3038
Langmuir 1996, 12, 3038-3044
Formation of Ultrathin Multilayer and Hydrated Gel from Montmorillonite and Linear Polycations Yuri Lvov,† Katsuhiko Ariga,† Izumi Ichinose,‡ and Toyoki Kunitake*,† Supermolecules Project, JRDC, Kurume Research Center Building, Kurume, Fukuoka 839, Japan, and Faculty of Engineering, Kyushu University, Fukuoka 812, Japan Received November 3, 1995. In Final Form: March 22, 1996X Organized ceramic/polymer nanocomposites were designed from montmorillonite clay (Mont), cationic poly(ethylenimine) (PEI) or poly(dimethyldiallylammonium chloride) (PDDA) and proteins. Ordered multilayers with a controllable number of layers were prepared by alternate adsorption of delaminated montmorillonite (0.03 wt % dispersion) and polycations. Quartz crystal microbalance, X-ray diffraction, and scanning electron microscopy monitoring of the assembly have shown formation of ordered films with bilayer thickness of 3.3 nm for Mont/PEI and 3.6 nm for Mont /PDDA. Increased viscosity near the substrate was recognized at higher concentrations of Mont (1 wt %). At these and higher concentrations super hydrated (water content 99.4 wt %) solid gels of Mont and polycations were produced by mixing of their aqueous dispersions. Its open card-house structure was revealed with a scanning electron microscopy study.
Introduction Construction of organic/inorganic nanostructured materials is an important target of modern materials research. Formation of nanocrystalline particles under Langmuir monolayers has been intensively studied to prepare ordered multilayers composed of CdS, ZnS, PbSe and silver nanoparticle layers “sandwiched” by amphiphile bilayers.1,2 Highly ordered organic/inorganic superlattices were formed by casting bilayer-forming amphiphiles with subsequent dipping in aqueous cadmium halides.3a Another method of preparing nanocomposite films is casting of mixtures of metal oxide particles and amphiphile molecules. In such processes multilayers of alumina particles and amphiphile bilayers were produced,3b and Li- or Al-double-metal hydroxide was alternated with organic acids.4 The intercalation of organic polymers into layered ceramics provides an access to novel polymer-ceramic nanocomposites. They exhibit unique physical and mechanical properties attributed to synergism of the individual components.2,5 Their properties are not necessarily the same as those found in dispersion of single ceramic layers in bulk polymer matrix or in solution. The selfassembling nature of the components yields two-dimensional submicrometer lamella which are stacked by hundreds in the third dimension.5 If we can build-up similar architectures in stepwise manners rather than in the all-in-once manner, even more elaborate nanocomposites will become available. There have been reported a variety of publications on organic/inorganic ceramics (and clay) systems,5-10 but only a few of them are devoted to production of controlled multilayers with molecular precision. Recently Kotov et †
Supermolecules Project. Kyushu University. X Abstract published in Advance ACS Abstracts, May 15, 1996. ‡
(1) Fendler, J. H. In Thin Films, v.20. Organic thin films and surfaces: directions for the nineties; Ulman, A., Eds.; Academic Press: San Diego, New York, Boston, 1995; pp 11-40. (2) Ozin, G. Adv. Mater. 1993, 4, 612. (3) (a) Ichinose, I.; Kimizuka, N.; Kunitake, T. J. Phys. Chem. 1995, 99, 3736. (b) Tsutsumi, N.; Sakata, K.; Kunitake, T. Chem. Lett. 1992, 1465. (4) Dutta, P.; Robins, D. Langmuir 1994, 10, 1851. (5) (a) Giannelis, E. J. Mater. 1992, March, 28. (b) Mehrotra, V.; Giannelis, E. Solid State Ionic 1992, 51, 115. (c) Via, R.; Ishii, H.; Giannelis, E Chem. Mater. 1993, 5, 1694.
