CN103657513B - Gemini-type ampholytic surfactant based on perfluoroolefine and preparation method thereof - Google Patents

Abstract

The invention discloses a class of amphoteric gemini surfactants based on perfluorononene and perfluorohexene and a preparation method thereof. The method uses perfluorononene or perfluorohexene as a raw material to obtain perfluoroalkenyloxybenzenesulfonyl chloride through condensation with phenol and chlorosulfonation, and then reacts with 2-dimethylaminoethylamine or 3-dimethylamine Condensation of propylamine to obtain perfluoroalkenyloxybenzenesulfonamide with tertiary amine in the amine part, and then combine with 1,2-dihaloethane, 1,3-dihalopropane, 1,4-dihalobutane, 1, Condensation of 5-dihalopentane, 3-oxa-1,5-dihalopentane or 1,3-dihaloacetone (halogen is chlorine or bromine), and finally with sodium ethylene sulfonate, sodium chloroethyl sulfonate , sodium bromoethylsulfonate or 1,3-propane sultone condensation to obtain amphoteric gemini fluorosurfactants with excellent surface activity and low critical micelle concentration. This kind of amphoteric gemini fluorosurfactant has outstanding characteristics such as simple synthesis operation, low cost, high surface activity and low concentration, and has broad market prospects.

Description

A class of perfluoroolefin-based gemini-type amphoteric surfactants and preparation method thereof

technical field

The invention relates to a class of fluorosurfactants, in particular to a class of perfluoroolefin-based gemini-type amphoteric surfactants, and a preparation method thereof.

Background technique

Surfactant is an important fine chemical product, which has multiple functions such as wetting, emulsifying, foaming, defoaming, and dispersing, and is known as industrial monosodium glutamate. Surfactants are generally classified into anionic, cationic, amphoteric and nonionic.

Commonly used surfactants, whose hydrophobic groups are hydrocarbon groups, are likely to contain other elements such as oxygen, sulfur, nitrogen, chlorine, bromine, etc. in the molecule. Such commonly used surfactants are called ordinary surfactants. Surfactants containing fluorine, phosphorus, silicon, boron and other elements in the surfactant molecule are called special surfactants.

Fluorosurfactant is a special kind of surfactant. It has high thermal stability, high chemical stability, high surface activity, and both oil and water repellency. Therefore, it has a wide range of uses in fire protection and oil production. The industrial synthesis methods of fluorosurfactants mainly include electrolysis, fluoroolefin telomerization and fluoroolefin oligomerization.

The most widely used fluorosurfactants are perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) and their derivatives, but the impact of these compounds on the environment and human health has been widely concerned. Various experiments have shown that PFOA and PFOS hardly undergo any observable degradation under various physical and chemical conditions, and are currently one of the most difficult to degrade organic pollutants in the world, with high bioaccumulation and human multi-organ toxicity , countries have restricted or banned the use.

Hexafluoropropylene oligomers, especially hexafluoropropylene dimers and hexafluoropropylene trimers, are cheap and avoid the disadvantages of serious pollution by electrolysis. Fluorosurfactants derived from them are a class of potential PFOA and PFOS alternatives. However, hexafluoropropylene oligomer has a branched chain structure, and its surface activity cannot reach the level of PFOA, PFOS and their derivatives, which greatly limits the application of this type of substitute.

Contents of the invention

In order to overcome the defects of insufficient surface activity of existing hexafluoropropylene oligomer derivatives surfactants, the present invention prepares a surfactant based on perfluorononene (hexafluoropropylene trimer) and perfluorohexene (hexafluoropropylene Dimer) Gemini amphoteric surfactant, the general formula is

Wherein R _f is perfluorononenyl or perfluorohexenyl; m is 2 or 3; n is 2 or 3; R is (CH ₂ ) ₂ or (CH ₂ ) ₃ or (CH ₂ ) ₄ or (CH ₂ ) ₅ or (CH ₂ ) ₂ O(CH ₂ ) ₂ or CH ₂ COCH ₂ .

