Capillary electrokinetics has been employed to measure the streaming current in solutions of alkali metal chlorides at electrolyte concentrations of 0.1 and 1 M. The results obtained have confirmed the earlier conclusion that there are no hydrodynamically immobile layers near a solid molecularly smooth surface. It has been shown that the magnitudes of the current and the calculated electrokinetic potential decrease in a series LiCl, NaCl, KCl, RbCl, and CsCl. In this series, the crystallographic radius increases and the radius of a hydrated ion decreases. The obtained dependences have led to a conclusion that the cations located near a negatively charged surface are unhydrated.

The kinetics of adsorption of two polylysine samples with different molecular weights from aqueous solutions on the surface of fused quartz has been studied by the capillary electrokinetics method. It has been found that the adsorption of positively charged polylysine on the negatively charged quartz surface decreases the absolute value of the initial ζ potential; moreover, in the case of polylysine with the higher molecular weight, it leads to the reversal of the surface charge. The constant values of the ζ potential and the times for which these values are reached depend on the molecular weight and concentration of polylysine. The rate of variations in the ζ potential is highest during the first several minutes of the solution contact with the surface. A decrease in the electric conductivity during polylysine adsorption has, for the first time, been observed, thus leading to a hypothesis that large aggregates are formed on the quartz surface. The measurement of the deformability of the layers has shown that, under our experimental conditions, dense undeformable layers are formed. Polylysines are irreversibly adsorbed on the quartz surface.

The electrokinetic properties and selectivity of an acetyl cellulose membrane with respect to 0.0001 mol/L sodium chloride solutions in water–ethanol mixtures have been studied. The electrical conductivity, streaming potential, and filtration and selectivity characteristics of the membrane have been measured. It has been found that, in solutions with alcohol contents of 4 and 12%, the membrane selectivity with respect to sodium chloride is increased and decreased relative to that in an aqueous solution, respectively. No correlation between the membrane selectivity and its surface charge has been observed. The membrane has been found to possess a slight selectivity (20–26%) with respect to ethanol. It has been hypothesized that the solvation enthalpy of electrolyte ions changes differently in a free solution and membrane pores at different contents of ethanol in the mixtures, thereby affecting the membrane selectivity.

Streaming current has for the first time been measured in solutions of salts with double- and triple-charged cations at high electrolyte concentrations by the method of capillary electrokinetics. It has been shown that the streaming current is proportional to an applied pressure. At an electrolyte concentration of 1 M, the diffuse layer is completely absent. In this case, the existence of the streaming current indicates that there are no hydrodynamically immobile layers near a smooth solid surface.

Electrokinetic properties of an asymmetric nanofiltration membrane modified with a cationic polyelectrolyte (styrene–dimethylaminopropylmaleimide copolymer) and the selectivity of the membrane with respect to solutions of differently charged electrolytes have been studied. The modification has been carried out by filtering the polyelectrolyte from the side of the selective layer and the opposite side of the membrane. It has been found that the membrane selectivity to sodium sulfate and magnesium chloride solutions increases with polyelectrolyte concentration in the solution used for membrane modification from the side of the selective layer. A decreased selectivity to sodium sulfate and an unchanged selectivity to magnesium chloride have been observed for the membrane modified from the substrate side.

The method of capillary electrokinetics has for the first time been used to measure streaming current at high electrolyte concentrations. It has been shown that the streaming current is proportional to the applied pressure. At an electrolyte concentration of 1 M, the thickness of the diffuse layer is comparable with the size of a water molecule (0.3 nm); i.e., there is almost no diffuse layer. The existence of the streaming current in this case indicates that there are no hydrodynamically immobile layers near a smooth solid surface.

