Adsorption of Naphthenic Acids onto Mineral Surfaces Studied by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D)

In the present research, the adsorption of naphthenic acids from n-dodecane solutions onto hydrophilic surfaces of the most common minerals in petroleum reservoirs (silica, alumina, and calcite) has been investigated using the quartz crystal microbalance with dissipation monitoring (QCM-D). The wettability and surface roughness of the mineral surfaces applied for the QCM-D measurements were determined with contact angle and atomic force microscopy (AFM) measurements. Corrections for changes in bulk density and viscosity of the naphthenic acids solutions as a function of concentration are necessary to determine the actual adsorption levels for the QCM-D measurements and have been accounted for in the adsorption calculations. The adsorption affinity of the naphthenic acids to the mineral surfaces is ranked as calcite > alumina > silica. The adsorption data for all systems can be adequately well described by the Langmuir isotherm and adsorption free energies (Delta G degrees) calculated from the fitted Langmuir isotherms indicate that the binding between naphthenic acids and mineral surfaces is of a physical nature. Plots of change in surface area per molecule (sigma) as a function of the variation of concentration indicate that the adsorption process of the naphthenic acids changes as a function of surface coverage and suggest that the naphthenic acids chains undergo conformational alterations during the adsorption. The surface area per molecule (sigma) of the adsorbed naphthenic acids onto the mineral surfaces is calcite < alumina < silica for all concentrations, which supports the adsorption affinity ranking.