TRANSPORT PROPERTIES OF LIQUID N-ALKANES

Mutual and tracer diffusion coefficients supplementing existing data are presented for the systems n-C5H14 + n-C12H26, n-C7H16 + n-C16H34, n-C8H18 + n-C16H34, and n-C10H22 + n-C16H34 deriving from Gouy interferometric and diaphragm cell measurements. Based upon these and other reported data, the transport behavior of n-alkanes can be summarized as follows. (1) The Darken equation holds; i.e., D12 = (x1D2* + x2D1* d ln f1x1/d ln x1 (This is consistent with Van Geet and Adamson's finding for the system n-C8H18 + n-C12H26.2) (2) The activation energy for the tracer diffusion of any n-alkane is the same in any n-alkane medium at a given density. In a medium of a given density, the size of the moving segment is independent of the chain length. (3) For all binary n-alkane mixtures, D12/(d ln f1x1/d ln x1) is a linear function of density, ρ. All these lines extrapolate to zero mobility at ρ = 0.84, corresponding to a supercooled melt of an n-paraffin of very high molecular weight. The slopes of these lines are linear functions of √n1n2, where n1, n2 are the numbers of carbon atoms in the two components. (4) The tracer diffusion coefficient of any linear alkane Cn in any n-alkane medium (either pure liquid or mixture) is determined only by n and the density of the medium at a given temperature. (5) Deviation from linearity of plots of D12 against ρ can be used to determine the thermodynamic terms, d ln F1x1/d ln x1, hence also activity coefficients and excess free energies. (6) Viscosities of n-alkanes (pure liquids or mixtures) are simple monotonie functions of density. These rules describe accurately the transport behavior of those n-alkanes which have been studied and should define those properties for any liquid n-alkane system or melt. Rules 2 and 4 are slightly modified versions of the application of the principle of congruence first used in this fashion by Van Geet and Adamson.