Adsorption of Biomolecules to Ice and Their Effects upon Ice Growth. 2. A Discussion of the Basic Mechanism of “Antifreeze” Phenomena

Different fish, insects, and plants have evolved a range of peptides and glycopeptides that adsorb to ice from water solution and dramatically influence ice growth in ways that benefit the organisms. These “antifreeze” biomolecules are fairly large, the smallest so far discovered having a MW of 1600, and they prevent ice crystal growth from water at values of supercooling up to several degrees Celsius. Dealing with their interaction with the ice/water interface, one is faced with the choice of treating it as an adsorption-equilibrium phenomenon, in which the surface concentration adjusts easily to the solution concentration, or as irreversible adsorption, as if the molecules are very small particles that can achieve essentially permanent, equilibrium configurations at the ice/water interface. The two viewpoints each have areas of validity and usefulness, but the particle viewpoint is argued here to be the more insightful for the problem of understanding how the antifreezes produce their effects upon ice growth and recrystallization. Small particles have been found to have size thresholds below which they migrate with a growth interface and above which they are engulfed, in experiments carried out at controlled crystal growth rates. Part 1 (Knight, C. A.; Wierzbicki, A.; Laursen, R. A.; Zhang, W. Cryst. Growth Des. 2001, 1, 429−438) presents evidence that adsorption to ice that allows the ice to grow but without engulfing the adsorbed molecules is quite common. By analogy with small particles, much of this probably represents permanent adsorption at the interface, but migration of the adsorbate with the interface as the ice grows. For adsorption−inhibition of crystal growth to be effective, a necessary property of “antifreeze” molecules is that they be unable to migrate. Accordingly, an essential property for a material to produce freezing hysteresis may be that it fits almost perfectly onto ice. A nearly perfect fit may be needed to prevent entry of water molecules into the interface, where they can exert the systematic “force of crystallization” that causes the migration. While most modeling studies have focused on the strength and type of bonding of antifreeze molecules to ice surfaces, this “snugness” of the ice−antifreeze contact may be as, or even more, important.