José A. Manzanares

Equilibrium swelling properties of polyampholytic hydrogels

The role of counter ions and ion dissociation in establishing the equilibrium swelling of balanced and unbalanced polyampholytic hydrogels has been investigated experimentally and theoretically. The swelling dependence on both the net charge offset and the external bath salt concentration has been examined using an acrylamide based polyampholytic hydrogels. By careful consideration of the swelling kinetics, we illustrate the effects of ion dissociation equilibria and counter ion shielding in polyampholytic hydrogels near their balance point where both polyelectrolyte and polyampholyte effects are present. The theory considers a Flory type swelling model where the Coulombic interactions between fixed ions in the hydrogel resemble those of an ionic solid with a Debye screening factor. Theoretical predictions from this model are in qualitative agreement with our experimental results.

Ion-exchange fibers and drugs: an equilibrium study

The purpose of this study was to investigate the mechanisms of drug binding into and drug release from cation-exchange fibers in vitro under equilibrium conditions. Ion-exchange groups of the fibers were weakly drug binding carboxylic acid groups (-COOH), strongly drug binding sulphonic acid groups (-SO(3)H), or combinations thereof. Parameters determining the drug absorption and drug release properties of the fibers were: (i) the lipophilicity of the drug (tacrine and propranolol are lipophilic compounds, nadolol is a relatively hydrophilic molecule), (ii) the ion-exchange capacity of the fibers, which was increased by activating the cation-exchange groups with NaOH, (iii) the ionic strength of the extracting salt (NaCl), which was studied in a range of 1.5 mM to 1.5 M, and finally (iv) the effect of divalent calcium ions (CaCl(2)) on the release of the model drugs, which was tested and compared to monovalent sodium ions (NaCl), and combinations thereof. It was found that the lipophilic drugs, tacrine and propranolol, were retained in the fibers more strongly and for longer than the more hydrophilic nadolol. The more hydrophilic nadolol was released to a greater extent from the fibers containing strong ion-exchange groups (-SO(3)H), whereas the lipophilic drugs were attached more strongly to strong ion-exchange groups and released more easily from the weak (-COOH) ion-exchange groups. The salt concentration and the choice of the salt also had an effect: at lower NaCl concentrations more drug was released as a result of the influence of both electrostatic and volume effects (equimolar drug:salt ratio). Incorporation of CaCl(2) in the bathing solution increased drug release considerably as compared to NaCl alone. The equilibrium distribution of the drug species between the fiber and external solution phases was also simulated and it was found that the theoretical modelling proposed describes adequately the basic trends of the behavior of these systems.

Controlled Transdermal Iontophoresis by Ion Exchange Fiber

The objective of this study was to assess the transdermal delivery of drugs using iontophoresis with cation- and anion-exchange fibers as controlled drug delivery vehicles. Complexation of charged model drugs with the ion-exchange fibers was studied as a method to achieve controlled transdermal drug delivery. Drug release from the cation-exchange fiber into a physiological saline was dependent on the lipophilicity of the drug. The release rates of lipophilic tacrine and propranolol were significantly slower than that of hydrophilic nadolol. Permeation of tacrine across the skin was directly related to the iontophoretic current density and drug concentration used. Anion-exchange fiber was tested with anionic sodium salicylate. The iontophoretic flux enhancement of sodium salicylate from the fiber was substantial. As the drug has to be released from the ion-exchange fiber before permeating across the skin, a clear reduction in the drug fluxes from the cationic and anionic fibers were observed compared to the respective fluxes of the drugs in solution. Overall, the ion-exchange fibers act as a drug reservoir, controlling the release and iontophoretic transdermal delivery of the drug.

Ion-exchange fibers and drugs: a transient study

The objective of this study was to theoretically model and experimentally measure the kinetics and extent of drug release from different ion-exchange materials using an in-house-designed flow-cell. Ion-exchange fibers (staple fibers and fiber cloth) were compared with commercially available ion-exchange materials (resins and gels). The functional ion-exchange groups in all the materials were weak -COOH or strong -SO3H groups. The rate and extent of drug release from the fibers (staple fiber>fiber cloth) was much higher than that from the resin or the gel. An increase in the hydrophilicity of the model drugs resulted in markedly higher rates of drug release from the fibers (nadolol>metoprolol>propranolol>tacrine). Theoretical modelling of the kinetics of ion exchange provided satisfactory explanations for the experimental observations: firstly, a change in the equilibrium constant of the ion-exchange reaction depending on the drug and the ion-exchange material and, secondly, a decrease in the Peclet number (Pe) with decreasing flow-rate of the drug-releasing salt solution.

The physical description of elementary surface phenomena: Thermodynamics versus mechanics

A unified treatment of elementary surface phenomena based on the formalism of thermodynamics is presented and compared to more familiar treatments based on the formalism of Newtonian mechanics. Emphasis is put on the surface free energy concept rather than on surface tension, not only because the former is more fundamental, but also because the latter may mislead if pushed too far. The examples discussed (Young–Laplace and Young–Dupre equations, and capillary rise) can be easily described with the help of the Helmholtz function, and clearly show some of the advantages of the thermodynamic approach. In particular, several misleading results appearing in elementary treatments can be avoided by using this approach. It is concluded that: (i) thermodynamics and physical chemistry courses should favor the formalism of thermodynamics rather than mechanics when dealing with surface phenomena; and (ii) when the mechanical approach is still preferred, some weak points in the standard derivations (e.g., the existenc...

