The Rate of Water Equilibration in Vapor-Diffusion Crystallizations: Dependence on the Distance from the Droplet to the Reservoir

The rate of water equilibration in hanging-drop vapor-diffusion experiments was studied as a function of the distance separating the hanging drop from the surface of the reservoir solution. Hanging drops of 1.00 M NaCl were allowed to partially equilibrate with reservoirs of 2.00 M NaCl at room temperature. Over the range of droplet-reservoir distances examined, 7.6-119.4 mm, the larger the distance that separated the droplet and reservoir, the slower the droplet equilibrated with the reservoir. The variation of the rate of equilibration with droplet-reservoir distance was non-linear; the rate was most sensitive to variations in the droplet-reservoir separation at short separations. A mathematical model of the equilibration kinetics was developed that fits the experimental data. The model is based on the assumption that the rate-limiting step in vapor-diffusion equilibration is transit of water across the vapor space. A simple device to vary the rate of water equilibration, and thereby optimize macromolecular crystal growth conditions, is described.