Underwater Performance of Eco-Friendly Choline-Based Ionic Liquid Coatings Applied on Stone Surfaces

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Methodology

• The description of the formulations, application conditions, and curing times is sufficiently detailed; however, it would be advisable to specify viscosity/actual coating consumption and environmental conditions during curing. Additionally, it would be useful to quantify, as far as possible, the fraction of coated area/loss of coating based on the UV and SEM-BSE images, for example through image analysis, to support the qualitative interpretations regarding heterogeneity in limestone.
• The characterization protocols are appropriate and rely on established standards. Replication is adequate, but the manuscript does not detail statistical analyses beyond means and standard deviations, nor does it formulate prior quantitative hypotheses, which limits the statistical rigor of the comparisons. It would be desirable to include a more explicit statistical analysis (ANOVA–Analysis of Variance or significance tests) to compare treatments in terms of ΔE, contact angles, and absorption coefficients, indicating significance levels and effect sizes.

Results and Analysis

• In the durability tests, the UV images show nearly complete fluorescence retention in marble up to 6 months and the appearance of non-luminescent areas in limestone from 3 months onwards for some formulations, which the authors relate to porosity heterogeneity and preferential absorption in more open zones. This interpretation is qualitatively supported by the SEM-BSE micrographs, but the analysis remains descriptive; it would be desirable to quantify the fraction of coated area and correlate it with porosity profiles (e.g., via MIP or µCT).
• Overall, the results are presented clearly through figures and tables, but a more robust quantification of temporal degradation is missing (kinetics of signal loss, comparison between “static” conditions and microcosms).

Discussion and Conclusions

• The conclusions that the coatings are promising for limestone and marble in underwater environments are supported by the data presented for the studied timeframes. However, the extrapolation to “broader industrial applications” in port infrastructures and aquatic environments, as well as the implicit reference to long-term antifouling strategies, should be expressed with greater caution, given that real biofouling is not evaluated and neither longer-term periods nor complete environmental cycles are addressed.
• It would be advisable to clarify that the main contribution is a physicochemical and durability validation in laboratory/microcosm settings, and to identify biological evaluation (microbial adhesion, biofilm formation, testing in real marine conditions) as essential future steps.
• Statements regarding potential for “broad industrial applications” and long-term behavior should be qualified, making it clear that the data are limited to 6 months under laboratory and microcosm conditions without real biological colonization.
• Future work should explicitly include biofouling tests in natural marine conditions (e.g., controlled open-sea immersions) and an assessment of potential IL release into the environment (ecotoxicity) under degradation scenarios.

Writing and Style

• The introduction is extensive and, although it provides solid context on biofouling and IL properties, it could benefit from slight condensation and from a more explicit transition toward the specific question of durability in highly porous limestone.
• The terminology for the formulations (NES, ES, TE, NES+I, ES+I, etc.) is well defined, but it would be useful to include an early table or diagram enabling the reader to follow the combinations more easily without repeatedly consulting Table 1.
• Consider including a graphical scheme summarizing the formulations, the types of tests (surface properties, UV, static washout, microcosms), and the experimental timeline, to improve overall understanding of the design.

References

• The discussion could be strengthened by incorporating additional references on methodologies for assessing coating durability under real or semi-controlled marine conditions (e.g., “field exposure racks,” immersion/dry cycles, natural biofouling), to better contextualize the limitations of the microcosms used.
• It would be useful to cite more explicitly studies that quantitatively relate porosity heterogeneity to the penetration and retention of consolidants or coatings in calcareous stones, beyond the TiO₂ case mentioned.

Comments for author File: Comments.pdf

Author Response

We sincerely thank you for your thorough evaluation of our manuscript and for your constructive and detailed comments. Your careful assessments and suggestions have improved the clarity, scientific comprehension, and overall quality of the work. We have carefully considered all comments and revised the manuscript accordingly, as detailed in the point-by-point responses below. All changes to the text are highlighted in yellow.

 

Methodology

• The description of the formulations, application conditions, and curing times is sufficiently detailed; however, it would be advisable to specify viscosity/actual coating consumption and environmental conditions during curing. Additionally, it would be useful to quantify, as far as possible, the fraction of coated area/loss of coating based on the UV and SEM-BSE images, for example through image analysis, to support the qualitative interpretations regarding heterogeneity in limestone.

