# Cosmetic creams with caprylic acid-based Natural Eutectic Solvents: Stability, rheology and user perception

Alexis Verger, Roxane Grard, Charlotte Pradel, Xavier Perse, Salima Bouderbala, Alexandra Despres, Leslie Boudesocque-delaye, Emilie Munnier

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Alexis Verger, Roxane Grard, Charlotte Pradel, Xavier Perse, Salima Bouderbala, et al.. Cosmetic creams with caprylic acid-based Natural Eutectic Solvents: Stability, rheology and user perception. International Journal of Cosmetic Science, 2025, 10.1111/ics.70051. hal-05510895

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
ORIGINAL ARTICLE

International Journal of
Cosmetic Science

SOCIÉTÉ FRANÇAISE DE
COSMOLOGIE

<figure><img src="image_2.png" /></figure>

# Cosmetic creams with caprylic acid-based Natural Eutectic Solvents: Stability, rheology and user perception

Alexis Verger<fnref n="1" /> | Roxane Grard<fnref n="1" /> | Charlotte Pradel<fnref n="1" /> | Xavier Perse<fnref n="1" /> |

Salima Bouderbala<fnref n="1" /> | Alexandra Despres<fnref n="2" /> | Leslie Boudesocque-Delaye<fnref n="3" /> |

Emilie Munnier<fnref n="1" /> ID

## Correspondence

Emilie Munnier, UPR CNRS 4301 CBM, Département NMNS, Université de Tours, 37200 Tours, France.

Email: emilie.munnier@univ-tours.fr

## Funding information

Conseil Régional du Centre-Val de Loire, ARD CVL Cosmétosciences, Grant/Award Number: 2020-00141275; Agence Nationale de la Recherche, Grant/Award Number: ANR-22-CE43-0006-04

## Abstract

**Objective:** This study investigates the impact of hydrophobic Natural Eutectic Solvents (NES) on the physicochemical and sensory characteristics of cosmetic creams. The aim is to assess their potential as innovative, sustainable ingredients introduced directly into the oily phase of an oil-in-water emulsion prior to emulsification.

**Methods:** Four NES formulations, composed of natural components such as caprylic acid, lauric acid, menthol and 1,3-propanediol were incorporated at 10% wt. into a COSMOS-compliant cream chassis. The resulting creams were evaluated for physical stability using static multiple light scattering (Turbiscan®), rheological behaviour via shear and oscillatory tests, and sensory attributes through a trained panel.

**Results:** Among the four NES-containing creams, three formulations demonstrated satisfactory physical stability over 30 days under accelerated ageing conditions. Menthol-based NES induced phase separation, correlating with increased droplet size and higher Turbiscan Stability Index values. All creams exhibited shear-thinning behaviour, with viscosity profiles and yield stress varying depending on NES composition. Sensory analysis revealed that NES significantly influenced key attributes such as odour intensity, spreading, pick-up and firmness.

**Conclusion:** Hydrophobic NES can be successfully integrated into the internal phase of cosmetic creams, influencing both their structural and sensory properties. These findings highlight their potential as multifunctional and eco-friendly ingredients in cosmetic formulation, supporting the development of more sustainable, more performant skincare products.

## KEYWORDS

chemical analysis, emulsion, formulation/stability, hydrophobic natural deep eutectic solvents, rheology, sensory evaluation

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *International Journal of Cosmetic Science* published by John Wiley & Sons Ltd on behalf of Society of Cosmetic Scientists and the Société Française de Cosmétologie.

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<footnote n="1">¹Université de Tours, UPR CNRS 4301 CBM, Département NMNS, Tours, France</footnote>

<footnote n="2">²RCP-Design Global, Tours, France</footnote>

<footnote n="3">³Université de Tours, UR 7502 SIMBA, Faculté de Pharmacie, Tours, France</footnote>
Résumé

**Objectif:** Cette étude examine l'impact des solvants eutectiques naturels (Natural Eutectic Solvents, NES) hydrophobes sur les caractéristiques physicochimiques et sensorielles des crèmes cosmétiques. L'objectif est d'évaluer leur potentiel en tant qu'ingrédients innovants et durables introduits directement dans la phase huileuse d'une émulsion huile dans l'eau avant l'émulsification.

**Méthodes:** Quatre formulations de NES, constituées de composants naturels tels que l'acide caprylique, l'acide laurique, le menthol et le 1,3-propanediol, ont été incorporées à 10% en poids dans un châssis de crème conforme à COSMOS. La stabilité physique des crèmes résultantes a été évaluée à l'aide d'une diffusion statique de lumière multiple (Turbiscan®), le comportement rhéologique par des tests de cisaillement et d'oscillation, et les attributs sensoriels par le biais d'un comité d'évaluation formé.

**Résultats:** Parmi les quatre crèmes contenant des NES, trois formulations ont démontré une stabilité physique satisfaisante sur 30 jours dans des conditions de vieillissement accélérées. Les NES à base de menthol ont induit une séparation de phase, corrélée à une taille de gouttelettes accrue et à des valeurs plus élevées de l'indice de stabilité de Turbiscan. Toutes les crèmes ont montré un comportement de rhéofluidification, avec des profils de viscosité et des contraintes de rendement variables en fonction de la composition en NES. L'analyse sensorielle a révélé que les NES influençaient de manière significative des caractéristiques clés telles que l'intensité des odeurs, la propagation, la récupération et la fermeté.

