JP2013227403A - Heptamethine cyanine compound - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heptamethine cyanine compound having excellent fastness such as resistance to moist heat, water resistance and heat resistance, and further having near infrared-absorbing characteristics, and to provide a dye composition thereof.SOLUTION: There are provided a heptamethine cyanine compound having near infrared-absorbing characteristics and represented by general formula (1), and an oily or aqueous dye composition containing the compound. (In formula (1), Rand Rare each independently halogen, 1-5C alkyl, alkoxy, phenyl or nitro; p and q are each an integer of 0-2; Rand Rare each independently 1-5C alkyl; and D is a linking group for forming a heptamethine cyanine compound).

Description

  The present invention relates to a novel heptamethine cyanine compound.

  Heptamethine cyanine compounds with benzoxazole nuclei bonded to both ends of the methine chain are widely used as near-infrared absorbing dyes, and are used in a wide range of applications such as oil-based inks, near-infrared absorbing filters, and near-infrared absorbing films. . In general, the properties required for near-infrared absorbing dyes vary depending on the application, but it is particularly strongly required that the colored material is fast against light, heat, and the like.

  In general, a heptamethine cyanine compound has a high molecular extinction coefficient (mass extinction coefficient) and is excellent in near-infrared absorptivity, but has disadvantages such as poor heat fastness, light resistance, heat resistance, water resistance and the like. For this reason, there is a need for a heptamethine cyanine compound having a high molecular extinction coefficient and a high robustness, but no heptamethine cyanine compound having these performances has been found. Non-patent document 1 describes a heptamethine cyanine compound having an iodine ion as a counter ion, but as a result of the study by the present inventors, the heptamethine cyanine compound described in non-patent document 1 is resistant to heat and moisture. Fastness such as light resistance, heat resistance and water resistance was insufficient. Further, Patent Document 1 describes a heptamethine cyanine compound having a tristrifluoromethanesulfonylmethide anion, but does not describe a heptamethine cyanine compound in which a benzoxazole nucleus is bonded to both ends of a methine chain.

Dyes and Pigments 53 (2002) 143. JP 2008-088426 A

  An object of the present invention is to provide a novel heptamethine cyanine compound having near infrared absorption properties excellent in fastness such as moisture resistance, light resistance, heat resistance and water resistance, and a dye composition using the compound as a dye. And

  As a result of intensive studies to solve the problems as described above, the present inventors have found that the ptametin cyanine compound having a specific structure has drastically improved fastness such as moisture and heat resistance as compared with the prior art. The headline and the present invention have been completed.

（式（１）においてＲ_１及びＲ_２はそれぞれ独立にハロゲン原子、炭素数１〜５のアルキル基、アルコキシ基、フェニル基又はニトロ基を表し、ｐ及びｑは０〜２の整数を表し、Ｒ_３及びＲ_４はそれぞれ独立に炭素数１〜５のアルキル基を表し、Ｄはヘプタメチンシアニン化合物を形成するための連結基を表す）、
（２）Ｄが式（２）〜式（４）から選ばれる一種であることを特徴とする（１）に記載の化合物

（式中、Ｙは水素原子、ハロゲン原子、フェニル基、ジフェニルアミノ基又は炭素数１〜４のアルキル基を示し、＊は結合部位を示す）、
（３）Ｒ_１及びＲ_２がそれぞれ独立に、塩素原子、エチル基、メチル基、メトキシ基、エトキシ基、フェニル基またはニトロ基であることを特徴とする（１）または（２）に記載の化合物、
（４）Ｒ_３及びＲ_４がそれぞれ独立に、メチル基、エチル基、ｎ−ブチル基、メトキシエチル基又はｎ−ブトキシエチル基であり、Ｙが塩素原子または水素原子であることを特徴とする（１）乃至（３）のいずれか一つに記載の化合物、
（５）ｐ及びｑが０であることを特徴とする（４）に記載の化合物、
（６）（１）乃至（５）のいずれか一つに記載の化合物と少なくとも１種の油溶性有機溶媒を含有する油性染料組成物、
（７）（１）乃至（５）のいずれか一つに記載の化合物及び水性媒体を含有する水性染料組成物、
に関する。 That is, the present invention
(1) a compound represented by the following general formula (1),

