Free energy profiles of adsorption of surfactant micelles at metal-water interfaces

We have studied adsorption behaviour of cationic and uncharged surfactant molecules and their micelles at metal-water interfaces via all-atom molecular dynamics (MD) simulations. Our simulations reveal that unaggregated surfactant molecules adsorb strongly on to the metal surface without any free energy barrier. The adsorption behaviour of micelles is quite different. Micelles of cationic surfactants experience a long-range free energy barrier to adsorption, which is attributed to the presence of a corona of counterions and hydration water around these micelles, which gets disturbed as the micelles approach the surface. Micelles of uncharged surfactants do not have a corona of counterions around them and therefore show a barrierless free energy profile of adsorption. The micelles of both cationic and uncharged surfactants strongly adsorb by disintegrating at the metal surface. In the disintegrated state, the molecules comprising the micelles re-arrange to attain either a lying down configuration in which the molecular axis is parallel to the surface or a standing up configuration in which the molecular axis is perpendicular to the surface.