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Microwave-Assisted Intercalation of 1-Alkanols and 1,ω-Alkanediols into r-Zirconium Phosphate. Evidence of Conformational Phase Transitions in the Bimolecular Film of Alkyl Chains Umberto Costantino,† Riccardo Vivani,† Vı´teˇzslav Zima,*,‡ Ludvı´k Benesˇ,‡ and Kla´ra Mela´nova´‡ Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto, 8-06123 Perugia, Italy, and Joint Laboratory of Solid State Chemistry, University of Pardubice, Studentska´ 84, 532 10 Pardubice, Czech Republic Received July 5, 2001. In Final Form: November 26, 2001 A series of 1-alkanols and 1,ω-alkanediols with chain lengths ranging from 2 to 18 and 2 to 10 carbon atoms, respectively, was intercalated into R-Zr(HPO4)2‚H2O by exposing it, intimately dispersed with the guest species, to microwave radiation. The intercalation of 1-hexanol in the presence of different amounts of 1-propanol was also investigated. Thermogravimetric analysis showed that the intercalation compounds contained as a maximum 2 mol of alkanol or 1 mol of alkanediol per 1 mol of zirconium phosphate. The correlation between the interlayer distance of the samples, determined at room temperature, and the number of carbon atoms in the alkyl chain indicates that the alkanol molecules are arranged as an ordered bimolecular film in the interlayer region of the host. Differential scanning calorimetry analysis and X-ray powder diffraction patterns of representative samples taken at different temperatures showed a phase transition in which the bimolecular film undergoes a change from an all-trans conformation to a conformation in which the O-C1-C2-C3 torsion angle changes from 180 to 136°. The temperature at which this phase transition occurs was found to increase with the increasing number of carbon atoms in the alkyl chain. Similar effects were observed with the 1,ω-alkanediol intercalates.
Introduction R-Zr(HPO4)2‚H2O (hereafter R-ZrP) is a layered compound, generated by the packing of two-dimensional macromolecular units that are weakly interacting with each other. Due to the presence of ionogenic P-OH groups on the surface of the layers and to the easy accessibility of the interlayer region, this compound is able to act as a host for cationic or molecular polar guest species that are accommodated in the interlayer region by means of intercalation reactions. Host-guest chemistry of R-ZrP has been a consolidated topic of reviews1-3 and chapters of books,4-6 and a growing interest has recently been paid to the intercalation of guest species having special functionalities in the field of photochemistry,7 molecular and chiral recognition8 and biocatalysis.9 However, some fundamental aspects of the * To whom correspondence should be addressed. † Universita ` di Perugia. ‡ University of Pardubice. (1) Alberti, G.; Casciola, M.; Costantino, U.; Vivani, R. Adv. Mater. 1996, 8, 291-303. (2) Alberti, G.; Costantino, U. J. Mol. Catal. 1984, 27, 235-250. (3) Clearfield, A. Chem. Rev. 1988, 88, 125-148. (4) Alberti, G.; Costantino, U. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Oxford University Press: Oxford, 1991; Vol. 5, p 136. (5) Clearfield, A. In Progress in Intercalation Research; Mu¨llerWarmuth, W., Scho¨llhorn, R., Eds.; Kluwer Academic Publishers: Dordrecht, 1994; Vol. 17, p 240. (6) Clearfield, A.; Costantino, U. In Comprehensive Supramolecular Chemistry; Alberti, G., Bein, T., Eds.; Pergamon: Oxford, 1996; Vol 7, p 107. (7) Hoppe, R.; Alberti; G.; Costantino, U.; Dionigi, C.; Schulz-Ekloff, G.; Vivani, R. Langmuir 1997, 13, 7252-7257. (8) Cao, G.; Garcia, M. E.; Alcala`, M.; Burges, L. F.; Mallouk, T. E. J. Am. Chem. Soc. 1992, 114, 7574-7575. (9) Kumar, C. V.; Chaudari, A. J. Am. Chem. Soc. 2000, 122, 830837.
