Ab initio molecular dynamics simulation of the structure and proton transport dynamics of methanol-water solutions

Joseph A. Morrone, Kiryn E. Haslinger, Mark Tuckerman

Research output: Contribution to journalArticle

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 (XM = 0.25 and XM = 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 XM = 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 (∼120 ps), allowed the diffusion constant of the charge defect to be computed, yielding a value of D = 4.2 × 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.

Original languageEnglish (US)
Pages (from-to)3712-3720
Number of pages9
JournalJournal of Physical Chemistry B
Volume110
Issue number8
DOIs
StatePublished - Mar 2 2006

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Methanol
Molecular dynamics
Protons
methyl alcohol
molecular dynamics
protons
Water
Computer simulation
water
simulation
coordination number
Defects
Proton transfer
defects
Neutron diffraction
neutron diffraction
Kinetic theory
Deuterium
Solvation
kinetic theory

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Ab initio molecular dynamics simulation of the structure and proton transport dynamics of methanol-water solutions. / Morrone, Joseph A.; Haslinger, Kiryn E.; Tuckerman, Mark.

In: Journal of Physical Chemistry B, Vol. 110, No. 8, 02.03.2006, p. 3712-3720.

Research output: Contribution to journalArticle

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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 (XM = 0.25 and XM = 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 XM = 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 (∼120 ps), allowed the diffusion constant of the charge defect to be computed, yielding a value of D = 4.2 × 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.",
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