
We study, by simulations and analytic approach, the behaviour of water at the
interface with proteins or in hydrophobic nanoconfinement. For water molecules
adsorbed on the protein surface, we calculate the temperature dependence of
the relaxation time of the dynamics of the hydrogen bond (HB) network, finding
two dynamic crossovers, (i) at approximately 252 K and (ii) at approximately
181 K. We show how the two crossovers relate to the presence of two specific
heat maxima. The first is caused by fluctuations in the HB formation, and the
second, at a lower temperature, is due to the cooperative reordering of the HB
network [1]. For water between hydrophobic walls, at nanoscopic separation, we
study how the diffusion constant parallel to the walls depends on the
microscopic structure of water. At low temperature, water diffusion can be
associated with the number of defects in the hydrogen bond network [2].
However, the number of defects solely does not account for the peculiar
diffusion of water, with maxima and minima along isotherms. We calculate a
relation that quantitatively reproduces the behavior of diffusivity, focusing
on the high-temperature regime. We clarify how the interplay between breaking
of hydrogen bonds and cooperative rearranging regions of 1-nm size gives rise
to the diffusion extrema in nanoconfined water and offers a possible
explanation for the ultrafast transport of water in nanochannels with size
smaller than 1 nm [3]. We finally find a dramatic decrease of compressibility,
thermal expansion coefficient, and specific heat for water confined in
disordered nanochannels [4].
References
1) M. G. Mazza, K. Stokely, S. E. Pagnotta, F. Bruni, H. E. Stanley, and G.
Franzese, Proc. Natl. Acad. Sci. USA 108, 19873 (2011).
2) F. de los Santos, and G. Franzese, J. Phys. Chem. B 115, 14311 (2011).
3) F. de los Santos, and G. Franzese, Phys. Rev. E 85, 010602(R) (2012).
4) E. G. Strekalova, M. G. Mazza, H. E. Stanley, G. Franzese, Phys. Rev. Lett.
106, 145701 (2011).