J. Phys. Chem. B 2007, 111, 13151-13155
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ARTICLES Emulsion Stabilization and Inversion Using a pH- and Temperature-Sensitive Amphiphilic Copolymer Fre´ de´ ric Marchal,† Angelina Roudot,‡ Nade` ge Pantoustier,‡ Patrick Perrin,‡ Jean Daillant,† and Patrick Guenoun*,† SerVice de Chimie Mole´ culaire, LIONS, Baˆ timent 125, C.E.A. Saclay, F-91191 Gif-sur-YVette Cedex, France, and Laboratoire de Physico-chimie des Polyme` res et des Milieux Disperse´ s, UniVersite´ Pierre et Marie Curie, ESPCI, CNRS UMR 7615, 10 rue Vauquelin, F-75005 Paris, France ReceiVed: June 26, 2007; In Final Form: September 14, 2007
We describe how a versatile amphiphilic diblock copolymer can form oil-in-water (o/w) or water-in-oil (w/o) emulsions depending on pH and temperature. At high pH and temperature, this copolymer is mostly hydrophobic and forms w/o emulsions. Its spontaneous curvature is greatly increased upon pH and/or temperature lowering (due to protonation and/or hydration, respectively), which allows the formation of o/w emulsions. Conductivity measurements and confocal fluorescence micrographs evidence the two kinds of structures obtained over a wide range of pH and temperature. We also show how the emulsion type can be reversibly switched along a temperature scan under stirring. The lower stability of the w/o emulsions as compared to the o/w ones is attributed to a lack of electrostatic repulsion. The importance of the copolymer architecture and conformation with regards to droplet stability is discussed.
1. Introduction Controlling emulsion inversion is an important challenge for both theoretical and practical motivations.1-4 It consists of turning an oil-in-water (o/w) emulsion into a water-in-oil (w/ o) emulsion (or vice versa), preferentially by some external trigger. This operation is the basis of the widely used phase inversion temperature (PIT) method,5-7 which yields particularly fine emulsions without high input of energy. On the applied side, tunable emulsions are found in various applications such as drugs,8 cosmetics,9,10 or oil recovery.1,11,12 Although the oil/water ratio may also be of importance,4,13,14 it is known from common experience that the emulsion type is mostly determined by the nature of the surfactant. Several useful empirical concepts have emerged throughout the last century to confirm this link. The ability for a surfactant to favor one particular type of emulsion has been related to its preferential partitioning in one of the phases,15 its hydrophilic-lipophilic balance (HLB),16 the phase behavior of the corresponding ternary system,5,6,17 or its spontaneous curvature.2,18,19 However, understanding how the spontaneous curvature at the molecular level governs the curvature of emulsion droplets is not straightforward, because of the different length scales involved. Indeed, while the radius of a typical micelle lies in the nanometer range (no more than few nanometers with traditional surfactants, and several tens of nanometers for block copolymers), most emulsion droplets belong to the micrometer range. This means that, whatever the emulsion type, surfactant molecules “see” the * Corresponding author. Phone: 33(0)169087433. E-mail: patrick.
[email protected]. † LIONS. ‡ Universite ´ Pierre et Marie Curie.
interface as a locally flat membrane. To link both scales, Kabalnov and coauthors2,3 pointed out that curvature frustrations at the molecular level become significant when droplets undergo coalescence. According to those authors, a spontaneously curved surfactant stabilizes preferentially droplets of similar curvature, as it tends to prevent formation of the strongly oppositely curved channels that would allow coalescence. This argument implies that one way of inverting an emulsion basically consists of inverting the spontaneous curvature of the emulsifier. Reports on successful attempts of controlling emulsion inversion are relatively scarce. Such inversions are the result of modifications in the affinity between emulsifier and water or oil and can be induced by more or less intrusive methods. Addition of cosurfactants,9 and changes in temperature,14 pH,12,20 or ionic strength21 are typically used, but more subtle triggers such as light have also been successfully exploited.22 Even more unusual is the use of emulsifiers that can respond to two different stimuli, as discussed in the present study. We made use of an amphiphilic diblock copolymer as a stabilizer of invertible emulsions. Polymers can be convenient emulsion stabilizers, as reported in various experimental23-25 and theoretical studies,26,27 but they can also be efficient emulsion reversers,28 because each polymeric chain contains a large number of stimuli-responsive units. This means that the response to the stimuli is greatly amplified as compared to what is achieved with traditional surfactant molecules. Our polymer is composed of two blocks: the first one is a polystyrene (PS) block and is therefore hydrophobic, whereas the second one mostly consists of 2-(dimethylamino)ethyl methacrylate (DMAEMA) units but is actually a PS-st-PDMAEMA statistical block because it contains up to 32% styrene units (Figure 1). The main
10.1021/jp0749827 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/31/2007
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Figure 2. Sensitivity of a DMAEMA unit to pH (left) and temperature (right). This corresponds to a decrease in hydrophilicity from left to right. Figure 1. Structure of the PS-b-(PS-st-PDMAEMA) copolymer. Note that the 20 PS units and 44 PDMAEMA units of the second block are statistically distributed along the chain.
