Surface and intercalate chemistry of the layered silicates. Part VI. Tetrahydrofuran intercalates of a series of cation-exchanged montmorillonites

Abstract

Intercalates of tetrahydrofuran (THF) with nine distinct cation-exchanged montmorillonites have been prepared and may be divided into two classes depending upon the magnitude of the basal spacing, d₀₀₁. In one class (with NH₄⁺, Na⁺, Co²⁺, Ni²⁺, and Cu²⁺)d₀₀₁ lies close to 14.6 Å; in the other (with Ca²⁺, Sr²⁺ and Ba²⁺) the spacing is close to 19.0 Å, while Li⁺-exchanged montmorillonites are capable of forming both types of intercalate. The orientation and position of the organic molecules for six of the intercalates (NH₄⁺, Na⁺, Co²⁺, Ni²⁺, Cu²⁺, and Sr²⁺) have been determined form one-dimensional electron density maps constructed from the results of X-ray diffraction measurements. The lower-spacing intercalates have one layer of organic molecules in the interlamellar region with the THF perpendicular to the sheets. The number of THF molecules per formula unit [idealised, Al₄Si₈O₂₀(OH)₄] ranges from 1.3 (NH₄⁺) to 2.3 (Ni²⁺), the corresponding figures per exchanged cation being 2.6 (NH₄⁺) to 9.4 (Ni²⁺). The higher spacing intercalates have a double layer of THF molecules inclined at ca. 55° to the sheets. There are, for example, 4.8 molecules per formula unit of the Sr²⁺-exchanged clay. Electron spectroscopy demonstrates that the copper-exchanged clay and intercalate change from Cu²⁺ to Cu⁺ under the influence of the incident, low energy, X-rays.