolamine, triallylamine, tripropargylamine, quinuclidine, N-ethyl-N-propyl amine, piperidine, azepane, diethanolamine, diallylamine, benzylamine or hexylamine; or

(c) dimethyl piperidinium, methyl ethyl piperidinium, methyl propyl piperidinium, methyl triethylammonium, tetraethyl ammonium or propyltriethyl ammonium.

One embodiment provides novel compounds comprising a fatty acid and a counterion as described herein.

Brief Description of the Figures

Figure 1 illustrates the structure of certain fatty acids.

Figure 2 illustrates the retention of molecules based on molecular weight and critical area. A retention of 100% indicated that the molecule did not permeate the membrane at any level and a retention of 0% indicated that the molecule readily permeated the membrane and was not retained, a) The plot of retention versus molecular weight for 35 samples is shown, b) The plot of retention versus critical area is shown for the molecules in part a).

Figure 3 shows a schematic of the apparatus that was used in most of the experiments described herein. Molecules A and B were initially added to the solvent upstream of the membrane and only molecule A permeated to the downstream solvent.

Figure 4 shows the distance between nitrogen and the terminal carbon (or oxygen) for four different amines.

Figure 5 illustrates energy-minimized space filling models for each fatty acid and fatty acid salt with triisobutylamine. One image shows the fatty acid to emphasize any curvature. The other image shows a view of the critical area of each fatty acid salt with triisobutylamine. The images for a) elaidic acid, b) stearic acid, c) oleic acid, d) linoleic acid, e) linolenic acid, f) vaccenic acid, and g) petroselinic acid are shown.

Detailed Description

Alkyl as used herein in includes straight and branched saturated hydrocarbon chains.

Alkenyl as used herein includes straight and branched hydrocarbon chains that comprise one or more carbon-carbon double bonds.

Alkynyl as used herein includes straight and branched hydrocarbon chains that comprise one or more carbon-carbon triple bonds. Cycloalkyl such as a (C₃-C₈)cycloalkyl or (C₃-C )cycloalkyl as used herein refers to a saturated or partially unsaturated cyclic hydrocarbon.

Optionally substituted alkyl, optionally substituted alkenyl and optionally substituted alkynyl include alkyl, alkenyl and alkynyl groups optionally substituted with one or more (e.g. 1 , 2, 3, 4, 5, or more) groups independently selected from, oxo (=0), halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (CrC^alkyl, (C₂-C )alkenyl, (C₂- C6)alkynyl and (C₃-C₇)cycloalkyl.

Optionally substituted cycloalkyl groups include cycloalkyl groups optionally substituted with one or more (e.g. 1, 2, 3, 4, 5, or more) groups independently selected from (C _rC₆)alkyl, (C _!-C₆)alkenyl, (C _!-C₆)alkynyl, oxo (=0), halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (Ci-C )alkyl, (C₂-C₆)alkenyl, (C₂-C₆)alkynyl and (C₃-C₇)cycloalkyl.

Halo or halogen as used herein includes fluoro, chloro, bromo and iodo.

The term "enriched" as used herein with term retentate means that the ratio of one or more fatty acids versus other fatty acids in the retentate is greater than the corresponding ratio of the permeate and/or the mixture from which the retentate and permeate were derived. In the same manner the term "enriched" as used herein with term permeate means that the ratio of one or more fatty acids versus other fatty acids in the permeate is greater than the corresponding ratio in the retentate and/or the mixture from which the retentate and the permeate were derived. Accordingly, the permeate may be enriched in one or more fatty acids when compared to the corresponding retentate or the mixture from which the permeate was derived. Likewise, the retentate may be enriched in one or more fatty acids when compared to the corresponding permeate or the mixture from which the retentate was derived.

Membranes

Membranes include semipermeable materials which can be used to separate components of a mixture into a permeate that passes through the membrane and a retentate that is rejected or retained by the membrane. One particular type of membrane is an organic solvent nanofiltration membrane. An organic solvent nanofiltration membrane is a membrane that is compatible with organic solvents and separates molecules in a specific size range. In one embodiment the organic nanofiltration membrane separates molecules with molecular weights from 50 to 1000 g mol^"1. Organic solvent nanofiltration membranes include but are not limited to those membranes based on polydicyclopentadiene, polyimide, polyaniline and polyacrylate which polymeric materials can be nanoparticulate. Examples include highly cross-linked polydicyclopentadiene (PDCPD), Duramem® (membrane), Puramem®

(membrane), and Starmem® (membrane).

One particular membrane is highly cross-linked polydicyclopentadiene (PDCPD) (Long, T. R.; Gupta, A.; Miller II, A. L.; Rethwisch, D. G.; Bowden, N. B. J Mater. Chem. 2011, 21, 14265; Gupta, A.; Rethwisch, D. G.; Bowden, N. B. Chem. Commun. 2011, 46, 10236, and US Patent Application Number 13/546,252, all of which references are hereby incorporated in their entirety). These membranes were fabricated by polymerizing 5,000 molar equivalents of dicyclopentadiene with one molar equivalent of the Grubbs first generation catalyst to yield solid, dense membranes. These membranes do not have well- defined pores such as zeolites; rather, when they are swollen in organic solvents, they possess openings between the polymer chains that small molecules may diffuse through. The distribution in size of the openings is polydisperse and on the nanometer to sub-nanometer size scale. The flux of a large number of molecules through PDCPD membranes were investigated and it was discovered that the membranes were highly selective to retain molecules with critical areas above 0.50 nm² as shown in Figure 2, but molecules with critical areas below 0.38 nm permeated the membranes (Gupta, A.; Rethwisch, D. G.; Bowden, N. B. Chem. Commun. 2011, 46, 10236.). The critical areas in Figure 2 were defined as the smallest rectangular cross-sectional areas for each molecule in its lowest energy state. These values were measured in silico as reported in prior publications (Gupta, A.; Rethwisch, D. G.; Bowden, N. B. Chem. Commun. 2011, 46, 10236.) The molecules that were retained had values for their flux that were at least four to five orders of magnitude lower than those for molecules that did permeate. These membranes did a poor job of separating molecules based on their molecular weights as shown in Figure 2a, but when the critical area for each molecule was plotted against retention a clear difference was observed. PDCPD membranes are a new type of size-selective membrane that separates organic molecules with molecular weights up to 1,000 g mol^" based on their critical areas.

As used herein the term "highly crosslinked" as applied to a polydicyclopentyldiene matrix includes martices wherein the ratio of crosslinked double bonds to uncrosslinked double bonds is at least about 3:2. In one embodiment of the invention the ratio of crosslinked double bonds to uncrosslinked double bonds is about 2:3. In one embodiment of the invention the ratio of crosslinked double bonds to uncrosslinked double bonds is at least about 7:3. In another embodiment of the invention the ratio of crosslinked double bonds to uncrosslinked double bonds is at least about 4:1.

As used herein, the term "matrix" means a regular, irregular and/or random arrangement of polymer molecules such that on a macromolecular scale the arrangements of molecules may show repeating patterns, or may show series of patterns that sometimes repeat and sometimes display irregularities, or may show no pattern. On a scale such as would be obtained from TEM, SEM, X-Ray or FTNMR, the molecular arrangement may show a physical configuration in three dimensions like those of networks, meshes, arrays, frameworks, scaffoldings, three dimensional nets or three dimensional entanglements of molecules. The matrix may be non-self supporting. The matrix is in the form of a thin film with an average thickness from about 5 nm to about 200 μηι. In usual practice, the matrix is grossly configured as an ultrathin film or sheet.

