**3. Mass transfer within simple emulsions. Solid ripening in W/O emulsions**

An example is given by the mass transfer between yet solid droplets and still liquid ones due to under cooling phenomena. This transfer not very well known but that has to be taken into account when during the storage of the emulsion, the temperature reaches values below the melting ones, is referred as solid ripening as far the equilibrium state in these conditions is all the dispersed material solid (Clausse et al. 1999b). The other type is encountered when a material is added in the continuous phase of an emulsion. It is expected to diffuse and to react chemically with the material of the dispersed droplets. At the end a stable solid material is also obtained. Formation of solid hydrates in petroleum industry is a typical example of such a situation. It is this kind of transfer that is described thereafter.

An example of such solid ripening giving rise to the formation of a hydrate is illustrated by the study of trichlorofluoromethane (CCl3F) hydrate formation in W/O emulsions. CCl3F is a volatile liquid poorly soluble in water and forms a hydrate under mild conditions at 8.5°C and 1 bar. Therefore, CCl3F appeared to be a good candidate in order to mimic the conditions of gas hydrate formation in W/O emulsions as a model system (Jakobsen et al. 1996; Fouconnier et al. 1999, 2006). The solid hydrate phase is formed inside the dispersed droplets as the result of a chemical reaction between CCl3F molecules and water molecules present in the droplets. Actually, CCl3F molecules are initially dissolved in the oily

Mass Transfers Within Emulsions Studied by

was then proposed as illustrated in figure 3.

the hydrate is completely dissociated.

emulsions and the effect of salts upon the hydrate formation.

example, how to determine the quantity of hydrate formed.

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 749

during the cooling of the emulsions or by storing them at low temperatures for a long period. The conditions of CCl3F hydrate formation by DSC were different from those used by Jakobsen. There was no agitation and furthermore the emulsions understudied were really stable. During the cooling of the emulsions, it was only observed the crystallization of aqueous droplets as bell shape signals at temperatures below -40°C as the droplets

Therefore, in order to evidence the conditions of the CCl3F hydrate formation in W/O emulsions, a study using DSC and X-ray diffraction has been undertaken (Fouconnier et al. 2006). It has been concluded that a high degree of undercooling results in a spontaneous ice formation in the droplets during cooling of the emulsions. It has been showed by X-ray diffraction that the hydrate formed during subsequent heating of the emulsions as soon as the ice was beginning to melt. A mechanism of hydrate formation based on the shell model

During the heating of the emulsion, when the ice begins to melt, the droplet can be viewed as a core of ice surrounding by a saline solution. During the progressive melting of ice, the hydrate forms leading to the formation of a shell around the droplet. When the ice is completely melts, the hydrate formation is stopped and a shell of hydrate phase is in equilibrium with a concentrated saline solution since salt does not participate to the hydrate formation. Finally, when the hydrate dissociates, the remaining saline solution is progressively diluted until reaching the initial composition of the dispersed solution when

From this simplified model it was then possible to study the hydrate formation within W/O

It is described in next paragraph how to interpret the heating of DSC curves and for

Fig. 3. Schematic representation of the droplet composition at different temperatures

contained salt. Hydrate formation was not detected during the cooling.

continuous phase and diffuse through the W/O interface to be encapsulated by water molecules into a crystalline hydrate phase of structure II. In this chapter, it is described how to study the CCl3F hydrate formation by DSC.

#### **3.1 Emulsion preparation**

The emulsions were prepared using Exxol D80 (a mixture of aliphatic and cycloaliphatic hydrocarbons from C10 to C13) and mixed with trichlorofluoromethane as the continuous oil phase (Fouconnier et al., 2002). The emulsifier, berol 26 (tetraoxyethylene nonyl-phenyl ether, C9-Ph-E04), was added to the oil phase at a concentration of 4% by volume. The amount of CCl3F is incorporated in the oil in stoechiometric proportion according to the hydrate formation reaction: CCl3F + 17 H20 → CCl3F(H2O)17. The dispersed phase was composed of aqueous saline solutions of calcium chloride at different concentrations by weight. Nevertheless, in order to diminish the evaporation of volatile CCl3F and also to prevent the emulsion breaking, a primary emulsion made of Exxol D80, berol 26 and the saline solution was firstly prepared. The dispersion was obtained at 10000 rpm by means of a homogenizer Polytron PT 3000 during 10 minutes. The CCl3F was finally added under gentle mixing to the emulsion stored at 0°C and kept at this temperature until utilization. The final emulsions were 60/40 water to oil ratio.

#### **3.2 Mechanisms of hydrate formation in W/O emulsions**

Jakobsen et al. were the first to study CCl3F hydrate formation in W/O emulsions by dielectric spectroscopy (Jakobsen et al., 1996). During experiments, the emulsions were under constant stirring and the hydrate formation was detected with an induction period at 4.5°C but took place spontaneously at 3°C. The induction period was attributed to an insufficient undercooling, a reduced contact between CCl3F and water molecules, the energy added to the system by stirring and inhomogeneous mixing during the initial period due to the addition of CCl3F in the emulsion. The shift of temperature between 8.5°C and 3°C was attributed to the freezing depression point due to the presence of NaCl in the dispersed aqueous phase and the effect of undercooling in emulsions as it has been described in section 2.

The authors modeled the kinetics of the CCl3F formation as followed.

$$\text{CCl}\_3\text{F}\_{\text{(oil)}} \rightarrow \text{CCl}\_3\text{F}\_{\text{(aquous phase)}} \tag{a}$$

$$\text{CCl}\_3\text{F} + 17\text{ H}\_2\text{O} \rightarrow \text{CCl}\_3\text{F.(H}\_2\text{O})\_{17} \tag{b}$$

$$\text{CCl}\_3\text{F} + 17\,\text{H}\_2\text{O} \text{ \(\text{\(\text{\(}H}\_2\text{O}\)}\_{\text{\(\(\text{\(}H}\_2\text{O}\)}\text{)}\_{\text{\(\(}H\)}) \tag{6}$$

The reaction (a) represents the diffusion of CCl3F from the oil phase to the aqueous dispersed phase. The reaction (b) represents the slow formation of the hydrate considering that cavities of structure II are forming. The reaction (c) represents the autocatalytic formation of the hydrate.