S0743-7463(95)01002-X CCC: $12.00
al.9 demonstrated the formation of Langmuir monolayers of hectorite plates treated by amphiphiles and its deposition on solid substrates with the help of a horizontal lift method. Kleinfeld and Ferguson10 applied for the first time the electrostatic layer-by-layer adsorption of oppositely charged components to produce ultrathin ceramic/ polycation multilayers. The principle of the alternate adsorption was invented for oppositely charged colloidal particles in 1966 in the pioneering work of Iler.11 Mallouk et al.12 later developed alternate adsorption of Zr4+ ions and diphosphonic acid, and Decher and co-workers13 extended this technique to linear polycations and polyanions. The procedure is described as follows (Figure 1a): A solid substrate with the charged planar surface is immersed in a solution containing oppositely charged polyions, and the polyion layer is adsorbed (step A). Since adsorption is carried out at relatively high concentrations of polyelectrolyte, a number of ionic groups remain exposed to the solution interface, and thus the surface charge is effectively reversed. After being rinsed in water, the solid substrate is immersed in a solution containing oppositely charged polyions or clay particles. Again a charged polymer layer is adsorbed, but now the original surface charge is restored (step B). By repetition of both steps (A, B, A, B, ...) in a cyclic fashion, alternating multilayer assembly may be obtained. Figure 1b illustrates an alternate adsorption procedure where charged solid plates are assembled at every other step. Recently, the procedure was extended (6) Bunker, B.; Rieke, P.; Tarasevich, B.; Campbell, A.; Fryxell, G.; Graff, G.; Song, L.; Liu, J.; Virden, J.; McVay, G. Science 1994, 264, 48. (7) Nakamura, T.; Thomas, J. J. Phys. Chem. 1986, 90, 641. (b) Carminati, S.; Carniani, C.; Miano, F. Colloids Surf. 1990, 48, 209. (c) Newman, A. C. Chemistry of Clays and Clay Minerals; Mineralogical Society: London, 1987. (8) Isayama, M.; Sakata, K.; Kunitake, T. Chem. Lett. 1993, 1283. (9) Kotov, N.; Meldrum, F.; Fendler, J.; Tombacz, E.; Dekany, I. Langmuir 1994, 10, 3797. (10) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (11) Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569. (12) (a) Lee, H.; Kepley, L.; Hong, H.-G., Mallouk T. J. Am. Chem. Soc. 1988, 110, 618. (b) Yang, H.; Aoki, K.; Hong, H.-G.; Sackett, D.; Arendt, M.; Yau, S.-L.; Bell, C.; Mallouk, T. J. Am. Chem. Soc. 1993, 115, 11855. (c) Bell, C.; Arendt, M.; Gomez, L.; Schmehl, R.; Mallouk, T. J. Am. Chem. Soc. 1994, 116, 8374. (13) (a) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (b) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160. (c) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481.
© 1996 American Chemical Society
Ultrathin Multilayer and Hydrated Gel
Figure 1. Simplified schemes of alternate polyion assembly: (a) linear polyanions and linear polycation; (b) linear polyanion and plate-shaped macrocation; (c) linear polyanion and cationic protein. Chart 1
to introduction of globular proteins as charged layers, providing ordered multicomponent protein films14a-d and semiconductor nanoparticles14e as shown in Figure 1c. The aim of this work is to establish the use of clay montmorillonite as polyanion in the alternate adsorption process (Chart 1). Linear polymeric ions are almost always effective as a component of this process. However, other types of polyions can pose problems, as is shown by attempted alternate assembly of proteins.14d The globular shape of aqueous proteins was not suitable for direct protein assembly. Since montmorillonite is a planar macroion, its behavior in the assembly process would be different from those of linear polymer ions. The use of charged inorganic sheets has been briefly discussed in the past;10 however, their behavior is not well characterized. 1. Experimental Section Materials. Poly(diallyldimethylammonium chloride) (PDDA, Aldrich), sodium poly(styrenesulfonate) (PSS, Aldrich), and poly(ethylenimine) (PEI, Wako) were commercially available and used without further purification at a concentration of 1.5 mg/ mL (Chart 1). The pH of the solutions was adjusted by adding aqueous HCl. PDDA is always a charged linear polycation and PEI (pKa ) 10.8) is a highly branched polycation. PSS (pKa ) (14) (a) Lvov, Y.; Ariga, K.; Kunitake, T. Chem. Lett. 1994, 2323. (b) Kong, W.; Wang, L.; Gao, M.; Zhou, H.; Zhang, H.; Li, W.; Shen, J. J. Chem. Soc., Chem. Commun. 1994, 1297. (c) Keller, S.; Kim, H.-N.; Mallouk, T. J. Am. Chem. Soc. 1994, 116, 8817. (d) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (e) Kotov, N.; Dekany, I.; Fendler, J. J. Phys. Chem. 1995, 99, 13065.
