Ts. Radeva

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.

On the low frequency dispersion of the electrooptic effects

SummaryThe paper studies the factors which could be responsible for the difference between the high and low frequency electrooptic effects, the neglect of which could lead to erroneous conclusions about the particle electric properties. The presented considerations are illustrated by experimental data for an aqueous palygorskite suspension. New possibilities are demonstrated for the obtaining of information from the low frequency dispersion of the electrooptic effects. On the basis of the experimental data concerning the dispersion behaviour of palygorskite, the hypothesis is suggested for the existence of a slow longitudinal electric polarizability acting at frequencies of the applied field below 60 Hz.ZusammenfassungIn der Arbeit wurden die Faktoren diskutiert, welche für die Unterschiede der elektrooptischen Effekte bei hohen und niedrigen Frequenzen verantwortlich sind. Auf Grund experimenteller Daten mit wäßrigen Palygorskitsuspensionen wird eine langsam longitudinale elektrische Polarisierbarkeit postuliert, die bei den angewandten Feldstärken bei Frequenzen unter 60 cm-1 wirksam wird.

Comparison of electric light scattering and birefringence for polydisperse suspensions

SummaryA review is made of the existing theoretical expressions, describing the change of the scattered light intensity and the birefringence by application of an electrical field to a colloid solution.Furthermore, the geometrical functions for disc-shaped particles are calculated in this article for the case of electric light scattering and are. presented graphically. In addition, expressions are derived for the average value of the electrical polarizability (γ) for the case of polydisperse solutions.ZusammenfassungEs wird ein überblick über die existierenden theoretischen Beziehungen gegeben, die die Änderung der Intensität des Streulichtes und der Doppelbrechung von Kolloidsuspensionen im elektrischen Feld beschreiben.Weiterhin sind die geometrischen Funktionen für scheibchenförmige Teilchen in dieser Arbeit für den Fall der elektrooptischen Lichtstreuung berechnet und grafisch dargestellt. Außerdem werden Beziehungen für den Mittelwert der elektrischen Polarisierbarkeit (γ) für den Fall polydisperser Suspensionen abgeleitet.

Structure of polyelectrolyte layers on colloidal particles at different ionic strengths

Abstract The structure of polystyrenesulfonate (PSS) layer adsorbed onto ellipsoidal β-ferric hydrous oxide (β-FeOOH) particles is examined using electro-optics. Changes in the thickness are investigated by varying the ionic strength of the solution for rinsing the adsorbed PSS layer. An increase in the layer thickness is found after rinsing with water, which turns to a decrease when salt (NaCl) is added to the rinsing solution. No changes are registered if formation and rinsing of the layer is carried out at constant ionic strength. Compression of the PSS layer is also found after adsorption of an oppositely charged polyethylenimine (PEI) on the layer rinsed with water or in the presence of 10 −4 M NaCl. Complex-formation between the two oppositely charged polyions on the particle surface is supposed to explain the behavior of the polyelectrolyte bilayer.

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.

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.

Effect of the surface-active substance cetylpyridinium chloride on the electric light scattering of bentonite

Abstract The changes in the electric properties and the stability of an aqueous suspension of sodium bentonite on addition of the cationic surface-active substance cetylpyridinium chloride (CPC) is studied by electric light scattering. The CPC concentration varies from 10 −5 to 10 −3 mol l −1 , and the suspension weight concentration is 5 · 10 −6 kg l −1 . The dimensions of the bentonite particles are followed by electron microscopy and the decay of the electro-optic effect. The particle dimensions are greatest at the isoelectric point (10 −4 mol l −1 CPC), determined microelectrophoretically. The greater value of the electro-optic effect in the kHz range for suspensions to which CPC is added in comparison with untreated suspensions is explained by the increase of the particle dimensions, which leads to an increase in the particle electric polarizability. The electric polarizability per unit interfacial area of the particles decreases, which is related to CPC adsorption on the particles. The greater dispersion for suspensions with CPC at frequencies below 100 Hz, also manifest in large negative effects, is explained by the increase of the ratio of permanent dipole moment to induced dipole moment.

Unusual dispersion of the electro-optically determined electric polarizability of bentonite dispersed particles in the range 10 kHz — 1 MHz

An unusual dispersion of the electric light scattering by aqueous calcium bentonite is observed. The electro-optic effect rises with the increase of the frequency in the range 10 kHz — 1 MHz. Parallel measurements are made on suspensions of sodium bentonite and zetlizer kaolinite at different ionic strengths and particle dimensions. It is shown that the dispersion for calcium bentonite does not greatly depend on the concentration of the indifferent electrolyte (sodium chloride with concentrations 10−4 and 5.10−4 M/l) or on the suspension concentration. Several possibilities for explaning this unusual dispersion are considered.

Electric light scattering from polytetrafluoroethylene suspensions

Abstract Aqueous suspensions of polytetrafluoroethylene (PTFE) rod-like particles have been studied by electric light scattering. The investigation of the suspensions with particle concentrations 0.05, 0.1, 1 and 4 wt.% shows that for the last two concentrations the electric light scattering effect changes its sign from positive to negative when going to lower frequencies at low electric field strengths. This “anomalous” frequency behaviour is observed in semi-dilute suspensions in which the rotational volumes of the rods begin to overlap (the overlap concentration C ∗ =0.52 wt.%). An increase in the ionic strength in dilute suspension (0.05% PTFE) from 5 × 10−5 to 5 × 10−3 M NaCl leads to a decrease in the amplitude of the effect and displacement of its relaxation frequency towards higher frequencies. These results confirm the concept that polarization of the electric double layers of the non-interacting particles is a basic cause for the kilohertz electro-optic effect.

Electric light scattering by suspensions of anisotropic, slightly anisodiametric latex particles

Abstract The electric light scattering effect has been measured in aqueous suspensions of almost monodisperse, slightly elongated polytetrafluorethylene particles, and the influence of particle concentration and ionic strength investigated. The absence of dependence of the electric light scattering on particle concentration in the range 0.022–1.1% (w/w) at constant ionic strength is considered as evidence that there are no collective effects in these suspensions. The decrease of the kilohertz (KHz) effect with increase of ionic strength (from 5 × 10 −5 to 5 × 10 −3 M NaCl) and displacement of its relaxation frequency towards higher frequencies confirmed the concept for the interfacial origin of the electro-optic effect in dilute suspensions.