3216
Langmuir 2001, 17, 3216-3222
Separation of Adsorbed Formamide and Intercalated Formamide Using Controlled Rate Thermal Analysis Methodology Ray L. Frost,*,† Ja´nos Kristo´f,‡ Erzse´bet Horva´th,§ and J. Theo Kloprogge† Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane Queensland 4001, Australia, Department of Analytical Chemistry, University of Veszpre´ m, H8201 Veszpre´ m, PO Box 158, Hungary, and Research Group for Analytical Chemistry, Hungarian Academy of Sciences, H 8201 Veszpre´ m, PO Box 158, Hungary Received December 1, 2000. In Final Form: February 28, 2001 Controlled rate thermal analysis (CRTA) has been used to separate adsorbed formamide from intercalated formamide in formamide-intercalated kaolinites. This separation is achieved by removal of the sample at the end of the controlled isothermal desorption step. The temperature of this isothermal desorption is dependent on the use of open or closed crucibles in the thermal analysis unit but is independent of the formamide/water ratio. X-ray diffraction shows that the formamide-intercalated kaolinite remains expanded after formamide desorption with a d(001) spacing of 10.09 Å. Further heating to 300 °C results in the deintercalation of the formamide-intercalated kaolinite. DRIFT spectroscopy shows differences between the infrared spectra of the adsorbed and formamide-intercalated kaolinites. An intense band observed at 3629 cm-1 is attributed to the inner surface hydroxyls hydrogen bonded to the formamide. The adsorbed formamide-intercalated kaolinites contain adsorbed water and show intensity in the 1705 cm-1 band, which is absent in the CRTA-treated formamide-intercalated kaolinites.
Introduction Kaolinite and halloysite can be expanded with small organic compounds1-4 and with salts of short-chain fatty acids.5-9 This expansion occurs along the C-axis, is known as intercalation,3,4 and is readily observed by X-ray diffraction (XRD).7-9 Kaolinite has a d(001) spacing of around 7.2 Å and expansion with formamide results in the basal spacing increasing from ∼7.2 to 10.09 Å. When this intercalation occurs, changes are observed in the spectra of both the kaolinite and the inserting molecule.11-14 Upon intercalation, additional infrared (IR) bands are observed for the formamide-intercalated kaolinites at 3629 and 3606 cm-1.1,11,15 The 3629 cm-1 band is attributed to the hydroxyl stretching frequency of the inner surface * Corresponding author. Ph: +61 7 3864 2407. Fax: +61 7 3864 1804. E-mail:
[email protected]. † Queensland University of Technology. ‡ University of Veszpre ´ m. § Hungarian Academy of Sciences. (1) Ledoux, R. L.; White, J. L. J. Colloid Interface Sci. 1966, 21, 127. (2) Olejnik, S.; Posner, A. M.; Quirk, J. P. Clay Miner. 1970, 8, 421. (3) Lagaly, G. Philos. Trans. R. Soc. London 1984, A311, 315. (4) Raussell-Colom, J. A.; Serratosa, J. M. Reactions of clays with organic substances. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; Mineralogical Society, Longman Scientific and Technical; London, U.K., 1987. (5) Frost, R. L.; Tran, T. H. T.; Kristof, J. Clay Miner. 1997, 32, 587. (6) Frost, R. L.; Kristof, J. Clays Clay Miner. 1997, 45, 551. (7) Frost, R. L.; van der Gaast, S. J. Clay Miner. 1997, 32, 293. (8) Frost, R. L.; Kristof, J.; Kloprogge, J. T.; Tran, T. H. T. Am. Mineral. 1998, 83, 1182. (9) Frost, R. L.; Kristof, J.; Paroz, G. N.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 208, 478. (10) Frost, R. L.; Lack, D. A.; Kloprogge, J. T.; Tran, T. H. T. Clays Clay Miner. 1999, 48, 297. (11) Frost, R. L.; Forsling, W.; Holmgren, A.; Kloprogge, J. T.; Kristof, J. J. Raman Spectrosc. 1998, 29, 1065. (12) Frost, R. L.; Tran, T. H. T.; Rintoul, L.; Kristof, J. Analyst 1998, 123, 611. (13) Frost, R. L.; Kristof, J.; Paroz, G. N.; Tran, T. H.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 204, 227. (14) Frost, R. L.; Kristof, J.; Paroz, G. N.; Tran, T. H.; Kloprogge, J. T. J. Colloid Interface Sci. 1998, 208, 216.
