Non-linear vibrational sum frequency spectroscopy of atmospherically relevant molecules at aqueous solution surfaces

Surface vibrational sum frequency spectroscopy has been shown to be a powerful surface probe of molecules adsorbed at solid and liquid surfaces. Studies described herein apply this method to studying heterogeneous air/aqueous solution interfaces to understand surface adsorption and structure of several solute molecules adsorbed at aqueous surfaces. The molecules examined at aqueous solution surfaces include Dimethyl sulfoxide (DMSO), methane sulfonic acid (MSA) and acetone. These results reveal that small soluble molecules such as these organize in different ways at the surface of aqueous solutions. This surface organization has implications for atmospheric chemical processes since adsorption at the surface of atmospheric aerosols affects bulk chemical concentrations.     

The line adsorption equation: the one-dimensional counterpart of the Gibbs adsorption equation

The thermodynamic development for multiphase contact lines is analogous to that for surfaces or interfaces. However, for one of the most important equations in surface thermodynamics, the Gibbs adsorption equation, the one-dimensional analogue is missing. This paper derives such an analogue, the line adsorption equation. Similarly to the Gibbs adsorption equation, the line adsorption equation is derived from Gibbsian thermodynamics. For a three-phase, three-component contact line system (e.g. an oil lens on the surface of an aqueous solution), the line concentrations (excesses) of two immiscible solvents can be made vanish by appropriately placing the dividing line. Consequently, the line concentration of the solute can be evaluated through the line tension change with the volume concentration of the solute. Such an evaluation provides information about molecular adsorption at the contact line, which is important in physical chemistry of lines, but difficult to obtain by any other means.     

A molecular dynamics study of the dielectric properties of aqueous solutions of alanine and alanine dipeptide

Molecular dynamics simulations were used to compute the frequency-dependent dielectric susceptibility of aqueous solutions of alanine and alanine dipeptide. We studied four alanine solutions, ranging in concentration from 0.13-0.55 mol/liter, and two solutions of alanine dipeptide (0.13 and 0.27 mol/liter). In accord with experiment we find a strong dielectric increment for both solutes, whose molecular origin is shown to be the zwitterionic nature of the solutes. The dynamic properties were analyzed based on a dielectric component analysis into solute, a first hydration shell, and all remaining (bulk) waters. The results of this three component decomposition were interpreted directly, as well as by uniting the solute and hydration shell component to a "suprasolute" component. In both approaches three contributions to the frequency-dependent dielectric properties can be discerned. The quantitatively largest and fastest component arises from bulk water [i.e., water not influenced by the solute(s)]. The interaction between waters surrounding the solute(s) (the hydration shell) and bulk water molecules leads to a relaxation process occurring on an intermediate time scale. The slowest relaxation process originates from the solute(s) and the interaction of the solute(s) with the first hydration shell and bulk water. The primary importance of the hydration shell is the exchange of shell and bulk waters; the self-contribution from bound water molecules is comparatively small. While in the alanine solutions the solute-water cross-terms are more important than the solute self-term, the solute contribution is larger in the dipeptide solutions. In the latter systems a much clearer separation of time scales between water and alanine dipeptide related properties is observed. The similarities and differences of the dielectric properties of the amino acid/peptide solutions studied in this work and of solutions of mono- and disaccharides and of the protein ubiquitin are discussed.     

Molecular details of ovalbumin-pectin complexes at the air/water interface: a spectroscopic study

To stabilize air-water interfaces, as in foams, the adsorption of surface-active components is a prerequisite. An approach to controlling the surface activity of proteins is noncovalent complex formation with a polyelectrolyte in the bulk phase. The molecular properties of egg white ovalbumin in a complex with pectin in the bulk solution and at air/water interfaces were studied using drop tensiometry (ADT) and time-resolved fluorescence anisotropy techniques. The complex formation of ovalbumin with pectin in the bulk resulted in the formation of a compact structure with a different spatial arrangement depending on the protein/pectin ratio. Complex formation did not provide an altered protein structure, whereas the conformational stability was slightly increased in the complex. In excess pectin, an overall condensed complex structure is formed, whereas at limited pectin concentrations the structure of the complex is more "segmental". The characteristics of these structures did not depend on pH in the 7.0 to 4.5 regime. Interaction with pectin in the bulk solution resulted in a significantly slower adsorption of the protein to the air/water interface. The limited mobility of the protein at the interface was found for both ovalbumin and ovalbumin-pectin complexes. From both the rotational dynamics and total fluorescence properties of the protein in the absence and presence of pectin, it was suggested that the complex does not dissociate at the interface. Ovalbumin in the complex retains its initial "aqueous" microenvironment at the interface, whereas in the absence of pectin the microenvironment of the protein changed to a more nonpolar one. This work illustrates a more general property of polyelectrolytes, namely, the ability to retain a protein in its microenvironment. Insight into this property provides a new tool for better control of the surface activity of complex biopolymer systems.     

