Do hydration forces play a role in thin film drainage and rupture observed in electrolyte solutions?

The mechanism that controls bubble coalescence in electrolyte solutions remains unresolved. The problem is difficult as sensitive dynamic thin film processes are critical. Here we discuss the relationship between film dynamics, specific-ion effects and the combining rules that codify electrolyte effects on bubble coalescence. The relationship with Hofmeister effects is explored, revealing that these very different manifestations of specific ion effects ultimately have the same origin, being the interfacial positioning of ions, which for the air–water interface correlates with the empirically derived α and β assignments used in the combining rules. Ion hydration is important as it strongly influences the interfacial positioning of ions and therefore ultimately bubble coalescence, however dynamic events determine if a collision results in coalescence and therefore we conclude that hydration forces play no role in bubble coalescence in electrolyte solutions.     

The Standard Partial Molar Volumes of Ions in Solution. Part 2. The Volumes in Two Binary Solvent Mixtures with No Preferential Solvation

Standard partial molar volumes of ions were obtained from literature data on 1:1 electrolytes in mixtures of propylene carbonate (PC) with acetonitrile (MeCN) and of water (W) with methanol (MeOH) at 298.15 K. The hypothesis was examined that when the solvents in the mixtures do not differ too much in their polarity and/or hydrogen-bonding ability, only negligible preferential solvation occurs in the solvent shell around the ion where electrostriction takes place. Given the solvent-independent intrinsic volume of an ion, the electrostriction, calculated by the shell-by-shell method, permits the examination of this proposition. This hypothesis was indeed validated by the calculated standard partial molar ionic volumes in the dipolar aprotic mixtures and in the protic aqueous methanol.     

Rubidium-87 NMR studies of rubidium salts and complexes with 18-crown-6 and cryptand-222 in solutions

Rubidium-87 NMR measurements were used to study the behavior of the Rb⁺ ion in water, methanol, and propylene carbonate solutions. In aqueous solutions the ⁸⁷Rb chemical shift varies linearly with the mean activity of the salt. In methanol and propylene carbonate solutions the relationship is linear only at high salt concentrations. The resonance lines are broad and vary from ∼ 150 Hz (at half height) in water to ∼ 1000 Hz in propylene carbonate. Additions of macrocyclic ligands ¹⁸C6 and C222 to Rb⁺ solutions in the three solvents result in further broadening of the resonance line so that variations in the resonance frequency cannot be measured with a reasonable precision.     

Specific background electrolytes for nonaqueous capillary electrophoresis

The use of organic solvents or mixture of solvents in capillary electrophoresis is gaining wider attention. The electroosmotic flow mobility of eight organic solvents (acetonitrile, acetone, dimethylformamide, dimetylsulphoxide, propylene carbonate, methanol, ethanol, n-propanol) and of mixtures of several solvents (methanol-acetonitrile, methanol-propylene carbonate, acetonitrile-propylene carbonate) has been studied. The influence of 1,3-alkylimidazolium salts in different solvents on the separation of different analytes has been investigated. Some of these salts have shown usefulness for matrix-assisted laser desorption ionization matrices and off-line analysis of electrophoresis fractions. It also appears that nonaqueous capillary electrophoresis with 1,3-alkylimidazolium salts as background electrolytes is suitable for separation small inorganic ions.     

Heteroconjugation-based capillary electrophoretic separation of phenolic compounds in acetonitrile and propylene carbonate

A mixture of methyl- and hydroxy-substituted phenols was separated by capillary electrophoresis in pure acetonitrile and propylene carbonate. Interactions between undissociated phenolic compounds and the background electrolytes were investigated. In the present work, benzyltriethylammonium chloride, tetrabutylammonium acetate, and two room temperature-molten salts, 1-butyl-3-methyl imidazolium trifluoroacetate and 1-butyl-3-methyl imidazolium heptafluorobutanoate, were used as background electrolytes. The formation of a negative complex between background electrolyte anion and neutral phenolic compound was observed and the formation constant calculated. The formation constants for anion-analyte complexes were approximately the same in propylene carbonate and in acetonitrile. In both solvents the formation constants were the highest for acetate and the lowest for trifluoroacetate. The separation of analytes was slightly influenced by the nature of the solvent: in acetonitrile the resolution between peaks was higher for 1,3-dihydroxyphenol and 1,3,5-trihydroxyphenol, in propylene carbonate 3-methylphenol and phenol were better separated. It was demonstrated that traces of water influence the mobilities of anion-phenol complexes in propylene carbonate.     

