An analysis of molecular packing and chemical association in liquid water using quasichemical theory

Abstract

We calculate the hydration free energy of liquid TIP3P water at 298 K and 1 bar using a quasi-chemical theory framework in which interactions between a distinguished water molecule and the surrounding water molecules are partitioned into chemical associations with proximal (inner-shell) waters and classical electrostatic-dispersion interactions with the remaining (outer-shell) waters. The calculated free energy is found to be independent of this partitioning, as expected, and in excellent agreement with values derived from the literature. An analysis of the spatial distribution of inner-shell water molecules as a function of the inner-shell volume reveals that water molecules are preferentially excluded from the interior of large volumes as the occupancy number decreases. The driving force for water exclusion is formulated in terms of a free energy for rearranging inner-shell water molecules under the influence of the field exerted by outer-shell waters in order to accommodate one water molecule at the center. The results indicate a balance between chemical association and molecular packing in liquid water that becomes increasingly important as the inner-shell volume grows in size.

Modeling molecular and ionic absolute solvation free energies with quasichemical theory bounds

“…These results clearly illustrate the lack of dependence of the QCT free energy on the conditioning radius λ. 63 The inclusion of charged interactions for the water and ion cases results in a large increase in the separation of the upper and lower bounds for µ ex OS,LR (λ). For water, Fig.…”

“…The result for the excess chemical potential is independent of the choice of λ, as we will see in our modeling results, and has been observed in previous calculations. 63 There may be judicious choices for λ, though, that can lead to helpful approximations that enhance the efficiency and utility of the theory.…”

“…Recently, quasi-chemical theory has been further applied to the calculation of the aqueous solvation free energies of water 63,64 and small nonpolar molecules such as perfluoromethane (CF 4 ) 50 and methane. 14 These studies have found that the absence of close-contact repulsion in the OS long-ranged part of the free energy induced by the QCT partitioning leads to near-Gaussian energy distributions.…”

Ion-water clusters, bulk medium effects, and ion hydration

“…Given the equilibrium constant of the above reaction is K n and the density of water molecules is ρ w , we immediately obtain [21][22][23][24]42,43 …”

“…This factor accounts for the free energy of creating an empty coordination sphere in the solvent and thus accounts for the packing (steric) contributions to µ ex α . The quasichemical form is obtained by noting that x 0 is specified by chemical equilibria of the form [21][22][23][24]42,43 …”