Clathrate hydrates of tetrabutylammonium carboxylates and dicarboxylates

Abstract

Phase diagrams of the binary aqueous systems with tetra-n-butylammonium (TBA) carboxylates ((C₄H₉)₄NC_nH_{2 n+1}CO₂, where n=0÷5) and dicarboxylates ([(C₄H₉)₄N]₂(CH₂)_nC₂O₄, where n=1÷3) including some branched carboxylate anions, have been studied in the field of crystallization of clathrate hydrates. Monocrystals of many hydrates have been prepared and their composition, densities, melting points and X-ray data have been determined. In the set of TBA carboxylate hydrates the stability increases towards TBA propionate or butyrate hydrates (for different structures) and then it decreases as sizes of anions grow, This is explained by additional stabilization of the framework caused by small cavities being occupied until the hydrophobic part of the anion is able to go in the cavity. In the set of hydrates of TBA dicarboxylates the change of the stability is easily accounted for by the modelling of the inclusion of dicarboxylate ions in the cavities, within known structures, different ways of hydrophilic inclusion being taken into account.