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The adsorption and mobility of aqueous and aqueous-hydrocarbon pore fluids between charged surfaces are fundamental questions in surface and colloid science, and are directly related to many practical industrial, particularly the petroleum industry for the oil and gas recovery. The ambient and high temperature/pressure (T/P) sedimentary basin conditions have significant impact on many interfacial processes involved in the pore fluids, such as solvation of interlayer cations by water molecules, ion exchange between different species, and the competition of surface sorption between solvated ions, water molecules and hydrocarbon species. While several prominent studies^1-3 have revealed many insights into the structure and dynamics of aqueous and aqueous-hydrocarbon fluids, the swelling mechanism of clay from dehydrated states is still an unresolved issue. More fundamental questions regarding the adsorption structures of water, ion, and hydrocarbon species near charged clay surfaces still deserve further investigations.   

In many previous studies, the dynamics of clay sheet in pore fluids is usually ignored. Most previous investigations required full constraint of the atomic positions. Our starting point in this project is to use more realistic CVFF (consistent valence force field)⁴ and more reasonable atomic charges for polar systems⁵ to focus on the adsorption structure of water and ion near charged clay surfaces. Using liquid-vapor canonical-ensemble molecular dynamics (LV-NVT-MD) simulations⁶, we first reinvestigated the density distributions of water molecules and ions near mica surfaces at very thick aqueous layers (the osmotic swelling case), and compared with the X-ray reflectivity experimental results⁷. This is a controversial issue in several previous studies. In this report, we summarize main findings based on three scenarios: (1) D = 3.0 nm water film confined between two mica surfaces, in which K⁺ ions are completely exchanged by H₃O⁺ hydroniums⁸; (2) the same water-clay system with K⁺ ions not being exchanged; and (3) based on the early speculation from hydration force experiments⁹, in which hydronium ions might penetrate into clay lattice, we assume that proton H⁺ can form flexible M-H bond, where M represents either a tetrahedral Al substitution or the -OH group below the ditrigonal cavities on the mica surface.

The most significant finding is, for the H₃O aqueous film, the locations of the first three peaks of the total O density (approximately equivalent to the total electron density in the X-ray experiment) are very close to those in the X-ray experiment⁷. In the K⁺ aqueous film, potassium ions are more diffused into inner layers, indicating that most potassium ions are fully hydrated, rather than partially hydrated observed in the early work¹⁰. In the scenario (3), in which proton H⁺ is assumed to form flexible M-H bond with either tetrahedral Al substitution or the -OH group below mica ditrigonal cavity, we find that none of them yields the water adsorbed peak and the first hydration peak. Only one oxygen peak about 0.2 nm away from the mica surface was obtained (not shown in this report). This indicates that the early experimental speculation that H⁺ ion may penetrate into mica ditrigonal cavity⁹ seems not possible.

Using the more realistic CVFF force parameters and atomic charges, we recently studied methane hydrate between K⁺-Montmorillonite surfaces. Our primary results demonstrate that methane tends to migrate to montmorillonite surface to form inner-sphere complex, while potassium ions are fully hydrated to form outer-sphere complex. These results are consistent with other studies¹, indicating that montmorillonite surfaces are more hydrophobic.

This research grant is currently fully supporting one PhD student. The mechanism of clay swelling from dehydrated state, and the impact of high T/P conditions, as well as the uptake of hydrocarbon chain molecules on the adsorption structure and dynamics of a variety of aqueous-hydrocarbon fluids will be investigated in the coming years.

 ¹  G. Sposito, N. T. Skipper, R. Sutton, et al., Proceedings of the National Academy of Sciences of the United States of America 96, 3358 (1999).

²  N. T. Skipper, P. A. Lock, J. O. Titiloye, et al., Chemical Geology 230, 182 (2006).

³  R. T. Cygan, S. Guggenheim, and A. F. K. van Groos, Journal of Physical Chemistry B 108, 15141 (2004).

⁴  H. Heinz, H. Koerner, K. L. Anderson, et al., Chemistry of Materials 17, 5658 (2005).

⁵  H. Heinz and U. W. Suter, Journal of Physical Chemistry B 108, 18341 (2004).

⁶  Y. S. Leng, Journal of Physics-Condensed Matter 20, 354017 (2008).

⁷  L. Cheng, P. Fenter, K. L. Nagy, et al., Physical Review Letters 87, 156103 (2001).

⁸  B. J. Gertner and J. T. Hynes, Faraday Discussions, 301 (1998).

⁹  R. M. Pashley, Journal of Colloid and Interface Science 83, 531 (1981).

¹⁰ Y. S. Leng and P. T. Cummings, Journal of Chemical Physics 125, 104701 (2006).