Carlos F. González-Fernández

Application of network thermodynamics to the computer modelling of nonstationary diffusion through heterogeneous membranes

Abstract Nonstationary diffusion through multilayer membranes has been studied using network thermodynamics together with the electrical circuit simulation routine, SPICE. A network model for a system made up of a multilayer membrane between two external phases is proposed. The concentration profiles in the composite membrane and the decay of the osmotic pressure across various associations (bi- and multilayer) of individual membranes connected in series has been simulated and calculated, thus doing away with the need for highly complex mathematical resolutions of the differential diffusion equations. The effect that a rearrangement of the spatial order of these membranes has on both the osmotic pressure and the activity profile is commented upon.

The uptake of a reactive dye by bleached cotton and its effect on the electrical properties of the interface: I. Electrokinetic properties

An experimental investigation on streaming potential, sorption, and electrical conductance of porous plugs of bleached cotton in aqueous solutions of a reactive dye, Remazol brilliant blue R (RBB-R), at varying temperatures is described. The absolute value of the (negative) zeta potential of cotton decreases appreciably when temperature is increased at concentrations of RBB-R in solution up to ca. 10-4 mole/liter, the reverse trend being observed at higher dye concentrations. Similar behavior was deduced for the surface charge density of the fiber. The uptake of dye by the fiber is favored by the increase in temperature, this fact being in accordance with the chemical nature for the bonds involved. However, physical contributions, mainly H-bonds, cannot be neglected at the lowest temperatures studied. The effective dyeing surface has been found to increase linearly with temperature. Surface conductanc, ks, predominates in the total electrical conductance of the system, k, at low concentrations of dye in solution, while it is the bulk component, kb, which predominates in the higher concentration range.

The effect of previous convective flux on the nonstationary diffusion through membranes. Network simulation

Abstract Isothermal diffusion-convection of binary solutions through both homogeneous and heterogeneous membranes has been studied using a network thermodynamic method. A network model for a system made up of a membrane between two external phases is proposed. With this network model and using the electrical circuit simulation program SPICE, the decay of the osmotic pressure across the membrane, taking into account the influence of the volume flow history on the initial concentration profile, has been simulated and calculated for both equal and unequal external concentrations.

The effect of tannic acid on the electrical properties of the interface and the non-linear streaming potential of cellulose in a cationic dye solution

An experimental investigation on streaming potentials of porous plugs of cellulose in both the linear and the non-linear range is described. The variation of electrokinetic coefficients with the concentration of a cationic dye in solution has been studied and the influence of mordant on the electrical properties of the interface of cellulose/cationic dye solution systems has been analysed. The second-order electrokinetic coefficients have been interpreted in terms of modifications of generalized coefficients produced by changes in thermodynamic forces.

A network thermodynamic method for numerical solution of the Nernst-Planck and Poisson equation system with application to ionic transport through membranes

Simple techniques of network thermodynamics are used to obtain the numerical solution of the Nernst-Planck and Poisson equation system. A network model for a particular physical situation, namely ionic transport through a thin membrane with simultaneous diffusion, convection and electric current, is proposed. Concentration and electric field profiles across the membrane, as well as diffusion potential, have been simulated using the electric circuit simulation program, SPICE. The method is quite general and extremely efficient, permitting treatments of multi-ion systems whatever the boundary and experimental conditions may be.

The adsorption of n-alkylammonium chlorides at the aqueous solution-air interface

Abstract Data on the surface tension of n -alkylammonium chloride solutions at various surfactant concentrations and at neutral and basic pH are given. By applying the Gibbs adsorption equation to the γ vs C curves, the adsorption isotherms of surfactants at the air-aqueous solution interface were obtained. These correspond to adsorption isotherms of the Langmuir type. Adsorbed monolayers obey the adsorption isotherm which is derived assuming immobility of the surfactants in the air-aqueous solution interface. This adsorption isotherm leads to consistent values for the standard free energy of adsorption. Finally, the hypothesis of the formation of stable complexes alkylamine-alkylammonium ion as responsible in determining the effect of pH on the interfacial density of these surface active substances is discussed.

The uptake of a reactive dye by bleached cotton and its effect on the electrical properties of the interface: II. Kinetics and thermodynamics

Abstract Data on the kinetics and thermodynamics of absorption of Remazol brilliant blue R on bleached cotton at different temperatures are presented. It is established that the increase in temperature of the system increases the dye content of the fiber. The experimental time—sorption isotherms were found to be represented by the exponential kinetic equation Mt = Meq(1 - e−kt), where Mt and Meq refer to the amounts of reactive dye uptaken by the cellulosic fiber at time t and at equilibrium, respectively, and k refers to the rate constant. The empirical rate constant k has been found to decrease with temperature. Values of half-dyeing time at different temperatures are presented. The sorption equilibria of RBB-R by bleached cotton are described by a Freundlichs isotherm type equation. Changes of enthalpy and entropy related to the process of dyeing were calculated from the Clausius—Clapeyron equation. Isosteric heats of dyeing have been found to be positive and of the order of 20 to 105 kJ/mole. The global change of entropy is positive and of the order of 85 to 430 J/mole K. The best thermodynamic conditions for the uptake of dye by the fiber appear at the highest temperatures, as the standard free energy of dyeing takes then the more negative values. From the results obtained the conclusion can be derived that the absorption of RBB-R by bleached cotton in the investigated temperature interval takes place with establishment of strong chemical bonds although physical contributions cannot be neglected.

Effect of sulphonate content of direct cotton dyes on the non-linear electrokinetic behaviour of cellulose plugs

Abstract The influence of the sulphonate groups in direct cotton dyes on the non-linear effects in streaming potential measurements has been investigated. The second-order coefficient L 211 is independent of dye concentration below ca. 5 × 10 −4 M and increases above this value. The lower the sulphonate content, the higher is the second-order coefficient L 211 . The magnitude of the coefficient L 221 is determined by both the intensity of the flow of liquid in the capillaries and the surface conductance. The latter is mainly due to adsorption of dye on the fibre, the sequence for this being monosulphonic > tetrasulphonic > pentasulphonic dye. The second-order electrokinetic coefficients have been interpreted in terms of modifications of generalized coefficients, produced by the changes in thermodynamic forces.

Electrokinetic transport of aqueous solutions of electrolytes through fibrous systems. Non-linear phenomenological relations

Abstract An experimental investigation on streaming potentials of porous plugs of cellulose and glass fibre in the non-linear range is described. The variation of the second order electrokinetic coefficients, L 211 and L 221 , as a function of fibre concentration in the porous pad has been studied. The electroviscous effect accounts for the decrease of these coefficients as packing density increases. The dependence of L 211 and L 221 on the concentration of the various electrolytes used is somewhat similar to that of the first order coefficients L 21 and L 22 (s), respectively. The explanation for this effect might be found in the influence exerted by the hydrodynamic conditions prevailing in the capillaries on the structure of the electric double layer and on the surface conductance.