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Theory of Solvation

Microscopic fields in liquid dielectrics

We have developed an analytical model of the field inside a cavity in a uniformly polarized dipolar liquid. The microscopic theory shows that Maxwell's equations of continuum electrostatics are realized through a singularity in the microscopic response function representing a non-decaying longitudinal polarization wave. The appearance of this solution depends on the order of continuum and thermodynamic limits taken in the microscopic equations. Fields in microscopic cavities are much different from macroscopic predictions approaching with increasing cavity size a new continuum expression derived from the microscopic equations. Numerical Monte Carlo simulations never reach the standard continuum limit and instead converge to a new continuum solution (see EPL'08).

 

Equilibrium solvation in polar solvents

 

We have developed a microscopic theory of solvation in polar solvents aiming at the solvation thermodynamics of large molecules (see JCP 120 (2004) pp. 7532-7556). The theory combines the input from the solute in terms of its atomic coordinates and partial charges with the microscopic input from the solvent in terms of k-dependent correlation functions of the solvent polarization. Two projection of the dipolar polarization are important in the solvent response. Therefore, the microscopic polarization response is given in terms of the longitudinal (L) and transverse (T) structure factors (Figure). A convenient parameterization of the structure factors of polar solvents through the solvent dipole moment, polarizability, density, effective diameter, and static and high-frequency dielectric constants has been developed. The solvent and solute input are combined in the k-integral of the response function with the solute electric field which gives the solvation free energy (Figure).

Solvation dynamics in polar solvents


The microscopic model of polar solvation has been extended to solvation dynamics. The Figure shows the calculations for the Stokes shift correlation function of quinoxaline in MTHF close to its glass transition temperature (see JCP 122 (2005), in press). The calculations indicate that longitudinal response is the dominant component in the microscopic solvation dynamics for solutes of arbitrary shape and charge distribution.



Equilibrium solvation in quadrupolar solvents

A theory of equilibrium solvation in quadrupolar solvents has been developed. It is an extension of the formulation in polar solvents on the case of the second solvent multipole. The solvent quadrupolar response is now represented by three structure factors (Figure). The solvation free energy is calculated by k-integration of the gradient of the solute electric field with the response function built on the quadrupolar structure factors. The theory is applied to electron transfer and solvation in non-dipolar solvents.