Freeze Concentration of Solutes at the Ice/Solution Interface Studied by Optical Interferometry

Mach−Zender optical interferometry was used to measure the solute concentration profile in solution near the ice/solution interface under quasi-steady state conditions in order to gain a quantitative understanding of freeze concentration. At low ice growth velocities, the freeze concentration behavior of a range of solutes, including sucrose, a globular protein, and some low molecular weight polymers, was accurately described by the quasi-steady state approximation to the diffusion equation. Increasing deviations from this approximation occurred at higher growth speeds and for larger macromolecules, where diffusion was presumably inhibited by chain entanglement that led to the development of significant concentration gradients parallel to the ice/solution interface. In mixtures of noninteracting solutes, the concentration profiles of the individual components could be distinguished separately. The behavior of a helical antifreeze glycopeptide could not be explained in terms of its diffusion behavior, however. Its equal amount of incorporation into the ice phase at all growth speeds led to the conclusion that it interacted with the ice interface by virtue of possessing significant crystallographic similarities with the ice lattice. The description of freeze concentration at the ice/solution interface by the quasi-steady state approximation is therefore limited to low molecular weight (nonentangled) solutes, low growth speeds, and the absence of substantial interactions between the ice phase and the solute.