Alkali-labile surfactants

The ester bond is the typical linkage to use in the design of alkali-labile surfactants. The concept is by no means new. Poly(ethylene glycol) (PEG) esters of fatty acids have been around for a long time. They are produced by ethoxylation of the fatty acid the product obtained is a mixture of roughly... 

Jaeger et al. have studied the kinetics of hydrolysis of cationic ketal-based surfactants [41], A comparison was made between acid hydrolysis of surfactants in nonaggregated form and in the form of either micelles or vesicles. (Ketal surfactants with one hydrophobic tail formed micelles and those with two hydrophobic tails formed vesicles.) It was found that both types of aggregation caused about two orders of magnitude reduction of the hydrolysis rate. Aggregation is evidently a way to protect these acid-labile cationic species from acid hydrolysis just as aggregation is a way to speed up alkaline hydrolysis of cationic alkali-labile surfactants, such as esterquats. 

Amphiphiles with an acid- or alkali-labile bond constitute the most widely explored routes to achieve cleavable surfactants. However, other approaches have also been taken. For instance, several types of surfactants with UV-labile bonds have been synthesized and evaluated. Photochemical cleavage yields non-surface-active species and the concept is attractive because it allows an extremely fast breakdown of the surfactant to occur. 

The possible procedures involving titration with SDS or BEC all depend on pH control, and are therefore not applicable to mixtures containing alkali-labile quats. It is better to assume that such mixtures will necessitate separation, and the guidelines given below assume that alkali-labile quats are absent, and that only one quat and one amphoteric are present, although the astute analyst will be able to work out schemes for some mixtures containing more than one surfactant of each class. 

Apart from the product classes discussed above, which include the most important types of cleavable surfactants, several more or less exotic examples of surfactants with limited half-life have been reported. For instance, isethi-onate esters with very high degrees of alkali lability have been developed. These products, made by esterification of an alkylpolyoxyethylene carboxylic acid with the sodium salt of isethionic acid, have been claimed to be partially cleaved when applied to the skin [68]. Cleavable quaternary hydrazinium surfactants have been explored as amphiphiles containing a bond which splits very easily. The surfactants are cleaved by nitrous acid under extremely mild conditions [69]. Ozone cleavable surfactants have been developed as examples of environmentally benign amphiphiles. These surfactants, which contain unsaturated bonds, break down easily during ozonization of water, which is a water purification process of growing importance. Both normal surfactants [70] and geminis [71] have been synthesized and tested in ozonolysis. The latter surfactants contained unsaturation either in the hydrophobic tails or in the linker unit. 

---