A-sulfo

A purified fatty acid is recommended for the preparation of a pure a-sulfo acid. Purified palmitic acid (m.p. 60.8-61.4°, neutralization equivalent 256.2) is prepared by twice recrystallizing a good commercial grade of palmitic acid from acetone at 0°, using a solvent ratio of 10 ml. to 1 g. However, the reaction may be applied to commercial saturated higher fatty acids, if the iodine number is sufficiently low. The checkers obtained similar results with recrystaUized Neo-Fat 1-56 (Armour and Company, Chicago, 111.) or Eastman white label palmitic acid. 

Other direct methods for the sulfonation of the higher fatty acids are by the use of sulfur trioxide vapor or by the use of chlorosulfonic acid. Indirect methods are also available for the preparation of a-sulfo fatty acids and their salts from an a-bromo fatty acid made by the Hell-Volhard-Zelinsky reaction. The bromo compound may be converted directly to the sodium salt of a sulfonic acid through the Strecker reaction or may be converted to the mercaptan and oxidized to the sulfonate. Sulfonation of the lower fatty acids has been studied by Backer and co-workers. ... 

Polyacrylamide may be converted into a sulfo-methyl derivative by condensation with formaldehyde and sodium bisulfite under basic conditions. 

As another example, a synergistic enhancement of tensile strength has been reached in blends of a sulfo-nated polyacrylonitrile terpolymer (SPAN) with a polyurethane (PU) cationomer [23]. Maximum enhancement was achieved at a blend composition of 30/70 (PU/ SPAN). At this blend composition, the tensile strength was raised from an initial value of 78.5 MPa to 196 MPa and the strain-to-fracture was at its highest. The en-... 

This reaction was studied in the gaseous phase at 120°C with a sulfo-nated organic ion exchanger as catalyst (p. 27), under both the competitive system (Villa) (2 esters + 1 alcohol) and the system (VUIb) (1 ester + 2 alcohols)... 

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). 

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). 

However, it could be expected that the share of the latter group will rise to the same extent as the rising importance of environmental digestibility. It is very possible that in the future the C16/C18 ester sulfonates will partly replace the alkylbenzenesulfonates produced from petrochemical raw material [6,7]. N. R. Smith [8] expects the a-sulfo methyl esters to be an alternative to ethylene-based surfactants. An increase in the production of surfactants based on ethylene is problematic, because in industrial countries ethylene production is occurring at 95% of capacity and more. 

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 a-sulfo fatty acid esters are obtained by a-sulfonation of the fatty acid esters, and in basic mediums they form the corresponding salts. 

Nagayama et al. [36] studied a-sulfonation using nuclear magnetic resonance (NMR). They reported the presence of two intermediates. The first intermediate is the adduct of S03 to the carbonyl oxygen formed at low temperatures. In contrast to the mechanism of Stein et al., they did not propose a rearrangement of this intermediate but a second addition of S03 to the activated a-hydrogen to give the second intermediate. The reaction of the intermediate with sodium hydroxide can lead to the disodium salt if the neutralization is immediate or to the sodium a-sulfo fatty acid ester if the neutralization is delayed. 

Neutralization leads to the salt of the a-sulfo fatty acid ester, but only if the neutralization step is delayed. If the neutralization is immediate the a-sulfonated anhydride forms a disalt of the a-sulfo fatty acid as a byproduct [38]. The production of the disalt is also effected by the ratio between S03 and the ester. A high surplus of S03 would shorten the reaction time, but the amount of disalt in the end product would increase. For 90°C and 30 min an optimal S03/es-ter ratio is 1.2 1 [37]. 

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... 

Besides these normal technical products, many other different types of a-sulfo fatty acid esters have been described in the literature. For example, Weil et al. prepared a-sulfopalmitates and stearates with higher alcohols [19] and also monoesters of polyhydric alcohol [39] and of hexitols and sucrose [40] for their special properties. In addition to the sodium salt, Stirton et al. used other cations, such as Li, NH4, K, Mg, and Ca, to study the relationship between the structure and the surfactant properties [30]. 

In the literature a number of different techniques for the preparation of a-sulfo fatty acid esters can be found. There is equipment for small-scale and commercial scale sulfonation. Stirton et al. added liquid sulfur trioxide dropwise to the fatty acids dispersed or dissolved in chloroform, carbon tetrachloride, or tetrachoroethylene [44]. The molar ratio of S03/fatty acid was 1.5-1.7 and the reaction temperature was increased to 65 °C in the Final stage of sulfonation. The yield was 75-85% of the dark colored a-sulfonated acid. The esterification of the acid was carried out with either the a-sulfonic acid alone, in which case the free sulfonic acid served as its own catalyst, or with the monosodium salt and a mineral catalyst. 

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]. 

Because of their preferential use as detergents, the main interest in the physicochemical properties of the salts of a-sulfo fatty acid esters is related to their behavior in aqueous solution and at interfaces. In principle these are surface-active properties of general interest like micelle formation, solubility, and adsorption, and those of interest for special applications like detergency, foaming, and stability in hard water. 