Modeling of surface vs. bulk ionic conductivity in fixed charge membranes

A two-region model for describing the conductivity of porous fixed charge membranes is proposed. In the surface region, the conductivity is due to the mobile positive ions (counterions) around the negative fixed charges. In the pore center region, the conductive properties resemble those of the external electrolyte solution because the fixed charges are assumed to be effectively neutralized by the counterions in the surface region. Activation energies and surface diffusion coefficients are estimated by assuming that the counterion jump from a fixed charge group is the rate limiting process for surface transport. The barrier energy for this jump is calculated using a simple electrostatic model with two microscopic parameters, the sum of the counterion and fixed charge hydration radii and the local dielectric constant. The bulk conductivity is obtained from experimental data. The total membrane conductivity and the counterion transport number are then calculated as functions of the external solution concentration for several pore radii and membrane fixed charge concentrations. The results are compared with those given by the Donnan model for homogeneous membranes and by the numerical solution of a continuous model based on the Poisson–Boltzmann equation extended to finite size ions. The study of the membrane conductivity for a series of electrolytes allows to distinguish clearly between the mechanism characteristic of the bulk ionic conductivity and that of surface conductivity. The surface conductivity is found to be significant for narrow pores at low external solution concentrations.

Effects of temperature and ion transport on water splitting in bipolar membranes

Abstract We have considered the effects of temperature and ion transport on water splitting in bipolar membranes. The Nernst-Planck equations are employed for the ion fluxes, and the water splitting phenomenon is accounted for by means of either the Onsager theory of the Second Wien Effect or the Chemical Reaction model. Comparison between theory and experiment can be done on the basis of the experimental data by Zabolotskii et al. [Soviet Electrochem., 20 (1985) 1238] . Special attention is paid to the form of the current-voltage curves as well as to the variation of the current carried by the H+ and OH− ions with the total electric current. It is found that the predictions derived from the Chemical Reaction model are generally in better agreement with experimental results than those obtained from the Second Wien Effect theory. Finally, we have applied the transport model to a fouled anion exchange membrane exhibiting a bipolar-like structure in order to study the influence of the fixed charge concentration in the fouling film on water splitting.

Membrane potential of bipolar membranes

Abstract The membrane potential of a bipolar membrane composed of a cation-exchange layer in series with an anion-exchange layer is analyzed theoretically and experimentally. The theoretical approach is based on an extension of the Nernst-Planck equations of monopolar charged membranes to the case of the two ion-exchange layers in series and the diffusion boundary layers adjacent to the bipolar membrane. The experimental results concern the transient behavior, stirring effects and concentration dependence of the membrane potential, and can be explained by means of the above theoretical approach. It is shown that membrane potential measurements can contribute significantly to a better electrochemical characterization of bipolar membranes.

Current-voltage curves for ion-exchange membranes. Contributions to the total potential drop

Abstract The effect of concentration polarization on the current-voltage (I-V) curves for ion-exchange membranes has been studied theoretically. Firstly, some of the hypotheses found in the literature concerning this field are reviewed. Secondly, we calculate numerically the different contributions to the total potential drop and assess the role of the local electroneutrality condition and the electroosmotic flow and coion flux on the I-V curves. Under the Nernst-Planck/Poisson/Donnan formalism and the conditions assumed, plateau regions in the I-V curves are obtained, thus showing that other “side effects” (water splitting, particular hydrodynamic conditions in the depleted layer, etc.) are responsible for the S-shaped I-V curves reported for most ion-exchange membranes. The analysis carried out can be regarded as an extension of the previous study by Spiegler (Desalination, 9 (1971) 367).

Contact Potentials, Fermi Level Equilibration, and Surface Charging

This article focuses on contact electrification from thermodynamic equilibration of the electrochemical potential of the electrons of two conductors upon contact. The contact potential difference generated in bimetallic macro- and nanosystems, the Fermi level after the contact, and the amount and location of the charge transferred from one metal to the other are discussed. The three geometries considered are spheres in contact, Janus particles, and core-shell particles. In addition, the force between the two spheres in contact with each other is calculated and is found to be attractive. A simple electrostatic model for calculating charge distribution and potential profiles in both vacuum and an aqueous electrolyte solution is described. Immersion of these bimetallic systems into an electrolyte solution leads to the formation of an electric double layer at the metal-electrolyte interface. This Fermi level equilibration and the associated charge transfer can at least partly explain experimentally observed different electrocatalytic, catalytic, and optical properties of multimetallic nanosystems in comparison to systems composed of pure metals. For example, the shifts in the surface plasmon resonance peaks in bimetallic core-shell particles seem to result at least partly from contact charging.