Improving Colloidal Stability of Sepiolite Suspensions: Effect of the Mechanical Disperser and Chemical Dispersant

Round 1

Reviewer 1 Report

Optimizing the dispersibility of clays in water is not a trivial task, due to the multitude of parameters that can be changed (e.g. mechanical methods used, dispersant concentration, pH and initial concentration of material to be dispersed). Thus, in order to contribute to the understanding of the process of dispersing sepiolite in water, the authors report a systematic study of the dispersibility of two different types of sepiolite, where the effect three mechanical methods (magnetic stirring, high speed shearing and ultrasonication) and four molecular dispersants were assessed. The manuscript is generally well-written. However, there are several aspects that require further clarification.

After preparing the suspensions, the samples are classified according to their stability. However, the authors should define accurately what is considered a stable suspension. From the manuscript, the reader infers that a stable suspension is when no phase separation is observed (by visual inspection), but it is not defined if it is right after the mechanical treatment, or a few hours later (the time scales must be well defined in the manuscript).

In the section 3.2.3, it is not well explained which type of interactions (electrostatic interactions, hydrogen bonding, or van der Waals forces?) are established between the dispersants and sepiolite, and how the pH affects these interactions. Also, what is the effect the dispersant on the zeta potential of sepiolite (as compared with sepiolite without dispersant), and how affects the stability?

The conclusion should be written is a clearer way, in particular after line 443.

Other minor questions:

1. It is not clear why the authors claim to use crossed polarizes to observe the samples in the microscope. Crossed polarizers are used to observe birefringent materials (e.g. liquid crystals), which does not seem to be case of the sepiolite.
   
   
2. In figure 6, what is the age of the samples?
   
   
3. The paragraph starting in line 303 should be rewritten in a clearer way, and perhaps divided in several paragraphs.
   
   
4. In table 3 and 4, what is the uncertainty of the sizes of the particles obtained by the DLS. In the size distribution there is more than one population?
   
   
5. In line 355, the authors claim that particles with smaller size lead to more stable suspensions. This relation is not clear.
   
   
6. In table S1 and S2 what is the meaning of Zeta deviation? Is this any kind of uncertainty?

Author Response

-Reviewer #1:     

Optimizing the dispersibility of clays in water is not a trivial task, due to the multitude of parameters that can be changed (e.g. mechanical methods used, dispersant concentration, pH and initial concentration of material to be dispersed). Thus, in order to contribute to the understanding of the process of dispersing sepiolite in water, the authors report a systematic study of the dispersibility of two different types of sepiolite, where the effect three mechanical methods (magnetic stirring, high speed shearing and ultrasonication) and four molecular dispersants were assessed. The manuscript is generally well-written. However, there are several aspects that require further clarification.

Reply: We are thankful to reviewer for the comments on our manuscript and for having recognized the general interest of the work. Based on the reviewer comments the manuscript was improved accordingly.

 

After preparing the suspensions, the samples are classified according to their stability. However, the authors should define accurately what is considered a stable suspension. From the manuscript, the reader infers that a stable suspension is when no phase separation is observed (by visual inspection), but it is not defined if it is right after the mechanical treatment, or a few hours later (the time scales must be well defined in the manuscript).

Reply: We agree with the reviewer. The time scale is very important since a sepiolite suspension can remain stable after a few hours, but not after a few days, or after a few days but not after several months from its preparation. The time scale considered for the evaluation of the suspension’s stability was added to manuscript and it was 90 days (section 2.3. It was considered that a suspension is stable, without signs of phase separation by visual inspection, if it remains stable for at least this 90 days-period.

 

In the section 3.2.3, it is not well explained which type of interactions (electrostatic interactions, hydrogen bonding, or van der Waals forces?) are established between the dispersants and sepiolite, and how the pH affects these interactions. Also, what is the effect the dispersant on the zeta potential of sepiolite (as compared with sepiolite without dispersant), and how affects the stability?

Reply: The types of interactions involved in each system, which we believe are mainly hydrogen bonding and van der Waals forces, with CMC or alginate, were specified in the manuscript (section 3.2.3). This discussion was previously done for the polyacrylate system and now extended to the other three systems.

 

The conclusion should be written is a clearer way, in particular after line 443.

Reply: The conclusion was rewritten in order to clarify the text.

 

Other minor questions:

 

It is not clear why the authors claim to use crossed polarizes to observe the samples in the microscope. Crossed polarizers are used to observe birefringent materials (e.g. liquid crystals), which does not seem to be case of the sepiolite.

