I. P. Sergeeva

Examination of the surface of quartz capillaries by electrokinetic methods

Abstract The electrokinetic and surface charge of “fresh” quartz capillaries is equal to 1.2 ÷ 2.3 μC/cm2, and it is independent of the concentration of KCl solution (for C ⩾ 103 M). Long-time contact with water and KCl solutions leads to the gel-layer formation on the capillary surface, and accordingly to the changes in surface charge and potential. The liquid flow in the capillary under the action of a sufficiently high pressure drop can shear and tear off the surface gel layer. This effect is accompanied by the observed changes in the ζ-potential values.

Influence of cationic surfactant on the surface charge of silica and on the stability of aqueous wetting films

Abstract Interference and ellipsometric methods were used to measure the equilibrium thicknesses of wetting films (50–70 nm) of aqueous solutions of CTAB and NaCI. The experimental isotherms of disjoining pressure of wetting films agree with the results of calculations performed according to the theory of electrostatic forces acting between film surfaces whose potentials differ in value. The ranges of the stability, metastability, and instability of the films were determined. The rupturing of metastable films is accompanied by a transition from complete to partial wetting.

Zeta potential measurements with fibre plugs in 1:1 electrolyte solutions

Abstract The aim of this paper is correct calculation of zeta potential values from streaming potential measurements in aqueous electrolyte solutions. To determine correct zeta potential values it is very important to measure the total electric conductivity exactly and to consider the electrode polarization. The streaming potential measurements of various fibre samples in KCl solution of different concentrations were carried out by means of two different measuring devices working in very different pressure ranges. The calculated zeta potential values show that the measuring method gives well reproducible results independent of the applied pressure and the kind of device. In addition, we discuss the possibility of, and the reason for, a maximum observed sometimes in the dependence of zeta potential values on the electrolyte concentration in the case of 1 : 1 electrolytes by means of measurements of a fibre diaphragm and by using computer simulations based on a Gouy—Chapman—Stern—Grahame model.

Modification of quartz surfaces using cationic surfactant solutions

Abstract Two models of Langmuir-type adsorption of cationic surfactant on quartz surfaces were developed. The monolayer model considers a simultaneous absorption of cations on charged and neutral sites. The two-layer model assumes that absorption takes place on charged sites only, and a second layer is formed as a result of adsorption of cations on hydrophobic tails of preadsorded ions. Computer treatment of experimental dependencies of the surface charge of quartz capillaries on the concentration of cethyltrimethylammonium bromide (CTAB) solutions and pH values results in potentials of adsorption on charged sites Φ 1 =−15 kT, and on non-charged ones Φ 2 =−11 kT. Both models give the same values of potentials, probably due to close values of Φ 2 for non-charged sites on quartz surfaces and for hydrophobic tails for preadsorbed cations. Atomic force microscopy of adsorded CTAB layers favors the monolayer mechanism of adsorption, no patches of molecules were observed.

Electrokinetic study of polymer surfaces

Abstract On the basis of electrokinetic measurements in thin quartz capillaries coated with micrometer-thick polystyrene (PS) layers, values of ζ-potentials and of surface charge σ in different electrolyte solutions were calculated. Potentials of adsorption of K+, Ba2+, La3+, and Cl- ions on a hydrophobic PS surface were computed with equations of localized adsorption. For hydrophilic copolymers of PS with polyacrylic acid (PA) the mechanism of charge formation is connected with dissociation of COOH groups. For 4% of PA in copolymer, the surface charge increases to 4 μC/cm2 at pH 11, which is 10-fold as compared with PS. A further increase in PA content (up to 40%) causes the formation of charged surface “brush” layers. Dissociation of COOH groups within a brush layer leads to very high values of surface charge (up to −160 μC/cm2) and surface conductivity. The relative value of surface conductivity decreases from κ s κ v = 3.2 to 1 when the concentration of KC1 solution increases from 10−4 to 10−1N.

Adsorption layers of poly(ethylene oxide) on quartz studied using capillary electrokinetics

Abstract The adsorption of poly(ethylene oxide) (PEO) molecules decreases the electrokinetic potential of a quartz surface. The corresponding change in electrostatic forces may influence the interaction between colloidal particles in polymer solutions. The equilibrium between flowing PEO solution and adsorption layers in thin quartz capillaries is established after 1–2 h. The kinetics may be controlled by the attachment and reconformation of polymer molecules in the adsorbed layers. The saturated adsorption layers of PEO are easily deformed under the action of the shear stress of the flowing solution. The effect of shear stress requires further investigation using joint measurements of the adsorption and the hydrodynamic and electrokinetic thicknesses of the adsorption layers. The thin quartz capillaries represent a simple model system with a well-characterized geometry and a molecularly smooth surface. Capillary electrokinetics may effectively be used for the investigation of electrosurface phenomena in polymer solutions.

