Application of a Two-Structure Model

In this section we formulate the general aspect of the application of the simplest mixture model approach to water. We shall use an exact two-structure model as introduced in section 7.9. In the following subsection, we shall illustrate the application to solutions of an interstitial model for water, and in section 7.13 we shall discuss the application of a more general MM approach to this problem. 

Consider a system of Nw water molecules and Ns solute molecules at a given temperature T and pressure P. (We later confine ourselves to dilute solutions, Ns Niv in this section, the treatment is more general.) We henceforth assume that T and P are constant and therefore omit them from our notation. 

Let Nl and Nh be the average number of L and H molecules obtained by any classification procedure (section 5.13). For the purpose of this section, we need not specify the particular choice of the two species therefore, our treatment will be very general. For concreteness, one may think of a TSM constructed from the vector Xc (section 7.9), i.e., 

Consider the volume as an example of an extensive variable. The total differential of the volume (7, P constant) is 

As we have noted before, is not really an independent variable, and since we have kept T, P, and constant in (7.12.9), there remains only one independent variable. Ns- In fact, recognizing that Ni, is a function of T, P, Nw and N, through (7.12.5) or (7.12.6), we can rewrite (7.12.9) as 

The idea that a solute changes the structure of the solvent is very old. As an example of the application of this idea, we refer to Chad well s explanation of the following puzzling observation (Chadwell, 1927) The addition of solutes such as ether or methyl acetate to water was found to decrease the compressibility of the system, in spite of the fact that the compressibilities of these pure solutes are about three times larger than the compressibility of pure water. It has been postulated that water contains two species, say monomeric water molecules and polymers of water molecules. Addition of a solute causes a shift toward the component that has a lower compressibility hence a qualitative explanation of the experimental observation is provided. Similar attempts to explain the effect of solute on viscosity, dielectric relaxation, self-diffusion, and many other properties have been suggested in the literature. 

There are, essentially, two fundamental questions that have been the subject of extensive research. The first is concerned with the type of structure that water molecules are assumed to form around the solute molecules. Progress in this field was mainly due to comparison of the thermodynamics of dissolution of gases in water with the thermodynamics of gas-hydrate formation [see, for example, Glew (1962), Namiot (1961), and a review by Ben-Naim (1972e)]. The second problem is concerned with the mechanism by which a simple solute such as argon enhances the structure of the solvent.  

Perhaps one of the most striking pieces of evidence that a simple solute has a significant structural effect on water comes from a comparison of the 

The effect of the solute on the structure of water has been investigated by numerous authors. In most studies, one assumes an ad hoc mixture model for water and then examines the shift in chemical equilibrium between the various species involved. [Examples of such studies are Frank and Quist (1961), Namiot (1961), Grigorovich and Samoilov (1962), Yashki-chev and Samoilov (1962), Wada and Umeda (1962 ), Nemethy and Sche-raga (1962), Ben-Naim (1965a,b), Mikhailov (1968), Mikhailov and Ponomareva (1968), Frank and Franks (1968).] 

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In this section we present an example of the application of a two-structure model, based on the exact MM approach to the theory of liquids (section 5.13). Then we extract a particular MM which can be viewed as an approximation of the general exact MM approach. The latter, because of its simplicity and solvability, is useful in the study of some thermodynamic aspects of both pure water and aqueous solutions of simple solutes. 

We recently reported a structure-activity model for variations In target organs (12) and are currently examining the possible application of the quantitative structure-activity approach to the problem of specles-to-specles differences In susceptibility toward nltrosamlne carcinogenesis (19). These two topics will be discussed In the remainder of this presentation. 

Although many satisfactory VCD studies based on the gas phase simulations have been reported, it may be necessary to account for solvent effects in order to achieve conclusive AC assignments. Currently, there are two approaches to take solvent effects into account. One of them is the implicit solvent model, which treats a solvent as a continuum dielectric environment and does not consider the explicit intermolecular interactions between chiral solute and solvent molecules. The two most used computational methods for the implicit solvent model are the polarizable continuum model (PCM) [93-95] and the conductor-like screening model (COSMO) [96, 97]. In this treatment, geometry optimizations and harmonic frequency calculations are repeated with the inclusion of PCM or COSMO for all the conformers found. Changes in the conformational structures, the relative energies of conformers, and the harmonic frequencies, as well as in the VA and VCD intensities have been reported with the inclusion of the implicit solvent model. The second approach is called the explicit solvent model, which takes the explicit intermolecular interactions into account. The applications of these two approaches, in particular the latter one will be further discussed in Sect. 4.2. 

The reader might well wonder what the authors are smoking because this heading covers all of chemistry True indeed, so all we can possibly present in this short section is a review of the ways in which the fundamental structural forms represented by the elements are modified by heteroatoms and how the introduction of heteroatoms can generate new types of structure. Again, in the simplest molecular forms the application of the two-center-two-electron bond model plus 8/18-electron rules will be our focus however, places where this guide fails will be noted. 

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