Adsorption and solvation of ethanol at the water liquid-vapor interface: a molecular dynamics study

Unraveling the Surface Activity of Ethanol-Water Mixtures through Experiments and Molecular Dynamics Simulations

Ethanol's complete miscibility in water makes it a widely used solvent in various applications, such as organic compound synthesis, paint manufacture, chromatography, and cosmetics preservation. Studies suggest that ethanol's concentration at interfaces can be higher than in the bulk due to its amphiphilic nature, especially at lower concentrations, making it a surface-active agent. Accordingly, ethanol plays a crucial role in controlling the emulsion stability, foam formation, heat transfer, and coating adhesion. However, the precise concentration ranges up to which ethanol's surface activity dominates its interfacial properties, and the underlying molecular mechanism is not fully understood in the literature. In this context, our foamability experiments, coupled with film stability experiments conducted via ethanol drop impact on varying concentration ethanol-water mixture pools, indicate that the surface-active nature of ethanol is observed up to a maximum of 10% molar ethanol concentration in water. We next employ all-atom molecular dynamics simulations to reveal that the surface tension and other interfacial properties are most significantly affected only up to the molar concentration in the range of 0-10% of ethanol in water. This observation is further supported by free energy analyses, indicating that the stabilization free energy of an ethanol molecule at the interface becomes comparable to that in the bulk region beyond this concentration range. The transition from surface-active to a behavior resembling a homogeneous solution occurs when the molar concentration of ethanol in water exceeds 10%. This transition is attributed to distinctive alterations in the number and strength of ethanol-water hydrogen bonds. These findings provide valuable insights into the interfacial molecular structure, which can be suitably exploited to modulate interfacial properties and dynamic behavior in a wide array of industrial and scientific applications.

Investigating the Vapor-Phase Adsorption of Aroma Molecules on the Water-Vapor Interface using Molecular Dynamics Simulations

Surfactants are amphiphilic additives primarily used to reduce the surface tension of water and manipulate its wettability on various surfaces. Recent reports suggest that volatile surfactants, such as aroma molecules, diffuse more quickly to the interface from the vapor-phase than conventional surfactants typically used in the aqueous phase. The ability to adsorb from the vapor phase, in addition to their use as cosurfactants, expands the potential applications of volatile surfactants, particularly in situations where adding surfactants from the liquid phase is difficult. Here, we present a molecular level understanding of the adsorption kinetics of linalool, a common aroma molecule, on the water interface using molecular dynamics simulations. We note that the value of surface tension while adsorption from vapor and liquid phases is dependent only on the surface coverage. A minimum surface tension of 32 ± 1.8 mN/m is obtained in both cases at a maximum surface coverage of 4.88 μmol/m2 at 300 K. We observe the extent of decrease of the H-bonds between linalool-water and linalool-linalool molecules at various surface coverages to explain the mechanism of surface tension reduction. We solve Gibb's adsorption equation to establish a correlation between the surface coverage of linalool and the corresponding bulk concentration in experiments. We investigate the free energy profile of linalool's adsorption behavior at different surface coverages and temperatures. Our report suggests that linalool adsorption onto the water interface is an enthalpy-driven process primarily dependent on the strength of the interaction between the hydroxyl group of linalool and water molecules. These insights are crucial for selecting a suitable aroma molecule for various applications that target the vapor-phase adsorption mechanism.

A Kinetic Model for Predicting Trace Gas Uptake and Reaction

A model is developed to describe trace gas uptake and reaction with applications to aerosols and microdroplets. Gas uptake by the liquid is formulated as a coupled equilibria that links gas, surface, and bulk regions of the droplet or solution. Previously, this framework was used in explicit stochastic reaction-diffusion simulations to predict the reactive uptake kinetics of ozone with droplets containing aqueous aconitic acid, maleic acid, and sodium nitrite. With the use of prior data and simulation results, a new equation for the uptake coefficient is derived, which accounts for both surface and bulk reactions. Lambert W functions are used to obtain closed form solutions to the integrated rate laws for the multiphase kinetics; similar to previous expressions that describe Michaelis-Menten enzyme kinetics. Together these equations couple interface and bulk processes over a wide range of conditions and do not require many of the limiting assumptions needed to apply resistor model formulations to explain trace gas uptake and reaction.

