The effect of hydrogen bonding on the diffusion of water in n-alkanesand n-alcohols measured with a novel single microdroplet method

摘要

While the Stokes–Einstein (SE) equation predicts that the diffusion coefficient of a solute will beinversely proportional to the viscosity of the solvent, this relation is commonly known to fail forsolutes, which are the same size or smaller than the solvent. Multiple researchers have reported thatfor small solutes, the diffusion coefficient is inversely proportional to the viscosity to a fractionalpower, and that solutes actually diffuse faster than SE predicts. For other solvent systems, attractivesolute-solvent interactions, such as hydrogen bonding, are known to retard the diffusion of a solute.Some researchers have interpreted the slower diffusion due to hydrogen bonding as resulting fromthe effective diffusion of a larger complex of a solute and solvent molecules. We have developed andused a novel micropipette technique, which can form and hold a single microdroplet of water whileit dissolves in a diffusion controlled environment into the solvent. This method has been used toexamine the diffusion of water in both n-alkanes and n-alcohols. It was found that the polar solutewater, diffusing in a solvent with which it cannot hydrogen bond, closely resembles small nonpolarsolutes such as xenon and krypton diffusing in n-alkanes, with diffusion coefficients ranging from12.5 X 10~(-5)cm~2/ s for water in n-pentane to 1.15 X 10(-hexadecane. Diffusioncoefficients were found to be inversely proportional to viscosity to a fractional power, and diffusioncoefficients were faster than SE predicts. For water diffusing in a solvent (n-alcohols) with whichit can hydrogen bond, diffusion coefficient values ranged from 1.75 X 10~(-5)cm~2/ s in n-methanol to0.364 X 10~(-5)cm~2/ s in n-octanol, and diffusion was slower than an alkane of correspondingviscosity. We find no evidence for solute-solvent complex diffusion. Rather, it is possible that thesmall solute water may be retarded by relatively longer residence times (compared tonon-H-bonding solvents) as it moves through the liquid.