Langmuir, Vol. 12, No. 12, 1996 3039 1) is a linear polyanion. Horse heart myoglobin and chicken egg white lysozyme (both from Sigma) were used at pH 5, where they are positively charged. Glucose oxidase Aspergillus niger (GO, Wako) was used in water solution at a concentration of 1-2 mg/ mL. Ultrapure water with the specific resistance better than 18 MΩ cm was obtained by reverse osmosis followed by ion exchange and filtration (Yamato-WQ500, Millipore, Japan). Montmorillonite (Mont) used is a high-purity sodium montmorillonite (Kunipia F, Kunimine Kogyo, Japan) with the composition Na0.66(OH)4Si8(Al3.34Mg0.66)O20. It was dispersed by ultrasonication for 3 min in water and adjusted to 0.3 mg/mL, pH 6.5. The average dimensions of well-dispersed montmorillonite particles are 1 × 500 × 500 nm (according to SEM and light scattering data supplied by Kunimine Kogyo). The montmorillonite sheets are negatively charged and are balanced by interlamellar sodium cations that become dissociated in water. Quartz Crystal Microbalance Technique. The quartz crystal microbalance (QCM) technique is based on the tendency of a piezoelectric crystal to change its natural oscillation frequency when additional mass deposition or depletion on the crystal electrodes takes place.15,16 A QCM device produced by USI System, Japan, was used in order to control the assembly process. The quartz resonator frequency was stable for several hours at (2 Hz. All experiments were carried out in an air-conditioned room at ca. 22 °C. Resonators used were covered by evaporated silver electrodes on both faces and its resonance frequency was 9 MHz (AT-cut). The QCM technique was operated in two modes: (1) The resonator was immersed for a given period of time in a polyelectrolyte solution and dried in nitrogen stream and frequency changes were measured. (2) In the in situ monitoring of adsorption, only one side of the resonator was in contact with the surface of the solution. This arrangement developed by Ebara and Okahata16 made possible on-line control of the assembly process. In the first stage a well-defined precursor film with a thickness of ca. 10 nm was assembled from PEI and PSS onto resonators, quartz slides, or silicon wafers. The precursor films contained three to five polyion layers in the alternate mode: PEI/PSS, and the terminal layer was “positive” PEI (or PDDA). Then the resonator or silicon wafer were alternately immersed for 20 min in aqueous dispersions of Mont clay and in aqueous PEI or PDDA with intermediate water washing. This process was periodically interrupted for the purpose of measuring QCM resonance frequency. The following relationship is obtained between adsorbed mass M (g) and frequency shift ∆F (Hz) by taking into account characteristics of quartz resonators used15,16
∆F ) (-1.83 × 108)M/A
(1)
where A is the apparent area of quartz microbalance placed between QCM electrodes and is 0.16 ( 0.01 cm2 in our system. Then, one finds that a 1 Hz change in ∆F corresponds to 0.9 ng in weight. The thickness increase on one side of the resonator is given by14d
d(nm) ) 0.027(-∆F(Hz))/F
(2)
where F is densities of adsorbed materials: 2.5 g/cm3 for Mont and 1.2 g/cm3 for the polycation.17 Because of uncertainties in the surface area and density, the calculated thickness is reliable to (10%. The validity of eq 2 is supported by SEM observation of the film, as discussed below. Other Measurements. A resonator or a silicon wafer with an assembled film was cut and coated with 20 Å thick Pt by use of an ion coater (Hitachi E-1030 ion sputter, 10 mA/10 Pa) under argon atmosphere. Scanning electron micrographs were obtained with a Hitachi S-900 instrument at an acceleration voltage of 25 kV. (15) Sauerbrey, G. Z. Phys. 1959 155, 206. (16) (a) Ebara, Y.; Okahata, Y. Langmuir 1993, 9, 574. (b) Ebara, Y.; Okahata, Y. J. Am. Chem. Soc. 1994, 116, 11209. (17) (a) Polymer Handbook; Brandrup, J., Immergut, E., Eds.; WileyInterscience Publishers: New York, Chichester, Brisbane, Toronto, 1975; A part 5. (b) Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 1988.
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Lvov et al.