Tuning Contact Angles of Aqueous Droplets on Hydrophilic and Hydrophobic Surfaces by Surfactants

Adsorption of small amphiphilic molecules occurs in various biological and technological processes, sometimes desired while other times unwanted (e.g., contamination). Surface-active molecules preferentially bind to interfaces and affect their wetting properties. We use molecular dynamics simulations to study the adsorption of short-chained alcohols (simple surfactants) to the water–vapor interface and solid surfaces of various polarities. With a theoretical analysis, we derive an equation for the adsorption coefficient, which scales exponentially with the molecular surface area and the surface wetting coefficient and is in good agreement with the simulation results. We apply the outcomes to aqueous sessile droplets containing surfactants, where the competition of surfactant adsorptions to both interfaces alters the contact angle in a nontrivial way. The influence of surfactants is the strongest on very hydrophilic and hydrophobic surfaces, whereas droplets on moderately hydrophilic surfaces are less affected.

Uptake of N2O5 by aqueous aerosol unveiled using chemically accurate many-body potentials

The reactive uptake of N2O5 to aqueous aerosol is a major loss channel for nitrogen oxides in the troposphere. Despite its importance, a quantitative picture of the uptake mechanism is missing. Here we use molecular dynamics simulations with a data-driven many-body model of coupled-cluster accuracy to quantify thermodynamics and kinetics of solvation and adsorption of N2O5 in water. The free energy profile highlights that N2O5 is selectively adsorbed to the liquid-vapor interface and weakly solvated. Accommodation into bulk water occurs slowly, competing with evaporation upon adsorption from gas phase. Leveraging the quantitative accuracy of the model, we parameterize and solve a reaction-diffusion equation to determine hydrolysis rates consistent with experimental observations. We find a short reaction-diffusion length, indicating that the uptake is dominated by interfacial features. The parameters deduced here, including solubility, accommodation coefficient, and hydrolysis rate, afford a foundation for which to consider the reactive loss of N2O5 in more complex solutions.

Influence of drug/lipid interaction on the entrapment efficiency of isoniazid in liposomes for antitubercular therapy: a multi-faced investigation

Isoniazid (INH) is one of the primary drugs used in tuberculosis treatment and its encapsulation in liposomal vesicles can both improve its therapeutic index and minimize toxicity. Here we consider mixtures of hydrogenated soy phosphatidylcholine-phosphatidylglycerol (HSPC-DPPG) to get novel biocompatible liposomes for INH delivery. We determined INH encapsulation efficiency by coupling for the first time UV and Laser Transmission Spectroscopy and we showed that HSPC-DPPG liposomes can load more INH than expected from simple geometrical arguments, thus suggesting the presence of drug-lipid association. To focus on this aspect, which has never been explored in liposomal formulations, we employed several complementary techniques, such as dynamic and static light scattering, calorimetry and surface pressure measurements on lipid monolayers. We find that INH-lipid interaction increases the entrapment capability of liposomes due to INH adsorption. Moreover, the preferential INH-HSPC dipole-dipole interaction promotes the modification of lipid ordering, favoring the formation of HSPC-richer domains in excess of DPPG. Our findings highlight how investigating the fundamental aspects of drug-lipid interactions is of paramount importance for the optimal design of liposomal nanocarriers.

