UNIAXIAL-STRESS AND SOL CONCENTRATION-DEPENDENCE OF THE STRUCTURE OF A DRESSED MACROION IN A DILUTE ELECTROLYTE SOLUTION

Vermiculite clay gels in 0.03 M butylammonium chloride solution were s tudied by neutron diffraction in the a range 0.02-2.2 Angstrom(-1), pe rmitting a determination of both the distance between the clay platele ts (the d spacing) and the intermediate-range structure of the interla yer solution. The structure was investigated as a function of the frac tion of the clay in the condensed matter system (r) and the applied un iaxial pressure along the swelling axis (p). The d spacing of about 17 0 Angstrom at zero pressure and r similar to 0.01 reduces dramatically with both increasing pressure and increasing r value without changing the interlayer structure significantly. For r = 0.39 a remarkably low d value of about 50 Angstrom was obtained for a gel in equilibrium wi th a crystalline phase. The existence of such an equilibrium is a coun terexample to DLVO theory but well accounted for by Sogami theory. Ind ependent of the pressure, r value, and salt concentration, the clay su rfaces are covered by an about 6 Angstrom thick layer of water molecul es, and the majority of the butylammonium chains sit outside this wate r layer and form an approximately 4-5 Angstrom thick layer located 12- 16 Angstrom from the center of the clay platelets, giving us a picture of a dressed macroion. Continuum electrical theories of clay swelling can thus only be valid at distances greater than the 35-40 Angstrom t hickness of the clay platelet plus adsorbed layers of water molecules and counterions. Further, the results suggest that 35 Angstrom is abou t the lowest possible d spacing that can be achieved for the gel phase by either increasing the uniaxial pressure toward infinity or using a sufficiently high sol concentration.