Reply: The cross polarizers were used to enhance the contrast and improve the quality of the images. We are aware that this technique is mainly used for birefringent materials, but also can help to improve the quality of the images in other systems.

 

In figure 6, what is the age of the samples?

Reply: The images (photographs and micrographs) were taken 90 days after sample preparation. This information was added to manuscript.

 

The paragraph starting in line 303 should be rewritten in a clearer way, and perhaps divided in several paragraphs.

Reply: The reviewer’s suggestion was followed and the paragraph was divided and some parts rewritten.

 

In table 3 and 4, what is the uncertainty of the sizes of the particles obtained by the DLS. In the size distribution there is more than one population?

Reply: In general, the uncertainty is low because the standard deviation among the six repetitions done for each sample is also quite small, for most of the cases below 20 nm. In order to better clarify the size distribution shape, we reported the PDI, which gives us a good idea about the particle size distribution for each sample. Also, the values reported are not the mean size or z-average because we are in presence of suspensions with relative polydispersity, which is the expected, in our opinion, due to the nature of the materials studied. We reported the Di50 of the samples which give us the value of the size comprising 50% of the particles in each sample and this parameter is not highly affected by the presence of more than one peak.

 

In line 355, the authors claim that particles with smaller size lead to more stable suspensions. This relation is not clear.

Reply: Particles of bigger size will sediment faster than particles of smaller size. For example, if we are using a simple magnetic stirrer the particle size obtained is mostly in the micron range, and thus, the suspensions obtained are very unstable and a sediment containing almost all the sepiolite particles is formed in few hours, Figure 6 suspensions a) and b). However, if we are using ultrasonication the particle size is in the range of nm, as we can see in tables 3 and 4 (Di50). Particles in the nm range do not tend to sediment quickly and we can obtain stable suspensions for long periods of time (at least 90 days, as observed), even without chemical dispersing agent.

 

In table S1 and S2 what is the meaning of Zeta deviation? Is this any kind of uncertainty?

Reply: Yes, the zeta deviation reported is the uncertainty of the zeta determination. Each measurement has an uncertainty associated: the reported zeta potential value is the average value obtained for the six repetitions done for each sample.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript "Improving colloidal stability of sepiolite suspensions: Effect of the mechanical disperser and chemical dispersant" presents the preparation and characterisation of water-based suspensions of two kinds of sepiolite and analyses the effects of different mechanical treatments and dispersant agents. The work is of general interest in the field of sepiolite composites preparation and it presents numerous data acquired with many different characterisation techniques. In my opinion the quality of the scientific work is good but presentation needs serious improvements. In particular the zeta potentials data are not sufficiently discussed, and some of the paragraphs in the results part are not clear.

 

In the following some specific aspects to be addressed:

 

1. Introduction is well written and clear. Only the literature cited in the last paragraph is a bit misleading: did the authors refer to a specific subset of sepiolite composites? If not there are many other papers concerning preparation and characterisation of sepiolite-polymer composites (see for examples the reviews "Polymer nanocomposites from modified clays: Recent advances and challenges" Kotal, M., Bhowmick, A.K. 2015,Progress in Polymer Science, and "Clay composites for thermal energy storage: A review" Voronin, et al 2020 Molecules). I recommend rephrasing in order to limit the filed of the literature cited, if this is the case, or include more references showing the broad applications of sepiolite-polymer composites.

 

2. Section 3.1 line 249 the authors state that dispersions of sepiolite 2 has much higher viscosity than those of sepiolite 1. Do they measure viscosity? It would be good to report viscosity data, since they could affect DLS measurements.

 

3. In section 3.2 it is not clear how the stability of suspensions was evaluated. For example how long after the preparation have the pictures of figure 6 been taken? Are all the homogenous phases obtained stable for the whole period of observation (60 days as reported in the methods? Or 3 months as stated in section 3.2.2 )? If yes specify. In the discussion, please distinguish between the obtainments of homogenous/well dispersed phases and their stability in time. For example rephrase description of figure 6 in terms of homogeneity instead of stability, and avoid expressions like “become stable (only one phase) pag 11 line 319”

 

4. Throughout section 3.2 , the zeta potential data are not discussed much. In particular there is not an interpretation on how those data can indicate the stability of the suspensions. How do they support the conclusions?

 

5. Paragraph 3.2.2. The reported percentages are not of direct comprehension, I suggests to report the expression “6 suspensions over the 15 investigated” instead of “40% of the suspensions”. Moreover, is the phrase on the particle re-aggregation supported by the presented data? If yes please specify.

