THE EFFECT OF SOLVATION ON THE CONFORMATION OF FREELY JOINTED REPULSIVE TRIMERS

The effect of solvation on the conformation of a dense one-component f luid composed of freely jointed repulsive trimers is calculated theore tically and compared to results from computer simulation. All monomers in the fluid interact with one another via the purely repulsive, shif ted-truncated Lennard-Jones potential. We are able to confirm an earli er suggestion that when the conformation of nonpolar flexible molecule s is calculated with a site-site solvation potential and the reference interaction site model (RISM) theory, better results are obtained wit h the recently derived Percus-Yevick (PY) style solvation potential th an the hypernetted-chain (HNC) style or Gaussian-fluctuation solvation potential. Interestingly, although RISM-based theory predicts well th e general shape of the probability distribution of trimer conformation s, it cannot recover a small feature observed in the simulations and e xpected on simple physical grounds. At high density, there are three p referred conformations of the trimer, corresponding to the three ways in which a flexible trimer can be fitted into a dense fluid with local ly hexagonally close-packed (hcp) symmetry. RISM-based theories predic t a preference for only two conformations of the trimer. To predict th e preference for all three conformations, information about three-site correlations in the fluid must be incorporated into the theory. We pr esent a theory that includes this information and is thereby capable o f reproducing the results of our simulations.