Surface modification of bentonites. V. Sol-gel transitions of Ca-montmorillonite in the presence of cationic end-capped poly(ethylene oxides)

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The colloidal state of aqueous Ca2+-montmorillonite dispersions was modified with three types of cationic end-capped poly(ethylene oxides). The macromolecules were not only adsorbed at the external surfaces but were also intercalated into the dispersed montmorillonite particles. The amounts adsorbed and the basal spacings (∼1.7 nm) were comparable to the corresponding Na+-montmorillonite dispersions. The poly(ethylene oxides) protruding out of the interlayer spaces or adsorbed at the external surface determined the colloidal behaviour of the dispersions. The phase diagrams (sol-gel diagrams) of the Ca2+-montmorillonite dispersions differed from those of the corresponding Na+-montmorillonite because of the different colloidal states of both dispersions (Ca2+-montmorillonite particles vs. delaminated Na+-montmorillonite). The phase diagrams of the poly(ethylene oxide) containing Ca2+-montmorillonite dispersions showed fields of sol and flocs. Attractive gels were formed in a few cases only (TMA+-PEO 1500 and 4000, THA2+-PEO 1500, 4000 and 20,000) and with distinctly lower gel strength than in the presence of Na+ ions. On the basis of the sol-gel diagrams, conditions (type and concentration of poly(ethylene oxides), montmorillonite content) can be selected which lead to peptization of the Ca2+-montmorillonite particles into stable colloidal dispersions (sols). Addition of cationic poly(ethylene oxides) can enhance the salt tolerance up to 1000 mmol/l NaCl.
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