Diffuse Double Layer Potential Research Articles

The voltammetric responses of nanometer-sized electrodes in weakly supported electrolyte: A theoretical study

The effect of the supporting electrolyte concentration on the interfacial profiles and voltammetric responses of nanometer-sized disk electrodes have been investigated theoretically by combining the Poisson–Nernst–Planck (PNP) theory and Butler–Volmer (BV) equation. The PNP-theory is used to treat the nonlinear couplings of electric field, concentration field and dielectric field at electrochemical interface without the electroneutrality assumption that has been long adopted in various voltammetric theories for macro/microelectrodes. The BV equation is modified by using the Frumkin correction to account for the effect of the diffuse double layer potential on interfacial electron-transfer (ET) rate and by including a distance-dependent ET probability in the expression of rate constant to describe the radial heterogeneity of the ET rate constant at nanometer-sized disk electrodes. The computed voltammetric responses for disk electrodes larger than 200 nm in radii in the absence of the excess of the supporting electrolyte using the present theoretical scheme show reasonable agreements with the predications of the conventional microelectrode voltammetric theory which uses the combined Nernst–Planck equation and electroneutrality equation to describe the mixed electromigration-diffusion mass transport without including the possible effects of the diffuse double layer (Amatore et al. [25]). For electrodes smaller than 200 nm, however, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. It is shown that the deviations are mainly resulted from the overlap between the diffuse double layer and the concentration depletion layer (CDL) at nanoscale electrochemical interfaces in weakly supported media, which will result in the invalidation of the electroneutrality condition in CDL, and from the radial inhomogeneity of ET probability at nanometer-sized disk electrodes.

On the origin of electrostatic and steric repulsion in oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers

Oil-in-water emulsion films were used to investigate the origin of electrostatic and steric surface forces between two oil droplets in aqueous solution of nonionic polymeric surfactants. The Thin Liquid Film–Pressure Balance Technique was used. Equivalent thickness of oil-in-water emulsion films stabilized by PEO-PPO-PEO triblock copolymers (Pluronic ® F108 and P104) vs. pH was measured at constant capillary pressure and ionic strength. The isoelectric point at the negatively charged film interfaces, corresponding to zero diffuse double layer potential was found. This demonstrates that the origin of the electrostatic repulsion in the emulsion films studied is associated to preferential adsorption of OH − ions. Disjoining pressure isotherms for the emulsion films obtained at different pH were fitted to de Gennes’ scaling theory for steric interaction between polymer ‘brush’ layers with the ‘brush’ thickness as a fitting parameter. The fit obtained was satisfying and yielded PEO-‘brush’ thickness of 9.6 nm. These results allowed the conclusion that in absence of diffuse double layer forces the stabilization of oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers was due to ‘brush-to-brush’ steric repulsion.

The double layer potentials at the toluene–aqueous interface in the presence of CTAB/NaBr. Implications for Ion-Selective Electrodes

The interfacial tension isotherms for cetyltrimethylammnonium bromide (CTAB) in the presence of sodium bromide at the aqueous–toluene interface are discussed. In accordance with previous observations for an aqueous–air interface, the presence of a non-surface active electrolyte (NaBr) has strong influence on the interfacial activity of CTAB at the aqueous–toluene interface. The results were analysed within the framework of the Surface quasi-Two Dimensional Electrolyte model (STDE), assuming that the non-surface active bromide counterions partially penetrate the Stern layer formed by cetyltrimethylammonium cations adsorbed at the interface. This analysis allowed for calculation of the electrical double layer (EDL) potentials (Stern potential and the diffuse double layer potential). The calculated EDL potentials were compared with the membrane potentials generated in the Ion-Selective Electrodes (ISE) containing neither ionophore nor ion-exchanger, when exposed to aqueous solutions of CTAB.