Experimental evidence that halogen bonding catalyzes the heterogeneous chlorination of alkenes in submicron liquid droplets

A key challenge in predicting the multiphase chemistry of aerosols and droplets is connecting reaction probabilities, observed in an experiment, with the kinetics of individual elementary steps that control the chemistry that occurs across a gas/liquid interface. Here we report evidence that oxygenated molecules accelerate the heterogeneous reaction rate of chlorine gas with an alkene (squalene, Sqe) in submicron droplets. The effective reaction probability for Sqe is sensitive to both the aerosol composition and gas phase environment. In binary aerosol mixtures with 2-decyl-1-tetradecanol, linoleic acid and oleic acid, Sqe reacts 12-23× more rapidly than in a pure aerosol. In contrast, the reactivity of Sqe is diminished by 3× when mixed with an alkane. Additionally, small oxygenated molecules in the gas phase (water, ethanol, acetone, and acetic acid) accelerate (up to 10×) the heterogeneous chlorination rate of Sqe. The overall reaction mechanism is not altered by the presence of these aerosol and gas phase additives, suggesting instead that they act as catalysts. Since the largest rate acceleration occurs in the presence of oxygenated molecules, we conclude that halogen bonding enhances reactivity by slowing the desorption kinetics of Cl2 at the interface, in a way that is analogous to decreasing temperature. These results highlight the importance of relatively weak interactions in controlling the speed of multiphase reactions important for atmospheric and indoor environments.

The molecular structure of the surface of water-ethanol mixtures

Mixtures of water and alcohol exhibit an excess surface concentration of alcohol as a result of the amphiphilic nature of the alcohol molecule, which has important consequences for the physico-chemical properties of water-alcohol mixtures. Here we use a combination of intensity vibrational sum-frequency generation (VSFG) spectroscopy, heterodyne-detected VSFG (HD-VSFG), and core-level photoelectron spectroscopy (PES) to investigate the molecular properties of water-ethanol mixtures at the air-liquid interface. We find that increasing the ethanol concentration up to a molar fraction (MF) of 0.1 leads to a steep increase of the surface density of the ethanol molecules, and an increased ordering of the ethanol molecules at the surface. When the ethanol concentration is further increased, the surface density of ethanol remains more or less constant, while the orientation of the ethanol molecules becomes increasingly disordered. The used techniques of PES and VSFG provide complementary information on the density and orientation of ethanol molecules at the surface of water, thus providing new information on the molecular-scale properties of the surface of water-alcohol mixtures over a wide range of compositions. This information is invaluable in understanding the chemical and physical properties of water-alcohol mixtures.

Vapor-Induced Liquid Collection and Microfluidics on Superlyophilic Substrates

Liquid manipulation on solid surfaces has attracted a lot of attention for liquid collection and droplet-based microfluidics. However, manipulation strategies mainly depend on chemical modification and artificial structures. Here, we demonstrate a feasible and general strategy based on the self-shrinkage of the droplet induced via specific vapors to efficiently collect liquids and flexibly carry out droplet-based reactions. The vapor-induced self-shrinkage is driven by Marangoni flow originating from molecular adsorption and diffusion. Under a specific vapor environment, the self-shrinking droplet exhibits unique features including reversible responsiveness, high mobility, and autocoalescence. Accordingly, by building a specific vapor environment, the thin liquid films and random liquid films on superlyophilic substrates can be recovered with a collection rate of more than 95%. Moreover, the vapor system can be used to construct a high-efficiency chemical reaction device. The findings and profound understandings are significant for the development of the liquid collection and droplet-based microfluidics.

Molecular Mechanisms and Atmospheric Implications of Criegee Intermediate-Alcohol Chemistry in the Gas Phase and Aqueous Surface Environments

Criegee intermediates and alcohols are important species in the atmosphere. In this study, we use quantum chemistry and Born-Oppenheimer molecular dynamics (BOMD) simulations to investigate the reaction between methanol/ethanol and Criegee intermediates (anti- or syn-CH₃CHOO) in the gas phase and at the air-water interface. Reactions at the interface are found to be much faster than those in the gas phase. When water molecules are available, loop structures can be formed to facilitate the reaction. In addition, nonloop reaction pathways characterized by the formation of hydrated protons, although with a low possibility, are also identified at the air-water interface. Implications of our results on the fate of Criegee intermediates in the atmosphere are discussed, which deepen our understanding of Criegee intermediate-alcohol chemistry in humid environments.

Control of solutal Marangoni-driven vortical flows and enhancement of mixing efficiency

HYPOTHESIS

In droplet microfluidics applications, flow control and mixing in a small volume without any active external devices is a challenge. Vapor-mediated solutal Marangoni flows can be effectively generated by applying the vapor of a volatile liquid, which can be possibly controlled, and can eventually be used in a mixing enhancement device.