Basicity comparison for di-substituted 4-nitropyridine derivatives in polar non-aqueous media

Acid dissociation, as well as cationic homoconjugation equilibria have been studied potentiometrically in systems involving four di-substituted 4-nitropyridines and conjugate cationic acids in the polar non-aqueous solvents - aprotic protophobic acetonitrile (AN) and propylene carbonate (PC), the amphiprotic methanol (MeOH), and in the aprotic protophilic dimethyl sulfoxide (DMSO). The influence of solvent effect on the obtained acidity constants has been discussed. The acidity constants (expressed as pK_a values) were compared with those previously determined in another polar protophobic aprotic solvent - acetone (AC), and obtained for the unsubstituted pyridine (Py). A comparison of the acid dissociation constants determined in all media studied has proved that the strength of the cationic acids increases on going from acetonitrile through propylene carbonate, acetone, and methanol to dimethyl sulfoxide. Furthermore, the values of acidity constants in the non-aqueous media have shown that in all the solvents studied they change according to the substituent effects. It has been also found that substituted 4-nitropyridine derivatives studied exhibit no tendency towards cationic homoconjugation in acetonitrile, propylene carbonate, and methanol and dimethyl sulfoxide. Moreover, it has been demonstrated that the acid dissociation constants determined by potentiometric titration method in all the solutions investigated correlate well with the calculated energy parameters of the protonation reactions in the gaseous phase.     

Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions

Bubble coalescence behavior in aqueous electrolyte (MgSO(4), NaCl, KCl, HCl, H(2)SO(4)) solutions exposed to an ultrasound field (213 kHz) has been examined. The extent of coalescence was found to be dependent on electrolyte type and concentration, and could be directly linked to the amount of solubilized gas (He, Ar, air) in solution for the conditions used. No evidence of specific ion effects in acoustic bubble coalescence was found. The results have been compared with several previous coalescence studies on bubbles in aqueous electrolyte and aliphatic alcohol solutions in the absence of an ultrasound field. It is concluded that the impedance of bubble coalescence by electrolytes observed in a number of studies is the result of dynamic processes involving several key steps. First, ions (or more likely, ion-pairs) are required to adsorb at the gas/solution interface, a process that takes longer than 0.5 ms and probably fractions of a second. At a sufficient interfacial loading (estimated to be less than 1-2% monolayer coverage) of the adsorbed species, the hydrodynamic boundary condition at the bubble/solution interface switches from tangentially mobile (with zero shear stress) to tangentially immobile, commensurate with that of a solid-liquid interface. This condition is the result of spatially nonuniform coverage of the surface by solute molecules and the ensuing generation of surface tension gradients. This change reduces the film drainage rate between interacting bubbles, thereby reducing the relative rate of bubble coalescence. We have identified this point of immobilization of tangential interfacial fluid flow with the "critical transition concentration" that has been widely observed for electrolytes and nonelectrolytes. We also present arguments to support the speculation that in aqueous electrolyte solutions the adsorbed surface species responsible for the immobilization of the interface is an ion-pair complex.     

Effect of solvents on properties of polymer gel-electrolyte based on polyester diacrylate

New polymer gel electrolytes based on polyester diacrylates and LiClO₄ salt solutions in organic solvents are developed for lithium ion and lithium polymer batteries with a high ionic conductivity up to 2.7 × 10^?3 Ohm^?1cm^?1 at the room temperature. To choose the optimum liquid electrolyte composition, the dependence is studied of physico-chemical parameters of new gel electrolytes on the composition of the mixture of aprotic organic solvents: ethylene carbonate, propylene carbonate, and λ-butyrolacton. The bulk conductivity of gel electrolytes and exchange currents at the gel electrolyte/Li interface are studied using the electrochemical impedance method in symmetrical cells with two Li electrodes. The glass transition temperature and gel homogeneity are determined using the method of differential scanning calorimetry. It is found that the optimum mixture is that of propylene carbonate and λ-butyrolacton, in which a homogeneous polymer gel is formed in a wide temperature range of ?150 to +50°C.     

