**1. Introduction**

Emulsions in cosmetic, pharmaceutical and food industry rely on biocompatible formulations. Among the various ingredients, surfactants are needed to provide long term stability and suitable features (drop size and distribution, tactile and sensorial attributes, etc.) to the emulsified products. Aiming at these applications and, more generally, to pursue the objectives related to green and sustainable development, surfactants derived from plant products are more and more utilized, in substitution of synthetic and even biocompatible molecules.

The present paper addresses the interfacial properties and emulsification of water-oil system for two surfactants derived from vegetable sources, namely saponin and citronellol glucoside (CG) as compared to the classical biocompatible synthetic surfactant Tween 80.

As the biocompatible oil phase the Medium Chain Triglyceride (MCT) oil is used. MCT is a coconut-oil derivate which, owing to its purity and high and well-defined content of saturated triglycerides, which are metabolized in an easy way by the body, is largely utilized in pharmaceutics and is receiving increasing attention from nutritionists. Applications are therefore also devised by food and cosmetic industry. Emulsions of MCT find, in particular, interesting applications in the development of effective parenteral nutrition formulations, nutraceutics and for the encapsulation of active ingredients and drugs [1–4]. The formulation of fully biocompatible emulsions of MCT represents therefore a valuable target for many applications.

Saponin is a natural surfactant mainly extracted from plants [5,6] characterized, in general, by a hydrophobic aglycone structure with hydrophilic sugar residues. Many species of saponin exist, mostly classified on the basis either of the differences in the aglycone structure or the number of linked sugar chains [7]. The adsorption properties of saponin at water-air interface in relation to its ability to stabilize foams have been widely investigated [8–11]. In particular, it has been evidenced that the appreciable surface activity of saponin at water-air interface together with high value of the dilational viscoelasticity modulus, which has been found to be of the order of hundreds mN/m [8], make this compound a rather effective stabilizer of foams, already in pure water, without any further additives. Recently, saponin is intensively studied also as an emulsifying and emulsion stabilizing agent [12–15].

Citronellol glucoside (CG) is the β-d-glucoside of citronellol, an unsaturated monoterpene widespread in plant essential oils and a common flavor used in cosmetics and body care, as well as an aroma compound of food (Citrus sensation). Its glucoside CG is also occurring in crop plants like grape as a storage form of aroma. Recently, it became available by biotechnological production and can serve a dual role as a flavor precursor split by enzymes (e.g., in oral cavity) and as a non-ionic tenside or emulsifier in consumer products. CG is less complex in comparison to saponin with respect to chemical structure(s) and chemical identity. The aglycon of CG is a monoterpene (C10, branched alkene), while that of saponin is a triterpene (C30, substituted ring structures); the glycosidic parts are monosaccharides in the case of CG, while they are oligosaccharides in case of saponin, containing also an ionic carboxy-function. Furthermore, saponin is a mixture of triterpene glycosides.

Tween surfactants belong to a class of synthetic non-ionic surfactants which are considered non-toxic and to weakly interact with electrolytes and, for that, widely employed in those fields requiring bio-compatibly as emulsion stabilizers, both for domestic use and industrial application [16,17]. Moreover, they are often used as model surfactants in fundamental studies on emulsification and coalescence [18]. In particular, Tween 80 (polyoxyethylene sorbitan monooleate) is a synthetic non-ionic tenside or emulsifier and is composed of a central sorbitol (anhydride) structure derivatized from polyoxyethylene chains and the oleate ester group, the latter representing the lipophilic part of the molecule. It is in wide use in food, cosmetics and pharmacy technology.

Even though surfactants are key ingredients for emulsifiers, there is still a lack of deterministic approaches for the formulation of emulsions with desired characteristics, on the basis of the properties of concerned surfactant adsorption layers. This represents a challenging task, requiring systematic investigations contributing to clarify how the properties measured for single interfaces correlate for example with the stability and the drop size evolution of corresponding emulsions.

Based on these premises, a study on the interfacial properties of the above described surfactants at water-MCT oil interfaces, in relation to their ability to stabilize emulsions, is here presented. The interfacial characterization is carried out by interfacial tension and dilational viscoelasticity measurements, with the aim of assessing the adsorption properties of the different surfactants from the point of view of both thermodynamics, determining the equilibrium relation between the bulk concentration and the adsorption or adsorption isotherm and dynamics, evaluating the dilational rheology response of the adsorbed layers. These properties are then correlated with the behavior of the respective emulsions, monitored during their evolution and analyzed in terms of their structure by optical microscopy observation.

The mitigation of potential adverse impacts of emulsifiers on environment and health is pursued not only by the use of biocompatible surfactants but also by the reduction of their amount in the formulations. At low surfactants concentrations however, the process of emulsification in micro- and sub-micro-sized droplets results in a significant depletion of the surfactant concentration in the liquid phases, caused by the adsorption at the huge amount of liquid-liquid interfacial area produced. The proper correlation between interfacial properties and emulsion features needs then to account for these depletion effects. These aspects, however, have not received so far sufficient attention in the available studies, while they are explicitly accounted for and discussed in the present one.
