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Formation of Aqueous Colloidal Dispersions of Exfoliated γ-Zirconium Phosphate by Intercalation of Short Alkylamines G. Alberti,* S. Cavalaglio, C. Dionigi, and F. Marmottini Department of Chemistry, University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy Received April 24, 2000. In Final Form: July 6, 2000 A good exfoliation of layered γ-zirconium phosphate in aqueous medium at room temperature has been obtained by intercalation of short alkylamines such as methyl-, dimethyl- and ethyl-amine. No appreciable exfoliation was observed when butylamine or longer alkylamines were intercalated, while intermediate behavior was obtained with the intercalation of propylamine. Since a good exfoliation of this layered compound is an essential step for its functionalization by topochemical reactions with phosphonic or phosphinic acids, the amine intercalation compounds and the various steps of the intercalation processes were investigated in order to obtain information on the mechanism of the exfoliation and a model of this mechanism is proposed. The use of the colloidal dispersion of exfoliated lamellae for the preparation of auto-consistent and flexible membranes as well as thin supported films of γ-zirconium phosphate is described.
Introduction Many layered solids can be exfoliated in single lamellae, or in packets of a few lamellae, with the formation of stable colloidal dispersions.1,2 The exfoliation of layered solids is a subject of interest not only because it is related to the interactions between adjacent lamellae of layered solids but also for the particular shape of the colloidal particles. These are bidimensional colloids having the same surface area as that of platelike crystals (typically ranging from a few µm2 to a few mm2) from which they are obtained while the thickness is extremely small (typically 0.5-1.2 nm). This subject has recently been reviewed.3 Practical uses of these colloidal dispersions include the preparation of self-assembled layered structures,4-6 self-consisting films or materials covered with very thin pellicles of layered compounds,7 thin proton conductor membranes,8 high surface area catalysts,3,9 micro- and meso-porous materials,10 and intercalated compounds with large organic cations such as dyes.11,12 Concerning layered zirconium phosphates, the exfoliation of R-type, R-Zr(O3P-OH)2‚H2O, is a well-known process and has been accomplished by preswelling, usually obtained by the intercalation of propylamine7 or methylamine.13 The exfoliation of γ-zirconium phosphate (γ* To whom correspondence should be addressed. (1) Alberti, G., Bein, T., Eds. Two and Three-Dimensional Inorganic Networks; Pergamon: New York, 1996. (Vol. 7 of the series Comprehensive Supramolecular Chemistry; Lehn, G. M., Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Eds.) (2) Jacobson, A. J. Mater. Sci. Forum 1994, 1, 152. (3) Jacobson, A. J. In ref 1, Chapter 10 and references therein. (4) Mallouk, T. E. In ref 1, Chapter 6 and references therein. (5) Keller, S. W.; Kim, H. N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (6) Clearfield, A. Prog. Inorg. Chem. 1998, 47, 371 and references therein. (7) Alberti, G.; Casciola, M.; Costantino, U. J. Colloid Interface Sci. 1985, 107, 256. (8) Alberti, G.; Casciola, M.; Massinelli, L.; Palombari, R. Ionics 1996, 2, 179. (9) Clearfield, A.; Costantino, U. In ref 1, Chapter 4 and references therein. (10) Alberti, G.; Marmottini, F.; Murcia-Mascaro´s, S.; Vivani, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1594. (11) Hoppe, R.; Alberti, G.; Costantino, U.; Dionigi, C.; Schulz-Ekloff, G.; Vivani, R. Langmuir 1997, 13, 7252. (12) Nazar, L. F.; Jacobson, A. J. J. Mater. Chem. 1994, 4, 1419.
