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Gelled Polymerizable Microemulsions. 1. Phase Behavior Cosima Stubenrauch,*,† Renate Tessendorf,†,‡ Reinhard Strey,‡ Iseult Lynch,§ and Kenneth A. Dawson§ Schools of Chemical and Bioprocess Engineering and of Chemistry and Chemical Biology, UniVersity College Dublin, Belfield, Dublin 4, Ireland, and Institut fu¨r Physikalische Chemie, UniVersita¨t zu Ko¨ln, Luxemburger Strasse 116, 50939 Ko¨ln, Germany ReceiVed March 9, 2007. In Final Form: April 22, 2007 Microemulsions are gaining increasing importance as templates since a great deal is known about how to tune their structure and the size of the domains. The concept of synthesizing a bicontinuous high surface area polymer is well-known, by “arresting” the oil (water) phase and polymerizing the water (oil) phase. However, a general route for the 1:1 replication of the bicontinuous structure has not been found yet. Our approach to achieving this goal entails arresting the oil phase by gelling it, i.e., by forming an organogel, and polymerizing the aqueous phase. The ternary base system water-n-dodecane-Lutensol AO5 (technical-grade nonionic n-alkyl polyglycol ether with an average molecular structure of C13/15E5) was chosen, and the organogelator 12-hydroxyoctadecanoic acid (12-HOA) as well as a polymerizable aqueous phase containing the monomer N-isopropylacrylamide (NIPAm) and the cross-linker N,N′-methylenebisacrylamide (BisAm) were added. To understand the influence of adding 12-HOA to the oil and NIPAM + BisAm to the aqueous phase on the phase behavior, phase diagrams were determined after each compositional change. The respective phase diagrams are presented and discussed in terms of their potential use as templates for new high surface area polymers.
1. Introduction Microemulsions (thermodynamically stable complex fluids containing oil, water, and surfactant) can serve as templates to obtain high surface area materials, as they are structured on a nanometer scale with domain sizes of ξ ≈ 5-50 nm, which corresponds to surface areas of 300-30 m2 g-1. Two out of countless possible microemulsion structures are discrete particles of one phase dispersed in the second (oil droplets in water or vice versa) or a bicontinuous structure consisting of two equal subphases, namely, oil and water.1,2 The tuning parameter for the structure is the curvature H of the surfactant monolayer. The structure and the size of the microemulsion domains can be controlled very easily, so microemulsions are gaining increasing importance as templates. In general, using microemulsions as templates should lead to a material with a high surface area and a structure equal to the structure of the template. This material would be suitable for a range of applications that require rapid and responsive sampling, selective separations, high-throughput catalytic processing, or enhanced chemical activity. The successful use of droplet microemulsions as templates for metallic nanoparticles3,4 automatically suggests the use of microemulsions as templates for polymers of various nanostructures. However, there are considerable practical problems in trying to replicate microemulsions via polymerization. First, surfactant monolayers are not rigid enough to preserve the original shape during polymerization. Second, structural changes in a microemulsion are much faster (∼1 µs) than the polym* To whom correspondence should be addressed. E-mail: cosima.
[email protected]. Phone/fax: +353-1-716-1923/1177. † School of Chemical and Bioprocess Engineering, University College Dublin. ‡ Universita ¨ t zu Ko¨ln. § School of Chemistry and Chemical Biology, University College Dublin. (1) Sottmann, T.; Strey, R. In Fundamentals of Interface and Colloid Science; Lyklema, J., Ed.; Elsevier: Amsterdam, 2005; Vol. 5, pp 5.1-5.96. (2) Strey, R.; Jahn, W.; Porte, G.; Bassereau, P. Langmuir 1990, 6, 1635. (3) Capek, I. AdV. Colloid Interface Sci. 2004, 110, 49. (4) Uskokovic´, V.; Drofenik, M. Surf. ReV. Lett. 2005, 12, 239.