intercalation chemistry of R-ZrP still need to be elucidated. Owing to this, the detailed study of intercalation reactions of R-ZrP with relatively simple classes of guest species, such as n-alkylamines or n-alkanols, is of interest. While much work has been published about alkylamine intercalation,6 very little investigation has been accounted for alkanol intercalation compounds. It is known that the compound does not swell spontaneously when dispersed in methanol or ethanol or other weakly basic solvents. Furthermore, intercalation of these molecular species requires procedures designed to overcome the activation energy of the intercalation process. Costantino10 prepared some intercalation compounds with alcohols and glycols by contacting sodium salt forms of R-ZrP with guest species acidified with strong mineral acids. Tomita and coworkers11 observed the formation of alcohol intercalation compounds when R-ZrP microcrystals were boiled in pure liquid alkanols. However, the laborious procedures of intercalation, as well as the low stability of the compounds obtained prevented a systematic investigation of the phases obtained. Great interest has recently been paid to new ways of synthesizing both organic and inorganic compounds based on the application of microwave radiation. In the presence of microwaves, interesting compounds such as KVO3, CuFe2O4, BaWO4, and YBa2Cu3O7 were prepared more easily and in a shorter time than with the classical methods. In the field of intercalation chemistry, this method was used to obtain intercalation compounds of (10) Costantino, U. J. Chem. Soc., Dalton Trans. 1979, 402. (11) Hasegawa, Y.; Kontani, S.; Tomita, I. J. Inclusion Phenom. 1993, 16, 329-337. (12) Mela´nova´, K.; Benesˇ, L.; Zima, V.; Kalousova´, J.; Votinsky´, J. J. Inclusion Phenom. 1999, 33, 391-402.
10.1021/la011021p CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002
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Figure 1. Microwave apparatus: (a) inlet of radiation from microwave generator; (b) waveguide (sectioned in the picture to show the interior); (c) reaction vessel; (d) absorber of microwave radiation and cooler.
VOPO4 with pyridine and its derivatives13 and with 1-alkanols or 1,ω-alkanediols.14 Very likely, the absorption of microwaves by the water molecules present in the interlayer region of the host causes their rapid removal and the exfoliation of the host. After microwave exposure and partial cooling of the reaction mixture, theVOPO4 layers reaggregated taking the molecules of the new guest between them. This method was also used for the intercalation of alcohols and diols into NbOPO4 and NbOAsO4, isostructural with VOPO4.15 Therefore, it was of interest to attempt a direct intercalation of alkanols and glycols in R-ZrP under a microwave field. This paper reports the preparation of R-ZrP intercalates with 1-alkanols, with the number of C atoms of the alkyl chain (nC) ranging from 2 to 18, and 1,ω-alkanediols (with nC ) 2-10) and the characterization of the materials obtained. The probable arrangement of these guests in the interlayer region is based on the examination of X-ray powder diffraction patterns, thermogravimetry, and differential scanning calorimetry of many intercalation compounds of the homologous series of guests. Experimental Section Materials and Reagents. Well-crystallized R-ZrP, of formula R-Zr(HPO4)2‚H2O, was obtained according to the method proposed by Alberti and Torracca.16 A clear solution was prepared by dissolving 10.1 g of ZrOCl2‚8H2O, 8 mL of hydrofluoric acid (40% w/w), and 92 mL of H3PO4 (85% w/w) in 160 mL of water. The solution was heated at 80 °C for 4 days, maintaining a constant volume by continuously adding water. Decomposition of zirconium fluorocomplexes caused the precipitation of a microcrystalline powder (average size 5 µm) of R-ZrP, that was washed with deionized water and dried in air. Alkanols and glycols were “Fluka” reagents and were used without any further purification. Preparation Procedures. The microwave apparatus used for carrying out the intercalation reactions was specially constructed by Radan