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 increasing chain length were calculated by measuring the friction from short time nonequilibrium molecular dynamics simulations. The frictional constant was calculated from the exponentially decaying distance between 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. Unfortunately, both methods required the same computational input even for large values of n. The tracer diffusion coefficient is found to obey an algebraic law D ~ n-α for n <= 100, where n is the chain length of the solvent. The value of α =1.1 is in perfect agreement with experimental tracer diffusivities determined 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 found. The tracer diffusion coefficient converges to a constant value in correspondence with the converging melt density