Langmuir 2001, 17, 6905-6909
6905
UV-Visible Light: A Novel Route to Tune the Type of an Emulsion Iolanda Porcar, Patrick Perrin,* and Christophe Tribet Ecole Supe´ rieure de Physique et Chimie de Paris (ESPCI), CNRS, Universite´ Pierre et Marie Curie (UPMC), UMR 7615, Physico-Chimie des Polyme` res, Paris, France Received June 22, 2001. In Final Form: July 30, 2001 The light-induced control of the type (oil or water continuous medium) of emulsions stabilized by an appropriate combination of two polyelectrolyte surfactants is described in this paper. Due to its balanced hydrophilic-lipophilic properties, the first polymeric emulsifier (denoted BHL) has the ability to stabilize both direct and inverse emulsions and play the role of the main emulsifier of the n-dodecane-water system. The second one is a photoresponsive amphiphilic polymer (PR polymer) used at such low concentrations that the sole presence of the PR polymer as an emulsifier leads to the formation of unstable emulsions. The chromophore groups randomly distributed along the PR polymer backbone enable the adjustment of its HL properties upon irradiation by near-UV light. We demonstrate that the change in hydrophobicity of the PR polymer induced by irradiation can be suitably used to tune the type of the liquid-liquid dispersion containing both polymers. Consequently, UV-visible light is presented as a novel trigger to control the emulsion type.
Introduction Most often, the kinetically stable liquid-liquid dispersions of two immiscible fluids are either water in oil (W/O, inverse emulsion) or oil in water (O/W, direct emulsion) emulsions although more complex morphologies such as multiple emulsions can also be prepared under specific conditions.1-3 Flipping an emulsion from one type to the other is a difficult and important challenge. For instance, oil-based muds, which are complex inverse emulsions, are used as drilling fluids in oil and gas extraction to carry the drill cuttings out of the well. For environmental reasons, cuttings must be separated out from the mud and cleaned. Breaking up or inverting the emulsion can certainly help to meet these requirements.4 In addition, emulsions are often required to possess excellent but controlled stability. In cosmetic applications, direct emulsions which display excellent storage stability but break quickly upon contact with human skin to release a uniform oil layer are interesting moisturizing products. The salt at the surface of the skin5 triggers the breaking of emulsions. It is also well-known that the demulsification6-13 of water in crude oil is an integral and important * Corresponding author. Postal address: Patrick Perrin, ESPCILPM, 10, rue Vauquelin, 75005 Paris, France. (1) Becher, P. Encyclopedia of Emulsion Technology; Dekker: New York, 1983 (Vol. 1); 1985 (Vol. 2); 1988 (Vol. 3). (2) Grossiord, J. L.; Seiller, M. Multiple emulsions: Structure, Properties and Applications; Edition de Sante´: Paris, France, 1998. (3) Garti, N.; Bisperink, C. Curr. Opin. Colloid Interface Sci. 1998, 3, 657. (4) Monfreux, N. Ph.D. Dissertation, Universite´ Pierre et Marie Curie, Paris, France, 1998. (5) Lochhead, R. Y.; Hemker, W. J.; Castaneda, J. Y. Soap, Cosmet., Chem. Spec. 1987, May, 28. (6) Fordedal, H.; Sjoblom, J. J. Colloid Interface Sci. 1996, 181, 589. (7) Fordedal, H.; Midttun, O.; Sjoblom, J.; Kvalheim, O. M.; Schildberg, Y.; Volle, J. L. J. Colloid Interface Sci. 1996, 182, 117. (8) Taylor, S. E. Colloids Surf. 1988, 29, 29. (9) Mohammed, R. A.; Bailey, A. I.; Luckham, P. F.; Taylor, S. E. Colloids Surf., A 1994, 83, 261. (10) Bhardwaj, A.; Hartland, S. J. Dispersion Sci. Technol. 1994, 15, 133. (11) Graham, D. E.; Stockwell, A.; Thompson, D. G. Spec. Publ.sR. Soc. Chem. 1983, 45, 73. (12) Sharma, I. C.; Haque, I.; Srivastava, S. N. Colloid Polym. Sci. 1982, 260, 616.
