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Temperature-dependent stability of water-in-undecanol emulsions
Colloids and Surfaces A: Physicochem. Eng. Aspects 224 (2003) 241 /249 www.elsevier.com/locate/colsurfa
Temperature-dependent stability of water-in-undecanol emulsions B.P. Binks, C.P. Whitby * Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK Received 10 March 2003; accepted 12 June 2003
Abstract The stability of water-in-undecanol emulsions to sedimentation and coalescence as a function of temperature is described. By monitoring the release of water and oil by the emulsions, the stability of emulsions prepared in the absence of stabilisers and in the presence of either hydrophobic silica particles or octanoic acid molecules is characterised. We show that the destabilisation phenomena observed for the solid-stabilised emulsions reflects the breakdown of emulsions prepared in the absence of stabilisers. The extent of coalescence in the solid-stabilised emulsions undergoes a transition at a temperature about 10 8C above the melting point of undecanol and the stability of the emulsion to sedimentation passes through a maximum at about the same temperature. We attribute the low stability of these emulsions to coalescence at temperatures up to 10 8C above the melting point of undecanol to the formation of a solid-like layer of alcohol at the drop surface, similar to the surface crystallisation phenomena of medium chain length alcohols at planar oil /water interfaces. The addition of octanoic acid to these emulsions does not affect the surface crystallisation. # 2003 Elsevier B.V. All rights reserved. Keywords: Emulsion; Undecanol; Stability; Hydrophobic silica; Octanoic acid
1. Introduction Surfactant-free emulsions can be stabilised by a variety of solid particles including inorganic oxides, latex and carbon [1]. Solid particles are thought to stabilise emulsions by accumulating at the oil /water interface and providing a physical barrier to drop coalescence and also possibly by
* Corresponding author. Tel.: /44-1482-46-5417; fax: /441482-46-6410. E-mail address: [email protected] (C.P. Whitby).
forming networks in the continuous phase [1]. The type and stability of the emulsion formed is very system dependent, varying with the system composition and the way in which the emulsion is prepared. For systems consisting of equal volumes of oil and water and a single particle type however, predictions can be made about emulsion type and stability based simply on the oil /water interfacial tension (gow) and the particle wettability [2,3]. One way to express particle wettability is through the angle, u , the particles make with the oil /water interface and although this parameter cannot be measured directly (unlike the oil /water interfacial
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tension) it can be estimated from contact angle measurements on solids with a similar surface chemistry to the particles [3] or calculated theoretically [4]. The emulsion type is determined by which liquid phase more readily wets the particles and therefore becomes the continuous phase of the emulsion. Emulsion stability is related to the energy by which the particles are attached to the oil /water interface, given by E pR2 gow (19cos u)2
(1)
where R is the radius of the particles and the sign in the brackets is negative for u less than 908 and positive for u greater than 908 [5,2]. Thus the energy of attachment passes through a sharp maximum at u /908. The magnitude of the maximum is proportional to gow. For particles of extremely high or low wettability, the energy of attachment is very low and thus emulsions containing very hydrophobic or very hydrophilic silica particles are very unstable to coalescence as particles are easily displaced from the interface when drops collide [2]. While it has been argued that the wettability of silica particles is determined by their inherent hydrophobicity [2], the wettability of particles of intermediate hydrophobicity is sensitive to the oil type [3]. Silica particles are more hydrophobic at polar oil /water interfaces than at non-polar oil / water interfaces. Thus it is difficult to stabilise emulsions of polar oils and water using particles of intermediate hydrophobicity [3]. In an unusual case, the instability of emulsions formed from undecanol and water was observed to be markedly dependent on temperature [3] and it is the behaviour of this particular system that this paper is concerned with. Based on the low undecanol / water interfacial tension (estimated to be 9.5 mN m 1) and the high contact angle of the particles at the undecanol /water interface (estimated to be 1608), Binks and Lumsdon [3] predicted that water /undecanol emulsions stabilised by nanoparticles of intermediate hydrophobicity would be oil-continuous and of low stability. They found that the emulsions prepared were indeed water-inoil emulsions with a high viscosity (‘gel-like’) which broke down completely into the separate
phases over time [3]. The emulsions contracted from the walls of the container, maintaining the shape of the vessel and releasing oil and water. The time scale of the destabilisation process depended on both the water volume fraction (fw) and the temperature, with the stability of the emulsion to sedimentation passing through a maximum as a function of temperature. The emulsion breakdown phenomenon was thought to be similar to coalescence in emulsions of highly viscous oils in water [6,7], with the coalescence of the water drops governed by the viscosity of the continuous phase, which was higher at high water volume fractions. The temperature dependence of the sedimentation could not be explained however. The unusual breakdown phenomena may be linked to the temperature dependent phase transitions observed in fatty alcohol monolayers at the alcohol /water [8 /11] and air /water [12 /20] interfaces. For medium to long chain length alcohols (C9 /C16), the tension of the alcohol /water interface is a linear function of temperature up to 10 K above the melting point of the bulk alcohol and a parabolic function at higher temperatures [8 /10]. There is a similar discontinuity in the surface tension of alcohol monolayers at the air /water interface, and based on the discontinuity in the ellipsometric angle at the same temperature, Berge and Renault [12] demonstrated that the monolayer undergoes a first order phase transition at this temperature. Using X-ray diffraction measurements, Renault et al. [13] found that below the transition temperature the monolayer had a structure similar to the structure of the bulk alcohol at temperatures just below the melting point, while above the transition temperature the monolayer had a liquid-like structure. It is likely that the discontinuity in the interfacial tension of alcohol / water systems is also due to the melting of a solidlike layer of alcohol at the interface since at temperatures close to the solidification point of water-saturated undecanol, a solid layer of alcohol was observed to form at the planar interface between bulk water and alcohol [11]. The aim of the work described here is to investigate whether the breakdown of viscous water-in-undecanol solid-stabilised emulsions could be related to the phase transition in the
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undecanol layer at the planar alcohol /water interface. Thus the breakdown of water-in-undecanol emulsions was studied as a function of temperature (T ). Recently, it was found that the temperature at which the phase transition occurs of fatty alcohol monolayers at the air /water surface can be controlled by the addition of short chain carboxylic acids, which in their protonated form adsorb to the monolayer and alter the surface crystallisation [21 /23]. Therefore the influence of adding a solid particle emulsifier or a surfactant molecule (octanoic acid) emulsifier on the emulsion breakdown behaviour was investigated.
