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

Abstract

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.

Elastic Modulus of a Polymer Nanodroplet: Theory and Experiment

“…For mica (an extensive account of its surface properties can be found in the work of Hu et al) we have γ S d = 30 mJ·m –2 , γ S p = 90 mJ·m –2 , and γ S = 120 mJ·m –2 , with γ S simply being the sum of the dispersion − and polar contributions, in fair agreement with independently reported values of mica’s total surface tension. ,, …”

“…For mica (an extensive account of its surface properties can be found in the work of Hu et al 117 ) we have γ S d = 30 mJ•m −2 , γ S p = 90 mJ•m −2 , and γ S = 120 mJ•m −2 , with γ S simply being the sum of the dispersion 118−120 and polar 121 contributions, in fair agreement with independently reported values of mica's total surface tension. 119,120,122 The spreading parameter, eq 16, written in terms of Fowkes's model, eq 20, and the previous assumption γ P ≃ γ P d , illustrates that the interaction between mica and PB is governed by dispersion forces: S = −2γ P d + 2(γ S d γ P d ) 1/2 . A similar conclusion would be the product of an analysis from the perspective of the vOCG 102 model (developed later in this article): the occasional double bond of PB would render it with an ever-so-slightly basic character, while siloxane and other groups make the mica surface mostly basic, 123 meaning that any acid−base interaction cross terms between the two will be negligible compared to the dominant Lifshitz−van der Waals terms.…”

Measurement of the surface free energy of calcium-montmorillonite

“…The results of Bailey & Price (1970) and Slabaugh (1981) show modifications of the surface properties of micas and bentonites, respectively, when an organic compound coats the mineral; these complexes behave like low-energy solids, i.e. they are water-repellent.…”