Organic modification of synthetic montmorillonite by long- and short-chain quaternary ammonium salt to enhance the thickening ability

Abstract

As natural montmorillonite has a very high phase transition temperature, natural organophilic montmorillonite as a thickening agent has been used to prepare high temperature grease. The thickening property of organophilic montmorillonite is influenced by montmorillonite composition and nature of the organic compounds. In order to improve the thickening ability of organophilic montmorillonite to obtain a stable and excellent lubricating performance of montmorillonite grease, synthetic montmorillonite with the uniform composition is an effective solution. Organophilic montmorillonite as a thickener usually uses a single long-chain quaternary ammonium salt to modify montmorillonite, which limits to further improve thickening ability. To address these issues, a new modification strategy was applied by selecting a very short chain quaternary ammonium salts and a long chain quaternary ammonium salts to co-modify synthetic montmorillonite. Synthetic organophilic montmorillonite was obtained through the co-intercalation of tetramethylammonium chloride (TMAC) and octadecyl trimethyl ammonium chloride (OTAC). Due to the huge difference in alkyl chains between TMAC and OTAC, the specific surface area and the number of mesopores of organophilic montmorillonite increased significantly. Molecular dynamics simulation further proved that TMAC entered the interlayer space and caused changes in the arrangement of interlayer organics. The interlayer space, organic content, and specific surface area can be effectively controlled by adjusting the molar ratio of TMAC and OTAC. The results revealed that molar ratio of TMAC and OTAC 2:8 was optimal, and the thickening ability of organophilic montmorillonite was equivalent to 5 times that of only OTAC. Tribological tests showed the grease exhibited good anti-wear ability and stable friction reduction at 180 °C. Due to the convenience of the synthetic method used and its superior rheological and tribological performance, this study may provide new insights into colloid, interface chemistry, and materials science to prepare organophilic montmorillonite.