complexes of organic compounds · complexes of organic halloysite with compounds r. m. carr and hwa...

Clay Minerals (1971/, 9, 153.

C O M P L E X E S OF O R G A N I C

H A L L O Y S I T E W I T H C O M P O U N D S

R. M. C A R R AND H W A C H I H *

Chemistry Department, University of Otago, New Zealand

(Received 12 May 1971)

ABSTRACT: More than twenty new organic complexes of halloysite have been prepared and one hundred and twenty-seven compounds were tested. Results are compared with those of earlier studies and use is made of recent findings concerning particle morphology and interstratification effects to interpret the results. The compounds which form complexes with halloysite are polar, and are usually either acids or bases. Their molecules usually contain two functional groups (preferably ---OH and/or --NH2), are relatively small and have one functional group per two carbon atoms, but do not usually include cyclic or aromatic types. The best explanation of the existence of these metastable complexes seems to lie in the ability of some organic molecules to form hydrogen bonds with the halloysite structure, about which insufficient knowl- edge is available to allow quantitative evaluations to be made.

I N T R O D U C T I O N

Organic complexes of halloysite were first studied systematically by MacEwan (1946, 1948) and no major studies of a similar nature have since been reported. MacEwan demonstrated that several highly polar organic liquids including simple alcohols, diols, alcohol ethers and some diamines formed single-layer complexes with hydrated halloysite. On the other hand metahalloysite, which does not hydrate directly, was shown to form a complex with ethane 1:2 diol.

Two greatly expanded halloysites were reported by Caill~re et al. (1950) and these were obtained by using propanol containing a small quantity of water and from a solution containing 10% saturated barium hydroxide and 90% ethanol, the complexes having basal spacings of 14 A and 17 A respectively. It is probable that the latter complex is a mixed organic-salt type. This type of structure is possible since halloysite readily forms complexes with ionic compounds including ammonium and potassium salts (Wada, 1959, 1961).

The present study confirms most of MacEwan's findings and demonstrates the

*1 Reservoir, 5th Avenue, Penang, Malaysia.

154 R. M. Carr and Hwa Chih

existence of many more complexes which can be prepared not only from organic liquids but also from solids.

E X P E R I M E N T A L

Materials Halloysites from two localities, Te Puke and 'Kauri' Mountain, New Zealand,

were used. Both are almost fully hydrated and impurities which are present mainly in the Te Puke clay include very small amounts of quartz and cristobalite, a little organic matter and traces only of iron. Electron microscopic examination indicates a tabular morphology for the Te Puke clay whereas the 'Kauri' sample contains many tubular particles.

Organic chemicMs were mainly of ordinary laboratory grade purity and no special effort was made to purify them.

Equipment X-ray diffraction patterns were obtained with Philips' equipment using Ni filtered

Cu Ka radiation. Infra-red spectra were obtained from a Perkin-Elmer model 421 grating spectro-

photometer. Electron micrographs were obtained from a Siemens' instrument using samples

sedimented onto collodian discs.

Procedure Small samples (<0.5 g) of halloysite were exposed to organic liquids or solutions

of organic solids in centrifuge tubes. The tubes were periodically agitated to ensure good contact. Many complexes are formed after 1 or 2 hr but some require several days to attain equilibrium. All organic compounds were held in contact with halloysite samples for at least 3 days and many were held for 1 week. Exposure to liquid or solution was terminated either by centrifugation and decantation of supernatant liquid, or by direct withdrawal of a sample of the clay. The clay material was then transferred to a glass slide and prepared for X-ray diffraction. In those cases where volatile organic liquids were used, excess liquid was always present on the sample slide during exposure to the X-ray beam. Using this simple technique it was possible to obtain a trace of the profile of the basal peak before even the most volatile of the liquids used evaporated. In some cases (see methanol and acetaldehyde in Table 3), changes in the basal peak began to take place immediately the samples 'dried out'.

For infra-red examination complexes were mulled in the complexing liquid or with nujol, and held between sodium chloride plates.

Thermal and vacuum stability tests were conducted by placing X-ray diffraction slides in an oven at 100 ~ for 1 hr, and in a vacuum dessicator until the excess organic liquid vaporized, respectively.

Organic complexes of halloys#e 155

In other tests some complexes were exposed for many hours to distilled water in centrifuge tubes, the water being changed by decantation following centrifugation several times.

All products were subjected to X-ray diffraction and results are expressed in terms of MacEwan's (1948) A value, viz. clearance space occupied by the organic material measured as the difference between the observed d00~ basal spacing and the d00~ for metahalloysite which is taken as being 7"2 A.

