Synergistic Effects of Sodium Lauryl Sulfate and Lauryl Dimethylamine Oxide Blends on Foam Properties and Skin Irritation Reduction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments to Authors:

Ms. Ref. No.: cosmetics-4056255

Overview and Suggestions for Authors

The manuscript details the synergistic interaction of two surfactants, SLS and LDAO, emphasizing CMC measurement, foaming properties, and the zein test. The topic is highly relevant to cosmetic and detergent formulation. The work is technically sound and measures relevant properties for cosmetic development, but the analysis needs to be more thorough with proper explanation before the manuscript can be accepted.

 

Comments

1. Figure 1. The quality of the graph needs to be improved. The y-axis should be plotted on a logarithmic scale of concentration, as is standard for tensiometric plots, and it is preferable to keep the concentration in molar units since the molecular weights of both surfactants are well known.
2. Table 1: In addition to these parameters, other interfacial parameters such as surface excess and minimum area per head group are essential for calculations.
3. CMC values should be analyzed based on Clint's rationale, and the synergistic effect should be discussed more thoroughly.
4. It would be scientifically valuable if the authors analyzed their data using the Rosen equation to examine the synergistic interaction.
5. The explanation linking reduced Zein solubilization to lower free monomer concentration and more stable mixed micelles is reasonable but remains qualitative. Please clarify the reason behind this connection.
6. Figure 5: The 60:40 ratio produces maximum foam volume, but this optimum is not explained mechanistically.
7. Estimations of errors for all parameters should be provided (CMC, Zein number, and foam volume).

Author Response

Overview and Suggestions for Authors:

The manuscript details the synergistic interaction of two surfactants, SLS and LDAO, emphasizing CMC measurement, foaming properties, and the zein test. The topic is highly relevant to cosmetic and detergent formulation. The work is technically sound and measures relevant properties for cosmetic development, but the analysis needs to be more thorough with proper explanation before the manuscript can be accepted.

Response:

Thank you to the reviewer for your constructive comments and for appreciating the work carried out. We acknowledge the need for further data discussion and are committed to enhancing the manuscript to meet the required standards for acceptance. We think that, after the modifications proposed for the 3 reviewers that include extending data discussion, including new explanations and equations, the technical quality of the article has improved significantly.

Comments:

1. Figure 1. The quality of the graph needs to be improved. The y-axis should be plotted on a logarithmic scale of concentration, as is standard for tensiometric plots, and it is preferable to keep the concentration in molar units since the molecular weights of both surfactants are well known.

Response:

Thank you for the comment. Following your recommendation we have changed the Figure 1 including a logarithmic scale in x-axis for the concentration, as well as using molar units for the concentration and CMC. We have also modified table 3, including now the CMC in molar units.

2. Table 1: In addition to these parameters, other interfacial parameters such as surface excess and minimum area per head group are essential for calculations.

Response:

Thank you for your comment. We totally agree that those kinds of parameters are very interesting and could give a lot of value to our work. However, we think that going very deep in parameters related to thermodynamics or molecular aggregation could be out of the scope of the article. Moreover, addressing them properly would require a much more specific and detailed study, focusing exclusively on those parameters.

Nevertheless, we are already working on this, using different equations to calculate the concentration on the micelle and the interface, as well as obtaining the interaction parameters to quantify the synergy on the different mixtures. Our aim is to prepare in the coming months, another article targeted at a journal more related to molecular liquids, interfacial phenomena, etc.

 

3. CMC values should be analysed based on Clint's rationale, and the synergistic effect should be discussed more thoroughly.

Response:

Thank you for your comment. Indeed, we have the calculations of the mixture’s CMC using Clint’s model. Taking your advice, we have added a column in Table 3 with the Ideal CMC calculated with the Clint’s model in order to visualize better how far it is the real CMC obtained from the ideal one. As well, we included the Clint’s equation and its description in section 2.2.2 (lines 172-177).

Consequently, the interpretation of the results obtained was included in the discussion section (lines 244 -246).

4. It would be scientifically valuable if the authors analysed their data using the Rosen equation to examine the synergistic interaction.