hydroxyl group hydrogen bonded to the carboxyl group of the formamide. The 3606 cm-1 band is ascribed to water in the interlayer.15 Upon intercalation, changes are also observed in both the hydroxyl deformation modes and the carboxyl bands. Thermal analysis is normally carried out as a dynamic experiment with a constant and continuous heating rate. Such experimentation is not able to determine phase changes, which occur at close temperature intervals. New thermoanalytical techniques, which can separate thermal processes, have been developed.16-18 These techniques enable these closely overlapping phase phenomena to be separated. The method is known as constant rate thermal analysis (CRTA) and depends on the rate of mass loss, such that no heating occurs when the phase change occurs. Such thermoanalytical experiments are known as isothermal thermogravimetric analysis (ITGA) or quasiisothermal TGA. One of the problems associated with the spectroscopic analysis of the intercalated kaolinites is the uncertainty of the nature of the intercalate. The formamide-intercalated kaolinite may contain both adsorbed water and adsorbed formamide. Thus, the infrared spectra of the intercalated kaolinites probably determine more than one phase, that is, the formamide-intercalated kaolinite and the formamide-adsorbed kaolinite. The technique of CRTA thermal analysis enables the separation of the adsorbed from the intercalated kaolinite. This is achieved through stopping the experiment between the desorption step and the deintercalation step. (15) Cruz, M.; Laycock, A.; White, J. L. Perturbation of OH groups in intercalated donor-acceptor complexes. I. Formamide, methyl formamide, and dimethyl formamide kaolinite complexes. In Proceedings of the International Clay Conference, Tokyo, 1969; Israel University Press: Jerusalem, 1969;p 775. (16) Paulik, F.; Paulik, J. Thermochim. Acta 1986, 100, 23. (17) Kristo´f, J.; Frost, R. L.; Kloprogge, J. T.; Horva´th, E.; Ga´bor, M. J. Therm. Anal. Calorim. 1999, 56, 885. (18) Kristof, J.; Horvath, E.; Frost, R. L.; Kloprogge, J. T. J. Therm. Anal. 2000, 63, 279.
10.1021/la001686b CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001
Separation of Adsorbed from Intercalated Formamide
Langmuir, Vol. 17, No. 11, 2001 3217
Thermoanalytical studies showed that the removal of intercalated formamide below 300 °C is a complex process and that the liberation of formamide from the complex takes place in two overlapping stages.17 Further research showed that a higher amount of formamide can be connected to the clay if intercalation is carried out in the presence of water.18 The complexity of the thermal decomposition patterns and the subtleties of the vibrational spectroscopic (FT-IR and Raman spectrometric) data require a detailed study of the thermal decomposition mechanism. The objective of this paper is to describe the differences between the adsorbed and formamide-intercalated kaolinite using a combination of thermal analysis, X-ray diffraction, and infrared spectroscopic techniques. Experimental Methods The Kaolinite Intercalates. The kaolinite used in this study is from Kira´lyhegy, Hungary. This mineral has been characterized both by X-ray diffraction and by Raman spectroscopy. The kaolinite is a low-defect kaolinite with a Hinckley index of ∼1.35. The kaolinite was intercalated by mixing 300 mg of the kaolinite with 5 cm3 of a 100% formamide or 50% formamide aqueous solution for 80 h at room temperature. The excess solution was removed by centrifugation, and the intercalated kaolinites were kept in a desiccator before thermoanalytical, XRD, and vibrational spectroscopic analyses. Thermogravimetric Analysis. Thermal decomposition of the intercalates was carried out in a Derivatograph PC-type thermoanalytical instrument (Hungarian Optical Works, Budapest, Hungary) in a nitrogen atmosphere at a preset, constant decomposition rate of 0.20 mg/min. The samples were heated in an open platinum crucible at a rate of 1 °C/min up to 300 °C. With the