Clay Minerals (1983) 18, 459-472.

S Y N T H E T I C I M O G O L I T E : P R O P E R T I E S , S Y N T H E S I S , A N D P O S S I B L E A P P L I C A T I O N S

V. C. F A R M E R , M. J. A D A M S , A. R. F R A S E R AND F. P A L M I E R I *

The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 2Q J, UK, and *Istituto di Chimiea Agraria, Universita Delgi Studi di Napoli, Portiei, Italy

(Received 17 May 1983)

A B S T R A C T : The unique properties of imogolite are closely related to its structure, which is a tube of 23-27 /~ outer diameter and ~ 10 A inner diameter, with an AIOH outer surface and SiOH inner surface. Acid dispersions contain the long, positively-charged tubes as isolated units or small bundles, which form bulky gels in alkali, and flocculate with negatively-charged colloids, polyvalent anions, and long-chain anionic detergents. Sorption properties are associated with the 10 A intra-tube pores and with inter-tube channels of variable dimensions. Surface acidity is less than that of layer-silicate clays. The chemical and mechanical stability, biological activity, film- and fibre-forming characteristics, and conditions of synthesis are reviewed, on the basis of both new and published findings. Areas of potential application are indicated.

Imogolite is a hydrous aluminium silicate with a unique tubular structure and novel properties; as such, it has no immediate application in any commercial process where layer-silicate clays now play a role. The properties of the isolated product and of its dispersions can be rationally related to its structure, and this relationship is reviewed here. Emphasis is placed on those features which might find practical applications. Problems of synthesis and isolation of imogolite are also discussed.

The material has only recently become available, with some difficulty, in either its natural or synthetic form, and many of its properties have as yet been only tentatively explored. The distinctive character of the natural mineral was first established by Yoshinaga & Aomine (1962), who identified it as a fibrous component of the acid-dispersible clay from the Imogo pumice layer, which is widespread in Kyushu Island, Japan. Subsequently, it has been shown to occur in almost pure form as macroscopic gelatinous coatings in several pumice deposits in Japan (Wada, 1977) and as a trace component of podzolized soils on non-volcanic parent materials (Tait et al., 1978; Farmer, 1982). Elucidation of its structure by Cradwick et aL (1972) led to its synthesis (Farmer, Fraser & Tait, 1977), and a variety of procedures has now been described (Farmer & Fraser, 1979; British Patent 1 574 954 and US Patent 4 252 779). The occurrence and properties of natural imogolite are very fully reviewed by Wada (1977).

S T R U C T U R E

The structure proposed by Cradwick et al. (1972) is a tube whose wall consists of a single continuous gibbsite sheet with the inner hydroxyl surface of the gibbsite replaced by O3SiOH groups (Fig. 1). Such a structure has the empirical formula (HO)3A1203SiOH,

�9 The Macaulay Institute for Soil Research, 1983

460 V. C. Farmer et al.

Gibbsite b ~ - ' - - im 2w/n ~' ( a ) ogolite "'

01 ~ - ~ " O ib#o::,~::~_

b 1. ( b ) ~ ~

oo si o o AI O0 0 Q e OH

FIG. 1. (a) Mode of attachment of an 03SiOH group to the face of a gibbsite sheet, causing it to curl to form an imogolite tube. (b) Section of imogolite tubes in hexagonal close packing.

Properties o f synthetic imogolite 461

FIG. 2. Electron diffraction patterns of oriented preparations of synthetic (a) and natural (b) imogolite. From Wada et al. (1979).

which is also the sequence of atoms encountered on passing from the outer to the inner surface of the tubular model. Natural samples have compositions in the range A1203(SiOz)l.0_l.2(HzO)v3_3.0 (Wada, 1977). The outer and inner Van Der Waals diameters of the tube are ~23 /k and ~10 /k for natural material, and some 10-15% greater for synthetic material. This proposed structure accounts well for the general features of electron diffraction patterns (Fig. 2).

X-ray diffraction patterns of wet, oriented films of synthetic imogolite (Fig. 3) show a series of broad reflections corresponding to the scattering of individual tubes. On drying the films at 100-200~ the tubes associate in parallel bundles to give an additional sharp reflection at ~23 A associated with the ~27 A centre-to-centre tube separation (Farmer & Fraser, 1979). At 300 ~ the inter-tube reflection shifts to ~20.5 /k, and the broad reflections disappear irreversibly, indicating partial breakdown in the tube structure. At 350~ dehydroxylation gives an X-ray amorphous structure, in which part of the aluminium has moved into tetrahedral environments (Russell et al., 1969). Natural imogolite exhibits similar behaviour, but all reflections occur at spacings about 15% smaller, corresponding to the smaller tube diameter.