Local fluctuations in solution mixtures

An extension of the traditional Kirkwood-Buff (KB) theory of solutions is outlined which provides additional fluctuating quantities that can be used to characterize and probe the behavior of solution mixtures. Particle-energy and energy-energy fluctuations for local regions of any multicomponent solution are expressed in terms of experimentally obtainable quantities, thereby supplementing the usual particle-particle fluctuations provided by the established KB inversion approach. The expressions are then used to analyze experimental data for pure water over a range of temperatures and pressures, a variety of pure liquids, and three binary solution mixtures - methanol and water, benzene and methanol, and aqueous sodium chloride. In addition to providing information on local properties of solutions it is argued that the particle-energy and energy-energy fluctuations can also be used to test and refine solute and solvent force fields for use in computer simulation studies.     

Dynamics of encapsulation and controlled release systems based on water-in-water emulsions: negligible surface rheology

A nonequilibrium thermodynamic model based on the interfacial transport phenomena (ITP) formalism was used to study deformation-relaxation behavior of water-in-water emulsions. The ITP formalism allows us to describe all water-in-water emulsions with one single theory. Phase-separated biopolymer solutions, hydrogel beads, liposomes, polymersomes, colloidosomes, and aqueous polymer microcapsules are all limiting cases of this general theory with respect to rheological behavior of the bulk phases and interfaces. Here we have studied two limiting cases of the general theory, with negligible surface rheology: phase-separated biopolymer solutions and hydrogel beads. We have determined the longest relaxation time for a small perturbation of the interfaces in these systems. Parameter maps were calculated which can be used to determine when surface tension, bending rigidity, permeability, and bulk viscoelasticity dominate the response of a droplet or gel bead. In phase-separated biopolymer solutions and dispersions of hydrogel beads six different scaling regimes can be identified for the relaxation time of a deformation. Hydrogel beads may also have a damped oscillatory response to a deformation. The results presented here provide new insight into the complex dynamics of water-in-water emulsions and also suggest new experiments that can be used to characterize the interfacial properties of these systems.     

On the determination of partial molar polarizations and dipole moments of solutes from multicomponent solutions alone: experimental and model development using deutero-labeled organic compounds

A general inverse problem methodology is introduced to determine the partial molar polarizations and the dipole moments of individual solutes from multicomponent solutions alone. A model quaternary system consisting of three deuterated solutes, for example, acetone-d6, acetonitrile-d3, and dimethylformamide-d7 in cyclohexane at 298.15 K and 0.1013 MPa, was studied. Following an experimental design protocol, multicomponent solutions in the range of concentration 0.0006 < x(solute i) < 0.0085 were prepared using a semi-batch procedure by injecting one solute at a time. In situ FTIR spectroscopic measurements of these quaternary solutions were performed together with simultaneous condensed-phase bulk measurements of density, refractive index, and relative permittivity. Three different numerical approaches were used to determine the individual limiting solute molar polarizations from the multicomponent solutions. These limiting molar polarizations were then used to calculate the individual solute dipole moments using the Debye formula. In addition, direct dipole moment calculations were performed using the Guggenheim-Smith formula where individual solute parameters were obtained from multivariate analysis of the multicomponent solution data. Response surface models played a central role in many of the inverse problems. The results of the various methods are compared. In general, the dipole moments of all solutes from multicomponent solutions were in good agreement with those determined from independent binary experiments. Additionally, numerical sensitivity analysis was performed in order to identify the significant contributions to dipole moment uncertainty. The general approach introduced in the present contribution can be applied to a wide range of systems.     

