Maria Stoimenova

Electro—optic investigation of oxide suspensions. Mechanism of formation of the induced dipole moment

Abstract A model system (aqueous suspension of aluminium oxide particles) is used for the investigation of the mechaism of formation of the induced dipole moment, causing the large low-frequency electro—optic effects exhibited by colloid suspensions. The results demonstrate that the induced moment in the kilohertz frequency region is created only by the diffuse part of the surface electric double layer. The relaxation frequency of the induced moment shows a distinct dependence on the diffuse layer charge density. A surface conductivity mechanism is in accordance with the presented results.

Eletro-optic investigation of oxide suspensions. On the nature of “permanent dipole moment”

Abstract A model system (an aqueous suspension of aluminium oxide) is used for the study of the negative low-frequency electro-optic effect (usually interpreted as a “transverse permanent dipole moment”). It is demonstrated that the effect is related to the single particle (not a collective effect) and correlates well with its electrophoretic mobility. These results indicate the electrokinetic nature of the negative electro-optic effect.

Orientationally induced conservative dichroism in dilute disperse systems

Abstract The conservative dichroism induced by orientation of a disperse system in an external field is described theoretically in the Rayleigh and Rayleigh—Debye—Gans approximations. The formulas derived in the paper yield the possibility for analyzing the effect and for estimating its contribution to the total dichroism and birefringence of the system.

Orientational optic effects from above rayleigh particles

Abstract The general approach of van de Hulst for description of the orientationally induced optic effects in disperse systems through the components of the scattering matrix is used for the theoretical treatment and comparison of the effects from above Rayleigh particles. The orientational birefringence, dichroism, and light-scattering variation, described in the approximations of Stevenson and Ikeda, are analyzed qualitatively and the modification of some basic features of the effects is demonstrated. Some advantages of the orientational light scattering are pointed out, concerning the sensitivity of the method.

Electrical Properties of Polyelectrolyte Layers Adsorbed on Colloidal Particles at Different Ionic Strength

The investigation presents results on the low-frequency electrical polarization of β-FeOOH particles with adsorbed layers from sodium salts of poly(4-styrene sulfonate), poly(acrylic acid), and carboxymethyl cellulose, obtained by electric light scattering. The adsorption is realized in aqueous NaCl solutions of different concentration, and the suspensions of the coated particles are then rinsed to low conductivity. Several electro-optical parameters are sensitive to the conformation of the adsorbed polyelectrolyte layer. The particle electrical polarizability increases drastically with the concentration of NaCl (correlating with significant increase of the adsorbed amount of polyelectrolyte), while their electrophoretic mobility remains practically unchanged. Two time scales are involved in the stepwise relaxation of the electrical polarizability. The faster process is attributed to movement of ions in the polymer layer, restricted by the coiled conformation to smaller distances. The contribution of the Debye atmosphere outside the polymer layer remains almost unchanged, which correlates with the low sensitivity of the electrophoretic mobility to the increasing amount of adsorbed polyelectrolyte. Abrupt increase in a narrow interval of salt concentrations is observed both for the low-frequency component of the particle polarization and for the hydrodynamic layer thickness, indicating changes in the surface electric state, most probably due to swelling of the adsorbed polymer layer.

Low-frequency dispersion of the electro-optic effects from monocationic forms of palygorskite

Abstract The electro-optic effects from palygorskite suspensions exhibit frequency dispersion below 100 Hz. The relation of this effect to the counterion distribution in the particle double electric layer (DEL) is investigated, using for this purpose the properties of the ions of the liotropic series of the alkali metals to be adsorbed at different extents on the particle surface. The presented experimental data demonstrate the relation of the low-frequency dispersion to the counterions of the compact part of the DEL. For explanation of the obtained results a hypothesis is proposed for a slow polarizability of the Stern layer.

Electrically induced concentration fluctuations in Escherichia coli suspensions

Abstract Biological cells are permanently exposed to electric fields by self generated or external electrical sources. It is important to understand whether and how very weak electric fields influence the electric properties of biological cells. In the present study, we demonstrate by the electro-optic method the induced concentration fluctuations in suspensions of Escherichia coli cells subjected to alternating electric fields of low intensity (20–240 V cm−1) and low frequency (0.2–100 Hz). A frequency window is formed where concentration fluctuations are already significant, but the sample is still stable enough under the action of the field and permits the application of longer pulses. Our assumption is that, in this range, conditions are created for the accumulation of considerable induced local density variations and the latter could be responsible for the appearance of instability and electroporation of E. coli cells at similar conditions.

Electro-optic information on polymer adsorption and its influence on the electric double layer of colloid particles

Abstract On a model system (aqueous suspension of γ-Al 2 O 3 , by means of electro-optics, the formation of an adsorption layer of polyacrylamide on the surface of colloid particles is followed. The variations of several electro-optic parameters (particle relaxation time, induced dipole moment, relaxation frequency, negative electro-optic effect) are in close correlation with the rise of polymer adsorption and with the conformational changes in the adsorption layer. Considerations are presented concerning the nature of electro-optic effects and their changes by the adsorbed polymer. Advantages are demonstrated of the electro-optic study of polymer adsorption.

Electro-Optic Investigation of Oxide Suspensions: Surface Charge Sign Reversal through Specific Adsorption

Abstract Conclusions present in previous papers on the nature of low-frequency electro-optic effects are verified for more drastic variations in the particle double electric layer. The effects are followed during sign reversal of surface charge resulting from specific adsorption of ions. The results support the view that the large electro-optic effects manifested in the kilohertz and the hertz frequency regions are both related to parameters of the diffuse part of double electric layer: the kilohertz effect—to particle surface conductivity, and the negative hertz effect—to the value and profile of electrokinetic potential. The form of the frequency curves follows the changes in the ratio of those effects, hence of diffuse layer parameters.

Transient electric light scattering by large strongly elongated disperse particles

Abstract The theory of transient electric light scattering is extended to larger particles, fulfilling the restrictions of the Rayleigh-Debye-Gans approximation. The equations derived are valid for rigid, strongly elongated, monodisperse particles, orientated under the simultaneous action of both their permanent and induced dipole moments. The equation for the rise of the effect gives the possibility the particle electric moments to be determined. The equation describing the decay of electric light scattering differs from the one given by Wippler. It offers a method for obtaining the particle rotational diffusion coefficient. For small particles the equations derived coincide with the formulas of Nishinari-Yoshioka, Benoit, and Tinoco, describing transient electric birefringence. The dependence of the coefficient on particle size in the formulas presented shows the limits of application of the small particle theory.