Impact of Fatty Acid Composition of Polyglycerol Esters on the Emulsifying Performance in Cosmetic Formulations
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis paper addresses the interesting question of forming concentrated nanoemulsions for cosmetic formulations by using decaglycerol esters as surfactants. The study itself was mainly focusing on determining the colloidal stability of formed nanoemulsions by doing different standard ageing tests. The surfactants were prepared by the authors and were systematically varied with respect to their acyl chains contained in order to yield optimised nanoemulsion stability.
From the practical approach to generate stable emulsion gels this work has been done in a comprehensive and systematic way, but from the more scientific point I see a major shortcoming in the lack of information regarding the surfactants employed here. This lack of knowledge about the molecular composition of the surfactants studied makes it largely impossible to deduce an interpretation of the observations in terms of the molecular architecture of the surfactants used and thereby to generate an understanding of how the molecules have to be designed in order to achieve the desired goal of being optimised for emulsion stabilisation. Accordingly, I am very split in my assessment of this work.
For being useful in a scientific way it would be necessary that the authors would properly characterise the structures of the surfactants that they work with. This would be possible, but one would require comprehensive work input and thereby also lead to a completely different article, one of much higher scientific value.
In contrast, one could argue that from a purely empirical argument this work still gives valuable insights into the formulation of colloidally stable emulsions with glyceride surfactants. In the end, this is an editorial decision with respect to how they see the scope and the claim of this journal.
However, even for sticking to the basic approach of this manuscript that does not give deeper insights into the relevance of the molecular features of the surfactants, still some major issues would have to be addressed:
- PITd in eq. 1 is never defined.
- polyglycerol-10 is used as a basis for most surfactants here, but it is never specified further what this compound is in terms of molecular composition. Here as much information as possible must be given in order to be able to assess what the later on synthesised surfactants may look like.
- For Table 1, if there is a percentage of ester given, what then is the rest contained in the product? What is the content of diesters, triesters, etc.?
- in 3.1.3 less defined polyglycerol esters were obtained by reacting with a triglyceride containing a mix of chains. I would assume the same properties could be obtained by simply mixing purer surfactants accordingly together. Did the authors consider doing so, as in this way they could gain an insight into the relevance of the individual chain lengths on their properties?
- the cmcs reported in Table 4 are strange with respect to their relative changes. An increase of the cmc with increasing chain length would mean that the hydrophobic effect of the longer chain is less pronounced. How could this be explained and why is this the only case where such a behaviour would be observed, while in hundreds of other systems one observes always the opposite?! I would assume that here simply the interpretation of the authors is wrong and that one sees here effects of having diesters that then lead to lower cmcs. Another origin of this discrepancy could also be that in the surface tension measurements no real equilibrium value was achieved, which can easily happen in such surfactant mixtures. Of course, here again a thorough characterisation of the molecular composition of the surfactants would be required. In any case here a good explanation must be given, why a behaviour is reported that is clearly in disagreement with established surfactant science. For sure the statement “Consequently, the more polyglycerol-10 esters are composed of short-351 chain fatty acids, the lower their CMC will be, and vice versa.” cannot remain in this form in the manuscript.
- the cmc values derived from the surface tension curves are given at various places. However, here also the surface tension curves should be shown in the SI, as that would allow to gain valuable insights into the behaviour of the surfactants.
- the cmc values are given with very high precision. How was the error determined, which is given for these values?
- the DLS measurement in Figure 6 is not really explained and justified. Was here a dilute, low viscous emulsion measured or a HIPE emulsion gel? And if it was a gel, how was the autocorrelation function interpreted for such a non-ergodic system? This measurement definitely needs some good further explanation in order to know what one can learn from it.
In general, the authors have a good coverage of the literature, but it might be pointed out that high internal phase formulations have already been studied for a long time, like for instance described in https://doi.org/10.1016/S0021-9797(89)80053-0.
Finally, the manuscript should also become carefully revised with its use of the English language.
In summary, it is an editorial decision of this paper without proper characterisation of the surfactant molecules can be considered for publication. However, even if then there would be a larger number of points listed above that would require a major revision of the manuscript.
