Langmuir 1998, 14, 5977-5979
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Amphiphilic Copolymers: A New Route To Prepare Ordered Monodisperse Emulsions P. Perrin Ecole Supe´ rieure de Physique et Chimie de Paris (ESPCI), CNRS, Universite´ Pierre et Marie Curie (UPMC), UMR 7615, Physico-Chimie des Polyme` res, 10, rue Vauquelin, 75005 Paris, France Received June 4, 1998. In Final Form: July 23, 1998 Highly ordered macroporous materials with uniform pore size of the micrometer scale are known to have important technological uses such as photonic band-gaps and optical stop-bands. However, they can only be produced through adequate treatment of ordered monodisperse macroemulsions. Here, we report how a small amount of a new polymeric emulsifier leads to the rapid formation of a crystalline array of oil cells surrounded by a thin layer of aqueous polymer solution using a simple shear in-situ emulsification procedure. The three-dimensional periodic structure is demonstrated by Bragg diffraction and optical microscopy.
An emulsion is a heterogeneous system consisting of at least two immiscible liquids dispersed in one another in the form of droplets of diameter exceeding 0.1 µm. Two consequences arise from the thermodynamic instability of these systems: First, a shear flow sufficiently high to overcome the interfacial tension between the two fluids is required to deform and break large droplets into smaller ones. Second, it is essential to add emulsifiers to prevent droplets from merging by coalescence. The resulting metastable dispersions give rise to a broad range of industrial applications such as foods, pharmaceuticals, or cosmetics to name but a few. However, it is important to realize that whatever the method of preparation, all emulsions generally exhibit a broad droplet size distribution. As shown recently,1 emulsification in viscoelastic media can be used to directly produce monodisperse emulsions, but this is the only exception to the rule. Furthermore, in the actual context where most applications are being submitted to stringent toxicity requirements, the amount of surfactant required to generate sufficiently high viscoelasticity (concentration larger than 40% (w/w) in the continuous phase) is large. Viscous oils can also be used to increase the viscoelasticity of the system. However, this technique restricts the field of applications. Consequently, new methods are required to generate ordered monodisperse liquid-liquid dispersions. Although techniques exist for producing a porous threedimensional periodic solid with pore diameters smaller than approximately 40 nm,2-7 there is no general method to create such materials with pore diameters comparable to optical wavelengths. The latter would have important technological uses as materials with photonic bandgaps8-10 and optical stop-bands.11 A prerequisite for the (1) Mason, T. G.; Bibette, J. Phys. Rev. Lett. 1996, 77, 3481. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (3) Beck, J. S.; et al. J. Am. Chem. Soc. 1992, 114, 10834. (4) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (5) Sayari, A. Chem. Mater. 1996, 8, 1840. (6) Khushalani, D.; Kuperman, A.; Ozin, G. A. Adv. Mater. 1995, 7, 842. (7) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (8) Yablonovitch, E. J. Opt. Soc. Am. 1993, B10, 283. (9) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Modeling the Flow of Light ; Princeton University Press: Princeton, NJ, 1995.
production of such ordered macroporosity in solid materials is the formation of ordered monodisperse macroemulsions. As shown recently in a very elegant way,12 specific treatments of ordered liquid-liquid dispersions yield solid porous materials with monodisperse spherical pores after removal of the emulsion droplets. In this work, we report a novel route for creating an almost perfect crystal of micrometer size, soft, deformable entities, using simple techniques and unusual ingredients. More specifically, we describe an approach to the preparation of ordered monodisperse macroemulsions with a method combining the use of very small quantity (4% (w/ w) in the continuous phase) of an easy-to-synthesize, nontoxic, new emulsifier and a simple shear in-situ emulsification procedure. The polymeric surfactant is a hydrophobically modified poly(sodium acrylate) and its chemical structure reads as follows:
n is the number of carbon atoms of the hydrophobic grafted moiety and t is the degree of grafting in mol %. Although polymers with a broad range of t and n were synthesized, the discussion in this paper is restricted to the macrosurfactant with t ) 10% and n ) 12. The hydrophobic alkyl chains chemically grafted onto the negatively charged polymer backbone are randomly distributed and the molecular weight of the polymer is 50 000 g/mol. Details of the grafting reaction synthesis13 as well as the viscometric properties of aqueous polymer solutions14 were reported previously. A drop of a 4% (w/w) concentrated polymer aqueous solution (deionized water with a Milli-Q system from (10) Soukoulis, C. Photonic Band Gap Materials; Kluwer: Dordrecht, 1996. (11) Flaugh, P. L.; O’Donnell, S. E.; Asher, S. A. Appl. Spectrosc. 1994, 38, 847. (12) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (13) Wang, T. K.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1989, 20, 577. (14) 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.
