Determining the origin of shape transitions in micellar systems at the atomic level: a computational study

Abstract

Specific ion effects are a general phenomenon in ionic micellar systems. Cationic micelles with inorganic counterions are usually spherical, but some aromatic anions induce sphere-to-rod transitions in these aggregates. Aromatic anion-induced micellar shape transitions have been related to counterion insertion into the hydrophobic core, ion-pairing and dehydration at the interface. Trifluoromethanesulfonate (triflate, Tf) also induces drastic changes on the properties of cationic micelles, therefore, shape transitions induced by anions are not a feature solely attributable to aromatic counterions. Models to describe counterion adsorption at the micellar interface have been developed, but the important aspects related to micellar shape transitions are usually overlooked. Thus, the current description of counterion/monomer interactions cannot predict micellar shape changes. Since specific interactions can be included in Monte Carlo simulations, this methodology is adequate to assay counterion/monomer interactions at the atomic level. We propose a comparative study of anion effects upon cationic micelles, using different counterions. The data will be analyzed and the results may provide valuable information to understand the driving forces of shape transitions in cationic micelles induced by added salts. Comprehension of specific ion effects on shape changes of surfactant aggregates at the atomic level can be an important model to arrive at a more general understanding of salt effects on the shape of colloids and membranes in aqueous solution. (AU)