(In Formula (1), R ₁ and R ₂ each independently represent a halogen atom, an alkyl group having 1 to 5 carbon atoms, an alkoxy group, a phenyl group, or a nitro group, p and q represent an integer of 0 to 2, R ₃ and R ₄ each independently represent an alkyl group having 1 to 5 carbon atoms, and D represents a linking group for forming a heptamethine cyanine compound).
(2) The compound according to (1), wherein D is a kind selected from the formulas (2) to (4)

(In the formula, Y represents a hydrogen atom, a halogen atom, a phenyl group, a diphenylamino group or an alkyl group having 1 to 4 carbon atoms, and * represents a binding site),
(3) R ₁ and R ₂ are each independently a chlorine atom, an ethyl group, a methyl group, a methoxy group, an ethoxy group, a phenyl group or a nitro group, wherein (1) or (2) Compound,
(4) R ₃ and R ₄ are each independently a methyl group, an ethyl group, an n-butyl group, a methoxyethyl group or an n-butoxyethyl group, and Y is a chlorine atom or a hydrogen atom. (1) to the compound according to any one of (3),
(5) The compound according to (4), wherein p and q are 0,
(6) An oil-based dye composition containing the compound according to any one of (1) to (5) and at least one oil-soluble organic solvent,
(7) An aqueous dye composition containing the compound according to any one of (1) to (5) and an aqueous medium,
About.

  When the compound of the present invention forms an oily or aqueous dye composition and the dyed colored body is processed, the compound exhibits characteristics superior in fastness to conventional products. That is, the heptamethine cyanine compound having near-infrared absorption characteristics of the present invention can be suitably used for a dye-colored product, and can be applied to a wide range of uses such as a near-infrared absorption filter and ink-jet ink.

  The compound of the present invention is represented by the formula (1).

In Formula (1), R ₁ and R ₂ each independently represent a halogen atom, an alkyl group having 1 to 5 carbon atoms, an alkoxy group, a phenyl group, or a nitro group, p and q represent an integer of 0 to 2, ₃ and R ₄ each independently represents an alkyl group having 1 to 5 carbon atoms, and D represents a linking group for forming a heptamethine cyanine compound.

Examples of the halogen atom in R ₁ and R ₂ in Formula (1) include a fluorine atom, a chlorine atom, or a bromine atom.

Examples of the alkyl group in R ₁ and R ₂ of the formula (1) include an alkyl group having 1 to 5 carbon atoms, such as a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, Examples include i-butyl group, t-butyl group, n-amyl group, i-amyl group, t-amyl group and the like. These alkyl groups may have a substituent, such as a 3- to 6-membered saturated heterocyclic group (for example, a tetrahydrofuryl group), a phenyl group which may have an alkoxy group, a carbamoyl group. , Alkoxy groups (for example, methoxy group, ethoxy group, n-butoxy group and the like), alkoxyalkoxy groups (for example, ethoxyethoxy group, methoxyethoxy group, methoxymethoxy group and the like) and the like.

Examples of the alkoxy group for R ₁ and R ₂ in the formula (1) include a group in which the alkyl group in the alkyl group which may have the substituent for R ₁ and R ₂ is bonded to an oxygen atom.

The phenyl group in R ₁ and R ₂ in the formula (1) may have a substituent, and examples of the substituent include a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, a hydroxy group, a hydroxyethyl group, and a hydroxy group. Propyl group, hydroxybutyl group, 2-sulfoethyl group, carboxyl group, carboxyethyl group, cyano group, cyanoethyl group, methoxy group, methoxyethyl group, ethoxy group, ethoxyethyl group, propoxy group, propoxyethyl group, trifluoromethyl group , Pentafluoroethyl group, carbamoyl group and the like.

Preferably in R < ₁ > and R < ₂ > of Formula (1), a chlorine atom, an ethyl group, a methyl group, a methoxy group, an ethoxy group, a phenyl group, and a nitro group are mentioned.

In R ₁ and R ₂ of Formula (1), the substitution numbers p and q are 0 to 2, and when substituents R ₁ and R ₂ are present, the substitution positions are not particularly limited. The number of substitutions p and q is preferably 0.