Values for A.mln(calc ) w e r e obtained from 'Catalin' molecular models. Molecular sizes obtained from these models are approximately 0-5 A too small due to discrepancies in the probable van der Waals atomic dimensions. Accordingly all sizes obtained from the models were adjusted and the values listed represent the probable smallest distance through each molecule. It must be noted that the molecular orientation required for this minimum value may not be the same as that obtaining in the complex.

C R I T E R I A FOR C O M P L E X F O R M A T I O N

Assuming a simple, direct relationship between doo~ and ~ values it follows that definite complex formation can be claimed only when

~product > AH20

(A H2o is approximately 2'8 A in the present study). This criterion is implicit in MacEwan's discussion of his experimental results.

Recent studies (Harrison & Greenberg, 1962; Churchman, 1970) on the removal of interlayer water from hydrated halloysite, have shown that this dehydration proceeds through a whole series of intermediate phases, thus confirming MacEwan's (1947) conclusions based on analytical data. Churchman obtained profiles of the X-ray peaks which characterize the interlayer water capacities of halloysites over the complete range from fully-hydrated to dehydrated, and demonstrated that an interstratification mechanism gave the only satisfactory explanation of the profile shapes. This proposed mechanism may be used as the basis for a new criterion of complex formation for halloysites. In the halloysite-metahalloysite series of structures, the end-members only give 001 X-ray peaks which have reasonably sharp and symmetrical profiles, and then only if the materials have high crystalline order. All partially hydrated forms give broad asymmetric X-ray peaks spreading across the region between the two end-member peaks. Higher order reflections are obtained from samples with symmetrical 001 peaks, but are not present in those which exhibit broad and for asymmetric 001 profiles. Since hydrated halloysite is the water complex of the species, it follows that organic complexes will have similar X-ray diffraction properties. Therefore a new criterion for complex formation can be formulated as follows: an halloysite is fully complexed when (a) its 001 X-ray reflection is sharp and symmetrical; (b) higher order reflections, viz. 003 and some- times 002, are present. The condition Ar > AH20 is a useful guide only, and where the X-ray reflections do not conform to the new criterion, indicates a ten-


dency only to complex formation. This new criterion has been used for montmorillonite-organic complexes (Brindley & Ray, 1964) where interstratification effects had been well characterized.

As a consequence of the interstratification mechanism, no conclusion can be reached concerning a partially complexed halloysite with a clearance space smaller than that for water. In such a case, chemical analysis of the product in principle, would resolve this problem, but in practice is well-nigh impossible owing to the difficulty of sample purification arising from the relative instability of all complexes. Since the new criterion of complex formation is of more general application than the simple 'clearance' space consideration, it will be used in the interpretation of all experimental results. Hence the presence or absence of higher order X-ray reflections from the 001 lattice planes is noted for all products under the heading 'X-ray reflections' in the'tables of results given below. Where higher order reflections were not observed due to the interfering presence of reflections from either crystalline complexing substance or from impurities, the shape of the 001 reflection is noted. In some cases where the position of the 001 reflection from the halloysite sample remained unchanged during exposure to the organic substance, changes were observed in both the intensity and the symmetry of the reflection. In such cases, even if both the intensity and the symmetry increased, the evidence for complex formation is equivocal since the organic reagent may have catalysed some ordering or even slight rehydration of the halloysite without necessarily having occupied interlayer sites itself.

R E S U L T S AND DISCUSSION

All of the organic liquids used by MacEwan (1948) were tested with samples of Te Puke halloysite and the results are listed in Table 1. In general four types of behaviour are observed: (1) complex formation; (2) a tendency to complex formation as indicated by diffuse 001 reflections with basal spacings greater than that of hydrated halloysite, e.g. butyl cellosolve; (3) dehydration, as shown by the monohydric alcohols, diethylamine and acetone; (4) no complex.

The propane 1.2 diamine complex immediately formed on exposure of the clay to the liquid but, within minutes, began to expand from a value of 4.2 ,~ to one of 10 A (after 30 min). This was probably caused by water absorption.

The complexes formed from ethane 1.2 diol, propane 1.3 diol, glycerol and diethylene glycol appear to be stable to heat and to vacuum whereas those formed from methyl cellosolve and ethane 1.2 diamine are stable to vacuum only. All other complexes appear to be relatively unstable to heat and vacuum.