Small-angle X-ray reflectivity experiments were performed with a Siemens D500 diffractometer at J. Gutenberg University, Mainz, Germany. In the reflectivity geometry (Θ-2Θ scanning), the intensity distribution normal to the sample surface was measured. At this stage of the data analysis, no attempts were made to calculate the full density profiles because of rather poor X-ray patterns that contained only two fringes and no Bragg reflections. We estimated the total film thickness from X-ray Kiessig fringes. The interference between beams reflected from the air/polyion-film interface and from the polyion/siliconsubstrate interface leads to Kiessig fringes. We used the following formula18 for the thickness analysis
(1 - δ/sin2 Θk) sin Θk ) kλ/2d
(3)
where δ is deviation from the refractive index, n ) 1 - δ, λ is wavelength of the incident beam (0.154 nm), and 2Θ is the scattering angle. The thickness d was calculated to an accuracy of (2 nm using the fringes with k ) 2 and 3 for two samples: PEI/PSS/PEI + (Mont/PEI)m, m ) 10 and 11. The δ value was assumed to be 3 × 10-6.18 Differential scanning calorimetry (DSC) of Mont/PEI gels was performed with a DSC 120 / SSC5200 system (Seiko Inst., Japan) in the range of -20 to + 10 °C with a scanning speed of 0.5 °C /min.
2. Results Assembly of Mont/PEI and Mont/PDDA Multilayers. Figure 2 shows QCM monitoring of montmorillonite assembly onto resonators. The first three steps in Figure 2a represent formation of a precursor film with the terminal “positive” PEI layer. Then alternate Mon/PEI layers were assembled. One can see a linear change in -∆F with steps of 100 Hz for Mont and 100 Hz for PEI. The total frequency shift (-∆F) for the alternate adsorption cycle is 200 ( 10 Hz. Interestingly, the mass increase in the Mont adsorption remained unchanged regardless of whether we used its dispersion of either 0.03, 0.15, or 1 wt %. This indicates that after conversion of the surface charge to a negative one upon clay adsorption, an additional Mont attachment is prevented by electrostatic repulsion. At some steps we omitted the drying process after Mont adsorption and conducted the following assembly cycles immediately. However, the step of growth was the same in all cases (see cycles 14-16). It is obvious that the assembly process is based on saturated adsorption of the two components and drying is not obligatory for the film growth. We performed this assembly cycle independently six times and the results were reproducible. The use of PDDA in place of PEI gave essentially the same results as given in Figure 2b. The step of growth for Mont/ PDDA was -∆F ) 240 ( 10 Hz: 140 Hz for Mont and 100 Hz for PDDA. Additionally we assembled a more complicated film with {Mont/PEI/PSS/PEI}. When this cycle was repeated four times, we obtained an identical growth step of -∆F ) 280 ( 20 Hz. This figure is composed of 100 Hz for a Mont layer and 180 Hz for the triple PEI/PSS/PEI interlayer. We can increase the distance between the two clay sheets from 2.2 to 4.0 nm by this architecture. The polycation “glue” approach to get together negative montmorillonite clay sheets and negatively charged glucose oxidase globules with the unit cell: GO-/PEI/Mont-/PEI was described earlier.14d These methods open a way to construct artificially orchestrated protein systems that can carry out complex enzymatic reactions. We obtained layer thicknesses of 1.1 nm for each Mont adsorption and 2.2 nm each PEI adsorption by using eq (18) (a) Rietord, F.; Benattar, J.; Bosio, L.; Robin, P.; Blot, C.; Kouchkovsky, R. J. Phys. 1987, 48, 679. (b) Tippmann-Krayer, P.; Mo¨hwald, H.; Lvov, Y. Langmuir 1991, 7, 2298.
Figure 2. Frequency shift (-∆F) due to alternate Mont/PEI adsorption: (a) Mont/PEI and (b) Mont/PDDA; (9) Mont adsorption steps, (O) PEI adsorption, and (+) PDDA adsorption. Steps 13-15 were continued without intermediate drying, but linear mass increases were clearly noticed.