EXPERIMENTS

We investigated and controlled vapor-mediated solutal Marangoni flows by varying the local surface tension. We systematically tested the effects of different volatile liquids and their vapor concentration on the flow pattern. Furthermore, by varying the number of vapor sources, we generated and controlled multiple vortices, and analyzed them by particle image velocimetry (PIV). The proposed method was applied to a mixing enhancement application.

FINDINGS

We show that in addition to the surface tension of the volatile liquid, the vapor concentration also influenced the local surface tension along the interface, which in turn changed the internal flow velocity. To predict the flow velocity and oscillatory frequency of the solutal Marangoni flow, we developed a theoretical model based on scaling analysis that showed a good agreement with the experimental results. We believe that the current study will motivate low-cost and portable sample flow control and mixing systems in the near future.

Moses Effect: Splitting a Sessile Droplet Using a Vapor-Mediated Marangoni Effect Leading to Designer Surface Patterns

In this work, we showcase a mechanism of rapid and focused solvent depletion using vapor-mediated interaction that can nonintrusively cleave a sessile water droplet reminiscent of Moses parting the Red Sea. The Marangoni effect is induced by the differential adsorption of vapor from a nearby pendant droplet of ethanol, leading to an exponential increase in surface velocity inside the water droplet. The Marangoni convection leads to the drainage of liquid from the central section of the water droplet and consequently splits it. By encoding the position of the ethanol (vertical as well as horizontal) droplet, an array of liquid motion is observed (split, shift, and slosh) in the water droplet. This method is further extended to nanocolloidal systems, where the liquid motion can be exploited to generate a wide gamut of deposit patterns ranging from uniform precipitate to sporadic islands without resorting to the more traditional evaporation-driven capillary flows ("coffee stains") or custom engineering of the shape of the nanoparticles. We further provide a detailed exposition of the physical mechanisms responsible for the splitting of the liquid drop and consequent particle deposition. The concept can be extended to liquid actuation in open channel microfluidic chips and surface patterning as in medical diagnostics, optoelectronics, and thermal management.

Comparing Alchemical Free Energy Estimates to Experimental Values Based on the Ben-Naim Formula: How Much Agreement Can We Expect?

The solvation free energy (SFE) plays a key role in thermodynamics. One well-established method for computing the SFE is through an alchemical transformation. However, experimental SFEs are generally determined according to the Ben-Naim equations relying on vapor pressure or density ratios. It is important to establish whether, or to what extent, typical alchemical-based free energy computations provide results comparable to experimental SFEs. In this work, we mimic experimental measurements by simulating the liquid-vapor coexistence of water without alchemical operations. The SFEs measured through vapor pressure and density ratios are used to validate the SFEs obtained through alchemical transformations. It is shown that proper consideration of the nonideal behavior of the vapor is important to ensure that the alchemical SFEs are consistent with the Ben-Naim SFEs. Alchemical transformations in the vapor phase should be performed in addition to solution phase transformations for strongly interacting solutes, such as those with low boiling temperatures and large second virial coefficients. A formula based on the virial expansion of pressure is proposed to provide a better estimate of the true SFE from the simulated vapor pressures. The proposed formula is also applicable to experimental determinations of SFE when the pressure-based route is used.

Controlling self-assembly and buckling in nano fluid droplets through vapour mediated interaction of adjacent droplets

HYPOTHESIS

Sessile droplets of contrasting volatilities can communicate via long range (∼O (1) mm) vapour-mediated interactions which allow the remote control of the flow driven self-assembly of nanoparticles in the drop of lower volatility. This allows morphological control of the buckling instability observed in evaporating nanofluid droplets.

EXPERIMENTS

A nanofluid droplet is dispensed adjacent to an ethanol droplet. Asymmetrical adsorption induced Marangoni flow (∼O (1) mm/s) internally segregates the particle population. Particle aggregation occurs preferentially on one side of the droplet leaving the other side to develop a relatively weaker shell which buckles under the effect of evaporation driven capillary pressure.