Jakobsen et al. showed that the diffusion of CCl3F molecules from the oil phase to the aqueous phase is the rate-limiting step. They also speculated that the surfactant can act as a barrier to the diffusion of the hydrate forming specie.

We also studied a similar dispersed system by DSC, CaCl2 was also used, and it is worth noting that in our conditions of study it was impossible to detect the hydrate formation

continuous phase and diffuse through the W/O interface to be encapsulated by water molecules into a crystalline hydrate phase of structure II. In this chapter, it is described how

The emulsions were prepared using Exxol D80 (a mixture of aliphatic and cycloaliphatic hydrocarbons from C10 to C13) and mixed with trichlorofluoromethane as the continuous oil phase (Fouconnier et al., 2002). The emulsifier, berol 26 (tetraoxyethylene nonyl-phenyl ether, C9-Ph-E04), was added to the oil phase at a concentration of 4% by volume. The amount of CCl3F is incorporated in the oil in stoechiometric proportion according to the hydrate formation reaction: CCl3F + 17 H20 → CCl3F(H2O)17. The dispersed phase was composed of aqueous saline solutions of calcium chloride at different concentrations by weight. Nevertheless, in order to diminish the evaporation of volatile CCl3F and also to prevent the emulsion breaking, a primary emulsion made of Exxol D80, berol 26 and the saline solution was firstly prepared. The dispersion was obtained at 10000 rpm by means of a homogenizer Polytron PT 3000 during 10 minutes. The CCl3F was finally added under gentle mixing to the emulsion stored at 0°C and kept at this temperature until utilization.

Jakobsen et al. were the first to study CCl3F hydrate formation in W/O emulsions by dielectric spectroscopy (Jakobsen et al., 1996). During experiments, the emulsions were under constant stirring and the hydrate formation was detected with an induction period at 4.5°C but took place spontaneously at 3°C. The induction period was attributed to an insufficient undercooling, a reduced contact between CCl3F and water molecules, the energy added to the system by stirring and inhomogeneous mixing during the initial period due to the addition of CCl3F in the emulsion. The shift of temperature between 8.5°C and 3°C was attributed to the freezing depression point due to the presence of NaCl in the dispersed aqueous phase and the effect of undercooling in emulsions as it has been described in

CCl3F(oil) → CCl3F(aqueous phase) (a)

CCl3F + 17 H2O → CCl3F.(H2O)17 (b)

 CCl3F + 17 H2O ℜ CCl3F.(H2O)17 (c) The reaction (a) represents the diffusion of CCl3F from the oil phase to the aqueous dispersed phase. The reaction (b) represents the slow formation of the hydrate considering that cavities of structure II are forming. The reaction (c) represents the autocatalytic

Jakobsen et al. showed that the diffusion of CCl3F molecules from the oil phase to the aqueous phase is the rate-limiting step. They also speculated that the surfactant can act as a

We also studied a similar dispersed system by DSC, CaCl2 was also used, and it is worth noting that in our conditions of study it was impossible to detect the hydrate formation

to study the CCl3F hydrate formation by DSC.

The final emulsions were 60/40 water to oil ratio.

**3.2 Mechanisms of hydrate formation in W/O emulsions** 

The authors modeled the kinetics of the CCl3F formation as followed.

barrier to the diffusion of the hydrate forming specie.

**3.1 Emulsion preparation** 

section 2.

formation of the hydrate.

during the cooling of the emulsions or by storing them at low temperatures for a long period. The conditions of CCl3F hydrate formation by DSC were different from those used by Jakobsen. There was no agitation and furthermore the emulsions understudied were really stable. During the cooling of the emulsions, it was only observed the crystallization of aqueous droplets as bell shape signals at temperatures below -40°C as the droplets contained salt. Hydrate formation was not detected during the cooling.

Therefore, in order to evidence the conditions of the CCl3F hydrate formation in W/O emulsions, a study using DSC and X-ray diffraction has been undertaken (Fouconnier et al. 2006). It has been concluded that a high degree of undercooling results in a spontaneous ice formation in the droplets during cooling of the emulsions. It has been showed by X-ray diffraction that the hydrate formed during subsequent heating of the emulsions as soon as the ice was beginning to melt. A mechanism of hydrate formation based on the shell model was then proposed as illustrated in figure 3.

During the heating of the emulsion, when the ice begins to melt, the droplet can be viewed as a core of ice surrounding by a saline solution. During the progressive melting of ice, the hydrate forms leading to the formation of a shell around the droplet. When the ice is completely melts, the hydrate formation is stopped and a shell of hydrate phase is in equilibrium with a concentrated saline solution since salt does not participate to the hydrate formation. Finally, when the hydrate dissociates, the remaining saline solution is progressively diluted until reaching the initial composition of the dispersed solution when the hydrate is completely dissociated.

From this simplified model it was then possible to study the hydrate formation within W/O emulsions and the effect of salts upon the hydrate formation.

It is described in next paragraph how to interpret the heating of DSC curves and for example, how to determine the quantity of hydrate formed.

Fig. 3. Schematic representation of the droplet composition at different temperatures

Mass Transfers Within Emulsions Studied by

emulsion without CCl3F was also studied.

the water-CaCl2 equilibrium curve.

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 751

remaining saline solution when ice is completely melted, it is possible to carry out a mass balance using the temperatures of ice-aqueous solution equilibrium and by using the water-CaCl2 equilibrium curve. Such mass balance is illustrated by the study of an emulsion containing a dispersed CaCl2 aqueous solution of 20 wt%. For comparison, the equivalent

Figure 5a presents the DSC curves for emulsions with and without CCl3F. Figure 5b shows

Fig. 5. a) DSC curves of emulsions with and without CCl3F containing initially saline solutions of 20 wt% CaCl2. b) Equilibrium curve of the water-CaCl2 binary system

solution of 20 wt% used for the emulsion preparation.

respectively.

