Solvation Dynamics at the Interface between Water and Self-assembled Monolayers

Molecular dynamics computer simulations are used to study the solvation dynamics following an electronic transition in a chromophore adsorbed at the interface between water and chlorine-terminated self-assembled monolayers. By varying the composition of the monolayer and the degree of chlorination, the role of interface polarity and structure on the relaxation time can be elucidated. The relaxation times are found to be faster at smooth interfaces and at interfaces in which the degree of chlorination is larger. Both the water and the self-assembled molecules contribute to the dynamics. However, we find that most of the water contribution to the relaxation is due to water molecules in the first solvent shell while the monolayer contribution is mostly from chlorine-terminated hydrocarbon molecules in the outer solvation shell of the chromophore. The water relaxation is faster than the monolayer relaxation in each system, but it is always slower than that of bulk water. The validity of linear response theory is discussed, and comparisons are made between the relaxation in the systems studied here, simulations of liquid/liquid interfaces, and experimental solvation dynamics data.