Hydrophobic hydration and the anomalous partial molar volumes in ethanol-water mixtures

Thermodynamic Analysis of Capillary and Electric Field Effects on Liquid-Vapor Equilibrium: A Study on the Water-Ethanol Mixture

This study investigates phase equilibrium manipulation in nonideal mixtures through a combined capillary and external electric field approach. Utilizing thermodynamic principles, an expression is established for estimating the equilibrium liquid mole fraction in a confined system subjected to a localized electric field within a capillary that is filled with a liquid phase in equilibrium with its vapor counterpart. Applied to a water-ethanol system, the model suggests large shifts in the equilibrium liquid mole fraction of water due to the electric field and capillary effects. These findings reveal that while the capillary's influence remains negligible for radii exceeding 10 nm, capillaries of smaller dimensions, when exposed to electric fields of around 300 MV/m, can amplify the equilibrium liquid water mole fraction by up to 55%. This suggests the potential for phase equilibrium control through larger capillaries and lower electric fields, while intriguing complexities arise at very small radii.

Thermodynamic Study of the Electric Field Effect on Liquid-Vapor Mixture at Equilibrium: An Analysis on a Water-Ethanol Mixture

In this paper, the effect of electric fields on phase equilibria through polarization is investigated. A relation is derived for the chemical potential of a system, where the electric field is localized over a liquid phase mixture in equilibrium with a vapor phase mixture. This relation is then applied to a water-ethanol mixture to explore the effect of polarization-based electric fields on the liquid phase composition. It is observed that the quadratic dependence on electric field strength produces little effect below field strengths of approx. 10 MV/m. However, above this field strength, the mole fraction of water in the liquid phase grows rapidly, increasing by a factor of 8 for a water vapor phase fraction of 0.2 and a field strength of 500 MV/m, which approaches the dielectric breakdown strength of water. Nonetheless, this field strength could be achievable with microfluidic experimental setups.

An atomistic explanation of the ethanol-water azeotrope

Ethanol and water form an azeotropic mixture at an ethanol molecular percentage of ∼91% (∼96% by volume), which prohibits ethanol from being further purified via distillation. Aqueous solutions at different concentrations in ethanol have been studied both experimentally and theoretically. We performed cylindrical micro-jet photoelectron spectroscopy, excited by synchrotron radiation, 70 eV above C1s ionization threshold, providing optimal atomic-scale surface-probing. Large model systems have been employed to simulate, by molecular dynamics, slabs of the aqueous solutions and obtain an atomistic description of both bulk and surface regions. We show how the azeotropic behaviour results from an unexpected concentration-dependence of the surface composition. While ethanol strongly dominates the surface and water is almost completely depleted from the surface for most mixing ratios, the different intermolecular bonding patterns of the two components cause water to penetrate to the surface region at high ethanol concentrations. The addition of surface water increases its relative vapour pressure, giving rise to the azeotropic behaviour.

Induction and detection of pressure waves by pulsed thermal lens technique in water-ethanol mixtures

The mode-mismatched dual-beam thermal lens technique is widely applied in the characterization of optical and thermo-physical properties of solids and liquids. The technique has also been used to investigate transient acoustic waves induced by pulsed laser excitation at the nanosecond time scale. In this paper, we developed a semi-analytical model to describe the transient acoustic wave that allows a fitting procedure to get the physical properties of fluid samples. The method was used to investigate samples with different mixtures of ethanol and water, and quantitative information of piezo-optic coefficient and sound speed are evaluated for the fluid mixtures.

Molecular heterogeneity in aqueous cosolvent systems

Aqueous cosolvent systems (ACoSs) are mixtures of small polar molecules such as amides, alcohols, dimethyl sulfoxide, or ions in water. These liquids have been the focus of fundamental studies due to their complex intermolecular interactions as well as their broad applications in chemistry, medicine, and materials science. ACoSs are fully miscible at the macroscopic level but exhibit nanometer-scale spatial heterogeneity. ACoSs have recently received renewed attention within the chemical physics community as model systems to explore the relationship between intermolecular interactions and microscopic liquid-liquid phase separation. In this perspective, we provide an overview of ACoS spatial segregation, dynamic heterogeneity, and multiscale relaxation dynamics. We describe emerging approaches to characterize liquid microstructure, H-bond networks, and dynamics using modern experimental tools combined with molecular dynamics simulations and network-based analysis techniques.

