Application of Terahertz Time-Domain Spectroscopy to Study the Microheterogeneities of Solutions: A Case Study of Aqueous Sugar Solutions
Round 1
Reviewer 1 Report (Previous Reviewer 1)
The author has incorporated the suggestions and significantly improved the manuscript. Particularly noteworthy is the inclusion of the difference spectra plot, Figure 3, which adds a substantial level of depth to this study.
Line 434 in the conclusion section I am still not very happy with the word "for the first time".
Author Response
The phrase "for the first time" was removed from the Conclusion.
Reviewer 2 Report (Previous Reviewer 3)
Accept after minor revision (corrections to minor methodological errors and text editing
Comments for author File: 
Comments.pdf
Minor editing of English language required
Author Response
I read the author's respond and still have some questions.
(1) “Figure 3. Difference spectra of DS of molecular solutions and solutions containing MH.” should be revised “Figure 3. Difference spectra of DS of molecular solutions containing MH.”
Author’s response:
I am ready to agree that my caption to this figure may be unclear. However, the caption proposed by the reviewer is also not entirely accurate. I changed it as follows:
“DS difference spectra: Subtracting the spectrum of a solution containing MH from the spectrum of a molecular solution.”
(2) The authors declare that “From the difference spectra of Figure 3, it can be seen that for all three sugars, with a decrease in frequency, molecular solutions exhibit an increase in dielectric losses (ε’’) compared with solutions containing MH……”, what does "it" mean in this sentence? Is it the curve in Figure 2 or the curve in Figure 3? “Compared with solutions containing MH” means what? If it means the curve in Figure 3, the spectra for all three sugars didn’t go down in frequency such as around 90 cm-1 and molecular solutions didn’t exhibit an increase in dielectric losses (ε’’).
Author’s response:
I thank the reviewer for pointing out inaccurate formulations. This sentence has been modified as follows:
The difference spectra shown in Figure 3 indicate that with a decrease in frequency, starting from about 40 cm-1, the molecular solution exhibits a stronger increase in dielectric losses than the solution containing MH.
(3) It is necessary for the authors to provide absorption spectra.
Author’s response:
Now Figure 2 contains the transmission spectra, as well as the refractive index spectra, from which DS were calculated.
A number of English edits were made in the text.
Reviewer 3 Report (Previous Reviewer 2)
The revised manuscript, compared to the original manuscript, gives a better clarification and presentation of the difference of complex dielectric spectra for glucose, fructose, sucrose solutions. The manuscript is improved, and my comments in the previous review reports are addressed.
I suggest publication after some minor modifications.
(1) I feel that Fig. 2 and Fig. 3 can be combined into one figure. The data are relevant, and it would be clearer to see them in the single page.
(2) In Line 332 the author mentions the difference in ε’’ for fructose solutions at 20 cm-1 is 0.15. However, from Fig. 3 the ε’’ difference is only 0.05. Please clarify.
The English writing is good.
Author Response
The revised manuscript, compared to the original manuscript, gives a better clarification and presentation of the difference of complex dielectric spectra for glucose, fructose, sucrose solutions. The manuscript is improved, and my comments in the previous review reports are addressed.
I suggest publication after some minor modifications.
(1) I feel that Fig. 2 and Fig. 3 can be combined into one figure. The data are relevant, and it would be clearer to see them in the single page.
Author’s response:
These figures were combined into one figure 3.
(2) In Line 332 the author mentions the difference in ε’’ for fructose solutions at 20 cm-1 is 0.15. However, from Fig. 3 the ε’’ difference is only 0.05. Please clarify.
Author’s response:
This is my typo. I didn't mean 20, but 10 cm-1. This mistake has been fixed.
A number of English edits were made in the text.
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
Title:
1. The title of the article lacks clarity and readability. If I wrote the paper I might pick slightly modified title for this article such as
“Application of Terahertz Time-Domain Spectroscopy to Study the Microheterogeneities of Solutions: A Case Study of Aqueous Sugar Solutions".
Introduction:
1. Line 118: “The most sensitive method to hydrate shells is terahertz (THz) spectroscopy “. This sentence is misleading. It can be modified as “THz spectroscopy is one of the efficient and sensitive tool that can successfully probe the hydration shell of molecules”
2. Line 120: “In our work” should be replaced by “In our previous work”
Methods:
1. Line 153, ? is wavenumber or frequency?
2. Did the author use two cuvettes (50/ 100 micron thick), or a 50/100 micron thick spacer in between two quartz windows?
Results:
1. Figure 1 ensures that there are two size distributions. The larger size is perhaps for molecular aggregations which are not present after filtration. However all the solutions contain only 0.5 mol% of sugar. Can someone expect aggregation in such a low concentration?
