Local Structures of Methanol–Water Binary Solutions Studied by Soft X-ray Absorption Spectroscopy (original) (raw)
Related papers
Journal of Electron Spectroscopy and Related Phenomena, 2004
We have examined the influence of the intermolecular interaction on the local electronic structure by using X-ray absorption and emission spectra of liquid methanol, water, and their mixtures (in molar ratios of 9:1 and 7:3). We find a strong involvement of hydrogen bonding in the mixing of water and methanol molecules. The local electronic structure of water and methanol clusters, where water cluster is bridging within a 6-member open-ring structured methanol cluster, is separately determined. The experimental findings suggest an incomplete mixing of water-alcohol systems and a strong self-association between methanol chain and water cluster through hydrogen bonding. The enhancement of joint water-methanol open-ring structure owes the explanation to the loss of entropy of the aqueous solutions.
Journal of Physical Chemistry A, 2005
We utilized X-ray absorption spectroscopy (XAS) and X-ray Raman scattering (XRS) in order to study the ion solvation effect on the bulk hydrogen bonding structure of water. The fine structures in the X-ray absorption spectra are sensitive to the local environment of the probed water molecule related to the hydrogen bond length and angles. By varying the concentration of ions, we can distinguish between contributions from water in the bulk and in the first solvation sphere. We show that the hydrogen bond network in bulk water, in terms of forming and breaking hydrogen bonds as detected by XAS/XRS, remains unchanged, and only the water molecules in the close vicinity to the ions are affected.
The Journal of chemical physics, 2016
While methanol and ethanol are macroscopically miscible with water, their mixtures exhibit negative excess entropies of mixing. Despite considerable effort in both experiment and theory, there remains significant disagreement regarding the origin of this effect. Different models for the liquid mixture structure have been proposed to address this behavior, including the enhancement of the water hydrogen bonding network around the alcohol hydrophobic groups and microscopic immiscibility or clustering. We have investigated mixtures of methanol, ethanol, and isopropanol with water by liquid microjet X-ray absorption spectroscopy on the oxygen K-edge, an atom-specific probe providing details of both inter- and intra-molecular structure. The measured spectra evidence a significant enhancement of hydrogen bonding originating from the methanol and ethanol hydroxyl groups upon the addition of water. These additional hydrogen bonding interactions would strengthen the liquid-liquid interaction...
Surface relaxation in liquid water and methanol studied by x-ray absorption spectroscopy
The Journal of Chemical Physics, 2002
X-ray absorption spectroscopy is a powerful probe of local electronic structure in disordered media. By employing extended x-ray absorption fine structure spectroscopy of liquid microjets, the intermolecular O-O distance has been observed to undergo a 5.9% expansion at the liquid water interface, in contrast to liquid methanol for which there is a 4.6% surface contraction. Despite the similar properties of liquid water and methanol ͑e.g., abnormal heats of vaporization, boiling points, dipole moments, etc.͒, this result implies dramatic differences in the surface hydrogen bond structure, which is evidenced by the difference in surface tension of these liquids. This result is consistent with surface vibrational spectroscopy, which indicates both stronger hydrogen bonding and polar ordering at the methanol surface as a consequence of ''hydrophobic packing'' of the methyl group.
Probing the Local Structure of Liquid Water by X-ray Absorption Spectroscopy†
The Journal of Physical Chemistry B, 2006
It was recently suggested that liquid water primarily comprises hydrogen-bonded rings and chains, as opposed to the traditionally accepted locally tetrahedral structure (Wernet et al. Science 2004, 304, 995). This controversial conclusion was primarily based on comparison between experimental and calculated X-ray absorption spectra (XAS) using computer-generated ice-like 11-molecule clusters. Here we present calculations which conclusively show that when hydrogen-bonding configurations are chosen randomly, the calculated XAS does not reproduce the experimental XAS regardless of the bonding model employed (i.e., rings and chains vs tetrahedral). Furthermore, we also present an analysis of a recently introduced asymmetric water potential (Soper, A. K. J. Phys.: , which is representative of the rings and chains structure, and make comparisons with the standard SPC/E potential, which represents the locally tetrahedral structure. We find that the calculated XAS from both potentials is inconsistent with the experimental XAS. However, we also show the calculated electric field distribution from the rings and chains structure is strongly bimodal and highly inconsistent with the experimental Raman spectrum, thus casting serious doubt on the validity of the rings and chains model for liquid water. † Part of the special issue "Charles B. Harris Festschrift".
