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

This paper presents a structural study of a four-component clay-polymer-sal t-water system, consisting of n-butylammonium vermiculite, poly(ethylene ox ide) (PEO), n-butylammonium chloride, and heavy water, using neutron diffra ction and H/D isotope substitution of the butylammonium and PEO chains. The PEO molecules, salt ions, and water molecules are located in the interlaye r regions between parallel and regularly spaced clay platelets. The results show that the added PEO does not cause any significant alteration in the d istribution of butylammonium ions, when compared with the corresponding thr ee-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 la yer 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 e thylene oxide segments partly displace water molecules immediately adjacent to the clay surfaces. Thus, the clay surfaces are covered by, first, one l ayer of adsorbed ethylene oxide segments and water molecules, second, anoth er molecular layer of water, and, third, the layer of butylammonium ions. F rom the ordered structure around each, clay platelet, we obtain a picture o f an approximately 30 Angstrom thick dressed macroion. The polymer segments that are not adsorbed onto the clay surfaces are rather homogeneously dist ributed 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.