Functional group contributions to partial molar compressibilities of alcohols in water

A recently developed methodology for evaluating solute functional group con tributions to the partial molar compressibility of solution [Lockwood, D. M .; Rossky, P. J. J. Phys. Chem. B. 1999, 103, 1982] is used to examine solv ation of methanol and ethanol in water. Additive contributions to the compr essibility are resolved for the methyl and hydroxyl groups of each solute, and the results are shown to be the same for both solutes within statistica l error. Further, the effect of each functional group on the solvent is fou nd to be localized in the vicinity of that functional group, explaining the apparent independence of functional group contributions observed experimen tally by other workers. The difference in partial molar compressibility bet ween methanol and ethanol can be attributed to localized solvent perturbati on by the methylene group in ethanol. For the potential functions employed, compressibilities calculated via classical molecular dynamics simulations are in best agreement with experiments performed at temperatures higher tha n those at which the simulations are performed. Implications of calculated alcohol group contributions for studies of more complex solutes, including biomolecules, are discussed.