The temperatures of ice-saline solution equilibrium were determined as described previously and are T = -28ºC for the emulsion containing CCl3F and T = -18ºC for the emulsion without CCl3F. These temperatures are then reported on the equilibrium curve of the water-CaCl2 system. Therefore, T = -28ºC corresponds to a hydrate phase in equilibrium with a saline solution of 25 wt% of CaCl2 and T = -18ºC corresponds to the dispersed saline

On the other hand, the mass fraction ϕ of aqueous phase of the emulsion was needed to carry out the mass balance. According to the formulation of the emulsion, the mass fraction ϕ is given by the following relation where maqueous, E and moil phase, E are the mass of the dispersed saline solution and the mass of the oil phase (CCl3F + berol + Exxol D80)

#### **3.3 DSC measurements and determination of the amount of hydrate formed**

Figure 4 report the DSC heating curves of two emulsions with and without CCl3F after being cooled down since -90ºC. Both emulsions were prepared with aqueous solutions of CaCl2 at a concentration of 6.8 wt%. On the DSC curves, the signal I at -52ºC is attributed to the eutectic fusion of the water-calcium chloride binary system. Signals II and III represent the progressive melting of ice as it has been described in section 2. An additional signal IV only appears for the emulsion containing CCl3F and it is attributed to the dissociation of hydrate.

Fig. 4. DSC curves of emulsions with and without CCl3F containing initially saline solutions of 6.8 wt% CaCl2

It has been observed from the DSC and X-ray diffraction analysis that the hydrate dissociates progressively like melting of ice in presence with salt as the line 1\* of the figure 2 shows. Therefore, the temperature of hydrate dissociation has been determined as the solidliquid equilibrium temperatures of water-salt systems (Fouconnier et al. 2002).

The temperatures of the end of progressive ice melting (TII, TIII) and the hydrate dissociation temperature (TIV) are obtained by taking the intersection between the baseline of DSC curves and a line parallel to the greatest slope line of the eutectic peak, going through the node of each endothermic peak. The corresponding temperatures are given by the projection of the intersection points (dotted lines) on the temperature axis. Doing so, the temperatures of the end of the melting of ice (or solid-aqueous solution equilibrium temperature) are of - 4.7ºC and -4ºC for the emulsion with and without CCl3F respectively. The temperature of the hydrate dissociation is 7.5ºC. It can be observed that the temperature of the end of ice melting in reference to the emulsion containing CCl3F is slightly lower than the one of the emulsion without CCl3F. This shift of temperature is attributed to the formation of hydrate. When hydrate forms, water molecules are engaged in the hydrate structure resulting in the concentration of the saline solution inside the emulsion droplets.

Once stated how to determine the temperatures of the hydrate dissociation as function of salt concentration and by considering that the hydrate phase is in equilibrium with a

Figure 4 report the DSC heating curves of two emulsions with and without CCl3F after being cooled down since -90ºC. Both emulsions were prepared with aqueous solutions of CaCl2 at a concentration of 6.8 wt%. On the DSC curves, the signal I at -52ºC is attributed to the eutectic fusion of the water-calcium chloride binary system. Signals II and III represent the progressive melting of ice as it has been described in section 2. An additional signal IV only appears for the emulsion containing CCl3F and it is attributed to the dissociation of hydrate.

Fig. 4. DSC curves of emulsions with and without CCl3F containing initially saline solutions

It has been observed from the DSC and X-ray diffraction analysis that the hydrate dissociates progressively like melting of ice in presence with salt as the line 1\* of the figure 2 shows. Therefore, the temperature of hydrate dissociation has been determined as the solid-

The temperatures of the end of progressive ice melting (TII, TIII) and the hydrate dissociation temperature (TIV) are obtained by taking the intersection between the baseline of DSC curves and a line parallel to the greatest slope line of the eutectic peak, going through the node of each endothermic peak. The corresponding temperatures are given by the projection of the intersection points (dotted lines) on the temperature axis. Doing so, the temperatures of the end of the melting of ice (or solid-aqueous solution equilibrium temperature) are of - 4.7ºC and -4ºC for the emulsion with and without CCl3F respectively. The temperature of the hydrate dissociation is 7.5ºC. It can be observed that the temperature of the end of ice melting in reference to the emulsion containing CCl3F is slightly lower than the one of the emulsion without CCl3F. This shift of temperature is attributed to the formation of hydrate. When hydrate forms, water molecules are engaged in the hydrate structure resulting in the

Once stated how to determine the temperatures of the hydrate dissociation as function of salt concentration and by considering that the hydrate phase is in equilibrium with a

liquid equilibrium temperatures of water-salt systems (Fouconnier et al. 2002).

concentration of the saline solution inside the emulsion droplets.

of 6.8 wt% CaCl2

**3.3 DSC measurements and determination of the amount of hydrate formed** 

remaining saline solution when ice is completely melted, it is possible to carry out a mass balance using the temperatures of ice-aqueous solution equilibrium and by using the water-CaCl2 equilibrium curve. Such mass balance is illustrated by the study of an emulsion containing a dispersed CaCl2 aqueous solution of 20 wt%. For comparison, the equivalent emulsion without CCl3F was also studied.

Figure 5a presents the DSC curves for emulsions with and without CCl3F. Figure 5b shows the water-CaCl2 equilibrium curve.

Fig. 5. a) DSC curves of emulsions with and without CCl3F containing initially saline solutions of 20 wt% CaCl2. b) Equilibrium curve of the water-CaCl2 binary system

The temperatures of ice-saline solution equilibrium were determined as described previously and are T = -28ºC for the emulsion containing CCl3F and T = -18ºC for the emulsion without CCl3F. These temperatures are then reported on the equilibrium curve of the water-CaCl2 system. Therefore, T = -28ºC corresponds to a hydrate phase in equilibrium with a saline solution of 25 wt% of CaCl2 and T = -18ºC corresponds to the dispersed saline solution of 20 wt% used for the emulsion preparation.