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ferred into stoppered, graduated vessels which were placed in a thermostated water bath (set to the required temperature). The emulsion type was inferred by observing whether an emulsion drop dispersed, or remained as a drop, when added to a volume of water or oil. The conductivity of some of the emulsions prepared (containing background electrolyte) was determined using a digital Jenway conductivity meter 4310 with Pt/Pt black electrodes. The stability of the water-in-oil emulsions to sedimentation and coalescence was assessed by monitoring the change with time of the position of the oil /emulsion and water /emulsion interfaces respectively.
2. Experimental 3. Results and discussion 2.1. Materials Water was first passed through an Elga reverse osmosis unit and then a Milli-Q reagent water system. Undecan-1-ol (with a purity of 99% and a nominal melting point of 11 8C) was obtained from Aldrich and columned twice to remove polar impurities. The melting point of the alkanol was measured to be (14.09/0.3) 8C by differential scanning calorimetry (Perkin /Elmer DSC7). nOctanoic acid (with a purity of 99%) was obtained from Sigma. The partially hydrophobic monodisperse silica particles were a gift from Rhodia Services, with a primary particle diameter of 25 nm. They were prepared by grafting n -octyltriethoxysilane onto hydrophilic silica (purchased from Clariant). The particles were transferred from the synthesis media into water by dialysis. The aqueous dispersion of particles was strongly flocculated, with flocs about 40 mm in size which sedimented rapidly. In some experiments, a background electrolyte (sodium chloride, Prolabo, analytical grade) was added to the dispersion. 2.2. Methods The emulsions were prepared using a Janke and Kunkel Ultra-Turrax homogeniser with an 18 mm head operating at 13 500 rpm for 2 min to emulsify mixtures of the alcohol and the aqueous dispersion of particles. The emulsions were rapidly trans-
During the emulsification (2 min in total) of equal volumes of undecanol and water alone, a viscous white-gray emulsion forms rapidly (within 5 s). The emulsion consists of solid-like flocs of white material and a little thin clear fluid (presumably oil). When a small sample is added to oil and stirred gently, the flocs partly break up (settling once the stirring is halted). However the emulsion cannot be dispersed in water by gentle agitation. It is concluded that the emulsion is oil continuous since the emulsion conductivity is low and comparable to that of undecanol. The emulsion is unstable and as water and oil are released, the white material partly contracts from the walls of the vessel. In the absence of stabilisers, or in the presence of poor stabilisers such as octanoic acid, complete phase separation occurs. In the presence of a good stabiliser like partially hydrophobic silica particles, the emulsion does not break down completely. The destabilisation and, in particular, the rate of sedimentation is extremely temperature dependent. For poorly stabilised emulsions this was difficult to quantify except in the initial stages, for reasons that are discussed later. Fig. 1 shows the time taken for 10% of the oil to be released following sedimentation from 50 vol.% water-in-undecanol emulsions as a function of temperature. No data is shown for poorly stabilised emulsions equilibrated at 18 8C, since it could not be measured accurately (com-
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are stored) tend to be more stable than those within the bulk. Thus it is difficult to assess the extent of coalescence and sedimentation after the initial stages of phase separation since the contraction of the gelled emulsions is non-homothetic [7]. Overall, the time taken for complete phase separation follows the same trend as the initial rate of sedimentation, passing through a maximum of about 6 h at 23 8C (shown later). It should be noted that the times measured could vary by as much as 15% for individual emulsions prepared and equilibrated under the same conditions. Emulsions stored in plastic vessels appear to contract in a more uniform manner and those equilibrated at intermediate temperatures take longer to phase separate than those stored in glass (shown later).
Fig. 1. Time taken for 10% of the oil to be resolved from 50 vol.% water-in-undecanol emulsions prepared in the absence of stabilisers (circles), and in the presence of 0.7 wt.% hydrophobic silica in the aqueous phase (squares), 1.9 mM octanoic acid at pH 2.4 (triangles) and 1.9 mM octanoic acid at pH 9.5 (inverted triangles).
plete phase separation took only 3 min). Irrespective of the type of stabiliser, the initial stability of the emulsions to sedimentation passes through a maximum at temperatures between 23 and 25 8C. The magnitude of the increase in the stability of the emulsions to sedimentation on the addition of stabilisers is largest around these temperatures also. The difference in temperature at which the rate of sedimentation is slowest for different stabilisers is not believed to be significant. These results will now be discussed in more detail for the different kinds of emulsions. 3.1. Emulsions prepared in the absence of stabilisers The breakdown of water-in-undecanol emulsions prepared in the absence of a stabiliser does not occur steadily. While sedimentation and coalescence begin almost immediately at low temperatures (5/23 8C), there is often a short delay before coalescence is observed in emulsions equilibrated at higher temperatures. Water drops close to the walls of the glass vessels (in which the emulsions
3.2. Emulsions stabilised by hydrophobic silica particles The addition of partially hydrophobic silica particles stabilises the emulsions against complete breakdown and while the contraction is still not uniform, the extent to which the water and oil phases resolve from the emulsion can be assessed throughout the destabilisation. In Fig. 2, the fractions of water and oil resolved from 50 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% hydrophobic silica are shown as a function of time at a selection of the temperatures studied. At temperatures below 23 8C, coalescence and sedimentation occur quickly, with large proportions of water and oil being released respectively. Between 23 and 25 8C, sedimentation is much slower than at low temperatures and only a small amount of coalescence occurs when the drop volume fraction (fd) is about 0.75. At higher temperatures, the rate of sedimentation increases; however only a small amount of coalescence occurs (again only when fd is about 0.75). The initial rate of sedimentation has the same temperature dependence as that for emulsions in the absence of particles as shown in Fig. 1. Although the time taken for the emulsion to become stable to sedimentation is temperature dependent, the extent of sedimentation is approximately the same at all temperatures. The extent of coalescence, however, is much higher at low temperatures
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Fig. 2. Stability of 50 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% hydrophobic silica to sedimentation and coalescence as a function of time at different temperatures.
