Alexandar M. Zhivkov

Electro-optics of metal particles: electric birefringence of gold rods

The electro-optical response of aqueous suspensions of gold rods of diameter and length of 15 and 260 nm respectively, stabilized with poly(vinylpyrrolydone) was studied by electric birefringence. The electric polarizability of gold rods was calculated from the birefringence measurements as 1/10 29 Fm 2 and corresponds to that deduced from dichroism (van der Zande ’98). This value is of the same order of magnitude as the electric polarizabilities determined for non-metallic rods of similar dimensions (length about 300 nm and axial ratio about 20) such as TMV, palygorskite, oxides, etc. However, distinct differences in the optical anisotropy and the dispersion behaviour in stationary electric fields between non-metallic and metallic rods were observed. The optical anisotropy was determined (g1/g2)/0.7, which is two orders of magnitude higher than that for non-metallic rods of similar size and form. Moreover, an unexpected oscillating dispersion behaviour of the steady-state electric birefringence effect and a slow build up of the electro-optical effect compared to the decay were observed. # 2002 Elsevier Science B.V. All rights reserved.

Adsorption of carboxymethyl cellulose on alumina particles.

The polyelectrolyte adsorption on colloid particles is often used for stabilization or flocculation of water suspensions. The aim of this work is to study the adsorption of carboxymethyl cellulose (CMC) on alumina (γ-Al2O3) colloid particles. The particles and polymer are chosen because of the capability of the metal-oxide ampholyte surface and the weak polyelectrolytes to alter their charge by pH. The measurements are done at pH 6.0 where the CMC carboxylic gropes are almost fully dissociated and the alumina surface is positively charged. The high linear charge density of the polyelectrolyte chain provides Na(+) counterions condensation on the COO(-) groups. The main employed method is the electric light scattering based on particle orientation in sinusoidal electric field. The electric polarizability and the relaxation time after field switching off (both depending on the particle charge and size) are used as criteria for polymer adsorption and particle aggregation. Micro-electrophoresis is applied as additional techniques indicating the sign and density of the surface charge. The results obtained give the conditions (time dependence, particle and polymer concentrations) where the CMC adsorption is complete and the suspension is stable.

High frequency electric polarizability of bacteria E. coli: Dependence on the medium ionic strength

There are two main kinds of electric polarizability of bacteria: surface charge dependent (SChD) and Maxwell-Wagner (MW) polarizability. The aim of this article is to distinguish SChD and MW components on the external bacteria surface. An electro-optic method (electric turbidity) was used to study the polarizability of E. coli fixed by formaldehyde at the frequency range 20 kHz to 20 MHz. According to the literature the SChD polarization disappears at such high frequencies and MW one gives the main contribution. However we found unexpected dependence on the outer medium electrolyte concentration, which cannot be explained by MW polarization. The results show that the polarizability decreases by ionic strength increasing in the same way as the double electric layer thickness does. Such behaviour is a characteristic for SChD polarizability, which allows us to conclude that this component has the main contribution on the external bacteria surface at the experimental frequencies mentioned.

Influence of ethanol on the high frequency electric polarizability of E. coli

The interface electric polarizability of bacteria (charge dependent (ChD) and Maxwell-Wagner (MW) polarizabilities) gives information about their electric charge, determined by the structure and functional state. It is well known that the polarizability could be changed significantly by adding some substances to the suspension, and can be measured using an electro-optical (EO) method. There are some literature data, according to which the adding of ethanol decreases the electric polarizability of the cells. However the reason for the change in this parameter is not clear, as well as which component (ChD and/or MW) of polarizability has the main contribution. Generally the present work shows that the effect of ethanol is connected to the change of the internal (cytoplasm) MW polarizability and is mainly caused by increasing the cell membrane permeability. This results in an ionic flow through the membrane, which velocity and direction depends on the relative values of the inner (cytoplasm) and the outer medium ionic strength.

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.

