JPS60238329A - Manufacture of fluorine-containing cation-exchange membrane - Google Patents

Abstract

PURPOSE:To introduce a carboxylic acid group in high efficiency, and to obtain a useful ion-exchange membrane having carboxilic acid group, by reacting a fluorine-containing polymer membrane having a specific functional group with a mineral acid, and hydrolyzing the reaction product. CONSTITUTION:A fluorine-containing polymer membrane is made to react with an iodine compound in the presence of a radical initiator to obtain a raw membrane of a fluorine-containing polymer having -CFI- group and/or -CF2I group. The obtained membrane is made to react with a mineral acid, and then hydrolyzed to obtain a carboxyl-containing membrane. The raw membrane is the one wherein -CFI- or -CF2I groups are present in the region within 1mu from at least one surface of the membrane, the amount of the functional group is 4-40wt% in terms of iodine, the fluorine atom is bonded with -CFI- or -CF2I-, the thickness is 0.05-2mm., and a dimension is >=1cm (e.g. perfluorocarbon polymer).

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a method for producing a fluorine-containing cation exchange membrane.
The purpose of this method is to perform a carboxyl group introduction reaction on a fluorine-containing polymer membrane having a 2I group to impart carboxyl groups. The object of the present invention is to obtain a fluorine-containing cation exchange membrane.

Fluorine-containing polymer membranes have a wide range of uses, simply as membrane materials with excellent heat resistance and chemical resistance, but they can also be used to add new functions to these special properties. Many attempts have been made to develop useful products with high added value, such as ultrafiltration membranes, reverse osmosis membranes, and ion exchange membranes. In these cases, there are various methods such as counting the pore size distribution as in ultrafiltration membranes, and adding new functions by bonding ion exchange functional groups. Today, the brine electrolysis industry is converting to electrolysis methods that do not use mercury due to the problem of environmental pollution caused by mercury.When using membranes in such an electrolytic atmosphere, fluorine-containing membranes are preferred from the viewpoint of heat resistance and chemical resistance. It is preferable to use a Bar7/I/Oli type film material. For example, in the case of a water-permeable diaphragm, it has extremely good oxidation resistance, but it also has strong oxidation resistance. There are various proposals for this hydrophilic treatment, but it is preferable that a cation exchange group is bonded to the fluorine-containing polymer membrane.

Also important are ion exchange membranes that are substantially water-impermeable. Furthermore, in the case of ultrafiltration membranes, reverse osmosis membranes, etc., the fact that cation exchange groups are bonded thereto affects the selectivity for the substance to be filtered, the salt rejection rate, etc.

As such cation exchange groups, sulfonic acid groups are generally used due to their thermal stability and other reasons, and carboxyl groups are rarely used, especially from the viewpoint of thermal stability during molding of fluorine-containing polymer compounds. do not have. Today, such a fluorine-containing polymer film with a cation exchange group bonded to it is produced by hydrolyzing a copolymer film of perfluoroalkyl vinyl ether sulfonyl fluoride and tetrafluoroethylene to form a sulfonic acid group. There is something I did. Although this is extremely stable against oxidizing agents, when it is used as a diaphragm for the above-mentioned salt electrolysis, etc., there are some difficulties in electrolytic performance.

In addition, there are many cases in which a polymeric membrane having carboxyl groups instead of sulfonic acid groups is required, and from this perspective we have proposed the method described in Japanese Patent Application Laid-open No. 50-96472. In JP-A-50-108182, he proposed an ion-exchange membrane with carboxyl groups as cation-exchange groups, and in JP-A-51-122677, he proposed the conversion of a membrane with sulfonic acid groups to a membrane that rains carboxyl groups. One method has been completed. Many inventions of this keyaki were made later, in JP-A No. 51-1
30495 also describes a method of copolymerizing a monomer having a carboxyl group with tetrafluoroethylene, and also JP-A No. 1852-24175 and JP-A No. 52-24176.
, it has been proposed to convert a sulfonylpalite group into a carboxyl group by reduction treatment.

