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Gelled Polymerizable Microemulsions. 2. Microstructure Cosima Stubenrauch,† Renate Tessendorf,†,‡ Anna Salvati,§ Daniel Topgaard,| Thomas Sottmann,‡ Reinhard Strey,‡ and Iseult Lynch*,§ SFI-Strategic Research Cluster in Solar Energy ConVersion, Centre for Synthesis and Chemical Biology (CSCB), School of Chemical and Bioprocess Engineering, and School of Chemistry and Chemical Biology, UniVersity College Dublin, Belfield, Dublin 4, Ireland, Institut fu¨r Physikalische Chemie, UniVersita¨t zu Ko¨ln, Luxemburger Strasse 116, 50939 Ko¨ln, Germany, and Physical Chemistry 1, Lund UniVersity, P.O. Box 124, SE-221 00 Lund, Sweden ReceiVed March 24, 2008. ReVised Manuscript ReceiVed April 28, 2008 Using bicontinuous microemulsions as templates opens a new field for the design of novel structures and thus novel materials, but has significant challenges due to the very small composition and temperature windows in which microemulsions are bicontinuous. In previous work we had shown that we can take a ternary base system (water-ndodecane-C13/15E5), add monomer and cross-linker (N-isopropylacrylamide and N,N′-methylenebisacrylamide) to the water phase, and add a gelator (12-hydroxyoctadecanoic acid) to the oil phase while remaining in the one-phase region of the phase diagram. It was also possible to allow the gelator to form an organogel by changing the temperature such that we crossed the sol-gel line, which fell within the one-phase region. In this work, we show conclusively that addition of the monomers and the gelator does not affect the microemulsion microstructure and that, even in the gelled state, the polymerizable microemulsion is indeed bicontinuous. 1H NMR self-diffusion, conductivity, and small-angle neutron scattering measurements all confirm the bicontinuous nature of the gelled polymerizable microemulsion.
1. Introduction Bicontinuous microemulsions are nanostructures that occur in mixtures of oil, water, and surfactant, where the two liquid phases are considered to be continuous at the same time. They exist when the curvature of the surfactant film is exactly balanced, i.e., when it has a zero mean and a negative Gaussian curvature. A small shift in composition (or temperature) may result in formation of an oilin-water or water-in-oil microemulsion, where only one of the phases is continuous and the other forms discrete droplets. Characterization of microemulsion microstructures is difficult and requires a combination of several techniques.1 The experimental techniques reported to have been used to characterize microemulsion microstructure include electrical conductivity, viscosity, differential scanning calorimetry (DSC), small-angle neutron and small-angle X-ray scattering (SANS and SAXS), nuclear magnetic resonance (NMR), freeze-fracture electron microscopy (FFEM), and cryo field emission scanning electron microscopy (cryo-FESEM). Each technique provides differents often complementarysinformation regarding the type and structure of microemulsions. Unequivocal characterization of the bicontinuous nature of a microemulsion requires the simultaneous observation of three different properties: (a) the same relative self-diffusion coefficients for both the oil and the water components due to the two phases being continuous; (b) a continuous change of the electrical conductivity starting at high values [oil-in-water (o/w) microemulsion] and ending in very low values [water-in-oil (w/o) microemulsion]; (c) SANS spectra with a characteristic peak at low q values. * To whom correspondence should be addressed. E-mail: Iseult@ fiachra.ucd.ie. † School of Chemical and Bioprocess Engineering, University College Dublin. ‡ Universita¨t zu Ko¨ln. § School of Chemistry and Chemical Biology, University College Dublin. | Lund University. (1) Moulika, S. P.; Paulb, B. K. AdV. Colloid Interface Sci. 1998, 78, 99.
In our previous work we developed a polymerizable bicontinuous microemulsion system, where the oil phase was gelled.2 The objective of that study was to find a template that is strong enough to preserve the original structure during the polymerization of the water phase. The system chosen was the ternary base system water-n-dodecane-Lutensol AO5 (technical grade nonionic n-alkyl polyglycol ether with an average molecular structure of C13/15E5), the gelator for the oil phase was 12-hydroxyoctadecanoic acid (12-HOA), and the polymerizable aqueous phase contained the monomer N-isopropylacrylamide (NIPAm) and the cross-linker N,N′-methylenebisacrylamide (BisAm).2 Detailed phase diagrams were elucidated to study the effect of each of the components on the phase behavior of the microemulsion, with the following patterns emerging: Addition of NIPAm + BisAm to the aqueous phase of the ternary base system water-n-dodecane-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-ndodecane-C13/15E5 caused a shift of the one-phase region to lower temperatures and a slight increase of the surfactant efficiency. 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. The result of these phase behavior studies was the specification of the conditions under which a clear, gelled phase located in the one-phase region of the water/NIPAm/BisAm-n-dodecane/12HOA-C13/15E5 microemulsion is formed.2 However, two important questions remained from the previous study, which need to be resolved prior to undertaking the polymerization of the water (2) Stubenrauch, C.; Tessendorf, R.; Strey, R.; Belkoura, L.; Lynch, I.; Dawson, K. A. Langmuir 2007, 23, 7730.
10.1021/la800918g CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
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phase. The first is to confirm that the template, i.e., the gelled polymerizable microemulsion, is indeed bicontinuous below the sol-gel transition of the gelator. This is crucial, given that we intend to use the bicontinuous structure as a template to form high-surfacearea polymeric materials. The second is to understand the nature and structure of the organogel in the bicontinuous microemulsion, as these two structures have very different length scales. The gelator 12-HOA is known to self-assemble into long fibrils that are connected via spatially extended (pseudo)crystalline microdomains (nodes).3 The single fibrils have a diameter of 20-30 nm depending on the solvent, while the mesh sizes are in the range of micrometers depending on the amount of gelator. On the other hand, the domain sizes of a microemulsion are typically in the range 10-50 nm. Thus, we would like to understand the detailed structural arrangements in the gelled polymerizable bicontinuous microemulsion system used here. In this work, we present results from 1H NMR self-diffusion (Fourier transform pulse gradient spin echo, FTPGSE), conductivity, and SANS measurements performed on the ternary base system alone, on systems containing the monomer and the cross-linker, and on systems containing all of the components required to finally polymerize the gelled microemulsion. The complementary information we obtain from the different techniques shows very conclusively that the microstructure is indeed bicontinuous.
