The effect of chemisorbed and physisorbed water on the interfacial structure and dynamics in silica-filled polydimethylsiloxane (PDMS) based composites have been investigated. Toward this end, we have combined molecular dynamics simulations and experimental studies employing dynamic mechanical analysis and nuclear magnetic resonance analysis. Our results suggest that the polymer-silica contact distance and the mobility of interfacial polymer chains significantly decreased as the hydration level at the interface was reduced. The reduced mobility of the PDMS chains in the interfacial domain reduced the overall, bulk, motional properties of the polymer, thus causing an effective 'stiffening' of the polymer matrix. The role of the long-ranged Coulombic interactions on the structural features and chain dynamics of the polymer were also examined. Both are found to be strongly influenced by the electrostatic interactions as identified by the bond orientation time correlation function, local density distribution and radial distribution functions. These results have important implications for the design and life performance behavior of nanocomposite silica-siloxane materials. (C) 2004 Elsevier Ltd. All rights reserved.