The above compounds use perfluorononenyloxyphenyl or perfluorohexenyloxyphenyl as the lipophilic part, quaternary ammonium and sulfonate as the hydrophilic part, and a methylene chain (CH ₂ ) _n (n=2,3,4,5) or 3-oxapentylidene chain (CH ₂ ) ₂ O(CH ₂ ) ₂ or 2-oxopropylene as the connecting bridge; lipophilic part and hydrophilic The water moieties are linked by sulfonamide linkages. Owing to having possessed above-mentioned structure characteristic, make described compound have very high surface activity, surface tension is between 15.2mN/m and 20.2mN/m, close to the level of PFOA and PFOS; Critical micelle concentration is lower, between Between 6.0x10 ^-5 mol/L and 2.3x10 ^-4 mol/L.

The invention applies the principle and design method of the gemini surfactant to the perfluoroalkene-based fluorosurfactant, and prepares the perfluoroalkene-based gemini-type amphoteric fluorosurfactant.

The preparation of the surfactant of the present invention is obtained by condensation of perfluorononene or perfluorohexene with phenol, chlorosulfonation, or condensation with phenol, sulfonation, chlorination, or condensation with sodium p-hydroxybenzenesulfonate, chlorination Perfluoroalkenyloxybenzenesulfonyl chloride as raw material. Perfluoroalkenyloxybenzenesulfonyl chloride is condensed with 2-dimethylaminoethylamine or 3-dimethylaminopropylamine to obtain a perfluoroalkenyloxybenzenesulfonamide intermediate (A) with a tertiary amine in the amine part, and then with 1,2-dihaloethane, 1,3-dihalopropane, 1,4-dihalobutane, 1,5-dihalopentane, 3-oxa-1,5-dihalopentane, 1 , 3-dihaloacetone condensation to obtain a bis-quaternary ammonium salt type intermediate (B) (halogen is chlorine or bromine), and finally a series of gemini-type amphoteric fluorosurfactants are obtained by N-sulfoalkylation reaction under alkaline conditions (C).

In the preparation method used in the present invention, the reaction of preparing perfluoroalkenyloxybenzenesulfonyl chloride from perfluorononene and perfluorohexene has been disclosed in literature, such as US Patent No. 4,788,321. The condensation reaction mainly involves the selection of solvent and acid-binding agent. The solvent generally uses a strong polar aprotic solvent, such as N,N-dimethylformamide, acetonitrile, tetrahydrofuran, etc. Acid-binding agents generally use triethylamine, tri-n-butylamine, pyridine and the like. Perfluoroalkenyloxybenzenesulfonyl chloride can also be prepared from perfluoroalkenyloxybenzene through sulfonation to prepare perfluoroalkenyloxybenzenesulfonic acid, and then chlorinated by a chlorinating agent such as phosphorus pentachloride. It can also be prepared from perfluoroalkenyloxybenzene Alkene or perfluorohexene is condensed with sodium p-hydroxybenzenesulfonate, and then chlorinated by a chlorinating agent. These methods have also been disclosed in the literature, such as European Patent EP0420235, Cao Yougui, etc. (Fine Chemical Industry 2012, 29 (2) , 174-177, Shen Haimin et al. (Pesticide 2007, 46(8), 520-522, 534).

In the preparation method used in the present invention, the perfluoroalkenyloxybenzenesulfonamide intermediate (A) is prepared by condensing perfluoroalkenyloxybenzenesulfonyl chloride with 2-dimethylaminoethylamine or 3-dimethylaminopropylamine The amination reaction has been disclosed in the literature, such as Chinese patent CN1743062 and Shi Hongxin et al. (Journal of Zhejiang University of Technology 2009, 37 (6), 619-622). It is characterized in that the hydrogen chloride produced by the reaction can be absorbed by adding a tertiary amine acid-binding agent such as triethylamine, pyridine, etc., or can be absorbed by the tertiary amine contained in the amine part itself, and after the reaction is completed, it can be absorbed with an inorganic base such as sodium hydroxide , sodium carbonate, etc., to release the tertiary amine for the next reaction.

In the preparation method used in the present invention, the quaternization reaction of the bisquaternary ammonium salt type intermediate (B) prepared by the condensation of the perfluoroalkenyloxybenzenesulfonamide intermediate (A) and the dihaloalkane is useful in the synthesis of gemini surfactants. It has been reported, but it has not been reported for the synthesis of gemini fluorosurfactants with perfluorononene or perfluorohexene as lipophilic groups. Dihaloalkanes can be brominated or chlorinated hydrocarbons, brominated hydrocarbons have higher reactivity, but chlorinated hydrocarbons are less expensive.

