Unconventional vesicle-to-ribbon transition behaviour of diacyl glycerol amino acid based surfactants in extremely diluted systems induced by pH-concentration effects

Abstract

A vesicle-to-ribbon transition has been observed in extremely dilute 1-O-(L-arginyl)-2, 3-O-di-decanoyl-sn-glycerol dichlorohydrate (1010R) surfactant systems by means of static light scattering. At concentration as low as 0.005 mM the solutions scattered significantly. From the angular dependence and molecular weight of the aggregates a vesicular structure is suggested. Increasing surfactant concentration in water induces the vesicle-to-ribbon transition at concentrations as low as 0.5 mM. This transition is accompanied by a strong decrease of scattered intensity and change in angular dependence. Lowering the pH at a fixed concentration can induce the same transition. Both parameters change the protonation of the surfactant, inducing an increase in preferred surfactant head-group area. Those findings are congruent with the observed surface tension behaviour as a function of surfactant concentration and suggests an explanation for the widely different critical micellar concentration (c.m.c.) as determined by different techniques.