erization reaction (∼1 ms per step).5 In other words, the template structure changes continuously during the polymerization because the system has enough time to respond to the compositional and volume changes caused by the consumption of monomer.6 In the worst case, a phase separation occurs during the polymerization, as reported by Holtze, who tried to encapsulate water droplets in a polymer matrix but obtained a polymer and an expelled aqueous phase instead.7 In the best case, a polymer with structures much larger than those of the original template (i.e., the microemulsion) is obtained. A prominent example of the “best case” is the polymerization of oil-in-water droplet microemulsions to obtain latex nanoparticles. Usually the resulting particles are 5-10 times larger than the templating microemulsions.6,8,9 However, in the case of polymerizing bicontinuous microemulsions, the increase in structure size can be as much as 50-fold, and so far only macroporous gels with small surface areas have been reported.6,10 In other words, polymerizing bicontinuous microemulsions with domain sizes of ξ ≈ 10-50 nm leads to polymers with pore sizes of 0.5-2.5 µm. How can we address this problem? As mentioned above, the problem is mainly due to the different time scales of structural changes in a microemulsion during polymerization. Thus, the challenge is to slow or even “arrest” 11,12 the structural changes (5) One way of synthesizing metallic nanoparticles via microemulsions is to prepare two microemulsions of equal size, one containing a metal salt and the other one the reducing agent. Mixing these two microemulsions immediately leads to the reduction of the metal salt and thus to the formation of the respective metallic nanoparticles. The reason why the final nanoparticle size nearly equals the initial droplet size is the fact that the exchange of reactants is faster than the structural changes of the microemulsion (∼1 µs). It is only in this case that the microstructure can be replicated. (6) Hentze, H. P.; Co, C. C.; McKelvey, C. A.; Kaler, E. W. Top. Curr. Chem. 2003, 226, 197. (7) Holtze, C. H. W. Ph.D. Thesis, MPI Golm, Germany, 2004. (8) Lade, O.; Beizai, K.; Sottmann, T.; Strey, R. Langmuir 2000, 16, 4122. (9) Co, C. C.; de Vries, R.; Kaler, E. W. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001; pp 455-469. (10) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1995, 36, 2637. (11) Joosten, J. G. H.; Gelade, E. T. F.; Pusey, P. N. Phys. ReV. A 1990, 42, 2161. (12) Dawson, K. A. Curr. Opin. Colloid Interface Sci. 2002, 7, 218.
10.1021/la700685g CCC: $37.00 © 2007 American Chemical Society Published on Web 06/09/2007
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that occur during the polymerization. One way to “capture” the template’s structure is the use of polymerizable surfactants. In this case the time scales of structural change and polymerization are expected to be similar as it is the surfactant itself that is polymerized. Mixtures of polymerizable surfactants were used to template micelles, lyotropic mesophases, and water-in-oil microemulsions, and it was shown that the structure of the templates was retained after polymerization.13 However, this approach cannot be used for the replication of a bicontinuous microemulsion. In this case not only the separating surfactant layer but also one of the subphases (oil or aqueous phase) needs to be polymerized. The only successful 1:1 replication of a bicontinuous microemulsion to date was achieved using bicontinuous microemulsion glasses as templates.14,15 The microemulsion glass was obtained by replacing water with a concentrated sugar solution. This sugar-based microemulsion contains a liquid monomer, and photopolymerization led to an almost 1:1 replica of the template.15 The advantage of this route is the fact that the original microemulsion glass can easily be removed after the polymerization by simple dissolution of the sugar template. Moreover, no organic solvents are involved in the process, only easily recyclable components. However, there are also three important disadvantages: the technique is restricted to the synthesis of oil-soluble polymers only, a nearly water-free polar phase is needed which requires a complicated dehydration procedure, and due to the glassy, highly viscous state of the polar phase, studying the phase behavior of the respective microemulsions is very time-consuming. In the present paper we suggest an alternative, more flexible route toward the synthesis of high surface area polymers using bicontinuous microemulsions as templates. The principal idea is gelation instead of glassification of the templating phase and polymerization of the remaining low-viscous phase (see section 3 for details). If this route turns out to be successful, it will open up a completely new research area, as it can be used for the synthesis of both oil- and water-soluble polymers. In other words, gelled microemulsions have the potential to become a general route for the synthesis of high surface area polymers. In this work, the primary interest is in water-soluble polymers which could be used in medical and pharmaceutical applications. For example, responsive macro- or microporous polymers are commonly the material of choice for controlled drug release. A prominent example of such a polymer is poly(N-isopropylacrylamide) (p-NIPAm). An advantage of p-NIPAm is its structural versatility as it can be easily modified with a wide variety of functional groups, making it responsive to a large number of external stimuli.16 In response to environmental changes (e.g., temperature, pH, light, electrical current) a coil-globule phase transition takes place, which results in dramatic changes in both the volume and the surface area (swelling-shrinking transition). Having access to responsive nanoporous p-NIPAm with a high surface area would allow us to accelerate the response significantly. Moreover, the responsiveness could be used to alter the spacing between the surface groups and to act as an “on-off” switch for surface processes that require intimate contact between neighboring species. With biologically active surface groups we would automatically be in the arena of biomimetic materials. Our aim in the present paper is to describe a synthetic route toward high surface area p-NIPAm. To achieve this, it was first necessary to find a suitable ternary base system. The system of (13) Summers, M.; Eastoe, J.; Davis, S.; Du, Z.; Richardson, R. M.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2001, 17, 5388. (14) Gao, F.; Ho, C.-C.; Co, C. C. J. Am. Chem. Soc. 2004, 126, 12746. (15) Gao, F.; Ho, C.-C., Co, C. C. Macromolecules 2006, 39, 9467. (16) Lynch, I.; Dawson, K. A. Macromol. Chem. Phys. 2003, 204, 443.