Ltd. Pardubice (Czech Republic). It operates at a frequency of 2450 ( 30 MHz with the total generator output of 800 W, of which 30-50 W is absorbed by the reaction mixture. The metal waveguide with a square cavity (5.2 × 5.2 cm) is equipped with an opening (2 cm diameter) through which the reaction vessel is inserted into the waveguide axis (Figure 1). The following general procedure was used for the intercalation reactions of R-ZrP with alcohols and diols. About 0.25 g of microcrystalline powder was suspended in about 7 g of dry liquid alcohol or glycol, or intimately mixed with them in cases when they were solid, in a 15 mL glass flask equipped with a reflux condenser and placed in the waveguide of the microwave (13) Chatakondu, K.; Green, M. L. H.; Mingos, D. M. P.; Reynolds, S. M. J. Chem. Soc., Chem. Commun. 1989, 1515-1517. (14) Benesˇ, L.; Mela´nova´, K.; Zima, V.; Kalousova´, J.; Votinsky´, J. Inorg. Chem. 1997, 36, 2850-2854. (15) Benesˇ, L.; Mela´nova´, K.; Zima, V.; Kalousova´, J.; Votinsky´, J. J. Solid State Chem. 1998, 141, 64-69. (16) Alberti, G.; Torracca, E. J. Inorg. Nucl. Chem. 1968, 30, 317318.
Costantino et al. generator under stirring and at a controlled temperature. In such a way, intercalates with alkanols and diols with less than seven carbon atoms in the chain were prepared. Zirconium phosphate intercalated with alcohols and diols with longer carbon chains were prepared by two different procedures. The first one involves exposing a mixture of the host and the alkanol to be intercalated, containing a small amount of 1-propanol, to microwave radiation under reflux and for the required time. 1-Propanol was removed from the reaction mixture by heating with microwaves in an open test tube. In the second procedure, the intercalation compound of zirconium phosphate and 1-propanol was prepared by the method described above and then 1-propanol in the intercalate was replaced with desired longer alcohol by treatment in a microwave field. After the mixture was cooled, the solid product that formed was filtered off. When solid alcohols or glycols were used, the suspension was separated from the melt by hot filtration. Analytical Procedures and Instrumentation. The determination of the guest/host molar ratio of the obtained intercalation compounds requires a specific treatment. The excess of liquid that wets the microcrystals cannot be washed out with water or other solvents that can replace the guest; furthermore, the vapor pressure of the intercalated species is generally high, and spontaneous deintercalation occurs when the compound is allowed to dry in air. The excess of liquid in the intercalation compounds containing low-boiling-point alkanols (C2, C3, C4) and glycols (C2, C4) was removed by taking advantage of the isopiestic equilibrium. They were allowed to dry in a vacuum desiccator containing a solution of the alkanol (glycol) with urea. Intercalation compounds with high-boiling-point alkanols (C8, C9, C12, C16) were carefully washed with n-hexane. In both cases, the guest/host molar ratio was determined by thermogravimetric analysis (TGA) of the dry samples performed with a Stanton 781 thermoanalyzer, operating at a heating rate of 5 °C/min under an air flow. Differential scanning calorimetry (DSC) measurements of some intercalation compounds were performed with a PerkinElmer DSC4 apparatus, previously calibrated with indium standard. X-ray powder diffraction (XRPD) patterns of the intercalates, wetted with a small amount of the guest alcohol, were obtained with an X-ray diffractometer (HZG-4, Germany) using Ni-filtered CuKR radiation. The CuKR2 intensities were removed from the original data. Silicium (a ) 5.430 55 Å) was used as internal standard. Diffraction angles were measured from 2θ ) 1.5° to 2θ ) 50°. XRPD patterns at different temperatures, from 20 to 130 °C, were taken on samples heated on a corundum plate equipped with a thermocouple.17 The measurements in the 3-65 °C range were carried out on a thermostated aluminum sample holder equipped with a thermocouple; plastic foil was used as protection from air moisture and thermal deintercalation.
