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.