Nematic virial coefficients of very long hard molecules and Onsager theory

Author: Velasco E.   Padilla P.  

Publisher: Taylor & Francis Ltd

ISSN: 1362-3028

Source: Molecular Physics, Vol.94, Iss.2, 1998-06, pp. : 335-339

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Abstract

The virial coefficients B2-B5 of a fluid of hard molecules interacting via a hard Gaussian overlap potential have been obtained by Monte Carlo integration. Molecular elongations ranging from 1 to 105 are considered. Virial coefficients are computed as a function of the nematic order parameter S , using a simple representation for the orientational distribution function. The calculations cover the entire order parameter range and include the limiting cases S = 0 (randomly oriented molecules) and S = 1 (completely parallel molecular arrangements). In analogy with results from previous studies for spherocylinders, the virial coefficients in the case S = 0 are seen to vanish in the limit of infinitely long molecules (K ->8), though the asymptotic regime sets in rather slowly. Similar asymptotic behaviour is observed even for relatively high values of S , which implies that, for the values where the transition to the nematic is expected to occur and well w ithin the nematic range, the covergence properties of the virial expansion must be rather insensitive to the nematic order parameter. This result may indicate that Onsager theory for the isotropic-nematic transition, a virial expansion truncated at second order, is exact in the limit .