Energetic Origin of Proton Affinity to the Air/Water Interface

Subpicosecond Molecular Rearrangements Affect Local Electric Fields and Auto-Dissociation in Water

Molecular simulations of auto-dissociation of water molecules in an 81,000 atom bulk water system show that the electric field variations caused by local bond length and angle variations enhance proton transfer within ∼600 fs prior to auto-dissociation. In this paper, auto-dissociation relates to the initial separation of a proton from a water molecule to another, forming the H33O+ and OH- ions. Only transfers for which a proton's initial nearest covalently bonded oxygen remained the same for at least 1 ps prior to the transfer and for which that proton's new nearest acceptor oxygen remained the same for at least 1 ps after the transfer were evaluated. Electric fields from solvent atoms within 6 Å of a transferring proton (H*) are dominant, with little contribution from farther molecules. However, exclusion of the accepting oxygen in such electric field calculations shows that the field on H* from the other solvent atoms weakens as the time to transfer becomes less than 600 fs, indicating the primary importance of the accepting oxygen on enabling auto-dissociation. All resultant OH- and H3O+ ion pairs recombined at times greater than 1 ps after auto-dissociation. A concentration of 8.01 × 1017 cm-3 for these ion pairs was observed. The simulations indicate that transient auto-dissociation in water is more common than that inferred from dc-conductivity experiments (10-5 vs 10-7) and is consistent with the results of calculations that include nuclear quantum effects. The conductivity experiments require the rearrangement of farther water molecules to form hydrogen-bonded "water wires" that afford long-range and measurable proton transport away from the reaction site. Nonetheless, the relatively large number of picosecond-lived auto-dissociation products might be engineered within 2D layers and oriented external fields to offer new energy-related systems.

Adsorption Energetics of Amino Acid Analogs on Polymer/Water Interfaces Studied by All-Atom Molecular Dynamics Simulation and a Theory of Solutions

Energetics of adsorption was addressed with all-atom molecular dynamics simulation on the interfaces of poly(2-methoxyethyl acrylate) (PMEA), poly(methyl methacrylate) (PMMA), and poly(butyl acrylate) (PBA) with water. A wide variety of adsorbate solutes were examined, and the free energy of adsorption was computed with the method of energy representation. It was found that the adsorption free energy was favorable (negative) for all the combinations of solute and polymer, and among PMEA, PMMA, and PBA, the strongest adsorption was observed on PMMA for the hydrophobic solutes and on PMEA for the hydrophilic ones. According to the decomposition of the adsorption free energy into the contributions from polymer and water, it was seen that the polymer contribution is larger in magnitude with the solute size. The total free energy of adsorption was correlated well with the solvation free energy in bulk water only for hydrophobic solutes. The roles of the intermolecular interaction components such as electrostatic, van der Waals, and excluded-volume were further studied, and the electrostatic component was influential only in determining the polymer dependences of the adsorption propensities of hydrophilic solutes. The extent of adsorption was shown to be ranked by the van der Waals component in the solute-polymer interaction separately over the hydrophilic and hydrophobic solutes, with the excluded-volume effect from water pointed out to also drive the adsorption.

Solvation energetics of proteins and their aggregates analyzed by all-atom molecular dynamics simulations and the energy-representation theory of solvation

Solvation is a controlling factor for the structure and function of proteins. This article addresses the effects of solvation from an energetic perspective for the fluctuations and cosolvent-induced changes in protein structures and the equilibrium of aggregate formation for a peptide. A theoretical framework to analyze the solvation effects with an explicit solvent is introduced by adopting the energy-representation theory of solvation, and the connection of the solvation free energy to the protein structure and the aggregation tendency is quantitatively described in combination with all-atom molecular dynamics simulations. The interaction components that govern the solvation effects on the structural variations of proteins are further identified through correlation analysis, and a computational scheme to assess the shift of an aggregation equilibrium due to the addition of a cosolvent is provided.