**Conclusion:** Les NES hydrophobes peuvent être intégrés avec succès dans la phase interne des crèmes cosmétiques, influençant à la fois leurs propriétés structurelles et sensorielles. Ces résultats soulignent leur potentiel en tant qu'ingrédients multifonctionnels et respectueux de l'environnement dans la formulation cosmétique, soutenant ainsi le développement de produits de soins de la peau plus durables et plus performants.

INTRODUCTION

In recent years, the cosmetic industry has undergone a significant transformation, driven by an increasing consumer demand for sustainable, eco-friendly products that not only offer effective skincare but also demonstrate a commitment to minimizing environmental impact. This shift is part of a larger societal movement toward sustainability, where modern consumers are more conscious of their ecological footprint and seek products that align with their values. Transparency, ecological responsibility and product safety are now central to consumers' purchasing decisions, fostering a new wave of innovation within the cosmetic sector [1]. In response to this evolving demand, researchers and formulators are actively exploring novel ingredients and technologies that adhere to the principles of green chemistry while maintaining or enhancing product performance. Cosmetic formulations

labelled as ‘green’ or ‘sustainable’ often rely on natural, renewable and environmentally friendly ingredients. These products aim to reduce the use of synthetic chemicals, minimize carbon footprints and incorporate biodegradable materials, setting new standards for ethical consumer goods [2], in accordance with several of the 17 UN sustainable development goals. Beyond the environmental aspect, there is a growing awareness of the potential health risks associated with the use of synthetic compounds in daily skincare products. Consumers are increasingly concerned about the safety of ingredients that may pose risks to their skin health or overall well-being. This dual focus on sustainability and safety has propelled the cosmetic industry to prioritize the development of innovative formulations that meet these evolving expectations without compromising efficacy or consumer sensory experience [3]. In this context, Natural Eutectic Solvents (NES) have emerged as a promising class of ingredients
that align with the principles of green chemistry and sus-
tainable formulation. NES are composed of natural com-
pounds such as organic acids, amino acids, fatty acids and
sugars, which, when combined in specific molar ratios,
form stable eutectic mixtures. The melting point of these
mixtures is lower compared to that of each ingredient.
This is due to favourable van der Waals interaction, ex-
tensive hydrogen bonding and positive entropy of mixing
among components [4]. These solvents exhibit unique
physicochemical properties, setting NES apart from con-
ventional solvents. Notably, NES are biodegradable, bio-
compatible and often derived from renewable resources,
making them an attractive option for eco-conscious for-
mulators aiming to reduce the environmental impact of
their products [5-7]. Less studied than their hydrophilic
equivalents, hydrophobic NES (HNES), demonstrate their
ability to dissolve poorly water-soluble active compounds
[4], such as vitamins [8], essential oils [9] and antioxidants
[10], enhancing their stability and bioavailability. HNES
can be composed of ionic or neutral compounds of various
natures, notably quaternary ammonium salts, but those
compliant with European cosmetic regulation are usually
composed of medium chain middle-length fatty acids (e.g.
caprylic acid, capric acid and lauric acid), terpenes (men-
thol and thymol) and alkane diols (propane diol and oc-
tane diol) and more rarely of amino acids [4, 11]. Recently,
HNES have shown great promise in the cosmetic field for
the preparation of ready-to-use extracts containing apolar
molecules of interest like lycopene, astaxanthin esters,
carotenoids or free fatty acids from vegetal or microalgal
biomasses [12-14].

To examine the formulability of such ready-to-use
HNES-based extracts, the present study aims to evaluate
for the first time the impact of a selection of hydrophobic
NES on the stability, rheological behaviour and sensory
properties of a cosmetic cream. Specifically, the effects of
four NES combinations made of potential cosmetic ingre-
dients from the three principal classes of HNES neutral
components were explored. For comparison purposes,
since all NES tested were based on the medium-chain
fatty acid caprylic acid (C8), a medium-chain triglycer-
ide (MCT) was chosen as the control. MCTs are widely
used in cosmetic formulations due to their reproducibil-
ity, safety and regulatory acceptance, making them a rel-
evant benchmark for assessing the specific contribution
of NES to cream properties. The control MCT was a stan-
dard mixture composed predominantly of caprylic acid
(C8, 50-80%) and capric acid (C10, 20-50%) in the form
of purified triglycerides, used as the main component of
the oily phase. Combinations were proposed based on the
previous works of Boudesocque-Delaye and colleagues.
Indeed, Wils et al. and Hilali et al. demonstrated the
interest of C8-based combinations, at the same or very