We have added further details regarding the curing procedure. Environmental conditions were reported as “room temperature” because temperature and relative humidity were not monitored during the experiments. Regarding the actual product consumption, the weight gain after application was not recorded. Based on our experience, the variability associated with the applied amount is very limited under the adopted application protocol and does not significantly influence the parameters measured in this study.

We also performed image analysis using ImageJ software to quantify the reduction of UV emission and, in turn, coating loss. Data have been reported as mean percentages of three replicates for each treatment. We added a paragraph (2.6 quantification of IL detachment after washout) to materials and methods. Statistical analyses (ANOVA, TOST similarity test) have been computed to detect similarities among different conditions (see Table 3 and Supplementary material).

 

• The characterization protocols are appropriate and rely on established standards. Replication is adequate, but the manuscript does not detail statistical analyses beyond means and standard deviations, nor does it formulate prior quantitative hypotheses, which limits the statistical rigor of the comparisons. It would be desirable to include a more explicit statistical analysis (ANOVA–Analysis of Variance or significance tests) to compare treatments in terms of ΔE, contact angles, and absorption coefficients, indicating significance levels and effect sizes.

We applied Welch’s t-test on colorimetric and capillary adsorption data to detect differences between samples with binder alone and with binder+IL. We added a paragraph to materials and methods (2.8 statistical analyses) and reported p-values in supplementary material.

 

Results and Analysis

• In the durability tests, the UV images show nearly complete fluorescence retention in marble up to 6 months and the appearance of non-luminescent areas in limestone from 3 months onwards for some formulations, which the authors relate to porosity heterogeneity and preferential absorption in more open zones. This interpretation is qualitatively supported by the SEM-BSE micrographs, but the analysis remains descriptive; it would be desirable to quantify the fraction of coated area and correlate it with porosity profiles (e.g., via MIP or μCT).

As reported in the previous reply, we performed ImageJ analyses to quantify the percentage of coating loss on washed probes. Thank you for this suggestion, we think that by a μCT analysis this point can be furtherly clarified in future research to correlate quantification to the porosity of material. Overall, the results are presented clearly through figures and tables, but a more robust quantification of temporal degradation is missing (kinetics of signal loss, comparison between “static” conditions and microcosms).

 

Discussion and Conclusions

• The conclusions that the coatings are promising for limestone and marble in underwater environments are supported by the data presented for the studied timeframes. However, the extrapolation to “broader industrial applications” in port infrastructures and aquatic environments, as well as the implicit reference to long-term antifouling strategies, should be expressed with greater caution, given that real biofouling is not evaluated and neither longer-term periods nor complete environmental cycles are addressed.
• It would be advisable to clarify that the main contribution is a physicochemical and durability validation in laboratory/microcosm settings, and to identify biological evaluation (microbial adhesion, biofilm formation, testing in real marine conditions) as essential future steps.
• Statements regarding potential for “broad industrial applications” and long-term behavior should be qualified, making it clear that the data are limited to 6 months under laboratory and microcosm conditions without real biological colonization.
• Future work should explicitly include biofouling tests in natural marine conditions (e.g., controlled open-sea immersions) and an assessment of potential IL release into the environment (ecotoxicity) under degradation scenarios

We thank the reviewer for all valuable comments regarding the “Discussion and Conclusions” section, with which we fully agree. In response, we have revised the manuscript to enrich the discussion of the data, providing clarifications on the controlled laboratory conditions used in our experiments. This allows for a more explicit description of the experimental setup and the factors influencing our results.

Furthermore, we have thoroughly revised the “Conclusions” section to clarify statements regarding “broader industrial applications” and “long-term antifouling strategies.” We now explicitly indicate that real biofouling tests and extended environmental cycles were not evaluated and should be considered as essential steps for future experiments under marine conditions. The new sentences focus not only on the scope of our research and its results (see, for example, lines 484-486 - “…these findings allow the identification of the most promising coatings that can be considered suitable for further experiments in real marine conditions…”) but also on limitations of the current study and further experimental step (see, for example, lines 486-488 – “…long-term durability can be evaluated, the influence of micro- and macrofoulers can be studied and their antifouling effect can be tested. Furthermore, ecotoxico-logical studies on marine model species will be essential …”.