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Figure 1. Molecular structures of the gelator, the surfactant, the monomer, and the cross-linker (from the top to the bottom).
choice became H2O-n-dodecane-Lutensol AO5 (technicalgrade nonionic n-alkyl polyglycol ether with an average molecular structure of C13/15E5). After the phase behavior of the base system was studied, water was replaced by a polymerizable aqueous phase containing the monomer N-isopropylacrylamide (NIPAm) and the cross-linker N,N′-methylenebisacrylamide (BisAm). The influence of this exchange on the phase behavior was carefully investigated. Finally, we added the organogelator 12-hydroxyoctadecanoic acid (12-HOA) to the oil phase and again investigated the phase behavior of the resulting system. On the basis of these systematic studies, we are now able to specify the conditions under which a clear, gelled phase, located in the onephase region of our model microemulsion, is formed. It is this microemulsion that will be used as the template to synthesize water-soluble, responsive p-NIPAm with as large a surface area as possible. 2. Experimental Section 2.1. Materials. The gelator 12-HOA, the monomer NIPAm, and the cross-linker BisAm were purchased from Acros Organics. According to the manufacturer, these chemicals have a purity of 99%. BisAm and 12-HOA were used as received; NIPAm was recrystallized twice from n-hexane. The technical-grade surfactant Lutensol AO5 (nonionic n-alkyl polyglycol ether with an average molecular structure of C13/15E5) was donated by BASF. n-Dodecane (purity of 99%) was purchased from Sigma-Aldrich. The water was purified by a Milli-Q system or alternatively doubly distilled. The molecular structures are shown in Figure 1. 2.2. Phase Diagram of the Organogel. A binary mixture of n-dodecane + 12-HOA was weighed into a test tube and sealed with a polyethylene stopper. The sol-gel transition temperature was determined in a water bath with a precision of (1 K. As the sol-gel transition temperature depends on the gelator concentration a dilution series was carried out. The mass fraction of 12-HOA in the oil phase is given by β (all compositional definitions are given in section 2.3). The transition temperatures obtained for different gelator concentrations are shown in Figure 2.
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3. General Concept
Figure 2. Sol-gel transition temperature T of the binary system n-dodecane-12-HOA as a function of the gelator mass fraction β. 2.3. Phase Diagrams of the Microemulsions. The masses of the components water (A), oil (B), surfactant (C), monomer (NIPAm), cross-linker (BisAm), and gelator (12-HOA) are denoted m(A), m(B), m(C), m(NIPAm), m(BisAm), and m(12-HOA), respectively. The compositions of the samples are given by the mass fraction of one component in the total, such that the overall oil fraction is given by the mass of oil in the water plus oil mixture, as R)
m(B) m(A) + m(B)
(1)
The overall mass fraction of the surfactant is γ)
m(C) m(total)
(2)
The mass fraction of NIPAm + BisAm in the aqueous phase is ψ)
m(NIPAm) + m(BisAm) m(A) + m(NIPAm) + m(BisAm)
(3)
The mass fraction of 12-HOA in the oil phase is β)
m(12-HOA) m(B) + m(12-HOA)
(4)
Various amounts of the components were weighed into test tubes containing a magnetic stirring bar and sealed with polyethylene stoppers. All samples were prepared at a 1:1 water-to-oil ratio, i.e., at R ) 0.5. The mass fraction ψ of NIPAm + BisAm in the aqueous phase was also kept constant throughout the study, namely, at ψ ) 0.07. For microemulsions containing NIPAm and BisAm the aqueous phase was prepared separately to ensure that the monomers were completely dissolved, and required amounts of this aqueous phase were added. For microemulsions containing the gelator, the gelator was first dissolved in n-dodecane and then the gelator-containing oil phase was weighed into the test tube. The resulting mixture was heated until the sample had a low viscosity, i.e., until the sol-gel transition temperature was reached, and stirred for several minutes. To speed the gelation process, the homogenized sample was then cooled in a freezer until it was completely gelled, before being taken out of the freezer and put into the water bath for phase studies. The phase behavior of all microemulsions was studied as a function of the temperature T and the total surfactant concentration γ. Phase boundaries were determined with a precision of (0.5 K. The resulting phases were characterized by visual inspection in transmitted light. Crossed polarizers were used to detect the presence of anisotropic phases. All phase boundaries are found to be reversible.