Results and Discussion Preparation and Characterization of Materials. The evolution of the intercalation process of 1-propanol carried out under microwave radiation was followed by performing XRPD analysis of the reaction mixture at various irradiation times. It is assumed that the fraction of the 1-propanol intercalate in the reaction mixture, w, is equal to the ratio between the intensity of the diffraction maximum of the intercalate and the intensity of the (002) line of the host. Figure 2 shows the dependence of w (in %) as a function of the reaction time for 1-propanol intercalation. The formation of the product was very rapid at the beginning of the exposure to the microwave radiation, but the reaction slowed after about 10 min, to reach 87% of intercalation after 1 h. However, during the intercalation of 1-propanol, crystallization water of R-ZrP was released from the interlayer space. This water competes with the (17) Benesˇ, L. Sci. Pap. Univ. Pardubice, Ser. A 1996, 2, 151-155; Chem. Abstr. 1997, 126, 310616v
Intercalation of Alkanols into Zirconium Phosphate
Figure 2. Amount of 1-propanol intercalate, w, as a function of time, t, of exposure to microwave radiation.
alkanol in the intercalation reaction and it is not possible to synthesize pure 1-propanol intercalate in one reaction step. In light of these results, the following procedure was adopted to achieve the full intercalation of 1-propanol into R-ZrP. The finely ground host, mixed with the liquid alcohol, was exposed to microwave radiation for 10 min and filtered off, and the solid, mixed with a new dose of 1-propanol, was again exposed to the microwave radiation for another 10 min. Under these conditions, the guest/ host molar ratio of 2 was obtained, in agreement with previous studies.10,11 In this compound, we can assume that each alkanol molecule interacts with one P-OH group of the host layer by its hydroxyl polar head. Since the free area around each phosphate group on the layer18 is 24 Å2 and the cross-sectional area of the n-alkyl chain19 is evaluated to be 18.6 Å2, there is enough room for each phosphate to interact with the OH group of the alkanol. On the other hand the free space allowed is not sufficient to host two guest molecules coming from adjacent layers. A noninterpenetrated bimolecular film of intercalated guests is therefore expected to form. Intercalation of alcohols with chains shorter than that for 1-propanol was unsuccessful under microwave radiation probably because of the low boiling points of the guests that prevented the system from reaching the temperature required to start the reaction. Intercalations of alcohols with chains longer than 1-propanol are slowed due to the increased steric demands of the guests. Alcohols with more than six carbon atoms cannot be intercalated directly into R-ZrP even under microwave radiation. However, it was found that the presence of a small amount of 1-propanol in the reaction mixture favors the intercalation of longchain alkanols and glycols. It seems that 1-propanol acts as a catalyst in the intercalation reaction. 1-Propanol is probably first taken up at the edge of the crystallites with a consequent increase in the interlayer distance from 7.6 to about 16 Å and then replaced by the other guest molecule, which is present in a much higher concentration. This mechanism is very similar to the catalytic effect of a small amount of sodium ions in the exchange of cations with a larger ionic radius by R-Zr(HPO4)2‚H2O.20 Alternatively, intercalation compounds of alkanols with more (18) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311-3314. (19) Kitaigorodsky, A. I. In Molecular Crystals and Molecules; Loebl, E. M., Ed.; Academic Press: New York, 1973; p 61. (20) Schuck, G.; Melzer, R.; Sonntag, R.; Lechner, R. E.; Bohn, A.; Langer, K.; Casciola, M. Solid State Ionics 1995, 77, 55-62.