On the other hand, the mass fraction ϕ of aqueous phase of the emulsion was needed to carry out the mass balance. According to the formulation of the emulsion, the mass fraction ϕ is given by the following relation where maqueous, E and moil phase, E are the mass of the dispersed saline solution and the mass of the oil phase (CCl3F + berol + Exxol D80) respectively.

Mass Transfers Within Emulsions Studied by

Population of **droplets#1**

> Simple Emulsion#1

Fig. 6. Schematic representation of mixed emulsion preparation

decrease of the amount of pure droplets by transfer (Figure 7).

tetradecane to hexadecane droplets in O/W mixed emulsion.

Mixed Emulsion with 2 populations of **droplets#1**and **droplets#2** +

Simple Emulsion#2

The mass transfers due to a gradient composition between the different phases present in the emulsion lead to a modification of the composition of the droplets by dilution and the

> Composition Ripening from droplets#1 to droplets#2

**t0 t end**

Fig. 7. Schematic representation of the droplet composition with time in mixed emulsion

**4.1 DSC measurements and determination of water mass transfer in W/O mixed** 

The goal of the work reported (Clausse et al., 1995; 1999; 2008; Sacca et al., 2008) is to evidence the composition ripening that takes place in W/O mixed emulsions and to measure the rate of water exchange between pure water droplets and aqueous solute (urea or NaCl) droplets dispersed in oil medium. The effect of the emulsion stabilizing agent, as emulsifier or particles, on the kinetics of water transfer was investigated and discussed.

Many studies evidenced the composition ripening that takes place in mixed emulsion by measuring the rate of mass exchange between two kinds of water droplets with solute dispersed in oil media or two populations of hydrocarbon droplets dispersed in aqueous phase. In the case of this study, we propose to describe in this chapter the pure water mass transfer to aqueous solute solution droplets in W/O mixed emulsion and the transfer of

(W/O) emulsions.

**emulsions** 

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 753

transfer mechanisms across liquid membranes (Li, 1968; Colinart et al., 1984; Noble & Stern, 1995). For instance, these systems allow to model transport processes between two oil phases separated by an aqueous membrane in the case of oil-in-water (O/W) emulsions and between two aqueous phases separated by an organic membrane in the case of water-in-oil

> Population of **droplets#2**

> > Mixed Emulsion

Simple Emulsion with 1 population of **droplets#(1+2)**

$$\varphi = \frac{m\_{\text{quaves\\_phase},E}}{m\_{\text{coll\ phase},E} + m\_{\text{quours\\_phase},E}} = 0.624 \tag{3}$$

The emulsion sample was also weighted before introducing it inside the calorimeter head. In this example, the mass sample was ms = 0.2111g. From this data it was possible to determine the mass of the aqueous phase inside the emulsion sample as

$$m\_{\text{aquous phase},S} = m\_S \times \mathcal{op} = 0.1317 \,\text{g} \tag{4}$$

which finally gave 0.1054 g of water in the emulsion sample.

Finally, by knowing that the saline solution concentrated from 20 wt% to 25 wt% due to hydrate formation, it can be deduced that 75% of mass water does not participate to the reaction. Therefore, the number of mole of water engaged in the hydrate structure can be deduced from the equation (5).

$$m\_{water} = \frac{m\_{water\,\,energy}}{M\_{water}} = \frac{0.1054 \times (1 - 0.75)}{18} = 1.4638 \times 10^{-3} mol\tag{5}$$

The mass of hydrate crystallized mhydrate during the progressive melting of ice is calculated via equation (6) considering structure II of hydrate (CCl3F.(H2O)17).

$$m\_{hydrate} = \frac{n\_{water\,\,energy}}{17} \times M\_{hydrate} = 3.8190 \times 10^{-2}\,\text{g}\tag{6}$$

where Mhydrate = 443.5 g.mol-1 is the molar weight of the CCl3F hydrate.

Furthermore, it was also possible to determine the hydrate dissociation energy. The hydrate dissociation peak was integrated via the calorimeter software. Therefore, the corresponding energy released during the dissociation of hydrate divided by the mass calculated as described before, gave the specific dissociation energy of the hydrate. In this example, the specific dissociation energy was determined as 166.1 J.g-1 at -10ºC (Fouconnier et al. 2002).

The CCl3F hydrate model has been also used to understand the mechanism of hydrate formation in emulsion (Dalmazzone et al. 2002). It has been demonstrated that DSC is a suitable technique to detect the hydrate formation via the solid-liquid transitions involved and to predict the hydrate formation zone by the determining the temperature of hydrate dissociation as a function of salt concentration.

This model has been used to study methane hydrate formation in drilling muds (Dalmazzone et al. 2002). It has been demonstrated that the CCl3F hydrate formation can modeled gas hydrate formation in applied systems. Studies of gas hydrate formation by DSC can be found in literature (Koh et al, 2002; Karrat & Dalmazzone, 2003; Lachance et al., 2008; Dalmazzone et al. 2009, Davies et al., 2009) and it must be also emphasized that DSC has been developed to be directly used in offshore in order to predict the zone of gas hydrate formation (Le Parlouër et al. 2004).

#### **4. Mixed emulsions**

Mixed emulsions are obtained by gently mixing two simple emulsions containing droplets of different composition (Figure 6). Mixed emulsions are well-suited model systems to study transfer mechanisms across liquid membranes (Li, 1968; Colinart et al., 1984; Noble & Stern, 1995). For instance, these systems allow to model transport processes between two oil phases separated by an aqueous membrane in the case of oil-in-water (O/W) emulsions and between two aqueous phases separated by an organic membrane in the case of water-in-oil (W/O) emulsions.