Author Response
General Remark : “For being useful in a scientific way it would be necessary that the authors would properly characterise the structures of the surfactants that they work with. This would be possible, but one would require comprehensive work input and thereby also lead to a completely different article, one of much higher scientific value.”
Response : Thank you for your remark. A substantial number of revisions have been made to the manuscript in order to provide as much information as possible regarding the molecular structure of the surfactants. We hope that these additions meet your expectations and help to better highlight the work carried out in this study.
Comments 1 : “PITd in eq. 1 is never defined.”
Response 1 : We agree with this comment. We have, accordingly, added the definition of PITd page 3 lines 137 to 140. “The PITd of a substance under investigation is the slope of the equation describing the variation of the phase inversion temperature between the water/octane/C10E4 system and this system with the addition of studied substance.” If you would like more information about the PIT-slope method, you can read the article doi: 10.1111/ics.12516.
Comments 2 : “polyglycerol-10 is used as a basis for most surfactants here, but it is never specified further what this compound is in terms of molecular composition. Here as much information as possible must be given in order to be able to assess what the later on synthesised surfactants may look like.”
Response 2 : Thank for pointing this out. You are correct that no information on the polyglycerol-10 used was initially included, as the objective of this work was to investigate the influence of the fatty‑acid composition of polyglycerol esters, and the same polyglycerol was used for all prototypes. However, we agree that knowing the molecular composition of the polyglycerol-10 provides valuable context for interpreting the nature of the surfactants subsequently synthesized. For this reason, we have now added these data to the manuscript, as the corresponding analyses had indeed been performed. The analytical method has been included in the Materials and Methods section (section 2.2.2, ‘Polyglycerol‑10 oligomers distribution’, lines 153–167). The oligomer distribution of the commercial polyglycerol‑10 used in this study is presented in the Results section (section 3.1.1, lines 233–248). It is important to note that this type of molecule is particularly challenging to analyze due to its high polydispersity, and no analytical standards are available for such compounds.
Comments 3 : “For Table 1, if there is a percentage of ester given, what then is the rest contained in the product? What is the content of diesters, triesters, etc.?”
Response 3 : In Table 1, the percentage of ester reported corresponds to the proportion of esters obtained at the end of the synthesis relative to the total amount of product recovered. For example, for the PG10‑C16 prototype, once the reaction is complete, meaning that all free fatty acid has reacted with the polyglycerol, the final product consists of approximately 90% esters and 10% unreacted polyglycerol‑10.
The distribution of monoesters, diesters, triesters, and higher esters has also been added to the manuscript in Table 1 (page 6). This analysis was performed using an internal method, and the results should be interpreted with caution, as the characterization of such compounds is particularly challenging due to the very large molecular diversity generated during synthesis. Indeed, fully solubilizing all compounds in the analytical solvents is extremely challenging due to their wide range of polarities.
Comments 4 : “in 3.1.3 less defined polyglycerol esters were obtained by reacting with a triglyceride containing a mix of chains. I would assume the same properties could be obtained by simply mixing purer surfactants accordingly together. Did the authors consider doing so, as in this way they could gain an insight into the relevance of the individual chain lengths on their properties?”
Response 4 : Thank you for this comment. This is indeed the central point we aimed to highlight in this work.
We first examined polyglycerol esters synthesized with a single type of fatty acid, referred to as ‘PG10‑1FA’, which correspond to ‘pure’ polyglycerol esters. As shown in section 3.1.1, such surfactants were unable to produce stable nano‑HIPEs.
We then evaluated combinations of these ‘pure’ esters, as described in section 3.1.2. For example, blending PG10‑C14 with PG10‑C16. In this case, each ester was synthesized individually and then combined at different ratios in the nano‑HIPE formulations. Although all ratios and all possible combinations were tested, none yielded nano‑HIPEs stable at room temperature, 40 °C, 50 °C, and under freeze–thaw cycles. Nevertheless, the results were already improved compared with using each ester alone, as shown in section 3.1.1.
This led us to synthesize polyglycerol esters using oils (triglycerides) containing mixtures of fatty acids, with the objective of introducing structural disorder and generating mixed diesters and triesters, as detailed in section 3.1.3. This stepwise approach ultimately allowed us to identify how variations in fatty‑acid chain length influence the properties of polyglycerol esters.
