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 hexa decyltrimethylammonium (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 concen trations (>100 mM). Cl- and Br- form slightly oblate micelles near their re spective critical micelle concentrations. Addition of Br- causes a transfor mation to cylindrical micelles whereas addition of Cl- does not. The differ ence 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 har d/soft (unpolarizable/polarizabile) nature of the ions. The ability to effe ct the sphere-to-cylinder transformation correlates with the availability o f a soft anionic atom in the counterion. Presumably, this soft anionic atom promotes partitioning of the ion from the water to the micelle surface, th ereby lowering the electrostatic repulsion between headgroups and effecting the sphere-to-cylinder transformation. Surface micelles are observed at co ncentrations below the critical micelle concentration. For CTABr, AFM can d etect surface micelles at concentrations greater than half the critical mic elle concentration.