Fig. 6. Schematic representation of mixed emulsion preparation

752 Mass Transfer - Advanced Aspects

 , , ,

*oil phase E aqueous phase E*

The emulsion sample was also weighted before introducing it inside the calorimeter head. In this example, the mass sample was ms = 0.2111g. From this data it was possible to determine

, 0.1317 *m m aqueous phase S S* = × =

Finally, by knowing that the saline solution concentrated from 20 wt% to 25 wt% due to hydrate formation, it can be deduced that 75% of mass water does not participate to the reaction. Therefore, the number of mole of water engaged in the hydrate structure can be

18

*n mol*

The mass of hydrate crystallized mhydrate during the progressive melting of ice is calculated

Furthermore, it was also possible to determine the hydrate dissociation energy. The hydrate dissociation peak was integrated via the calorimeter software. Therefore, the corresponding energy released during the dissociation of hydrate divided by the mass calculated as described before, gave the specific dissociation energy of the hydrate. In this example, the specific dissociation energy was determined as 166.1 J.g-1 at -10ºC (Fouconnier et al. 2002). The CCl3F hydrate model has been also used to understand the mechanism of hydrate formation in emulsion (Dalmazzone et al. 2002). It has been demonstrated that DSC is a suitable technique to detect the hydrate formation via the solid-liquid transitions involved and to predict the hydrate formation zone by the determining the temperature of hydrate

This model has been used to study methane hydrate formation in drilling muds (Dalmazzone et al. 2002). It has been demonstrated that the CCl3F hydrate formation can modeled gas hydrate formation in applied systems. Studies of gas hydrate formation by DSC can be found in literature (Koh et al, 2002; Karrat & Dalmazzone, 2003; Lachance et al., 2008; Dalmazzone et al. 2009, Davies et al., 2009) and it must be also emphasized that DSC has been developed to be directly used in offshore in order to predict the zone of gas

Mixed emulsions are obtained by gently mixing two simple emulsions containing droplets of different composition (Figure 6). Mixed emulsions are well-suited model systems to study

*m m m*

ϕ

the mass of the aqueous phase inside the emulsion sample as

which finally gave 0.1054 g of water in the emulsion sample.

*water engaged*

*m*

*water*

via equation (6) considering structure II of hydrate (CCl3F.(H2O)17).

*n*

17 *water engaged hydrate hydrate*

where Mhydrate = 443.5 g.mol-1 is the molar weight of the CCl3F hydrate.

*M*

deduced from the equation (5).

*water*

dissociation as a function of salt concentration.

hydrate formation (Le Parlouër et al. 2004).

**4. Mixed emulsions** 

0.624 *aqueous phase E*

ϕ

0.1054 (1 0.75) <sup>3</sup> 1.4638 10

× − <sup>−</sup> <sup>=</sup> <sup>=</sup> = × (5)

<sup>2</sup> 3.8190 10

*m M g* <sup>−</sup> = × =× (6)

<sup>=</sup> <sup>=</sup> <sup>+</sup> (3)

*g* (4)

The mass transfers due to a gradient composition between the different phases present in the emulsion lead to a modification of the composition of the droplets by dilution and the decrease of the amount of pure droplets by transfer (Figure 7).

Fig. 7. Schematic representation of the droplet composition with time in mixed emulsion

Many studies evidenced the composition ripening that takes place in mixed emulsion by measuring the rate of mass exchange between two kinds of water droplets with solute dispersed in oil media or two populations of hydrocarbon droplets dispersed in aqueous phase. In the case of this study, we propose to describe in this chapter the pure water mass transfer to aqueous solute solution droplets in W/O mixed emulsion and the transfer of tetradecane to hexadecane droplets in O/W mixed emulsion.

### **4.1 DSC measurements and determination of water mass transfer in W/O mixed emulsions**

The goal of the work reported (Clausse et al., 1995; 1999; 2008; Sacca et al., 2008) is to evidence the composition ripening that takes place in W/O mixed emulsions and to measure the rate of water exchange between pure water droplets and aqueous solute (urea or NaCl) droplets dispersed in oil medium. The effect of the emulsion stabilizing agent, as emulsifier or particles, on the kinetics of water transfer was investigated and discussed.

Mass Transfers Within Emulsions Studied by

t=35min; e) t=65min

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 755

I II

a b

t=5min

t=35min

dq/dt

1mCaVs

**0 10 20 30 40 50 % Urea (wt/wt)**

Fig. 9. Freezing temperature of urea solution versus urea concentration. Te: equilibrium freezing temperature of the urea solution in bulk; T\*: most probable freezing temperature of

the micro-sized droplets of urea solution dispersed in oil media


c

d

e

 **0 -10 -20 -30 -40 -50 -60 -70 -80**

**T(°C)**

t=65min

T(°C)

Fig. 8. DSC cooling curves of W/O mixed emulsion with pure water droplets and water+urea droplets dispersed in oil media at successive time intervals c) t=5min; d)

#### **4.1.1 Emulsion preparation**

The W/O mixed emulsions, for which water transfer is expected, were obtained by a simple manual mixing of equal masses of two W/O simple emulsions. First, W/O simple emulsions were made of 20 wt% (or 30 wt%) of pure water droplets or 20 wt% (or 30 wt%) of aqueous solute solution droplet dispersed in continuous phase consisting in variable mixtures of paraffin oil and pure vaseline paste. The aqueous solution consists in a urea solution (concentration of 20 wt% or 30 wt%) or a NaCl solution (concentration of 20 wt%). The emulsification was realized with a high speed homogeneizer at 50°C-60°C due to the higher viscosity of the aqueous solutions compared to pure water. To study the influence of the stabilizing agent, the W/O simple emulsions were stabilized employing different surfactants in the oil continuous phase, as the lipophilic lanolin emulsifier (8 wt%) and the nonionic Span 80 surfactant (3 wt%). To study the influence of solid particles, the W/O simple emulsions were solely stabilized by hydrophobic Aerosil R711 silica particles (0.1 wt% to 3 wt%). Then, the W/O mixed emulsion was obtained by mixing equal masses of the W/O simple emulsions prepared in the same conditions and with the same stabilizing agent. The W/O resultant mixed emulsion is a mixture of 10wt% (or 15 wt%) of pure water and 10 wt% (or 15 wt%) of aqueous solute solution droplets dispersed in oil media.