To use microemulsions as templates, the respective phase behavior needs to be studied initially. At constant pressure, the phase behavior of ternary systems can be represented in an upright phase prism with an isothermal Gibbs triangle, H2O (A)-oil (B)-nonionic surfactant (C) as the base and the temperature T as the ordinate.17 To investigate the phase behavior, sections are made through the phase prism, usually at constant water-to-oil ratio R or at constant surfactant concentration γ (see ref 1 and references therein). In the present study we are primarily interested in the former section, i.e., in phase diagrams measured at equal amounts of water and oil (R ) 0.5) as a function of the temperature T and the surfactant concentration γ. A schematic drawing of such a section is shown in Figure 3. Due to the shape of the three-phase body and the attached one-phase region, the phase diagram is known as a “fish” cut.17 At low temperatures an oil-in-water microemulsion coexists with an excess oil phase (denoted by 2), whereas at high temperatures a water-in-oil microemulsion in contact with an excess water phase (2h) is observed. At intermediate temperatures and low surfactant concentrations (γ0 < γ < γ˜ ) a bicontinuous microemulsion coexists with an excess water and an excess oil phase (3). By increasing γ, more water and oil can be solubilized by the microemulsion phase, as is indicated by the test tubes in Figure 3. At the characteristic surfactant concentration γ˜ the system is able to completely solubilize water and oil; hence, a one-phase microemulsion appears (1). The point where the threeand the one-phase regions meet is called the X ˜ point, which is defined by the coordinates γ˜ and T˜ . This point is of the utmost importance as it represents the minimum surfactant concentration needed to form a one-phase microemulsion and is thus a measure of the efficiency of a microemulsion. A number of methods have been applied to shed light upon the relation between the phase behavior and the microstructure of microemulsions, and a general understanding was finally obtained.1,18,19 Of importance for the present study is the fact that for R ) 0.5 the structure of the microemulsion is bicontinuous at concentrations and temperatures next to the X ˜ point. At T < T˜ and γ > γ˜ the system tends to form oil droplets in a continuous aqueous phase, while at T > T˜ and γ > γ˜ water droplets in a continuous oil phase are the preferred structure as is illustrated in Figure 3. In other words, having the phase diagram for R ) 0.5, one knows the conditions under which the microstructure is bicontinuous, which, in turn, is our targeted template and polymer structure. The final aim is to provide a synthetic route for both oil- and water-soluble polymers with surface areas that are as large as possible. Once this route is found, one can start tuning the properties of the resulting polymers according to specific needs. While the general route is clear, namely, formulating a bicontinuous microemulsion in which the templating phase is in an “arrested” state while the polymerization takes place in the remaining low viscous phase, the experimental details are not known yet. One experimental approach was given above, namely, arresting the aqueous phase by replacing water with sugar and polymerizing the oil phase.14,15 However, due to the limitations of this procedure we consider a different route to be the most promising. Instead of using a microemulsion glass, the aim is to use a microemulsion gel as the template. In other words, the water (oil) phase is arrested via gelation and not via glassification, thereby producing a hydrogel (an organogel). This approach has (17) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654. (18) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (19) Burauer, S.; Belkoura, L.; Stubenrauch, C.; Strey, R. Colloids Surf., A 2003, 228, 159.
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Figure 3. (Left) Section through the phase prism of a ternary system, H2O-alkane-CiEj, at a 1:1 water-to-oil ratio (R ) 0.5) as a function of the surfactant mass fraction γ and the temperature T. The X ˜ point is defined by the surfactant concentration γ˜ for which at T˜ the one-phase region (1) is reached. γ0 corresponds to the monomeric solubility of the surfactant in the solvents. 2, 3, and 2h denote water-, surfactant-, and oil-rich microemulsions in equilibrium with the corresponding excess phase(s). The test tubes show the number of coexisting phases at different T and γ values. The figure was taken from ref 19 and slightly modified. (Right) Three typical microstructures formed in the one-phase region: oil-in-water droplets (T < T˜ ), water-in-oil droplets (T > T˜ ), and bicontinuous microemulsions (T ≈ T˜ ).
4. Results and Discussion
Figure 4. Scheme of the general concept to use microemulsions as templates for new high surface area polymers.
three advantages over the use of microemulsion glasses. First, it can be used for the synthesis of both oil- and water-soluble polymers. The only requirement is to identify an appropriate gelator for the oil and the water phases, respectively. Second, only 1-4 wt % gelator is needed, thus avoiding the complicated dehydration procedure required in the case of the sugar template mentioned above. Third, as will be shown below, the phase behavior of the templating microemulsion is much easier to study, especially at temperatures above the sol-gel transition. After having found a suitable ternary base system, the general steps of the proposed templating route are as follows (see Figure 4): (1) replace water (oil) by a polymerizable aqueous (oil) phase, (2) arrest the oil (aqueous) phase via the formation of an organogel (a hydrogel), (3) polymerize the aqueous (oil) phase, and (4) remove the organogel (hydrogel). While the first two steps require intense phase studies, the last two are not difficult from an experimental point of view. However, the characterization of the templating gelled microemulsion and of the resulting polymer is challenging. In the following sections the first very promising results toward the synthesis of watersoluble polymer gels with a high surface area are presented.
