Phase Transitions and Electrochemical Properties of Ionic Liquids and Ionic Liquid—Solvent Mixtures

Round 1

Reviewer 1 Report

In this review, the authors summarized recent advances in experimental, computational, and theoretical studies of ILs and IL-solvent mixtures. The structures of this review are well organized and the authors presented a thorough discussion of phase transitions and electrochemical properties of ILs and IL-solvent mixtures. This review can be accepted for publication after minor revisions.

1) The authors discussed ILs and ILS in this review, but I can see that only the "ionic liquids-solvent mixtures" is highlighted in the title. The authors may slightly change the title to include the "ionic liquids".

2) The "ionic liquid-solvent mixtures" is abbreviated as ILS, which is somewhat similar to ILs (an abbreviation for ionic liquids). In addition, I can see that "ionic liquid-solvent mixtures" is frequently used in the content (for example at the 25^th line, the 46^th line, the 216^th line, etc.). So the authors may use the full name of “ionic liquid-solvent mixtures” in the whole manuscript.

3) Line 140, what is the full name of “[BMM][PF6]”? Is the cations the “[BMIM]”?

4) Lines 434 and 435, “see (9/4)” should be “see Eq. (9/4)”.

5) Fig. 10, the authors missed some parameters as I can see there are some “?” marks

6) The authors may provide some discussion about the heterogeneous structures and dynamics in ILs and IL-solvent mixtures as these heterogeneities are directly correlated with their functionalities in electrochemical applications.

Author Response

In this review, the authors summarized recent advances in experimental, computational, and theoretical studies of ILs and IL-solvent mixtures. The structures of this review are well organized and the authors presented a thorough discussion of phase transitions and electrochemical properties of ILs and IL-solvent mixtures. This review can be accepted for publication after minor revisions.

1) The authors discussed ILs and ILS in this review, but I can see that only the "ionic liquids-solvent mixtures" is highlighted in the title. The authors may slightly change the title to include the "ionic liquids".

Our response

We have changed the title as suggested by the Referee.

2) The "ionic liquid-solvent mixtures" is abbreviated as ILS, which is somewhat similar to ILs (an abbreviation for ionic liquids). In addition, I can see that "ionic liquid-solvent mixtures" is frequently used in the content (for example at the 25th line, the 46th line, the 216th line, etc.). So the authors may use the full name of “ionic liquid-solvent mixtures” in the whole manuscript.

Our response

We have changed ‘ILS’ to ‘IL-solvent mixtures’.

3) Line 140, what is the full name of “[BMM][PF6]”? Is the cations the “[BMIM]”?

Our response

It was a misprint, in the revised manuscript it is corrected, and the full name is given (in red font)

4) Lines 434 and 435, “see (9/4)” should be “see Eq. (9/4)”.

Our response

We have made the requested change.

5) Fig. 10, the authors missed some parameters as I can see there are some “?” marks

Our response

Fig. 10 (in the revised version Fig.11) is replaced by a corrected one. All parameters are given in the caption or in the figure.

6) The authors may provide some discussion about the heterogeneous structures and dynamics in ILs and IL-solvent mixtures as these heterogeneities are directly correlated with their functionalities in electrochemical applications.

Our response

We have made the requested addition in the second added paragraph in Sec. 3.2 in the revised manuscript .

Author Response File: Author Response.pdf

Reviewer 2 Report

Reviewer’s report on the manuscript entitled, “Phase transitions and electrochemical properties of Ionic

Liquids - solvent mixtures” by Carolina Cruz, and Alina Ciach, (Manuscript ID: molecules-1242035)

This review reports on advances in ionic liquids and ionic liquid + solvent mixtures as is related to electrochemical, thermodynamics, and structural properties. The authors begin by providing a reasonable description of classical electrochemical underpinnings. However, as the manuscript continues, the information provided suffers from over-generalization with respect to systems discussed.

Overall, this review is nicely organized, well-written and identifies a variety of relevant topics with respect to the interface of electrochemistry and ionic liquids. What is consistently missing is a critical review of the literature with regard to the discussion of specific IL systems that support their points. A few representative illustrations are given below of points for improvement:

• In nearly the entire document, there is (for the most part) only a general mention of "ILs" and "ILS"s – discussion of specific IL identities is lacking. So then, it is rather difficult for a reader to use this review in a specific way except to find a handful of references that relate to the specific topic mentioned. In a few places, there is reference to a specific cation type, and then only to imidazolium ILs, seen on pgs. 5, 7. But even these refer only generally to the imidazolium class. In a couple of references listed, the titles do mention specific cation/anion pairs used in the reported study(ies). But this is insufficient for a literature review article.

