pubs.acs.org/Langmuir © 2009 American Chemical Society
Two-Component Thermoreversible Hydrogels of Melamine and Gallic Acid† Abhijit Saha, Bappaditya Roy, Ashesh Garai, and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700 032, India Received January 14, 2009. Revised Manuscript Received February 26, 2009 A new two-component hydrogel of melamine and gallic acid is reported for three different compositions of the components. Optical and scanning electron microscopy indicate the fibrillar network structure of the gel, and the DSC study indicates a reversible first-order phase transition in the system. The storage modulus (G’) versus frequency plot is linear and invariant at 35 and 50 °C but not at 70 °C, where it is in the sol state. The rheological melting point of 58 °C is close to the gel melting point obtained from DSC. The system shows a strong influence of pH on gelation. An FTIR study indicates H-bond formation between the >CdO group of gallic acid and the -NH2 group of melamine. 1H NMR spectra indicate the presence of π-π stacking in the gel. The UV-vis peak positions of gallic acid remain unaffected during complexation; however, the normalized absorption intensity is higher in the GM13 sol compared to that of other complexes. The photoluminescence (PL) spectra of the gels are interesting, showing a two-order increase in intensity in the gel state compared to that in the sol state. Three different structures of the complexes are proposed for the three different compositions of the components.
Introduction The self-assembly of small molecules or macromolecules in a solution may entrap a large amount of solvent, immobilizing *Corresponding author. E-mail:
[email protected]. † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue.
(1) Molecular Gels: Materials with Self Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Spinger: Dordrecht, The Netherlands, 2006. (2) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201. (3) Hirst, A. R.; Smith, D. K. Chem.;Eur. J. 2005, 11, 5496. (4) (a) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644. (b) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008, 37, 109. (5) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821. (6) de Loos, M.; Feringe, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2005, 3615. (7) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (8) (a) Ishi-I, T.; Shinkai, S. Top. Curr. Chem. 2005, 258, 119. (b) Kiyonaka, S.; Sugiyasu, K.; Shinkai, S.; Hamachi, I. J. Am. Chem. Soc. 2002, 124, 10954. (c) Mukhopadhyay, P.; Iwashita, Y.; Shirakawa, M.; Kawano, S.-i.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2006, 45, 1592. (9) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491. (10) (a) Suzuki, M.; Yumoto, M.; Kimura; Shirai, H.; Hanabusa, K. Chem.-Eur. J. 2003, 9, 348. (b) Suzuki, M.; Yumoto, M.; Kimura; Shirai, H.; Hanabusa, K. Tetrahedron Lett. 2004, 45, 2947. (11) (a) Yang, Z.; Xu, B. J. Mater. Chem. 2007, 17, 2385. (b) Wang, Q.; Yang, Z.; Zhang, X.; Xiao, X.; Chang, C. K.; Xu, B. Angew. Chem., Int. Ed. 2007, 46, 4285. (12) (a) Smith, A. M.; Williams, R. J.; Tang, C.; Coppo, P.; Collins, R. F.; Turner, M. L.; Saiani, A.; Ulijn, R. V. Adv. Mater. 2007, 20, 37. (13) Lee, K. Y.; Mooney, D. J. Chem. Rev. 2001, 101, 1869. (14) Ellis-Behnke, R. G.; Liang, Y.-X.; You, S.-W.; Tay, D. K. C.; Zhang, S.; So, K.-F.; Schneider, G. E. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 5054. (15) Friggeri, A.; Feringa, B. L.; van Esch, J. H. J. Controlled Release 2004, 97, 241. (16) Tiller, J. C. Angew. Chem., Int. Ed. 2003, 42, 3072. (17) (a) Yagai, S.; Monma, Y.; Kawauchi, N.; Karatsu, T.; Kitamura, A. Org. Lett. 2007, 9, 1137. (b) Yagai, S.; Nakajima, T.; Kishikawa, K.; Kohmoto, S.; Karatsu, T.; Kitamura, A. J. Am. Chem. Soc. 2005, 127, 11134. (18) Vemula, P. K.; John, G. Chem. Commun. 2006, 2218. (19) (a) Leong, W. L.; Batabyal, S. K.; Kasapis, S.; Vittal, J. J. Chem.; Eur. J. 2008, 14, 8822. (b) Kamikawa, Y.; Kato, T. Langmuir 2007, 23, 274. (20) (a) Manna, S.; Saha, A.; Nandi, A. K. Chem. Commun. 2006, 4285. (b) Saha, A.; Manna, S.; Nandi, A. K. Langmuir 2007, 23, 13126. (c) Saha, A.; Manna, S.; Nandi, A. K. Chem. Commun. 2008, 3732. (21) Thermoreversible Gelation of Polymers and Biopolymers; Guenet, J. M., Eds.; Academic Press: London, 1992.
