Fatty ester sulfonates

Description. These surfactants are sometimes known under their abbreviated names FES for fatty ester sulfonate, MES for methyl ester sulfonate, or ASME for a-sulfo methyl ester. There are two types of sulfo fatty acid esters ... 

Corrosion inhibitors partial esters of succinic acid, fatty acids, sulfonates, phenates, amine phosphates. 

The mechanism for sulfonation of hydrogenated fatty esters is accepted as a two-stage process. A rapid sequence of reactions leads to the formation of intermediates having approximately 2 1 stoichiometry of sulfur trioxide to ester. In the subsequent slower and higher temperature aging step, the SO is released for further reactions and the starting material conversion proceeds to completion (133). 

Sodium fatty acid ester sulfonates are known to be highly attractive as surfactants. These have good wetting abiHty and exceUent calcium ion stabiHty as weU as high detergency without phosphates, and are used in powders or Hquids. They can also be used in the textile industry, emulsion polymerization, cosmetics, and metal surface fields. Moreover, they are attractive because they are produced from renewable natural resources and their biodegradabiHty is almost as good as alkyl sulfates (134—137). 

Fatty JicidFster Sulfonates. Fatty acid ester sulfonates (FAES) are generally produced from methyl esters, ie, methyl ester sulfonate (MES) and prepared via sulfonation, followed by bleaching and neutralization, in a relatively difficult and complex process ... 

Reactions of the hydrocarbon chain in alkanoic acids include a-sulfonation and halogenation (51—54). The a-sulfonated fatty ester salts have excellent lime-dispersing properties and are valuable surface-active agents. 

From both economic and ecological points of view, substances derived from esters of monocarboxylic acids sulfonated in the a position form an interesting class of surfactants [1]. The general formula of these a-sulfomonocarboxylic esters, also called a-sulfo fatty acid esters or, in short, a-ester sulfonates, is Rj-CH(S03Me)-C00-R2 (with Rj and R2 = alkyl groups, Me = alkali metal). 

Ester sulfonates will become more and more interesting in the future because the raw materials for their preparation are fatty acid esters which can be prepared from oils and fats, and thus from renewable resources. They can be used as possible substitutes for surfactants based on petrochemicals. 

Also the a-ester sulfonates are less important today. In the Federal Republic of Germany, for example, the total production of surfactants was about 700,000 t/a in 1993. For a more detailed analysis of different types of surfactants, use must be made of data collected before the unification of Germany. In 1988 the consumption of surfactants in detergents was about 227,500 t/a, the consumption of anionic surfactants was about 116,000 t/a and less than 1000 t/a of a-sulfo fatty acid esters [5] (the values refer to German Detergent Law). 

Compared with the fatty alcohol sulfates, which are also oleochemically produced anionic surfactants, the ester sulfonates have the advantage that their raw materials are on a low and therefore cost-effective level of fat refinement. The ester sulfonates are produced directly from the fatty acid esters by sulfona-tion, whereas the fatty alcohols, which are the source materials of the fatty alcohol sulfates, have to be formed by the catalytic high-pressure hydrogenation of fatty acids esters [9]. The fatty acid esters are obtained directly from the fats and oils by transesterification of the triglycerides with alcohols [10]. 

In spite of all their merits, ester sulfonates have not been used to a great extent as yet [12]. The reason is that a practicable process for producing a-sulfo fatty acid esters with good qualities is difficult because of the formation of byproducts with worse properties (e.g., the disalts of the a-sulfo fatty acids). 

The starting substances for the production of a-ester sulfonates are the triglycerides of animal and vegetable fats and oils. The transesterification of the glycerides with alcohol leads directly to fatty acid alkyl esters and glycerin [7]. 

For technical applications, the production of ester sulfonates from the (purified) sulfo fatty acids involves too much effort, especially because the relevant fatty acid esters can be produced directly from the triglycerides of fats and oils by transesterification. The only possible way to produce ester sulfonates is the sulfonation of fatty acid esters. 

