Surface structure of kaolinite from the detailed analysis of gas/solid adsorption isotherms

Abstract

Monolayer region, nitrogen, adsorption isotherms were measured on a size-fractionated, Georgia kaolinite, a North Carolina pyrophyllite, and attempted on a Brazilian gibbsite. Two isotherms were measured on kaolinite: one at 77.15°K and the other at 89.52°K. Together, these were used to substantiate the validity of a recently developed isotherm-analysis technique called CAEDMON. This technique provides a way to obtain the distribution of adsorptive energies on a heterogeneous substrate. The distribution was interpreted in terms of surface structure and the following results were found.

A single dioctahedral sheet of gibbsite is unstable when isolated. It requires the presence of adjacent layers of either water or other gibbsite layers to exist. Approximately half of the kaolinite surface is undistinguishable from the pyrophyllite surface. This surface was well characterized on the kaolinite sample used in this work, and it is believed to be the oxygen surface formed by the tetrahedral silica sheet. The other half of the surface, believed to be the gibbsite-like, dioctahedral sheet, has a greater adsorptive potential for nitrogen than the silica sheet, but was not well characterized on the present sample. The gibbsite-like surface of this kaolinite was partially dehydrated under a vacuum at 125°C and partially dissolved at pH 3.5 in the size-fractionation process. This alteration of the gibbsite-like surface was unintentional, and was revealed only by the CAEDMON substrate analysis technique. In addition, a new, lower, structural dehydration, decomposition temperature for kaolinite was reported: it is 200°C. Finally, the failure of the B.E.T. equation to adequately describe adsorption data on homotattic substrates was explained in terms of registry of the adsorbate by the substrate.