(174g) Cavity Thermodynamics in Organic Solvents

Solution thermodynamics is dominated in large part by the process of creating a solute sized cavity in the solvent. Scaled-Particle Theory (SPT) presents fundamental relationships between the thermodynamics of cavity formation and spontaneous solvent fluctuations. We have studied using Molecular Dynamics simulations a number of organic solvents the statistics of cavity formation. These simulation results are then compared to a revised version of Reiss's classic SPT that takes into account the molecular shape of the solvent. Carbon atom densities in the vicinity of the cavity surface, i.e., the contact density, have been evaluated for these solvents. Systematic changes in the contact density for homologous aliphatic hydrocarbons observed from simulation are correctly reproduced from our molecular SPT. Moreover, the molecular SPT accurately predicts the ordering of contact densities for polyethylene oligomer melts of increasing length, that would not be captured by Reiss's classic SPT which ascribes differences in solvents simply to differences in an empirical van der Waals solvent radius. Our molecular theory, on the other hand requires only one van der Waals radius for the methyl/methylene carbons and is then predictive for a wide range of solvents without resorting to post hoc empirical parameterizations.