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it and producing physical gels.1-24 The high surface area of the nanostructures developed in the self-assembly process is the main cause of physical gelation. Recently, studies on small molecular gelators have gained considerable momentum, and organogelators have attracted a great deal of attention compared to hydrogelators. However, the hydrogels produced spontaneously by the self-assembly of small molecules are of great interest for various applications such as tissue engineering,13,14 drug delivery,15,16 pollutant capture and release,8b templated nanoparticle synthesis, and so forth.18 For the potential use of hydrogelators in biomedical applications, recently two-component systems producing hydrogels through supramolecular complex formation have attracted much interest.3,20,24 The bicomponent hydrogels have some benefit over one-component small-molecular hydrogels because the supramolecular bonding between the components is very labile25,26 and can be used for the end use of the gel. Because of the nine hydrogen bonding sites present in melamine (Scheme 1), it is widely used as a gelling component20,24,27 by combining with a complementary molecule. Our recent work on the hydrogels of melamine and riboflavin20 and that of Anderson et al.24 on the hydrogels of melamine and uric acid are very encouraging in the search for other new complementary molecules that can produce a supramolecular complex with melamine and hence a gel. The planar, rigid melamine molecule with a good H-bonding ability can produce stable π stacking, yielding the fibrillar network structure in its gel. Hydrogen bond formation is the primary force of complexation, the π stacking of the complex causes the secondary structure, and finally the aggregation of (22) te Nijenhuis, K. Adv. Polym. Sci. 1997, 130, 1. (23) Reversible Polymeric Gels and Related Systems; Russo, P. S., Ed.; American Chemical Society: Washington, DC, 1987. (24) Anderson, K. M.; Day, G. M.; Paterson, M. J.; Byrne, P.; Clarke, N.; Steed, J. W. Angew. Chem., Int. Ed. 2008, 47, 1058. (25) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, Germany, 1995. (26) Supramolecular Polymers; Cifferi, A., Ed.; Marcel Dekker: New York, 2000. (27) Gamez, P.; Reedijk, J. Eur. J. Inorg. Chem. 2006, 29.
Published on Web 03/13/2009
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Letter Scheme 1. Chemical Structures and H-Bondinga,b
a (A) Chemical structure of gallic acid and melamine and visual image of a 2% (w/v) hydrogel of the GM11 complex. b (B) Schematic illustration of possible modes of H-bonding in GM complexes having different molar proportions of the components.
the π-stacked complexes produces the fibrillar network structure forming the two-component hydrogel.20 In the search for other complementary molecules that can produce hydrogels with melamine, gallic acid (G, 3,4,5-trihydroxy benzoic acid) is a promising candidate because the molecule (Scheme 1) has a carboxyl group and a phenolic OH group that can produce H bonds to melamine. Besides, gallic acid is a cheap, nontoxic monomer that occurs from the metabolism of plants.28 In this letter, we present the behavior of the melamine-gallic acid hydrogel produced at different compositions of the components. From the FTIR studies, attempts are made to understand the H-bonding interaction between the components, and 1H NMR and photoluminescence (PL) spectra are used to understand the mode of self-assembly through the π-stacking process. This system is important because new hydrogels are produced in a simple way without the tedious synthesis of any complex organic molecule and without the use of any amphipiles, surfactant, or charged species.