The chemical and physical properties of imogolite are consistent with this structure.

D I S P E R S I O N S

Imogolite can be dispersed in acid solution, presumably by adsorption of protons on its outer aluminium hydroxide surface, but not in alkaline solutions, as the ionizable SiOH groups are exposed only on the inner surface of the tube. The highest degree of dispersion of individual tubes is achieved by synthesis, which gives water-clear acid dispersions.

462 V. C. Farmer et al.

23.0

100~

/20.5 l

16.4 9.4

w e t

A FIG. 3. X-ray diffraction of an oriented film of synthetic imogolite in the wet and oven-dry

(IO0~ states, and after heating to 300~

Natural imogolite is best dispersed by ultrasonic treatment at pH 3.5--4.5 of gel films which have not been allowed to dry out (Horikawa, 1975a), but such dispersions always display opalescence, and are thought to contain bundles of tubes. Freeze-dried imogolite prepared from acid dispersions retains much of its ability to redisperse in water, but neutralized dry preparations disperse only partially in acid.

The physical properties of the dispersions are controlled by interactions between the long tubes of the dispersed imogolite. Electrophoresis studies (Horikawa, 1975a) show that the dispersed phase begins to flocculate around pH 7, and is completely immobilized at pH 9-10 (Fig. 4). This behaviour is reflected in the properties of a 0-1% dispersion, which is freely mobile at pH 4, begins to clot around pH 7 and sets to a coherent gel in dilute ammonia. Even in acid dispersions, however, the movement of air bubbles is impeded by tube entanglements. Gels containing 0.1% solids are weak and exhibit considerable syneresis on breakage, but the resultant gelled slurry can support sand in suspension. Gels with 0-5% solids scarcely bleed liquid.

4

3

Properties of synthetic imogolite

- 2

7

0 E >-

I

lO

40 +

IM

I I I I I I I 4 5 6 7 8 9 1 0

p H

(a)

5

4

3

- 2

E cj >: - l i

> 1

2

3

I 4

/m ~m \

.......... j ik~

I I 1 I I I 1 4 5 6 7 8 9 10

p H

(b)

FiG. 4. (a) Electrokinetic behaviour of imogolite (IM), montmorillonite (M), and mixed dispersions containing 10, 20 and 40% of imogolite. (b) Electrokinetic behaviour of imogolite (IM), kaolinite (K) and mixed dispersions containing 10, 20, 30 and 40% imogolite. From

Horikawa (1976).

463

There is little information on the rheology of imogolite dispersions, whose properties are likely to be highly dependent on tube length, tube dispersion, and adsorbed ions. Egashira (1977) reported that a 0.1% imogolite sol of pH 5.2 had a viscosity comparable to a 1% montmorillonite suspension. Wells et al. (1980) worked with a partially gelled dispersion of pH 7, and found very marked hysteresis on successive cycles of shearing, with no thixotropy (Fig. 5). The mechanical shearing of the gel caused tube fracture and tube agglomeration. In more acid dispersions tube agglomeration seems less likely, but tube fracture could still occur on vigorous shearing.

Imogolite sols can be flocculated by salts; because of the positive charge on the tube surface, polyvalent anions are more effective than monovalent, and long-chain anionic detergents are more effective than long-chain cationic detergents (Horikawa & Hirose, 1975). Sodium laurylsulphonate confers floatability by air bubbles on imogolite in 0-01% dispersions at 8.4 x 10 -4 M at pH 4 and 1.7 x 10 -4 M at pH 9 (Horikawa, 1975b).

