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.