close ratios, in the extraction of molecules of interest for
cosmetics like phycoproteins, carotenoids and polyun-
saturated fatty acids from microalgae [12, 15, 16]. These
combinations were also chosen to approach the impact
of hydrophobicity and the type of bond network inside
the NES. First, a combination of two medium-chain fatty
acids caprylic acid/lauric acid (4:1, mol/mol, C8C12),
characterized by a dense van der Waals bond network
was chosen for its very high hydrophobicity. Then combi-
nations of caprylic acid and 1,3-propanediol, at 2 differ-
ent ratios (1:2, mol/mol, C8P<sub>1:2</sub> and 5:1, mol/mol, C8P<sub>5:1</sub>)
were chosen to evaluate the impact of the variation of
hydrophobicity on the stability of emulsions. Next, a
combination with a terpene was evaluated: caprylic
acid/menthol (1:1, mol/mol, C8M). This study aims to
investigate how these NES combinations influence the
consumer experience by modifying the properties of cos-
metic creams, specifically focusing on product stability,
rheological behaviour and sensory attributes. The four
HNES were introduced in the internal oily phase of a
COSMOS-compliant chassis cream, at a level of 10%wt.,
simulating the introduction of a NES-based extract con-
taining active molecules. The obtained creams were then
studied in terms of stability, rheological behaviour and
sensory properties to evaluate the formulability of this
new type of ingredient.

MATERIALS AND METHODS

Chemicals

Glycerin, 99+, lauric acid (C12), 99%, caprylic acid (C8), 99% and 1–3, propanediol, 98% were purchased from Thermo Scientific (Waltham, USA). Xanthan gum and menthol were obtained from COOPER (Melun, France). Amigum® (sclerotium gum), was a kind gift from Alban Muller (Fontenay-sur-Eure, France). Labrafac® Lipophile WL 1349 (caprylic/capric triglyceride, MCT) was acquired from Gattefossé (Saint-Priest, France). Lipoid P75-3® (hydrogenated lecithin), was a kind gift from Lipoid (Ludwigshafen, Germany). Benzyl alcohol and dehydroacetic acid were purchased from Fisher Scientific (Illkirch, France). Water was purified using a Milli-Q system (Millipore Corporation, Bedford, MA, USA).

NES formulation

NES C8C12 (caprylic acid/lauric acid, 4:1, mol/mol), C8P<sub>1:2</sub> (caprylic acid/1,3-propanediol, 1:2, mol/mol), C8P<sub>5:1</sub> (caprylic acid/1,3-propanediol, 5:1, mol/mol), C8M (caprylic acid/menthol, 1:1, mol/mol) were prepared by
<table><caption>TABLE 1 Quantitative composition of the creams with and without hydrophobic NES.</caption><thead><tr><th rowspan="2">Ingredients</th><th rowspan="2">Function</th><th colspan="2">%wt. in the analysed samples</th></tr><tr><th>Control</th><th>NES creams</th></tr></thead><tbody><tr><td>NES (%wt.)</td><td>Solvent</td><td>0</td><td>10</td></tr><tr><td>Xanthan gum (%wt.)</td><td>Thickener</td><td>0.25</td><td>0.25</td></tr><tr><td>Sclerotium gum (%wt.)</td><td>Thickener</td><td>0.75</td><td>0.75</td></tr><tr><td>Glycerol (%wt.)</td><td>Humectant</td><td>10</td><td>10</td></tr><tr><td>Hydrogenated lecithin (%wt.)</td><td>Emulsifier</td><td>3</td><td>3</td></tr><tr><td>Caprylic/capric triglycerides (%wt.)</td><td>Emollient</td><td>25</td><td>15</td></tr><tr><td>Dehydroacetic acid/benzyl alcohol mix (%wt.)</td><td>Antimicrobial agent</td><td>1</td><td>1</td></tr><tr><td>Water (% wt.)</td><td>Solvent</td><td colspan="2">q.s 100</td></tr></tbody></table>

<table><caption>TABLE 2 Formula numbering.</caption><thead><tr><th>NES</th><th>Ingredient code</th><th>Cream code</th></tr></thead><tbody><tr><td>Medium chain triglyceride</td><td>MCT</td><td>Control</td></tr><tr><td>C8/C12 (4:1, mol/mol)</td><td>C8C12</td><td>C_C8C12</td></tr><tr><td>C8/1,3-propanediol (5:1, mol/mol)</td><td>C8P<sub>5:1</sub></td><td>C_C8P<sub>5:1</sub></td></tr><tr><td>C8/1,3-propanediol (1:2, mol/mol)</td><td>C8P<sub>1:2</sub></td><td>C_C8P<sub>1:2</sub></td></tr><tr><td>C8/Menthol (1:1, mol/mol)</td><td>C8M</td><td>C_C8M</td></tr></tbody></table>

mixing components together. The mixtures were heated at 50°C and stirred by magnetic stirring for 1 h until a colourless liquid was obtained. After returning to room temperature (for about 1 h 30), NES was stored for a maximum of 7 days at 20°C in an oven before introduction into the creams. Density values were determined with a DMA 501 instrument (Anton Paar) at 25°C. Viscosity was measured using a Kinexus pro+ rheometer (Netzsch) with a sand-blasted Couette geometry. After loading, samples were allowed to relax and acclimatize for a minimum of 5 min prior to measurement. Apparent viscosity values were recorded by subjecting the samples to increasing shear rates ranging from 0.1 to 1000 s⁻¹ for a duration of 5 min at 25 points per decade.

