Abstract: The effects of electrolytes (NaCl, Na₂SO₄ and MgCl₂) and cationic surfactant (cetyltrimethylammonium bromide) on colloidal properties of mixed metal hydroxide (MMH)/montmorillonite (Mt) suspension were studied through measurements of electrophoretic mobilities, rheology and pH. The results show that all the electrolytes and surfactant studied in this work decreased Bingham yield point (YP) of MMH/Mt system, but exerted different effects on fluid loss (FL) and pHof MMH/Mt system. The results are interpreted as means that the cationic ions of electrolytes mainly displace the charge-balancing ions of Mt particles, such as H⁺ and Na⁺, so enhance their aggregation, leading to the decrease of pH and the increase of FL, but the anionic ions of these electrolytes mainly displace the charge-balancing ions of MMH particles, such as OH^- and Cl^-, so weaken the aggregation effects of MMH on Mt particles, therefore, cause the increase of pH and the decrease of FL. In another word, the overall effects of electrolytes on MMH/Mt system depend on the relative ability of anionic and cationic ions to exert the opposite effects on FL and pH. As for cetyltrimethylammonium bromide (CTAB), the adsorption of CTAB on Mt particles does not only displace H⁺ and cause the aggregation of Mt particles but also reverses their wetting ability, therefore, CTABdecrease FL and YP of MMH/Mt system to a larger extent than inorganic electrolytes although it can only decrease pH slightly.

Key words: Al-Mg mixed metal hydroxide, Na-montmorillonite, Rheology