The forces induced between two molecularly smooth solid surfaces when they approach each other to nanometer-scale distances and then subjected to shear in an aqueous electrolyte environment are ubiquitous in many diverse systems, and the underlying mechanism is at the heart of numerous proposed technologies. These forces, in the form of either hydrophilic or hydrophobic interactions, strongly depend on the role of nanoconfined water and the chemistry of surfaces, which are universally thought to be important to determine the dynamic properties of the related systems, such as the stability of colloid dispersions, the swelling of clays, nanofluidity in ion channels and through crowded intracellular environment, friction and lubrication in micro/nano electro-mechanical systems (M/NEMS), and the role of biological membranes in protein folding. Here, based on our previous work1,2 and as a first attempt, we have developed a specific molecular dynamics (MD) ensemble which enabling the confined system to connect to the bulk and then performed MD simulations to study the normal approach process ¾ the mechanism of hydration force. We investigate the squeeze-out mechanism of water molecules, the structure change of hydration layer and phase transition mechanism from liquid phase to bilayer ice2, and the normal spring force variations during the approach. Beginning from D = 2.44 nm water film, very strong repulsive hydration force has been observed between the two negative charged mica surfaces. The force oscillation is compared with recent and early surface force balance (SFB) experimental results3.

1 Y. S. Leng and P. T. Cummings, Physical Review Letters 94 (2), 26101 (2005). 2 Y. S. Leng and P. T. Cummings, Journal of Chemical Physics 124, 74711 (2006); Y. S. Leng and P. T. Cummings, Journal of Chemical Physics (submitted) (2006). 3 J. N. Israelachvili and R. M. Pashley, Nature 306 (5940), 249 (1983); U. Raviv and J. Klein, Science 297 (5586), 1540 (2002).