Solvation dynamics in reverse micelles: the role of headgroup-solute interactions

We present molecular dynamics simulation results for solvation dynamics in the water pool of anionic-surfactant reverse micelles (RMs) of varying water content, w(0). The model RMs are designed to represent water/aerosol-OT/oil systems, where aerosol-OT is the common name for sodium bis(2-ethylhexyl)sulfosuccinate. To determine the effects of chromophore-headgroup interactions on solvation dynamics, we compare the results for charge localization in model ionic diatomic chromophores that differ only in charge sign. Electronic excitation in both cases is modeled as charge localization on one of the solute sites. We find dramatic differences in the solvation responses for anionic and cationic chromophores. Solvation dynamics for the cationic chromophore are considerably slower and more strongly w(0)-dependent than those for the anionic chromophore. Further analysis indicates that the difference in the responses can be ascribed in part to the different initial locations of the two chromophores relative to the surfactant interface. In addition, slow motion of the cationic chromophore relative to the interface is the main contributor to the longer-time decay of the solvation response to charge localization in this case.