 

Writing and Style

• The introduction is extensive and, although it provides solid context on biofouling and IL properties, it could benefit from slight condensation and from a more explicit transition toward the specific question of durability in highly porous limestone.

We modified the last part of the introduction accordingly.

• The terminology for the formulations (NES, ES, TE, NES+IL, ES+IL, etc.) is well defined, but it would be useful to include an early table or diagram enabling the reader to follow the combinations more easily without repeatedly consulting Table 1.
• Consider including a graphical scheme summarizing the formulations, the types of tests (surface properties, UV, static washout, microcosms), and the experimental timeline, to improve overall understanding of the design.

We thank the reviewer for these two helpful suggestions on the clarity of the experimental design and formulations. We have added new figures to the manuscript to facilitate the understanding.

Specifically, a new Figure 1 has been introduced to provide a clear and comprehensive overview of the experimental design. In addition, a new Figure 2 illustrates how the different formulations were applied to the stone substrates. Finally, Table 1 explain the correspondence between treatment abbreviations and the specific formulation combinations.

Together, these additions allow readers to more easily follow the formulation terminology, experimental workflow, and testing timeline without the need for repeated cross-referencing.

 

References

• The discussion could be strengthened by incorporating additional references on methodologies for assessing coating durability under real or semi-controlled marine conditions (e.g., “field exposure racks,” immersion/dry cycles, natural biofouling), to better contextualize the limitations of the microcosms used.

We thank the reviewer for this comment. We completely revised the “Conclusion” section for better clarify the real limitations of our experimental setups used and data obtained. In addition, we include references on further experimental activities needed: long-term exposure, real marine environment conditions, formation and inhibition of fouling, ecotoxicological studies.

• It would be useful to cite more explicitly studies that quantitatively relate porosity heterogeneity to the penetration and retention of consolidants or coatings in calcareous stones, beyond the TiO₂ case mentioned.

We added some references about this issue.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Does the introduction provide sufficient background information and include all relevant references?

Yes, the introduction provides sufficient background information; however, it could be improved to reach an optimal level from a more demanding perspective. It could be strengthened by including a sentence such as: “However, there are no standardized or protocol-based studies on …”. In addition, the introduction could be improved by incorporating a more detailed mechanical–structural discussion of the substrate.

Are the methods adequately described?

Yes. The methods are described appropriately, are reproducible, and comply with international standards. However, the quantitative justification of certain parameters could be improved, such as UV irradiance (500 W/m²), aging time (507 h), and fixed IL concentration (1.14 mmol/m²), particularly in relation to environmental equivalences or previously published studies.

Furthermore, it would be useful to clarify whether any statistical criteria were applied, or alternatively, to explicitly state if no statistical analysis was performed.

Are the conclusions supported by the results?

Yes. The conclusions are supported by the results and are presented in a manner consistent with the experimental evidence. The only limitation concerns the industrial projection, which cannot be supported based on long-term behavior (beyond one year) or performance in open-sea environments.

Comments on the Quality of English Language

Could the English be improved to express the research more clearly?

Yes. The English is generally correct; however, its clarity and technical precision should be improved. Certain passages would benefit from more concise sentence structures and a more consistent use of technical terminology in order to convey the research objectives, methodology, and results more effectively.

Author Response

We sincerely thank you for your thorough evaluation of our manuscript and for your constructive and detailed comments. Your careful assessments and suggestions have improved the clarity, scientific comprehension, and overall quality of the work. We have carefully considered all comments and revised the manuscript accordingly, as detailed in the point-by-point responses below. All changes to the text are highlighted in yellow.

 

Comments and Suggestions for Authors

Does the introduction provide sufficient background information and include all relevant references?

Yes, the introduction provides sufficient background information; however, it could be improved to reach an optimal level from a more demanding perspective. It could be strengthened by including a sentence such as: “However, there are no standardized or protocol-based studies on …”. In addition, the introduction could be improved by incorporating a more detailed mechanical–structural discussion of the substrate.

We thank the reviewer for the comment. There is no strong evidence of literature describing standardized protocols to study the durability of antifouling coating applied on stone materials. We added some sentences to the introduction accordingly.

 

Are the methods adequately described?

Yes. The methods are described appropriately, are reproducible, and comply with international standards. However, the quantitative justification of certain parameters could be improved, such as UV irradiance (500 W/m²), aging time (507 h), and fixed IL concentration (1.14 mmol/m²), particularly in relation to environmental equivalences or previously published studies.