Interaction of oil droplets with silica surfaces

Abstract Measurements of contact angles, emulsion film thicknesses, movement of oil droplets in thin capillaries and surface charges are used for investigation of oil droplet interaction with silica surfaces in aqueous solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB) of various concentrations from 10−7 and 10−3 M. As model systems tetradecane and silicone oil, polished optical quartz plates and quartz capillaries were used. Surface charges were obtained from measurements of streaming potentials in thin capillaries. In this case a paraffin wax coating on the inner capillary surface was used as the oil/solution interface. Emulsion film stability and oil adhesion to silica surfaces are determined by electrostatic and hydrophobic attraction forces influenced by addition of cationic surfactants. Strong adhesion takes place when the quartz surface is hydrophobized owing to CTAB adsorption, and electrical potentials of emulsion film interfaces become different in magnitude. In the case under investigation, the strongest adhesion of oils takes place at 10−5 M CTAB. In general, the results of these experiments offer the possibility of controlling oil adhesion, depending on the surface properties of porous bodies and oils.

Layer-to-layer adsorption of oppositely charged polyelectrolytes: The effect of molecular mass: 2. Bilayer systems

The effect of molecular mass on the formation of a bilayer structure upon the layer-to-layer adsorption of a cationic polyelectrolyte (poly(dimethyldiallylammonium chloride), molecular mass M = 500000 and 100000-200000 Da) and an anionic polyelectrolyte sodium (poly(acrylate), M = 30000 and 2100 Da) on the surface of fused quartz is studied by the capillary electrokinetic method. The time required to reach constant adsorption values and the structure of bilayer systems depend on the ratio between molecular masses of the cationic and anionic polyelectrolytes. The deformability of the bilayer system significantly exceeds that of the first layer in the case when the second layer is formed from an anionic polyelectrolyte with a lower molecular mass, thus suggesting the loosening of the first adsorption layer of the cationic polyelectrolyte. The adsorption of the anionic polyelectrolyte with higher molecular mass insignificantly affects the density of the first layer. Variation in the deformability of the layer with time (its aging) depends on the ratio between molecular masses of the polyelectrolytes.

Layer-to-layer adsorption of oppositely charged polyelectrolytes: The effect of molecular mass: 1. Formation of the first layer

The formation and properties of adsorption layers of poly(dimethyldiallylammonium chloride) with different molecular masses on the surface of fused quartz are studied by the capillary electrokinetics method. It is shown that the value of ζ potential depends on the flow rate of liquid determined by the pressure drop. Such behavior can be explained by the deformation of the adsorption layer. At low rates of liquid flow, constant values of adsorption and time needed to achieve these values decrease for the samples of lower molecular masses, which is probably related to the more closely packed structure and, hence, to the lower deformability of the adsorption layers, as well as to the shortest times during which conformational rearrangements proceed in the layer. The time of conformational changes in the adsorption layer significantly exceeds the time of adsorption. The adsorption of cationic polyelectrolyte is irreversible. It is found that the compaction of adsorption layers increases with time; the rate of compaction of layers of a low-molecular-mass polyelectrolyte is higher and the layers of a high-molecular-mass polyelectrolyte retain the residual deformability even for six days. The measurements of the filtration of polyelectrolyte solutions through thin quartz capillaries allow the thickness of adsorption layers and their deformation under pressure to be estimated.

Layer-by-layer adsorption of polyelectrolyte and surfactant and adsorption of their complexes on solid surface

The capillary electrokinetics method (measurement of streaming potential and current in a capillary with a radius of 5–7 μm made of fused quartz) is employed to study the structure formation at interfaces between quartz and solutions containing a cationic polyelectrolyte (poly(diallyldimethylammonium chloride) with molecular mass M = 100000−200000) and an anionic surfactant (sodium dodecyl sulfate). The kinetics of surface layer formation is studied upon the layer-by-layer adsorption of the components and the adsorption of their complexes at the same component ratios. It is established that the formation time and the electrokinetic potentials of the surface layers are almost independent of the procedure of their formation. In the case of the layer-by-layer adsorption, the first layers of the polyelectrolyte appear to be virtually undeformed, thus indicating that molecules with a planar conformation prevail in the adsorption layer. Surfactant adsorption enhances the deformation (layer loosening), which decreases with time (layer aging). Layers formed from the complexes have a denser (less deformable) structure. Variations in the electrokinetic potentials of the layers during the long-term pumping of a background electrolyte solution through a capillary witnesses the prevailing desorption of the anionic surfactant, with the desorption being noticeably more pronounced for the layers resultant from the adsorption of the complexes.