Molecular dynamics study of solvation of coumarin-314 at the water/air interface

Molecular dynamics experiments have been carried out to study equilibrium and dynamical aspects of orientational correlations and solvation of Coumarin-314 lying at the aqueous/air interface. Stable interfacial solvation states for the probe are characterized by an orientation of its molecular plane mostly parallel to the interface. At ambient temperature, spontaneous flipping transitions take place at characteristics time intervals of approximate to0.5-1 ns; the pathway between the stable states involves a transition state in which the molecular dipole points perpendicularly to the interface. In-plane dynamics of the probe is characterized by a sequence of diffusive-like segments interrupted by the flipping episodes. whereas out-of-plane motions include large amplitude oscillations modulated by small-amplitude, fast rocking of the probe. Using nonequilibrium simulations, we also studied the solvation response of the surface following a vertical excitation of the probe. The overall solvation relaxation is slightly slower than that found in bulk, due to a lesser extent of rotational relaxation. Spatial characteristics of the surface solvation response is analyzed in terms of contributions from different individual solute-site responses. Dynamical results are also compared to predictions from linear theories.