Zooming in on the role of surfactants in droplet coalescence at the macroscale and microscale

Coalescence of dispersed micrometer-scale droplets is an essential step toward the separation of emulsions. The thin film between droplets must form, drain, and rupture for coalescence to occur. In surfactant-stabilized emulsions, the film drainage and droplet coalescence processes are known to be hindered because of reduced interfacial mobility. However, a clear correlation between this mobility and the underlying surfactant transport and interfacial response to shear and dilatational deformations is undercharacterized. For microscale droplets, the effect of surfactant transport to the interface and along the interface is often difficult to isolate from other bulk effects on emulsion stability. In this work, we review surfactant-mitigated coalescence in both macroscale and microscale experiments, highlighting the importance of interfacial curvature and length scales when establishing a correlation between coalescence theory and film mobility.     

Selective coalescence of bubbles in simple electrolytes

Simple ions in electrolytes exhibit different degrees of affinity for the approach to the free surface of water. This results in strong ion-specific effects that are particularly dramatic in the selective inhibition of bubble coalescence. I present here the calculation of electrostatic interaction between free surfaces of electrolytes caused by the ion accumulation or depletion near a surface. When both anion and cation are attracted to the surface (like H+ and Cl- in HCl solutions), van der Waals attraction facilitates approach of the surfaces and the coalescence of air bubbles. When only an anion or cation is attracted to the surface (like Cl- in NaCl solutions), an electric double layer forms, resulting in repulsive interaction between free surfaces. I applied the method of effective potentials (evaluated from published ion density profiles obtained in simulations) to calculate the ionic contribution to the surface-surface interaction in NaCl and HCl solutions. In NaCl, but not in HCl, the double-layer interaction creates a repulsive barrier to the approach of bubbles, in agreement with the experiments. Moreover, the concentration where ionic repulsion in NaCl becomes comparable in magnitude to the short-range hydrophobic attraction corresponds to the experimentally found transition region toward the inhibition of coalescence.     

Effect of alcohols on the initial growth of multibubble sonoluminescence

The effect of alcohols on the initial growth of the multibubble sonoluminescence (MBSL) intensity in aqueous solutions has been investigated. With increasing concentrations of the alcohols, the number of pulses required to grow the MBSL intensity to a steady state (N(crit)) increases (relative to that of water) initially to a maximum for all the alcohols used in this study, followed by a decrease for methanol and ethanol. The cause of the initial increase in N(crit) is attributed to the inhibition of bubble coalescence in the system. This inhibition in bubble coalescence results in a population of bubbles with a smaller size range and thus a larger number of pulses is required to grow the bubbles to their sonoluminescing size range. It is suggested that the decrease in the N(crit) at higher alcohol concentrations may be caused by an increase in the bubble growth by rectified diffusion.     

Limiting conductances of electrolytes and the walden product in mixed solvents in a phenomenological approach

The applicability of the Quint-Viallard conductivity equation to the representation of electrical conductivities in mixed solvents is examined. The concept of the modified Walden product is introduced, and the benefits compared with the ordinary Walden product are discussed. The universal curve of limiting conductances for all electrolytes (or for all ions) in a given pair of solvents is introduced and examined in a number of mixtures which include methanol, ethanol, 1-propanol, tert-butyl alcohol, 1,4-dioxane, N, N-dimethylformamide, sulfolane, tetrahydrofuran, and ethylene carbonate with water. Also examined are nonaqueous mixtures of acetone-ethanol, acetone-1-propanol, dimethyl sulfoxide-propylene carbonate, acetonitrile-methanol, acetonitrile-carbon tetrachloride, and acetonitrile-propylene carbonate. Many electrolytes were involved in the evaluation of the universal curves, but the majority are alkali-metal halides, tetraalkylammonium halides, tetraalkylammonium tetraphenylborides, and potassium xanthates (inorganic and organic acids are treated separately). If in a given mixed solvent system the limiting conductance of electrolyte is unknown, the universal curve permits estimating its value and gives an indication about the quality of performed conductivity measurements. The existence of universal curves of limiting conductances indicates that the properties of electrolytes in pure solvents are, to a great extent, preserved also in the mixture of solvents due to the simple dilution effect.     