2 (Table 1). In the case of the Mont/PDDA assembly, the thickness of the Mont unit layer is slightly larger (1.4 nm). The layer thickness of a montmorillonite crystal is shown to be 0.96-1.55 nm depending on the extent of hydration (see product manual of Kunipia F, Kunimine Kogyo). It is clear from these data that essentially a single layer of Mont is adsorbed on the cationic surface in each adsorption step. The difference in thickness of the Mont layers on PEI and PDDA may be caused by different hydration due to a different surface charge of these polycations. A SEM photograph of a film containing 16 alternate layers of Mont and PEI is presented in Figure 3. The film is stacked from Mont plates having 1 nm thickness and ca. 500 nm width. PEI with 2 nm thickness is located between its sheets. The film contains large sections with constant thickness of 30 ( 5 nm. The uniformity of the film thickness is remarkable, if we take into account that the diameter of a Mont sheet is ca. 15 times larger than the film thickness (i.e., a nonflat location of Mont would disturb drastically the film). Two types of defects are visible in Figure 3. There are scrambles due to adsorption of non-delaminated Mont particles (A). The second type of defect, probably, is due to overlapping of edges of Mont lamella (B). X-ray reflectivity curves for the {PEI/PSS/PEI + (Mont/ PEI)10-11} films were poor; they contained only two Kiessig fringes, and no Bragg reflections. From positions of these
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Langmuir, Vol. 12, No. 12, 1996 3041
Table 1. Characteristics of Multilayer Assembly of Clays and Polycations step of growth for clay + polycation, nm
τ, clay (10%, min
τ, polycations (10%, min
Mont/PEI
1.1 + 2.2 ) 3.3
1.8
1.2
Mont/PDDA Hect/PDDA (ref 10)
1.4 + 2.2 ) 3.6 bilayer 3.2-3.8
1.5 ≈0.1
2 ≈0.1
bilayer thickness, nm (from X-ray diffraction) 3.6 ( 0.4 (from Kiessig fringes) (the spacing is 1.45 from Bragg reflection)
Table 2. Physical States of Aqueous Mixtures of Mont and PEI at Different Concentrations: A, Solid Gel; B, Soft Gel; C, Viscous Liquid; D, Liquid with Minor Inclusions; E, Soluble Complex
Figure 3. Scanning electron micrographs of {PEI/PSS/PEI + (GOD/PEI)8} film on a silicon wafer.
Figure 4. In situ monitoring of the frequency change (F) during consecutive adsorption of PEI and Mont without interrupting the process for drying. At the last stage of the assembly an increased Mont concentration (1 wt %) was used.
fringes film thickness was calculated following eq 3. For {PEI/PSS/PEI + (Mont/PEI)10} film the thickness is 45 ( 2 nm, and for {PEI/PSS/PEI + (Mont/PEI)11} it is 48.6 ( 2 nm. We can estimate then the thickness of a Mont/PEI bilayer as 3.6 ( 0.4 nm. This value is close to the value of 3.3 nm calculated from QCM measurements as given in Table 1. Adsorption Kinetics in Alternate Assembly. The kinetic process of alternate Mont/polycation adsorption was followed by using an in situ QCM technique. Figure 4 shows consecutive PEI/Mont adsorption steps. The first step is adsorption of PEI onto the Mont terminal layer as shown by open circles. The subsequent adsorption of Mont onto the terminal PEI layer is given by filled circles. The third process is adsorption of PEI onto the Mont layer. One can see that adsorption in the first three steps reaches saturation in 5-6 min. The characteristic time (firstorder rate) of adsorption, τ, is 1.8 ( 0.2 min for Mont and 1.2 ( 0.2 min for PEI. The adsorption kinetics for the
Mont concn (wt)
0.05%
0.1%
0.5% 1% 2% 4%
E E E C
E E C C
PEI concentration (wt) 0.2% 0.4% 0.8% 1.6% E A A B
E E A A
E E C A
E E D C
3.2% E E D C
Mont/PDDA system was approximately the same. These adsorption rates are faster than those (τ ) 2-4 min) measured earlier for linear polycations and polyanions or charged proteins.13c,14d The last branch in Figure 4 shows a different process. At this stage we increased the concentration of Mont to 1 wt %. The adsorption did not reach saturation in this case, and more than one layer of Mont plates was undoubtedly adsorbed. When we removed the resonator from the solution surface after the last run, a tail of viscous gel followed the resonator. Apparently, Mont plates formed a more complex structure on the PEI surface, and we need to use low Mont concentrations in order to produce a single, uniform layer in each adsorption step. Nevertheless, in alternate assembly of Mont (1 wt % dispersion)/ PEI with careful sample washing, at every adsorption circle a film structure similar to that of 0.03 wt % Mont was obtained. Gel Formation and Protein Inclusion. The preceding results at a high Mont concentration are indicative of formation of a Mont/PEI gel in bulk water medium. Thus, we prepared stock solutions of Mont by sonication (see above) and PEI