Surface tension of general electrolyte solutions

A simple analytic expression is derived for the surface tension of a solution of general electrolytes including symmetrical and unsymmetrical electrolytes. Following the theory of Levin and Flores-Mena (Europhys Lett (2001) 56:187), we have introduced an ion-free layer of thickness just below the Gibbs dividing surface at the air/electrolyte solution interface. We use the linearized Poisson-Boltzmann equations for the mean potential and for the local fluctuation potential around an ion in the electrolyte solution, together with the Laplace equation for these potentials in the ion-free layer. It is found that the contribution of the mean potential as well as the tangential Maxwell stress along the interface vanishes for low potentials. Experimental data by Matubayasi et al (J Colloid Interf Sci (1999) 209:398, ibid (1999) 209:403, ibid (2001) 243:444) are analyzed with the present theory in order to estimate the values of for several electrolytes.     

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.     

Surface tension of liquid marbles

Effective surface tension of liquid marbles was measured by three independent experimental techniques: vibration, shape analysis, and maximal marble height. Marbles obtained with various powders: polyvinylidene fluoride, polytetrafluoroethylene, polyethylene, and lycopodium, were studied. The effective surface tension depends strongly on the kind of powder coating the marble. The capillary interaction between particles coating the marble was involved for qualitative interpretation of the reported data.     

Effect of temperature on the surface tension of diluted aqueous solutions of 1,2-hexanediol, 1,5-hexanediol, 1,6-hexanediol and 2,5-hexanediol

Surface tension of aqueous solutions of 1,2-hexanediol (1,2HD), 1,5-hexanediol (1,5HD), 1,6-hexanediol (1,6HD), and 2,5-hexanediol (2,5HD) was measured as a function of composition using the method of capillary rise at 283.15, 288.15, 293.15, 298.15, 303.15 and 308.15 K with emphasis in the very dilute region.     

Experiments and model for the surface tension of (MDEA + [Bmim][BF4]) and (MDEA + [Bmim][Br]) aqueous solutions

The surface tension (γ) of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]), 1-butyl-3-methylimidazolium bromide ([Bmim][Br]), (N-methyldiethanolamine(MDEA) + [Bmim][BF₄]) and (MDEA + [Bmim][Br]) aqueous solutions were measured by using the BZY-1 surface tension meter. The temperature ranged from (293.2 to 323.2) K. The mass fraction of MDEA ranged from 0.35 to 0.45. A thermodynamic equation was proposed to model the surface tension of (MDEA + ionic liquids) (ILS) aqueous solutions and the calculated results agreed well with the experiments. The effects of temperature, mass fractions of MDEA and ILS on the surface tension were demonstrated on the basis of experiments and calculations.     

The ionic product of water in highly concentrated aqueous electrolyte solutions

Summary The ionic product of water, , has been determined in aqueous NaCl (0.5–5.0M), KCl (3.0M), NaNO₃ (3.0 and 5.0M), and KNO₃ (2.5M) at 25 °C from high-precision potentiometric titrations carried out in cells with liquid junction using either glass or hydrogen electrodes. Measurements ofK _w provide a set of self-consistent data that can be used in the estimation of activity coefficient changes and liquid junction potentials in the study of extremely concentrated electrolyte solutions. Where comparison is possible, results obtained by hydrogen electrode measurements are in excellent agreement (ca ± 0.005 inpK _w) with other reliable experimental values and the predictions of thePitzer activity-coefficient model. The glass electrode results are, as expected, routinely lower (by 0.03–0.05pK _w units), owing to interference by Na⁺ ions. This effect virtually disappears in solutions of potassium salts. Comparison of the experimental results with thePitzer predictions shows that knowledge of the ternary interaction parameters is essential to account for specific ionic effects in the concentration dependence ofpK _w.On leave from the Abteilung für Physikalische Chemie und Theoretische Hüttenkunde, Montanuniversität Leoben, A-8700 Leoben, Austria     

Thermodynamic study of interfacial tension between oil and aqueous phases composed of ionic liquids and brine