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Figure 3. XRPD patterns of R-Zr(HPO4)2 intercalated with (a) 1-octanol, (b) 1-nonanol, and (c) 1-hexadecanol. Table 1. Basal Spacings (d), Taken at 25 °C, of the Intercalation Compounds of r-Zr(HPO4)2 with n-Alkanols with an Increasing Number of Carbon Atoms (nC) in the Chaina nC
d (Å)
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
14.07b 16.24 18.60 20.76 23.00 24.95 27.38 27.17 29.01 30.88 32.84 34.57 36.65 38.38 40.46 42.08 43.98
dt (Å)
composition Zr(HPO4)2‚2C3H7OH Zr(HPO4)2‚2C4H9OH
25.33 (12 °C) 29.36 (35 °C)
Zr(HPO4)2‚1.9C8H17OH Zr(HPO4)2‚1.8C9H19OH Zr(HPO4)2‚1.8C12H23OH
44.3 (80 °C)
Zr(HPO4)2‚1.8C16H33OH
a The interlayer distances obtained after phase transition by a heating or cooling process (dt) are also reported (taken at the temperatures indicated in parentheses). Column 4 reports the compositions of selected intercalates calculated from the TGA data. b Data from ref 10.
than six carbon atoms were prepared by using the 1-propanol intercalate as a precursor, contacting it with the desired guest (see Experimental Section). Figure 3 shows the XRPD patterns of some of the intercalation compounds obtained. They have a good degree of crystallinity and the (00l) lines are very evident up to the fifth order. The compositions of some of the intercalates obtained are reported in Table 1, while Figure 4 shows the TG curves of some representative samples. As expected, the maximum experimental alkanol/host molar ratio is close to 2. Arrangement of 1-Alkanol in the Interlayer Region. Even though the XRPD data of the intercalation compounds are of good quality, they are not sufficient to attempt a full structural characterization. However, useful information about the arrangement of the guest molecules in the interlayer region of the prepared compounds can be obtained by a comparative analysis of XRPD data, especially when prior information, such as composition and structure of the host, is available and when it is reasonable to assume that the intercalation process does
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Costantino et al. Table 2. Linear Regression of the Dependence of Basal Spacing, d, on the Number of Carbon Atoms, nCa samples
A
sdA
1-alkanols C2-C8 2.206 0.020 1-alkanols C9-C18 1.875 0.010 1,n-alkanediols 0.945 0.022
R m (deg) 2 2 1
59.6 47.1 47.6
B
sdB
R
9.684 0.107 0.9998 10.289 0.139 0.9999 8.538 0.144 0.9981
a The inclination angle of the carbon chain, R, derived by the slope A, is also indicated. d ) AnC + B (A ) mnC‚1.27 sin R). sdA ) standard deviation of the slope A; m ) number of guest layers in the interlayer space; R ) inclination of the carbon chain calculated from the slope A; sdB ) standard deviation of the intersection B; R ) correlation coefficient.
Figure 4. TG curves of R-Zr(HPO4)2 intercalated with (a) 1-octanol and (b) 1-hexadecanol.
Figure 6. Thermal changes of the basal spacing, d, in 1-octanol (C8), 1-nonanol (C9), and 1-hexadecanol (C16) intercalated R-Zr(HPO4)2.
Figure 5. Dependence of basal spacing, d, on the number of carbon atoms, nC, in the aliphatic chain of the intercalated alcohols: empty circles, values of d observed at room temperature; solid circles, values for the thermally treated samples (see text).
