Melting of ice in porous glass: why water and solvents confined in small pores do not crystallize?

Abstract

The melting of ice in porous glass having different distribution of pores sizes is analyzed in details. One shows that confined water crystallizes only partially and that an interface layer, between the ice crystallites and the surface of the pore, remains liquid. Properties of this non crystalline interface at low temperature is studied by NMR and DSC. Both methods lead to an interface thickness h of the order of 0.5 nm, this explains why water do not crystallize when the dimension of confinement is less than a critical length $d^{\ast }\sim 1$ nm. The variation of the melting enthalpy per gram of total amount of water with the confinement length is explained taking into account two effects: a) the presence of this layer of water at the interface and b) the linear variation of the melting enthalpy $\Delta H_{m}$ with the melting temperature Tm. From the data of the literature one draws the same conclusions concerning other solvents in similar porous materials. Also one points out the important role of the glass temperature Tg in preventing the crystallization of the liquids confined in small pores and/or between the crystallites and the surface of the pores