Surface Tension Of Salt Solutions Research Articles

Surface Penetration without Enrichment: Simulations Show Ion Surface Propensities Consistent with Both Elevated Surface Tension and Surface Sensitive Spectroscopy.

Molecular dynamics was used to investigate ion surface propensities in NaCl, KBr, and CsI solutions with an MP2-based force field. Although NaCl is found to be strongly repelled from the liquid-vapor interface, softer ions, such as I-, penetrate closely to the interface. Despite the surface penetration, the concentration of CsI near the interface is still lower than that in the bulk, thus leading to no surface enrichment. The salt concentration is found to affect relative surface propensities of the ions. More significant surface penetration is observed at higher salt concentrations. Softer ions at higher concentrations form a complex multilayer arrangement that can not be characterized as a simple surface bilayer. The simulated ion distributions explain the spectroscopic evidence of surface perturbation by soft ions with a negative surface excess consistent with an increased surface tension of salt solutions.

Ion-specific effects on surface potential and surface tension of water solutions explained via volume exclusion effects

Abstract The competition of ions for available neighboring hydrating water molecules (the exclusion volume effect) is important at high ionic strengths. Because of the differences in the hydration numbers of various ions, the volume exclusion effects might be responsible for the observed specific ion effects (the Hofmeister series) at large electrolyte concentrations. The exclusion volume is taken into account in the framework of the Structure Making/Structure Breaking (SM/SB) model, in which the interface with a thickness of a few Angstroms is assumed to be depleted of SM ions (typically, the cations, such as Na+, K+ or Li+), while the SB ions (such as H+, OH−, Cl−, Br−, I−) are adsorbed on the air/water interface via the Langmuir adsorption equations. Whereas at low electrolyte concentrations, the volume exclusion has negligible effect on both Zeta potential and surface tension, for ionic strengths above 0.1 M it leads to ion specificity for both of them, and this can explain the Hofmeister series in the behavior of surface tension at large electrolyte concentrations. Whereas the surface tension of salt solutions depends on the hydration number of SB anions, their equilibrium constants might be partially responsible for the ion specificity.

How does the electronic continuum model perform in the prediction of the surface tension of salt solutions?

The electronic continuum (EC) model uses a scaling of the charges of the ions in order to model implicitly the polarization into nonpolarizable models. This scaling procedure is applied here to two standard nonpolarizable force fields to investigate the salt concentration dependence of the surface tension and density of NaCl aqueous solutions. The composition of the interface and the orientation of the water molecules at the water surface are reported for different combinations of force fields.

Prediction of the concentration dependence of the surface tension and density of salt solutions: atomistic simulations using Drude oscillator polarizable and nonpolarizable models

Molecular simulations using Drude oscillator polarizable and nonpolarizable models for water and ions are carried out to predict the dependence of the surface tension on salt concentration. The polarizable water and ion models are based only on the classical Drude oscillators. The temperature dependence of the surface tension of water is examined for different water models. The dependence of salt densities on salt concentration is investigated through the nonpolarizable and Drude oscillator polarizable models. Finally, the reproduction of the surface tension of salt solution over a large range of concentrations is analyzed through a number of combinations between ions and water force fields. The structure of the interface is then discussed as a function of polarization effects. We establish here the inability of the Drude oscillator polarizable force fields to reproduce the salt concentration dependence of surface tension of NaCl aqueous solutions.

Are standard values the best choice? A critical statement on rheological soil fluid properties viscosity and surface tension

The rheological properties of a fluid in terms of viscosity η and surface tension γ are often neglected in studies on hydrostatic and hydrodynamic phenomena. However, their measurement seems to provide us with more benefit than it takes time and effort. Especially regarding transport of fluids in soil, the exact definition of the fluid properties is necessary for predicting water transport correctly.In this paper we review and summarize the available scientific knowledge on rheological soil fluid properties and supplement it with our own investigations on viscosity and surface tension of salt and soil solutions. Our results showed neither a clear linear relationship between η and salt concentration in aqueous solutions nor to the kind of cation or anion of the dissolved salt whereas the surface tension of salt solutions generally increased linearly with molar concentration though at different rates. As an example, a 1M MgSO4 solution doubled η but increased γ only by 3%, whereas 1M MgCl2 caused 4% increase in γ, but still 50% increase in η. Viscosity of soil solutions depended on soil–water ratio as well as fertilization of the soil. The largest deviation to standard η of water was about 6% at 40% gravimetric water content.Concluding from the literature review and our own findings, we recommend to additionally measure rheology (namely viscosity and surface tension) of the soil solution in order to improve modeling of hydrostatic and hydrodynamic phenomena in soils by utilizing more realistic fluid properties which might deviate significantly from the usually employed standard values. The mathematical error cumulates with increasing salt concentration as a deviation of 10% to the standard value also causes an error of 10% in linear relationships derived from that parameter, e.g. the equation of Hagen–Poiseuille.Future investigations should focus on the manifold single effects as well as on the interactions of different dissolved components in the soil solution on viscosity and surface tension and how these are influenced by temperature, pressure (i.e. shear rate) and concentration.

