Polar Solvation and Solvation Dynamics in Supercritical CHF3: Results from Experiment and Simulation

On the validity of linear response approximations regarding the solvation dynamics of polyatomic solutes

Gaussian statistics and linear response predictions of the nonequilibrium solvation dynamics are tested for numerous solute/solvent combinations.

Combining non-equilibrium simulations and coarse-grained modelling allows for a fine-grained decomposition of solvation dynamics

The time-dependent Stokes shift is shown to be a localized and short-ranged effect in ionic liquids.

Solvation shell dynamics studied by molecular dynamics simulation in relation to the translational and rotational dynamics of supercritical water and benzene

The solvation shell dynamics of supercritical water is analyzed by molecular dynamics simulation with emphasis on its relationship to the translational and rotational dynamics. The relaxation times of the solvation number (tau S), the velocity autocorrelation function (tau D), the angular momentum correlation function (tau J), and the second-order reorientational correlation function (tau 2R) are studied at a supercritical temperature of 400 degrees C over a wide density region of 0.01-1.5 g cm(-3). The relaxation times are decomposed into those conditioned by the solvation number n, and the effect of the short-ranged structure is examined in terms of its probability Pn of occurrence. In the low to medium-density range of 0.01-0.4 g cm(-3), the time scales of water dynamics are in the following sequence: tau D>tau S approximately or > tau J approximately or > tau 2R. This means that the rotation in supercritical water is of the "in-shell" type while the translational diffusion is not. The comparison to supercritical benzene is also performed and the effect of hydrogen bonding is examined. The water diffusion is not of the in-shell type up to the ambient density of 1.0 g cm(-3), which corresponds to the absence of the transition from the collision to the Brownian picture, whereas such transition is present in the case of benzene. The absence of the transition in water comes from the fast reorganization of the hydrogen bonds and the enhanced mobility of the solvation shell in supercritical conditions.

Solvation and Solvatochromism in CO2-Expanded Liquids. 2. Experiment−Simulation Comparisons of Preferential Solvation in Three Prototypical Mixtures

Electronic absorption and emission spectra of 10-bis(phenylethynyl)anthracene (PEA) and coumarin 153 (C153) are measured as functions of composition along the bubble-point curve at 25 degrees C in CO2-expanded cyclohexane (c-C6H12), acetonitrile (CH3CN), and methanol (CH3OH). The nonlinear dependence of the spectral frequencies on composition suggests substantial preferential solvation of both solutes by the liquid components of these mixtures. Estimates of enrichment factors (local mole fraction of a component divided by its bulk value) based on the assumption that spectral shifts are linearly related to local composition are quite large (approximately 10) in the cases of the C153/CH3CN + CO2 and C153/CH3OH + CO2 systems at high xCO2. Computer simulations of anthracene, the chromophore of PEA, and C153 in these three CO2-expanded liquids are used to clarify the relationship between local composition and spectral shift. A semiempirical model consisting of additive electrostatic and dispersive interactions is able to capture the main features observed experimentally in all six solute/solvent combinations. The simulations show that the commonly used assumption of a linear relation between spectral shifts and local compositions grossly exaggerates the extent of preferential solvation in these mixtures. The collective nature of electrostatic solvation and the composition dependence of the solute's coordination number are shown to be responsible for the breakdown of this assumption.

Characterization of the solvation dynamics of an ionic liquid via molecular dynamics simulation

The solvation dynamics of ionic liquids have been the subject of intense experimental study but remain poorly understood. We present the results of molecular dynamics simulations of the solvation dynamics of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate in response to photoexcitation of the fluorescent dye coumarin-153. We reproduce the time-resolved fluorescence Stokes shift using linear response theory, then use novel statistical techniques to analyze cation and anion contributions to the signal. We find that the solvation dynamics are dominated by collective ionic motion and characterize the time scale for various features of the collective response. Further, we use the Steele analysis [Mol. Phys. 61, 1031 (1987)] to characterize the contributions to the observed Stokes shift made by translational and rovibrational degrees of freedom. Our results indicate that in contrast to molecular liquids, the rovibrational response is trivial and the observed fluorescence response arises almost entirely from ionic translation. Our results resolve previously open questions in the literature about the nature of the rapid dynamics in room-temperature ionic liquids and offer insight into the physical principles governing ionic liquid behavior on longer time scales.

Solvation Dynamics of C153 in Supercritical Fluoroform: A Simulation Study Based on Two-Site and Five-Site Models of the Solvent

Molecular dynamics (MD) simulations of a probe solute (coumarin C153) in supercritical fluoroform are used to study time-dependent solute-solvent interactions. We study the dynamics of solvent reorganization in response to electronic excitation of C153 at a temperature of 1.03 T(c) (the critical temperature) and a series of densities above and below the critical density. Simulations of a two-site and five-site models of fluoroform are presented and compared. The time-dependent solvation response after solute electronic excitation is studied in the two cases, and the five-site results present an earlier onset of exponential decay that is closer to what is expected to be the experimental response. This is confirmed by comparison to experiment. In addition to obtaining the solvation response from nonequilibrium MD trajectories, approximate solvation responses were obtained from equilibrium time correlations of the fluctuations in the solvation energy change in the presence of ground- and excited-state solutes. For the five-site model, the equilibrium excited-state response shows stronger density dependence than the ground-state one. The nonequilibrium response appears to have an intermediate decay rate between the two equilibrium functions. The solute-partial-charge-solvent-induced-dipole interaction was also taken into account by means of a perturbative approach, which improved the agreement with experimental measurements available at densities corresponding to 1.4-1.6 rho(c) (where rho(c) the critical density). From the comparison between the two models, it is possible to conclude that an atomistic description is necessary for correctly representing the portion of solvation dynamics that is related to reorientation. This consideration is supported by providing results for orientational time correlation functions and by comparing the correlation times with the experimental ones.