2. Experimental Part 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. 2.2. Phase Diagrams. In our previous study the experimental procedure to measure phase diagrams is explained in detail.2 Of importance for the present study are the definitions to characterize the samples: the masses of the individual 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 composition of the samples is given in mass fractions. Thus, it holds that the mass fraction of oil in the water plus oil mixture is
R)
m(B) m(A) + m(B)
(1)
the overall mass fraction of the surfactant in the sample 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)
and the mass fraction of 12-HOA in the oil phase is:
β)
m(12-HOA) m(B) + m(12-HOA)
(4)
All phase diagrams were measured at a 1:1 water-to-oil ratio (R ) 0.5) and at a constant mass fraction, ψ, of NIPAm + BisAm in the aqueous phase (ψ ) 0.07). To gel the microemulsion, various amounts of gelator were added to the oil phase and the phase behavior was (3) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133.
studied as a function of the temperature, T, and the total surfactant concentration, γ, for each gelator concentration, i.e., each β value. 2.3. 1H NMR Self-Diffusion (FTPGSE). FTPGSE-NMR experiments were performed on a Bruker AV-200 spectrometer operating at a 200.13 MHz 1H resonance frequency. The magnet is equipped with a Bruker DIFF-25 gradient probe capable of delivering gradients of 9.6 T m-1 in the z direction. The temperature was controlled with a precision of (0.1 °C with a B-VT 3000 temperature unit. To reduce convection in the samples (caused by the unavoidable temperature gradient in the magnet), small volumes of the samples were measured in 4 mm NMR glass tubes. In particular, for the measurement of the pure solvents and their solutions, a series of experiments as a function of the diffusion time, ∆, were performed to verify that convection was not affecting the diffusion coefficient obtained over the investigated temperature range. The gelled microemulsions were heated until the sample had a low viscosity, i.e., until the sample temperature was above the sol-gel transition. The low-viscosity, homogeneous liquid sample thus obtained was quickly transferred into an NMR tube and cooled in a freezer until completely gelled. Finally, all samples were equilibrated at the required temperature in a water bath prior to measurement and then transferred into the magnet equilibrated at the same temperature. A pulsed gradient spin echo pulse sequence was used, and echo decays were analyzed with the Stejskal-Tanner equation4
[
(
I ) I0 exp -(γgδ)2 ∆ -
δ D ) I0 exp[-kD] 3
)]
(5)
where γ ) 2.675 × 108 s-1 T-1 is the magnetogyric ratio of the proton, g is the gradient strength, δ is the length of the gradient pulse, ∆ is the time between the onset of the gradient pulses (i.e., the diffusion time), and D is the self-diffusion coefficient. The experiment was performed by recording NMR spectra as a function of g while δ and ∆ are kept constant. All parameters were set such that a complete decay of the signal intensity was obtained at the highest value of g. By plotting the intensities of the peaks corresponding to the protons of the different components in the samples versus k, a nonlinear least-squares fit of eq 5 to the data yields D. The errors of the D values were calculated via a Monte Carlo simulation by introducing noise to the experimental data and repeating the fit on the so-generated set of values. This procedure was repeated 1000 times, leading to a mean D value and a corresponding standard deviation.5 Prior to the study of the gelled microemulsion, the self-diffusion of the individual compounds in the water and oil bulk phases, respectively, was measured to be able to interpret the results for the gelled microemulsion. In particular, the following samples were studied: (1) binary mixtures of n-dodecane + 12-HOA at different concentrations and temperatures to assign the peaks to the different components and to compare the oil self-diffusion in the presence of the gelator to the bulk self-diffusion of the oil at the same temperature; (2) a binary mixture of water + NIPAm at 7 wt % NIPAm (ψ ) 0.07) at different temperatures to assign the peaks to NIPAm and to compare the water self-diffusion in the presence of NIPAm to the bulk self-diffusion of water at the same temperature; (3) a binary mixture of water + C13/15E5 at 7.5 wt % C13/15E5 (γ ) 0.075) at T ) 49.8 °C to assign the peaks to C13/15E5. The gelled bicontinuous microemulsion sample studied is marked in Figure 3 (top). Measurements were performed with 0.5 °C steps through the temperature range in which a one-phase microemulsion is formed, namely, between T2-1 ) 34.0 °C and T1-2 ) 37.4 °C. The slight shift of the phase boundaries to lower temperatures compared to the phase diagram shown in Figure 3 (top) is explained in section 3.1. Figure 1 shows the 1H NMR spectrum of the gelled microemulsion at 35 °C. The peaks are assigned as follows: the peak at 4.7 ppm is from water, the peaks at 7.9, 6.4, and 5.8 ppm are from NIPAm, the 3.7 ppm peak is from the oxyethylene headgroup of the surfactant, the intense peak at 1.3 ppm is due to the oil and the (4) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288. (5) Alper, J. S.; Gelb, R. I. J. J. Phys. Chem. B. 1990, 94, 4747.
Gelled Polymerizable Microemulsions
Figure 1. 1H NMR spectrum of the gelled microemulsion.
Figure 2. Decay of relative peak areas, I/I0, as a function of k (see eq 5). For the signals observed at 3.7, 4.7, 5.7 (data not shown), 6.4, and 7.9 (data not shown) ppm, a monoexponential decay is observed, while the decay is biexponential for the signals at 1.0 and 1.3 ppm (see the text for further details). As the I/I0(k) curves obtained for 5.7, 6.4, and 7.9 ppm were the same, only the data for 6.4 ppm are shown.
surfactant -CH2- groups (probably containing a small contribution from the gelator), and the peak at 1.0 ppm is from the -CH3 group of the oil, surfactant, and NIPAm (and again a small contribution from the gelator). The low mobility of the gelator 12-HOA in the gelled state results in broad resonance lines, which makes the gelator virtually invisible in both the 1H NMR spectrum and the diffusion experiment. BisAm, which is also present in the water phase in the microemulsion, was neglected due to its very low concentration in comparison to that of NIPAm and the large similarity of its structure to that of NIPAm. Figure 2 shows an example of the intensity decays of each of the peaks in the microemulsion as a function of k. For a better comparison, the intensities are normalized by dividing them by the respective maximum intensity, I0, which is measured at g ) 0. The characteristic peak of the EO group allowed us to calculate the surfactant selfdiffusion coefficient in the gelled microemulsion. As mentioned above, there was no peak attributed to the oil alone, and thus, the diffusion of the surfactant, determined from the EO peak, was used to deconvolute the contribution of the surfactant molecules to the -CH2- and -CH3 peaks from that of the oil. Thus, the self-diffusion coefficient for the oil molecules could be obtained by fitting the area of the -CH2- and -CH3 peaks with a biexponential decay, where one of the two components was fixed as the self-diffusion coefficient of the surfactant, calculated from the peak of the EO groups. The contributions to the signal arising from the gelator and BisAm were always neglected (as explained above).
