Anionic Surfactant

7 Typical anionic surfactants are sodium lauryl sulphate or sodium lauryl ether sulphate.

From: Handbook of Nonwovens, 2007

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Surface Tension

Bastian E. Rapp, in Microfluidics: Modelling, Mechanics and Mathematics, 2017

20.3.2.1 Anionic Surfactants

Anionic surfactants are all surfactants that carry a negatively charged head group. In principle, any negatively charged functional group that can complex with a counterion may lend itself to be a cationic surfactant. This section lists the most commonly used ones.

Carboxylic Acids. The most commonly used anionic surfactants are based on aliphatic carboxylic acids, which can be derived from naturally occurring animal and plant fats (see Fig. 20.14). Fats as they occur in nature are usually triesters of carboxylic acids and glycerol. These fats can be hydrolyzed using potassium hydroxide or sodium hydroxide, a process that is commonly referred to as saponification (see Fig. 20.13). The term refers to the fact that the resulting substances are what is generally referred to as soap. The carboxylic acids are retrieved as their sodium or potassium salts, which makes them anionic surfactants. They are the main ingredient of soap. Because they can be obtained from fats, these aliphatic carboxylic acids are often referred to as fatty acids.

Fig. 20.13. Saponification reaction to obtain anionic surfactants (soap) by hydrolysis of naturally occurring triglycerides, which are animal or plant fats.

Fig. 20.14. Commonly used anionic surfactants based on carboxylic acids.

Commercially, the most relevant surfactant is sodium stearate, which is the sodium salt of stearic acid. As stated, it is obtained by saponification of many animal fats. Several carboxylic surfactants are based on bile acid obtained from the bile of animals, among them cholic acid and its salt (usually the sodium salt) as well as deoxycholic acid, which has one less alcohol group. These surfactants are only weakly anionic, which makes them suitable for use in cell biology. Another commonly used substance for cell lysis is sodium lauroyl sarcosinate. It is also used in numerous cosmetics, shampoos, and soaps.

Several carboxylic poly(ethylene glycol) ethers can also be used as surfactants, e.g., glycolic acid ethoxylate 4-tert-butylphenyl ether (with an aromatic hydrophobic tail) or glycolic acid ethoxylate laurylphenly ether (with an alkylphenyl hydrophobic tail) as well as glycolic acid ethoxylate oleyl ether (with a purely alkyl tail).

You may come across a group of commonly used fluorinated surfactants called Zonyl FSA fluorosurfactants. These are usually used in form of their lithium salts.

Sulfonic Acids. The second important class of anionic surfactants is based on sulfonic acid. Fig. 20.15 shows a selection of the most important compounds of this class.

Fig. 20.15. Commonly used anionic surfactants based on sulfonic acids.

One of the most important surfactants of this class is sodium dodecyl sulfate, which is the sodium salt of dodecyl sulfonic acid. It is commonly used in biology and biochemistry for denaturing proteins, i.e., for destroying the quaternary and tertiary structure. This is an important prerequisite for gel electrophoresis, which is often conducted after a denaturing step using sodium dodecyl sulfate. The gel is mostly made from polyacrylamide, which is why this type of gel electrophoresis is then referred to as sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). It is also possible to combine dodecyl sulfonic acid with an ammonium ion, resulting in ammonium dodecyl sulfate. In order to increase water solubility, a polyether can be inserted into sodium dodecyl sulfate, resulting in sodium lauryl ether sulfate. Another commonly used anionic surfactant is dioctyl sodium sulfosuccinate. Besides aliphatic sulfonic acids, many aromatic sulfonic acid surfactant are in use, with sodium dodecylbenzenesulfonate being the most commonly used one.

In many applications, fluorinated anionic surfactants are being used, e.g., as stain repellent additive in cloth and surface treatment. Fluorinated surfactants are usually environmentally critical as their half-life time is very high and they tend to accumulate in organism, which is why they are referred to as bioaccumulating. One of the most commonly used surfactants was perfluorooctane sulfonic acid, but it is being replaced by perfluorobutane sulfonic acid, which has a much shorter half-life time and does not seem to be bioaccumulating.

As for the carboxylic acids, several sulfonic acid ethers can also be used, e.g., 3-sulfopropyl ethoxylate lau-rylphenly ether (with an alkylphenyl hydrophobic tail); it is mostly used as its potassium or sodium salt.

