Study of Nonpolar Solvation Dynamics in Supercritical Lennard−Jones Fluids in Terms of the Solvent Dynamic Structure Factor

Abstract

The nonpolar solvation dynamics in Lennard−Jones fluids is discussed in terms of the relationship with the dynamic structure factor of neat solvent, using the theoretical expression that describes the solvation correlation function as the superposition of solvent dynamic structure factors at various wavenumbers. Several expressions for the coupling strength between the state transition of the solute and the solvent density modes are examined with respect to their abilities to predict the static fluctuation. In the present theoretical model, it is found that the difference between the ground- and excited-state solute−solvent interactions can be adequately taken as the coupling strength with the solvent density mode. Employing this expression for the coupling, the solvent fluctuation around k ≅ σ -1 (σ stands for the diameter of the solvent) contributes dominantly to the static fluctuation in all the densities investigated. This corresponds to the feature of the solvation dynamics in mixed solvents, in which the effective wavenumber is determined to be 1.14σ-1 from the proportionality between the solvation rates and the diffusion coefficients in the higher-density region. The half decay time (t 1/2) of the dynamic structure factor at this wavenumber shows similar density dependence to that of the solvation correlation function obtained in our previous work. The half decay time of the dynamic structure factor is correlated with the static structure factor. This supports our previous proposal that the curvature of the free energy surface along the solvation coordinate is essential to the solvation dynamics. The agreement between the present theory and the simulation is further improved by taking the motion of the solute into account.

Equilibrium solvation in quadrupolar solvents

“…Several effective medium approximations have been proposed in the literature to deal with the problem of the local density profile. 10 They replace the solute-solvent interaction potential in the orientational component of the chemical potential of solvation (v 0s θ(r) in Eq. ( 13)) with an effective solute-solvent coupling including the information about the solute-solvent density correlations (direct solute-solvent correlation function in the surrogate 6,10,77 and density functional 9 theories or vertex in the modecoupling formulation 11 ).…”

“…10 They replace the solute-solvent interaction potential in the orientational component of the chemical potential of solvation (v 0s θ(r) in Eq. ( 13)) with an effective solute-solvent coupling including the information about the solute-solvent density correlations (direct solute-solvent correlation function in the surrogate 6,10,77 and density functional 9 theories or vertex in the modecoupling formulation 11 ). However, all such approximations are heavily dependent on the detailed knowledge of the solute-solvent distribution function which is available only for solutes of simple shape.…”

“…The understanding of kinetics of chemical reactions in non-polar or, more generally, non-dipolar solvents poses the necessity to describe thermodynamic 1,2,3,4,5,6 and dynamic 7,8,9,10,11 aspects of non-polar solvation. 12 The overall solvation free energy in a non-dipolar solvent (zero permanent dipole) can be approximately separated into the contribution from the solute repulsive core (cavitation energy), the attractive dispersion solvation, and electrostatic contributions from induced dipoles and permanent multipoles starting from the quadrupole moment.…”

Solvation Structure and Dynamics of cis- and trans-1,2 Dichloroethene Isomers in Supercritical Carbon Dioxide. A Molecular Dynamics Simulation Study.

“…The local solvation structure and especially the local density augmentation (LDA) effects of several, mainly dilute, supercritical solutions have been investigated by employing experimental techniques, − statistical mechanical theories − and molecular simulations. − These studies have revealed important information about the solvation in SCFs.…”

“…Using these techniques several authors have provided important information about the local density inhomogeneities (LDI) in pure SCF, ,− as well as in SC solutions. − Furthermore, in our previous studies we have focused on the static and dynamic behavior of aforementioned LDIs and residence dynamics in pure SCFs, ,− and the effect of intermolecular interactions upon them …”