ZrPO4‚O2P(OH)2‚2H2O, hereafter γ-ZrP) with propylamine does not give good results,14 but a very good exfoliation has recently been obtained using 1:1 water/acetone mixtures at 60-90 °C.15 The mechanism of exfoliation was attributed to the formation of acetone intercalated phases with consequent breaking of interlayer hydrogen bonds. However, due to the high volatility of acetone, the characterization of the acetone-intercalated phases was very difficult. Owing to the structural similarity of dimethylamine (DMA) and acetone, it was thought of interest to investigate whether γ-ZrP exfoliation could be also obtained by intercalation of this amine. If so, because of the stability of DMA-intercalated phases at room temperature, useful information on the exfoliation mechanism of γ-ZrP was expected from the characterization of these phases. During a parallel investigation on intercalation processes of n-alkylamines in γ-ZrP it was found that a good exfoliation of this layered compound can also be obtained with methyl- and ethyl-amine (hereafter MeA and EthA). Unstable colloidal dispersions were obtained with propylamine, but no exfoliation was observed with amines containing more than four carbon atoms (nc) in the alkylchain. The intercalation mechanism of long chain amines (nc from 3 to 18) has been reported and discussed elsewhere.14 It seems appropriate to report the results obtained for MeA and EthA intercalation here, since a comparison with the intercalation behavior of DMA facilitates a general discussion of the γ-ZrP exfoliation by the intercalation of short amines. Experimental Section Chemical. C. Erba RPE-ACS reagents were used without further purification. Preparation of γ-Zirconium Phosphate. The monoammonium form, ZrPO4‚O2P(OH‚ONH4), was prepared by thermal decomposition of zirconium fluorocomplexes, as previously described.16 This salt form was then converted into hydrogen form with HCl. To avoid formation of R-ZrP, it is advisable to use (13) Alberti, G.; Marmottini, F. J. Coll. Interface Sci. 1993, 157, 513. (14) Alberti, G.; Marmottini, F.; Cavalaglio, S.; Severi, D. Langmuir 2000, 16, 4165. (15) Alberti, G.; Dionigi, C.; Giontella, E.; Murcia-Mascaro´s, S.; Vivani, R. J. Coll. Interface Sci. 1997, 188, 27.
10.1021/la0006061 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/08/2000
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HCl concentrations lower than 1 M and temperatures lower than 20 °C. A typical conversion is as follows: 0.5 g of solid are dispersed in 100 mL of 0.5 M HCl and the dispersion is maintained for 24 h, under stirring, at 15-18 °C. After centrifugation, the treatment with 0.5 M HCl is again repeated. The γ-ZrP in hydrogen form obtained with this procedure is then washed twice, directly in the centrifuge tube, with 0.01 M HCl (60 mL g-1 of solid) and then dried at 100 °C for a day (to remove HCl traces). The anhydrous γ-ZrP is finally rehydrated to γ-ZrPO4‚O2P(OH)2‚2H2O (interlayer distance: 1.22 nm) by storage over a saturated BaCl2 solution (average particle size: 1-10 µm). Titrations. (a) Without Added Salt. γ-ZrP (0.200 g) was dispersed in 40 mL of deionized water and titrated, at room temperature, with 0.1 M aqueous solution of amine using an automatic titrator (Titralab VIT90), under a nitrogen stream. (b) In the Presence of Added Salt. The titrations were performed as above in the presence of the corresponding alkylammonium chloride. (c) At Constant pH. The titrations were performed as above at 20 °C, in closed vessels, previously fluxed with nitrogen, with the titrator in pH-stat mode. As previously discussed,14 the amount of intercalated amine in the pH range 3-8 can be assumed to be equal to the amount of added amine. The amount of intercalated amine is here reported as mol of amine taken up per formula weight of γ-ZrP (hereafter, simply mol/mol). Intercalation Procedure with Gaseous Amines. γ-ZrP (0.500 g) was put on a watch glass (diameter 7 cm) and then placed in a vacuum desiccator (volume 1.5 dm3) in which gaseous MeA in equilibrium with 20 mL of concentrated MeA (40% in w/w) was present. The time of contact was 30 days at room temperature. The same procedure was followed for the intercalation of EthA. In this case, the concentration of the EthA solution placed in the desiccator was 50% in w/w. Exfoliation Procedure. γ-ZrP (1.00 g) was dispersed, at room temperature, in 180 mL of distilled water, then 3.0 m moles of dimethylamine (DMA) in 20 mL of water were slowly added under stirring. The colloidal dispersion thus obtained was very stable (at least 3 weeks at room temperature). A similar procedure was used to obtain exfoliation by intercalation of MeA and EthA, but 6.0 mmol of MeA (or 5.0 mmol of EthA) in 20 mL of water were added. Instrumental. X-ray powder diffraction (XRPD) patterns were recorded with a Philips PW 1710 automatic diffractometer using Ni-filtered Cu KR radiation. The samples were previously conditioned at constant relative humidity (r.h.). Thermogravimetric curves were obtained with a Stanton Redcroft STA 780 thermoanalyzer.