We added additional information on the aging test in paragraph 2.3, and discussed the limitations as well.  The IL concentration has been established in our previous research; we added references in Materials and Methods section.

 

Furthermore, it would be useful to clarify whether any statistical criteria were applied, or alternatively, to explicitly state if no statistical analysis was performed.

Thank you for the suggestion. We performed a statistical analysis to increase the robustness of our data. Statistical approach was described in the materials and methods (paragraph 2.8) and results have been discussed throughout the manuscript. See supplementary material for more details on statistical results.

 

Are the conclusions supported by the results?

Yes. The conclusions are supported by the results and are presented in a manner consistent with the experimental evidence. The only limitation concerns the industrial projection, which cannot be supported based on long-term behavior (beyond one year) or performance in open-sea environments.

We thank the reviewer for this observation. We agree that a speculative connection of our data with possible industrial applications must be better described. In particular, the long-term performance of the coatings beyond the duration of our study and under real marine conditions remains untested. We have revised the manuscript (see lines 481 to 496) to explicitly highlight this limitation and to indicate that further investigations will be necessary to assess durability, antifouling efficacy and no ecotoxicological effects over extended periods in open-sea environments.

Comments on the Quality of English Language

Could the English be improved to express the research more clearly?

Yes. The English is generally correct; however, its clarity and technical precision should be improved. Certain passages would benefit from more concise sentence structures and a more consistent use of technical terminology in order to convey the research objectives, methodology, and results more effectively.

We read the manuscript and improved the terminology used. Sentences have been slightly modified to be more concise and clearer.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper investigates the performance of a choline-based ionic liquid (IL) coating applied on underwater stone surfaces, aiming to evaluate its potential as an environmentally friendly anti-biofouling coating. The work is systematically conducted, covering multiple dimensions such as physical characterization of the coating, chemical stability, and medium-to-long-term underwater durability, with a reasonably designed experimental approach. The writing is generally clear, and the data provide substantial support. However, there remains room for improvement in clarifying experimental details.

Specific Comments and Suggestions

1. The description of "non-fluorescent areas" post-elution in the text is primarily based on qualitative observation. It is suggested that the authors attempt to further quantify the degree of "localized detachment" of the IL from the limestone surface using methods such as image analysis.

 

2. The assertion in the contact angle results that "high hydrophilicity is beneficial for antifouling" is advised to be further contextualized. High hydrophilicity is not invariably a positive factor in all antifouling systems. It is recommended to clarify.

 

3. A clearer justification for the chosen IL content (1.14 mmol/m²) is recommended. While this value is applied across various systems, it remains unclear whether it represents an optimized result.

 

4. Although the parameters for the UV aging experiment are provided fairly comprehensively, the approximate equivalent real-world environmental exposure time-framecorresponding to the 507 hours of artificial aging is not specified. It is suggested that the authors indicate the rough real-world exposure duration range.

 

5. The criterion for the judgment of "no significant degradation" in the IR analysis needs to be more explicit. It is recommended to specify whether this is based on peak position, peak intensity ratios.

 

6. The text uses a color change magnitude of ΔE < 5 as an acceptable threshold. It is suggested to supplement the discussion with the reference standard or literature.

 

7. While SEM and UV results indicate a weakening of the IL signal in some areas, it is unclear whether this represents complete loss or migration into internal pores. A more rigorous distinction and discussion between the phenomena of "washout" and "redistribution" is recommended.

 

8. It is advised to include in the discussion a consideration of the differences between the hydrodynamic conditions in the elution experiments and those in a real marine environment.

 

 

9. The current conclusions appear slightly optimistic. It is advisable to explicitly state that this system still requires further validation under real marine conditions and over longer timescales to maintain scientific prudence.

Author Response

We sincerely thank you for your thorough evaluation of our manuscript and for your constructive and detailed comments. Your careful assessments and suggestions have improved the clarity, scientific comprehension, and overall quality of the work. We have carefully considered all comments and revised the manuscript accordingly, as detailed in the point-by-point responses below. All changes to the text are highlighted in yellow.