Now, in the method of polymerizing 7-mers containing carboxyl groups to form a film-like material, there is a high possibility of decarboxylation at the molding temperature unless special measures are taken.
In the method of No. 1851-122677, the compound having an amino group and a carboxyl group or a functional group easily converted into a carboxyl group to be bonded to sulfonylpalite is mainly hydrocarbon-based, and even after fluorination treatment, Fluorination &
! Good has its limits. In addition, the method of converting a sulfonyl halide group into a carboxyl group by reduction treatment has a low reaction rate, and the sulfonyl halide group often decomposes and converts into another inert group other than a carboxyl group. When used as an ion exchange membrane, a rise in the membrane's instantaneous resistance is unavoidable. Apart from the above, we conducted research on a method for more efficiently converting a sulfonyl halide group into a carboxyl group, and as a result, we found that in the presence of a radical initiator in a fluorine-containing polymer film, as described above, -CFI- group and/or -CF2I by reacting with an iodine compound
A thick film of a fluorine-containing polymer film having a group is obtained, then a mineral acid is reacted with the raw film of the fluorine-containing polymer film, and carboxyl groups are imparted by subsequent hydrolysis. We arrived at fluorine cations.

The thick fluorine-containing polymer film used in the present invention is -C
There are no particular limitations as long as it represents an FI- group and/or a -CF3I group. However, preferably, -C
FI- group and/or -CF2I group are present, and the functional group thereof is present in an amount of 4% to 40% by weight as iodine, and furthermore, a fluorine atom and a -CFI- group and/or -C
A film-like material in which F2I groups are bonded uniformly or non-uniformly and generally has a thickness of 0.05 m to 2 m and a length of 1 m or more in one direction is preferably used. When a thick film containing less than 4% (by weight) of iodine is used, the electrical resistance of the fluorine-containing cation exchange membrane obtained by the present invention tends to increase; When a thick film containing more than 40% by weight is used, the strength and electrochemical properties of the fluorine-containing cation exchange membrane tend to decrease.

In addition, since the -CFI- group has a lower conversion yield to a carboxyl group than the -CF group, it is preferable that the iodine contained in the group is at least 10 mol% or more.
It is desirable that 50 mol% or more is present as a -CF21 group. -CFI- group and/or -CF2 in this case
The method of introducing I group is I) easily -CFI- group and/or -
A method of reacting an iodine compound with a fluorine-containing polymer film having a functional group that can be converted into a CF2I group, for example, a method of reacting an iodine compound with a fluorine-containing polymer film having a sulfonyl halide group; In the case of a molecular compound, especially a fluorine-containing polymer membrane having double bonds, ICl1. The desired fluorine-containing polymer membrane can be produced by carrying out an addition reaction of I2 etc. in the presence or absence of a suitable solvent.

In addition, in the case of a fluorine-containing polymer membrane that does not have unsaturated bonds, there are methods to react in the same manner as above. Among these, the sulfonyl halide group in the case of ■) is sulfonyl fluoride.

Refers to sulfonyl chloride group, sulfonyl bromide group, sulfonyl iodide, etc. Also, the fluorine atom is -
It is important that the F2I group is bonded to the α-position carbon to which the =crt- group is bonded, and more preferably it is a perfluoro group. Specifically, perfluor 0 (
A copolymer of perfluoroalkyl vinyl ether sulfonyl fluoride and tetrafluoroethylene, whose main component is 3,6-shioxer 4-methyl-7-octensulfonyl fluoride), formed into a membrane or hydrolyzed. These include sulfonic acids converted into sulfonyl fluorides and sulfonyl chlorides.

Further, in I) and II), when the iodine compound is reacted in the presence of a radical initiator, heat and light may be used.

Alternatively, it is preferable to coexist with ionizing radiation.

That is, in the presence of a vapor of elemental iodine, a method of irradiating ultraviolet rays, a method of heating, a method of irradiating with ionizing radiation, a method of dissolving elemental iodine in an organic solution (1ω, alcohol, carbon disulfide), etc. Particularly favorable results can be obtained when a method of heating to a temperature above .degree. C. or a method of irradiating with visible light or ultraviolet light in the presence of a photosensitizer is used in combination. Generally, when heat is used, it is carried out at a temperature of 0° C. or higher, preferably 50° C. or higher, within a temperature range in which the fluorine-containing polymer film does not decompose. When using light, ultraviolet light is preferred, but it is not necessarily ultraviolet light, and visible light may be used in the presence of a sensitizer. In this case, conventionally known photosensitizers can be used without any restriction, and can be appropriately selected depending on the purpose.