not appreciably change the structure of the host. Table 1 reports the interlayer distance (d) of the intercalates obtained, taken at room temperature, while Figure 5 shows its dependence on the number of carbon atoms in the alkyl chain of the guest. Generally speaking, the linear correlation of this plot can be interpreted as follows. The constant increase of the interlayer distance for each addition of one carbon atom in the alkyl chain indicates that the guest molecules have the same arrangement in the different compounds. Therefore, the alkyl chains can be reasonably assumed to be in an all-trans conformation. The increment in the interlayer distance for each C-C bond corresponds to the slope of the straight line correlation of the above plot, and it can be related to the inclination angle, R, of the alkyl chain axis to the plane of the layers by
d(Å) ) mnC‚1.27 sin R + B
(1)
where m is the number of guest layers in the interlayer space (in this case m ) 2, as deduced from the composition, see Table 1) and 1.27 Å is the increment in length of the alkyl chain, supposed to be in a all-trans conformation, per each additional carbon atom. B is a constant and represents the thickness of the structural residue when nC ) 0. In the present case, B corresponds to the interlayer
distance of R-ZrP with two hydroxyl heads per mole of zirconium in the interlayer region. Unexpectedly, also in reference to already published data,6,10 the interlayer distances reported in Figure 5 can be grouped into two linear sets, with a discontinuity point occurring when going from 1-octanol to 1-nonanol intercalates. When eq 1 is applied to these data, the two different correlation straight lines indicate two different arrangements of the intercalated guests. From C2 to C8 (set 1) a tilting angle, R1 ) 59.6°, and a residual thickness, B1 ) 9.68 Å, are calculated; from C9 to C18 (set 2), R2 ) 47.1° and B2 ) 10.29 Å are the values derived (see Table 2). This peculiar behavior cannot be related to compositional differences. For example, 1-octanol and 1-nonanol intercalates have about the same alkanol amount, but the interlayer distance of the 1-nonanol intercalate is even smaller than that of the 1-octanol intercalate. The relationships between the two different arrangements were further elucidated by investigating the thermal behavior of the intercalation compounds with 1-octanol, 1-nonanol, and 1-hexadecanol. Figure 6 shows the dependence of the interlayer distance of these intercalates as a function of the temperature. It can be noted that the samples belonging to set 2 (1-nonanol and 1-hexadecanol intercalates), heated at a proper temperature, undergo a solid-state phase transition with an increase of their interlayer distance of about 1 Å. The new interlayer distances obtained (represented as solid circles in Figure 5 and reported in Table 1, column 3) are close to those calculated on the basis of the extrapolation of the linear regression of set 1. The temperature at which the transition occurs increases with increasing number of carbon atoms. For example, 1-nonanol intercalate shows
Intercalation of Alkanols into Zirconium Phosphate
Langmuir, Vol. 18, No. 4, 2002 1215 Table 3. Comparison between the Observed Basal Spacings (dobs) of Samples from C9 to C16 and Those Calculated (dcalc) by Equation 2 When n1 ) 2 (See Text)
Figure 7. DSC curve of 1-hexadecanol intercalated R-Zr(HPO4)2.
a phase transition in the 30-35 °C interval, while for 1-hexadecanol intercalate, a similar effect is observed in the 50-85 °C temperature region. Therefore, intercalates with alkanols shorter than 1-nonanol are expected to show this phase transition at temperatures lower than 30 °C. Figure 6 shows that 1-octanol intercalation compound, cooled from room temperature to 0 °C, undergoes a phase transition associated with a decrease of the basal spacing at about 12 °C. In this case, the new interlayer distance is very close to that predicted by the straight line interpolating the data of set 2. These phase transitions are reversible, as observed by combining XRPD patterns taken at different temperatures and DSC measurements. Figure 7 shows the DSC curve for the 1-hexadecanol intercalate. During heating, the sample shows three endothermic peaks. The peak with the highest endothermic effect at about 84 °C corresponds to the transition observed by XRPD. All the DSC effects are reversible showing exothermic peaks during the cooling of the sample. We can presume the existence of low- and a hightemperature phases for all the alkanol intercalates of R-ZrP that have a different arrangement of the guest molecules. At room temperature, compounds belonging to set 1 (shorter