Adsorption of cytochrome c on montmorillonite nanoplates: Protein concentration dependence

Cytochrome c [cytC] is a mitochondrial hemoprotein functioning as electron carrier in the respiratory chain of the biological cells. Being adsorbed on colloid particles cytC can be introduced in the cells by phagocytoses. In the present work we study the adsorption of cytC on montmorillonite (MM) particles combining the electro-optic and electrophoretic techniques. MM particles were chosen as nanoplates having negative pH-independent charge and high ratio surface/mass. The measurements were done at pH 6.5 where cytC globule is positively charged. The main employed method is the electric light scattering based on orientation of colloid particles in sinusoidal electric field. Interfacial electric polarizability was obtained from the degree of orientation at steady-state and the particle size - from the relaxation time after the field switching off. Microelectrophoresis was used to monitor the alteration of the surface charge at protein adsorption. The cytC-concentration dependence of the polarizability and the mobility shows out that the total (net) charge of cytC-MM complex turns its sign from negative to positive, the isoelectric point appears at 5:3 mg/mg (0.135 mol/kg) cytC/MM and saturated protein adsorption is reached at additional twofold increasing of cytC/MM ratio. The suspension is stable at low and high protein concentrations, at intermediate ones aggregation arises.

Influence of cytoplasm electrolyte concentration on Maxwell-Wagner polarizability of bacteria E. coli.

The electric polarizability of bacteria is considered in the literature to have a surface charge dependent (ChD) and a Maxwell-Wagner (MW) mechanism. We distinguish experimentally both the types of interface polarizability by the frequency of the electric field and the medium electrolyte concentration. It was shown in a previous work ( Zhivkov , A. M. ; Gyurova , A. Y. Colloids Surf., B 2008 , 66 , 201. ) that the ChD component is shown up on the outer bacteria surface even at megahertz frequencies. The MW polarizability is studied in the present work in the range from 20 kHz to 20 MHz by change in the inner (cytoplasm) electrolyte concentration. The ion transport through the cytoplasmic membrane of alive and fixed by formaldehyde E. coli K12 is accelerated by adding of ethanol in low concentration. The frequency dependence and the kinetics of the electric polarizability and the size of the bacteria are investigated by conservative electric dichroism, based on the alteration of the optical density at orientation of the cells in electric field. The conclusion is that the internal MW component has the main contribution to the change in the total bacteria polarizability, as well as the external MW and the internal ChD components are not shown up.

Electro-optical characterization of aqueous laponite suspensions

Electric birefringence measurements have shown that 0.01 wt.% laponite suspensions are very polydisperse, including both higher size aggregates than the expected 30 nm diameter plate-like particles and smaller particles with diameter below 7 nm. Kerr constant and electric polarizabilty seems to be in reasonable agreement with the results for higher size plate-like clay particles. At higher concentrations (0.1 and 1 wt.%) anomalous signals are observed. The usefulness of this approach to elucidating the gelation properties of laponite is discussed.

Electric polarizability dispersion of alumina particles with adsorbed carboxymethyl cellulose

When the linear charge density of a polyelectrolyte chain reaches some critically high level, an electrostatic adsorption of part of the counterions appears, that is the so-called counterion condensation. There are contradictory opinions in the literature about the condensed counterion mobility along the polymer chain: the analytical theories predict some mobility, but the experimental research does not give an unequivocal answer. The present experimental investigation aims to verify the reports for the condensed counterion migration in a sinusoidal electric field. By using electric light scattering we investigate an aqueous suspension of γ-alumina particles after the complete adsorption of carboxymethyl cellulose on their surface; the probable migration of condensed counterions must contribute to the electric polarizability (when the frequency is under a given critical value) leading to a higher degree of particle orientation in the applied electric field. We compare the frequency dependences of the polarizability at two polyelectrolyte concentrations in the suspension: under and above the recharging point (appearing due to adsorption of the negatively charged polyelectrolyte on the positively charged surface) where the total polarizability is equal, but the ratio between the quantities of the diffuse and the condensed counterions is different. A procedure for determination of the counterion shares is invented; it uses the measured electrophoretic mobility and the calculated fraction of the condensed ions. The results indicate the absence of the polarizability component caused by the condensed counterions; i.e. they do not manifest their presence when an external electric field is applied. We have concluded that the condensed ions are immobile in a sinusoidal field with moderate intensity in the frequency range of 10 Hz to 1 MHz.

pH-dependence of electric light scattering by water suspension of purple membranes

The electro-optical behaviour of purple membranes (PM) water suspension in pH range 3.3–6.2 was studied by electric light scattering method. At low degree of orientation the steady-state electro-optical effect (EOE) increases in pH range 3.8–5.6, but an abnormal behaviour of the pH-dependence is observed outside this range. At full orientation EOE has two peaks at pH 3.6 and 5.8, related to changes in membrane geometry. Relaxation time increases in pH range 3.9–5.5, which corresponds to membrane area increase with 20%. The resonance character of pH-dependence around the peaks was explained by reversible alterations in PM curvature, related to asymmetrical increase of the area of both membrane surfaces.