Michel Grisel

Release of limonene from polysaccharide matrices: viscosity and synergy effect

A detailed study of limonene release behaviour from water-based low-concentration polysaccharide solutions and weak gels is reported. On the one hand, rheological measurements made it possible to characterize xanthan (X) and galactomannan [guar gum (G) and carob gum (C)], alone or mixed together. On the other hand, release behaviour of limonene from the previous matrices was quantified using a solid phase micro extraction technique combined with gas chromatography analysis. All of the results indicated large differences in release behaviour, depending on both the matrix composition and the solution viscosity. Moreover, when synergistic interactions between the polysaccharides were developed in the media (X/C mixtures), the results clearly indicated that the release was greatly reduced, thus showing that the volatile release behaviour was not only governed by steric phenomena but also resulted from the occurrence of specific interactions that remain to be elucidated.

Impact of emollients on the spreading properties of cosmetic products: a combined sensory and instrumental characterization.

This study deals with the impact of emollients on the spreading properties of cosmetic products using a combined sensory-instrumental approach. To that purpose, three esters and one silicone were selected and incorporated separately into an oil phase. Different cosmetic o/w emulsions were then prepared with these different oil phases. Both of them were analyzed by instrumental techniques and in vivo sensory analyses. A significant effect of the emollient used was established in emulsions and in oil phases as well. Concerning emulsions, results reveal a clear correlation between in vivo spreading evaluation and friction coefficient parameters measured by texture analyzer, despite a fairly low correlation coefficient (Pearson coefficient=-0.78). Concerning oil phases, characterization of spreading was done by monitoring the contact angle relaxation of a drop of solution after deposition on a flat PMMA surface whereas sensory procedure was based on spontaneous spreading of oil phases onto the skin. Finally, good correlations between in vivo sensory analysis and instrumental measurements of both oils and emulsions were found, thus promising the possible development of predictive tools to evaluate spreadability.

Hydrophobically modified xanthan: an amphiphilic but not associative polymer.

Hydrophobic octyl moieties have been grafted in various densities onto the carboxylic acid functions of xanthan under its ordered conformation. The outcoming amphiphilic and associative properties were studied by fluorescence spectroscopy and rheology. Results showed that the conformation of xanthan is not affected by the chemical modification and remains the same as the native one. Additionally, xanthan derivatives do not show any viscoelastic enhancement; nevertheless, their dynamics is strongly slowed down: the higher the grafting density, the slower the relaxation. We proved that hydrophobically modified xanthan, even being amphiphilic, does not exhibit any additional associating properties compared to the unmodified xanthan. The high stiffness of xanthan helices does not allow the derivatives to adopt the organization usually observed for flexible amphiphilic polymers. On the basis of these observations, a model depicting such a singular behavior is proposed.

Stretching properties of xanthan and hydroxypropyl guar in aqueous solutions and in cosmetic emulsions

Filament stretchability of xanthan gum (XG) and hydroxypropyl guar (HPG) was investigated in aqueous solutions (0.125, 0.25, 0.5, 1, 1.2 and 1.5% w/w) and in O/W emulsions using a texture analyzer. Additionally, rheological characterizations were carried out on the systems and shear and oscillation parameters were used to interpret stretching properties. XG solutions exhibited a solid-like behavior with rheological parameters much higher than for HPG one whatever the concentration. Filament stretching values of XG solutions were superior to HPG for concentration below 1% w/w and then became comparable for higher concentrations. No meaningful relationship was found between rheological and stretching values. Synergy was observed for all XG/HPG mixtures at 0.125, 0.25 and 0.5% influencing both the rheological and the filament stretching values. The 25/75 XG/HPG ratio showed the maximum synergistic effect at all concentrations while the filament stretchability was enhanced in a wider range of ratios. XG and HPG did not present the same behavior in emulsions. No clear synergistic effect was observed and XG markedly influenced the emulsion filament stretching.