 

6. Paragraph 3.2.3. line 387: On the other hand, poorer results were obtained with  alginate, being this biopolymer less effective to stabilize sepiolite suspensions in water (under the  tested conditions). 
Was this behaviour known before? If not, please rephrase ( for example … the data indicate that this biopolymer is less effective to stabilize… )

 

7. I suggest to revise the language, in particular verb tenses. As general rule, use present perfect to describe findings that are always true: for example, line 222 are instead of were, line 222 are instead of were, line 234 are present instead of appeared, line 260 is instead of was, lines 277-280, ecc.

Other refuses:

Line 209 “It was found some Al and Fe”  change in “Al and Fe were found”

Line 218  “attributed  due to release” choose attributed or due

Line 231 “occurs a band” change in “a band occurs”

Line 403 “it has been reported the aggregation of alginate polymer and gelation” change in “the aggregation and gelation of alginate polymer has been reported”

Line 425 “Is important” change “It is important”

 

Moreover address some issues in the tables and figures captions:

 

Caption of Table 2. Please specify which physicochemical data are reported, how they are measured and define the abbreviations (D (4,3), D10, D50, D90).

Caption of Figure 6. Please specify how the images in the insets were obtained (it is written in the main text but the caption should be self-explaining).

Caption of Table 3 and 4. Define D0.5

 

Author Response

-Reviewer #2:     

The manuscript "Improving colloidal stability of sepiolite suspensions: Effect of the mechanical disperser and chemical dispersant" presents the preparation and characterisation of water-based suspensions of two kinds of sepiolite and analyses the effects of different mechanical treatments and dispersant agents. The work is of general interest in the field of sepiolite composites preparation and it presents numerous data acquired with many different characterisation techniques. In my opinion the quality of the scientific work is good but presentation needs serious improvements. In particular the zeta potentials data are not sufficiently discussed, and some of the paragraphs in the results part are not clear.

Reply: We are grateful to reviewer for the comments on our manuscript and for having recognized the general interest of the work. Based on the reviewer comments the manuscript was improved accordingly.

 

In the following some specific aspects to be addressed:

Introduction is well written and clear. Only the literature cited in the last paragraph is a bit misleading: did the authors refer to a specific subset of sepiolite composites? If not there are many other papers concerning preparation and characterisation of sepiolite-polymer composites (see for examples the reviews "Polymer nanocomposites from modified clays: Recent advances and challenges" Kotal, M., Bhowmick, A.K. 2015,Progress in Polymer Science, and "Clay composites for thermal energy storage: A review" Voronin, et al 2020 Molecules). I recommend rephrasing in order to limit the filed of the literature cited, if this is the case, or include more references showing the broad applications of sepiolite-polymer composites.

Reply: In fact, the idea was to refer a specific subset of sepiolite composites, i.e., composites incorporating the chemical dispersants addressed in the present work (poly(sodium acrylate), CMC, alginate) or other polysaccharides in general (starch, nanofibrillated cellulose). The reviewer’s suggestion was followed and the scope of the composites under consideration was better specified by rephrasing the last paragraph of the text.

 

Section 3.1 line 249 the authors state that dispersions of sepiolite 2 has much higher viscosity than those of sepiolite 1. Do they measure viscosity? It would be good to report viscosity data, since they could affect DLS measurements.

 Reply: We totally agree with reviewer. The viscosity of the suspensions could affect the DLS measurements. However, it is important to note that the DLS measurements were done with a very low sepiolite concentration, 0.1wt% (see section 2.4, second paragraph), to avoid undesired effects, as viscosity interference or possible entanglements or association between sepiolite crystals. In a future study we intend to present some viscosity data of selected formulations used to enhance the sepiolite dispersion.

 

In section 3.2 it is not clear how the stability of suspensions was evaluated. For example how long after the preparation have the pictures of figure 6 been taken? Are all the homogenous phases obtained stable for the whole period of observation (60 days as reported in the methods? Or 3 months as stated in section 3.2.2 )? If yes specify. In the discussion, please distinguish between the obtainments of homogenous/well dispersed phases and their stability in time. For example rephrase description of figure 6 in terms of homogeneity instead of stability, and avoid expressions like “become stable (only one phase) pag 11 line 319”

 Reply: The images (photographs and micrographs) of figure 6 were taken 90 days after sample preparation. This information was added to manuscript. The homogeneous phases reported in the manuscript (Table 3 and Table 4, where no clear phase separation was observed) were the ones obtained stable for at least 90 days, the whole period of observation. The observation period was also corrected in section 2.3 (materials and methods). The description of figure 6 was rewritten in terms of “homogeneity” and the expression “only one phase” was replaced by “homogenous suspension”.