Langmuir, Vol. 24, No. 16, 2008 8475 2.4. Conductivity. Various amounts of the components were weighed into a thermostated cell containing a magnetic stirring bar and two electrodes, which were dipped into the sample. The electrical conductivity measurements were carried out using a Wayne Kerr precision component analyzer, model 6425, using a fixed spacing parallel platinum plate cell, whose constants were determined by calibration with liquids of known conductivity. Measuring the conductivity (κ0) of the pseudobinary reference system H2O/NIPAm/ BisAm-Lutensol AO5 was not possible as this system forms lyotropic mesophases in the relevant concentration range. Thus, the conductivity of the microemulsion, κ, cannot be compared with the conductivity of the pseudobinary system, κ0. The samples were thermostated by an MGW Lauda Thermostat, model RC 6, with a precision of (0.1 °C. 2.5. Small-Angle Neutron Scattering. The microstructure of the gelled and the non-gelled microemulsions was studied by SANS with the KWS-2 spectrometer at the Forschungsneutronen-quelle Heinz Maier-Leibnitz (FRM-II, Garching, Germany). For measurements in bulk contrast, water was exchanged by the same volume of heavy water (D2O). Thus, the mass fractions were R ) 0.475 and ψ ) 0.068. Note that the phase boundaries of the one-phase region shift toward lower temperatures on exchanging H2O by D2O and were thus remeasured after each compositional change. SANS measurements were carried out in the middle of the one-phase region. The microemulsions were heated, homogenized, and rotated into 0.2 mm path QS quartz cells before gelation. The quartz cells were placed in a thermostated sample holder in the neutron beam. For the scattering experiments at the KWS-2 spectrometer mean wavelengths of λmean ) 7.5 Å and λmean ) 12 Å with ∆λ/λ ) 0.20 were used. q ranged from 0.002 to 0.15 Å-1. The collimation was adjusted to not limit the resolution. Two sample-to-detector distances were measured (8.0 and 2.0 m) with the two-dimensional detector on axis. The data from the two-dimensional detectors were normalized and radially averaged according to the standard procedures provided by the Ju¨lich Centre for Neutron Science. Each data set was put on absolute scale by measuring the incoherent scattering of H2O. Data sets from the different distances overlapped without scale adjustment. A few data points of the lowest and highest q values were cut from each set.
3. Results and Discussion 3.1. Phase Diagram of the Gelled Microemulsion. In our previous study we measured the phase diagrams of the microemulsions consisting of H2O/NIPAm/BisAm-n-dodecane/12HOA-C13/15E5.2 For the present studies a constant water-to-oil ratio (R ) 0.5) and a constant mass fraction of NIPAm + BisAm in the aqueous phase (ψ ) 0.07) was chosen. This mass fraction of monomer was chosen as it is known from our previous work that this concentration is sufficient to create a robust cross-linked hydrogel.6 As our aim is to synthesize a bicontinuous hydrogel, this seemed a reasonable starting point. However, additional experiments at double the monomer concentration are also being undertaken, as a higher polymer density may be preferable to withstand the subsequent cleaning steps post-polymerization. To gel the microemulsion, the gelator 12-HOA was added to the oil phase and the phase behavior was studied as a function of the temperature, T, and the total surfactant concentration, γ, for gelator concentrations between β ) 0 and β ) 0.041. In Figure 3 the resulting T(γ) diagrams of microemulsions with gelator mass fractions of β ) 0.031 (Figure 3 (top, middle right, bottom right)), β ) 0.018 (Figure 3 (middle left)), and β ) 0 (Figure 3 (bottom left) are shown for the sake of clarity (see ref 2 for further details). The phase diagrams show that with decreasing gelator content the one-phase region shifts to higher temperatures while the sol-gel boundary shifts to lower temperatures. As can be seen in Figure 3 (top), the sol-gel transition at β ) 0.031 is mainly above the one-phase region; i.e., most parts (6) Lynch, I.; Gorelov, A. V.; Dawson, K. A. Phys. Chem. Chem. Phys. 1999, 1, 2103.
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Table 1. Experimentally Determined Self-Diffusion Coefficients of Bulk Water (D0,H2O) as Well as of NIPAm (DNIPAm) and Water (DH2O(NIPAm)) in the Binary Water + NIPAm System (7 wt % NIPAm, Same Concentration As Used in the Microemulsions)a T/°C
D0,H2O/10-9 m2 s-1
DNIPAm/10-10 m2 s-1
DH2O(NIPAm)/10-9 m2 s-1
DH2O(NIPAm)/D0,H2O
D0,H2O(literature)b/10-9 m2 s-1
34.5 35.0 35.5 36.0 36.5 37.0
2.78 ( 0.02 2.80 ( 0.01 2.83 ( 0.01 2.85 ( 0.01 2.85 ( 0.03 2.93 ( 0.01
7.47 ( 0.12 7.56 ( 0.07 7.76 ( 0.06 7.75 ( 0.06 7.96 ( 0.12 8.34 ( 0.08
2.33 ( 0.03 2.36 ( 0.01 2.44 ( 0.02 2.44 ( 0.03 2.49 ( 0.02 2.50 ( 0.01
0.838 0.843 0.862 0.856 0.874 0.853
2.87 2.91 2.94 2.97 3.01 3.04
a The ratio of the self-diffusion coefficients for water in the 7 wt % NIPAm solution and in the bulk has been calculated. For comparison, values calculated according to ref 12 are also given. Measurements were performed with δ ) 0.5 ms and ∆ ) 20 ms in the region of interest (34.5-37 °C) with 0.5 °C steps. No convection has been detected. The errors of the self-diffusion coefficients are calculated as explained in the text. b Calculated according to ref 12.
of the one-phase region are gelled. Thus, for studying the microstructure of the gelled microemulsion, we decided to work with the phase diagram obtained for β ) 0.031. Unfortunately, an extensive anisotropic liquid crystalline (LC) phase is formed in the region between the 2-1 and the 1-2j phase boundaries, which substantially limits the γ and T ranges of the isotropic one-phase region and thus the range where studies of the microstructuresand finally the polymerizationscan be carried out. The phase diagrams shown in Figure 3 were used to determine the experimental conditions for all measurements carried out in this study. Two comments have to be made. First, in the case of technical grade surfactants the phase boundaries usually shift to higher or lower temperatures if a new batch is taken as each batch differs slightly in composition. Thus, we remeasured the phase boundaries whenever we changed the batch. Second, for the SANS measurements water was exchanged by heavy water (D2O), which is well-known to effect the phase behavior. The white triangles represent the new phase boundaries of the D2Ocontaining sample, and the gray tilted squares represent the temperature at which the SANS experiments were performed. All samples were prepared as close as possible to the fishtail point, where the bicontinuous structure is located. 3.2. Self-Diffusion in the Gelled Microemulsion. It is wellknown that the self-diffusion coefficients of oil and water in a microemulsion are close to that of the respective bulk phase if they form the continuous phase. This is indeed the case for water in o/w droplet microemulsions and oil in w/o droplet microemulsions. However, in a bicontinuous microemulsion, where both the oil and water phases are continuous, free diffusion is possible in two dimensions only and restricted in the third direction due to the presence of the surfactant layer. Thus, the theoretical self-diffusion coefficients of water and oil in a bicontinuous microemulsion are 2/3 of the corresponding bulk values.7,8 Knowing the self-diffusion coefficients of the two solvents, i.e., of water and oil, one can easily discriminate between droplet and bicontinuous microemulsions.7–11 Having determined the selfdiffusion coefficients of water and oil in a microemulsion, the connectivity of the solvents and therefore the type of microstructure can be deduced by plotting the relative self-diffusion coefficients, D/D0, versus the tuning parameter of the system, which is the temperature, T, in the present case. The reference value D0 is the bulk self-diffusion coefficient of the solvent in the respective nonstructured system. In our case the reference values are the self-diffusion coefficient of water in the binary system water + monomer (neglecting the cross-linker) and the (7) Bodet, J. F.; Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J. Phys. Chem. 1988, 92, 1898. (8) Lindman, B.; Shinoda, K.; Olsson, U.; Anderson, D.; Karlstro¨m, G.; Wennerstro¨m, H. Colloids Surf. 1989, 38, 205. (9) Lindman, B.; Olsson, U. Ber. Bunsen-Ges. 1996, 100, 344, and references therein. (10) Stubenrauch, C.; Findenegg, G. H. Langmuir 1998, 14, 6005. (11) Reimer, J.; Sod¨erman, O.; Sottmann, T.; Kluge, K.; Strey, R. Langmuir 2003, 19, 10692.
