The vermiculite used was obtained from the "Santa Olalla" deposit (Huelva, SW Spain). The vermiculite-decylammonium complex was prepared according to the ...
Clay Minerals (1988) 23, 379-390
D E C O M P O S I T I O N OF A L K Y L A M M O N I U M C A T I O N S I N T E R C A L A T E D IN V E R M I C U L I T E J. L. P I ~ R E Z - R O D R I G U E Z ,
E. M O R I L L O *
AND C. M A Q U E D A *
lnstituto de Ciencia de Materiales, CSIC, Sevilla, and *lnstituto de Recursos Naturales y Agrobiologla de Sevilla, CSIC, Apartado 1052, 41080 Sevilla, Spain (Received October 1987; revised 19 April 1988)
A B S T R A C T : Decomposition of decylammonium cations on vermiculite surfaces as a result of acidity has been studied. The vermiculite-decylammonium complex was treated with solutions of HC1 in water and butanol, and solutions of chlordimeform in butanol. X-ray powder diffraction and IR spectroscopy data showed the presence after treatment of ammonium cations in addition to aliphatic chain compounds in the interlamellar space. The ammonium cation is produced by breaking of the C-N bond of alkylammonium due to the low pH of the medium and, when using but/tool as solvent, to the dissociation of the scarce water molecules on the clay surface. The influence of the different solutions on the period of decomposition of alkylammonium cations is discussed. Vermiculite-decylammonium complex treated with HC1 solutions for a year released structural Mg z+ due to the acidity of the medium. This Mg2+ partially displaced ammonium cations from the interlamellar space.
The phenomenon of alkylammonium decomposition on montmorillonite surfaces has been studied by several workers. Calvet et al. (1964) showed that alkylammonium cations adsorbed on montmorillonite surfaces are decomposed at temperatures considerably below the corresponding theoretical decomposition temperatures of amines or their chlorhydrates. Above 100~ the adsorbed alkylammonium cations decompose into N H I (which remains on the clay), organic residues and hydrocarbons, as the result of the catalytic rupture of C - N bonds. The catalytic effect was attributed to protons in the water adsorbed on the montmorillonite surface. Durand et al. (1972) re-examined these experiments using an inert atmosphere and long reaction periods. They studied the thermal transformation of alkylammonium cations adsorbed on the surface of montmorillonite in various conditions of hydration. The reactions observed were mainly transalkylations, for which a mechanism of acid catalysis was proposed, requiring a high degree of dissociation o f the water remaining on the surface of the clay. There are differences between the thermal transformation of alkylammonium montmorillonite in an inert atmosphere and the corresponding transformation in presence of oxygen. In the latter, a faster increase in the N H ~ content was observed, and transalkylation processes did not operate. In a previous paper (P6rez-Rodriguez et al., 1985) we have observed the decomposition of interlamellar alkylammonium in vermiculite using butanol solutions of a pesticide, chlordimeform. The aim of this p a p e r is to establish whether this decomposition was the result of the interlamellar acidity. 9 1988 The Mineralogical Society
380
J. L. P&ez-Rodriguez et al. EXPERIMENTAL
The vermiculite used was obtained from the "Santa Olalla" deposit (Huelva, SW Spain). The vermiculite-decylammonium complex was prepared according to the method proposed by Lagaly & Weiss (1969). Technical grade chlordimeform (N'-(4-chloro-2-methylphenyl)-N,N-dimethyl methanoimidamide hydrochloride) was used. This pesticide is soluble in water (50 wt%) and ionizes completely giving the chlordimeform cation and chloride anion, by the following process:
I H20 1 "CH3
. .H +. .... ".... ~CH3 C1--rx ~_) )--l'q = CH - ~,IQ CH 3
+ C1-
The vermiculite-decylammonium complex was treated with 25 mmol/l butanol solution of chlordimeform, and with HCI (37.%o)in water and butanol. The treatments were carried out at 60~ in stoppered, continuously shaken glass tubes. The solutions were changed weekly for the first 100 days of treatment and thereafter left in contact with the samples without further change for a year. The basal spacings were determined by X-ray diffraction (XRD) from several orders of the (00/) reflections. The samples Were washed several times with the corresponding solvent used in the previous adsorption process, air dried on glass slides, and examined as oriented films. The infrared (IR) absorption spectra were recorded from 4000 cm -1 to 400 cm -~ using a Perkin Elmer double beam spectrophotometer on samples prepared as KBr discs. Total N was determined using the Kjeldahl method, by digestion of the sample with conc. H2SO4 and a catalyst (Se + K2SO4) , and displacement of N (as NH~) with NaOH (50~o w/v) in a vapour stream. The NH~ ions from the interlamellar space of the vermiculite were determined by displacement of these cations with MgO. In both cases the distilled fraction was collected on H3BO 3 (4% w/v), and the excess was titrated with 0.02 N HC1. Likewise, the interlamellar Mg 2+ was determined by saturation of samples with 1 N ammonium acetate solutions at pH 7. The Mg 2+ displaced was determined by atomic absorption spectrophotometry. The C content in the interlamellar space of the vermiculite was measured using a Perkin Elmer CNH elemental analyser (240-C). RESULTS AND DISCUSSION
Treatment with butanol solutions of chlordimeform The XRD patterns of the vermiculite-decylammonium complex, and this complex treated with butanol solutions of chlordimeform for two weeks are shown in Fig. 1. The complex
381
Alkylammonium cations in vermiculite
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J . L . P~rez-Rodriguez et al.