Within the limits of experimental error half of the results of this investigation are essentially in agreement with those of MacEwan, i.e. the discrepancy in 6 values is ~0-3 A. For three of the remaining liquids: methanol, propane 1.3 diol, and butane 1.3 diol, noticeably smaller values were observed in this study. Four other results: 1.4 dioxane, butyl cellosolve, propane 1.2 diamine, and nitrobenzene, may be more concordant than they appear, due to the difficulty of interpreting the

Organic complexes of halloysite 157

precise meaning of MacEwan ' s 'diffuse or double line'. However, the observed instabil i ty of the propane 1.2 d iamine complex would certainly cause the appearance of a diffuse or even double X-ray line. In the present study complexes were not obtained with ethanol, acetone, acetaldehyde and acetonitrile in direct contrast to

MacEwan ' s observations.

TABLE 1. Comparison with MacEwan's results

X-ray Reagent reflections A observed /Xmin (calc) A MacEwan

Methanol 001 asym 2"5 Ethanol 001 d asym < 0'3 1-propanol 001 d asym < 0' 3 1-butanol 001d asym < 0"3 Ethane 1.2 diol 001,002, 003 3"6 Propane 1.3 diol 001, 003 3"8 Butane 1.3 diol 001d asym 3.5d Glycerol 001, 003 4'0 1.4 dioxane 001 asym n.c. Methyl cellosolve 001, 003 3.8 Ethyl cellosolve 001, 003 3"7 Butyl cellosolve 001 d asym d Diethylene glycol 001, 003 3.9 Triethylene glycol 001, 003 3-8 2-chlorethanol 001, 003 3" 6 Ethane 1.2 diamine 001, 003 4-5 Propane 1.2 diamine 001,002 4.2 Diethylamine 001 d asym 0"3- Acetone 001 d asym 1 "2d Acetaldehyde 001 asym n.c. Acetonitrile 001 d asym 0.5 Nitromethane 001, 003 3.0 Nitrobenzene 001 asym n.c.

3'7 3"4 4.0 2"8d 4.0 0"3 4"0 0"3 4.0 3"7 4"0 4-4 4-0 4"0d 4"4 3"8 4.2 d 4'4 3"5 4"4 3'6d 4"4 3"ld 3"9 3"6 4-0 3"5 4.0 3'6 4-0 4"5 4-5 d 4.0 0"3 4"0 3"9 4'0 3"6 3-7 3"4 3"7 3'3 3-7 d

d = diffuse (this work and MacEwan) or double peak (MacEwan) asym = asymmetric peak shape n.c. = no complex

The above differences betweeen the results of the two investigations could be due to kinetic factors arising from durat ion of t reatment and the nature of the clay mineral . It has been noted elsewhere that clay samples were held in contact with the organic liquids for periods of up to seven days whereas MacEwan used 1 hr treatments. In view of the probable metastabil i ty of hydrated halloysites and all other complexes (see below), it is possible that an halloysite sample treated with organic material for several days may reach a state of apparent equi l ibr ium with a composit ion different from that of a sample treated for 1 hr only. This type of explanation could be used to acount for the smaller • values obtained with three


alcohols since the values were observed to change with time and then reach apparent equilibrium. It should be noted that in some cases experimental results were not reproducible. For example, in one experiment cyclohexanol formed a complex with a a value of 4'4 A, but in a subsequent experiment the listed value of 0-7 A was noted. Such erratic behaviour could be due to kinetic differences arising from a variety of causes including metastability of the product.

The structural nature of the halloysite is a second factor which can have significant effects. In experiments with 'Kauri' halloysite it was observed that ethane 1.2 diol forms a complex more slowly than it does with the Te Puke clay and appears to reach equilibrium at a smaller A value. Further, solids, e.g. malonic acid and /3-alanine, which formed complexes slowly with Te Puke halloysite (see below) did not cause any significant changes in the 'Kauri' clay. These differences most probably arise from Kinetic effects of tabular and tubular morphology on interlayer changes. The postulate that a tubular halloysite ('Kauri') will undergo c-axis ex- pansion less readily than a tabular clay (Te Puke) seems feasible and is supported by the above experimental observations, and confirms the conclusion (Nagasawa, 1969) that 'the halloysite with smaller b-dimensions tends to hold the interlayer water more firmly and to assume a tubular form more easily'. Measurement of the 06 spacing shows that the 'Kauri' halloysite has a smaller b-dimension than the Te Puke sample. It is suggested, therefore, that the reported difference in the behaviour of ethanol, acetaldehyde, acetone and acetonitrile may arise from differences in the halloysites which were tested, MacEwan's clay mineral being more reactive than either the 'Kauri' or the Te Puke varieties.