Coalescence in foams and emulsions: Similarities and differences

Coalescence in emulsions and foams is far from being understood, despite many years of investigations. The phenomenon is not easy to be characterized because it is extremely rapid and coupled to several others, gravity effects, leading to vertical motion of drops/bubbles and ripening, leading to their growth. Coalescence implies the rupture of films between drops/bubbles and involves contributions from hydrodynamics, surface rheology, surface forces, and thermal fluctuations. Different coalescence scenarios were identified and are described. There are close similarities between emulsion and foam behavior, as remarked earlier by several researchers. Ivan Ivanov, to whom this article is dedicated, was one of them. He and his group pioneered parallel studies in both emulsions and foams, aiming to clarify coalescence mechanisms. As discussed in this review, such an approach proved very successful and deserves to be continued in the future.     

Evaluating the electrochemical properties of PEO‐based nanofibrous electrolytes incorporated with TiO2 nanofiller applicable in lithium‐ion batteries

In the present work, nanofibrous composite polymer electrolytes consist of polyethylene oxide (PEO), ethylene carbonate (EC), propylene carbonate (PC), lithium perchlorate (LiClO₄), and titanium dioxide (TiO₂) were designed using response surface method (RSM) and synthesized via an electrospinning process. Morphological properties of the as‐prepared electrolytes were studied using SEM. FTIR spectroscopy was conducted to investigate the interaction between the components of the composites. The highest room temperature ionic conductivity of 0.085 mS.cm^?1 was obtained with incorporation of 0.175 wt. % TiO₂ filler into the plasticized nanofibrous electrolyte by EC. Moreover, the optimum structure was compared with a film polymeric electrolyte prepared using a film casting method. Despite more amorphous structure of the film electrolyte, the nanofibrous electrolyte showed superior ion conductivity possibly due to the highly porous structure of the nanofibrous membranes. Furthermore, the mechanical properties illustrated slight deterioration with incorporation of the TiO₂ nanoparticles into the electrospun electrolytes. This investigation indicated the great potential of the electrospun structures as all‐solid‐state polymeric electrolytes applicable in lithium ion batteries.     

Evaluation of the concentration dependence of transference numbers for dilute electrolyte solutions at various temperatures

At various temperatures the concentration dependence of transference numbers of electrolytes in methanol, ethanol, 1-propanol, acetonitrile, propylene carbonate, and water is investigated with the help of equations which take into account non-coulombic effects. A general rule for predicting the sign and magnitude of the concentration dependence of transference numbers is established. Literature data are reevaluated with the help of these equations and consistency of experimental data with the prediction is examined. Reference data for ion limiting conductances from experimental investigations are proposed which may be used for the judgment as to the reliability of empirical reference electrolyte methods.Dedicated to Professor Viktor Gutmann on the occasion of his 65th birthday.     

The effect of surface-active solutes on bubble coalescence in the presence of ultrasound

The sonication of an aqueous solution generates cavitation bubbles, which may coalesce and produce larger bubbles. This paper examines the effect of surface-active solutes on such bubble coalescence in an ultrasonic field. A novel capillary system has been designed to measure the change in the total volume resulting from the sonication of aqueous solutions with 515 kHz ultrasound pulses. This volume change reflects the total volume of larger gas bubbles generated by the coalescence of cavitation bubbles during the sonication process. The total volume of bubbles generated is reduced when surface-active solutes are present. We have proposed that this decrease in the total bubble volume results from the inhibition of bubble coalescence brought about by the surface-active solutes. The observed results revealed similarities with bubble coalescence data reported in the literature in the absence of ultrasound. It was found that for uncharged and zwitterionic surface-active solutes, the extent of bubble coalescence is affected by the surface activity of the solutes. The addition of 0.1 M NaCl to such solutes had no effect on the extent of bubble coalescence. Conversely, for charged surface-active solutes, the extent of bubble coalescence appears to be dominated by electrostatic effects. The addition of 0.1 M NaCl to charged surfactant solutions was observed to increase the total bubble volume close to that of the zwitterionic surfactant. This suggests the involvement of electrostatic interactions between cavitation bubbles in the presence of charged surfactants in the solution.     

