SOLVENT EVAPORATION RATES IN THE CLOSED CAPILLARY VAPOR DIFFUSION METHOD OF PROTEIN CRYSTAL-GROWTH

The rate of transport of water vapor from the evaporator end to the condenser end through the intervening air space in closed protein crystal growth capillaries of the Plaas-Link design has been analyzed both theoretically and experimentally. The theory is based upon the assumptions that (1) the concentration of the precipitating agent exceeds that of all other solutes and (2) the process determining the rate of approach to equilibrium is the diffusion of water vapor through the air space. According to the theory, the rate of equilibration depends upon ten parameters. These include the absolute temperature, the vapor pressure of pure water, the vapor pressure lowering coefficient of the precipitating agent, the diffusion coefficient of water vapor through air, the length of the air space, the cross sectional area of the capillary, the total numbers of moles of water and of precipitant, and also their differences at the beginning of the experiment. Equilibration data taken with aqueous ammonium sulfate in tubes oriented with the evaporation flux both parallel and antiparallel the ambient gravity are essentially identical and in quantitative agreement with the theory.