Assessing the Impact of Solvent Selection on Vibrational Sum-Frequency Scattering Spectroscopy Experiments

The development of vibrational sum-frequency scattering (S-VSF) spectroscopy has opened the door to directly probing nanoparticle surfaces with an interfacial and chemical specificity that was previously reserved for planar interfacial systems. Despite its potential, challenges remain in the application of S-VSF spectroscopy beyond simplified chemical systems. One such challenge includes infrared absorption by an absorptive continuous phase, which will alter the spectral lineshapes within S-VSF spectra. In this study, we investigate how solvent vibrational modes manifest in S-VSF spectra of surfactant stabilized nanoemulsions and demonstrate how corrections for infrared absorption can recover the spectral features of interfacial solvent molecules. We also investigate infrared absorption for systems with the absorptive phase dispersed in a nonabsorptive continuous phase to show that infrared absorption, while reduced, will still impact the S-VSF spectra. These studies are then used to provide practical recommendations for anyone wishing to use S-VSF to study nanoparticle surfaces where absorptive solvents are present.

Atmospheric Spectroscopy and Photochemistry at Environmental Water Interfaces

The air-water interface is ubiquitous in nature, as manifested in the form of the surfaces of oceans, lakes, and atmospheric aerosols. The aerosol interface, in particular, can play a crucial role in atmospheric chemistry. The adsorption of atmospheric species onto and into aerosols modifies their concentrations and chemistries. Moreover, the aerosol phase allows otherwise unlikely solution-phase chemistry to occur in the atmosphere. The effect of the air-water interface on these processes is not entirely known. This review summarizes recent theoretical investigations of the interactions of atmosphere species with the air-water interface, including reactant adsorption, photochemistry, and the spectroscopy of reactants at the water surface, with an emphasis on understanding differences between interfacial chemistries and the chemistries in both bulk solution and the gas phase. The results discussed here enable an understanding of fundamental concepts that lead to potential air-water interface effects, providing a framework to understand the effects of water surfaces on our atmosphere.

Spectroscopic BIL-SFG Invariance Hides the Chaotropic Effect of Protons at the Air-Water Interface

The knowledge of the water structure at the interface with the air in acidic pH conditions is of utmost importance for chemistry in the atmosphere. We shed light on the acidic air-water (AW) interfacial structure by DFT-MD simulations of the interface containing one hydronium ion coupled with theoretical SFG (Sum Frequency Generation) spectroscopy. The interpretation of SFG spectra at charged interfaces requires a deconvolution of the signal into BIL (Binding Interfacial Layer) and DL (Diffuse Layer) SFG contributions, which is achieved here, and hence reveals that even though H 3 O + has a chaotropic effect on the BIL water structure (by weakening the 2D-HBond-Network observed at the neat air-water interface) it has no direct probing in SFG spectroscopy. The changes observed experimentally in the SFG of the acidic AW interface from the SFG at the neat AW are shown here to be solely due to the DL-SFG contribution to the spectroscopy. Such BIL-SFG and DL-SFG deconvolution rationalizes the experimental SFG data in the literature, while the hydronium chaotropic effect on the water 2D-HBond-Network in the BIL can be put in perspective of the decrease in surface tension at acidic AW interfaces.

Free-energy analysis of physisorption on solid-liquid interface with the solution theory in the energy representation

Physisorption of urea on its crystal in contact with water was subject to energetics analysis with all-atom molecular dynamics simulation. The transfer free energy of urea to an adsorption site was treated in the framework of the energy-representation theory of solutions, which allows a fast computation of the free energy in an inhomogeneous environment with solid-liquid interface. The preference of adsorption was then compared between the (001) and (110) faces, and it was found that the physisorption is more favorable on (001) than on (110) in correspondence to the hydrogen bonding between the adsorbed urea and the crystal urea. Among the terrace configurations of adsorption, the attractive interaction governs the preferable site with a minor role of the repulsive interaction. The effect of an edge was also treated by examining the terrace and step and was shown to be strongly operative on the (110) face when the CO group of the adsorbed urea points toward the edge. The present work demonstrates that the solution theory can be a framework for analyzing the energetics of physisorption and addressing the roles of the crystal and liquid at the interface through the systematic decomposition of free energy.