Interesting interactions between imogolite sols and negatively-charged layer-silicate sols are shown by electrophoresis studies (Horikawa, 1976). In mixed imogolite- montmorillonite suspensions, mobility falls to zero or near zero with 20-40% imogolite in the mixture (Fig. 4), and the sols are flocculated. In mixed imogolite-kaolinite suspensions at pH 4-5, 1% imogolite is sufficient to cause complete flocculation, and 10% or more imogolite gives stable positively-charged sols, in which imogolite acts as a protective agent (Fig. 4). At pH 9-10, 10% imogolite flocculates 90% kaolinite, and 20% imogolite flocculates 80% montmorillonite, although charge neutralization of the montmorillonite is not quite complete at this ratio. Particle association between imogolite and the

464 V . C . F a r m e r et al.

16

12 #_

Jz ~o 8

I 1 I I I 1 2 0 0 0 4 0 0 0

Shear rate D/s

FIG. 5. Shear-stress relationships on repeated cycles for a 0.23% imogolite dispersion at pH 7. From Wells et al. (1980).

layer-silicates is shown by electron microscopy (Horikawa et al., 1976), even at pH 10, where no positive charge would be expected on the imogolite surface.

In spite of their negative surface charge, kaolinite and halloysite sols do not acquire floatability with long-chain cationic detergents. The above findings indicate that floatability with sodium laurylsulphonate might be conferred on such sots by addition of a few per cent of an imogolite dispersion, and adjusting to pH 9-10.

S O L I D S

The imogolite structure should exhibit three types of porosity: (a) intra-tube pores of ~ 10 /k, (b) inter-tube spaces in parallel arrays, which vary with hydration state and collapse on drying, and (c) irregular pores between bundles of tubes in a cross-linked network of fibre bundles. The calculated intra-tube pore volume for natural material is at most 0.10 cm s g-% based on an efficient packing of water molecules to give an effective tube diameter of 11 ~ (Cradwick et al., 1972). Nitrogen adsorption, which would be expected to fill the intra-tube pores and the finer inter-bundle pores, gives a measured porosity of 0. 145 cm 3 g-1 for natural material, and 0.112 cm s g-1 for synthetic material (Adams, 1980). The lower pore volumes of the synthetic material may reflect a greater content of non-tubular material, infilling of tubes with excess aluminium hydroxide, or differences in tube alignment.

Properties of synthetic imogolite TABLE 1. Effective pore volumes (Vo) and heats of adsorption (q) for various molecules on synthetic imogolite, derived from gas chromato- graphic studies (Adams, 1980). Heats of adsorption on silica gel, q(SiOH), are included for comparison.

Adsorbate I1o, (cm 3 g-l) q(kJ mole-l) * q(SiOH)

n-Butane 0.061 36.18 26.4 n-Pentane 0.057 49.05 30.5 n-Hexane 0.067 57.82 36.4 Cyclohexane 0.040 45.78 - - Benzene 0.050 66.65 43.1 Water 0-083 Perfluorotributylamine 0.0065

465

Gas chromatographic studies of synthetic imogolite (Table 1) indicate that the intra-tube pores are accessible to hydrocarbons at least up to the size of benzene and cyclohexane ( ~ 6 /~ diameter) but exclude perf luorotr ibutylamine (~ 10.2 A diameter). Inefficient packing of the hydrocarbons in the tubes reduces measured pore volumes to 0 . 0 4 0 - 0 . 0 6 7 cm 3 g- l , compared with 0.083 cm 3 g-1 for water measured under the same conditions. The heats of adsorpt ion of the hydrocarbons are about 50% greater than for the same species on silica gel, which provides a similar SiOH surface, but a much larger

pore size than imogolite.

60 f O adsorption

�9 desorption

50

Ki-G / ~ ) ................ 905-Ac

4 0

>-

o o

"~ 30 ?

10

0 I I I I I 0 02 0 4 0-6 Q-8 1-0

Relat;ve pressure FIG. 6. Water-adsorption isotherms on two natural imogolite samples, Ki-G and 905-Ac, and on

Ca- and Na-montmorillonite. From Wada & Yoshinaga (1969).

466 V.C. Farmer et al.

Water adsorption isotherms (Fig. 6) on natural imogolite indicate filling of the intra-tube pores at ~ 5% relative humidity, followed by entry into inter-bundle and inter-tube spaces, giving a total water content of 22-25% at 50% relative humidity. Differences in hydration level between natural specimens have been ascribed to differences in tube alignment (Yoshinaga, 1968). Better aligned specimens (Im-G and 905-Ac in Fig. 7) exhibit more intense inter-tube X-ray diffraction maxima, have lower hydration levels, and begin to dehydroxylate at lower temperatures (200 ~ compared with 250~ than more poorly aligned specimens (Ki-G and Ka-G). After heating to 250~ intertube pores collapse irreversibly (Wada & Henmi, 1972). The inter-tube space appears to be accessible to polar organic molecules such as 2-methoxyethanol (ethylene glycol monomethyl ether) (Egashira & Aomine, 1974), and can incorporate neutral salts with suppression of the inter-tube diffraction maximum in oven-dried specimens (observed for BaC12 by the authors, and reported by Wada & Henmi (1972) for quaternary ammonium salts).

