Surface Tension Of Electrolyte Solutions Research Articles

Effect of hydrated shell layers on surface tension of electrolyte solutions: Insights from interpretable machine learning

To explore the effects of hydration shell layer on the surface tension of electrolytes solution and to build an effective prediction model, a machine learning based model is proposed to accurately predict and explain the surface tension of electrolytes solution. The model combines machine learning (ML) algorithms, force filed parameters of molecular dynamics simulations and radial distribution function (RDF) to accurately capture the structure feature for electrolytes solution. The prediction performed an extremely low average relative deviation. SHapleyAdditive explanation (SHAP) method is used to indicate the features order of importance from strong to weak. It noted that the second hydration shell on the influence of surface tension may beyond the first hydration shell. This work provides a method for one-step acquisition of surface tension data that not only to accurately predict physical and chemical properties of materials, but also to extend the application of molecular dynamics simulations, providing enlightening insights for detecting underlying physical mechanisms.

Hofmeister Series: Insights of Ion Specificity from Amphiphilic Assembly and Interface Property.

Hofmeister series (HS), ion specific effect, or lyotropic sequence acts as a pivotal part in a number of biological and physicochemical phenomena, e.g., changing the solubility of hydrophobic solutes, the cloud points of polymers and nonionic surfactants, the activities of various enzymes, the action of ions on an ion-channel, and the surface tension of electrolyte solutions, etc. This review focused on how ion specificity influences the critical micelle concentration (CMC) and how the thermoresponsive behavior of surfactants, and the dynamic transition of the aggregate, controls the aggregate transition and gel formation and tunes the properties of air/water interfaces (Langmuir monolayer and interfacial free energy). Recent progress of the ion specific effect in bulk phase and at interfaces in amphiphilic systems and gels is summarized. Applications and a molecular level theoretical explanation of HS are discussed comprehensively. This review is aimed to supply a fresh and comprehensive understanding of Hofmiester phenomena in surfactants, polymers, colloids, and interface science and to provide a guideline to design the microstructures and templates for preparation of nanomaterials.

Molecular dynamics simulations study for the effect of cations hydration on the surface tension of the electrolyte solutions

In this work, the influence of different cations (Na+, K+, Ca2+ and Mg2+) on the surface tension of electrolyte solution was investigated using molecular dynamics simulations (MDs). The structure of the water-ion system was discussed in terms of ion-oxygen, ion-hydrogen radial distribution function (RDF) and hydration numbers of different electrolyte ions. It was found that the water-ion system existed in the form of ionic hydration shells. Besides, the interactions between the ions and water molecules were illustrated by the hydration energy, which provided the extra driving force to enlarge the surface tension of the ionic solution compare to the water. These results commendably explained the phenomenon that the greater the hydration energy of the ions, the greater the surface tension of these ionic solution.

The Jones-Ray effect reinterpreted: Surface tension minima of low ionic strength electrolyte solutions are caused by electric field induced water-water correlations

The surface tension of electrolyte solutions exhibits a minimum at millimolar electrolyte concentrations and then rises with increasing concentration. This minimum, known as the Jones-Ray effect, has been hotly debated over the past ∼80years. If not considered as an artifact, it is typically ascribed to a phenomenological rare binding site for ions or ion pairs. Here, we propose an alternative underlying mechanism, namely that the hydrogen bond network of water responds to the collective electrostatic field of ions by increasing its orientational order, supported by recent surface tension measurements of NaCl solutions in H2O and D2O, and second harmonic scattering experiments in combination with ion resonant second harmonic reflection experiments. Recent thermodynamic and purely electrostatic treatments of the surface tension provide support for this interpretation. In addition, concerns related to possible artifacts influencing the measurements are quantified experimentally.

