(698c) Mechanism of Adsorption of Non-Ionic Surfactants on Calcite

Soumik Das¹, Fardin Khabaz¹, Quoc Nguyen² and Roger T. Bonnecaze^[1]*

¹ McKetta Department of Chemical Engineering, The University of Texas at Austin, Austin, TX, 78712

²Hildebrand Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, TX, 78712

Nonionic surfactants that do not form oil-water microemulsions have been found to be effective in making oil-wet carbonates water wet while and having a relatively high oil-water interfacial tension. The mechanisms of the adsorption and wettability alteration are not understood at a molecular level. Molecular dynamics (MD) simulations are used to investigate the mechanism and configuration of adsorption of non-ionic surfactants on calcite. For the simulations, we use calcite as a representative carbonate surface. The presence of a thin intercalated water layer between surfactant and the surface indicates a weak physical interaction with calcite. Consequently, these systems exhibit very little adsorption in the monomer regime and most of the adsorption of the surfactant is predicted to occur in its micellar form. We present energy arguments in support of this theory by measuring the free-energy change associated with monomer and micellar adsorption. The energy landscape exhibits a long-range adsorption barrier and favorable near-surface local energy minimum. Simulations for different surfactants reveal the how the length of the hydrophilic tail and the nature of the hydrophobic unit affects adsorption. In addition determination of the energy of adsorption at different temperatures provides comprehensive picture of adsorption of non-ionic surfactants that is in very good qualitative agreement with experimental observations.

* rtb@che.utexas.edu