### **4.1.2 DSC measurements - results and discussion**

In the case of W/O mixed emulsion constituted at time zero of two kinds of water droplets, two broad exothermic peaks are observed on DSC curves, corresponding to the crystallization of pure water droplets and of aqueous solution of solute droplets.

In the case of W/O mixed emulsions containing pure water droplets and aqueous urea solution droplets stabilized by lanolin surfactant, the DSC cooling curves (Figure 8) indicate two solidification peaks corresponding to the freezing signal I of pure droplets at T\* = -39°C (Figure 8a) and the freezing signal II of aqueous urea solution droplets with a concentration of 30 wt% at T\* = -60°C (Figure 8b). DSC cooling curves point out a noticeable decrease with time of the signal I area characteristic of pure water droplets freezing (Figure 8c;d). In addition, DSC cooling curves show a shift with time towards higher temperature of the signal II corresponding to the solidification of water+urea droplets. Therefore these results evidence that there is no urea transfer and that water has been transported from the pure water droplets towards the water+urea droplets, causing their dilution, according to the calibration curve of this system reported on Figure 9. Finally, only one signal at around T = -48°C is observed 65 min after the mixing and no more evolution has been observed after that time (Figure 8e). This unique signal assumed that there are no more pure water droplets whereas the water+urea droplets are still present and their dilution having reached a maximum. From the knowledge of the dependence of the water+urea droplets freezing temperature versus the urea composition (Figure 9), it was deduced that the unique signal observed at T\* = -48°C is characteristic of the freezing of 15 wt% urea solution droplets. This final composition is in agreement with the initial W/O mixed emulsion obtained by mixing equal masses of each W/O simple emulsion containing 30% pure water of droplets and 30% of aqueous urea solution droplets. The evolution of pure water moles numbers in mixed emulsion was deduced from the surface area of the solidification signal I of pure water (using the Equation 2) (Figure 10).

It appears that the transfer is rather fast at the beginning in agreement which the fact that the gradient of concentration is maximum at t=0. Afterwards the experimental results obtained from four different emulsions are scattered in a relative large range that shows the problem of reproducibility. More controlled size droplets would certainly improve the reproducibility.

The W/O mixed emulsions, for which water transfer is expected, were obtained by a simple manual mixing of equal masses of two W/O simple emulsions. First, W/O simple emulsions were made of 20 wt% (or 30 wt%) of pure water droplets or 20 wt% (or 30 wt%) of aqueous solute solution droplet dispersed in continuous phase consisting in variable mixtures of paraffin oil and pure vaseline paste. The aqueous solution consists in a urea solution (concentration of 20 wt% or 30 wt%) or a NaCl solution (concentration of 20 wt%). The emulsification was realized with a high speed homogeneizer at 50°C-60°C due to the higher viscosity of the aqueous solutions compared to pure water. To study the influence of the stabilizing agent, the W/O simple emulsions were stabilized employing different surfactants in the oil continuous phase, as the lipophilic lanolin emulsifier (8 wt%) and the nonionic Span 80 surfactant (3 wt%). To study the influence of solid particles, the W/O simple emulsions were solely stabilized by hydrophobic Aerosil R711 silica particles (0.1 wt% to 3 wt%). Then, the W/O mixed emulsion was obtained by mixing equal masses of the W/O simple emulsions prepared in the same conditions and with the same stabilizing agent. The W/O resultant mixed emulsion is a mixture of 10wt% (or 15 wt%) of pure water

and 10 wt% (or 15 wt%) of aqueous solute solution droplets dispersed in oil media.

crystallization of pure water droplets and of aqueous solution of solute droplets.

In the case of W/O mixed emulsion constituted at time zero of two kinds of water droplets, two broad exothermic peaks are observed on DSC curves, corresponding to the

In the case of W/O mixed emulsions containing pure water droplets and aqueous urea solution droplets stabilized by lanolin surfactant, the DSC cooling curves (Figure 8) indicate two solidification peaks corresponding to the freezing signal I of pure droplets at T\* = -39°C (Figure 8a) and the freezing signal II of aqueous urea solution droplets with a concentration of 30 wt% at T\* = -60°C (Figure 8b). DSC cooling curves point out a noticeable decrease with time of the signal I area characteristic of pure water droplets freezing (Figure 8c;d). In addition, DSC cooling curves show a shift with time towards higher temperature of the signal II corresponding to the solidification of water+urea droplets. Therefore these results evidence that there is no urea transfer and that water has been transported from the pure water droplets towards the water+urea droplets, causing their dilution, according to the calibration curve of this system reported on Figure 9. Finally, only one signal at around T = -48°C is observed 65 min after the mixing and no more evolution has been observed after that time (Figure 8e). This unique signal assumed that there are no more pure water droplets whereas the water+urea droplets are still present and their dilution having reached a maximum. From the knowledge of the dependence of the water+urea droplets freezing temperature versus the urea composition (Figure 9), it was deduced that the unique signal observed at T\* = -48°C is characteristic of the freezing of 15 wt% urea solution droplets. This final composition is in agreement with the initial W/O mixed emulsion obtained by mixing equal masses of each W/O simple emulsion containing 30% pure water of droplets and 30% of aqueous urea solution droplets. The evolution of pure water moles numbers in mixed emulsion was deduced from the surface area of the solidification signal I of pure water

It appears that the transfer is rather fast at the beginning in agreement which the fact that the gradient of concentration is maximum at t=0. Afterwards the experimental results obtained from four different emulsions are scattered in a relative large range that shows the problem of reproducibility. More controlled size droplets would certainly improve the reproducibility.

**4.1.2 DSC measurements - results and discussion** 

(using the Equation 2) (Figure 10).