4.1. Phase Diagram of the Ternary Base System. Before measurement of the phase diagram of the ternary base system, the three components of this system had to be identified. Finding a suitable ternary system means identifying a suitable oil and a suitable surfactant, assuming that water is the polar phase. As in the present paper we aim to describe how to use microemulsions efficiently as templates, the steps involved in obtaining the final templating microemulsion are described in detail. The first challenge was to find an oil with which an organogel can be formed. The search for a suitable oil automatically includes the search for a suitable gelator. Although countless organogelators are known,20 our choice was limited by two requirements: First, the resulting organogel had to be clear, as the turbidity of the sample is used to distinguish between a onephase and a two- or three-phase region. If the gelled oil phase were turbid simply because the organogel is turbid, it would not be possible to locate the one-phase microemulsion region visually. Second, the addition of the gelator must not destroy the microemulsion. Phase studies with Kraton G-1650, a commercially available triblock copolymer often used for the formation of organogels, revealed that microemulsions are not formed in the presence of this polymer. This could simply be due to the fact that the polymer is too large to fit into the unpolar microdomains of the microemulsion.21 Thus, as polymers cannot be used as gelators for our purpose, a low molecular mass gelator22 had to be found. A promising candidate was the gelator 12HOA. Depending on the concentration of 12-HOA and on the type of solvent (aromatic solvent, alkane, chlorinated solvent, etc.), the respective organogels are transparent or turbid.23 The (20) Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2006. (21) A similar phenomenon has been observed with amphiphilic block copolymers, the presence of which induces a phase separation if the polymer block(s) are larger than the domain size(s) of the microemulsion (Frank, C.; Strey, R.; Schmidt, C.; Stubenrauch, C. J. Colloid Interface Sci. 2007, 312, 76). (22) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133. (23) Terech, P. Colloid Polym. Sci. 1991, 269, 490.
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Figure 5. “Fish diagrams” of the ternary base system H2O-ndodecane-C13/15E5 (dark gray circles) and the polymerizable system H2O/NIPAm/BisAm-n-dodecane-C13/15E5 (black circles) with an NIPAm + BisAm content of 7 wt % (ψ ) 0.07) in the aqueous phase. All diagrams were measured at R ) 0.5. The distorted shape of the phase diagram is due to the use of a technical-grade surfactant.
final oil of choice was dodecane, and the organogel of choice was dodecane + 12-HOA, which fulfilled the requirements for formation of a clear gel and the possibility to formulate a microemulsion using this binary system as the oil phase. Having chosen the oil, we still needed to find a suitable surfactant. As large amounts of surfactant were expected to be needed for systematic phase studies, we searched for a technicalgrade surfactant to reduce the costs. Moreover, the surfactant had to be chosen such that the one-phase region of the microemulsion is located at temperatures below the sol-gel transition of the organogel. As a rough estimate we took the sol-gel transition temperature of the binary organogel (see Figure 2) and thus needed to find a technical-grade surfactant that, together with water and dodecane, forms a one-phase microemulsion at T < 65 °C. (Note that it was not known to what extent the gelation in general and the sol-gel transition temperature in particular would be affected if the organogel were no longer a binary system but part of the microemulsion.) The surfactant of choice was the technical-grade nonionic n-alkyl polyglycol ether Lutensol AO5 (BASF), the overall molecular structure of which is C13/15E5. This surfactant was chosen as the phase behavior of a similar technical-grade surfactant, namely, Marlowet EM50 (Sasol), with an overall molecular structure of C12/13E5, has already been studied, and it was found that the one-phase region of water-n-dodecane-Marlowet EM50 is located at T < 55 °C.24 Choosing a surfactant with an even longer hydrophobic group was expected to lead to a shift to lower temperatures. The ternary base system chosen is H2On-dodecane-C13/15E5. As explained in section 3, the phase diagram of primary interest is the so-called fish diagram, particularly the one-phase region behind the X ˜ point where polymerizations will be performed. Thus, only parts of the total fish diagram were measured in all cases. The respective part of the ternary base system is shown in Figure 5. Comparing Figure 5 with Figure 3, one clearly sees that the shape of the experimentally determined phase diagram is heavily (24) Sottmann, T.; Lade, M.; Stolz, M.; Schoma¨cker, R. Tenside, Surfactants, Deterg. 2002, 39, 20.
