Aqueous Solvation of Methane from First Principles

Abstract

Structural, dynamical, bonding, and electronic properties of water molecules around a soluted methane molecule are studied from first principles. The results are compatible with experiments and qualitatively support the conclusions of recent classical molecular dynamics simulations concerning the controversial issue on the presence of "immobilized" water molecules around hydrophobic groups: the hydrophobic solute slightly reduces (by a less than 2 factor) the mobility of many surrounding water molecules rather than immobilizing just the few ones which are closest to methane, similarly to what was obtained by previous first-principles simulations of soluted methanol. Moreover, the rotational slowing down is compatible with the one predicted on the basis of the excluded volume fraction, which leads to a slower hydrogen bond exchange rate. The analysis of simulations performed at different temperatures suggests that the target temperature of the soluted system must be carefully chosen, in order to avoid artificial slowing-down effects. By generating maximally localized Wannier functions, a detailed description of the polarization effects in both solute and solvent molecules is obtained, which better characterizes the solvation process.

Methane Hydration‐Shell Structure and Fragility

“…The influence of oily molecules on the structure of liquid water is a subject of wide‐ranging importance and ongoing speculation, dating back at least to the seminal studies of Henry Frank and Marjorie Evans who proposed that “ water forms frozen patches or microscopic icebergs around such solute molecules .” The quintessential oily solute, methane (CH 4 ), has been the subject of numerous theoretical,– molecular dynamics (MD), and ab initio (AIMD) simulations, although its low solubility poses significant experimental challenges. The few prior experimental studies of methane in water have reached conflicting conclusions, ranging from neutron scattering measurements that found “ no evidence for enhanced water structure ” to recent isotopic infrared difference measurements that found “ evidence that supports, unequivocally, the iceberg view of hydrophobicity .” This last conclusion is inconsistent with MD and AIMD, predictions, that find no frozen patches or icebergs around oily molecules, although MD hydration thermodynamic predictions– accurately reproduce the entropic anomalies that originally motivated the Frank and Evans iceberg hypothesis . Moreover, both theoretical predictions and experimental fs‐IR and NMR measurements find slower water dynamics around oily solutes, with no evidence of icebergs.…”

“…The few prior experimental studies of methane in water have reached conflicting conclusions, ranging from neutron scattering measurements that found “ no evidence for enhanced water structure ” to recent isotopic infrared difference measurements that found “ evidence that supports, unequivocally, the iceberg view of hydrophobicity .” This last conclusion is inconsistent with MD and AIMD, predictions, that find no frozen patches or icebergs around oily molecules, although MD hydration thermodynamic predictions– accurately reproduce the entropic anomalies that originally motivated the Frank and Evans iceberg hypothesis . Moreover, both theoretical predictions and experimental fs‐IR and NMR measurements find slower water dynamics around oily solutes, with no evidence of icebergs. Here we quantify the influence of methane on the structure of liquid water by performing Raman multivariate curve resolution (Raman‐MCR) hydration‐shell vibrational spectroscopic measurements and MD simulations from −10 °C to 300 °C (at 30 MPa).…”

Energy benchmarks for methane-water systems from quantum Monte Carlo and second-order Møller-Plesset calculations

“…5,6 In addition, methane molecules in aqueous solution are pertinent to the study of the hydrophobic interactions that are so important throughout biology. [7][8][9][10] Computer simulations based on empirical force fields have been an important tool for investigating processes such as the nucleation and growth of methane hydrate and the dissolution of methane gas in water, 2,5,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] and simulations based on density functional theory (DFT) 6,[28][29][30][31][32] have also been valuable for the study of methane-water systems. Energy benchmarks from accurate quantum-mechanical calculations are essential for testing both force fields and DFT methods.…”

Thermodynamic and structural signatures of water-driven methane-methane attraction in coarse-grained mW water

“…While it is challenging to study the association process in experiments, it has been extensively studied through molecular simulations. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] The water models in these studies encompass ab initio modeling using density functional theory (DFT), 16,17 classical fully atomistic models, [18][19][20][21][22][23][24][25][26] anisotropic coarse-grained models, 30,32 and isotropic coarse-grained models. 31 Three features of the free energy of methane-methane interaction have been well established by these studies: (i) a global minimum in the free energy when the methane pair is in contact, which is referred to as contact pair (CP), (ii) a second lowest free energy state when the methane pair is separated by a shell of water, which a) Author to whom correspondence should be addressed.…”