Understanding Calcite Wettability Alteration through Surface Potential Measurements and Molecular Simulations

Mineral wettability and wettability alteration are important factors that determine the distribution and mobility of oil during the recovery process. Because wettability is dependent on many factors (e.g., hydrocarbon composition, mineralogy, and pH), predicting mineral wettability is often difficult. The goal of this study is to look at changes that occur on the mineral itself, specifically changes in the surface structure and surface potential, using experimental methods complemented by quantum-mechanical calculations to better understand the molecular-level processes involved in wettability alteration. Nanoscale surface imaging is combined with Kelvin probe force microscopy (KPFM) to characterize changes in topography and surface potential for water-wet (hydrophilic) and oil-wet (hydrophobic) calcite surfaces, using the surfactant hexamethyl­disilazane (NHSi₂(CH₃)₆, HMDS) to render the calcite surface oil-wet. KPFM measurements show that HMDS adsorbs preferentially on step edges of the calcite surface and is coupled by an increase in surface potential, which suggests a decrease in electron density in the valence band wherever HMDS is adsorbed. Density functional theory (DFT) calculations of HMDS adsorption on calcite confirm an increase in the surface potential of oil-wet calcite and show that Ca corner sites are associated with the most favorable HMDS adsorption energies. Coadsorption of H⁺ and OH^– with HMDS is more likely to occur at edges and Ca kink sites and indicates that this surfactant may be an effective wettability modifier at a range of pH conditions. This study is the first application of KPFM to mineral wettability and demonstrates that with further development KPFM can be a powerful tool to study interactions between specific functional groups and surface sites modifying the surface’s electronic structure and wettability

Three New Silver Uranyl Diphosphonates: Structures and Properties

The hydrothermal reaction of uranium trioxide and methylenediphosphonic acid in the presence of silver nitrate resulted in the formation of three new uranyl coordination polymers: AgUO₂[CH₂(PO₃)­(PO₃H)] (<b>Ag-1</b>), [Ag₂(H₂O)_1.5]­{(UO₂)₂[CH₂(PO₃)₂]­F₂}·(H₂O)_0.5 (<b>Ag-2</b>), and Ag₂UO₂[CH₂(PO₃)₂] (<b>Ag-3</b>). All consist of uranyl pentagonal bipyramids that form two-dimensional layered structures. <b>Ag-1</b> and <b>Ag-3</b> possess the same structural building unit, but the structures are different; <b>Ag-3</b> is formed through edge-sharing of F atoms to form UO₅F₂ dimers. The pH and silver cation have significant effects on the structure that is synthesized. Raman spectra of single crystals of <b>Ag-1</b>, <b>Ag-2</b>, and <b>Ag-3</b> reveal <i>v</i>₁ UO₂²⁺ symmetric stretches of 816 and 829, 822, and 802 cm^–1, respectively. Electronic structure calculations were performed using the projector augmented wave (PAW) method with density functional theory (DFT) to gain insight into the nature of bonding and electronic characteristics of the synthesized compounds. Herein, we report the syntheses, crystal structures, Raman spectroscopy, and luminescent behavior of these three compounds

Electrochemical and Spectroscopic Evidence on the One-Electron Reduction of U(VI) to U(V) on Magnetite

Reduction of U­(VI) to U­(IV) on mineral surfaces is often considered a one-step two-electron process. However, stabilized U­(V), with no evidence of U­(IV), found in recent studies indicates U­(VI) can undergo a one-electron reduction to U­(V) without further progression to U­(IV). We investigated reduction pathways of uranium by reducing U­(VI) electrochemically on a magnetite electrode at pH 3.4. Cyclic voltammetry confirms the one-electron reduction of U­(VI) to U­(V). Formation of nanosize uranium precipitates on the magnetite surface at reducing potentials and dissolution of the solids at oxidizing potentials are observed by in situ electrochemical atomic force microscopy. XPS analysis of the magnetite electrodes polarized in uranium solutions at voltages from −0.1 to −0.9 V (E⁰_U(VI)/U(V)= −0.135 V vs Ag/AgCl) show the presence of only U­(V) and U­(VI). The sample with the highest U­(V)/U­(VI) ratio was prepared at −0.7 V, where the longest average U–O_axial distance of 2.05 ± 0.01 Å was evident in the same sample revealed by extended X-ray absorption fine structure analysis. The results demonstrate that the electrochemical reduction of U­(VI) on magnetite only yields U­(V), even at a potential of −0.9 V, which favors the one-electron reduction mechanism. U­(V) does not disproportionate but stabilizes on magnetite through precipitation of mixed-valence state U­(V)/U­(VI) solids