SURFACE THERMODYNAMIC PROPERTIES OF SYNTHETIC HYDROTALCITE COMPOUNDS

The contact angles of test liquids on the surface of several synthetic hydrotalcite compounds were determined by thin layer wicking. These contact angles were used to solve the Young equation for polar materials to yield the Lifshitz-van der Waals apolar surface tension component, gamma(LW), and the polar Lewis acid/base parameters, gamma(circle plus) and gamma(circle minus), of the samples. These values indicate that the synthetic hydrotalcite compounds of this study are monopolar (or nearly so) materials with a relatively modest gamma(LW) (21-27 mJ/m(2)), and they are Lewis bases (gamma(circle minus) = 17-32 mJ/m(2)) with a small or no Lewis acid character (gamma(circle plus) congruent to 0). The observed variation of the surface tension components as a function of the chemical composition of the samples is determined by the type of interlayer anion (CO32- vs SO42-) and the magnitude of the layer charge (determined by the value of the Mg2+/(Mg2+ + Al3+) ratio. The fact that the hydrotalcite layer has a net positive charge, in contrast to the negative charge of 2:1 phyllosilicates, appears to play little or no role in determining the acid/base character of these layer structures.