A combined droplet train and ambient pressure photoemission spectrometer for the investigation of liquid/vapor interfaces

We describe a combined ambient pressure photoelectron spectroscopy/droplet train apparatus for investigating the nature and heterogeneous chemistry of liquid/vapor interfaces. In this instrument a liquid droplet train with typical droplet diameters from 50-150 mum is produced by a vibrating orifice aerosol generator (VOAG). The droplets are irradiated by soft X-rays (100-1500 eV) in front of the entrance aperture of a differentially pumped electrostatic lens system that transfers the emitted electrons into a conventional hemispherical electron analyzer. The photoemission experiments are performed at background pressures of up to several Torr, which allows the study of environmentally important liquid/vapor interfaces, in particular aqueous solutions, under equilibrium conditions. The exposure time of the droplet surface to the background gases prior to the XPS measurement can be varied, which will allow future kinetic measurements of gas uptake on liquid surfaces. As an example, a measurement of the surface composition of a chi = 0.21 aqueous methanol solution is presented. The concentration of methanol at the vapor/liquid interface is enhanced by a factor of about 3 over the bulk value, while the expected bulk value is recovered at depths larger than about 1.5 nm.     

Different Views on the Stability of Protein Conformations and Hydrophobic Effects

Apparently contradictory statements about the thermodynamics of aqueous protein solutions and of hydrophobic effect are quoted and discussed. Some credibility is found in the divergent points of view and it is pointed out that they focus attention on different aspects of the complicated conditions in aqueous solutions, some of which are more important than others for the stability of protein conformations.The importance of characteristics of solvent water is emphasized, in particular (1) the strong mutual cohesion of water molecules, and (2) structural changes of water induced by (nonpolar) solute molecules. It is stressed that consideration of only one of these effects and an inexpedient choice of standard states are origins of confusion in the literature about aqueous systems. A simple approach to hydrophobic effects considering both of the above mentioned effects, is proposed.     

Polyelectrolyte/surfactant mixtures in the bulk and at water/oil interfaces

Stabilization of emulsions by mixed polyelectrolyte/surfactant systems is a prominent example for the application in modern technologies. The formation of complexes between the polymers and the surfactants depends on the type of surfactant (ionic, non-ionic) and the mixing ratio. The surface activity (hydrophilic–lipophilic balance) of the resulting complexes is an important quantity for its efficiency in stabilizing emulsions. The interfacial adsorption properties observed at liquid/oil interfaces are more or less equivalent to those observed at the aqueous solution/air interface, however, the corresponding interfacial dilational and shear rheology parameters differ quite significantly. The interfacial properties are directly linked to bulk properties, which support the picture for the complex formation of polyelectrolyte/surfactant mixtures, which is the result of electrostatic and hydrophobic interactions. For long alkyl chain surfactants the interfacial behavior is strongly influenced by hydrophobic interactions while the complex formation with short chain surfactants is mainly governed by electrostatic interactions.     

Structure, dynamics, and the free energy of solute adsorption at liquid-vapor interfaces of simple dipolar systems: molecular dynamics results for pure and mixed Stockmayer fluids

We have performed molecular dynamics simulation studies of the structural, thermodynamic, and dynamical properties of liquid-vapor interfaces of pure and binary Stockmayer fluids of different polarity. The density profiles, the width of the liquid-vapor interface, and the orientational structure of the interfaces are calculated to characterize the structural aspects of the interfaces. Among the thermodynamic properties, we have computed the surface tension and also the free energy of transfer of a charged solute across the liquid-vapor interface for both pure and mixed fluids. Among the dynamical properties of the interfaces, we have calculated the time dependence of the velocity and angular velocity autocorrelation functions, continuous and intermittent survival probabilities, mean square displacements, diffusion coefficients, and also the dipole correlation functions and orientational relaxation times of interfacial solvent molecules. It is found that the width of the interfaces decreases with increase of concentration of the more polar component. The dipole vectors of the interfacial molecules tend to align parallel to the surfaces and this alignment is enhanced with increasing dipole moment of the fluid molecules. Also, the surface tension shows an increasing trend with increase of dipole moment of the molecules. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. In general, the molecules at the interfaces are found to rotate and translate in the parallel direction at a somewhat faster rate than the bulk molecules. Also, on increase of concentration of the more polar component, the diffusion and orientational relaxation of interfacial molecules are found to show a weaker slowing down than those of the bulk molecules, which can be attributed to the preferential presence of the more polar component in the bulk liquid regions. The temporal behavior of the interfacial survival probabilities reveals a decrease of the survival times with increasing polarity, which can be attributed to a corresponding decrease in the interfacial thickness. Results are presented for both continuous and intermittent survival times and the origins of their differences are discussed. The free energy calculations reveal no minimum at the interfaces for adsorption of a charged solute, which shows that the ions would prefer to stay in the interior of the liquid phases, rather than at interfaces, for these model dipolar systems.     

