Impact of Modified Seawater on Zeta Potential and Morphology of Calcite and Dolomite Aged with Stearic Acid

Zeta potential measurements and microscopic surface characterization and imaging were conducted on calcite and dolomite crystals aged in stearic acid model oil and exposed to different synthetic brines representing different potential scenarios of injected seawater from the Arabian Gulf. Calcite particles were negatively charged in deionized water and maintained negative surface charges in all tested brines, except in diluted Arabian Gulf seawater that contained higher concentration of Ca²⁺ and Mg²⁺ ions. Dolomite particles were positively charged in deionized water as well as in all tested brines, except in diluted Arabian Gulf seawater that contained four times higher concentration of SO₄^2- ions. Scanning electron microscopy and atomic force microscopy experiments on cleaved calcite and dolomite chips showed different morphological changes when both samples were aged in model oil and then treated with brines. Calcite surface dissolution was observed in addition to stearic acid deposition. Surface elemental analysis using energy-dispersive spectroscopy showed Mg²⁺ and SO₄^2- ions adsorb preferably on locations where stearic acid is deposited. The finding that stearic acid was adsorbing more strongly on dolomite than on calcite could indicate why the tested brines were less efficient to change the zeta potential of the dolomite systems. The current study concludes that manipulating the concentration of potential-determining ions present in the Arabian Gulf seawater, especially Mg²⁺ and SO₄^2- ions, will alter the surface charges of aged calcite and dolomite samples as well as their surface morphology.