In the preparation method used in the present invention, the hydrogen on the sulfonamide nitrogen of the bisquaternary ammonium salt type intermediate has higher acidity, and it is easy to react with alkali to generate nitrogen negative ions, and then react with sulfoalkylation reagents to obtain amphoteric gemini surface activity. agent, the specific reaction conditions can be adjusted according to the reaction raw materials. The sulfoalkylating agent is generally sodium ethylene sulfonate, sodium chloroethyl sulfonate, sodium bromoethyl sulfonate or 1,3-propane sultone, etc.

The compound with general structure (C) obtained in the present invention is characterized by infrared spectrum, mass spectrum, elemental analysis and other detection means, combined with the raw materials used in the reaction and reaction conditions, etc., consistent with the structure shown. The surface properties of the products also show that they have the characteristics of a gemini structure.

The amphoteric gemini fluorosurfactant prepared by the present invention has high surface activity, can significantly reduce surface tension by adding a small amount, has excellent dispersibility, rewetability, etc., and can be used in fabric finishing, washing industry, oil field tertiary oil recovery, Fire protection, chrome mist suppression, metal surface treatment, etc.

Detailed ways

In order to further illustrate the present invention, it will be described below in conjunction with specific examples.

Example 1 Preparation of N-(3-dimethylaminopropyl) perfluorononenyloxybenzenesulfonamide (A)

Add 62.3 g (0.1 mol) of perfluorononenyloxybenzenesulfonyl chloride, 10.2 g (0.1 mol) of 3-dimethylaminopropylamine and 100 mL of tetrahydrofuran into a four-necked flask, and dropwise add 20.2 g of triethylamine ( 0.2mol), about 40min to drop, react at 60°C for 5h, pour the reaction solution into water, separate the organic phase, extract three times with tetrahydrofuran, combine the organic phases, and precipitate to obtain a light yellow solid with a yield of 94.6%.

Example 2 Preparation of perfluorononenyloxy-based bisquaternary ammonium salt intermediate (B)

Take 34.4g (0.05mol) of the product obtained in Example 1 and 80mL of toluene and heat it to reflux, add dropwise 3.2g (0.025mol) of 1,3-dichloroacetone and 20mL of toluene solution under stirring, reflux for 10 h, and cool to room temperature. Filter, wash with toluene, and dry in vacuo to obtain a white solid with a yield of 92.2%.

Example 3 Preparation of amphoteric gemini surfactant (C) based on perfluorononene

75.2g (0.05mol) of the product obtained in Example 2 and 120mL of N,N-dimethylformamide were heated to 80°C, and 25.3g (0.12mol) of sodium bromoethanesulfonate, 4.8g (0.12mol) of sodium bromoethanesulfonate and 4.8g (0.12mol) of A solution of sodium hydroxide and 50 mL of water was incubated for 14 hours, cooled to room temperature, diluted with water, filtered, washed with water and methanol successively, and vacuum-dried to obtain an off-white solid with a yield of 94.9%.

Infrared spectrum characteristic peaks (cm ^-1 ): 1600 (benzene ring C=C), 1495 (benzene ring C=C), 1245 (S=O), 1232 (aryl-O), 1184 (CF), 1135 ( CF), 1130 (S=O), 1040 (S=O), there is no NH peak around 3330. Elemental analysis: C ₄₇ H ₄₄ F ₃₄ N ₄ O ₁₃ S ₄ theoretical value C 34.26%, H 2.67%, F 39.25%; found value C 34.18%, H 2.55%, F 39.04%. Mass spectrum: Molecular weight 1646.

Example 4 Preparation of N-(2-dimethylaminoethyl)perfluorohexenyloxybenzenesulfonamide (A)

Add 47.3g (0.1mol) of perfluorohexenyloxybenzenesulfonyl chloride, 8.8g (0.1mol) of 2-dimethylaminoethylamine and 100mL of acetonitrile into a four-necked flask, and drop 20.2g of triethylamine at 20°C (0.2mol), about 40 minutes to drop, 60 ° C reaction for 5 hours, the reaction solution was poured into water, the organic phase was separated, extracted twice with acetonitrile, the organic phase was combined, and precipitated to obtain a light yellow solid with a yield of 96.3%.

