Specific Permselectivity and Electrochemical Properties of Homogeneous Bilayer Membranes with a Selective Layer Made of DADMAC and EMA Copolymer

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Authors talk about the Specific Permselectivity and Electrochemical Properties of Homogeneous Bilayer Membranes with Selective Layer Made of 3 DADMAС and EMA Copolymer.

The creation of a charge-selective bilayer membrane with enhanced selectivity towards 17 monovalent metal cations is indeed a good research and authors have done a good job on the literature review and experimental results discussion.

However, the only suggestion would be to include a discussion paragraph with a schematics or major findings of the paper and discuss the results and its consequences, effects in detail. 

Author Response

>>Authors talk about the Specific Permselectivity and Electrochemical Properties of Homogeneous Bilayer Membranes with Selective Layer Made of DADMAС and EMA Copolymer.

The creation of a charge-selective bilayer membrane with enhanced selectivity towards monovalent metal cations is indeed a good research and authors have done a good job on the literature review and experimental results discussion.

 

Answer: We would like to express our sincere gratitude to the reviewer for their meticulous attention to our manuscript and for the constructive feedback provided. 

 

>>However, the only suggestion would be to include a discussion paragraph with a schematics or major findings of the paper and discuss the results and its consequences, effects in detail.

 

Answer: We tried to provide a brief discussion about our findings in the conclusions section. We added two new or heavily edited paragraphs in the beginning of the conclusions.

The following text was added: “This study investigates the electrochemical and physicochemical properties of indi-vidual layers and composite bilayer ion-exchange membranes obtained by applying a previously developed DADMAC and EMA copolymer to the surface of a PFSA membrane. The bilayer membranes exhibit high selectivity for monovalent metal cations, particularly sodium and lithium. The specific permselectivity coefficient of the MK-2 bilayer mem-brane for sodium cations increases by more than six times (from 0.8 to 4.8) under the ex-perimental conditions used in this paper. Another significant finding is a five-fold in-crease (from 0.3 to 1.3) in the specific permselectivity coefficient for lithium ions when separated from cobalt ions.

A disadvantage of the membranes obtained is a significant decrease in the limiting current density, which is explained by the formation of a limiting state not in the depleted diffusion layer adjacent to the membrane surface, but at the internal boundary of the cati-on exchanger and anion exchanger. This phenomenon imposes certain limitations on the range of applicable current densities and should be considered in the further development of processes and devices using the developed charge-selective membranes.”

Reviewer 2 Report

Comments and Suggestions for Authors

The paper investigated  theoretically and experimentally,the overall and partial current-voltage characteristics, as well as external and internal diffusion limiting currents of bilayer membranes with a thin anion-exchange layer  based of the copolymer of N,N-diallyl-N,N-dimethylammonium chloride  and  ethyl methacrylate  on the surface of a membrane substrate made from polyfluorosulfonic acid (PFSA). The novelty  of this study is related the use of a  bilayer charge-selective membranes with high selectivity for monovalent metal cations. The abstract is clear and introduction is explanatory. Also calculations were performed using a previously developed mathematical model qualitatively and quantitatively to confirm the obtained experimental data. Experimental part is not very clear. It is not mentioned how the bilayer is obtained. What technique was used for casting the anion-exchange membrane on PFSA. It was used a special device, or another method (like spin coating)?  The expression "to obtain with selective layer thicknesses of 6 µm and  24 µm"  is general. Why you have chosen these thicknesses and how do these influence the properties of these membranes? Results and discussion are well written. Conclusions are clear. The paper can be accepted for publication after corrections.

 

Author Response

>>The paper investigated theoretically and experimentally,the overall and partial current-voltage characteristics, as well as external and internal diffusion limiting currents of bilayer membranes with a thin anion-exchange layer  based of the copolymer of N,N-diallyl-N,N-dimethylammonium chloride  and  ethyl methacrylate  on the surface of a membrane substrate made from polyfluorosulfonic acid (PFSA). The novelty  of this study is related the use of a  bilayer charge-selective membranes with high selectivity for monovalent metal cations. The abstract is clear and introduction is explanatory. Also calculations were performed using a previously developed mathematical model qualitatively and quantitatively to confirm the obtained experimental data.

Answer: We extend our sincere gratitude to the reviewer for their diligent attention to our article and for the positive feedback provided.

 

>>Experimental part is not very clear. It is not mentioned how the bilayer is obtained. What technique was used for casting the anion-exchange membrane on PFSA. It was used a special device, or another method (like spin coating)?