In the present study the authors were unable to prepare complexes of methanol and ethanol by direct contact with hydrated halloysite. The asymmetric profile of the 001 peak from the sample treated with methanol indicated that dehydration had started but did not proceed beyond a stage represented by a A value of 2.5 A. The possibility of partial complex formation cannot be dismissed in the absence of information concerning the nature of the interlayer material, but is most unlikely since the other alcohols tested showed apparent dehydrating effects causing extensive collapse of the interlayer structure. MacEwan claimed the formation of a complex with ethanol and then proceeded to argue that a dehydrating effect should occur. In view of the molecular size of ethanol relative to MacEwan's observed value, the short duration of the complex-forming experiment, and the results of the present study where apparent extensive dehydration was observed in several experiments with both types of clay, MacEwan was justified in being slightly suspicious of his own claim.

It was concluded from the above results that, in general, organic liquids of high polarity and/or containing two functional groups (preferably hydroxyl or amino) are able to form complexes with halloysite. Accordingly more compounds including several solids with similar properties were tested and the results are given in Table 2. In experiments where solids were used, aqueous solutions were prepared wherever possible, diethyl ether and acetone being used in a few cases only.

Organic complexes of halloysite TABLE 2. New complexes

159

X-ray Reagent reflections /x observed A rain (calc)

Ethanol Crotyl alcohol Propane 1.2 diol 2-butyne 1.4 diol in diethyl ether Formaldehyde Acetyl acetone Cyclohexane 1.2 dione in acetone Acetic acid Propionic acid Lactic acid Malonic acid in water Formamide Acetamide Urea Semicarbazide hydrochloride in water Methylamine hydrochloride in water Ethylamine hydrochloride in water Diethylene triamine Hydrazine hydrate Hydroxylamine hydrochloride Aniline hydrochloride Ethanolamine Isopropanolamine N-acetylethanolamine Triethanolamine Glycine in water /3-alanine in water Betaine hydrochloride in water Sarcosine in water

001dd 001, 003 001,002, 003 001, 003 001, 003 001, 003 001,002d, 003d 001 003 001 003 001 003 001 003 001 003 001 003 001 003 001 003 001 003 001 003 001 003 001 003 001 003 001dd 001, 003 001, 003 001, 003 001, 003 001, 003 001, 003 O01dd O01dd

3.5 4.0 4.3 4-0 4.4 4.2 3.5 4.0 3.1 3-0 4.0 4.0 5.4 5.0 4.4 4.0 4.9 4.0 4.6 4.2 3.5 3-9 3.0 3.0 3.9 4.0 3.8 3.0 3.2 3.6 4.5 3.6 5.5 3.6 3.8 4.5 3.3 3.9 3.0 3.6 7.8 3.6 3.5 4-0 3.8 4-5 4.0 4-0 4-0 4-9 3.2 4.0 4-5 4.0 5.9 5.7 4.8 4-0

dd = extremely diffuse N.B. Weiss & Russow (1963) reported complexes of halloysite with formamide, hydrazine,

and urea.

The complex with e thanol was fo rmed by first p repar ing the po tass ium acetate complex which was then exposed to e thanol for 10 days , when a diffuse X- ray reflection was observed together with a sharp, intense reflection f rom the basa l planes of the acetate complex.

The the rmal and vacuum stabil i t ies of the complexes fo rmed from solids were not tested since these are proper t ies concerned with vola t i l i ty and so have l i t t le relevance to most solids. I t was found tha t complexes fo rmed from crotyl alcohol , p ropa ne 1.2 diol, fo rmamide , lact ic acid, d ie thylene t r iamine , t r ie thanolamine , and N-ace ty l - e thanolamine are s table to bo th hea t and vacuum. V a c u u m stabi l i ty only was shown by the i sop ropano lamine complex while those fo rmed f rom formaldehyde , acetyl acetone, acetic acid, p rop ion ic ac id and hydraz ine hydra te were all the rmal ly unstable and showed varying degrees of v a c u u m instabil i ty.


The complexes formed with urea, formamide and hydrazine hydrate were washed several times with water to give products which were shown to be fully-hydrated halloysites from their low-angle X-ray diffraction profiles. Both Kauri and Te Puke varieties (i.e. tubular and tabular) behaved similarly thus supporting Weiss and Russow's (1963) observations. I t was also observed that the rehydrated halloysites would rapidly lose their interlayer water when allowed to dry at room temperature.