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Application of UV–VIS spectrophotometry for chemical analysis

Bhim Prasad Kafle, in Chemical Analysis and Material Characterization by Spectrophotometry, 2020

5.4.5.3.4.1 General discussion

Anionic surfactants are currently the types most used, being incorporated in the majority of detergent and cleaning-product formulas in daily use. Linear-chain alkyl benzenesulfonate types, are the most popularly used synthetic anionic surfactants. In the majority of the detergent and cleaning-products, anionic surfactants have been extensively used for over 40 years. These surfactants pass into sewage-treatment plants, where they are partially aerobically degraded and partially adsorbed to sewage sludge that is applied to land. Finally, they are dumped into the waterways and onto soil, where they constitute some of the main factors affecting the natural ecosystem. Therefore, it is important to determine the concentration of anionic surfactants with accuracy and have quick and simple procedures to monitor their biodegradation over time.

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Synthetic Chemistry of the Inorganic Ordered Porous Materials

Z.-A. Qiao, Q.-S. Huo, in Modern Inorganic Synthetic Chemistry (Second Edition), 2017

15.9.1.2 Anionic Surfactants and Co-Structure-Directing Agent (CSDA)

The anionic surfactant can be used as template directly to synthesize mesoporous materials such as SnO2, Al2O3, and Ga2O3 through a SI+ interaction pathway. A new route (SN+I) is the self-assembly of anionic surfactants and inorganic precursors by using aminopropylsiloxane or quaternized aminopropylsiloxane as the CSDA [151,183]. The negatively charged headgroups of the anionic surfactants interact electrostatically with the positively charged ammonium sites of the CSDAs. The alkoxysilane groups of the CSDA co-condense with tetraalkoxysilane and are subsequently assembled to form the silica framework. Fig. 15.20 is a scheme of this synthetic route.

Figure 15.20. A scheme of CSDA.

Reproduced with permission from S. Che, A.E. Garcia-Bennett, T. Yokoi, K. Sakamoto, H. Kunieda, O. Terasaki, T. Tatsumi, Nat. Mater. 2 (2003) 801. Copyright 2003 of the Nature Publishing Group.

In the synthetic system with anionic surfactant N-myristoyl-l-glutamic acid as template and N-trimethoxylsilylpropyl-N,N,N-trimethylammonium chloride as CSDA, the packing of the micelle was controlled by simply adjusting the neutralization degree of the C14GluA surfactant. Different mesophases ranging from tetragonal P42/mnm (cage type, AMS-9), cubic Fd3m (cage type, AMS-8), to 2D hexagonal p6mm (cylindrical, AMS-3), and a bicontinuous double diamond cubic Pn3m mesophase (AMS-10) were obtained by decreasing the amount of NaOH that was added into the reaction system [184]. AMS-10 may exhibit a lower curvature close to bicontinuous cubic Ia-3d from the sequence of the mesophases. Changing the degree of ionization of the surfactant results in changes of the surfactant packing parameter g, which leads to different mesostructures. Furthermore, variation of the charge density of positively charged amino groups of the CSDA also gives rise to different values of g [185].

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Determination of Surfactants in Cosmetics

M.C. Prieto-Blanco, ... D. Prada Rodríguez, in Analysis of Cosmetic Products, 2007

Anionic surfactants.

The main anionic surfactants, alkyl sulfates and alkyl sulfonates as well as ethoxylate derivatives were analysed. C12 and C14 homologues of alkyl sulfates were separated in shampoo using a nonaqueous system with methanol and indirect UV detection with p-toluenesulfonate, which is highly sensitive and has similar mobility to alkyl sulfates. Other anionic surfactants, such as alkyl sulfonates and linear alkyl sulfonates are separated, but they are not applied to cosmetic products (Salimi-Moosavi and Cassidy, 1996).

Heinig et al. (1996a) developed methods in aqueous and nonaqueous medium for three types of anionic surfactants: alkyl sulfates, alkyl ethoxy sulfates and alkyl sulfonates. Several separation parameters, such as chromophore buffer type, concentration, pH, non-absorbing buffers and organic modifiers were examined. Anionic surfactant with low electrophoretic mobility, like sulfonates and alkyl ethoxy sulfates could be separated and good peak shapes displayed using dodecylbenzene sulfonate, whereas p-hydroxybenzoate is more suitable for surfactants like alkyl sulfates that have higher mobility. Nonaqueous methods provide better resolution and detection limits than aqueous methods but the latter give better area reproducibility. Sodium lauryl sulfate in toothpaste and face wash was determined using an aqueous method, while alkyl (C12–14) ethoxy sulfate were separated in shampoo and face wash using a nonaqueous method. The authors recommended using these methods for product control, considering that electropherogram may be a fingerprint of surfactant formulations.

