Interfacial Energies of Monomolecular Films of Fatty Acids Deposited on Mica in Aqueous and Nonaqueous Media—the Strength of Hydrophobic Interactions

An investigation into the energy per unit area of interface formed between monomolecular layers of fatty acids adsorbed on a mica surface and polar and nonpolar liquids is described. The results are compared with values predicted from Young's relation, γsl = γsv − γlvcosθ, where the γ's represent interfacial energies between solid, liquid, and vapor phases, and θ is the contact angle. This relation is known to hold for systems in which the phases remain homogeneous right up to the interface. It is shown that for nonpolar liquids the predicted and measured values are the same, while for water an additional term amounting to 7.5 erg/cm2 of interface must be introduced in Young's relation to obtain equality. This additional energy has been associated with hydrophobic interactions and from model building a value of 213 cal/mol of water ordered at the interface is obtained. The results are in good agreement with values obtained by Frank and Evans. The experiments shed additional light on the action of urea on hydrophobic interactions. In a concentrated solution of urea (> 8M), the predicted and measured values of the solid/liquid interfacial energy are again equal. This leveling effect can only be interpreted as showing that the liquid phase is homogeneous and suggests that urea causes a breakdown of the ordered structure of the water in the neighborhood of the hydrophobic interface. This conclusion is consistent with current views of the denaturant action of urea on proteins and other biomolecules.