TABLE 1 CMC of Different Sodium a-Sulfo Fatty Acid Alkyl Esters... 

FIG. 1 Critical micelle concentration as a function of the number of carbon atoms in the hydrophobic rest of sodium a-sulfo fatty acid methyl esters. Methods O, surface tension +, conductivity A, solubilization of a dye x, solubility (all without electrolyte) , surface tension with a constant electrolyte concentration of 5 x 10"2 mol/L. (From Ref. 57.)... 

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. 

The Gibbs equation allows the amount of surfactant adsorbed at the interface to be calculated from the interfacial tension values measured with different concentrations of surfactant, but at constant counterion concentration. The amount adsorbed can be converted to the area of a surfactant molecule. The co-areas at the air-water interface are in the range of 4.4-5.9 nm2/molecule [56,57]. A comparison of these values with those from molecular models indicates that all four surfactants are oriented normally to the interface with the carbon chain outstretched and closely packed. The co-areas at the oil-water interface are greater (heptane-water, 4.9-6.6 nm2/molecule benzene-water, 5.9-7.5 nm2/molecule). This relatively small increase of about 10% for the heptane-water and about 30% for the benzene-water interface means that the orientation at the oil-water interface is the same as at the air-water interface, but the a-sulfo fatty acid ester films are more expanded [56]. 

The ability to disperse the calcium soap formed from a given amount of sodium oleate has been studied for a number of a-sulfo fatty acid esters with 14-22 carbon atoms [28,30]. In principle, the lime soap dispersion property increases with the number of C atoms and the dissymmetry of the molecule. Esters with 14 C atoms have no dispersion power and in the case of esters with 15-17 carbon atoms the least symmetrical are the better lime soap-dispersing agents. However this property does not only depend on the symmetry but on the chain length of the fatty acid group. For example, methyl and ethyl a-sulfomyristate have better dispersing power than dodecyl propionate and butyrate. The esters with 18 and more carbon atoms are about equal in lime soap dispersion power. Isobutyl a-sulfopalmitate is the most effective agent under the test conditions. 

The applications of a-sulfo fatty acid esters are widely spread as for other surfactants. They can be used in detergents, cleansers, and cosmetic products as well as in the building industry and for the production of synthetic materials and agrochemicals. The main properties for these applications are surface activity, wetting ability, hard water stability, lime soap dispersion power, and good human and environmental safety profiles. 

Powder and liquid detergents can be used in washing machines. The a-sulfo fatty acid esters are used in both product types, but their solubility in water has... 

When a-sulfo fatty acid esters are used as the major active component in detergents they can cause problems because of their foaming properties. In European horizontal drum-type automatic washers they produce too much foam, and in the rinse cycle of the American and Japanese pulsator-type washers the foam cannot be completely rinsed out [38]. The problem of inefficient rinsing can be solved by the addition of soap [63] or sulfonated unsaturated fatty acid esters [64]. For European applications special foam inhibitors are needed. 

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 liquid detergent for dishes, vegetables, and fruit comprises the sodium salt of a-sulfo coconut acid ethyl ester (20%), an alkylamine oxide (5%), citric acid (0.5%), ethanol (50%), and water [78]. 

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]. 

A phosphate-free detergent with excellent detergency and improved foaming and rinsing properties consists of 10% sodium a-sulfo hardened beef tallow fatty acid methyl ester, 10% sodium dodecyl sulfate, 5% a-sulfomyristic acid disodium salt, 10% zeolite, 10% sodium silicate, 10% sodium carbonate, 10% cellulose, 40% Flauber s salt, and 4% water [80]. 

A heavy granular detergent can be produced by mixing a detergent composition with powdered or granular sodium carbonate. A typical detergent comprises 8% sodium a-sulfo hardened palm oil fatty acid methyl ester, 2% di-Na a-sulfopalmitate, 10% Na coconut oil alcohol sulfate, 2% polyethylene... 

For an enzyme-containing detergent the enzyme is added to a detergent composition with esters and disalts of a-sulfo fatty acids. The detergent granular comprises 7 % sodium a-sulfo hardened palm oil fatty acid methyl ester, 5% disodium salt of a-sulfo hardened palm oil fatty acid, 10% sodium a-olefin (C14 18) sulfonate, 10% zeolite, 5% sodium carbonate, 5% water, and 55% sodium sulfate [82]. 

A detergent, imparting better flexibility to cotton cloth is produced by adding 1% sodium a-sulfo hardened tallow fatty acid methyl ester and 9% disodium a-sulfo hardened palm oil fatty acid to a mixture of 5% sodium n-dodecyl-benzenesulfonate, 5% a-C16-olefmsulfonate, 3% dimethyldistearylammonium chloride, 15% zeolite, 10% sodium silicate, 10% sodium carbonate, 2% soap, 35% Na2S04-7H20, and 5% water [84],... 

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. 

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