We carried out surface force measurements between surfactant-bilayers having variable hydrophobicity that could be reversibly modulated by light activation, and have obtained a possibly universal interaction potential for the attractive interaction between surfaces whose �hydrophobicity factor� varies from f=0 (fully hydrophobic interaction corresponding to an interfacial energy of ~50 mJ/m2 for two hydrocarbon surfaces in water) to f=1 (no hydrophobic interaction, just a pure DLVO interaction). For f>1, the derived equation appears to also naturally cover hydrophilic interactions, i.e., the additional repulsive �hydration� force between naturally hydrophilic surfaces or surfaces rendered hydrophilic by the adsorption (or ion exchange) of hydrated ions, where again the magnitude of the repulsion depends on the degree of hydration. A universal exponential decay length of ~1.0 nm is proposed for both the attractive (hydrophobic) and repulsive (hydration) forces with f varying from 0 to about 2 as we go from a purely hydrophobic to a purely hydrophilic (or highly hydrated) surface with more than one layer of bound water molecules. The model is shown to quantitatively account for many interfacial phenomena including the various instabilities and turning points in complex interaction potentials, such as the long range forces, short range forces, hydrophobic breakthrough point, and final hydrophobic adhesion as two bilayers approach, spread (thin), rupture, hemi-fuse, and are then separated from adhesive contact; as well as the repulsive force measured between hydrated (mica) surfaces. 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.

Development of a general interaction potential for hydrophobic and hydrophilic interactions

246th National Meeting (2013)

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