Molecular Origin of the Prepeak in the Structure Factor of Alcohols

Prepeak in the structure factor of alcohols is known for a half century and was attributed to one of two mechanisms (i) self-assembly in aggregates and (ii) existence of spatial heterogeneity. Although both explnations are often argued the molecular origin is yet unclear. In this work, molecular dynamics simulation of neat alcohols and their mixtures in the presence of an apolar liquid in bulk and in confined phases is performed to unveil and to clarify the origin of the prepeak at the molecular scale. Unambiguously, we show that the existence of the prepeak is the result of the self-assembly in clusters leading to long-range correlations rather than the spatial heterogeneity. We also establish that the confinement of neat liquids at the nanoscale does not erase the clustering and the prepeak but strongly reduce the spatial heterogeneity. Regarding the binary alcohol/toluene mixtures, we highlight the possibility to erase the clustering and the spatial heterogeneity from nanoconfinement inducing the formation of a core-shell structure. By tuning the interfacial chemistry and pore size, we shed light on the possibility to control the spatial heterogeneity, the self-assembly, and the microphase separation.

Hydrophobic solvation increases thermal conductivity of water

The interaction of water with small alcohols can be used as a model for understanding hydrophobic solvation of larger and more complex amphiphilic molecules. Despite its apparent simplicity, water/ethanol mixtures show important anomalies in several of their properties, like specific heat or partial molar volume, whose precise origin are still a matter of debate. Here we report high-resolution thermal conductivity, compressibility, and IR-spectroscopy data for water/ethanol solutions showing three distinct regions of solvation, related to changes in the H-bond network. Notably, the thermal conductivity shows a surprising increase of ≈3.1% with respect to pure water at dilute concentrations of ethanol (x = 0.025), which suggests a strengthening of H-bond network of water. Our results prove that the rate of energy transfer in water can be increased by hydrophobic solvation, due to the cooperative nature of the H-bond network.

Temperature, Pressure, and Concentration Derivatives of Nonpolar Gas Hydration: Impact on the Heat Capacity, Temperature of Maximum Density, and Speed of Sound of Aqueous Mixtures

The hydrophobic effect is an umbrella term encompassing a number of solvation phenomena associated with solutions of nonpolar species in water, including the following: a meager solubility opposed by entropy at room temperature; large positive hydration heat capacities; positive shifts in the temperature of maximum density of aqueous mixtures; increases in the speed of sound of dilute aqueous mixtures; and negative volumes of association between interacting solutes. Here we present a molecular simulation study of nonpolar gas hydration over the temperature range 273.15-373.15 K and a pressure range -500 to 1000 bar to investigate the interrelationships between distinct hydrophobic phenomena. We develop a new free energy correlation for the solute chemical potentials founded on the Tait equation description of the equation-of-state of liquid water. This analytical correlation is shown to provide a quantitatively accurate description of nonpolar gas hydration over the entire range of thermodynamic state points simulated, with an error of ∼0.02 k_BT or lower in the fitted chemical potentials. Our simulations and the correlation accurately reproduce many of the available experimental results for the hydration of the solutes examined here. Moreover, the correlation reproduces the characteristic entropies of hydration, temperature dependence of the hydration heat capacity, perturbations in the temperature of maximum density, and changes in the speed of sound. While negative volumes of association result from pairwise interactions in solution, beyond the limits of our simulations performed at infinite dilution, we discuss how our correlation could be supplemented with second virial coefficient information to expand to finite concentrations. In total, this work demonstrates that many distinct phenomena associated with the hydrophobic effect can be captured within a single thermodynamically consistent correlation for solute hydration free energies.

Near-infrared spectroscopic study of molecular interaction in ethanol-water mixtures

Given the importance of ethanol-water mixtures in many chemical and biological processes, the molecular interaction in ethanol-water binary system was studied using near-infrared (NIR) spectroscopy. Excess spectra (in form of excess absorption coefficient) and Gaussian fitting were applied to analyze low concentration ethanol-water mixtures, ranging from 0 to 10% (v/v). With the knowledge of aquaphotomics, six kinds of water species were identified for 0-10% ethanol-water system, and it was indicated that water can be a sensitive probe for analyzing the structural changes and the interactions in the solutions. The excess spectra and two-dimensional (2D) correlation spectroscopy were introduced for high concentration mixtures (10-100%) analysis and found that the intermolecular hydrogen bonding strength between ethanol and water reaches to the maximum at 40% ethanol concentration which may be related to some abnormal properties of alcoholic solutions reported previously. In 40-100% mixtures, ethanol molecules tend to initiate the self-association which leads to the weakening of the interaction between ethanol and water. This paper not only deepens the understanding of the structure and dynamics of alcoholic solution, but also opens a new perspective in molecular interaction analysis in aqueous system by understanding the roles of water.

