Measurement of the Low-Frequency Dielectric Properties of Colloidal Suspensions: Comparison between Different Methods.

A careful analysis of the main factors that affect the low-frequency dielectric measurements of conducting liquid samples is carried out. The influence of the type of the measurement cell, the calibration method, and the type of the instrument used, on the spectra obtained using the variable electrode spacing technique, is investigated. Permittivity and conductivity measurements in the 10 Hz to 10 MHz range are reported for low (sigma approximately 0.01 S/m) and high (sigma approximately 0.7 S/m) conductivity samples, both electrolyte solutions and polystyrene particle suspensions. Two measurement cells are evaluated: one made of glass currently used at Granada and the other made of acrylic currently used at Tucumán. Two calibration methods, the classical Short/Open correction and the quadrupolar technique (similar to the Short/Open/Load correction), are contrasted, and two impedance analyzers, the HP 4284 A and the HP 4192 A, are compared. Copyright 2000 Academic Press.

Calculation of the Dielectric Increment and Characteristic Time of the LFDD in Colloidal Suspensions of Spheroidal Particles.

The alpha-dispersion amplitude of suspensions of colloidal particles is usually calculated from the low-frequency asymptotic of the frequency-dependent solution of the electrodiffusion equations. Since these equations written in spheroidal coordinates do not separate, no theoretical results exist for the low-frequency dielectric properties of suspensions of spheroidal particles. In order to sidestep this problem, we used another method which relates the dielectric properties to the energy stored in the system (Grosse, C., Ferroelectrics 86, 171 (1988)) which, at low frequencies, mainly corresponds to the Gibbs free energy associated to the field-induced electrolyte concentration changes outside the double layer (Grosse, C. and Shilov, V. N., J. Colloid Interface Sci. 193, 178 (1997)). This method permits us to calculate the static permittivity by solving a purely static problem, which makes it possible to calculate analytically the alpha-dispersion amplitude of suspensions of spheroidal particles since the electrodiffusion equations do separate in the static case. We also calculate the characteristic time of the alpha-dispersion from the dispersion amplitude and the static and high-frequency values of the dipolar coefficient. The analytical results obtained are presented and discussed for both prolate and oblate geometries, and for parallel, perpendicular, and random orientations of the particles with respect to the applied field. Copyright 1999 Academic Press.

Electrophoretic Mobility of Colloidal Particles in Weak Electrolyte Solutions.

The analytical theory of the thin double layer concentration polarization in suspensions of colloidal particles is generalized to the case of weak electrolyte solutions, i.e., when the dissociation-recombination equilibrium and rate constants have both finite values. It is shown that under the action of a static applied field, regions near the particle appear where there is departure from the dissociation-recombination equilibrium. The resulting ion and ion-pair sources have a strong bearing on their flows, leading to a change of the electrolyte concentration gradients around the particle. This phenomenon also modifies the value of the particle electrophoretic mobility, which is dependent on the concentration polarization. At constant ionic strength, the theoretical maximum of the electrophoretic mobility versus zeta potential curve can substantially surpass in weak electrolyte solutions the corresponding value attained in strong electrolytes. Copyright 1999 Academic Press.

Broad Frequency Range Study of the Dielectric Properties of Suspensions of Colloidal Polystyrene Particles in Aqueous Electrolyte Solutions.

The dielectric properties of suspensions of spherical polystyrene particles with radii of 34 and 50 nm, in aqueous electrolyte solutions were measured in the frequency range of 10 kHz to 30 GHz using HP 4192A impedance and 8753A network analyzers, and double path interferometers. The samples were prepared from the stock suspensions with a high particle concentration (volume fraction of 0.4), which were used either unchanged or diluted with increasing amounts of water down to a concentration of 0.05. The permittivity and total loss spectra showed three well-defined relaxation regions, which could successfully be interpreted using an existing model [Grosse, C., J. Phys. Chem. 92, 3905 (1988)] (corrected by including a term of counterion diffusion on the particle surface). A model spectral function was fitted to the data using seven free parameters: the static conductivity of the electrolyte solution, three parameters associated with the relaxation of the electrolyte solution, and just three parameters for the description of both the counterion and the Maxwell-Wagner relaxations. For different particle concentrations, the values of the surface conductivity (obtained from the Maxwell-Wagner parameters) remained essentially constant, while the low-frequency relaxation parameters exhibited very good agreement with theoretical predictions in Delgado, A. V., Arroyo, F. J., Gonzales-Caballero, F., Shilov, V. N., and Borkovskaya, Yu. B., Colloids Surf. A, in press. Copyright 1998 Academic Press.

