**4.2.1 Emulsion preparation**

758 Mass Transfer - Advanced Aspects

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

I

II

t = 10 minutes

t = 1 hour 15 minutes

t = 3 hours 15 minutes

t = 20 hours

t = 23 hours


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

observed after 1 hour of evolution.

0


a

b

c

d

e

f

g

hours 15 min; e) t = 3 hours 15 min; f) t = 20 hours; g) t = 23 hours



**unités arbitraires**




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 phase.

Mass Transfers Within Emulsions Studied by

Total oil transfer between droplets at t=6h

t = 4h t = 3h t = 2h t = 0

Heat flow (Exothermic→)

signal I of pure tetradecane (using the Equation 2) (Figure 17).

Total oil transfer between droplets at t=31h30

t = 4h

t = 24h t = 12h t = 8h

t = 29h

Heat flow (Exothermic→)

time intervals

t = 0

Temperature (°C)

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 761

droplets is in agreement with the value expected. The results indicate the time to reach the total oil transfer is 16 hours with 2% of Brij 35 and only 3 hours and 30 minutes with 4% of Brij 35 (not represented), 24 hours with 2% of Tween 20 (Figure 15a right) and only 6 hours with 4% of Tween 20 (Figure 15a left). The result shows that the mass transfer is achieved after 31 h and 30 minutes of evolution when droplets are dispersed in aqueous phase containing 2% of NaCl and 2% of Tween 20 (Figure 15b). The evolution of pure tetradecane moles numbers in mixed emulsion was deduced from the surface area of the solidification

> Total oil transfer between droplets at t=24h

0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20

t = 20h

t = 16h t = 8h

Heat flow (Exothermic→)

Temperature (°C) 0 -2 -17 Temperature (°C) 0 -2 -17

(a)

(b) Fig. 15. DSC curves showing the solidification of O/W mixed emulsion containing 15 wt% of n-tetradecane droplets and 15 wt% of n-hexadecane droplets dispersed in an aqueous phase with a) 4 wt% (left) and 2 wt% (right) of Brij 35, b) 4 wt% (left) and 2 wt% (right) of Tween 20 surfactant and c) 2 wt% of NaCl and 2 wt% of Tween 20 surfactant, at successive

t = 0

#### **4.2.2 DSC measurements – results and discussion**

In the case of O/W mixed emulsion constituted at time zero of two populations of oil droplets of different nature, two broad exothermic peaks are observed on DSC curves, corresponding to the crystallization of each population of oil droplets.

In the case of O/W emulsions containing n-tetradecane droplets and n-hexadecane droplets stabilized by a surfactant, the DSC curves (Figure 14) indicate a first broad exothermic signal I at T\* = -2°C for the solidification of the n-hexadecane droplets, a second broad exothermic signal II at T\* = -17°C corresponding to the freezing of the n-tetradecane droplets, and a narrow exothermic peak at Tc = -22°C related to the crystallization of the aqueous continuous phase. DSC curves (Figure 15) show an evolution of the two oil solidification peaks with time: an area reduction of the solidification signal II of pure tetradecane droplets and a displacement of the solidification signal I of n-hexadecane towards lower temperatures. The area reduction of the solidification peak II of pure n-tetradecane droplets is related to the amount decrease of pure n-tetradecane droplets with time. The displacement of the solidification signal I of n-hexadecane droplets towards lower temperature is related to the composition modification of the n-hexadecane droplets by the n-tetradecane dilution with time, according to the calibration curve reported on Figure 16. These evolutions evidence a preferential and global oil exchange from the tetradecane droplets towards the n-hexadecane droplets. In addition, the DSC curves reveal only one solidification peak and no change in the crystallization temperature of the dispersed phase obtained after some hours of emulsion evolution. This unique signal suggests that the mass transfer between the oils droplets is complete. The crystallization temperature of the last unique signal is observed at around T\* = -10°C and corresponds to a composition of 50% in mass of n-hexadecane in the droplets according the calibration curve (Figure 16). The initial O/W mixed emulsion containing the same mass ratio of tetradedacne/n-hexadecane, the resulting composition due to a mixture between the tetradecane and the n-hexadecane droplets is 50% of n-hexadecane. Therefore, the final n-hexadecane composition within the

Fig. 14. DSC curves of solidification of O/W mixed emulsion with n-tetradecane droplets and n-hexadecane droplets dispersed in aqueous phase

#### Mass Transfers Within Emulsions Studied by Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 761

760 Mass Transfer - Advanced Aspects

In the case of O/W mixed emulsion constituted at time zero of two populations of oil droplets of different nature, two broad exothermic peaks are observed on DSC curves,

In the case of O/W emulsions containing n-tetradecane droplets and n-hexadecane droplets stabilized by a surfactant, the DSC curves (Figure 14) indicate a first broad exothermic signal I at T\* = -2°C for the solidification of the n-hexadecane droplets, a second broad exothermic signal II at T\* = -17°C corresponding to the freezing of the n-tetradecane droplets, and a narrow exothermic peak at Tc = -22°C related to the crystallization of the aqueous continuous phase. DSC curves (Figure 15) show an evolution of the two oil solidification peaks with time: an area reduction of the solidification signal II of pure tetradecane droplets and a displacement of the solidification signal I of n-hexadecane towards lower temperatures. The area reduction of the solidification peak II of pure n-tetradecane droplets is related to the amount decrease of pure n-tetradecane droplets with time. The displacement of the solidification signal I of n-hexadecane droplets towards lower temperature is related to the composition modification of the n-hexadecane droplets by the n-tetradecane dilution with time, according to the calibration curve reported on Figure 16. These evolutions evidence a preferential and global oil exchange from the tetradecane droplets towards the n-hexadecane droplets. In addition, the DSC curves reveal only one solidification peak and no change in the crystallization temperature of the dispersed phase obtained after some hours of emulsion evolution. This unique signal suggests that the mass transfer between the oils droplets is complete. The crystallization temperature of the last unique signal is observed at around T\* = -10°C and corresponds to a composition of 50% in mass of n-hexadecane in the droplets according the calibration curve (Figure 16). The initial O/W mixed emulsion containing the same mass ratio of tetradedacne/n-hexadecane, the resulting composition due to a mixture between the tetradecane and the n-hexadecane droplets is 50% of n-hexadecane. Therefore, the final n-hexadecane composition within the

**19 17 16 15 13 11 10 8 7 5 4 2 1 -1 -3 -4 -6 -7 -9 -10 -12 -14 -15 -17 -18 -20 -21 -23 Temperature (°C)**

Fig. 14. DSC curves of solidification of O/W mixed emulsion with n-tetradecane droplets

**Crystalisation of the aqueous phase**

**tetradecane crystalization peak**

**<sup>I</sup> II**

**IV**

**4.2.2 DSC measurements – results and discussion** 

**Heat flow (Exotheramic ->)**

**n-hexadecane crystalization peak**

and n-hexadecane droplets dispersed in aqueous phase

corresponding to the crystallization of each population of oil droplets.

droplets is in agreement with the value expected. The results indicate the time to reach the total oil transfer is 16 hours with 2% of Brij 35 and only 3 hours and 30 minutes with 4% of Brij 35 (not represented), 24 hours with 2% of Tween 20 (Figure 15a right) and only 6 hours with 4% of Tween 20 (Figure 15a left). The result shows that the mass transfer is achieved after 31 h and 30 minutes of evolution when droplets are dispersed in aqueous phase containing 2% of NaCl and 2% of Tween 20 (Figure 15b). The evolution of pure tetradecane moles numbers in mixed emulsion was deduced from the surface area of the solidification signal I of pure tetradecane (using the Equation 2) (Figure 17).

Fig. 15. DSC curves showing the solidification of O/W mixed emulsion containing 15 wt% of n-tetradecane droplets and 15 wt% of n-hexadecane droplets dispersed in an aqueous phase with a) 4 wt% (left) and 2 wt% (right) of Brij 35, b) 4 wt% (left) and 2 wt% (right) of Tween 20 surfactant and c) 2 wt% of NaCl and 2 wt% of Tween 20 surfactant, at successive time intervals

Mass Transfers Within Emulsions Studied by

successive time intervals

al., 1998; 1999; Elwell et al., 2004).

**4.3 Model of kinetics of composition ripening** 

and separated by a plane liquid membrane made of oil (Figure 19).

**0**

**0,5**

**1**

**1,5**

**2**

**2,5**

**Heat Flow (W/g)**

**3**

**3,5**

Differential Scanning Calorimetry (DSC) - Application to Composition Ripening and Solid Ripening 763

**t = 13days**

**t = 8days**

**t = 72h**

**t = 24h**

**t = 0**

**-10 0 10 20 Temperature (°C)**

Fig. 18. DSC curves showing the solidification of solid-stabilized O/W mixed emulsion containing 20 wt% of n-tetradecane droplets and 20 wt% of n-hexadecane droplets, at

This work shows a preferential mass transfer from the tetradecane to the n-hexadecane droplets in agreement with the literature. Many studies evidenced the importance of the solubility of the oil dispersed phase in water on the direction and the rate of oil exchange through the continuous water phase (Taisne et al., 1996; Binks et al., 1998; 1999). Indeed, it was demonstrated that the Tween 20 surfactant enhances the solubility of the tetradecane rather than the hexadecane (McClement et al., 1995; Weiss et al., 2000). The results show that the characteristic time scale for oil exchange between tetradecane and n-hexadecane droplets kinetics can span a wide range from seconds to several hours in presence of surfactant. The kinectics of composition ripening seems to depend on parameters of emulsion formulation as the surfactant type, the surfactant concentration, and the amount of salt and the presence of solid particles in the aqueous phase. The results show that the rate of oil exchange between droplets is faster when the non-ionic surfactant concentration is higher, in agreement with the literature (McClement et al., 1993c; Binks et al., 1998; 1999). On the contrary, the amount of salt added into the continuous phase slows down the rate of oil transfer as it was evidenced by McClement et al. (McClement et al., 1993c), and the presence of solid particles seems to block the oil exchange (Drelich et al., 2011). These results suggest that oil transfer may enhanced by the excess of surfactant micelles in the continuous phase. Mechanism of micelle transportation and solubilization of the oil through the continuous phase was proposed in the literature (McClement et al., 1992; 1993a; 1993b, 1993c; Binks et

The mechanism of composition ripening has to be considered when a composition gradient exists within the emulsion under study. To model the mass transfer, the mixed emulsions are pictured as: two oil phases of different nature compartmented and separated by a plane liquid aqueous membrane or two aqueous phases of different composition compartmented

Fig. 16. Calibration curve giving the most probable crystallization temperature as a function of n-tetradecane/n-hexadecane oil droplets dispersed in aqueous phase

Fig. 17. Ratio y of pure tetradecane moles numbers non-transferred in O/W mixed emulsion with n-tetradecane droplets and n-hexadecane droplets dispersed in aqueous surfactant phase versus time

In the case of O/W emulsions stabilized by silica particles, the DSC curves (Figure 18) show no evolution of the solidification signals of oil droplets observed during a time scale of 13 days. These results suggest that no modification of the droplets oil composition occurred during this time scale. These evolutions evidence silica particles not permit to a mass transfer between tetradecane and n-hexadecane.

Fig. 16. Calibration curve giving the most probable crystallization temperature as a function

**Time (h)** 0 5 10 15 20 25

Fig. 17. Ratio y of pure tetradecane moles numbers non-transferred in O/W mixed emulsion with n-tetradecane droplets and n-hexadecane droplets dispersed in aqueous surfactant

In the case of O/W emulsions stabilized by silica particles, the DSC curves (Figure 18) show no evolution of the solidification signals of oil droplets observed during a time scale of 13 days. These results suggest that no modification of the droplets oil composition occurred during this time scale. These evolutions evidence silica particles not permit to a mass

of n-tetradecane/n-hexadecane oil droplets dispersed in aqueous phase

1.2

1

0.8

0.6

**y(mol/mol)**

0.4

0.2

0

transfer between tetradecane and n-hexadecane.

phase versus time

Fig. 18. DSC curves showing the solidification of solid-stabilized O/W mixed emulsion containing 20 wt% of n-tetradecane droplets and 20 wt% of n-hexadecane droplets, at successive time intervals

This work shows a preferential mass transfer from the tetradecane to the n-hexadecane droplets in agreement with the literature. Many studies evidenced the importance of the solubility of the oil dispersed phase in water on the direction and the rate of oil exchange through the continuous water phase (Taisne et al., 1996; Binks et al., 1998; 1999). Indeed, it was demonstrated that the Tween 20 surfactant enhances the solubility of the tetradecane rather than the hexadecane (McClement et al., 1995; Weiss et al., 2000). The results show that the characteristic time scale for oil exchange between tetradecane and n-hexadecane droplets kinetics can span a wide range from seconds to several hours in presence of surfactant. The kinectics of composition ripening seems to depend on parameters of emulsion formulation as the surfactant type, the surfactant concentration, and the amount of salt and the presence of solid particles in the aqueous phase. The results show that the rate of oil exchange between droplets is faster when the non-ionic surfactant concentration is higher, in agreement with the literature (McClement et al., 1993c; Binks et al., 1998; 1999). On the contrary, the amount of salt added into the continuous phase slows down the rate of oil transfer as it was evidenced by McClement et al. (McClement et al., 1993c), and the presence of solid particles seems to block the oil exchange (Drelich et al., 2011). These results suggest that oil transfer may enhanced by the excess of surfactant micelles in the continuous phase. Mechanism of micelle transportation and solubilization of the oil through the continuous phase was proposed in the literature (McClement et al., 1992; 1993a; 1993b, 1993c; Binks et al., 1998; 1999; Elwell et al., 2004).