Microstructure of nonideal methanol binary liquid mixtures

The nonideality of binary mixtures is often related to the nature of the interactions between both liquids and of the heterogeneity at the nanoscale-named microstructure. When one of the liquids is a hydrogen bonds former and the second is aprotic, the progressive diluting of the hydrogen-bonding network leads to a clustering and nanophases. By considering two mixtures, toluene-methanol and cyclohexane-methanol, the nonideality and its connection with the structure at the nanoscale and the intermolecular interactions are numerically investigated. Contrary to the toluene that is fully miscible in methanol, cyclohexane presents a high range of immiscibility which makes it a relevant system to study the nucleation (local segregation) and its propagation. In both mixtures, the deviation from the ideal behavior is observed. In the case of the toluene-methanol mixture, the initial hydrogen-bonding network corresponding to a homogenous structure is locally broken due to the favorable toluene-methanol interactions leading to the spatial heterogeneity at the origin of the nonideality. In the range of miscibility of the cyclohexane-methanol mixtures, the formation of hydrophobic nanophases of larger size is observed due to the unfavorable interactions between both components leading to a self-organizing of cyclohexane molecules. The immiscibility of cyclohexane and methanol are then correlated to the formation of nanophases and their propagation, which are also at the origin of the spatial heterogeneity. In the pure methanol, we highlight the disconnection between the clustering and the heterogeneity. We shed light on the fact that the prepeak observed in the structure factor is independent of the degree of heterogeneity, but is connected to the presence of cyclic clusters.

Hydrogen Bond Networks in Binary Mixtures of Water and Organic Solvents

We here present an approach for the optical in situ characterization of hydrogen bond networks (HBNs) in binary mixtures of water and organic solvents (OSs), such as methanol, ethanol, and acetonitrile. HBNs are characterized based on (i) the analysis of experimental molar Raman spectra of the mixture, (ii) partial molar Raman spectra of the mixture constituents, and (iii) computed ideal molar Raman spectra of the mixture. Especially, the consideration of the partial molar Raman spectra provides insights into the development of hydrogen bonds of molecules of one species with their neighbors. The obtained Raman spectra are evaluated with respect to the centroid of the symmetric stretching vibration Raman signal of water and to the hydroxyl stretching vibration of alcohols. We show the influence of composition and temperature on the development of the HBN of the mixtures, the HBN of water, and the HBN of the OS molecules.

Spectroscopic and Thermodynamic Study of Biopolymer Adsorption Phenomena in Heterogeneous Solid-Liquid Systems

Molecular selective adsorption processes at the solid surface of biopolymers in mixed solvent systems are poorly understood due to manifold interactions. However, the ability to achieve adsorptive fractionation of liquid mixtures is posited to relate to the role of specific solid-liquid interactions at the adsorbent interface. The hydration of solid biopolymers (amylose, amylopectin, cellulose) in binary aqueous systems is partly governed by the relative solvent binding affinities with the biopolymer surface sites, in accordance with the role of textural and surface chemical properties. While molecular models that account for the surface area and solvent effects provide reliable estimates of hydration energy and binding affinity parameters, spectroscopic and thermal methods offer a facile alternative experimental approach to account for detailed aspects of solvation phenomena at biopolymer interfaces that involve solid-liquid adsorption. In this report, thermal and spectroscopic methods were used to understand the interaction of starch- and cellulose-based materials in water-ethanol (W-E) binary mixtures. Batch adsorption studies in binary W-E mixtures reveal the selective solvent uptake properties by the biomaterials, in agreement with their solvent swelling in pure water or ethanol. The nature, stability of the bound water, and the thermodynamic properties of the biopolymers in variable hydration states were probed via differential scanning calorimetry and Raman spectroscopy. The trends in biopolymer-solvent interactions are corroborated by dye adsorption and scanning electron microscopy, indicating that biopolymer adsorption properties in W-E mixtures strongly depend on the surface area, pore structure, and accessibility of the polar surface groups of the biopolymer systems, in agreement with the solvent-selective uptake results reported herein.

