Effect of the side chain of N-acyl amino acid surfactants on micelle formation: An isothermal titration calorimetry study

The enthalpy of micelle formation, critical micelle concentration (CMC), and aggregation number of micelles for seven amino acid type surfactants, sodium N-dodecanoyl-glycinate (C12Gly), -l-alaninate (C12Ala), -l-valinate (C12Val), -l-leucinate (C12Leu), -l-phenylalaninate (C12Phe), -l-glutaminate (C12Gln), and -l-threoninate (C12Thr), were obtained at three temperatures ranging from 288.15 to 308.15 K by isothermal titration calorimetry. The enthalpies of micelle formation for these surfactants were positive at 288.15 K and decreased with increasing temperature to ultimately become negative above 308.15 K. The CMC of the amino acid surfactants decreased monotonously with increasing hydrophobicity of the amino acid residue without exception, while with the exception of the C12Phe system, the enthalpy of micelle formation increased. The micellization behavior of C12Phe also deviated from that of other amino acid systems in terms of the heat capacity and the aggregation number. These results suggest the presence of a strong intra-micelle interaction between the side chains of phenylalanine.