THE EFFECT OF UNIAXIAL-STRESS ON THE SWELLING OF N-BUTYLAMMONIUM VERMICULITE

The swelling of an n-butylammonium vermiculite in solutions of n-butylammonium chloride has been studied by neutron diffraction as a fraction of uniaxial stress, P, and the concentration of the soaking solution, C(ex). The swollen phase was studied in a range of concentrations of the external solution of between 0.2 M and 10(-3) M, for which the zero stress c-axis spacings, L(min), were 80 angstrom and 550 angstrom respectively. The reduction of these separations application of different stresses to the gels was determined in eight different concentrations, up to uniaxial pressures of 1 atm. These are direct measurements of the swelling pressure, P, as a function of the interlayer distance, L. At each concentration a plot of ln P vs. L is linear to the limit of the experimental accuracy. The negative gradients of these plots, kappa-in, are not equal to the kappa values in the external soaking solution, kappa-ex. The relation kappa-in x (min) = 9 is found to be approximately valid for all concentrations studied. The intercepts of the plots are used to calculate the surface potential psi-0. psi-0 is approximately constant across the whole concentration range studied, its average value being equal to 70 m V.