COMPRESSION STUDY ON LAMELLAR PARTICLES DISPERSED IN ELECTROLYTE-SOLUTIONS

The behaviour of hydrophilic layered silicate (sodium, hydrogen and calcium montmorillonite) particles dispersed in dilute sodium chloride solutions was studied in a centrifugal field. The compressibility of the sediments and the reversibility of deformation were measured in compressive-decompressive cycles over the rotation rate range 2000-60000 rev min-1. Series of excess pressure and volume fraction data relating to the same state of dispersion were calculated. Because of the flat-to-flat ordered structure, a volume fraction as high as 0.81 was attained at the highest pressure. Small hysteresis loops were observed. Distances between lamellae and basal spacings were calculated from the volume fraction data of the solid components. The basal spacing data at the highest compression closely approximated the basal spacings obtained from X-ray diffraction measurements on solid samples of different monocationic montmorillonites. The structure of the interlayer space could collapse to one layer of water when it contained hydrogen and sodium ions, and to two layers of water when it contained calcium ions. Owing to the Ca(s)/Na ion exchange process that takes place spontaneously in Ca-montmorillonite suspensions containing 0.1 mol dm-3 NaCl, the compressibility of the Ca-montmorillonite sediment was almost the same as that of Na-montmorillonite. For monocationic systems the experimental pressure vs distance functions exhibited a relatively good agreement with the swelling pressure vs distance curves calculated from the electrostatic forces between two similar flat plates. The elastic modulus calculated from the experimental pressure vs distance curves increased with increasing compression up to the order of 10(7) N m-2, corresponding to a glass-like material.