Vol 56, No 11 (2016)

Information about current advances in the field of multisensory systems based on ion exchange membranes for analysis of multicomponent aqueous-organic media of various assignments is summarized. The most promising membrane nanomaterials for chemical sensors are considered. The possibilities for directed variation of characteristics of chemical sensors via modification of perfluorosulfonic cation exchange membranes are analyzed. Particular attention is given to investigations concerning the determination of biologically active compounds in polyionic solutions at different pH values.

The effect of the modification of Nafion membranes with nanoparticles of acid salts of heteropoly acids has been described. Ion conductivity at high and low relative humidity (RH), diffusion permeability, and mechanical properties of hybrid materials have been studied. It has been shown that the membrane modification with the acid salts of heteropoly acids makes it possible to increase the ionic conductivity both at high and low humidity, with the conductivity being determined by the amount of charge carriers at the surface of the dopant particles. The most significant effect of dopant incorporation is observed at low humidity. The conductivity of the sample Nafion + 5 wt % of Cs_xH_{4 – x}SiW₁₂O₄₀ at room temperature is σ = 4.1 mS/cm and it is more than two times as high as the conductivity of the initial Nafion membrane at RH = 30%. By modifying with the acid salts of heteropoly acids, the diffusion permeability of the hybrid membranes is reduced as compared to the initial membrane, thereby suggesting an increase in cation transport selectivity along with an increase in proton conductivity. Introduction of small amounts of the dopant do not cause deterioration of the mechanical properties of the membrane.

A procedure for the fabrication of profiled cation-exchange bilayer membranes with the homogenized surface based on the commercial membrane MK-40 has been developed. The surface morphology and membrane microstructure have been studied by atomic-force microscopy, electron microscopy, and standard contact porosimetry. The concentration dependences of the electrical conductivity and diffusion permeability of the profiled and bilayer profiled membranes have been studied. It has been shown that the application of an MF-4SK film on the surface of the profiled membrane results in a decrease in its diffusion permeability and some increase in specific conductivity. Based on the data obtained, the transport and structure parameters have been calculated in terms of the microheterogeneous model to assess the influence of the modification on the properties of the support membrane. The current–voltage characteristics of the membranes have been measured in sodium chloride solutions, and it has been shown that profiling leads to an increase in the limiting current by 40%. The investigation of mass transfer of ions in the channels formed by the support and modified membranes has shown that under intense current regimes, the mass transfer coefficient through the profiled bilayer membrane is one and a half times that through the initial profiled membrane.

The effect of ionic–molecular composition of reconstituted milk solutions on the response of potentiometric ion-selective electrodes and cross sensitive Donnan-potential sensors based on MF-4SС membranes in H- and K-forms has been studied. The multivariate data were processed and interpreted using the radar chart and principle component analysis. The multisensory system based on membrane sensors with a software and hardware complex for recognition of the reconstituted milk samples containing from 0.40 to 8.46 wt % of dried milk has been developed.

High molecular weight metathesis poly(5-ethylidene-2-norbornene) (PENB) has been synthesized in the presence of the 1st generation Grubbs catalyst at a high monomer/catalyst ratio (3000/1 and higher). The yields of the corresponding polymer have been more than 90% at all monomer/catalyst ratios and molecular weights (M_w) of PENB have been higher than 3 × 10⁵. The gas permeability of different gases (He, H₂, O₂, N₂, CO₂, and CH₄) through PENB films has been studied, and diffusivity and selectivity coefficients have been determined. It has been found that PENB is more permeable than unsubstituted metathesis polynorbornene and cycloalkyl-substituted polynorbornene dicarboximides, but it is less permeable than some Si-containing polynorbornenes. Despite the fact that PENB is more permeable than metathesis polynorbornene and cycloalkyl-substituted polynorbornene dicarboximides, it is close to these polymers in ideal selectivities for some gas pairs.

In relation to the demand for asymmetric porous hollow fiber membranes to be used in gas–liquid membrane contactors designed for operation in organic media, polysulfone membranes of this type have been prepared and subsequently modified to impart oleophobic properties to their surface. The structure and properties of the membranes have been characterized using various techniques, such as optical and scanning electron microscopy, and by measuring contact angles and the permeability of helium, carbon dioxide, and hexane. The surface properties of the membranes have been modified by etching with a mixture of hydrogen peroxide and sulfuric acid or coating with a perfluorinated acrylic copolymer. In the latter case, modified membrane samples have shown a significant reduction in wettability with both water and organic liquids. The hexane permeability data indicate the absence of hexane flow through the membrane modified with perfluorinated acrylic copolymer until a gauge pressure of about 1 atm. The results of the study lead to the conclusion that these membranes can find use in gas–liquid membrane contactors, e.g., for the removal of dissolved gases from liquid hydrocarbons.

Cellulose composite membranes have been fabricated by casting a cellulose solution in N-methylmorpholine oxide on a nonwoven polyester support. The membranes have been tested for nanofiltration of aprotic solvents. The solvent permeability has changed from 0.11 ± 0.02 to 2.5 ± 0.4 kg/(m² h bar) in the following order: DMSO > NMP > DMFA > THF > acetone, which can be attributed to a decrease in viscosity of the fluids. The rejection of the anionic dyes Orange II (MW 350) and Remazol Brilliant Blue R (MW 626) has been found to range within 15–85% and 42–94%, respectively, on the solvent nature. Sorption experiments have revealed a noticeable difference between certain solvents in interaction with the membrane material: a lower degree of cellulose swelling in THF (37%) and a higher degree in DMSO (230%). In addition, it has been found that the rejection of solutes by the composite membranes correlates with the degree of cellulose swelling. A rejection of ≥90% has been achieved for Remazol Brilliant Blue R, which has the larger molecule, at a cellulose swelling ratio of 100% or higher. Thus, it has been concluded that polymer swelling leads to narrowing the porous structure of the cellulose layer of the composite membrane and, hence, improvement in separation parameters.