In one embodiment the invention provides a composite membrane comprising a highly crosslinked polydicyclopentyldiene matrix on a porous support backing material. The porous support backing material can comprise a polymeric material containing pore sizes which are of sufficient size to permit the passage of permeate therethrough. Examples of porous support backing materials which may be used to prepare composite membranes of the invention include polymers such as polysulfones, polycarbonates, microporous

polypropylenes, polyamides, polyimines, polyphenylene ethers, and various halogenated polymers such as polyvinylidine fluoride.

Fatty acids

The term "fatty acid" as used herein refers to an aliphatic carboxylic acid. The aliphatic group of the fatty acid is a hydrocarbon chain of about 4-28 carbons and can be straight, branched, saturated or unsaturated (e.g., comprising one or more carbon-carbon double bonds). Fatty acids include saturated fatty acids (e.g., fatty acids wherein the aliphatic group is saturated, for example (C₄-C₂₈)alkyl) such as (C₄-C₂₈)alkylC0₂H and unsaturated fatty acids (e.g. fatty acids wherein the aliphatic group has one or more one carbon-carbon double bonds, for example (C₄-C₂₈)alkenyl) such as (C₄-C₂₈)alkenylC0₂H. Unsaturated fatty acids include monounsaturated fatty acids (fatty acids wherein the aliphatic group has one carbon-carbon double) and polyunsaturated fatty acids (fatty acids wherein the aliphatic group has two or more carbon-carbon double bonds).

Fatty acids include but are not limited to oleic acid, linolenic acid, vaccenic acid, petroselinic acid, elaidic acid, palmitic acid, stearic acid, omega 3 fatty acids (e.g. linolenic acid, eicosapetnaenoic acid and docosahexaenoic acid), omega 6 fatty acids (e.g. linoleic acid and arachidonic acid ) and omega 9 fatty acids.

The term "c/s-fatty acid" refers to a an unsaturated fatty acid that has at least one cis carbon-carbon double bond in the aliphatic group (e.g. cis-( C₄-C₂₈)alkenylC0₂H). Examples of cis-fatty acids include but are not limited to oleic acid, linoleic acid, linolenic acid, vaccenic acid, petroselinic acid, eicosapetnaenoic acid, docosahexaenoic acid and arachidonic acid.

The term "trans-fatty acid" refers to an unsaturated fatty acid that has at least one trans carbon-carbon double bond and no cis carbon-carbon double bonds in the aliphatic group (e.g. trans-(C₄-C₂g)alkenylC0₂H). Examples of trans-fatty acids include but are not limited to elaidic acid.

Counterions

The fatty acids discussed herein form complexes with counterions. These complexes comprise the fatty acid and a counterion wherein the counterion is associated with the carboxyl portion of the fatty acid. As used herein the term "complex" includes any structure comprising a fatty acid and a counterion; such complexes include salts. As used herein the term "associated" includes any interaction (e.g. ionic, electrostatic, bonding, etc.) between the fatty acid and the counterion. In one embodiment each fatty acid is associated with one counterion. In one embodiment each fatty acid in the mixture of fatty acids being separated is associated with the same counterion.

Counterions as used herein include molecules that modify the critical area of the fatty acid in a manner such that fatty acids can be separated by a membrane. In one embodiment the counterion is considered to be positively charged and is associated with the carboxylate anion of the fatty acid (such complexes includes fatty acid salts). In one embodiment the positively charged counterion is in a protonated form wherein the proton is derived from the fatty acid thereby forming a complex between the positively charged, protonated counterion and the fatty acid anion. In one embodiment the counterion is a positively charged amine. In one embodiment the protonated amine counterion can be derived from the interaction of an amine with the acidic hydrogen of the fatty acid. In one embodiment the positively charged counterion is a fully substituted amine such as a tetrasubstituted amine or quaternary amine. In one embodiment the fully substituted amine such as a tetrasubstituted amine or quaternary amine forms a complex with the fatty acid anion. In one embodiment the counterion is heteroaryl counterion or bicyclic heterocycle counterion. In one embodiment the heteroaryl counterion or bicyclic heterocycle counterion is a protonated positively charged counterion wherein the proton is derived from the interaction of the heteroaryl or heterocycle with the acidic hydrogen of the fatty acid. In one embodiment the heteroaryl counterion or bicyclic heterocycle counterion is a protonated positively charged counterion. In one embodiment the heteroaryl counterion is substituted on a ring nitrogen with substituents other than hydrogen whereby the ring nitrogen is fully substituted (e.g., one or two substituents) and thus has a positive charge. In one embodiment the bicyclic heterocycle counterion is substituted on a ring nitrogen with substituents other than hydrogen whereby the ring nitrogen is fully substituted (e.g., one or two substituents) and thus has a positive charge.

In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.03 nm²to about 0.55 nm². In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.03 nm² to about 0.50 nm . In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.03 nm² to about 0.45 nm². In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.03 nm to about 0.40 nm . In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.03 nm to about 0.35 nm . In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical

2 2

area of about 0.06 nm to about 0.55 nm . In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.06 nm² to about 0.50 nm². In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical

2 2

area of about 0.06 nm to about 0.45 nm . In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.06 nm² to about 0.40 nm². In one embodiment the heteroaryl counterion or the bicyclic heterocycle counterion has a critical area of about 0.06 nm to about 0.35 nm .

In one embodiment the heteroaryl counterion comprises a monocyclic or bicyclic aromatic ring of from about 5-10 ring atoms which ring comprises about 1 to 9 carbon atoms and about 1-4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the rings, wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted with any substituents to provide a counterion with any of the above described critical areas. The sulfur and nitrogen atoms may also be present in an oxidized form provided the ring is aromatic

In one embodiment the bicyclic heterocycle counterion comprises a saturated or partially unsaturated bicyclic ring of about 6 to 12 ring atoms, which ring comprises about 3 to 10 carbon atoms and about 1-4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the rings, wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted with any substituents to provide a counterion with any of the above described critical areas. The sulfur and nitrogen atoms may also be present in an oxidized form.

In one embodiment the heteroaryl counterion comprises a monocyclic or bicyclic aromatic ring of from about 5-10 ring atoms which ring comprises about 1 to 9 carbon atoms and about 1-4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the rings, wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted with the below described substituents to provide a counterion with any of the above described critical areas. The sulfur and nitrogen atoms may also be present in an oxidized form provided the ring is aromatic

In one embodiment the bicyclic heterocycle counterion comprises a saturated or partially unsaturated bicyclic ring of about 6 to 12 ring atoms, which ring comprises about 3 to 10 carbon atoms and about 1-4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the rings, wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted with the below described substituents to provide a counterion with any of the above described critical areas. The sulfur and nitrogen atoms may also be present in an oxidized form