The acid strength (Ho) of the imogolite surface (Fig. 8), as measured by Hammett indicators, is low (6-8 to 4.0) at relative humidities >30%, but increases on drying and reaches values of 1.5 to - 5 . 6 when strictly anhydrous. There is no further increase on heating through dehydroxylation, but in the range 500-980~ the acid strength becomes similar to metakaolinite ( - 5 . 6 to - 8 . 2 ) before declining to a low value when the dehydroxylate phase crystallizes to mullite.

It is uncertain whether imogolite has a small ion e~change capacity (Wada, 1977), or simply acts as a sorbent for salts, acids and bases from solution. Studies on synthetic

J3 >_

E

g

o

4 0

3 0

2 0

10

x I m G

B Ki-G

z~ K a G

O 905-Ac

50% r.h.

P20s

1 O0 2 0 0 3 0 0 4 0 0 5 0 0

Temperature (~

FIG. 7. Thermogravimetric curves for different natural imogolite specimens. 'Im-G', 'Ki-G', 'Ka-G' and '905-Ac', denote imogolite specimens from the Imaichi, Kitakami, and Kanuma

pumice beds and the Uemura soil, respectively. From Yoshinaga (1968).

Properties of synthetic imogolite

F-V-'q E 2 S 2 1 ~ F T q r--T--q Ho 4.0 to 6.8 3.3 to4-O 2 . 0 t o 3.3 1-5 t o 2 . 0 - -5 .6 to 1.5

r---r--q 8-2 to - -5 .6

467

H(AI) [ Gb

HIA, Is I 4 I 3 1 2 ~ ' ] Im NalSI 4 I 3 I 2 I "7

A/ImH(At) 1 5 1 4 3 I 2 I I I Na1514 I 3. 2 I ]

A H(A016] I 3 2 I~1 aa I 5. I 4 2 1 ,J

.(A,I [ ~ 5 J Mt

N, I I 4 3 I l l

Kt "(A')I61 5 I NaI61 S ,j

I 1 I I I I I I I i 1 0 10 20 30 40 50 60 70 80 90 100

(r.h. %)

FIG. 8. Acid strength (Ho) of surfaces of gibbsite (gb), imogolite (Im), mixed allophane and imogolite (A/Im), allophane (A), montmorillonite (Mt) and kaolinite (Kt) in hydrogen (H/AO

and sodium (Na) saturated forms. From Henmi & Wada (1974).

! J J

~ 0 " 2 / [ - KH2PO4 pH 4

E 01 ~ pH-5 I

1 2 3 I a mol crri z

I I I

KHzPO4

/ /

I I I 1 0 20 3 0

FIG. 9. Salt adsorption on synthetic and natural imogolite from aqueous solutions. Adsorption of H3BO3, sodium oxalate, KH2PO 4 and cetylpyridinium chloride (CPCI) were measured with background electrolyte (0-01-0-1 M NaCI or KC1). Data for oxalate (Parfitt et al., 1977) and phosphate (Parfitt et al., 1974) are for natural imogolite; other data are original and for synthetic

imogolite.

468 V.C. Farmer et al.

imogolite by the authors have shown that its salt sorption capacity lies in the range 0.15-0.30 m moles g 1 for both strongly and weakly adsorbed salts (Fig. 9), which appear to compete for the same sites. Thus, adsorption of 0.14 mole g-1 phosphate from 2-7 mM phosphate solution completely inhibits adsorption of nitrate from 5 mM solution, from which 0.1 m mole g-i is adsorbed in the absence of phosphate. Similarly, cations appear to compete for sorption sites, but ion size or ion hydration appears to play a greater role than ion charge in competitive adsorption. Thus Ba 2§ is preferred to Mg 2§ and K § in ratios of 1.6 and 2.0 respectively when added in equal concentration (25 mM) as chlorides. Unlike the layer-silicates, imogolite has a low affinity for bulky organic cations, such as cetylpyridinium chloride (Fig. 9) or paraquat (1 : 1-dimethyl-4 : 4-bipyridinium chloride).

