The influence of a variety of media on solid/fluid interfacial energies has been measured by a cleavage technique. The solid used was mica, chosen because of its near perfect cleavage and ideal bulk properties. Solid/vapour and solid/liquid interfacial energies, ysv and y s ~, were measured by cleaving specimens in the form of strips, first, in an atmosphere of the vapour and then with the specimens completely immersed in the corresponding liquid. Samples of mica coated with monomolecular films of fatty acids were constructed in such a way that separation took place between the oppositely oriented films. Polar and non-polar liquids and vapours were used in these experiments. The results allow an investigation into the validity of Young's equation for contact angles which are zero or positive. For well-behaved systems in which no adsorption takes place during the cleavage, the relation is valid. For fatty acid films in an aqueous environment, it is necessary to introduce an additiona1 term into the relation to preserve the equality, thus

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.