Hydrogen bonding in water nanoconfined between graphene surfaces: a molecular dynamics simulation study

Molecular dynamics simulations are done to study the structure and dynamics of hydrogen bonds (HBs) in water, nanoconfined between parallel graphene surfaces, at a constant parallel component of pressure, 101.3 kPa, and at constant temperatures, ranging from 300 to 390 K. The results indicate that layering of water molecules beside the surfaces strongly influences the structure of HBs. Very close to the surfaces, due to the geometrical restrictions, the hydrogen atoms of water preferentially orient toward the surfaces, and hence, scarify their HB network. The number of HBs per donor, compared to the corresponding bulk value, is reduced in the organized water layers beside the surfaces. In contrary, their number is increased at distances corresponding to the density profile minima, due to the formation of HBs between donors and acceptors in the neighboring organized layers. An analysis of the temperature dependence of the number of HBs shows that the HBs closer to the surfaces are weaker than those in the bulk water. Besides, the entropy change for HB breakage in the pore is lower than that for the bulk water. The short-time behavior of HB dynamics, with a characteristic time a parts per thousand 0.1 ps, is shown to be insensitive to the surface proximity. However, an examination of the long-time relaxation of HBs indicates that HBs closer to the surface relax slower than those in the bulk. Such a slower relaxation is shown to be connected to the slower diffusive motion of water, depending on the surface proximity. Consequently, the rate constants for forward (HB breakage) and backward (HB formation) reactions in the pore are smaller than the corresponding bulk values. In all cases, the effect of surfaces is shown to extend up to a parts per thousand 1.0 nm (interphase thickness) from the surfaces.