Interactions in mixed micellar systems comprising chiral cationic amino acid based and conventional anionic surfactants

Abstract

The formation of mixed micelle in water between a chiral cationic amino acid based surfactant, l-alaninehydrochloride dodecylester (l-ADDE), and conventional anionic surfactants (AS), sodium decyl sulfate (SDS), sodium dodecyl sulfate (SDDS) and sodium dodecylbenzene sulfonate (SDBS), has been investigated by conductivity, density and dynamic light scattering (DLS) measurements at 25 °C. The critical micelle concentrations (CMC), degree of counterion dissociation (α) and standard Gibbs energy of micellization (ΔGmic°) have been determined for both pure and mixed systems, and the results have been compared with each other. Physicochemical parameters included the critical micelle concentration of ideal mixtures (CMCideal), the mole fraction of l-ADDE in ideal (XL−ADDEideal) and real mixed micelles (XL−ADDE), and interaction parameter (β) have been calculated by theoretical models to reveal the properties of mixed micelles. l-ADDE/SDS mixed system exhibited behavior changing from synergistic to antagonistic interactions with increasing l-ADDE concentration, whereas a reverse case was observed in l-ADDE/SDBS mixed system, i.e., from antagonistic to synergistic interactions. l-ADDE/SDDS mixed system showed antagonistic interactions over all the mole fraction range studied. The change of the apparent molal volume upon micellization (∆Vφmic) values determined from density measurements revealed that mixed micelles packed more tightly than their corresponding pure micelles. In addition, the sizes of the aggregates in binary mixed systems were found to be larger than those of the pure ones indicating the transformation from the small micelles to large aggregated structures. The increment in size of aggregates may be suggested with the vesicular structures.