at different concentrations and examined properties of their mixtures. When equal volumes of 1 wt % aqueous Mont and 0.2 wt % aqueous PEI were mixed quickly (in 5-10 s) the whole mixture turned a uniform gel. The gel formation was observed only for limited mixing ratios of the two components. When we mixed Mont and PEI solutions in other ratios, the mixtures remained liquid or contained minor inclusions. Table 2 presents a systematic study of the concentration effect on the gel formation. One can see that the gel formation is favored in part. The gel is not formed with 0.5 wt % Mont irrespective of the PEI concentration. It is formed for combination of concentrations of 1% (Mont)-0.2% (PEI), 2%-0.4%, and 4%-0.8%. The amount of charges for negative clay and positive PEI are approximately equal under these conditions.20 Thus, uniform superhydrated gels were formed in neutral mixtures. The presence of this neutralization point may indicate recharging of the Mont surface at higher concentrations, preventing clay flocculation (and the lack of charge compensation prevents flocculation at lower concentrations). Again, as in the case of layer-by-layer adsorption, (19) Horn, D. In Polymeric Amines and Aminoions; Goethals, E., Ed.; Pergamon Press: Tarrytown, NY, 1980; pp 333-355. (20) In these calculations we have taken into account PEI and Mont concentrations, molecular weights, and charge of monomers, which was -0.66 of the elementary charge unit for Mont {(OH)4Si8(Al3.34Mg0.66)O20} and +0.2 for PEI {-CH2-CH2-NH-} in according to its ionization degree at pH 7.19
3042 Langmuir, Vol. 12, No. 12, 1996
Figure 5. A 1 cm piece of gel composed from 1 wt % of Mont and 0.2 wt % PEI solutions.
we are dealing with recharging of a Mont surface. Maximal flocculation (called in the paper “solution destabilization”) was found for kaolin/PEI,21a,b when a net charge of the mixture was close to zero. Increasing or decreasing of PEI concentration prevented flocculation. The amount of PEI at this point was 2-4 mg for 1 g of kaolin at pH 7-10. It is much less than in our case (200 mg of PEI on 1 g of Mont). This discrepancy comes from the fact that we used delaminated Mont sheets, but kaolin particles had diameters of 0.1-1 µM in the flocculation work. The surface charge of kaolin must be much less than that of Mont. Even when pressed against a solid substrate, the gel kept its shape unchanged for hours (Figure 5). Its shape is not droplet-like, but shows some random edges. The concentration range of the gel formation is most sharply defined at low component concentrations. Mixing of 1 wt % Mont and 0.2 wt % PEI forms a gel, but the range is more smeared at higher component concentrations, and the mixtures remain liquid outside these ranges. The water content in the gel is as high as 99.4 wt % at 1 wt % Mont and 0.2 wt % PEI, so the gel increases its volume by ca. 400 times as compared with that of dry components. When kept in a closed bottle (to avoid drying) the gel was stable and uniform for at least 9 months. Upon heating up to 100 °C, it partially melts with water separation. Under mechanical pressures water was also separated. The Mont gel is not destroyed in 0.1 M HCl and is stable in 0.1 M NaCl solutions. Nevertheless, the homogeneous gel texture may be destroyed by organic solvent with low dielectric constants. Addition of 20 wt % of ethyl alcohol changes the gel structure with penetration of alcohol into gel. Many solid particles appeared and the gel became more fluid. Similar shrinkage of organic polyelectrolyte gels was recorded by Tanaka et al.22a,b As the dielectric constant of the polyion gel medium decreases, trapped counterions form ion pairs and decrease the surface charge, making gels shrink.22c Interestingly, we can incorporate water-soluble proteins in these gels. Aqueous solutions of myogobin or lysozyme, 2 mg/mL, positively charged at pH 5 were mixed with aqueous PEI, with the final PEI concentration of ca. 0.2 wt %. A gel was formed when this cationic solution was mixed with 1 wt % aqueous Mont in equal volume. In the case of myoglobin, the gel has a red-brown color. Correspondingly, negatively charged glucose oxidase was dissolved in 1 wt % aqueous Mont suspension (in such a way that final GO concentration was of 2 mg/mL). The gel was formed by its mixing with an equal volume of 0.2% PEI or PDDA. The resulting gel was solid and had (21) (a) Alince, B.; van de Ven, T. G. M. J. Colloid Interface Sci. 1993, 155, 465. (b) Gregory, J. J. Colloid Interface Sci. 1976, 55, 35. (c) Stenius, P.; Ja¨rnstro¨m, L.; Rigdahl, M. Colloid Surf. 1990, 51, 219. (22) (a) Tanaka, T. Phys. Rev. Lett. 1978, 40, 820. (b) Shibayama, M.; Tanaka, T. J. Chem. Phys. 1992, 97, 6842. (c) Khokhlov, A.; Kramarenko, E. Macromol. Theory Simul. 1994, 3, 45.
