**Mass Transfers Within Emulsions Studied by Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening**

D. Clausse1, A. Drelich1 and B. Fouconnier2

*1EA 4297 Transformations Intégrées de la Matière Renouvelable, Université de Technologie de Compiègne; 2Departamento de Ingeniería de Procesos y Hidráulica, Universidad Autónoma Metropolitana; 1France 2México* 

### **1. Introduction**

742 Mass Transfer - Advanced Aspects

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In this chapter mass transfer within simple, mixed and multiple emulsions will be considered. Simple emulsions are made of either water droplets dispersed within an oil phase (W/O) or oil droplets dispersed within an aqueous phase (O/W). Mixed emulsions are obtained by mixing gently to avoid coalescence, two simple emulsions the composition of the droplets being different. Finally, multiple or double emulsions are obtained by dispersing either a simple emulsion (W/O) in an aqueous phase to get what is referred as W/O/W emulsion or by dispersing a simple emulsion (O/W) in an oil phase to get an O/W/O emulsion.

These systems are known to be instable and their evolution towards bulk separated phases is the result of coalescence of the droplets due to different main mechanisms as Ostwald ripening, flotation, aggregation, sedimentation or creaming. This evolution can be considerably reduced by adding in their formulations either surfactants or/and particles. Thus, kinetic stability can be obtained. In that case other mechanisms can be studied, mechanisms that traduce an evolution of the systems as well but breakdown of the emulsions is not the result of the evolution understudy. It is this kind of evolution due to mass transfer that will be studied in this chapter. The bibliography will be done in each part dealing with the different kinds of emulsions considered.

First mass transfer in simple emulsions will be described. For these systems a not very well known mass transfer can be the result of the coexistence of still liquid droplets and yet solid ones at a temperature below the solid/liquid equilibrium temperature. This situation is the result of nucleation phenomena that create a delay for the formation of a solid germ that will induce the solidification of the droplets at different temperatures during the regular cooling of the emulsion. Another phenomenon leading to mass transfer in these systems is obtained by adding a material in the oil phase in a W/O emulsion, material that will diffuse and react

Mass Transfers Within Emulsions Studied by

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 745

liquid is said to be under cooled. The degree of undercooling is defined by ΔT = Tm –Tc. For water, this degree is around 20°C for bulk water of a few mm3 and around 40°C for a population of microsized droplets. Another point to stress is that the shape of the signals is also different. This can be attributed to the way the material solidifies, very rapidly in a bulk

sample or progressively in a dispersed phase as it has been indicated previously.

Fig. 1. Cooling curves for water dispersed in an emulsion (a) and for bulk water (b)

From the electrical power dq/dt registers by the calorimeter it is possible to deduce the enthalpy power dh/dt involved by the freezing or melting of the sample. On using rather low scanning rate temperature less than 2K/min, dh/dt can be approximated by dq/dt. Therefore by a previous calibration done with pure materials, the area of the signal correctly delimited permits to determine ΔH, the total heat involved in the liquid-solid transition. This quantity divided by the energy involved per mass unit Δh permits to know the mass m involved in the transition. Δh for the freezing is different for the melting one due to the net influence of the temperature specially for water due to a rather high difference between the values of the heat capacities of ice and liquid water. Nevertheless this quantity can be estimated from data on heat capacities values and as far comparison of areas of signals are done, this point is not a problem by itself. Furthermore when it is possible the amount of material involved in the transition can be determined by the area of the melting signal at a known temperature at which Δh is found in the literature. Should a mass transfer induces a change of the initial mass m(t=0) of a material, the following of the area A(t) of either the

chemically with the water of the dispersed droplets. Following this process, an example of hydrate formation will be described.

In mixed emulsions the transfer is due to the difference of composition between the two populations of droplets. The phase wherein they are dispersed plays the role of a liquid membrane and should this membrane be permeable to one of the material present in the droplets a mass transfer occurs as it can be observed in a direct osmosis device.

In multiple emulsions, the situation is very similar of the one observed in mixed emulsions, except that the phases involved in the mass transfer are the dispersed droplets in the globules (primary emulsion) and the continuous phase wherein the globules are dispersed. A difference in the composition between these two kinds of phases lead to a mass transfer, the material of the globules playing in that case the role of a liquid membrane.

Different techniques have been used to detect this kind of mass transfer in emulsions. They are based on the phenomena linked with the mass transfer, mainly: solidification and changes of the composition and the sizes of the phases involved in the mass transfer. Therefore classical techniques as spectroturbidimetry, light scattering, conductivimetry and rheology have been used. In this chapter the results obtained by using a technique that has been developed for charactering emulsions and their evolution due to mass transfer will be thoroughly described. The referred technique called DSC for Differential Scanning Calorimetry is described in the next section. Afterwards the results dealing with simple, mixed and multiple emulsions will be described.
