Ab Initio Molecular Dynamics Simulation of the Structure and Proton Transport Dynamics of Methanol−Water Solutions

Abstract

Ab initio molecular dynamics simulations are employed to study the structural and proton transport properties of methanol-water mixtures. Structural characteristics analyzed at two different methanol mole fractions (X(M) = 0.25 and X(M) = 0.5) reveal enhanced structuring of water as the methanol mole fraction increases in agreement with recent neutron diffraction experiments. The simulations reveal the existence of separate hydrogen-bonded water and methanol networks, also in agreement with the neutron diffraction data. The addition of a single proton to the X(M) = 0.5 mixture leads to an anomalous structural or Grotthuss-type diffusion mechanism of the charge defect in which water-to-water, methanol-to-water, and water-to-methanol proton transfer reactions play the dominant role with methanol-to-methanol transfers being much less significant. Unlike in bulk water, where coordination number fluctuations drive the proton transport process, suppression of the coordination number of waters in the first solvation shell of the defect diminish the importance of coordination number fluctuations as a driving force in the structural diffusion process. The charge defect is found to reside preferentially at the interface between water and methanol networks. The length of the ab initio molecular dynamics run (approximately 120 ps), allowed the diffusion constant of the charge defect to be computed, yielding a value of D = 4.2 x 10(-5) cm2/s when deuterium masses are assigned to all protons in the system. The relation of this value to excess proton diffusion in bulk water is discussed. Finally, a kinetic theory is introduced to identify the relevant time scales in the proton transfer/transport process.

Water at hydrophobic interfaces delays proton surface-to-bulk transfer and provides a pathway for lateral proton diffusion

“…The hydronium ion forms three H-bonds with surrounding water molecules [the so-called 'Eigen structure' (34)]. Similar asymmetric amphiphile-like solvation patterns through both hydrophilic and hydrophobic interactions have previously been found for the excess proton in methanolwater solutions (35,36). Its electronic density is significantly polarized by the first and second solvation shells (Fig.…”

Collision dynamics and uptake of water on alcohol-covered ice

“…Commonly methanol is considered to be completely miscible in water, but observations that the entropy increase due to mixing methanol and water is less than expected for ideal solutions suggests incomplete mixing on a molecular scale (Dixit et al, 2002). Numerical simulations (Dougan et al, 2004;Morrone et al, 2006) and analysis of careful measurements (Guo et al, 2003;Soper et al, 2006) reveal networks of enhanced structure built from hydrogen-bonded water and methanol. Ultimately the behavior of methanol in water and on water surfaces depends sensitively on the balance between the molecules hydroxyl group which can participate in hydrogen bonding and its hydrophobic methyl group (Souda et al, 2003) and the specific topology of molecular level structuring (Morrone et al, 2006).…”

“…Numerical simulations (Dougan et al, 2004;Morrone et al, 2006) and analysis of careful measurements (Guo et al, 2003;Soper et al, 2006) reveal networks of enhanced structure built from hydrogen-bonded water and methanol. Ultimately the behavior of methanol in water and on water surfaces depends sensitively on the balance between the molecules hydroxyl group which can participate in hydrogen bonding and its hydrophobic methyl group (Souda et al, 2003) and the specific topology of molecular level structuring (Morrone et al, 2006). Studies, specific to methanol layers on ice surfaces have observed stable monolayer coverages (Winkler et al, 2002;Jedlovszky et al, 2006).…”

Mechanism of Conformational Transition of Silk Fibroin in Alcohol‐water Mixtures

“…Methanol and ethanol are miscible with water in any proportion; however, extensive studies suggest an incomplete mixing of alcohol-water at microscopic level and a strong self-association between alcohol and water molecules through hydrogen bonding. 26,27 In water-rich region, the tetrahedral-like water clusters remain. With increasing alcohol concentration, the water network will be gradually ruptured because there are less water molecules to form the network and the water molecules tend to form hydrogen bonds with alcohol molecules in the zigzag chain-like structure of alcohol evolved.…”