COMPRESSION STUDY ON LAMELLAR PARTICLES DISPERSED IN ELECTROLYTE-SOLUTIONS

The behaviour of hydrophilic layered silicate (sodium, hydrogen and ca lcium montmorillonite) particles dispersed in dilute sodium chloride s olutions was studied in a centrifugal field. The compressibility of th e sediments and the reversibility of deformation were measured in comp ressive-decompressive cycles over the rotation rate range 2000-60000 r ev min-1. Series of excess pressure and volume fraction data relating to the same state of dispersion were calculated. Because of the flat-t o-flat ordered structure, a volume fraction as high as 0.81 was attain ed at the highest pressure. Small hysteresis loops were observed. Dist ances between lamellae and basal spacings were calculated from the vol ume fraction data of the solid components. The basal spacing data at t he highest compression closely approximated the basal spacings obtaine d from X-ray diffraction measurements on solid samples of different mo nocationic montmorillonites. The structure of the interlayer space cou ld collapse to one layer of water when it contained hydrogen and sodiu m ions, and to two layers of water when it contained calcium ions. Owi ng to the Ca(s)/Na ion exchange process that takes place spontaneously in Ca-montmorillonite suspensions containing 0.1 mol dm-3 NaCl, the c ompressibility of the Ca-montmorillonite sediment was almost the same as that of Na-montmorillonite. For monocationic systems the experiment al pressure vs distance functions exhibited a relatively good agreemen t with the swelling pressure vs distance curves calculated from the el ectrostatic forces between two similar flat plates. The elastic modulu s calculated from the experimental pressure vs distance curves increas ed with increasing compression up to the order of 10(7) N m-2, corresp onding to a glass-like material.