Heat Transfer Fluids Based on Amino-Functionalized Silica Dispersed in 1,2-Propylene Glycol and in 50-50 Aqueous 1,2-Propylene Glycol

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

It is not clear why the authors want to study PG based nanofluids. They first claim EG is not particularly toxic, and then they claim they want to study PG because of the toxicity of EG.

The authors claim all the solutions show similar stabitilty after one month. However, from the pictures of disperstions after one month in Fig. 3, they clearly show different color and there exhbits precipitation at the bottom. The authors need to provide more systematic characterization.

Comments on the Quality of English Language

none

Author Response

It is not clear why the authors want to study PG based nanofluids. They first claim EG is not particularly toxic, and then they claim they want to study PG because of the toxicity of EG.

We changed the wording to clarify the goal. The current version is

Moreover, it is not flammable, moderately toxic, and inexpensive. This is why EG and its mixtures with water are attractive alternatives to water as components of heat transfer nanofluids. Increasingly higher requirements regarding product safety inspire the research towards less toxic alternatives of common heat transfer fluids.

The authors claim all the solutions show similar stabitilty after one month. However, from the pictures of disperstions after one month in Fig. 3, they clearly show different color and there exhbits precipitation at the bottom. The authors need to provide more systematic characterization.

We agree that the alternation visible as coloration is a serious limitation of our heat transfer fluids. We clearly stated this in the main text. To further emphasize this limitation we added the following in the conclusions.  

 

Yellowish coloration of dispersions aged at 80^oC indicate physico-chemical changes in the system, which are not detected by z potential or particle size measurements.

Reviewer 2 Report

Comments and Suggestions for Authors

The presented paper concerns nanofluids' stability and electrokinetic properties based on a mixture of propylene glycol and water with functionalized silica, so the present title could be misleading since there were no studied thermal properties. Similarly, the statement in the conclusions that the fluid is suitable for use in heat transfer systems without any tests of thermal properties is only an optimistic assumption. The authors should present experimental results of thermal properties to confirm this assumption. Moreover, there are a few minor issues that require attention:

1. Authors should use "hydrodynamic diameter" instead of "particle size" for results obtained from zetasizer. Additionally, in lines 95-99 Authors state that using a zetasizer is not always appropriate to the studied system, but in their case is good and presents the actual size of the particle. Such a statement should be explained and supported by other, more relevant techniques of particle size characterization.

2. In line 88 is a reference error.

3. How electrical conductivity was designated or calculated?

4. In legends of figures 4 - 13 should be added units of temperature.

5. What was the load of amino-functionalized silica in the base fluid?

Author Response

The presented paper concerns nanofluids' stability and electrokinetic properties based on a mixture of propylene glycol and water with functionalized silica, so the present title could be misleading since there were no studied thermal properties. Similarly, the statement in the conclusions that the fluid is suitable for use in heat transfer systems without any tests of thermal properties is only an optimistic assumption.

Regarding the title. A phrase “heat transfer fluids” is used to facilitate the literature searches.   We believe that our research may be interesting for the community engaged in heat transfer fluids rather than for other researchers. Moreover the phrase “heat transfer fluids” is a short and understandable description of our systems. Multiple use of more detailed description of the system like “dispersions of amino-functionalized silica in aqueous PG” would make the text boring and difficult to read.

 

The authors should present experimental results of thermal properties to confirm this assumption. 

We do not have any left overs of our powder to conduct extra experiments. Our assumption that the dispersion will have a higher heat conductance than solvent alone is based on hundreds of studies with different solids dispersed in different liquids. Each time the thermal conductivity of dispersion was higher than that of pure solvent, so why should our systems behave differently? Nevertheless we added the following explanation.

The thermal conductivity of our dispersions was not studied experimentally but basing on the research cited in Table 2 and on multiple studies involving solvents other than PG and different solid particles it is supposed that the thermal conductivity of our dispersions is higher than that of pure solvents

 

Moreover, there are a few minor issues that require attention:

1. Authors should use "hydrodynamic diameter" instead of "particle size" for results obtained from zetasizer.

Actually we report radius rather than diameter. We replaced apparent particle radius by apparent hydrodynamic radius in the figure captions and several times in the main text.  