**4.1.1 Emulsion preparation** 

Fig. 8. DSC cooling curves of W/O mixed emulsion with pure water droplets and water+urea droplets dispersed in oil media at successive time intervals c) t=5min; d) t=35min; e) t=65min

Fig. 9. Freezing temperature of urea solution versus urea concentration. Te: equilibrium freezing temperature of the urea solution in bulk; T\*: most probable freezing temperature of the micro-sized droplets of urea solution dispersed in oil media

Mass Transfers Within Emulsions Studied by

(Figure 12).

system, around 60min.

1

0.8

0.6

**y(mol/mol)**

0.4

0.2

0

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 757

In the case of W/O mixed emulsions containing pure water droplets and aqueous NaCl solution droplets stabilized by lanolin surfactant, the DSC cooling curves (Figure 11) indicate a solidification signal I at T\* = -39.5°C corresponding to freezing of pure water droplet (Figure 11f) and a solidification signal II relative to the freezing of aqueous NaCl solution droplets with a concentration of 20 wt% at T\* = -67°C (Figure 11g). With time, DSC cooling curves show the area of the solidification signal I of pure water droplets decreases whereas the freezing signal II appears to broaden first and then to be more and more narrow (Figure 11a-e). Therefore these results evidence the decrease of the amount of pure water droplets in mixed emulsion by a dilution of the water+NaCl droplets due to water coming from the pure water droplets. When the DSC cooling curves do not change any longer after 82 min, the signal I has practically disappeared and a well defined signal II observed at around T\* = -51°C is noticeable (Figure 11e). This unique signal evidence that the complete water mass transfer is achieved: no more pure water droplets are still present and the water+NaCl droplets are diluted as much as possible. From the knowledge of the phase diagram of the water+NaCl emulsified system, and the melting temperature of the final droplets population, it was deduced that the unique signal observed at T\* = -51°C is characteristic of the freezing of 10 wt% aqueous NaCl solution droplets. This final composition is in agreement with the formulation of the W/O mixed emulsion. The evolution of the percentage of pure water moles numbers in mixed emulsion was deduced from the surface area of the solidification signal I of pure water (using the Equation 2)

In that case as well, it appears that the transfer is rather fast at the beginning. Furthermore, the time involved to reach equilibrium is very close to the one observed for the water+urea

In the case of W/O mixed emulsions containing pure water droplets and aqueous urea solution droplets stabilized by hydrophobic silica particles instead of a surfactant, the DSC curves (Figure 13) indicate a solidification signal I at T\* = -37°C very close to what was observed for the freezing of pure droplets (Figure 13a) and a solidification signal II at T\* = -52.6°C characteristic of the freezing of aqueous urea solution droplets with a

0 30 60 90 120 150

Fig. 12. Ratio y of pure water moles numbers non-transferred in W/O mixed emulsion with

pure water droplets and water+NaCl droplets dispersed in oil media, versus time

Time (min)

Test A Test B

Fig. 10. Ratio y of pure water moles numbers non-transferred in W/O mixed emulsion with pure water droplets and water+urea droplets dispersed in oil media versus time

Fig. 11. DSC cooling curves of W/O mixed emulsion with pure water droplets and water+NaCl droplets dispersed in oil media at successive time intervals a) t = 2 min; b) t = 25 min; c) t = 43 min; d) t = 62 min; e) t = 82 min

Test 1 Test 2 Test 3 Test 4

**Time (min)**

0 20 40 60 80 100 120

Fig. 10. Ratio y of pure water moles numbers non-transferred in W/O mixed emulsion with

pure water droplets and water+urea droplets dispersed in oil media versus time

a)

*dq dt*

b)

c)

d)

e)

b) t = 25 min; c) t = 43 min; d) t = 62 min; e) t = 82 min

f) g)

Fig. 11. DSC cooling curves of W/O mixed emulsion with pure water droplets and water+NaCl droplets dispersed in oil media at successive time intervals a) t = 2 min;


 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

**y(mol/mol)**

In the case of W/O mixed emulsions containing pure water droplets and aqueous NaCl solution droplets stabilized by lanolin surfactant, the DSC cooling curves (Figure 11) indicate a solidification signal I at T\* = -39.5°C corresponding to freezing of pure water droplet (Figure 11f) and a solidification signal II relative to the freezing of aqueous NaCl solution droplets with a concentration of 20 wt% at T\* = -67°C (Figure 11g). With time, DSC cooling curves show the area of the solidification signal I of pure water droplets decreases whereas the freezing signal II appears to broaden first and then to be more and more narrow (Figure 11a-e). Therefore these results evidence the decrease of the amount of pure water droplets in mixed emulsion by a dilution of the water+NaCl droplets due to water coming from the pure water droplets. When the DSC cooling curves do not change any longer after 82 min, the signal I has practically disappeared and a well defined signal II observed at around T\* = -51°C is noticeable (Figure 11e). This unique signal evidence that the complete water mass transfer is achieved: no more pure water droplets are still present and the water+NaCl droplets are diluted as much as possible. From the knowledge of the phase diagram of the water+NaCl emulsified system, and the melting temperature of the final droplets population, it was deduced that the unique signal observed at T\* = -51°C is characteristic of the freezing of 10 wt% aqueous NaCl solution droplets. This final composition is in agreement with the formulation of the W/O mixed emulsion. The evolution of the percentage of pure water moles numbers in mixed emulsion was deduced from the surface area of the solidification signal I of pure water (using the Equation 2) (Figure 12).

In that case as well, it appears that the transfer is rather fast at the beginning. Furthermore, the time involved to reach equilibrium is very close to the one observed for the water+urea system, around 60min.

In the case of W/O mixed emulsions containing pure water droplets and aqueous urea solution droplets stabilized by hydrophobic silica particles instead of a surfactant, the DSC curves (Figure 13) indicate a solidification signal I at T\* = -37°C very close to what was observed for the freezing of pure droplets (Figure 13a) and a solidification signal II at T\* = -52.6°C characteristic of the freezing of aqueous urea solution droplets with a

Fig. 12. Ratio y of pure water moles numbers non-transferred in W/O mixed emulsion with pure water droplets and water+NaCl droplets dispersed in oil media, versus time

Mass Transfers Within Emulsions Studied by

**emulsion** 

phase.

also studied and discussed.