Schmid et al. studied in detail the sulfonation reaction of fatty acid methyl esters with sulfur trioxide [37]. They measured the time dependency of the products formed during ester sulfonation. These measurements together with a mass balance confirmed the existence of an intermediate with two S03 groups in the molecule. To decide the way in which the intermediate is formed the measured time dependency of the products was compared with the complex kinetics of different mechanisms. Only the following two-step mechanism allowed a calculation of the measured data with a variation of the velocity constants in the kinetic differential equations. 

In the first step an S03 molecule is inserted into the ester binding and a mixed anhydride of the sulfuric acid (I) is formed. The anhydride is in a very fast equilibrium with its cyclic enol form (II), whose double bonding is attacked by a second molecule of sulfur trioxide in a fast electrophilic addition (III and IV). In the second slower step, the a-sulfonated anhydride is rearranged into the ester sulfonate and releases one molecule of S03, which in turn sulfonates a new molecule of the fatty acid ester. The real sulfonation agent of the acid ester is not the sulfur trioxide but the initially formed sulfonated anhydride. In their detailed analysis of the different steps and intermediates of the sulfonation reaction, Schmid et al. showed that the mechanism presented by Smith and Stirton [31] is the correct one. 

Most of the technically produced a-sulfo fatty esters are prepared from unbranched saturated fatty acid esters that are derived from 8 22 carboxylic acids and Cj-C3 alcohols. In particular the C12 (lauric), C14 (myristic), C16 (palmitic), and C18 (stearic) acids are interesting because the ester sulfonates... 

Even if the fatty acid esters have been sulfonated under optimal conditions the ester sulfonates are dark-colored [12,33] so the sulfonated product has to be bleached. The second pretreatment is the neutralization of the acid product to obtain the salt of the a-sulfo fatty acid ester. Different techniques have been published in the literature. Kapur et al. suggested bleaching with 3-4 wt % NaOCl (15 wt % solution) after neutralization with a 30% aqueous solution of sodium hydroxide. This technique is for small-scale sulfonation [46]. 

Bistline and Stirton compared the CMC values of ester sulfonates with cyclic ester groups [54]. The phenyl esters have higher values than benzyl and cyclohexyl esters. The influence of the structure of the ester group decreases with increasing chain length of the hydrophobic fatty acid group. The cyclic esters of a-sulfostearic acid, for example, have nearly the same CMC values. 

Fujiwara et al. used the CMC values of sodium and calcium salts to calculate the energetic parameters of the micellization [61]. The cohesive energy change in micelle formation of the a-sulfonated fatty acid methyl esters, calculated from the dependency of the CMC on the numbers of C atoms, is equivalent to that of typical ionic surfactants (Na ester sulfonates, 1.1 kT Ca ester sulfonates, 0.93 kT Na dodecyl sulfate, 1.1 kT). The degree of dissociation for the counterions bound to the micelle can be calculated from the dependency of the CMC on the concentration of the counterions. The values of the ester sulfonates are also in the same range as for other typical ionic surfactants (Na ester sulfonates, 0.61 Ca ester sulfonates, 0.70 Na dodecyl sulfate, 0.66). 

For the disodium salts of the a-sulfo fatty acids it was found that the Krafft points are higher than those of the a-ester sulfonates. Because the CMC is also higher for the disalts they are more soluble than ester sulfonates at low temperatures but less soluble at higher temperatures [58]. 

The surface and interfacial tension of a great number of ester sulfonates has been measured by Stirton et al. [26-28,30]. The values of the surface tension of 0.2% solutions at 25 °C are in the range from 25 to 50 mN/m and from 2 to 20 mN/m for the interfacial tension. In the group with the same number of C atoms the pelargonates and laurates have the lowest values. Among the esters of the same a-sulfo fatty acid, the surface and interfacial tension decreases with increasing molecular weight of the alcohols. Surface tension values also depend on the cation. For the alkali salts the values decrease from lithium to sodium to potassium. 

For long-chain alcohol esters it is interesting to see that the interfacial tension between a 0.01 wt % aqueous solution and octane or xylene has a minimum for ester sulfonates with a total 22 carbon atoms in the fatty acid chain and the ester chain [60]. The balance in length between the two chains has only a poor effect. Thus, a-sulfonated fatty acid esters with a total number of 22-26 carbon atoms in the molecule have excellent interfacial activities. To attain the same magnitude in the interfacial tension between linear alkylbenzenesulfonate (LAS) solution and octane, the required concentration of LAS is 0.1 wt %. This is 10 times the concentration needed for a-sulfonated fatty acid esters [60]. 