The regularities of adsorption of a cationic polyelectrolyte, poly(diallyldimethylammonium chloride), on the surface of fused quartz are studied at different values of solution pH by capillary electrokinetics. It is shown that the polyelectrolyte adsorption on a negatively charged surface depends on the value of the surface charge and increases with its growth. At a low charge value (pH 3.8), the polyelectrolyte adsorption increases the quartz surface charge. The driving forces of the adsorption are both electrostatic interaction and forces of nonelectrostatic nature, probably hydrophobic interactions and a change in entropy due to the displacement of counterions from a double layer. The adsorption of poly(diallyldimethylammonium chloride) on quartz from alkaline and neutral solutions is irreversible, which indicates the key role of the electrostatic interaction. At low values of the surface charge, the nonelectrostatic interactions play the main role, thereby resulting in polyelectrolyte desorption.

Capillary electrokinetics has been employed to study the effect of salt cation charge on cationic polyelectrolyte adsorption by the negatively charged surface of fused quartz. It has been found that polyelectrolyte adsorption values decrease with a rise in the cation charge and increase with salt concentration. Salt cation charge influences the conformation of polyelectrolyte molecules in an adsorbed layer. High values of the charge reversal suggest a significant entropy contribution to the adsorption mechanism.

A procedure has been proposed for measuring the surface potential of hollow-fiber membranes by the streaming-potential method under the conditions of a tangential flow of a solution. The zeta-potential and surface charge of nanofiltration hollow-fiber polyacrylonitrile membranes have been measured. The measurements have been performed for membranes with different porosities, which were obtained by partial drying of initial humid membranes. The porosity has been determined from the electrical conductivity of a membrane. An equation has been proposed for calculating the charge transfer by a solution flow in a porous layer. It has been shown that the use of the proposed equation makes it possible to obtain more correct values of the membrane surface potential.

The capillary electrokinetics method (measurements of streaming potential and current in original and hydrophobized fused quartz capillaries with radii of 5–7 μm) is employed to study the formation of adsorption layers upon contact with solutions containing a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). It is shown that polyelectrolyte adsorption causes the charge reversal of both hydrophilic and hydrophobic surfaces, with a smaller amount of the substance being adsorbed on the hydrophobic than on the hydrophilic surface. The adsorption on both surfaces increases with the polymer solution concentration. The cationic polyelectrolyte adsorption on the pure quartz surface occurs mainly due to the electrostatic attraction, while, in the case of the hydrophobic surface, the contribution of hydrophobic interactions increases. The study of the layer deformability shows that, on the hydrophilic surfaces, the layer ages and its structure depends on the polymer solution concentration. On the modified surface, the deformation of even freshly formed layers is slight, which suggests that a denser layer is formed on the hydrophobic surface. In contrast to the hydrophilic surface, the polyelectrolyte is partly desorbed from the hydrophobic surface.

A physicochemical model is proposed to describe electrokinetic phenomena in capillaries and pores the surface of which is coated with a charged porous adsorption layer. The use of this model makes it possible to explain experimental data on the surface potentials of polyelectrolyte adsorption layers upon their deformation resulting from solution flow. The commonly used Smoluchowski equation is shown to lead to large errors in the determination of the potential and charge of the surface of an adsorption layer.

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.

The flow of KCl solutions through thin quartz capillaries coated with an adsorbed layer of a cationic polyelectrolyte (CPE), poly(dimethyldiallylammonium chloride) (molecular mass M = 500000), is studied. It is found that the adsorption layer is soft and its thickness depends on shear stress generated by the liquid flow through the capillary. The hydrodynamic thickness of the CPE adsorption layer is 80–90 nm at low flow rates of a solution, and it decreases to values comparable with the experimental error at high flow rates. The dried adsorption layer appears to be hydrophobic (the advancing contact angle is about 80°); in these capillaries, the flow rate of a KCl solution is increased that can be interpreted as a solution slip on the surface of CPE adsorption layer. The long-term contact of the dried CPE adsorption layer with KCl solution, probably, results in the swelling of the adsorption layer, which is accompanied by a decrease in the contact angle and ζ potential of the adsorption layer surface as calculated from the streaming potential of the same solution.