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Figure 1. DMA titration curves obtained in the absence of added salt (full curve) and in the presence of 0.1 and 1 M dimethylammonium chloride (dashed and dotted curve, respectively). The exfoliation was only observed in the titration without added salt and the arrow indicates the point at which the maximum degree of exfoliation was obtained.
Figure 2. XRPD patterns of samples having a different degree of DMA intercalation.The samples were obtained by titration in the presence of 0.1 M dimethylammonium chloride and were previously conditioned at 75% relative humidity.
DMA Titration Curves. A comparison of the exfoliation ability of MeA, EthA, and DMA showed that the pH of the solution at which exfoliation takes place increases in the order DMA < EthA < MeA (see later). Because of the structural analogy between DMA and acetone, the DMA titration curves will be discussed first. Three typical DMA titration curves in the absence and in the presence of added salt (0.1 and 1.0 M) are shown in Figure 1 (full, dashed, and dotted curve, respectively). Note the presence of two distinct plateaux, which can be attributed to an intercalation that occurs in two distinct steps. The first step, corresponding to the uptake of about 0.4 mol/mol, occurred at a pH lower than 4. This amount of amine must be easily accommodated between the layers since the uptake was very fast (see later) and the XRPD patterns indicated that the intercalation process took place without an appreciable variation of the original interlayer distance d of γ-ZrP. Additional amine was intercalated with more difficulty. Thus, there is first a steep slope in the titration curves, then a second intercalation process
starts at higher pH value which is strongly dependent on the concentration of the added salt. The exfoliation of γ-ZrP was only observed in the titration curves without added salt. However, the phases formed at various stages of the intercalation can be better characterized when obtained from titration curves in the presence of added salt. The XRPD patterns of some typical samples having different degrees of intercalation indicated that this process occurred with a phase transition associated with a discontinuous d variation. As an example, Figure 2 shows that, for samples conditioned at 75% r.h., d increased from 1.22 to 1.5 nm. The gradual conversion of the original phase into the final phase was completed when about 1 mol/mol was taken up. Because of the coexistence of two solid phases, the pH value was expected to remain constant during the transformation of the 1.22 nm phase into the phase with larger interlayer distance.17 The presence of a maximum in the titration curves can be related to the fact that, after the initial formation of this latter phase, the enlargement of the more external part of the interlayer region reduces the activation energy for the diffusion of the amine in the interlayer space. Thus, the rate of uptake was expected to increase and the intercalation process could continue even at a pH value appreciably lower than that corresponding to the maximum of the curve which depends on the rate of titration (i.e., from the experimental conditions of titration chosen for the titrator). To obtain kinetic information on the DMAuptake, the titration was carried out with the titrator
(16) Alberti, G.; Giontella E.; Murcia-Mascaro´s, S.; Vivani, R. Inorg. Chem. 1998, 37, 4672.
(17) Alberti, G.; Costantino, U. In ref 1, Chapter 1 and references therein.
Results and Discussion
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Figure 3. Curve of DMA uptake vs time obtained at pH 7.65. Figure 5. XRPD patterns of the phase γ-ZrPO4‚O2P(OH)2‚ DMA‚nH2O conditioned at different relative humidity.
