Diffusion of Squalene in Nonaqueous Solvents

Capillary flow techniques have been used to determine the translational diffusion constant, D, of squalene in seven alkanes and five cyclohexanes. The alkanes are n-hexane, n-octane, n-decane, n-dodecane, n-tetradecane, 2,2,4,4,6,8,8-heptamethylnonane (isocetane), and 2,6,10,14-tetramethylpentadecane (pristane). The cyclohexanes are cyclohexane, n-butylcyclohexane, n-hexylcyclohexane, n-octylcyclohexane, and n-dodecylcyclohexane. When combined with published data in CD₂Cl₂, ethyl acetate, n-hexadecane, squalane, n-octane-squalane mixtures, and supercritical CO₂, the 35 diffusion constants and viscosities, η, vary by factors of ∼230 and ∼500, respectively. A fit to the modified Stokes-Einstein equation (MSE, D/T = A _SE/η ) gives an average absolute percentage difference (AAPD) of 7.72% between the experimental and calculated D values where p and A _SE are constants, T is the absolute temperature, and the AAPD is the average value of (10²) (|D _calcd - D _exptl|/D _exptl). Two other MSE fits using subsets of the 35 diffusion constants may be useful for (a) estimating the viscosity of the hydrophobic core of lipid droplets, where squalene is a naturally occurring component, and (b) providing estimates of the D values needed to design extraction processes by which squalene is obtained from plant oils. The Wilke-Chang equation also was considered and found to give larger AAPDs than the corresponding MSE fits.