self-diffusion coefficient of oil in the binary system oil + gelator, which will be denoted DH2O(NIPAm) and Doil(12-HOA), respectively. For the three limiting cases of o/w droplets, w/o droplets, and bicontinuous structures, one obtains the following relations: (a) o/w droplets, D/D0(H2O) . D/D0(oil), (b) w/o droplets, D/D0(H2O) , D/D0(oil), (c) bicontinuous, D/D0(H2O) ≈ D/D0(oil). The first two relations hold as the droplets are smaller than the micrometer scale distance a molecule diffuses during the 10 ms time scale defined by the diffusion NMR experiment. Due to the small size of the microemulsion droplets, the experiment is not sensitive to the molecular displacement within the droplets but only to the translation of the entire droplet. 3.2.1. Reference for Water and Oil Diffusion. 3.2.1.1. Water Diffusion. The water + NIPAm system was investigated at ψ ) 0.07 as a function of the temperature to determine the selfdiffusion of water in a sample that has the same composition as the aqueous phase in the gelled microemulsion. This value was then used to calculate the relative self-diffusion coefficients, which will be called obstruction factors in the following discussion. In addition, the self-diffusion of pure water was measured as a function of temperature. Table 1 shows the selfdiffusion coefficients of water (DH2O(NIPAm)) and NIPAm (DNIPAm) in the binary water + NIPAm system as well as that of bulk water (D0,H2O). The latter values are in very good agreement with the listed literature values [D0,H2O(literature)].12 The ratio between the two water diffusion coefficients (DH2O(NIPAm)/ D0,H2O) is also given. As can be seen in Table 1, the water self-diffusion is strongly affected by the presence of NIPAm molecules and a 10-20% reduction of the self-diffusion coefficient was found compared to that of bulk water. This can be explained by the particular properties of NIPAm, which is known to form large hydration shells due to structuring of water around the isopropyl part of the monomers at high temperatures in the polymerized or gelled state.13 While no data on this phenomenon are found in the literature for the monomeric form, it is reasonable to infer a similar trend. From these results, it is clear that the diffusion coefficient of water in the binary system water + NIPAm, i.e., DH2O(NIPAm), is the proper reference state to calculate the obstruction factors for the microemulsion. 3.2.1.2. Oil Diffusion. The oil + 12-HOA system was investigated at β ) 0.003 as a function of the temperature to determine the self-diffusion of oil in a sample with a composition similar to that of the oil phase in the gelled microemulsion and thus to calculate the respective obstruction factor. As the gelator is surface-active, it partitions between the bulk oil phase and the interface in the corresponding microemulsion (see ref 2 for details). Thus, the final concentration of 12-HOA in the oil phase is smaller than the total 12-HOA content in the system. An (12) Holz, M.; Heil, S. R.; Sacco, A. Phys. Chem. Chem. Phys. 2000, 2, 4740. (13) Kogure, K.; Nanami, S.; Masuda, Y.; Toyama, Y.; Kubota, K. Colloid Polym. Sci. 2005, 283, 1163–1171.
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Table 2. Experimentally Determined Self-Diffusion Coefficients of Oil in the Bulk (D0,oil) and in the Organogel (Doil(12-HOA)) (0.3 wt % 12-HOA) and Their Ratio (Doil(12-HOA)/D0,oil)a T/°C
D0,oil/10-9 m2 s-1
Doil(12-HOA)/10-9 m2 s-1
Doil(12-HOA)/D0,oil
D0,oil(literature)b/10-9 m2 s-1
34.5 35.0 35.5 36.0 36.5 37.0
0.94 ( 0.11 0.98 ( 0.04 1.00 ( 0.07 1.01 ( 0.01 1.01 ( 0.08 1.05 ( 0.10
0.97 ( 0.08 0.97 ( 0.05 0.99 ( 0.03 0.99 ( 0.05 1.00 ( 0.04 1.02 ( 0.11
1.029 0.995 0.986 0.979 0.988 0.971
0.962 0.971 0.979 0.988 0.996 1.005
a For comparison, values calculated according to ref 12 are also given. Measurements were performed with δ ) 0.5 ms and ∆ ) 20 ms in the region of interest (34.5-37 °C) with 0.5 °C steps. No convection has been detected. In all cases, the self-diffusion coefficients are calculated from the area of the two peaks at 1.3 and 1.0 ppm. The errors of the self-diffusion oefficients are calculated as explained in the text. b Calculated according to ref 12.
indicator for this partitioning is not only the shift of the phase diagrams but also the observation that about 10 times less gelator is needed to form a gel in the binary system compared to the 12-HOA amount needed to form a gel in the microemulsion. Given that our final microemulsion contained 3 wt % (β ) 0.031) gelator, the binary system containing 0.3 wt % 12-HOA was chosen to study the oil self-diffusion in the presence of 12-HOA. We will come back to this point below when discussing the diffusion coefficient of 12-HOA (D12-HOA). For comparison, the self-diffusion of pure oil was also measured as a function of temperature. Table 2 shows the self-diffusion coefficients of oil (Doil(12-HOA)) in the binary oil + 12-HOA system and in the bulk (D0,oil) as well as the ratio of these two values. As was the case for D0,water, the D0,oil values are in very good agreement with the respective literature values [D0,oil(literature)].12 The gelator signal could not be detected, which can be easily explained if one argues that the 12-HOA molecules form solid fibrils and thus cannot diffuse freely in the solution. It should be noted that the oil diffusion in the organogel is comparable to that of pure dodecane, even though the sample is in a macroscopically gelled state. The reason for this observation is the fact that the theoretical obstruction factor for a 3% solution is about 0.985, D/D0 ) 1/(1 + volume fraction of gelator/2),14 which explains why the oil diffusion is not affected by the presence of the gelator. Moreover, a 3-D network of crystalline fibrils obstructs the diffusion less than freely moving 12-HOA molecules would as the total volume needed for the gel network is smaller than that for monomerically dissolved molecules. 3.2.2. Diffusion in the Gelled Microemulsion. Knowing the positions of the peaks for the oil, water, surfactant, and NIPAm (see section 2.3), the self-diffusion coefficients for each of the components in the gelled polymerizable microemulsion were determined. Measurements were carried out with the gelled microemulsion (R ) 0.5, β ) 0.031, γ ) 0.11, ψ ) 0.07) as a function of the temperature to “cross” the one-phase region of the gelled microemulsion, which is between 34.0 and 37.4 °C (see Figure 3 (top)). The results are shown in Figure 4, where the calculated self-diffusion coefficients for each component are plotted versus T. 3.2.2.1. DH2O and Doil. As can be seen in Figure 4, the water self-diffusion coefficient decreases while the oil self-diffusion coefficient increases with increasing temperature. This result demonstrates that in the investigated temperature range the structure of the microemulsion changes from water-continuous to oil-continuous. 3.2.2.2. DNIPAm. Another indicator for a structural change from a water-continuous to an oil-continuous microemulsion is the decrease of the self-diffusion coefficient of the NIPAm molecules. NIPAm has a significant solubility in water (.22.5 wt %) and