(5/21 8C). This is illustrated in Fig. 3, where the final or equilibrium fractions of oil and water resolved from the solid-stabilised emulsions are shown as a function of temperature. Given the exceptionally long times required for the emulsions to stabilise at temperatures between 23 and 27 8C (see Fig. 2) it is argued that the coalescence which occurs under these conditions is augmented by collapse of some of the lowermost emulsion drops under the pressure of the drops above. The extent of coalescence in the majority of solid-stabilised emulsions is typically low, due to the adsorbed layer of particles at the oil /water interface preventing the close approach of drops [1]. At temperatures of 23 8C and above, these systems behave like typical Ramsden emulsions. At lower temperatures, the presence of the particles only halts the coalescence to a small, but
important, extent. This is improved by increasing the concentration of particles in the emulsion. Also shown in Fig. 3 is data for an emulsion stabilised by a higher concentration (3 wt.%) of hydrophobic silica (filled symbols). Increasing the particle concentration does not alter the extent of sedimentation; however the rate of sedimentation is slower (it takes about 66 min for 80% of the water to be released compared to 7 min at the lower particle concentration). In Fig. 4, the fractions of water and oil resolved from solid-stabilised water-in-undecanol emulsions at a lower water volume fraction (fw /0.2) are shown as a function of time, at three different temperatures. The extent of sedimentation and the trend in the temperature dependence of the rate of sedimentation are similar to that for emulsions at fw /0.5 (assuming that the emulsion equilibrated
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Fig. 3. Final fractions of oil (from sedimentation) and water (from coalescence) released from 50 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% (open symbols) and 3 wt.% (filled symbols) hydrophobic silica as a function of temperature.
at 25 8C continued sedimenting until about 80% of the water was released). The fraction of water released at 18 8C is lower than that at fw /0.5, due to the lower drop volume fraction which reduces the frequency of drop collisions and thus the opportunities for drop coalescence. The temperature dependence of the coalescence in solid-stabilised water-in-undecanol emulsions is highly unusual. The transition in the stability of the emulsions to coalescence occurs at temperatures about 10 8C above the measured melting point of the alcohol (14.09/0.3 8C). All the emulsions are prepared by homogenisation using a rotor /stator head which heats the emulsion to relatively high temperatures during its operation. The emulsions are then placed in a water bath to equilibrate at the required temperature and it seems likely that as the emulsions cool, a solidlike layer of undecanol forms at the surface of the water drops, at temperatures up to 23 8C, just as it would form at a planar interface. It is possible that some of the undecanol crystallises in the solid-like layer of alcohol which forms. These crystals may bridge the thin films between water drops at close separations resulting in coalescence, just as low densities of interfacial fat crystals increase the
Fig. 4.
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Fig. 4. Stability of 20 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% hydrophobic silica to sedimentation and coalescence as a function of time at different temperatures.
extent of coalescence in water-in-triglyceride oil emulsions [24]. The presence of solid silica particles at the drop surfaces provides a steric barrier to the close approach of the water drops, thus reducing the coalescence caused by the crystals. At temperatures of 25 8C and above, the layer of undecanol at the drop surfaces is fluid-like and the presence of solid particles at the interface reduces the frequency of drop collisions and thus the extent of coalescence. The temperature dependence of the sedimentation behaviour has yet to be accounted for. The rate at which an isolated spherical drop in a dilute emulsion, of radius r , sediments (ns) is given by Stokes’ law [25] ns
2r2 (ro r)g 9ho
(2)
where g is the acceleration due to gravity, ro and ho are the density and shear viscosity of the continuous phase respectively and r is the density of the drop phase. Assuming that the emulsion drop diameter is fixed, the rate of sedimentation can be reduced by decreasing the density difference between the two phases or by increasing the viscosity of the continuous phase. In Table 1, the viscosity and density of undecanol and the density of water at different temperatures are given. With increasing temperature, the magnitude of the density difference between undecanol and water increases, and the viscosity of undecanol decreases, and so the rate of sedimentation should increase. This may explain why the stability of the water-in-
undecanol emulsions to sedimentation decreases as the temperature increases from 25 to 35 8C. At temperatures below 25 8C, the presence of undecanol crystals (and presumably solid particles released by drop coalescence) could increase the viscosity of the continuous phase. This effect, however, would become more marked at lower temperatures, implying that the rate of sedimentation should decrease with decreasing temperature. However, the assumption that the drop diameter remains unchanged may not be correct. It is unclear why the hydrophobic precipitated silica particles used in these experiments are able to stabilise the emulsions against complete phase separation, unlike those used (hydrophobic fumed silica) in the previous study [3]. There is some evidence for the adsorption of undecanol onto hydrophobic silica [29]. It is possible that crystallisation may occur in adsorbed alcohol monolayers on solid surfaces, since changes in the monolayer of dodecanol adsorbed on a hydrophobic surface from a solid to liquid-like structure have been observed at a temperature about 18 8C above the melting point of the bulk alkanol [30]. Perhaps the presence of these crystals causes further disruption to the thin films formed between drops at close separations, encouraging coalescence to occur. Supposing that the degree of hydrophobisation on the two types of silica is different, the extent of adsorption of the undecanol onto the two silicas will be different, thus affecting the extent to which this behaviour might contribute to the emulsion destabilisation. 3.3. Emulsions stabilised by octanoic acid molecules Bonosi et al. [21] report that the temperature at which the phase transition in fatty alcohol mono-
Table 1 Viscosities and densities of undecanol and water at different temperatures. Temperature (8C)
Viscosity of undecanola (mPa s)
Density of undecanola (g cm 3)
Density of waterb (g cm 3)
20 25 35
16.952 13.830 9.380
0.8325 0.8291 0.8223
0.99 823 0.99 707 0.99 406
a b
Taken from Refs. [26,27]. Taken from Ref. [28].
