COLL 488 |
| Peter G. Vekilov, Oleg Galkin, and Mrinal Shah. Department of Chemical Engineering, University of Houston, Houston, TX 77204-4004 |
| A homogeneous phase of off-critical composition quenched below its limit of stability may be either metastable, i.e., the generation of a new phase is delayed by a free energy barrier, or, at deep quenches, unstable. The structure and dynamics of phases emerging upon crossing the metastability boundary have seen theoretical controversy, yet experimental evidence is scarce. We monitor in real time and in real space the generation of a dense liquid phase in high-concentration solutions of the protein lysozyme, a poor fit to mean-field assumptions, after temperature T quenches into thermodynamically-defined metastable and unstable regions. We show that the evolution of the structure factor during nucleation is similar to that during spinodal decomposition and reveals no peculiarity predicted upon crossing the metastability boundary. We also show that the wavelength of fastest new-phase growth in the metastable regime has evolution different from than in the unstable regime, and the response of the new-phase growth rate to increasing undercooling DT changes sign at a certain high DT. Kinetic definitions of the metastability/instability boundary using these two facts yield values within ~ 1.5 K, i.e., the boundary appears as an area rather than a line, which is near and above the thermodynamic prediction. Delay times for the appearance of the new phase in the unstable regime are significant, i.e., new-phase growth is hindered by kinetic barriers. While our results agree with predictions of the non-mean-field theories of phase transformations, the experimentally observed behaviour is richer than the one envisioned by theory. |
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Symposium in Memory of Arthur W. Adamson
8:30 AM-11:50 AM, Thursday, April 1, 2004 Marriott -- Orange County 4, Oral
Division of Colloid and Surface Chemistry |