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distorted. This distortion appears whenever the system contains more than one surfactant or whenever other surface-active components are present. Hence, it is a general feature of technicalgrade surfactants. The reason for the distortion is well-known and understood. In the case of the chosen technical-grade C13/15E5, it is the broad distribution of the ethoxylated head groups that causes the distortion toward high temperatures T at low surfactant concentration γ. The smaller the head group, the less hydrophilic the surfactant and thus the higher its oil solubility. The addition of oil and water to the microemulsion (i.e., a decrease of γ) leads to an extraction of the more oil soluble molecules from the interface, rendering the remaining surfactant mixture effectively more hydrophilic than the base system. In other words, the smaller the γ, the more hydrophilic the surfactant mixture which forms the interfacial film. According to the general properties of nonionic microemulsions (see, e.g., Figure 5.8 in ref 1), a shift of the phase diagram toward higher temperatures is expected with increasing hydrophilicity of the surfactant or the surfactant mixture. This is exactly what is seen in Figure 5. More detailed discussions can be found in refs 1 and 25-28. Coming back to the results for the ternary base system, one sees that the coordinates of the X ˜ point are γ˜ ) 0.08 and T˜ ) 55 ( 1 °C; i.e., the minimum amount of surfactant required to solubilize equal amounts of water and oil is around 8 wt % at a temperature of 55 °C. Surprisingly, T˜ is approximately the same as that observed for the short-chain homologue Marlowet EM50, while γ˜ is lower than that of Marlowet EM50 (γ˜ ) 0.11) as expected. We will come back to these coordinates in connection with the phase diagrams obtained for the other microemulsion systems. 4.2. Phase Diagram of the Polymerizable Microemulsion. Having measured the ternary base system, water was replaced by a polymerizable aqueous solution containing the monomer NIPAm and the cross-linker BisAm. The respective phase diagram of the new microemulsion system was studied at a fixed NIPAm + BisAm content of 7 wt % (ψ ) 0.07) in the aqueous phase. This concentration was chosen because in one of our previous studies p-NIPAm hydrogels with ∼7 wt % monomer were synthesized and the properties of the resulting macroporous hydrogels were studied.16 Thus, a reference system for comparing the properties of our new nanoporous hydrogels would be available if the synthesis of the latter were successful. Additionally, the concentration of polymer must be sufficient to form a single connected network upon polymerization. The phase diagram of the polymerizable microemulsion is also shown in Figure 5. Comparing the ternary base system (ψ ) 0) with the polymerizable system, one sees that the phase diagram of the latter is shifted toward higher T and higher γ compared to that of the base system. Consequently, the coordinates of the X ˜ point are also affected. In the presence of 7 wt % NIPAm + BisAm the minimum amount of surfactant required to solubilize equal amounts of water and oil is γ˜ ) 0.13 at T˜ ) 55.0 °C, while it is γ˜ ) 0.08 at T˜ ) 55.0 °C for the base system. Thus, the surfactant efficiency slightly decreases, while the so-called mean temperature T˜ remains the same. The addition of NIPAm + BisAm to the aqueous phase decreases the efficiency because the hydrophilicity of the aqueous phase decreases. The same observation has been made for microemulsions containing formamide in the aqueous phase.29 In ref 29 not only a slight increase in γ˜ but also a (25) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (26) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 10313. (27) Yamaguchi, S.; Kunieda, H. Langmuir 1997, 13, 6995. (28) Stubenrauch, C.; Paeplow, B.; Findenegg, G. H. Langmuir 1997, 13, 3652. (29) Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 742.
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significant increase in T˜ was reported. The fact that T˜ stays constant in the present system is due to the distorted shape of the one-phase region (see the discussion in section 4.1), i.e., due to the use of a technical-grade surfactant, which was not the case in ref 29. However, although T˜ is not changed, the addition of NIPAm + BisAm to the aqueous phase shifts both phase boundaries of the one-phase region (2-1 and 1-2h) about 10 K toward higher temperatures, in agreement with the observations made for the formamide-containing system. 4.3. Phase Diagrams of Gelled Polymerizable Microemulsions. After replacement of water by a polymerizable aqueous phase and measurement of the corresponding phase diagram, the next step was to arrest the oil phase, i.e., to form an organogel. As described above, the challenge was to find a suitable gelator such that the resulting organogel is clear (to be able to perform reliable phase studies) and that the addition of the gelator does not destroy the microemulsion. The gelator 12-HOA was found to fulfill these requirements and was thus used to gel the oil phase of the microemulsion. The required gelator concentration was estimated from the phase diagram of the binary system. As can be seen in Figure 2, at β > 0.01 the sol-gel transition takes place at T > 60 °C, i.e., above the one-phase region of the polymerizable microemulsion. To be on the safe side, we added 1.8 wt % (β ) 0.018) gelator to the oil phase and measured the resulting phase diagram. The result