The amounts of salt adsorbed correspond to 0 .8-1.7 conventional molecules per unit length of tube (8-4 A) and so greatly exceed the number of edge sites at tube ends. The affinity of imogolite for oxalate and phosphate is less than that of gibbsite, where sorption occurs by displacement of hydroxyl from edge sites (Parfitt et al., 1977), and adsorption of phosphate or oxalate on imogolite does not appear to involve displacement of hydroxyl, as there is little pH shift when their salts are added to an imogolite suspension. Addition of BaC12 to imogolite at saturation level suppresses the inter-tube X-ray reflection, indicating that the sites of salt adsorption are inter-tube. Inter tube adsorption is consistent with the flocculating effects of salts, particularly those with polyvalent anions, on imogolite dispersions.

The site of the strong adsorption of boric acid has not been explored, but possibly involves interaction with SiOH groups.

F I L M A N D F I B R E F O R M A T I O N

Self-supporting filims, in which the tubes lie parallel to the film plane, are readily prepared by evaporating washed NH3-gelled dispersions on to polyethylene sheet.

Unmodified imogolite dispersions have no fibre forming capacity, but fibres ranging from 0.1 gm to 0.1 mm in diameter have been prepared by adding polyethylene oxide (PEO) to synthetic imogolite dispersions. The finest fibres were obtained from a dispersion containing 74 mM A1 (nominally 0.74% anhydrous imogolite) and 1% PEO (M.W. 4 x 106), by drawing in warm air. For the coarsest fibres, 5% PEO (M.W. 6 x 105) was added to the 0.74% imogolite dispersion, and the fibres drawn in an ammonia atmosphere, which caused the imogolite component to gel. The PEO component decomposed and vaporized during examination in the electron microscope, or on heating to 150~ leaving the imogolite tubes highly aligned parallel to the fibre, as shown by electron diffraction from the finest fibres. The fibres had little strength and became brittle, although remaining coherent, on heating at temperatures up to 1000~ The decomposition product from salt-free dispersions at 1000 ~ was mullite, but glassy products were obtained from preparations containing sodium perchlorate.

S T A B I L I T Y

The imogolite structure is easily damaged by dry grinding (Henmi & Yoshinaga, 1981). Its thermal stability is low, and partial dehydroxylation begins around 200~ in some samples (Fig. 7), although major structural changes require temperatures in excess of 250~ It is readily decomposed by Al-complexing agents, such as 0.2 M oxalate at pH 3,

Properties of synthetic imogolite 469

but is stable to non-complexing acids at pH values down to 2.5, provided that the equilibrium levels of AI and Si are exceeded in the solution. Under near-neutral leaching conditions it decomposes to gibbsite or boehmite, unless sufficient silica is present in the leaching water (6/~g cm -3 at 20~ and 18 ~tg cm a at 100~ Farmer, Smith & Tait, 1979; Farmer & Fraser, 1982). It begins to dissolve in cold alkali at pH values > 12 (Horikawa, 1975a; Farmer et al., 1983), and is dissolved or modified by hot alkali (Farmer, Smith & Tait, 1977).

B I O L O G I C A L A C T I V I T Y

Although imogolite is fibrous under the electron microscope, its bulk isolated form is not fibrous in the optical microscope. It occurs widely in soils in Japan, and is likely to be a component of natural acidic waters (Farmer, 1982), but no report of any associated health hazards has appeared. In in vitro tests, it is non-toxic to macrophage cells, indicating absence of fibrogenic activity, but is highly toxic to lung cell cultures, which is an indicator of carcinogenic activity after intrapleural injection (personal communication from Dr. R. Davies, MRC Pneumoconiosis Unit, Penarth). Although this does not necessarily correlate with mesothelioma induction by inhalation, care should be exercised in handling freeze-dried imogolite, which is readily dispersed in air. Imogolite forms a complex with yeast RNA which is reported to attenuate tumour cells (Maekawa & Momii, 1972).