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Fig. 5. Time taken for complete phase separation of 50 vol.% water-in-undecanol emulsions in glass vessels as a function of temperature. The emulsions were prepared in the absence of stabilisers (circles) or with the aqueous phase containing 1.9 mM octanoic acid at pH 2.4 (triangles) or 1.9 mM octanoic acid at pH 9.5 (inverted triangles) or 10 mM octanoic acid at pH 2.4 (filled triangles) or with the oil phase containing 100 mM octanoic acid and the aqueous phase at pH 2.5 (diamonds).
layers at the air /water interface occurs can be controlled by the addition of short chain length carboxylic acids to the water. In aqueous solutions at pH less than the pKa of the acid, the molecules are predominantly in the protonated form and readily adsorb in the alcohol monolayer affecting the surface crystallisation by reducing the temperature at which the transition occurs. Thus emulsions were prepared from undecanol and aqueous solutions of 1.9 mM octanoic acid at high or low pH and equilibrated at different temperatures to determine if the presence of octanoic acid would cause a shift in the temperature-dependent stability behaviour of the emulsions. The emulsions have a similar morphology to those prepared in the absence of stabilisers and phase separate over time in a similar fashion. As was shown in Fig. 1, the temperature dependence of initial sedimentation of emulsions containing octanoic acid is unchanged at either low or high pH in the dispersed phase. The overall
emulsion stability increases in the presence of octanoic acid as the emulsions take longer to phase separate than those prepared in its absence as shown in Fig. 5. This suggests that during the homogenisation of the two liquids, octanoic acid (in either protonated or charged form) is adsorbed at the alcohol /water interface and acts like a surfactant, partially stabilising the water drops. The presence of such molecules at the interface does not however perturb the crystallisation occurring in the monolayer of alcohol at the surface of the drops. (Also shown in this figure is data for emulsions stored in plastic vessels. As mentioned earlier, due to the different interactions between the alcohol, water and plastic, as these emulsions contract they are less attached to the walls of the vessel.) Increasing the concentration of octanoic acid in the aqueous or oil phase of the emulsions does not change these finding as shown in Fig. 5. The concentration of the carboxylic acid used exceeded its solubility in water and also in undecanol, making it likely that a substantial proportion of the molecules would be located at the interface. Again however, the presence of the acid did not appear to affect the crystallisation occurring in the monolayer. It is proposed that acid molecules adsorbed in alcohol monolayers at air /water interfaces act as impurities which as solidification occurs are rejected and may diffuse back into the aqueous phase [21 /23]. It was estimated that under these conditions the proportion of octanoic acid molecules in the monolayer is about 14% [23]. In the case of the emulsions being prepared here, however, it seems likely that most of the acid molecules are located at the alcohol/water interface. Given the number of molecules available, it is possible that they may account for a significant fraction of the surface area of the drops and thus it is their role as a surfactant which dominates the behaviour at the interface.
4. Conclusions In summary, the unusual temperature-dependent stability of water-in-undecanol emulsions is thought to be related to the formation of a solid-
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like layer of alcohol at the alcohol /water interface at temperatures up to 10 K above the melting point of the alcohol. At these low temperatures, crystals of the alcohol may form in the interfacial region, and the presence of these crystals may enhance coalescence by bridging the thin films formed between drops at close separations. The addition of solid particles may prevent the close approach of the drops, thus reducing the extent of coalescence. The addition of octanoic acid, although slightly stabilising the emulsions, does not alter the temperature at which the transition in stability of the emulsions is observed. The temperature dependence of the sedimentation in these emulsions cannot yet be explained.
Acknowledgements We thank Rhodia Services Inc. (France) for provision of a grant to fund CPW. We would also like to thank P.D.I. Fletcher for useful discussions.
References [1] B.P. Binks, Curr. Opin. Colloid Interf. Sci. 7 (2002) 21. [2] B.P. Binks, S.O. Lumsdon, Langmuir 16 (2000) 8622. [3] B.P. Binks, S.O. Lumsdon, Phys. Chem. Chem. Phys. 2 (2000) 2959. [4] B.P. Binks, J.H. Clint, Langmuir 18 (2002) 1270. [5] S. Levine, B.D. Bowen, S.J. Partridge, Colloids Surf. 38 (1989) 325. [6] J. Philip, L. Bonakdar, P. Poulin, J. Bibette, F. LealCalderon, Phys. Rev. Lett. 84 (2000) 2018. [7] J. Philip, J.E. Poirier, J. Bibette, F. Leal-Calderon, Langmuir 17 (2001) 3545. [8] M. Aratono, T. Takiue, N. Ikeda, A. Nakamura, K. Motomura, J. Phys. Chem. 96 (1992) 9422.
