Electrodialysis (ED) of calcium-containing solutions is a widely applied process in water treatment, dairy industry etc. The lower performance of ion-exchange membranes in these processes (in comparison, e.g. with ED of sodium-containing solutions) is known, but the reasons for this effect are still unclear. We study the behavior of two sulfonated cation-exchange membranes: a CMX (Astom, Japan) and a CJMC-5 (Hefei Chemjoy Polymer Materials, China). For both membranes, the bulk properties (conductivity, diffusion permeability, counterion permselectivity) change when the membrane is soaked in a 0.02 N CaCl2 solution, but these changes are reversible: when the membranes are again equilibrated with a NaCl solution, the bulk properties restore and become the same as before. The exposure of the membranes to an overlimiting current does not affect the bulk properties. However, it is not the case for the properties controlled by the membrane surface. In particular, the length of the limiting current plateau of the current-voltage curve increases and electroconvection (visualized using Rhodamine 6G) decreases with increasing the time of the membrane exposure to overlimiting current in a CaCl2 solution. The zeta potential is about -30 mV for a "fresh " membrane, and +40 mV after exposure of this membrane to an overlimiting current in a 0.02 N CaCl2 solution; the both measurements were made in the same NaCl solution. The changes in surface-controlled properties appear to be irreversible. This effect is explained by specific adsorption of Ca2+ ions on the surface of sulfonated membranes promoted by a strong electric field at the interface. This explanation is supported by simulations of the distribution of the electric potential and ion concentrations in the interfacial electrical double layer.

Chronopotentiometry using pulses of a constant current of density j is a powerful method for characterizing ionexchange membranes (IEMs). We report on the influence of coion transport on the shape of chronopotentiogram (ChP) and the consequent new possibility of quantifying coion transport based on ChP analysis. We show that in the case where the bathing solution contains Ca2+ or Mg2+ ions, the ChPs of the homogeneous (CMX) and heterogeneous (MK-40) cation-exchange membranes at overlimiting current densities have a maximum (a peak), which appears a few seconds after the transition time. The time required to reach a stationary state is of the order of d(2)/(D) over bar (2) (where d is the membrane thickness and (D) over bar (2) is the coion diffusion coefficient in the membrane); this time under our experiment conditions is about 300-400 s. We show that the cause of the maximum is the increase in coion transfer caused by the current-induced concentration polarization of the bathing solution. This increase in coion transfer results in increasing the limiting current density j(lim), which at j = const leads to a reduction in the resistance of the depleted diffusion layer over time and the appearance of a maximum on the ChP. 1D mathematical modeling is based on the Nernst-Planck-Poisson equations. The main assumption inspired by the works of Levich and Amatore is that the apparent electrolyte diffusion coefficient in the depleted solution increases with increasing electroconvection. The stationary value of this diffusion coefficient is found from the I-V curve. The only fitting parameter is the critical potential difference, which refers to the onset of intensive electroconvection.

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.

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.

The esteemed author of the Comment presents a calculation of the number of charges per unit pore sur� face based on the classical theory of the electrical dou� ble layer. Such calculations were performed previously as well. We called attention for the first time to the dif� ference between the charge magnitudes determined from the streaming potential and pore conductivity when studying glass reverseosmosis membranes (1). Analogous results were later obtained by different authors when investigating polymer ultrafiltration and reverseosmosis membranes, porous glasses, rocks, etc. This fact may be related to the difference between the total charge of a pore and the electrokinetic charge, which is always lower than the total charge. Moreover, it may be associated with the discreteness of charges, which is not taken into account in the classi� cal theory of the electrical double layer. Therefore, it is quite unclear why the low charge magnitude in fine pores determined from the streaming potential must, in the opinion of the author of the Comment, serve as the evidence of the absence of these pores. As can be seen from the title of the article, the main goal of our work was to study the electrosurface prop� erties (surface conductivity, charge, and isoelectric point) of latent tracks in a poly(ethylene terephtha� late) film irradiated with highenergy ions. We did not investigate the mechanism of gas flow through the latent tracks. Of course, some model is required for studying the electrosurface properties. We selected the model of a cylindrical channel the wall of which is covered with a porous, loose layer. If a liquid flow inside this layer should be considered to be the surface diffusion is, in our opinion, a question of terminology. The presence of the porous layer on the pore sur�

Poly(ethylene terephthalate) films irradiated by high-energy ions without additional etching have been studied. It has been shown that tracks of heavy high-energy ions in poly(ethylene terephthalate) may be considered to be single through capillaries suitable for studying electrophysical properties. It has been revealed that the surface electrical conductivities of electrolyte solutions (KCl, LiCl, MgCl 2 , and BaCl 2 ) in the tracks of nonetched membranes are substantially higher, while the surface charges are noticeably lower, than those of etched membranes.

Исследованы пленки из полиэтилентерефталата после облучения высокоэнергетическими ионами без дополнительного травления. Показано, что треки тяжелых высокоэнергетических ионов в полиэтилентерефталате можно рассматривать как единичные сквозные нанокапилляры, пригодные для исследования электрофизических свойств. Обнаружены значительное повышение поверхностной электропроводности растворов электролитов (KCl, LiCl, MgCl2 и BaCl2) в треках непротравленных мембран и, в то же время, значительно меньшие значения поверхностного заряда по сравнению с протравленными мембранами.

The electrosurface characteristics are studied for poly(ethylene terephthalate) (PET) track membranes (TMs) with pore radii of 6.5–60 nm, which are used for ultra- and microfiltration. The data obtained enable one to indirectly assess the structure of tracks and variations in the pore space structure of TMs with an increase in the pore radii. Higher porosity values obtained for TMs from the data on their electrical resistance in comparison with those derived from the filtration data lead one to state that the PET pore surface has a loose structure. The thermal treatment of TMs makes the porosity values determined by the methods of electrical resistance and filtration closer to one another. The regularities of variations in the isoelectric point, ζ potential, and surface charge suggest that the properties and structure of PET pore surface depend on the pore radius. The data obtained may be used to predict the separating power of TMs.

Изучены электроповерхностные характеристики трековых мембран (ТМ) из полиэтилентерефталата (ПЭТФ) с радиусами пор от 6.5 до 60 нм, используемых для ультра и микрофильтрации. Результаты исследований позволяют косвенно судить о структуре трека и об изменении структуры порового пространства ТМ по мере увеличения радиуса пор. Более высокие значения пористости ТМ, определенные из данных по электрическому сопротивлению мембран, по сравнению со значениями пористости, полученными из фильтрационных данных, позволяют утверждать, что поверхность пор ПЭТФ имеет рыхлую структуру. При термообработке ТМ происходит сближение значений пористости, определенных из данных по электрическому сопротивлению и фильтрации. Закономерности изменений изоэлектрической точки, -потенциала и поверхностного заряда свидетельствуют о том, что свойства и структура поверхности пор ПЭТФ зависят от их радиуса. Полученные результаты могут быть использованы для прогнозирования разделительной способности ТМ.

The diffusion of alkali metals and ammonium chlorides through an OPMN-KM3 composite nanofiltration membrane is studied as a function of the electrolyte concentration on both sides of the membrane. A model is proposed for calculating diffusion coefficients that takes into account changes in the electrolyte concentration in selective pores of membrane. The diffusion coefficients are calculated both using the proposed model and by Fick’s equation. The effect of the orientation of the membrane with respect to the diffusion flux on the diffusion coefficients of salts is analyzed.