Solute interaction

Taken together, these solvent-solute interactions make up the solvent polarity, which is represented well by Hildebrand s solubility parameter (1950). 

For dilute solutions, solute-solute interactions are unimportant (i.e., Henry s law will hold), and the variation of surface tension with concentration will be linear (at least for nonelectrolytes). Thus... 

Bardeen C J and Shank C V 1994 Ultrafast dynamics of the solvent-solute Interaction measured by femtosecond four-wave mixing LD690 In n-alcohols Chem. Phys. Lett. 226 310-16... 

Berg M and Vanden Bout D A 1997 Ultrafast Raman echo measurements of vibrational dephasing and the nature of solvent-solute interactions Acc. Chem. Res. 30 65-71... 

From the standpoint of thermodynamics, the dissolving process is the estabHsh-ment of an equilibrium between the phase of the solute and its saturated aqueous solution. Aqueous solubility is almost exclusively dependent on the intermolecular forces that exist between the solute molecules and the water molecules. The solute-solute, solute-water, and water-water adhesive interactions determine the amount of compound dissolving in water. Additional solute-solute interactions are associated with the lattice energy in the crystalline state. 

In the concluding chapters we again consider assemblies of molecules—this time, polymers surrounded by solvent molecules which are comparable in size to the repeat units of the polymer. Generally speaking, our efforts are directed toward solutions which are relatively dilute with respect to the polymeric solute. The reason for this is the same reason that dilute solutions are widely considered in discussions of ionic or low molecular weight solutes, namely, solute-solute interactions are either negligible or at least minimal under these conditions. 

Our primary interest in the Flory-Krigbaum theory is in the conclusion that the second virial coefficient and the excluded volume depend on solvent-solute interactions and not exclusively on the size of the polymer molecule itself. It is entirely reasonable that this should be the case in light of the discussion in Sec. 1.11 on the expansion or contraction of the coil depending on the solvent. The present discussion incorporates these ideas into a consideration of solution nonideality. 

In addition to an array of experimental methods, we also consider a more diverse assortment of polymeric systems than has been true in other chapters. Besides synthetic polymer solutions, we also consider aqueous protein solutions. The former polymers are well represented by the random coil model the latter are approximated by rigid ellipsoids or spheres. For random coils changes in the goodness of the solvent affects coil dimensions. For aqueous proteins the solvent-solute interaction results in various degrees of hydration, which also changes the size of the molecules. Hence the methods we discuss are all potential sources of information about these interactions between polymers and their solvent environments. 

We assume that the observed interference is the cumulative effect of the contributions of the individual polymer molecules and that solute-solute interactions do not enter the picture. This effectively limits the model to dilute solutions. This restriction is not particularly troublesome, since our development of the Rayleigh theory also assumes dilute solutions. 

In applying the Debye theory to concentrated solutions, we must extrapolate the results measured at different concentrations to C2 = 0 to eliminate the effects of solute-solute interactions. 

We begin with Eq. (10.89)-that is, we neglect solute-solute interactions-and write... 

Practical Solubility Concepts. Solution theory can provide a convenient, effective framework for solvent selection and blend formulation (3). When a solute dissolves in a solvent, a change in free energy occurs as a result of solvent—solute interactions. The change in free energy of mixing must be negative for dissolution to occur. In equation 1,... 

The solvophobic model of Hquid-phase nonideaHty takes into account solute—solvent interactions on the molecular level. In this view, all dissolved molecules expose microsurface area to the surrounding solvent and are acted on by the so-called solvophobic forces (41). These forces, which involve both enthalpy and entropy effects, are described generally by a branch of solution thermodynamics known as solvophobic theory. This general solution interaction approach takes into account the effect of the solvent on partitioning by considering two hypothetical steps. Eirst, cavities in the solvent must be created to contain the partitioned species. Second, the partitioned species is placed in the cavities, where interactions can occur with the surrounding solvent. The idea of solvophobic forces has been used to estimate such diverse physical properties as absorbabiHty, Henry s constant, and aqueous solubiHty (41—44). A principal drawback is calculational complexity and difficulty of finding values for the model input parameters. 