Figure 4. Weight loss curve of the phase γ-ZrPO4‚O2P(OH)2‚ DMA‚nH2O conditioned at 75% relative humidity. The heating rate was 5 °C/min.
operating in pH stat mode. The experiment was performed at pH 7.65, i.e., just a little below the maximum of the correspondent titration curve in the absence of added salt. As shown in Figure 3, the rate was initially very fast (until an amine uptake of about 0.3 mol/mol), then slowed and became extremely slow after the intercalation of about 0.4 mol/mol (additional 0.1 mol/mol were taken up in about 160 h). Finally, in correspondence with the initial formation of the enlarged phase, the process suddenly became very fast and 0.5 mol/mol were taken up in less than 1 h. These results support the above suppositions about the presence of the maximum in the titration curve. The thermal decomposition of the phase γ-ZrPO4‚ O2P(OH)2‚DMA‚nH2O conditioned at 75% r.h. was investigated. Figure 4 shows the weight loss curve of this phase. The total loss at 1000 °C was 30.4%, and the final product, as confirmed by XRPD, was ZrP2O7 (formula weight (fw) 265). Water and DMA losses were not well separated. Nevertheless, by considering that the fw of the compound can be calculated to be (265)[100/(100-30.4)] ) 380.75 and that the fw of ZrPO4‚O2P(OH)2‚DMA is 328, we obtain: n ) (380.75-328)/18 ) 2.93. Thus, the loss associated with the hydration water must be (2.93)(18)[100/380.75] ) 13.85%, while the loss associated with DMA is (45)[100/380.75] ) 11.82%. The remaining 4.73% can be attributed to the water condensation of the H2PO4 group. From the inflection of the curve (a loss of about 12%) it can be deduced that the greater part of hydration water was lost at 100 °C. Note that the loss of the DMA takes place in two steps. Since the intercalation of DMA also occurred in two steps, it seems that the amine intercalated in the second step (second plateaux of the titration curve) is lost at temperatures considerably lower than those of the amine intercalated in the first step. When the phase γ-ZrPO4‚O2P(OH)2‚DMA‚2.9 H2O was conditioned at high r.h., additional water was taken up
Figure 6. MeA titration curves obtained in the absence and in the presence of 1 M methylammonium chloride (full and dashed curve, respectively). The exfoliation is only observed in the absence of added salt and the arrow indicates the point at which the maximum degree of exfoliation was obtained.
with considerable d enlargement. As an example, the XRPD patterns of samples conditioned at 90 and 100% r.h. are shown in Figure 5. It was also found that when the sample was dispersed in distilled water, hydration was so high that an infinite swelling occurred (i.e., a colloidal dispersion was obtained). These data confirm that the exfoliation in the intercalation processes performed in the presence of added salt was prevented by the presence of the electrolyte. It was also observed that no good exfoliation can be obtained if the DMA content of γ-ZrP samples is e0.8 mol/mol. It can be concluded that the phase giving rise to exfoliation must have a composition near to γ-ZrP‚DMA. Similar considerations can be made for the titration curve without added salt (Figure 1, full curve). In this case the same intercalation processes discussed above take place but, due to the absence of the salt, the corresponding steps occur at higher pH values. The curve also exhibited a more marked maximum after which a gradual colloidization of γ-ZrP was observed. The arrow indicates the point at which the greatest part of the original microcrystals was transformed into a colloidal dispersion. No additional information on the exfoliation process can be obtained from the titration of the second acid proton of γ-ZrP since the exfoliation (in the absence of added salt) was already completed when about one DMA mol/ mol was taken up. Therefore, no further investigations of the DMA uptake at pH a higher than 8.4 were carried out. MeA and EthA Titration Curves. MeA titration curves are shown in Figure 6, while XRPD patterns of some typical samples with different degrees of MeA intercalation are shown in Figure 7a. The intercalation processes seem to be similar to those obtained with DMA. In brief, in the presence of 0.1 M
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Figure 8. XRPD patterns of (a) γ-ZrP‚1.6 MeA and (b) γ-ZrP‚ EthA phases obtained through contact (30 h) with gaseous amines. The samples were then conditioned at 90% relative humidity.