S0743-7463(98)00654-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/16/1998
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Figure 1. Following the emulsification procedure. (A) Droplets align in the shear direction (arrows). Optical microscope scale (white bar) is 10 µm. (B) As the sample is sheared, ordered monodisperse droplets are formed as shown from Bragg diffraction rings.
Figure 2. Ordered monodisperse macroemulsions. The diffuse rings with six spots (A) reveals the presence of ordered layers of hexagonal close-packed planes of oil cells (B). Optical microscope scale is 10 µm, and shear direction is indicated by the black arrow.
Millipore) is placed between two glass slides together with an excess of oil (n-dodecane). The pH of the polymer solution is about 8 to 9 since the polyelectrolyte is fully neutralized. The viscosity of the polymer solution, measured with a controlled stress rheometer (Carrimed CS-100), is about 500 and 1000 mPa s respectively at shear rates of 1000 and 400 s-1. The sample is sheared manually by moving the slides back and forth relative to one another. The white color characteristic of the formation of an emulsion appears rapidly while the oil excess is eliminated from the liquid-liquid dispersion. Instead of the in-situ emulsification procedure, one can carry out the experiment starting with a polydisperse premixed emulsion. The alignment of the droplets in the shear direction in the course of the emulsification process is clearly shown in Figure 1A. After a few more oscillations of the slides, laser light (wavelength) 632.8 nm) coming from a beam perpendicular to the sample shear plane is scattered in such a way that Bragg diffraction rings are formed indicating ordering of the droplets as a result of the shearinduced narrow droplet size distribution (Figure 1B). With further shearing of the sample, the formation of a diffuse ring with six bright spots (Figure 2A) reveals the presence of ordered layers of hexagonal close-packed planes of oil cells (Figure 2B).15 The size of the cells can be estimated from light scattering pattern and optical microscopy photographs. The methods are in good agreement and give a value of the cell diameter equal to 3 µm in this particular experiment. In this report, the effects of
changing shear rate and gap spacing were not investigated systematically. However, it seems clear that decreasing the gap and/or increasing shear rate gives smaller size droplets in agreement with recent observations.1 The slides were moved with oscillation amplitude and rate respectively of approximately 1 and 1 cm s-1, which roughly corresponds to a shear rate of 400 s-1 with a 50 µm gap. During the emulsification process, pressure is applied on the slides while shearing the sample thereby decreasing regularly the space available to the emulsion sample. An estimate of the gap spacing (sample thickness) is optically obtained by focusing through the sample. The vertical displacement of the microscope stage is determined from the microscope fine focus, which is graduated in increments of 2 µm. Bragg scattering rings have appeared at gap spacing values of about 80 µm while lower gap spacing values ranging from 20 to 30 µm were required to obtain a Bragg diffuse ring with six spots. Hence, no more than 10 ordered crystalline planes were stacked between the glass slides. We have demonstrated that this type of emulsifier is certainly one of the most appropriate for the direct production of monodisperse emulsions. Actually, the design of the molecule offers the advantage of combining the three stabilization mechanisms of a direct emulsion: electrostatic and steric repulsions of the droplets as well as the controlled gelation of the external phase preventing the droplets from getting close to each other.16,17 The latter
(15) Paulin, S. E.; Ackerson, B. J.; Wolfe, M. S. Phys. Rev. E 1997, 55, 5812.
(16) Perrin, P.; Lafuma, F. J. Colloid Interface Sci. 1998, 197, 317. (17) Perrin, P.; Lafuma, F.; Audebert, R. Prog. Colloid Polym. Sci. 1997, 105, 2228.