Simultaneous correlation of hydrophobic interactions in HIC and protein solubility in aqueous salt solutions and mixed solvents

The chromatographic retention in hydrophobic and reversed phase chromatography and the solubility of proteins display some common features. The chromatographic retention, as well as the solubility, is modulated by the thermodynamic properties of the solute in the fluid phase. The retention measurements at linear conditions provide information of the solution properties of the protein at infinite dilution, and the solubility measurements produce the supplementary information about the solution properties at the saturation limit. This provides a useful approach to simultaneous correlation of the chromatographic retention and the solubility.The experimental data, used for the correlation, comprise retention measurements of lysozyme on different HIC adsorbents using an aqueous ammonium sulphate eluant, an aqueous ammonium sulphate eluant with an admixture of ethanol, as well as published solubility data.The chromatographic retention data and the corresponding solubility data have been correlated using a chemical potential model derived from Kirkwood's theory of solutions of charged macro-ions and zwitterions in electrolyte solutions. The model correlated the chromatographic retention factor and the solubility data within the precision of the measurements. The model was applied in a pH range from 4 to 11. It was demonstrated experimentally, as well as theoretically, that an admixture of ethanol to the aqueous eluant changes the thermodynamic retention factor on various adsorbents identically when compared to the thermodynamic retention factor in an ethanol free eluant.     

Benchmarking the thermodynamic analysis of water molecules around a model beta sheet

Water molecules play a vital role in biological and engineered systems by controlling intermolecular interactions in the aqueous phase. Inhomogeneous fluid solvation theory provides a method to quantify solvent thermodynamics from molecular dynamics or Monte Carlo simulations and provides an insight into intermolecular interactions. In this study, simulations of TIP4P‐2005 and TIP5P‐Ewald water molecules around a model beta sheet are used to investigate the orientational correlations and predicted thermodynamic properties of water molecules at a protein surface. This allows the method to be benchmarked and provides information about the effect of a protein on the thermodynamics of nearby water molecules. The results show that the enthalpy converges with relatively little sampling, but the entropy and thus the free energy require considerably more sampling to converge. The two water models yield a very similar pattern of hydration sites, and these hydration sites have very similar thermodynamic properties, despite notable differences in their orientational preferences. The results also predict that a protein surface affects the free energy of water molecules to a distance of approximately 4.0 Å, which is in line with previous work. In addition, all hydration sites have a favorable free energy with respect to bulk water, but only when the water–water entropy term is included. A new technique for calculating this term is presented and its use is expected to be very important in accurately calculating solvent thermodynamics for quantitative application. © 2012 Wiley Periodicals, Inc.     

Estimation of Adsorption Capacity for Dissociating and Non Dissociating Aromatic Compounds on Activated Carbon with Different Models

The process of adsorption of two dissociating and two non-dissociating aromatic compounds from dilute aqueous solutions on an untreated commercially available activated carbon (B.D.H.) was investigated systematically. All adsorption experiments were carried out in pH controlled aqueous solutions. The experimental isotherms were fitted into four different models (Langmuir homogenous Models, Langmuir binary Model, Langmuir-Freundlich single model and Langmuir-Freundlich double model). Variation of the model parameters with the solution pH was studied and used to gain further insight into the adsorption process. The relationship between the model parameters and the solution pH and pK_a was used to predict the adsorption capacity in molecular and ionic form of solutes in other solution.A relationship was sought to predict the effect of pH on the adsorption systems and for estimating the maximum adsorption capacity of carbon at any pH where the solute is ionized reasonably well.N₂ and CO₂ adsorption were used to characterize the carbon. X-ray Photoelectron Spectroscopy (XPS) measurement was used for surface elemental analysis of the activated carbon.     