S Y N T H E S I S

It is difficult to evaluate the purity and quality of imogolite preparations. Measurement of the volume of centrifuged gel (1400 g for 10 min) precipitated by ammonia from 10 cm 3 of a solution diluted to 1 mM or 2.5 mM gives a measure of some combination of yield and tube length, but not of purity. The presence of non-tubular products (aluminium hydroxides and proto-imogolite allophane) can probably be fully evaluated only by electron microscopy of a high enough resolution to reveal the tube structure of imogolite and spherical shells of allophane (Wada et al., 1979). An indication of purity is, however, provided by IR spectroscopy, as the sharpness of the 1R absorption bands of the product is reduced by the presence of non-tubular allophane (Fig. 10), and both crystalline and non-crystalline aluminium hydroxides contribute additional features in the OH-stretching region. Any opalescence that develops during synthesis or isolation of the product indicates the formation of boehmite or gibbsite. Non-tubular allophane can be selectively extracted from the product, as it is more sensitive to chemical attack by complexing agehts under controlled conditions--for example, extraction by 0.3 M sodium citrate and 1 M sodium bicarbonate at 80~ for 20 min (Farmer et al., 1983). Moreover, tube growth is essentially a crystallization process and as such is highly sensitive to minor changes in procedure, which affect the number and nature of active seeds in the reacting solution. Accordingly, the progress of tube formation should be followed by gel-volume yields during the synthesis until optimum yields are obtained.

Synthesis is most easily achieved in dilute solutions. Freshly formed hydroxyaluminium species readily combine with orthosilicic acid to form a reactive proto-imogolite sol (Farmer, Fraser & Tait, 1979), which forms imogolite on heating at 100~ provided sufficient excess silica is present to prevent hydrolysis to aluminium hydroxide (~0.3 mra Si(OH) 4 at 100~ (Farmer, Smith & Tait, 1979)). Thus dilute imogolite sols are readily

470 fT. C. Farmer et al.

1 I I I l

NAT

I I I I I OM -I

FIG. 10. IR spectra of synthetic proto-imogolite allophane (PROTO), synthetic imogolite (SYN) and natural imogolite (IM), From Farmer & Fraser (1979).

prepared by refluxing, or incubating at 95-100~ solutions containing 2-5 mM A1(C104)3 and 1.55 mM Si(OH)4, adjusted to pH 5 and then immediately re-acidified to give final added concentrations of 1 mM HC104 and 2 mM acetic acid (modified from Farmer, Fraser & Tait, 1977). Any opalescence should be allowed to clear by standing overnight before heating. Nitrate or chloride may be substituted for the perchlorate, akhough chloride inhibits the rate of imogolite formation. A 2 mM Si(OH) 4 stock solution can b e prepared by hydrolysing Si(OEt)4 with vigorous stirring in water, or by passing a sodium silicate soltttion through a cation exchange column in the acid form. Typically, this procedure gives 2.5-3.2 cm 3 gel from 10 cm 3 of the solution after 1-3 days, or longer with chloride.

Increasing the reagent concentrations in the above procedure may decrease gel yield and increase the proportion of non-tubular products, although a satisfactory product has been obtained with 10 mM Al(CIO4) a by adding Si(OEt)4 directly to the perchlorate solution and immediately adjusting the pH to 4.5 (about OH/A1 = 2) with vigorous stirring until hydrolysis of the ethoxide is complete. On standing overnight the pH fell to 3.6 before beginning incubation at 97~ More concentrated salt-free imogolite dispersions (0.3%) can be prepared from these synthetic preparations by precipitating the imogolite gel with ammonia, centrifuging and washing repeatedly with weak ammonia till salt-free, then washing with water to near neutrality, and redispersing in any suitable acid (acetic, HCI, HNO 3, HC104).

Higher concentrations of imogolite up to 50--70 mM in AI can be prepared by direct synthesis using proto-imogolite sols of reduced or zero-salt content. A convenient procedure (Farmer & Fraser, 1979) involves hydrolysing a mixture of 75 m mol Si(OEt)4 with 150 m mol A1 s-butoxide in 950 cm 3 solution containing 75 m mol HC1Q, with

Properties of synthetic imogolite 471

vigorous stirring continued until the initial gelatinous precipitate is completely dispersed (3 h) and then with gentle stirring for 18 h. After removing any residual precipitate by centrifuging, aliquots of the slightly opalescent solution are diluted to 10-50 mM in A1, and heated to 95-100~ to determine which concentration gives maximum gel depth after 2-4 days' digestion. Optimum gel yields are often obtained at 20 mM A1 (3 cm 3 gel/10 cm 3 sol at 2.5 mM A1) but higher concentrations and greater gel volumes have been obtained by adding successive aliquots of the concentrate to the digestion mixture at daily intervals, increasing the A1 concentration in 20-mM steps up to 50--60 mM A1. By this means, gel volumes of 5-6 cm a from 10 cm a of 2.5 mM A1 dispersions have been obtained at 65 mM A1 after 7 days' digestion. The products, unfortunately, frequently contain aluminium hydroxide species, which may arise, at least in part, from hydrolysis products in the commercial A1 butoxide.