Examples of the alkyl group in R ₃ and R ₄ in the formula (1) include an alkyl group having 1 to 5 carbon atoms, such as a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, Examples include i-butyl group, t-butyl group, n-amyl group, i-amyl group, t-amyl group and the like. These alkyl groups may have a substituent, such as a 3- to 6-membered saturated heterocyclic group (for example, a tetrahydrofuryl group), a phenyl group that may have an alkoxy group, Examples include carbamoyl group, alkoxy group (for example, methoxy group, ethoxy group, n-butoxy group, etc.), alkoxyalkoxy group (for example, ethoxyethoxy group, methoxyethoxy group, methoxymethoxy group, etc.), among which methyl group, ethyl Group, n-butyl group, methoxyethyl group or n-butoxyethyl group is preferred.

  The linking group for forming the heptamethine cyanine compound as D in the formula (1) is the above formula (2) to the formula (4) [wherein Y is a hydrogen atom, a halogen atom, a phenyl group, diphenylamino. A group or an alkyl group having 1 to 4 carbon atoms, and * represents a bonding site].

  Examples of the halogen atom in Y in the formulas (2) to (4) include a fluorine atom, a chlorine atom, and a bromine atom, and the alkyl group having 1 to 4 carbon atoms includes a methyl group, an ethyl group, and an n-propyl group. , I-propyl group, n-butyl group, i-butyl group, t-butyl group and the like. Y is preferably a hydrogen atom, a chlorine atom or a phenyl group, and particularly preferably a hydrogen atom or a chlorine atom.

  The compound of the present invention can be synthesized, for example, by the following method with reference to the method described in Patent Document 1.

(式中、Ｒ_１、Ｒ_３及びｐは前記と同じ意味を表す)で表される化合物２モルと、下記式（６）

(式中、Ｄは前記と同じ意味を表す)、又は下記式（７）

(式中、Ｄは前記と同じ意味を表す)、
で表される化合物１モル及びトリス（トリフルオロメタンスルホニル）メチド酸又はそのナトリウム塩、カリウム塩等の塩１モルを、必要に応じ酢酸ナトリウム、酢酸カリウム、ピペラジン、ピペリジン、トリエチルアミン等の塩基触媒の存在下で、エタノール、無水酢酸あるいは無水酢酸と氷酢酸の混合物のような有機溶媒中で、例えば、５０〜１４０℃で通常１０分〜１０時間、好ましくは３０分〜４時間加熱し、縮合させ対称シアニン化合物（式（１）におけるＲ_１とＲ_２、Ｒ_３とＲ_４、ｐとｑがそれぞれ同一である化合物）を合成することができる。 Formula (5)

(Wherein R ₁ , R ₃ and p represent the same meaning as described above) 2 mol of the compound represented by the following formula (6)

(Wherein D represents the same meaning as described above), or the following formula (7)

(Wherein D represents the same meaning as described above),
1 mol of the compound represented by the formula (1) and 1 mol of tris (trifluoromethanesulfonyl) methydic acid or a salt thereof such as sodium salt or potassium salt thereof, if necessary, presence of a base catalyst such as sodium acetate, potassium acetate, piperazine, piperidine, triethylamine Under heating in an organic solvent such as ethanol, acetic anhydride or a mixture of acetic anhydride and glacial acetic acid, for example, usually 10 minutes to 10 hours at 50 to 140 ° C., preferably 30 minutes to 4 hours, condensed and symmetrical A cyanine compound (a compound in which R ₁ and R ₂ , R ₃ and R ₄ , and p and q in formula (1) are the same) can be synthesized.

(式中、Ｒ_２、Ｒ_４及びｑは前記と同じ意味を表す、ただし式（５）及び式（８）においてＲ_１とＲ_２、Ｒ_３とＲ_４、ｐとｑの組み合わせのうち少なくとも一つは同一ではない)の化合物１モルを用いて、同条件下において合成することができる。 On the other hand, when synthesizing an asymmetric cyanine compound, 1 mol of the compound of the above formula (5) is used, and the following formula (8)

(Wherein R ₂ , R ₄ and q represent the same meaning as described above, except that at least one of the combinations of R ₁ and R ₂ , R ₃ and R ₄ , and p and q in formula (5) and formula (8)). Can be synthesized under the same conditions using 1 mole of the compound (one is not the same).