Thermodynamic Modeling of Surface Tension of Aqueous Electrolyte Solution by Competitive Adsorption Model

Thermodynamic modeling of surface tension of different electrolyte systems in presence of gas phase is studied. Using the solid-liquid equilibrium, Langmuir gas-solid adsorption, and ENRTL activity coefficient model, the surface tension of electrolyte solutions is calculated. The new model has two adjustable parameters which could be determined by fitting the experimental surface tension of binary aqueous electrolyte solution in single temperature. Then the values of surface tension for other temperatures in binary and ternary system of aqueous electrolyte solution are predicted. The average absolute deviations for calculation of surface tension of binary and mixed electrolyte systems by new model are 1.98 and 1.70%, respectively.

Ion interactions with the air-water interface using a continuum solvent model.

Explaining and predicting the distribution of ions at the air-water interface has been a central challenge of physical chemistry for nearly a century. In essence, the problem amounts to calculating the change in the solvation energy of an ion as it approaches the interface. Here, we generalize our recently developed model of ionic solvation energies to calculate this interaction. The change in the Born energy as well as the static polarization response of the ion is included by using the conductor-like screening model (COSMO), which treats the ions quantum mechanically. Approximate expressions for the dispersion repulsion, cavity attraction, and surface potential contributions are also included. This model reproduces the surface tensions of electrolyte solutions and is consistent with ab initio molecular dynamics (MD) simulation. The model provides clear physical insight into iodide's adsorption. Unlike alternative models, no parameters are deliberately adjusted to reproduce surface tensions, and all of the important contributions to the interactions are included. Solving this problem has important direct implications for atmospheric chemistry and bubble properties. It also has important indirect implications for the more complex interactions of ions with protein and mineral surfaces. These play a fundamental role in a vast number of biological and industrial processes. The model is conceptually simple and has low computational demand, which facilitates its extension to these important applications.

Surface tension of electrolyte solutions: A self-consistent theory

We study the surface tension of electrolyte solutions at the air/water and oil/water interfaces. Employing field-theoretical methods and considering short-range interactions of anions with the surface, we expand the Helmholtz free energy to first order in a loop expansion and calculate the excess surface tension. Our approach is self-consistent and yields an analytical prediction that reunites the Onsager-Samaras pioneering result (which does not agree with experimental data), with the ionic specificity of the Hofmeister series. We obtain analytically the surface-tension dependence on the ionic strength, ionic size and ion-surface interaction, and show consequently that the Onsager-Samaras result is consistent with the one-loop correction beyond the mean-field result. Our theory fits well a wide range of concentrations for different salts using one fit parameter, reproducing the reverse Hofmeister series for anions at the air/water and oil/water interfaces.

Soil-water contact angle as affected by the aqueous electrolyte concentration

Electrolyte concentration changes the surface tension of the water-air interface, which is expected to affect the solid-water contact angle. In the present study, we focused on examining the effects of aqueous electrolyte concentration on the contact angle of soil samples with different hydrophobicities using sodium chloride (NaCl) and calcium chloride (CaCl2) solutions. Japanese Andisol and silica sand were hydrophobized using stearic acid to obtain different hydrophobicities. The contact angle of all the samples increased with increasing electrolyte concentration. This was attributed to the increasing surface tension of the solutions. The response of contact angle to the electrolyte concentration was positively correlated with soil surface free energy fitting into a linear relationship in Andisol, but not in silica sand. Surface tension of electrolyte solutions increases linearly with electrolyte concentration up to high salinities, and the contact angle can be expected to increase with increasing electrolyte concentration in a corresponding pattern. The relationship observed in the present study was non-linear, where more prominent increase of contact angle was observed at low electrolyte concentrations. The response of contact angle to the increasing electrolyte concentration was almost negligible at concentrations higher than 0.06 mol L−1, [ionic strength (I): 0.06 mol L−1] for NaCl, and 0.1 mol L−1, (I: 0.3 mol L−1) for CaCl2 solutions. The lowered solid-liquid interfacial free energy and its effects on contact angle with increasing adsorption of ions at the interface was considered to be the reason for decreasing slopes of the curves. Initial increase in contact angle with increasing ionic strength of CaCl2 was lower than that of NaCl. This might be because the adsorption of ions on soil surfaces, and the consequent effects on lowering the free energy of the solid-liquid interface and the contact angle, are more pronounced for calcium ions (Ca2+) than for sodium ions (Na+).