Effect of electrolytes on surface tension of ionic surfactant solutions

Abstract An improved model of ionic surfactant adsorption was developed by considering penetration of counterions into the interfacial Stern layer. Accordingly, the surface layer was treated as quasi two-dimensional electrolyte in which the electroneutrality condition was not fulfilled. Moreover, in the model the finite size of surfactant head-groups and counterions were taken into account as well as lateral electrostatic interactions. The model was confronted with experimental data obtained for cetyltrimethylammonium bromide in different electrolytes. It was found, that the model correctly reflected the variation in surface tension isotherms with the ionic strength of the solution. Contrary to previous models of ionic surfactant adsorption, our model allowed to account for the strong dependence of adsorption on type of counterion present in solution. Using the model a dependence of Stern layer potential and diffuse double layer potential on the concentration of surfactant in solution also was predicted.

The significance of stagnant layer conduction in electrokinetics

This paper examines the consequences of the occurrence of conduction processes in the stagnant fluid layer between the surface of a colloidal particle and the shear plane when it is undergoing an electrokinetic process. It is shown that the most widely used model system for studying electrokinetics (viz. monodisperse polystyrene sulfate latex) displays significantly different behaviour in different experiments depending on the details of the post-polymerisation clean-up procedure. Nonetheless, the results provide important insights into how electrokinetic results should be interpreted. A procedure is suggested which should allow a more realistic evaluation of the diffuse double layer potential from electrokinetic results without the need for extensive conduction studies.

A comparison of electroacoustic and microelectrophoretic zeta potential data for titania in the absence and presence of a poly (vinyl alcohol) adlayer

The recently developed technique of electroacoustics can measure the zeta potential in relatively concentrated (0.1–50 vol%) particle dispersions that cannot be analyzed by conventional microelectrophoresis. In this research, the MATEC Applied Sciences ESA 8000 electroacoustic apparatus is used to determine the zeta potential of aqueous colloidal anatase titania in the absence and presence of 88 mol% poly (vinyl alcohol), PVA, which adsorbs strongly to the titania surface. For both the bare titania and the titania/PVA adlayer systems the electroacoustically determined zeta potential data are in agreement with data collected on the Bank Bros. Mk. II microelectrophoresis apparatus, provided that the electroacoustic data are corrected for the effect of particle inertia.For all pH values the presence of a PVA adlayer acts to decrease the magnitude of the zeta potential. Since the PVA does not significantly change the surface charge density of the titania, this decrease is interpreted as an outward shift in the shear plane location. A diffuse double layer potential profile is then used to calculate an effective adlayer thickness from the zeta potential data. The calculated adlayer thickness is found to increase with nominal polymer molecular weight.

Contribution of surface potential to resting potential in mouse neuroblastoma cells: Estimation with fluorescent dyes and from shift of threshold potential for Ca-Spike

The contribution of the changes in the diffuse double layer potential (surface potential) to the resting membrane potential of the mouse neuroblastoma (N-18 clone) was estimated with the use of fluorescent dyes and from the shift of the threshold potential of the Ca-spike. The membrane potential monitored by Rhodamine 6G was unchanged with variations of external K + concentration ([K +] 0) below about 10 mM, and changed linearly with log [K +] 0 above this concentration, similar to the changes observed with a microelectrode. This result indicates that the independence of the membrane potential of [K +] 0 at low [K +] 0 is not brought about by the artifacts caused by insertion of a microelectrode into the cell. The changes in the surface potential in response to various polyvalent cations (Mg 2+, Ca 2+, Mn 2+, Cd 2+, and La 3+) were monitored by the fluorescence of ANS. The behavior of the surface potential thus observed was closely correlated with that of the membrane potential for the respective polyvalent cations. The surface potential changes in response to increase in [Ca 2+] 0 from 1.8 to 100 mM and in [Mg 2+] 0 from 0.8 to 100 mM were 88% and 85% in magnitude compared with the corresponding changes in the membrane potential, respectively. The effect of variation of [Ca 2+] 0 on the threshold potential of the Ca-spike was compared with that of [Ca 2+] 0 on the resting membrane potential. The magnitude of the surface potential change determined by the shift of the threshold potential was comparable to the magnitude of the membrane potential change. The results in the present study suggest that the surface potential change produced at the membrane surface of N-18 cells is detected as a transmembrane potential change without a large reduction in magnitude.

Inactivation of the alamethicin-induced conductance caused by quaternary ammonium ions and local anesthetics.

