134 Fundamentals of ice formation during rapid cooling and warming

Abstract

We have examined ice formation in aqueous solutions containing common cryoprotectants during cooling and warming at rates from ∼10 K/s to >10,000 K/s. For all solutes tested, the critical cooling rate below which ice forms varies exponentially with solute concentration. The exponential’s characteristic concentration for each solute correlates with the solute’s Stokes radius. We have developed a simple statistical mechanical extension of classical nucleation theory that assumes solutes must be excluded from the critical nucleus. This theory provides an excellent account of our data, and relates the characteristic concentration for each solute to the solute radius and the critical nucleation radius of ice in pure water. This microscopic theory of glass formation in the presence of solutes has consequences for cryobiology, for cloud physics, and for general glass forming systems. Ice formation on warming is of comparable or greater importance to ice formation on cooling in determining survival of cryopreserved samples. Measurements of solutions of several common cryoprotectants show that the critical warming rates below which ice transiently forms on warming are typically one to three orders of magnitude larger than critical cooling rates. Critical warming rates also depend upon cooling rates, due to the presence of small ice fractions in nominally vitrified samples. Current phenomenological models of ice nucleation and growth with current best estimates for their input parameters derived from data at large concentrations and small cooling rates overestimate both critical warming rates and critical cooling rates by orders of magnitude in the low concentration, high rate regime. In vitrification protocols, minimizing concentrations of potentially damaging cryoprotectants while minimizing ice formation thus requires ultrafast warming rates as well as fast cooling rates to minimize the required warming rates. Source of funding: NIH, NSF. Conflict of interest: None declared. ret6@cornell.edu