Figure 7. XRPD patterns of some typical γ-ZrP samples having different degrees of MeA and EthA intercalation (a and b, respectively). The samples were conditioned at 90% relative humidity.
added salt, the intercalation of about 0.5 mol/mol of MeA occurred easily at pH < 3, while additional 0.4 mol/mol were taken in the pH range 3-8. Since XRPD analysis showed that no appreciable enlargement of the interlayer distance of the original γ-ZrP took place, it can be concluded that, due to the small size of MeA, about 0.9-1.0 mol/mol can be inserted in the half-cavities of γ-ZrP. For additional MeA-uptake, a phase transition with an appreciable d-variation took place corresponding to the last plateau of the titration curve and the phase transition was completed for an intercalation of about 1.6 mol/mol. This phase exhibited a d value of 1.48 nm when conditioned at 90% r.h. Since the degree of crystallinity was very low, the preparation was attempted by the intercalation of gaseous MeA. The XRPD pattern of the compound obtained (Figure 8a), showed that a slightly more crystalline phase can indeed be prepared with this procedure. A colloidal dispersion of exfoliated γ-lamellae was obtained when this phase was stirred in water. In the titration curve performed in the absence of added salt a gradual exfoliation was observed for MeA intercalation greater than 1.2-1.3 mol/mol. The arrow indicates the point at which the greater part of the layered compound was exfoliated. Note that, due to the small size of MeA, only a small maximum is present in the titration curve. Titration curves with EthA in the absence and in the presence of added salt are shown in Figure 9, while XRPD patterns of some typical samples at different degrees of intercalation are shown in Figure 7b. About 0.4 mol/mol of EthA were intercalated in the half-cavities at a pH lower than 3 without appreciable enlargement of the interlayer distance. As in the case of DMA intercalation, the phase transition with enlargement of d spacing occurred when the amine uptake was greater than 0.5
Figure 9. EthA titration curves obtained in the absence and in the presence of 0.1 M ethylammonium chloride (full and dashed curve, respectively). The exfoliation is only observed in the absence of added salt and the arrow indicates the point at which the maximum degree of exfoliation was obtained.
mol/mol, although at a lower pH. The interlayer distance of the phase γ-ZrP‚EthA conditioned at 90% r.h. was 1.90 nm. This interlayer distance increased to 4.50 nm at 100% r.h., while colloidal dispersions were obtained by stirring the sample in deionized water. As in the case of MeA, a better degree of crystallinity was obtained when the intercalated phase was obtained from gaseous EthA (Figure 8b). This phase also gave a colloidal dispersion of exfoliated γ-lamellae when stirred in deionized water. Finally, it can be pointed out that the evidence of the maximum present in the titration curves of short alkylamines increases in the same order of the steric hindrance to the amine diffusion in the interlayer space of γ-ZrP (i.e., MeA < EthA < DMA). Taking into account that steric hindrance to the diffusion and activation energy of the intercalation process are strictly related each other, this result gives further evidence to our supposition that the presence of the maximum is due to an initial high activation energy for intercalation process of short alkylamines. Intercalation and Exfoliation Mechanism. From the data obtained here some general considerations on the intercalation of amines with short alkylchains in γ-ZrP
Aqueous Colloidal Dispersions
and on the exfoliation mechanism of this layered compound can be made. In a recent paper14 on the intercalation of alkylamines (nc g 3) in γ-zirconium phosphate, it was concluded that the distribution of these molecules in the interlayer region is essentially regulated by the tendency for the protonated heads (-NH3+) to distribute themselves as far as possible in the interlayer region in order to reduce their electrical repulsion. At the same time, apolar alkylchains tend to interact with each other through van der Waals forces. This contrasts with the electrical repulsion of the protonated heads. In the case of long alkylamines (nc g 6), the interaction between chains becomes prevalent and the amines are therefore forced to occupy adjacent sites even at relatively low pH values. This is in agreement with the presence of a single step in the intercalation process, which occurs at low pH values. The present study confirms that, in the case of very short alkylamines (nc e 2), which have small interactions, the intercalation process is essentially regulated by the