Additionally, in lines 95-99 Authors state that using a zetasizer is not always appropriate to the studied system, but in their case is good and presents the actual size of the particle.

There is no such a statement in the text.

Such a statement should be explained and supported by other, more relevant techniques of particle size characterization.

This is not about a limitation of the technique, but about a property of the system. The following explanation was added

The model used in interpretation of the results obtained by dynamic light scattering and its limitations are explained in the user manual of Malvern Zetasizer and in handbooks of colloid chemistry and they will not be discussed here.

2. In line 88 is a reference error.

corrected

3. How electrical conductivity was designated or calculated?

The following was added

Malvern Zetasizer was also used to measure the electric conductance of dispersions and of solutions w/o solid particles.

4. In legends of figures 4 - 13 should be added units of temperature.

added

5. What was the load of amino-functionalized silica in the base fluid?

Nanofluids were prepared by dispersion of 45 mg of amino-functionalized silica in 150 mL of solvent

This is clearly stated in the original Ms.

Reviewer 3 Report

Comments and Suggestions for Authors

The submission by Kalbarczyk et al. describes the preparation of heat transfer fluids based on amino-functionalized silica and 1,2- propylene glycol. They claim that the positive zeta potential and the radii of the silica particles remained unchanged after heating up to 80°C for up to 28 days. While the results presented suggest potentially interesting properties from this class of materials, the manuscript has many flaws which preclude publication at this time. I have described my major concerns below:

·  The Introduction section is too short and lacks important information about the storyline. The authors should briefly describe the background of nanofluids and the relevance of using nanoparticles in heat transfer fluids. What are the advantages of using acidic heat transfer fluids when it is well-known that they can be problematic (e.g., corrosion)?

· The authors should include the synthesis procedure of the amino-functionalized silica in the Materials and Methods section.

· In the Results and Discussion section the authors should draw connections between the different findings (e.g., Is acetic acid or HCl better for acidifying the heat transfer fluids based on the result?).

· What could be the reason that HCl has a higher degree of dissociation in 50% EG than in 50% PG?

· The authors should include a figure (like Figure 2) in the manuscript showing the electric conductance of acetic acid in 50% EG and in 50% PG as a function of acetic acid concentration.

·  What was the yellow deposit observed in the dispersions after aging? Did the authors try to analyze it? It is well-known that deposit formation in heat transfer fluids is disadvantageous for several reasons (e.g., reduced heat transfer efficiency).

· What is the reason that the zeta potential values are much higher in 100% PG than in 50% PG?

· The authors did not include the evolution of zeta potential of amino-functionalized silica in 50% PG without acid. Why?

· The authors did not include the evolution of particle radius in dispersions of amino-functionalized silica in 50% PG without acid. Why?

· Based on the results (Figure 6 and 11), the amino-functionalized silica particles are very stable in 100% PG without the addition of any acid. Do the authors think that adding any acid is still justified?

·  Did the authors measure the specific heat capacity of the prepared heat transfer fluids?

Comments on the Quality of English Language

Original is misspelled in Figure 1.

Author Response

The submission by Kalbarczyk et al. describes the preparation of heat transfer fluids based on amino-functionalized silica and 1,2- propylene glycol. They claim that the positive zeta potential and the radii of the silica particles remained unchanged after heating up to 80°C for up to 28 days. While the results presented suggest potentially interesting properties from this class of materials, the manuscript has many flaws which preclude publication at this time. I have described my major concerns below:

• The Introduction section is too short and lacks important information about the storyline. The authors should briefly describe the background of nanofluids and the relevance of using nanoparticles in heat transfer fluids. What are the advantages of using acidic heat transfer fluids when it is well-known that they can be problematic (e.g., corrosion)?

The rationale for using dispersions rather than pure solvents as heat transfer fluids was discussed in multiple excellent reviews. Therefore we only added a short phrase.

