Colloids and Surfaces A: Physicochemical and Engineering Aspects, 71 (1993) 269-276 Elsevier Science Publishers B.V., Amsterdam 269 Compression study on hydrophobic layered silicates dispersed in organic liquid mixtures Etelka Tombicz, Istvin DeCr and Imre Dtkhny zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH Department of Colloid Chemistry, Attila Jdzsef University, Aradi Vbrtanuk tere I, H-6720 Szeged, Hungary (Received 10 July 1992; accepted 19 December 1992) Abstract The behaviour of n-octadecylammonium montmorillonite particles dispersed in pure methanol and pure benzene or their mixtures was studied under compressive and decompressive conditions by using an analytical ultracentrifuge. Series of pressure and volume fraction data relating to the same state of dispersion were calculated. On the supposition of a flat- to-flat ordered structure of the silicate layers in the sediment, both the distance between lamellae and the basal spacing values could be calculated from the volume fraction data of the solid component. Conclusions were drawn from comparisons of determined basal spacings with experimental basal spacings obtained from X-ray diffraction measure- ments for the same systems and with the calculated values of the most probable arrangement of the alkyl chains between the silicate layers. As the compression is increased, the pendent long alkyl chains on the opposing layers can interpenet- rate. At the highest compression, the chains lie flat on the silicate layers, similarly to their structure in the dried state. On decrease of the deformation force, the alkyl chains rise and reach the tilt angle of 55’ calculated from the molecular geometry of the organocomplex. The silicate layers with pendent alkyl chains can move further away as liquid transport from the supematant leads to a solid/liquid (S/L) adsorption equilibrium at lower compression. Reversible changes in basal spacing during a compressive-decompressive cycle can take place at the azeotropic composition of liquid mixture adsorption, when the decrease in the excess free energy function of the adsorption exchange process calculated from the excess isotherm of methanol-benzene adsorption is a maximum. Keywords: Compression; layers; silicates. Introduction Different compression techniques for the meas- urement of internal pressure as a function of volume fraction have been applied to characterize concentrated dispersions of plate-like or non-plate- like particles. A specially designed compression cell was used by Ottewill and his co-workers [l-6]. In this cell an external pressure was applied to a dispersion confined between a semipermeable filter and a rubber membrane. The volume fraction or the distance of separation could be measured as a Correspondence to: E. Tomb&z, Dept. of Colloid Chemistry, Attila Jozsef University, Aradi Vertanuk tere 1, H-6720 Szeged, Hungary. function of excess osmotic pressure. A method similar in principle to Ottewill’s was described by Homola and Robertson [7]. The centrifuge tech- nique, previously used mainly to study emulsion stability, was extended by Gilinyi et al. [8,9] to determine volume fraction vs osmotic pressure functions in practically the entire volume fraction range. Electrostatically stabilized aqueous disper- sions of spherical [1,2,4,5,7-91 and plate-like [1,3,4] particles were studied at different neutral electrolyte concentrations. Non-aqueous disper- sions have received far less attention. Only a few papers [2,6,7] have been found on investigations of sterically stabilized polymer latices in non-polar media. The behaviour of these systems was quite different from that observed for electrostatically 0927-7757/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved