Effect of fatty acids, water composition and pH on the wettability alteration of calcite surface

Fatty acids in presence of water film alter calcite surface to oil-wet. The wettability alteration is dependent on the structure of the fatty acids, water composition and pH. Long chain fatty acid (stearic acid), strongly adsorbs onto the calcite surface from “oil phase” (n-C10) in oil/water/calcite system as indicated by contact angle measurements. On the other hand, short chain fatty acid (heptanoic acid) adsorbs on the calcite surface to a lesser extent, which agrees with “Traube's rule”.Adsorption of carboxylate anion is influenced by the ionic composition of the water, ionic strength and pH. The presence of Mg2+ and SO42− ions is shown to increase the water-wetness of the calcite. The degree of wetting is dependent on the pH. At pH < 7 in stearic acid/n-decane/water/calcite system, both ions reduce the contact angle. Higher reduction is obtained in presence of SO42− compared to Mg2+. The measured advancing contact angles are 95°, 108° at pH 5 and 88°, 82° at pH 7 in presence of SO42− and Mg2+, respectively. Further increase of the pH in the presence of Mg2+ causes more reduction of contact angle (68° at pH 10); while in the presence of SO42− ions an inflection point occurred where the advancing contact angle increased to 100°. In other words, the calcite surface became slightly more oil-wet.