2. The author stated that “the low-size peak in the distributions in Figure 1 refers to individual sugar molecules with some hydrate shell.” – To me this is an important statement, please provide proper references.
3. Line 309: Please clarify the procedure:
The DS of the aqueous phase solutions of 0.5% ...... are fitted with a model provided by equation3. The fitted parameters are shown below.
4. What is n, % in table 1? Is it the free water fraction as calculated with equation: Please clarify.
Discussion:
5. Please rewrite line 31.
As follows from Figure 1, before and after filtration, we have different solutions that differ in the presence of MH.
6. Iine 324: In figure 2, only Fructose shows a mild difference in the real component. Other data are exactly overlapped with each other.
7. Line 333: “a decrease in Δε1 indicates an increase in the binding of water molecules.” – needs proper references.
8. Line 338-339: “The last parameter describes the contribution of intermolecular vibrations of water molecules bound by hydrogen bonds to the overall dielectric response.” – Proper references are missing.
9. Line 350-352: “That is, the structure of water inside the MH of fructose solution is not homogeneous, and contains both areas of increased water connectivity and areas of destroyed structure”
What does it mean? I really cannot understand what is the structure of water inside the MH of fructose?
10. Line 353: “no manifestations of the destroyed structure of water in MH of glucose and sucrose solutions not detected.”
Line 355: “no manifestations of the destroyed 353 structure of water in MH of glucose and sucrose solutions not detected.”
– Meaning is not clear for these sentences.
11. Line 374, what is generalized hydrate water?
12. Figure 3 must be modified. Some suggestions: put the numbering a, b, c inside the respective curves, increase the font size of the a parameters, increase the point size of the data points.
13. In the conclusion the author claimed that this is the first study regarding MH, by THz spectroscopy. However that is not correct. For an example, Chakraborty et al., J. Mol. Liq., 2023, 382, 121998 this paper shows the difference in the hydrogen bonded structure in micro heterogeneous and phase-separated mono alcohol aqueous solutions.
General comments:
The Definition and Presentation of "Microheterogeneities" in the Article:
In this article, it is important for the author to provide a clear definition of the term “microheterogeneities”. Anything that is not homogeneous are considered heterogeneous. However it can be structural or dynamical or morphological heterogeneity. In my understanding, the author could use the terms “aggregation” or “condensate” instead of heterogeneity which would make it easier for the reader to understand the message of the paper.
Suggested Improvements for the Clarity and Supporting Evidence:
This paper can be published if the conclusions are supported by additional experiments or computational studies. In my opinion only figure 1 conclusively distinguishes the presence of MH in the solutions. However, the most crucial data are presented in figure 2, which does not clearly demonstrate distinct behavioural differences.
Suggested Request for Text Revision:
The text in the paper is currently quite challenging to comprehend in terms of scientific content. Nevertheless, I believe that rectifying this issue can be accomplished with relative ease. I kindly request the author to consider rewriting the text in a more accessible manner.
The structure of some sentences are little complex, and hence hard to understand the underline meaning. Those can be easily improved. Apart from the structures it is fine.
Reviewer 2 Report
This manuscript uses the terahertz time-domain spectroscopy, for the first time, to study the microheterogeneities of glucose, fructose, and sucrose solutions. The sugar hydration parameters are calculated from the dielectric spectra of different solutions, in the forms of microheterogeneities and molecular solutions. The water bindings in all three solutions are decreased in the microheterogeneities form. For the microheterogeneities of fructose solution, the author observed the additional decrease in the number of hydrogen bonds between water molecules and an increase in the number of free water molecules. This work demonstrates that terahertz time-domain spectroscopy can provide new information about the microheterogeneities of aqueous solutions.
This work is interesting and solid. There are a number of issues or questions that I believe should be addressed before publication.
(1) While it is true that the crystalline sugar film is different from the sugar in the solution, why the amorphous sugar film is equivalent to the sugar in the solution in terms of the DS?
(2) Is the terahertz radiation that interacts with the sample collimated radiation or focused radiation? If it is a focused radiation, the spectra difference of two samples with two sample thicknesses may not be equal to the spectrum of the 50.2-um sample only.
(3) It would increase the readability for broader readers if you could provide the frequency values in the unit of Hz besides the unit of wavenumber.
(4) The unit of the calculated f in Line 200 seems not right to me. After multiplying three items, I got cm^6% but not vol%.
(5) Line 96: what is “HPLC”?
(6) Line 108: what are SANS and SAXS?
(7) What is “об%” in Line 203?
The manuscript is well written.
Reviewer 3 Report
Comments for author File: Comments.pdf
