(209c) Low-Temperature Water Does Not Freeze in Nanotubes

Low-Temperature Water does not Freeze in Nanotubes

Mahdi Khademi and Muhammad Sahimi

Mork Family Department of Chemical Engineering & Materials
Science, University of Southern California, Los Angeles, California 90089-1211,
USA

There is intense interest in
studying the properties of confined water at very low temperatures below its freezing
point in the bulk. The challenge is partly motivated by water's critical
importance to biological phenomena, and the question of how microorganisms
survive at very low temperatures. The cellular structure of such microorganisms
contains nanochannels and nanopores and, thus, what happens to water in such
nanoscale structures has significant effect on the future of preserving cells
and live microorganisms. We have carried out an extensive study of water
dynamics inside a nanotube at and below the freezing temperature of water under
the bulk conditions. In particular we have evaluated various correlation
functions, such as the cage correlation (CC) function, the radial distribution
function, and the velocity autocorrelation function. The CC function follows a stretched exponential function, which
has important implications for diffusion of water in nanochannels,
conformational dynamics of proteins in ?crowded? cellular environments, and
transport in confined media, as well as the validity of the Stokes-Einstein
formula relating the viscosity to self-diffusivity. We show, based on molecular dynamics (MD) simulation, that water
molecules inside nanotubes do not follow typical freezing behavior seen in
nature under the bulk conditions. Our simulations include both carbon and
silicon-carbide nanotubes. We conclude the observed behavior is strongly
related to size of the nanotubes, rather than their chemical structure. We
argue that due to spatial limitations and steric hindrance inside small enough
nanotubes, there is not enough room for the formation of hexagonal ice and,
therefore, water inside such nanotubes attains a lower energy at temperatures
below the typical freezing point without ice formation.