Measurement of molecular diffusivities of liquid alkane systems
"An unsteady-state porous frit method of diffusion coefficient measurement, recently developed in this laboratory, was improved and expanded upon. The experimental technique has been refined and documented. The unsteady-state diffusion of an initially 0.5 N NaCl solution into pure water (effective diffusivity -1.480 x 10-5 cm2/sec) was employed as the calibration standard. An overall calibration precision of ±4% was obtained. Diffusion times of two hours or less were used for the non-aqueous diffusivity measurements. The validity of this measurement technique was confirmed by the agreement of the measured self-diffusion coefficient of n-heptane (25⁰C) with literature values. A numerical simulation of the frit diffusion process was developed and permitted such effects as solvent withdrawal and concentration dependency of the diffusion coefficient to be studied. A method of evaluating the overall effective diffusivity of the sodium chloride calibration standard for the specific conditions employed in this study was also developed. A time-averaged solvent volume approximation was employed for the data analysis. Numerical simulation confirmed the validity of both the NaCl effective diffusivity and the solvent volume approximations. Diffusivities of a number of n-alkane-n-alkane and n-alkane-n-alcohol systems were measured at temperatures of 20⁰, 25⁰, 30⁰ and 40⁰C. A carbon-14 tracer technique was used in conjunction with the frit method. In most cases diffusivities were determined as the average of duplicate experimental measurements; agreement of the two measurements was generally ±10%. The measured diffusivities permitted an investigation to be made concerning the diffusion mechanism of straight-chain molecules. Comparison of the data with the Stokes-Einstein and Eyring diffusion models indicated that during diffusion n-alkane molecules are oriented lengthwise parallel to the direction of flow. The group Dμ/T was found to be constant for the n-alkanes but showed a marked temperature dependence for the n-alkane-n-alcohol systems. Contrary to a previously proposed theory, the data indicated that the ratio of n-alkane diffusivities in an n-alkane solvent is not equal to the inverse ratio of solute carbon numbers. The data were compared to numerous prediction correlations. Several correlations were found to be reasonably accurate for n-alkane diffusion. With the exception of one modified Eyring expression, all correlations failed to predict the n-alkane-n-alcohol diffusivities with any degree of accuracy. A previously unreported failing of an accepted diffusivity prediction relation was observed for this class of binary systems"--Abstract, pages ii-iii.
Missouri University of Science and Technology
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