Ionizing radiation uses α, β, γ, and X-rays, and the irradiation rate is 11M.
The optimal dose is selected within the range of 0.1 to 30 Mrad, but it must be carried out within a range that does not cause significant decomposition of the fluorine-containing polymer film or decrease in mechanical strength. Trout. In addition, conventionally known organic and inorganic radical initiators can be used without restriction, and in the case of single-organic radical initiators, hydrocarbon-based ones, fluorine-containing ones, perfluorinated ones, etc. can be selected as appropriate. and the half-life of decomposition is 1 at temperatures above 40°C.
A time of 0 hours or longer is preferred and can be used without any restriction.

Specifically, benzoyl peroxide, α-l-azohisisobutyronitrile, lauryl peroxide, ditertiary butyl peroxide, N2F2. (CF3
COO)2 etc. are exemplified. Further, when the radical initiator is used in combination with heat, light, ionizing radiation, etc., an even greater effect appears, but the conditions are selected so that the reaction proceeds most efficiently.

The iodine compound used here may be elemental iodine in the form of a gas, a solution, or a solid, and the concentration is not particularly limited, but it is usually used from 0.00% to a saturated solution.

One of the solvents used in the solution state is selected so that it can dissolve iodine, and also for the purpose of counting the degree of reaction to the polymer film when carrying out the reaction. For example, if you want to cause a reaction only near the surface of a polymer film, use a material that has poor affinity for the polymer film so that only the surface layer reacts, and even the inside of the film. When it is desired to proceed with the reaction, it is preferable to use a solvent that has a good affinity for the membrane-like dry matter and can effectively swell the membrane-like substance. Inorganic and organic iodine salts are used as other iodine compounds, and in the case of inorganic iodine salts, the cation is an alkali metal salt.

All alkaline earth metals, 1M iodine soil, #J salt, etc. are not particularly limited. More specific examples include sodium iodide, potassium iodide, lithium iodide, cesium iodide, and calcium iodide.

Magnesium iodide, strontium iodide.

Cobalt iodide, nickel iodide, iron iodide.

Copper iodide and the like are preferred. In addition, as cations for organic iodine salts, organic substances bonded with so-called onium bases such as primary, secondary, and tertiary amines, quaternary ammonium bases, arsonium bases, phosphonium bases, stibonium bases, and sulfonium bases are used. It will be done. The organic chain of the amine is not limited to saturated or unsaturated straight-chain bipartite chain alkyl groups, cyclic ones, those having an aromatic ring, those having a heterocyclic ring, etc. More specific examples include alkylammonium iodine salts such as tetramethylammonium iodo.

Hydroiodide of aniline, hydroiodide of diethylamine, hydrogen iodide of triethanolamine, hydroiodide of triethylamine.

etc. are suitable. In addition, iodine compounds that are not in the form of salts, but those that are bonded by covalent bonds may be effective in some cases. For example, iodotyrosine. Now, when these iodine compounds are reacted in the presence of the energy source mentioned above, there are many unclear points about what kind of reaction takes place, and it has not been fully elucidated, but we believe that the following reaction proceeds. It is estimated based on the current situation and various analyses.

-CF=CF2+ 12 Sakaki-CFI-CF2I (1)
-CF2-CF3+ I2→-CF2CF2I+IF
(2) -CF2So□X〇Work2→-CF 2I+SO
□↑IX, (3) Also, depending on the selection of reaction conditions, -CF2CF3+ I2-) -CF2CF3 may occur as a side reaction.
+IF (4)-CF2CF3 + I2→-CI2C
F3 +IF (5)-CF2SO2X+■2→-CF
2So2I+IX (6)→-CF2XOGF, C
1, Br) (7) is also considered to be progressing, and in the case of the reaction (5, 6, 7), sulfonic acid groups are generated again due to hydrolysis, or other undesirable reactions occur. This is not preferred from the perspective of the purpose of the present invention, but it is preferable to select and carry out conditions such that reactions (1) (2 and 3) proceed as much as possible. Next, after carrying out the reaction under conditions that produce as many compounds of the form shown in formulas (1) and (3) as possible, in the present invention, a carboxyl group introduction reaction is carried out to impart a carboxyl group. It is characterized by

A typical example of the carboxyl group introduction reaction is freezing.

