Ion-free Layer Research Articles

Surface tension and surface Δχ-potential of concentrated Z+:Z− electrolyte solutions

Schmutzer’s model for the surface of aqueous electrolyte solutions is generalized to Z+:Z− salts. The thickness of the ion-free layer is calculated from the thickness of the “hydrophobic gap” at the water surface (1.38Å) and the radii of the ionic hydration shells. The overlap between the adsorption and the diffuse double layers is accounted for. The proposed model predicts the dependence of the surface tension σ and the surface Δχ-potential on the electrolyte concentration cel in agreement with the available data, without adjustable parameters. The Hofmeister effect on σ for salts of the same valence type is explained with their ion-specific activity coefficients. The negative value (toward air) of the Δχ-potential of most 1:1 electrolytes originates from the dipole moment of the water molecules at the surface. The negative χ-potential due to water dipoles is inversely proportional to the dielectric permittivity ε of the solution. Since ε diminishes as cel increases, most 1:1 electrolyte solutions exhibit a more negative χ-potential than pure water (Δχ<0). The Hofmeister series of Δχ of 1:1 salts (ΔχLiCl≈ΔχNaCl<ΔχKCl<ΔχKF) follows the corresponding series of ε (εLiCl≈εNaCl<εKCl<εKF). The theory allows the estimation of the surface potential χ0 of pure water from the experimental data for electrolyte solutions; the result, χ0≈−100mV, confirms the value currently accepted in the literature.

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 δ 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 δ 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.

Comparison of Cationic Excesses in the Presence of Perchlorate and Chloride Anions

Cationic surface excesses of Li+, Na+, Mg2+, and Al3+ are obtained from electrocapillary curves corresponding to aqueous perchlorate solutions. They are compared with earlier reported data on chlorides, obtained under similar conditions. The influence of ionic sizes on the ion-free layer thickness is discussed in the framework of Gouy−Chapman and the generalized UDCA theory. Differences between the surface excesses of a given cation in the presence of perchlorate and chloride find a plausible explanation in terms of the anion distance of closest approach to the electrode.

Application of the MPB theory to the analysis of the ion-free layer thickness

The concept of ion-free layer thickness is examined on the basis of contemporary theories of the electrical double layer. A comparison between surface excesses derived from Modified Poisson-Boltzmann (MPB and URMPB) and mean field (GC and UDCA) theories is performed. The influence of unequal distances of closest approach for cations and anions in the analysis of experimental surface excesses is cancelled to a significant extent by inclusion of self-atmosphere and exclusion volume effects. An application is made to the mercury/aqueous LiCl, NaCl and KCl interfaces.

Generalization of the UDCA theory and its application to the analysis of the ion-free layer thickness

The modified Gouy-Chapman theory with two planes of closest approach is generalized for any electrolyte stoichiometry, considering also the influence of the interfacial dielectric constant. Application is made to some experimental systems to derive the cation and anion distances of closest approach from the ion-free layer thickness.

Interfacial tensions at alkane-aqueous electrolyte interfaces

Surface tensions of aqueous electrolytes, and their interfacial tensions against n-dodecane, have been determined at 20°C. The salts studied were LiCl, NaCl, KCl, KBr, NaBr, KI and Na2SO4 at concentrations up to about 1 mol kg–1. For the alkali metal chlorides and Na2SO4 the surface and interfacial tension increments are similar for a given electrolyte. The corresponding increments for KBr, NaBr and KI however are found to differ considerably. The results are discussed in terms of both electrostatic theory and dispersion force theory of interfaces. With respect to the latter, it is found that if due allowance is made for the presence of an ion-free layer of water at the interface, an approximate approach in which only a single dominant interaction frequency is considered, gives results in reasonable accord with experiment.

Comparative studies of phosphate adsorption at the mercury-solution interface

Summary Large differences between directly measured interfacial tensions and those derived from capacitance curves were observed at anodic rational potentials in solutions of Na 2 HPO 4 and Na 3 PO 4 . These discrepancies associated with sticking of the mercury meniscus in the Lippmann electrometer at extreme anodic potentials were attributed to the probable formation of highly insoluble compounds such as (Hg 2 ) 3 (PO 4 ) 2 , Hg 2 HPO 4 and HgO. The charge due to specifically adsorbed HPO 4 2− anions was calculated by comparing experimental relative surface excesses with those of the GCS diffuse layer theory. The p.z.c. for Na 2 HPO 4 and Na 3 PO 4 solutions over a wide range of concentrations was found to vary within about only 1 mV. No significant anion specific adsorption was detected for Na 2 HPO 4 until negative or positive q m values were attained. Results of q − ′ calculations were compared with previously published work on NaH 2 PO 4 (ref. 1) and Na 2 SO 4 . The presence of an ion-free layer of adsorbed water molecules was considered as being able to exert an appreciable influence on the calculated values of q − ′, and this concept was employed to obtain “corrected data” for anion specific adsorption according to the method reported for NaH 2 PO 4 by Parsons and Zobel 1 . Capacitance results in Na 3 PO 4 solutions were not thermodynamically analysed because of the different nature and charge of the anions in solution. The adsorbability of the three phosphate anions on positively charged mercury was empirically related to the high degree of structure-ordering of the solvent as indicated by their large positive B coefficients for the Jones and Dole equation. Considerations of molar fluidity persuaded us to qualitatively suggest that increasing specific adsorption may occur at anodically polarized Hg in the order: PO 4 3− 4 2− 2 PO 4 − . Ancillary information on homogeneous ion-solvent interactions led us to speculate on a possible simplified molecular model of the interface to account for the observed double layer properties of HPO 4 2− . Structural effects at the metal-solution interface due to the combined influences of ionic and electrode fields appear to be consistent with the suggested modelistic interpretation despite its crude nature and the obvious pitfalls of extending macroscopic thermodynamic measurements to speculate on the micro-physical entities involved in the double layer.

Dielectric coefficients in the interphase between electrode and ionic solution

Fundamental electrostatic concepts are coupled with considerations of a quasi-dipolar solid model to account for the dependency of effective dielectric coefficients, ε, of specifically and non-specifically adsorbed electrolytes over a wide concentration range. Attention is given in quantitative detail to the variation of ε as a function of temperature, ionic strength capacitance at the point of zero charge, and the thickness of the ion-free layer. Applications to published experimental capacitance data for non-specifically adsorbed electrolytes show good agreement in most cases with different theoretical approaches suggested by the theory of structure at charged interfaces due to Bockris, Devanathan and Müller and by an earlier treatment of the double layer by Conway, Bockris and Ammar.

Ionic hydration and the thermodynamic cationic surface excess at the mercury-aqueous electrolyte interface

Interfacial tension measurements of several concentrated aqueous electrolytes have been made using a modified form of a capillary electrometer which is described. The measurements indicate the presence of an ion-free layer adjacent to the mercury surface, the thickness of which appears to depend on the degree of hydration of the ions concerned.

Ionic hydration and the thermodynamic cationic surface excess at the mercury-aqueous electrolyte interface

Interfacial tension measurements of several concentrated aqueous electrolytes have been made using a modified form of a capillary electrometer which is described. The measurements indicate the presence of an ion-free layer adjacent to the mercury surface, the thickness of which appears to depend on the degree of hydration of the ions concerned.