Experimental Section Materials. Gallic acid (G) and melamine (M) were purchased from Aldrich. A mixture of gallic acid and melamine in different mole ratios, such as 1:1 to make the GM11 gel, was made in a glass tube, and water was added to make the total complex concentration of 2% (w/v). It was then sealed, heated to solubilize the components homogeneously, and quenched to room temperature to produce hydrogels. Then the hydrogels (28) Kricheldorf, H. R.; Stukenbrock, T. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2347.
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were vaccuum dried at room temperature to produce GM xerogels. Two other compositions of the complex (e.g., GM31 and GM13) were also prepared by taking the 3:1 and 1:3 molar compositions of G and M, respectively. Microscopy. To understand the network morphology of the gel, small portions of the hydrogels of GM11, GM13, and GM31 complexes of different concentrations, produced at 25 °C, were placed on a glass coverslip. They were dried in air at room temperature and in vacuum and then they were observed through an FESEM instrument (JEOL, JSM 6700F) operating at 5 kV after the surface was coated with platinum. The morphology of the gel is also studied by observation through an optical microscope (Leitz, Biomed) under perfectly crossed polarization and taking the picture through a digital camera (Leica D-LUX 3). Spectroscopy. The FTIR spectra of pure G, M, and GM xerogels were recorded using the KBr pellets of the samples in an FTIR-8400S instrument (Shimadzu). The UV-vis spectra of the samples were recorded on a Hewlett-Packard UV-vis spectrophotometer (model 8453). Fluorescence studies of RM31 hydrogel samples prepared in a sealed cuvette were carried out in a Horiba Jobin Yvon Fluoromax 3 instrument. Each gel sample was contained in a quartz cell of 1 cm path length and was excited at 256 nm. Emission scans were recorded from 300 to 500 nm using a slit width of 5 nm for emission and 3 nm for excitation with a 1 nm wavelength increment having an integration time of 0.5 s. 1H NMR spectra of pure G, M, and GM11 sol and gel of 2% w/v hydrogels in D2O were recorded via a Bruker DPX 300 instrument at 300 MHz (where the concentration of G was the same for all samples). Rheology. Rheological experiments were performed with an AR 2000 advanced rheometer (TA Instruments) using parallel Langmuir 2009, 25(15), 8457–8461
Letter plate geometry in a Peltier plate. The plate diameter was 25 mm, and the plate gap was 500 μm. The experiments were performed from 5 to 70 °C. Two types of experiments were performed: (i) by frequency sweep and (ii) by temperature ramp methods. The frequency sweep experiments were made at 35, 50, and 70 °C where at the two former temperatures the system is in the gel state and at 70 °C the system is in the sol state. The temperature ramp experiments were performed between 10 and 65 °C at a constant frequency of 1 Hz.
Results In Scheme 1, molecular structures of melamine and gallic acid are presented, and a representative visual image of gel formation in an inverted test tube is also shown. It is to be noted that the gel is not transparent; rather it is white in color. The polarized optical micrograph (Figure 1) of the GM11 gel clearly indicates the presence of fibrils dispersed in the gel. To understand the morphology more clearly, the SEM images of the xerogels of GM31, GM11, and GM13 are presented in Figure 2, which indicates different morphologies for GM31, GM11, and GM13 samples. The presence of a fibrillar network structure is evident in GM11 and GM13 gels, but in the GM31 gel, the morphology is rodlike. The fibrils of GM13 are intertwined, and those of GM11 are somewhat twisted. The rods of GM31 are produced from the pilling of fibrils as evidenced from the inset of Figure 2c. The thicknesses of the fibrils are 0.19 ( 0.3 μm for GM11, 0.23 ( 0.04 μm for rods of GM31, and 0.09 ( 0.01 μm for GM13 xerogels. In Supporting Information Figure 1a, the DSC results of the GM11 gel (2% w/v) are presented for heating, cooling, and reheating after keeping the system at 5 °C for 10 min. During the first heating, a broad endotherm is obtained at 63.5 °C whereas during cooling an exotherm is obtained at 9.8 °C. In the second heating, the endothermic peak appears at 43.5 °C, indicating the thermorevesible nature of the gel. This is