Counterion effects on adsorbed micellar shape: Experimental study of the role of polarizability and charge

We have used atomic force microscopy (AFM) to measure the shape of micelles adsorbed to the interface between hydrophilic silica and solutions of hexadecyltrimethylammonium (CTA(+)) ions in the presence of various counterions. Spherical CTA(+) surface micelles are adsorbed from solutions containing salts of CH3CO2-, CO32-, SO42-, SO32-, and HSO32-, even at high salt concentrations (>100 mM). Cl- and Br- form slightly oblate micelles near their respective critical micelle concentrations. Addition of Br- causes a transformation to cylindrical micelles whereas addition of Cl- does not. The difference between Br- and Cl- is similar to the behavior observed previously in bulk solution. Addition of S2O32- or CS32- or HS-/S2- transforms spherical micelles to cylinders. We rationalize these effects on the basis of the hard/soft (unpolarizable/polarizabile) nature of the ions. The ability to effect the sphere-to-cylinder transformation correlates with the availability of a soft anionic atom in the counterion. Presumably, this soft anionic atom promotes partitioning of the ion from the water to the micelle surface, thereby lowering the electrostatic repulsion between headgroups and effecting the sphere-to-cylinder transformation. Surface micelles are observed at concentrations below the critical micelle concentration. For CTABr, AFM can detect surface micelles at concentrations greater than half the critical micelle concentration.