This approach was also applied using stearin, not only triglycerides. As shown in Table 3 (page 8), combining PG10‑C16 with PG10‑C18 yields a nano‑HIPE that is stable only at room temperature, but not under freeze–thaw cycles nor at elevated temperatures (40 °C or 50 °C). In contrast, reacting polyglycerol‑10 with stearin (a 50/50 mixture of C16 and C18 fatty acids) produces the PG10‑C16C18 prototype, which forms gels that remain stable at room temperature, 40 °C, and 50 °C, although still not under freeze–thaw conditions (Table 4, page 9). These results demonstrate that grafting different fatty acids onto the same polyglycerol backbone leads to a significantly more efficient surfactant than simply blending ‘pure’ polyglycerol esters
Comments 5 , 6 & 7 : “- the cmcs reported in Table 4 are strange with respect to their relative changes. An increase of the cmc with increasing chain length would mean that the hydrophobic effect of the longer chain is less pronounced. How could this be explained and why is this the only case where such a behaviour would be observed, while in hundreds of other systems one observes always the opposite?! I would assume that here simply the interpretation of the authors is wrong and that one sees here effects of having diesters that then lead to lower cmcs. Another origin of this discrepancy could also be that in the surface tension measurements no real equilibrium value was achieved, which can easily happen in such surfactant mixtures. Of course, here again a thorough characterisation of the molecular composition of the surfactants would be required. In any case here a good explanation must be given, why a behaviour is reported that is clearly in disagreement with established surfactant science. For sure the statement “Consequently, the more polyglycerol-10 esters are composed of short-351 chain fatty acids, the lower their CMC will be, and vice versa.” cannot remain in this form in the manuscript.
- the cmc values derived from the surface tension curves are given at various places. However, here also the surface tension curves should be shown in the SI, as that would allow to gain valuable insights into the behaviour of the surfactants.
- the cmc values are given with very high precision. How was the error determined, which is given for these values?”
Response 5, 6 & 7 : Thank you for these very relevant remarks. These results were also unexpected considering the current knowledge in the field, which is why the measurements were repeated several times (4–5 replicates per prototype). The protocol was strictly followed, equilibrium was ensured before each measurement, and the standard deviations obtained were very low (0.69–2.86 mg/L), confirming the high repeatability of the data.
We attempted to formulate a plausible hypothesis to explain why the CMC increased as the average fatty‑acid chain length increased, contrary to what is generally reported in the literature, but we agree that this explanation is not yet robust. Polyglycerol‑10 esters appear to be highly complex structures due to the wide distribution of oligomers (from PG2 to at least PG12, linear/cyclized/branched), the diversity of ester species (monoesters, diesters, triesters, etc.), and the range of fatty‑acid chain lengths. This molecular heterogeneity likely causes deviations from the classical CMC trends observed for simpler surfactants.
This discrepancy can also be seen in the review by Mingyu Zhang and Guangyan Zhang, Recent advances in the properties and applications of polyglycerol fatty acid esters (2025, https://doi.org/10.3390/polym17070879), where the authors state that the CMC decreases with increasing fatty‑acid chain length. Yet, in their Table 2 (page 8), PG10 mono‑caprylate/caprate shows a CMC of 10–30 mg/L, while PG10 monolaurate has a CMC of 30 mg/L.
As we are currently unable to provide a solid mechanistic explanation for our observations, we have decided to remove these CMC data from the manuscript, should this be acceptable to you. We are continuing our investigations to better understand this phenomenon and hope to return in the future with stronger evidence supporting our conclusions.
Comments 8 : “the DLS measurement in Figure 6 is not really explained and justified. Was here a dilute, low viscous emulsion measured or a HIPE emulsion gel? And if it was a gel, how was the autocorrelation function interpreted for such a non-ergodic system? This measurement definitely needs some good further explanation in order to know what one can learn from it.”