part of crude oil production.14 The understanding of the mechanisms governing the stabilization/breaking and inversion process of emulsions is thus of major importance in the oil industry as well. These few practical examples show the importance to developing emulsion systems, which exhibit the appropriate stability properties and emulsion type for a specific application. Experimentally, this can be suitably achieved by changing physicochemical parameters, referred to as field variables, such as temperature (ethoxylated emulsifiers are well-known to be temperature sensitive), oil chain length, type or size of both the hydrophobic and hydrophilic moieties of the emulsifier, and electrolyte type and concentration.15-17 In this paper, we demonstrate that light can be used to trigger the O/W-breaking-W/O sequence of emulsions. In addition, a suitable combination of amphiphilic polyelectrolytes is used to stabilize the dispersions instead of the more classical (small-molecule) surfactants. The motivation for this arises mainly from the possible toxicity of amphiphilic small molecules, which has recently stimulated a growing interest in replacing them by polymers. Also, only a few studies dealing with the breaking and type of emulsions stabilized by ion-containing polymeric surfactants are available in the literature.17 For instance, Mathur et al.18 have recently described an emulsion system whereby the stabilizing properties can be reversibly switched on and off using poly(methacrylic acid) (PMAA)-grafted-poly(ethylene oxide) (PEO) based copolymers. A small change in pH triggers the stabilization/destabilization process of the emulsions. The study (13) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Lu, J. R. J. Colloid Interface Sci. 1990, 139, 128. (14) Breen, P. J.; Wasan, D. T.; Kim, Y. H.; Nikolov, A. D.; Shetty, C. S. Emulsions and emulsion stability. In Emulsions and Emulsion Stability; Sjoblom, J., Ed.; Surfactant Science Series, Vol. 61; Marcel Dekker: New York 1996; p 237-285. (15) Davis, H. T. Colloids Surf., A 1994, 91, 9. (16) Kabalnov, A.; Wennerstrom, H. Langmuir 1996, 12, 276. (17) Perrin, P.; Millet, F.; Charleux, B. Emulsions Stabilized by Polyelectrolytes. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; Surfactant Science Series, Vol. 99; Marcel Dekker: New York, 2001; p 363-445. (18) Mathur, A. M.; Drescher, B.; Scranton, A. B.; Klier, J. Nature 1998, 392, 370.
10.1021/la010955a CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001
6906
Langmuir, Vol. 17, No. 22, 2001
Porcar et al. Scheme 1
is based on the idea that polymers present the unique feature of undergoing a sharp conformation transition upon a small modification of the bulk solution properties near a critical point. For instance, many temperatureand pH-responsive systems have been based on a transition between soluble/insoluble or extended/collapsed states of polymers for switching macroscopic properties such as viscosity and turbidity. Interestingly, the transition undergone by adsorbed macromolecules might result in a marked change in the interfacial properties, which in turn can be used to adjust emulsion breaking and/or emulsion type.18 This approach seems highly promising in a context where the vast resources of polymer chemistry and physics are far from being fully exploited in the research field of emulsions. A new trigger, UV-visible light, is presented in this work as a novel route to control the emulsion type. To the best of our knowledge, we describe a new application of photoresponsive macromolecules.19 Experimental Section Emulsion System. Oil (n-dodecane) and water were emulsified using an appropriate mixture of two hydrophobically modified polyelectrolytes. The main or primary emulsifier is a poly(sodium acrylate) grafted with n-dodecylacrylamide groups:
τ is the molar fraction of hydrophobes. Although polymers with a wide range of degree of hydrophobic modification (1 e τ% e 80) were synthesized4,20,21 and used as an emulsifier,22-25 the experiments described in this paper are restricted to the polymeric surfactant with τ ) 60% and a molecular weight of 50 000 g/mol (balanced hydrophilic-lipophilic (BHL) polymer). By analogy with the corresponding low hydrophobically modified polymers (typically τ e 20%),26 the chemically grafted alkyl chains of the BHL polymer are likely to be “randomly” distributed along the poly(sodium acrylate) backbone. We reported on the ability of the BHL polymer to control the type of emulsion by changing external field parameters such as pH and ionic strength23 and to stabilize both direct and inverse concentrated emulsions.25 (19) Iric, M. Adv. Polym. Sci. 1993, 110, 50. (20) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1989, 20, 577. (21) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-Soluble Polymers: Synthesis, Solution Properties and Applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991; p 218. (22) Perrin, P.; Lafuma, F. J. Colloid Interface Sci. 1998, 197, 317. (23) Perrin, P.; Monfreux, N.; Lafuma, F. Colloid Polym. Sci. 1999, 276, 945. (24) Perrin, P. Langmuir 1998, 14, 5977; 2000, 16, 4774. (25) Perrin, P. Langmuir 2000, 16, 881. (26) Magny, B. Ph.D. Dissertation, Universite´ Pierre et Marie Curie, Paris, France, 1992.