Separating viscoelastic and compressibility contributions in pressure-area isotherm measurements

Monolayers of surface active molecules or particles play an important role in biological systems as well as in consumer products. Their properties are controlled by thermodynamics as well as the mechanical properties of the interface itself. For insoluble species forming Langmuir monolayers, surface pressure-area isotherms are typically used to characterize the thermodynamic state. A Langmuir trough equipped with a Wilhelmy plate is often used for such measurements. However, when Langmuir interfaces are compressed and become more structured, the elastic response of these interfaces can interfere with the measurement of the surface pressure-area isotherm, even when the compression speed is slow. Recent reports of compression data for highly elastic interfaces revealed a dependence of the apparent surface pressures on the geometry of the measurement trough. In the present work, this dependence is investigated by considering adequate constitutive models. Since deformations in such compression experiments can be large, linearized versions of the Kelvin–Voigt model do not suffice. We develop a framework for quasi-linear constitutive models by choosing suitable non-linear strain tensors, adequately separating the shear and dilatational effects in a frame invariant manner. The proposed constitutive models can be used as building blocks to describe viscoelastic behavior as well. The geometry dependence in isotherm measurements is then shown to be a consequence of varying contributions of the isotropic surface pressure and extra shear and dilatational elastic stresses. Using these insights, an approach is proposed to obtain the intrinsic surface pressure-area isotherms for elastic interfaces. As a case study, experimental data on graphene oxidesheets at the air–water interface is investigated to evaluate the proposed model.     

Activity of water in aqueous systems; a frequently neglected property

In this critical review, the significance of the term 'activity' is examined in the context of the properties of aqueous solutions. The dependence of the activity of water(l) at ambient pressure and 298.15 K on solute molality is examined for aqueous solutions containing neutral solutes, mixtures of neutral solutes and salts. Addition of a solute to water(l) always lowers its thermodynamic activity. For some solutes the stabilisation of water(l) is less than and for others more than in the case where the thermodynamic properties of the aqueous solution are ideal. In one approach this pattern is accounted for in terms of hydrate formation. Alternatively the pattern is analysed in terms of the dependence of practical osmotic coefficients on the composition of the aqueous solution and then in terms of solute-solute interactions. For salt solutions the dependence of the activity of water on salt molalities is compared with that predicted by the Debye-Hückel limiting law. The analysis is extended to consideration of the activities of water in binary aqueous mixtures. The dependence on mole fraction composition of the activity of water in binary aqueous mixtures is examined. Different experimental methods for determining the activity of water in aqueous solutions are critically reviewed. The role of water activity is noted in a biochemical context, with reference to the quality, stability and safety of food and finally with regard to health science.     

Physicochemical and Rheological Properties of Novel Magnesium Salt-Polyacrylamide Composite Polymers

Two series of novel inorganic-organic composite polymers have been prepared through physical blending of magnesium chloride and magnesium hydroxide respectively with polyacrylamide aqueous solution. The physicochemical properties of the magnesium salt-polyacrylamide composite polymers were tuned by varying the ratio between the magnesium salt (e.g., magnesium chloride and magnesium hydroxide) and polyacrylamide. Characterizations of magnesium salt-polyacrylamide composite polymers were carried out via FTIR and TEM. Parameters such as solution conductivity and viscosity were also taken into account to characterize the physicochemical properties of the composite polymer aqueous solutions. Magnesium chloride-polyacrylamide (MCPAM) composite polymer aqueous solutions have a higher conductivity compared to magnesium hydroxide-polyacrylamide (MHPAM) composite polymer aqueous solutions. The viscosities of the MHPAM composite polymer aqueous solutions were found higher than MCPAM composite polymer aqueous solutions. The rheological properties of the composite polymer aqueous solutions were investigated using steady-state flow and oscillatory frequency sweep within the linear viscoelastic region. Shear-thinning effect was observed for both composite polymer systems when the shear rate increases. In oscillatory frequency sweep tests, both composite polymer systems show that the viscoelastic behaviors depend strongly on the magnesium salt concentrations. Viscous behavior was found to be dominant for both composite polymer systems.     

Enrichment of deuterium oxide at hydrophilic interfaces in aqueous solutions

The structure of water at aqueous interfaces is of the utmost importance in biology, chemistry, and geology. We use neutron reflectivity and quartz crystal microbalance to probe an interface between hydrophilic quartz and bulk liquid solutions of H2O/D2O mixtures. We find that near the interface the neutron scattering length density is larger than in the bulk solution and there is an excess adsorbed mass. We interpret this as showing that there is a region adjacent to the quartz that is enriched in D2O and extends 5-10 nm into the solution. This suggests caution when interpreting results where D2O is substituted for H2O in aqueous interfacial chemistry.