Aggregation behaviors of alkyl α-keto acids in water

Abstract

Single-tailed α-keto acids are a kind of fatty acid derivatives and weakly acidic amphiphiles. Only the aggregation behavior of α-ketooctanoic acid (KC8) in water was investigated so far, and thus the current understanding of their aggregation behavior, such as the effect of alkyl chain length, is still limited. It is essential to investigate the aggregation behavior of alkyl α-keto acids such as α-ketodecanoic acid (KC10) and α-ketododecanoic acid (KC12), to fully understand the features of α-keto acids. The aggregation behavior of KC10 and KC12 in water was investigated by changing their concentrations (C), using equilibrium surface tension, cryogenic transmission electron microscopy, dynamic light scattering, small-angle X-ray scattering, and deuterium nuclear magnetic resonance, and compared with that of KC8. A C-dependent stepwise aggregation was observed for the KC10 and KC12 systems. With increasing C, vesicles, precipitates, micelles, and lamellar phase (or α-gel phase) form successively in the systems, where precipitates and micelles both coexist with vesicles, similar to the case of KC8. With an increase in chain length, the pKa of α-keto acids increases while their critical concentrations of stepwise aggregations decrease, arising from the enhancement of the interactions between alkyl chains. The aggregation behavior observed can be explained by the existence of multispecies (including the “acid-soap” dimers) and the interaction between alkyl chains.