A heteroaryl counterion as used herein is a refers to a single aromatic ring (moncyclic heteroaryl counterion) or a bicyclic aromatic ring (bicyclic heteroaryl counterion) that has carbon atoms in the ring, one or more nitrogen atoms in the ring and that can optionally have one or more oxygen or sulfur atoms in the ring and wherein the ring is optionally substituted as described below and in general has a formal positive charge. Thus, the heteroaryl counterion includes aromatic rings of from about 1 to 9 carbon atoms and about 1 -4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the rings wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted as described below. The sulfur and nitrogen atoms may also be present in an oxidized form provided the ring is aromatic. In one embodiment the heteroaryl counterion is a monocyclic heteroaryl counterion of from about 1 to 6 carbon atoms and about 1-4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the ring wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted as described below. The rings of the bicyclic heteroaryl counterion are generally fused rings. The heteroaryl counterions are optionally substituted with one or more (e.g., 1, 2, 3, 4, 5, 6 or more) groups selected from (Q-C^alkyl, (Cj-C₆)haloalkyl (C₂-C₆)alkenyl, (C₂- C₆)alkynyl, (C₃-C7)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (C C₆)alkyl, (C₂-C₆)alkenyl, (C₂-C₆)alkynyl and (C₃-C₇)cycloalkyl. Such heteroaryl counterions for example, include but are not limited to pyridyl, pyrimidinyl, oxazolyl, thiazolyl, quinolyl, and indolyl each of which is optionally substituted as described above. A bicyclic heterocycle counterion as used herein is a refers to a saturated or partially unsaturated ring that has carbon atoms in the rings, one or more nitrogen atoms in the rings and that can optionally have one or more oxygen or sulfur atoms in the rings and wherein the ring is optionally substituted as described below and in general has a formal positive charge. Thus, the bicyclic heterocycle counterion includes bicyclic rings of from about 3 to 10 carbon atoms and about 1-4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the rings wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted as described below. The sulfur and nitrogen atoms may also be present in an oxidized form. In one embodiment the bicyclic heterocycle counterion includes bicyclic rings of from about 5 to 9 carbon atoms and about 1-4 heteroatoms selected from the group consisting of oxygen, nitrogen and sulfur in the rings wherein at least one of the heteroatoms is nitrogen and wherein the ring is optionally substituted as described below. The rings of the bicyclic heterocycle can be connected to each other by fused, bridged or spiro bonds. The bicyclic heterocycle counterions are optionally substituted with one or more (e.g., 1, 2, 3, 4, 5, 6 or more) groups selected from (Ci-C₆)alkyl, (C!-C6)haloalkyl (C₂-C )alkenyl, (C₂-

C₆)alkynyl, (C₃-C7)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (C₁-C₆)alkyl, (C₂-C₆)alkenyl, (C₂-C₆)alkynyl and (C₃-C₇)cycloalkyl. Such heteroaryl counterions for example, include but are not limited to quinuclidine which is optionally substituted as described above.

One embodiment provides for separating a mixture of two or more fatty acids associated with counterions comprising contacting a membrane with a first mixture comprising two or more different fatty acids associated with counterions, so that the mixture is fractionated into a permeate comprising one or more different fatty acids associated with counterions and a retentate comprising one or more different fatty acids associated with counterions, wherein at least one of the permeate or retentate is enriched in one or more different fatty acids associated with counterions wherein the counterion is:

(a) heteroaryl or bicyclic heterocycle, wherein the heteroaryl or bicyclic heterocycle is optionally substituted with one or more (e.g., 1, 2, 3, 4, 5, 6 or more) groups selected from (C_rC₆)alkyl, (C,-C₆)haloalkyl (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (C₁-C )alkyl, (C₂- C₆)alkenyl, (C₂-C6)alkynyl and (C3-C₇)cycloalkyl;

(b) dipropylamine, diisobutyl amine, dibutylamine, N-ethylpiperidine,

triethanolamine, triallylamine, tripropargylamine, quinuclidine, N-ethyl-N-propyl amine, piperidine, azepane, diethanolamine, diallylamine, benzyl amine or hexylamine; or

Solvents

Any suitable organic solvent can be used with the fatty acids in the separations described herein. For example, suitable solvents may include protic and aprotic organic solvents (e.g. methanol, benzene, toluene, methylene chloride, chloroform,

carbontetrachloride, tetrahydrofuran, pentane, hexanes, dimethylformamide or acetonitrile) or mixtures thereof. Specific Embodiments of the Invention

It is to be understood that the following embodiments of the invention can be combined with one or additional embodiments of the invention as described herein.

In one embodiment the invention provides a method wherein the permeate is enriched in one or more different fatty acids associated with counterions.

In one embodiment the invention provides a method wherein the retentate is enriched in one or more different fatty acids associated with counterions.

In one embodiment the invention provides a method wherein the permeate is enriched in one or more different fatty acids associated with counterions and the retentate is enriched in one or more different fatty acids associated with counterions.

In one embodiment the invention provides a method wherein the membrane is an organic solvent nanofiltration membrane.

In one embodiment the invention provides a method wherein the organic solvent nanofiltration membrane comprises polydicyclopentadiene, polyimide, polyaniline or polyacrylate. In one embodiment the invention provides a method wherein the organic solvent nanofiltration membrane comprises polydicyclopentadiene.

In one embodiment the invention provides a method wherein the organic solvent nanofiltration membrane comprises a highly crosslinked polydicyclopentadiene matrix.

In one embodiment the invention provides a method wherein the ratio of crosslinked double bonds to uncrosslinked double bonds in the highly cross-linked polydicyclopentadiene matrix is at least about 2:3.

In one embodiment the invention provides a method wherein the ratio of crosslinked double bonds to uncrosslinked double bonds in the highly cross-linked polydicyclopentadiene matrix is at least about 3:2.

In one embodiment the invention provides a method wherein the ratio of crosslinked double bonds to uncrosslinked double bonds in the highly cross-linked polydicyclopentadiene matrix is at least about 4: 1.

In one embodiment the invention provides a method wherein the membrane is a part of an assembly that comprises two or more membranes.

In one embodiment the invention provides a method wherein membrane is part of a spiral wound module.

In one embodiment the invention provides a method wherein the membrane separates molecules based on their critical areas.

In one embodiment the invention provides a method wherein each fatty acid of the first mixture, permeate and retentate is associated with one counterion and wherein the counterions are identical.

In one embodiment the invention provides a method wherein the counterion has a critical area that allows one or more different fatty acids associated with the counterion of the first mixture to permeate the membrane at a higher rate than one or more different fatty acids associated with the counterion of the first mixture so that the permeate is enriched in one or more different fatty acids associated with the counterion.

In one embodiment the invention provides a method wherein the counterion has a critical area that impedes the permeation of one or more different fatty acids associated with the counterion of the first mixture through the membrane so that the retentate is enriched in one or more fatty acids associated with the counterion.

In one embodiment the invention provides a method wherein the counterion has a critical area that allows one or more fatty acids associated with the counterion of the first mixture to permeate the membrane so that the permeate is enriched in one or more fatty acids associated with the counterion.

In one embodiment the invention provides a method wherein the counterion has a critical area that prevents one or more fatty acids associated with the counterion of the first mixture to permeate the membrane so that the retentate is enriched in one or more fatty acids associated with the counterion.

In one embodiment the invention provides a method wherein the heteroaryl counterion is a 5 or 6-membered monocyclic heteroaryl counterion and the bicyclic heterocycle counterion is a 5 to 8 membered bicyclic heterocycle counterion.

In one embodiment the invention provides a method wherein the heteroaryl counterion is a 5 or 6-membered monocyclic heteroaryl counterion.

In one embodiment the invention provides a method wherein the heteroaryl counterion is a 5 or 6-membered monocyclic heteroaryl counterion wherein one of the ring nitrogen of the heteroaryl counterion is substituted with one or more (C₁-Cg)alkyl and wherein the heteroaryl counterion is optionally substituted with one or more groups selected from (C_\- C₆)alkyl, (C C₆)haloalkyl (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (C₁-C )alkyl, (C₂- C₆)alkenyl, (C₂-C₆)alkynyl and (C₃-C₇)cycloalkyl.

In one embodiment the invention provides a method wherein the heteroaryl counterion is a 6-membered monocyclic heteroaryl counterion wherein one of the ring nitrogen of the heteroaryl is substituted with a (Ci-Cg)alkyl, and wherein the heteroaryl is optionally substituted with one or more groups selected from (C₁-C₆)alkyl, (C₁-C6)haloalkyl (C₂-

C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (Ci-C )alkyl, (C₂-C )alkenyl, (C₂-C₆)alkynyl and (C₃- C₇)cycloalkyl.

In one embodiment the invention provides a method wherein the heteroaryl counterion comprises a pyridyl. In one embodiment the invention provides a method wherein the heteroaryl counterion is pyridyl wherein the pyridyl ring nitrogen is substituted with a (C₁-C )alkyl and wherein the pyridyl is optionally substituted with one or more groups selected from (C_!-C₆)alkyl, (Cj- C₆)haloalkyl (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (Q-C^alkyl, (C₂-C₆)alkenyl, (C₂- C )alkynyl and (C₃-C )cycloalkyl.

