Adsorption of stearic acid by allophane

Adsorption of stearic acid by allophane

Chemical Geology, 68 (1988) 199-206 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands 199 [2] ADSORPTION OF STEARIC ACID B...

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Chemical Geology, 68 (1988) 199-206 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands



ADSORPTION OF STEARIC ACID BY ALLOPHANE S. YARIV 1, L. HELLER-KALLAI ~ and Y. DEUTSCH ~ ~Institute of Chemistry, The Hebrew University o/ Jerusalem, Jerusalem 91904 (Israel) 2Institute o/Earth Sciences, The Hebrew University o/Jerusalem, Jerusalem 91904 (Israel) ~The Geological Survey of Israel, Jerusalem 95501 (Israel) (Received August 17, 1987; revised and accepted December 31, 1987 )

Abstract Yariv, S., Heller-Kallai, L. and Deutsch, Y., 1988. Adsorption of stearic acid by allophane. Chem. Geol., 68: 199-206. IR spectra and DTA-TG curves showed that molten stearic acid was partially converted to the ionic form on adsorption by allophane. The process was not affected by grinding the samples. In this respect allophane is unique among the Al-rich clays, which generally cause little or no conversion unless the clay-carboxylic acid associations are well ground. Stearic acid was adsorbed on the surface of allophane through water molecules. On heating, water was lost together with some of the acid; the remaining acid became coordinated to surface A1. In both open and semi-closed systems stearate ions were retained to higher temperatures than the acidic form. Allophane catalysed oxidation of part of the organic matter.

1. I n t r o d u c t i o n

Fatty acids are common constituents of soils and sediments and are among the most studied biological markers. A distinction is generally made between unbound and bound fatty acids and the latter may be further subdivided into bound and tightly bound acids. The composition and distribution patterns of these fractions are used to reconstruct the diagenetic and paleoenvironmental history of the organic matter. The relative amounts of the various fractions and the distribution patterns depend upon the method of extraction, which in some cases included a demineralisation step (for a recent review see Mendoza et al., 1987). Clay minerals may play an important part by adsorbing carboxylic acids, thus decreasing their


extractability, and may affect the course of diagenesis by acting as catalysts for decarboxylation, cracking and polymerisation reactions (Johns, 1979; Aizenshtat et al., 1984; HellerKallai et al., 1984). The nature of the interaction between different clay minerals and carboxylic acids is therefore of considerable interest. This paper forms part of a series dealing with this subject (Yariv and Heller-Kallai, 1984; Heller-Kallai, 1986). Laboratory simulation of natural processes is problematic, not only because of the elevated temperatures used to reduce reaction times, but also because of the choice of reaction conditions. The question may be raised whether an open, semi-closed or closed system most closely represents natural processes (Heller-Kallai, 1985).

© 1988 Elsevier Science Publishers B.V.

200 In the present series organo-clay associations were heated in open systems and in the form of alkali halide disks, which simulate a semi-closed environment. It was previously established that the interaction of carboxylic acids with alkali halides is negligible compared with that of the clay minerals (Yariv and HellerKallai, 1984). Stearic acid was chosen because of its relative abundance and the moderate length of its hydrocarbon chain. Allophanes are important constituents of soils derived from volcanic ash. They are unstable on burial and are therefore not expected to occur in significant amounts in older sediments. They may, however, be regarded as models for other non-crystalline minerals, in particular A1- and Fe-oxides and -hydroxides.

2. E x p e r i m e n t a l 2.1. Materials The sample of allophane, from Bedford, Indiana, U.S.A., was from the collection of Grim (1968). Following Parfitt et al. (1980), the strong IR absorption at 1020 cm - 1 of this sample indicates a Si/A1 ratio close to 1. According to Holdridge and Vaughan (1957) the DTA curve suggests that the sample is associated with a minor impurity, probably halloysite, which is associated with the allophane in Bedford (Davis et al., 1950). The IR spectrum of the sample after dehydration of the allophane does, indeed, show weak OH stretching vibrations characteristic of halloysite. Stearic acid was supplied by Merck ®, Darmstadt, F.R.G.

2.2. Procedure Allophane-stearic acid associations were obtained by mixing 1 part of allophane with 5 parts of stearic acid by weight. The mixtures were heated in closed vessels at 100°C for 72 hr., cooled and washed 5 times with hexane to remove excess acid.