Drastic Compensation of Electronic and Solvation Effects on ATP Hydrolysis Revealed through Large-Scale QM/MM Simulations Combined with a Theory of Solutions

Hydrolysis of adenosine triphosphate (ATP) is the "energy source" for a variety of biochemical processes. In the present work, we address key features of ATP hydrolysis: the relatively moderate value (about -10 kcal/mol) of the standard free energy, ΔG_hyd, of reaction and the insensitivity of ΔG_hyd to the number of excess electrons on ATP. We conducted quantum mechanical/molecular mechanical simulation combined with the energy-representation theory of solutions to analyze the electronic-state and solvation contributions to ΔG_hyd. It was revealed that the electronic-state contribution in ΔG_hyd is largely negative (favorable) upon hydrolysis, due to the reduction of electrostatic repulsion accompanying the breakage of the P-O bond. In contrast, the solvation effect was found to be strongly more favorable on the reactant side. Thus, we showed that a drastic compensation of the two opposite effects takes place, leading to the modest value of ΔG_hyd at each number of excess electrons examined. The computational analyses were also conducted for pyrophosphate ions (PPi), and the parallelism between the ATP and PPi hydrolyses was confirmed. Classical molecular dynamics simulation was further carried out to discuss the effect of the solvent environment; the insensitivity of ΔG_hyd to the number of excess electrons was seen to hold in solvent water and ethanol.

Why is Benzene Soluble in Water? Role of OH/π Interaction in Solvation

The XH/π interaction (X = C, N, or O) plays an essential role in a variety of fundamental processes in condensed phase, and it attracts broad interests in the fields of chemistry and biochemistry in recent years. This issue has a direct relevance to an intriguing phenomenon that a benzene molecule exhibits a negative solvation free energy of -0.87 kcal/mol in ambient water though it is a typical nonpolar organic solute. In this work, we developed a novel method to analyze the free energy δμ due to the electron density fluctuation of a solute in solution to clarify the mechanism responsible for the affinity of benzene to bulk water. Explicitly, the free energy δμ is decomposed into contributions from σ and π electrons in π-conjugated systems on the basis of the QM/MM method combined with a theory of solutions. With our analyses, the free energy δμ(π) arising from the fluctuation of π electrons in benzene was obtained as -0.94 kcal/mol and found to be the major source of the affinity of benzene to water. Thus, the role of π electrons in hydration is quantified for the first time with our analyses. Our method was applied to phenyl methyl ether (PME) in water solution to examine the substituent effects of the electron donating group (EDG) on the hydration of a π-conjugated system. The delocalization effect of the π electrons on hydration was also investigated performing the decomposition analyses for ethene and 1,3-butadiene molecules in water solutions. It was revealed that the stabilization due to δμ(π) for butadiene (-0.76 kcal/mol) is about three times as large as that for ethene (-0.26 kcal/mol), which suggests the importance of the delocalization effect of the π electrons in mediating the affinity to polar solvent.

Ions at hydrophobic interfaces

We review the present understanding of the behavior of ions at the air-water and oil-water interfaces. We argue that while the alkali metal cations remain strongly hydrated and are repelled from the hydrophobic surfaces, the anions must be classified into kosmotropes and chaotropes. The kosmotropes remain strongly hydrated in the vicinity of a hydrophobic surface, while the chaotropes lose their hydration shell and can become adsorbed to the interface. The mechanism of adsorption is still a subject of debate. Here, we argue that there are two driving forces for anionic adsorption: the hydrophobic cavitational energy and the interfacial electrostatic surface potential of water. While the cavitational contribution to ionic adsorption is now well accepted, the role of the electrostatic surface potential is much less clear. The difficulty is that even the sign of this potential is a subject of debate, with the ab initio and the classical force field simulations predicting electrostatic surface potentials of opposite sign. In this paper, we will argue that the strong anionic adsorption found in the polarizable force field simulations is the result of the artificial electrostatic surface potential present in the classical water models. We will show that if the adsorption of anions were as large as predicted by the polarizable force field simulations, the excess surface tension of the NaI solution would be strongly negative, contrary to the experimental measurements. While the large polarizability of heavy halides is a fundamental property and must be included in realistic modeling of the electrolyte solutions, we argue that the point charge water models, studied so far, are incompatible with the polarizable ionic force fields when the translational symmetry is broken. The goal for the future should be the development of water models with very low electrostatic surface potential. We believe that such water models will be compatible with the polarizable force fields, which can then be used to study the interaction of ions with hydrophobic surfaces and proteins.

