Electrostatic and induction effects in the solubility of water in alkanes.

Experiments show that at 298 K and 1 atm pressure, the transfer free energy, μ, of water from its vapor to liquid normal alkanes CH (n=5…12) is negative. Earlier it was found that with the united-atom TraPPE model for alkanes and the SPC/E model for water, one had to artificially enhance the attractive alkane-water cross interaction to capture this behavior. Here we revisit the calculation of μ using the polarizable AMOEBA and the non-polarizable Charmm General (CGenFF) forcefields. We test both the AMOEBA03 and AMOEBA14 water models; the former has been validated with the AMOEBA alkane model while the latter is a revision of AMOEBA03 to better describe liquid water. We calculate μ using the test particle method. With CGenFF, μ is positive and the error relative to experiments is about 1.5 kT. With AMOEBA, μ is negative and deviations relative to experiments are between 0.25 kT (AMOEBA14) and 0.5 kT (AMOEBA03). Quantum chemical calculations in a continuum solvent suggest that zero point effects may account for some of the deviation. Forcefield limitations notwithstanding, electrostatic and induction effects, commonly ignored in consideration of water-alkane interactions, appear to be decisive in the solubility of water in alkanes.