Measurement of real free energies of transfer of individual ions from water to other solvents with the jet cell

Resolution of the activities of solutions of electrolytes into the individual ionic contributions cannot be carried out rigorously and requires the introduction of extrathermodynamic assumptions which have inherent uncertainties. The most commonly used approaches are basically similar in that they are based on the assumed solvent independence of the difference in the enthalpy or Gibbs energy of transfer of pairs of model solutes, e.g., tetraphenylarsonium and tetraphenylborate ions, or ferricinium ion and ferrocene. In this work we follow an alternative approach pioneered by Parsons involving measurement in the jet (Kenrick) cell of outer-potential differences between solutions of the same electrolyte in two solvents. These potential differences provide the real free energies of transfer of individual ions which, in turn, differ from the usual Gibbs energies of transfer by the work required to transfer the ion through the dipolar layers at the two solvent-gas interfaces. One objective of this work was to improve the reliability of real free energy of transfer measurements, which are experimentally demanding, to within ca. ±0.5 kJ-mol^–1 in order to match typical uncertainties in Gibbs transfer energies of electrolytes. This goal was met, in most instances, by careful evaluation of experimental parameters (particularly jet pressure). A major improvement over previous measurements was made by adding a supporting electrolyte which allowed stable potentials to be obtained at test electrolyte concentrations as low as 10^–4M. Real free energy changes are reported for the transfer of silver ion from water to methanol, ethanol, acetonitrile, propylene carbonate and dimethyl sulfoxide, as well as for the transfer of chloride ion from water to methanol and ethanol. Reliable data of this kind may lead to improved understanding of either the properties of the surfaces of solvents or the interactions of model solutes with solvents, depending on which of the two fields develops most.     

Calculation of the equilibrium constants and cooperative parameters of formation of Cu(II) chloride complexes in nonaqueous solvents

Copper(II) distributions over chloride complexes in various organic solvents were analyzed in terms of a modified matrix model. The equilibrium coordination constants of a first ligand and the corrections for the mutual influence between the ligands during the complexation were calculated. It was demonstrated that displacement of the solvent molecule by a chloride ion from the inner coordination sphere of the Cu(II) ion is always of anticooperative character. In MeCN, addition of a chloride ion to a copper ion follows the simplest additive scheme of coordination of the ligand with equivalent coordination vacancies. Possible reasons for nonadditive complexation in DMF, DMSO, trimethyl phosphate, and propylene carbonate are discussed.     

Solvation of the potassium ion complexed with dibenzo-30-crown-10. Transfer activity coefficients between methanol and various solvents

The stability constants of the potassium complex with dibenzo-30-crown-10 have been determined from potentiometric or solubility measurements in the solvents: methanol, iso-propanol, n-butanol, propylene carbonate, acetonitrile and dimethylsulfoxide. The solubility of the ligand in these solvents has also been determined and the transfer activity coefficients of the potassium complex for transfer from methanol to solvent (S), _SM(KL⁺), have been computed. Although solid state studies indicate that dibenzo-30-crown-10 completely surrounds the potassium ion and shields it from water, the transfer activity coefficient of the potassium complex is found to be highly solvent dependent. Dibenzo-30-crown-10 is thus less effective for the removal of the solvation sphere of the potassium ion than previously estimated.     

New cyclic carbonate solvent for lithium ion batteries: trans-2,3-butylene carbonate

A new cyclic carbonate useful for lithium ion batteries with graphitic carbon anode is presented. Although it is structurally very similar to propylene carbonate, it is much less reactive toward graphite than propylene carbonate. The decrease in the reactivity can be explained in terms of its unique geometry that hinders its co-intercalation into the layer spacing of graphite structure. We demonstrate that electrolytes containing this solvent exhibit a satisfactory initial efficiency and discharge performance at low temperature.