Response 8 : We agree with this comment. The DLS measurements were performed directly on the nano‑HIPE gel (the transparent gel) without any dilution. When such systems are diluted, they become white, and our instrument cannot perform reliable measurements on opaque samples. Achieving sufficient transparency would require extensive dilution, which would alter the system and therefore the droplet size. We attempted DLS measurements on diluted nano‑HIPEs, but the repeatability was very poor.
The instrument used does not provide the autocorrelation function, so the reported value should be considered indicative rather than fully quantitative. To verify the measurements, we performed optical microscopy at ×400 magnification. No droplets were observed at this magnification, which confirms that the system is indeed submicron in size.
The figure has been updated, it can be found on page 12, figure 7.
Comments 9 : “In general, the authors have a good coverage of the literature, but it might be pointed out that high internal phase formulations have already been studied for a long time, like for instance described in https://doi.org/10.1016/S0021-9797(89)80053-0.”
Response 9 : Thank you for pointing this out. High–internal‑phase formulations have indeed been known for a long time, as well as the challenges associated with their fabrication and stabilization, given their extreme sensitivity. The purpose of this work is not to highlight nano‑HIPEs themselves, but rather to demonstrate that it is possible to develop highly efficient natural surfactants, efficient enough to stabilize systems that are among the most demanding in terms of processability and stability, such as high‑internal‑phase formulations.
Comments 10 : “Finally, the manuscript should also become carefully revised with its use of the English language.”
Response 10 : Thank you for your remark. The titles of the figures and tables have been improved, and the entire manuscript has been revised by a native speaker. We hope these changes address your concerns. Thank you again for your constructive comments. Kind regards.
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Authors,
Thank you for the opportunity to read your research work.
- Apparently, the title is not fully compatible with the manuscript.
- Please consider revising the Abstract. It was difficult to follow. Some abbreviations were not initially explained.
- The introduction section was too long.
- Avoid using "solar emulsion" to reference sunscreens.
- Item 2.5 was poorly described. Packaging material, volumes or masses, and assays must be presented in detail.
- Figure 6 must be improved.
- It was not clear the composition of all samples (emulsions and other mixtures).
Overall, despite the length of the manuscript, it was not deeply discussed. A general restructuring must be considered.
Author Response
Thank you for your interesting comments.
Comments 1 : “Apparently, the title is not fully compatible with the manuscript.”
Response 1 : Thank you for pointing this out. We agree with this comment. Therefore, we have changed the title from “Impact of synthesis process and fatty acid composition on the emulsifying performance of polyglycerol esters in solar and nano-emulsions” to “Impact of fatty acid composition of polyglycerol esters on the emulsifying performance in cosmetic formulations”. Indeed, what this study demonstrates is that the synthesis parameters influence surfactant performance because they modify the relative ester composition. In other words, the synthesis conditions directly affect the composition of polyglycerol esters, which is why the term ‘synthesis process’ was removed from the title.
Comments 2 : “Please consider revising the Abstract. It was difficult to follow. Some abbreviations were not initially explained.”
Response 2 : Agree. We have, accordingly, modified the abstract by explaining all the abbreviations, adding data and rephrasing the methodology and conclusions to make then clearer to emphasize this point. The changes can be observed from line 14 to line 22. The section on CMC measurements was also removed following comments from another reviewer.
Comments 3 : “The introduction section was too long.”
Response 3 : Thank you for this comment. The introduction was indeed too long. It has been revised and shortened, decreasing from 951 words initially to 793 words now, while preserving a coherent structure: the current state and challenges of the cosmetic sector; the need to develop highly efficient natural ingredients capable of stabilizing complex emulsions; the complex emulsions investigated in this work (nano‑HIPEs and mineral sunscreens); the available natural surfactant options; and finally the detailed outline of the article.
Comments 4 : “Avoid using "solar emulsion" to reference sunscreens.”
Response 4 : Thank you for pointing this out. The term ‘solar emulsion’ has been replaced throughout the manuscript with the word ‘sunscreens’.
Comments 5 : ”Item 2.5 was poorly described. Packaging material, volumes or masses, and assays must be presented in detail.”