quasi-isothermal, quasi-isobaric heating program of the instrument, the furnace temperature was regulated precisely to provide a uniform rate of decomposition in the main decomposition stage. Some of the samples were heated in a labyrinthtype crucible to provide a self-generated atmosphere and quasiisobaric conditions during decomposition. X-ray Diffraction. XRD analyses were carried out on a Philips wide angle PW 1050/25 vertical goniometer equipped with a graphite diffracted beam monochromator. The d spacing and intensity measurements were improved by application of a selfdeveloped computer-aided divergence slit system enabling constant sampling area irradiation (20 mm long) at any angle of incidence. The goniometer radius was enlarged from 173 to 204 mm. The radiation applied was Cu KR1 from a long fine focus Cu tube, operating at 40 kV and 40 mA. The oriented samples were measured at 50% relative humidity in stepscan mode with steps of 0.02° 2θ and a counting time of 2 s. Measured data were corrected with the Lorentz polarization factor (for oriented specimens) and for their irradiated volume. Diffuse Reflectance Infrared Spectroscopy. Diffuse reflectance Fourier transform infrared spectroscopic (commonly known as DRIFT) analyses were undertaken using a Bio-Rad 60A spectrometer. Scans (512) were obtained at a resolution of 2 cm-1 with a mirror velocity of 0.3 cm/s. Spectra were coadded to improve the signal-to-noise ratio. Approximately 3 wt % kaolinite or formamide-intercalated kaolinite was dispersed in 100 mg of oven-dried spectroscopic grade KBr with a refractive index of 1.559 and a particle size of 5-20 µm. Reflected radiation was collected at ∼50% efficiency. Background KBr spectra were obtained, and spectra were expressed as ratios to the background. The diffuse-reflectance accessory used was designed exclusively for Bio-Rad FTS spectrometers. It is of the so-called “praying monk” design and is mounted on a kinematic baseplate. It includes two four-position sample slides and eight sample cups. The cup (3 mm deep, 6 mm in diameter) accommodates powdered samples mixed with KBr using an agate mortar and pestle in 1-3% concentration. The collection efficiency of this adaptor is approximately 50%. The reflectance spectra expressed as Kubelka-Mink units versus wavenumber curves are very similar to absorbance spectra and can be evaluated accordingly. The advantage of using DRIFT measurements over the pellet
Figure 1. Thermal decomposition curves of formamideintercalated kaolinite under dynamic heating conditions. technique is that in this case the likely interference of the mulling agent (intercalation of KBr in a liquid phase under pressure) can be avoided. Spectral manipulation such as baseline adjustment, smoothing, and normalization was performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH). Band component analysis was undertaken using the Jandel “Peakfit” software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a LorentzGauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss-Lorentz ratio was maintained at values greater than 0.7, and fitting was undertaken until reproducible results were obtained with squared correlations of r2 greater than 0.995. Graphics are presented using Microsoft Excel.
Results and Discussion Thermoanalytical Results. The thermal decomposition curves (TG) of formamide-intercalated and formamide/water-intercalated kaolinite recorded under dynamic heating conditions are shown in Figure 1. For the formamide-intercalated kaolinite, the differential analysis curves (DTA) show two endotherms at 65 and 150 °C ascribed to the loss of water and formamide, respectively. A weak inflection is also observed at 560 °C, which is
3218
Langmuir, Vol. 17, No. 11, 2001
Frost et al.
Figure 3. X-ray diffraction of low-defect kaolinite (a) intercalated with 100% formamide, (b) CRTA treated intercalated with 100% formamide, (c) intercalated with 50% formamide/ 50% water, and (d) CRTA treated intercalated with 50% formamide/50% water.