Marked differences in the rates of complex formation were observed; e.g. malonic acid and acetic acid form complexes much more slowly than acetamide, urea, and ethanolamine. Four of the compounds: ethanol, aniline hydrochloride, betaine hydrochloride and sarcosine, showed a tendency only to complex formation. The type of inconsistency observed with cyclohexanol (see above) appeared again with resorcinol, 2.4 dinitrophenylhydrazine, m-nitrophenol, o-aminophenol, picric acid, pyridine, aspartic acid, and cysteine. All attempts to prepare these complexes a second time failed.

It has been observed both in this and other studies (Churchman, 1970) that the hydrated halloysite prepared by washing the potassium acetate complex is extremely unstable. Since this observation indicates a change in relative stabilities it is possible that the rehydrated phase may form complexes with organic compounds more readily than do naturally hydrated samples. Therefore a quantity of rehydrated haUoysite was prepared and treated with several compounds which had already been tested. These compounds included some which had given spurious results, some which showed a tendency only to form complexes, some of which behaved differently from MacEwan's observations and some which unexpectedly did not form complexes. The results are listed in Table 3.

TABLE 3. Rehydrated halloysite

Reagent and Exposure time X-ray Behaviour at room (days-l) reflections /', obs A min (calc) temperature

2.4 dinitrophenyl hydrazine 8 001dd asym 2"8 4"5 Resorcinol 8 001dd asym 2"8 3"7 Picric acid 8 001dd asym 0'8 3"7 Pyridine 4 001,002, 003 5"0 3"7

Butane 1.3 diol Aniline hydrochloride Methanol Acetaldehyde Acetone 1.4 dioxane Acetonitrile Oxalic acid Pyruvic acid Dicyandiamide

8 001dd 3'4 4'0 8 001 003 2.9 3.6 4 001 003 3"5 3'7 8 001 2'8 4"0 8 001 2.8 4-0 8 001dd 0"5 4"2

10 001, 003 3"3 3,7 4 001d asym 2"7 2,5 4 001d 5"1 3,7 8 001dd asym 2"8 3'3

Extensively dissociated after 20 hr

Dissociated after 2 hr Dissociated rapidly Dissociated rapidly Dissociated after 2 hr

Dissociated after 2 hr

Organic complexes of halloysite 161 Four of the substances--pyridine, aniline hydrochloride, methanol and acetoni-

trile--formed complexes in which higher orders of the basal reflections were visible, but all of these phases, when exposed to the atmosphere at room temperature, showed loss of interlayer material. Acetaldehyde and acetone yielded phases with sharp, symmetrical basal reflections but without higher orders, and with remarkably low a values. Both phases dissociated rapidly at room temperature. Butane 1.3 diol, oxalic acid, pyruvic acid, and dicyandiamide all showed a tendency only to form complexes. It is of interest to note that the complex-forming process appears to proceed more slowly with the rehydrated clay mineral than with naturally hydrated halloysite.

The above results for methanol, acetonitrile, acetaldehyde and acetone support the view that discrepancies between MacEwan's observations and those listed in Table 1 may arise from differences between the halloysites which were used.

Stability It is generally accepted that the interlayer forces existing in both the halloysites

and also the kaolin family of clay minerals in general, are those attributed to hydrogen bonding and van der Waals interactions. Hydrogen bonding is widespread owing to the polar nature of the silicate sheets and the availability of hydrogen and oxygen atoms at the interlayer surfaces. In view of the fact that both the hydrogen bond and the van der Waals interactions are of low energy, it is not surprising that the complexes of halloysite are all relatively unstable. The stability with respect to temperature and vacuum seems to parallel the relative volatilities of the parent liquids, and therefore some marked differences were observed cf. ethane 1.2 diol and acetic acid. A more conclusive demonstration of relative instability was provided by the ease with which complexed halloysites can' be rehydrated to a phase which rapidly loses its interlayer water when allowed to dry at room temperature.

The complex-forming process in hydrated halloysites may be regarded as a competition for the organic material between halloysite and water as follows:

halloysite-organic + water . . . . . . AG1 7

halloysite-water + organic

halloysite + water-organic . . . . . . AG2

AG1 and AG2 are the free energy changes associated with each reaction. When --AG1 /> 0 > --AG~ complex formation will proceed.

If --AG2 7> --aG1 irreversible dehydration of the halloysite will proceed, e.g. with short-chain monohydric alcohols.