In mixtures of ester sulfonates and disalts the remission decreases as the content of disalts increases (Fig. 10). But small amounts of disalts do not have a strong negative effect on the detergency of the ct-sulfo fatty acid esters [581. 

A special application of a-sulfo fatty acid esters is for highly concentrated hard surface cleaners which are used after dilution in a pail of water. Normally hydrotropes are needed to increase the solubility of the surfactants in water and assure clear, homogeneous, and storage-stable products. If the LAS, which are typical surfactants in hard surface cleaners, are replaced by ester sulfonates the hydrotrope can be deleted from the formula [62]. 

A great number of detergent compositions using a-sulfo fatty acid esters are given in the patent literature. The a-ester sulfonates are major or minor components, besides other surfactants and builders [68-76]. Some examples of laundry and dishwashing detergents are given below. 

A soap-based powder can be produced in combination with ester sulfonates. Thirty-five percent of a sodium soap mixture (5% lauric acid, 5% myristic acid, 52% palmitic acid, 21% stearic acid, 12% oleic acid, and 5% linoleic acid) is mixed with 15% sodium a-sulfo palm oil fatty acid methyl ester, 3% lauric acid ethoxylate, 5% sodium silicate, 17% sodium carbonate, 20% Na2S04- 10H2O, and 5% water [79]. 

The addition of an ester sulfonate of an unsaturated fatty acid improves the rinsing properties in washing tests. A mixture of 50% sodium methyl a-sulfostearate and 50% sodium salt of sulfonated methyl oleate (30 70 mono-sulfonate/polysulfonate ratio) was used [83]. 

Generally speaking, up to now the importance of a-sulfo fatty acid esters in cosmetic products has been low [1 p. 367], In the future they may become more interesting because of their mildness. a-Sulfomethyl laurate and most other ester sulfonates are mild to the skin also, they are not human skin sensitizers or primary skin irritants. Tests have shown that a-sulfomethyl laurate is mild enough to be in bath products, such as bubble bath [62]. Three patents for different applications are given to show how ester sulfonates can be used in cosmetics. 

Salts of a-sulfo fatty acid esters can work as emulsifying agents for the preparation of asphalt emulsions and asphalt-latex emulsions. The ester sulfonates improve the storage stability of the emulsions [101,102]. In the manufacture of lightweight gypsum products air bubbles have to be mixed into the slurries. The use of salts of sulfonated C10 l8 fatty acid alkyl esters as foaming agents produces uniformly distributed fine bubbles [103]. Salts of C10 16 fatty acid alkyl ester sulfonates can also be added to cement mixtures to prevent slump loss of the mixtures [104]. 

For a further separation of the sulfonated surfactants the latter are heated for 4 h with 2 N HC1. The methyl ester sulfonates are split into methanol and a-sulfo fatty acids, which form disodium salts after neutralization with NaOH. The product mixture from acid hydrolysis can be separated by extraction with petroleum ether. For example, the fatty alcohols formed from fatty alcohol sulfo-... 

The amount of the ester sulfonates, besides the mono- and disalt of the a-sulfo fatty acid, can be calculated by two titrations, one in the acid and one in the basic range. In the basic range both sulfonates and carbocylate functionalities are negatively charged and titrated with the cationic surfactant hyamine. In acid medium the RCOOH group is protonated and no longer available for the titration. Since hyamine-methylene blue (acid conditions) titrates only sulfonate and hyamine-phenol red (basic conditions) determines both sulfonates and carbo-cylates, substraction of the titration value with phenol red from the double value of the titration with methylene blue yields only the a-sulfo fatty acid ester. This is the only species of the three which has merely the sulfonate function [106]. 

Finally, ion chromatography can be used to determine the a-sulfo fatty acid esters. The chromatographic column is a nonpolar poly sty rene/divinylbenzene column and the ion pair reagent is 0.005 M ammonia. In order to reduce the elution time, acetonitrile is added as a modifier with increasing concentration. This gradient technique makes it possible to separate simultaneously ester sulfonates and disalts by chain length. Determination is achieved by standards with defined chain length [107]. 

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