In one embodiment the invention provides a method wherein the heteroaryl counterion is a compound of formula I:

-y

wherein R is a (C₁-C₈)alkyl; each R is independently is selected from (C₁-C₆)alkyl, (C₁-C₆)haloalkyl (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (C_t-C^alkyl, (C₂-C₆)alkenyl, (C₂- C )alkynyl and (C₃-C₇)cycloalkyl; and n is 0, 1, 2, 3, 4 or 5.

In one embodiment the invention provides a method wherein the heteroaryl counterion is a compound of formula la:

la

wherein R^l is a (C₁-C₈)alkyl.

In one embodiment the invention provides a method wherein heteroaryl counterion is

N-methypyridinium, N-ethylpyridinium or n-propylpyridinium.

In one embodiment the invention provides a method wherein the bi cyclic heterocycle counterion comprises a 5 to 8 membered bicyclic heterocycle counterion.

In one embodiment the invention provides a method wherein the bicyclic heterocycle counterion is an unsubstituted 5 to 8 membered bicyclic heterocycle.

In one embodiment the invention provides a method wherein the bicyclic heterocycle counterion is quiniclidine.

In one embodiment the invention provides a method wherein the counterion is a heteroaryl counterion or a bicyclic heterocycle counterion.

In one embodiment the invention provides a method wherein the counterion is dipropylamine, diisobutyl amine, dibutylamine, N-ethylpiperidine, triethanolamine, triallylamine, triporpargylamine, quinuclidine, N-ethyl-N-propyl amine, piperidine, azepane, diethanolamine, diallylamine or hexylamine.

In one embodiment the invention provides a method wherein the counterion is the protonated (e.g., mono or deprotonated), positively charged form of dipropylamine, diisobutyl amine, dibutylamine, N-ethylpiperidine, triethanolamine, triallylamine, triporpargylamine, quinuclidine, N-ethyl-N-propyl amine, piperidine, azepane, diethanolamine, diallylamine or hexylamine.

In one embodiment the invention provides a method wherein the counterion is dimethyl piperidinium, methyl ethyl piperidinium, methyl propyl piperidinium, methyl triethylammonium, tetraethyl ammonium or propyltriethyl ammonium.

In one embodiment the invention provides a method wherein counterion is a protonated counterion.

In one embodiment the invention provides a method wherein counterion is a protonated counterion wherein the proton is derived from the fatty acid.

In one embodiment the invention provides a method wherein counterion is a protonated counterion wherein the proton is derived from the fatty acid and wherein the protonated counterion and the anion of the fatty acid form a complex.

In one embodiment the invention provides a method wherein the first mixture comprises:

a) at least one cis-fatty acid associated with a counterion and

b) at least one trans-fatty acid associated with a counterion or at least one saturated fatty acid associated with a counterion.

In one embodiment the invention provides a method wherein the first mixture comprises at least one cis-fatty acid associated with a counterion and at least one saturated fatty acid associated with a counterion. In one embodiment the invention provides a method wherein the first mixture comprises two or more different cis-fatty acids associated with counterions.

In one embodiment the invention provides a method wherein the permeate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of less than or equal to about 0.07 nm .

In one embodiment the invention provides a method wherein the permeate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of less than or equal to about 0.23 nm .

In one embodiment the invention provides a method wherein the permeate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of less than or equal to about 0.21 nm .

In one embodiment the invention provides a method wherein the permeate is enriched in at least one trans-fatty acid associated with a counterion or one saturated fatty acid associated with a counterion.

In one embodiment the invention provides a method wherein the permeate is enriched in at least one saturated fatty acid associated with a counterion.

In one embodiment the invention provides a method wherein the permeate is enriched in at least one cis-fatty acid associated with a counterion.

In one embodiment the invention provides a method wherein the retentate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of greater than or equal to about 0.19 nm².

In one embodiment the invention provides a method wherein the retentate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of greater than or equal to about 0.21 nm².

In one embodiment the invention provides a method wherein the retentate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of greater than or equal to about 0.29 nm².

In one embodiment the invention provides a method wherein the retentate is enriched in at least one cis-fatty acid associated with a counterion. In one embodiment the invention provides a method wherein the first mixture comprises soybean oil wherein the fatty acid components of the soybean oil are associated with counterions.

In one embodiment the invention provides a method wherein the first mixture comprises palmitic acid, stearic acid, linolenic acid, oleic acid and linoleic acid each associated with a counterion.

In one embodiment the invention provides a method wherein the permeate is enriched in palmitic acid and stearic acid each associated with a counterion.

In one embodiment the invention provides a method wherein the retentate is enriched in linolenic acid, oleic acid and linoleic acid each associated with a counterion.

In one embodiment the invention provides a method wherein the first mixture comprises, linolenic acid, oleic acid and linoleic acid each associated with a counterion.

In one embodiment the invention provides a method wherein the permeate is enriched in oleic acid associated with a counterion.

In one embodiment the invention provides a method wherein the retentate is enriched in linolenic acid and linoleic acid each associated with a counterion.

In one embodiment the invention provides a method wherein the retentate is enriched in linolenic acid associated with a counterion.

In one embodiment the invention provides a method wherein the retentate is enriched in linoleic acid each associated with a counterion.

In one embodiment the invention provides a method wherein the permeate is removed one or more times during the separation.

In one embodiment the invention provides a method wherein the permeate is removed one or more times during the separation and replaced with a solvent.

In one embodiment the invention provides a method wherein the permeate is removed continuously.

In one embodiment the invention provides a method wherein the first mixture comprises a solvent.

In one embodiment the invention provides a method wherein the solvent comprises one or more protic or aprotic organic solvents. In one embodiment the invention provides a method wherein the solvent comprises toluene, hexane, methanol, methylene chloride, tetrahydrofuran, dimethylformamide, chloroform, benzene or acetonitrile or mixtures thereof.

In one embodiment the invention provides a method wherein the solvent comprises toluene, hexane, methanol or methylene chloride or mixtures thereof.

In one embodiment the invention provides a method wherein the solvent comprises methanol and methylene chloride.

In one embodiment the invention provides a method wherein pressure is applied to the first mixture to increase the flux of the first mixture through the membrane.

In one embodiment the invention provides a novel compound comprising a fatty acid with a counterion as described herein.

The invention will now be illustrated by the following non-limiting Example.

Example 1. Separation of Fatty Acids

Materials.

Dicyclopentadiene, fatty acids, amines and solvents were purchased at their highest purity from Aldrich or Acros and used as received.

Fabrication of PDCPD membranes.

A 20 mg mL^"1 solution of Grubbs first generation catalyst (benzylidene- bis(tricyclohexylphosphine)dichlororuthenium, bis(tricyclohexylphosphine)-benzylidine ruthenium(IV) dichloride) was made using 1,2-dichloroethane. A sample of this solution (0.72 mL, 6.0 x 10^" mmol of catalyst) was added to 12 mL of dicyclopentadiene and heated to 40 °C. Heat was used to keep dicyclopentadiene (melting point 33 °C) a liquid. This solution was immediately placed between two glass slides with 100 μπι thick paper as spacers along the edges. The sample was heated to 50 °C for 2 h and then removed from the glass slides. All PDCPD membranes used as described herein were fabricated according to this method. Separation experiments.

Some of the fatty acids shown in Figure 1 were used in the examples herein. These fatty acids all possessed 18 carbons and represented some of the most common fatty acids found in vegetable oils. Linolenic acid and linoleic acid which are two essential fatty acids that are needed within the body but that humans are not able to synthesize are also included in the examples described herein.

PDCPD membranes were fabricated as described above and shown in Scheme 1.