Molecular Dynamics Simulations of the Surface Tension and Structure of Salt Solutions and Clusters

Sodium halides, which are abundant in sea salt aerosols, affect the optical properties of aerosols and are active in heterogeneous reactions that cause ozone depletion and acid rain problems. Interfacial properties, including surface tension and halide anion distributions, are crucial issues in the study of the aerosols. We present results from molecular dynamics simulations of water solutions and clusters containing sodium halides with the interatomic interactions described by a conventional force field. The simulations reproduce experimental observations that sodium halides increase the surface tension with respect to pure water and that iodide anions reach the outermost layer of water clusters or solutions. It is found that the van der Waals interactions have an impact on the distribution of the halide anions and that a conventional force field with optimized parameters can model the surface tension of the salt solutions with reasonable accuracy.

Enthalpy−Entropy Contributions to Salt and Osmolyte Effects on Molecular-Scale Hydrophobic Hydration and Interactions

Salts and additives can significantly affect the strength of water-mediated interactions in solution. We present results from molecular dynamics simulations focused on the thermodynamics of hydrophobic hydration, association, and the folding-unfolding of a hydrophobic polymer in water and in aqueous solutions of NaCl and of an osmolyte trimethylamine oxide (TMAO). It is known that addition of NaCl makes the hydration of hydrophobic solutes unfavorable and, correspondingly, strengthens their association at the pair as well as the many-body level (Ghosh, T.; Kalra, A.; Garde, S. J. Phys. Chem. B 2005, 109, 642), whereas the osmolyte TMAO has an almost negligible effect on the hydrophobic hydration and association (Athawale, M. V.; Dordick, J. S.; Garde, S. Biophys. J. 2005, 89, 858). Whether these effects are enthalpic or entropic in origin is not fully known. Here we perform temperature-dependent simulations to resolve the free energy into entropy and enthalpy contributions. We find that in TMAO solutions, there is an almost precise entropy-enthalpy compensation leading to the negligible effect of TMAO on hydrophobic phenomena. In contrast, in NaCl solutions, changes in enthalpy dominate, making the salt-induced strengthening of hydrophobic interactions enthalpic in origin. The resolution of total enthalpy into solute-solvent and solvent-solvent terms further shows that enthalpy changes originate primarily from solvent-solvent energy terms. Our results are consistent with experimental data on the hydration of small hydrophobic solutes by Ben-Naim and Yaacobi (Ben-Naim, A.; Yaacobi, M. J. Phys. Chem. 1974, 78, 170). In combination with recent work by Zangi, Hagen, and Berne (Zangi, R.; Hagen, M.; Berne, B. J. J. Am. Chem. Soc. 2007, 129, 4678) and the experimental data on surface tensions of salt solutions by Matubayasi et al. (Matubayasi, N.; Matsuo, H.; Yamamoto, K.; Yamaguchi, S.; Matuzawa, A. J. Colloid Interface Sci. 1999, 209, 398), our results highlight interesting length scale dependences of salt effects on hydrophobic phenomena. Although NaCl strengthens hydrophobic interactions at both small and large length scales, that effect is enthalpy-dominated at small length scales and entropy-dominated for large solutes and interfaces. Our results have implications for understanding of additive effects on water-mediated interactions, as well as on biocompatibility of osmolyte molecules in aqueous solutions.

Correlation Between Thermal Aggregation and Stability of Lysozyme with Salts Described by Molar Surface Tension Increment: An Exceptional Propensity of Ammonium Salts as Aggregation Suppressor

Protein aggregation is a critical problem for biotechnology and pharmaceutical industries. Despite the fact that soluble proteins have been used for many applications, our understanding of the effect of the solution chemistry on protein aggregation still remains to be elucidated. This paper investigates the process of thermal aggregation of lysozyme in the presence of various types of salts. The simple law was found; the aggregation rate of lysozyme increased with increasing melting temperature of the protein (T (m)) governed by chemical characteristics of additional salts. Ammonium salts were, however, ruled out; the aggregation rates of lysozyme in the presence of the ammonium salts were smaller than the ones estimated from T (m). Comparing with sodium salts, ammonium salts increased the solubility of the hydrophobic amino acids, indicating that ammonium salts adsorb the hydrophobic region of proteins, which leads to the decrease in aggregation more effectively than sodium salts. The positive relation between aggregation rate and T (m) was described by another factor such as the surface tension of salt solutions. Fourier transform infrared spectral analysis showed that the thermal aggregates were likely to form beta-sheet in solutions that give high molar surface tension increment. These results suggest that protein aggregation is attributed to the surface free energy of the solution.