The first fundamental step in determining the physicochemical properties of an equilibrium system is to determine the activity coefficient of electrolyte and non-electrolyte ions. Based on understanding the importance of activity coefficient in thermodynamic systems in this study, in order to predict interfacial tension between oil and aqueous phases composed of ionic liquids and brine, a modified thermodynamic equation based on concentration and coefficient of activity of ionic liquids is defined. For this study, the Extended UNIQUAC model is desired and its adjustable parameters are optimized with Genetic + PSO algorithm. The modified model has practical features such as investigating the effect of concentrations of salts in the water of oil fields formation on the interfacial tension of the system, investigating the effect of concentrations of various organic compounds such as ionic liquids on the interfacial tension of the system and investigating the interaction energy between organic and inorganic ions. In this study, the optimization of the modified thermodynamic equation to predict the interfacial tension of solutions containing [C₈Py][Cl], [C₁₈Py][Cl], [C₁₂mim][Cl] and [C₁₈mim][Cl] with the presence of brine and distilled water is investigated. Also, the effect of ionic strength of the solution in 32 equilibrium systems on interfacial tension is investigated. According to the optimization results of this study, the design of a computer program can be considered to predict the interfacial tension with the presence of ionic liquids and salts.     

Dynamic surface tension,critical micelle concentration,and activity coefficients of aqueous solutions of nonyl phenol ethoxylates

Dynamic surface tensions, σ(t) for aqueous solutions of nonyl phenol ethoxylates (NPEOs) at the temperature 298.15 K were measured using a Lauda drop volume tensiometer. The non-ionic surfactants analyzed in this work were Tergitol NP-9, NP-35 and NP-40. By using the classical Ward and Torday equation, the diffusion coefficient for each bulk surfactant concentration was calculated. The equilibrium surface tension values were determined by extrapolating the dynamic surface tension to t → ∞ on the σ(t) vs. t^−1/2 curves. These values were used to determine the critical micelle concentrations (CMC) of the surfactant aqueous solutions as well as to calculate the infinite dilution activity coefficient of the surfactant, following a model that combines the Volmer surface equation of state and the Gibbs adsorption equation.     

Chromatographic behaviour of aromatic compounds on gel filtration Media

Summary Highly cross-linked gels utilized as gel filtration media exhibit adsorption effects towards aromatic or apolar compounds. The adsorption is linearly dependent on salt concentration. We investigated the effect of various salts spanning the Hofmeister series on the adsorption of three model compounds, Adenosine 5′-phosphate, ε-dinitrophenyllysine and tryptophan. It was found that salts exhibiting marked salting-out properties, such as potassium phosphate and sodium sulfate, were the most effective in enhancing the adsorption were the most effective in enhancing the adsorption of the aromatic molecules onto the gel. On the contrary, in the presence of salts with marked salting-in properties, such as Kl or KSCN, the linear dependence of elution parameters for the same compounds was negatively correlated and adsorption decreased with salt concentration. However, a number of salts of intermediate character in the Hofmeister series, such as NaCl or NH₄Cl, did not affect the elution behaviour of the chosen model compounds. The different effects observed are probably due to differential hydration of the molecules in the presence of the salts and to the consequent different hydrodynamic behaviour of the solute when interacting with the hydration layers of the gel matrix.     

Surface tension measurements in the study of molecular complexes

Surface tension measurements can be used to investigate molecular complex formation in liquid solutions for strong and weak complexes. The association constant and epthalpy for triethylamine-iodine, hexamethylbenzene-tetracyanoethylene and ethanol-iodine in cyclohexane are 4·55×10³, 218 and 0.93M⁻¹ at 25° C and 12·5, 7·7 and 5·1 kcal/mol respectively. These values compare well with those reported in the literature by other methods.     

Structure and dynamics at the surface of a concentrated aqueous solution of CsF

A molecular dynamics simulation of a liquid layer of a concentrated CsF solution in water has been performed in order to compare the results with those obtained in an experimental study of our group. The main result of the experiment was the existence of a monolayer of nearly pure water constituting the surface and a homogeneous mixture constituting the bulk of the system. The simulation reveals the same phenomena which can be explained by the circumstance that the ions near the surface mostly keep their first solvation shell intact. The water molecules belonging to these shells and being placed on the vapor side constitute this monolayer. The density profiles of the ions indicate that the Cs ions penetrate further into the surface than the F ions. The orientational structure of the first shell of water molecules around an ion is the same for ions in the surface and ions in the bulk in contrast to the dynamics which is altered. The spectra of the librational motion are shifted to lower frequencies. In addition to that the spectra belonging to libration which involves motion of the dipole moment develop a peak in the low frequency range irrespective of whether the water molecules are bonded to Cs or to F ions. This can be correlated with an overall preferred orientation of the water molecules in the surface which is most pronounced for the dipole moment. The calculation of the diffusion coefficients shows that the top surface layer of nearly pure water is a region of enhanced and extremely anisotropic mobility. The mean residence time of water molecules in the surface in the first shell of an ion is reduced according to the enhanced mobility.     

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.     