A thick film of a fluorine-containing polymer film having -CFI- groups and/or -CF2I groups as functional groups is reacted with mineral acid to convert the functional groups into functional groups having ester bonds, and then hydrolyzed. It's a method.

That is, specifically, allantoic membrane sulfonic acid.

1 fluorosulfonic acid 1 fuming sulfuric acid, concentrated sulfuric acid 1 mineral acid such as oleum, concentrated nitric acid, concentrated hydrochloric acid, particularly preferably chlorosulfonic acid or fluorosulfonic acid, generally within the range of 50° C. to the decomposition temperature of the membrane, particularly preferably: Process within the range of 100° to 250°C. Next, a hydrolysis treatment is carried out in an alkaline aqueous solution such as caustic soda or soda carbonate, generally at a temperature ranging from room temperature to reflux, to convert the ester obtained by the mineral acid treatment into a carboxyl group.

The allantoic membrane that undergoes the carboxyl group-introducing reaction has been described above, and a typical preferred example thereof is a membrane-like allantoic membrane of barfluor U carbon.

The carboxyl group introduction reaction described above is a polymer reaction, unlike the conventional case of low molecular weight compounds in which a perfluoroalkyl group and iodine are bonded, so the reaction is accelerated in some cases and may also cause steric hindrance.

In some cases, the speed is slowed down by diffusion of reaction reagents and differences in membrane swelling properties depending on the solvent used. Also, the amount of reaction reagent used is present in the thick film - CFI+.

It is not necessarily necessary to convert all of the -CF2I groups into carboxyl groups, and depending on the purpose of use of the cation exchange membrane obtained in the present invention, the surface layer portion of one or both sides of the membrane or a certain thickness may be converted into carboxyl groups. Crawling may be preferable. Therefore, the amount of the reaction reagent used should be 0.1 mol% or more based on the iodine present in the film,
It is necessary to take different considerations than when carrying out a conventional reaction with a low-molecular-weight compound having a perfluoroalkyl group and iodine. In addition, the reaction time cannot be determined unconditionally depending on the reaction temperature, the concentration and molar ratio of the reaction reagents, the reaction conditions such as the solvent used, and the intended use of the membrane obtained after the reaction, but it is generally between a few seconds and 100 hours. You can select it as appropriate depending on the purpose of use.

In the present invention, it is important to efficiently introduce <-CFI- group 1-CF2I group into the membrane and to efficiently convert it into a carboxyl group. A preferred embodiment of the fluorine-containing cation exchange membrane obtained by the present invention is that at least 1 micron of the surface layer has a 0.05 mm per gram dry membrane (
HW) or more carboxyl groups, and the surface layer has at least 0.2 millimeters per gram dry membrane (H type) or more cation exchange groups. Thickness exchange capacity is 0.5 mm! J toffi
/, Durham dry membrane (H type), the electrical resistance of the membrane increases and may even exceed 20Ω-d, in which case it is no longer of industrial significance. Furthermore, when the carboxyl group is present in the surface layer in an amount less than 0.2 millimeters per gram dry film (HW), the effect of the presence of the carboxyl group is weakened, and the industrial significance tends to be lost. Namely 1. The effect of improving the fixed ion concentration of the membrane is weakened.

Furthermore, the carboxyl groups may be uniformly dispersed throughout the film, may be present biased toward the surface layer, or may be present only on one surface layer. Furthermore, there are cases where only carboxyl groups are present, in which case at least 0.05
It is desirable that an amount of greater than or equal to milliequivalents/gram dry film be present. At the same time, sulfonic acid groups may coexist, and the distribution of the sulfonic acid groups and carboxylic acid groups may exist in a gradient such that they cross each other from the blood on the opposite side of the membrane. Of course, layers containing carboxylic acid and sulfonic acid may exist in a fused state. At the same time, by impregnation polymerization or other methods, sulfonic acid groups,
A cation exchange group other than a carboxylic acid group may coexist, and specifically, a phosphoric acid group, a sulfite group, a sulfuric acid ester group, and a phosphoric acid ester group.

There is no problem even if a hydroxyl group, an acid amide group bonding a dissociated hydrogen atom through an acid amide bond, etc. are present together.