The photoresponsive (PR) polymer is a M ) 150 000 g/mol poly(sodium acrylate) randomly grafted with 7 mol % of azobenzene chromophores (see Scheme 1).27 As shown above, the transazobenzene group (apolar isomer) can be converted into the cis isomer (polar isomer) upon irradiation at 365 nm.27,28 The cis to trans reverse isomerization occurs either in darkness or upon visible irradiation.27,28 The presence of chromophore pendant groups along the charged backbone of the second polymer enables the adjustment of its amphiphilic properties upon irradiation by near-UV light. Emulsion type was then investigated as a function of electrolyte concentration, BHL/PR polymer ratio, and irradiation. In particular, the experimental conditions under which the light-induced sweep of the O/W-breaking-W/O sequence was achieved are described in this paper therefore proving the photoresponsive character of the emulsion system. Method of Preparation of Emulsion Samples. Two percent (weight of polymer/weight of solvent) concentrated BHL polymer solutions were prepared by swelling the polymer for 18 h in 4 mL of NaNO3 Milli-Q water (Millipore). An appropriate volume (varying from 60 to 200 µL) of a nonirradiated (i.e., dark-adapted for 24 h) or irradiated 0.5 wt % aqueous solution of the PR polymer (polymer solutions with a thickness of 3 mm were vertically irradiated at 365 nm for 30 min) was then added to the BHL polymer solution. The mixture of the two polymers was left in the dark under magnetic stirring for 90 min. Four milliliters of n-dodecane were then added to the aqueous mixture. The oil and aqueous phases are then kept at rest for another 90 min. Emulsions are finally prepared by mixing the two phases for 3 min at 24 000 rpm using a Heidolph DIAX 900 homogenizer. The emulsion type was determined by observing the dilution of the emulsion in both oil and water. A drop of a direct (respectively inverse) emulsion is immediately dispersed in water (respectively in oil) which is not the case in oil (respectively in water).
Results and Discussion The emulsion type of a large number of samples was then investigated as a function of both sodium nitrate and PR polymer concentrations (Figure 1). In the absence of the PR polymer, CPR ) 0%, direct and inverse emulsions are obtained at salt concentrations smaller and larger than 1.1 M, respectively. The oil and water continuous emulsion domains are still observed in the presence of the nonirradiated PR polymer. At low CPR concentrations (e0.01%), the emulsion type diagram remains unchanged. As the PR polymer concentration increases, the boundary between the two emulsion type domains shifts to higher salt concentrations. As an example, for polymer concentrations of 0.018 and 0.025%, the flipping from a direct to an inverse emulsion occurs at salt concentrations of 1.6 and 2.7 M, respectively. We now describe the effect of (27) Porcar, I.; Sergot, P.; Tribet, C. Evidence for Photoresponsive Cross-links in Solution of Azobenene-modified Amphiphilic Polymers. In Stimuli-Responsive Water-Soluble and Amphiphilic Polymers; McCormick, C. L., Ed.; ACS Symposium Series 780; American Chemical Society: Washington, 2001; p 82. (28) Zimmerman, G.; Chow, L. Y.; Paik, U. J. J. Am. Chem. Soc. 1958, 80, 3528.