Acylglutamates, anionic surfactants used in cosmetics due to their lack of irritation and good biodegradability, can be analysed by CE method. The effect of the pH and concentration of buffer and organic solvents on (C12–18) N-acyl-L-glutamate separation was examined. Methanol was more effective than acetonitrile in minimizing the variation of mobility and zone width caused by changes in sample concentration. The method applied to cosmetic lotion enabled N-lauroyl-L-glutamate to be separated from preservatives like methylparaben (Kunimasa and Kameyama, 1999).

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Proceedings of the International Conference on Colloid and Surface Science

H. Hoffmann, ... J. Hao, in Studies in Surface Science and Catalysis, 2001

2.1.2.2 C12(EO)2.5SO3H + C14DMAO

Cationic-anionic surfactant systems can also be prepared by adding the acids of alkylsulfonates oralkylsulfates to zwitterionic surfactants [15]. The acids protonate the zwitterionic surfactants what results in the formation of a cationic surfactant that combines with theanionic surfactant to the cat-anionic surfactant systems. In fig. 4 the phase diagram for 100 mM mixtures of tetradecyldimethylamineoxide and dodecylethoxysulfuric acid(DES) is shown. With increasing mole-fraction of DES one finds first a L1-phase, then a two phase L1/Lα-region, a single Lα-phase, aprecipitate phase, a single Lα-phase,a two phase L1/Lα-region and finally again a L1-phase. The precipitate dissolves for T = 50 ° C and forms a Lα-phase. The two Lα-phases to the left and to the right of the precipitate region show very weak birefringence. The two phases differ however in their rheological properties. The Lα-phase on the left hand side is an uncharged phase and for this reason has low viscoelastic properties while the Lα-phase on the right hand side is ionically charged and has strong viscoelastic properties. Theoretically it is possible to use the dodecylsulfuric acid in combination with C14DMAO for the preparation of the cat-anionic surfactant system. Preliminary experiments have shown however that this combination forms a precipitate over a much wider mixing ratio. It is thus more favourable to use the DES for the preparation of the system. The acid form of the DES was prepared from sodium dodecylethoxysulfate (SDES) by ion exchange process. As already indicated by its extremely weak birefringence, the Lα-phase that is prepared by mixing the two surfactants consists of vesicles as is shown in a micrograph in fig. 5. In contrast to the vesicles in fig. 1 the vesicles in fig. 5 are smaller in size and do not contain large multilamellar vesicles.

Fig. 4. Phase diagram of a 100 mM solution of C14DMAO with a 100 mM solution of the acid of dodecylethoxysulfate (DES). Note the two Lα-phases to the left and right of the precipitation zone.

Fig. 5. FF-TEM micrograph of a Lα-phase with multilamellar vesicles in the system C14DMAO/C12(EO)xSO3H with the composition 100 mM C14DMAO / 30 mM C12(EO)xSO3H.

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Biobased Surfactants: Overview and Industrial State of the Art

Douglas G. Hayes, George A. Smith, in Biobased Surfactants (Second Edition), 2019

1.5.1 Anionic Surfactants

Anionic surfactants are the most frequently used category of surfactants due to their employment in laundry and personal care products. Some of the oldest and most common surfactants are fatty acid soaps (dating back to 2800 BCE in ancient Babylonia (Willcox, 2000)), formed via saponification of FFA. Recently, nanostructured soft matter has been prepared from fatty acid salts that employ organic cations (Fameau et al., 2011, 2013; Fameau and Zemb, 2014). Methyl ester sulfonates (MES, Fig. 1.1) are perhaps the most widely used biobased surfactant, with its major application being in powder and liquid laundry detergents as a replacement of the fossil fuel-derived surfactants, particularly LAS. The synthesis, properties, and applications of MES are thoroughly described in Chapter 9.

