Hydrophobic effect solvation

Modem understanding of the hydrophobic effect attributes it primarily to a decrease in the number of hydrogen bonds that can be achieved by the water molecules when they are near a nonpolar surface. This view is confirmed by computer simulations of nonpolar solutes in water [15]. To a first approximation, the magnimde of the free energy associated with the nonpolar contribution can thus be considered to be proportional to the number of solvent molecules in the first solvation shell. This idea leads to a convenient and attractive approximation that is used extensively in biophysical applications [9,16-18]. It consists in assuming that the nonpolar free energy contribution is directly related to the SASA [9],... 

In cases where the solvation energies are large, as for example when ionic compounds dissolve in water, these hydrophobic effects, based on adverse changes in entropy, are swamped. Dissolving such compounds can be readily accomplished due to the very large energies released when the ions become hydrated. 

Some cosolvents can diminish the hydrophobic effect of water. For example, ethanol in water increases the solubility of hydrocarbons by helping to solvate them in preference to the poor solvation by water alone. 

It is worth pointing out that the equilibrium positions in many esterification reactions in hydrophobic solvents are quite favourable, so that high yields can be obtained even if the water activity is close to 1. In these cases, it is the effective solvation of the ester product in the medium that is a main driving force the reaction. This is further discussed below. 

Other energetic components associated widi the solvation process include non-electrostatic aspects of hydrogen bonding and solvent-structural rearrangements like the hydrophobic effect. Despite many years of study, the fundamental physics associated with both of these processes remains fairly controversial, and physically based models have not been applied with any regularity in the context of continuum solvation models. 

Part of the motivation behind so straightforward an approach derives from its ready application to certain simple systems, such as the solvation of alkanes in water. Figure 11.8 illustrates the remarkably good linear relationship between alkane solvation free energies and their exposed surface area. Insofar as the alkane data reflect cavitation, dispersion, and the hydrophobic effect, this seems to provide some support for the notion that these various terms, or at least their sum, can indeed be assumed to contribute in a manner proportional to solvent-accessible surface area (SASA). 

If counter ions are adsorbed only by electrostatic attraction, they are called indifferent electrolytes. On the other hand, some ions exhibit surface activity in addition to electrostatic attraction because of such phenomena as covalent bond formation, hydrogen bonding, hydrophobic and solvation effects, etc. Because of their surface activity, such counter ions may be able to reverse the sign of because the charge of such ions adsorbed exceeds the surface charge. 

In the case of contact forces, the magnitude of the enthalpy term is relatively small. The entropic contribution, however, can be significant. When a nonpolar compound is in an aqueous solution, the water molecules form a highly ordered solvent shell around the nonpolar portions of the compound (Scheme 9.3). This phenomenon is called the hydrophobic effect. Once the compound buries itself into a binding site on a target, some solvating water molecules will... 

Pratt and co-workers have proposed a quasichemical theory [118-122] in which the solvent is partitioned into inner-shell and outer-shell domains with the outer shell treated by a continuum electrostatic method. The cluster-continuum model, mixed discrete-continuum models, and the quasichemical theory are essentially three different names for the same approach to the problem [123], The quasichemical theory, the cluster-continuum model, other mixed discrete-continuum approaches, and the use of geometry-dependent atomic surface tensions provide different ways to account for the fact that the solvent does not retain its bulk properties right up to the solute-solvent boundary. Experience has shown that deviations from bulk behavior are mainly localized in the first solvation shell. Although these first-solvation-shell effects are sometimes classified into cavitation energy, dispersion, hydrophobic effects, hydrogen bonding, repulsion, and so forth, they clearly must also include the fact that the local dielectric constant (to the extent that such a quantity may even be defined) of the solvent is different near the solute than in the bulk (or near a different kind of solute or near a different part of the same solute). Furthermore... 

This cohesiveness produces a high surface tension, and all but the most polar or ionic solutes experience a hydrophobic force that drives them onto the stationary phase and causes retention. Most chemists are not as familiar with hydrophobic effects as with polar interactions, since the former have only recently been discussed in the chromatographic literature (1-6). The hydrophobic effect results from the strong attractive forces between water molecules. The "structure" of the water must be distorted or disrupted when a solute is dissolved. We can think of the solute as forming a cavity in the water. Highly polar or ionic solutes can themselves interact strongly with water, compensating for this distortion, and are thus easily solvated. 

---