Reaction Coordinate for Ice Crystallization on a Soft Surface.

The control of assembly and crystallization of molecules is becoming increasingly important in chemistry, engineering, and materials sciences. Crystallization is also central to understand natural processes that include the formation of atmospheric ice and biomineralization. Organic surfaces, biomolecules, and even liquid/vapor interfaces can promote the nucleation of crystals. These soft surfaces present significant structural fluctuations, which have been shown to strongly impact the rate of crystallization. This raises the question of whether degrees of freedom of soft surfaces play a role in the reaction coordinate for crystal nucleation. Here we use molecular simulations to investigate the mechanism of ice nucleation promoted by an alcohol monolayer. Our analysis indicates that while the flexibility of the surface strongly depresses its ice nucleation ability, it does not play a role in the coordinate that controls the transformation from liquid to ice. We find that the variable that drives the transformation is the size of the crystalline cluster, the same as that for the homogeneous crystallization. We argue that this is a general result that arises from the separation of time scales between surface fluctuations and the crossing of the transition state barrier for crystallization.