表１−２

表１−３

Although the specific example shown by the said Formula (1) of this invention is shown in the following Table 1-1-Table 1-3, this invention is not limited to these.
Table 1-1

Table 1-2

Table 1-3

  The oily or aqueous dye composition of the present invention contains the compound of the present invention having near infrared absorption characteristics, an oil-soluble organic solvent in the case of an oily dye composition, and an aqueous medium in the case of an aqueous dye composition. In the oily or aqueous dye composition of the present invention, the compound of the present invention is preferably contained in an amount of 0.2 to 40% by mass, and more preferably 0.5 to 20% by mass. Further, in the oily or aqueous dye composition of the present invention, a near-infrared absorbing dye other than the formula (1) may be added as necessary for the purpose of preparing the near-infrared region. Examples of near-infrared absorbing dyes that can be added include diimonium-based, phthalocyanine-based and nickel dithiol-based dyes; water-soluble near-infrared absorbing dyes such as acidic dyes, reactive dyes, direct dyes, cationic dyes, and basic dyes; Examples include oil-soluble near-infrared absorbing dyes such as dispersible dyes and solvent-based dyes; organic pigments, carbon black, and the like, which are added in a dissolved or dispersed state in a solvent.

  The aqueous dye composition of the present invention can be prepared by dispersing the compound of the present invention having near infrared absorption characteristics in an aqueous medium. Examples of the aqueous medium include water or a water-soluble organic solvent. Examples of water-soluble organic solvents include alcohols such as methanol, ethanol, propanol, isopropanol, butanol, isobutanol, t-butanol, pentanol, benzyl alcohol; ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol Polyhydric alcohols such as polypropylene glycol, glycerin, trimethylolpropane, 1,3-pentanediol, 1,5-pentanediol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, tri Ethylene glycol monoethyl ether, triethylene glycol monobutyl ether Dipropylene glycol monomethyl ether of glycol derivatives; ethanolamine, diethanolamine, triethanolamine, amines morpholine; 2-pyrrolidone, NMP, 1,3-dimethyl - imidazolidinone, and the like.

  The oil-based dye composition of the present invention can be prepared by dissolving or dispersing the compound of the present invention having near infrared absorption characteristics in at least one oil-soluble organic solvent. Examples of the oil-soluble organic solvent used include alcohols such as ethanol, pentanol, octanol, cyclohexanol, benzyl alcohol, and tetrafluoropropanol; ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, propylene glycol mono Glycol derivatives such as ethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, triethylene glycol monoethyl ether, ethylene glycol diacetate, propylene glycol diacetate; ketones such as methyl ethyl ketone and cyclohexanone; butylphenyl ether, benzyl Ethers such as ether and hexyl ether; , Esters such as butyl acetate, ethyl benzoate, butyl benzoate, ethyl laurate, butyl laurate; polar organic solvents such as acetonitrile, DMF, dimethyl sulfoxide, sulfolane, NMP, 2-pyrrolidone, etc. These solvents may be used alone or in combination of two or more.

  Dispersants used in oil dye compositions include sodium dodecylbenzenesulfonate, sodium laurate, formalin condensate of naphthalene sulfonic acid, formalin condensate of alkyl naphthalene sulfonic acid, formalin condensate of creosote oil sulfonic acid, poly Ammonium salt of oxyethylene alkyl ether sulfate, ammonium salt of polyoxyethylene alkyl phenyl ether sulfate, polyoxyalkyl ether phosphate ester salt and other known anionic surfactants, vinyl naphthalene derivatives, α, β-ethylenically unsaturated carboxylic acid Aliphatic alcohol esters, styrene, styrene derivatives, acrylic acid, acrylic acid derivatives, methacrylic acid, methacrylic acid derivatives, maleic acid, maleic acid derivatives, maleic anhydride, maleic anhydride A block copolymer consisting of at least two monomers selected from oleic acid derivatives, itaconic acid, itaconic acid derivatives, fumaric acid, fumaric acid derivatives, etc., or a random copolymer, or a high salt thereof. A molecular dispersing agent etc. are mentioned, It is preferable to use these 1 or more types between 10-100 mass% with respect to the compound of this invention. In addition to these dispersants, known nonionic surfactants such as polyoxyethylene sorbitan fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, copolymers of ethylene oxide and propylene oxide, and silicones In addition, a known acetylene-based antifoaming agent can be added at the time of pigment dispersion and / or after pigment dispersion.