TABLE 1: Degrees of Polymerization (DP) and Number Average Molar Masses (Mn) of the Polymers Used in This Study polymer
DP
Mn (g/mol)
PDMAEMA PS-st-PDMAEMA PS-b-(PS-st-PDMAEMA)
197 40-st-85 41-b-(20-st-44)
30 800 17 600 13 100
interest of the DMAEMA units lies in their combined pHresponsiveness and thermosensitivity,29,30 which allows the spontaneous curvature of the copolymer to be adjusted. In the following, we describe emulsification of water and toluene using this copolymer surfactant. 2. Experimental Section Polymers Synthesis and Characterization. The polymers used in this study are synthesized by atom transfer radical polymerization (ATRP), and details on the synthesis will be the focus of an upcoming paper. Size exclusion chromatography (SEC) indicates a polydispersity index (Ip ) Mw/Mn) of 1.06 for the PS block of PS-b-(PS-st-PDMAEMA). The whole copolymer has an overall Ip of 1.27. Despite a good blocking efficiency (around 80%, as determined by comparison of theoretical and experimental number average molar masses), this Ip value is relatively high. This is classically observed with homopolymerization of DMAEMA even by ATRP31 and may be due to retention of DMAEMA units in the SEC column. The precise composition of the second block is determined by 1H NMR spectrometry, and sampling of the reaction medium during polymerization of this second block reveals the statistical repartition of PS and PDMAEMA units. Two additional polymers are prepared and used in this study: a PS-stPDMAEMA copolymer having similar statistics and a PDMAEMA homopolymer. Degrees of polymerization (DP) and number average molar masses (Mn) of the three polymers are gathered in Table 1. Cloud points of the homopolymer and statistical copolymer in water are determined by visual inspection of glass tubes immersed in a thermostated bath. Emulsions Preparation and Characterization. A series of acidic solutions are prepared by dissolving appropriate amounts of HCl in Milli-Q water (Millipore). 0.1% (weight of polymer/ weight of water) are added to equal volumes of acidic solution and toluene (Sigma Aldrich, >99.9%) in glass test tubes. The tubes are kept in a thermostated bath for about 20 min. Once the temperature of interest is reached, emulsions are obtained by mixing the two phases (total volume of a few millilitres) for 10 s at 25 000 rpm using an IKA Ultra-Turrax T8.01. The emulsion type is then determined by measuring the conductivity by means of a CDM210 conductivity meter (Radiometer Analytical). At constant pH, whenever the emulsion type changes with temperature, conductivity is monitored under progressive heating and cooling and constant stirring to refine
the location of the inversion and test its reversibility. Several temperature cycles with decreasing heating/cooling speed are required to pinpoint the inversion temperature as accurately as possible. Emulsion type is finally confirmed using confocal fluorescence microscopy. Small amounts of Rhodamine B or Sudan Red are added to the emulsion, which is then observed between cover glasses through an Olympus Fluoview FV1000 confocal microscope. 3. Results and Discussion The pH-sensitivity of PDMAEMA is due to the ability of its tertiary amine group to be protonated, while its thermosensitivity is caused by disruption of the hydrogen bonds between water and the carbonyl and amine groups, as illustrated in Figure 2. Consequently, when studying the solubility of a PDMAEMA homopolymer as a function of temperature and pH, a cloud point curve is found (Figure 3a) between a zone of solubility and a zone of insolubility at high pH and high temperature (i.e., low degree of protonation and high degree of dehydration). Clouding occurs at temperatures consistent with values measured elsewhere30 for PDMAEMA homopolymers of similar degrees of polymerization. As seen in Figure 3a, this cloud point curve is shifted toward lower pH and temperature when hydrophobic styrene units are introduced in the PDMAEMA chain to mimic the structure of the hydrophilic block of PS-b-(PS-st-PDMAEMA). A further lowering of the curve is expected when using our copolymer, which contains a whole PS block in addition to the PS-st-PDMAEMA statistical block, but this cannot be measured because this copolymer is insoluble as unimers in water, whatever the pH and the temperature. Nevertheless, toluene being a good solvent of polystyrene, our PS-b-(PS-st-PDMAEMA) copolymer can be solubilized in the presence of both water and toluene, regardless of the pH and the temperature, which allows us to emulsify the two phases. The emulsion types obtained are positioned in the temperature-pH map shown in Figure 3b. Because the copolymer is insoluble in the absence of toluene, actual pH values of the aqueous phases are not directly accessible and are therefore measured after addition of toluene. The electrode is immersed in the lower aqueous phase before emulsification, and pH is measured under very moderate stirring to avoid emulsification. A frontier between the zone of o/w emulsions and that of w/o emulsions emerges rather evidently from the set of data represented in Figure 3b. As expected, emulsions are of the o/w type when the copolymer is highly protonated (roughly below pH 6), and of the w/o type when it is unprotonated (roughly above pH 7.5). Yet most interestingly, for intermediate protonation ratios, the thermosensitivity of our copolymer becomes discernible. A comparable versatility was observed by Binks et al.32 around pH 8 and 60 °C with PDMAEMA-stabilized polystyrene latex particles used as “Pickering” emulsifiers. In our case, between about pH 6 and pH 7.5, the copolymer charge appears low enough to allow emulsion inversion via dehydration upon heating. The location of this zone of partial protonation
Emulsion Stabilization and Inversion
