Examination of chain length effects on the solubility of alkanes in near-critical and supercritical aqueous solutions

Molecular dynamics simulations of infinitely dilute aqueous solutions of bu tane and octane were conducted in both the subcritical and supercritical re gions. The free energy of hydration of each alkane species was calculated a nd compared with predictions from a recently proposed equation of state. Th e results suggest that near the critical temperature of water (along a give n isobar), the trends in the free energy of hydration for a series of acycl ic alkanes reverse with increasing temperature. This implies that larger ch ains (such as octane) have a greater solubility than smaller chains (such a s butane). The effects of alternative combining rules other than the standa rd Lorentz-Berthelot relationships on the free energy of hydration were als o examined.