Substituent Effects in Kaolinite Flotation Using Dodecylamine: Experiment and DFT Study

Round 1

Reviewer 1 Report

This manuscript presents DFT results on the adsorption of three substituted cationic surfactants on kaolinite. The results were analyzed as a function of solvent accessible surfaces, head group charge, number of bonds between surfactants and kaolinite, as well as other parameters related to the electronic structure of the molecules.

 

The paper present some interesting results but a great number of important issues should be clarified and improved before publication. Another weakness found is the not always easy readability of the text.

 

I recommend rejection of the paper in its current form.

I strongly encourage authors a truly huge revision of the text, as well as more calculations to draw firmer conclusions and publish their results in a new article.

 

Below I have provided numerous remarks to improve the text.

 

Minor points:

 

1. Introduction, line 39: define DRN, DEN and DPN to keep the text homogeneous. Do the same for other abbreviations appearing in the introduction.

2. Introduction, Line 46: What does -0.045 nm mean? Do the authors mean particles smaller than 0.045 nm?

 

3. Materials and methods, line 90, 91. Remove 2.1 and 2.1.1 or use headings.

 

4. Materials and methods, Figure 1: Is BHDMC well represented by this molecular model?  I think the H of one hydroxyl group is missing.

 

5. Materials and methods, line 123: Visual module? Do the authors mean Builder Module?

 

6.  Materials and methods, line 112: What does -0.074 nm mean? Do the authors mean particles smaller than 0.074 nm?

 

7. Materials and methods, line 132-139: Could you please reference the methods? Do the same thing for CASTEP, lines 152-157.

 

8. Materials and methods, Figure 2: Assign colors to atoms. The figure does not show a complete crystal structure. It seems to be cut and is not very clear. I suggest the authors use the complete crystal structure here, as they refer it in the text and add a new figure below showing the two different surfaces separately. This new figure showing the two surfaces could accompany the paragraph in which they explain the building of these two surfaces

 

9. Materials and methods, lines 165-168. “Use the clear surface function in the software to cut the kaolinite crystal cells along 165 001 face, obtain 001 face and 001 (-) face respectively, and construct the supercell. In order 166 to eliminate the influence of periodic structure, add a vacuum layer of 50 Å on the surface 167 of kaolinite”. I suggest the authors rewrite this sentence. It seems to be taken out of a user manual. Do the same for “add the collector into the flotation cell, open the inflation valve 2 minutes later, and start collecting foam products for 3 minutes. Finally, the foam product was dried, weighed and the yield of kaolinite was calculated” (lines 191-194)

 

10. 3.1.1. Molecular surface area: lines 198-210. This paragraph is unnecessary. I think the authors need not explain what molecular surfaces are. A reference would be enough.

 

11. 2.3.2. Calculation method. What are the dimensions of the periodic cells used to study adsorption? What is the thickness of the surface? Are surfactant molecules in adjacent cells interacting?

 

 

Major points:

 

12. 3.1.1. Molecular surface area: lines 218-221. The authors calculate the molecular surface of only one molecule. Why do they mention the molecular surface at the critical micelle concentration in this paragraph? They need more molecules to calculate it. How is this area at CMC related to the one they have calculated for a single molecule?

 

13. 3.1.2. Mulliken charge distribution, line 240. What do the authors mean with positive electricity?

 

14. 3.1.2. Mulliken charge distribution, lines 241-245. The authors state that “zeta potential of silicate minerals is negative at a wide range of pH values, while ammonium salt cationic surfactants are mainly adsorbed on the surface of silicate minerals by electrostatic action. Therefore, the introduction of benzyl and alcohol hydroxyl groups into the head group of amine surfactants can improve the electrostatic interaction between the reagents and kaolinite”.

 

From the above paragraph and from table 5, one can deduce that BHMDC is best adsorbed on the kaolinite surface, as its headgroup charge is more positive.

From Table 8, the authors reach the same conclusion.  BHMDC interacts strongly with kaolinite.

Experiments also favor BHMDC at high concentrations, but at low concentrations, BHDA performs better than BHMDC and DDA (first point in the graph).

What is the concentration of the models used in the calculations? These data are needed to make comparisons. In other words, how are the models used in the calculations related to figure 8 in terms of concentration?

 

On the other hand, dipole moment, adsorption energy and HOMO-LUMO results point to DDA as the best collector.

 

Could you explain these discrepancies? Results seems to depend on the parameter chosen to characterize the molecules and point in different directions.  I am not sure I have understood you results. They seem contradictory. I think it is very important to relate your simulation models (concentration?), with real results, as the latter depend on concentration, at least for low quantity of surfactants, for which BHDA works better.

 

 

15. Another important and final remark is that the authors base their conclusions on one single calculation. In real systems, the surfactants molecules could adsorb on the surface with different configurations (even if some of them are preferred due to the interactions that establish surfactants and kaolinite), in which case, the results may change. This work is incomplete without exploring the potential energy surface of these structures, focusing on those configurations with lower energy. Only in that case, they will be able to make firm conclusions.

Comments for author File: Comments.docx

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

In this work, the authors combined experiment and theory to study kaolinite flotation using dodecylamine. The DFT method was used to study the adsorption of DDA, BHDA and DHMDC on kaolinite with HOMO and LUMO orbital. The flotation was studied by experiment mean. The flotation was studied experimentally. The article presents thorough scientific data. My comments are:

-          The kaolinite sample's chemical composition, X-Ray diffraction, and BET surface measurement must be compared with the previous publication (ref 13) by the author.

-          The abbreviations BHDA and BHMDC do not correspond to the reagent name. The calculation does not include a chloride atom for Dodecyl dihydroxyethyl methyl ammonium chloride.

-          In Figure 8, despite having a higher recovery yield than BHDA and DDA at high concentration, BHMDC has the lowest results at low concentration. The authors must address this issue.

-          The authors should explain the reason for using three reagents in the introduction.

-          The calculation has a limited set of possible adsorption types under investigation.

-          To compare the experiment and theory in figure 8, the calculation system should have a higher number of probe molecules. Reference 13 presents calculation results for kaolinite and DDA with up to 10 molecules.

-          The authors should give the information regarding the calculation of heat of adsorption energy (DH). At which temperature is this calculated?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have  responded all my questions.

Reviewer 2 Report

The MS has been corrected as suggested.