These membranes were highly cross-linked by the Grubbs catalyst. Dicyclopentadiene has two carbon-carbon pi bonds; one is highly strained (approximately 25 kcal/mole of ring strain) and other is less strained (approximately 7 kcal/mole of ring strain). The ring opening metathesis polymerization of the highly strained pi bond yielded polymer and the ring opening of the less strained ring yielded cross-links (Long, T. .; Gupta, A.; Miller II, A. L.;

Rethwisch, D. G.; Bowden, N. B. J Mater. Chem. 2011, 21, 14265; Gupta, A.; Rethwisch, D.

G.; Bowden, N. B. Chem. Commun. 2011, 46, 10236; Amendt, M. A.; Chen, L.; Hillmyer, M. A. Macromolecules 2010, 43, 3924; Amendt, M. A.; Roerdink, M.; Moench, S.; Phillip, W.

A. ; Cussler, E. L.; Hillmyer, M. A. Aust. J. Chem. 2011, 64, 1074; Kovacic, S.; Krajnc, P.; Slugovc, C. Chem. Commun. 2010, 46, 7504; Lee, J. K.; Gould, G. L. J Sol-Gel Sci. Technol. 2007, 44, 29; Ren, F.; Feldman, A. K.; Carnes, M.; Steigerwald, M.; and Nuckolls, C.

Macromolecules 2007, 40, 8151 ; Rule, J. D.; Moore, J. S. Macromolecules 2002, 35, 7878). In prior work it was shown that 83% of the less strained pi bond was ring opened which led to a highly cross-linked matrix (Long, T. R.; Gupta, A.; Miller II, A. L.; Rethwisch, D. G.;

Bowden, N. B. J Mater. Chem. 2011, 21, 14265; Gupta, A.; Rethwisch, D. G.; Bowden, N.

B. Chem. Commun. 2011, 46, 10236). Scheme 1 shows the polymerization of

dicyclopentadiene by the Grubbs first generation catalyst yielded a highly cross-linked solid polymeric slab. Scheme 1

The experimental apparatus for studying the permeation of fatty acids is shown in Figure 3. A 100 micron-thick PDCPD membrane was fabricated and placed between two glass reservoirs. Solvent was added to either side of the membrane. On one side of the membrane molecules that were to be studied were added with the solvent. This side was defined as the upstream side of the membrane. On the downstream side of the membrane only solvent was added. The molecules diffused from the upstream side to the downstream side of the membrane. The solvent on both sides of the membranes were constantly mixed using stir bars to ensure uniform concentrations on each side of the membrane. After a period of time, typically 24 h, well-defined aliquots of solvent from both sides of the membranes were removed and the solvent was evaporated. An internal standard of toluene or tetraethylene glycol was added prior to analysis by 1H NMR spectroscopy to allow the absolution concentrations of each molecule downstream and upstream of the membranes to be measured.

Separation of oleic acid from linoleic and linolenic acids.

The permeation of oleic and linoleic acid salts made from nine different amines were studied. Initially a mixture of oleic acid (0.426 mmol), linoleic acid (0.426 mmol), an amine (0.852 mmol) and dichloromethane (25 mL) was added to the upstream side of the

membrane. The nine amines that were studied are shown in Table 1. Dichloromethane (25 mL) was added to the downstream side of the membrane. The ratio of the concentration of the molecule in the solvent on the downstream side (S_d) to the upstream side (S_u) was measured every 24 h. The results of these experiments are shown in Table 1. Table 1

Permeation of oleic acid and linoleic acid with different amines

through PDCPD membranes.

Oleic acid S_d/S_u Linoleic acid S_d/S_u

Entry Amine 24 h 48 h 72 h 24 h 48 h 72 h Separation

factor

1 triethylamine 0.24 0.44 0.99 0.16 0.37 0.84 1.2

2 tripropylamine 0.021 0.06 0.15 0 0 0.03 5.0

3 dipropylamine 0.46 0.76 1.00 0.16 0.3 0.50 2.0

4 diisobutylamine 0.16 0.27 0.57 0.08 0.18 0.21 2.7

5 Dibutylamine 0.06 0.15 0.30 0.03 0.08 0.12 2.5

6 N-ethylpiperidine 0.22 0.36 0.71 0.09 0.18 0.30 2.4

7 triethanolamine 0.07 0.17 0.33 0 0 0.04 8.3

8 triallylamine 0.32 0.57 0.76 0 0.10 0.16 4.8

9 tripropargylamine 0.27 0.85 0.97 0 0.10 0.21 4.6

The data in Table 1 shows that the flux of the oleic acid salt differed from the linoleic acid salt depending on the amine used. The difference in flux is quantified by a "separation factor". This value is the ratio of the value at 72h for Sd/Su for the oleic acid salt divided by the value at 72h for Sd/Su for the linoleic acid salt. The separation factor provides a means to quantify the difference in flux for these two salts, and its use indicates that the difference in flux is dependent on the choice of amine.

A trend occurs for entries 2, 7, 8, and 9 in Table 1. The four amines are all tertiary and have similar sizes and shapes. In fact, the distance (measured through space) of the N atom to the terminal carbon (or oxygen for triethanolamine) vary only from 0.383 nm to 0.348 nm as seen in Figure 4. The flux of both the oleic and linoleic acid salts through the PDCPD membranes decreased as the amine used progressed from tripropylamine to triethanolamine to triallylamine to tripropargylamine. This trend in flux was similar to the trend observed for the difference in distance between nitrogen and the terminal carbon (or oxygen). The four amines also led to differences in the flux of oleic and linoleic acid salts as shown by the separation factor. Table 2 shows the separation of oleic acid and linolenic acid salts using eight different amines. These experiments were completed in a similar manner to those reported in Table 1. Oleic acid, linoleic acid, an amine, and methylene chloride were added to the upstream side of the membrane and only methylene chloride was added to the downstream side. Similar to Table 1 , a "separation factor" was used to quantify separation between the oleic acid salt and linoleic acid salt.

Table 2

Permeation of oleic and linolenic acid salts with different amines through

PDCPD membranes.

Oleic acid S_d/S„ Linolenic acid S_d/S„

Amine

Entry 24 h 48 h 72 h 24 h 48 h 72 h Separation

Factor

1 triethylaraine 0.09 0.33 1.04 0 0.24 0.52 2

2 tripropylamine 0.028 0.07 0.13 0 0 0.02 6.5

3 dipropylamine 0.41 0.76 0.99 0.09 0.16 0.20 5.0

4 diisobutylamine 0.10 0.37 0.57 0 0.08 0.10 5.7

5 dibutylamine 0.06 0.18 0.27 0 0.04 0.07 3.9

6 triethanolamine 0.14 0.21 0.25 0 0 0.02 12.5

7 triallylamine 0.32 0.47 0.79 0 0.09 0.12 6.6

8 tripropargylamine 0.21 0.71 1.02 0 0.12 0.17 6

Separation of linoleic and linolenic acid salts.