Answer: We used a direct pouring of the copolymer solution onto the surface of the membrane-substrate. The membrane-substrate was also produced using casting method in which we cast a solution of PFSA ion-polymer onto the glass plate. The ammount of the PFSA solution was enough to obtain a thick layer (210 microns) of PFSA membrane. The PFSA and copolymer solutions were based on the same solvent – isopropyl alchohol. We propose that application of copolymer solution onto substrate-membrane results in a partial dissolution of the upper layer of the PFSA membrane thus leading to at least partial entaglement of the copolymer and PFSA polymer chains.

We added the procedure to the section 2.3 Membranes preparation (line 109-120).

The added text: “The bilayer membranes were prepared using the following procedure: A layer of PFSA solution, sufficient for forming a 210 µm substrate membrane, was applied to a pre-cleaned and degreased glass surface. Solvent evaporation was carried out at 25 °C and ambient pressure. Once the substrate membrane was formed, a copolymer solution was applied to its surface. Because both layers of the bilayer membrane were prepared from solutions with the same solvent, we propose that applying the copolymer solution to the substrate membrane leads to partial dissolution of the PFSA membrane's upper layer. This results in at least partial entanglement of the copolymer and PFSA polymer chains. Solvent evaporation then took place at 25 °C and ambient pressure for 24 hours. This process produced a dense layer of the copolymer on the cation-exchange membrane surface. Modified membranes with selective layer thicknesses of 6 µm and 24 µm were subsequently designated as MK-1 and MK-2, respectively.”

 

>>The expression "to obtain with selective layer thicknesses of 6 µm and  24 µm"  is general. Why you have chosen these thicknesses and how do these influence the properties of these membranes?

Answer: We selected these thicknesses because, in our previous works, we have already developed bilayer membranes with similar thicknesses of the modifying layer.

 

Results and discussion are well written. Conclusions are clear. The paper can be accepted for publication after corrections.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors developed homogeneous bilayer membranes by applying a thin anion-exchange layer of DADMAC and EMA to a PFSA membrane substrate. They investigated the membranes' current-voltage characteristics, diffusion-limiting currents, and parameters such as specific conductivity, sorption, and diffusion permeability. The bilayer membranes exhibited selectivity for monovalent metal cations, with sodium cation selectivity improving more than sixfold. The experimental data were validated using a four-layer mathematical model with quasi-equilibrium boundary conditions, highlighting the effectiveness of the bilayer design in mixed NaCl and CaCl₂ solutions.

General Considerations:

Upon reading the paper, it appears that the theoretical framework dominates the experimental results. Although some material characterizations were performed, it remains unclear how the material achieves permselectivity (from a chemical point of view).

The introduction is not well-focused, leaving the potential applications of this material ambiguous. Only three lines are dedicated to the study's purpose, whereas a more detailed description of the rationale behind the material's development is necessary.

In the "Membranes Preparation" section, I do not understand why the Authors included lines 90 to 98. This content belongs in the results section, where the surface chemistry of the materials should be further investigated, providing insights into the final chemical composition.

The citation in line 146 is missing and should be replaced with the correct reference.

Detailed Considerations:

1. No experimental support was provided beyond FTIR characterization regarding the material's chemical composition, nor was the synthesis process emphasized. How do the authors expect the materials to interact between them (polymers and supports, polymer/polymer)? Is the process similar to the layer-by-layer technique based on charge interactions? Is crosslinking between materials expected? If other analytical techniques (EDX, XPS, etc.) are not available, FTIR/ATR analysis of the monomer powders before deposition and evaluation of changes post-deposition would be useful. Control experiments are also necessary, analyzing the support materials before and after functionalization.

2. From line 127, the authors state, "The thicknesses of the modifying films and membranes were measured using an Absolute Digimatic MDH Mitutoyo electronic micrometer, with an accuracy of 1 μm." Could further details be provided? Additional material characterizations (e.g., AFM)?

3. In the IR spectrum, the intensity axis should be labeled as "a.u."

4. Figures 4 and 5 could be combined into a single panel, as could Figures 6, 7, and 8.

5. To demonstrate actual permselectivity, testing in a real-world matrix is crucial. In my opinion, the publication of this work depends on proving the material's effectiveness in real scenarios. For instance, analyzing real saline waters (such as seawater or simulated solutions following known literature recipes) and assessing the target salt content before and after incubation with the solutions (ICP-MS) would be essential.

Comments on the Quality of English Language

English is fine. No particular problems were detected

Author Response

Reviewer #3

The authors developed homogeneous bilayer membranes by applying a thin anion-exchange layer of DADMAC and EMA to a PFSA membrane substrate. They investigated the membranes' current-voltage characteristics, diffusion-limiting currents, and parameters such as specific conductivity, sorption, and diffusion permeability. The bilayer membranes exhibited selectivity for monovalent metal cations, with sodium cation selectivity improving more than sixfold. The experimental data were validated using a four-layer mathematical model with quasi-equilibrium boundary conditions, highlighting the effectiveness of the bilayer design in mixed NaCl and CaCl₂ solutions.