  Examples of the method for dispersing the pigment into the fine particles include a method using a sand mill (bead mill), a roll mill, a ball mill, a paint shaker, an ultrasonic disperser, a microfluidizer, and the like. Among these, a sand mill (bead mill) is preferable. In the grinding of pigments in sand mills (bead mills), it is preferable to use beads with small diameters and to treat them under conditions that increase the grinding efficiency by increasing the filling rate of beads. It is preferable to remove the elementary particles by separation or the like.

  The dye composition of the present invention may contain a surface adjusting agent, antiseptic / antifungal agent, pH adjusting agent and the like as other additives. As surface conditioning agents, polysiloxane or polydimethylsiloxane surfactants, and as antiseptic / antifungal agents, sodium dehydroacetate, sodium benzoate, sodium pyridinethione-1-oxide, zinc pyridinethione-1-oxide 1,2-benzisothiazolin-3-one, amine salts of 1-benzisothiazolin-3-one, etc., as pH adjusters, alkali hydroxides such as sodium hydroxide, potassium hydroxide, lithium hydroxide, Tertiary amines such as triethanolamine, diethanolamine, dimethylethanolamine, diethylethanolamine and the like can be mentioned, and each can be added as necessary.

  In addition, in the oily or aqueous dye composition of the present invention, a polyamide system, a polyurethane system, or a polyester system that is compatible with the medium in the composition within a necessary range for the purpose of improving the fixability of the dye to the object to be colored. It is preferable to contain an epoxy or polyacrylic resin. Further, for the purpose of improving the fixability, a monomer, oligomer, polymerization initiator or the like having an ethylenically unsaturated group may be contained within a necessary range. The oily or aqueous dye composition of the present invention can be prepared by dissolving or dispersing and mixing the above components in a solvent.

  The compound of the present invention is used in various paints, water-based inks, oil-based inks, ink-jet inks, and coloring compositions for near-infrared absorbing filters as oil-based dye compositions or water-based dye compositions. The oil-based dye composition and the aqueous dye composition are used for materials to be colored such as plain paper, coated paper, plastic film, and plastic substrate. Examples of a method for applying the dye composition of the present invention to a material to be colored include various printing methods such as offset printing, letterpress printing, flexographic printing, and ink jet printing, and coating methods using a spin coater, a roll coater, and the like.

  EXAMPLES Hereinafter, although an Example demonstrates this invention concretely, this invention is not limited to these Examples. In Examples, “part” means “part by mass” unless otherwise specified. In addition, evaluations such as resistance to moist heat and water resistance were evaluated by measuring the absorbance of the dye-colored product using a spectrophotometer “Shimadzu Corporation UV-3150”.

Example 1 (Synthesis of Compound No. 1 in Table 1-1)
In a 100 ml four-necked flask, 50 parts of ethanol, 4.2 parts of 3-ethyl-2-methylbenzoxazolium iodide, 2 parts of glutaconaldehyde dianil hydrochloride, potassium salt of tristrifluoromethanesulfonylmethide 3. 3 parts and 3 parts of triethylamine were charged and stirred at 60 ° C. for 3 hours. The precipitated crude product was collected by filtration, washed with diethyl ether and water in this order and dried. 0.3 parts of 1 was obtained.
Maximum absorption wavelength 695nm (dichloromethane)