FINDINGS

The inter-droplet distance is varied to demonstrate the effect on the precipitate shape (flatter to dome shaped) and the location of the buckling (top to side). In addition to being a simple template for hierarchical self-assembly, the presented exposition also promises to enhance mixing rates (∼1000 times) in droplet-based bioassays with minimal contamination.

Dilution of whisky - the molecular perspective

Whisky is distilled to around 70% alcohol by volume (vol-%) then diluted to about 40 vol-%, and often drunk after further slight dilution to enhance its taste. The taste of whisky is primarily associated with amphipathic molecules, such as guaiacol, but why and how dilution enhances the taste is not well understood. We carried out computer simulations of water-ethanol mixtures in the presence of guaiacol, providing atomistic details on the structure of the liquid mixture. We found that guaiacol is preferentially associated with ethanol, and, therefore, primarily found at the liquid-air interface in mixtures that contain up to 45 vol-% of ethanol. At ethanol concentrations of 59 vol-% or higher, guaiacol is increasingly surrounded by ethanol molecules and is driven to the bulk. This indicates that the taste of guaiacol in the whisky would be enhanced upon dilution prior to bottling. Our findings may apply to other flavour-giving amphipathic molecules and could contribute to optimising the production of spirits for desired tastes. Furthermore, it sheds light on the molecular structure of water-alcohol mixtures that contain small solutes, and reveals that interactions with the water may be negligible already at 89 vol-% of ethanol.

Molecular level properties of the free water surface and different organic liquid/water interfaces, as seen from ITIM analysis of computer simulation results

Molecular dynamics simulations of the interface of water with four different apolar phases, namely water vapour, liquid carbon tetrachloride, liquid dichloromethane (DCM) and liquid dichloroethane (DCE) are performed on the canonical ensemble at 298 K. The resulting configurations are analysed using the novel method of identification of the truly interfacial molecules (ITIM). Properties of the first three molecular layers of the liquid phases (e.g. width, spacing, roughness, extent of the in-layer hydrogen bonding network) as well as of the molecules constituting these layers (e.g., dynamics, orientation) are investigated in detail. In the analyses, particular attention is paid to the effect of the polarity of the non-aqueous phase and to the length scale of the effect of the vicinity of the interface on the various properties of the molecules. The obtained results show that increasing polarity of the non-aqueous phase leads to the narrowing of the interface, in spite of the fact that, at the same time, the truly interfacial layer of water gets somewhat broader. The influence of the nearby interface is found to extend only to the first molecular layer in many respects. This result is attributed to the larger space available for the truly interfacial than for the non-interfacial molecules (as the shapes of the two liquid surfaces are largely independent of each other, resulting in the presence of voids between the two phases), and to the fact that the hydrogen bonding interaction of the truly interfacial water molecules with other waters is hindered in the direction of the interface.

Pyrene fluorescence at air/sodium dodecyl sulfate solution interface

The dependence of pyrene fluorescence spectra on the concentration of sodium dodecyl sulfate (SDS) was observed, where the solution was prepared from water saturated with pyrene. The values of the I(1)/I(3) ratio from the bulk solution and from the upper meniscus region in an optical cell were similar but decreased rapidly around the critical micelle concentration (cmc) of SDS, indicating that pyrene molecules preferred to be solubilized in the micelles having a lower dielectric constant. The fluorescence intensity of the excimer indicated the concentration of pyrene molecules at the air/solution interface or the surface activity of pyrene molecules. In addition, the intensity from the meniscus region is much larger than that from the bulk at the concentrations below the cmc, whereas there was no difference in the intensity between the bulk and the meniscus above 8 mmol dm(-3) of SDS. The analysis of the fluorescence intensity from the excimer strongly suggests the presence of molecular aggregates that are favorable to the pyrene molecules just like the micelles in the bulk, making them less movable.