It must be noted that the term stability, with reference to complexed halloysites, is relative only. The irreversibility of the removal of interlayer water from halloysite, the marked instability of rehydrated halloysites, and the results of hydrothermal investigations (Churchman, 1970), all indicate that hydrated halloysites together with all other complexes are probably metastable species, metahalloysite being the


stable form. The irreversibility of the complex-forming process has been demonstrated in another fashion in the present study by the failure of metahalloysites to form complexes with ethane 1.2 diol, ethane 1.2 diamine, and ethanolamine. The metahalloysites were prepared from Te Puke haUoysite by (a) air-drying at 42 ~ for 24 hr; (b) vacuum drying at room temperature over concentrated sulphuric acid for 4 days. MacEwan's observation that metahalloysite may give a complex with ethane 1.2 diol could have been due either to kinetic factors described elsewhere or (more likely) to the use of an incompletely dehydrated halloysite in his experiments.

Nature o] the organic compounds

MacEwan's experiments indicated that polar organic liquids only would form complexes with halloysite and the results of the present study, in general, confirm this view. However, a detailed examination of all results reveals some anomalies. Many compounds other than those listed in Tables 1 and 2 were tested. Those which did not form complexes together with those whose behaviour was uncertain, i.e. those which gave spurious results and those which showed a tendency only, are: methanol, ethanol, l-propanol, 1-butanol, cyclohexanol, butane 1.3 diol, 3-hydroxy 3-methyl butan-2-one, pyruvic aldehyde, acetaldehyde, propionaldehyde, crotyl aldehyde, acetone, methyl ethyl ketone, biacetyl, dimethyl ether, diethyl ether, butyl cellosolve, 1.4 dioxane, formic acid, oxalic acid, succinic acid, glutaric acid, o-amino n-butyric acid, ~,-amino n-butyric acid, tartaric acid, ascorbic acid, fructose, glucose, sucrose, acetonitrile, acrylonitrile, methyl acetate, ethyl acetate, vinyl acetate, hydrazine sulphate, methylamine, diethylamine, triethylamine, dicyandiamide, lysine hydrochloride, histidine hydrochloride, cystine, creatine, tyrosine, leucine, sarcosine, valine, proline, hydrOxy-proline, serine, aspartic acid, cysteine, cysteine hydrochloride, betaine hydrochloride, arginine hydrochloride, ornithine hydrobromide, anthranilic acid, hippuric acid, ethane 1.2 dithiol, propane 1.3 dithiol, dimethyl sulphoxide, chlorobenzene, p-dichlorobenzene, nitrobenzene, p-nitro- toluene, o-chloronitrobenzene, phenol, resorcinol, catechol, hydroquinone, p-nitrophenol, m-nitrophenol, o-aminophenol, p-nitrobenzaldehyde, benzoic acid, m- hydroxybenzoic acid, o-nitrobenzoic acid, p-nitroanisole, aniline, aniline hydrochloride, p-nitroaniline, o-phenylene diamine, 4-aminopyridine, pyridyl 2-aldehyde, pyridyl 4-aldehyde, quinoline, 8-hydroxy quinoline, 2.4 dinitrophenyl hydrazine.

It is obvious that both the complex-forming compounds and those which do not form complexes include molecules of both high and low polarity. For example acetaldehyde, acetone, 1-propanol and 1-butanol with dielectric constants in the range 17-22 and formic acid with a value of 58"5 do not form complexes readily, whereas complex-forming acetic acid, ethane 1.2 diol, glycerol, and water have dielectric constants of 6-15, 37-7, 42"5, and 80"4 respectively.

Again it may be expected that dipole moments could be used as a measure of polarity (except in symmetrical molecules such as 1.4 dioxane) and hence of complex-


forming ability. Complex formers such as nitromethane and acetamide have high dipole moments comparable with those of acetonitrile, dimethyl sulphoxide, nitrobenzene, and o-chloronitrobenzene which do not form complexes.

Many of the compounds tested can be described as acids and bases and therefore a relationship could exist between dissociation constants in aqueous solution and the ability to form complexes with halloysite. Once again it was impossible to make predictions on this basis.