Hofmeister Effects in Surface Tension of Aqueous Electrolyte Solution

The surface tension of electrolyte solutions shows marked specific ion effects. We here show an important role for both ionic solvation energies and ionic dispersion potentials in determining this ion specific surface tension of salt solutions. The ion self-free energy changes when an ion moves from bulk solution into the interfacial region, with its decreasing water density profile. We will show that the solvation energies of different ions correlate very well with the surface tension of salt solutions. Inclusion of this distance-dependent self-free energy contribution brings qualitative agreement with experiments and the right Hofmeister series. This is so not only for surface tension changes but also for measured surface potentials. The inclusion of ionic dispersion interaction potentials further improves the agreement with experiments. We discuss how further progress in the theory of the surface tension of salts can be achieved.

Energy of an ion crossing a low dielectric membrane: the role of dispersion self-free energy

The Born charging equation predicts that the permeability of a cell membrane to ions by the solubility-diffusion mechanism depends on the ionic radius and on the dielectric constant of the membrane. However, experiments, for example, on red blood cells and on lysosome membranes, show that the permeability depends strongly on the choice of salt anion in a way that cannot be accommodated by differences in ionic size. We demonstrate that one step towards understanding this ion specificity is to take account of the previously ignored dispersion self-free energy of the ion. This is the quantum electrodynamic analogue of the (electrostatic) Born self-energy of an ion. We show that the dispersion self-free energy contribution can be and often is of the same order of magnitude as the Born contribution. To understand the observed specificity, it is essential to take into account of both ionic size and ionic polarizability. In parallel and to reinforce these observations, we also give simple estimates for how self-free energy changes that occur when an ion moves into the air–water interface region (which has a density profile for water molecules) can influence the surface tension of salt solutions. Consistency can be found between the Hofmeister sequences observed in ion permeation and in surface tension of electrolytes when these previously ignored self-free energies are included properly.

Calculation of surface tension of salt solutions: effective polarizability of solvated ions

The Onsager–Samaras version of Wagner's theory of surface tension of solutions of 1:1 salts has been modified to include ion-induced dipole terms in the image potential. Effective solvated ionic polarizabilities are used, which are large and negative owing to local dielectric saturation around the ions. The modified theory agrees with the order of magnitude of the variation among various alkali halides (e.g., Δσ for 0.1 molar aqueous KI is about 70% of the corresponding value for KCl). Polarizabilities were estimated from Hasted's dielectric decrements. Keywords: surface tension, salt solutions, symmetric electrolytes, effective polarizability, theory.

The thermodynamic equation of state for insoluble monolayers. II. Ionized films

A thermodynamic equation of state for the Π–A curves of insoluble monolayers is extended to ionized surfactant species. It is shown that at infinite dilution on a pure water subphase, the limiting law ΠA=νskT (where νs is the number of ions into which the surfactant dissociates) is obtained. For monolayers spread on salt solutions with a common counterion, it is shown that the adsorption of soluble salt appears explicitly in the equation of state. If the Gibbs dividing surface is placed such that this adsorption is taken as zero, the form of the equation becomes similar to, and can be applied in the same manner as, that used for uncharged films. In some cases, this permits fitting experimental Π–A curves with surface-pressure independent activity efficients. When this is not possible, it suggests that the relative salt and water adsorptions change as the film is compressed. Additional experimental information will be required to evaluate this possibility, and to permit application of the general equation of state for multicomponent systems. The application of this formalism to the surface tension of salt solutions without spread monolayers is also considered briefly.

Discussion of “Effects of Electrostatic Fields on the Surface Tension of Salt Solutions” [G. M. Schmid, R. M. Hurd, and E. S. Snavely, Jr., (pp. 852–856, Vol. 109, No. 9)

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Closure to “Discussion of ‘Effects of Electrostatic Fields on the Surface Tension of Salt Solutions’ [G. M. Schmid, R. M. Hurd, and E. S. Snavely, Jr., (pp. 852–856, Vol. 109, No. 9)

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Effects of Electrostatic Fields on the Surface Tension of Salt Solutions

The surface tension of air/ solution interfaces with applied electrostatic fields of up to 6700 v/cm was measured using a surface energy balance. The concentration was 0–3.5%. It was found that fields of either polarity lower the surface tension of all investigated solutions. Magnitude of the surface tension changes and dependence on direction of the field lead to the conclusion that the potential drop at air/water interfaces is smaller than previously thought, that the water dipole orientation is only slight, and that the hydrogen in the oriented water molecule is facing the air. The experiments point out that the effect of the field is predominantly on the sodium ion. It is shown that the difference in the energy for adsorption at the interface between cations and anions cannot be neglected. Calculations on the basis of a modified Gibbs equation on the assumption of a diffuse double layer at the interface leads to unreasonably high surface deficiencies. Neglecting the diffuse double layer and reckoning on the basis of an inner double layer only leads to values of the same order of magnitude as those calculated from the Gibbs equation alone.