In checking for specific mention of other IL cation families, this reviewer notes that there is sparse, if any, mention of other common cation classes, particularly,

Phosphonium – 0

Ammonium – 0

Pyrrolidinium – 2 mentions

Pyridinium – 0,

and then only in the stated reference title.

In this regard, while the review does point out research associated with the varying topics chosen for this review, the reader is left to do the work of specific application to identify whether a specific IL has been included or discussed. In this case, using a search engine likely would be more effective and would afford the searcher the ability to include specific search terms to find the desired information.

• Explanations are at times too vague. As one example, on lines 105-106, the authors state that “Several works…” provided “…insights…”. The reader is left with the questions What ILs?”, “What experiments?” and “What insights?” While the authors very briefly state one example, in lines 109 ff more questions surface from phrases such as “… all the ILs” and “…substantial differences…”. Then, the idea of a characteristic length is mentioned but not described. It would be useful (and appropriate) for readers to know what the relationship is between the “characteristic length” and “ion dimensions”. Given the variety of IL cation/anion pairs available, and the substantial differences in ion dimensions, this statement would benefit greatly from the inclusion of specific information. For example, the 1,3-dimethylimidazolium cation dimension is substantially different than the C₁₆-methylimidazolium cation. Or, another example is the often used trihexyltetradecylphosphonium cation. Similarly, anions vary widely. So, some qualification here seems necessary.

In contrast, the paragraph starting at line 190 is informative and provides appropriate details to aid the reader’s understanding.

• Overall, Section 5 seems to be more thorough coverage of the stated topic and is more representative of anticipated content coverage in a “review”. That said, there is still a notable lack of IL system specificity. For example, in Section 5.1 although the sub-section title suggests that information is presented for IL + solvent systems, there is no mention of any IL + solvent. And while there is mention of one specific imidazolium IL study (BMIM-PF₆), the context is neat IL, on a gold surface. There is no mention/discussion of solvent systems.

Line 409 continues the pattern of generic reference to IL-solvent mixtures. Section 5.2 seems to focus more on the idea of capillary ionization potential in the context of IL-rich and IL-poor systems but there no indication for which IL systems this is relevant.

• In the abstract, the authors state that they report on “…advances…” but it isn’t clear from the text exactly what are the advances, nor do the concluding remarks identify any advances. Moreover, the conclusion is overly terse and general, and does not provide readers with a reasonable capsulization of the work presented. As is often the case, readers parse their time by examining abstracts and conclusions before investing time to read the entire document. With sensitivity to the readership, the manuscript could be substantially improved in this regard.

 

The above few points are representative of the general nature of the discussion in this manuscript and highlights the need to call out and discuss specific systems with respect to the selected topics. The topics chosen seem to be reasonable and worthy of review and discussion. To this reviewer, it seems that the point and purpose of writing a review article is to provide the readership with a thorough literature review that compiles, and perhaps may provide an evaluation of, a breadth of specific content. Thus, discussion of specific IL examples would seem to be much more appropriate for the purposes of discussing a review topic.

On the whole, the concept of this review is notable and would provide a useful resource to the readership that works at the interface of electrochemistry and ionic liquids. To that end, the authors should consider integrating specific information about the ILs and ILS systems that underpin the points they have chosen to discuss in this work.

Author Response

This review reports on advances in ionic liquids and ionic liquid + solvent mixtures as is related to electrochemical, thermodynamics, and structural properties. The authors begin by providing a reasonable description of classical electrochemical underpinnings. However, as the manuscript continues, the information provided suffers from over-generalization with respect to systems discussed.

Overall, this review is nicely organized, well-written and identifies a variety of relevant topics with respect to the interface of electrochemistry and ionic liquids. What is consistently missing is a critical review of the literature with regard to the discussion of specific IL systems that support their points. A few representative illustrations are given below of points for improvement:

Our response

In the revised manuscript, 4 new paragraphs and a new figure (Fig.1) are added in sec.3.1, and 2 new paragraphs and some more additions in sec.3.2, with description of specific ILs (in red font). In addition, we add information about examples of substances for which the discussed properties were observed in several places (in red font).

• In nearly the entire document, there is (for the most part) only a general mention of "ILs" and "ILS"s – discussion of specific IL identities is lacking. So then, it is rather difficult for a reader to use this review in a specific way except to find a handful of references that relate to the specific topic mentioned. In a few places, there is reference to a specific cation type, and then only to imidazolium ILs, seen on pgs. 5, 7. But even these refer only generally to the imidazolium class. In a couple of references listed, the titles do mention specific cation/anion pairs used in the reported study(ies). But this is insufficient for a literature review article.