X-ray absorption spectrum of liquid water from molecular dynamics simulations: Asymmetric model
Physical Review B, 2006
We address the question of the local structure in liquid water by computing the x-ray absorption ͑XA͒ spectrum explicitly averaging over configurations from classical and Car-Parrinello molecular dynamics ͑MD͒ simulations. We confirm the prediction by Wernet et al. ͓Science 304, 995 ͑2004͔͒ that the resulting spectra are not representative for liquid water due to a too large fraction of molecules with two donating hydrogen bonds. The H-bond criterion given by Wernet et al. is, however, not sufficient to predict the XA spectrum for general structures from MD simulations. Only by selecting specific single-donor MD configurations is it possible to represent the liquid water spectrum; this results in an improved and more restrictive geometrical connection between local structure and spectral features in the simulation. The configurations, with one strong and one weak donating hydrogen bond, need to be even more asymmetric than previously assumed. This bonding situation is strongly underrepresented in the simulations.
Zeitschrift für Naturforschung A, 2000
The structure of clusters in methanol-water solutions in its dependence on the methanol mole fraction xM has been investigated by mass spectrometry on clusters isolated from submicron droplets by adiabatic expansion in vacuum and by X-ray diffraction on the bulk binary solutions. The mass spectra have shown that the average hydration number, (nm), of m-mer methanol clusters decreases with increasing xM , accompanied by two inflection points at xM = ~0.3 and ~0.7. The X-ray diffraction data have revealed a similar change in the number of hydrogen bonds per water and/or methanol oxygen atom at ~2.8 Å. On the basis of both results, most likely models of clusters formed in the binary solutions are proposed: at 0 < xM < 0.3 the tetrahedral-like water cluster is the main species, at 0.3 < xM < 0.7 chain clusters of methanol molecules gradually evolve with increasing methanol content, and finally, at xM > 7 chain clusters of methanol become predominant. The present results a...
Probing the hydrogen-bond network of water via time-resolved soft X-ray spectroscopy
Physical Chemistry Chemical Physics, 2009
We report time-resolved studies of hydrogen bonding in liquid H2O, in response to direct excitation of the O-H stretch mode at 3 mu m, probed via soft x-ray absorption spectroscopy at the oxygen K-edge. This approach employs a newly developed nanofluidic cell for transient soft xray spectroscopy in liquid phase. Distinct changes in the near-edge spectral region (XANES) are observed, and are indicative of a transient temperature rise of 10K following transient laser excitation and rapid thermalization of vibrational energy. The rapid heating occurs at constant volume and the associated increase in internal pressure, estimated to be 8MPa, is manifest by distinct spectral changes that differ from those induced by temperature alone. We conclude that the near-edge spectral shape of the oxygen K-edge is a sensitive probe of internal pressure, opening new possibilities for testing the validity of water models and providing new insight into the nature of hydrogen bonding in water.
Three-Dimensional Structure in Water−Methanol Mixtures
The Journal of Physical Chemistry A, 1997
The diverse properties of hydrogen-bonded liquids and solutions must manifest their unique local structures. An unambiguous three-dimensional picture of the local ordering in these liquid systems is not accessible through radial distribution functions, the usual outputs of computer simulation, or experimental studies. In this work we employ spatial distribution functions to analyze the three-dimensional local structure in watermethanol solutions. Molecular dynamics simulations are performed at room temperature for five watermethanol liquid mixtures scanning the entire range of compositions. The effects of the alcohol on water structure and water on methanol structure are considered in detail. The results are compared to previous simulations and discussed from the point of view of various solvation models. Large structural changes are observed, many of which are not apparent from simple radial analysis. In water-rich solution we confirm a high degree of ordering, characterized by a very strong preference for tetrahedral arrangements, where the water molecules appear most highly localized around the hydroxyl group of the methanol solute. Strongly hydrated methanol molecules adopt rather specific relative positions that most readily accommodate the ordering within their hydration cages. In methanol-rich solution the local structure very closely resembles that of pure methanol. We find that rather long equilibration periods appear to be necessary to obtain accurate structural information in computer simulations of these complex systems.