2.3. Experiments in semi-closed systems To simulate a semi-closed environment the allophane-stearic acid associations were heated in alkali halide disks. These were prepared by grinding 200 mg of NaC1 or KC1, or 150 mg CsC1 with 3 mg of the clay-stearic acid association for 5 min. before pelleting. IR spectra were recorded on a Perkin-Elmer ® 595 IR spectrometer. The disks were crushed and repelleted; this process was repeated 3 times. The disks were then heated for 30 days at 115°C. This treatment removed adsorbed water and loosely bound stearic acid. The spectra obtained served as a basis of reference for subsequent changes caused by the thermal treatments. The disks were heated for 30 days at 190°C and for 1 day each at the higher temperatures given in the text. At various stages they were cooled, pressed without crushing and IR spectra were recorded.

2.4. Experiments in an open system Mixtures of allophane with stearic acid were heated at 10°C min. -1 in a Redcroft ®Stanton ® thermal apparatus, in a stream of either N2 or air and DTA and TG curves were recorded. Variously heated samples were removed for IR spectroscopy.

3. R e s u l t s and d i s c u s s i o n 3.1. Infra-red ( I R ) spectra of allophane-stearic acid associations heated in alkali halide disks The spectra of allophane-stearic acid associations in the form of alkali halide disks ( NaC1, KC1 and CsC1) showed broad bands characteristic of stearic acid and of stearate anions. Grinding the disks did not cause further conversion of stearic acid into stearate ions. This is in contrast to stearic acid associations with talc and pyrophyllite (Heller-Kallai et al., 1986), sepiolite and palygorskite (Yariv and Heller-Kallai, 1984 ) and kaolinite in which the bands due to the ionic form were either absent


CI~~ NoCl 115° CsCI 115°


o z

CsGI 250 ° z










v ~,

CH~ m~

r_CsCl °


t #

some of the water. With CsC1 a new shoulder appeared at 1660 c m - 1, associated with a small band at 1430 cm-1. It seems that the C O O H groups were hydrated at room temperature, but that most of the water was lost from the CsC1 disk on heating at 115°C. The COOH groups then became directly coordinated to A1, as was previously observed with pyrophyllite ( HellerKallai et al., 1986). NaC1 and KC1 disks retained water more firmly and a weak 1660 c m - 1 band, though probably present after heating at 190 ° C, could be distinguished only after heating at 250 ° C, when sufficient water had been lost. The band disappeared entirely after heating at 300 ° C, whereas with the CsC1 disk a weak absorption at 1660 c m - 1persisted. It seems that acid directly coordinated to A1 is more firmly held on heating than acid bound to the clay through hydration water. With NaC1 and KC1 disks, which retained water to higher temperatures than CsC1, the acid evaporated together with the water. Only minor amounts of acid remained available for direct coordination with A1.

3.1.2. COO--absorptions. The asymmetric and I ~000





I I 250,3


I - - I 2200

1 1800




~ I 1600


I 1400

Fig. 1. Selected features of IR spectra of allophane-stearic acid associations (a = NaC1 disk, heated 115 ° C; b = CsC1 disk, heated 115 ° C; c = NaC1 disk, heated 250 ° C; d -- CsC1 disk, heated 250°C; e=NaC1 disk, heated 3 5 0 ° C ; / = C s C l disk, heated 350 ° C ).

or very weak before grinding and were enhanced by grinding the disks. With allophane grinding merely led to intensification of the bands and to greater amounts of adsorbed water. Fig. 1 shows selected features of the spectra obtained after heating the disks.

3.1.1. COOH absorptions. The C O O H stretching vibration appears as a shoulder at 1710 cm -1. The CO, OH band constitutes a satellite to the symmetric C O O - band, extending up to 1405 cm -1. On heating at 115°C part of the acid escaped from NaC1 and KC1 disks, together with

symmetric bands appear at 1600-1575 and 1470-1460 cm -1, respectively. In addition shoulders were observed at 1560 and 1540 c m - 1. The symmetric band is not diagnostic, as it overlaps bands due to CO, OH and CH. The shape and location of the asymmetric band, which is much stronger than the COOH absorption, depends on the hydration state of the sample. It is broad initially, overlapping the water bending vibration at 1630 cm-1. After heating the disks at 115 °C for 6 days sufficient water was lost from the CsC1 and KC1 disks to give bands with distinct maxima at 1583 and 1587 cm-1, respectively, but the NaC1 disk still showed a broad band ranging from 1598 to 1578 cm-1. After heating at 190°C for 35 days the C O 0 - bands were broad, with a maximum at 1580 cm -1, independent of the alkali halide. Table I shows changes in the extinction of the C O O - , CH2 and CO2 absorptions after heating