Example 5 Preparation of bis-quaternary ammonium salt intermediate (B) based on perfluorohexenyloxy

Take 26.2g (0.05mol) of the product obtained in Example 4 and 80mL of acetonitrile and heat to reflux, add dropwise 5.1g (0.025mol) of 1,3-dibromopropane and 10mL of acetonitrile solution under stirring, reflux for 16h, cool to room temperature, filter , washed successively with acetonitrile and cyclohexane, and vacuum-dried to obtain an off-white solid with a yield of 93.8%.

Example 6 Preparation of amphoteric gemini surfactant (C) based on perfluorohexene

62.5g (0.05mol) of the product obtained in Example 5 and 130mL of N-methylpyrrolidone were heated to 80°C, and 26.6g (0.16mol) of sodium chloroethanesulfonate, 16.6g (0.12mol) of potassium carbonate and 45mL of The water solution was incubated for 13 hours, cooled to room temperature, diluted with water, filtered, washed with water and ethanol successively, and vacuum-dried to obtain a white solid with a yield of 90.4%.

Infrared spectrum characteristic peaks (cm ^-1 ): 1599 (benzene ring C=C), 1494 (benzene ring C=C), 1243 (S=O), 1234 (aryl-O), 1187 (CF), 1134 ( CF), 1129 (S=O), 1040 (S=O), there is no NH peak around 3330. Elemental analysis: C ₃₉ H ₄₂ F ₂₂ N ₄ O ₁₂ S ₄ theoretical value C 35.89%, H 3.22%, F 32.06%; measured value C 35.77%, H 3.11%, F 31.94%. Mass spectrum: Molecular weight 1304.

Example 7 Preparation of N-(2-dimethylaminoethyl)perfluorononenyloxybenzenesulfonamide (A)

Add 62.3g (0.1mol) of perfluorononenyloxybenzenesulfonyl chloride, 8.8g (0.1mol) of 2-dimethylaminoethylamine and 100mL of tetrahydrofuran into a four-necked flask, and add 20.2g of triethylamine dropwise at 20°C (0.2mol), about 40 minutes to drop, 60 ° C for 5 hours, the reaction solution was poured into water, the organic phase was separated, extracted three times with tetrahydrofuran, the organic phase was combined, and precipitated to obtain a light yellow solid with a yield of 91.5%.

Example 8 Preparation of bis-quaternary ammonium salt intermediate (B) based on perfluorononenyloxy

Take 33.7g (0.05mol) of the product obtained in Example 7 and 80mL of toluene and heat it to reflux, add dropwise 3.2g (0.025mol) of 1,4-dichlorobutane and 20mL of toluene solution under stirring, reflux for 10 h, and cool to room temperature , filtered, washed with toluene, and dried in vacuo to obtain a white solid with a yield of 90.8%.

Example 9 Preparation of amphoteric gemini surfactant (C) based on perfluorononene

73.8g (0.05mol) of the product obtained in Example 8 and 120mL of N,N-dimethylacetamide were heated to 80°C, and 15.6g (0.12mol) of sodium ethylene sulfonate and 4.8g (0.12mol) of hydrogen sulfonate were added dropwise while stirring. A solution of sodium and 50 mL of water was incubated for 15 hours, cooled to room temperature, diluted with water, filtered, washed with water and methanol successively, and vacuum-dried to obtain an off-white solid with a yield of 92.4%.

Infrared spectrum characteristic peaks (cm ^-1 ): 1601 (benzene ring C=C), 1495 (benzene ring C=C), 1243 (S=O), 1235 (aryl-O), 1185 (CF), 1134 ( CF), 1128 (S=O), 1040 (S=O), there is no NH peak around 3330. Elemental analysis: C ₄₆ H ₄₄ F ₃₄ N ₄ O ₁₂ S ₄ theoretical value C 34.12%, H 2.72%, F 39.93%; measured value C 34.00%, H 2.58%, F 39.74%. Mass spectrum: Molecular weight 1618.