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[9] M. Aratono, T. Takiue, N. Ikeda, A. Nakamura, K. Motomura, J. Phys. Chem. 97 (1993) 5141. [10] J. Glinski, G. Chavepeyer, J.K. Platten, C. De Saedeleer, J. Colloid Interf. Sci. 158 (1993) 382. [11] M. Salajan, J. Glinski, G. Chavepeyer, J.K. Platten, J. Colloid Interf. Sci. 164 (1994) 387. [12] B. Berge, A. Renault, Europhys. Lett. 21 (1993) 773. [13] A. Renault, J.F. Legrand, M. Goldmann, B. Berge, J. Phys. II France 3 (1993) 761. [14] B. Berge, O. Konovalov, J. Lajzerowicz, A. Renault, J.P. Rieu, M. Vallade, J. Als-Nielsen, G. Gru¨bel, J.F. Legrand, Phys. Rev. Lett. 73 (1994) 1652. [15] G.A. Sefler, Q. Du, P.B. Miranda, Y.R. Shen, Chem. Phys. Lett. 235 (1995) 347. [16] R. Braun, B.D. Casson, C.D. Bain, Chem. Phys. Lett. 245 (1995) 326. [17] C. Alonso, D. Blaudez, B. Desbat, F. Artzner, B. Berge, A. Renault, Chem. Phys. Lett. 284 (1998) 446. [18] T. Toyomasu, T. Takiue, N. Ikeda, M. Aratono, Langmuir 14 (1998) 7313. [19] M.G. Mun˜oz, L. Luna, F. Monroy, R.G. Rubio, F. Ortega, Langmuir 16 (2000) 6657. [20] D. Vollhardt, G. Emrich, S. Siegel, R. Rudert, Langmuir 18 (2002) 6571. [21] F. Bonosi, A. Renault, B. Berge, Langmuir 12 (1996) 784. [22] P.-F. Lenne, A. Valance, F. Bonosi, B. Berge, C. Misbah, Europhys. Lett. 38 (1997) 301. [23] P.-F. Lenne, F. Bonosi, A. Renault, E. Bellet-Amalric, J.F. Legrand, J.-M. Petit, F. Rieutord, B. Berge, Langmuir 16 (2000) 2306. [24] D. Johansson, B. Bergensta˚hl, E. Lundgren, J. Am. Oil Chem. Soc. 72 (1995) 939. [25] G.G. Stokes, Philos. Mag. 1 (1851) 337. [26] Z. Shan, A.-F.A. Asfour, Fluid Phase Equilibria 143 (1998) 253. [27] Z. Shan, A.-F.A. Asfour, J. Chem. Eng. Data 44 (1999) 118. [28] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 55th ed., CRC Press, Ohio, 1974. [29] F. Mugele, S. Baldelli, G.A. Somorjai, M. Salmeron, J. Phys. Chem. B 104 (2000) 3140. [30] M.S. Johal, E.W. Usadi, P.B. Davies, J. Chem. Soc., Faraday Trans. 92 (1996) 573.
Temperature-dependent stability of water-in-undecanol emulsions B.P. Binks, C.P. Whitby * Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK Received 10 March 2003; accepted 12 June 2003
Abstract The stability of water-in-undecanol emulsions to sedimentation and coalescence as a function of temperature is described. By monitoring the release of water and oil by the emulsions, the stability of emulsions prepared in the absence of stabilisers and in the presence of either hydrophobic silica particles or octanoic acid molecules is characterised. We show that the destabilisation phenomena observed for the solid-stabilised emulsions reflects the breakdown of emulsions prepared in the absence of stabilisers. The extent of coalescence in the solid-stabilised emulsions undergoes a transition at a temperature about 10 8C above the melting point of undecanol and the stability of the emulsion to sedimentation passes through a maximum at about the same temperature. We attribute the low stability of these emulsions to coalescence at temperatures up to 10 8C above the melting point of undecanol to the formation of a solid-like layer of alcohol at the drop surface, similar to the surface crystallisation phenomena of medium chain length alcohols at planar oil /water interfaces. The addition of octanoic acid to these emulsions does not affect the surface crystallisation. # 2003 Elsevier B.V. All rights reserved. Keywords: Emulsion; Undecanol; Stability; Hydrophobic silica; Octanoic acid
1. Introduction Surfactant-free emulsions can be stabilised by a variety of solid particles including inorganic oxides, latex and carbon [1]. Solid particles are thought to stabilise emulsions by accumulating at the oil /water interface and providing a physical barrier to drop coalescence and also possibly by
* Corresponding author. Tel.: /44-1482-46-5417; fax: /441482-46-6410. E-mail address: [email protected] (C.P. Whitby).
forming networks in the continuous phase [1]. The type and stability of the emulsion formed is very system dependent, varying with the system composition and the way in which the emulsion is prepared. For systems consisting of equal volumes of oil and water and a single particle type however, predictions can be made about emulsion type and stability based simply on the oil /water interfacial tension (gow) and the particle wettability [2,3]. One way to express particle wettability is through the angle, u , the particles make with the oil /water interface and although this parameter cannot be measured directly (unlike the oil /water interfacial
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tension) it can be estimated from contact angle measurements on solids with a similar surface chemistry to the particles [3] or calculated theoretically [4]. The emulsion type is determined by which liquid phase more readily wets the particles and therefore becomes the continuous phase of the emulsion. Emulsion stability is related to the energy by which the particles are attached to the oil /water interface, given by E pR2 gow (19cos u)2
(1)
where R is the radius of the particles and the sign in the brackets is negative for u less than 908 and positive for u greater than 908 [5,2]. Thus the energy of attachment passes through a sharp maximum at u /908. The magnitude of the maximum is proportional to gow. For particles of extremely high or low wettability, the energy of attachment is very low and thus emulsions containing very hydrophobic or very hydrophilic silica particles are very unstable to coalescence as particles are easily displaced from the interface when drops collide [2]. While it has been argued that the wettability of silica particles is determined by their inherent hydrophobicity [2], the wettability of particles of intermediate hydrophobicity is sensitive to the oil type [3]. Silica particles are more hydrophobic at polar oil /water interfaces than at non-polar oil / water interfaces. Thus it is difficult to stabilise emulsions of polar oils and water using particles of intermediate hydrophobicity [3]. In an unusual case, the instability of emulsions formed from undecanol and water was observed to be markedly dependent on temperature [3] and it is the behaviour of this particular system that this paper is concerned with. Based on the low undecanol / water interfacial tension (estimated to be 9.5 mN m 1) and the high contact angle of the particles at the undecanol /water interface (estimated to be 1608), Binks and Lumsdon [3] predicted that water /undecanol emulsions stabilised by nanoparticles of intermediate hydrophobicity would be oil-continuous and of low stability. They found that the emulsions prepared were indeed water-inoil emulsions with a high viscosity (‘gel-like’) which broke down completely into the separate
