Ionic Strength Effect in the Equilibrium and Rheological Behavior of an Amphiphilic Triblock Copolymer at the Air/Solution Interface

Round 1

Reviewer 1 Report

The paper presents systematic studies on the effect of NaCl addition on the equilibrium surface tension of PEO-PPO-PEO triblock copolymer (Pluronic F68) solutions. In addition, dilatational rheology of Gibbs monolayers under corresponding conditions was study, as well. The main conclusion of the presented work is that the addition of salt affects the surface tension of solutions of considered compound while its impact on the dilatational rheological response is of less importance. I found the paper interesting and well written. I have only several minor remarks, which should be addressed before the paper acceptance.

1.  Was there any specific reason to use the term “vapor/solution” interface instead of air/solution interface? (term "vapor" suggests different kind of gas in an ambient atmosphere).

2.  It should be clearly highlighted (mainly in the conclusions) that the observed effect of interfacial tension decrease is noticeable only for relatively high NaCl concentrations (ionic strengths), which are rare in the common laboratory experiments.

3. Fig. 1 – why not compare  the experimentally obtained isotherm with the data from Refs 39 and 55?

4. First line of the conclusion section “This workevaluates” – space is missing.

Author Response

The paper presents systematic studies on the effect of NaCl addition on the equilibrium surface tension of PEO-PPO-PEO triblock copolymer (Pluronic F68) solutions. In addition, dilatational rheology of Gibbs monolayers under corresponding conditions was study, as well. The main conclusion of the presented work is that the addition of salt affects the surface tension of solutions of considered compound while its impact on the dilatational rheological response is of less importance. I found the paper interesting and well written. I have only several minor remarks, which should be addressed before the paper acceptance.

1. Was there any specific reason to use the term “vapor/solution” interface instead of air/solution interface? (term "vapor" suggests different kind of gas in an ambient atmosphere).

Following the recommendation of the reviewer, we have decided to substitute vapor/solution for air/solution.

2. It should be clearly highlighted (mainly in the conclusions) that the observed effect of interfacial tension decrease is noticeable only for relatively high NaCl concentrations (ionic strengths), which are rare in the common laboratory experiments.

Following the reviewer recommendation, we have included a brief statement in the result and discussion section, and a second one in the conclusion section.

3. Fig. 1 – why not compare the experimentally obtained isotherm with the data from Refs 39 and 55?

There is a comment in the text about the agreement between previous results in the literature and the one obtained in our study. We consider that the inclusion of the isotherm obtained previously in our work do not provide any new information to the discussion.

4. First line of the conclusion section “This workevaluates” – space is missing.

We have corrected.

We thank to the reviewer for the comments, they were very useful for improving the quality of the manuscript.

Reviewer 2 Report

In their article "Ionic strength effect in the equilibrium and rheological  behavior of an amphiphilic triblock copolymer at the vapor/solution interface" the authors have studied using three surface dilatational rheometry methods the rheological response of a PEO-PPO copolymer at the air/water interface. Moreover, they have studied the effect of NaCl on the surface dilatational rheology.

The article is clearly written, and the results are sound. I believe that the manuscript can be a nice addition to the existing literature of dilatational rheology of PEO-PPO copolymers at the a/w interface.

In this respect, what I missed was a more thorough comparison with past literature studies, not in view of validating the experimental results (which is already done and the results are meaningful), but rather in view of a structure-property correlation. For example, the dilatational rheology of different PEO-PPO copolymers was recently studied by Sagis and Kroeger et al., while past studies exist from Miller, Hansen, among others. What would be of most interest to the reader, including myself, would be how the structure of the copolymer affect the rheological response and the salting in effect. I realize that this is beyond the scope of this study, but would definitely be added value to the manuscript.

Some minor remarks:

1.     Section 2.2.1. What was the size of the Wilhelmy plate (both platinum and paper) that was employed?

2.     Section 2.2.1. What was the area of the trough? Where was the plate placed and at what angle relative to the barrier movement? The trough typically shows a mixed flow where not pure dilatation but also shear is present, especially at large ΔA.

