Solvation and Hydrophobic Effect

Classically, the effects of varying solvent composition are decomposed into electrostatic and hydrophobic effects. Recent reexamination of the classic Tanford experiments on contributions to change in solvation free energy differences (Auton, Holthauzen, and Bolen 2007), along with consideration of what constitutes hydro-phobicity (Chandler 2005) brings into question this taxonomy. Given the work reviewed above, we prefer to use a more natural set of variables, electrostatic and van der Waals interactions, which are conveniently part of the underlying intermolecular interaction models, to show the mechanistic trends of solution (de)stabilization. 

Intra-molecular ordering in synthetic polymers also merits consideration vis-a-vis biopolymers. In the natural macromolecules, the superposition of hydrogen bonding, ion-pair formation and hydrophobic interactions is well-known, and the native structures are viewed as accomodations to these and other solvation and coulombic effects. With synthetic polymers, one may focus on a few of these phenomena selectively, and much may be gained from the judicious use of the synthetic macromolecules as analogs to the natural ones. 

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],... 

Solvation and especially hydration are rather complex phenomena and little is known about them. Depending on the kind of molecular groups, atoms or ions interacting with the solvent, one can differ between lyo- or hydrophilic and lyo-or hydrophobic solvation or hydration. Due to these interactions the so-called liquid structure is changed. Therefore it seems to be unavoidable to consider, at least very briefly, the intermolecular interactions and the main features of liquids, especially water structure before dealing with solvation/hydration and their effects on the formation of ordered structures in the colloidal systems mentioned above. 

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

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