Structural properties and fragile to strong transition in confined water.

We derive by computer simulation the radial distribution functions of water confined in a silica pore modeled to reproduce MCM-41. We perform the calculations in a range of temperatures from ambient to deep supercooling for the subset of water molecules that reside in the inner shell (free water) by applying the excluded volume corrections. By comparing with bulk water we find that the first shell of the oxygen-oxygen and hydrogen-hydrogen radial distribution functions is less sharp and the first minimum fills in while the oxygen-hydrogen structure does not significantly change, indicating that the free water keeps the hydrogen bond short range order. The two body excess entropy of supercooled water is calculated from the radial distribution functions. We connect the behavior of this function to the relaxation time of the same system already studied in previous simulations. We show that the two body entropy changes its behavior in coincidence with the crossover of the relaxation time from the mode coupling fragile to the strong Arrhenius regime. As for bulk water also in confinement, the two body entropy has a strict connection with the dynamical relaxation.