Figure 2. Controlled rate thermal analysis of formamideintercalated kaolinite.
attributed to kaolinite dehydroxylation. The thermogravimetric curve shows two weight losses at 150 and 130 °C. The differential thermogravimetric curves (DTG) are more informative, and three inflections are observed at 65, 150, and 536 °C. These minima are attributed to the loss of water, formamide, and kaolinite hydroxyls. For the kaolinite intercalated with the water/formamide mixture, the DTA shows three endotherms at 65, 130, and 157 °C. The TGA curves show two weight losses at 50 and 150 °C. Again the DTGA curves are more informative, showing three inflections at 50, 130, and 157 °C. The 130 °C inflection in the DTGA curves for the water/formamideintercalated kaolinite is not observed for the formamideintercalated kaolinite. The thermoanalytical curves of the formamide-intercalated Kira´lyhegy kaolinite recorded under quasiisothermal conditions are given in Figure 2. Under the controlled rate conditions, two weight losses are observed: (a) at 118 °C under quasi-isothermal conditions and (b) at 153 °C. Thus, under dynamic conditions only a single weight loss was observed, but under quasiisothermal conditions now two weight losses are observed. According to TG-MS studies, the mass loss stage at 67 °C belongs to the removal of 3.27 wt % of water. Bonded formamide is released in two stages under quasiisothermal conditions at 118 °C (21.57 wt %) and under nonisothermal conditions at 153 °C (4.24 wt %).
Significant differences can be observed in the thermal behavior of differently bonded formamide molecules. Whereas the first type of bonded formamide is lost under isothermal conditions at 118 °C, the second type of formamide molecule exhibits a drastically different behavior on heating. The observation that the temperature remained spontaneously constant during the first formamide mass loss stage indicates an equilibrium reaction. Stabilization of sample temperature in an equilibrium reaction also means the constant concentration (partial pressure) of gaseous decomposition products in the space among the particles in the open crucible used and the establishment of an equilibrium between the opposite diffusion processes of formamide vapor leaving the sample and nitrogen replacing formamide vapor. The liberation of strongly bonded formamide takes place under nonisothermal conditions in a similar fashion and at practically the same temperature (156 and 153 °C) indicating the similar mechanism of formamide bonding to the clay structure. Because the best separation of the overlapping formamide decomposition stages can be obtained at 130 °C, heating was stopped in a separate experiment at 130 °C. In this experiment, the formamide-intercalated kaolinite was removed and analyzed by X-ray diffraction and infrared spectroscopy. XRD Results. Figure 3 displays the X-ray diffraction pattern of the formamide-intercalated kaolinite. Basal spacings were calculated from degrees two theta measurements using Bragg’s law (nλ ) 2d sin θ). The XRD traces of the formamide- and formamide/water-intercalated kaolinites (a and c) show identical d(001) spacing of 10.2 Å. Trace (b) is that of the CRTA-treated formamideintercalated kaolinite removed from the thermoanalytical instrument at 130 °C. Importantly, the XRD pattern of the CRTA-treated formamide-intercalated kaolinite is identical to that of the untreated formamide-intercalated kaolinite. This proves that the CRTA-treated formamideintercalated kaolinite is still completely expanded at 130 °C. Upon heating the intercalated kaolinite to 300 °C, the formamide is lost and the structure collapses to its original d spacing of ∼7.2 Å (Figure 3d). The basal spacings are not the same with or without the formamide. The basal spacing of the nonexpanded clay is 7.2 Å; upon intercalation with formamide, the kaolinite expands to 10.2 Å. Upon CRTA treatment to remove adsorbed formamide, the clay still remains expanded at 10.2 Å. The adsorbed formamide is less strongly bound to the kaolinite hydroxyl sheets than the chemically bound formamide in the intercalation complex. DRIFT Spectroscopy. Figure 4 displays the DRIFT spectra of the untreated low-defect kaolinite and the
Separation of Adsorbed from Intercalated Formamide
Langmuir, Vol. 17, No. 11, 2001 3219
Table 1. Results of the Band Component Analysis of the DRIFT Spectra of the Hydroxyl Stretching Region of Kaolinite Intercalated with 100% Formamide and with 50% Formamide/50% Water Mixtures and Their CRTA-Treated Products kaolinite low defect 100% formamide-intercalated kaolinite 50% formamide/water-intercalated kaolinite 100% formamide-intercalated kaolinite CRTA treated 50% formamide/water-intercalated kaolinite CRTA treated
band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
Figure 4. DRIFT spectroscopy of the NH and OH stretching region of (a) low-defect kaolinite, (b) intercalated with 100% formamide, (c) CRTA treated intercalated with 100% formamide, and (d) CRTA treated intercalated with 50% formamide/ 50% water.