202 TABLE I Relative values of the extinction of COO- (1580 em-1), CH2 (2920 cm-1) and C02 (2320 cm-1) bands after heating at different temperatures Temperature




(~c) C00-

CH 2

CO 2






CO 2











190 250 300 350

0.55 0.40 0.37 0.06

0.64 0.71 0.73 0.33

1.25 2.87 3.21 3.05

1.04 0.63 0.62 0.08

0.85 0.84 0.82 0.43

3.04 4.24 3.93 1.69

0.76 0.62 0.56 0.07

1.03 0.97 0.86 0.60

3.25 4.60 3.57 1.09

the disks at different temperatures. In view of the problems inherent in quantitative infra-red studies (band overlap and baseline tracing) the values given should be regarded merely as trends. It appears from Table I that the intensity of the C O O - band decreased steadily with temperature. This decrease in intensity was not always associated with a corresponding decrease in intensity of the CH2 absorptions, suggesting that some decarboxylation occurred on heating (see Section 3.1.3). After heating at 350 ° C the C O O - absorption became very weak andbroad, with maxima at ~ 1600, ~ 1606-1570 and ~ 1606-1585 cm -1 with the CsC1, KC1 and NaC1 disks, respectively.

3.1.3. CH2 absorptions and decarboxylation reactions. A decrease in intensity of the CHz band at 2920 c m - 1reflects loss of organic matter from the system (Table I). Organic matter escaped from NaC1 and KC1 disks on heating at 190°C, probably due to evaporation of free or loosely bonded acid. With the CsC1 disk water was lost at a lower temperature and most of the acid that persisted at 115°C was bonded directly to A1 ( see Section 3.1.1 ). In consequence, unlike with NaC1 and KC1 disks, the intensity of the CH2 absorptions did not decrease between 115 ° and 190°C with the CsC1 disk. Between 190 ° and 300 °C the intensity of the CHz absorptions remained approximately constant with all three disks, while that of the C O O - bands decreased. This indicates that the ions were partly decar-

boxylated, but that the hydrocarbon chains were preserved inside the disks. After heating at 350 °C the intensity of the CH2 bands was considerably reduced, while that of absorptions at 2960 and 2870 c m - 1was greatly enhanced (Fig. 1 ), indicating a large increase in the C H J C H 2 ratio. The inference that decarboxylation occurred is supported by the appearance of an intense band at 2320 cm -1, assigned to adsorbed carbon dioxide. This band, which was detectable after heating at 115°C, increased in intensity in the temperature range of 115-250 ° or 115-300°C and decreased after heating at higher temperatures (Table I ). Apparently decarboxylation of the acid occurred before that of the ionic form, as is well illustrated by the spectra obtained with the KC1 disk after heating at 115 ° and 190°C (Table I): the intensity of the C O O - absorptions remained unchanged, that of the CHe absorptions was reduced by ~ 15%, the CO2 band at 2320 c m - 1 was greatly intensified and the shoulder at 1700 cm-1, assigned to C O O H groups, became very weak. These observations are compatible with the interpretation that some acid evaporated while some was decarboxylated. The CO2 formed was adsorbed and trapped in the disk. On further heating the ions were decarboxylated, as shown by the reduced intensity of the C O O - absorption and a concomitant increase in intensity of the CO2 band up to a maximum at 250-300°C, when COz was presumably released from the

203 09 I 90



i 3:-





~ 5,3/

C \




"-~ c














Fig. 3. TG curves in N2 atmosphere (a-d as in Fig. 2).

• 2(;,:

5 ~)00



[ 500


l EDC,

L 7oc





Fig. 2. DTA curves in N2 atmosphere (a=stearic acid; b = allophane; c, d = stearic acid and allophane in ratios 1 : 1 and 1 : 5, respectively ).

disks. The intensity of the CH2 bands did not change significantly between 190 ° and 300 ° C, indicating that the hydrocarbon chains were trapped in the disks.

3.2. Differential thermal analysis (DTA) and thermogravimetry (TG) of mixtures of aUophane and stearic acid Figs. 2 and 3 show the DTA and TG curves, respectively, of stearic acid, allophane, and of mixtures of stearic acid and allophane in the ratios 1 : 1 and 1:5, recorded in an inert atmosphere. Fig. 4 shows corresponding DTA curves recorded in air. The TG curves obtained in the oxidising atmosphere were similar to those recorded in the inert atmosphere. The figures clearly demonstrate that the curves obtained from the mixture of allophane with stearic acid differ from the sums of the individual components.