A polarizable continuum model for molecules at diffuse interfaces

In this work we illustrate an extension of the polarizable continuum model to describe solvation effects on molecules at the interface between two fluid phases (liquid/liquid, liquid/vapor). This extension goes beyond the naive picture of the interface as a plane dividing two distinct dielectrics, commonly employed in continuum models. The main feature of the model is the use of a diffuse interface with an electric permittivity depending on the position. This characteristic clearly allows the study of simple interfaces as well as more complex membrane or multilayer structures. Moreover the smooth variation of the permittivity in the diffuse interface, in contrast to the sharp boundary between two regions, overcomes the numerical divergences due to charges placed at the boundary. The implementation of the model relies on the integral equation formalism, which allows one to calculate the reaction field acting on a molecule immersed in a dielectric environment once the proper Green's function is known. In the present case, such a Green's function is obtained numerically, allowing a large flexibility in the choice of the dielectric permittivity profile. The applications have been selected with the aim of illustrating the capabilities of the model; its present limitations are also discussed.

Energy transfer at a gas-liquid interface: kinematics in a prototypical system

A detailed characterization of collisional energy transfer at a liquid surface not only provides a framework for the interpretation of experimental studies but also affords insight into energy feedback mechanisms that may be important in multiphase combustion processes. We address this problem by performing simulations of a prototypical Lennard-Jones system, investigating the dependence of the energy transfer and incident-atom trapping probability on the liquid temperature, on the mass and angle of incidence of the impinging atom, and on the strength of the gas-liquid interaction. In general, in agreement with the results of experiments, these calculations point to the dominance of kinematic effects in determining the gross energy transfer, but they also attest to the important role played by surface roughening in the enhancement of energy transfer that accompanies an increase in the liquid temperature.

Structure of the Liquid−Vapor Interface of Water−Methanol Mixtures as Seen from Monte Carlo Simulations

Monte Carlo simulation of the vapor-liquid interface of water-methanol mixtures of different compositions, ranging from pure water to pure methanol, have been performed on the canonical (N, V, T) ensemble at 298 K. The analysis of the systems simulated has revealed that the interface is characterized by a double layer structure: methanol is strongly adsorbed at the vapor side of the interface, whereas this adsorption layer is followed at its liquid side by a depletion layer of methanol of lower concentration than in the bulk liquid phase of the system. The dominant feature of the interface has been found to be the adsorption layer in systems of methanol mole fractions below 0.2, and the depletion layer in systems of methanol mole fractions between 0.25 and 0.5. The orientation of the molecules located at the depletion layer is found to be already uncorrelated with the interface, whereas the methanol molecules of the adsorption layer prefer to align perpendicular to the interface, pointing straight toward the vapor phase by their methyl group. Although both the preference of the molecular plane for a perpendicular alignment with the interface and the preference of the methyl group for pointing straight to the vapor phase are found to be rather weak, the preference of the methyl group for pointing as straight toward the vapor phase as possible within the constraint imposed by the orientation of the molecular plane is found to be fairly strong. One of the two preferred orientations of the interfacial water molecules present in the neat system is found to disappear in the presence of methanol, because methanol molecules aligned in their preferred orientation can replace these water molecules in the hydrogen-bonding pattern of the interface.

Computer Simulation Investigation of the Water−Benzene Interface in a Broad Range of Thermodynamic States from Ambient to Supercritical Conditions