## Cream formulation

A COSMOS (COSMetic Organic and Natural Standard) certifiable chassis cream formula based on a direct emulsion (o/w) was selected to measure the formulability of 10% wt. of a hydrophobic NES. They were replaced by an equivalent weight of caprylic/capric triglyceride in the control formulations. This formula is reported in Table 1.

Creams were produced at room temperature with a DAC 150.1 CM148 mixer (Hauschild SpeedMixer; Hamm, Germany). First, the aqueous phase was prepared by placing the components (glycerin, water and the mix of thickeners) in a SpeedMixer® mixing cup containing the Hauschild pulverizing ring set, then mixing for 4 min at 1800 rpm. Then, the emulsion was made by adding directly in the same cup, on top of the aqueous phase, the components of the oily phase (medium chain triglycerides, depending on the sample hydrophobic NES, the preservative and the emulsifier) and mixing for 2 min at 1800 rpm. Each assay was performed in triplicate (n=3). Creams were named C_C8C12, C_C8P<sub>5:1</sub>, C_C8P<sub>1:2</sub> and C_C8M following the correspondence in the Table 2.

They were stored in a climatic chamber (Memmert, Büchenbach, Germany) at 40°C with 75% humidity for 30 days for stability studies.

## Static Multiple Light Scattering-based stability analysis

Static Multiple Light Scattering-based stability analyses were done with the Turbiscan® instrument (Microtrac, Toulouse, France). TSI (Turbiscan® Stability Index), a value calculated by the Turbiscan® software, approximates the stability of the emulsion. The droplet size is also calculated thanks to the software. The analysis was made 24 h after the preparation on each sample and at D30 in a 4 mL vial.

## Rheology

Rheological characterization was conducted using a Kinexus pro+ rheometer (Netzsch, Germany) at a
<table><caption>TABLE 3 Viscosity and density of the internal phases of the creams (25°C).</caption><thead><tr><th>Cream</th><th>Composition of the dispersed phase</th><th>η (Pa·s)</th><th>Density</th></tr></thead><tbody><tr><td>Control</td><td>MCT 100% wt.</td><td>0.032±0.003</td><td>0.944±0.038</td></tr><tr><td>C_C8C12</td><td>C8C12 40%wt./MCT 60% wt.</td><td>0.014±0.001</td><td>0.929±0.046</td></tr><tr><td>C_C8P<sub>5:1</sub></td><td>C8P<sub>5:1</sub> 40%wt./MCT 60% wt.</td><td>0.015±0.001</td><td>0.937±0.037</td></tr><tr><td>C_C8P<sub>1:2</sub></td><td>C8P<sub>1:2</sub> 40%wt./MCT 60% wt.</td><td>0.021±0.001</td><td>0.942±0.047</td></tr><tr><td>C_C8M</td><td>C8M 40%wt./MCT 60% wt.</td><td>0.016±0.001</td><td>0.928±0.046</td></tr></tbody></table>

temperature of 25°C. All measurements were performed in triplicate, with a fresh sample loaded for each run. After loading, samples were allowed to relax and acclimatize for a minimum of 15 min prior to measurement.

## Shear test

To assess the flow properties of the cream systems, continuous shear flow tests were conducted using a sanded cone-plate geometry (4° angle, 40 mm diameter). Apparent viscosity values were recorded by subjecting the samples to increasing shear rates ranging from 0.1 to 1000 s⁻¹ for a duration of 150 s.

## Oscillation tests

The deformation sweep experiment was performed at a frequency of 1 Hz and a strain ranging from 0.1% to 100%. The frequency sweep experiment was carried out at a strain of 0.5% belonging to the linear regime and the frequency varied from 50 Hz to 0.1 Hz.

## Sensory analysis - descriptive sensory profiles

Preliminary sensory evaluation of the cream formulations was carried out fresh samples 24 h after their preparation. Sensory profiles were carried out by a panel of five trained subjects selected based on their sensory skills, their ability to describe a product, their perceptions, their ability to memorize, their motivation and their availability. Ten sensory attributes were evaluated for each cream: spreading, stringiness, freshness, soaping, visual and tactile penetration, stickiness, odour intensity, pick-up, colourshade and firmness; and a score was attributed from 0 to 10. Score limits were determined by the references of the EBITouch® kit (EBI, Cergy, France) [17], except for odour intensity, where 0 corresponds to an empty jar and 10 an internal laboratory reference.

## Statistical analysis

Results are expressed as mean values ± standard deviation (SD). The Mann-Whitney test was used for statistical analysis using the GraphPad Prism 8.0 software. Viscoelastic modulus results and relaxation times were obtained individually for each replicate using the Rspace software supplied with the rheometer and then averaged using R software.

