Stability and Application Properties of Surfactant-Free Cosmetic Emulsions: An Instrumental Approach to Evaluate Their Potential

Round 1

Reviewer 1 Report

Rather pleasant manuscript.

A small note. Fig.1, 3, 7, 10. Please add what the graphic deviations in the figures correspond to (is it standard deviation?) 

Author Response

A small note. Fig.1, 3, 7, 10. Please add what the graphic deviations in the figures correspond to (is it standard deviation?)

Thank you, we have specified in the captions of the images that the standard deviation is indicated in the graphs

Reviewer 2 Report

Titel - 'emulsifier free' is not correct. further few suggestions see attached

Comments for author File: Comments.pdf

Author Response

Titel Stability and application properties of emulsifier-free cosmetic emulsions should read – surfactant free emulsions – despite the term ‘emulsifier-free’ emulsions is in use it is not correct, when speaking about Pickering emulsions, when particles are used as emulsifiers, dissolved, later adsorbed, polymers can also served as emulsifiers.

Thank you. We changed the title as suggested.

Introduction

39 ‘To achieve thermodynamic stability, surfactants are often used in mixed systems or in combination….’ colloidal dispersions, including emulsions are thermodynamically unstable in any case, therefore it should read ‘kinetic stability’ or to improve stability …therefore at 44 it should read emulsions formulation with kinetic stability

55 ‘prevents irreversible instability phenomena’ – more accurately – minimize, or almost prevents

66 ‘Unpleasant sensory feelings must be avoided in a cosmetic product, in order not to affect the patient's acceptability’ – be concise – cosmetic or pharmaceutical

We have corrected the introduction section as indicated.

2.2 methods

158 “Suppelmentary Materials”

159 ‘40° - 45° C’ should read 40 – 45 °C

163 40°C – by convention always use X °C

180 50mm – by convention always leave space between number and unit same applies to percent, i.e., 100 %

We have corrected as indicated.

174 ‘To predict physical instability phenomena, samples were subjected to strong mechanical stress conditions by using the centrifuge MPW-56 of MED, at 3000 RPM - for describing the definite stress conditions give the centrifugal acceleration, which depends on centrifuge geometry

We modified it by expressing the value of relative centrifugal force RCF, which takes into account the radius of the centrifuge.

Results

217 ‘Lower percentages of oily phase determined a reduction of the amount of interface available for the adsorption of zinc particles and phase separation occurred.’ this is nonsense – assuming droplet size is constant, the amount necessary to fully cover the interface increases with content of dispersed phase – surplus amount of emulsifier does not lead usually to destabilization (except depletion flocculation). Usually increase in disperse phase concentration leads to higher stability up-to the phase inversion point, due to increase in viscosity. Also applies to 258 ff

We modified the sentences as follows.

Line 227: “Lower percentages of oily phase determined a reduction of the viscosity of the system leading to phase separation and flocculation of zinc oxide particles”

Line 268: “The consequent increase in the percentage of aqueous phase resulted in a decrease in the viscosity of the system, accelerating the onset of instability phenomena.”

232 ff ‘However, these systems were characterized by a pronounced sticky and greasy after-feel due to the high concentration of lipids..’ so then, what is the rational to test further with 40 % oil, i.e., AZ15 O40?

The powders A (starch) and Z (zinco oxide) used alone were not able to reduce the greasiness given by the high percentage of oily phase. We investigated whether the binary association A-Z mitigated this sensory unpleasant effect.

278 ‘has been extensively studied in the scientific literature [x]’ Reference?

Reference n. 23

284 ‘As revealed by the results of the mechanical stress test in centrifuge conducted at 3000 and 4800 rpm, emulsions with xanthan gum were not stable’ – it should be mentioned, that dispersions stabilized by xanthan and alike, usually have high stability at lower stress conditions but have pronounced shear thinning properties!

We have added this information in line 290: “…which are used in the cosmetic formulation for their efficient viscosifying and texture properties, showing high stability at low stress conditions and pronounced shear thinning properties”

277 succynoglycan – should read succino…

468 ‘The system was stable when formulated with esters of long-chain saturated fatty acids’ – according to classification should read medium chain long chain is usually > C12

We modified as indicated.

4 Conclusions

‘emulsifier-free’

for supplement also correct 40 °C x % w/w qb. 100 4500 rpm (S4)

S2 ‘ Z was dispersed in oil phase at 45°c 45 °C

ZO series 20 – 40

why use 0.1 and in other tables 0,1

Thank you. We modified as suggested.

