Research on the Particle Growth Process of Colloidal Silica Derived from the Sol-Gel Process Using Active Silicic Acid Solutions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript submitted by K. Higuchi et al. with the title „Research on the particle growth process of colloidal silica derived from the sol-gel process using active silicic acid solutions“ presents a detailed study on the influence of different synthesis parameters on the sol-gel preparation of SiO₂ particles. In contrast to the Stöber method, the alternative synthesis via active silicic acid solutions is known to result in particles of higher density, but the tuning of particle sizes is more complicated and much less knowledge exists on the variation of the primary particle size. In this respect, this study is of interest, as the influence of different reaction parameters is studied and discussed. However, in the present form the manuscript would be of little value to the scientific community, as the study is not clearly laid out, the text is often confusing and the data and gained insights are not presented in a concise and easy-to-follow way.

The main problem of this study is that the targets and the experimental approach to reach these targets are not laid out clearly. Whilst the introduction describes the general problems well, it ends abruptly with only one sentence stating that the study aims to investigate the silanol content. However, a correlation between experimental conditions and the silanol content in form of the reaction equations is only stated on p. 6 in the discussion section so that the reader only then can really understand what is the purpose of the variations of the experimental conditions.

What adds to the confusion are very rudimental captions for Tables and a non-uniform way of presenting the data. Captions like “Data in the literature” or “Comparison from the view point of particle shape” are not of much help for the reader and lack scientific precision. The results for samples 4-8 are presented in Table 2 with the samples arranged in columns and the different parameters as lines, but in Table 3 for samples 9+10 this is reverted and a third parameter for the pH is stated… Also Table 5 seems not to be of much help for understanding. All of these need to be revised and labelled properly so that the readers understand their purpose. For example, in Table 1 the label “Items to confirm their influence” is not clear to the reader – that should be changed so that it becomes clear that Samples 1-3 show a variation in the minimum pH (maybe clearer than “the lowest pH” as the authors use, also this would help avoiding strange phrases such as “the optimum lowest pH”), etc. Additionally, whilst providing proper citations, the authors partially present literature data in the tables and a “projected” literature value – this might be discussed in the text, but to present such a re-calculated value in a table appears scientifically doubtful.

The main target parameter, the primary particle size, was determined based on the specific surface area. As the SEM images however show that partially, some particles appeared to be fused (e.g. Fig. 4, Exp. 4,5), these data must at least be compared to particle sizes determined from the SEM images and possible errors/deviations discussed.

The authors also stay unclear about parameters such as the particle growth rate. They only calculated a “relative particle growth rate”, which is volume-based, but do not state how they actually define the growth rate. The statement “The particle growth rate is expected to be proportional to the silica deposition rate.” highly depends on the definition of the growth rate – if it is volume-based than that statement is clear by definition (if the growth of particles occurs only via “silica deposition”). Also, wouldn’t it be possible to calculate (under certain assumptions) a growth rate and state these values rather than just relative values?

The cited literature at present appears insufficient and other studies on the silicic acid-based synthesis of silica and investigations of the reaction mechanisms need to be considered to derive a more general understanding of the reaction process (e.g. DOI 10.1007/s11051-005-9047-4, 10.1007/s10934-011-9549-7, 10.1016/j.mee.2009.10.005, 10.1007/s10973-010-0697-9).

Comments on the Quality of English Language

The language of the manuscript needs improvement. Whilst the language quality overall is fine, there are many errors and complicated/imprecise phrases used. For example, the authors generally speak of the “denseness” of the silica particles, but in the conclusions section they use the term “density”. They state (line 312) that “the addition amount of initial amine” is an influencing factor – but it should be specified more clearly as “the amount of initially added amine”.

Author Response

Thank you for reviewing our manuscript. We sincerely appreciate your valuable feedback.

Please find attached our responses to your comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript (Solids-3554864) describes a series of investigations into the effects of initial acid concentration and pH on the particle growth behavior of active silicic acid solutions.  The goal of this research is to better understand how particle grow behavior depends on pH and acidity and use this understanding to optimize the preparation of colloidal silicas useful to industrial processes.

I believe the results reported here are of interest to the scientific community and recommend publication after some revisions.

