(243b) Development of a General Interaction Potential for Hydrophobic and Hydrophilic Interactions

By performing surface force measurements, we have derived an interaction potential for the contribution of the attractive hydrophobic interaction as two surfactant bilayers approach, stress (causing the bilayers to thin and expose hydrophobic area), rupture, fuse, and are then separated from adhesive contact. The model quantitatively describes the complex interaction potential during bilayer hemi-fusion, including the long range forces, short range forces, hydrophobic breakthrough point, and final hydrophobic adhesion at contact. Between the bilayers, the hydrophobic interaction potential depends on the amount of exposed hydrophobic area, which progressively increases as the bilayers are further stressed. Generalization of the interaction potential results in a non-dimensional number, the “Hydra parameter,” that can account for the amount of hydrophobicity at a given interface. The Hydra parameter, H_y, varies from H_y = 1 for a fully hydrophobic interaction (corresponding to an interfacial energy of 50 mJ/m² for a hydrocarbon surface in water) to H_y = 0 for no hydrophobic contribution. For H_y < 0, the equation also naturally accounts for hydrophilic interactions, i.e., the additional repulsive steric-hydration force between hydrophilic surfaces, where now the magnitude of the repulsion depends on the degree of hydration. An exponential decay length of ~1 nm is proposed for both the attractive (hydrophobic) and repulsive (hydration) forces. The interaction potential is tested for a variety of surfaces and parameters: surfactant bilayers and monolayers and self-assembled hydrophobic polymer surfaces will be discussed, as will the effects on the hydrophobicity of these surfaces by adjusting salt concentration, pH, specific ions, and solvent quality. A new picture is emerging that suggests that both hydrophobic and hydrophilic interactions can be described by a single unifying equation, suggesting a similar mechanism for both interactions, for example, a depleted water density (and/or decrease of the hydrogen bonding order parameter) near hydrophobic surfaces, and an increased water density (and/or hydrogen bonding order parameter) near hydrophilic surfaces.