Now, when carboxylic acid groups are efficiently introduced into the membrane in this way, the water content decreases and the defined ion concentration increases as described above, and when both sides are treated in this way, it is difficult to use either method. Also, when only one side is treated in this way, salts, bases, and acids are The amount of diffusion of ions decreases, Donnan rejection increases, and current efficiency can be improved.

The content of the present invention will be explained in more detail in Examples below, but the present invention is not limited to these Examples.

Next, regarding the evaluation of membrane properties, the electrical resistance was 3.5N-N.
aCA! and 6.0N-NaOH on both sides of the membrane,
This is a value measured by 100 cycles of alternating current at 5°C. The exchange capacity was determined by immersing the membrane in acid form in a certain amount of 1N-NaOH and leaving it for 2 hours, and then reducing the amount of NaOH neutralized by the acid in the membrane to 0. IN-MCI! It was back titrated and expressed as the weight per dry Ml, 9 (H type). The water content was measured at room temperature after boiling in pure water at 100°C for 30 minutes, and was expressed as a percentage of the H-type dry film IJ. For the electrolysis experiment, an electrolytic cell with an effective tortoise volume of 1d7F+' was used, and the anode was a normal metal anode made of titanium lath coated with titanium dioxide and ruthenium dioxide.
A soft iron wire mesh was used as the cathode. The membrane was supported by the anode, and the gap between the cathode and the membrane was maintained at approximately 3 mm. The temperature during electrolysis was maintained at 80 to 90° C., and pure water was supplied to the cathode chamber to constantly obtain NaOH at a constant concentration. A saturated alkali solution was supplied to the anolyte, and in the case of saline, the solution was discharged at approximately 3.0 to 35N. In addition, Ca+1 and M! in the brine used! ! "The sum of both was kept below 1 ppm.

Example 1 Tetrafluoroethylene and barfluoro tff (3°6-
After copolymerizing Sioxer (4-methyl-7-octensulfonyl fluoride), the resulting film (thickness: 0.15 mm) was immersed in a 10% NaOH methanol solution and hydrolyzed. The membrane was treated to form a perfluorosulfonic acid type cation exchange membrane. The exchange vessel was a 0.91 meq/gram dry membrane (Type H). This was immersed in 20% nitric acid at 80°C for 16 hours to convert it to the sulfonic acid form, and then 13
The sulfonic acid groups were converted to sulfonyl chloride groups by immersion reaction at 0° C. for 72 hours. In order to see the rate of conversion of this to sulfonyl chloride, the sex appeal resistance was measured in 1.0N-HCJ with 1000 cycles of alternating current.
．． It was about 45,000 Ω-d at 0°C.

This membrane was then soaked in an ethanolic saturated solution of iodine.

The film was immersed in a solution containing 2 parts of ditertiary butyl peroxide in an autoclave at room temperature, left for 16 hours to fully impregnate the film, and then heated in an oil bath at 130°C for 24 hours. . After cooling, the membrane was taken out and washed thoroughly with ethanol, and then with carbon disulfide.The infrared spectrum of the surface reflection was taken, and the absorption of 1420cIrL-', which corresponds to sulfonyl chloride, disappeared. Ta. Therefore, when we determined the amount of iodine in this membrane using fluorescent X-rays, the ion exchange capacity was 0.85.
The equivalent of milliequivalents/gram of dry membrane was bound.

This quantitative determination was performed using an anion exchange membrane NEO8EPTA A.
V-4T (8 strongly basic anion exchange membrane manufactured by Tokuyama Soda, exchange membrane 12.2 mm/g dry membrane) was mixed with 1.0 mm of sodium iodide. An anion exchange membrane which had been immersed in an ON aqueous solution and had been completely converted into an iodine ion type by repeatedly changing the solution was used as a standard sample, and the amount of iodine bonded to the membrane was calculated based on the ratio to this. Next, this iodine-bonded perfluorinated film-like molecular compound was placed in a large excess of oleum containing 30% excess of sulfur trioxide at 130°C for 40 minutes.
After soaking for an hour, it was left to cool, and the results were 98% and 80%.

The sample was immersed in 40% diluted sulfuric acid, followed by water, and finally in a methanol solution of 101 NaOH. When the surface of the obtained film-like material was observed using a reflection infrared absorption spectrum, strong absorption at about 1690 au-' was observed. In addition, absorption of sulfonic acid of about 1060 cm-'' was ruled as a weak shoulder.