formamide CRTA-treated and untreated formamideintercalated kaolinite. The figure clearly illustrates the changes in the NH and OH stretching region of the kaolinite spectra through intercalation with formamide and through the CRTA treatment. The differences in the DRIFT spectra between the formamide and CRTA-treated formamide-intercalated kaolinites are quite remarkable. The spectra of the CRTA-treated samples show intensity in the 3300-3500 cm-1 region, whereas the nontreated formamide-intercalated kaolinite shows intensity in the 3100-3300 cm-1 region. This difference is attributed to the nature of the samples. The spectrum labeled d is that of a sample which contains both intercalated and adsorbed formamide. Spectra b and c are that of the formamideintercalated kaolinite only after CRTA treatment. The band component analyses of the DRIFT spectra of the hydroxyl stretching region for the formamide-intercalated and CRTA-treated formamide-intercalated kaolinites are reported in Table 1. The kaolinite shows five bands at 3693, 3681, 3667, 3651, and 3619 cm-1. The first four bands are attributed to the kaolinite inner surface hydroxyls, and the band at 3619 cm-1 is attributed to the inner hydroxyl. The relative intensities of these bands are 28.0, 9.3, 8.6, 31.9, and 21.1%, respectively. Even though the results of the X-ray diffraction show that the kaolinite was fully expanded, the DRIFT spectra of the hydroxyl stretching region of the formamide-intercalated kaolinite shows that some intensity remains in the hydroxyl stretching bands of the inner surface hydroxyls compared with that of the CRTA-treated formamide-intercalated kaolinites. The bandwidth of the 3619 cm-1 band attributed to the inner hydroxyl of the kaolinite band is 9.1 cm-1. The widths of the four inner surface hydroxyls are broader with
ν1
ν2
ν3
3693 28.0 19.2 3692 6.0 25.0 3694 11.2 21.2 3695 9.0 24.2 3695 6.0 28.5
3667 8.6 11.8 3670 2.9 19.3 3670 5.5 21.2
3651 31.9 18.4 3648 25.6 35.7 3648 24.4 34.7
ν6
3628 18.6 14.0 3628 14.6 14.9 3629 91.0 40.0 3629.5 94.0 40.0
ν5
ν7
3619 21.1 9.1 3621 0.5 16.0 3621 4.0 18.8 not observed
absent 3606 46.4 53.0 3605 43.8 55.5
not observed
bandwidths of 19.2, 15.9, 11.8, and 18.4 cm-1. Upon intercalation of the kaolinite with formamide, additional bands are observed at 3606 and 3629 cm-1. The relative intensities of these bands are 35.3 and 11.0%. The bandwidths of these bands are 47.9 and 11.5 cm-1. Bandwidths can be helpful in the assignment of bands: for example, broad bands may be associated with water vibrations, and narrow bands with hydroxyl stretching frequencies. These bands at 3606 and 3629 cm-1 are attributed to the intercalated water and to the inner surface hydroxyl groups hydrogen bonded to the formamide. Bands are observed at 3694, 3671, 3649, and 3620 cm-1. The relative intensities of these bands are 17.7, 5.2, 28.8, and 16.0%. Remarkable differences are observed in the intensities of the hydroxyl stretching region of the CRTA-treated formamide-intercalated kaolinites. Only two bands are resolved at 3695 and 3629 cm-1. The relative intensities of these bands are 91.0 and 9.0% for the 100% CRTAtreated formamide-intercalated kaolinite and 94 and 