Toward a unified picture of the water self-ions at the air-water interface: a density functional theory perspective

The propensities of the water self-ions, H3O(+) and OH(-), for the air-water interface have implications for interfacial acid-base chemistry. Despite numerous experimental and computational studies, no consensus has been reached on the question of whether or not H3O(+) and/or OH(-) prefer to be at the water surface or in the bulk. Here we report a molecular dynamics simulation study of the bulk vs interfacial behavior of H3O(+) and OH(-) that employs forces derived from density functional theory with a generalized gradient approximation exchange-correlation functional (specifically, BLYP) and empirical dispersion corrections. We computed the potential of mean force (PMF) for H3O(+) as a function of the position of the ion in the vicinity of an air-water interface. The PMF suggests that H3O(+) has equal propensity for the interface and the bulk. We compare the PMF for H3O(+) to our previously computed PMF for OH(-) adsorption, which contains a shallow minimum at the interface, and we explore how differences in solvation of each ion at the interface vs in the bulk are connected with interfacial propensity. We find that the solvation shell of H3O(+) is only slightly dependent on its position in the water slab, while OH(-) partially desolvates as it approaches the interface, and we examine how this difference in solvation behavior is manifested in the electronic structure and chemistry of the two ions.

Electrokinetics of polar liquids in contact with nonpolar surfaces

Zeta potentials of several polar protic (water, ethylene glycol, and formamide) as well as polar aprotic (dimethyl sulfoxide) liquids were measured in contact with three nonpolar surfaces using closed-cell electroosmosis. The test surfaces were chemisorbed monolayers of alkyl siloxanes, fluoroalkyl siloxanes, and polydimethylsiloxanes (PDMS) grafted on glass slides. All these liquids exhibited substantial electrokinetics in contact with the nonpolar surfaces with these observations: the electrokinetic effect on the fluorocarbon-coated surface is the strongest and on a PDMS grafted surface, the effect is the weakest. Even though these hygroscopic liquids contain small amounts of water, the current models of charging based on the adsorption of hydroxide ions at the interface or the dissociation of pre-existing functionalities (e.g., silanol groups) appear to be insufficient to account for the various facets of the experimental observations. The results illustrate how ubiquitous the phenomenon of electrokinetics is with polar liquids contacting such apparently passive nonpolar surfaces. We hope that these results will inspire further experimental and theoretical studies in this important area of research that has potential practical implications.

Brønsted basicity of the air-water interface

Differences in the extent of protonation of functional groups lying on either side of water-hydrophobe interfaces are deemed essential to enzymatic catalysis, molecular recognition, bioenergetic transduction, and atmospheric aerosol-gas exchanges. The sign and range of such differences, however, remain conjectural. Herein we report experiments showing that gaseous carboxylic acids RCOOH(g) begin to deprotonate on the surface of water significantly more acidic than that supporting the dissociation of dissolved acids RCOOH(aq). Thermodynamic analysis indicates that > 6 H(2)O molecules must participate in the deprotonation of RCOOH(g) on water, but quantum mechanical calculations on a model air-water interface predict that such event is hindered by a significant kinetic barrier unless OH(-) ions are present therein. Thus, by detecting RCOO(-) we demonstrate the presence of OH(-) on the aerial side of on pH > 2 water exposed to RCOOH(g). Furthermore, because in similar experiments the base (Me)(3)N(g) is protonated only on pH < 4 water, we infer that the outer surface of water is Brønsted neutral at pH ∼3 (rather than at pH 7 as bulk water), a value that matches the isoelectric point of bubbles and oil droplets in independent electrophoretic experiments. The OH(-) densities sensed by RCOOH(g) on the aerial surface of water, however, are considerably smaller than those at the (>1 nm) deeper shear planes probed in electrophoresis, thereby implying the existence of OH(-) gradients in the interfacial region. This fact could account for the weak OH(-) signals detected by surface-specific spectroscopies.

Structure and stability of charged clusters

When a cluster or nanodroplet bears charge, its structure and thermodynamics are altered and, if the charge exceeds a certain limit, the system becomes unstable with respect to fragmentation. Some of the key results in this area were derived by Rayleigh in the nineteenth century using a continuum model of liquid droplets. Here we revisit the topic using a simple particle-based description, presenting a systematic case study of how charge affects the physical properties of a Lennard-Jones cluster composed of 309 particles. We find that the ability of the cluster to sustain charge depends on the number of particles over which the charge is distributed-a parameter not included in Rayleigh's analysis. Furthermore, the cluster may fragment before the charge is strong enough to drive all charged particles to the surface. The charged particles in stable clusters are therefore likely to reside in the cluster's interior even without considering solvation effects.