Effect of Divalent and Monovalent Salts on Interfacial Dilational Rheology of Sodium Dodecylbenzene Sulfonate Solutions

Round 1

Reviewer 1 Report

 

The manuscript deals with the influence of salts on surface properties of surfactant solutions and contains detailed experimental data on the surface tension and dilational viscoelasticity modulus of the mixed solutions of SDBS with five inorganic salts. The reliability of experimental data is doubtless and they allow the authors to discuss the effects of surfactant concentration, adding salts, surface pressure and of different ions on the dilational modulus of surfactant solutions. The data can be interesting for the broad readership of Colloids and Interfaces because the dilational modulus is an important parameter and the surfactants with inorganic electrolytes are frequently used in many industries dealing with foaming, emulsification and coating.  The manuscript is well-organized and the language is clear enough. Therefore the manuscript is recommended for publication in Colloids and Interfaces.

The only remark relates to the interpretation of the bell shape dependence of the dilational modulus on the surfactant concentration and surface pressure. This shape is well described by the Lucassen – van den Tempel model, which takes into account the only relaxation process – the surfactant exchange between the surface layer and bulk phase. Therefore, the claim that the shape can be “attributed to the transition between two relaxation processes, i.e., rearrangement, change of orientation or conformation of the surfactant molecules or their aggregation at the interface to the molecular exchange of surfactant molecules from bulk phase to the interface” (p. 14, ls. 402-405) is incorrect and must be eliminated from the manuscript.

Author Response

We appreciate the time and constructive comments of all reviewers.

Thank you for noting this mistake. Yes, the Lucassen and van den Tempel model and consideration of the surfactant molecular exchange as the only influential relaxation process are correct for simple surfactants and counterions (such as the ones understudy). However, here we meant to highlight that in the regime of low SDBS concentrations, the elasticity increases with the bulk concentration due to the increase of surface concentration. However, for high concentrations, the diffusion becomes fast and dampens the surface tension gradient created by the perturbation.

We corrected this part accordingly:

“In this study, because the chemical structure of SDBS molecules and counterions is very simple, relaxation caused by conformational changes is unlikely and the surfactant exchange between the bulk and the surface remains the only main relaxation process affecting the surface rheology.

With increasing the SDBS bulk concentration, the surface coverage, and the molecular exchange increase. The stimulated surface coverage enhances the surface viscoelasticity while the induced molecular exchange decreases the viscoelasticity. At low SDBS concentrations, the molecular exchange is relatively less significant, thus the surface elasticity increases with the bulk concentration due to the increase of surface concentration.”

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript written by the authors discuss the interaction of anionic surfactant with salt and focuses on the micellization shift towards lower concentration and corresponding viscosity change of such solutions. Increasing the salt concentration reduces the critical micellar concentration (CMC). Further, authors varied the counterion and observed that different counterions have an immense effect on the CMC shift and rheology of such solutions, known as ionic-specific effects. Overall such investigations are not new, where the investigation with respect to viscosity changes have not been addressed in the literature so far. In total the manuscript is well written, in my opinion the most important information have been discussed but in the following I still have two points to the authors:

 

1. My major critique is the lack of explanation of the viscosity change with respect to micelle geometry and surfactant slat ratio variation. Yes, lowering the CMC by salts increases the number of micelles. But how the structure of the micelle changes, especially pronounced for the divalent ions. Is it more spherical or more elongated? Does the micellar size increase or the number of aggregation changes? It is known that shape of the macromolecular structure is drastically influencing the viscosity of such solution, not salt itself. Structure and salt concentration are correlating with each other. Thus a deeper investigation into micellar structure seems to be very important here, ideally presented from experimental investigations. This can be quickly done by light scattering or with a better structural picture from small angle x-ray or neutron scattering.

 

2. Figures are self-explanatory, but a labelling into a,b,c... and mentioning the necessary information in the figure caption should be done for figure 1, figure 4.

Author Response

We appreciate the reviewer's time and constructive feedback.

please see below our response/comments:

1- Thanks for the suggestion. The focus of this paper is on the interfacial properties of the SDBS+salt solutions therefore it is out of focus to investigate the effect of the size and structure of micellar aggregates on the bulk viscosity of the solution. We also need to note that all the dilational viscoelasticity measurements conducted in this study are at SDBS concentrations below the CMC because at high concentrations (above the CMC), the viscoelasticity modulus tends to approach zero.  

However, due to the importance of the effect of salts on the change in the size and structure of micellar aggregates, as highlighted by the reviewer, we have added the following paragraph along with a new schematic in Figure 2b in the revised paper to provide a brief information on this concept.

“It should be highlighted that salts not only promote the growth rate of micelles (reduces the CMC) but they also increase the size and shape of micelles [31,32]. Sood and Aggarwal [31] showed that for SDBS and salt solutions at a concentration twice the observed CMC of the mixed system, the hydrodynamic diameter of the micelles increases drastically by approximately 25 nm and 43 nm with the addition of 10 mM NaCl and MgCl₂, respectively. The associated change in the packing parameter P_m defined as P_m= υ/(A₀ l) (υ and l represent the volume and length of the hydrophobic chain of the micelle and A₀ is micellar surface area per head group) also confirmed the change in the shape of the micelles from spherical to cylindrical at higher salt concentrations. This was consistent with an earlier study of Cheng and Gulari in 1982 showing the prolate ellipsoid or the rod shape micellar aggregates using quasielastic light scattering [32], as depicted in Figure 2b.”

 

2- Regarding the second comment, all figures and figure captions have been corrected as per the reviewer’s suggestion.

Author Response File: Author Response.docx

Reviewer 3 Report

The work is clearly performed in a professional and serious manner, the controls placed on the experiments are sufficient to trust the results, the interpretations of the data are in general appropriate, the list of references is broad, appropriate and up-to-date. I'm sure this paper will be interesting for readers. The work is recommended for publication following consideration of the authors of only a few very minor points:

1. The “Ion-specific effects are omnipresent in colloidal systems. Although interfacial properties of sin-gle/mixture surfactant layers have been extensively studied during the last two decades, there is no systematic study on mixed surfactant and salt systems, demonstrating the effect of type, valency, and concentration of the added salt on the interfacial dilational viscoelasticity (E) of the system.” in the abstract can be put into the introduction. The abstract should include the main points of the paper. Generally speaking, it should include the research purpose, research methods, results and final conclusions. The abstract should be as concise as possible, and the background information of the research should be deleted.

2. p.4, line 145: “air-water interface”： The air-water interface is generally expressed as surface tension rather than interfacial tension. Please check whether the statement is correct.

Author Response

We appreciate the reviewer's time for reviewing the manuscript and the constructive feedback. Please see below our response/comment:

1- The abstract is corrected as per the reviewer's suggestion.

2- To remain consistent with the literature, we replaced the terms “interfacial tension” and “interfacial rheology” with “surface tension” and “surface rheology” in the revised paper.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

All suggestions and necessary criticism have been discussed in the revised manuscript. Publish as is.