Structure formation of alkylammonium montmorillonites in organic media

Using adsorption, X-ray diffraction (XRD), theological, electrokinetic, dielectric and calorimetric measurements, the mechanism for the gel formation of the alkylammonium derivatives of the Pyzhevsky montmorillonite (Ukraine) (OM) in organic liquids of different polarity was investigated. The influence of modification, type of the organic cations, length of the alkyl chain, type of organic liquid and polar additive as well as traces of water on the swelling volume, enthalpy of immersion, zeta potential and the Bingham yield stress of the organo-gels were evaluated. There exists a certain correlation between the swelling volume, the ability for gel formation and the basal spacing of OM (d(001)), which depends on the size of the intercalated cation. Thermodynamical analysis indicates that the degree of swelling and the gel strength are determined by the energy of the layer cohesion, energy of the solvation, the electrostatic repulsion and entropy effects. The thickening of OM dispersions and gel formation is optimal when the surface coverage by alkylammonium ions reaches theta = 0.85-1.0. The lyophilicity and zeta potential are then high but not too strong to cause complete peptization. The linear increase of theological parameters of the organo-gels in benzene, toluene, p-xylene, o-xylene, chlorobenzene, benzonitrile and nitrobenzene is related to the increase of the polarization of the liquid molecules and the transition from positive to negative zeta -potentials and the enthalpy of immersion. The effect of protonic and aprotonic compounds on the rheological, electrokinetic and thermodynamic properties of the hydrocarbon gels of octadecyl benzyldimethylammonium montmorillonite was also studied. For homologue alcohols, this influence decreases with the increase of chain length, corresponding to the decrease of the adsorption and the dielectric permittivity (epsilon). The influence of the aprotonic compounds on the rheological properties and on the enthalpy of immersion does not correlate with epsilon, but this influence is larger when the polarization and electron donor ability of the polar molecules are higher. The maximal gel formation of the hydrocarbon-OM dispersions requires a certain amount of water, which corresponds to a monomolecular coverage of the polar surface centers. This effect is explained by the strong orientation of the adsorbed water molecules, which creates giant dipole moments on the particles and H-bonds between the particles. (C) 2001 Elsevier Science B.V. AII rights reserved.