Effect of NaCl on Aggregation Number, Microviscosity, and Cmc of N-Dodecanoyl Amino Acid Surfactant Micelles
Microviscosity, micellar aggregation number (N), and critical micelle concentration (cmc) were determined in solutions of sodium N-dodecanoyl-glycinate (Lau-Gly), -valinate (Lau-Val), -leucinate (Lau-Leu), and -phenylalaninate (Lau-Phe) as a function of NaCl using fluorescent probes (auramine, pyrene). The microviscosity increased with increasing concentration of NaCl and reached a saturated value. In addition, two breakpoints were found on the curve of microviscosity vs NaCl concentration and by comparison with the data of N and solution viscosity. The first and second breakpoints were explained, respectively, as the beginnings of micellar growth and intermicellar interaction. The value of N increased steeply above the first breakpoint and the solution viscosity began to increase at the second breakpoint. The NaCl concentrations at the first and second breakpoints increased in the order Lau-Phe < Lau-Leu < Lau-Val approximately Lau-Gly. The absolute value of the slope of the Corrin-Harkins plot of the cmc was smaller for Lau-Leu and -Phe than for Lau-Gly and -Val. Larger absolute values of the slope (1.2 approximately 2), which suggested the contribution of a salting out effect on cmc, were observed at high NaCl concentration for Lau-Gly and -Val. The micellar growth and intermicellar interaction occurred at lower NaCl concentration with increased size of the amino acid residue when the amino acid of the amino acid surfactant was larger than valine; that is, the hydrophobicity of the amino acid residue was the dominant factor rather than steric hindrance.
Miyagishi
S
Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa, 920, Japan
Akasohu
W
Hashimoto
T
Asakawa
T