Exploring the structure of inter-platelet galleries in organically modified montmorillonite using the small-angle X-ray scattering interface distribution function approach

Small-angle X-ray scattering interface distribution function (IDF) analysis is successfully applied to evaluate the two- and three-phase inter-platelet gallery structure in synthetic montmorillonite (MMT) modified with a mixture of chemically grafted and physically adsorbed chains of a typical organic modifier, octadecyl trimethyl ammonium bromide. Two distinctly different states of adsorbed chains in the inter-platelet galleries are identified. The first one corresponds to a fully cation-exchanged structure with an inter-platelet distance of about 21 Å. In this case the organic modifier is in a liquid-like state, and the system does not exhibit any structural or thermal transitions between ambient temperature and 433 K. When the number of adsorbed chains exceeds the cation-exchange capacity of the MMT platelets the inter-platelet spacing is increased to 43 Å. In this case, the variable-temperature IDFs reveal partially crystalline order in the inter-platelet galleries, which vanishes at the melting point of the alkyl tails confined in the galleries.