Nine amines were used to study the separation of linoleic and linolenic acid as shown in Table 3. Results are shown in Tables 1 and 2. Initially a mixture of linoleic acid (0.426 mmol), linolenic acid (0.426 mmol), amine (0.852 mmol) and dichloromethane (25 n L) were added to the upstream side of the membrane. Dichloromethane (25 mL) was added to the downstream side of the membrane. The ratio of the concentration of the molecule in the solvent on the downstream side (Sj) to the upstream side (S_u) was measured every 24 h. Table 3

Permeation of linoleic acid and linolenic acid with different

through PDCPD membranes

Linoleic acid S_d/S_u Linolenic acid S_d/S_u

Amine

Entry 24 h 48 h 72 h 24 h 48 h 72 h Separation factor

1 diisopropylamine 0.36 0.58 0.77 0.09 0.18 0.25 3.1

2 quinuclidine 0.33 0.58 1.01 0.25 0.49 0.9 1.1

3 N-ethyl-N-propyl 0.35 0.57 0.83 0.11 0.18 0.28 3.0 amine

4 Piperidine 0.43 0.65 1.00 0.3 0.61 0.92 1.1

5 azepane 0.4 0.59 0.95 0.31 0.5 0.86 1.1

6 diethanolamine 0.23 0.43 0.66 0.05 0.13 0.21 3.1

7 diallylamine 0.33 0.58 0.77 0.08 0.16 0.23 3.3

8 hexylamine 0.47 0.82 1.00 0.37 0.69 1.00 1

9 benzylamine 0.5 0.8 1.01 0.4 0.75 0.93 1.1

The results in Table 3 demonstrate that optimization of the amine can lead to separation of polyunsaturated fatty acids from one another. The amines shown in entries 1, 3, 6, and 7 lead to the largest separation factors for the fatty acid salts. Interestingly, the amines that were successful at separating linoleic from linolenic acid were disubstituted. Other amines that were investigated, including primary amines in entries 8 and 9, lead to little or no separation between the fatty acid salts. These results demonstrate that linoleic and linolenic acid salts can have different flux based on the careful selection of amines.

Separation of oleic, linoleic, and linolenic acids using quaternary amines.

Amines are known to possess offensive odors and are considered hazardous.

Accordingly, quaternary amines were investigated for separating separate oleic, linoleic and linolenic acids. Quaternary amines have negligible vapor pressure because they are salts and have very high boiling temperatures. Quaternary amines were prepared based on piperidine, pyridine and triethylamine to optimize the separations of the three fatty acids. The structures of the some of these amines are shown in Scheme 2. To have the quaternary amine as the counterion for the fatty acids it was necessary to add a base such as NaOH. All of these quaternary amines have positively charged N atoms that can act as counterions to negatively charged fatty acids.

Initially a mixture of oleic acid (0.426 mmol), linoleic acid (0.426 mmol), linolenic acid (0.426 mmol), NaOH (1.28 mmol), amine (1.28 mmol) and dichloromethane: methanol (80:20, 25 mL) was added to the upstream side of the membrane. Dichloromethane:

methanol (80:20, 25 mL) was added to the downstream side of the membrane. The ratio of the concentration of the molecule in the solvent on the downstream side (S_d) to the upstream side (S_u) was measured every 24 h. The results of these permeation experiments are shown in

Table 4.

Scheme 2. Structures of quaternary amines that were studied.

methyl pyridinum iodide ethyl pyridinum iodide propyl pyridinum iodide

dimethyl piperidinium iodide methyl ethyl piperidinium iodide methyl propyl piperidinium iodide methyl trie

thylammonium iodide tetraet y ammonium iodide

Table 4

Permeation of oleic acid, linoleic acid, and linolenic acid with different quaternary

through PDCPD membranes

Oleic acid SJS_a linoleic acid S_<j S_u linolenic acid SJS_a

Entry Quaternary amines

24 h 48 h 72 h 24 h 48 h 72 h 24 h 48 h 72 h

1 methyl pyridinium iodide 0.3₅ 0.67 0.33 0.64 0.28 0.62

2 ethyl pyridinium iodide 0.36 0.63 1.14 0.24 0.48 0.83 0.14 0.33 0.56

3 Propyl pyridinium iodide 0.37 0.₅7 0.95 0.17 0.41 0.92 0.13 0.26 0.35

4 Dimethyl piperidinium iodide 0.3 0.73 1.07 0.29 ₀.53 1.03 0.18 0.49 1.04

5 methyl ethyl piperidinium iodide 0.27 0.56 0.96 0.07 0.23 0.26 0.04 0.08 0.16

6 methyl propyl piperidinium iodide 0.26 0.61 0.74 0.06 0.06 0.15 0 0.07 0.06

7 Methyl triethylammonium iodide 0.24 0.46 0.77 0.12 0.29 0.61 0.09 0.17 0.31

8 Tetraethyl ammonium iodide 0.14 0.43 0.89 0.24 0.2 0.31 0.06 0.13 0.21

9 Propyltriethyl ammonium iodide 0.16 0.34 0.72 0.08 0.13 0.2 0.01 0.02 0.07 Table 4 shows that oleic acid had a faster permeation through the membrane than linoleic or linolenic acids for some amines. For instance, in entry 9 oleic acid had a faster permeation than either polyunsaturated fatty acid. Another result from Table 4 is that small differences in the structure of the amine may lead to large difference in permeation. For instance, going from dimethylpiperidinium (entry 4) to methyl ethyl piperidinium (entry 5) led to large differences in the permeation of the polyunsaturated fatty acid salts but the permeation of the oleic acid salts was considerably faster. The comparison of entries 7, 8, and 9 are also informative. In all three entries the flux of the oleic acid salt was similar and faster than the flux of the linoleic and linolenic acid salts. In contrast, the flux of the linoleic and linolenic acid salts decreased going from entries 7 to 8 to 9. These results show that quaternary amines can be used to differentiate the flux of oleic acid from linoleic and linolenic acids, and that these same amines can be used to differentiate the flux of linoleic from linolenic acid salts. Thus, purification of a mixture of oleic, linoleic, and linolenic acids is possible. Separation of stearic acid and oleic acid with triisobutylamine through PDCPD membranes under pressure with different solvents.

The separate fatty acid salts using pressure was investigated. In these experiments the membrane was placed horizontally in the apparatus and stearic acid (0.5 mmol), oleic acid (0.5 mmol), triisobutylamine (1 mmol) and solvent (100 mL) were added to the upstream side. Solvent (20 mL) was added to the downstream side of the membrane to swell both sides. After swelling for 30 min, a pressure of 75 psi was applied to the upstream side. The solution permeated to the downstream side within 30 min (Table 5). Two different solvents were used and both were used to separate oleic acid from linoleic acid. In both experiments most of the stearic acid (96% and 97% of the total amount of stearic acid added at the beginning of the experiments) was found in the solvent downstream of the membrane. In contrast, only 25% of the oleic acid initially added to the experiment was found in the downstream solvent. The remainder of the acids were found in the upstream solvent and within the membrane.

Table 5

Separation of stearic acid and oleic acid with different solvents under a pressure of 75 psi.

Entry Solvent Stearic acid in Oleic acid in Time (min)

downstream downstream

solvent solvent

(%) (%)

1 DCM : hexanes 96 25 20 min

(70/30)

2 Toluene: hexanes 97 25 30 min

(70/30)

Measurement and comparison of critical areas.

Differences in partitioning coefficients do not explain the differences in permeation of the fatty acid salts, so the differences in permeation must have been due to the differences in flux. In cross-linked polymer matrixes the diffusion, D, of a molecule depends exponentially on the energy of activation, E_a (kcal mol^"1) according to the equation D = D₀exp(-E_a/RT) (Crank, J. The mathematics of diffusion; Clarendon Press: Oxford, 1970). Molecules that are much smaller than the pores in a matrix can diffuse rapidly because the polymer matrix does not have to rearrange to allow them to diffuse. Molecules that are on the same size as the pores or larger than the pores diffuse slowly because the polymer matrix must deform and the value for E_a is large. In practice, the rate of diffusion in cross-linked polymers has been shown to be heavily dependent on the cross-sectional areas of molecules. For instance, in 1982 Berens and Hopfenberg plotted the log of diffusion versus the square of diameter for 18 molecules that permeated poly( vinyl chloride), polystyrene, and polymethymethacrylate (Berens, A. R.; Hopfenberg, H. B. J. Mem. Sci. 1982, 13, 283). The diffusion of He

(diameter squared = 6.66 x 10² nm²) was ten orders of magnitude faster than the diffusion of

1 2 *

neopentane (diameter squared = 3.36 x 10^" nm ). PDCPD was a highly cross-linked polymer matrix and the rate of diffusion of molecule was expected to depend on their critical areas. In prior work it was shown that molecules above a critical area of 0.50 nm² did not permeate PDCPD membranes but molecules with critical areas below 0.38 nm did permeate.

