Surfactants, cleavable

K. Holmberg, In K. Holmberg (Ed.), Cleavable Surfactants, Novel Surfactants, Surfactant Science Series, Vol. 74, Marcel Dekker, New York, USA, 1998, p. 333. 

Hydrolyzable surfactants are, as the name implies, surfactants that break down by hydrolysis. Implicit in the phrase is that the surfactant is designed in such a way that hydrolysis will be simpUfied. Hydrolyzable surfactants are sometimes seen as synonymous with cleavable surfactants, which is a phrase commonly seen in the literature. In a strict sense there is a difference between the two, however, since surfactants may be cleaved by means other than hydrolysis. Hydrolyzable surfactants are therefore just a fraction, although a major fraction, of cleavable surfactants. 

The interest in cleavable surfactants has increased rapidly in recent years and the topic has been covered in review papers during the last decade [1-4]. This chapter begins with a relatively thorough discussion about the incentive for hydrolyzable surfactants, continues with a discussion about biodegradation of surfactants, which is important for understanding the concept of hydrolyzable surfactants, and then gives an account of the development of hydrolyzable surfactants with an emphasis on recent results. 

In addition to environmental concerns, there are other driving forces for the development of cleavable surfactants. One such incentive is to avoid complications such as foaming or formation of unwanted, stable emulsions after the use of a surfactant formulation. Use of a cleavable surfactant instead of... 

Finally, surfactants that break down into non-surface active products in a controlled way may find use in speciahzed applications, such as in the biomedical field. For instance, cleavable surfactants that form vesicles or microemulsions can be of interest for drug dehvery, provided the metabolites are nontoxic. 

Most cleavable surfactants contain a hydrolyzable bond. Chemical hydrolysis is either acid- or alkaU-catalyzed and many papers discuss the surfactant breakdown in terms of either of these mechanisms, hi the environment, bonds susceptible to hydrolysis are often degraded by enzymatic catalysis, but only few papers dealing with cleavable surfactants have dealt with such processes in vitro. Other approaches that have been taken include incorporation of a bond that can be destroyed by UV irradiation or use of a bond which is cleaved when exposed to ozone. 

Stjerndahl M, Limdherg D, Holmherg K (2003) Cleavable surfactants. In Holmberg K (ed) Novel Siufactants, 2nd ed. Marcel Dekker, New York... 

The ammonium salts (417) <90TL449> and (418) <93TL6985> have been introduced as cleavable surfactants which hydrolyze to give surfactants of higher critical micelle concentration. Dioxolane-... 

Contact angle rqiresents the effect that the cleaved surfactants have on water repellency of nylon fabrics (Yamauchi et al., 1996). Lin et al. (2006) evaluated contact angles of aqueous solutions upon the cleavable silicone surfactant after plasma treatment at various powers. The result indicates that the PEG 6000 sUicone surfactant imparted a higher water repellency than did the other cleavable surfactant that is, the use of... 

Obviously, betaine-type cleavable surfactants of the acetal type hydrolyse in acid media (72). The dode-cyl derivative decomposes completely after 2 hours 40 minutes in a 2% HCl solution, after 24 hours at pH 1, and only after more than one week at pH 3. 

A second incentive for the development of cleavable surfactants is to avoid complications such as foaming or formation of unwanted, stable emulsions after use of a surfactant formulation. Cleavable surfactants present the potential for elimination of some of these problems. If the weak bond is present between the polar and the non-polar part of the molecule, cleavage will lead to one water-soluble and one water-insoluble product. Both moieties can usually be removed by standard work-up procedures. This approach has been of particular interest for surfactants used in preparative organic chemistry and in various biochemical applications. 

Figure 17.8. Preparation of (a) 1,3-dioxolane and (b) 1,3-dioxane surfactants from a long chain aldehyde and a 1,2-and a 1,3-diol, respectively. (From K. Holmberg, Cleavable surfactants, in Novel Surfactants (Ed. K. Holmberg), Surfactant Science Series 74, Marcel Dekker, New York, 1998)...

Acetal surfactants have been found to resemble traditional surfactants in terms of their physico-chemical properties. However, it has been reported that the CMC values for acetal-containing surfactants are somewhat lower than those of the corresponding conventional surfactants. Furthermore, the efficiency of the surfactants, expressed as the concentration required to produce a 20 mN/m reduction in surface tension, was higher for the cleavable surfactants. Evidently, the acetal linkage connecting the hydrophobic tail and the polar head-group gives a contribution to the surfactant... 

Figure 17.11. Preparation of a cleavable surfactant containing two polyoxyethylene chains (R is a long-chain alkyl)...

Figure 17.14. Acid-catalysed hydrolysis of a second-generation cleavable surfactant. (From D. A. Jaeger, SupramoL Chem., 5, 27 (1995))...

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. 

Ozone-cleavable surfactants have also 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. It is likely that other mechanisms of surfactant breakdown in the environment will be explored in the future in the design of new surfactants with good environmental characteristics. 

A completely different way to avoid the problem of residual surfactant in the end-product is to use cleavable surfactants, i.e. surfactants that spontaneously break down at some stage. This concept has been discussed above in Section 2. 

Holmberg, K., Cleavable surfactants, in Reactions and Synthesis in Surfactant Systems, Texter, J. (Ed.), Marcel Dekker, New York, 2001. 

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