V. Peikov

Adsorption of polyacrylic acid on α-Al2O3 colloid particles as studied by electro-optics

Abstract The adsorption of polyacrylic acid (PAA) on partially soluble α-Al 2 O 3 particles is studied by electric light scattering and electrophoresis at pH 5 and constant ionic strength in the presence of aluminium ions. The stability and the thickness of adsorbed polymer layer were determined from the changes of relaxation times of particle disorientation obtained from transient electric light scattering. The electrophoretic mobility changes sharply from positive to negative in a narrow concentration range the isoelectric point (iep) being at an adsorption density of Γ PAA =2.5 mg m −2 . The changes of electro-optical effect, proportional to particles surface electric polarizability, with PAA concentration follow the changes of the electrophoretic mobility. A minimum of polarizability is observed at the iep. The frequency dependence of electro-optical effect giving information on the different polarization mechanisms is shifted to lower frequencies above the iep indicating a decreased mobility of counterions whose movement is responsible for polarizability. The thickness of the adsorption layer increases above the iep. This increase and the decreased counterions mobility can be an indication to conformational changes in the adsorbed polymer layer.

Kinetics of reaction-limited cluster aggregation studied by electric light-scattering

Abstract Earlier experimental results from an electric light scattering study of the slow coagulation of monodisperse spherical α-Fe 2 O 3 particles in the presence of a bivalent electrolyte (MgSO 4 ) are discussed on the basis of fractal growth theory. A more realistic fractal model for the structure of aggregates is introduced. The kinetics of the changes of three different types of average relaxation time (related to average characteristic sizes) are monitored. The experimental results indicate a reaction-limited cluster aggregation process, i.e. slow coagulation. The slopes of the semilogarithmic dependences of the different types of average relaxation time on the aggregation time are approximately equal owing to the self-similarity of the aggregates. At the highest electrolyte concentration used, a process of cross-over to fast coagulation is observed. In this regime, the measured fractal dimension D of about 1.6 ± 0.3 is in a good agreement with that of the diffusion-limited cluster aggregation models.

Polarized electric light scattering study of interactions in α-FeOOH suspensions

Abstract The changes of electro-optical behaviour with increase in particle concentration are followed by the electric light scattering method to study the effects of interactions (optical and electrical) in α-FeOOH aqueous suspensions. Different polarizations of the initial and scattered light are used as well as light of two different wavelengths. The peculiarities of the electro-optical effects (the decrease and the negative values as well as a decreased particle relaxation time) observed at a high particle concentration are common for many other disperse systems. A comparison between different experimentally measured electro-optical quantities shows that the electro-optical behaviour observed with increase in particle concentration is connected with the effects of optical interactions in the suspensions. For the present system, the concentration limit below which the electro-optical effect is not influenced by existing effects of multiple light scattering is 0.01 g l −1 .

ELECTRO-OPTICAL DETERMINATION OF α-FeOOH PARTICLE SIZES

ABSTRACT Electric light scattering method is used for revealing the presence of aggregates in very diluted aqueous α-FeOOH suspensions. Three theoretical models are used for the geometrical form of aggregates. The comparison between the electro-optical results -theoretical and experimental - shows that even in the most diluted suspensions exist aggregates with irregular form. Using the comparison between electron microscopy and electro-optical results the geometric shape of the aggregates is determined.

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.