is presented in Figure 6 (top). The comparison of the phase diagrams measured at β ) 0.018 and β ) 0 leads to three important general observations. First, the location of the fish and thus of the targeted bicontinuous structure is significantly affected by the addition of 12-HOA, which shows again that phase studies are indispensable. Second, the one-phase region is shifted to lower temperatures and lower surfactant concentrations, which means that the efficiency of the microemulsion is increased. In the presence of 1.8 wt % gelator the minimum amount of surfactant required to solubilize equal amounts of water and oil is γ˜ ) 0.10 at T˜ ) 45.0 °C, while it is γ˜ ) 0.13 at T˜ ) 55.0 °C for the gelator-free system. This observation can be explained by the fact that the gelator is a surface-active fatty acid and thus has an effect similar to that of adding a long-chain alcohol to a microemulsion.26 Third, the sol-gel transition takes place at temperatures that are ∼40 °C lower compared to those of the binary organogel at the same β. A possible explanation for the latter observation could be that the gelator is distributed among the aqueous phase, the oil phase, and the interface such that the effective gelator concentration in the oil phase is much smaller than 1.8 wt %. However, according to Figure 2, at β ) 0.004 (0.4 wt %) an organogel is formed at T < 47 °C. To explain our observation, around 80% of the total gelator concentration must be dissolved in the aqueous phase and/or adsorbed at the interface, which is very unlikely. Another explanation could be that the gel is not able to form within the confined geometry of a bicontinuous microemulsion. While the domain sizes of the latter are in the nanometer range, the mesh sizes of a gel are in the micrometer range. In any case, the amount of gelator is too low to form an organogel either because the gelator was mainly dissolved in the aqueous phase or because the mesh sizes of the gel would have been too large (the lower the amount of gelator, the larger the mesh size must be). Indeed, an increase of the gelator concentration to β ) 0.025 shifted the sol-gel transition in the right direction, i.e., to higher temperatures (see Figure 6 (top)). Simultaneously, the one-phase region is shifted toward lower temperatures, resulting in γ˜ ) 0.09 and T˜ ) 39.0 °C as coordinates of the new X ˜ point. A further increase of the gelator concentration to β ) 0.029 again shifted both the sol-gel transition to higher temperatures and the one-phase region
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Figure 6. “Fish diagrams” of the polymerizable system H2O/NIPAm/ BisAm-n-dodecane-C13/15E5 (black circles) and the gelatorcontaining polymerizable system H2O/NIPAm/BisAm-n-dodecane/ 12-HOA-C13/15E5. All diagrams were measured at R ) 0.5 and ψ ) 0.07. Three different gelator concentrations were studied: β ) 0.018, 0.025, and 0.029. While the sol-gel transition is located below the one-phase region for the two lower β values (top), it occurs within the one-phase microemulsion region at β ) 0.029 (bottom). The distorted shape of the phase diagram is due to the use of a technical-grade surfactant.
to lower temperatures (γ˜ ) 0.12 and T˜ ) 38.0 °C) and thus finally resulted in the phase diagram originally aimed at. As can be seen in Figure 6 (bottom), the sol-gel transition takes place at temperatures between the 2-1 and the 1-2h phase boundaries. Thus, parts of the one-phase region are gelled. If the trends observed so far were systematic, a further increase of the gelator concentration would lead to a sol-gel transition that takes place above the one-phase microemulsion phase; i.e., the whole onephase region is gelled. Indeed, adding higher amounts of 12HOA to the oil phase shifts the sol-gel transition to higher temperatures. The phase diagrams measured for β ) 0.031 and 0.041, respectively, are seen in Figure 7. Comparing Figures 6 and 7, one clearly sees that the temperatures of the sol-gel transition increase with increasing gelator concentration as was the case for the binary organogel (see Figure 2). The sol-gel transitions for the two highest gelator concentrations (β ) 0.031 and 0.041) are indeed mainly above the one-phase region. However, the coordinates of the X ˜ point and thus the efficiency of the microemulsion seem to have reached
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additional gelator is only used for the formation of the gel network, which is stronger the higher the gelator concentration. We conclude by adding that, unfortunately, in most of the gelled regions an anisotropic liquid crystalline (LC) phase was observed, which substantially limits the γ and T range of the isotropic one-phase region and thus the range where polymerization can be carried out. To increase the γ and T range of the isotropic one-phase region, alternative gelators need to be tested. As 12-HOA forms crystalline fibrils and nodes in organogels,22,30 it is very likely that the presence of 12-HOA induces the formation of ordered liquid crystalline phases.31 Thus, a gelator that has no or a lower tendency to form crystalline microdomains or a suitable branched surfactant that suppresses the formation of LC phases needs to be found, and extensive phase studies need to be carried out. If it turns out that this is not feasible, polymerizing in the LC region could be a second option as the resulting polymer should also have a high surface area. This, however, is not the primary goal. In any case, extensive phase studies are still required.