 

Comments and Suggestions for Authors

This paper investigates the performance of a choline-based ionic liquid (IL) coating applied on underwater stone surfaces, aiming to evaluate its potential as an environmentally friendly anti-biofouling coating. The work is systematically conducted, covering multiple dimensions such as physical characterization of the coating, chemical stability, and medium-to-long-term underwater durability, with a reasonably designed experimental approach. The writing is generally clear, and the data provide substantial support. However, there remains room for improvement in clarifying experimental details.

Specific Comments and Suggestions

1. The description of "non-fluorescent areas" post-elution in the text is primarily based on qualitative observation. It is suggested that the authors attempt to further quantify the degree of "localized detachment" of the IL from the limestone surface using methods such as image analysis.

Thank you the reviewer for the suggestion to improve the manuscript. We performed image analysis using ImageJ software. The spots of IL detachment have been quantified as %Area from UV images. The analysis was described in the materials and methods section (paragraph 2.6), and percentages of coating detachment have been reported (see table 3). We also performed statistical analysis (ANOVA, TOST similarity test) to detect statistical differences among experimental conditions, lithotype and time of exposure to seawater.

 

2. The assertion in the contact angle results that "high hydrophilicity is beneficial for antifouling" is advised to be further contextualized. High hydrophilicity is not invariably a positive factor in all antifouling systems. It is recommended to clarify.

We agree with the reviewer that high hydrophilicity is not universally beneficial for antifouling performance, and that no direct evidence supporting such a correlation is provided in our study. Therefore, the original statement could be misleading if interpreted as a general rule. In response to this comment, we have revised the manuscript by removing this assertion.

 

3. A clearer justification for the chosen IL content (1.14 mmol/m²) is recommended. While this value is applied across various systems, it remains unclear whether it represents an optimized result.

The IL concentration has been established in our previous research, we added references in Materials and Methods section.

 

4. Although the parameters for the UV aging experiment are provided fairly comprehensively, the approximate equivalent real-world environmental exposure time-frame corresponding to the 507 hours of artificial aging is not specified. It is suggested that the authors indicate the rough real-world exposure duration range.

We added additional information on the aging test in paragraph 2.3, and discussed the limitations as well.

 

5. The criterion for the judgment of "no significant degradation" in the IR analysis needs to be more explicit. It is recommended to specify whether this is based on peak position, peak intensity ratios.

We clarified this point in Results and Discussion paragraph.

 

6. The text uses a color change magnitude of ΔE < 5 as an acceptable threshold. It is suggested to supplement the discussion with the reference standard or literature.

We added some references about this threshold.

 

7. While SEM and UV results indicate a weakening of the IL signal in some areas, it is unclear whether this represents complete loss or migration into internal pores. A more rigorous distinction and discussion between the phenomena of "washout" and "redistribution" is recommended.

Thank you the reviewer for the suggestion. The “redistribution” occurs as the stone is treated, this induces a differential amount of product between more porous (more product) and less porous areas (less product). As the washout occurs, the thinner layers of coating would leave the lesser porous areas uncovered. We clarified this point into the text and added some sentences in the section 3.2 (line 679-697).

 

8. It is advised to include in the discussion a consideration of the differences between the hydrodynamic conditions in the elution experiments and those in a real marine environment.

We thank the reviewer for this suggestion. The hydrodynamic conditions are an essential factor of our two experimental setups employed for durability tests. Thus, we better clarify the importance of hydrodynamic force in discussion section, and we modified conclusions to highlight how the real marine hydrodynamic conditions is an essential further step to validate our data.  

 

9. The current conclusions appear slightly optimistic. It is advisable to explicitly state that this system still requires further validation under real marine conditions and over longer timescales to maintain scientific prudence.

We agree that the current conclusions may appear optimistic and have therefore revised the manuscript to emphasize that the proposed IL-based coatings still require further validation under real marine conditions and over extended timescales. This clarification ensures that our laboratory findings are presented in the “conclusions” as a strong indication to select the most suitable coatings for further validation, thus accurately reflecting the scope and limitations of the study.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

In this second round of review, I consider that the authors have adequately and thoroughly addressed the recommendations made in my initial report. New figures have been incorporated, which improve the clarity and quality of the presentation of the results, and recent and relevant bibliographic references have been added, strengthening the theoretical framework and context of the work. Likewise, adjustments have been introduced in the writing and structure of the article, contributing to a clearer and more coherent presentation of the content.