Influence of the interaction potential and of the temperature on the thermodiffusion (Soret) coefficient in a model...

In this paper the thermodiffusive behavior of an equimolar binary mixture subject to repulsive potentials of the form (sigma/r)(n) is investigated by using nonequilibrium molecular dynamics (NEMD) and the thermodiffusion (Soret) coefficient, S(T), is computed in a wide range of temperatures. With the aim to contribute to the study of the dependence of the Soret coefficient on the interaction potential, the exponent n of the potential is varied from 1 to 12, that is from a pseudocoulombian to a pseudohard-sphere interaction. The steady state equation is integrated for the composition function under reasonable assumptions and it is shown that in some cases the request for it to be linear cannot be satisfied. For this reason nonlinear functions are used to fit the NEMD composition data. The simulations indicate a negligible dependence of S(T) on the composition (in the composition range here considered) while the dependence on the temperature is more marked. The computed values of S(T) as a function of the temperature are fitted with analytical functions. It is found that with n> or =3 (medium and short range interaction) the model system behaves like a dilute gas mixture with the Soret coefficient varying with the temperature almost like 1/T. In the case of n=1 (long range interaction), S(T) has a more complex dependence on T: in particular it shows a change of sign. The analytical fitting functions, S(T)(T), are used in the integrated steady state equation thus obtaining the steady state composition profile and its comparison with the NEMD results indicates the grounding of the approach here proposed.