Calculation of the Static Permittivity of Suspensions from the Stored Energy

It is shown that the static permittivity of suspensions of charged particles in electrolyte solution can be deduced from the field-induced change of the Gibbs free energy stored outside the double layer. This energy is related to the changes of the electrolyte concentration around the particle and is obtained solving a purely static problem. The method is first verified deducing well-known results corresponding to suspensions of spherical particles. It is then used to calculate the static permittivity of suspensions of spheroidal particles, leading to a new result that cannot be analytically obtained using the classical approach. Copyright 1997 Academic Press. Copyright 1997Academic Press

Cellular membrane potentials induced by alternating fields.

Membrane potentials induced by external alternating fields are usually derived assuming that the membrane is insulating, that the cell has no surface conductance, and that the potentials are everywhere solutions of the Laplace equation. This traditional approach is reexamined taking into account membrane conductance, surface admittance, and space charge effects. We find that whenever the conductivity of the medium outside the cell is low, large corrections are needed. Thus, in most of the cases where cells are manipulated by external fields (pore formation, cell fusion, cell rotation, dielectrophoresis) the field applied to the cell membrane is significantly reduced, sometimes practically abolished. This could have a strong bearing on present theories of pore formation, and of the influence of weak electric fields on membranes.

Pressure responses of hypertensive patients treated with thiazides, beta blockers and clonidine during a psychological experimental stress situation.

A total of 61 hypertensive patients treated during more than six months with antihypertensive drugs and showing a satisfactory evolution and normal basal arterial pressure values were studied. Patients were subjected to a psychological stress test in order to evaluate the effectiveness of the pharmacological treatment during and after an emotional situation. Systolic arterial pressure (SAP) and diastolic arterial pressure (DAP) were measured in three instances: before (I), during (II) and after the test (III). Patients were divided in two groups: a) moderate hypertensive subjects (N = 38) treated with thiazides and beta-blockers and b) severe hypertensive subjects (N = 23) treated with the same drugs plus clonidine. A group of 26 normotensive subjects was used as control. Reactivity was determined comparing SAP and DAP values in instances I and II, whereas the recovery phase was evaluated by comparing instances I and III. SAP and DAP reactivity were highest in severe hypertensive patients and lowest in normal subjects. SAP and DAP recovery was partial in severe hypertensive subjects and complete in the other two groups. All reactivity and recovery differences were statistically significant. These results suggest that antihypertensives treatment that normalized basal arterial pressure values in these patients was not sufficiently effective to protect them during a psychological stress situation.

Pressure responses of hypertensive patients treated with thiazides, beta blockers and clonidine during a psychological experimental stress situation.

A total of 61 hypertensive patients treated during more than six months with antihypertensive drugs and showing a satisfactory evolution and normal basal arterial pressure values were studied. Patients were subjected to a psychological stress test in order to evaluate the effectiveness of the pharmacological treatment during and after an emotional situation. Systolic arterial pressure (SAP) and diastolic arterial pressure (DAP) were measured in three instances: before (I), during (II) and after the test (III). Patients were divided in two groups: a) moderate hypertensive subjects (N = 38) treated with thiazides and beta-blockers and b) severe hypertensive subjects (N = 23) treated with the same drugs plus clonidine. A group of 26 normotensive subjects was used as control. Reactivity was determined comparing SAP and DAP values in instances I and II, whereas the recovery phase was evaluated by comparing instances I and III. SAP and DAP reactivity were highest in severe hypertensive patients and lowest in normal subjects. SAP and DAP recovery was partial in severe hypertensive subjects and complete in the other two groups. All reactivity and recovery differences were statistically significant. These results suggest that antihypertensives treatment that normalized basal arterial pressure values in these patients was not sufficiently effective to protect them during a psychological stress situation.