Electrical electrolyte concentration dependence

Equation 46 suggests that, maintaining pi constant, q, must depend linearly on if only a first-order electroviscous effect exists, and an increase in the electrolyte concentration implies a decrease in the thickness, 1/k, of the electrical double layer. 

Studies of the adsorption of surface active electrolytes at the oil-water interface provide a convenient method for testing electrical double layer theory and for determining the state of water and ions in the neighborhood of an interface. The change in the surface amount of the large ions modifies the surface charge density. For instance, the surface ionic area of 100 per ion corresponds to 16, /rC/cm. The measurement of the concentration dependence of the changes of surface potential were also applied to find the critical concentration of formation of the micellar solution [18]. 

It is assumed that the quantity Cc is not a function of the electrolyte concentration c, and changes only with the charge cr, while Cd depends both on o and on c, according to the diffuse layer theory (see below). The validity of this relationship is a necessary condition for the case where the adsorption of ions in the double layer is purely electrostatic in nature. Experiments have demonstrated that the concept of the electrical double layer without specific adsorption is applicable to a very limited number of systems. Specific adsorption apparently does not occur in LiF, NaF and KF solutions (except at high concentrations, where anomalous phenomena occur). At potentials that are appropriately more negative than Epzc, where adsorption of anions is absent, no specific adsorption occurs for the salts of... 

The second period, from 1890 to around 1920, was characterized by the idea of ionic dissociation and the equilibrium between neutral and ionic species. This model was used by Arrhenius to account for the concentration dependence of electrical conductivity and certain other properties of aqueous electrolytes. It was reinforced by the research of Van t Hoff on the colligative properties of solutions. However, the inability of ionic dissociation to explain quantitatively the properties of electrolyte solutions was soon recognized. 

Here subscripts a and c denote anode and cathode respectively, iref is the reference exchange current density, y is the concentration dependence exponent, [ ] and [ ]ref represent the local species concentration and its reference concentration, respectively. Anode transfer current, Ra, is the source in the electric potential equations at the anode/electrolyte interface with positive sign on membrane (electrolyte) side and negative sign on solid (anode) side. Similarly, near the cathode interface, the source on membrane (electrolyte) side is negative of the cathode transfer current, Rc and that on solid (cathode) side is positive of Rc. The activation over-potentials, in Equations (5.35) and (5.36) are given by... 

Later work (8,15) showed that the value of a increased with increasing electrolyte concentration and that it could be correlated with the electrophoretic mobility. Table IX shows that the electrophoretic mobility measured using the Micromeritics Mass Transport cell increased with increasing a as observed for IT, Na+, and Ba++ counterions. These results also show that the distribution of the counterions in the electric double layer is critically dependent upon the nature of the counterion, e.g.,... 

Fig. 9. A rotation spectrum is produced by observing the motion of a cell in a rotating electric field of constant amplitude and plotting the rotation speed of the cell against frequency of the field. In solutions of low conductivity, the cell rotates in the opposite direction to the field (anti-field rotation) at low frequencies. This rotation reaches a peak when the field frequency corresponds to the charge relaxation time of the membrane. The position of this peak therefore contains information about membrane permittivity and conductivity. As the frequency increases further, the rate of cell spinning falls, becoming zero at about 1 MHz. Above this frequency, the cell starts to spin with the field (co-field rotation) and a second peak is reached. The frequency at which this peak occurs depends in practice mainly on the conductivity of the interior of the cell. It may be used for non-destructive determination of cytosolic electrolyte concentration.

Combined with densities, molecular weights, and transference numbers (fractions of the current carried by the various ionic constituents), the conductivity yields the relative velocities of the ionic constituents under the influence of an electric field. The mobilities (velocity per unit electric field, cm2 s-1 V-1) depend on the size and charge of the ion, the ionic concentration, temperature, and solvent medium. In dilute aqueous solutions of dissociated electrolytes, ionic mobilities decrease slightly as the concentration increases. The equivalent conductance extrapolated to zero electrolyte concentration may be expressed as the sum of independent equivalent conductances of the constituent ions... 

Secondly, selectivity is not always achievable. For example, permselectivity of ion-exchanging polymer films fails at high electrolyte concentration. We have shown that even if permselectivity is not thermodynamically found, measurements on appropriate time scales in transient experiments can lead to kinetic permselectivity. To rationalise this behaviour we recall that the thermodynamic restraint, electrochemical potential, can be split into two components the electrical and chemical terms. These conditions may be satisfied on different time scales. Dependent on the relative transfer rates of ions and net neutral species, transient responses may be under electroneutrality or activity control. 

The electrical properties of polyelectrolyte complexes are more closely related to those of biologically produced solids. The extremely high relative dielectric constants at low frequencies and the dispersion properties of salt-containing polyelectrolyte complexes have not been reported for other synthetic polymers. Neutral polyelectrolyte complexes immersed in dilute salt solution undergo marked changes in alternating current capacitance and resistance upon small variations in the electrolyte concentration. In addition, their frequency-dependence is governed by the nature of the microions. As shown in... 

The course of h(Cci) dependence indicating the decrease in equilibrium thickness up to the transition to NBF as well as the course of n(Ii) isotherm with a distinct barrier transition, reveal the electrostatic character of the forces acting in the film. Thus, double electric layer can be estimated, knowing that n / = pc+T vw The capillary pressure pa was measured experimentally while Tlvw was calculated from Eq. (3.89). The potential was determined within the electrolyte concentration range of 5-10 4 to 10 3 mol dm 3 (Fig. 3.48) in which the films were relatively thick, yielding a value of (po = 36 3 mV. In this respect films stabilised with the zwitterionic lipid DMPC are very similar to those stabilised with non-ionic surfactants [e.g. 100,186,189] (see also Section 3.4.1.1). The low ( -potential leads to the low barrier in the FI(Ii) isotherm which can easily be overcome at relatively low electrolyte concentrations and low pressure values. 

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