Unprecedented tunable hydrophobic effect and anion recognition triggered by AIE with Hofmeister series in water

An unprecedented tunable hydrophobic effect in self-assembly of a small cationic organic fluorophore(NI-TPy~+)-based with aggregation-induced emission(AIE) property was realized in aqueous solution.The amplification of hydrophobicity was found to be significantly dependent upon the increasing aggregate s of NI-TPy~+,which enable d the study of the hydrophobic binding of chaotropic anions with the Hofmeister series.     

Surface tension of fluoroalkanes in a liquid phase

The parachor assigned to fluorine atoms in fluorinated alkanes was examined. The values vary systematically with the number of fluorine atoms in the molecule. Partially fluorinated alkanes show large values, while perfluoroalkanes give a smaller value which is constant for all the perfluoroalkanes examined in this work. The molecular structure seems to be important for the intermolecular interaction of this series of compounds, and may be responsible for the deviation from the parachor additivity rule.     

液体的表面吸附:表面相厚度和热效应的理论研究

本文定量研究了6种纯液体的表面张力与温度的关系,进一步预测了这些液体从体相到表面相的相变过程中所放出的热量,并提出放热的根源是分子在表面相更有序的排列引起了熵的减少。本文还研究了CaCl_2和K_2CO_3水溶液的表面张力随浓度的变化关系,理论模拟结果与实验数据非常一致;同时,在给定β值的情况下,还对16种强电解质溶液的表面层厚度进行了估算,对所揭示的溶液表面层增厚现象给出了理论解释。     

The liquid–gas interface of oxygen–nitrogen solutions: 1. Surface tension

The capillary method of surface tension measurement has been used to measure the surface tension of oxygen–nitrogen solutions in the temperature range from 80 to 132 K. At temperatures below the nitrogen critical temperature (T_c = 126.2 K) the capillary constant and the surface tension of solutions are smaller than their additive values and vary linearly with the temperature. Experimental data are compared with the results of calculating the surface tension by the theories of Pinnes and Rowlinson.     

Transference numbers of sodium chloride in concentrated aqueous solutions and chloride conductances in several concentrated electrolyte solutions

Stable chlorine electrodes with low bias potentials have been developed by introducing 25% Ir+75% Pt electrodes and an improved gas line. With their use in cells with transference, cation constituent transference numbers have been measured at 25°C in NaCl solutions from 1.7 to 6 modal. These results agree well with four other sets of data in the literature but disagree with two further sets based on emf determinations with Ag/AgCl electrodes. A table of best NaCl transference numbers is proposed. The conductances of the chloride ion-constituent in concentrated NaCl, KCl, and HCl solutions are compared.     

Surface tension of liquid metals and alloys — Recent developments

Surface tension measurements are a central task in the study of surfaces and interfaces. For liquid metals, they are complicated by the high temperatures and the consequently high reactivity characterising these melts. In particular, oxidation of the liquid surface in combination with evaporation phenomena requires a stringent control of the experimental conditions, and an appropriate theoretical treatment. Recently, much progress has been made on both sides. In addition to improving the conventional sessile drop technique, new containerless methods have been developed for surface tension measurements. This paper reviews the experimental progress made in the last few years, and the theoretical framework required for modelling and understanding the relevant physico-chemical surface phenomena.     

Solvent effects on chemical processes. 3. Surface tension of binary aqueous organic solvents

The surface tension of binary solvents is modelled by analogy to solvation effects arising from solvent-solute interactions. Competitive exchange equilibria are postulated between solvent component I (water) and solvent component 2 (organic cosolvent) for solute, which in this case is air; the solvation shell is thus the surface phase. A quantitative relationship is given between surface tension and mole fractions x₁ and x₂, the model parameters being exchange equilibrium constants K₁ and K₂. The equation is analyzed, it is applied to literature surface tension data, and it is compared with an earlier model from this laboratory. Curve-fits are very good, and the parameters appear to possess physical significance.     

Surface tension of concentrated cellulose solutions in 1-ethyl-3-methylimidazolium acetate

Four sources of cellulose with different molecular weights were dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate at 100 °C over a 10 h period. The solution densities were determined and these results were subsequently utilised to access the influence of dissolved cellulose on surface tension properties of cellulose/ionic liquid solutions. Surface tension measurements revealed increasing molecular weight and concentration reduced surface tension while temperature increases showed the opposite effect. These results are consistent with that of repulsive polymer-wall interactions near the interface in good solvent conditions. The semi-flexible nature of this carbohydrate in solution can help explain deviations of these results when compared to ideal flexible chains.