THE SURFACE FREE-ENERGIES OF TALC AND PYROPHYLLITE

The components of the interfacial surface tension of talc and pyrophyllite were determined by measuring the rate of the capillary rise of a number of liquids through thin, sedimented deposits of the powdered minerals. The rate of capillary rise of a liquid in a powder is related to the contact angle between the liquid and the solid by the Washburn equation. The contact angles thus derived were used to determine the apolar (Lifshitz-van der Waals) component, gamma-LW, and the polar, electron-acceptor and electron-donor parameters, gamma+ and gamma- respectively, of the Lewis acid/base component of the total interfacial surface energy using the Young equation. The values of gamma-LW for talc and pyrophyllite (31.5 and 34.4 mJ/m2) are slightly smaller than for smectite clay minerals (e.g., the value for hectorite is 39.9 mJ/m2), the electron donor parameter values are roughly comparable for talc and pyrophyllite (gamma+ = 2.4 and 1.7 mJ/m2) as are the values of the electron acceptor parameter (gamma- = 2.7 and 3.2 mJ/m2). The well-known hydrophobicity of these two minerals is due to the remarkably small value (for silicate minerals) of gamma-(gamma+ is normally small or zero for silicates and many other oxides). The small values of both gamma+ and gamma- mean that the Lewis acid/base interactions between talc or pyrophyllite and highly polar water molecules are very weak. In contrast, low-charge smectites, the minerals most similar chemically and structurally to talc and pyrophyllite, have much greater values of gamma- (greater-than-or-equal-to mJ/m2) and are hydrophyllic.