The behaviour of organic substances is systematically described in terms of the properties of functional groups, and examination of the above results indicates the possible existence of an empirical correlation. In general most of the complex- forming molecules contain two or more functional groups, e.g. diols, triols, alcohol ethers, aminoalcohols, diamines, amides, amine salts,/3-diketones, and amino acids. A few monfunctional compound~ including some carboxylic acids and nitromethane also form complexes. The list of non-complex formers contains many monofunc- tional compounds including the monohydric alcohols, ethers, aldehydes, ketones, nitriles and amines together with aromatics of aU kinds, sugars, some amino acids, and a few miscellaneous difunctional molecules. The complex-formers do not contain carbon chains of length greater than C~; cyclohexane 1.2 dione being the only exception. Several compounds which readily form complexes contain more than four carbon atoms but in each case, e.g. ethyl cellosolve, triethylene glycol; continuous carbon chain units have a length of two or three atoms only and are linked by other atoms. The significance of the role of functional groups and of the carbon chain length can be summarized as follows: in general an organic compound will be likely to form a complex with halloysite provided it has a maximum chain length of C4 and has one functional group for every two carbon atoms in the molecule. This rule has many exceptions, e.g. pyruvic acid, oxalic acid, succinic acid, dicyandiamide, sarcosine, betaine hydrochloride; but appears to be valid for diols, diamines, and aminoalcohols. The behaviour of ring compounds, e.g. sugars and aromatics, does not follow the above rules so, empirically, the rule should be extended to exclude cyclic and aromatic molecules in general.

Finally, a discussion of the reasons for the formation of halloysite organic complexes must include consideration of hydrogen bonding which is present in the best known complex, hydrated halloysite. Almost without exception the compounds which form complexes either are known to exhibit hydrogen bonding in their liquid and solid states or are capable of forming hydrogen bonds with the silicate-hydroxyl

layers in halloysite.

Hydrogen bonding and structural considerations An attempt was made to study the bonding existing in complexes by measuring

their absorption of infra-red radiation. Such studies are complicated in halloysites by the difficulties encountered in the preparation of pure samples. All spectra were taken from samples containing an excess of the complexing liquid and consequently


only the hydroxyl absorption region, 3600-3700 cm -1, could be readily interpreted. Two prominent absorptions at 3620 cm -1 and 3690 cm -1 are characteristic of the Te Puke hydrated halloysite hydroxyl region (Churchman, 1970), and on dehydration the intensity of the latter appears to increase a little, while that of the former remains unchanged. Spectra taken from several complexes were characterized by a decrease in the intensity of the 3690 cm -1 absorption and such an effect arises either from a decrease in the number of hydrogen bonds or from perturbation and orientation changes in hydroxyl groups (Cruz, Laycock & White, 1969). In the absence of pure, fully complexed halloysites no further information, apart from the observation that changes in hydrogen bonding take place during complexing, could be obtained from infra-red absorption studies.

Hydrogen bond formation is associated with bond shortening effects and these can normally be deduced from structural data of the kind presented in Tables 1 and 2. However, uncertainty concerning the relative orientation both of silicate sheets and of hydroxyl groups within halloysite particles makes detailed calculations uncertain. Furthermore the • values determined from X-ray diffraction data do not provide direct information concerning the orientation of interlayer molecules. A comparison of the experimental A values with ~ rain (talc) indicates probable bond shortening in several complexes but, owing to the uncertainties present in assign- ment of van der Waals radii, a smaller LXobs~rvedmay not necessarily be caused by bond shortening. In those cases where Aob~rv~ d > ~(mi~alc) it may be concluded that in the interlayer molecular orientation is different from that assumed in the calculation. It seems reasonable to assume that hydrogen bonding is present in all complexes, but it is dangerous to speculate on the nature of the atoms involved in the bonds when so many assumptions have to be made concerning atomic sizes and structural orientation.

MacEwan attempted to analyse his results quantitatively and demonstrated the presence of C - - H . . . . O hydrogen bonds from bond length calculations. It was assumed that the carbon atoms are half-way between the structural sheets but such an assumption can be justified only when bond shortening is at a maximum value thus precluding other orientations. In the course of his analysis MacEwan assigned a van der Waals radius of 1-35 A to hydrogen atoms in contact with the oxygen and hydroxyl layers of halloysite, but later used a value of 1-2 A for H . . . . H van der Waals contacts between organic molecules within the interlayer space. No reason is given for either of these assumptions, viz. varying hydrogen size, and lack of bonding between molecules; and hence it appears that the arbitrary assign- ments of hydrogen radii are necessary in order to make the explanation fit the experimental data. Furthermore, MacEwan's calculation of the size of a hydro- carbon chain (end-on orientation) is in error by 0-45 A, and all other calculated A values appear to be either too large or too small by an amount equal to 0.5 A. In the present study the more generally accepted van der Waals radius for hydrogen of 1.35 A has been used in the determination of ~n(~a~c)" Hence it appears that MacEwan's conclusions concerning C - - H . . . . O hydrogen bonds are based on some doubtful assumptions and calculations and hence are unlikely to be valid.


