Use of the DSC for the characterization of emulsions.

Emulsions are systems made up of a liquid dispersed in the form of droplets within another immiscible liquid called the "continuous phase". There are two main types of emulsion:

• water-in-oil (w/o) emulsions, composed of droplets of an aqueous phase dispersed in a continuous oily phase;

• oil-in-water (o/w) emulsions, composed of droplets of an oily phase dispersed in a continuous aqueous phase.

Extremely complex emulsions can be found, such as mixed and multiple emulsions, and those stabilized by surfactants and/or particles ("Pickering emulsions"). These systems are found in all sectors of industry: food, cosmetics, pharmaceuticals, petroleum, chemicals, polymers... Given the complexity of their formulation, most emulsions are particularly difficult to characterize by conventional physical methods, which generally require sample dilution prior to analysis.

One of the essential characteristics of emulsions is their stability. This stability can be measured by a number of techniques, generally based on the analysis of droplet size distribution within the emulsion. Indeed, it is generally accepted that the smaller the droplet size, the more stable the emulsion will be over time. Most of these techniques are easily applicable to dilute oil-in-water emulsions (light scattering, Coulter counter, optical microscopy...), but are unfortunately difficult to use in the case of opaque, concentrated water-in-oil emulsions, especially if they also contain suspended solids. Even optical microscopy, considered a "universal" technique in the field of emulsions, is very complicated to implement in the case of systems containing different types of solids. A very rapid means of determining the stability of opaque, concentrated invert emulsions is the "bottle test", which involves visually recording the quantity of aqueous phase separated as a function of time. While this test provides very interesting qualitative and quantitative information on emulsion stability and the quality of the separated aqueous phase, it remains highly empirical.

In the field of emulsion characterization, attention must be paid to the problem of sample dilution, which can lead to disturbances in droplet surface properties and droplet interactions. Dilution can lead to flocculation or osmotic shock effects, inducing changes in droplet size distribution. In fact, few experimental techniques can be used without dilution. These include dielectric spectroscopy, rheology, conductimetry, and more recent techniques such as those based on acoustic waves, focused beam reflectance, microwave attenuation, NMR and, finally, differential scanning calorimetry (DSC).

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