also true for other GM systems (Supporting Information Figure 1b,c). The difference in the melting peak positions in the first and second heating is due to the probable annealing of the gel (of the first heating) while keeping the temperature at room temperature (30 °C) after its preparation outside of DSC for 4 to 5 days. The annealing causes an improved gel structure than that prepared at 5 °C in DSC, with a higher melting temperature. The lower gelation temperature than that of gel melting indicates hysteresis, which is a common property in the first-order phase transition. Thus, the system shows fibrillar network structure and a reversible first-order phase transition indicating the formation of the thermoreversible gel.21,29
Figure 1. Polarized optical micrograph of a 2% GM11 (w/v) hydrogel. Langmuir 2009, 25(15), 8457–8461
Figure 2. FESEM images of 2% (w/v) xerogels of (A) GM11, (B) GM13, and (C) GM31 (inset containing 3-fold enlarged picture of rods) hydrogels. To get further evidence of gel formation, the rheological measurements of the GM11 system are presented in Figure 3a, b. The G0 (storage modulus) versus frequency plot (Figure 3a) is linear and invariant with angular frequency (ω) (G0 (ω) ≈ ω0,0) at 35 and 50 °C, but the G0 versus frequency plot is not linear and is invariant at 70 °C. The exponent values of G0 (calculated from the slope of the log-log plot in the lowfrequency range (e5 rad/s) for temperatures of 35, 50, and 70 °C are 0.01, 0.02, and 1.08, so at 35 and 50 °C, the system is a gel, but at 70 °C, the system is in the sol state, produced by breaking the network structure.21,22,29-32 In Figure 3a, the G00 (loss modulus) plot of the GM11 gel with temperature is presented, and it is also invariant with frequency. Also, it is below the G0 plot at 35 and 50 °C, but at 70 °C, the (29) Daniel, C.; Dammer, C.; Guenet, J. M. Polym. Commun. 1994, 35, 4243. (30) Garai, A.; Nandi, A. K. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 28. (31) Garai, A.; Nandi, A. K. J. Nanosci. Nanotechnol. 2008, 8, 1842. (32) Tiitu, M.; Hiekkataipale, P.; Hartikainen, J.; Makela, T.; Ikkala, O. Macromolecules 2002, 35, 5212.
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Figure 4. Comparison of WAXS patterns of G, M, GM31, GM13 and GM11 xerogels of 2% (w/v) hydrogels.
Figure 3. (A) log-log plots of storage (G0 ) and loss (G00 ) moduli vs angular frequency for the 2% (w/v) GM11 hydrogel at the indicated temperatures. (B) Oscillatory viscoelastic temperature ramp curve of the storage and loss moduli of the 2% (w/v) GM11 hydrogel at a constant frequency of 1 Hz.
loss modulus is above the G0 plot, showing a crossover point at 30 rad/s indicating pseudosolid behavior of the sol above that frequency.30,31 In Figure 3b, the G0 versus temperature plot of the GM11 system crosses the loss modulus versus temperature plot at 58 °C and shows a sharp fall after that temperature. Thus, the melting point of the gel is at 58 °C, and it is very close to that of the DSC melting peak (62.5 °C). Therefore, these rheological experiments confirm the formation of a thermoreversible gel in the GM-water systems.21,22,29-32 To elucidate the cause of gel formation in this bicomponent system, FTIR spectroscopy is performed. From the FTIR spectra (Supporting Information Figure 2), it is evident that the >CdO peak at 1701 cm-1 of G disappears in the gel, indicating H-bonding interaction between the >CdO group of G and the -NH2 group of melamine. The 3000-3500 cm-1 peaks correspond to -NH vibrations, and these vibrations are completely broadened for GM31 system. On the other hand for GM13 and GM11 systems there are some residual intensity, indicating free (i.e., noninteracting -NH2 groups) in these compositions. So from the FTIR spectra it may be concluded that H-bonding between >CdO group and -NH protons are responsible for supramolecular complex formation. The 1240 cm-1 peak of G is for the C-O vibration, and it becomes shifted to higher-energy vibration (i.e., to 1244 and 1255 cm-1 for GM13 and GM31, respectively, indicating that the C-O group vibration becomes restricted, probably for H-bonding with N-H protons). In the GM11 system, the peak at 1240 cm-1 is totally lost, probably for the same reason discussed above. There is also 8460 DOI: 10.1021/la900156w