Long alkyl chain quaternary ammonium ions (QA), the local anesthetics (LA) tetracaine and lidocaine, imipramine, and pancuronium cause inactivation of the alamethicin-induced conductance in lipid bilayer membranes. The alamethicin-induced conductance undergoes inactivation only when these amphipathic compounds are added to the side containing alamethicin. The concentration of QA required to cause a given amount of inactivation depends on the length of the hydrocarbon chain and follows the sequence C9 greater than C10 greater than C12 greater than C16. LA and imipramine, in contrast to QA or pancuronium, are able to promote appreciable inactivation only if the pH of the alamethicin-free side is equal to or lower than the pK of these compounds. The membrane permeability to QA, LA, or imipramine is directly proportional to the alamethicin-induced conductance and is larger than the one for potassium. The observed steady state and time-course of the inactivation are well described by a model similar to that proposed by Heyer et al. (1976. J. Gen. Physiol. 67:703--729) and extended for any value of the diffuse double layer potential and for LA and imipramine. In this model QA, LA, or imipramine are able to permeate through the membrane only when the alamethicin-induced conductance is turned on. The amphipathic compounds then bind to the other membrane surface, changing the transmembrane potential and turning the conductance off. For a given concentration of QA, LA, or imipramine the extent of inactivation depends on two factors: first, the binding characteristics of these compounds to the membrane surface and second, their ability to permeate through the membrane when the alamethicin-induced conductance is turned on. The several possible mechanisms of permeation of the amphipathic molecules tested are discussed.

Surface charge, surface dipoles and membrane conductance

The conductance of lipid membranes in the presence of nonactin is changed by the adsorption of small amounts of ionic and zwitterionic surfactants. The conductance changes are, in many instances, not accounted for by the variation in surface charge or diffuse double layer potential as calculated from Gouy-Chapman theory. The changes are, however, all accurately accounted for by the variation in total potential across the membrane interface. This potential includes contributions from surface dipoles and specific adsorption, as well as any diffuse double layer effects not included in the Gouy-Chapman theory. The total potential changes were inferred from Volta or compensational potential changes at bulk oil (and monolayer)/aqueous solution interfaces. Surface charge densities were found by standard thermodynamic methods involving the use of the Gibbs equation. Electrokinetic potentials for the appropriate surfaces were also measured and, in general, agreed well with the diffuse double layer potentials calculated from the Gouy-Chapman theory.

Adsorption of potential-determining ions at the silica—aqueous electrolyte interface and the role of some cations

The specific surface charge density on a silica sample of known surface area was measured in the presence of LiCl, NaCl, KCl, CsCl and (C 2 H 5 ) 4 NCl. A counter-ion charge-surface charge balance was set up and analytically checked. Above pH∼7, the surface charge is much higher than on silver iodide or mercury. At a fixed ionic strength it increases in the order, (C 2 H 5 ) 4 N + <Li + <Na + <K + <Cs + . Notwithstanding the high surface charge, the diffuse double-layer potential, δ , is not particularly high. An explanation of these facts has been given on the basis of the presence of negative groups and cation penetration inside the pores of the gel layer, the extent of which depends to a large extent on the cation volume as compared to pore size.

Surface properties and flow behavior of kaolinite. Part I: Electrophoretic mobility and stability of kaolinite sols

The electrophoretic mobility of kaolinite sols has been studied as a function of electrolyte concentration in NaHCO 3-Na 2CO 3 buffer at pH 10.4. The positive charges which occur on the edges of the kaolinite particles in acid solution are no longer present at this pH and the particles are negatively charged owing to defects in the crystal lattice. Most of the exchangeable cations are evidently quite firmly attached to the particles in a Stern layer. The calculated ζ-potentials are considered to give a measure of the potential ( Ψ δ ) at the outer edge of the Stern layer (i.e., the diffuse double layer potential), and this is used to calculate the potential energy of repulsion between the particles as a function of distance. Rapid coagulation of the sol by a uni-univalent electrolyte at this pH occurs at an ionic strength of about 0.016 g. ion/l.; this is consistent with a value for the van der Waals' constant, A, of 2 × 10 −12 erg, in agreement with the values previously suggested by Street and Buchanan, Ottewill and Watanabe, and Overbeek.