In checking for specific mention of other IL cation families, this reviewer notes that there is sparse, if any, mention of other common cation classes, particularly,

Phosphonium – 0

Ammonium – 0

Pyrrolidinium – 2 mentions

Pyridinium – 0,

and then only in the stated reference title.

In this regard, while the review does point out research associated with the varying topics chosen for this review, the reader is left to do the work of specific application to identify whether a specific IL has been included or discussed. In this case, using a search engine likely would be more effective and would afford the searcher the ability to include specific search terms to find the desired information.

Our response

In the revised version, we add several paragraphs with description of the most common IL cation families, and provide examples of substances that were recently investigated in the context of structure, double layer capacitance and phase transitions (in red font).

• Explanations are at times too vague. As one example, on lines 105-106, the authors state that “Several works…” provided “…insights…”. The reader is left with the questions What ILs?”, “What experiments?” and “What insights?” While the authors very briefly state one example, in lines 109 ff more questions surface from phrases such as “… all the ILs” and “…substantial differences…”. Then, the idea of a characteristic length is mentioned but not described. It would be useful (and appropriate) for readers to know what the relationship is between the “characteristic length” and “ion dimensions”. Given the variety of IL cation/anion pairs available, and the substantial differences in ion dimensions, this statement would benefit greatly from the inclusion of specific information. For example, the 1,3-dimethylimidazolium cation dimension is substantially different than the C16-methylimidazolium cation. Or, another example is the often used trihexyltetradecylphosphonium cation. Similarly, anions vary widely. So, some qualification here seems necessary.

In contrast, the paragraph starting at line 190 is informative and provides appropriate details to aid the reader’s understanding.

Our response

The first vague sentence is removed in the revised manuscript, and the second one is rephrased. Moreover, we add additional information in 2 paragraphs added in sec.3.2 (in red font).

• Overall, Section 5 seems to be more thorough coverage of the stated topic and is more representative of anticipated content coverage in a “review”. That said, there is still a notable lack of IL system specificity. For example, in Section 5.1 although the sub-section title suggests that information is presented for IL + solvent systems, there is no mention of any IL + solvent. And while there is mention of one specific imidazolium IL study (BMIM-PF6), the context is neat IL, on a gold surface. There is no mention/discussion of solvent systems.

•  

Line 409 continues the pattern of generic reference to IL-solvent mixtures. Section 5.2 seems to focus more on the idea of capillary ionization potential in the context of IL-rich and IL-poor systems but there no indication for which IL systems this is relevant.

Our response

Section 5 summarizes results obtained from a model based on general thermodynamic considerations. In the revised manuscript we add a paragraph above sec. 5.1 (in red font) with examples of IL-solvent mixtures that exhibit bulk phase diagrams that agree with the model predictions. The theoretical results of sec. 5.1, 5.2 are expected to concern in particular the systems mentioned in the added paragraph.

 

• In the abstract, the authors state that they report on “…advances…” but it isn’t clear from the text exactly what are the advances, nor do the concluding remarks identify any advances. Moreover, the conclusion is overly terse and general, and does not provide readers with a reasonable capsulization of the work presented. As is often the case, readers parse their time by examining abstracts and conclusions before investing time to read the entire document. With sensitivity to the readership, the manuscript could be substantially improved in this regard.

 Our response

We thank the referee for this suggestion. In the revised manuscript, the abstract and the conclusions are extended. The additions are in red font.

The above few points are representative of the general nature of the discussion in this manuscript and highlights the need to call out and discuss specific systems with respect to the selected topics. The topics chosen seem to be reasonable and worthy of review and discussion. To this reviewer, it seems that the point and purpose of writing a review article is to provide the readership with a thorough literature review that compiles, and perhaps may provide an evaluation of, a breadth of specific content. Thus, discussion of specific IL examples would seem to be much more appropriate for the purposes of discussing a review topic.

On the whole, the concept of this review is notable and would provide a useful resource to the readership that works at the interface of electrochemistry and ionic liquids. To that end, the authors should consider integrating specific information about the ILs and ILS systems that underpin the points they have chosen to discuss in this work.

 Our response

We thank the referee for this suggestion. The revised manuscript is substantially extended, and a discussions of properties of specific ILs is added on a rather general level as well as in relation to the discussed phenomena associated with phase transitions and electrochemical properties of ILs and IL-solvent mixtures.

Round 2

Reviewer 2 Report

This is a much improved draft and is suitable for publication.