The manuscript is written as a compromise between a recipe for industrial processes chemists and a scientific article.  By this I mean there are a lot of broad conclusions based on very little data that leave me thinking, “this might be the explanation but there are many other factors that should be considered before I am convinced this is the correct explanation”.  Additionally, there are some ambiguities that should be addressed before publication.  More specifically:

1. The acidity (amount of acid added initially) and pH are critical parameters in the research described here. Unfortunately, the descriptions in the text and the experimental procedures confuse me as to the pH of what solution is being described (the initial active silicic acid solution (ASAS in manuscript) or the colloidal silica reaction solution (ASAS is added to a water tetramethyl ammonium hydroxide (TMAH) solution to induce seed particle formation and growth).  This ambiguity could be easily addressed by being more specific when in the text by stating “the pH of the ASAS solution or reaction solution”.
2. While I have investigated silicates in the past, I am not familiar with the part of the procedure involving multiple additions of the ASAS solution to the reaction solution. I suggest that authors add a bit more description of this.  My “guess” is that after the first ASAS addition, water and other volatiles are removed by distillation which reduces the reaction solution volume thus allowing subsequent additions of ASAS solutions.  But this is a guess. 

At the end of the manuscript (p.10; 293) there is the sentence, “Therefore the addition of acid to control PPD is only necessary for the first preparation of ASAS.”  This is one of the important conclusions/recommendations of the research reported on here.  Thus, I think the authors should make it as clear as possible how these sequential additions of the ASAS solution work in both their experiments and in the context of process parameters.

3. I suggest that authors consider moving equations (1) and (2) to much earlier in the article and discussing generally the role of acid on the hydrolysis and condensation reactions on the formation of silica seed particles and subsequent growth into colloidal silica particles. This would give readers a much clearer idea of what is actually going on and what is being investigated in their studies.  I admit this is my bias – I like to have some idea of what is going on “underneath” (i.e. fundamental chemical reactions) the 10 experiments presented in Table 1 where all I see are the phenomenological variables that are changed.
4. Table 3: I suggest that the authors change the column heading from “Items to confirm their influence” to simply “Comments”.  Furthermore, it might help to group (using the comments column?) different experiments based on what variable is being investigated in a set of experiments.  (This might be easier said than done as different combinations of experiments show the affects of different experimental variables, so consider this a suggestion).  Additionally, in Table 3 the use of ibid. is incorrect.  Please change this.
5. Minor point: units on SSA just below Table 3 do not seem correct.  Please check.
6. The notion that the ASAS solution is “unstable” is somewhat peripheral to the main topics of the article but this also is important in any application of the results reported on here. It would be helpful to me as a reader for the authors to provide a bit more context about this instability.  For example, the authors describe how they attempted to prevent overheating of the the ASAS solution after the addition of TMOS.  It would be helpful to monitor the solution temperature as TMOS is added and provide an actual temperature range for the mixing procedure due to this concern.
7. Minor point: on p. 1 of the manuscript there is a reference to figure 1 in the txt (line 38).  Figure 1 has nothing to do with what is being discussed in the txt at this point.  The authors should look at this.
8. On page 5 of the manuscript the first text reference to the Si NMR integration data is made (presented in Table 2, p. 4). However, the experimental context for these data is not presented until p. 8 of the manuscript.  Thus, a reader such as I is left wondering (a lot) about these data until late in the article.  The experimental details of the SSNMR measurements should be moved forward in the manuscript into the materials and methods section so that the reader has some idea of how NMR integration values were obtained.  (I’m not doubting the author’s procedures, I just think that it would help readers to have more confidence in these data if they had experimental details earlier in the manuscript.)
9. On p.5 of the manuscript the authors refer to an “ancient” report in the literature (reference 8 from 1950) as to how they determined the “acid concentration” of the solutions in this study. The “acid concentration” and its relationship to pH are probably the most important parameters investigated in this study.  I suggest the authors provide more description of this parameter directly in the article.
10. On p. 7, about line 193, there are a series of sentences that the word “may” several times. The use of this word in this context is to be avoided.  The reader is left wondering if anything that is stated has any validity at all.  “It may be true, but it may not be true.”  I urge the authors to rewrite this passage
11. On page 8, line 219, the sentence “Exp. 7 corresponds to example 3 in the literature.” I suggest the authors not make the reader go consult example 3 in the literature to see what Exp. 7 corresponds to.  Just use a few words to tell the reader what Exp. 7 is and cite lit. reference [3].
12. 8, line 230, suggest change from “seemed not to be substantial” to “in reasonable agreement”.
13. The authors use the word “augment” in several places in the manuscript (line 275; p.8). Suggest authors consider using increase in most of these instances.