# RESULTS

## NES and NES-based oily phases characterization

The viscosity measurements of the Natural Eutectic Solvents (NES) at 25°C revealed differences among the tested formulations, indicating the impact of their respective compositions on rheological behaviour. All NES exhibited Newtonian flow, characterized by a constant low viscosity independent of the applied shear rate, comprised between 0.008 (C8C12) and 0.028 Pa·s (MO), when capric/caprylic triglyceride (MCT) used as the internal phase in the control cream shows a viscosity of 0.032 Pa·S (Table S1). This low viscosity is usual for neutral-components-based HNES [4]. Their density is comprised between 0.904 and 0.981, depending on the components and ratios between those components, when MCT shows a density of 0.944 (Table S1). Every NES was miscible with MCT. Each NES-enriched cream contains 10%wt. NES, which represents 40%wt. of the oil phase. This high proportion of NES can modify significantly the density and viscosity of the MCT-based dispersed phase, as presented in Table 3.

All NES-based oily phases show low viscosities, all inferior to the reference MCT. Two groups seem to appear: oily phases enriched with NES mainly composed of C8 show lower viscosity compared to the oily phase containing the NES mainly composed of 1,3 propane diol. Concerning densities, oily phases based on NES containing alkane diol show a density very close to the control, whereas the other ones show a slightly inferior density.
## NES-enriched cream analysis

Creams were analysed 24h after preparation (D1) and after 30 days in an oven at 40°C with 75% relative humidity (D30). After this month of storage in accelerated ageing conditions, only four of the five formulations were physically stable: C_C8M showed a phase separation for the three samples stored. Only D1 results are presented for this formulation.

## Static Multiple Light Scattering-based analysis

The stability of the creams was evaluated using the Turbiscan® instrument, which provided the Turbiscan Stability Index (TSI) and droplet size measurements at 24h (D1) and after 30 days (D30). The TSI is a calculated value helping to identify dispersions more susceptible to being physically unstable very soon after their preparation. It is admitted that the lower the TSI, the more stable the dispersion. Considering droplet size, the calculation from the SMLS results leads to overestimated values compared to optical microscopy, for instance, but permits comparison of samples analysed in the same conditions. The results are summarized in Table 4.

After 30 days in accelerated conditions, one cream among the five prepared exhibited a phase separation: C_C8M. Indeed, this cream showed the highest TSI (2.10) and droplet size (17.60 µm, as calculated from de SMLS results) at D1. Two other creams presented a TSI >1, C_C8P<sub>1.2</sub> and C_C8P<sub>5.1</sub> but were visually stable after 1 month in accelerated ageing conditions.

<table><caption>TABLE 4 Turbiscan Index Stability (TSI) and droplets diameter of creams with or without hydrophobic NES.</caption><thead><tr><th rowspan="2">Samples</th><th>TSI</th><th colspan="2">Droplets diameter (µm)</th></tr><tr><th>D1</th><th>D1</th><th>D30</th></tr></thead><tbody><tr><td>Control</td><td>1.00±0.1</td><td>9.28±0.21</td><td>9.56±0.18</td></tr><tr><td>C_C8C12</td><td>0.83±0.60</td><td>6.71±1.15</td><td>9.02±1.43</td></tr><tr><td>C_C8P<sub>5.1</sub></td><td>1.17±0.12</td><td>8.52±0.95</td><td>9.12±0.85</td></tr><tr><td>C_C8P<sub>1.2</sub></td><td>1.10±0.19</td><td>11.74±0.58</td><td>8.75±0.93</td></tr><tr><td>C_C8M</td><td>2.10±0.60</td><td>17.60±0.14</td><td>N/A</td></tr></tbody></table>

Note: Data represent the mean ± SD.

<table><caption>TABLE 5 Viscosity values (Pa·s) measured at different shear rates (s⁻¹) during flow tests of the studied creams.</caption><thead><tr><th>Samples</th><th>η (0.1)</th><th>η (1)</th><th>η (10)</th><th>η (100)</th><th>η (1000)</th></tr></thead><tbody><tr><td>Control</td><td>221.10±12.47</td><td>29.14±2.11</td><td>4.08±0.09</td><td>0.67±0.02</td><td>0.12±0.01</td></tr><tr><td>C_C8C12</td><td>101.67±8.69</td><td>23.27±2.13</td><td>3.05±0.21</td><td>0.49±0.03</td><td>0.12±0.01</td></tr><tr><td>C_C8P<sub>5.1</sub></td><td>126.93±22.97</td><td>24.84±0.74</td><td>3.10±0.02</td><td>0.50±0.01</td><td>0.11±0.01</td></tr><tr><td>C_C8P<sub>1.2</sub></td><td>163.20±11.55</td><td>22.67±1.32</td><td>2.96±0.13</td><td>0.53±0.01</td><td>0.11±0.00</td></tr><tr><td>C_C8M</td><td>231.50±8.67</td><td>30.23±0.85</td><td>4.22±0.12</td><td>0.70±0.02</td><td>0.13±0.01</td></tr></tbody></table>

In terms of droplet size, the control formulation remained relatively stable, with a slight increase from 9.28 µm (D1) to 9.56 µm (D30) that is not considered significant considering the method of determination. NES-containing creams displayed greater variability in droplet size evolution. C_C8C12, C_C8P<sub>5.1</sub> and C_C8P<sub>1.2</sub> showed initial droplet sizes superior to or inferior to the control, but after 30 days showed values very close to the control, around 9 µm, suggesting that a time of equilibration is necessary to observe the systems.