Impact of coated TiO2-nanoparticles used in sunscreens on two representative strains of the human microbiota: Effect of the particle surface nature and aging

The impact of two differently coated TiO2-nanoparticles (NPs) was evaluated on two representative bacteria from the cutaneous microbiota (Staphylococcus aureus and Pseudomonas fluorescens) in conditions of use. Particles were coated in order to exhibit either hydrophilic or hydrophobic behavior. A first exposure scenario within some fresh/aged model emulsions was developed in order to measure both the impact of the NPs presence and their surface nature during the emulsions conservation. Thanks to this protocol, it was demonstrated that, during aging, the hydrophobic NPs modified the physicochemical characteristics of the emulsions, such as the pH or the colloids sizes, and favored the development of potential pathogenic bacteria. A second scenario was then envisaged, aiming to mimic the exposition of the skin, especially of the cutaneous bacteria, to NPs. Tested NPs were extracted from emulsions with different aging, and results highlighted the importance of both the NP coating nature and their history in emulsion. The different NPs impacts on the bacteria growth were discussed and linked to their surface properties modifications during aging, as polarity and charges. Finally, through two exposition scenarios, this work highlights the major impact of the NPs surface properties on bacteria.

Chemical modification of xanthan in the ordered and disordered states: an open route for tuning the physico-chemical properties

The impact of the chain stiffness on physicochemical properties has been studied by chemical modification of xanthan under both ordered and disordered conformations. Corresponding rheological properties were studied and results showed that amphiphilic xanthan exhibited completely different behaviors depending on its conformation during modification. Xanthan, when modified under ordered conformation, exhibits similar behavior to non-modified one, only the chain relaxation being strongly slowed down. Therefore, the high stiffness of xanthan helices does not allow hydrophobic moieties to associate. Oppositely, xanthan modified under its disordered conformation displayed a chemical gel-like behavior without any relaxation of the chain within the studied frequency range nor with temperature, which is unexpected for this length of alkyl chains. These different viscoelastic properties can be correlated to the regioselectivity of the grafting; the latter can be controlled by the conformation of xanthan during modification, thus by the synthesis conditions.

Influence of the emollient on emulsions containing lamellar liquid crystals: from molecular organization towards applicative properties

The sensory perception of cosmetic emulsions is complex as it is governed by an important number of parameters like the choice of raw materials, their interactions, the structural organisation of the system, etc. The aim of the present work was to go further in the interpretation of the emollient‐surfactant interactions, towards the emulsions applicative properties. For this purpose, two systems containing liquid crystals of the lamellar type were formulated, differing only in the selected emollient.

Hydrophobically modified xanthan: Thickening and surface active agent for highly stable oil in water emulsions

Oil-in-water (O/W) emulsions were prepared using exclusively a hydrophobically modified xanthan (HMX) under ordered conformation without any surfactant. A series of HMX differing in grafting density were utilized at different concentrations. It was demonstrated that HMX stabilizes O/W emulsions where pristine xanthan do not. Indeed, in some cases, emulsions with HMX proved to be homogeneous and stable over months. Emulsions microstructure and rheological properties were investigated over time as a function of HMX concentration and grafting density in order to determine the mechanisms governing this efficient stability. The higher the HMX concentration is, the better the emulsions stability. However, unexpectedly, increasing the HMX grafting density leads to faster destabilization. The whole set of results demonstrate that the stability of the emulsions against coalescence and creaming is explained by partitioning of HMX molecules between bulk and interface, thus split between viscosifying the aqueous phase and stabilizing the interface.

Determination of specific interactions between aroma compounds and xanthan/galactomannan mixtures

Abstract The gas/liquid partition coefficient (K) of aroma compounds was measured in pure polysaccharide solutions and in polysaccharide mixtures, using the Phase Ratio Variation method. With this method, the retention in 0.1% and 0.02% polysaccharide concentrations was determined. The results showed that in semi-dilute regime (0.1%) all solutions and mixtures induced a high retention of limonene and hydrophobic esters, thus confirming hydrophobic interactions between these compounds and the polymer chains. In dilute pure polysaccharide solutions (0.02%), a decrease in retention was noticed for the esters and a ‘salting out’ effect for limonene. Inversely, at 0.02%, mixtures of polysaccharides showed a good retention of aroma compounds explained by the well-known synergistic interactions between xanthan and carob gums.