 

General Considerations:

>>Upon reading the paper, it appears that the theoretical framework dominates the experimental results. Although some material characterizations were performed, it remains unclear how the material achieves permselectivity (from a chemical point of view).

Answer: The bilayer membranes developed in this study exhibit significant selective permeability for monovalent ions. This selective permeability arises not from specific chemical interactions between the ions and the ionogenic matrix of the membrane (as we suggested in a previous study with bulk-modified membrane [Achoh A. et al. The Effect of Bulk Modification of the MF-4SK Membrane with Phosphorylated Hyper-Branched Dendrimer Bolthorn H20 on the Mechanisms of Electroconvection/Dissociation of Water and Specific Selectivity to Divalent Ions // Electrochem. Multidisciplinary Digital Publishing Institute, 2024. Vol. 5, № 1. P. 84–106.]), but rather from the so-called electrostatic barrier effect. The thin modifying layer applied to the substrate membrane contains ionogenic groups with a positive charge (quaternary ammonium bases), similar to the cations of the salts in the solution. Despite the fact that these cations are co-ions for the modifying layer, its thinness prevents significant resistance to their electrodiffusion. However, cations with higher charges are strongly repelled by the ionogenic groups within the modifying layer, resulting in a concentration that is an order of magnitude lower than that of monovalent cations. Nevertheless, the mobilities (diffusion coefficients) of these ions within the membrane do not differ substantially, as indicated by the data in Table 1 of the article (where the diffusion coefficient of monovalent ions is also higher). As a result, the fluxes of monovalent and divalent ions within the modifying layer differ significantly, and primarily monovalent ions are transported through the bilayer membrane.

We have added the following text to section 3.3: “The thin modifying layer applied to the substrate membrane surface contains positively charged inogenic groups (quaternary ammonium bases). Although these cations act as co-ions for the modifying layer, its thinness prevents significant resistance to their elec-trodiffusive flux. However, cations with higher charges are strongly repelled by the inor-ganic groups within the ionic channels of the modifying layer, resulting in a concentra-tion-thin that is considerably lower than that of univalent cations [26]. Nevertheless, the diffusion coefficients of these ions within the modifying layer do not differ substantially, as indicated by the data in Table 1. Therefore, the resulting flux of monovalent ions is greater through the modifying layer and the bilayer membrane as a whole compared to the flux of divalent ions.”

 

>>The introduction is not well-focused, leaving the potential applications of this material ambiguous. Only three lines are dedicated to the study's purpose, whereas a more detailed description of the rationale behind the material's development is necessary.

Answer: The introduction section was updated to better show of the rationale behind the material's development.

The following text was added (lines 67-81): “However, these studies employed water-soluble polyelectrolytes (sulfonated polysty-rene and protonated polyallylamine) for modifying the cation-exchange membrane. The protonated polyallylamine suggested in [21] exhibits several limitations. Its quaternary ammonium groups undergo deprotonation in alkaline media, converting to amino groups. Additionally, the water-solubility of polyallylamine results in its leaching from the support membrane surface. These combined factors diminish the stability of the charge-selective membranes [21].

A viable alternative to the aforementioned protonated polyallylamine is the DADMAC-EMA copolymer previously developed by the authors of this work [22]. This polyelectrolyte is water-insoluble, and the quaternary ammonium group formed by the pyrrolidinium heterocyclic fragments exhibits stability in alkaline environments. Physi-co-chemical properties of the copolymer in ternary solutions have not been previously in-vestigated, making it a relevant subject for verifying the previously developed four-layer mathematical model. Also, it posses good adhesion towards polyfluorosulfonic acid films (Nafion analogue) as they both can be casted from the same solvent.”

 

>>In the "Membranes Preparation" section, I do not understand why the Authors included lines 90 to 98. This content belongs in the results section, where the surface chemistry of the materials should be further investigated, providing insights into the final chemical composition.

Answer: The text in lines 90-98 of the original manuscript describes the preparation of the bilayer membranes studied in this work. Following feedback from reviewers, this section has been expanded with more details on the membrane preparation procedure.

The synthesized copolymer, used in this work, was previously prepared and characterized using various physicochemical methods by the authors. Detailed information can be found in [Bondarev D., Melnikov S., Zabolotskiy V. New homogeneous and bilayer anion-exchange membranes based on N,N-diallyl-N,N-dimethylammonium chloride and ethyl methacrylate copolymer // J. Memb. Sci. Elsevier B.V., 2023. Vol. 675, № February. P. 121510.].

The PFSA membrane has also been extensively studied in previous works by both the authors of this article and other researchers.