## Rheology

The rheological properties of the creams were evaluated to compare the effects of different NES on structure and texture. These measurements provide critical insights into the flow behaviour, structural integrity and application characteristics of the formulations, ensuring consistency in production and consumer experience. Rheological studies were conducted at 25°C. This choice of temperature aligns with actual usage conditions of the formulation, ensuring the data reflects practical application. It also assesses product stability and user comfort [18]. Continuous shear flow tests were conducted using a Kinexus pro+ rheometer and viscosity values were recorded at increasing shear rates (0.1, 1, 10, 100 and 1000 s⁻¹), as summarized in Table 5. The viscosity curves for the control and NES-containing formulations are illustrated in Figure 1.

The viscosity profiles of all formulations demonstrated a non-Newtonian, shear-thinning behaviour, where viscosity decreases as shear rate increases. This indicates that the creams become less resistant to flow under applied stress, a desirable property for ease of application. This rheological profile is characteristic of many cosmetic emulsions and ensures ease of application, as viscosity decreases under shear stress [19]. Among the stable formulations, at low shear rates (0.1 s⁻¹), the control sample exhibited the highest viscosity (221.10 Pa·s), significantly (p-value: 0.003) higher than all NES formulations, indicating a thicker texture and slower flow under low-stress conditions. C_C8P<sub>1.2</sub> had the highest viscosity (163.20 Pa·s), followed by C_C8P<sub>5.1</sub> (126.93 Pa·s) and C_C8C12 (101.67 Pa·s). The observed differences may
<figure><img src="image_4.png" /><figcaption>FIGURE 1 Flow properties of cream C\_C8C12 (blue), C\_C8P\_5:1 (yellow), C\_C8P\_1:2 (red), C\_C8M (green) and control (black).</figcaption></figure>

stem from the intrinsic viscosity of the NES and their impact on the three-dimensional network of the emulsion [20]. NES that interact more strongly with the aqueous phase and emulsifiers may enhance structuring, leading to higher viscosity [21]. It is interesting to note that the finally unstable formulation exhibited higher viscosity than the control, probably linked to the larger droplet size or to interactions between an ingredient of the NES and the thickener present in the formula. However, at higher shear rates (100–1000 s⁻¹), the viscosity values of all formulations converged, indicating that during spreading, the perceived differences between formulations would be minimal [22].

The strain sweep test results (Table 6) highlight the linear viscoelastic region (LVR) and flow threshold (yield stress) of the formulations. The control formulation exhibited the highest flow threshold (38.83 Pa), followed by C\_C8M (33.34), indicating greater structural resistance before yielding. C\_C8C12 and C\_C8P\_5:1 had intermediate flow thresholds (28.14 Pa and 27.07 Pa, respectively), while C\_C8P\_1:2 exhibited the lowest threshold (23.19), demonstrating a lower resistance to flow. This suggests that NES formulations require less stress to begin flowing, aligning with their lower viscosity values. From a practical standpoint, this could impact product spreadability and consumer perception, making the formulations feel lighter during application [23, 24].

The frequency sweep tests (Figure 2) evaluated the storage modulus (G') and loss modulus (G'') to determine the viscoelastic behaviour of the creams. In all formulations, G' > G'', indicating that the creams behave as viscoelastic solids, meaning they retain structural integrity while displaying some flow characteristics. However, only C8P\_1:2 showed a measurable relaxation time (0.025 s), suggesting stronger molecular interactions within the formulation. This could indicate a higher degree of internal structuring

<table><caption>TABLE 6 Rheological data (means ± SD) obtained from the strain sweep tests.</caption><thead><tr><th>Samples</th><th>Linear viscoelastic region (LVR) (Pa)</th><th>Flow threshold (Pa)</th></tr></thead><tbody><tr><td>Control</td><td>2.43 ± 0.18</td><td>38.83 ± 0.99</td></tr><tr><td>C\_C8C12</td><td>2.09 ± 0.25</td><td>28.14 ± 1.20</td></tr><tr><td>C\_C8P\_5:1</td><td>1.82 ± 0.44</td><td>27.07 ± 0.53</td></tr><tr><td>C\_C8P\_1:2</td><td>2.71 ± 0.49</td><td>23.19 ± 2.13</td></tr><tr><td>C\_C8M</td><td>3.57 ± 0.39</td><td>33.34 ± 1.51</td></tr></tbody></table>

or network elasticity, which may contribute to long-term stability [25]. The control formulation exhibited higher viscosity and a higher flow threshold, suggesting greater resistance to deformation. Among the NES formulations, C8P\_1:2 demonstrated the highest viscosity and strongest elasticity, suggesting a more structured internal network [26]. Despite these differences, the shear-thinning behaviour and viscoelastic properties remained consistent across all formulations, ensuring a uniform consumer application experience. These findings underscore the potential of NES to influence the textural and stability characteristics of emulsions, providing useful insights for optimizing cosmetic formulations.

