Solute-solvent interactions difference

Solute-solvent interactions differ from solvent-solvent interactions. 

Solution that does not obey Raoult s law at a certain concentration -> Solute-solvent interactions differ from solvent-solvent interactions. 

The preceding empirical measures have taken chemical reactions as model processes. Now we consider a different class of model process, namely, a transition from one energy level to another within a molecule. The various forms of spectroscopy allow us to observe these transitions thus, electronic transitions give rise to ultraviolet—visible absorption spectra and fluorescence spectra. Because of solute-solvent interactions, the electronic energy levels of a solute are influenced by the solvent in which it is dissolved therefore, the absorption and fluorescence spectra contain information about the solute-solvent interactions. A change in electronic absorption spectrum caused by a change in the solvent is called solvatochromism. 

For those dilute mixtures where the solute and the solvent are chemically very different, the activity coefficient of the solute soon becomes a function of solute mole fraction even when that mole fraction is small. That is, if solute and solvent are strongly dissimilar, the relations valid for an infinitely dilute solution rapidly become poor approximations as the concentration of solute rises. In such cases, it is necessary to relax the assumption (made by Krichevsky and Kasarnovsky) that at constant temperature the activity coefficient of the solute is a function of pressure but not of solute mole fraction. For those moderately dilute mixtures where the solute-solute interactions are very much different from the solute-solvent interactions, we can write the constant-pressure activity coefficients as Margules expansions in the mole fractions for the solvent (component 1), we write at constant temperature and at reference pressure Pr ... 

A discussion of the different types of solute-solute and solute-solvent interactions acting in homogeneous catalysis by transition metal complexes. E. Cesarotti, R. Ugo and L. Kapan, Coord. Chem. Rev., 1982,43, 275-298 (47). 

One important parameter in the solvent characterization is the solvent polarity, which can be defined in different ways [24]. Polarity is connected with solute-solvent interactions, and these depend on the solute. Thus, there are five types of molecular interactions ... 

The dielectric constant and refractive index parameters and different functions of them that describe the reactive field of solvent [45] are insufficient to characterize the solute-solvent interactions. For this reason, some empirical scales of solvent polarity based on either kinetic or spectroscopic measurements have been introduced [46,47]. The solvatochromic classification of solvents is based on spectroscopic measurements. The solvatochromic parameters refer to the properties of a molecule when its nearest neighbors are identical with itself, and they are average values for a number of select solutes and somewhat independent of solute identity. 

As the same types of intermolecular forces are involved, there is no qualitative difference between solute-solvent interactions and the recognition of a compound by a bio (macro) molecular compound. 

In a recent paper. Mo and Gao [5] used a sophisticated computational method [block-localized wave function energy decomposition (BLW-ED)] to decompose the total interaction energy between two prototypical ionic systems, acetate and meth-ylammonium ions, and water into permanent electrostatic (including Pauli exclusion), electronic polarization and charge-transfer contributions. Furthermore, the use of quantum mechanics also enabled them to account for the charge flow between the species involved in the interaction. Their calculations (Table 12.2) demonstrated that the permanent electrostatic interaction energy dominates solute-solvent interactions, as expected in the presence of ion species (76.1 and 84.6% for acetate and methylammonium ions, respectively) and showed the active involvement of solvent molecules in the interaction, even with a small but evident flow of electrons (Eig. 12.3). Evidently, by changing the solvent, different results could be obtained. 

MD simulations in expHcit solvents are stiU beyond the scope of the current computational power for screening of a large number of molecules. However, mining powerful quantum chemical parameters to predict log P via this approach remains a challenging task. QikProp [42] is based on a study [3] which used Monte Carlo simulations to calculate 11 parameters, including solute-solvent energies, solute dipole moment, number of solute-solvent interactions at different cutoff values, number of H-bond donors and acceptors (HBDN and HBAQ and some of their variations. These parameters made it possible to estimate a number of free energies of solvation of chemicals in hexadecane, octanol, water as well as octanol-water distribution coefficients. The equation calculated for the octanol-water coefficient is ... 

Solute motions follow a quite different pattern. The solute is perturbed after a given number of motions of the solvent. Hypothetically, the solute could remain motionless, which would correspond to place the system of coordinates in the solute molecule. However, from a statistical point of view it is better to allow some motion given the fact that if a solvent molecule is displaced, only one solute-solvent interaction is perturbed whereas if it is the solute that moves all the solute solvent interactions are modified. 

In the third model, solvent molecules act as a bulk medium and significandy modify the solute properties. In this type, solute-solvent interaction is modeled using the continuum approach [8-11]. A variety of models have been proposed in the literature to treat solvent molecules in different situations. 

Supercritical solutions are characterized by very low solvent densities. As a result, they possess the interesting feature that solubility is determined more by solute-solute than solute-solvent interactions. Thus we were able to express the solubilities of naphthalene and a series of indole derivatives in four different supercritical solvents (C2H4, C2H6, C02 and the highly polar CHF3) in the same functional format, only the numerical coefficients varying from one to another.57 Solute-solvent interactions do occur,58 but solubility can be represented quite... 

This Chapter has outlined several different approaches to the computational determination of solution properties. Two of these address solute-solvent interactions directly, either treating the effects of individual solvent molecules upon the solute explicitly or by means of a reaction field due to a continuum model of the solvent. The other procedures establish correlations between properties of interest and certain features of the solute and/or solvent molecules. There are empirical elements in all of these methods, even the seemingly more rigorous ones, such as the parameters in the molecular dynamics/Monte Carlo intermolecular potentials, Eqs. (16) and (17), or in the continuum model s Gcavitation and Gvdw, Eqs. (40) and (41), etc. 

The QSAR technique, widely developed by Kamlet, Taft and coworkers38,98 for the prediction of specific solute-solvent interactions, has been used to predict the different solute-solvent contributions to property variations of compounds. The influence of solvent on the relative basicity of dipolar trimethylamines has been recently studied a descriptor was developed to describe a unique solute-solvent interaction involving dipolar amines99. 

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