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mechanism is a direct consequence of the associating behavior of this series of polymers. The main property of hydrophobically modified polymers arises from the development of high viscosity above a given polymer concentration known as the critical aggregate concentration (cac ) 1% (w/w) for this particular polymer), which depends mainly on polymer hydrophobicity and hence on t and n.14,16,17 At concentrations higher than the cac, intermolecular hydrophobic association occurs leading to the formation of a transient network and hence to an increase in the viscosity of the solution. Even at low polymer concentration, the presence of hydrophobic interchain aggregates gives the dispersion sufficiently high viscosity to create ordered monodisperse emulsions through shear-induced fragmentation of the droplets. As discussed above, increasing the length of the grafts (n) and/or the degree of grafting (t) decreases the cac of the copolymers. Hence it is reasonably possible to synthesize more hydrophobic copolymers (but still water soluble) to produce ordered monodisperse macroemulsions at even lower concentration than 4% (w/w). For instance, a polymer (M ) 50 000) with t ) 10% and n ) 18 (or t ) 20% and n ) 12) could lead to the formation of ordered monodisperse emulsions at concentrations as low as 1 or 2% (w/w). Increasing the molecular weight of the polymer is an even simpler possibility for obtaining a three-dimensional periodic structure in the dispersion at a lower emulsifier concentration. Hence, amphiphilic associating copolymers achieve emulsion organization at much lower concentrations than conventional surfactants. Furthermore, unlike small-molecule surfactants, grafted polymers have the advantage that their amphiphilic properties can be nearly continuously tuned during synthesis by adjusting molecular weight, composition, or architecture.18 For a given polymer structure, they can also be monitored by means of external parameters such as ionic strength or pH. In general, the oil-water-hydrophobically modified poly(sodium acrylate) system described here thus represents only a special case of a more general category of structuring molecules that could be used under suitable conditions to produce ordered monodisperse macroemulsions regardless of oil or emulsion type. As an example, we believe that this important step toward the design of efficient unconventional, nonpolluting19 emulsifiers could be extended without major difficulty to the case of biodegradable long (18) Perrin, P.; Monfreux, N.; Lafuma, F. Colloid Polym. Sci., in press. (19) Toxicity tests were performed at the Schlumberger-Dowell Research Center on Fluids Chemistry in St Austell (Cornwall, United Kingdom). According to the British law for oil field applications in the North Sea, the copolymer was found to be nontoxic.
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Figure 3. Proposed film structure. In this qualitative model, hydrophobic groups (thick lines) extending into oil droplets are responsible for the adsorption of the emulsifier at the interface. Since the polymer concentration is above cac, the presence of oil-containing interchain hydrophobic aggregates (circles) is a reasonable assumption. The randomly distributed hydrophobic alkyl chains and the negatively charged hydrophilic backbone are respectively represented by thick and thin lines. Short lines are negative charges.
molecules (modified natural polymers an example) in order to broaden the field of applications. This experiment also gives unexpected striking evidence of the higher emulsifying potential of macromolecules over small-molecule surfactants even under packing constraints since the polymer chains are confined in the thin water layer separating oil droplets (Figure 2B). At the present time, the structure of the liquid film protecting the droplets from coalescence is still unknown. Nevertheless, recent studies have clearly demonstrated the exceptional resistance of the film to breaking.16,17 Qualitatively, the arrangement of the polymer chains in the interstitial phase, suggested in Figure 3, is certainly a reasonable model. Note that there is no attempt within this simple description (Figure 3) to give any quantitative information regarding the morphology of the aqueous phase (examples are the density of hydrophobic aggregates in the film or surfaces occupied by grafts at interfaces). Following the same idea, this work also provides a convenient model system for fundamental studies of copolymer adsorption at liquid-liquid interfaces. Acknowledgment. The author thanks F. Lafuma, I. Iliopoulos, F. Lequeux, A. Khokhlov, P. G. De Gennes, P. Chaikin and J. Prost for helful discussions and critical reading of the manuscript. To the memory of Dr. Audebert who has intensively contributed to the development of associating polymers. LA980654+