The Stokes-Einstein equation has already been presented. It was noted that its vahdity was restricted to large solutes, such as spherical macromolecules and particles in a continuum solvent. The equation has also been found to predict accurately the diffusion coefficient of spherical latex particles and globular proteins. Corrections to Stokes-Einstein for molecules approximating spheroids is given by Tanford. Since solute-solute interactions are ignored in this theory, it applies in the dilute range only. 

The development of micellar liquid chromatography and accumulation of numerous experimental data have given rise to the theory of chromatographic retention and optimization methods of mobile phase composition. This task has had some problems because the presence of micelles in mobile phase and its modification by organic solvent provides a great variety of solutes interactions. 

Proper condensed phase simulations require that the non-bond interactions between different portions of the system under study be properly balanced. In biomolecular simulations this balance must occur between the solvent-solvent (e.g., water-water), solvent-solute (e.g., water-protein), and solute-solute (e.g., protein intramolecular) interactions [18,21]. Having such a balance is essential for proper partitioning of molecules or parts of molecules in different environments. For example, if the solvent-solute interaction of a glutamine side chain were overestimated, there would be a tendency for the side chain to move into and interact with the solvent. The first step in obtaining this balance is the treatment of the solvent-solvent interactions. The majority of biomolecular simulations are performed using the TIP3P [81] and SPC/E [82] water models. 

When the silica surface is in contact with a solvent, the surface is covered with a layer of the solvent molecules. If the mobile phase consists of a mixture of solvents, the solvents compete for the surface and it is partly covered by one solvent and partly by the other. Thus, any solute interacting with the stationary phase may well be presented with two, quite different types of surface with which to interact. The probability that a solute molecule will interact with one particular type of surface will be statistically controlled by the proportion of the total surface area that is covered by that particular solvent. 

From the point of view of solute interaction with the structure of the surface, it is now very complex indeed. In contrast to the less polar or dispersive solvents, the character of the interactive surface will be modified dramatically as the concentration of the polar solvent ranges from 0 to l%w/v. However, above l%w/v, the surface will be modified more subtly, allowing a more controlled adjustment of the interactive nature of the surface It would appear that multi-layer adsorption would also be feasible. For example, the second layer of ethyl acetate might have an absorbed layer of the dispersive solvent n-heptane on it. However, any subsequent solvent layers that may be generated will be situated further and further from the silica surface and are likely to be very weakly held and sparse in nature. Under such circumstances their presence, if in fact real, may have little impact on solute retention. 

Retention is controlled by solute interactions with both the mobile phase and the stationary phase and each will be discussed in this chapter. Interactions in the mobile... 

Solute Interacting with Second Layer of Solvent (B) by Sorption... 

Figure 11. The Different Types of Solute Interaction that can Occur on a Silica Surface Containing a Solvent Bi-layer...

Where there are multi-layers of solvent, the most polar is the solvent that interacts directly with the silica surface and, consequently, constitutes part of the first layer the second solvent covering the remainder of the surface. Depending on the concentration of the polar solvent, the next layer may be a second layer of the same polar solvent as in the case of ethyl acetate. If, however, the quantity of polar solvent is limited, then the second layer might consist of the less polar component of the solvent mixture. If the mobile phase consists of a ternary mixture of solvents, then the nature of the surface and the solute interactions with the surface can become very complex indeed. In general, the stronger the forces between the solute and the stationary phase itself, the more likely it is to interact by displacement even to the extent of displacing both layers of solvent (one of the alternative processes that is not depicted in Figure 11). Solutes that exhibit weaker forces with the stationary phase are more likely to interact with the surface by sorption. 

Now, if the solutes interact with themselves more strongly than they do with the stationary phase, then their presence will increase the interaction of further solute with the stationary phase mixture. This gives an isotherm having the shape shown in Figure 10. This type of isotherm is called a Freundlich isotherm, the expression for... 

Thermal changes resulting from solute interactions with the two phases are definitely second-order effects and, consequently, their theoretical treatment is more complex in nature. Thermal effects need to be considered, however, because heat changes can influence the peak shape, particularly in preparative chromatography, and the consequent temperature changes can also be explored for detection purposes. 

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