Molecular structure and dynamics of nano-confined aqueous solutions: Computer simulations of clay, cement, and polymer membranes

Molecular-scale knowledge of the thermodynamic, structural, and transport properties of water confined by interfaces and nano-pores of various materials is crucial for quantitative understanding and prediction of many natural and technological processes, including carbon sequestration, water desalination, nuclear waste storage, cement chemistry, fuel cell technology, etc. Experimental nanoscale studies of such systems are not always feasible, and their results often require considerable interpretation in the efforts to extract surface- and confinement-specific quantitative information from the measurements. Computational molecular modeling significantly complements such efforts and provides invaluable molecular-scale background for better understanding of the specific effects of the substrate structure and composition on the structure, dynamics and reactivity of interfacial and nano-confined aqueous solutions. Based on the successful development and implementation of the CLAYFF force field (Cygan et al., 2004), we have recently performed a series of molecular dynamics simulations of aqueous interfaces with several representative inorganic and organic nanoporous materials (Ahn et al., 2008; Kalinichev et al., 2007, 2010; Wang et al., 2006, 2009) in order to better understand and quantify the effects of the substrate composition and structure on the properties of interfacial and nano-confined aqueous solutions. Individual H2O molecules and hydrated ions at interfaces simultaneously participate in several dynamic processes, which can be characterized by different, but equally important time- and length- scales. The first molecular layer of interfacial water at all substrates is often highly ordered, indicating reduced translational and orientational mobility of the H2O molecules. However, this ordering can not be simply described as "ice-like", but rather resembles the behavior of supercooled water or amorphous ice, although with significant substrate-specific variations. These results help to interpret experimental NMR, IR, X-ray, and neutron scattering measurements performed for the same systems. Ahn, W.-Y., Kalinichev, A.G., Clark, M.M. (2008) Effects of background cations on the fouling of polyethersulfone membranes by natural organic matter: Experimental and molecular modeling study. J.Membr.Sci., 309,128-140. Cygan R.T., Liang J.-J., Kalinichev A.G. (2004) Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. Journal of Physical Chemistry B, 108, 1255-1266. Kalinichev A.G., Wang, J., Kirkpatrick R. J. (2007) Molecular dynamics modeling of the structure, dynamics and energetics of mineral-water interfaces: Application to cement materials. Cement and Concrete Res., 37, 337-347. Kalinichev, A.G., Kumar, P., Kirkpatrick, R.J. (2010) Effects of hydrogen bonding on the properties of layered double hydroxides intercalated with organic acids: MDF computer simulations. Philos. Mag., 90, 2475-2488. Wang J., Kalinichev A.G., Kirkpatrick R.J. (2006) Effects of substrate structure and composition on the structure, dynamics and energetics of water on mineral surfaces: MD modeling study. Geochim. Cosmochim. Acta, 70, 562-582. Wang J., Kalinichev A.G., Kirkpatrick R.J. (2009) Asymmetric hydrogen bonding and orientational ordering of water at hydrophobic and hydrophilic surfaces: A comparison of water/vapor, water/talc, and water/mica interfaces. J.Phys.Chem.C, 113, 11077-11085