a very low solubility in oils such as hexane.15 As no data could be found regarding the solubility of NIPAm in dodecane, we measured it via density and found that the solubility of NIPAm in dodecane is indeed negligible (not detectable). In other words, NIPAm diffusion can only take place in the aqueous phase, and thus, its decreased diffusion coefficient reflects the structural changes of the microemulsion. Comparing the DH2O with DNIPAm values in Figure 4, one sees that the temperature effect on the NIPAm self-diffusion coefficients is slightly smaller than the effect on the water self-diffusion coefficients (decrease of the D value by 86% and 72%, respectively). The most likely explanation is the fact that the increase in temperature increases diffusion, which opposes the decrease induced by the microemulsion structural changes. 3.2.2.3. Dsurf. As seen in Figure 4, the surfactant diffusion is much slower than that of all other components and slightly increases with increasing temperature (increase of the D value by 20%). Assuming that all surfactant molecules are adsorbed at the oil-water interface, i.e., neglecting the monomeric solubility of the surfactant in the oil and aqueous phases, only lateral diffusion along the interface can take place. Moreover, the surfactant has the highest molecular weight of all components. These two differences explain the slower diffusion compared to that of the other three components. However, a structural change from water-continuous to oil-continuous should lead to a maximum if one plots Dsurf versus the temperature. At low temperatures oil-in-water droplets are formed, while at high temperatures water-in-oil droplets are formed. In both cases, the surfactant diffuses as slowly as the discontinuous phase, i.e., the droplets. At intermediate temperatures a bicontinuous microemulsion is formed and the surfactant molecules experience a faster (lateral) diffusion compared to the droplet diffusion.9,16 The most likely explanation of why such a maximum is not observed is that the structure is bicontinuous over the narrow temperature interval studied. In other words, at the lower phase boundary no oil droplets and at the upper phase boundary no water droplets are formed, but bicontinuous phases of positive and negative mean curvatures, respectively, are formed. Another explanation could be the same as given above for NIPAm, namely, an increase in Dsurf with increasing temperature which overcompensates for the decrease that one expects if a bicontinuous phase transforms to water-in-oil droplets. To answer this question conclusively, further studies are needed. 3.2.2.4. D12-HOA. Finally, the gelator could not be detected although the concentration needed to gel the oil phase in the microemulsion was, as explained above, 10 times higher than the concentration needed to gel the binary system oil + 12-HOA. In our previous work we speculated that gelator molecules are probably adsorbed at the interface, which reduces the concentration of 12-
(14) Holmberg, K.; Jon¨sson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons, Ltd.: New York, 2003.
(15) Dowding, P. J.; Vincent, B.; Williams, E. J. Colloid Interface Sci. 2000, 221, 268. (16) Anderson, D. M.; Wennerstro¨m, H. J. Phys. Chem. 1990, 94, 8683.
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Figure 3. Phase diagrams of the studied samples. (Top) NMR sample: T(γ) cut of the system H2O/NIPAm/BisAm-n-dodecane/12-HOA-C13/15E5 at R ) 0.50, β ) 0.031, and ψ ) 0.075 (black and white circles). The stars represent the sol-gel boundary of the microemulsion. Self-diffusion NMR measurements were carried out at γ ) 0.110 as a function of the temperature (vertical line). (Middle) Conductivity samples: (Left) T(γ) cut of the system H2O/NIPAm/BisAm-n-dodecane/12-HOA-C13/15E5 at R ) 0.50, β ) 0.018, and ψ ) 0.075 (black circles). The stars represent the sol-gel boundary of the microemulsion. Conductivity measurements were carried out at γ ) 0.109 as a function of the temperature (vertical line). (Right) Same as the top panel presented on a different y scale. Conductivity measurements were carried out at γ ) 0.105 as a function of the temperature in the one-phase region only (vertical line). (Bottom) SANS samples: (Left) T(γ) cut of the system H2O/NIPAm/BisAm-n-dodecane/12-HOAC13/15E5 at R ) 0.50, β ) 0, and ψ ) 0.075 (black circles). The white triangles mark the new phase boundaries with D2O, and the gray tilted square marks the temperature at which the SANS measurement was carried out. (Right) Same as the top panel presented on a different y scale. The white triangles mark the new phase boundaries with D2O, and the gray tilted square marks the temperature at which the SANS measurement was carried out.
HOA in the oil phase, and thus, a larger amount of 12-HOA is required to form an organogel.2 If that argument were true, one would expect a signal for the “interfacial” part of 12-HOA and a self-diffusion coefficient similar to that of the surfactant. This, however, was not observed, which forces us to revise the argument given previously. Still, some 12-HOA molecules will be adsorbed
at the interface, but the amount is obviously very small (too small to be detectable via NMR). Thus, the major part of the gelator has to be in the oil phase. The fact that the gelator is not “visible” in an FTPGSE experiment means that it is not diffusing and thus that it can only be located in the fibrils and nodes of the gel network. In other words, the gel network in the oil phase of the microemulsion
Gelled Polymerizable Microemulsions
Figure 4. Self-diffusion coefficients of water (DH2O), oil (Doil), the monomer NIPAm (DNIPAm), and the surfactant C13/15E5 (Dsurf) in the gelled microemulsion as a function of temperature. The sample composition and the temperature scan are indicated in Figure 3 (top). The break of the y axis is between 0.45 × 10-10 and 1.5 × 10-10 m2 s-1.
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maintained in the presence of the gelator, i.e., in the gelled microemulsion. A closer inspection reveals that the temperature of the intersection point is indeed exactly in the middle of the two phase boundaries, which were determined to be T2-1 ) 34.0 °C and T1-2 ) 37.4 °C (see section 2.3). The value of the obstruction factor, however, is unusually low, which will be discussed in the following paragraph. As mentioned above, the theoretical value for the intersection point is D/D0 ) 2/3, which corresponds to a system with a minimal surface of zero mean curvature (H ) 0). In fact, for a number of systems obstruction factors of approximately 2/3 at the intersection point have been found,17–19 but somewhat lower values (about 0.4) have also been reported.20–22 Deviations from the theoretical upper limit of 2/3 have been attributed to defects in the structure, i.e., to deviations from H ) 0.19,20,23 In particular, local tubular structures are discussed as defects which significantly reduce the obstruction factor. For the present system an obstruction factor of ∼0.33 was found at the intersection point, leading us to conclude that the bicontinuous structure of the present system has a number of irregularities, such as restricting necks and local tubules, which cause a significant reduction of the self-diffusion coefficients. Note that the present study is the first in which obstruction factors in gelled microemulsions have been measured. As we do not have any other values for comparison, we cannot exclude that the low obstruction factors are “typical” for a gelled microemulsion, i.e., that obstruction factors in gelled microemulsions are generally lower compared to those of the respective low-viscosity microemulsions. We conclude this section by mentioning that we neglected a small contribution in our calculations of the obstruction factor for water. Water diffusion is hindered because of the hydration of the surfactant’s headgroup, which can be accounted for by
DH2O ) pfree Dfree(H2O) + phydr Dhydr
Figure 5. Obstruction factors of water and oil in the gelled microemulsion as a function of the temperature. The reference systems were binary mixtures with compositions similar to those of the aqueous (DH2O(NIPAm)) and the oil (Doil(12-HOA)) phases of the microemulsion. The intersection point is at T ) 35.7 °C and an obstruction factor of 0.325. The sample composition and the temperature scan are indicated in Figure 3 (top). Solid lines are guides to the eye.