6.0% for the CRTA-treated 50% formamide-intercalated kaolinite. The band at 3629 cm-1 is attributed to the inner surface hydroxyls hydrogen bonded to the formamide. The band at 3695 cm-1 is assigned to the nonbonded inner surface hydroxyls. In the NH stretching region, bands are observed at around 3460, 3365, 3344, 3227, and 3148 cm-1.19,20 The bands are broad, and because of the complexity of the spectra in the NH and OH stretching regions, the bands are difficult to resolve with precision. What is significant, however, is that the bands in the 3344, 3227, and 3148 cm-1 region are attributable to adsorbed formamide and the bands at 3460 and 3365 cm-1 are attributable to the intercalated formamide. Another means of studying the changes in the kaolinite surface structure upon intercalation is the study of the hydroxyl deformation modes. These bands are centered on 913 cm-1.21 Figure 5 displays the hydroxyl deformation modes of kaolinite, the formamide-intercalated kaolinite, and the CRTA-treated formamide-intercalated kaolinites. The band component analysis of the DRIFT spectra of the formamide-intercalated and CRTA-treated formamideintercalated kaolinites are reported in Tables 2-4. For kaolinite, three deformation bands are observed at 913, 924, and 939 cm-1 with relative intensities of 57.7, 11.0, and 31.0%. These bands are attributed to the hydroxyl deformation modes of the inner hydroxyl and the inner surface hydroxyl groups, respectively.21 Upon intercalation (19) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Graselli, J. G. The Handbook of infrared and Raman characteristic frequencies of organic molecules; Academic Press: San Diego, CA, 1991. (20) Bandekar, J. Biochim. Biophys. Acta 1992, 1120, 123. (21) Frost, R. L. Clays Clay Miner. 1998, 46, 280.
3220
Langmuir, Vol. 17, No. 11, 2001
Frost et al.
Table 2. Results of the Band Component Analysis of the DRIFT Spectra of the NH Stretching Region of Kaolinite Intercalated with 100% Formamide and with 50% Formamide/50% Water Mixtures and the CRTA-Treated Formamide-Intercalated Kaolinites kaolinite
spectrum
100% formamide-intercalated kaolinite
DRIFT
50% formamide/50%water-intercalated kaolinite
DRIFT
100% formamide-intercalated kaolinite CRTA treated
DRIFT
50% formamide/50% water-intercalated kaolinite CRTA treated
DRIFT
ν1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
ν3
ν4
ν5
ν6
3460
ν2
3365 broad
3344
3227 broad
3184 broad
3460 33.7 40.9 3462 51.1 50.4 3462 48.0
3365 broad
3344 53.5 59.9 3332 18.0 45.4 3335 22.0 60.5
3248 1.9 24.8
3167 7.7 36.4
3365 31.5 72.8 3371 14.8 68.3
Table 3. Results of the Band Component Analysis of the DRIFT Spectra of the Hydroxyl Deformation Region of the Formamide Intercalates and the CRTA-Treated Intercalates kaolinite low defect 100% formamide-intercalated kaolinite 50% formamide/50% water-intercalated kaolinite 100% formamide-intercalated kaolinite CRTA treated 50% formamide/50% water-intercalated kaolinite CRTA treated
Figure 5. DRIFT spectroscopy of the OH deformation region of (a) low-defect kaolinite, (b) intercalated with 100% formamide, (c) CRTA treated intercalated with 100% formamide, and (d) CRTA treated intercalated with 50% formamide/50% water.