 

Throughout section 3.2 , the zeta potential data are not discussed much. In particular there is not an interpretation on how those data can indicate the stability of the suspensions. How do they support the conclusions?

 Reply: It is known that low absolute zeta potential values are a driving force for flocculation and instability of the particles in solution. Thus, an increase in the absolute value obtained by pH increment or by addition of chemical dispersants will be favourable to enhance the stability of the suspensions. This trend is clearly observed in our systems with an increase in the number of stable suspensions at pH 8.0 and 12.0 compared with that at pH 3.0. Also, the addition of chemical dispersants such as polyphosphate or CMC led to higher zeta potential values and resulted in more stable suspensions. These observations were added to manuscript.

 

Paragraph 3.2.2. The reported percentages are not of direct comprehension, I suggests to report the expression “6 suspensions over the 15 investigated” instead of “40% of the suspensions”. Moreover, is the phrase on the particle re-aggregation supported by the presented data? If yes please specify.

 Reply: The reviewer’s suggestion was followed and the expression was changed. Additionally, the increase in pH resulted in a trend toward the increase of the absolute zeta potential value and it is known that higher zeta potential values are favourable for the particle’s dispersion. This information was added to the paragraph in section 3.2.2, as well as a reference supporting this information.

 

Paragraph 3.2.3. line 387: On the other hand, poorer results were obtained with  alginate, being this biopolymer less effective to stabilize sepiolite suspensions in water (under the  tested conditions). 
Was this behaviour known before? If not, please rephrase ( for example … the data indicate that this biopolymer is less effective to stabilize… )

 Reply: From the best of our knowledge this behaviour was not known before. The reviewer’s suggestion was accepted and the phrase was rewritten.

 

I suggest to revise the language, in particular verb tenses. As general rule, use present perfect to describe findings that are always true: for example, line 222 are instead of were, line 222 are instead of were, line 234 are present instead of appeared, line 260 is instead of was, lines 277-280, ecc.

Reply: The reviewer’s suggestion was accepted.

 

Other refuses:

 

Line 209 “It was found some Al and Fe”  change in “Al and Fe were found”

Reply: The suggestion was accepted.

 

Line 218  “attributed  due to release” choose attributed or due

Reply: The phrase was corrected.

 

Line 231 “occurs a band” change in “a band occurs”

Reply: The phrase was corrected.

 

Line 403 “it has been reported the aggregation of alginate polymer and gelation” change in “the aggregation and gelation of alginate polymer has been reported”

Reply: The phrase was rewritten according to the reviewer’s comment.

 

Line 425 “Is important” change “It is important”

Reply: The phrase was changed.

 

Moreover address some issues in the tables and figures captions:

Caption of Table 2. Please specify which physicochemical data are reported, how they are measured and define the abbreviations (D (4,3), D10, D50, D90).

Reply: The physicochemical data reported are particle size and specific surface area of the sepiolite raw materials (powders). The parameters considered were defined in the footnote of the table. D (4,3) is weighted mean value of particle size by volume, D10 is the point in the size distribution, up to and including which, 10% of the total volume of material in the sample is ‘contained’, D50 is then the size point below which 50% of the material is contained (weighted in volume), D90 is that size below which 90% of the particle size of the material is contained (by volume). The abbreviations (D (4,3), D10, D50, D90) were modified to Dv(4,3), Dv10, Dv50 and Dv90 to become the abbreviations more precise. Additionally, a few changes were made in the paragraph below regarding discussion of size results in table 2, in order to complete the discussion.

 

Caption of Figure 6. Please specify how the images in the insets were obtained (it is written in the main text but the caption should be self-explaining).

Reply: The insets in Figure 6 were obtained by microscopic observation of the suspensions. This information was added to Figure 6 legend.

 

Caption of Table 3 and 4. Define D0.5

Reply: The D_0.5 is the size distribution point (diameter), by intensity, below which 50% of the material particle size is contained. Also, it can be denoted D50. In order to avoid misunderstandings, the D0.5 in tables 3 and 4 was changed to Di50. The definition was added to the footnote of the tables.

Author Response File: Author Response.pdf