Figure 3. (a) Cloud point curves of a pure PDMAEMA homopolymer and PS-st-PDMAEMA statistical copolymer. A polymer concentration of 1% (wt/wt) is used so that the cloud points are sufficiently perceptible. (b) Temperature-pH map showing the separation between o/w (squares) and w/o (triangles) emulsions obtained with PS-b-(PSst-PDMAEMA)-water-toluene. Solid symbols materialize the result of independent conductivity measurements, whereas series of hollow symbols at constant pH represent the results of temperature scans such as the one detailed in Figure 4.
is consistent with a pKa value of 7.0 found for the conjugate acid form of PDMAEMA, as determined by titration of a PDMAEMA homopolymer.30 In this intermediate region, temperature scans are performed to refine the location of the inversion and test its reversibility. A typical scan, carried out at a pH value of 6.3, is presented in Figure 4 and is reported in Figure 3b. The conductivity (κ) of an emulsion is proportional to the conductivity of its continuous phase. At low temperatures, water is continuous, which is why κ increases with temperature, but as soon as the emulsion is inverted, toluene becomes continuous and κ falls to about zero. When heating at a rate of 3 °C per min, the inversion is detected around 23 °C; however, when cooling down at the same rate the emulsion is not inverted back before 8 °C. Such hysteresis is occasionally observed when dealing with adsorbing polymers33 and can be significantly narrowed by performing slower heating-cooling cycles. The following cycles indeed reveal a much more reversible inversion, occurring at about 17 °C, as reported in the phase diagram of Figure 3b. It should be noted that whatever the heating or cooling rate, no intermediate value of conductivity was measured around the inversion point, which means that no hypothetical intermediate structure between the two dispersions of opposite curvature could be detected this way. The reversibility of this inversion observed in Figure 3b and Figure 4 obviously arises from the reversibility of the copolymer cloud point. The shape
J. Phys. Chem. B, Vol. 111, No. 46, 2007 13153 of the inversion line is indeed very similar to the shape of the cloud point curves shown in Figure 3a. As expected, this line is located at lower pH and temperature than the cloud point curves of PS-st-PDMAEMA and PDMAEMA, because our diblock is more hydrophobic. Looking now at the stability of the emulsions, a major distinction ought to be made between o/w and w/o systems. Whereas the o/w emulsions obtained here are particularly stable (despite a relatively low input of energy), the w/o emulsions display significantly less stability. Indeed, o/w emulsions exhibit relatively little creaming and exist for at least several months, while it takes only few hours to reduce the w/o emulsions to a thin layer between the two phases. As seen in Figure 5, in this latter case the resulting upper toluene phase is slightly opalescent and occupies a larger volume than the water phase, suggesting that the copolymer forms a slightly water-swollen w/o microemulsion. This striking difference in emulsion stability can be accounted for by the fact that, contrary to w/o droplets, o/w emulsion droplets benefit from an electrostatic stabilization due to the presence of the charged PDMAEMA brush on the outer side of the interface. In the latter situation, keeping in mind Kabalnov et al.’s picture,2,3 the formation of a channel between two droplets is hindered by this bulky outer electrostatic brush, which can explain why coalescence is not observed. On the contrary, in the case of the w/o type, this denser brush (although not electrostatic at high pH) points toward the inside of the droplets and is therefore able to promote the formation of channels. More stability can be provided to the w/o emulsions by the two following means. First, a significant improvement is achieved if the copolymer is introduced in the toluene phase a few hours before emulsification is carried out. Heating and stirring allow a full dissolution of the copolymer, which can then stabilize the water droplets more efficiently. Second, Ostwald ripening can be slightly limited by introducing NaCl into the aqueous phase: improved stability is apparent immediately after emulsification. However, in the latter case, the emulsion becomes as unstable as a nonsalted one after a few hours. This might mean that the smallest droplets ripen at first, whereas the resulting ones are too big (see Figure 6) to be sensitive to Ostwald ripening, which is driven by differences in Laplace pressure (i.e., inversely proportional to the droplet radius). Our observations are, however, insufficient to allow a more complete understanding of the destabilization process. Confocal fluorescent micrographs of our o/w and w/o emulsions are shown in Figure 6. We make use of a hydrophilic (Rhodamine) and a lipophilic dye (Sudan Red), which allow an unambiguous distinction between dispersed and continuous phase. Adsorption of the fluorophore at the droplet interface on Figure 6a and c shows that Rhodamine is actually rather amphiphilic, but observation through an optical microscope without introduction of dye shows that the dyes have no visible effect on the structures. Both types of droplets display a fairly high polydispersity, with an average diameter of 10-20 µm for the oil droplets, and less than 10 µm for the water droplets. This size disparity is somewhat surprising, considering that bigger droplets are usually less stable. This must mean that the electrostatic stabilization is particularly efficient. As expected from the poor stability of the w/o emulsions, Figure 6c and d shows highly packed droplets on the verge of coalescence, or actually undergoing coalescence. This high packing is amplified by the fact that toluene dries out quite rapidly during sample preparation. The apparent deformation of the w/o droplets is likely to be caused by the proximity of the cover glass.