chains) are in their high-temperature phases, while compounds of set 2 (longer chains) are in their low-temperature phases. This phenomenon can be explained by the following considerations: We can assume that the oxygen atoms of the intercalated alkanols are firmly held by hydrogen bonds among three P-OH groups so that the O-C1 bond is roughly perpendicular to the host layer, and the remaining fragment possesses the most convenient conformation, that is, all-trans. In such an arrangement the chains should be tilted by 55° to the host layers. For the high-temperature phases, a slightly larger R1 value (59.6°) was found. A weak repulsion of hydrophobic alkyl groups from the hydrophilic surface of the host layers should justify this difference, forcing the chain to assume an angle slightly higher than that calculated. For low-temperature phases, alkyl chains are more closely packed probably because the attractive hydrophobic forces between them should tend to minimize the interlayer volume by changing the alkyl chain inclination to R2 ) 47.1°. This change could be easily achieved by
nC
dobs (Å)
dcalc (Å)
nC
dobs (Å)
dcalc (Å)
9 10 11 12 13
27.17 29.01 30.88 32.84 34.57
27.22 29.10 30.97 32.85 34.72
14 15 16 17 18
36.65 38.38 40.46 42.08 43.98
36.60 38.47 40.35 42.22 44.10
rotating the C1-C2 bond, that is, by changing the O-C1C2-C3 torsion angle (thereafter referred to as β). From geometric considerations, the β value that best fits the experimental data is 136°. According to this hypothesis, in the low-temperature phases the alkyl chains should present a nonuniform inclination angle to the layer plane, that is, 59.6° from O to C2 and 47.1° for the rest of the chain. This description is also supported by the fact that the two straight lines interpolating the interlayer distances of the high- and low-temperature phases intersect at nC ) 2. This means that the common residue constituted by an R-ZrP layer and two alkanol fragments up to C2 has the same thickness for the two phases. A further confirmation of this description can be obtained by the following considerations. The interlayer distances of the distorted system can be described by the equation
dcalc ) n1m‚1.27 sin R1 + B + (n - n1)m‚1.27 sin R2 (2) where dcalc is the calculated interlayer distance, n1 is the number of carbon atoms in the chain inclined at an angle R1, and n is the total number of carbon atoms in the chain, the other parameters assume the same meaning as in eq 1. It was found that the best fit between the calculated and the experimental d values is when n1 ) 2. Table 3 reports a comparison between the observed basal spacings of samples belonging to set 2 and those calculated according to eq 2. The phase transition can be justified by the fact that the van der Waals forces are weakened when an intercalate in the distorted arrangement is heated. Consequently, the repulsing hydrophobic interactions prevail, and the distorted arrangement changes to the all-trans arrangement with a higher interlayer distance. This process is schematically represented in Figure 8 for the 1-nonanol intercalate. Co-intercalation of Two Different Alkanols. As already described, the microwave-assisted intercalation of long-chain alkanols requires the presence of small amounts of 1-propanol. Therefore, it was of interest to investigate the nature of materials formed when the host, suspended in mixtures of 1-propanol and 1-hexanol, is exposed to microwave radiation. Figure 9 reports the interlayer distance of the obtained intercalation compounds as a function of the molar fraction of 1-hexanol, y, in the equilibrating mixtures. The discontinuous change of the interlayer distance indicates that the system is polyphasic. As expected, 1-propanol intercalate, with an interlayer distance of 16.3 Å, is obtained when y is zero. This phase continues up to y ) 0.3, without appreciable solubilization of 1-hexanol molecules in the interlayer region. At y values higher than 0.3, 1-hexanol partially displaces 1-propanol from the interlayer and an intercalation compound, containing 1-propanol and 1-hexanol in a roughly equimolar ratio, is formed. It is interesting to note that the observed interlayer distance, 19.7 Å, is very near to that calculated when assuming the formation of a phase with an ideal composition R-Zr(HPO4)2‚C3H7OH‚
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Figure 8. Schematic representation of the high-temperature all-trans and low-temperature distorted arrangements of 1-nonanol in the interlayer space of R-Zr(HPO4)2.
Figure 9. Dependence of the basal spacing, d, of the prepared solid phases on the composition, y, of the starting mixture 1-propanol-1-hexanol.
