Rotational Dynamics of Strongly Adsorbed Solute at the Water Surface

The orientational dynamics of nitrobenzene adsorbed at the water liquid/vapor interface as a model for the orientational dynamics of surface-active solute are studied using classical molecular dynamics computer simulations. By varying the charge distribution of the solute and by comparing the results with those in bulk water, we are able to determine the effects of dielectric and mechanical frictions on reorientation dynamics and to correlate the orientational dynamics with the specific hydration of the solute. As in our previous model studies, we find that the equilibrium orientational relaxation is much slower in the bulk than at the interface. Variations of the solute charge distributions show that, as the solute becomes more polar, the surface rotation slows and approaches the bulk behavior. The reorientation dynamics are quite anisotropic, with out-of-plane rotation faster than in-plane rotation. This anisotropy disappears when the solute-water electrostatic interactions are turned off.