Ice-liquid equilibrium meets nano-confinement

ESTEFANA GONZALEZ SOLVEYRA; EZEQUIEL DE LA LLAVE; GALO J. A. A. SOLER-ILLIA; MOLINERO, VALERIA; DAMIAN A. SCHERLIS PEREL

Holderness, New Hamshire

Conferencia; Gordon Research Conference on Water and Aqueous Solutions; 2012

Gordon Research Conference

In the present work, we use large-scale molecular dynamics simulations with the mW water model [1] to investigate the freezing, melting, and structure of water and ice in 2-4 nm diameter cylindrical nanopores. The use of hydrophilic silica-like and hydrophobic silica-like walls as well as CNTs with different pore diameters and pore fillings, allows us to address whether there is a correlation between the water-wall interaction, the wall roughness, the pore size and the pore filling, with the structure of water and ice in the pores and the temperature and enthalpy of melting and freezing of the confined water, with a specific focus on how water nucleates and grows ice in these nano-systems, and what is the structure of the crystallized water in the pores. Experiments and simulations results show that hydrophobic and hydrophilic nanopores with an atomically rough wall (e.g. silica and functionalized silica) result in the formation of ice I. [2,3] Above the melting temperature, these silica-like nanopores, both partially and completely filled, contain two water phases in coexistence: a condensed liquid plug and a surface-adsorbed phase. As the temperature is lowered, only the liquid plug crystallizes, producing ice I with stacks of hexagonal and cubic layers. The confined ice is wetted by a premelted liquid layer that persists in equilibrium with ice down to temperatures well below its melting point. [4] Nucleation in these pores is not facilitated by the surface and proceeds in an homogenous manner.[3] While liquid water in the rugged pores is not layered, water in the CNT is layered even at room temperature. On cooling, these layering results in the formation of multiwall ice structures.[5] We observe that the ?onion? ice/carbon interface is not mediated by a disordered layer, in contrast to the premelted (quasiliquid) layer existing in rugged hydrophilic and hydrophobic pores. We determine the temperature and enthalpy of melting as a function of the filling fraction and size of the pore. In agreement with experiments, we find that the melting temperature of the nanoconfined ice is strongly depressed with respect to the bulk Tm, it depends weakly on the filling fraction and is insensitive to the hydrophobicity of the pore wall. The enthalpy of melting ?when normalized by the actual amount of ice in the pore ? was found indistinguishable for the hydrophobic and hydrophilic pores, insensitive to the filling fraction and, within the error bars, the same as the difference in enthalpy between bulk liquid and bulk ice evaluated at the temperature of melting of ice in the nanopores. [4] [1]Molinero, V.; Moore, E. B. J Phys Chem B 2009, 113, 4008. [2]Jähnert, S.; Vaca Chávez, F.; Schaumann, G. E.; Schreiber, A.; Schönhoff, M.; Findenegg, G. H. In Physical Chemistry Chemical Physics 2008; Vol. 10, p 6039. [3]Moore, E. B.; De La Llave, E.; Welke, K.; Scherlis, D. A.; Molinero, V. In Physical Chemistry Chemical Physics 2010; Vol. 12, p 4124. [4]González Solveyra, E.; De La Llave?, E. In The Journal of ? 2011. [5]Bai, J.; Wang?, J. In Proceedings of the National ? 2006.