382
yields a basal spacing of 21.2 A (Fig. la) which, after treatment for one week with the chlordimeform-butanol solution, decreases to 12.6 A. Perfectly defined diffractions of higher orders appear after constant shaking for two weeks (Fig. lb) and also after nine weeks treatment with the sample being shaken several times a day. The decrease in the basal spacing from 21.2 A to 12-6 A indicates an important alteration of the interlamellar space, but it does not correspond to the formation of the vermiculitechlordimeform complex, which has a basal spacing of 14.6 A (Morillo et al., 1983). IR spectra of the untreated vermiculite-decylammonium complex and treated with chlordimeform solution in butanol for two weeks are shown in Fig. 2. The spectrum of the complex (Fig. 2a) shows the characteristic bands corresponding to the - C H 3 (2950, 1450 cm-X), - CH2 - (2920, 2850, 1460 cm -l) and - NH~ (3100, 1625, 1570, 1500 cm -1) groups of the alkylammonium ions. The vibrations at 1625 cm -1 and 1500 cm -1 correspond to the asymmetrical and symmetrical - N H deformation of the -NH~- groups. At 1570 cm -1, a shoulder appears corresponding to the symmetrical - NH deformation of the - NH~ groups
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1250
Alkylammonium cations in vermiculite
383
bonded to the basal oxygens by a stronger H bond (Serratosa et al., 1970). The band at 1625 cm -1 also corresponds to the OH deformation of water, because the OH-stretching band at 3400 cm -I indicates the presence of some interlamellar water. The spectrum of the complex treated with the chlordimeform solution in butanol for two weeks (Fig. 2b) shows many differences from that described. The absorption bands at 3250, 3040, 2800 and 1430 cm -1 are clearly characteristic of the ammonium ion (NH +) in the interlamellar space (Ahlrichs et al., 1972; Stone & Wild, 1978). Also, bands in the 2900-2800 cm -~ region and at i470 cm -~ of the C - H of CHz or CH3 groups are present in addition to water bands at 3420 and 1625 cm -1. These data show the presence of ammonium ions in the interlamellar space of vermiculite as well as aliphatic chain compounds, as the basal spacing (12.6 A) is higher than that of the ammonium-vermiculite (10.5/~ for our sample), and it is probably due to an interstratification between collapsed interlayers with NH~, and interlayers with decylammonium or other compounds derived from its decomposition. The ammonium ions in the vermiculite-decylammonium complex amounted to 10 mEq/100 g, increasing to 110 mEq/100 g after two weeks treatment with chlordimeformbutanol solution. As the cation exchange capacity (CEC) of the vermiculite is ~ 140 mEq/ 100 g (Justo Erbez, 1984), the amount of NH~ in interlamellar positions after treatment with chlordimeform-butanol represents 78~ of the CEC. These observations suggest decomposition of the alkylammonium ions by breaking of the C - N bond. This is attributed to the high acidity of the small amount of water present in the system which is strongly dissociated (Mortland et al., 1963), as the solvent for chlordimeform is butanol, and, accordingly, the proton mobility in the water-adsorbed layer is very high (Hecht et al., 1966). Chlordimeform chlorhydrate in water produces an acidic pH and as a result the small amount of water present makes the partial dissociation of the pesticide possible, and increases the acidity of the interlamellar space. The breaking of the C - N bond is partially favoured by the way in which - NH~ groups of the alkylammonium cations are located on the hexagonal (ditrigonal) "holes" (Johns & Sen Gupta, 1967), and as a result, the interaction between NH~- groups and the negative charge on the clay is strong, and the C - N bond is weakened. The organic species resulting from the alkylammonium decomposition are released partially to the solution or to the external surface, the remainder staying in the interlamellar space in a flat orientation, as shown by the basal spacing, 12.6 A. According to Durand et al. (1972) the aliphatic products formed by this process are alcohols which are transformed into a mixture of hydrocarbons, but in the present study these transformations have not been confirmed. The amount of interlamellar C in this sample after treatment with chlordimeform-butanol solutions is 2.8~, and if all this C is in the form of decylammonium cation, it would represent ~ 26 mEq/100 g, in contrast to 140 mEq/100 g of initial saturation. The vermiculite-decylammonium complex was treated with butanol for several weeks in order to observe the influence of this organic solvent in the decomposition of interlamellar decylammonium cations. The XRD patterns of complexes treated for ten weeks with butanol do not show any changes relative to the original sample. After ten weeks a shoulder appears at 1430 cm -~ on the IR spectrum, corresponding to N H + cations, indicating the beginning of alkylammonium decomposition in the interlamellar space. This decomposition must be due to the dissociation of interlamellar water but the process takes place slowly and takes longer than in the chlordimeform-butanol treatment.