phases over time [3]. The emulsions contracted from the walls of the container, maintaining the shape of the vessel and releasing oil and water. The time scale of the destabilisation process depended on both the water volume fraction (fw) and the temperature, with the stability of the emulsion to sedimentation passing through a maximum as a function of temperature. The emulsion breakdown phenomenon was thought to be similar to coalescence in emulsions of highly viscous oils in water [6,7], with the coalescence of the water drops governed by the viscosity of the continuous phase, which was higher at high water volume fractions. The temperature dependence of the sedimentation could not be explained however. The unusual breakdown phenomena may be linked to the temperature dependent phase transitions observed in fatty alcohol monolayers at the alcohol /water [8 /11] and air /water [12 /20] interfaces. For medium to long chain length alcohols (C9 /C16), the tension of the alcohol /water interface is a linear function of temperature up to 10 K above the melting point of the bulk alcohol and a parabolic function at higher temperatures [8 /10]. There is a similar discontinuity in the surface tension of alcohol monolayers at the air /water interface, and based on the discontinuity in the ellipsometric angle at the same temperature, Berge and Renault [12] demonstrated that the monolayer undergoes a first order phase transition at this temperature. Using X-ray diffraction measurements, Renault et al. [13] found that below the transition temperature the monolayer had a structure similar to the structure of the bulk alcohol at temperatures just below the melting point, while above the transition temperature the monolayer had a liquid-like structure. It is likely that the discontinuity in the interfacial tension of alcohol / water systems is also due to the melting of a solidlike layer of alcohol at the interface since at temperatures close to the solidification point of water-saturated undecanol, a solid layer of alcohol was observed to form at the planar interface between bulk water and alcohol [11]. The aim of the work described here is to investigate whether the breakdown of viscous water-in-undecanol solid-stabilised emulsions could be related to the phase transition in the
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undecanol layer at the planar alcohol /water interface. Thus the breakdown of water-in-undecanol emulsions was studied as a function of temperature (T ). Recently, it was found that the temperature at which the phase transition occurs of fatty alcohol monolayers at the air /water surface can be controlled by the addition of short chain carboxylic acids, which in their protonated form adsorb to the monolayer and alter the surface crystallisation [21 /23]. Therefore the influence of adding a solid particle emulsifier or a surfactant molecule (octanoic acid) emulsifier on the emulsion breakdown behaviour was investigated.
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ferred into stoppered, graduated vessels which were placed in a thermostated water bath (set to the required temperature). The emulsion type was inferred by observing whether an emulsion drop dispersed, or remained as a drop, when added to a volume of water or oil. The conductivity of some of the emulsions prepared (containing background electrolyte) was determined using a digital Jenway conductivity meter 4310 with Pt/Pt black electrodes. The stability of the water-in-oil emulsions to sedimentation and coalescence was assessed by monitoring the change with time of the position of the oil /emulsion and water /emulsion interfaces respectively.
2. Experimental 3. Results and discussion 2.1. Materials Water was first passed through an Elga reverse osmosis unit and then a Milli-Q reagent water system. Undecan-1-ol (with a purity of 99% and a nominal melting point of 11 8C) was obtained from Aldrich and columned twice to remove polar impurities. The melting point of the alkanol was measured to be (14.09/0.3) 8C by differential scanning calorimetry (Perkin /Elmer DSC7). nOctanoic acid (with a purity of 99%) was obtained from Sigma. The partially hydrophobic monodisperse silica particles were a gift from Rhodia Services, with a primary particle diameter of 25 nm. They were prepared by grafting n -octyltriethoxysilane onto hydrophilic silica (purchased from Clariant). The particles were transferred from the synthesis media into water by dialysis. The aqueous dispersion of particles was strongly flocculated, with flocs about 40 mm in size which sedimented rapidly. In some experiments, a background electrolyte (sodium chloride, Prolabo, analytical grade) was added to the dispersion. 2.2. Methods The emulsions were prepared using a Janke and Kunkel Ultra-Turrax homogeniser with an 18 mm head operating at 13 500 rpm for 2 min to emulsify mixtures of the alcohol and the aqueous dispersion of particles. The emulsions were rapidly trans-
During the emulsification (2 min in total) of equal volumes of undecanol and water alone, a viscous white-gray emulsion forms rapidly (within 5 s). The emulsion consists of solid-like flocs of white material and a little thin clear fluid (presumably oil). When a small sample is added to oil and stirred gently, the flocs partly break up (settling once the stirring is halted). However the emulsion cannot be dispersed in water by gentle agitation. It is concluded that the emulsion is oil continuous since the emulsion conductivity is low and comparable to that of undecanol. The emulsion is unstable and as water and oil are released, the white material partly contracts from the walls of the vessel. In the absence of stabilisers, or in the presence of poor stabilisers such as octanoic acid, complete phase separation occurs. In the presence of a good stabiliser like partially hydrophobic silica particles, the emulsion does not break down completely. The destabilisation and, in particular, the rate of sedimentation is extremely temperature dependent. For poorly stabilised emulsions this was difficult to quantify except in the initial stages, for reasons that are discussed later. Fig. 1 shows the time taken for 10% of the oil to be released following sedimentation from 50 vol.% water-in-undecanol emulsions as a function of temperature. No data is shown for poorly stabilised emulsions equilibrated at 18 8C, since it could not be measured accurately (com-
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are stored) tend to be more stable than those within the bulk. Thus it is difficult to assess the extent of coalescence and sedimentation after the initial stages of phase separation since the contraction of the gelled emulsions is non-homothetic [7]. Overall, the time taken for complete phase separation follows the same trend as the initial rate of sedimentation, passing through a maximum of about 6 h at 23 8C (shown later). It should be noted that the times measured could vary by as much as 15% for individual emulsions prepared and equilibrated under the same conditions. Emulsions stored in plastic vessels appear to contract in a more uniform manner and those equilibrated at intermediate temperatures take longer to phase separate than those stored in glass (shown later).