The advantages and disadvantages of dispersions rather than pure solvents as heat transfer fluids were discussed in [1] in other excellent reviews, and will not be detailed here. The dispersions have higher thermal conductivity than pure solvents, and high thermal conductivity is much desired in heat transfer fluids, but they are also more viscous, which is an adverse effect

Regarding the problems with corrosion we added the following

Acid addition improves the stability of dispersion, but acidic dispersions may also accelerate the corrosion of metals.

• The authors should include the synthesis procedure of the amino-functionalized silica in the Materials and Methods section.

we added the following

10 g of silica were heated at 85^oC in solution of 20 g of 3-aminopropyltriethoxysilane (APTES, Thermo Fisher Scientific) in 700 mL of toluene for 24 h. The particles were separated by centrifugation, washed 3 times with ethanol, and dried at 105^oC.

 

• In the Results and Discussion section the authors should draw connections between the different findings (e.g., Is acetic acid or HCl better for acidifying the heat transfer fluids based on the result?).

we added the following

The properties of dispersions acidified with HCl on the one hand and with acetic acid on the other were very similar.

 

 

• What could be the reason that HCl has a higher degree of dissociation in 50% EG than in 50% PG?

We do not claim anything like this. PG is more viscous thus lower conductivity

• The authors should include a figure (like Figure 2) in the manuscript showing the electric conductance of acetic acid in 50% EG and in 50% PG as a function of acetic acid concentration.

The conductivity of acetic acid is much lower and the results are more scattered than with HCl thus we do not like to show these results. Determination of dissociation degree was not the main goal of this study.

• What was the yellow deposit observed in the dispersions after aging? Did the authors try to analyze it? It is well-known that deposit formation in heat transfer fluids is disadvantageous for several reasons (e.g., reduced heat transfer efficiency).

The amount of the deposit is a fraction of 1 mg. We have not analyzed it, but we believe that it consists chiefly of silica. To further emphasize the presence of the deposit as an adverse effect we added the following in the conclusions.

Yellowish coloration of dispersions aged at 80^oC indicates physico-chemical changes in the system, which are not detected by z potential or particle size measurements

• What is the reason that the zeta potential values are much higher in 100% PG than in 50% PG?

We do not know. We added the following.

The z potentials observed in 100% PG were higher than in 50% PG, and we do not have any rational explanation for this effect. In general the surface charging in organic and in mixed water-organic solvents is a fascinating and still unexplored field [17,22].

 

• The authors did not include the evolution of zeta potential of amino-functionalized silica in 50% PG without acid. Why?

This is because even at room temperature such dispersions are unstable against coagulation

• The authors did not include the evolution of particle radius in dispersions of amino-functionalized silica in 50% PG without acid. Why?

This is because even at room temperature such dispersions are unstable against coagulation

 

• Based on the results (Figure 6 and 11), the amino-functionalized silica particles are very stable in 100% PG without the addition of any acid. Do the authors think that adding any acid is still justified?

Now we know that dispersions w/o acid have stable zeta potential and particle size, but we did not know this when we started the measurements.

• Did the authors measure the specific heat capacity of the prepared heat transfer fluids?

No, we would have presented the results if we had such.

Comments on the Quality of English Language

Original is misspelled in Figure 1.

corrected

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors didnot additional experimental support.

Author Response

The authors didnot additional experimental support.

 

As we explained in the response to the 1st review, we have used up our powder so we cannot perform additional experiments with the same lot of powder.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have satisfactorily addressed the majority of the comments. However, I would like to revisit the following remark:

"Additionally, in lines 95-99, the authors state that using a zetasizer is not always appropriate for the studied system, but in their case, it is suitable and presents the actual size of the particles."

As English is not my native language, it appears to me that if the intended meaning of these sentences was different, the last sentence should include a negation.

"Therefore, the apparent particle size reported below does not represent the actual size of the particles."

Please verify this with a native English speaker.

Author Response

we agree with the comment. This was our mistake and now it is corrected.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have adequately addressed my queries and made the necessary corrections to the manuscript. It is now suitable for publication.

Author Response

there are no critical comments in this review