An alternative procedure for preparing a proto-imogolite concentrate, which uses less expensive reagents and gives purer products, involves redispersing a fresh aluminium orthosilicate precipitate in acid (British Patent 1 574 954). A sodium silicate solution (1000 cm3), prepared from a fusion of 1 g SiO 2 (16.7 m mol Si) with 5 g Na2CO 3, is added with vigorous stirring to 200 cm 3 solution containing 11-3 g Al(NO3) 3. 9H20 (30 m mol) and 25 m mol HC104, and continuing stirring until the solution is clear (20-30 min). After 1 h, the aluminium silicate is precipitated at pH 7, isolated by centrifuging, and immediately redispersed with vigorous stirring in about 200 cm ~ solution containing 6 cm 3 M HC104 and 12.5 cm 3 M acetic acid. This concentrate usually gives maximum gel yields at 10 mM A1, but higher concentrations with higher gel volumes at 2.5 mM A1 can be obtained by stepwise addition of concentrate at 2-day intervals. The sodium silicate solution can also be prepared from 4.4 g Na2SiO 3. 9H20 + 32 cm 3 M Na2CO 3 in 1000 cm 3 water.

P O T E N T I A L A P P L I C A T I O N S

Although the starting materials and synthesis conditions are easily accessible and inexpensive, the high water content of the products (99.5%), even after precipitation as gels, adds greatly to the cost of producing an air-dry product. Any application of this material must be of high value, probably as a unique catalyst. As yet, no assessment of its catalytic properties has been made, but the low surface acidity, absence of exchangeable cations, unique, regular, internal and external surface, defined porosity, and its ease of modification by salt adsorption and low-temperature heat treatment, all suggest that its catalytic properties could be unique.

Imogolite dispersions have been more fully explored, and clearly do have unique properties which suggest applications as a reversible support for solids in suspension, controlled by its pH-dependent gel-sol characteristics, or as a flotation agent in association with long-chain anionic detergents. Here again problems may arise because of transport costs of the low-concentration sols, but dispersible products of lower water content may be possible. Further research, however, will surely open up new possibilities, as continues to occur in the much more fully explored field of layer-silicate clays.

REFERENCES

ADAMS M.J. 0980) Gas chromatography adsorption studies on synthetic imogolite. J. Chromatography 188, 97-106.

472 V . C . F a r m e r et al,

CRADWlCK P.D.G., FARMER V.C., RUSSELL J.D. MASSOY C.R., WADA K. & YOSHINAGA N. (1972) Imogolite, a hydrated aluminium silicate of tubular structure. Nature Phys. Sci. 240, 187-189.

E6AS,IRA K. & AOMINE S. (1974) Effects of drying and heating on the surface area of allophane and imogolite. Clay Sei. 4, 231-242.

EGASHIRA K. (1977) Viscosities of allophane and imogolite clay suspensions. Clay Sei. 5, 87-95. FARMER V.C., FRASER A.R. & TAIT J.M. (1977) Synthesis of imogolite: A tubular aluminium silicate polymer.

J. Chem. Soe. Chem. Comm. 462-463. FARMER V.C., SMIV, B.F.L. & TAIT J.M. (1977) Alteration of allophane and imogolite by alkaline digestion.

Clay Miner. 12, 195-198. FARMER V.C. & FRASER A.R. (!979) Synthetic imogolite, a tubular hydroxyaluminium silicate. Pp. 547-553

in: International Clay Conference 1978 (M. M. Mortland and V. C. Farmer, editors). Elsevier, Amsterdam.

FAgMER V.C., FRASER A.R. & TArT J.M, (1979) Characterization of the chemical structures of natural and synthetic aluminosilicate gels by infrared spectroscopy. Geoehim. Cosmoehim Aeta 43, 1417-1420.

FARMER V.C., SMITh B.F.L. & TAIT J.M. (1979) The stability, free energy and heat of formation of imogolite. Clay Miner. 14, 103-107.

FARMER V.C. & FRASER A.R. (1982) Chemical and colloidal stability of sols in the AI203-Fe203-SiO2-H~O system: their role in podzolization. J. Soil Sei. 33, 737-742.

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