Interlayer structure of a clay-polymer-salt-water system

This paper presents a structural study of a four-component clay-polymer-salt-water system, consisting of n-butylammonium vermiculite, poly(ethylene oxide) (PEO), n-butylammonium chloride, and heavy water, using neutron diffraction and H/D isotope substitution of the butylammonium and PEO chains. The PEO molecules, salt ions, and water molecules are located in the interlayer regions between parallel and regularly spaced clay platelets. The results show that the added PEO does not cause any significant alteration in the distribution of butylammonium ions, when compared with the corresponding three-component system without added PEG. As in the three-component system, a major part of the butylammonium ions are located in a 4-5 Angstrom thick layer at a distance of 12-16 Angstrom from the center of the clay platelets. Rather than affecting the location of the butylammonium ions, some of the ethylene oxide segments partly displace water molecules immediately adjacent to the clay surfaces. Thus, the clay surfaces are covered by, first, one layer of adsorbed ethylene oxide segments and water molecules, second, another molecular layer of water, and, third, the layer of butylammonium ions. From the ordered structure around each, clay platelet, we obtain a picture of an approximately 30 Angstrom thick dressed macroion. The polymer segments that are not adsorbed onto the clay surfaces are rather homogeneously distributed in the interlayer region, at least in the present case with a high molecular weight PEG. Each polymer molecule adsorbs onto both clay surfaces and thereby induces a reduction of the interlayer spacing by a phenomenon known as polymer bridging flocculation.