Response 5 : Agree. Section 2.5 has been revised in accordance with your comment. The packaging and sample volumes have been added on line 219, page 5: ‘Glass pillboxes containing 30 mL of the different emulsion samples were stored at room temperature for six months, at 40 °C for three months, and at 50 °C for one month.’ In addition, the criteria used to determine whether the formulations were considered stable or not have been clarified on lines 221 to 223. A reference describing the parameters to be considered when assessing the stability of a cosmetic formulation has also been added on line 219.
Comments 6 : “Figure 6 must be improved.”
Response 6 : Indeed, Figure 6 (now Figure 7 after revisions) required improvement. It has therefore been updated, notably by adding units to the axes and providing explanations on how the measurement was performed. The modifications can be observed from line 452 to line 462 on page 12.
Comments 7 : “It was not clear the composition of all samples (emulsions and other mixtures)”
Response 7 : Thank you for pointing this out. The composition of the emulsions has been detailed by adding the wt% of each ingredient in the Materials and Methods section, in points 2.3 and 2.4. The composition of the surfactants has also been clarified by specifying the molar equivalents of polyglycerol‑10 and fatty acids used in the synthesis reactions. This information is provided in section 2.2, lines 124 to 126. In addition, the tables reporting the surfactant compositions have been revised to indicate that they refer to the fatty acid composition of the surfactants. Finally, chromatographic results on polyglycerol‑10 and on the relative composition (mono-, di-, triesters) of the first polyglycerol ester prototypes have been added to provide further insight into their structures (section 3.1.1, lines 233 to 248). Such analyses could not be performed on the polyglycerol esters containing mixtures of fatty acids due to the complexity of their structure and the very large number of different species present.
Comments 8 : “Overall, despite the length of the manuscript, it was not deeply discussed. A general restructuring must be considered.”
Response 8 : We have taken note of your remark. Based on your comments and those of the other reviewers, the entire manuscript has been revised. Several pieces of information have been added (including chromatographic analyses), and the section on CMCs has been removed, as it was not sufficiently relevant given the limited time available to fully interpret our results and provide a solid hypothesis. The crystallization phenomena of the surfactants have been detailed on page 9 in an effort to better support our conclusions. We hope these revisions will meet your expectations, and we remain at your disposal for any further information. Thank you. Kind regards.
Reviewer 3 Report
Comments and Suggestions for Authors1. Add some quantitative data in the abstract.
2. Add reference in section 2.2.2, 2.2.3, 2.2.4, 2.3, 2.5
3. What are the current limitations and research gaps in understanding the relationship between synthesis parameters, fatty acid composition, and emulsifying performance of polyglycerol esters in advanced emulsion systems?
4. What are the underlying mechanisms by which different synthesis methods and fatty acid chain characteristics affect the interfacial behavior and stability of polyglycerol esters in solar and nano-emulsions?
5. How do variations in synthesis routes and fatty acid profiles compare in determining the efficiency and stability of polyglycerol esters as emulsifiers in solar versus nano-emulsion applications?
6. Add future works in the conclusion.
Author Response
Comments 1 : “Add some quantitative data in the abstract.”
Response 1 : Agree. We have, accordingly, modified the abstract by explaining all the abbreviations, adding quantitative data ligne 21 and rephrasing the methodology and conclusions to make then clearer to emphasize this point. The changes can be observed from line 14 to line 22.
Comments 2 : “Add reference in section 2.2.2, 2.2.3, 2.2.4, 2.3, 2.5”
Response 2 : Thank you for pointing this out. We agree with this comment. We added reference for the section 2.2.4 Melting range and the section 2.5 Stability tests. However, it was not possible to add a reference for the sections concerning the chromatographic analyses (sections 2.2.2 and 2.2.3), as internal methods were used to perform them. No reference exists for these specific compounds, nor are there analytical standards available to compare our results with. Regarding the preparation of the sunscreen emulsion, the formulation laboratory developed a mineral O/W sunscreen emulsion specifically for this study, which is why it is also not possible to provide a reference for this part.
Comments 3 : “What are the current limitations and research gaps in understanding the relationship between synthesis parameters, fatty acid composition, and emulsifying performance of polyglycerol esters in advanced emulsion systems?”