5. Conclusion and Outlook
Figure 7. “Fish diagrams” of the gelator-containing polymerizable system H2O/NIPAm/BisAm-n-dodecane/12-HOA-C13/15E5 measured at β ) 0.031 (top) and β ) 0.041 (bottom). Both diagrams were measured at R ) 0.5 and ψ ) 0.07. In both cases the sol-gel transition is mainly located above the one-phase microemulsion region. The distorted shape of the phase diagram is due to the use of a technical grade surfactant. Table 1. X ˜ Point Coordinates of the Gelator-Containing Polymerizable System H2O/NIPAm/BisAm-n-Dodecane/ 12-HOA-C13/15E5 for All Studied Gelator Concentrations, i.e., β ) 0, 0.018, 0.025, 0.029, 0.031, and 0.042a
a
β
γ˜
T˜
0 0.018 0.025 0.029 0.031 0.041
0.13 0.10 0.09 0.12 0.09 0.09
55.0 45.0 39.0 38.0 39.0 39.0
All diagrams were measured at R ) 0.5 and ψ ) 0.07.
a limit. The X ˜ point coordinates of all gelator-containing systems are listed in Table 1. While small changes between β ) 0 and β ) 0.025 have significant effects on the location of the X ˜ point (mainly on T˜ ), an increase from β ) 0.031 to β ) 0.041 only affects the sol-gel transition. A possible explanation for this observation could be that at low β values the gelator is distributed between the bulk phases and the interface as was discussed above. Once the aqueous bulk phase and the interface are saturated, any
The phase diagrams of microemulsions containing the gelator 12-HOA in the oil phase and the monomer NIPAm and the cross-linker BisAm in the aqueous phase were studied. Addition of NIPAm + BisAm to the aqueous phase of the ternary base system water-n-dodecane-Lutensol AO5 (technical-grade nonionic n-alkyl polyglycol ether with an average molecular structure of C13/15E5) resulted in a shift of the one-phase region toward higher temperatures, due to the fact that the NIPAM + BisAm containing aqueous phase is more hydrophobic than pure water. Addition of the gelator 12-HOA to the oil phase of the system water/NIPAm/BisAm-n-dodecane-Lutensol AO5 caused a shift of the one-phase region to lower temperatures and a slight increase of the efficiency. The fatty acid is surface-active and thus has an effect similar to that of adding a long-chain alcohol to a microemulsion. Simultaneously, a gel is formed at low temperatures, while the sample has a low viscosity at temperatures above the sol-gel transition temperature. The sol-gel temperature increases with increasing amount of 12-HOA, and it was found that at a gelator mass fraction of β ) 0.029 (2.9 wt % 12-HOA in the oil phase) the sol-gel transition occurs within the one-phase region of the microemulsion, while it takes place above the one-phase region at β ) 0.031 and 0.041. In conclusion, we specified the conditions under which a clear, gelled phase located in the one-phase region of the water/NIPAm/BisAmn-dodecane/12-HOA-Lutensol AO5 microemulsion is formed. A detailed study of the microstructure is currently under way aiming at verifying that the gelled one-phase region is a gelled bicontinuous microemulsion. The complexity of the template structure arises from the fact that a microemulsion and a gel are structured on different length scales. While the domain sizes of the former are 10-50 nm, the mesh sizes of the gels are in the micrometer range. Once the gelled biocontinuous microemulsion structure is conclusively verified, the next step is to polymerize the aqueous phase, probably via a photoinitiation process. After the polymerization removal of the templating microemulsion gel will be achieved by simply raising the temperature above the sol-gel transition: the gel will be destroyed, and the gelatorcontaining phase can be removed by washing the sample, for example, with ethanol. Assuming that the gelled template has (30) Terech, P.; Rodriguez, V.; Barnes, J. D.; McKenna, G. B. Langmuir 1994, 10, 3406. (31) A similar phenomenon has been observed in mixtures of DNA and calcium zwitterionic lipids in which the presence of the stiff DNA strands induces the formation of a highly ordered rectangular columnar phase (McManus, J. J.; Raedler, J. O.; Dawson, K. A. J. Am. Chem. Soc. 2004, 126, 15966).
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a bicontinuous structure, it still remains to be determined whether the gel can arrest the structure during the polymerization so that the resulting polymer also has similar dimensions. Work to address this question is also under way. Acknowledgment. Financial support for this work was provided by the Marie Curie Research Training Networks “SelfOrganisation under Confinement (SOCON)” (Contract Number
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MRTN-CT-2004-512331) and “Arrested Matter” (Contract Number MRTN-CT-2003-504712), by the European Network of Excellence “Soft Matter Composites” (Contract Number NMP3-CT-2004-502235), by the Irish HEA (PRTLI Cycle 3), and by the Seed Funding Scheme of the University College Dublin. LA700685G