3.     Section 2.2.1. What was the volume of the droplet? What is the radius of the needle? What is the Worthington number and are size effects negligible? I believe that these are very important to state, as the main relaxation mechanism of the interfacial layers is, according to the authors, diffusion, which means that the apparent rheological response observed here will depend on size.

4.     Section 2.2.1. What amplitudes were employed in the oscillatory droplet and barrier measurements?

5.     Lines 151-155: the authors mention that the viscous moduli were not plotted as they were always much lower than the elastic moduli. Is this something that has been observed in the past in literature on similar systems?

6.     Line 171: Has the pseudo-plateau been observed in the past in literature? Maybe the authors can add a reference here.

7.     Figure 3: If I understand correctly, the authors have  plotted together the results of the oscillatory droplet and the oscillatory barriers. I think it might be worthwhile showing them separately to assess each method independently.

Author Response

In their article "Ionic strength effect in the equilibrium and rheological behavior of an amphiphilic triblock copolymer at the vapor/solution interface" the authors have studied using three surface dilatational rheometry methods the rheological response of a PEO-PPO copolymer at the air/water interface. Moreover, they have studied the effect of NaCl on the surface dilatational rheology.

The article is clearly written, and the results are sound. I believe that the manuscript can be a nice addition to the existing literature of dilatational rheology of PEO-PPO copolymers at the a/w interface.

In this respect, what I missed was a more thorough comparison with past literature studies, not in view of validating the experimental results (which is already done and the results are meaningful), but rather in view of a structure-property correlation. For example, the dilatational rheology of different PEO-PPO copolymers was recently studied by Sagis and Kroeger et al., while past studies exist from Miller, Hansen, among others. What would be of most interest to the reader, including myself, would be how the structure of the copolymer affect the rheological response and the salting in effect. I realize that this is beyond the scope of this study, but would definitely be added value to the manuscript.

We thank to the reviewer for the comment, and as mentioned this is beyond the scope of this study. The text has proper comparison with previously reported results, and the inclusion of additional discussion only make the text longer, but it does not provide a true value to the proposed discussion.

Some minor remarks:

1. Section 2.2.1. What was the size of the Wilhelmy plate (both platinum and paper) that was employed?

We have added the information required about the size of the Wilhelmy plates.

2. Section 2.2.1. What was the area of the trough? Where was the plate placed and at what angle relative to the barrier movement? The trough typically shows a mixed flow where not pure dilatation but also shear is present, especially at large ΔA.

We know about the possible role of shear deformations in oscillatory barrier experiments. In our experiments, the deformation amplitude was maintained relatively low to minimize the possible impact of shear response coupled to dilational one. The information required for the reviewer has been included.

3. Section 2.2.1. What was the volume of the droplet? What is the radius of the needle? What is the Worthington number and are size effects negligible? I believe that these are very important to state, as the main relaxation mechanism of the interfacial layers is, according to the authors, diffusion, which means that the apparent rheological response observed here will depend on size.

We have added the information required for the reviewer in the revised version of the manuscript.

4. Section 2.2.1. What amplitudes were employed in the oscillatory droplet and barrier measurements?

We have added the amplitude for the performed experiments.

5. Lines 151-155: the authors mention that the viscous moduli were not plotted as they were always much lower than the elastic moduli. Is this something that has been observed in the past in literature on similar systems?

This has been previously observed in the literature, we have included a statement in the revised version, and references:

6. Line 171: Has the pseudo-plateau been observed in the past in literature? Maybe the authors can add a reference here.

We have included additional information about the presence of the pseudo-plateau and its potential origin.

7. Figure 3: If I understand correctly, the authors have plotted together the results of the oscillatory droplet and the oscillatory barriers. I think it might be worthwhile showing them separately to assess each method independently.