This view is supported by Greenland's (1965) demonstration that no importance can be attributed to C - - H . . . . O interactions in montmorillonite organic complexes.

The importance of the presence of the amino and alcohol functional groups together with their frequency of occurrence in the interlayer space, has been noted. Water molecules are smaller than most of the other complexing molecules and in hydrated halloysite each molecule probably forms one hydrogen bond with the silicate structure (Brindley, 1961). It is probable that organic complexes tend to contain a maximum number of hydrogen bonds and, since most of the molecules are larger than water, the presence of two or more appropriate functional groups per molecule must facilitate the formation of a large number of hydrogen bonds. Most of the compounds which were tested are among those which are known to form hydrogen bonds, and yet many apparently do not form complexes with halloysite. Until more information is known about the interlayer space environment, it is impossible to speculate on the precise reasons for the observed behaviour of these substances.

C O N C L U S I O N S

Several conclusions relating not only to organic complexes but also to the nature of halloysite itself have been reached and they are summarized below: (1) The interstratification mechanism for the removal of interlayer water from halloysite forms the basis of a better criterion for complex formation than that based on simple geometrical considerations involving clearance spaces. (2) The metastability of all complexes including hydrated halloysite is postulated; metahalloysite being the only stable form of the clay mineral. (3) The physical nature of the halloysite, i.e. tabular or tubular morphology, seems to play an important role in the kinetics of complex formation. (4) The results confirm Nagasawa's findings (qv) concerning the relationship between b-axis dimensions, bonding of interlayer water, and particle morphology. (5) Support is given for the Radoslovich (1963) theory for the origin of tubular morphology (the b-axis dimensions of 'Kauri' and Te Puke halloysites increase on removal of interlayer water). (6) The compounds which form complexes with halloysites in general, are polar and are usually either acids or bases. Their molecules usually contain two functional groups, are relatively small, and have at least one functional group per two carbon atoms, but do not usually include cyclic and aromatic compounds. Complex formation is favoured by the presence of - O H and/or --NH2 functional groups. (7) It is postulated that the reason for complex formation lies in the ability of certain organic compounds to form a large number of hydrogen bonds with halloysite interlayer surfaces. Prediction of this ability is made impossibly difficult by lack of structural information concerning the precise orientation both of silicate sheets and of hydroxyl groups. Quantitative assessment of the mechanism of hydrogen bond formation is also difficult because of the doubt attached to assumptions concerning interlayer space geometry and the sizes of participating atoms, upon which such calculations are based.


A C K N O W L E D G M E N T S

The authors wish to thank the Director of the New Zealand Pottery and Ceramics Research Associa- tion for supplying the halloysite samples. The use of X-ray and infra-red equipment made possible by the generosity of the University Grants Committee (New Zealand) is acknowledged. One of us (R.M.C.) gratefully acknowledges permission to use the facilities of the Rothamsted Experimental Station, in particular those of the Pedology Department.

R E F E R E N C E S

BRINDLEY G.W. (1961) The X-ray Identification and Crystal Structures of Clay Minerals (G. Brown, editor), Chap. II, p. 71. Mineralogical Society, London.

BRtNDLEY G.W. & RAy S. (1964) Am. Miner. 49, 106-15. CAILL~RE S., GLAESER R., ESQEVlN J., & Ht~NIN S. (1950) C. R. Acad. Sei. Paris. 230, 308-10. CHURCHMAN J.B. (1970) Ph.D. thesis. University of Otago. CRuz M., LAYCOCK A. & WHITE J.L. (1969) Proe. Inter. Clay Conf. 1, 775-89. GREENLANt~ D.J. (1965) Soils Fertit. 28, 415-25. HARRISON J.L. & GREENnER~ S.S. (1962). Proe. Nat. Conf. Clays Clay Minerals: 9, 374-77. MACEWAN D.M.C. (1946) Nature, 157, 159-60. MAcEWAN D.M.C. (1947) Mineralog. Mag. 28, 36-44. MACEWAN D.M.C. (1948) Trans. Faraday Soe. 44, 349-67. NAGASAWA K. (1969)Proc. Inter. Clay Conf. 1, 15-30. RADOSLOVTCH E.W. (1963) Am. Miner. 48, 368-78. WADA K. (1959) Am. Miner. 44, 153-65. WADA K. (1961) Am. Miner. 46, 78-91. WEISS A. & Russow J. 0963) Proc. Inter. Clay Conf. 2, 69-74.

complexes of organic compounds · complexes of organic halloysite with compounds r. m. carr and hwa...

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