Thermodynamics of sodium maleate and chloride adsorption at an aqueous solution-gas interface

Surface tension of solutions of sodium chloride in water and sodium maleate in aqueous alkaline solutions in a wide concentration range at 298 and 303 K are measured. It is found that the dependences of surface tension of electrolyte solutions on concentration have extreme character. The obtained dependences are discussed from the standpoint of the effect of the nature of the solute and the processes of desolvation on the composition of surface layers and the surface activity of a solution. Total electrolyte adsorption isotherms are calculated using experimental surface tension and excess adsorption isotherms.

Surface Tension of Aqueous Electrolyte Solutions. Thermodynamics

A thermodynamic theory is developed for obtaining the enthalpic and entropic contributions to the surface excess Gibbs energy of electrolyte solutions from the dependence of the surface tension on concentration and temperature. For elaboration, accurate activity coefficients in solution as functions of concentration and temperature are required. The theory is elaborated for (1-1) electrolytes and applied to HClO(4), HNO(3), NaCl, NaBr, and LiCl, of which the first two adsorb positively and the other three negatively. One of the conspicuous outcomes is that in all cases, the surface excess entropies slightly decrease with electrolyte activity but remain close to that of pure water, whereas the enthalpy is different from that. The implication is that the driving force for positive or negative adsorption must have an enthalpic origin. This finding can be useful in developing and evaluating theoretical models for the interpretation of surface tensions of electrolyte solutions.

Inhibition of Bubble Coalescence by Electrolytes in Binary Mixtures of Dimethyl Sulfoxide and Propylene Carbonate

We have investigated the effect of solvent composition on inhibition of bubble coalescence by electrolytes in binary mixtures of dimethyl sulfoxide (DMSO) and propylene carbonate (PC). Unlike most mixtures, combinations of DMSO and PC exhibit minimal foaming over all compositions (with the strongest effect being at 25% PC by volume); thus, the influence of electrolytes can be investigated. Both LiBr and KSCN at moderate concentrations inhibit bubble coalescence at all solvent compositions. However, the concentration of electrolyte required to inhibit coalescence was in both cases a minimum at the PC(v/v) of 25%. The surface tension of electrolyte solutions in the mixed solvents indicates that the gradient in surface tension is not correlated with coalescence inhibition and therefore inhibition cannot be attributed to surface elasticity. We have also studied the inhibition of bubble coalescence by HCl at different solvent compositions. HCl is strongly inhibitory in DMSO but only weakly so in PC. We have found that HCl exhibits strong inhibition behavior at all mixtures studied. All electrolytes studied were most effective at inhibiting bubble coalescence at the PC(v/v) of 25%, indicating that the interactions between solvent molecules strongly determine the influence of electrolyte on coalescence inhibition. We propose that the formation of solvent complexes between DMSO and PC results in an increase in surface viscosity and the presence of electrolytes further amplifies this effect.

Effect of Ionic Strength on the Interfacial and Bulk Properties of Unsymmetrical Bolaamphiphiles

