Statistical Thermodynamics of Molecules with Orientation-Dependent Interactions in Homogeneous and Inhomogeneous Systems

Abstract

A lattice theory is presented for homogeneous and heterogeneous systems containing molecules with orientationdependent interactions. Correlations due to interactions are accounted for in first-order (quasi-chemical) approximation by allowing variations of the distribution of intermolecular contacts. To model heterogeneous systems, parallel lattice layers are allowed to be differently occupied. A partition function, written as a sum over distributions of molecules over orientations and locations and of intermolecular contacts, is derived for an arbitrary collection of monomeric species. Using a maximum term argument, self-consistent field equations are derived for the equilibrium distributions. These equations are solved numerically. Allowing lattice sites to be vacant, free-volume effects can be accounted for. Expressions are obtained for energy, entropy, chemical potentials, pressure, and surface tension. The capabilities of the method are illustrated by applying it to a number of specific systems, one of which exhibits a closed-loop coexistence curve. Another model in which one component is energetically anisotropic has an asymetric coexistence curve. Properties of coexisting phases and of the interface between them are investigated. With these models, it is found that orientational interactions may lead to a pronounced ordering in the interface and a negative interfacial entropy.

Nonfouling Characteristics of Dextran-Containing Surfaces

“…Such interaction between surface functional groups and surrounding water is dictated by the electron-donor/ electron-acceptor characteristics of the species involved. [18][19][20][21][22] Due to the complexity of the system which consists of a three dimensional array of hydrogen-bonded water and surface molecules, theoretical evaluation 18-22 of the chemical view has been a challenging task. Recently, some experimental reports have shed light on the governing effect of molecular interactions on biocompatibility, mostly protein resistance.…”

Molecular Modelling of Biological Membranes: Structure and Permeation Properties

“…It is not very likely that the apolar compounds that prevent the H-bonding network in water from developing can generate structures that overcompensate this loss. We strongly prefer the less-known interpretation of Besseling et al [5,6]. These authors argue that dissolving a small apolar compound has the effect that the H-bonding network is deformed (entropy loss).…”

“…, N. The distance between two atoms i, j is r ij , q is the (partial) charge on an atom, y is the angle defined by the coordinates (i, j, k) of three consecutive atoms, and f is the dihedral angle defined by the positions of four consecutive atoms, " o is the dielectric permittivity of vacuum, n is the dihedral multiplicity. The potential function, as given in equation (6), has many parameters that depend on the atoms involved. The first term accounts for Coulombic interactions.…”

“…The first and second terms in equation (6) are rather expensive in terms of computer power needed. They include the interaction of all atoms with all other Figure 4 (Plate 3).…”

“…In such force fields, polarisable atoms are usually given partial charges (cf. in equation (6) or Figure 3). These partial charges may be found from quantum mechanical analysis, and obviously these values differ per force field.…”