One challenge in the field of size-selective membranes is defining the critical area of a molecule. This is usually not attempted; rather, membranes are described as possessing a "molecular weight cutoff that is used to determine whether a new molecule will permeate (Fierro, D.; Boschetti-de-Fierro, A.; Abetz, V. J. Membr. Sci. 2012, 413-414, 91 ; Fritsch, D.; Merten, P.; Heinrich, K.; Lazar, M.; Priske, M. J. Membr. Sci. 2012, 401-402, 222;

Rundquist, E. M.; Pink, C. J.; Livingston, A. G. Green Chem. 2012, 14, 2197;

Sereewatthanawut, I.; Lim, F. W.; Bhole, Y. S.; Ormerod, D.; Horvath, A.; Boam, A. T.; Livingston, A. G. Org. Process Res. Dev. 2010, 14, 600; So, S.; Peeva, L. G.; Tate, E. W.; Leatherbarrow, R. J.; Livingston, A. G. Org. Process Res. Dev. 2010, 14, 1313; Szekely, G.; Bandarra, J.; Heggie, W.; Sellergren, B.; Ferreira, F. C. J. Membr. Sci. 2011, 381, 21 ; and van, d. G. P.; Barnard, A.; Cronje, J.-P.; de, V. D.; Marx, S.; Vosloo, H. C. M. J. Membr. Sci. 2010, 353, 70). The molecular weight cutoff is used although it is not meant to be a good predictor of what will permeate. It is well understood that molecular weight does not have a strong correlation with cross-sectional area. Rather, a molecular weight cutoff provides a simple, unambiguous method to suggest which molecules may permeate a membrane. The molecular weight of a molecule can be determined within minutes, but the critical area is much harder to determine and dependent on the method used.

The critical areas for the molecules used in this study were found using Spartan '08 VI .2.0. The free fatty acids were constructed and their energies were minimized in Spartan. Not surprising, the fatty acids were in the all-trans conformations. The fatty acids were rotated until the smallest rectangular cross-sectional area was found, and this value was labeled the critical area and reported in Table 5. The critical area was measured because this area was the smallest size for the pore that each molecule may diffuse through. The procedure to find the critical areas for the fatty acids salts was similar (triisobutylamine was used to form all of the salts). The energy of triisobutylamine was first minimized such that it could be docked in the same conformation with each fatty acid. Next, the energy of the fatty acid with the amine was minimized. The critical areas of the salts were found as described before.

Table 5.

Critical areas of fatty acids.

+

Critical area of Critical area

free fatty acid of fatty acid

(nm²) salt

Molecule (nm²)

Elaidic acid 0.12 0.38

Stearic acid 0.067 0.38

Oleic acid 0.21 0.59

Linoleic acid 0.34 0.97

Linolenic acid 0.36 0.94

Petroselinic acid 0.20 1.27

Vaccenic acid 0.24 0.47

In Figure 5 the seven fatty acid and fatty acids salts are shown in their energy minimized structures. The view on the right shows the orientation of the fatty acid salts that was used to find their critical area. The view on the left shows a side view of the fatty acids in the absence of amine to emphasize the curved structures of the cis-fatty acids. The cis-fatty acid salts had larger critical areas than the saturated and trans-fatty acid salts they were not completely eclipsed by the triisobutylamine. The omega ends of the cis-fatty acids were not eclipsed by triisobutylamine, and these ends were "hooks" that increased the critical areas of the cis-fatty acid salts.

It is important to note that there are other methods to measure critical areas. For instance, we defined the critical area as possessing a rectangular shape, but other shapes (i.e. sphere, square, oval, etc) can also be used and will give different values for the critical areas. The variation of the critical area depending on the method of its measurement is an important reason why many nanofiltration membranes use a molecular weight cutoff rather than a critical area cutoff. Although the absolute value for the critical areas may be debatable, it was clear from Figure 5 that cis-fatty acid salts had larger critical areas than the saturated and trans-fatty acid salts.

All publications, patents, and patent documents discussed herein are incorporated by reference herein, as though individually incorporated by reference. The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope of the invention.

Claims

1. A method for separating a mixture of two or more fatty acids associated with counterions comprising contacting a membrane with a first mixture comprising two or more different fatty acids associated with counterions, so that the mixture is fractionated into a permeate comprising one or more different fatty acids associated with counterions and a retentate comprising one or more different fatty acids associated with counterions, wherein at least one of the permeate or retentate is enriched in one or more different fatty acids associated with counterions wherein the counterion is:

(a) heteroaryl or bicyclic heterocycle;

(b) dipropylamine, diisobutyl amine, dibutylamine, N-ethylpiperidine,

2. The method of claim 1 , wherein the counterion is a heteroaryl counterion or a bicyclic heterocycle counterion.

3. The method of claim 1, wherein the counterion is dipropylamine, diisobutyl amine, dibutylamine, N-ethylpiperidine, triethanolamine, triallylamine, triporpargylamine, quinuclidine, N-ethyl-N-propyl amine, piperidine, azepane, diethanolamine, diallylamine or hexylamine.

4. The method of claim 1 , wherein the counterion is dimethyl piperidinium, methyl ethyl piperidinium, methyl propyl piperidinium, methyl triethylammonium, tetraethyl ammonium or propyltriethyl ammonium.

5. The method of any one of claims 1-4, wherein the permeate is enriched in one or more different fatty acids associated with counterions.

6. The method, of any one of claims 1-4, wherein the retentate is enriched in one or more different fatty acids associated with counterions.

7. The method of any one of claims 1-4, wherein the permeate is enriched in one or more different fatty acids associated with counterions and the retentate is enriched in one or more different fatty acids associated with counterions.

8. The method of any one of claims 1-4, wherein the membrane is an organic solvent nanofiltration membrane.

9. The method of claim 8, wherein the organic solvent nanofiltration membrane comprises polydicyclopentadiene, polyimide, polyaniline or polyacrylate.

10. The method of claim 9, wherein the organic solvent nanofiltration membrane comprises polydicyclopentadiene.

11. The method of claim 9, wherein the organic solvent nanofiltration membrane comprises a highly crosslinked polydicyclopentadiene matrix.

12. The method of claim 11 wherein the ratio of crosslinked double bonds to

uncrosslinked double bonds in the highly cross-linked polydicyclopentadiene matrix is at least about 2:3.

13. The method of claim 11 wherein the ratio of crosslinked double bonds to

uncrosslinked double bonds in the highly cross-linked polydicyclopentadiene matrix is at least about 3:2.

14. The method of claim 1 1 wherein the ratio of crosslinked double bonds to

uncrosslinked double bonds in the highly cross-linked polydicyclopentadiene matrix is at least about 4:1.

15. The method of any one of claims 1-14, wherein the membrane is a part of an assembly that comprises two or more membranes.

16. The method of any one of claims 1 -14, wherein membrane is part of a spiral wound module.

The method of any one of claims 1-14, wherein the membrane separates molecules on their critical areas.

18. The method of any one of claims 1-17, wherein each fatty acid of the first mixture, permeate and retentate is associated with one counterion and wherein the counterions are identical.

19. The method of any one of claims 1-18, wherein the counterion has a critical area that allows one or more different fatty acids associated with the counterion of the first mixture to permeate the membrane at a higher rate than one or more different fatty acids associated with the counterion of the first mixture so that the permeate is enriched in one or more different fatty acids associated with the counterion.

20. The method of any one of claims 1-18, wherein the counterion has a critical area that impedes the permeation of one or more different fatty acids associated with the counterion of the first mixture through the membrane so that the retentate is enriched in one or more fatty acids associated with the counterion.