384
J . L . P~rez-Rodrlguez et al.
Treatment with HCl-butanol and HCl-water solutions for six weeks In order to confirm the influence of acidity on the decomposition of alkylammonium cations in the vermiculite interlamellar space, several experiments were performed. The vermiculite-decylammonium complex was treated with aqueous and butanol hydrochloric solutions to obtain high acidity in the medium surrounding the sample, and to influence the acidity of the interlamellar space. These experiments were carried out under the same conditions as for the treatment with chlordimeform-butanol solutions. The XRD patterns of the vermiculite-decylammonium complex and this complex treated with HCl-butanol solution for six weeks are shown in Fig. 3. The 21.2 A basal spacing of the complex (Fig. 3a) disappears after two weeks treatment with HCl-butanol solution and there is no well-defined diffraction peak until after six weeks treatment (Fig. 3b), when a peak at 12-8 A appears. The IR spectra of the complex before and after treatment with HCl-butanol solution display many differences (Fig. 4a,b), such as the decrease in intensity of bands due to decylammonium cations, and the appearance of an absorption band at 1430 cm -1, corresponding to the N H + cations in the interlamellar space. The effects of this treatment on XRD and IR data are very similar to those observed for the treatment with chlordimeform-butanol solutions, and the conclusions are the same, namely, the alkylammonium ions are decomposed to yield interlamellar ammonium ions which are the main saturating cations of the vermiculite (the basal spacing decreases from 21.2/k to 12.8 A), and the aliphatic products of the decomposition partly remain in the interlayer space. However, there is a difference between the treatments: the time required to reach the decylammonium decomposition was two weeks for chlordimeform-butanol, and six weeks for HCl-butanol. These two different times may be due to the different acidity of the interlamellar space, depending on the amount of water (Theng, 1982; E1-Amany & Mill, 1984). If the treatment is made with HCl-butanol solution, in addition to the residual water of the vermiculite-decylammonium complex, some water is added with the HCI solution used (37~), and therefore the acidity is lower, and thus the decomposition of alkylammonium is less favoured. When the vermiculite-decylammonium complex is treated with the HCl-water solution, the results obtained are a little different to those for the HCl-butanol solution treatments. A peak at 12.8/k appears after six weeks treatment (Fig. 5b), but it is not very well defined, and other diffraction peaks appear due to higher spacings of layers in which the alkylammonium decomposition has not taken place, or has not been completed. Likewise, IR spectroscopic studies of samples treated with HCl-water solution for six weeks show the presence of the most characteristic band at 1430 cm -1 due to N H + cations (Fig. 6b). The influence of the amount of water on the decomposition of alkylammonium also manifests itself in these different treatments of the same sample. In the treatment with aqueous solution of HC1, the amount of water present in the system is much larger than that for the HCl-butanol solution treatment, and consequently the acidity of the system must be lower, with the result that decomposition of alkylammonium is less marked. For this reason, alkylammonium cations are not completely degraded after treatment for six weeks with HC1water solutions, and no well defined XRD peaks are obtained.
Alkylammonium cations in vermiculite
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J . L . Pkrez-Rodriguez et al.
386
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Treatment with HCl-butanol and HCl-water solutions for a year F o r the treatment of the vermiculite-decylammonium complex with HCl-water and HCIbutanol, the solutions were changed weekly for 14 weeks, and then the solutions were left in contact with the samples without further change for a year. F o r both treatments, the basal spacing of the vermiculite complex increased from 12.8 A after six weeks treatment to 14 A after one year (Figs. 3c and 5c). The interlamellar Mg z+ content in the complex after a year was 70 mEq/100 g for the treatment with HCl-butanol, and 95 mEq/100 g for the treatment with HCl-water.
Alkylammonium cations in vermiculite
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J.L. POrez-Rodriguez et al.
388
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