Building better water models using the shape of the charge distribution of a water molecule

The unique properties of liquid water apparently arise from more than just the tetrahedral bond angle between the nuclei of a water molecule since simple three-site models of water are poor at mimicking these properties in computer simulations. Four- and five-site models add partial charges on dummy sites and are better at modeling these properties, which suggests that the shape of charge distribution is important. Since a multipole expansion of the electrostatic potential describes a charge distribution in an orthogonal basis set that is exact in the limit of infinite order, multipoles may be an even better way to model the charge distribution. In particular, molecular multipoles up to the octupole centered on the oxygen appear to describe the electrostatic potential from electronic structure calculations better than four- and five-site models, and molecular multipole models give better agreement with the temperature and pressure dependence of many liquid state properties of water while retaining the computational efficiency of three-site models. Here, the influence of the shape of the molecular charge distribution on liquid state properties is examined by correlating multipoles of non-polarizable water models with their liquid state properties in computer simulations. This will aid in the development of accurate water models for classical simulations as well as in determining the accuracy needed in quantum mechanical/molecular mechanical studies and ab initio molecular dynamics simulations of water. More fundamentally, this will lead to a greater understanding of how the charge distribution of a water molecule leads to the unique properties of liquid water. In particular, these studies indicate that p-orbital charge out of the molecular plane is important.

Connections between the Anomalous Volumetric Properties of Alcohols in Aqueous Solution and the Volume of Hydrophobic Association

The partial molar volumes of alcohols in water exhibit a non-monotonic dependence on concentration at room temperature, initially decreasing with increasing concentration before passing through a minimum and rising to the pure liquid plateau. This anomalous behavior is associated with hydrophobic interactions. We report molecular simulations of short chain alcohols and alkanes in water to examine the volumetric properties of these mixtures at infinite dilution over a range of temperatures. Our simulations find this anomaly disappears at a crossover temperature, above which the solute volume only varies monotonically with concentration. A Voronoi volume analysis of solution configurations finds that solutes in clusters take up less space than individual solutes at low temperature and more space at elevated temperatures. These changes in cluster volumes are subsequently shown to correlate with the derivative of the solute partial molar volume with respect to solute concentration. The changes in solute volume upon nonpolar solute association impact the response of molecular-scale hydrophobic interactions for assembly with increasing pressure.

Evidence for confinement induced phase separation in ethanol-water mixture: a positron annihilation study

We report an experimental evidence for the phase separation of ethanol-water mixture confined in mesoporous silica with different pore size using positron annihilation lifetime spectroscopy (PALS). A bulk-like liquid in the core of the pore and a distinct interfacial region near the pore surface have been identified based on ortho-positronium lifetime components. The lifetime corresponding to the core liquid shows similar behavior to the bulk liquid mixture while the interfacial lifetime shows an abrupt rise within a particular range of ethanol concentration depending on the pore size. This abrupt increase is attributed to the appearance of excess free-volume near the interfacial region. The excess free-volume is originated due to microphase separation of confined ethanol-water primarily at the vicinity of the pore wall. We envisage that probing free-volume changes at the interface using PALS is a sensitive way to investigate microphase separation under nanoconfinement.

Hydrophobic fluorine mediated switching of the hydrogen bonding site as well as orientation of water molecules in the aqueous mixture of monofluoroethanol: IR, molecular dynamics and quantum chemical studies

Fluorination of ethanol changes orientation of water in its aqueous mixture.

Clustering mechanism of ethanol-water mixtures investigated with photothermal microfluidic cantilever deflection spectroscopy

The infrared-active (IR) vibrational mode of ethanol (EtOH) associated with the asymmetrical stretching of the C-C-O bond in pico-liter volumes of EtOH-water binary mixtures is calorimetrically measured using photothermal microfluidic cantilever deflection spectroscopy (PMCDS). IR absorption by the confined liquid results in wavelength dependent cantilever deflections, thus providing a complementary response to IR absorption revealing a complex dipole moment dependence on mixture concentration. Solvent-induced blue shifts of the C-C-O asymmetric vibrational stretch for both anti and gauche conformers of EtOH were precisely monitored for EtOH concentrations ranging from 20-100% w/w. Variations in IR absorption peak maxima show an inverse dependence on induced EtOH dipole moment (μ) and is attributed to the complex clustering mechanism of EtOH-water mixtures.

Physical Properties and Hydrogen-Bonding Network of Water-Ethanol Mixtures from Molecular Dynamics Simulations

While many numerical and experimental works were focused on water-ethanol mixtures at low ethanol concentration, this work reports predictions of a few physical properties (thermodynamical, interfacial, dynamical, and dielectrical properties) of water-ethanol mixture at high alcohol concentrations by means of molecular dynamics simulations. By using a standard force field a good agreement was found between experiment and molecular simulation. This was allowed us to explore the dynamics, structure, and interplay between both hydrogen-bonding networks of water and ethanol.