## Sensory analysis

Control cream is a white, firm and high-penetrating cream. Ten sensory attributes were evaluated for each stable cream to evaluate the impact of the introduction of 10% of HNES in the internal phase of a cream on the consumer experience. Only five of them revealed significantly different results from one NES-cream to another.

Odour intensity was significantly (p-value: 0.001) higher in all NES-containing formulations, each scoring 9, compared to the control, which had a score of only 1. This indicates that the presence of NES strongly influences the olfactory profile of the creams, due to the presence of caprylic acid in their composition. This could also be due to NES's ability to enhance the release of aromatic molecules [27]. No HNES tested modified the stickiness or stringiness of the cream: the scores of the 3 creams were all equal to the control, that is 4 and 6, respectively. Considering spreading, all NES increase the spreading score compared to the control (8 and 6, respectively). Creams C\_C8P\_5:1 and C\_C8C12 provide a sensation of freshness from 30 s after the application to 2 min. The scores obtained for the discriminatory attributes are summarized in Figure 3. Regarding colour shade, the control formulation received the lowest score (1), whereas C\_C8C12 (4:1) exhibited the highest score (9), followed by C\_C8P\_5:1 and C\_C8P\_1:2 (both 7), suggesting that caprylic acid is the major NES
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>FIGURE 2 Oscillatory frequency sweep test results for the formulation C\_C8C12 (a), C\_C8P\_5:1 (b), C\_C8P\_1:2 (c), C\_C8M (d) and control. G' and G'' moduli were represented by red curves (with circle marks) and black curves (with triangle marks) respectively.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>FIGURE 3 Sensory properties of cream C\_C8C12 (blue), C\_C8P\_5:1 (yellow), C\_C8P\_1:2 (red) and control (black).</figcaption></figure>

component increasing the colour of the cream. The pick-up property, which measures the adherence of the cream to the skin, varied significantly (p-value: 0.014) among formulations. C\_C8P\_5:1 (9) and C\_C8C12 (8) demonstrated the highest pick-up values, whereas C\_C8P\_1:2 showed a much lower value (2). This suggests that certain NES compositions improve the ability of the formulation to remain on the skin upon application, potentially due to their influence on interfacial interactions and film-forming properties [28]. Firmness assessments revealed that the control sample was the firmest, scoring 9 whereas the NES-based formulations were scored 1. Such properties may enhance consumer experience by making the creams easier to apply and more comfortable on the skin [29].

When creams are applied on the skin, a whitening effect can occur after 30 rotations. It is identical to the control for C\_C8C12 (2) and higher than the control for C\_C8P\_5:1. Interestingly, the introduction of a NES rich in hydrophilic 1,3 propanediol decreases the whitening effect of the formula, C\_C8P\_1:2 showing the smaller score with 1. Finally, tactile penetration was observed after the application of a specific amount of cream. Only C\_C8C12 had the same penetration capacities as the control (7). The two other creams showed a lower penetration capacity in the measurement conditions. The cream with the highest concentration of 1,3 propane diol, a well-known humectant, was the less penetrating cream (2), while C\_C8P\_5:1 showed an intermediate score of 5.

## DISCUSSION

This study investigated the effects of caprylic acid-based hydrophobic Natural Eutectic Solvents (HNES) on the stability, rheological behaviour and sensory characteristics of O/W cosmetic cream formulations. For the first time, HNES were introduced into the oily phase prior to emulsification at 10% wt. (corresponding to 40% of the dispersed phase). Indeed, hydrophobic or hydrophilic NES were already introduced at similar percentages in the continuous phase of W/O or O/W emulsions or cosmetic creams, but only after the emulsification step at the end of the production process [7, 28, 30]. Four different HNES were tested in the same chassis formula prepared with the same process: three composed mainly of caprylic acid (C8C12, C8P\_5:1, C8M), one composed of a majority of 1,3 propane
<figure><img src="image_8.png" /></figure>

diol, a hydrophilic component, combined with caprylic acid (C8P<sub>1:2</sub>). The findings reveal that 10% wt. HNES can significantly influence physical stability, viscosity and sensory perception of a cream, providing a novel perspective on their behaviour as structuring excipients, beyond their traditional role as extractants or solubilizers.

Among the tested systems, the C8/Menthol-based formulation (C8M) exhibited early phase separation, indicating insufficient physical stability. This instability correlated with a large initial droplet size (17.6 µm) and the highest Turbiscan Stability Index (TSI) observed among all formulations. Although the viscosity and density of the C8M-enriched oily phase were comparable to those of other NES systems, this result highlights that low viscosity alone is not a guarantee of stability and suggests that interfacial interactions or incomplete emulsification may have played a dominant role. Previous studies have successfully stabilized menthol-based HNES via high-energy emulsification techniques (e.g. Ultra-Turrax® homogenization), which may explain the observed destabilization in the current low-shear process [31]. Additionally, a potential partial demixing of the NES components cannot be excluded, especially if the eutectic system lacks thermodynamic stability at the oil-water interface.