of kaolinite with formamide, bands are observed at 905, 915, 938, and 974 cm-1. The relative intensities of these bands are 48.2, 24.0, 5.3, and 21.3%. Significantly, the intensity of the bands attributed to the hydroxyl deformation of the inner surface hydroxyls at 939 and 924 cm-1 are reduced in intensity to a total of 5.3%. Thus, as with the hydroxyl stretching frequencies, the intensity of the hydroxyl deformation vibrations has been reduced to
formamide band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
971 13.6 15.0 971 11.7 16.0 972 9.7 13.9 972 14.2 19.0
ν1-def
ν2-def
ν3-def
939 31.0 18.4
924 11.0 14.9
913 57.7 15.0 913 11.6 17.1 913 11.3 16.8
926 11.3 22.3 922 22.1 39.0
ν4-def
905 74.8 15.5 905 77.0 16.7 907 78.9 16.6 907 63.5 17.1
minimal intensity. An additional band at 905 cm-1 with 48.2% of the total intensity is attributed to the hydroxyl deformation vibration of the inner surface hydroxyls hydrogen bonded to the CdO of the formamide.19 The band at 974 cm-1 is attributed to an out-of-plane CH wag.19,20 For the CRTA-treated formamide-intercalated kaolinites, only two bands are observed at 922 and 907 cm-1, with relative intensities of 78.9 and 11.3% for the CRTA-treated 100% formamide-intercalated kaolinite and 63.5 and 22.1% for the CRTA-treated 50% formamide-intercalated kaolinite. No band was observed at 913 cm-1. The band at 922 cm-1 has been previously attributed to nonhydrogen-bonded inner surface hydroxyl groups.21 Figure 6 displays the 1200-1800 cm-1 region. This region is where the infrared spectra of the CdO stretching, NH deformation, HOH deformation, and CH bending modes are observed. The 1715 cm-1 band is attributed to the CdO stretching vibrations. The normal position of the CdO band is between 1640 and 1680 cm-1.19 This band is ascribed to an amide vibration which is a mix of the CdO and CN stretching vibrations in some mix of around 80/20.19,20 A substantial shift in this frequency is observed upon intercalation of the formamide into the kaolinite. The bands at 1695 and 1674 cm-1 are attributed to the NH deformation of a primary amide.19-21 These bands are amide-2 bands and are often described as a mix of the βNH2 and γCN vibrations in a 60/40 ratio.20 Two bands are observed at 1695 and 1674 cm-1. One band is perturbed (1695 cm-1), and the second is nonperturbed. Such observations suggest that one NH is bonded to the siloxane layer and the second NH of the amide group remains nonbonded. The band at 1630 cm-1 for the adsorbed formamide in the formamide-intercalated kaolinites corresponds to the HOH bending frequency of water. This frequency corresponds to the hydrogen-bonded water molecule. In this
Separation of Adsorbed from Intercalated Formamide
Langmuir, Vol. 17, No. 11, 2001 3221
Table 4. Results of the Band Component Analysis of the DRIFT Spectra of the 1000-1800 cm-1 Region of Kaolinite Intercalated with 100 % Formamide and with 50% Formamide/50% Water Mixtures and Their CRTA Treated Products formamide CdO stretch
kaolinite water 100% formamide-intercalated kaolinite 50% formamide/water-intercalated kaolinite 100% formamide-intercalated kaolinite CRTA treated 50% formamide/water-intercalated kaolinite CRTA treated
band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1 band center/cm-1 % area bandwidth/cm-1
Figure 6. DRIFT spectroscopy of the 1200-1700 cm-1 region of (a) low-defect kaolinite, (b) intercalated with 100% formamide, (c) CRTA treated intercalated with 100% formamide, and (d) CRTA treated intercalated with 50% formamide/50% water.