Sodium coco sulfate is a biobased homologue of the commonly employed surfactant sodium lauryl (dodecyl) sulfate (n-C12H25OSO3Na; SLS or SDS), with the acyl groups derived from coconut oil, palm kernel oil, or another high-lauric acid oil. Recently, the replacement of the sodium counterion of SLS with biobased choline ((CH3)2N+(CH2)2OH) has been shown to improve water solubility and lower the surfactant’s Krafft point temperature (Klein et al., 2013). Sodium laureth sulfate (sodium lauryl ether sulfate, n-C12H25(OCH2CH2)nOSO3Na, SLES) is a common surfactant in many personal care products and is a more effective foaming agent than SDS. Amide ether carboxylates, prepared from ethoxylation of the free OH end of fatty acid-monoethanolamine, followed by attachment of a COO-end group via sodium monochloro acetate, have good properties for dermatologic formulation: compatible with skin, biodegradable, low irritability with eyes and skin, good water solubility tolerance to water hardness, and good foam formation (Tsushima, 1997). Other biobased ionic surfactants include sodium methyl cocoyl taurate (a foam booster produced from medium-chain fatty acids and taurine, i.e., 2-aminoethanesulfonic acid, a common metabolite found in bile) and disodium coco sulfosuccinates (Pletnev, 2006).

A deficiency of LAS and SDS is their performance in hard water. Typically, chelating agents or “builders” are required to neutralize divalent cations such as Mg2 + and Ca2 +. The most common chelating agents are ethylenediaminetetraacetic acid (EDTA) and phosphates, which have negative environmental impacts (Doll and Erhan, 2009). Their bioaccumulation in waterways leads to a temporary surge of algal and bacterial growth, followed by oxygen depletion due to the thick “slime” layer that forms at the water-air interface that blocks oxygen and light transport from the air. This results in the loss of life for fish and aquatic wildlife and a source of drinking water. Biobased chelators with low ecotoxicity are currently under development (Doll and Erhan, 2009). Biobased sulfonate surfactants utilizing furans have recently been developed that perform better than LAS and SDS: lower Krafft point temperature and cmc, improved foaming behavior, faster wetting kinetics, and greater micelle stability in hard water (Park et al., 2016). The surfactant is synthesized by first forming the ketone 2-alkanoylfuran from furan, which can be derived from hemicellulose (xylose) (Perez and Fraga, 2014), and fatty acid anhydrides via Friedel-Crafts acylation, followed by sulfonation of the latter’s furan moiety (Fig. 1.8). Alternatively, the carbonyl group of 2-alkanoylfuran can be reduced using H2 and catalyst to produce 2-alkylfuran, followed by sulfonation (Fig. 1.8).

Fig. 1.8. Synthesis of furan-based surfactants.

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Molecular Structure and Phase Behavior of Surfactants

M. Miyake, Y. Yamashita, in Cosmetic Science and Technology, 2017

24.5.1 Molecular Structure of the Hydrophilic Groups and Their Phase Behavior

The phase behavior of anionic surfactants widely used in body soaps and shampoos has been compared within similar C12 alkyl chains and sodium salts. Fig. 24.8 shows the phase diagrams of sodium laurate (C12Soap),7 sodium dodecyl sulfate (SDS),8 sodium laurylbenzene sulfonate (LAS),9 and sodium dodecyl tetraoxyethylene sulfate (C12E4S)10 in surfactant/water two-component systems. The melting temperature of the solid phase for C12Soap is higher than that for SDS at all concentrations. The molecular packing of the carboxylate moiety, the head group of the soap, is more compact than the sulfate moiety of SDS because of the dipole–dipole interactions operative between the carbonyl groups. The molecular orientation of the soap is thus favored; the molecules are closely packed in the solid state, which results in a higher melting point for the soap. On the other hand, there is no solid phase for C12E4S and the melting point is below 0°C until the surfactant concentration is over 50 wt%. The crystallization of C12E4S is inhibited at low-surfactant concentrations, as the cross-sectional area per molecule (aS) of the sulfate group is expanded by the hydrated ethylene oxide (EO) units. This difference in their melting points is the reason why soap is mostly applied in solid products and alkyl ethoxylate sulfate (AES) in liquid products.

