Nonspherical molecules interaction models

Teja and Rice [134] have proposed another relationship to calculate the viscosity of mixtures of polar liquids. It is based on a corresponding states treatment for the mixture compressibility factors. This method is more accurate than the previous one for polar-polar mixtures, particularly for aqueous solutions. It uses two reference-solution models for nonspherical molecules and a single interaction parameter. For methanol-water systems, the accuracy was within 9% [5]. 

A variety of approaches have been used in the development of models for the interactions between nonspherical molecules, reflecting both the complexity of such interactions and the paucity of truly accurate quantitative information about them that could be used to assess the suitability of different models. Perhaps the most widely studied approach, especially over the last decade or so, is the interaction site formalism, in which the intermolecular pair potential is written as a sum of potentials between interaction sites on each molecule. We have... 

In Section II of this article formulas which will be used later are derived in an elementary way in Section III the convex cores of molecules are determined. Diffusion of heavy molecules is discussed in Section IV with use of the rigid convex model of molecules. The intermolecular potentials for nonspherical molecules are calculated in Section V from the second virial coefficients their applications to crystalline states are given in Section VI. Section VII treats the role of the electric multipole interactions for convex or quasi-convex molecules in crystals. 

Thus far, a propane molecule is treated as a spherical particle. This approximation is essential when the magnitude of the coupling between host and guest is examined. However, propane is actually anisotropic. Here, we examine more realistic models for propane and ethane together with argon, which are appropriately described by three, two and single LJ interaction sites whose parameters are listed in Table 3[25, 26, 27]. The nonspheri-cal propane consists of three interaction sites, two of which are equivalent and denoted by methyl , and the other is denoted by methylene . The nonspherical ethane consists of two interaction sites, both of which are equivalently denoted by methyl . 

Fig. 1.11 (a) A typical coarse-graining method replaces groups of atoms by ellipsoidal rigid bodies, (b) The Gay-Beme potential models the anisotropic interaction between nonspherical coarse-grained molecules potentials are described by the orientation of the molecules with respect to a fixed frame and the separation between their centers of mass... 

Our convex-core model of molecules is defined on the assumption that the intermolecular potential C/ is a function of only the shortest distance p between the cores. Hence, while it may introduce the principal notion of a nonspherical interaction, the dependence of the potential depth on the molecular orientation is not taken into consideration. 

Deviations of real molecules from the reference system may occur, e.g., due to attractive interactions (dispersion), formation of hydrogen bonds (association), or the nonspherical shape of the molecules (which can be understood as the formation of chains from spherical segments). These contributions are usually assumed to be independent of each other and are accounted for by different perturbation terms. Depending on the kind of considered perturbation and on the expression used for its description, different models have been developed. One of the first models derived from that idea was the Statistical-Associating-Fluid Theory (SAFT) (Chapman et al. [12, 13] Huang and Radosz [14, 15]). 

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