**Preface to "Ion and Molecule Transport in Membrane Systems"**

Membranes play an enormous role in our life. A biological cell membrane is an en-closing film that acts as a selective barrier, within or around the cell. Cell membranes control the fluxes of substances in and out of cells. Artificial membranes are widely used for the treatment of water and food solutions (milk, juices, wine, . . . ), fractionation of organic acids, bioactive compounds and nutrients, energy production and other applications. Artificial membranes largely mimic the structure and functions of biological membranes. Like cell membranes, many kinds of artificial membranes involve macromolecules consisting of a relatively long hydrophobic polymer chain and a hydrophilic "head" at its end. Such elements allow multiple types of interactions (hydrophobic-hydrophobic, dipole-dipole, ion-dipole, ion-ion) between them and water, which provides self-assembly resulting in formation of permselective thin films. The similarity in the structure leads to the similarity in the properties and the approaches to study the laws governing the behavior of both biological and artificial membranes. It is of interest that Kedem and Katchalsky deduced their famous equations for the description of transport processes in biological membranes [1,2]. Now, these equations are largely used for all types of membranes, in particular, for modelling ion and water transport in a promising technological process named Pressure Retarded Osmosis (PRO), which is employed for energy harvesting from salinity variations [3].

The idea of this special issue is to recollect the papers describing physico-chemical and chemico-physical aspects of ion and molecule transport, which are common for both biological and artificial membrane systems. The scope of the issue involves: Experimental studies and mathematical modeling providing new knowledge on the mechanisms of ion and molecule transport in artificial and living systems; Similarities in behavior of biological and artificial membranes; Biomimetic structural features of artificial membranes and their impact on membrane properties and performance for separation processes; Generalities and case studies in the field of material structure–properties relationships; Thermodynamics and irreversible thermodynamics description; Equilibriums and kinetics of transport processes in membrane systems; Coupling of ion and molecule transport with chemical reactions and catalysis; Impact of forced and natural convection on ion and molecule transport; Mechanisms of electric current-induced convection and its impact on ion and molecule transport across membranes; Concentration polarization and coupled effects occurring in membrane systems under the action of external pressure and electric driving forces; The physico-chemical and chemico-physical aspects of all kinds of separation, purification, and fractionation in membrane systems. In all cases, analysis of phenomena at the molecular level is encouraged.

Within this issue, there are papers devoted to studying thin mechanisms of ionic and molecular transport in cell membranes [4–6] as well as in artificial ones, which mimic biological membranes [7,8]. Y. Trofimov et al. reported the results of molecular dynamics simulations of water confined in the pore of a cell membrane, taking into account that microscopic properties of water near the molecular surface are radically different from those in the bulk [4]. K. Yue et al. [6] applied molecular dynamics to simulate the interactions of inhaled pollutant nanoparticles with the pulmonary surfactant monolayer. The review by M. Tingey et al. [5] evaluates the current tools and methodologies available to study the role of transmembrane proteins in some kinds of cell membranes. The team of Y. Zhang [7] described the mechanism of selective separation of volatile fatty acids (VFAs) using polymer inclusion membranes (PIMs) containing ionic liquids as the carrier. This process mimics the selective transport of some compounds (such as phenols, amino acids) by facilitated diffusion through cell membranes. W. Tien et al. [8] described the effects of cholesterol on water permittivity of biomimetic ion pair amphiphile bilayers, which are used to fabricate vesicles with various pharmaceutical applications.

The problems of preparation and properties of artificial membranes are considered in [9–13]. S.B. Aziz et al. [9] developed polymer blend electrolyte membranes based on chitosan, a biopolymer. It is shown that new membranes have a high performance in electrical double-layer capacitor applications, such as water desalination. The use of the Truhan model allowed detailed analysis of ion transport parameters of the chitosan-based polymer membrane [10]. B. Jaleh et al. [11] use deposition of TiO2 nanoparticles to improve the wettability of the O2 plasma activated polypropylene membrane. A new hydroxide exchange membrane was synthesized by A. Abbasi et al. [12]. Low zincate crossover and high discharge capacity of this kind of membranes make them promising to be used in zinc–air batteries, which represent an alternative to lithium–ion batteries for various applications. Novel anion-exchange membranes combining the advantages of densely functionalization architecture and crosslinking structure were fabricated by Q. Ge et al. [13]. A high ratio of hydroxide conductivity to water swelling suggests that these membranes have high potential for application in fuel cells. The analysis of some membrane properties affecting their overall performances is made in Refs. [14,15]. F. Luo et al. [14] examined trans-membrane potential across ion exchange membranes in order to evaluate their possible power efficiency, when applied in a "reverse electrodialysis heat engine". The impact of different ion compositions of a salt solution containing NaHCO3, Na2CO3, and NH4Cl electrolytes was examined. The performance of another membrane process, important in the recovery of fertilizers from wastewaters was studied by O. Rybalkina et al. [15]. Phosphorus transport through anion-exchange membranes in the course of electrodialysis of NaH2PO4 solution was investigated. It was shown that when H2PO4—ions enter the membrane, a part of these anions dissociates, hence parasitic transport of H+ ions occurs in the depleted solution, which essentially reduces the current efficiency of the process.

The analysis of interesting and diverse applications of artificial membranes is reported in Refs. [16-19]. An ionic-liquid-driven supported liquid membrane system was applied by J. Li et al. [16] for preparing a special kind of phosphors, which were characterized by good luminescent properties. The team of L. Bazinet for the first time realized simultaneous separation of peptides from salmon protein hydrolysate by three ultrafiltration membranes stacked in an electrodialysis system [17]. A thorough study of this green and ultra-selective process is presented. Another application concerning processing whey was investigated in references [18, 19]. G. Dufton et al. [18] used a special mode of electrodialysis where Pulsed Electric Fields (PEFs) are applied. It was found that the PEF mode, in which current pulses alternated with pauses of zero current, can increase the degree of both demineralization and deacidification of the whey, as well as reduce membrane scaling. Another kind of electric current pulses, called Pulsed Electrodialysis Reversal (PER), was applied by A. Merkel and A. Ashrafi [19] for demineralization of acid whey. They alternated relatively long direct current pulses with short reverse-polarity pulses to decrease the fouling onto membrane surface during ED. It was found that the fouling on the diluate side of both cation and anion exchange membranes in PER regime was reduced compared to the conventional ED.

The issues connected with the hydrodynamic conditions of mass transfer in membrane systems are considered in references [18–20]. G. Battaglia et al. [18] studied the effect of a trans-membrane pressure (TMP), which may arise in membrane stacks for electrodialysis (water desalination) or reverse electrodialysis (energy production by salinity gradient), on solution flow and mass transfer. A cognitive and helpful review of the effect of profiling ion-exchange membranes on the properties of electrodialysis was presented by S. Pawlowski, J. Crespo and S. Velizarov [19]. They were very convincing when showing that there is exciting potential for improving membrane performance due to the enormous degree of freedom in creating new profile geometries on a membrane surface. Some problems of electrokinetic instability of solution adjacent to an ion-exchange membrane are considered by Magnico [20]. The electrokinetic behavior of cation-exchange resin particles arranged in a well-defined geometrical structure has been studied by the team of Z. Slouka [21]. The understanding of this effect of coupling between water and ion transport is of utmost importance for improving the performance of electro-driven membrane separation processes.

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