Sulfosuccinate esters hydrolysis

Sulfur in oxidation state IV can be used to produce a variety of anionic snUbnates, as depicted in Scheme 1.5. Sodium bisulfite can be used to prepare sulfonates of a,b-unsaturated acids and esters, such as those prepared from maleic anhydride. The mechanism involves Michael addition to the activated double bond by the more nucleophilic sulfur atom, and is conducted in an aqueous two-phase system where, for example, a maleate half acid ester or diester is dispersed and heated under narrowly controlled pH conditions to minimize ester hydrolysis and avoid competitive hydroxide addition to the double bond. The resulting classes of surfactants include sulfosuccinates (which are in fact carboxylate sulfonate disalt surfactants) prepared from the maleic half acid esters of fatty alcohols or alcohol ethoxylates. Diesters of maleic are sulfonated by the same type of process to produce surfactants such as the ubiquitous dioctyl sulfosuccinate (DOSS) from the diester of 2-ethylhexyl alcohol and maleic anhydride. 

Sulfosuccinate esters can be determined by the ferric hydroxamate procedure (54). This is a general test for esters. The esters are reacted with hydroxylamine hydrochloride in the presence of base to form the hydroxamic acids (RCONHOH). The hydroxamic acids are then detected by the color formed when ferric chloride is added to the mixture. The competing reaction, base-catalyzed hydrolysis of the esters to form the carboxylic acid, is inhibited by running the hydroxlaminolysis at ice-bath temperature. Other esters will interfere with this determination, and the various sulfosuccinates can not be distinguished from each other. 

Figure 34. Hydrolysis rate of sodium maleate half ester and disodium sulfosuccinate half ester of SAE. Test conditions surfactant concentration—5 % aq. soln. temperature—40°C initial pH—7.0. ((-0-) sodium maleate half ester of C12.i>t SAE (3EO) (-A-) sodium maleate half ester of lauryl alcohol 3EO (-%-) disodium sulfosuccinate half ester of C12.1 t SAE (3EO) ( A ) disodium sulfosuccinate half ester of lauryl alcohol 3EO)...

A great variety of chemical reactions can be advantageously carried out in microemulsions [860-862]. In one of the first papers in this field, Menger et al. described the imidazole-catalyzed hydrolysis of 4-nitrophenyl acetate in water/octane microemulsions with AOT as an anionic surfactant [=sodium bis(2-ethyl-l-hexyl)-sulfosuccinate] [864]. The solubilized water, containing the imidazole eatalyst, is confined in spherical pools encased by surfactant molecules, which have only their anionic head groups (-SOb ) immersed in the aqueous droplets. When the ester, dissolved in water-insoluble organic solvents, is added to this water/octane/AOT/imidazole system, it readily undergoes the catalysed hydrolysis under mild reaction conditions (25 °C). 

Micelles can also be made in organic solvents. The usual amphiphile of choice is a branched dioctyl sulfosuccinate called AOT di-(2-ethylhexyl)-sodium-sulfosuccinate Aerosol T (Fig. 2.5.6). AOT micelles in organic solvents are inverted micelles in which the hydrocarbon chains point into the bulk medium, while the sulfonate headgroups stabilize the water droplets. The inverted micelles dissolve enzymes in isooctane, benzene, and similar solvents containing about 10% of water. Chymotrypsin dissolved in tiny water droplets remains, for example, very efficient in the hydrolysis of hydrophobic peptides (e.g., N-glutaryl-L-phenylalanine-p-nitroanilide) or in transesterification reactions of hydrophobic esters with amino acids. 

The monoesters of sulfosuccinic acid possess good washing ability and high foam stability, and are gentle to the skin. Therefore they are in widespread use in bodycare products such as shower gels and baby shampoos. However, their ester bonds are sensitive to hydrolysis in alkaline solutions. Thus, they cannot be used in common detergents. 

Aerosol OT (sodium diethylhexyl sulfosuccinate) is a surfactant known to form readily reverse micelles, in organic nonpolar solvents [4], Sulfosuccinates absorb slightly UV light at short wavelengths. However, the ester group is sensitive to hydrolysis, so they can only be used in near neutral solutions. 

In the second reaction step, the maleic acid ester is sulfated with an aqueous sodium sulfite solution to obtain the corresponding sulfosuccinate. Since the ester bond is sensible to hydrolysis, pH value of the aqueous sulfosuccinate must be carefully adjusted between 5 and 7.5 (Figure 13.8). 

Due to the ester linkage, all sulfosuccinates are sensitive to hydrolysis, especially under acidic conditions. 

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