HIGH-RESOLUTION STRUCTURAL STUDY OF AN ELECTRICAL DOUBLE-LAYER BY NEUTRON-DIFFRACTION

We present the first high-resolution study of the aqueous structure de veloped in the electrical double layer region of a colloidal clay syst em. Our samples were highly oriented vermiculite gels containing isoto pically labelled propylammonium counterions: C3H7NH3+ and C3D7NH3+ The gels were prepared with a clay layer spacing of 43.6 Angstrom. Time-o f-flight neutron diffraction was used to measure the scattering intens ity normal to the clay surfaces, from which the neutron scattering den sity was refined by an inverse Monte Carlo method. Diffraction data fr om two isotopically distinct samples were fitted simultaneously, there by allowing us to locate the labelled propyl groups unambiguously. Our results show that the counterions in our vermiculite gels are separat ed from the clay surfaces by two layers of partially ordered water mol ecules. In fact, the counterion density reaches a maximum at the cente r of the interlayer region. The measured structure is therefore at odd s with traditional primitive models of the electrical double layer. Su ch models place the maximum of the counterion density adjacent to the vermiculite clay surfaces, in the so-called Stern layer.