A NONEQUILIBRIUM SIMULATION METHOD FOR CALCULATING TRACER DIFFUSION-COEFFICIENTS OF SMALL SOLUTES IN N-ALKANE LIQUIDS AND POLYMERS

The tracer diffusion coefficients of methane in n-alkane liquids of in creasing chain length were calculated by measuring the friction from; shea time nonequilibrium molecular dynamics simulations. The frictiona l constant was calculated from the exponentially decaying distance bet ween two methane tracer molecules connected by a harmonic spring. The diffusion constants calculated with this method are in good agreement with those obtained from mean square displacement calculations. Unfort unately, both methods required the same computational input even for l arge values of n. The tracer diffusion coefficient is found to obey an algebraic law D similar to n(-alpha) for n less than or equal to 100, where n is the chain length of the solvent. The value of alpha=1.1 is in perfect agreement with experimental tracer diffusivities determine d with the Taylor dispersion method, the value being mainly determined by the tracer's size. For larger solvent chain lengths n>100 (tracer diffusion in polymer melts) a second regime of much slower decay is fo und. The tracer diffusion coefficient converges to a constant value in correspondence with the converging malt density. (C) 1998 American In stitute of physics.