Response:

Thank you for your comment. As explained in the previous comments, we have already been obtaining the interaction parameters for the SLS: LDAO system, as well as obtaining the micelle and interface concentration using non-ideal models. However, we think that the article is already long enough and besides, going deeper into this kind of parameters might be beyond the scope of the article.

Our plan is to continue working on the understanding of the synergies that take place among surfactants by quantifying parameters related to micelle formation and interface adsorption.

 

5. The explanation linking reduced Zein solubilization to lower free monomer concentration and more stable mixed micelles is reasonable but remains qualitative. Please clarify the reason behind this connection.

Response:

Thank you for the comment. We have extended the explanation regarding the connection among Zein solubilization and free monomer content/CMC in order to clarify the connection (lines 294-297).

 

6. Figure 5: The 60:40 ratio produces maximum foam volume, but this optimum is not explained mechanistically.

Response:

Thank you for the comment. As explained in the article, foam generation is related to interfacial parameters such as surface tension and CMC. Considering the improvements observed in these parameters when mixing SLS+LDAO, it can be expected and improvement in foaming. However, it is difficult to be able to explain why 60:40 has produced the best results. We are working on it, and we will study it from a mechanistically point of view in a near future.

7. Estimations of errors for all parameters should be provided (CMC, Zein number, and foam volume).

Response:

Thank you for your comment. Regarding foam generation, the standard deviation is included in Figure 5. In Figures 4 and 6 they have not been included to prevent overloading them with numerous curves, which could lead to confusion. However, as explained, each sample is measured in triplicate.

Regarding Figure 2 (Zein values) two replicates have been carried out and we have included in the graph the standard deviation.

Regarding Figure 1 (surface tension), measurements have been carried out twice. We are not including standard deviation in the figure to avoid cluttering it with too many curves, which could be confusing. However, standard deviation values are provided in Table 3 along with the CMC parameters.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

I congratulate the authors on their work, and I attach minor comments and clarifications.

Line 151: “required volume reached”: Was the required bond number respected for all the measurements?

Line 154: “The final value …”: When you say the final value, do you mean the value taken 5 seconds after droplet formation?

Line 182: Kjeldahl method: can you briefly describe the method you used to quantify nitrogen?

Line 200: “volume measurements were taken…”: How were these measurements done?

Figure 1: The x-axis of such plots is usually plotted in a logarithmic scale

Figure 1: What do the “X” on the curves refer to?

Figure 1: Are the results reproducible? You didn’t mention replicates in section 2.2.1

Lines 223-226: Would you consider calculating the surface coverage for each mixture?

Line 274: “Free monomers on the solution”: Replace “on” by “in”

Table 4: Why is the Zn_individual of LDAO decreasing with its increasing concentration?

Figure 4: Do you mean for the title of the x-axis to be “cycles” instead of “lectures”?

Line 323: You mean to refer to Figure 6 not 5

Author Response

Reviewer #2:

1. Line 151: “required volume reached”: Was the required bond number respected for all the measurements?

Response:

Thanks for your comment but it is not entirely clear for us what you mean by this question. Regarding the “required volume reached”, the volume needed to measure surface tension must be enough so that the gravity force comes into action, and so the higher the surface tension the higher the volume required. The volume selected must be the maximum the drop can withstand without falling. Once the drop reaches the volume selected, the measurement starts.

2. Line 154: “The final value …”: When you say the final value, do you mean the value taken 5 seconds after droplet formation?

Response:

Thank you for your comment. We have modified a bit the sentences explaining the tensiometer measurements in order to make it clearer. Once the drop volume is reached the equipment starts the measurement. It measures for 5 seconds at a frequency of 17 frames per second. Form each picture taken, a value of surface tension is calculated from the shape and dimensions of the drop. The equipment finally gives surface tension (mN/m) with a total of 5sec x17 fps values, and the average value is used. A total of 2 measurements per sample is carried out (lines 154-159).

 

3. Line 182: Kjeldahl method: can you briefly describe the method you used to quantify nitrogen?

Response:

Thank you for your comment. Kjeldahl method is explained in detail in the Zein test protocol. We have cited again the Zein protocol [33] in the line 192 to clarify that Kjeldahl method is explained in this document.