has to be much denser (smaller mesh sizes) compared to the network in the binary system. Drawing this conclusion is reasonable as the domain sizes in the microemulsion are very small and to “fit” a gel network into the oil domains very small mesh sizes are required. We hope to clarify this point by FFEM measurements in the near future. Figure 5 shows the calculated obstruction factors for oil and water in the gelled microemulsion, where Doil(12-HOA) and DH2O(NIPAm) are the reference values for the bulk phases measured at the same temperatures (see Tables 1 and 2). As can be seen in Figure 5, an increase of the temperature from the lower to the upper phase boundary leads to the characteristic change of the obstruction factors associated with a structural change from water-continuous to oil-continuous via a bicontinuous microemulsion.7–11 The intersection point occurs at T ) 35.7 °C and an obstruction factor of 0.325. At this intersection the obstruction factors of the two solvents are equal, which indicates the formation of a bicontinuous structure. Thus, we have clear evidence that the bicontinuous structure is
(6)
where pfree and phydr are the fractions of free and hydration water, respectively. DH2O is the average self-diffusion coefficient of water in the microemulsion, Dfree(H2O) is the self-diffusion coefficient of free water in the microemulsion, and Dhydr is the self-diffusion coefficient of the molecules to which the water is bound (i.e., Dhydr ) Dsurf). The resulting Dfree(H2O) values are expected to differ from DH2O(NIPAm), the self-diffusion coefficient of water in the reference system, only due to the microstructure of the microemulsion. If one were to take this correction into account, the intersection point would shift to a slightly higher obstruction factor and a higher temperature as the obstruction factor of water would be slightly larger. This effect, however, is very small and does not change the overall picture. See ref 10 for further details. 3.3. Conductivity of the Gelled Microemulsion. Electrical conductivity is used to discriminate between water-continuous and water-discontinuous structures, since the conductivity should be high in water-continuous microemulsions while it will be comparatively low in oil-continuous structures.24 It is known that the diffusion, Di, is directly proportional to the electrical (17) Fukuda, K.; So¨derman, O.; Lindman, B.; Shinoda, K. Langmuir 1993, 9, 2921. (18) Olsson, U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986, 90, 4083. (19) Lindman, B.; Shinoda, K.; Jonstro¨mer, M.; Shinohara, A. J. Phys. Chem. 1988, 92, 4702. (20) Olsson, U.; Nagai, K.; Wennerstro¨m, H. J. Phys. Chem. 1988, 92, 6675. (21) Shinoda, K.; Araki, M.; Sadaghiani, A.; Khan, A.; Lindman, B. J. Phys. Chem. 1991, 95, 989. (22) Carnali, J. O.; Ceglie, A.; Lindman, B.; Shinoda, K. Langmuir 1986, 2, 417. (23) Anderson, D. M.; Wennerstro¨m, H. J. Phys. Chem. 1990, 94, 8683. (24) Sottmann, T.; Strey, R. Fundamentals of Interface and Colloid Science; Elsevier: Amsterdam, 2005.
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Figure 6. Electrical conductivity as a function of the normalized temperature, Tnorm (eq 6), of the system H2O/NIPAm/BisAm-ndodecane/12-HOA-C13/15E5 with a mass fraction of 12-HOA in oil of β ) 0.018 (non-gelled microemulsion) and β ) 0.031 (gelled microemulsion). The sample compositions and the temperature scans are indicated in Figure 3 (middle).
conductivity, κ. Thus, it is possible to compare the conductivity of the gelled and non-gelled microemulsions to gain insight into the structure of the system H2O/NIPAm/BisAm-n-dodecane/ 12-HOA-C13/15E5. For the present microemulsion it was not possible to determine κ0 (see the explanation given in the Experimental Part). Two different gelator mass fractions, β, were chosen, namely, β ) 0.018 (non-gelled microemulsion) and β ) 0.031 (gelled microemulsion). As the compositions of the two samples were as close as possible, it is possible to compare the conductivities. The phase behavior of these microemulsions is shown in Figure 3 (middle), and the resulting conductivities are presented in Figure 6. Figure 6 shows the conductivity of the system H2O/NIPAm/ BisAm-n-dodecane/12-HOA-C13/15E5 with β ) 0.018 (black circles) and β ) 0.031 (gray tilted squares) as a function of the normalized temperature, Tnorm. Note that for the microemulsion with a gelator concentration of β ) 0.018 no jumps in the conductivity appear at the phase boundary. This is due to the continuous transition from water-continuous to oil-continuous microemulsion. The measurement was stopped before the system was completely water discontinuous. The measurements in the gelled microemulsion were only performed within the one-phase region as a phase separation in the two-phase region would not have taken place in an appropriate time. To compare the data, the temperature, T, is standardized with
Tnorm )
2(Texp - Tm(1Φ) (Tu(1Φ) - Tl(1Φ)
(7)
As can be seen in Figure 6, the conductivities in the one-phase regions of the two microemulsions are alike within experimental error. Moreover, the conductivity decreases continuously with increasing temperature, indicating a structural change from watercontinuous to oil-continuous structures. In conclusion, one can say that the conductivity measurements strongly support the NMR data and also indicate that the structures of the gelled and the non-gelled microemulsions are the same. Unfortunately, it is not possible to discuss these results further as κ0 is not known. 3.4. Small-Angle Neutron Scattering. SANS experiments provide an opportunity to study the statistical properties of the microstructure. In this work the focus was on the possible change in microstructure resulting from gelling the nonpolar phase of
Figure 7. SANS curves (double logarithmic plot of the intensity as a function of the scattering angle q, I(q)) of the system NIPAm/BisAm/ D2O-n-C12H26/12-HOA-C13/15E5 with R ) 0.478, β ) 0.0, ψ ) 0.068, and γ ) 0.124, measured at T ) 48.34 °C (non-gelled microemulsion) and R ) 0.471, β ) 0.031, ψ ) 0.068, and γ ) 0.111, measured at T ) 33.97 °C (gelled microemulsion). Solid lines are Teubner-Strey (eq 7, low q) and Porod (eq 8, high q) fits. The sample compositions and the temperatures are indicated in Figure 3 (bottom). Note that replacing H2O by D2O shifts the phase boundaries to lower temperatures as indicated in Figure 3 (bottom).
