Surface modification of bentonites: I. Betaine montmorillonites and their rheological and colloidal properties
Montmorillonite was modified by replacing the exchangeable cations with betaines (CH3)3N+ - (CH2)n - COO-M+ (n = 3, 5,7, 10) The betaine derivatives delaminated in water and formed a colloidal dispersion. Air-drying of this material yielded hard pieces which were difficult to redisperse. The dried material became redispersible in water when the Na ions (counterions to the carboxyl groups) were replaced by Li ions. Colloidal dispersions of this material were more stable against salts than Li+- or Na+-montmorillonite. Extremely high LiCl concentrations (>1 mol/l) were needed to coagulate the betaine derivatives (n>5) in the presence of diphosphate. The increased salt stability resulted from lyospheres around the silicate layers or thin packets of them which reduced the van der Waals attraction. Addition of organic solvents destabilized the dispersion by compressing the diffuse ionic layer (DLVO theory). The delaminated particles then aggregated to small flocs which settled very slowly. Neither band-type structures nor cardhouses were formed at conditions comparable to network formation and stiffening of Li- and Na-montmorillonite dispersions. Rheological measurements revealed the liquefying action of the betaines. Dispersions of butyrobetaine montmorillonite (15 g solid/l) revealed a relative viscosity (related to the dispersion medium water) rel ≊ 2. The longer chain derivatives showed a value slightly >1 whereas Li+-montmorillonite had rel = 8. Yield values were not formed at pH ≊ 7. Only at acidic conditions did the butyrobetaine montmorillonite dispersion showed a small yield value (<200 mPa).
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