Freezing in porous materials: the view from the nanoscale

Freezing in porous materials is of fundamental interest in understanding the effect of confinement, reduced dimension, and surface forces on the thermodynamics of liquids and solids. Freezing of liquids confined at the nanoscale is also relevant to applications involving lubrication, nanotribology, and fabrication of nanomaterials. Experimental and theoretical works on simple fluids and pore geometries have shown that the freezing temperature Tf is as a function of the reduced pore size H* = H/ (H is the pore width and  the size of the adsorbate molecule) and the ratio  of the wall/fluid (wf) to the fluid/fluid (ff) interactions. Tf is decreased compared to the bulk for  < 1, while it is increased for  > 1 (for review, see Coasne et al. Chem. Soc. Rev. 2013).

This simple picture fails to account for many experiments and molecular simulations on freezing in pores. For instance, owing to its specific nature, water departs from what is expected for simple fluids in pores of an ideal geometry; water crystallizes in hydrophobic pores while it remains liquid in hydrophilic pores.

In this work, I will first present molecular simulations on water crystallization in pores. I will show that water crystallization at the pore surface is suppressed because the number of hydrogen bonds formed is insufficient (even if hydrogen bonding with the host solid is considered), while crystallization in the pore center, unless for large pores, is hindered because curvature prevents the formation of a network of tetrahedrally coordinated molecules.

I will also present some results in which it was shown that, even for simple liquids, crystallization in disordered pores of a size smaller than 2 nm is suppressed because of surface disorder. At low temperatures, the confined frozen phase in such systems consists of a mixture of crystalline clusters and amorphous (liquid or solid) patches. We will see that for large pores crystallization in rough carbon pores, which is only partial as it is spatially restricted to the pore center, occurs through homogeneous nucleation.

These results shed light on solidification in pores by showing that there is a crossover between surface-induced and homogeneous crystallization upon increasing the surface disorder of the host material.