Origins of the Solvent Chain-Length Dependence of Gibbs Free Energies of Transfer

Experimentally measured partition coefficients show that the solubilities of small solutes in normal alkanes depend on the solvent chain length (n<INF>C</INF>). The causes for this n<INF>C</INF> dependence have not yet been unambiguously determined, and there is considerable controversy as to whether different interactions with methyl and methylene groups or entropic Flory−Huggins-like effects might play the major role. We have performed Gibbs-ensemble Monte Carlo simulations to study the vapor−liquid partitioning of methane in normal alkanes (with 6−12 carbon atoms) and related model solvents. The simulations show that the increase in solvent density with increasing n<INF>C</INF> is the main origin of the n<INF>C</INF> dependence for normal alkanes; that is, the solute molecule feels a different environment depending on the alkane chain length.