Figure 8. Schematic drawings of the structure of the binary organogel (left) and of the gelled microemulsion (right). See the text for further details.
a microemulsion. For the SANS experiments five bulk contrast samples (i.e., samples in which water was exchanged by heavy water, D2O) with increasing gelator mass fraction from β ) 0 up to β ) 0.031 were prepared. In all measurements the mass fraction of the oil phase (n-dodecane + 12-HOA) in the solvent mixture was kept constant, i.e., R ) 0.475, and the composition of the polymerizable aqueous solution was kept constant, at ψ ) 0.068. The mass fraction of the surfactant in the total mixture, γ (eq 2), was chosen to be as close to the fishtail point as possible as it is in this region where the bicontinuous phase is located. As the scattering curves of the different samples did not change significantly, in Figure 7 only two scattering curves are shown, namely, those measured at β ) 0 and β ) 0.031. Comparing the bulk scattering curves of the non-gelled (white circles) and the gelled (black circles) microemulsions, one sees no significant changes. A closer look reveals that the peak maximum of the non-gelled microemulsion is located at a slightly higher q value and is less intense than that of the gelled case. This is most probably due to the slightly higher surfactant mass fraction for the non-gelled microemulsion (γ ) 0.124 at β ) 0 compared to γ ) 0.111 at β ) 0.031) which had to be used to measure in the one-phase region. With increasing surfactant content, the microstructure gets smaller; i.e., the peak shifts to higher q values. As the intensity of the scattering peak is proportional to the
Gelled Polymerizable Microemulsions
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domain size (I ≈ φr3), the higher intensity of the peak of the gelled microemulsion can also be explained by the surfactant amount. To quantify the difference and/or similarities between the structure of a non-gelled and a gelled microemulsion, the data were fitted with the Teubner-Strey equation25,26
I(q) )
8πc〈η2 〉/ξTS a2 + c1q2 + c2q4
(8)
While the Teubner-Strey equation describes the small q values very well, the higher q values show a q-4 decrease which can be better described with the Porod equation26 22 S lim[I(q)] ) 2π(∆FS)2 q-4e-q t + Iincoh V
(9)
The correlation length, ξTS, the periodicity of the structure, dTS (distance between two polar or nonpolar domains), and the amphiphilicity factor, fa ) c1/(c2a2)1/2, result from the analysis of the Teubner-Strey fit. The amphiphilicity factor, fa, is the degree of organization of the microstructure and is close to -1 for strongly structured microemulsions.27 Moreover, the domain size, ξ, can be calculated according to
ξ)
dTS 2
(10)
Finally, via the q-4 decrease of the scattering curve, the specific internal interface, S/V, can be obtained. All data extracted from the SANS curves are listed in Table 3. Looking at Table 3 and Figure 7, one clearly sees that neither the listed values nor the scattering curves differ significantly. On closer examination of the amphiphilicity factor, fa, one sees that the structure of the non-gelled systems (β ) 0, 0.018, 0.025) has a higher order than that of the gelled systems (β ) 0.029, 0.031). A possible explanation is that the gelled microemulsions are not that homogeneous due to the gel fibers, which somehow interfere with the order. However, this difference is marginal. Thus, it is reasonable to conclude that the structures of the gelled and the non-gelled microemulsions are the same; i.e., gelling the nonpolar phase of a microemulsion does not affect the general pattern of microstructures in the one-phase region of a microemulsion. 3.5. Structure of the Gelled Microemulsion. To understand the structure of the gelled microemulsion, it is necessary to consider the structures of the two separated systems (gel and microemulsion) initially, as they are structured on completely different length scales. The situation becomes even more complicated if one considers that a microemulsion is a thermodynamically stable system, which is not the case for a gel. The main structural features of a gel and a microemulsion are summarized below, followed by a discussion of the possible structure of the gelled microemulsion. 3.5.1. Gels. Gellation is one of the processes that lead to longlived but nonequilibrium states. These states are considered to be dynamically arrested or nonergodic transitions.28 The concentration of the gelator is typically below 2 wt %. From a structural point of view the gelator forms a 3D network that immobilizes the liquid macroscopically. In the specific case of organogels formed with 12-HOA, the gelator self-assembles to form long fibrils connected via spatially extended (pseudo)crystalline microdomains (nodes).3 The single fibrils have a (25) Teubner, M.; Strey, R. J. Chem. Phys. 1987, 87, 3195. (26) Strey, R.; Winkler, J.; Magid, L. J. Phys. Chem. 1995, 99, 13232. (27) Schubert, K.-V.; Strey, R.; Kline, S. R.; Kaler, E. W. J. Chem. Phys. 1994, 101, 5343. (28) Dawson, K. A. Curr. Opin. Colloid Interface Sci. 2002, 7, 218–227.
diameter of 20-30 nm depending on the solvent, while the mesh sizes are in the range of micrometers depending on the amount of gelator. A schematic drawing is shown in Figure 8 (left). 3.5.2. Microemulsions. The domain size, ζ, of a bicontinuous microemulsion can be described as the average distance between two surfactant monolayers. It can be determined with SANS experiments and imaged with transmission electron microscopy (TEM). Extensive theoretical and experimental studies allow us to roughly estimate the domain size, ζ, of bicontinuous structures at a given total surfactant concentration if the composition of the sample and the area, aC, and the volume, VC, per surfactant molecule are known. All of the theoretical models agree in the functional form, predicting that
ξ)a
VC φAφB aC φC
(11)
with φA ) VH2O/(VH2O + Voil), φB ) Voil/(VH2O + Voil), and φC ) Vsurfactant/Vtotal [note that this holds for the specific internal interface S/V-1 ) φC-1(VC/aC)]. The absolute value of the prefactor, a, is representative of the geometrical model used to describe the bicontinuous structure. The model of Debye et al.29 predicts a ) 4, the Voronoi tessellation of Talmon and Prager30gives a ) 5.84, and the model of cubes by De Gennes and Taupin31 yields a ) 6. Experimentally, Sottmann et al.32 found that a ) 7.16 for bicontinuous structured microemulsions of the type water-n-alkane-CiEj. To estimate domain sizes for the system at hand, we used VC and aC of C12E5 at the water-dodecane interface, which were determined via SANS.32 As already mentioned, the volume fractions of water and oil, respectively, are φA ) 0.43 and φB ) 0.57, which correspond to equal mass fractions (R ) 0.5). These numbers allow us to calculate the domain size for any surfactant concentration. For a sample of composition R ) 0.5, ψ ) 0.073, β ) 0.031, and γ ) 0.121 one obtains ζ ) 21 nm if the amounts of NIPAm, BisAm, and 12-HOA are neglected and φC ) 0.10 is used. Despite these simplifications, this value is in perfect agreement with the SANS data (see Table 3), and we can conclude that the domain size in the non-gelled and the gelled microemulsions is ζ ) 22 ( 2 nm. 3.5.3. Gelled Microemulsions. On the basis of the considerations made above, we propose a structure for the gelled microemulsion which is a 3D network of fibrils interwoven with a 3D surfactant monolayer of zero (or close to zero; see the discussion of NMR results) mean curvature. A schematic drawing of such a scenario is shown in Figure 8 (right). The NMR, conductivity, and SANS data all confirm the bicontinuous nature of the gelled polymerizable microemulsion. The complexity of the template structure arises from the fact that a microemulsion and a gel are structured on different length scales. As discussed above, the domain sizes of the former are 10-50 nm, while the mesh sizes of the gels are in the micrometer range. Looking at the schematic drawing, one could argue that the mesh sizes of the gel network in the microemulsion have to be much smaller compared to those of a binary organogel to fit into the domains of the microemulsion. In other words, a larger amount of gelator would be needed to form the gel network in the microemulsion, which is indeed what we observed. This picture is further supported by the NMR measurements, according to which the gelator does not diffuse; i.e., the much larger amount of gelator is indeed needed for the formation of a gel network (29) Debye, P.; Anderson, H. R.; Brumberger, H. J. Appl. Phys. 1957, 28, 679. (30) Talmon, Y.; Prager, S. J. Chem. Phys. 1978, 69, 2984. (31) De Gennes, P. G.; Taupin, C. J. Phys. Chem. 1982, 86, 2294. (32) Sottmann, T.; Strey, R.; Chen, H.-S. J. Chem. Phys. 1997, 106, 6483.