Example 10 Preparation of N-(3-dimethylaminopropyl) perfluorohexenyloxybenzenesulfonamide (A)

Add 47.3 g (0.1 mol) of perfluorohexenyloxybenzenesulfonyl chloride, 10.2 g (0.1 mol) of 3-dimethylaminopropylamine and 100 mL of acetonitrile into a four-necked flask, and drop 20.2 g of triethylamine ( 0.2mol), about 40 minutes to finish dripping, 60 ° C reaction for 5 hours, the reaction solution was poured into water, the organic phase was separated, extracted twice with acetonitrile, the organic phase was combined, and the solvent was precipitated to obtain a light yellow solid with a yield of 94.8%.

Example 11 Preparation of bis-quaternary ammonium salt intermediate (B) based on perfluorohexenyloxy

Take 26.9g (0.05mol) of the product obtained in Example 10 and 80mL of acetonitrile and heat to reflux, add dropwise 3.6g (0.025mol) of bis(2-chloroethyl) ether and 10mL of acetonitrile solution under stirring, reflux for 14h, and cool to room temperature , filtered, washed successively with acetonitrile and cyclohexane, and dried in vacuo to obtain an off-white solid with a yield of 91.2%.

Example 12 Preparation of amphoteric gemini surfactant (C) based on perfluorohexene

61.0 g (0.05 mol) of the product obtained in Example 11 and 140 mL of dimethyl sulfoxide were heated to 90 ° C, and 12.2 g (0.10 mol) of 1,3-propane sultone, 16.6 g (0.12 mol) of carbonic acid A solution of potassium and 40 mL of water was incubated for 12 hours, cooled to room temperature, diluted with water, filtered, washed with water and ethanol successively, and vacuum-dried to obtain a white solid with a yield of 91.1%.

Infrared spectrum characteristic peaks (cm ^-1 ): 1599 (benzene ring C=C), 1494 (benzene ring C=C), 1242 (S=O), 1235 (aryl-O), 1187 (CF), 1135 ( CF), 1130 (S=O), 1040 (S=O), there is no NH peak around 3330. Elemental analysis: C ₄₄ H ₅₂ F ₂₂ N ₄ O ₁₃ S ₄ theoretical value C 37.99%, H 3.74%, F 30.07%; measured value C 37.81%, H 3.63%, F 29.90%. Mass spectrum: Molecular weight 1390.

Example 13 Preparation of bis-quaternary ammonium salt intermediate (B) based on perfluorononenyloxy

Take 34.4g (0.05mol) of the product obtained in Example 1 and 80mL of toluene and heat it to reflux, add dropwise 2.5g (0.025mol) of 1,2-dichloroethane and 20mL of toluene solution under stirring, reflux for 10 h, and cool to room temperature , filtered, washed with toluene, and dried in vacuo to obtain a white solid with a yield of 90.4%.

Example 14 Preparation of amphoteric gemini surfactant (C) based on perfluorononene

73.8g (0.05mol) of the product obtained in Example 13 and 120mL of N,N-dimethylformamide were heated to 80°C, and 25.3g (0.12mol) of sodium bromoethanesulfonate, 4.8g (0.12mol) of sodium bromoethanesulfonate and 4.8g (0.12mol) of A solution of sodium hydroxide and 50 mL of water was incubated for 12 hours, cooled to room temperature, diluted with water, filtered, washed with water and methanol successively, and vacuum-dried to obtain an off-white solid with a yield of 94.9%.

Infrared spectrum characteristic peaks (cm ^-1 ): 1600 (benzene ring C=C), 1496 (benzene ring C=C), 1246 (S=O), 1231 (aryl-O), 1185 (CF), 1135 ( CF), 1129 (S=O), 1040 (S=O), there is no NH peak around 3330. Elemental analysis: C ₄₆ H ₄₄ F ₃₄ N ₄ O ₁₂ S ₄ theoretical value C 34.12%, H 2.72%, F 39.93%; measured value C 34.00%, H 2.58%, F 39.74%. Mass spectrum: Molecular weight 1618.

Example 15 Preparation of bis-quaternary ammonium salt intermediate (B) based on perfluorohexenyloxy

Take 26.2g (0.05mol) of the product obtained in Example 4 and 80mL of tetrahydrofuran and heat it to reflux, add dropwise 5.8g (0.025mol) of 1,5-dibromopentane and 10mL of tetrahydrofuran solution under stirring, reflux for 28h, and cool to room temperature. Filter, wash with acetonitrile and cyclohexane successively, and dry in vacuo to obtain an off-white solid with a yield of 89.6%.