AbstractBy performing measurements of the equilibrium surface tensions of electrolyte solutions of three unsymmetrical bolaamphiphiles, each of which has an aromatic ring and a ω‐carboxyalkyl chain in its molecular structure, we conclude that the carboxyl and sulfonate groups at both ends of the molecule—which enter the solution while the hydrophobic chain extends into the air—can force these surfactants to adopt a looped configuration which looks like the letter “U” upside‐down, resulting in regular arrangements at the air/water interface and micelles in bulk solution. Surface tension measurements of these surfactants as a function of added salt reveal the limiting surface tension to be less sensitive to the ionic strength in LiCl solution and significantly sensitive to divalent Ca2+ and Mg2+ ions in hard water. This result reveals that the looped configuration of the molecule is the dominant factor in determining whether the molecules of this surfactant are sensitive to ionic strength or not. This paper also reports briefly on the effect of the interactions of divalent Ca2+ and Mg2+ ions with unsymmetrical bolaamphiphiles on the turbidity of hard water. These results suggest that the interactions of divalent Ca2+ and Mg2+ ions with carboxyl groups are strong, and the two breaks in the surface tension curves disappear. In 200 ppm hard water, the turbidity appears over just a small range of surfactant concentrations, showing that the presence of the Ca2+ and Mg2+ ions has little effect on the application of these surfactants.

ON THE NATURE OF IONS AT THE LIQUID WATER SURFACE

A qualitatively new understanding of the nature of ions at the liquid water surface is emerging. Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models. Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions. This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations. Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging.

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.

Direct experimental validation of the Jones–Ray effect

In 1937, Jones and Ray observed a minimum in the surface tension of electrolyte solutions at ∼ 1 mM concentrations. According to the Gibbs adsorption equation, a decreasing surface tension corresponds to an enhanced ion concentration at the interface, which is contrary to textbook descriptions based on Onsanger–Samaras theory. The ‘Jones–Ray Effect’ has since been essentially dismissed as an artifact of the indirect experimental method used, and remains today as a curiosity. Here, we present direct experimental confirmation of the enhanced anion concentration around 1 mM of alkali iodide solutions using resonance enhanced femtosecond second harmonic generation, and the extraction of a large and negative value for the surface excess free energy (−6.2 ± 0.2 kcal/mol).

Surface tension of electrolyte solutions

A simple analytic expression is derived for the excess surface tension of electrolyte solutions, which is in good agreement with the experimental data on NaCl in the concentration range up to as high as 1 M. This expression is consistent with the following two theories: (i) The recent theory of Levin and Flores-Mena (Europhys Lett (2001) 56:187), who demonstrated the important contribution of the formation of an ion free layer at the air–electrolyte solution interface, and (ii) the Onsager–Samaras theory (J Chem Phys (1934) 2:528) modified by taking into account the ion-free layer effect. It is shown that the excess surface tension consists of three parts: the contributions of the ion-free layer, the image interaction between the electrolyte ions and the ion-free layer, and the image interaction between the electrolyte ions and air.

Ion Binding and Ion Specificity: The Hofmeister Effect and Onsager and Lifshitz Theories

Present theories of colloid science do not account for specific ion effects as exemplified by the Hofmeister effect, surface tension of electrolyte solution interfaces, binding to micelles, effective charge in double layer interactions, and attractive interactions in low Hamaker constant systems. It is argued that specificity emerges naturally and can be rationalized if dispersion interactions acting on ions are included in the theory. These are in principle accessible from bulk solution properties. Specific ion adsorption due to dispersion interactions can be dominant even at charged interfaces especially at high salt concentrations (∼0.1Μ). The effects can be qualitatively different at air−water and oil−water surfaces. That part of extended Lifshitz theory for low Hamaker constant systems, in which the forces are mainly due to temperature and salt dependent interaction, is re-examined. It is shown to be at the same level of approximation as, and precisely equivalent to, the Onsager limiting law for the ...

Self-Consistent Theory of the Surface Tension of Electrolyte Solutions

A self-consistent treatment is given for the ion distribution near the surface of electrolyte solutions, by solving the Debye-Hueckel equation in the WKB approximation. The surface tension is calculated for electrolyte aqueous solutions with improvement of Onsager and Samaras' result, in comparison with the experiment. Kirkwood and Buff's method is reexamined to get a new and simple expression for the suface tension increase. The self-consistent treatment is shown to be crucial for obtaining a convergent result by means of different basic formulas.