Fig. 1. Time taken for 10% of the oil to be resolved from 50 vol.% water-in-undecanol emulsions prepared in the absence of stabilisers (circles), and in the presence of 0.7 wt.% hydrophobic silica in the aqueous phase (squares), 1.9 mM octanoic acid at pH 2.4 (triangles) and 1.9 mM octanoic acid at pH 9.5 (inverted triangles).
plete phase separation took only 3 min). Irrespective of the type of stabiliser, the initial stability of the emulsions to sedimentation passes through a maximum at temperatures between 23 and 25 8C. The magnitude of the increase in the stability of the emulsions to sedimentation on the addition of stabilisers is largest around these temperatures also. The difference in temperature at which the rate of sedimentation is slowest for different stabilisers is not believed to be significant. These results will now be discussed in more detail for the different kinds of emulsions. 3.1. Emulsions prepared in the absence of stabilisers The breakdown of water-in-undecanol emulsions prepared in the absence of a stabiliser does not occur steadily. While sedimentation and coalescence begin almost immediately at low temperatures (5/23 8C), there is often a short delay before coalescence is observed in emulsions equilibrated at higher temperatures. Water drops close to the walls of the glass vessels (in which the emulsions
3.2. Emulsions stabilised by hydrophobic silica particles The addition of partially hydrophobic silica particles stabilises the emulsions against complete breakdown and while the contraction is still not uniform, the extent to which the water and oil phases resolve from the emulsion can be assessed throughout the destabilisation. In Fig. 2, the fractions of water and oil resolved from 50 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% hydrophobic silica are shown as a function of time at a selection of the temperatures studied. At temperatures below 23 8C, coalescence and sedimentation occur quickly, with large proportions of water and oil being released respectively. Between 23 and 25 8C, sedimentation is much slower than at low temperatures and only a small amount of coalescence occurs when the drop volume fraction (fd) is about 0.75. At higher temperatures, the rate of sedimentation increases; however only a small amount of coalescence occurs (again only when fd is about 0.75). The initial rate of sedimentation has the same temperature dependence as that for emulsions in the absence of particles as shown in Fig. 1. Although the time taken for the emulsion to become stable to sedimentation is temperature dependent, the extent of sedimentation is approximately the same at all temperatures. The extent of coalescence, however, is much higher at low temperatures
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Fig. 2. Stability of 50 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% hydrophobic silica to sedimentation and coalescence as a function of time at different temperatures.
(5/21 8C). This is illustrated in Fig. 3, where the final or equilibrium fractions of oil and water resolved from the solid-stabilised emulsions are shown as a function of temperature. Given the exceptionally long times required for the emulsions to stabilise at temperatures between 23 and 27 8C (see Fig. 2) it is argued that the coalescence which occurs under these conditions is augmented by collapse of some of the lowermost emulsion drops under the pressure of the drops above. The extent of coalescence in the majority of solid-stabilised emulsions is typically low, due to the adsorbed layer of particles at the oil /water interface preventing the close approach of drops [1]. At temperatures of 23 8C and above, these systems behave like typical Ramsden emulsions. At lower temperatures, the presence of the particles only halts the coalescence to a small, but
important, extent. This is improved by increasing the concentration of particles in the emulsion. Also shown in Fig. 3 is data for an emulsion stabilised by a higher concentration (3 wt.%) of hydrophobic silica (filled symbols). Increasing the particle concentration does not alter the extent of sedimentation; however the rate of sedimentation is slower (it takes about 66 min for 80% of the water to be released compared to 7 min at the lower particle concentration). In Fig. 4, the fractions of water and oil resolved from solid-stabilised water-in-undecanol emulsions at a lower water volume fraction (fw /0.2) are shown as a function of time, at three different temperatures. The extent of sedimentation and the trend in the temperature dependence of the rate of sedimentation are similar to that for emulsions at fw /0.5 (assuming that the emulsion equilibrated
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Fig. 3. Final fractions of oil (from sedimentation) and water (from coalescence) released from 50 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% (open symbols) and 3 wt.% (filled symbols) hydrophobic silica as a function of temperature.
at 25 8C continued sedimenting until about 80% of the water was released). The fraction of water released at 18 8C is lower than that at fw /0.5, due to the lower drop volume fraction which reduces the frequency of drop collisions and thus the opportunities for drop coalescence. The temperature dependence of the coalescence in solid-stabilised water-in-undecanol emulsions is highly unusual. The transition in the stability of the emulsions to coalescence occurs at temperatures about 10 8C above the measured melting point of the alcohol (14.09/0.3 8C). All the emulsions are prepared by homogenisation using a rotor /stator head which heats the emulsion to relatively high temperatures during its operation. The emulsions are then placed in a water bath to equilibrate at the required temperature and it seems likely that as the emulsions cool, a solidlike layer of undecanol forms at the surface of the water drops, at temperatures up to 23 8C, just as it would form at a planar interface. It is possible that some of the undecanol crystallises in the solid-like layer of alcohol which forms. These crystals may bridge the thin films between water drops at close separations resulting in coalescence, just as low densities of interfacial fat crystals increase the
Fig. 4.
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Fig. 4. Stability of 20 vol.% water-in-undecanol emulsions stabilised by 0.7 wt.% hydrophobic silica to sedimentation and coalescence as a function of time at different temperatures.
