Donnan phenomena in membranes with charge due to ion adsorption. Effects of the interaction between adsorbed charged groups

A physical model for the modified Donnan phenomenon associated with ion adsorption on localized membrane sites is presented. This model accounts for the dependence of the concentration of adsorbed ions on electrolyte concentration and pH as it is influenced by the electrostatic interaction between adsorbed ions. The equilibrium thermodynamic concepts employed are based on the Donnan formalism for the ion equilibria between membrane and solution, and the Bragg–Williams approximation for an adsorption isotherm that incorported interaction between adsorbed ions. Our results include the concentration of charged groups in the membrane, the pH of the membrane phase solution, and the Donnan potential as functions of the pH and the electrolyte concentration of the external solution for different degrees of the electrostatic interaction. These magnitudes are of considerable interest in biopolymers, membranes, and conducting [email protected] ; [email protected]

Configurational entropy of microemulsions : The fundamental length scale

Phenomenological models have been quite successful in characterizing both the various complex phases and the corresponding phase diagrams of microemulsions. In some approaches, e.g., the random mixing model (RMM), the lattice parameter is of the order of the dimension of an oil or water domain and has been used as a length scale for computing a configurational entropy, the so‐called entropy of mixing, of the microemulsion. In the central and material section of this paper (Sec. III), we show that the fundamental length scale for the calculation of the entropy of mixing is of the order of the cube root of the volume per molecule—orders of magnitude smaller than the dimension of such a domain. This length scale is specifically the scale for the configurational entropy—not that which measures either the curvature of the interface, the ‘‘granularity’’ of the microemulsion, or the persistence length. Furthermore, we demonstrate, in general, that mixing entropy, evaluated in configuration space as opposed to phase space, will not be physically correct unless it is made to be consistent with the phase space evaluation. Following this core section, we give a one‐dimensional illustration of the problem (Sec. IV), and discuss the consequences of our general result with respect to the RMM (Sec. V). The RMM not only seriously underestimates the entropy of mixing but exhibits a dependence on composition that is qualitatively very different from the correct dependence. Furthermore, for oil or water rich compositions of the microemulsion, the correct mixing entropy reinforces effects that would normally be attributed to bending energy, i.e., it destabilizes the [email protected]

Demo 167. Destilación por enfriamiento con un frasco de Franklin vertical

Se presenta una demostración de aula que muestra que las fases líquida y vapor de una disolución que coexisten en equilibrio tienen distinta composición. Se muestra también que el disolvente y el soluto de una disolución en un frasco de Franklin vertical se pueden separar mediante destilación, concretamente condensando la fase vapor.A classroom demonstration is presented showing that the liquid and vapor phases of a solution that coexist in equilibrium have different composition. It is also shown that the solvent and solute of the solution in a vertical Franklin flask can be separated by distillation, specifically by condensing the vapor phase

Demo 168. Demostración de la ley de Raoult y de la nucleación heterogénea con un frasco de Franklin vertical

Se presenta una demostración de aula que muestra que la condensación del vapor se produce preferentemente sobre la superficie de partículas sólidas y que la presión de vapor de una disolución disminuye cuando aumenta la concentración de soluto.A classroom demonstration is presented showing that vapor condensation occurs preferentially on the surface of solid particles and that the vapor pressure of a solution decreases as the solute concentration increases

Demo 161. Resistencia eléctrica de un metal y de un semimetal. Bombillas de filamento incandescente: tungsteno vs. carbono

Se presenta una demostración de aula que muestra que la curva voltaje-intensidad del filamento de una bombilla incandescente es no lineal. Para un filamento de tungsteno se observa que la resistencia eléctrica aumenta con la temperatura del filamento, la cual aumenta con la potencia eléctrica consumida, pues esta se disipa por radiación según la ley de Stefan. Para un filamento de carbono se observa que la resistencia eléctrica disminuye con la temperatura, evidenciando así que no es un metal (sino un semimetal).A classroom demonstration is presented showing that the voltage-intensity curve of the filament of an incandescent light bulb is non-linear. For a tungsten filament, it is observed that the electrical resistance increases with the temperature of the filament, which increases with the electrical power consumed, since this is dissipated by radiation according to Stefan's law. For a carbon filament, it is observed that the electrical resistance decreases with temperature, thus showing that it is not a metal (but a semi-metal)

Demo 185. Miscibilidad parcial: Fenómeno de expulsión salina

Se presenta una demostración de aula que muestra que la adición de NaCl a una mezcla líquida de dos componentes miscibles (agua y propan-2-ol) hace que dejen de ser miscibles y se formen dos fases líquidas (ternarias), una acuosa rica en sal y la otra orgánica rica en alcohol. El objetivo es ilustrar el fenómeno de expulsión salina para evidenciar la importancia de las interacciones agua-electrólito frente a las interacciones agua-propan-2-ol.A classroom demonstration is presented showing that the addition of NaCl to a liquid mixture of two miscible components (water and propan-2-ol) causes them to become immiscible and to form two (ternary) liquid phases: one aqueous phase rich in salt and one organic phase rich in alcohol. The objective is to illustrate salting out and to demonstrate the importance of water-electrolyte interactions compared to water-propan-2-ol interactions

Demo 160. Miscibilidad parcial: un aumento de temperatura provoca un desplazamiento del equilibrio hacia estados de mayor entropía

Se presenta una demostración de aula que muestra que un aumento de temperatura provoca un desplazamiento del equilibrio hacia estados de mayor entropía usando para ello la variación de la miscibilidad con la temperatura.A classroom demonstration is presented showing that an increase in temperature causes a shift from equilibrium towards states of higher entropy using the variation of miscibility with temperature

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 [email protected] ; [email protected]

Modified Donnan phenomena in polyaniline with poly(vinyl sulphonate) chains

We develop a physical model, based on the modified Donnan phenomena ideas introduced previously by the authors, to describe the acid doping of the conducting polymer polyaniline. The theory is motivated by the experimental work of Asturias et [email protected] ; [email protected]

Proyecto de Innovación Docente del Laboratorio de Termodinámica

El proyecto de Innovación Educativa 2013 del Laboratorio de Termodinámica (PIE2103LT, UV-SFPIE_F013-146569) es un proyecto anual del tipo Finestra Oberta aprobado por la Resolución de 24 de julio de 2013 del Vicerectorat de Cultura i lgualtat de la Universitat de Valencia. Este documento incluye: Anexo 1: Experiencia virtual 1. Conductividad térmica Anexo 2: Experiencia virtual 2. Diagramas de fases sólido-líquido Anexo 3: Demostración de aula. Punto eutéctico profund