Example 2 (Synthesis of Compound No. 2 in Table 1-1)
In a 100 ml four-necked flask, 40 parts of ethanol, 2.1 parts of 3-ethyl-2-methylbenzoxazolium iodide, N-phenyl-2-chloro-3-[(phenylamino) methylene] -1-cyclohexene First, 1.3 parts of -1-methanimine hydrochloride, 1.6 parts of potassium salt of tristrifluoromethanesulfonylmethide, and 1.5 parts of triethylamine were charged and stirred at 60 ° C. for 3 hours. The precipitated crude product was collected by filtration, washed with diethyl ether and water in this order and dried. 0.4 part of 2 was obtained.
Maximum absorption wavelength 729nm (dichloromethane)

Synthesis Example 1 (Synthesis of binder resin (copolymer))
A 500 ml four-necked flask was charged with 160 g of methyl ethyl ketone, 10 g of methacrylic acid, 33 g of benzyl methacrylate and 1 g of α, α′-azobis (isobutyronitrile), and nitrogen gas was allowed to flow into the flask for 30 minutes while stirring. Then, it heated up to 80 degreeC and stirred for 4 hours as it was at 80-85 degreeC. After completion of the reaction, the reaction mixture was cooled to room temperature to obtain a colorless, transparent and uniform copolymer solution. This was precipitated in a 1: 1 mixed solution of isopropyl alcohol and water, filtered, and the solid content was taken out and dried to obtain a copolymer (A). The obtained copolymer (A) had a polystyrene equivalent weight average molecular weight of 18000 and an acid value of 152 (mgKOH / g).

Example 3 (Preparation of oil-based dye composition and dyed colored body)
In the following Table 2, the present compound No. 2 / Tetrafluoropropanol / The binder resin obtained in Synthesis Example 1 was mixed at a composition ratio of 0.25 part / 1 part / 0.8 part to prepare an oil-based dye composition. The obtained oil-based dye composition was spin-coated on a glass substrate and dried at 100 ° C. for 2 hours to prepare a dye-colored product 1.

In addition, the comparative example 1 in the following Table 2 uses a heptamethine cyanine compound of the following formula (100) described in Non-Patent Document 1 and similarly produces a dye colored body.

Humidity and heat resistance test The dyed colored body obtained by the above method was left in a constant temperature and humidity machine for 14 hours under the conditions of 85 ° C and 85% RH. The absorbance of each dye-colored product before and after the test was measured, and the residual ratio of the dye was determined by the following formula from the change in absorbance (OD value) in the near infrared region of 730 nm.
Dye remaining ratio = (absorbance after test / absorbance before test) × 100
Table 2 below shows the residual ratio of the dye in the wet heat resistance test.

Table 2
Residual rate Dye-colored product 1 6.6%
Comparative Example 1 1.2%

  As apparent from the results in Table 2 above, the dyed colored body 1 of the present invention has a residual rate of 6.6%, compared with the residual rate of 1.2% after the test of the dyed colored body of Comparative Example 1, which is Comparative Example 1. It shows a large value of 5 times or more, and is extremely excellent in heat and moisture resistance.

Claims (7)

Compound represented by the following general formula (1)

(In Formula (1), R ₁ and R ₂ each independently represent a halogen atom, an alkyl group having 1 to 5 carbon atoms, an alkoxy group, a phenyl group, or a nitro group, p and q represent an integer of 0 to 2, R ₃ and R ₄ each independently represent an alkyl group having 1 to 5 carbon atoms, and D represents a linking group for forming a heptamethine cyanine compound). The compound according to claim 1, wherein D is one selected from the formulas (2) to (4).

(In the formula, Y represents a hydrogen atom, a halogen atom, a phenyl group, a diphenylamino group or an alkyl group having 1 to 4 carbon atoms, and * represents a binding site). The compound according to claim 1 or 2, wherein R ₁ and R ₂ are each independently a chlorine atom, an ethyl group, a methyl group, a methoxy group, an ethoxy group, a phenyl group, or a nitro group. 2. R ₃ and R ₄ are each independently a methyl group, an ethyl group, an n-butyl group, a methoxyethyl group or an n-butoxyethyl group, and Y is a chlorine atom or a hydrogen atom. 4. The compound according to any one of 1 to 3. The compound according to claim 4, wherein p and q are 0. An oily dye composition comprising the compound according to any one of claims 1 to 5 and at least one oil-soluble organic solvent. An aqueous dye composition comprising the compound according to any one of claims 1 to 5 and an aqueous medium.