INTERMEDIATE INTERLAYER STRUCTURE IN BUTYLAMMONIUM VERMICULITES

Butylammonium vermiculite clay gels with c-axis spacings in the range 100-300 Angstrom were studied by neutron diffraction in the Q-range 0. 2-2.0 Angstrom(-1), permitting a determination of the interlayer struc ture in the intermediate range between 5 and 30 Angstrom. Differences between scattering patterns obtained from gels prepared with protonate d and deuterated butyl chains showed that the clay surfaces are covere d by a 6 Angstrom thick laver of water molecules, corresponding to two layers. The results partly validate the idea that clay swelling invol ves the formation of partially ordered water layers on the clay surfac es. The butylammonium chains sit between 11 and 14 Angstrom from the c enter of the clay platelets in a 3 Angstrom thick layer. Continuum ele ctrical theories of clay swelling can thus only be valid at distances greater than the 28 Angstrom thickness of the clay platelet plus adsor bed layers of water molecules and counterions. The possibility of usin g isotopic substitution in neutron diffraction experiments to obtain q uantitative information about ion adsorption at colloidal surfaces is discussed.