To evidence the similarity between the results obtained using independently the methods, we have added in the revised version to Figure 3, a new panel comparing the results for both methods.

We thank to the reviewer for the comments, they were very useful for improving the quality of the manuscript.

Reviewer 3 Report

This paper investigates the effect of ionic strength by addition of NaCl on the interfacial tension, interfacial storage modulus and rheology of Gibbs monolayers of Pluronic F-68. Overall, the article is well-written and this reviewer recommend publication after the following minor edits:

Line 54-57: The authors claim that salt addition does not impact the structure of the phase diagram, but modifies the specific area of each phase. It is unclear to the reviewer what they mean here. This needs to be expanded with clearer explanation and specific examples.

Line 79-87: The authors explain that they had previous work on Li+ ions in Pluronic. This work aims to use Na+ ions, but it is unclear what was the motivation. They explained that Li+ ions interact with PEO, making it unsuitable for testing ionic strength. Does this mean Na+ ion will not interact? The authors need to explain in detail why is Na+ ion selected for this study, and why it is a better candidate than Li+ ion.

Figure1: A schematic showing the Gibbs monolayers on the  vapor/solution interface will help with clarity and for readers to quickly grasp the work shown.

Figure3a, line 233-234: “even though for an ionic strength of 1M and polymer 233 concentration of 10-3 mM, a slight increase of the storage modulus is observed.” This is >10% increase in modulus, the authors need to explain what is the origin of this increase in storage modulus at high frequency when ionic strength is high.

Line 304-305: Authors claim that capillary wavelength and interfacial tension can be assumed to be directly correlated. The authors need to justify what is the basis of this assumption?

Line 477: Change “two” with “to”

Suggestion: The authors should show the change in interfacial tension with ionic strength without Pluronic.

Author Response

This paper investigates the effect of ionic strength by addition of NaCl on the interfacial tension, interfacial storage modulus and rheology of Gibbs monolayers of Pluronic F-68. Overall, the article is well-written and this reviewer recommend publication after the following minor edits:

Line 54-57: The authors claim that salt addition does not impact the structure of the phase diagram, but modifies the specific area of each phase. It is unclear to the reviewer what they mean here. This needs to be expanded with clearer explanation and specific examples.

We have provided additional explanation in the text to the issue raised by the reviewer.

Line 79-87: The authors explain that they had previous work on Li+ ions in Pluronic. This work aims to use Na+ ions, but it is unclear what was the motivation. They explained that Li+ ions interact with PEO, making it unsuitable for testing ionic strength. Does this mean Na+ ion will not interact? The authors need to explain in detail why is Na+ ion selected for this study, and why it is a better candidate than Li+ ion.

We have extended the discussion about the interest of Na+ and Li+ in the interaction with Pluronic.

Figure1: A schematic showing the Gibbs monolayers on the vapor/solution interface will help with clarity and for readers to quickly grasp the work shown.

We have added a sketch of the possible different conformations of Pluronic F-68 at the water/vapor interface.

Figure3a, line 233-234: “even though for an ionic strength of 1M and polymer 233 concentration of 10^-3 mM, a slight increase of the storage modulus is observed.” This is >10% increase in modulus, the authors need to explain what is the origin of this increase in storage modulus at high frequency when ionic strength is high.

We have introduced a brief comment about the potential origin of the increase of the elasticity with the ionic strength.

Line 304-305: Authors claim that capillary wavelength and interfacial tension can be assumed to be directly correlated. The authors need to justify what is the basis of this assumption?

Of course, both the real and imaginary part of the capillary wavenumber (wavelength and damping) are related through the dispersion equation with surface tension and real and imaginary elastic modulus in a non-trivial way. However, to avoid possible misinterpretations, we have removed the claim from the revised manuscript.

Line 477: Change “two” with “to”

We have corrected.

Suggestion: The authors should show the change in interfacial tension with ionic strength without Pluronic.

We have added values in the Appendix A.

We thank to the reviewer for the comments, they were very useful for improving the quality of the manuscript.