C6H13OH. The molecules of both alcohols combine in the interlayer region in such a way that the thickness of the bilayer is determined by the sum of the lengths of one molecule of 1-propanol and one molecule of 1-hexanol. A similar arrangement was observed in VOPO4 mixed alkanol intercalates.12 The mixed phase can solubilize 1-hexanol molecules and the interlayer distance increases slightly from 19.7 to 20.2 Å, when the 1-hexanol content of equilibrating mixtures increases from 0.35 to 0.65. With this latter composition, the mixed phase is saturated by 1-hexanol, and a further increase of y causes a sharp increase in the interlayer distance from 20.2 to 22.7 Å to form a 1-hexanol intercalation compound containing 1015% of 1-propanol solubilized in the interlayer. In the 0.75-1.0 compositional range, 1-hexanol replaces the solubilized 1-propanol and the interlayer distance of the pure 1-hexanol intercalate (23.0 Å) is finally obtained. The observed phenomena are in good agreement with the hypothesis that 1-propanol can act as a catalyst of the intercalation reactions of long-chain alkanols. Intercalation of 1,ω-Alkanediols. Several intercalation compounds of R-ZrP and diols were obtained under microwave radiation by using the 1-propanol intercalate as precursor. The guest/host molar ratio of the intercalate is very near to 1, in agreement with the previous data.10 Figure 10 shows the interlayer distance of the intercalates as a function of the number of carbon atoms present in the alkyl chain. A weak odd-even effect can be noted, in
Figure 10. Dependence of the basal spacing, d, on the number of carbon atoms, nC, in the aliphatic chain of the intercalated diols: empty circles, experimentally found values at ambient temperature; solid circle, value for thermally treated hexanediol intercalated R-Zr(HPO4)2.
which there is a larger increment of the interlayer distance on going from an odd to an even carbon atom compared to the increment observed on going from an even to an odd carbon. This effect may be interpreted with considerations similar to those reported in ref 21 and based on the strong tendency of the oxygen atom of the terminal OH groups to be in sp3 hybridization, so that the O-C bond is perpendicular to the layer plane. The average increment of the basal spacings for each additional carbon atom in the chain was obtained by interpolating the data of Figure 10 with a straight line. The linear regression, reported in Table 2, gives an intercept of 8.54 Å and a slope of 0.95 Å/carbon atom. This latter value indicates that the alkanediols are arranged in the interlayer region as a monolayer of extended molecules with their main axis tilted at a 47.6° angle with respect to the layer plane. Conclusion The application of microwave radiation to slurries containing R-Zr(HPO4)2‚H2O microcrystals, finely dispersed in 1-alkanols or 1,ω-alkanediols, is an efficient way to obtain the corresponding intercalation compounds even though intercalation of alcohols with more than six carbon (21) Alberti, G.; Costantino, U.; Marmottini, F.; Perego, G. J. Inclusion Phenom. 1989, 7, 549-560.
Intercalation of Alkanols into Zirconium Phosphate
atoms in the chain requires a pre-expansion of the interlayer distance of the original R-Zr(HPO4)2 by 1-propanol, after which the reaction proceeds with a displacement mechanism. A detailed study of these intercalation compounds led to the discovery of a thermally induced phase transition occurring at relatively low temperatures. The phase transition has been ascribed to a change in the conformation of the chain, and the transition temperature depends on the number of carbon atoms in the alkyl chain of the guest molecules. A similar behavior was found for the intercalation compounds of R-ZrP with 1,ω-alkanediols. Very likely, phase transitions similar to that found in the alkanol-zirconium phosphate system can be detected
Langmuir, Vol. 18, No. 4, 2002 1217
in other systems when the interlayer distance of the intercalates is carefully determined as a function of temperature. Acknowledgment. This work was carried out in the framework of the Agreement for Scientific Cooperation between the National Research Council of Italy and the Academy of Sciences of the Czech Republic. V.Z., L.B., and K.M. wish to thank the Grant Agency of the Czech Republic (Grant No. 202/01/0520); U.C. and R.V. wish to thank MURST Progetti Nazionali 1999. LA011021P