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Table 3. Measuring Temperature (T), Mass Fraction of the Gelator (β), Mass Fraction of the Surfactant (γ), Correlation Length (ζTS), Domain Size (ζ ) dTS/2), Periodicity of the Structure (dTS), Amphiphilicity Factor (fa), and Specific Internal Interface (S/V) of the System NIPAm/BisAm/D2O-12-HOA/n-C12H26-C13/15E5 at Constant r ) 0.47 and ψ ) 0.068 T/°C 48.34 33.85 32.46 30.09 33.97
β
γ
ζTS/nm
ζ/nm
dTS/nm
fa
(S/V)/nm-1
0.0 0.018 0.025 0.029 0.031
0.124 0.111 0.111 0.127 0.111
20.2 18.4 20.1 18.1 18.4
22.6 21.5 23.4 23.0 23.1
45.2 42.9 46.7 46.0 46.1
-0.77 -0.76 -0.76 -0.72 -0.73
3.3 × 10-2 5.0 × 10-2 4.5 × 10-2 5.0 × 10-2 3.5 × 10-2
and not for saturating the water/oil interface as we argued in our previous study. We hope to confirm our picture of a gelled microemulsion by FFEM measurements that are currently under way. To conclude, we point out that the structure of the present gelled microemulsion has nothing in common with the structure of so-called microemulsion-based gels. In the latter case waterin-oil microemulsions are used in which the aqueous cores are filled with gelatin. The formation of an infinite cluster consisting of percolated gelatin droplets leads to the gelation of the sample.29
4. Conclusions and Outlook In this paper we have conclusively shown that addition of a gelator (12-HOA) to the oil phase and monomer and cross-linker (N-isopropylacrylamide and N,N′-methylene bisacrylamide) to the aqueous phase of the ternary base system water-ndodecane-C13/15E5 does not alter the bicontinuous structure of the microemulsion, both above and below the sol-gel transition of the gelator (within the confined phase space where the base system is bicontinuous). This conclusion was drawn on the basis of self-diffusion, conductivity, and SANS measurements. Clearly, a gel and a bicontinuous microemulsion can coexist despite the large length scale differences and form a gelled microemulsion. We presented a possible structure which we hope to confirm with FFEM measurements that are currently under way. Having now verified that the gelled microemulsion does indeed have a bicontinuous structure, the next step in this work is to polymerize the aqueous phase, probably via a photoinitiation process. Following 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 gelator-containing phase can be removed by washing the sample, for example, with ethanol. We have now confirmed that the gelled template has indeed a bicontinuous structure; however, it remains to be determined whether the gel can arrest the structure during the polymerization so that the resulting polymer has dimensions similar to those of the templating microemulsion. This has been the challenge in previous attempts where the polymerization of bicontinuous microemulsions usually resulted in macroporous gels with small surface areas and structures about 50 times the original template size.33,34 The only successful oneto-one replication of a bicontinuous microemulsion to date was achieved by using bicontinuous microemulsion glasses as templates during the polymerization.35 The microemulsion glass was obtained by replacing water with sugar. This sugar-based microemulsion contains the liquid monomer whose photopolymerization led to an almost one-to-one replica of the template.35 The great 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,
only easily recyclable components and no organic solvents are involved in the process. However, there are also three important disadvantages: First, the technique is restricted to the synthesis of oil-soluble polymers only. In other words, we still need a route via which a glasslike oil phase can be obtained which, in turn, serves as a template for the polymerization of the aqueous phase. Note that water-soluble polymers with a high surface area are highly desirable for medical and pharmaceutical applications. Second, a nearly water-free polar phase is needed. In ref 35 a complicated dehydration procedure was used, which led to compositional gradients in the sample. An alternative approach was suggested by the same authors only recently, namely, dissolving premixed powders of sugar and surfactant into oil at elevated temperatures (365 K) and cooling the sample to a temperature below the glass transition temperature (which is ∼300 K).36,37 However, successful one-to-one replication using this sugar glass as a template has not been reported yet. Third, due to the glassy and thus highly viscous state of the polar phase, studying the phase behavior of the resultant microemulsions is very time-consuming. The authors themselves state that “the added complexity of the dehydration and photopolymerization steps is such that this approach may be suitable only for the preparation of specialty polymeric membranes for which no suitable solvents are available”. Using a gel instead of a glass has three advantages: First, it can be used for the synthesis of both oil- and water-soluble polymers, once an appropriate gelator for the oil or the water phase has been identified. Second, only 1-4 wt % gelator is necessary, thus avoiding the complicated dehydration procedure required in the case of the sugar glasses. Third, the phase behavior of the templating microemulsion is much easier to study, especially at temperatures above the sol-gel transition. To conclude, we point out that a glass is obviously strong enough to preserve the structure of the template during the polymerization. Does the same hold true for a gel? Work to answer this question is under way. Acknowledgment. Financial support for this work was provided by the Marie Curie Research Training Networks “SelfOrganisation under Confinement (SOCON)” (Contract Number 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), by the Seed Funding Scheme in University College Dublin, and by IRCSET (A.S.). The help of Dr. Henrich Frielinghaus during the SANS measurements is gratefully acknowledged. We are very thankful for countless illuminating discussions with Prof. Kenneth Dawson. LA800918G
(33) Hentze, H. P.; Co, C. C.; McKelvey, C. A.; Kaler, E. W. Top. Curr. Chem. 2003, 226, 197. (34) Palani Raj, W. R.; Sasthav, M.; Cheung, H. M. Polymer 1995, 36, 2637. (35) Gao, F.; Ho, C.-C.; Co, C. C. Macromolecules 2006, 39, 9467.
(36) Dave, H.; Gao, F.; Lee, J.-H.; Liberatore, M.; Ho, C.-C.; Co, C. C. Nat. Mater. 2007, 6, 287. (37) Co, C. C. Soft Matter 2008, 4, 658.