Interestingly, the C8C12-based formulation, which exhibited similar low viscosity in the internal phase (Table 3), yielded a cream that remained stable over 30 days under accelerated ageing conditions. This highlights that stability is not solely governed by viscosity but also by the specific nature of the molecular interactions within the NES and their compatibility with the emulsion system. The C8C12 cream demonstrated good pick-up and spreading performance and whitening and penetration scores similar to the control, despite its more pronounced odour and colour, attributable to the high content of caprylic acid.

The two eutectic systems combining caprylic acid and 1,3-propanediol (C8P<sub>5:1</sub> and C8P<sub>1:2</sub>) provided further insights into the effect of molar ratio and polarity on formulation behaviour. The internal phase based on C8P<sub>5:1</sub> was significantly less viscous than the one based on C8P<sub>1:2</sub> (p-value: 0.0475) and this difference persisted in the final cream viscosity. Despite their similar TSI values (both above 1), both creams remained visually stable after 30 days, suggesting that TSI alone may not be a reliable predictor of long-term emulsion stability when used independently. These formulations may have undergone early microstructural rearrangements without leading to macroscopic instability. A particularly notable result was the presence of a measurable relaxation time in the C8P<sub>1:2</sub> cream, not observed in the other formulations. This suggests the presence of a more elastic internal network with the capacity to dissipate mechanical energy, potentially favoring long-term physical stability. Such viscoelastic behaviour may be linked to stronger intermolecular

interactions between the more hydrophilic NES and the emulsifying agents or thickeners, leading to enhanced structural integrity.

From a sensory standpoint, all NES-containing creams significantly increased odour intensity, likely due to the volatile nature of caprylic acid and the potential for HNES to enhance the release of aromatic compounds [32]. However, firmness was consistently reduced across all NES-based formulations compared to the control, suggesting that NES tends to soften the cream matrix. While pick-up was improved in the C8P<sub>5:1</sub> and C8C12 creams, it was markedly reduced in the C8P<sub>1:2</sub> formulation. This contrast may stem from differences in interfacial film formation or phase polarity, affecting how the cream interacts with the skin upon application.

The whitening effect, an important visual property in cosmetic creams, varied among formulations. It was significantly (p-value: 0.017) enhanced in the C8P<sub>5:1</sub> cream, whereas C8P<sub>1:2</sub>—despite being structurally similar—exhibited the lowest whitening score. These findings highlight how even small changes in NES composition can significantly influence the optical behaviour of emulsions, possibly due to differences in droplet size distribution, interfacial thickness or refractive index contrast.

Finally, the penetration behaviour—defined here as the ability of the cream to disappear into the skin—also varied significantly (p-value: 0.018). While C8C12 matched the control, C8P<sub>1:2</sub> showed the lowest penetration, which could be attributed to the humectant nature of 1,3-propanediol. High water-retaining capacity may lead to slower absorption, creating a longer-lasting film on the skin surface.

Taken together, these results indicate that HNES offers a high level of tunability, not only for improving extraction or solubilization of actives, but also for fine-tuning the functional and sensory profiles of emulsified cosmetic formulations. Importantly, all tested NES are compliant with COSMOS certification standards, supporting their potential integration into green cosmetic product development. However, careful consideration of component ratios, emulsification processes and target sensory properties is required to fully leverage their multifunctional benefits. Further studies on skin may highlight a greater potential as the components of these NES show some cosmetic activities by themselves. Indeed, medium chain fatty acids like caprylic acid and lauric acid are described as microbiota regulators [12], lauric acid, menthol and propane-diol are described as penetration enhancers [33].

## CONCLUSION

This study confirms that hydrophobic Natural Eutectic Solvents (HNES), particularly those based on caprylic
acid, can serve as multifunctional ingredients in cosmetic cream formulations. Their incorporation prior to emulsification at 10% wt. influenced emulsion stability, rheology and sensory perception in a composition-dependent manner.

Stable systems like C8C12 and C8P<sub>5.1</sub> improved pick-up and spreadability, while C8P<sub>1.2</sub> offered enhanced internal structuring, albeit with reduced penetration. In contrast, the menthol-based formulation was destabilized under the same conditions, underscoring the importance of emulsification strategy and interfacial compatibility.

These results show that HNES can effectively modulate formulation properties beyond solubilization—offering tools to fine-tune cream texture, appearance and user experience. Their alignment with COSMOS standards makes them particularly attractive for sustainable product development in the cosmetic industry.

## AUTHOR CONTRIBUTIONS

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## ACKNOWLEDGEMENTS

Authors particularly thank Conseil Régional Centre Val de Loire (ARD CVL Cosmétosciences 2020-00141275) and Agence Nationale de la Recherche (DES4Skin project ANR-22-CE43-0006-04). Graphical abstract was created with BioRender.com.

## CONFLICT OF INTEREST STATEMENT

None.

## DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

## ORCID

Emilie Munnier https://orcid.org/0000-0002-6691-4484

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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** Verger A, Grand R, Pradel C, Perse X, Bouderbala S, Despres A, et al. Cosmetic creams with caprylic acid-based Natural Eutectic Solvents: Stability, rheology and user perception. Int J Cosmet Sci. 2025;00:1-11. https://doi.org/10.1111/ics.70051