3.2.1. DTA-TG in an inert atmosphere. The melting and boiling points of pure stearic acid are represented by endothermic peaks at furnace temperatures of 118 ° and 340°C, respectively (Fig. 2, a). The allophane shows two endotherms, at 132 ° and 495°C, respectively (Fig. 2, b). Holdridge and Vaughan (1957) attributed the small peak at the higher temperature of this sample to a contaminant, probably halloysite. The temperatures are furnace temperatures - - sample temperatures were 45-60 ° C lower, but as the curves are used only for comparison the actual sample temperatures are unimportant. The fact that the DTA curves of the allophane-stearic acid mixtures differ from the sum of those of allophane and stearic acid heated individually, indicates that chemisorption of stearic acid on allophane occurred in the course of heating. Moreover, part of the dehydration of allophane was delayed by the presence of stearic acid, as is clearly shown by the additional endotherms at 204°C obtained with samples containing stearic acid and allophane in the ratio 1:1 (Fig. 2, c and Fig. 4, c). This confirms that hydration water of allophane is involved in the allophane-stearic acid association. Similar conclusions may be reached from a




!i [


.b \

, "l p

\ \

f /







\ k






• ___


Ft F~N,~OE




Fig. 4. DTA curves in air ( a-d as in Fig. 2 ). For explanation of I - I I I see text.

T A B L E II Weight loss, % of sample (in brackets: weight loss, % of allophane ) Sample

Temperature range ( ° C ) RT-190




0 19.5 16 (19.1) 5.5 (11.0)

0 2.5 3.5 (4.3) 3.7 (7.4)

99 3.4 130 46.3

1 6 10.7 (12.8) 9.1 (18.2)

RT---room temperature; SA = stearic acid.


SA A SA-A, 1:5 SA-A, 1:1


study of the TG curves. Table II shows the weight losses in four temperature regions. With pure stearic acid no weight loss occurred below 245 ° C. The 1 : 5 stearic acid-allophane mixture gave rise to a TG curve resembling that of allophane, but the total weight loss below 245 °C exceeded the amount of water in the mixture. This indicates that, in the presence of allophane, some of the organic material was lost in this temperature range. The 1:1 allophane-stearic acid mixture showed a rapid weight loss below 140 ° C and a much more gradual one between 140 ° and 245 ° C. Below 190 ° C the weight loss of this mixture, referred to the amount of allophane present, was only 56% of that of allophane alone in the same temperature range. The total weight loss below 245 °C is less than the amount of water in the mixture, showing that, in the presence of stearic acid, water is strongly bonded to allophane. This bonded water amounts to at least 16% of the water initially present. By analogy with the 1 : 5 mixture it seems probable that some organic material was also lost below 245 ° C; the amount of water retained above 245°C therefore may even exceed 16%. This water evaporated together with most of the stearic acid between 245 ° and 370 ° C. The peak temperature of the corresponding endothermic reaction decreased with decreasing amounts of stearic acid in the system. (This was confirmed in independent experiments with different amounts of pure stearic acid.) The weight loss of the mixtures above 370°C, expressed as percentage of allophane, was greater than that of allophane alone, showing that some of the organic material was retained beyond the temperature at which the pure acid evaporated. 3.2.2. D T A - T G in air. Oxidation of pure stearic acid commenced at 230°C and the endotherm due to boiling of the acid overlaps several sharp exotherms. The TG curve shows that a small residue of organic matter persisted above the boiling point, probably in the form of charcoal. This organic residue gave rise to a small, very

205 broad exotherm at -~ 500 ° C, accompanied by a small weight loss, which was not observed in an inert atmosphere, and is therefore attributed to oxidation of the charcoal residue. In the presence of allophane, oxidation started at lower temperatures than with the pure acid, but the most intense exotherm occurred at a higher temperature (Fig. 4). It appears that allophane played a dual role in these reactions: (a) it acted as a catalyst, reducing the temperature at which oxidation commenced, as was previously observed with talc and pyrophyllite (Heller-Kallai et al., 1986); and (b) it delayed oxidation of part of the organic matter to higher temperatures.

