Nanoemulsions are obtained when the size of an emulsion globule reaches approximately 20-500 nm. The small droplet size can resist the physical destabilisation caused by gravitational separation, flocculation and/or coalescence. It also avoids the creaming process because the droplet's Brownian motion is enough to overcome the gravitational separation force [1
]. The size and polydispersity of nanoemulsions can affect properties such as particle stability, rheology, appearance, colour, texture and shelf life [3
]. In nanoemulsions, the most frequent instability phenomenon is Ostwald ripening [4
], which can be calculated according to the Lifshitz-Slezov and Wagner theory (LSW) using the following equation [6
Where ω is defined as the rate of change of the cube of the number average radius, D is the diffusion coefficient of the dispersed oil phase in the aqueous phase, γ is the interfacial tension between the two phases, c
∞ is the bulk solubility of the oil in the water and ρ is the oil density. k is a constant that has the value of 8/9 in the LSW.
Nanoemulsions are well characterised and are a promising drug delivery system with practical applications for pharmaceutical, cosmetic and chemical industry applications. They have been used in intravenous, oral and ocular drug administrations and have reduced drug side effects and improved the pharmacological effects of the drugs given [8–10, 4]. Nanoemulsions are primarily produced either by high-energy emulsification (e.g., high-pressure homogenisation) or by low-energy emulsification (using physicochemical properties of the components) . This work focuses on the latter method for nanoemulsion synthesis.
Rice (Oryza sativa) bran oil has unsaponifiable fractions that contain high levels of antioxidant-rich components, such as tocopherols/tocotrienols and gamma-oryzanol, which could be useful for topical formulations . The cosmetics industry has used rice bran oil in sunscreen formulations , in topical aging prevention products  and in treatments for skin diseases .
When skin is affected by diseases such as atopic dermatitis and psoriasis, it exhibits a compromised skin barrier function that causes increased transepidermal water loss [16–18]. Moisturisers can help improve the skin's function by relieving the cutaneous manifestations of these diseases [18–20]. Measuring the stratum corneum (SC) hydration degree gives important information about the biophysical properties and the function of the skin .
In vitro studies, such as the HET-CAM (Hen's Egg Test on the Chorioallantoic Membrane), are used to evaluate cosmetics products by immediately showing whether or not a solid or liquid substance irritates the hen's egg chorioallantoic membrane .
The aim of this study was to obtain nanoemulsions of rice bran oil and to evaluate their physical stability, irritating potential and in vivo moisturising activity.