random distribution of the protonated heads. The intercalation takes place first in the half-cavities and high pH values are then necessary to place the polar heads in more and more adjacent sites. Unfortunately, the degree of crystallinity of amine intercalated samples is not good enough to permit the determination of the position of the protonated amines in the interlayer region. Only a few plausible hypotheses based on the original structure of γ-ZrP and on the above experimental results can be proposed to explain the exfoliation of γ-ZrP by short amine intercalation. The rows of O2P(OH)2 groups present on each side of the γ-ZrP lamella create between each other parallel halfcavities18 (also called pockets) where 1 mol/mol of small neutral or cationic species may be inserted. In the original γ-ZrPO4‚O2P(OH)2‚2H2O, these half-cavities are filled by 1 mol/mol of water. In the process of intercalation of small amines, it is likely that their protonated form initially tends to be inserted in these half-cavities by replacing an equivalent amount of the water originally present. DMA molecules are too large to be inserted in the halfcavities. However, computer modeling elaborated with the help of the Hyperchem program, showed that the insertion becomes possible if a fraction of these halfcavities are slightly enlarged through a small rotation of the two O2P(OH)2 groups not involved in the bonds with >NH2+. This rotation causes a small reduction in the size of adjacent half-cavities where DMA can no longer be inserted. Consequently, only 50% of the half cavities can be filled (see Figure 10). Thus, this model is in agreement with the experimental fact that only about 0.5 mol/mol of DMA are taken up without variation of the interlayer distance. After the complete transformation of the original γ-ZrP into the phase ZrPO4‚O2P(OH)2‚0.5 DMA, the pH of the external solution must increase with further DMA addition. When the external pH value becomes sufficiently high, further DMA is forced to enter into the interlayer region and the phase transition +0.5 DMA
γ-ZrP‚0.5 DMA98γ-ZrP‚DMA takes place. However, since insertion in the remaining narrow half-cavities is no longer possible, the additional protonated DMA is expected to lie above the plane of the oxygens of -PO- charged groups of each layer. This charge distribution of the double film in the interlayer region makes each lamella positively charged in both its external (18) Poojary, D. M.; Shpeizer, B.; Clearfield, A. J. Chem. Soc., Dalton Trans. 1995, 111.
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Figure 10. A computer model of the phase γ-ZrPO4‚O2P(OH)2‚ 0.5 DMA. Note that additional DMA cannot be inserted in the half-cavities.
Figure 11. A computer model of the phase γ-ZrPO4‚O2P(OH)2‚ DMA. Note that a bifilm of protonated DMA is formed between the planes of the negative oxygens of the adjacent layers, explaining the exfoliation in water (see text).
sides, as schematically shown in Figure 11. Since the interlamellar hydrogen bonds are broken, this should provoke repulsion between adjacent lamellae. In agreement with the experimental results, several molecules of water are also intercalated between the layers to decrease this repulsion. According to the model of the advancing phase boundary, the intercalation process starts in the more external part of the interlayer region and proceeds toward the center.17 Since the exfoliation is due to the formation of the phase γ-ZrP‚DMA, the complete exfoliation of γ-ZrP cannot be expected when a small percentage of the original γ-ZrP phase, or the γ-ZrP‚0.5 DMA phase, is still present in the central part of the interlayer region. Thus, the full exfoliation of γ-ZrP is expected only when the peak at 1.22 nm in the XRPD pattern is completely absent. This has been experimentally confirmed. The gradual exfoliation can be therefore attributed to the fact that the size of the crystals is not uniform and therefore, as schematically shown in Figure 12, the homogeneous intercalation of DMA in the whole interlayer region is completed first in the smallest crystals. However, another explanation of the gradual exfoliation may be that the intercalation rate in the more external layers of the crystal is faster than
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Figure 12. A schematic model of gradual exfoliation based on the advancing phase boundary.The exfoliation cannot take place until the phase with smaller interlayer distance is present in the central part of the interlayer region. If the size of the layered microcrystals is not homogeneous, the phase transition is first completed in those of smaller size, thus explaining the gradual exfoliation.