Tuning Emulsions with UV-Visible Light
Langmuir, Vol. 17, No. 22, 2001 6907
Figure 2. Stability behavior of various emulsions containing the BHL polymer at a concentration of 2%: (crosses) without PR polymer, NaNO3 concentration is 1.2 M, W/O emulsion; (squares) PR concentration is 0.012%, [NaNO3] ) 1.5 M, i.e., right above the inversion point, irradiated sample, W/O emulsion; (filled circles) [PR] ) 0.012%, [NaNO3] ) 1.2 M, nonirradiated sample, sample A in the Figure 1 W/O emulsion; (hollow circles) [PR] ) 0.012%, [NaNO3] ) 1.2 M, irradiated sample, sample B in the Figure 1 O/W emulsion. The stability of the emulsion samples was assessed by measuring the volume of emulsions (emulsified volume ) V) remaining at different times of observation as shown above. The error on the volume is (5%. The inset pictures give an idea of the long term-stability of typical inverse (top) and direct (bottom) emulsions.
Figure 1. Effect on the emulsion type of the PR polymer concentration or a preirradiation of the aqueous phase (under a UV light source of 365 nm). O/W and W/O are dispersions of oil in water and water in oil, respectively. All samples were prepared as described in the text, i.e., by mixing equal volumes of n-dodecane and an aqueous solution containing 2 wt % of the BHL polymer with various amounts of NaNO3 and PR polymer as indicated above.
irradiation on the emulsion type. Interestingly, at sufficiently high polymer concentrations, typically CPR ) 0.012%, the irradiation under near-UV light (365 nm) causes the inversion point to move to even higher NaNO3 concentrations as compared to the corresponding nonirradiated emulsion sample. The effect of irradiation on the inversion point is clearly observed since the difference between the salt concentrations required to flip the irradiated and the nonirradiated systems from an O/W to a W/O emulsion is about CNaNO3 ) 0.5 M. For example, at CPR ) 0.012%, the presence of the nonirradiated PR polymer does not change the amount of salt needed to change the emulsion type. The transition from one type to the other occurs at about the same salt concentration as for the BHL polymer alone (CNaNO3 ) 1.1 M). In contrast, the critical electrolyte concentration is close to 1.5 M in the presence of the irradiated PR polymer at the same concentration. As a consequence, the irradiation can actually be used as a tool to control the emulsion type. Though the stability behavior was not considered in detail in this paper, we observed that in the vicinity of the inversion point, the coalescence phenomenon occurs much more rapidly in the case of the direct emulsions. For instance, the stability of emulsions containing 0.012% of the PR polymer was investigated at a constant salt concentration of 1.2 M (samples A and B in the Figure 1). As shown in Figure 2, the nonirradiated sample (A, filled circles) leads to the
formation of an oil continuous stable emulsion, while a direct emulsion which breaks rapidly is obtained upon irradiation at 365 nm (sample B, hollow circles). The emulsified volume (V%) of the inverse emulsion reaches a plateau value at V ) 65-70% which corresponds to dispersed phase volume fractions of 0.76-0.71 in the sedimented layer (Figure 2). The stabilities of the W/O emulsions were found to be similar, irrespective of sodium nitrate concentration, irradiation, or the presence of the PR polymer (Figure 2). The top inset photograph in Figure 2 provides an insight of the long-term stability of a typical inverse emulsion. Creaming occurs in the case of the direct emulsion. However, due to the rapid coalescence process, a stable cream layer with an oil volume fraction close to 0.65-0.75 could not be observed as shown from the inset bottom picture in Figure 2, which give an idea of the stability of a typical direct emulsion in the vicinity of the inversion point. Note that a creaming/sedimentation destabilization process is obviously expected since the size of the droplets near the inversion point is of the order of several micrometers (r ) 4 ( 2 µm). At this point, it is important to recall that the BHL polymer is the main emulsifier of our system. The photoresponsive polymer in the absence of the BHL polymer was not able to provide the emulsions with a long-term stability. Within the range of investigated CPR, the emulsions, which were water continuous whatever the sodium nitrate concentration and irradiation, broke rapidly after preparation (typically a few seconds). In contrast, the use of just the BHL polymer was sufficient to prepare both types of emulsion. A qualitative explanation for this was recently given by Kabalnov and Wennerstrom who recently revisited the oriented wedge theory.16 The complete analysis predicts that O/W and W/O emulsions form at positive and negative mean spontaneous curvature of the emulsifier sheetlike microstructure (H0), respectively, with a breaking point occurring at H0 ) 0. Consequently, direct and inverse emulsions will break if H0 decreases and increases, respectively. From a practical viewpoint, the parameters that would favor the formation of a direct (or inverse)
6908
Langmuir, Vol. 17, No. 22, 2001
emulsion must be activated to break an inverse (or direct) emulsion. In general, positive (or negative) H0 values correspond to surfactants with high (or low) HLB values. Although the concept of H0 is not obvious in the case of grafted copolymers, the value of H0 of the BHL polymer is likely to be close to zero since external field parameters such as ionic strength efficiently control the emulsion type (Figure 1). This view would tally with the observation that in the presence of a hydrophilic competitor (PR polymer) for the BHL polymer, the H0 displacement favors the formation of direct emulsions. It would explain the growth of the O/W domain upon both addition of PR polymer in the dark and irradiation, which decreases the PR hydrophobicity. At the molecular level, the displacement of the BHL polymer by the PR polymer would be a plausible cause of perturbation of the interfacial properties. Another tentative explanation would focus rather on whether the BHL polymer/PR polymer interactions in the bulk modify the activity of the BHL polymer itself. In an attempt to distinguish between the two hypotheses, we considered some bulk properties of polymers likely to be modified in the case of the polymer/polymer interaction such as the distortion of the visible spectrum29 and changes of the isomerization kinetics.30 At the present time, our measurements of a possible variation of the BHL polymer solubility in the presence of the PR polymer are not conclusive although further work is in progress. In addition, we found that the photochromic properties of the PR polymer were not sensitive to the presence of the BHL polymer. Both in the presence and absence of the BHL polymer, the UV absorption spectrum of the PR polymer exhibits the two characteristic absorption bands of the azobenzene groups centered at 347 and 440 nm.27,29 Upon an irradiation of the samples at 365 nm, the spectrum is distorted in the conventional manner (presence of two isosbestic points at 423 and 295 nm, drop of the absorption peak at 347 nm and increase around 440 nm) reflecting the usual displacement of the trans to cis isomers of azobenzene.27,29 Isosbestic points are consistent with the absence of any other states of the chromophores such as azobenzene/BHL polymer complexes or photodegraded groups. Not only was bulk isomerization not prevented, but the final stationary states and the kinetics of transitions remained essentially insensitive to the BHL polymer (Figure 3). Assuming a cis/trans extinction coefficient ratio equal to 0.055 at 347 nm,29 the stationary absorbance plateaus of absorbance reached upon a continuous irradiation at either 365 or 436 nm correspond to 20 or 80 mol % of trans isomers respectively, as generally reported for aqueous solutions of azobenzene-modified polymers (Figure 3).27,29 The two types of irradiation processes follow a first-order kinetic equation similar to that of the PR polymer alone.30 The corresponding kinetic constants measured in the presence and absence of the BHL polymer do not differ by more than the uncertainty (k365 ) (0.8 ( 0.1) × 10-2 s-1 and k436 ) (2.8 ( 0.2) × 10-2 s-1 for CPR ) 0.012%). After removal of the light source, the thermal relaxation toward the all-trans state takes about 1 day irrespective of the presence or absence of the BHL polymer (Figure 4). The results thus demonstrate that the photochromic properties of the aqueous mixture of the two polymers are similar to those of the PR polymer solution. The knowledge of the photoisomerization and relaxation rates allow the determination of the fraction (29) Morishima, Y.; Tsuji, M.; Kamachi, M.; Hatada, K. Macromolecules 1995, 28, 2867. (30) Haitjema, H. J.; Tan, Y. Y.; Challa, G. Macromolecules 1995, 28, 2867.
