On the thermodynamic driving force for polymer intercalation in smectite clays

The view that the intercalation of a polymer in an unmodified smectite clay is driven by the entropic increase that results from displacement of adsorbed water is re-assessed in the light of experiments that show rapid melt intercalation into a clay that has been heat-treated to remove gallery water. Dehydrated smectite clays with collapsed layers take up poly(ethylene glycol) from the melt in only I ks or from aqueous solution in under 18 ks, re-establishing the basal plane spacing to that for intercalation in untreated clay, 1.8 nm. Differential scanning calorimetry showed that the intercalation of poly(ethylene glycol) into montmorillonite is exothermic with an enthalpy change of - 153 J g(-1) based on the intercalated polymer and the heat of wetting for the internal surfaces of montmorillonite by poly(ethylene glycol) is -0.08 J m(-2). These results confirm the observation of re-expansion of heat-treated clays and imply that the reduction in free energy on intercalation results from a significant enthalpic change as well as an entropic change for clays with interlayer water, and primarily from an enthalpic change for clay in the absence of water.