In summary I think the results described here are valid.  As described above, I think parts of the manuscript could be rearranged and context added to make it easier to read and understand.

 

Publish after revision.

Author Response

Thank you for reviewing our manuscript. We sincerely appreciate your valuable feedback.

Please find attached our responses to your comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors report on the sol-gel synthesis of colloidal SiO2 from solutions of ASAS obtained by hydrolysis-condensation of TMOS, with the aim of controlling primary particle size and silica growth by changing amount of acid and pH, and providing solutions to technical problems in the production of colloidal silica.

I found this manuscript extremely difficult to read: the aim of the research is clear but Materials and Methods section does not contain a clear description of the experiments, and the description of the Results is lacking and even less clear.

What makes these problems worse is the fact that description and discussion of results always refers to aspects cited in not easily accessible literature, as 7 out of 11 references are patents.  Due to this issue, it is also not easy to assess the novelty and originality of the article.

Therefore, the article needs to be carefully revised, starting with providing clearer information in the introduction, referring also to the literature readily available to readers, considering that the parameters addressed in this research have long been studied by researchers in the field. In Section 4, catalyst-dependent hydrolysis-condensation-redissolution rates, and colloids growth should be discussed taking into account pivotal studies available in the literature.

The experiments must be described referring to Table 1; "the lowest pH" parameter must be clarified.

Methods section must be improved reporting the acquisition parameters, in particular for NMR. Parameters of CPMAS experiments must be optimized to assume that results could be considered quantitative with different proton environment (Figure 6); the use of hexamethylcyclotrisiloxane as an internal standard for Q band quantification also rises some questions. Please explain why peak integration gives the density of colloidal silica particles.

Please also consider the following points:

Were the results of Figure 2 only visually evaluated?

Errors should be added to PPD and SPD in Tables 2, 3, 6 and Figures 3 and 5.

Please add size bars to micrographs in Figure 4.

In Table 9 is it assumed as seed particle diameter the one of a POSS cage?

Particle growth rate (Table 6): according to equation 2 it looks like a growth factor not a rate.

Please improve and clarify the discussion on the silica deposition rate.

In the conclusions technical solutions for the production of colloidal silica are highlighted, but they are not effectively underlined in sections 3 and 4.

Comments on the Quality of English Language

English needs to be improved.

Author Response

Thank you for reviewing our manuscript. We sincerely appreciate your valuable feedback.

Please find attached our responses to your comments.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have clearly improved the manuscript according to the reviewer's suggestions.

I do not agree with all provided arguments - in particular, that the author state additional mentioned works are not relevant to their study and so they do not cite further literature - this low citation of literature renders their work highly specialized and possibly limits visibility and citations. Nevertheless, the work is now publishable.

Author Response

Comment: The authors have clearly improved the manuscript according to the reviewer's suggestions.

I do not agree with all provided arguments - in particular, that the author state additional mentioned works are not relevant to their study and so they do not cite further literature - this low citation of literature renders their work highly specialized and possibly limits visibility and citations. Nevertheless, the work is now publishable.

→Response: We sincerely thank you for your thoughtful review and for recognizing the improvements made to the manuscript. We truly value the time and care you’ve invested in evaluating our work.

Regarding the additional literature, we carefully considered your suggestion and, while we respectfully maintain that the referenced works fall outside the specific scope of our study. We understand and appreciate your concern about the potential impact of a more specialized focus. Your feedback has prompted valuable reflection, and we’re especially grateful for your support in recommending the manuscript for publication.

Reviewer 2 Report

Comments and Suggestions for Authors

see submitted pdf file with my comments 

Comments for author File: Comments.pdf

Author Response

Thank you very much for detailed and thoughtful feedback.

Please find our responses in the attached PDF document.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors improved the manuscript by taking into account most of the comments. 
In particular, reorganization of the presentation and discussion of the results allows for better understanding and easier reading of the article.

Author Response

Comment: The authors improved the manuscript by taking into account most of the comments. 
In particular, reorganization of the presentation and discussion of the results allows for better understanding and easier reading of the article.

→Response: Thank you very much for your positive feedback. We’re truly grateful that you took the time to engage thoughtfully with our work throughout the review process. We’re pleased to hear that the reorganization of the manuscript has improved its clarity and readability, as this was a key focus of our revisions. Your insights and support have been invaluable in shaping the final version of the paper, and we deeply appreciate your contribution.