A. Di Biasio

Effect of ions on counterion fluctuation in low-molecular weight DNA dielectric dispersions

The dielectric permittivity of aqueous solutions of low-molecular weight DNA (Mr = 3.2 X 10(5) ) in the presence of MgCl2 and AgNO3 has been measured in the frequency range from 5 kHz to 30 MHz, at a temperature of 25 degrees C. The DNA concentration was 3.5 X 10(-4) M in terms of phosphate and the salt concentration was varied from 1 X 10(-5) to 2 X 10(-4) M. The dielectric results have been analyzed in terms of two contiguous dielectric dispersions, and characteristic parameters have been discussed on the basis of polyelectrolyte theories which deal with counterion fluctuation. Some molecular parameters of the DNA molecule in electrolyte solutions are estimated.

The dielectric behavior of nonspherical biological cell suspensions: an analytic approach.

The influence of the cell shape on the dielectric and conductometric properties of biological cell suspensions has been investigated from a theoretical point of view presenting an analytical solution of the electrostatic problem in the case of prolate and oblate spheroidal geometries. The model, which extends to spheroidal geometries the approach developed by other researchers in the case of a spherical geometry, takes explicitly into account the charge distributions at the cell membrane interfaces. The presence of these charge distributions, which govern the trans-membrane potential DeltaV, produces composite dielectric spectra with two contiguous relaxation processes, known as the alpha-dispersion and the beta-dispersion. By using this approach, we present a series of dielectric spectra for different values of the different electrical parameters (the permittivity epsilon and the electrical conductivity sigma, together with the surface conductivity gamma due to the surface charge distribution) that define the whole behavior of the system. In particular, we analyze the interplay between the parameters governing the alpha-dispersion and those influencing the beta-dispersion. Even if these relaxation processes generally occur in well-separated frequency ranges, it is worth noting that, for certain values of the membrane conductivity, the high-frequency dispersion attributed to the Maxwell-Wagner effect is influenced not only by the bulk electrical parameters of the different adjacent media, but also by the surface conductivity at the two membrane interfaces.

d-glucose-induced alterations in the electrical parameters of human erythrocyte cell membrane

The alterations of the passive electrical parameters (the permittivity epsilon and the electrical conductivity sigma) of human erythrocyte cell membrane induced by the presence of glucose in the extracellular medium have been investigated by means of dielectric spectroscopy measurements. The membrane permittivity epsilon(s) and the permittivity epsilon(p) and electrical conductivity sigma(p) of the cytosol have been evaluated on the basis of a recent analytical model proposed by Prodan et al., 1983 [16], that takes into account the whole dielectric spectrum of a cell suspension, consisting of both the low-frequency alpha-dispersion and the high-frequency beta-dispersion. Our results show a marked increase of the membrane permittivity epsilon(s) close to a glucose concentration of 20mM. On the contrary, the electrical properties of the cytosol do not change appreciably. This finding strengthens the hypothesis that glucose interactions involve primarily the cell membrane and the mechanism of the transport is briefly discussed.

Effect of volume ion polarisations on Maxwell-Wagner dielectric dispersions

The effect of a volume distribution of charges around the dispersed particles in an uni-univalent electrolyte solution on the dielectric properties of the colloidal system has been taken into account and a simplified theory has been developed. The results show that the characteristic parameters of the Maxwell-Wagner dielectric dispersion are affected by these volume polarisation effects; the correction terms, depending on some parameters of the system are also reported and their magnitude evaluated in particular cases.

Quasi-elastic light scattering from large anisotropic particles: application to the red blood cells.

The usefulness of dynamic light scattering measurements for the determination of particle size and shape in suspensions of biological objects is well established. However, when the particle sizes are larger than the wavelength of the incident light, so that structural information on the scattering particles can be made available, the analysis of the correlation function of the scattered light requires a careful examination, owing to the problem involved in its theoretical description. In this note, dynamic light scattering techniques were employed for the determination of the size of human normal erythrocytes in physiological saline solution (isotonic solution) with the aim to discuss in detail the different effects that contribute to the scattered light intensity distribution and to evidentiate how correct information can be made available only when the anisotropic contribution to the translational diffusion coefficient of the erythrocyte cells is properly taken into account. In the case of erythrocytes, this effect produces deviation of the order of 8-10% in the evaluation of the cell size (the cell diameter and cell thickness) that can be accounted for by an appropriate dependence upon time of the autocorrelation function. Once the electric field autocorrelation function, where the intraparticle correlation function contains the correction for the anisotropic contribution, is considered, reasonable agreement was obtained between the data deduced from light scattering methods and those from scanning electron microscope pictures.

