Nanoemulsions: a new vehicle for skincare products

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Abstract

Nanoemulsions consist in very fine oil-in-water dispersions, having droplet diameter smaller than 100 nm. Compared to microemulsions, they are in a metastable state, and their structure depends on the history of the system. In the present work, nanoemulsions were prepared with a high shear device, which is less constraining than spontaneous emulsification procedures. Nanoemulsions are very fragile systems by nature. As they are transparent, the slightest sign of destabilisation appears visually. Two major sources of unstability were identified and extensively studied: Ostwald ripening and depletion induced floculation following the addition of thickening polymers. The control of these two mechanisms allowed the industrial production of a large variety of cosmetic products, from water-like fluids, to ringing gels obtained by increasing the oil phase content or by adding polymers. The nanoemulsions are easily valued in skin care due to their good sensorial properties (rapid penetration, merging textures) and their biophysical properties (especially their hydrating power).

Introduction

Nanoemulsions [1], [2], [3] consist in very fine emulsions, with a droplet diameter smaller than 100 nm. Unlike microemulsions, nanoemulsions are metastable systems, whose structure depends on the process used to prepare them. They can be prepared by spontaneous emulsification such as PIT emulsification [4] or phase inversion composition [5], or by using a high shear device [3], which allows a better control of the droplet size and a large choice of compositions.

Nanoemulsions are very fragile systems by nature. As they are transparent and usually very fluid, the slightest sign of destabilisation easily appears. They become opaque and creaming may be visible. We identified two major sources of instability in these systems that are discussed in this paper: one caused by the nature of the oil phase and another originating from the addition of polymers to thicken or to gel the nanoemulsion.

The very small size of the droplets (D∼50 nm) gives them characteristic properties, which can be easily valued in cosmetics. As they are transparent, they are related to freshness, purity, simplicity, water … They are easily absorbed by the skin. They can be sterilised by filtration. They lead to a large variety of products from water-like fluids to ringing gels.

Section snippets

Materials and methods

The nanoemulsions were stabilised by a mixture of two surfactants: PEG-8 isostearate/disodium stearoyl glutamate (9/1). The weight ratio of surfactant phase to oil phase was 0.25. The aqueous phase contained 18.75% w/w glycols. Different cosmetic oils with various chemical nature (triglycerides, esters, alcanes, silicones) and different molecular weights were formulated. The total weight content of oil within the emulsion was 25% at the highest.

To increase the viscosity of nanoemulsions,

Critical parameters for nanoemulsions stability

As a first approach, nanoemulsions were formulated using one oil only, and 15% w/w oil content. Most of the oils used allowed the production of transparent nanoemulsions (Fig. 1a). The droplet diameter measured by quasi-elastic light scattering was approximately 50 nm, what was confirmed by CryoTEM (Fig. 1b). However, some systems tended to whiten more or less quickly with time.

How to vary the texture of nanoemulsions?

A way to vary the texture of nanoemulsions is to increase their consistency, either by increasing the oil content, or by adding gelling or thickening agents.

Cosmetic properties of nanoemulsions

A great variety of products with different visual aspects, richness and skin feel are allowed with nanoemulsions: lotions, transparent milks, crystal-clear gels with different rheological behavior.

These products are easily valued in skincare products thanks to their good biophysical and sensorial benefits.

Conclusions

Owing to the potential of nanoemulsions in cosmetic products, an intensive study was performed in order to point out the role of some major constituents. For each system, destabilization mechanisms, such as ripening and flocculation, were analyzed; thus, the formulation and the stability of these new liquid/liquid dispersions are now precisely controlled.

The cosmetics performance of products based on nanoemulsions have already been evidenced.

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