case, the water is hydrogen bonded to the formamide. In the CRTA-treated formamide-intercalated kaolinites, no band at around 1630 cm-1 is observed, thus confirming the thermogravimetric analyses that water has been removed. No band is observed at 1709 cm-1 for the CRTAtreated formamide-intercalated kaolinites. This suggests that this vibration involves water in the vibrating unit. Thus, it is proposed that the 1709 cm-1 band arises from an interaction between the CdO and the OH of water. In the absence of water, this interaction no longer exists as is observed in the CRTA-treated kaolinites. An additional band is also observed for the formamide-intercalated kaolinite at 1290 cm-1. This and the band at 1317 cm-1 are assigned to the CH deformation vibrations. The two bands are attributed to the formamide-intercalated and the adsorbed formamide, respectively. The Difference Spectra. Figure 7 reports the difference spectra between the 100% formamide and CRTAtreated formamide-intercalated kaolinites. The spectra clearly show that a large amount of formamide is adsorbed on the kaolinite surface. TG-MS shows that 21.6% of the total mass is lost in the first stage of the CRTA isothermal desorption process. This represents a large amount of material, and the question arises as to where this adsorbed formamide is in the intercalated structure. There are a number of possibilities: (a) the formamide is adsorbed on the external kaolinite surfaces, (b) the formamide is adsorbed but not bonded to the internal kaolinite surfaces, (c) the formamide is a space filler between the kaolinite layers, and (d) the formamide acts with water as a space filling molecule between the kaolinite layers. Another further possibility is that the formamide is layered with water on the outside of the kaolinite surfaces. This seems
1709 24.7 38.6 1710 23.9 35.2
NH def
1675 58.4 30.0 1675 48.9 39.2 1671 76.4 24.4 1671 71.6 28.0
νH2O-1 ∼1630 100 45.0 1630 5.6 42.1 1630 5.5 36.9
ν
ν
ν
ν
1594 11.1 24.2 1595 21.7 44.1 1595 10.3 24.2 1595 13.5 26.3
1392 8.0 15.2 1392 8.0 15.2 1394 7.2 7.4 1395 7.7 7.7
1314 3.2 28.3 1314 3.2 28.3 1317 6.0 18.4 1317 7.2 19.1
1290 1.3 29.2 1290 1.3 29.2
Figure 7. DRIFT spectra of the (a) CRTA-treated formamideintercalated kaolinite, (b) the nontreated formamide-intercalated kaolinite, and (c) the difference spectra.
the most likely situation. The Birdwood kaolinite is very difficult to intercalate with formamide. Less than 23% of the kaolinite is intercalated. Yet, this formamideintercalated kaolinite shows a large isothermal decomposition at 118 °C, thus indicating that the formamide is adsorbed on the external surfaces of the kaolinite. The 1709 cm-1 vibration, which is an amide band, composed of the combination of the CdO (80%) and the C-N vibration (20%) is only evident in the adsorbed formamide. This brings into question the assignment of the 1709 cm-1 band, as it seems only to be observed in the presence of water. The amount of intercalated formamide is 4.24%. This means that only 1 in 4 of the inner surface hydroxyls are bonded to the formamide. Thus, a likely model is based on the formamide-water units space filling and adsorbed on the internal surfaces of the kaolinite. Conclusions Controlled rate thermal analysis has been used to separate adsorbed formamide from intercalated formamide in formamide-intercalated kaolinites. This separation is achieved by removal of the sample at the end of the controlled isothermal desorption of formamide. The differential analysis curves show two endotherms at 65 and 150 °C ascribed to the loss of water and formamide, respectively. A weak inflection is also observed at 560 °C, which is attributed to the dehydroxylation of the kaolinite. Under quasi-isothermal conditions, the liberation of loosely bonded formamide takes place in an equilibrium reaction at a constant temperature of 118 °C, independently of the type of kaolinite used and the amount of water present in the intercalation solution. The liberation
3222
Langmuir, Vol. 17, No. 11, 2001
of the strongly bonded formamide takes place in a nonisothermal, equilibrium process between ∼130 and 200 °C. When the kaolinite is 100% expanded, the percentage amount of loosely bonded formamide is 1.87 times higher if intercalation is carried out in a formamide/ water mixture rather than in pure formamide. X-ray diffraction shows that after removal of the loosely bonded formamide, the completely expanded structure of the intercalated kaolinite still exists; that is, no partial collapse of the structure results when some 70-80% of the reagent connected to the clay is removed. DRIFT spectroscopy shows that the spectra of the formamide-intercalated kaolinite before and after the quasi-isothermal step are different. Significant differences exist between the DRIFT spectra of the inserting molecule before and after the removal of the adsorbed formamide. The spectra of the kaolinite before the quasi-isothermal
Frost et al.
desorption of formamide show significant remaining intensity in the bands attributed to the inner surface hydroxyls. The band observed at 3629 cm-1 is assigned to the inner surface hydroxyls hydrogen bonded to the formamide. Several significant differences are apparent between the spectra of the adsorbed and formamideintercalated kaolinites. Acknowledgment. Financial support from the Hungarian Scientific Research Fund under Grant OTKA T25171 is also acknowledged. The financial and infrastructure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. LA001686B