Effect of short-range attractive interactions on the low-frequency electrical conductivity of water-in-oil microemulsions

In this paper we report on low-frequency electrical conductivity measurements in water-in-oil micro-emulsion systems, formed using four different organic solvents (n-pentane, n-octane, n-decane and carbon tetrachloride) at different water-to-surfactant molar ratios W, in the temperature range 5-60 °C (up to the stability limit of the single phase system). To evaluate the effects of short-range attractive interactions between surfactant-coated water droplets, the composition of the microemulsion has been appropriately adjusted by maintaining the distance between two adjacent droplets constant. The experimental data have been analyzed considering the currently stated charge fluctuation theories; the excess free energy associated with the whole charging process has been evaluated for the different solvents investigated. The results presented here give further support to the theory that inter-droplet interactions are due to the interdigitation of the surfactant tails and solvent phase molecules.

High-frequency dielectric polarization mechanism in water-in-oil microemulsions below percolation

In this note, we report high-frequency dielectric relaxations of water-in-oil microemulsion systems occurring in the frequency range from 1 MHz to 1.8 GHz. In the composition range and temperature interval investigated (water-to-surfactant molar ratio of 40.8 and fractional volumes from 0.05 to 0.30), the system behaves as a collection of isolated water droplets, well below the percolation threshold, where a more complex structural arrangement prevails. The observed dielectric spectra evidence a polarization mechanism occurring within the single water droplet, characterized by a dielectric strength of several units and a relaxation time of the order of 1-2 ns. The results presented here give support to the identification of the dielectric relaxation as being due to the orientational correlation of the surfactan head groups at the water-surfactant interface resulting in the formation of an apparent relaxing dipole moment. This polarization mechanism constitutes a novel contribution to the overall microemulsion dielectric spectra, whose features should be accurately investigated to gain a complete description of the electrical behaviour of these systems.

Effect of polymer adsorption on PEO-coated latex particles during salt-induced aggregation

Abstract The salt-induced aggregation of polystyrene particles in dilute aqueous solutions has been studied by means of dynamic light scattering measurements and the hydrodynamic radius of the resulting aggregates has been evaluated during the time evolution of the whole process. Poly(ethylene oxide) (PEO) polymer adsorbed on the particle surface at different amounts has been used to modify the inter-particle interactions resulting in the formation of clusters of increasing size or in the stabilization of the suspension, depending on the polymer molecular weight. The aggregation regime, i.e. a diffusion limited cluster aggregation (DLCA) occurring in the polymer-free latex suspension, is partially modified according to the polymer percentage adsorbed on the particle surface. At high polymer content, the polystyrene latex undergoes a complete steric stabilization. The deviation from a DLCA regime has been observed for different polymer contents and for polymers of different molecular weights, from 1.5 to 2000 kD. The alterations of the aggregation rates, induced by the polymer interactions, are presented and briefly discussed.

Passive electrical properties of biological cell membranes determined from Maxwell-Wagner conductivity dispersion measurements

Abstract The membrane electrical parameters of erythrocyte cells in physiological saline solutions have been evaluated from conductivity measurements at radiowave frequencies, by means of appropriate mixture equations. The dielectric model adopted takes into account the main structural characteristics of the membrane structure and is able to demonstrate the contributions of the polar head groups and the hydrophobic layers. The results show the effect of some metal alkali ions (Cs+, K+, Na+, Li+) on the transport processes across the erythrocyte membrane.

Electrical polarizability of differently shaped dielectric objects in the presence of localized interfacial charge distribution: a unifying scenario

We present the dielectric relaxation spectra of non-spherical shelled particles (namely cylindrical and toroidal particles) with a layer of localized electrical charges at the interfaces, uniformly dispersed in a continuous aqueous phase. Our numerical simulations extend over a wide frequency range, covering both the relaxation region due to the presence of localized layers of charges at the interface (α-relaxation) and the one due to the mismatch of the permittivity and conductivity of the adjoining phases (β-relaxation or the Maxwell–Wagner effect). Although there is a general formulation for determining the electrical polarizability α(ω) of arbitrarily shaped particles, based on the knowledge of the internal electric field distribution, the presence of the localized charge distribution makes its use formally rather elaborate. Contrarily, in this paper we show that, in the dipolar approximation, the polarizability α(ω), assuming the same functional dependence for spherical, cylindrical and toroidal particles, can be calculated in a very simple way and that the dielectric (and conductometric) properties of particle suspensions, adopting a simple inductive procedure, can be treated under a unifying scenario. Moreover, the method holds both for homogeneous and heterogeneous layered particles. A detailed prediction of the dielectric relaxations for both α- and β-processes in heterogeneous systems composed of spherical, cylindrical and toroidal particles is made for a wide range of electrical phase parameter values.