3.3. IR spectra of an aUophane-stearic acid mixture heated in air Thermal analysis alone does not differentiate between adsorbed stearic acid and stearate ions. To establish in which form the organic material occurred at various stages of the thermal analyses, aliquots of the 1:5 stearic acid-allophane mixture were heated to furnace temperatures of 250 °, 304 ° and 346°C in the thermoanalyser (points I, H and III, respectively, on curve d in Fig. 4 ) and IR spectra were recorded. After heating to point I, the onset of oxidation, the sample contained stearic acid and small amounts of stearate ions. After heating beyond the first strong exotherm (point H) the acid disappeared and only stearate ions remained. On further heating, up to point III, some stearate persisted but the intensity of the band was reduced. These observations confirm that allophane catalysed oxidation of stearic acid, but preserved some of the original material to higher temperatures by converting it into stearate ions, which were adsorbed on the surfaces of the mineral. 4. C o n c l u s i o n s

Clay minerals play a dual role in the diagenesis of fatty acids: they stabilise them by ad-

sorption but they may also act as catalysts promoting their degradation (Aizenshtat et al., 1984). Allophane is known to be a good anion adsorber (e.g., Wada, 1977). It is very effective in converting carboxylic acids into the ionic form, which is strongly adsorbed and has a greater thermal stability than the acid. This occurs readily on contact with the molten acid, in contrast to other clay minerals studied (talc, pyrophyllite, sepiolite and palygorskite) that require grinding or more elevated temperatures (Yariv and Heller-Kallai, 1984; Heller-Kallai et al., 1986). A preliminary survey showed that neither kaolinite nor halloysite resembled allophane in this respect. The more basic serpentines, in particular lizardite with a large surface area, caused ionisation of stearic acid similar to that with allophane, but these minerals are not expected to be abundant in environments rich in organic matter. Allophane and related compounds may, however, have a significant effect on the distribution patterns of unbound, bound and tightly bound carboxylic acids in soils and sediments. In the present series of experiments stearic acid was chosen as the model compound. It was found that when water was lost from a restricted system at relatively low temperatures (from the CsC1 disk) most of the acid was retained and was stabilised by forming direct bonds with A1 ions on the surfaces of allophane. Water lost at higher temperatures (from the more hydrophilic NaC1 and KC1 disks) carried most of the acid with it, probably by a mechanism resembling steam distillation. If a carboxylic acid of lower boiling point had been chosen, more of the acid would probably have been lost from all the disks together with the water, whereas higher boiling acids would, perhaps, have persisted also in the more hydrophilic environment after most of the water was lost. Factors of this type have to be taken into account when considering the implications of model reactions for the interpretation of geochemical processes.


References Aizenshtat, Z., Miloslavski, I. and Heller-Kallai, L., 1984. The effect of montmorillonite on the thermal decomposition of fatty acids under "bulk flow" conditions. Org. Geochem,, 7: 85-90. Davis, D.W., Rochow, T.G., Rowe, F.G., Fuller, M.L., Kerr, P.F. and Hamilton, P.G., 1950. Electron micrographs of reference clay minerals. Am. Pet. Inst., Columbia Univ., New York, N.Y., Proj. 49, Clay Miner. Stand., p. 9. Grim, R., 1968. Clay Mineralogy. McGraw-Hill, New York, N.Y., 2nd ed., 287 pp. Heller-Kallai, L., 1985. Do clay minerals act as catalysts in the thermal alteration of organic matter in nature? Problems of simulation experiments. Mineral. Petrogr. Acta, 29A: 3-16. Heller-Kallai, L., Aizenshtat, Z. and Miloslavski, I., 1984. The effect of various clay minerals on the thermal decomposition of stearic acid under "bulk flow" conditions. Clay Miner., 19: 779-788. Heller-Kallai, L., Yariv, S. and Friedman, I., 1986. Thermal analysis of the interaction between stearic acid and pyr-

ophyllite or talc - IR and DTA studies. J. Therm. Anal., 31: 95-106. Holdridge, D.A. and Vaughan, F., 1957. The kaolin minerals. In: R.C. Mackenzie (Editor), The Differential Thermal Investigation of Clays. Mineralogical Society, London, pp. 98-139. Johns, W.D., 1979. Clay mineral catalysis and petroleum generation. Annu. Rev. Earth Planet. Sci., 7: 183-198. Mendoza, Y.A., Giilaqar, F.O. and Buchs, A., 1987. Comparison of extraction techniques for bound carboxylic acids in Recent sediments. Chem. Geol., 62:307-319. Parfitt, R.L., Furkert, R.J. and Terno Henmi, 1980. Identification and structure of two types of allophane from volcanic ash soils and tephra. Clays Clay Miner., 28: 328-334. Wada, K., 1977. Allophane and imogolite. In: J.B. Dixon and S.B. Weed {Editors), Minerals in Soil Environments. Soil Sci. Soc. of Am., Madison, Wisc., pp. 628-629. Yariv, S. and Heller~Kallai, L., 1984. Thermal treatment of sepiolite- and palygorskite-stearic acid associations. Chem. Geol., 45: 313-327.