Diffusion of organic solutes in squalane

The translational diffusion constants, D, of 26 hydrocarbons have been determined in squalane (2,6,10,15,19,23-hexamethyltetracosane) at room temperature using capillary flow techniques. These new data and previously published room-temperature D values for the same solutes in some (or all) of the n-alkanes n-C-n-C constitute a study of solute diffusion in media spanning a 100-fold change in viscosity; at 23 °C, η= 0.31 cP for n-C, 3.2 cP for n-C, and 30 cP for squalane. The D values in the n-alkanes and squalane show deviations from the Stokes-Einstein relation, D = kT/(6πηr); the values of r, a solutes hydrodynamic radius, decrease as the viscosity increases. The deviations increase as the solute size decreases and are analyzed by fitting the diffusion constants to the modified Stokes-Einstein equation, D/T = A/ η. Fits involving the n-alkane-only and combined n-alkane-squalane D values give comparable results with values of p < 1 that increase as the solute size increases; p = 1 for the Stokes-Einstein limit. The deviations from Stokes-Einstein behavior also are discussed in terms of the relative sizes of the solutes, the n-alkanes, and squalane. © 2011 American Chemical Society.