the possibility of intermolecular H-bonding through the phenolic -OH groups. Evidence of it can be obtained from the FTIR spectra where the 3267 and 3351 cm-1 peaks are lost in the gels. The supramolecular complex produced by H-bonding can produce a secondary structure through π-π interaction. In Supporting Information Figure 3, the 300 MHz NMR spectra of G, the GM11 gel, and the GM11 sol in D2O are presented. It is apparent from the Figure that the 6.95 ppm peak of G is shielded to 6.92 ppm in the GM11 gel. The 6.95 ppm peak of G is due to the aromatic C-H protons, and it remains the same (6.96 ppm) for GM11 in the sol state. On gelation, π stacking occurs, causing a shielding of the aromatic protons and shifting the peak to 6.92 ppm. Thus, the π-stacking process is very much evident from the 1H NMR spectra.33 We have not observed any peaks from -COOH, -NH2, and -OH (phenolic) protons, probably because of the rapid exchange of protons with D2O. It is to be noted that the intensity of aromatic protons has decreased substantially as a result of gelation. The WAXS patterns of dried GM13, GM11, GM31, M, and G are presented in Figure 4. It is apparent from the Figure that the WAXS patterns of xerogels are different than those of G and M; the patterns are also different from each other. The results indicate that GM complexes produce different crystals than do the components and the crystal nature is also different in different compositions of the complexes. The different FTIR spectra and WAXS patterns of the complexes clearly indicate that they do not represent a state of mixed systems; rather, they are forming three different complexes. Pure gallic acid has UV-vis peaks at 228 and 260 nm, and the UV-vis peak positions of the sols remain intact during complex formation (Supporting Information Figure 4). The normalized spectra indicate that there is some change in the normalized intensity. GM 13 has the highest absorption intensity of all of the others having almost comparable intensity. The constancy of π-π* peak positions and the abrupt rise in intensity of GM13 compared to those of other complexes might be attributed to the symmetry deterioration of gallic acid in the GM13 complex compared to that of other complexes. From Scheme 1, it is evident that in the GM13 complex gallic acid is at the center and the three melamine molecules are at the periphery, increasing the dissymmetry of (33) Mitra, R. N.; Das, D.; Roy, S.; Das, P. K. J. Phys. Chem. B 2007, 111, 14107.
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Letter
the gallic acid significantly. This increased dissymmetry causes an increase in the transition probability34 and hence an enhancement in the intensity of absorption maximum. In other complexes, the symmetry of gallic acid is not strongly affected. From the inset of Supporting Information Figure 4, it is evident that the UV-vis peaks show a red shift upon increasing the concentration of the complex in the sol, but during gel formation, no shifting of the UV-vis peak occurs. The red shift of the UV-vis peaks with the concentration of the GM11 complex may be attributed for the decrease in the band gap for increased overlapping of π orbitals. We have excited the system at 256 nm, and in Supporting Information Figure 5a, the PL spectra of G, the GM11 sol, and the GM11 gel are presented. Gallic acid is PL-active, as is the GM11 sol. In the gel-formation process, the PL intensity increases dramatically in all three compositions, more than that for the pure G solution and also for GM sols (Supporting Information Figure 5b,c). The cause of such a large increase (2 orders of magnitude) may be due to hydrophobic core formation during gelation. Because of π stacking, hydrophobic cores are produced, introducing a hindrance to the deactivation of exitons with water and hence increasing the PL intensity abruptly.20 In Supporting Information Figure 5d, the PL spectra of GM11, GM31, and GM13 are compared at same G concentration. It is apparent from the Figure that GM31 and GM11 are comparable in PL intensity, but in GM13, the intensity is ∼14 times larger than the former intensities. The pH dependency of gel formation of this system is interesting. The systems produce gels at pH e7.0, but at higher pH (e.g., at pH 9), the systems do not produce a gel. Probably because of the ionization of gallic acid at higher pH, the GM complexes are not produced because of the lack of H-bonding.
