Adsorption of metal cations on mineral surfaces often controls their distribution in both natural and technological environments. The Callovo-Oxfordian (COx) formation, consisting largely of clay minerals like illite and smectite, is the location investigated in France as a site for geological nuclear waste disposal and storage. The uptake of radionuclides by layered clay minerals is the principal retention process for their diffusion. Hence, detailed molecular-scale understanding of the adsorption mechanisms of radionuclides on silicate minerals is essential, because it can significantly influence their mobility under the conditions of nuclear waste repositories. Cs+ ion is one of the important components of nuclear waste that is highly soluble in water and migrates easily in surface and sub-surface environments. The atomically smooth surface of muscovite mica, KAl2(Si3Al)O10(OH)2 is often used as an accurate model of illite clay. Experimental studies suggest that Cs+ ion adsorbs directly at the muscovite surface as an "inner sphere complex" [1]. We have investigated the structure and dynamics of Cs+ (exchanged for K+) and H2O molecules at the surface of muscovite at two different hydration levels by molecular dynamics (MD) computer simulations using fully flexible CLAYFF force field [2]. At the muscovite (001) surface, water molecules can donate 2 hydrogen bonds (to other H2O and/or to the surface O atoms) and accept 2 H-bonds (from other H2O). Water molecules can also partially replace surface cations, because their hydrogens bear some positive charge. Such surface-adsorbed H2O molecules have their negatively charged oxygen atoms exposed to the fluid phase and accessible for either H-bond acceptance from other H2Os or for their coordination of surface cations in the inner-sphere or outer-sphere configuration. Atomic density profiles of the surface species evidently support the presence of Cs+ as inner sphere complexes at the muscovite interface. Angular distributions of H2O molecular orientations with respect to the muscovite surface have also been studied, as well as the dynamical behaviour of surface species in terms of their self-diffusion coefficients, H-bonding time correlation functions, and residence times. The comparison of the surface behaviour at two different hydration states and the topological details of the interfacial H-bonding network provide new insight into the structure and dynamics of hydrated Cs+ at confined geometries. The MD simulation results are compared with available experimental data and the results of previous molecular simulations [3] to provide reliable molecular view of the hydrated Cs+ ions at the surface of illite. References [1] Kim, Y., Kirkpatrick, J.R., Cygan, R.T. Geochim. Cosmochim Acta, 60, 4059-4074 (1996). [2] Cygan, R.T., Liang, J.J., Kalinichev, A.G. J. Phys. Chem. B, 108, 1255-1266 (2004). [3] Wang, J.W., Kalinichev, A.G., Kirkpatrick, R.J., Cygan, R.T. J. Phys. Chem. B, 109, 15893-15905 (2005).