Dynamics of hydration of alcohols and diols in aqueous solutions

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 atomspecific 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 interactions, resulting in additional ordering in the liquid structures and leading to a reduction in entropy and a negative enthalpy of mixing, consistent with existing thermodynamic data. In contrast, the spectra of the isopropanol-water mixtures exhibit an increase in the number of broken alcohol hydrogen bonds for mixtures containing up to 0.5 water mole fraction, an observation consistent with existing enthalpy of mixing data, suggesting that the measured negative excess entropy is a result of clustering or micro-immiscibility. Published by AIP Publishing. [http://dx

Section: -17mentioning

confidence: 99%

Communication: Hydrogen bonding interactions in water-alcohol mixtures from X-ray absorption spectroscopy

Lam

Smith

Saykally

2016

“…NMR studies, femtosecond midinfrared spectroscopy measurements, and molecular dynamics simulations, all showed that the solvating water molecules possess significantly slower orientational dynamics than the molecules in bulk liquid water. [6][7][8][9][10][11][12][13][14][15] An interesting property of alcohols and other amphiphilic molecules is that these molecules can form aggregates in aqueous solution as a result of the hydrophobic interaction. This interaction plays an important role in processes such as protein folding and the formation of bilipid membranes.…”

Section: Introductionmentioning

confidence: 99%

Femtosecond midinfrared study of aggregation behavior in aqueous solutions of amphiphilic molecules

Petersen

Bakulin

Pavelyev

et al. 2010

Femtosecond midinfrared study of aggregation behavior in aqueous solutions of amphiphilic molecules Petersen, C.; Bakulin, A.A.; Pavelyev, V.G.; Pshenichnikov, M.S.; Bakker, H.J. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We study the spectral and orientational dynamics of HDO molecules in aqueous solutions of different concentrations of tertiary butyl alcohol ͑TBA͒ and trimethylamine-N-oxide ͑TMAO͒. The spectral dynamics is investigated with femtosecond two-dimensional infrared spectroscopy of the O-H stretch vibration of HDO: D 2 O, and the orientational dynamics is studied with femtosecond polarization-resolved pump-probe spectroscopy of the O-D stretch vibration of HDO: H 2 O. Both the spectral and orientational dynamics are observed to show bimodal behavior: part of the water molecules shows spectral and orientational dynamics similar to bulk liquid water and part of the water molecules displays a much slower dynamics. For low solute concentrations, the latter fraction of slow water increases linearly as a function of solute molality, indicating that the slow water is contained in the solvation shells of TBA and TMAO. At higher concentrations, the fraction of slow water saturates. The saturation behavior is much stronger for TBA solutions than for TMAO solutions, indicating the aggregation of the TBA molecules.

“…In both NMR and femtosecond midinfrared spectroscopy measurements, it was observed that the solvating water molecules show much slower orientational dynamics than the molecules in bulk liquid water. [6][7][8][9][10][11][12][13] The thermodynamic properties of hydrophobic hydration are strongly temperature dependent. [14][15][16][17][18][19] For small hydrophobic solutes, the free energy change associated with the solvation reaches a maximum at an elevated temperature, which implies that at this temperature the change in excess entropy has become zero, and the enthalpy change has become positive.…”

Section: Introductionmentioning

confidence: 99%

Strong temperature dependence of water reorientation in hydrophobic hydration shells

Petersen

Tielrooij

Bakker

2009

118

We study the temperature dependence of the orientational mobility of water molecules solvating hydrophobic molecular groups with femtosecond midinfrared spectroscopy. We observe that these dynamics show a strong temperature dependence. At temperatures Ͻ30°C the solvating water molecules show a reorientation time Ͼ10 ps, which is more than four times slower than in bulk water. With increasing temperature, the reorientation of the solvating molecules strongly accelerates and becomes much more equal to the reorientation rate of the molecules in the bulk liquid. These observations indicate that water molecules form relatively rigid solvation structures around hydrophobic molecular groups that melt at elevated temperatures.