Lvov et al.
slightly yellow color, corresponding to the color of GO solution. GO in the gel preserved its enzymatic activity as was demonstrated with the glucose oxidase substrate kit SK-3100 (Vector Lab. Inc., CA, USA). Microstructure of the Gel. Figure 6 shows SEM micrographs of initial Mont powder, hydrated gel, and dried gel. A bulk Mont powder before delamination is seen in Figure 6a as a solid. Figure 6b shows a Mont/PEI gel: single Mont plates with thickness of 1 nm are uniformly distributed with more or less periodical junctions between them. In this case the sample was quickly dried in vacuum just before SEM observation. The junctions are often starlike, as may be seen at a higher magnification (Figure 6c). Even after a slow (some days) drying of the gel in air (Figure 6d) the pattern remains more close to the gel structure than to the initial powder. Junctions between plates induced by PEI prevent Mont from reversion into tight packing. This structure reminds us of earlier “open card-house structure” for kaolin clay with face-edge plate association.21c Slow freezing of a gel down to -15 °C results in ice formation, and the subsequent ice melting destroyed the gel completely. The thermal behavior was studied more precisely with the help of differential scanning calirometry (DSC). The freezing point of water in the Mont/PEI gel is shifted to -12.5 °C and the melting point is located at -1.2 °C in the melting scan. The freezing point shift is much larger than what one can expect from molecular depression of the freezing point due to a dissolved substance (it cannot be more than -1 °C at concentrations of 1 wt %).17b The shift may be due to the structural reorganization of the water in the gel, being separated in small sections by Mont walls. The separation is estimated to be 500-1000 nm from Figure 7b, and water may show a behavior similar to that in narrow capillaries. Gel formation also takes place in aqueous mixtures of Mont/PDDA, but its concentration dependence is not clear. Gels were formed for PDDA concentrations from 0.1 to 4 wt % (higher concentrations were not analyzed) and 1 wt % Mont. SEM observation of the Mont/PDDA gel indicated morphologies similar to that of Mont/PEI gel. 3. Discussion Figure 7 presents a schematic illustration of the mixing behavior of Mont and polycations. The spatial organizations of Mont from different preparations are all stable, and they do not interconvert with each other. The multilayer films are water insoluble, and it is impossible to mechanically spread the gel as a thin film on the solid surface. Spontaneous formation of Mont/polycation gels in bulk solutions results from fixation of Mont plates together in random orientations and with water-filled voids. On the other hand, alternate fixation of the two components in densely-packed multilayers is realized by subsequent immersion of a solid support in solutions of Mont and polycation. It is essential to use a low concentration clay dispersion (less than 0.1 wt %) for controllable Mont multilayer architecture. When 0.03 wt % Mont solution is used, one Mont particle, with an average diameter of ca. 500 nm exists within a solution cube with 2000 nm edge. Then a Mont particle has room for sufficient undisturbed movement. At much higher concentrations, neighboring clay particles would compete in the space area for interaction with the underlying polycation, and the multiple-particle adsorption, if it occurs, would produce steric difficulties resulting in layer defects. The layerby-layer adsorption is still possible at higher concentrations (a few percent), but its mechanism is not clear. Careful sample washing after adsorption may remove
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Langmuir, Vol. 12, No. 12, 1996 3043
Figure 6. Scanning electron micrographs: (a) Mont powder; (b) gel formed from 1 wt % Mont/0.2 wt % PEI; (c) magnified view of the junction between Mont sheets in a gel; (d) gel dried in air during some days and then controlled by SEM.