Now, when we measured the color resistance of these films, when they were still sulfonic acid type films, it was 195 Ω-d, the exchange capacity was 0.91 meq/g dry film (HW), and the water content ff was 117%, which was calculated as follows. The fixed ion i11 degree is 5
The concentration was 2626 bottles. On the other hand, the membrane subjected to the method of the invention had an exchange capacity of 0.87 meq/g dry membrane (H type).

The water content was 12%, and the fixed ion concentration was 7.25 moles. However, in the film of the present invention, the electrical resistance increased to 3.85 Ω-d.

In addition, after reacting with iodine, reacting with fuming sulfuric acid, and then only hydrolyzing the membrane, we stained it with crystal violet and observed the thickness of the stained layer with a Wk microscope, and found that the areas marked "A" were extremely clearly stained from both sides of the membrane, and the center of the membrane was clearly stained. Part 4 was only slightly stained.

Next, when this membrane was used to electrolyze saturated saline at 20 A/dm, the cell voltage of the membrane containing only sulfonic acid groups was 3.52 V, and the 1 flow efficiency was 61% when 6ON-NaOH was obtained. However, the membrane produced by the method of the present invention has a cell voltage of 3
．． 98V, current efficiency is the same <8. ON-N a O
H was obtained and it was 94%. Also, NaC in NaOH
The former also has 45 ppp in terms of 48% NaOH.”
However, in the latter case, it was 21 ppm.

Example 2 When the same copolymer membrane of tetrafluoroethylene and perfluoroalkyl vinyl ether sulfonyl fluoride used in Example 1 was hydrolyzed, the exchange capacity was 0,
91 meq/g dry film (Hffi) P
A sulfonyl bromide film was formed by heating at 130° C. at about 80 o'clock in a mixture of B r 3 and PBr 3 at a ratio of 2:1. In order to check the degree of conversion from sulfonic acid to sulfonyl bromide, the electrical resistance was measured in 1.0N-HCAi at 1000 cycles of AC.
It was about 28,000 Ω-d at 25°C.

A solution of 3 parts of benzoyl peroxide dissolved in 100 parts of a saturated solution of iodine in ethanol was coated uniformly on one side of the film, and once it had penetrated into the film to some extent, both sides were covered with cellophane, and the 100 parts of iodine saturated solution in ethanol was applied. Heat treatment was performed at ℃ for 18 hours.

After cooling, the film was extracted with ethanol and carbon disulfide to remove adsorbed or attached iodine, and then exposed to fluorescent X.
! When iodine and sulfur were mixed with +J, S on the surface layer of the membrane
was reduced to about λ of the original gill, and iodine was bound in an amount equivalent to about 9 times the exchange capacity of the membrane. This was heated in fluorosulfonic acid at 100°C for 6 hours, then 10% Na
Hydrolysis was carried out by heating in an OH methanol solution.

The electrical resistance of this membrane was 2.63 Ω-d, and the fixed ion concentration was 6.5 m. When this was used to electrolyze saturated saline with the side where the carboxyl group is facing the cathode, 8.0N-NaOH was obtained and the current efficiency was 92%.
0I (The NaCl needle inside is 301 in terms of 48% NaOH)
)1)! It was 11.

When staining with matacrystal violet, only one surface layer was markedly stained, extending over about 15 microns, and the back side was not stained.

Example 3 Using the same barfluoroalkyl vinyl ether sulfoyl chloride type membrane used in Example 1, 1.0N-HC1
! The electrical resistance inside is approximately 45,000Ω-d at 25.0℃
Example 1
Similarly, after treatment with fuming sulfuric acid, treatment with 10% Na0)l aqueous solution, and further treatment with 10% NaOH methanol solution at 80°C.
The properties of the membrane after treatment were measured and further electrolysis of saturated saline solution was carried out. The results are shown in Table 1.

Claims (1)

[Claims] A fluorine-containing polymer membrane is reacted with an iodine compound in the presence of a radical initiator to obtain a fluorine-containing polymer membrane having -CFI- groups and/or -CF2I groups, and then the fluorine-containing polymer is 1. A method for producing a fluorine-containing cation exchange membrane, which comprises reacting a membranous material membrane with a mineral acid and then hydrolyzing it to impart carboxif groups.