**4.2.1 Emulsion preparation** 

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 759

This work shows a water mass transfer through the oil media from the pure water droplets to the aqueous droplets containing a solute, causing their dilution. The transfer mechanism found is in agreement with the solution-diffusion model. This mechanism supposed that water can be solubilized in the oil medium and can also diffuse in this oil medium due to the composition discrepancy between the droplets that creates a chemical potential gradient. According to thermodynamic considerations, water transfer is expected to transfer from pure water droplets (highest water chemical potential) towards water+solute droplets (lowest water chemical potential), and not the reverse. The results show that the characteristic time scale for water transport through the oil media can span about one hour in presence of surfactant. The kinectics of composition ripening seems to depend on parameters of emulsion formulation as the surfactant type and concentration, the solute type and concentration, and the presence of solid particles in the oil media. The mass transfer process is attributed to the great exchange area available in the emulsion and furthermore to the essential presence of surfactants. Although, the role of these parameters is not yet clearly established. These results evidence that the presence of silica particles in the oil media does not stop but slow down the water mass transfer, in comparison to a surfactant. These results suggest that the mechanism of mass transfer in presence of solid

particles might be different of the solution-diffusion model previously proposed.

**4.2 DSC measurements and determination of tetradecane mass transfer in O/W mixed** 

The objective of the work presented (Avendano-Gomez et al., 2000; Avendano-Gomez, 2002; Clausse et al., 2002b; Drelich, 2009) is to study the composition ripening that takes place in O/W mixed emulsions and to measure the rate of oil exchange between n-tetradecane and n-hexadecane droplets dispersed in aqueous phase. The influence of surfactant concentration, surfactant nature, amount of salt and presence of solid particles on the rate of oil exchange is

The O/W mixed emulsion was prepared by gently mixing two O/W simple emulsions. Firstly, O/W simple emulsions of tetradecane and n-hexadecane were prepared separately with a concentration of 40 wt% of the oil phase with the same surfactant type and concentration and homogenized with a high speed blender at 20 000 rpm. To study the influence of the surfactant, the O/W simple emulsions were stabilized employing different surfactant aqueous systems, as the non ionic surfactant Tween 20 and the ionic surfactant Brij 35. The different surfactant aqueous systems were prepared with two surfactant concentrations of 2 wt % and 4 wt%, corresponding to a higher concentration than their respective critical micellar concentration. To study the influence of the presence of salt, the O/W simple emulsions were prepared with aqueous solution containing an amount of NaCl (1 wt% and 2 wt%) added to the Tween 20 surfactant (2 wt%). To study the influence of solid particles, the O/W simple emulsions were stabilized by a mixture of hydrophilic Aerosil A200 (2 wt%) and hydrophobic Aerosil R711 (2 wt%) silica particles. Then, the O/W mixed emulsion was obtained by mixing equal masses of the O/W simple emulsions. The O/W resultant mixed emulsion is a mixture of 15 wt% of pure n-tetradecane and 15 wt% of pure n-hexadecane droplets dispersed in 70 wt% of surfactant aqueous

concentration of 20 wt% (Figure 13b). DSC curves show that the evolution of the solidification signal occurs similarly to what is observed in the case of W/O mixed emulsions stabilized by a surfactant. Therefore these results evidence that aqueous urea solution droplets are diluted by the transfer of water from pure water droplets which progressively disappear from the mixed emulsion, in agreement with the previous studies presented. DSC cooling curves show no modification of the unique solidification peaks observed from 3h 15min (Figure 13e) characteristic of the complete water mass transfer. Similar experiments were performed on W/O mixed emulsions prepared in the same condition but containing pure water droplets and aqueous urea solution droplets stabilized by the nonionic Span 80 surfactant. Same evolutions of the solidification signals are observed, but the unique solidification peaks resulting for complete water mass transfer is observed after 1 hour of evolution.

Fig. 13. DSC cooling curves of W/O mixed emulsion with pure water droplets and water+urea droplets dispersed in oil media and stabilized by hydrophobic silica particles at successive time intervals a) corresponding W/O simple emulsion of water+urea droplets at t=0; b) corresponding W/O simple emulsion of pure water droplets; c) t = 10min; d) t = 1 hours 15 min; e) t = 3 hours 15 min; f) t = 20 hours; g) t = 23 hours

This work shows a water mass transfer through the oil media from the pure water droplets to the aqueous droplets containing a solute, causing their dilution. The transfer mechanism found is in agreement with the solution-diffusion model. This mechanism supposed that water can be solubilized in the oil medium and can also diffuse in this oil medium due to the composition discrepancy between the droplets that creates a chemical potential gradient. According to thermodynamic considerations, water transfer is expected to transfer from pure water droplets (highest water chemical potential) towards water+solute droplets (lowest water chemical potential), and not the reverse. The results show that the characteristic time scale for water transport through the oil media can span about one hour in presence of surfactant. The kinectics of composition ripening seems to depend on parameters of emulsion formulation as the surfactant type and concentration, the solute type and concentration, and the presence of solid particles in the oil media. The mass transfer process is attributed to the great exchange area available in the emulsion and furthermore to the essential presence of surfactants. Although, the role of these parameters is not yet clearly established. These results evidence that the presence of silica particles in the oil media does not stop but slow down the water mass transfer, in comparison to a surfactant. These results suggest that the mechanism of mass transfer in presence of solid particles might be different of the solution-diffusion model previously proposed.

## **4.2 DSC measurements and determination of tetradecane mass transfer in O/W mixed emulsion**

The objective of the work presented (Avendano-Gomez et al., 2000; Avendano-Gomez, 2002; Clausse et al., 2002b; Drelich, 2009) is to study the composition ripening that takes place in O/W mixed emulsions and to measure the rate of oil exchange between n-tetradecane and n-hexadecane droplets dispersed in aqueous phase. The influence of surfactant concentration, surfactant nature, amount of salt and presence of solid particles on the rate of oil exchange is also studied and discussed.