 

4. Line 200: “volume measurements were taken…”: How were these measurements done?

Response:

A SITA foam tester is used to carried out the foam measurements as explained in 2.3 section. This equipment has multiple sensors to measure the height of the volume and translate this value into volume since the dimensions of the container are known.

 

5. Figure 1: The x-axis of such plots is usually plotted in a logarithmic scale

Response:

Thank you for your comment. We have modified the Figure 1 and included a logarithmic scale for the X axis (surfactant concentration), which has been now included in mmol/l units according to the suggestion of the Reviewer 1.

 

6. Figure 1: What do the “X” on the curves refer to?

Response:

Thanks for your comment. The X on Figure 1 refer to the CMC point, it is included in the legend, but to avoid any confusion, we have also added it to the text (line 220).

 

7. Figure 1: Are the results reproducible? You didn’t mention replicates in section 2.2.1

Response:

Thank you for your comment. Indeed, surface tension measurements have been carried out twice. We do not include standard deviation in Figure 1 because there are too many curves and it could be confusing. We have added a sentence indicating that two surface tension measurements are carried out per sample in section 2.2.1(lines 158-159). Besides, Table 3 includes now the standard deviation of the CMC.

 

8. Lines 223-226: Would you consider calculating the surface coverage for each mixture?

Response:

We thank you for this suggestion. However, calculating the surface coverage for each mixture is beyond the scope of the present work and would require additional assumptions that are not supported by the available data. Therefore, this analysis has not been included. Nevertheless, as mentioned in previous comments, we are already working on these types of parameters.

 

9. Line 274: “Free monomers on the solution”: Replace “on” by “in”

Response:

Corrected, thank you.

 

10. Table 4: Why is the Zn_individual of LDAO decreasing with its increasing concentration?

Response:

Thank you for your comment. In this case, since the values are very small, the difference is not significant and cannot be considered meaningful. The Zein test is a method that considers a surfactant not irritant when the Zein value is below 200mgN/100mL. LDAO is a non-irritant surfactant which, in very small concentrations (from 0,1 to 0,5%) barely solubilizes Zein. What is observed in Table 4 for LDAO are Zein values that can be considered almost negligible.

11. Figure 4: Do you mean for the title of the x-axis to be “cycles” instead of “lectures”?

Response:

Corrected, thank you.

 

12. Line 323: You mean to refer to Figure 6 not 5

Response:

Corrected, thank you.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Overall, the manuscript seems good and informative, please find my comments below:
1. The title is good.
2. In the introduction section, line 92-96 is to short. Please combine with other or add more sentences.
3. The table format, usually it only consists of top border and bottom border. Please revise it.
4. Please add reference in section 2.3.
5. In Figure 2, please add some standard deviation to the grap.
6. In section 3.2, line 260-264; 273-277; 305-309., please add more sentences/explanation. 
7.  The conclusion, better to make it one paragraph, and add some quantitative data.

Author Response

Reviewer #3:

1. The title is good.

Response:

Thank you.

 

2. In the introduction section, line 92-96 is too short. Please combine with other or add more sentences.

Response:

Thank you for your comment. We have added more sentences in lines 93-96.

 

3. The table format, usually it only consists of top border and bottom border. Please revise it.

Response:

Thank you for your comment, we have change it in all the tables.

 

4. Please add reference in section 2.3.

Response:

Thank you for your comment. We did not consider adding references in these sections since SITA foam tester is one of the most used devices to assess foam generation and stability. The conditions used, are not standard, there is no article that uses exactly the same ones. They are selected considering our knowledge and background with this equipment. Regarding the concentration of the solution analysed, a representative percentage of a real usage situation has been used (example hand dishwashing).

 

5. In Figure 2, please add some standard deviation to the graph.

Response:

Thank you, two replicates have been carried out, and we have changed the graph including them.

 

6. In section 3.2, line 260-264; 273-277; 305-309., please add more sentences/explanation. 

Response:

Thank you for your comment, some sentences have been added, as well as quantitative data, in order to clarify the data discussion regarding dermal irritation. (Lines 282-283, lines 293-297, lines 316-318)

 

7. The conclusion, better to make it one paragraph, and add some quantitative data.

Response:

Thank you for your comment, we have added some quantitative data in the conclusion and now it is only in one paragraph. (Lines 379-401).

Author Response File: Author Response.docx