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Figure 4. Conductivity (κ) of the emulsions obtained with PS-b-(PS-st-PDMAEMA)-water-toluene, under constant stirring and successive heatingcooling cycles at pH 6.3. Heating and cooling rates are progressively lowered to pinpoint the inversion temperature. Values of conductivity are very low because no extra salt was added, so as not to perturb the system.
Figure 5. Different PS-b-(PS-st-PDMAEMA)-water-toluene systems, 1 h after emulsification at 21 °C. Values of pH from left to right are: 4.6, 5.7, 7.2, 8.8.
after heating above 80 °C for a few hours and quenching. On the other hand, for sufficiently high pH and temperature, we obtain unstable w/o droplets and a w/o microemulsion once the emulsion is broken. This latter observation is also consistent with Winsor’s correlation17 between equilibrium phase behavior (microemulsion type) and the type of emulsion formed. However, theories15,16 that relate the emulsion type to the relative solubility of the emulsifier in each phase cannot be fully tested here because the solubility of our copolymer depends on pH and temperature. At sufficiently high pH, emulsions are of the w/o type, which is consistent with the fact that the neutral form of the copolymer is soluble in toluene and insoluble in water. To obtain a similar doubly invertible system, one can also contemplate using simply the statistical copolymer as a unique emulsifier. Preliminary tests show that this polymer does form an inverse emulsion despite the lack of PS block. However, the stability is poorer than with PS-b-(PS-st-PDMAEMA), which means that the PS block does play a role in anchoring the polymer efficiently at the water-toluene interface. Complementary experiments should provide a more accurate understanding of the copolymer conformation at the interface, but the architecture and the results collected so far strongly suggest that our diblock copolymer is located through the interface with the PS block extended on the toluene side, and the PS-stPDMAEMA block more or less extended on the water side, depending on pH and temperature. 4. Conclusion
Figure 6. Confocal fluorescent micrographs of PS-b-(PS-st-PDMAEMA)-water-toluene emulsions at room temperature: (a) o/w emulsion at pH 4.6 with Rhodamine B, (b) o/w emulsion at pH 4.6 with Sudan Red, (c) w/o emulsion at pH 8.8 with Rhodamine B, and (d) w/o emulsion at pH 8.8 with Sudan Red.
The present system (PS-b-(PS-st-PDMAEMA)-toluenewater) provides a good illustration of the possible correlation between spontaneous curvature at the molecular scale and curvature of micrometer-sized droplets. On the one hand, for sufficiently low pH and temperature, we obtain o/w droplets; at similar pH, it was previously shown34 that this PS-b-(PS-stPDMAEMA) copolymer can form frozen o/w micelles in water,
We have studied how the pH- and temperature-sensitivity of PDMAEMA can be exploited to produce doubly invertible emulsions. Although the w/o emulsions obtained here are not particularly stable, our PS-b-(PS-st-PDMAEMA)-toluenewater ternary system is of great interest, because it has the very rare ability to form both o/w and w/o emulsions reversibly. This kind of “smart” system is highly desirable in various applications, especially if the transition occurs in a convenient and easily accessible temperature range (between 0 and 20 °C for the present system). Subsequent work will be devoted in particular to trying to improve the stability of w/o emulsions. This might be achieved with a denser interface obtained with an actual PS-b-PDMAEMA diblock copolymer, that is, without residual PS units in the PDMAEMA block. References and Notes (1) Davis, H. T. Colloids Surf., A 1994, 91, 9.1. (2) Kabalnov, A.; Wennerstro¨m, H. Langmuir 1996, 12, 276. (3) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 1932.
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