Surface modification of bentonites: I. Betaine montmorillonites and their rheological and colloidal properties

Montmorillonite was modified by replacing the exchangeable cations with bet aines (CH3)(3)N+ - (CH2)(n) - COO-M+ (n = 3, 5, 7, 10) The betaine derivatives delaminated in water and formed a colloidal dispers ion. Air-drying of this material yielded hard pieces which were difficult t o redisperse. The dried material became redispersible in water when the Na ions (counterions to the carboxyl groups) were replaced by Li ions. Colloid al dispersions of this material were more stable against salts than Li+- or Na+-montmorillonite. Extremely high LiCl concentrations (>1 mol/l) were ne eded to coagulate the betaine derivatives (n>5) in the presence of diphosph ate. The increased salt stability resulted from lyospheres around the silic ate layers or thin packets of them which reduced the van der Waals attracti on. Addition of organic solvents destabilized the dispersion by compressing the diffuse ionic layer (DLVO theory). The delaminated particles then aggr egated to small flocs which settled very slowly. Neither band-type structur es nor cardhouses were formed at conditions comparable to network formation and stiffening of Li- and Na-montmorillonite dispersions. Rheological meas urements revealed the liquefying action of the betaines. Dispersions of but yrobetaine montmorillonite (15 g solid/l) revealed a relative viscosity (re lated to the dispersion medium water) eta(rel) approximate to 2. The longer chain derivatives showed a value slightly >1 whereas Li+-montmorillonite h ad eta(rel) = 8. Yield Values were not formed at pH approximate to 7. Only at acidic conditions did the butyrobetaine montmorillonite dispersion showe d a small yield value (<200 mPa).