Experimental studies of solvation dynamics in liquids invariably ask how changing a solute from its electronic ground state to an electronically excited state affects a solution’s dynamics. With traditional time-dependent-fluorescence experiments, that means looking for the dynamical consequences of the concomitant change in solute–solvent potential energy. But if one follows the shift in the dynamics through its effects on the macroscopic polarizability, as recent solute-pump/solvent-probe spectra do, there is another effect of the electronic excitation that should be considered: the jump in the solute’s own polarizability. We examine the spectroscopic consequences of this solute polarizability change in the classic example of the solvation dye coumarin 153 dissolved in acetonitrile. After demonstrating that standard quantum chemical methods can be used to construct accurate multisite models for the polarizabilities of ground- and excited-state solvation dyes, we show via simulation that this polarizability change acts as a contrast agent, significantly enhancing the observable differences in optical-Kerr spectra between ground- and excited-state solutions. A comparison of our results with experimental solute-pump/solvent-probe spectra supports our interpretation and modeling of this spectroscopy. We predict, in particular, that solute-pump/solvent-probe spectra should be sensitive to changes in both the solvent dynamics near the solute and the electronic-state-dependence of the solute’s own rotational dynamics.

Recent developments in the computer simulation studies of the effects of counterions on the properties of ionic surfactant systems in contact with aqueous solution phase are discussed. The article deals with three types of systems: normal micelles, monolayers at the air/aqueous solution interface, and reverse micelles, i.e., water-in-oil microemulsions.Highlights► Computer simulation of surfactant systems ► Effects of counterions on properties of ionic surfactant systems ► Normal micelles, monolayers, reverse micelles ► Effects of added salt and counterion type on aggregation properties.

The density dependence of the local structure and of collective dynamics of a polar fluid fluoroform along an isotherm at a temperature of 1.03 Tc, in the near-critical (NC) region, were studied by classical molecular dynamics (MD) simulations. In the case of local structure we focus on local density inhomogeneities and on orientational pair correlations that are relevant to dielectric properties and light scattering intensities. Our results show that the density dependence of the frequency shifts of fluoroform ν2 and ν3 modes correlates well with that of intermolecular dipole–dipole interactions. Our study of collective dynamics deals with dipole and polarizability anisotropy relaxation, experimentally accessible through far-infrared absorption, depolarized light scattering, and optical Kerr effect. Our MD simulations were performed using an all-atom nonpolarizable potential model of fluoroform. Contributions of induced dipoles to dielectric properties were included using first-order perturbation theory, and this approach was also used to include interaction-induced contributions to polarizability anisotropy relaxation. For interactions involving induced dipoles, we calculated and compared the results of a distributed polarizability model to a model with a single polarizable site located at the center-of-mass. Using a projection scheme that allows us to identify the contributions from different relaxation mechanisms, we found that dipole relaxation is dominated by collective reorientation, while in the case of polarizability anisotropy, relaxation processes related to translational dynamics make a major contribution over most of the fluid density range. The dielectric properties of fluoroform in the NC region were calculated and compared to the corresponding measurements. We found the dielectric constant and the far-infrared absorption spectrum to be in good agreement with experiments.

We present the results of a molecular simulation study of polarizability anisotropy relaxation of liquid acetonitrile confined in approximately cylindrical silica pores of diameters in the range of 20–40 Å. Grand Canonical Monte Carlo simulation is used to determine the density of acetonitrile in pores in equilibrium with the bulk liquid, and canonical-ensemble molecular dynamics is then used to calculate the trajectories of the filled pores prepared in this way. We find that the pores are wetting, partially due to hydrogen bonding between acetonitrile nitrogen and pore silanol groups and that acetonitrile molecules have preferential orientations relative to the interface. The mobility of molecules in interfacial regions is considerably reduced and dependent mainly on their proximity to the interface. We include the contributions of molecular and interaction-induced polarizabilities to the collective polarizability anisotropy relaxation. We find that this relaxation includes a slowly relaxing component absent from the corresponding process in bulk acetonitrile and that the amplitude of this component increases as the pore diameter decreases. These results are in agreement with optical Kerr effect experiments on acetonitrile in silica pores in a similar diameter range. Further analysis of our data indicates that collective reorientation and predominantly translational “collision-induced” polarizability dynamics both contribute to the slowly relaxing portion of polarizability anisotropy decay. We further find that pore anisotropy plays a role, giving rise to different relaxation rates of polarizability anisotropy components with a different mix of axial and radial character and that collective reorientation contributing to polarizability anisotropy relaxation is somewhat faster at long times than single-molecule orientational relaxation.

In this work, we present results from molecular dynamics simulations on the single-molecule relaxation of water within reverse micelles (RMs) of different sizes formed by the surfactant aerosol-OT (AOT, sodium bis(2-ethylhexyl)sulfosuccinate) in isooctane. Results are presented for RM water content w0 = [H2O]/[AOT] in the range from 2.0 to 7.5. We show that translational diffusion of water within the RM can, to a good approximation, be decoupled from the translation of the RM through the isooctane solvent. Water translational mobility within the RM is restricted by the water pool dimensions, and thus, the water mean-squared displacements (MSDs) level off in time. Comparison with models of diffusion in confined geometries shows that a version of the Gaussian confinement model with a biexponential decay of correlations provides a good fit to the MSDs, while a model of free diffusion within a sphere agrees less well with simulation results. We find that the local diffusivity is considerably reduced in the interfacial region, especially as w0 decreases. Molecular orientational relaxation is monitored by examining the behavior of OH and dipole vectors. For both vectors, orientational relaxation slows down close to the interface and as w0 decreases. For the OH vector, reorientation is strongly affected by the presence of charged species at the RM interface and these effects are especially pronounced for water molecules hydrogen-bonded to surfactant sites that serve as hydrogen-bond acceptors. For the dipole vector, orientational relaxation near the interface slows down more than that for the OH vector due mainly to the influence of ion–dipole interactions with the sodium counterions. We investigate water OH and dipole reorientation mechanisms by studying the w0 and interfacial shell dependence of orientational time correlations for different Legendre polynomial orders.