Reviewer 3 Report

Comments and Suggestions for Authors

The paper from Giovanni Tafuro and colleagues presents very interesting research aimed to formulate an oil-in-water fluid emulsion stabilized by the combination of organic and inorganic powders, in presence of a polysaccharidic rheological modifier. Nowadays when the cosmetics industry is looking for new ecological safe products, research on Pickering emulsions is particularly important.

The article is well organized, nevertheless I have some comments and aspects which could enhance the quality of your manuscript

1. In section materials there is no information about zea mays starch and zinc oxide characteristics i.e. particle size (nano or mikrometers?), bulk density, oil and water absorption capacity etc., especially that used zea mays starch is mixture of few chemical compounds instead pure starch.

2. Are the analyzed starch concentrations in line with the manufacturer's recommendations? For this group of raw materials (CELUS BI FEEL) you can find information that concentration in the product is in the range 1-3%. In this study  optimal concentration was 5%, and the concentrations up to 15% were analyzed.

3. Does the zinc oxide used in the tests have a declaration that it can act as a UV filter? Some producers provide information that, for example, 1% of the zinc oxide used in recipe increases the SPF factor by 1 degree. If so, why was it not included in the SPF calculation (table S1?)

Also in Table S1 I cant agree with term: labeled for SPF, because the SPF was determined only by SPF calculator. There wasn’t experimental part of it.  

4. How did Authors asses stickiness and greasiness of formulation? There is no information about sensory analysis or visual assessment of obtained emulsions.

5. Why Authors didn’t’ determined pH of formulations and didn’t perform accelerating aging of sample instead second cycle of centrifugation?

6. line 250 Figure 1b instead 11b

7. Line 278: scientific literature [x].  Shouldn't there be numbers instead of x?

Author Response

1. In section materials there is no information about zea mays starch and zinc oxide characteristics i.e. particle size (nano or mikrometers?), bulk density, oil and water absorption capacity etc., especially that used zea mays starch is mixture of few chemical compounds instead pure starch.

Thank you for your suggestion. We added a brief description of the powders in the Material section (line 117-124) based on the information reported in the technical data sheet.

2. Are the analyzed starch concentrations in line with the manufacturer's recommendations? For this group of raw materials (CELUS BI FEEL) you can find information that concentration in the product is in the range 1-3%. In this study optimal concentration was 5%, and the concentrations up to 15% were analyzed.

The concentration range shown in the technical data sheet refers to the use of the raw material as a texturizer, not as an emulsifier. In this study we have seen that higher concentrations are required to stabilize a surfactant-free emulsion.

3. Does the zinc oxide used in the tests have a declaration that it can act as a UV filter? Some producers provide information that, for example, 1% of the zinc oxide used in recipe increases the SPF factor by 1 degree. If so, why was it not included in the SPF calculation (table S1?)

The manufacturer does not provide specifications on the possible use of this raw material as a physical sun filter. For this reason, even if by its nature it should exert a minimum of protection against UV radiation, we have not considered it in the calculation of the protection factor.

Also in Table S1 I cant agree with term: labeled for SPF, because the SPF was determined only by SPF calculator. There wasn’t experimental part of it.

We changed the term labelled with “Final theoretical SPF” in Table S1. 

4. How did Authors asses stickiness and greasiness of formulation? There is no information about sensory analysis or visual assessment of obtained emulsions.

The stickiness and greasiness of formulations is correlated with the texture parameters of adhesiveness and stringiness, measured through an instrumental immersion/de-immersion test, as reported in literature. Several studies investigated the correlations between texture and sensory analysis. We added some of these studies in the bibliography (References n. 11 – 13 – 14 – 15)

5. Why Authors didn’t’ determined pH of formulations and didn’t perform accelerating aging of sample instead second cycle of centrifugation?

We determined the pH of the formulations, which was about 7.5 (we added a sentence in line 178).

We performed accelerating aging test on the samples with preservatives, filters and pigments as described in the methods section (line 182 – 185). The results are reported in lines 525-528, 561-563, and 620-622

6. line 250 Figure 1b instead 11b

Corrected

7. Line 278: scientific literature [x]. Shouldn't there be numbers instead of x?

Reference n. 23

Round 2

Reviewer 3 Report

I accept corrections and recommend manuscript for publication in present form.