However, in previous studies, we prepared bilayer membranes using a heterogeneous ion-exchange membrane as the substrate. Combining two homogeneous ion-exchange polymers with different chemical natures is a non-trivial task that was successfully addressed in this work. Therefore, we believe it is important to describe the procedure used to obtain homogeneous bilayer membranes.

 

>>The citation in line 146 is missing and should be replaced with the correct reference.

Answer: We are very sorry, but we cannot find a bad reference at line 146 (in the original manuscript). Line 146 follows section 2.5 IR spectra and is a blank line.

 

Detailed Considerations:

 

>>No experimental support was provided beyond FTIR characterization regarding the material's chemical composition, nor was the synthesis process emphasized. How do the authors expect the materials to interact between them (polymers and supports, polymer/polymer)? Is the process similar to the layer-by-layer technique based on charge interactions? Is crosslinking between materials expected? If other analytical techniques (EDX, XPS, etc.) are not available, FTIR/ATR analysis of the monomer powders before deposition and evaluation of changes post-deposition would be useful. Control experiments are also necessary, analyzing the support materials before and after functionalization.

Answer: Thank you for the comment. A commercial cation-exchange polymer (LF-4SK, JSC Plastpolymer, Russia) and an anion-exchange copolymer of DADMAH and EMA developed by the authors were used to prepare the bilayer membrane. The synthesis of the anion-exchange copolymer is not described here, as the copolymerization process and chemical structure of the polymer have been described in detail elsewhere [Bondarev, D.; Melnikov, S.; Zabolotskiy, V. New homogeneous and bilayer anion-exchange membranes based on N,N-diallyl-N,N-dimethylammonium chloride and ethyl methacrylate copolymer. J. Memb. Sci. 2023, 675, 121510, doi:10.1016/j.memsci.2023.121510.].

The cation-exchange and anion-exchange layers interact with each other through both electrostatic interactions between charged functional groups (–R₄N⁺ and –RSO₃^–) and adhesion interactions between the polymers (both polymers are soluble in isopropyl alcohol).

Based on the chemical structure of the polymers used and their operating conditions, intermolecular crosslinking through the formation of covalent bonds is not possible.

The preparation of the bilayer membrane does not involve any chemical interaction between the cation-exchange and anion-exchange layers. For this reason, the IR spectra of the membrane differ depending on the side being studied, but they do not differ from the spectra of the individual polymers (the DADMAC and EMA copolymer and PFSA). This is because the depth of the material penetrated by the IR spectrometer beam does not exceed 20 µm.

 

>>From line 127, the authors state, "The thicknesses of the modifying films and membranes were measured using an Absolute Digimatic MDH Mitutoyo electronic micrometer, with an accuracy of 1 μm." Could further details be provided? Additional material characterizations (e.g., AFM)?

Answer: The resolution of the available equipment is sufficient to determine the thickness of the modifying layer as either 6 or 24 micrometers. The thickness was determined by measuring the difference in average sample thickness (at 5 points) between the unmodified and modified areas of the bilayer membrane.

Unfortunately, the atomic force microscope available at our university does not allow us to study cross-sections of our materials. And studying the boundary between the modified and unmodified areas is difficult due to the small scanning area (1x1 mm2).

The following explanation was added to the text of the article (section 2.4, lines 143-145): “The thickness was determined by measuring the difference in average thickness of the sample (at 5 points) between the unmodified and modified areas of the bilayer membrane.”

 

>>In the IR spectrum, the intensity axis should be labeled as "a.u."

Answer: The figure was replaced in accordance with the above comment.

 

>>Figures 4 and 5 could be combined into a single panel, as could Figures 6, 7, and 8.

Answer: Thank you for yur comment. While some journals combine multiple figures into a single large panel, we do not believe this is appropriate for Figures 4 and 5 or Figures 6-8. These figures contain distinct sets of data, and their captions differ significantly. Combining them into a single panel would not enhance the clarity or readability of our article.

 

>>To demonstrate actual permselectivity, testing in a real-world matrix is crucial. In my opinion, the publication of this work depends on proving the material's effectiveness in real scenarios. For instance, analyzing real saline waters (such as seawater or simulated solutions following known literature recipes) and assessing the target salt content before and after incubation with the solutions (ICP-MS) would be essential.

Answer: Thank you for your comment. We also believe that testing in a real-world matrix is crucial for new materials. To test bilayer membrane effectivness we conducted an experiment using RMD device and simulated water of the Black Sea.

Numerous changes were done to the manuscript: new subsection 2.6 added, new subsection 3.4 added, conclusions were modified.

Please see the revised version of the manuscript for changes.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No comments.

Author Response

Thank you

Reviewer 3 Report

Comments and Suggestions for Authors

The Authors replied to all my comments

Author Response

>>The Authors replied to all my comments.

 

Thank you