Cleavable surfactants are of interest for several reasons. Above all, the development of surfactants with weak bonds deliberately built into the structure is driven by the need for improved biodegradability of amphiphiles. The breakdown may be catalyzed by enzymes, and biodegradation would be the normal mechanism in sewage plants. Alternatively, the surfactant may degrade by chemical means, e.g., induced by acid, alkali, ultraviolet (UV) light, heat, or ozone. Acid-and alkali-labile surfactants have attracted particular attention, and there is often a compromise between required stability at one stage and ease of breakdown at a subsequent stage. The paper reviews the main routes used to prepare cleavable surfactants and points out advantages and disadvantages of the different approaches. Emphasis is placed on the development during recent years. Cyclic and acyclic acetals, ketals, and ortho esters are the most important types of bonds for the preparation of acid-labile surfactants, whereas alkali-labile amphiphiles usually are based on ester bonds. The ester bond approach has been particularly important for cationic surfactants, and so-called ester quats have rapidly taken a large share of the traditional market for quats. Betaine esters constitute a special class of ester with very pronounced pH dependence. UV-labile surfactants based, for instance, on an azo bond, offer promise for the future.Paper no. S1174 in JSD 3, 81-91 (January 2000). KEY WORDS: Acetal, betaine ester, choline ester, cleavable surfactant, ester quat, isethionate ester, ketal, labile surfactant, ortho ester, UV degradation. SCHEME 17 SCHEME 16amples 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 (64).

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A new class of cleavable surfactants based on the ortho ester link is described. Ortho ester surfactants were synthesized from a short-chain ortho ester, a fatty alcohol, and a poly(ethylene glycol) monomethyl ether. The triple functionality of the ortho ester link results in a mixture of surface-active and nonsurface-active species. Structures were confirmed by 1 H and 13 C nuclear magnetic resonance. Hydrolysis characteristics at pH values ranging from 2 to 8 were determined. It was shown that, after a steep rise, hydrolysis curves flattened out before complete hydrolysis was reached. Ortho ester surfactants hydrolyze more rapidly than corresponding acetal surfactants. Hydrolysis rates are higher for purified samples and at higher temperature, while plateau levels are higher for the purified surfactant mixtures and at lower pH. The origin of these results is discussed. Data also indicate that the hydrolysis mechanism changes with pH. A test of the loss of emulsification capacity for an ortho ester surfactant with time at pH values from 2 to 11.5 and at 22 or 50°C was carried out, showing rapid breaking of the emulsion at mild acid conditions and a requirement of pH ≥ 10 for long-term stability. Surface tension and critical micelle concentration were determined for some of the surfactant mixtures. It was demonstrated that ortho ester surfactants possess excellent biodegradation properties. The new surfactants are promising candidates for use in industrial processes where temporary surfactant action is required.