Response 3 : Thank you for your question. At present, the existing literature on polyglycerol esters focuses exclusively on esters synthesized with pure fatty acids, meaning that only one type of fatty acid is used. To our knowledge, no studies have investigated polyglycerol esters synthesized from mixtures of fatty acids. Commercially available polyglycerol esters containing a single fatty acid (PG‑1FA) do exist; however, we have observed that such surfactants are only able to stabilize very simple emulsions. When more demanding systems require a more robust surfactant, these materials fail to provide sufficient stability and must be combined with an additional surfactant or co‑surfactant.
In this work, we investigate both the effect of esterifying several fatty acids onto the same polyglycerol backbone and the effect of combining two polyglycerol esters containing pure fatty acids (PG10‑1FAA + PG10‑1FAB) on formulation performance.
Furthermore, we did not identify any publications demonstrating the impact of synthesis parameters on the final ester composition and, consequently, on formulation performance. This study therefore provides new insights into how synthesis conditions shape the structure and functionality of polyglycerol esters. We show that the relative proportions of monoesters and diesters have a strong influence on emulsion stability.
Comments 4 : “ What are the underlying mechanisms by which different synthesis methods and fatty acid chain characteristics affect the interfacial behavior and stability of polyglycerol esters in solar and nano-emulsions?”
Response 4 : This is indeed an important question, and it is precisely what we aim to address in this article. The mechanism explaining why the fatty‑acid composition of polyglycerol esters—and therefore the synthesis parameters—plays a decisive role in the final surfactant performance is described on pages 8–9, lines 336–367. This explanation has been modified to ensure maximum clarity.
When polyglycerol esters contain only one type of fatty acid (e.g., PG10‑C12 or PG10‑C18), the crystalline structure of the surfactant becomes highly ordered, leading to the formation of large crystals at the droplet interfaces in emulsions. These large crystals can pierce the droplets and ultimately destabilize the emulsion.
In contrast, using polyglycerol esters containing several different fatty acids disrupts the crystalline packing, resulting in smaller crystals that no longer damage the droplets. Introducing multiple fatty acids increases the molecular diversity within the surfactant (e.g., monoesters‑FA1, monoesters‑FA2, diesters‑2×FA1, diesters‑2×FA2, diesters‑FA1‑FA2, triesters, etc.), which in turn reduces crystallinity and improves emulsion stability.
Comments 5 : “How do variations in synthesis routes and fatty acid profiles compare in determining the efficiency and stability of polyglycerol esters as emulsifiers in solar versus nano-emulsion applications?”
Response 5 : Regarding the sunscreen applications, they were included in this article solely to support our hypothesis concerning the role of partial glycerides. We initially hypothesized that the partial glycerides present in the MCT/S and HPKO/S prototypes enhanced the ability of these surfactants to stabilize emulsions, as these two prototypes produced stable nano‑HIPEs with all the cosmetic oils tested, whereas the C14/S prototype gave stable nano‑HIPEs with only two oils. The experiments performed with the sunscreen formulations allowed us to confirm this phenomenon. However, the influence of the synthesis routes and the fatty‑acid profiles wasn’t investigated in this part of the study. A dedicated study focusing specifically on sunscreen formulations is currently underway, and this is highlighted in the perspectives.
Comments 6 : “Add future works in the conclusion.”
Response 6 : Agree. Future research perspectives have been outlined in the conclusion section of the article (page 18, lines 709–717). These include a more detailed investigation of how the fatty‑acid composition of polyglycerol esters influences their ability to stabilize mineral sunscreen emulsions. Additional studies on the crystallinity of the various polyglycerol‑10 esters would also be valuable to better understand the mechanisms responsible for their instability under temperature variations (freeze–thaw cycles and/or storage at 50 °C). Finally, it would be worthwhile to further explore the preliminary CMC results (removed from the manuscript at the request of another reviewer) in order to establish a robust hypothesis explaining why the CMC of these polyglycerol‑10 esters increases as the average fatty‑acid chain length increases, a trend that contrasts with what is generally reported in the literature for simpler amphiphilic molecules.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsIn my opinion the authors have replied rather comprehensively and convincingly to the points raised in my report and accordingly one could go ahead with publication.