extent of coalescence in water-in-triglyceride oil emulsions [24]. The presence of solid silica particles at the drop surfaces provides a steric barrier to the close approach of the water drops, thus reducing the coalescence caused by the crystals. At temperatures of 25 8C and above, the layer of undecanol at the drop surfaces is fluid-like and the presence of solid particles at the interface reduces the frequency of drop collisions and thus the extent of coalescence. The temperature dependence of the sedimentation behaviour has yet to be accounted for. The rate at which an isolated spherical drop in a dilute emulsion, of radius r , sediments (ns) is given by Stokes’ law [25] ns
2r2 (ro r)g 9ho
(2)
where g is the acceleration due to gravity, ro and ho are the density and shear viscosity of the continuous phase respectively and r is the density of the drop phase. Assuming that the emulsion drop diameter is fixed, the rate of sedimentation can be reduced by decreasing the density difference between the two phases or by increasing the viscosity of the continuous phase. In Table 1, the viscosity and density of undecanol and the density of water at different temperatures are given. With increasing temperature, the magnitude of the density difference between undecanol and water increases, and the viscosity of undecanol decreases, and so the rate of sedimentation should increase. This may explain why the stability of the water-in-
undecanol emulsions to sedimentation decreases as the temperature increases from 25 to 35 8C. At temperatures below 25 8C, the presence of undecanol crystals (and presumably solid particles released by drop coalescence) could increase the viscosity of the continuous phase. This effect, however, would become more marked at lower temperatures, implying that the rate of sedimentation should decrease with decreasing temperature. However, the assumption that the drop diameter remains unchanged may not be correct. It is unclear why the hydrophobic precipitated silica particles used in these experiments are able to stabilise the emulsions against complete phase separation, unlike those used (hydrophobic fumed silica) in the previous study [3]. There is some evidence for the adsorption of undecanol onto hydrophobic silica [29]. It is possible that crystallisation may occur in adsorbed alcohol monolayers on solid surfaces, since changes in the monolayer of dodecanol adsorbed on a hydrophobic surface from a solid to liquid-like structure have been observed at a temperature about 18 8C above the melting point of the bulk alkanol [30]. Perhaps the presence of these crystals causes further disruption to the thin films formed between drops at close separations, encouraging coalescence to occur. Supposing that the degree of hydrophobisation on the two types of silica is different, the extent of adsorption of the undecanol onto the two silicas will be different, thus affecting the extent to which this behaviour might contribute to the emulsion destabilisation. 3.3. Emulsions stabilised by octanoic acid molecules Bonosi et al. [21] report that the temperature at which the phase transition in fatty alcohol mono-
Table 1 Viscosities and densities of undecanol and water at different temperatures. Temperature (8C)
Viscosity of undecanola (mPa s)
Density of undecanola (g cm 3)
Density of waterb (g cm 3)
20 25 35
16.952 13.830 9.380
0.8325 0.8291 0.8223
0.99 823 0.99 707 0.99 406
a b
Taken from Refs. [26,27]. Taken from Ref. [28].
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Fig. 5. Time taken for complete phase separation of 50 vol.% water-in-undecanol emulsions in glass vessels as a function of temperature. The emulsions were prepared in the absence of stabilisers (circles) or with the aqueous phase containing 1.9 mM octanoic acid at pH 2.4 (triangles) or 1.9 mM octanoic acid at pH 9.5 (inverted triangles) or 10 mM octanoic acid at pH 2.4 (filled triangles) or with the oil phase containing 100 mM octanoic acid and the aqueous phase at pH 2.5 (diamonds).
layers at the air /water interface occurs can be controlled by the addition of short chain length carboxylic acids to the water. In aqueous solutions at pH less than the pKa of the acid, the molecules are predominantly in the protonated form and readily adsorb in the alcohol monolayer affecting the surface crystallisation by reducing the temperature at which the transition occurs. Thus emulsions were prepared from undecanol and aqueous solutions of 1.9 mM octanoic acid at high or low pH and equilibrated at different temperatures to determine if the presence of octanoic acid would cause a shift in the temperature-dependent stability behaviour of the emulsions. The emulsions have a similar morphology to those prepared in the absence of stabilisers and phase separate over time in a similar fashion. As was shown in Fig. 1, the temperature dependence of initial sedimentation of emulsions containing octanoic acid is unchanged at either low or high pH in the dispersed phase. The overall
emulsion stability increases in the presence of octanoic acid as the emulsions take longer to phase separate than those prepared in its absence as shown in Fig. 5. This suggests that during the homogenisation of the two liquids, octanoic acid (in either protonated or charged form) is adsorbed at the alcohol /water interface and acts like a surfactant, partially stabilising the water drops. The presence of such molecules at the interface does not however perturb the crystallisation occurring in the monolayer of alcohol at the surface of the drops. (Also shown in this figure is data for emulsions stored in plastic vessels. As mentioned earlier, due to the different interactions between the alcohol, water and plastic, as these emulsions contract they are less attached to the walls of the vessel.) Increasing the concentration of octanoic acid in the aqueous or oil phase of the emulsions does not change these finding as shown in Fig. 5. The concentration of the carboxylic acid used exceeded its solubility in water and also in undecanol, making it likely that a substantial proportion of the molecules would be located at the interface. Again however, the presence of the acid did not appear to affect the crystallisation occurring in the monolayer. It is proposed that acid molecules adsorbed in alcohol monolayers at air /water interfaces act as impurities which as solidification occurs are rejected and may diffuse back into the aqueous phase [21 /23]. It was estimated that under these conditions the proportion of octanoic acid molecules in the monolayer is about 14% [23]. In the case of the emulsions being prepared here, however, it seems likely that most of the acid molecules are located at the alcohol/water interface. Given the number of molecules available, it is possible that they may account for a significant fraction of the surface area of the drops and thus it is their role as a surfactant which dominates the behaviour at the interface.
4. Conclusions In summary, the unusual temperature-dependent stability of water-in-undecanol emulsions is thought to be related to the formation of a solid-
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like layer of alcohol at the alcohol /water interface at temperatures up to 10 K above the melting point of the alcohol. At these low temperatures, crystals of the alcohol may form in the interfacial region, and the presence of these crystals may enhance coalescence by bridging the thin films formed between drops at close separations. The addition of solid particles may prevent the close approach of the drops, thus reducing the extent of coalescence. The addition of octanoic acid, although slightly stabilising the emulsions, does not alter the temperature at which the transition in stability of the emulsions is observed. The temperature dependence of the sedimentation in these emulsions cannot yet be explained.
Acknowledgements We thank Rhodia Services Inc. (France) for provision of a grant to fund CPW. We would also like to thank P.D.I. Fletcher for useful discussions.
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