  Organic molecules such as steroids or amino acids form crystals that can
facilitate the formation of ice -- arguably the most important phase transition
on earth. However, the origin of the ice nucleating ability of organic crystals
is still largely unknown. Here, we combine experiments and simulations to
unravel the microscopic details of ice formation on cholesterol, a prototypical
organic crystal widely used in cryopreservation. We find that cholesterol --
which is also a substantial component of cell membranes -- is an ice nucleating
agent more potent than many inorganic substrates, including the mineral
feldspar (one of the most active ice nucleating material in the atmosphere).
Scanning electron microscopy measurements reveal a variety of morphological
features on the surfaces of cholesterol crystals: this suggests that the
topography of the surface is key to the broad range of ice nucleating activity
observed (from -4 to -20 C\degree). In addition, we show via molecular
simulations that cholesterol crystals aid the formation of ice nuclei in a
unconventional fashion. Rather than providing a template for a flat ice-like
contact layer (as found in the case of many inorganic substrates), the
flexibility of the cholesterol surface and its low density of hydrophilic
functional groups leads to the formation of molecular cages involving both
water molecules and terminal hydroxyl groups of the cholesterol surface. We
argue that the phenomenal ice nucleating activity of steroids such as
cholesterol (and potentially of many other organic crystals) is due to (i) the
ability of flexible hydrophilic surfaces to form unconventional ice-templating
structures and (ii) the different nucleation sites offered by the diverse
topography of the crystalline surfaces.
