Surface Properties of Natural Calcite Filler Treated with Stearic Acid

In order to obtain hydrophobic material, the water suspension of natural limestone with the high content of calcite (>95 %), was treated with different amounts (0.5-4 %) of stearic acid dissolved in chloroform. Thermal analysis showed that at lower initial concentrations of stearic acid (up to 2 %), surfactant molecules are chemisorbed on calcite surface, while at higher initial concentrations acid molecules are additionally physisorbed on mineral surface. XRPD analysis of starting calcite as well as two modified products with 1.5 and 3 % of stearic acid indicated that modification of calcite surface with both amounts of stearic acid, did not cause any changes in the structure of calcite mineral. It was confirmed that long chain stearic acid, in presence of water, alters the calcite surface to strongly hydrophobic, where reaction between stearic acid and Ca ions occurs. Thus, the chemisorption of stearic acid on the calcite surface is responsible for retaining the lower surfactant layer on a charged surface while hydrophobic bonding causes formation of the upper surfactant layer. The optimal amount of stearic acid needed to cover the calcite surface with a monolayer of organic molecules lies between 1.5 % and 2 %.