Figure 24.8. Phase diagrams of typical anionic surfactants: (A) C12Soap,7 (B) SDS,8 (C) LAS,9 and (D) C12E4S.10

C12Soap, SDS, and C12E4S have liquid crystal phases at higher concentrations above the melting temperature. For C12Soap and SDS, the H1 phase appears at around 30–40 wt% of surfactant concentration, and the Lα phase at around 60–70 wt% of surfactant concentration. C12E4S exhibits an I1 phase and a V1 phase at the adjacent H1 phase, respectively. However, commercial AES does not form a cubic liquid crystal phase due to the EO units' distribution. The phase transition of C12Soap and SDS from H1 to Lα means that the CPP increases from 1/3–1/2 to 1/2, as the cross-sectional area per molecule decreases with the increasing surfactant concentration. The appearance of the I1 phase in the C12E4S system suggests that the CPP is 1/3 or lower, due to the bulky hydrophilic group consisting of the EO units and sulfate group, leading to CPP  1/3. This way, phase behavior is controlled by the CPP based on the molecular structure of the surfactant.

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Flow Analysis with Atomic Spectrometric Detectors

Angel Morales-Rubio, Miguel de la Guardia, in Analytical Spectroscopy Library, 1999

Surfactants

Gallego et al.201 determined anionic surfactants in waste waters by an indirect FAAS detection method. The method involves the extraction in IBMK of the ion pair surfactant-1,10-phenanthroline-copper. Determinations were made by measuring the copper level in the separated organic layer. Limits of detection of 45 ng/ml were reported and precisions were down 1%.

Cationic surfactants in natural, tap and waste waters were also determined202 by an indirect FAAS. The method involves the extraction into IBMK of the ion-pair surfactant-tetrathiocyanatecobaltate. As before, determinations were carried out by reading the AAS cobalt signal of the organic phase.

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Chemistry of chemical admixtures

G. Gelardi, ... R.J. Flatt, in Science and Technology of Concrete Admixtures, 2016

9.5.4.4 Taurates

Surfactants of the group of taurates (taurides) are rather mild anionic surfactants. They show good compatibility with non-ionic and other anionic surfactants, and provide good stability against hydrolysis as well as good lime soap-dispersing power. Because of this, they find application in air entrainers as ‘cosurfactants’ to support the efficiency of the main active surfactant.

A general representation of taurates is shown in Figure 9.37, where R mostly comes from saturated C12–C18 fatty acids, oleic acid or coconut fatty acid. According to Rosen (2004), the solubility, foaming, detergency and dispersing powers of the N-methyl derivatives (Figure 9.37 with R1 = CH3) are similar to those of the corresponding fatty acid soaps, and are effective in hard and soft water.

Figure 9.37. Chemical structure of an alkali metal salt of n-acyl-n-alkyl taurate.

Examples of commercially available products are sodium salts of oleic acid methyl taurate and coconut fatty acid methyl taurate.

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Body Care Cosmetics

T. Sakai, in Cosmetic Science and Technology, 2017

33.3.4.2 Mixed Surfactant System

It is well known that the addition of cationic surfactants to anionic surfactant solutions can drastically change the solution characteristics. These effects are attributable to the formation of a complex consisting of anionic surfactant and cationic surfactant molecules over a wide range of mixing ratios. This complex so formed has often been called a “catanionic surfactant.” The electrostatic neutralization between two opposite kinds of surfactants can provide a solution with nonionic characteristics and enhanced hydrophobicity. For example, there is a drastic decrease in the critical micelle concentration (CMC) and an increase in the detergency. Although these drastic changes in the properties can be achieved easily when quaternary ammonium salts or amine oxides are used as cationic surfactants, the systems become remarkably difficult to use due to the deposition of the hydrophobic complex or the excessive degreasing ability for cosmetic use. In liquid body wash systems, betaines, which are generally highly water soluble, are formulated commonly as cationic surfactants.

The skin irritation from surfactants might be related to the CMC. This is based on the hypothesis in physical chemistry that skin irritation might be caused by the adsorption and penetration of surfactant “monomers” into the skin and exacerbated by an increase in the monomer concentration (i.e., the CMC). There are a number of reports on this relationship for mixtures of anionic surfactants and betaines.14,15 In fact, the skin irritation from mixtures of anionic surfactants and betaines decreases with the reduction of the CMC. This mixture system has been applied to not only body cleansers but also a number of liquid detergents. However, the recent progress of studies on the relationship between skin irritation and surfactants has suggested that lowering the charge density of anionic surfactants with nonionic surfactants and betaines stabilizes the mixed micelles and prevents the monomers from being released.16 This is an ongoing discussion between chemists and biologists.

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