Facile Preparation and Tribological Properties of Water-Based Naphthalene Dicarboxylate Ionic Liquid Lubricating Additives

Taking 1,4-naphthalene dicarboxylic acid (1,4-ND), 2,3-naphthalene dicarboxylic acid (2,3-ND) and tetrabutyl ammonium hydroxide (N₄₄₄₄OH), tetrabutyl phosphonium hydroxide (P₄₄₄₄OH) as raw materials, water-based lubricant additives tetrabutyl ammonium 1,4-naphthalene dicarboxylate (1,4-NDN₄₄₄₄), tetrabutyl phosphonium 1,4-naphthalene dicarboxylate (1,4-NDP₄₄₄₄), tetrabutyl ammonium 2,3-naphthalene dicarboxylate (2,3-NDN₄₄₄₄), and tetrabutyl phosphonium 2,3-naphthalene dicarboxylate (2,3-NDP₄₄₄₄) were prepared in aqueous solution. The additives are not only simple to prepare, but also have good solubility in water. More importantly, they can significantly improve the friction reducing and anti-wear performances of water. On account of the mechanism analysis, it is estimated that the introduction of aromatic ring and more carboxyl groups to the additives play an important role for them to form stable lubrication protective films during the friction process and then to achieve excellent lubrication performance on steel/steel, copper/steel, and aluminum/steel friction pairs. Remarkably, the performances of 2,3-NDN₄₄₄₄ and 2,3-NDP₄₄₄₄ are superior to those of 1,4-NDN₄₄₄₄ and 1,4-NDP₄₄₄₄, which is ascribed to their stronger adsorption on the metal surfaces contributed by the two adsorption targets (two –COO⁻ groups). For 1,4-NDN₄₄₄₄ and 1,4-NDP₄₄₄₄, there is only one –COO⁻ group that can be simultaneously adsorbed on the metal surfaces from the perspective of space configuration. It weakens the adsorption and decreases the effectiveness of the lubrication protective films resulting in the reduced friction reducing and anti-wear properties of 1,4-NDN₄₄₄₄ and 1,4-NDP₄₄₄₄.