Example 16 Preparation of amphoteric gemini surfactant (C) based on perfluorohexene

63.9 g (0.05 mol) of the product obtained in Example 15 and 130 mL of N-methylpyrrolidone were heated to 80 ° C, and 26.6 g (0.16 mol) of sodium chloroethanesulfonate, 16.6 g (0.12 mol) of potassium carbonate and 45 mL of The solution in water was incubated for 13 hours, cooled to room temperature, diluted with water, filtered, washed with water and ethanol successively, and dried in vacuum to obtain a white solid with a yield of 92.5%.

Infrared spectrum characteristic peaks (cm ^-1 ): 1599 (benzene ring C=C), 1494 (benzene ring C=C), 1244 (S=O), 1234 (aryl-O), 1188 (CF), 1134 ( CF), 1129 (S=O), 1040 (S=O), there is no NH peak around 3330. Elemental analysis: C ₃₉ H ₄₂ F ₂₂ N ₄ O ₁₂ S ₄ theoretical value C 36.94%, H 3.45%, F 31.38%; measured value C 36.77%, H 3.31%, F 31.19%. Mass spectrum: Molecular weight 1332.

The chemical raw materials used in the above examples, solvents, etc. are all industrial grade products. The surface tension was measured with a surface tensiometer, and the critical micelle concentration was obtained by measuring the relationship between surface tension and concentration. The following are the surface tension and critical micelle concentration data of some surfactants prepared by the present invention.

The lowest surface tension of all the compounds prepared in the present invention is below 20.2mN/m, the lowest being only 15.2mN/m, which is similar to the lowest surface tension (≤16.0mN/m) of PFOA and PFOS derivatives. In comparison, the lowest surface tension of similarly structured but non-gemini species such as N-[3-(p-perfluorononenyloxybenzenesulfonamido)propyl]-N,N-dimethylglycine inner salt is 24.3mN/m, the critical micelle concentration is 6.0×10 ^-5 mol/L. It can be seen that the geminiization of perfluoroalkene surfactants can effectively improve the surface activity.

Those skilled in the industry should understand that the present invention is not limited by the above-mentioned embodiments, and the above-mentioned embodiments and the description in the specification are only for illustrating the principles of the present invention. On the premise of not departing from the spirit and scope of the present invention, there will be various changes and improvements in the present invention, and these all fall within the protection scope of the present invention.

Claims (2)

1. A class of Gemini amphoteric surfactants based on perfluoroolefins, the general formula is                                                  A compound, wherein R _f is perfluorononenyl or perfluorohexenyl; m is 2 or 3; n is 2 or 3; R is (CH ₂ ) ₂ or (CH ₂ ) ₃ or (CH ₂ ) ₄ Or (CH ₂ ) ₅ or (CH ₂ ) ₂ O(CH ₂ ) ₂ or CH ₂ COCH ₂ ; the above compounds use perfluorononenyloxyphenyl or perfluorohexenyloxyphenyl as the lipophilic part, and The quaternary ammonium and sulfonate are hydrophilic parts, and the lipophilic part and the hydrophilic part are linked by sulfonamide bonds. 2. A method for preparing the gemini-type amphoteric surfactant as claimed in claim 1 by perfluorononene or perfluorohexene, characterized in that: perfluorononene or perfluorohexene is condensed with phenol, chlorosulfur chlorination, or condensation with phenol, sulfonation, chlorination, or condensation with sodium p-hydroxybenzenesulfonate, chlorination to obtain perfluoroalkenyloxybenzenesulfonyl chloride, perfluoroalkenyloxybenzenesulfonyl chloride and 2-dimethylamino Condensation of ethylamine or 3-dimethylaminopropylamine to obtain the corresponding perfluoroalkenyloxybenzenesulfonamide, and then with 1,2-dihaloethane, 1,3-dihalopropane, 1,4-dihalobutane , 1,5-dihalopentane, two (2-haloethyl) ether or 1,3-dihaloacetone to react to obtain a double quaternary ammonium salt intermediate, wherein the halogen is chlorine or bromine, and finally under alkaline conditions Condensation with sodium ethylene sulfonate, sodium chloroethyl sulfonate, sodium bromoethyl sulfonate or 1,3-propane sultone yields a class of perfluoroolefin-based gemini-type amphoteric surfactants.