Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005-1827, USA.
Materials Science and Engineering, Ames Laboratory, Ames, Iowa 50011, USA.
J Chem Phys. 2017 Aug 21;147(7):074506. doi: 10.1063/1.4997916.
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.
实验表明，在 298 K 和 1 atm 压力下，水蒸气向正构烷烃 CH(n=5…12)液体转移的自由能 μ 为负值。此前发现，使用针对烷烃的 united-atom TraPPE 模型和针对水的 SPC/E 模型，必须人为增强烷烃-水的吸引力交叉相互作用，才能捕捉到这种行为。在这里，我们使用可极化 AMOEBA 和不可极化 Charmm General (CGenFF) 力场重新计算 μ。我们测试了 AMOEBA03 和 AMOEBA14 两种水模型；前者已通过 AMOEBA 烷烃模型进行了验证，而后者是对 AMOEBA03 的修订，以更好地描述液态水。我们使用测试粒子法计算 μ。使用 CGenFF，μ 为正值，与实验的误差约为 1.5 kT。使用 AMOEBA，μ 为负值，与实验的偏差在 0.25 kT（AMOEBA14）和 0.5 kT（AMOEBA03）之间。连续溶剂中的量子化学计算表明，零点能效应可能是导致部分偏差的原因。尽管存在力场限制，但在考虑水-烷烃相互作用时通常被忽略的静电和诱导效应似乎对水在烷烃中的溶解度起决定性作用。