in the bulk. Thus, the exfoliation may proceed layer by layer starting from the more external layers as in an onion. Further research is necessary to clarify which is the predominant process. Considerations similar to those above-reported for exfoliation through intercalation of DMA can be made for exfoliation through the intercalation of EthA. Only 0.5 mol/mol are inserted in the half cavities without appreciable change of the original interlayer distance. The additional protonated EthA is expected to lie above the plane of the oxygens of -PO- charged groups. Thus, the phase giving rise to exfoliation is γ-ZrP‚EthA. In the case of MeA, the only difference is that the steric hindrance of this small amine is less than that of DMA or EthA. Thus, unlike DMA and EthA, after the fast uptake of 0.5 mol/mol at low pH, additional MeA can be inserted, although at higher pH values, in the γ-ZrP half-cavities. Consequently, the γ-ZrP‚MeA phase does not exfoliate in water. However, when further MeA is added to the solution, the pH of the solution reaches high pH values at which the second proton of O2P(OH)2 groups can be titrated. Since the half cavities are completely filled, the additional protonated MeA is expected to lie above the plane of the oxygens of the -PO- charged groups, thus provoking repulsion between adjacent lamellae. The longer alkylchains of EthA prevent an appreciable amine insertion in the half cavities. On the other hand, both the distance between charged NH3+ polar heads belonging to adjacent layers and the lateral van der Waals interaction between alkylchains increase with the increasing length of the alkyl chain making the exfoliation of the layered compound more and more difficult. This model is in agreement with the fact that no good exfoliation has been found through intercalation of pro-
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pylamine while no exfoliation has been observed with amines that have longer alkylchains. Preparation of Auto-Consistent Membranes and Thin Supported Films. The colloidal dispersion obtained as described in the Experimental Section, is filtered on flat paper (e.g., Schleicher & Schuell 5893 blue ribbon) or plastic filters (e.g., Millipore RAWP 01), and air-dried at room temperature. Very compact and flexible selfconsistent membranes, with good mechanical properties, can be obtained. If necessary, the intercalated amine can be partially or totally eliminated, either by displacement with acid solutions, or by simple thermal treatment at 150-250 °C overnight. Very thin supported films (or pellicles) are formed when liquid films of colloidal dispersion are evaporated on surfaces of glass, plastic, and metals. Conclusions Useful information on the mechanism of exfoliation of γ-ZrP has been obtained by a detailed study of the intercalation of short alkylamines. Furthermore, the formation of colloidal dispersions in aqueous solutions opens new prospects for the preparation of organic derivatives of γ-ZrP. This large class of derivatives is at present almost exclusively obtained by an exfoliation process followed by a fast topotactic replacement of the original O2P(OH)2 with a variety of O2P(OH)R or O2PR2 groups.19 Thus, the preparation of the organic derivatives of γ-zirconium phosphate is one of the first examples, (maybe the first) in which a layered compound is first transformed into a colloidal dispersion and then, after chemical modifications obtained by fast topotactic reactions, again recomposed. Exfoliation with short alkylamines also provides an alternative to the water-acetone method for the preparation of self-consistent thin films of oriented γ-ZrP with good protonic conduction.8 The ease with which short alkylamines can be eliminated by thermal treatment opens the way to the preparation of silica-γ-ZrP composites with large specific surface area similar to silica-R-ZrP composites which exhibit high protonic conduction and good catalytic properties.20 Preliminary results in the preparation of these composites are very encouraging. Acknowledgment. This work was supported by MURST. We are grateful to Prof. R. Vivani for the elaboration of computer models. LA0006061 (19) Alberti, G. In ref 1, Chapter 5 and references therein. (20) Alberti, G.; Casciola, M.; Costantino, U.; Peraio, A.; Rega, T. J. Mater. Chem. 1995, 5, 1809.