The dependence of the properties of the water-benzene system on the thermodynamic conditions in a broad range of temperatures and pressures has been investigated by computer simulation methods. For this purpose, Monte Carlo simulations have been performed at 23 different thermodynamic states, ranging from ambient to supercritical conditions. The density profiles of the water and benzene molecules have been determined at each of the thermodynamic states investigated. Information on the dependence of the mutual solubility of the two components in each other as well as of the width of the interface on the temperature and pressure has been extracted from these profiles. The width of the interface has been found to increase with increasing temperature up to a certain point, where it diverges. The temperature of this divergence corresponds to the mixing of the two phases. The determination of the critical mixing temperature at various pressures allowed us to estimate the upper critical curve, separating the two-phase and one-phase liquid systems, of the phase diagram of the simulated water-benzene system. In analyzing the preferential orientation of the interfacial molecules relative to the interface, it has been found that the main orientational preference of the benzene molecules is to lie parallel with the plane of the interface, and the water molecules penetrated deepest into the benzene phase prefer to stay perpendicular to the interface, pointing by one of their O-H bonds almost straight toward the benzene phase, whereas the waters located at the aqueous side of the interface are preferentially aligned parallel with the interfacial plane. Although the strength of the observed orientational preferences decreases rapidly with increasing temperature, the preferred orientations themselves are found to be independent of the thermodynamic conditions. Remains of the orientational preferences of the molecules are found to be present up to temperatures as high as 650 K. The analysis of the relative orientation of the neighboring water-benzene pairs has revealed that the radius of the first hydration shell of the benzene molecules is independent of the thermodynamic conditions, even if the system consists of one single phase. It has been found that the nearest water neighbors of the benzene molecules are preferentially located above and below the benzene ring, whereas more distant water neighbors, belonging still to the first hydration shell, prefer to stay within the plane of the benzene molecule. In the two-phase systems the dipole vector of the nearest waters has been found to be preferentially perpendicular to the vector pointing from the center of the benzene molecule to the water O atom.

Molecular Dynamics Simulations of Atmospheric Oxidants at the Air−Water Interface: Solvation and Accommodation of OH and O3

A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass accommodation (alpha) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated toward the liquid surface when they are approximately 5 angstroms above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 to 140 ps, and the values of alpha for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atmospheric chemistry are discussed.

Influence of the water structure on the acetylcholinesterase efficiency

We have studied the catalytic efficiency of acetylcholinesterase (AChE) in various solutions with ion-disturbed water structure to explore the role that the water structure plays in the substrate-enzyme encounter. The extent of water structuring in the different aqueous solutions was determined by near-infrared spectroscopy. The influence of water structure on the degree of solvation and on the intramolecular mobility of AChE was investigated for different aqueous ionic solutions by small-angle x-ray scattering technique and depolarization fluorescence spectroscopy. It was found that the encounter process between AChE and acetylthiocholine was promoted in solutions with less structured water. In these solutions it was also found that AChE is less solvated coinciding with higher intramolecular mobility. The found experimental results suggest that the water structure may influence the substrate-enzyme encounter process by diminishing the AChE solvation shell and may help diffusion of the substrate through the gorge by enhancing the intramolecular mobility of AChE.

Structure of the Acetone Liquid−Vapor Interface as Seen from Monte Carlo Simulations

The orientational order of the molecules at the liquid-vapor interface of acetone has been investigated by computer simulation. To fully describe the orientational preferences of the acetone molecules, the bivariate joint distribution of two independent orientational parameters has been determined at different layers of the interface. The strength of the orientational ordering of the interfacial molecules has been found to be liquid-like rather than crystal-like. The obtained results have revealed that the interfacial acetone molecules have dual orientational preferences. The main symmetry axis of the molecules declines by about 50-70 degrees from the interface normal axis, pointing toward the liquid phase in both of the preferred orientations. However, the plane of the molecules in the orientation preferred on the liquid side of the interface is perpendicular to the interfacial plane, whereas the other preferred orientation, which is present on the vapor side of the interface, corresponds to the alignment obtained from this orientation by an almost 90 degrees rotation around the main symmetry axis. Because the population of the liquid side is higher than that of the vapor side of the interface, the first of the two preferred orientations is the dominant alignment over the entire interface, in good agreement with recent experimental findings (Chen, H.; Gan, W.; Wu, B. H.; Wu, D.; Zhang, Z.; Wang, H. F. Chem. Phys. Lett. 2005, 408, 284).

MOLECULAR BEAM STUDIES OF GAS-LIQUID INTERFACES

Molecular beam scattering experiments provide a way to disentangle the elementary steps involved in energy transfer and chemical reactions between gases and liquids. After surveying the history and recent progress in this field, we review studies of the kinematics of gas-liquid collisions and proton exchange of HCl, DCl, and HBr with supercooled sulfuric acid and liquid glycerol. These experiments help to clarify the role of the surface region in controlling trapping and interfacial- and bulk-phase reactions.