Micellar growth of N-dodecanoyl-alaninates with different counterions and its quantitative relation with some factors

Critical micelle concentrations (cmcʹs) and transition points of micellar growth were determined for Li+, Na+, triethanolammonium (NH(EtOH)3+), K+, Cs+, and NH4+ salts of N-dodecanoyl-alanine (Lau-Ala) by using a fluorescence probe method. Dependence of cmc on counterion concentration was expressed for 56 points of cmcʹs by the following equation with R2=0.988: log(cmc)=−0.524 log(counterion)−0.209(counterion)−3.010, regardless of the type of counterion. At high salt concentrations micellar growth was observed for K+, Cs+, and NH4+ salts of Lau-Ala, while Li+, Na+, and NH(EtOH)3+ salts did not exhibit micelle growth because of their low solubilities at high salt concentrations. The counterion concentrations at the transition points for K+ and Cs+ salts were very similar with each other, although the corresponding concentrations were lower for NH4+ salt than for these two salts. Micellar aggregation number was smaller for NH(EtOH)3+ salt than for the other salts. A semi-quantitative equation relating the surfactant and salt concentrations at the transition point to the acyl chain length of amino acid surfactant was proposed. Moreover, the influence of its amino acid side chain and the effect of counterion size on micellar growth were discussed, respectively.