Discussion The different WAXS patterns and different morphologies of the GM31, GM11, and GM13 gels are very interesting, suggesting different structures of the gels produced at different compositions. In Scheme 1, possible supramolecular complexes for the three different compositions are presented and there is the possibility of seven to nine H-bonds in the complexes (cf. FTIR spectra), so on average, there are 3545 Kcals/mol of energy involved in the complexation, making the supramolecular complexes stable. It is important to note that the GM11 complex has a linear shape (Scheme 1) that can produce good fibrils (Figure 2a). The GM13 and GM31 complexes have reverse structures: GM13 has a gallic acid center and GM31 has a melamine center. GM13 produces intertwined thin fibers; however, GM31 has rodlike morphology produced from the aggregation of fibrils. The structure of GM31 is probably more planar than that of GM13, causing easier π stacking to produce a greater concentration of fibrils that aggregate to a rodlike structure. In the lesser planar GM13 complex, π stacking may be somewhat hindered, causing intertwined fibrils. Thus, the different morphology (34) Kemp, W. Organic Spectroscopy; Macmillan Education: London, 1991; p 276.
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of the GM11, GM13, and GM31 complexes may be qualitatively explained from the different nature of the complexes. It is worthwhile to discuss why the GM13 gel has an ∼14-fold higher PL intensity than the GM31 and GM13 gels (Supporting Information Figure 5d). The reason is that the fibers of GM13 are very thin, so the deactivation of exitons is weaker through the adjacent fibrils, building the fibers. This behavior is similar to that of riboflavin-melamine gels, where the thicker fibers have a weaker PL intensity than do the thinner ones.20b In GM11 and GM31 gels, the thickness of the fibrils is 2 times larger than that of GM13. Here, the deactivation of exitons is greater than that for GM13 through the piled fibrils producing the fibers (GM11) or the rods (GM31). As the fiber thickness increases, the H-bonding interaction between the fibrils increases, decreasing the PL intensity by the deactivation of exitons through H-bonds. This would also cause a lower extinction coefficient (ε) for thicker fibers, giving lower PL intensity. The lower the symmetry, the fewer H-bonds present, causing a lower degree of supramolecular assembly, producing thinner fibrils and causing an increase in PL intensity. Therefore, all of the factors affecting the PL intensity are included in the thickness of the fibrils, in general. GM13 (395 nm) has a blue shift from the peak of GM11 and GM31 (420 nm), which is due to the decrease in the delocalization of exitons in the thinner fibrils, causing an increased energy barrier.
Conclusions A new two-component hydrogel of gallic acid and melamine is produced, showing a dramatic (two orders or magnitude) increase in PL intensity during gelation. The two components produce supramolecular complexes by H-bonding, and the complexes produce fibrils by the π-stacking process. The morphology and WAXS patterns of the xerogels are different for different compositions of the components, indicating different structures of the complexes arising from different modes of self-assembly. The UV-vis peak positions of gallic acid remain unaffected during complexation; however, the normalized absorption intensity is higher in the GM13 sol compared to that of other complexes. The dramatic increase in PL intensity has been attributed to hydrophobic core formation during gelation. At higher pH (e. g., pH 9) the systems do not produce a gel. Acknowledgment. We gratefully acknowledge the Department of Science and Technology, New Delhi (grant no. SR/S1/PC-32/2004), for financial support. A.S. and B.R. acknowledge CSIR, New Delhi, for providing their fellowship. Supporting Information Available: DSC thermograms of GM hydrogels. FTIR spectra of G, M, and xerogels of GM11, GM31, and GM13. 1H NMR spectra of pure G, M, the 2% (w/v) GM11 sol, and the GM11 gel. Normalized absorption spectra of G, GM11, GM31, and GM13. PL spectra of G, GM11, GM31, and GM13 2% (w/v) sols and gels. This material is available free of charge via the Internet at http://pubs.acs.org.
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