nonparallel, weakly-attached Mont plates from its substrate. Increased viscosity observed for 1 wt % Mont dispersion in the vicinity of the film surface could be caused by protrusion of PEI chains and its partial removal from the outermost layer of the film. The thickness of the polycation layer incorporated between Mont sheets is ca. 2.2 nm, i.e., four to five stacked polymer chains. It was reported that within 2 nm from ceramic surface crystalline ordering of polymer may be induced.5 We cannot judge the packing type of polycations
in the films, but we have the ability to vary the thickness of the polymeric interlayer between Mont sheets up to 10 nm or more by additional alternate adsorption of organic polyanion and polycation. Alternation of “hard” Mont plates and “soft” linear (or branched) polycations is important. Flexible linear polycations optimize electrostatic interaction and smooth defects and strengths between neighboring Mont sheets. Taking into account our earlier experience on protein/ polyion multilayer formation,14d we have tried to develop
3044 Langmuir, Vol. 12, No. 12, 1996
Figure 7. Preparative schemes of Mont/polycations nanocomposites.
the assembly procedure performing an alternate adsorption of negatively charged Mont sheets and positively charged myogobin or lysozyme (at pH 5). But in both cases only one monolayer of myogobin or lysozyme was adsorbed onto the Mont layer, and subsequent adsorption of montmorillonite or proteins have not taken place. Direct assembly of oppositely charged clay and proteins was difficult, probably because electrostatic attraction cannot be maximized with planar Mont macroions and protein globules. The density and location of charged units on the protein surface should be crucial in this respect. The “patched” nature of the protein surface charge23 is known to favor polyion anchoring. Flexible linear polyions can produce optimized electrostatic attraction, and they may penetrate in between protein molecules and act as electrostatic glue. A superlattice consisting of alternating Mont, PEI, and glucose oxidase monolayers was assembled.14d PEI in this film joined negative layers of montmorillonite and glucose oxidase. It demonstrates the possibility of bio/ceramic nanocomposite films. As well we have shown the method of protein incorporation in Mont/polycation gels, admixing negatively or positively charged proteins to clay suspension or polycation solution. It is of interest to compare our results with that of Kleinfeld and Ferguson on the alternation of hectorite (a mica-type synthetic silicate) and polycation PDDA.10 They
Lvov et al.
dispersed hectorite in water and used it as two-dimensional sheets of ca. 1 × 30 × 30 nm in combination with 0.2-5 wt % PDDA solution. The thickness of the hectorite/ PDDA bilayer was 3.2 nm for a 5 s adsorption and 3.8 nm for a 15 min adsorption. An X-ray study showed spacing of 1.45 nm, consistent with ABABAB alternation of 1 nm thick silicate sheets and a 0.5 nm polymer layer. The 2-fold difference between X-ray spacing and the step of growth measured by ellipsometry were explained by deposition of hectorite as bilayers within one adsorption cycle. This situation comes from the surrounding of hectorite pieces by long PDDA chains. Details of saturation and recharge processes are not clear for this case. In a very short immersion time, only anchoring of particles to the surface may take place, but further relaxation and mono- or bilayer formation probably takes place during the intermediate drying process. The minimal adsorption time for the hectorite/PDDA assembly was reported to be much less than the adsorption rate constant found in our kinetic experiments for Mont/PDDA. Related clay/polycation gel formation was described in a patent.24 The gel (with the suspension up to ca. 60-fold) was formed by mixing a 6 wt % solution of bentonite clay and 0.2 wt % polyacrylamide in a 4:1 ratio. These concentrations are much higher than those we used. The use of delaminated montmorillonite with a high surface charge permitted us to increase the extent of hydration. 4. Conclusions The assembly process of montmorillonite/polycation multilayers with precise control of its architecture is elaborated. We may produce exactly layered (one, two, three, four, five, or more) molecular films or solid gels, depending on the concentration of the two components and preparative methods. Resulting alternate films and highly-hydrated gels would give rise to useful functional materials. The methods of enzymes incorporation in clay/ polyion films and gel are demonstrated. Acknowledgment. We are thankful to Dr. M. Isayama (Fukuoka Industrial Technology Center, Japan) who provided us with the montmorillonite sample and a procedure for delamination, to Professor G. Decher (University of Strasbourg, Institute Charles Sadron) for the measurement of X-ray reflectivity, and to Professor A. Khokhlov (Moscow University, Russia) for useful remarks. LA951002D (23) (a) Park, J.; Muhoberac, B.; Dubin, P.; Xia, J. Macromolecules 1992, 25, 290. (b) Xia, J.; Dubin, P.; Kim, Y.; Muhoberac, B.; Klimkowski, V. J. Phys. Chem. 1993, 97, 4528. (24) Libor, O.; Nagy, G.; Szekely, T. Clay containing gels. Eur. Pat. Appl. EP 172938 A2 860305, 23 pages.