Porcar et al.
Figure 3. Reversible cis-trans isomerization of azobenzene as reflected by the 347 nm absorbance of PR polymer solutions maintained under irradiation. Samples were submitted to alternating wavelengths of 365 and 436 nm, in the absence (hollow circles) and in the presence (filled diamonds) of the BHL polymer (at 2 wt %). In both samples, the concentration of the PR polymer is 0.012% and the NaNO3 concentration is 1.2 M. Prior to the optical density measurements at 347 nm, the aqueous phase containing the BHL polymer prepared as detailed in the text was filtered through a 0.2 µm Millipore syringe filter to remove the insoluble part of the BHL polymer. The amount of the cis isomer (%) was calculated using the following equation: cis % ) (1 - OD(t)/OD(t0))/(1 - cis/trans). OD(t0) is the optical density of the 100% trans isomer (darkadapted sample), OD(t) is the optical density of the solution at time t, and the cis/trans extinction coefficient ratio, cis/trans, is 0.055 at 347 nm.
Figure 4. Relaxation in the dark after irradiation at 365 nm in the absence (hollow circles) and in the presence (filled diamonds) of the BHL polymer (at 2%). In both experiments, the concentration of the PR polymer is 0.012% and the NaNO3 concentration is 1.2 M. In the presence of the BHL polymer (2 wt %), the optical density values at 347 nm were measured after filtration through a 0.2 µm Millipore syringe filter.
of the cis isomer, fcis, present in the aqueous polymer mixture right before emulsification. First, under an irradiation at 365 nm for 30 min, the PR polymer has ample time to reach the stationary state corresponding to fcis ) 0.80 (Figure 3). Second, due to the incubation time in darkness of 2 × 90 min prior to emulsification, the relaxation curve as shown in the Figure 4 should lead to fcis about equal to 0.5 right before the emulsification. Consequently, a trans to cis conversion of only 50% of the azobenzene groups allows the type of emulsion to switch. In the future, it would certainly be of interest to perform a systematic study to determine with accuracy the minimum amount of cis isomer required to change the emulsion type.
Tuning Emulsions with UV-Visible Light
Conclusion We emphasize that the present observations, although lacking a clear interpretation, are of great importance for the development of future applications in responsive emulsions. They show that very slight modifications, on a molecular level and among a few percent of the total monomers, provide enough changes to switch the emulsion type. This first report on photoresponsiveness in emulsions demonstrates the high sensitivity of such systems which are able to distinguish the small photoinduced variation in dipolar moment (trans azo at 0 D to cis isomer at 3.1 D) of about 70 pendant groups along a 2000 monomer chain. With regard to applications, the low concentration of PR-polymer additives required (0.012 wt %) provides interesting versatility in terms of formulation. It should be possible to adjust the properties
Langmuir, Vol. 17, No. 22, 2001 6909
not only by using an extremely low amount of chemicals but also in keeping the absorbance at a low value. This point allows the stimulation of very thick samples (a few centimeters in depth) since the light can pass through. It will be of interest to further examine the effect of physicochemical parameters (polymer structure, degree of isomerization of the chromophores) and the polymer-polymer interactions which are essentially unknown at the present time. This may help in optimizing the modification of interfacial properties. It will also address the question of the reversibility of the system opening a route to cycle the emulsion type by a simple switch of the wavelength on the external light source. LA010955A