21. The method of any one of claims 1-20, wherein the heteroaryl counterion is a 5 or 6- membered monocyclic heteroaryl counterion and the bicyclic heterocycle counterion is a 5 to 8 membered bicyclic heterocycle counterion.

22. The method of any one of claims 1-20, wherein the heteroaryl counterion is a 5 or 6- membered monocyclic heteroaryl counterion.

23. The method of any one of claims 1-20, wherein the heteroaryl counterion is a 5 or 6- membered monocyclic heteroaryl counterion wherein one of the ring nitrogen of the heteroaryl counterion is substituted with one or more (C₁-C₈)alkyl and wherein the heteroaryl counterion is optionally substituted with one or more groups selected from (C₁-C₆)alkyl, (Cr C₆)haloalkyl (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H,

(C₂-C₆)alkenyl, (C₂- C )alkynyl and (C₃-C₇)cycloalkyl.

24. The method of any one of claims 1-20, wherein the heteroaryl counterion is a

6-membered monocyclic heteroaryl counterion wherein one of the ring nitrogen of the heteroaryl is substituted with a (Q-C^alkyl, and wherein the heteroaryl is optionally substituted with one or more groups selected from (Q-C^alkyl, (C₁-C₆)haloalkyl (C₂- C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (Q-C^alkyl, (C₂-C₆)alkenyl, (C₂-C₆)alkynyl and (C₃- C₇)cycloalkyl.

25. The method of any one of claims 1-20, wherein the heteroaryl counterion comprises a pyridyl.

26. The method of any one of claims 1 -20, wherein the heteroaryl counterion is pyridyl wherein the pyridyl ring nitrogen is substituted with a (d-C₈)alkyl and wherein the pyridyl is optionally substituted with one or more groups selected from (Q-C^alkyl, (C C₆)haloalkyl (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (C₁-C₆)alkyl, (C₂-C₆)alkenyl, (C₂-C₆)alkynyl and (C₃- C₇)cycloalkyl.

27. The method of any one of claims 1-20, wherein the heteroaryl counterion is a compound of formula I:

wherein R¹ is a

each R² is independently is selected from (CrC₆)alkyl,

(C C₆)haloalkyl (C₂-C₆)alkenyl, (C₂-C₆)alkynyl, (C₃-C₇)cycloalkyl, halo, -OR^a, -NR^a ₂ and -NR^a ₃ wherein each R^a is independently selected from H, (Q-C^alkyl, (C₂-C₆)alkenyl, (C₂- C₆)alkynyl and (C₃-C₇)cycloalkyl; and n is 0, 1, 2, 3, 4 or 5.

28. The method of any one of claims 1-20, wherein heteroaryl counterion is N- methypyridinium, N-ethylpyridinium or n-propylpyridinium.

29. The method of any one of claims 1-20, wherein the bicyclic heterocycle counterion comprises a 5 to 8 membered bicyclic heterocycle counterion.

30. The method of any one of claims 1-20, wherein the bicyclic heterocycle counterion is an unsubstituted 5 to 8 membered bicyclic heterocycle.

31. The method of any one of claims 1 -20, wherein the bicyclic heterocycle counterion is quiniclidine.

32. The method of any one of claims 1-31, wherein the first mixture comprises:

a) at least one cis-fatty acid associated with a counterion and

33. The method of any one of claims 1-31, wherein the first mixture comprises at least one cis-fatty acid associated with a counterion and at least one saturated fatty acid associated with a counterion.

34. The method of any one of claims 1-31, wherein the first mixture comprises two or more different cis-fatty acids associated with counterions.

35. The method of any one of claims 1-34, wherein the permeate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of less than or equal to about 0.12 nm².

36. The method of any one of claims 1-34, wherein the permeate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of less than or equal to about 0.07 nm².

37. The method of any one of claims 1 -34, wherein the permeate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of less than or equal to about 0.23 nm².

38. The method of any one of claims 1-34, wherein the permeate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of less than or equal to about 0.21 nm².

39. The method of any one of claims 1-36, wherein the permeate is enriched in at least one trans-fatty acid associated with a counterion or one saturated fatty acid associated with a counterion.

40. The method of any one of claims 1-36, wherein the permeate is enriched in at least one saturated fatty acid associated with a counterion.

41. The method of any one of claims 1-38, wherein the permeate is enriched in at least one cis-fatty acid associated with a counterion.

42. The method of any one of claims 1-41, wherein the retentate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of greater than or equal to about 0.19 nm².

43. The method of any one of claims 1-41, wherein the retentate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of greater than or equal to about 0.21 nm .

44. The method of any one of claims 1-41, wherein the retentate is enriched in at least one fatty acid associated with a counterion wherein the fatty acid has a critical area of greater than or equal to about 0.29 nm .

45. The method of any one of claims 1-44, wherein the retentate is enriched in at least one cis-fatty acid associated with a counterion.

46. The method of any one of claims 1-31 ,wherein the first mixture comprises soybean oil wherein the fatty acid components of the soybean oil are associated with counterions.

47. The method of any one of claims 1-31 wherein the first mixture comprises palmitic acid, stearic acid, linolenic acid, oleic acid and linoleic acid each associated with a counterion.

48. The method of claim 46 or claim 47 wherein the permeate is enriched in palmitic acid and stearic acid each associated with a counterion.

49. The method of any one of claims 46-48 wherein the retentate is enriched in linolenic acid, oleic acid and linoleic acid each associated with a counterion.

50. The method of any one of claims 1-31 wherein the first mixture comprises linolenic acid, oleic acid and linoleic acid each associated with a counterion.

51. The method of claim 50, wherein the permeate is enriched in oleic acid associated with a counterion.

52. The method of claim 50 or claim 51 , wherein the retentate is enriched in linolenic acid and linoleic acid each associated with a counterion.

53. The method of claim 50 or claim 51 wherein the retentate is enriched in linolenic acid associated with a counterion.

54. The method of claim 50 or claim 51 , wherein the retentate is enriched in linoleic acid associated with a counterion.

55. The method of any one of claim 1-54, wherein the permeate is removed one or more times during the separation.

56. The method of any one of claim 1-54, wherein the permeate is removed one or more times during the separation and replaced with a solvent.

57. The method of any one of claim 1-54, wherein the permeate is removed continuously.

58. The method of any one of claim 1-57, wherein the first mixture comprises a solvent.

59. The method of claim 58, wherein the solvent comprises one or more protic or aprotic organic solvents.

60. The method of claim 58, wherein the solvent comprises toluene, hexane, methanol or methylene chloride or mixtures thereof.

61. The method of claim 58, wherein the solvent comprises methanol and methylene chloride.

62. The method any one of claim 1 -61 , wherein pressure is applied to the first mixture to increase the flux of the first mixture through the membrane.

63. The method of any one of claims 17, 35-38 or 42-44, wherein the critical area of the first component is the lowest rectangular cross-sectional area of the first component in its energy minimized conformation and the critical area of the second component is the lowest rectangular cross-sectional area of the second component in its energy minimized conformation.

64. The method of any one of claims 1-4, wherein the membrane comprises

polydicyclopentadiene.

65. The method of claim 11 or claim 64, wherein the polydicyclopentadiene or the highly crosslinked polydicyclopentadiene has been prepared by ring-opening metathesis polymerization of dicyclopentadiene.

66. The method of claim 11 or claim 64, wherein the polydicyclopentadiene or the highly crosslinked polydicyclopentadiene has about four sp² hybridized carbons for every about ten carbons in the polymer.

67. The method of claim 1 1 or claim 64, wherein the polydicyclopentadiene or the highly crosslinked polydicyclopentadiene has four sp² hybridized carbons for every ten carbons in the polymer.