Solute-Solvent Correlations

The solute-solvent pair correlation function r ) gives the probability 

This semi-invariance of the ion s hydration structure manifests itself during ion transfer across the liquid/liquid interface. Depending on its size and charge, the ion is able to drag all or part of its hydration shell as it is transferred from the aqueous to the organic 

Closely related to this concept is the role played by the fluctuations in the solvent-ion interactions in driving small ions toward and away from the 

The ability of ions to interact with liquid surfaces in this manner has important implications for understanding several aspects of spectroscopy and dynamics involving ionic solutes, to be discussed below. 

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As illustrated in this section, recent advances in continuum theory of SD have produced results that agree well with experimental data for the solvation response in polar liquids. A missing piece in the recent applications of continuum theories to SD has been a comparison to molecular simulation. Such a comparison would be especially meaningful and informative in the case where MD simulation results are used to obtain e(to), given that the comparison would then be carried out for solvents with the same dielectric properties. Differences that might be uncovered between the results of simulation and theory would provide insight into the role of solute-solvent correlations in SD. 

Chialvo, A. J. Solute solute and solute solvent correlations in dilute near-critical ternary mixtures—Mixed-solute and en-trainer effects. J. Phys. Chem. 1993, 97, 2740-2744. 

There are several directions to extend the RISM-SCF/MCSCF method that are not described in the present chapter. One of such directions is a combination of ab initio MO theory with 3D-RISM, which is explained in chapter 4. The site-site treatment of the solute-solvent correlations involving the approximation of radial averaging constitutes a bottleneck of the RISM-SCF method, and thus lacks a 3D picture of the solvation structure for complex solutes. The SCF theory combined with the 3D-RISM is free from such a bottleneck. The test computation on the carbon monoxide in water provides a detailed hydration structure of water solvent as well as polarized CO electronic structure. [25] It is also found that the results from the original RISM-SCF/MCSCF method are in reasonable accord with those following from the 3D-RISM-SCF approach after reduction of the orientational dependence. This shows the RISM-SCF/MCSCF approach gives a proper picture for a solvation process. 

The SPC/E model [19] is used for water and the AMBER parameters are employed for amino acids. The interaction between site a in the solute molecule and site b in a water molecule is expressed by Eq.(3.17). The number density and the dielectric constant of water, which are used as part of the input data in the dielectrically consistent version of the RISM theory [11, 12], are taken from the experimental data at the standard temperature (298K) and pressure. The solute-solvent correlation functions are calculated by solving the basic equations for a solute molecule that is immersed in water at infinite dilution (see the Appendix). 

The site-site RISM/HNC theory has been coupled with the ab initio molecular orbital (MO) theory in a self-consistent field (SCF) calculation of the electronic and solvation structure of a solute molecule immersed in molecular solvent, referred to as the RISM-SCF method [59, 60, 61]. Since the site-site treatment of the solute-solvent correlations involves the approximation of radial averaging, it constitutes a bottleneck of the RISM-SCF method. Although this approach yields reasonable results for the thermodynamics of solvation for many solute species and solvents [62], it lacks a 3D picture of the solvation structure for complex solutes and oversimplifies the contribution to the solvation properties from highly directed electron orbitals of the solute molecule. 

This effect must not be confused with the cybotactic effects we have mentioned, nor with the hole in the solute-solvent correlation function gMs(t) (see Figure 8.5). The hole in the radial correlation function is a consequence of its definition, corresponding to a conditional property, namely that it gives the radial probability distribution of the solvent S, when the solute M is kept at the origin of the coordinate system. Cybotactic effects are related to changes in the correlation function gMs(t) (or better gMs(r> )) with respect to a reference situation. Surface proximity effects can be derived by the analysis of the gMs(r,fi) functions, or directly computed with continuum solvation methods. It must be remarked that the obtention of gMs(r) functions near the surface is more difficult than for bulk homogeneous liquids. Reliable descriptions of gMs(ri re even harder to reach. 

Chialvo, A. A. 1993b. Solute-solute and solute-solvent correlations in dilute near-critical ternary mixtures Mixed-solute and en trainer effects. Journal of Physical Chemistry. 97, 2740. Chialvo, A. A., S. Chialvo, J. M. Simonson, and Y. V. Kalyuzhnyi. 2008. Solvation phenomena in dilute multicomponent solutions. I. Formal results and molecular outlook. Journal of Chemical Physics. 128, 214512. 

Chialvo, A. A. (1993a) Solute-Solute and Solute-Solvent Correlations in Dilute Near-Critical Ternary Mixtures Mixed Solute and Entrainer Effects, Journal of Physical Chemistry 97, 2740-2744... 

If the density of solute species is finite, the total correlation functions in Eq. (70) can be easily sampled from simulations and thereafter the PMF can be computed straightforwardly. However, in the infinitely dilute limit of solute density, e.g., only two solute molecules are immersed in solvent, it is almost impossible to direcdy sample the solute—solute total correlation functions from simulation, and the solute-solute correlation function can then be calculated through the integral equation theories with or without the incorporation of simulation for computing the solute—solvent correlation functions. 

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

An exact description of the acidity of solutions and correlation of the acidity in various solvents is one of the most important problems in the theory of electrolyte solutions. In 1909, S. P. L. S0rensen suggested the logarithmic definition of acidity for aqueous solutions considering, at that time, of course, hydrogen instead of oxonium ions (cf. Eq. (1.4.11))... 

Finally, we note that we have mostly limited attention so far to the self-consistent reaction field limit of dynamical solvent polarization, which is the only one that has been generally implemented (see next Section). Nevertheless, there are problems where the solute-solvent dynamical correlation must be considered, and we will address that topic in Section 5. 

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