First Observation of Protomicelles in the System with a Non-Colloidal Surfactant

Round 1

Reviewer 1 Report

I have read with interest the manuscript “First Observation of Protomicelles in the System with a Non-Colloidal Surfactant.”

This is a high-quality spectrophotometric study of the system heptanol-Nile red (NR)-water. Interestingly and elegantly, the solubility measurements were carried out at a given chemical potential of NR, provided by a reservoir of the NR solid phase. Heptanol is a small-molecule amphiphilic substance (referred to as a non-colloidal surfactant) which, unlike usual surfactants, does not form micelles. It was shown that such non-colloidal surfactants can form micelle-like particles (“protomicelles”) by adsorption on the surface of dye molecules, The concept of protomicelles, earlier introduced by the authors (ref. [14 in the manuscript]) is fundamentally and broadly important beyond this study. Micelle-like aggregates can be formed in aqueous solutions by small amphiphilic molecules, such as low-molecular weight alcohols, by adsorption on other molecules, such as dyes.

I am happy to recommend this work for publication in Colloids and Interfaces.

 

However, I suggest the authors to consider additional relevant references. These additions and corresponding comments would enhance broader impacts of the work.

 

Non-colloidal surfactants are also referred in the literature to as “hydrotropes” [Kunz et al, Hydrotropes, Curr. Opin. Colloid Interface Sci. 2016, 22, 99−107; Subramanian et al. Mesoscale inhomogeneities in aqueous solutions of small amphiphilic molecules, Faraday Discuss. 167 (2013) 217; Buchecker et al., The impact of the structuring of hydrotropes in water on the mesoscale solubilization of a third hydrophobic component, Phys. Chem. Chem. Phys., 2017,19, 1806-1816].

One of the most remarkable hydrotrope, tertiary butanol, forms dynamic associates in water with the lifetime of a nanosecond. These dynamic associates were interpreted as pre-micelle structural fluctuations in aqueous solutions. In the presence of a hydrophobe, such as cyclohexane, mesoscale solubilized aggregates are formed. There aggregates have a profound similarity with protomicelles. Like protomicelles, they and they do not require a critical micelle concentration.

 

I also recommend checking misprints and typos. In Abstract, there is a sentence with an obvious typo (underlined): “It was found that heptanol protomicelles with a solubilization core in the as an NR molecule are formed in such a system”. Because of it, the meaning of the sentence is unclear.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript entitled "First Observation of Protomicelles in the System with a Non-Colloidal Surfactant" describes experimental observations and qualitative thermodynamic description of a way to improve the solubility of dye in water by using a relatively short alcohol (heptanol). The authors have worked a lot with similar systems before, so they know well the actual state of the art in the area, they even show their data from another publication - Fig. 2 is from Ref. 30 (which in fact is in a state "to be published"). The data are reliable and worthy publication. However the presentation and explanations should be significantly improved. 

The language is not good, and there are technical errors here and there, I am giving few examples:

- in the Abstract, the sentence "It was found that heptanol protomicelles with a solubilization core in the as an NR molecule are formed in such a system, so that in the ab-sence of micelles, the protomicelles take on the entire solubilization load." is very difficult to understand, especially with the present technical errors - must be rewritten.

- the first sentence of the Introduction part "The first reason why the study of joint aqueous solutions of dyes and surfactants was started was the ability of surfactants to monomerize hydrophobic dyes" is probably suitable for a presentation, but not for a paper in a journal. It should be better removed. 

- the last sentence of the part Results (on page 8) contains a word in Russian "где"

More serious for me are some other points:

- straight lines fit results - eqs. 5 and 6 on page 8 might be not show, instead a table comparing fits results of the data with heptanol (Fig. 6) and nonanol (Fig. 2) might be included for better analysis and comparison of the different systems.   

- the "Conclusion" part is rather a Discussion - should be renamed and another short and clear Conclusions part should be added.

At the end I would recommend the software and the procedure for peaks deconvolution, which is now in the section Results (1st paragraph on page 6) to be moved to Methodology part.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf