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W emulsions as a result of partial solidification
Colloids and Surfaces A: Physicochemical and Engineering Aspects 152 (1999) 23–29
Stability of W/O and W/O/W emulsions as a result of partial solidification D. Clausse *, I. Pezron, L. Komunjer Universite´ de Technologie de Compie`gne, Centre de Recherche de Royallieu, De´partement de Ge´nie Chimique, CNRS UPRES A 6067 Ge´nie des Proce´de´s, Division Thermodynamique et Physicochimie de Proce´de´s Industriels, BP 20529-60205 Compie`gne, Cedex, France Received 20 January 1998; accepted 4 November 1998
Abstract This communication reviews the drastic modifications that may occur within emulsions when they are submitted to temperature variations during manufacturing, storage, transport and use. Emphasis will be given to the modifications resulting from changes after solidification. Pathways of solidification of W/O and W/O/W emulsions will be presented. Due to the specific structure of the emulsions under study (simple or multiple) only partial solidification may be obtained. This situation, out of thermodynamic equilibrium, may lead to a water transfer across the oil phase. Resulting emulsion transformation and destabilization are examined. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Emulsion stability; Solidification; Mass transfer; Ripening
1. Introduction This paper deals with emulsion destabilization due to partial solidification, phenomenon that is scarcely considered although it can occur more frequently than thought. Solidification processes considered here are due to temperature changes that are more or less controlled. In the case of storage or transport of emulsified products, the environment parameters such as temperature may vary between −50 and +40°C depending on the country. Consequently the product may be altered without immediate noticeable change in the appearance of the emulsions. Our study has been limited to the solidification of the dispersed aqueous phase of a water in oil emulsion ( W/O) and * Corresponding author. Tel.: +33-44-23-44-39; fax: +33-44-23-19-80. E-mail address: [email protected] (D. Clausse)
to the solidification of the dispersed and bulk aqueous phases of a water in oil in water emulsion ( W/O/W ). The aqueous phases considered here are either pure water or salt solutions. The consequences of this solidification will be examined in the second part of the paper. Most of the results have been obtained by microcalorimetry using a differential scanning calorimeter. As this technique for studying emulsions has been described thoroughly elsewhere, experimental details can be found in references given in the text.
2. Solidification of the dispersed phase of a W/O emulsion The dispersed phase of a W/O emulsion is made of water droplets of a few micrometers diameter. From theoretical considerations, it is possible to
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D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
predict the way the solidification of the droplets will take place. This will be done for pure water droplets in the first part of this section. Some results obtained by microcalorimetry of such emulsions will also be given. In the second part, experimental results dealing with droplets of aqueous solution of sodium chloride will be described. They point out the possible tremendous influence of a solute on the solidification of the droplets and its consequences on the emulsion stability. 2.1. Pure water droplets It is well known that the temperature must be lowered well below 0°C (under atmospheric pressure) in order to induce water droplet solidification [1]. What may be less well known is that the droplets will solidify at different temperatures even in case of a monodisperse emulsion. This can be explained considering that the first solidification stage of a single droplet is the birth of a germ, defined as the smallest metastable solid particle that can be formed at a given temperature. The total solidification of the droplet is achieved by the growth of such germ. Indeed it can be shown that the temperature (T ) has to be well below 0°C for a spherical particle of ice acting as a germ to be in unstable equilibrium in an undercooled water droplet. The germ is small enough for capillary effects to be taken into account. The germ radius R is a function of the temperature T as shown in Eq. (1): ln (T/T )=−(2c · V )/(R · L ) (1) m i m where c is the interfacial tension between ice and undercooled water, V the ice molar volume, L i m the molar melting enthalpy of ice and T = m 273.15 K, the melting temperature of ice. If T is close to T , we may write: m (T −T )/T =(2c · V )/(R · L ) (2) m m i m From Eq. (2), it is possible to evaluate the value of R at any temperature although reliable values of c are not yet available. According to estimations made by Dufour and Defay [2], the following values can be given: T (°C ) R (cm)
0 2
−20 2.08×10−7
−40 1.03×10−7
It appears that the lower the temperature, the smaller the ice germ. Another point to consider is the kinetics of the formation of the germs themselves. For that purpose, a stationary nucleation rate J can be evaluated from the following simplified equation considering a model of germ formation from fluctuating ice embryos: J=A exp (−DW/kT )
(3)
where J is the number of germs formed per unit of time and of volume at temperature T; DW is the energy of formation of the germ: DW=(4/3)pcR2 and A is the pre-exponential factor taking into account, among other factors, the medium viscosity. Eq. (3) and Eq. (2) show that the closer the temperature T is to T (e.g. 0°C for water) the m smaller J is. Besides, J is expected to increase drastically when the temperature is far from T . m From this analysis, it is possible to predict the birth instant t of a germ in a droplet of volume g V according to Eq. (4):
P
tg VJ dt=1 (4) tm where t and t correspond to the times when the m g temperature reaches, respectively T (0°C ) and m T (germ formation temperature). If the experig ment is performed at a constant cooling rate T*, then T*=−dT/dt and Eq. (4) can be rewritten as:
P
−(V/T*)
Tg
J dT=1 (5) Tm Eq. (5) shows that the smaller the droplet volume, the lower the temperature at which there is a chance of formation of a germ in a single droplet. This is a general rule which can be applied to any sample. Let us study now the solidification of a big number of droplets each of them being considered as a single sample. It is generally considered that a droplet solidifies as soon as a germ forms within it. The formation of a germ is essentially a random phenomenon. Although Eq. (3) gives the number of germs formed per second and per volume unit, it does not indicate where in the volume the germs are formed. A definite volume of water is dispersed
D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
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[3] and hertzian spectrometry [4]. For a polydisperse emulsion characterized by a mean diameter of 1 mm, the temperature at which 50% of the droplets are frozen was found to be about −39°C. This analysis predicts that the droplet solidification can also be observed in time when the emulsion is maintained at any temperature below 0°C. Eq. (6) becomes: dN=J(N*−N )V dt
Fig. 1. Schematic histograms showing the transformation of water droplets into ice.
into the oil medium to make a W/O emulsion. This volume is split up into a large numbers of droplets and the formation of a germ in any one of these small droplets is a phenomenon for which only a probability can be drawn. This is why, considering a large number of identical droplets of the same radius cooled at the same rate, we will not observe simultaneous solidification of all the droplets. Some will solidify before others, so that their individual solidification temperatures will be dispersed around a mean value according to the analysis done thereafter. Considering N* the total number of water droplets of the same volume V, the number dN of droplets likely to solidify in the temperature interval dT is given by: dN=−J(N*−N )V(1/T*)dT
(6)
where T* is the cooling rate and N is the number of solidified droplets at temperature T. It appears that the proportion of solidified droplets, dN/N*, goes through a maximum as the number of liquid droplets (N*−N ) vanishes and the nucleation rate increases exponentially with decreasing temperature (Eq. (3)). The theoretical shape of the histograms describing the transformation of the droplets into ice can then be predicted ( Fig. 1). Actually such observation have been made on emulsions by calorimetry
(7)
Several attempts have been made to determine J [2]. It has been thought during a long time, according to theoretical predictions, that some isolated droplets would never solidify if the temperature was too far from a particular critical temperature close to −40°C because J would be nearly zero (Eq. (3)). Doubt has been raised and persists since total solidification have been observed after several months in the case of emulsions maintained at rather high temperatures [1,5,6 ]. It must then be remembered that, as soon as the temperature drops below 0°C, progressive solidification is possible, as can happen during any transport or storage of an emulsion. It has also been shown that a short thermal shock at −30°C significantly increases the solidification kinetics of droplets still undercooled at a higher temperature of −10°C [7]. For instance, 50% of the droplets are frozen within 14 h when they are cooled to −30°C before being maintained at −10°C, meanwhile for the same lapse of time no droplet is frozen when the emulsion is directly placed at −10°C. Similar effects are observed after a solidification/melting cycle of the emulsion: when, after a cooling stage at −20°C, the system is heated to a temperature higher than 0°C and then progressively cooled again, two characteristic solidification temperatures (at −33 and −39°C ) are observed instead of one (at −39°C ) [1]. The most surprising observation is that the number of droplets solidifying around −33°C was found to decrease when more and more cooling/heating cycles were performed. At the end of the evolution, the solidification of the droplets was found to occur in the same way as the one observed at the first cooling before cycles. Possible explanations for this phenomena will be discussed in Section 3.
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D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
2.2. Water+salt droplets Numerous studies have been made on dispersed water+salt solutions [8–10]. The presence of salt may induce a significant drop of the solidification temperature down to less than −50°C. Anyway, as in the case of pure water, solidification can be observed at higher temperatures. For example, it was observed by microcalorimetry that 70% of an emulsified aqueous solution of NaCl (molar fraction x=0.023) was solid after being maintained 156 h at −32°C. Furthermore, an unexpected behavior was observed during re-heating performed after the emulsion was maintained for different lapses of time at −32°C. Several examples of thermograms obtained are shown on Fig. 2. Fig. 2a, which can be taken as a reference, has been obtained during the heating stage after a full
Fig. 3. Thermograms obtained by differential scanning calorimetry of emulsions containing droplets of aqueous solution of sodium chloride and submitted to regular cooling.
Fig. 2. Thermograms obtained by differential scanning calorimetry of emulsions containing droplets of aqueous solution of sodium chloride and submitted to regular heating.
cooling of the emulsion. The eutectic melting is observed at −21°C followed by the progressive melting of the remaining ice which stops at −4.5°C in agreement with the equilibrium freezing point depression corresponding to the composition of the solution. Thermogram 2b was obtained after holding the emulsion for 1 h at −32°C. From the value of the signal area, which is proportional to the change of enthalpy during melting, it is possible to deduce that 16% of the dispersed solution has solidified. Thermogram 2c, after 156 h at −32°C (73% of the dispersed solution has solidified ), reveals unexpectedly a total melting very close to 0°C as if the salt had less influence. Thermograms 2d and 2e have been obtained, respectively during the re-heating of the system after a cooling stage following step 2c and after eight cooling/heating cycles following step 2c. There is a clear evidence of the shifting of the signals towards the ‘normal’ situation shown in Fig. 2a. This ‘back-up’ phenomenon is speeded-up (thermogram 2f ) when the
D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
emulsion is maintained during 16 h at 20°C before a new cooling/heating cycle. The thermograms obtained during the cooling stage of the emulsions (Fig. 3) are consistent with what is observed during the heating stages. Thermogram 3a, obtained during the first cooling stage of the emulsion, shows ice solidification in the droplets (at about −48°C ) and total solidification of the system (ice and salt crystals) around −72°C. In thermogram 3d, obtained during the cooling of the emulsion following cooling (thermogram 3a) and heating (thermogram 2c), early ice formation is observed around −40°C as observed for pure water droplets instead of −48°C observed for water+salt droplets. During the following cooling-heating cycles (thermograms: 2d (heating), 3e (cooling) and 2e (heating)), this phenomenon tends to disappear. After stabilization of the emulsion during 16 h at 20°C the thermograms obtained during cooling (3f ) and heating (2f ) are the same as those obtained during the first cooling (2a) and heating cycle (3a).
2.3. Solidification of the disperse and bulk aqueous phases of a multiple emulsion W/O/W Multiple water-in oil-in water ( W/O/W ) emulsions are obtained by dispersing a water in oil ( W/O) emulsion in a continuous aqueous phase [11]. Doing so, oil globules of diameter around 10 mm containing dispersed water droplets of about 1 mm diameter are obtained. When such an emulsion is submitted to a progressive cooling in a calorimeter, the solidification of the bulk aqueous phase wherein the oil globules are dispersed and of the dispersed water inside the globules are expected to occur at different temperatures as far as the volumes concerned are different: a few cube millimeters for the bulk (calorimeter cell volume) and a few cube micrometers for each droplet of the dispersed phase. It must be noticed that the bulk aqueous phase must be considered as a single sample and the dispersed aqueous phase as a very great number of samples. Therefore, the solidification temperature of the bulk is expected to change from one experiment to another whereas a mean solidification temperature will be obtained for the
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dispersed droplets as it is for a simple emulsion W/O. Indeed two different temperatures have been obtained: around −20°C for the bulk phase and −40°C for the dispersed phase as it was found for a simple emulsion [12].
3. Impact of solidification on the emulsion structure Under a thermal shock bringing the temperature of a W/O emulsion to −20°C for example, a part of the droplets will be frozen and the others constituted of undercooled water, as it was shown in the previous sections. It is likely that this partial solidification of the emulsion may lead to its destabilization since liquid water and ice can only coexist at 0°C. At this temperature the chemical potentials of ice m and undercooled water m are i w different according to Eq. (8) m −m =RT ln a i w i where a =P /P (P and P are the respective i i w i w vapor pressures of ice and undercooled water). Since m
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D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
When salt is present in the water droplets, a thermal shock bringing the emulsion to subambient temperatures could also lead to partial solidification of the dispersed droplets. Depending on the temperature, some droplets may be totally liquid, others totally solidified and others partially solidified and water transfer from the liquid droplets to the partly solidified droplets is theoretically possible. Indeed, as soon as ice is formed in a droplet the remaining solution concentration increases and the water chemical potential decreases. Therefore, the chemical potential of the liquid droplets is higher than the one corresponding to the remaining solution in the droplets wherein ice is present. The water transfer will induce a dilution of the remaining solution in the droplets and therefore the formation of more ice in the droplets to reach again the equilibrium composition. Besides, the concentration of the liquid droplets will increase and the water transfer will stop when the salt concentration in these droplets is equal to the concentration of the remaining solution within the droplets containing ice. According to this model, the partly solidified droplets will give rise to bigger droplets after melting and their salt concentration will be lower. This could explain the shift of the melting temperature towards 0°C as it was described in Section 2.2. Consequently after melting, less concentrated droplets will lead to early solidification during the next cooling. Thus, the emulsion at ambient temperature will be made of droplets with different salt concentrations. A transfer in the opposite direction can then take place from the biggest and less concentrated droplets to the smallest and the more concentrated ones; the recovering of the initial behavior after cooling/heating cycles and maintenance of the emulsion at ambient temperature can then be explained. From the knowledge of the liquid–solid NaCl–H O binary phase 2 diagram completed by the metastability zones [9,10], we calculated that considering a dispersed solution of molar fraction x =0.023 (T = 1 m −4.5°C ) maintained at −32°C, the composition of the solution in equilibrium with ice will be x =0.100. This situation induces a water transfer 2 from the still liquid droplets (x =0.023) towards 1
the droplets containing ice and a more concentrated solution (x =0.100) leading at ambient 2 temperature after melting to bigger droplets of concentration (x =0.012; T =−2°C ) together 3 m with smaller droplets of concentration (x =0.100; 2 T =−32°C ). This situation is out of equilibrium m and as it was stated previously, water transfer is expected in the reverse direction. Concerning multiple emulsions, it has been shown in Section 2.3 that a cooling of the emulsions down to −20°C may induce the spontaneous solidification of the bulk water wherein the oil globules containing still liquid water droplets are dispersed. Following the previous analysis on chemical potentials of ice and undercooled water, a water transfer can take place from the still liquid droplets in the oil globules towards the yet solid outer aqueous phase. The solidification energies detected by calorimetry are evidence of the transfer and allow determination of the mass of water transferred [11]. If the aqueous phases are made of pure water, the transfer is expected to stop when all the water has migrated from the inner droplets to the external phase of the oil globules. A simple O/W emulsion is then obtained. In the presence of a solute in the dispersed droplets inside the oil globules, the transfer will stop when the solute concentration in the water droplets has reached the one corresponding to the freezing depression temperature of −20°C.
4. Conclusions It was shown that partial solidification may take place within emulsions when the temperature during storage or transportation is brought to less than 0°C. Therefore water transfer is taking place and changes in the size distributions of emulsions are observed. The mean diameter of droplets in a W/O emulsion may become bigger and W/O/W emulsion may shift towards O/W emulsion. These results show that the temperature has to be controlled very carefully during storage and transportation of emulsified products.
D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
Acknowledgment The financial support of Poˆle Re´gional Picardie is gratefully acknowledged.
References [1] F. Broto, D. Clausse, J. Phys. C: Solid State Phys. 9 (1976) 4251. [2] L. Dufour, R. Defay, Thermodynamics of clouds, Academic Press, NY, 1963. [3] J.P. Dumas, D. Clausse, F. Broto, Thermochim. Acta 13 (1975) 261.
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[4] J. Lachaise, M. Clausse, J. Phys. D 8 (1975) 1227. [5] D. Clausse, L. Babin, F. Broto, M. Aguerd, M. Clausse, J. Phys. Chem. 87 (1983) 4030. [6 ] D. Clausse, J.P. Dumas, P.H.E. Meyer, F. Broto, J. Disp. Sci. Techn. 8 (1987) 1. [7] D. Clausse, F. Broto, Colloid Polym. Sci. 260 (1982) 641. [8] D. Clausse, in: E.A. Williams (Ed.), Advances in measurement and control of colloidal processesButterworthHeinemann, Guildford, UK, 1991.. [9] D. Clausse, I. Sifrini, J.P. Dumas, Thermochim. Acta 122 (1987) 123. [10] I. Sifrini, PhD Thesis, Pau University, France, 1983. [11] S. Raynal, I. Pezron, L. Potier, D. Clausse, J.L. Grossiord, M. Seiller, Colloids and Surfaces A: Physicochem. Eng. Aspects 91 (1994) 191. [12] L. Potier, S. Raynal, M. Seiller, J.L. Grossiord, D. Clausse, Thermochim. Acta 204 (1992) 145.
Stability of W/O and W/O/W emulsions as a result of partial solidification D. Clausse *, I. Pezron, L. Komunjer Universite´ de Technologie de Compie`gne, Centre de Recherche de Royallieu, De´partement de Ge´nie Chimique, CNRS UPRES A 6067 Ge´nie des Proce´de´s, Division Thermodynamique et Physicochimie de Proce´de´s Industriels, BP 20529-60205 Compie`gne, Cedex, France Received 20 January 1998; accepted 4 November 1998
Abstract This communication reviews the drastic modifications that may occur within emulsions when they are submitted to temperature variations during manufacturing, storage, transport and use. Emphasis will be given to the modifications resulting from changes after solidification. Pathways of solidification of W/O and W/O/W emulsions will be presented. Due to the specific structure of the emulsions under study (simple or multiple) only partial solidification may be obtained. This situation, out of thermodynamic equilibrium, may lead to a water transfer across the oil phase. Resulting emulsion transformation and destabilization are examined. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Emulsion stability; Solidification; Mass transfer; Ripening
1. Introduction This paper deals with emulsion destabilization due to partial solidification, phenomenon that is scarcely considered although it can occur more frequently than thought. Solidification processes considered here are due to temperature changes that are more or less controlled. In the case of storage or transport of emulsified products, the environment parameters such as temperature may vary between −50 and +40°C depending on the country. Consequently the product may be altered without immediate noticeable change in the appearance of the emulsions. Our study has been limited to the solidification of the dispersed aqueous phase of a water in oil emulsion ( W/O) and * Corresponding author. Tel.: +33-44-23-44-39; fax: +33-44-23-19-80. E-mail address: [email protected] (D. Clausse)
to the solidification of the dispersed and bulk aqueous phases of a water in oil in water emulsion ( W/O/W ). The aqueous phases considered here are either pure water or salt solutions. The consequences of this solidification will be examined in the second part of the paper. Most of the results have been obtained by microcalorimetry using a differential scanning calorimeter. As this technique for studying emulsions has been described thoroughly elsewhere, experimental details can be found in references given in the text.
2. Solidification of the dispersed phase of a W/O emulsion The dispersed phase of a W/O emulsion is made of water droplets of a few micrometers diameter. From theoretical considerations, it is possible to
0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 9 2 7- 7 7 57 ( 9 8 ) 0 08 5 3 -X
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D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
predict the way the solidification of the droplets will take place. This will be done for pure water droplets in the first part of this section. Some results obtained by microcalorimetry of such emulsions will also be given. In the second part, experimental results dealing with droplets of aqueous solution of sodium chloride will be described. They point out the possible tremendous influence of a solute on the solidification of the droplets and its consequences on the emulsion stability. 2.1. Pure water droplets It is well known that the temperature must be lowered well below 0°C (under atmospheric pressure) in order to induce water droplet solidification [1]. What may be less well known is that the droplets will solidify at different temperatures even in case of a monodisperse emulsion. This can be explained considering that the first solidification stage of a single droplet is the birth of a germ, defined as the smallest metastable solid particle that can be formed at a given temperature. The total solidification of the droplet is achieved by the growth of such germ. Indeed it can be shown that the temperature (T ) has to be well below 0°C for a spherical particle of ice acting as a germ to be in unstable equilibrium in an undercooled water droplet. The germ is small enough for capillary effects to be taken into account. The germ radius R is a function of the temperature T as shown in Eq. (1): ln (T/T )=−(2c · V )/(R · L ) (1) m i m where c is the interfacial tension between ice and undercooled water, V the ice molar volume, L i m the molar melting enthalpy of ice and T = m 273.15 K, the melting temperature of ice. If T is close to T , we may write: m (T −T )/T =(2c · V )/(R · L ) (2) m m i m From Eq. (2), it is possible to evaluate the value of R at any temperature although reliable values of c are not yet available. According to estimations made by Dufour and Defay [2], the following values can be given: T (°C ) R (cm)
0 2
−20 2.08×10−7
−40 1.03×10−7
It appears that the lower the temperature, the smaller the ice germ. Another point to consider is the kinetics of the formation of the germs themselves. For that purpose, a stationary nucleation rate J can be evaluated from the following simplified equation considering a model of germ formation from fluctuating ice embryos: J=A exp (−DW/kT )
(3)
where J is the number of germs formed per unit of time and of volume at temperature T; DW is the energy of formation of the germ: DW=(4/3)pcR2 and A is the pre-exponential factor taking into account, among other factors, the medium viscosity. Eq. (3) and Eq. (2) show that the closer the temperature T is to T (e.g. 0°C for water) the m smaller J is. Besides, J is expected to increase drastically when the temperature is far from T . m From this analysis, it is possible to predict the birth instant t of a germ in a droplet of volume g V according to Eq. (4):
P
tg VJ dt=1 (4) tm where t and t correspond to the times when the m g temperature reaches, respectively T (0°C ) and m T (germ formation temperature). If the experig ment is performed at a constant cooling rate T*, then T*=−dT/dt and Eq. (4) can be rewritten as:
P
−(V/T*)
Tg
J dT=1 (5) Tm Eq. (5) shows that the smaller the droplet volume, the lower the temperature at which there is a chance of formation of a germ in a single droplet. This is a general rule which can be applied to any sample. Let us study now the solidification of a big number of droplets each of them being considered as a single sample. It is generally considered that a droplet solidifies as soon as a germ forms within it. The formation of a germ is essentially a random phenomenon. Although Eq. (3) gives the number of germs formed per second and per volume unit, it does not indicate where in the volume the germs are formed. A definite volume of water is dispersed
D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
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[3] and hertzian spectrometry [4]. For a polydisperse emulsion characterized by a mean diameter of 1 mm, the temperature at which 50% of the droplets are frozen was found to be about −39°C. This analysis predicts that the droplet solidification can also be observed in time when the emulsion is maintained at any temperature below 0°C. Eq. (6) becomes: dN=J(N*−N )V dt
Fig. 1. Schematic histograms showing the transformation of water droplets into ice.
into the oil medium to make a W/O emulsion. This volume is split up into a large numbers of droplets and the formation of a germ in any one of these small droplets is a phenomenon for which only a probability can be drawn. This is why, considering a large number of identical droplets of the same radius cooled at the same rate, we will not observe simultaneous solidification of all the droplets. Some will solidify before others, so that their individual solidification temperatures will be dispersed around a mean value according to the analysis done thereafter. Considering N* the total number of water droplets of the same volume V, the number dN of droplets likely to solidify in the temperature interval dT is given by: dN=−J(N*−N )V(1/T*)dT
(6)
where T* is the cooling rate and N is the number of solidified droplets at temperature T. It appears that the proportion of solidified droplets, dN/N*, goes through a maximum as the number of liquid droplets (N*−N ) vanishes and the nucleation rate increases exponentially with decreasing temperature (Eq. (3)). The theoretical shape of the histograms describing the transformation of the droplets into ice can then be predicted ( Fig. 1). Actually such observation have been made on emulsions by calorimetry
(7)
Several attempts have been made to determine J [2]. It has been thought during a long time, according to theoretical predictions, that some isolated droplets would never solidify if the temperature was too far from a particular critical temperature close to −40°C because J would be nearly zero (Eq. (3)). Doubt has been raised and persists since total solidification have been observed after several months in the case of emulsions maintained at rather high temperatures [1,5,6 ]. It must then be remembered that, as soon as the temperature drops below 0°C, progressive solidification is possible, as can happen during any transport or storage of an emulsion. It has also been shown that a short thermal shock at −30°C significantly increases the solidification kinetics of droplets still undercooled at a higher temperature of −10°C [7]. For instance, 50% of the droplets are frozen within 14 h when they are cooled to −30°C before being maintained at −10°C, meanwhile for the same lapse of time no droplet is frozen when the emulsion is directly placed at −10°C. Similar effects are observed after a solidification/melting cycle of the emulsion: when, after a cooling stage at −20°C, the system is heated to a temperature higher than 0°C and then progressively cooled again, two characteristic solidification temperatures (at −33 and −39°C ) are observed instead of one (at −39°C ) [1]. The most surprising observation is that the number of droplets solidifying around −33°C was found to decrease when more and more cooling/heating cycles were performed. At the end of the evolution, the solidification of the droplets was found to occur in the same way as the one observed at the first cooling before cycles. Possible explanations for this phenomena will be discussed in Section 3.
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D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
2.2. Water+salt droplets Numerous studies have been made on dispersed water+salt solutions [8–10]. The presence of salt may induce a significant drop of the solidification temperature down to less than −50°C. Anyway, as in the case of pure water, solidification can be observed at higher temperatures. For example, it was observed by microcalorimetry that 70% of an emulsified aqueous solution of NaCl (molar fraction x=0.023) was solid after being maintained 156 h at −32°C. Furthermore, an unexpected behavior was observed during re-heating performed after the emulsion was maintained for different lapses of time at −32°C. Several examples of thermograms obtained are shown on Fig. 2. Fig. 2a, which can be taken as a reference, has been obtained during the heating stage after a full
Fig. 3. Thermograms obtained by differential scanning calorimetry of emulsions containing droplets of aqueous solution of sodium chloride and submitted to regular cooling.
Fig. 2. Thermograms obtained by differential scanning calorimetry of emulsions containing droplets of aqueous solution of sodium chloride and submitted to regular heating.
cooling of the emulsion. The eutectic melting is observed at −21°C followed by the progressive melting of the remaining ice which stops at −4.5°C in agreement with the equilibrium freezing point depression corresponding to the composition of the solution. Thermogram 2b was obtained after holding the emulsion for 1 h at −32°C. From the value of the signal area, which is proportional to the change of enthalpy during melting, it is possible to deduce that 16% of the dispersed solution has solidified. Thermogram 2c, after 156 h at −32°C (73% of the dispersed solution has solidified ), reveals unexpectedly a total melting very close to 0°C as if the salt had less influence. Thermograms 2d and 2e have been obtained, respectively during the re-heating of the system after a cooling stage following step 2c and after eight cooling/heating cycles following step 2c. There is a clear evidence of the shifting of the signals towards the ‘normal’ situation shown in Fig. 2a. This ‘back-up’ phenomenon is speeded-up (thermogram 2f ) when the
D. Clausse et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 152 (1999) 23–29
emulsion is maintained during 16 h at 20°C before a new cooling/heating cycle. The thermograms obtained during the cooling stage of the emulsions (Fig. 3) are consistent with what is observed during the heating stages. Thermogram 3a, obtained during the first cooling stage of the emulsion, shows ice solidification in the droplets (at about −48°C ) and total solidification of the system (ice and salt crystals) around −72°C. In thermogram 3d, obtained during the cooling of the emulsion following cooling (thermogram 3a) and heating (thermogram 2c), early ice formation is observed around −40°C as observed for pure water droplets instead of −48°C observed for water+salt droplets. During the following cooling-heating cycles (thermograms: 2d (heating), 3e (cooling) and 2e (heating)), this phenomenon tends to disappear. After stabilization of the emulsion during 16 h at 20°C the thermograms obtained during cooling (3f ) and heating (2f ) are the same as those obtained during the first cooling (2a) and heating cycle (3a).
2.3. Solidification of the disperse and bulk aqueous phases of a multiple emulsion W/O/W Multiple water-in oil-in water ( W/O/W ) emulsions are obtained by dispersing a water in oil ( W/O) emulsion in a continuous aqueous phase [11]. Doing so, oil globules of diameter around 10 mm containing dispersed water droplets of about 1 mm diameter are obtained. When such an emulsion is submitted to a progressive cooling in a calorimeter, the solidification of the bulk aqueous phase wherein the oil globules are dispersed and of the dispersed water inside the globules are expected to occur at different temperatures as far as the volumes concerned are different: a few cube millimeters for the bulk (calorimeter cell volume) and a few cube micrometers for each droplet of the dispersed phase. It must be noticed that the bulk aqueous phase must be considered as a single sample and the dispersed aqueous phase as a very great number of samples. Therefore, the solidification temperature of the bulk is expected to change from one experiment to another whereas a mean solidification temperature will be obtained for the
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dispersed droplets as it is for a simple emulsion W/O. Indeed two different temperatures have been obtained: around −20°C for the bulk phase and −40°C for the dispersed phase as it was found for a simple emulsion [12].
3. Impact of solidification on the emulsion structure Under a thermal shock bringing the temperature of a W/O emulsion to −20°C for example, a part of the droplets will be frozen and the others constituted of undercooled water, as it was shown in the previous sections. It is likely that this partial solidification of the emulsion may lead to its destabilization since liquid water and ice can only coexist at 0°C. At this temperature the chemical potentials of ice m and undercooled water m are i w different according to Eq. (8) m −m =RT ln a i w i where a =P /P (P and P are the respective i i w i w vapor pressures of ice and undercooled water). Since m
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When salt is present in the water droplets, a thermal shock bringing the emulsion to subambient temperatures could also lead to partial solidification of the dispersed droplets. Depending on the temperature, some droplets may be totally liquid, others totally solidified and others partially solidified and water transfer from the liquid droplets to the partly solidified droplets is theoretically possible. Indeed, as soon as ice is formed in a droplet the remaining solution concentration increases and the water chemical potential decreases. Therefore, the chemical potential of the liquid droplets is higher than the one corresponding to the remaining solution in the droplets wherein ice is present. The water transfer will induce a dilution of the remaining solution in the droplets and therefore the formation of more ice in the droplets to reach again the equilibrium composition. Besides, the concentration of the liquid droplets will increase and the water transfer will stop when the salt concentration in these droplets is equal to the concentration of the remaining solution within the droplets containing ice. According to this model, the partly solidified droplets will give rise to bigger droplets after melting and their salt concentration will be lower. This could explain the shift of the melting temperature towards 0°C as it was described in Section 2.2. Consequently after melting, less concentrated droplets will lead to early solidification during the next cooling. Thus, the emulsion at ambient temperature will be made of droplets with different salt concentrations. A transfer in the opposite direction can then take place from the biggest and less concentrated droplets to the smallest and the more concentrated ones; the recovering of the initial behavior after cooling/heating cycles and maintenance of the emulsion at ambient temperature can then be explained. From the knowledge of the liquid–solid NaCl–H O binary phase 2 diagram completed by the metastability zones [9,10], we calculated that considering a dispersed solution of molar fraction x =0.023 (T = 1 m −4.5°C ) maintained at −32°C, the composition of the solution in equilibrium with ice will be x =0.100. This situation induces a water transfer 2 from the still liquid droplets (x =0.023) towards 1
the droplets containing ice and a more concentrated solution (x =0.100) leading at ambient 2 temperature after melting to bigger droplets of concentration (x =0.012; T =−2°C ) together 3 m with smaller droplets of concentration (x =0.100; 2 T =−32°C ). This situation is out of equilibrium m and as it was stated previously, water transfer is expected in the reverse direction. Concerning multiple emulsions, it has been shown in Section 2.3 that a cooling of the emulsions down to −20°C may induce the spontaneous solidification of the bulk water wherein the oil globules containing still liquid water droplets are dispersed. Following the previous analysis on chemical potentials of ice and undercooled water, a water transfer can take place from the still liquid droplets in the oil globules towards the yet solid outer aqueous phase. The solidification energies detected by calorimetry are evidence of the transfer and allow determination of the mass of water transferred [11]. If the aqueous phases are made of pure water, the transfer is expected to stop when all the water has migrated from the inner droplets to the external phase of the oil globules. A simple O/W emulsion is then obtained. In the presence of a solute in the dispersed droplets inside the oil globules, the transfer will stop when the solute concentration in the water droplets has reached the one corresponding to the freezing depression temperature of −20°C.
4. Conclusions It was shown that partial solidification may take place within emulsions when the temperature during storage or transportation is brought to less than 0°C. Therefore water transfer is taking place and changes in the size distributions of emulsions are observed. The mean diameter of droplets in a W/O emulsion may become bigger and W/O/W emulsion may shift towards O/W emulsion. These results show that the temperature has to be controlled very carefully during storage and transportation of emulsified products.
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Acknowledgment The financial support of Poˆle Re´gional Picardie is gratefully acknowledged.
References [1] F. Broto, D. Clausse, J. Phys. C: Solid State Phys. 9 (1976) 4251. [2] L. Dufour, R. Defay, Thermodynamics of clouds, Academic Press, NY, 1963. [3] J.P. Dumas, D. Clausse, F. Broto, Thermochim. Acta 13 (1975) 261.
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[4] J. Lachaise, M. Clausse, J. Phys. D 8 (1975) 1227. [5] D. Clausse, L. Babin, F. Broto, M. Aguerd, M. Clausse, J. Phys. Chem. 87 (1983) 4030. [6 ] D. Clausse, J.P. Dumas, P.H.E. Meyer, F. Broto, J. Disp. Sci. Techn. 8 (1987) 1. [7] D. Clausse, F. Broto, Colloid Polym. Sci. 260 (1982) 641. [8] D. Clausse, in: E.A. Williams (Ed.), Advances in measurement and control of colloidal processesButterworthHeinemann, Guildford, UK, 1991.. [9] D. Clausse, I. Sifrini, J.P. Dumas, Thermochim. Acta 122 (1987) 123. [10] I. Sifrini, PhD Thesis, Pau University, France, 1983. [11] S. Raynal, I. Pezron, L. Potier, D. Clausse, J.L. Grossiord, M. Seiller, Colloids and Surfaces A: Physicochem. Eng. Aspects 91 (1994) 191. [12] L. Potier, S. Raynal, M. Seiller, J.L. Grossiord, D. Clausse, Thermochim. Acta 204 (1992) 145.