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III. Nature of Some Clay-Organic Complexes and Reactions

III. Nature of Some Clay-Organic Complexes and Reactions

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96



M . M . MORTLAND



interlamellar regions of the montmorillonite at least at the low pH. The

infrared data of Schnitzer and Kodama ( 1967) indicated that the adsorbed

fulvic acid was mainly in an undissociated form. While they gave no

specific mechanism it seems to the author, in view of the water present,

that probably a water bridge between the carbonyls of the fulvic acid and

mainly A13+on the exchange complex would be a likely interaction. It has

been shown by Yariv et al. (1966) that benzoic acid is coordinated

through water molecules to the more highly polarizing exchangeable

cation in the interlayer space of montmorillonite, but directly coordinated to N H i and K+. On dehydration of the complex, benzoic acid became directly coordinated to all the exchangeable cations investigated.

The effect of exchangeable cation on organic acid adsorption by clays

has been demonstrated by Yariv et al. ( 1966). They showed by infrared

absorption that the amount of benzoate ion in the clay complex depended

upon the kind of exchangeable cation. The conclusion of this must be

that the benzoate ion is associated primarily with the interlayer cation,

not with the silicate lattice. They also observed that the infrared spectra

of benzoate salts of the cations involved were not the same as in the clay

complex, thus the benzoate does not form a discrete crystalline phase but

presumably lies in the interlayer space. In addition, Yariv et al. (1966)

showed that the 00 1 spacing of montmorillonite-benzoic acid complex

varied depending upon the nature of the exchangeable cation. They conclude that benzoic acid is adsorbed to montmorillonite through the water

bridge described above, by direct ion-dipole interaction depending upon

the kind of cation and the hydration status of the system, or as benzoate

anion associated with polyvalent cations. Kodama and Schnitzer ( 1968)

also show a marked effect of exchangeable cation on fulvic acid adsorption on montmorillonite which may be explained in the same way as

described above for benzoic acid. In later work, Kodama and Schnitzer

( 1969) suggested that adsorption of fulvic acid on montmorillonite is related to the ease with which the fulvic acid can displace water from between the silicate layers. This is in accord with the above discussion in

that the fulvic acid and water compete for ligand positions around the

exchangeable cation either for direct coordination or by bridging through

directly coordinated water molecules.

The adsorption of amino acids by clays has received a great deal of

attention in the past. Adsorption as the cation, of course, takes place

below the isoelectric point of the amino acids. When present in the

cationic form, the amino acid may then be adsorbed by ion exchange with

other cations on the exchange complex of the clay (Greenland et al.,

1965a; Cloos et al., 1966; Sieskind, 1963; Fripiat et al., 1966; and



CLAY-ORGANIC



COMPLEXES A N D INTERACTIONS



97



earlier work referred to by Greenland, 1965a, in his review). When adsorbed at the clay surface as a cation, the amino acids resist displacement

when washed with water or other solvent but may be replaced with salt

solutions (Greenland et al., 196Sa; Cloos et al., 1966). In addition to

adsorption by ion exchange, amino acids may become cationic after adsorption and protonation at a clay surface capable of supplying protons.

As discussed in an earlier section, these protons may arise from exchangeable H + or from water associated with highly electronegative

exchangeable cations. Fripiat et al. (1966) have observed by infrared

absorption the zwitterion as' well as the cationic form of some amino

acids and peptides adsorbed on montmorillonite.

Greenland et al. (1965a) have shown that other forces also contribute

to the adsorption process, which they describe as an interaction between

the dipoles of the amino acids and the exchangeable cation and charged

surface sites onethe clay, and of dispersion forces between the surface and

the amino acid molecules. Evidence given for this was the adsorption

without protonation or cation exchange by Na+ and Ca2+-montmorillonite

at or near the isoelectric point of the amino acids (Greenland et al.,

196Sb). Cloos et al. ( I 966) propose an adsorption process which involves

salt formation between polyvalent exchangeable metal ions with the functional groups of the amino acid. This would be similar to cation bridging

effects proposed by several workers and extensively reviewed by Greenland ( I96Sa), in which polyvalent exchangeable cations form a bridge

between the anion and the clay surface. As mentioned above, Yariv et al.

( 1966) using infrared methods noted benzoate anion in montmorillonite

when benzoic acid was adsorbed by clays, particularly when polyvalent

cations were occupying the exchange sites.

The function of exchangeable cations in amino acid adsorption by

clays would seem to be paramount and would include the following interactions: ( 1 ) determine the proton supplying power of the clay surface and

therefore the possibilities of protonation of the amino acid and thereby

its cationic nature; (2) direct coordination of polar groups (carbonyl or

amino) to the exchangeable cation: (3) indirect coordination through

water bridges composed of water molecules directly coordinated to the

cation: (4) bridging by polyvalent cations where an amino acid anion

neutralizes one of the positive charges on the exchangeable cation.

The applicability of the above discussion to peptides and proteins is

obvious, and the same kinds of interaction would be expected. Several

workers referred to by Greenland in his review (1965a) have shown that

both coulombic and physical forces are involved in the adsorption of

proteins by clays. Certain other factors become important in polymers as



98



M. M. MORTLAND



shown by Greenland et al. (1965b) in studies on several amino acids and

their peptides. They found adsorption on acid montmorillonite resulted in

protonation of amino acids and their peptides as would be expected.

Under neutral conditions, the free energy of adsorption was related to

molecular weights, dielectric constant, and shapes of the adsorbed

molecules. They found for adsorption of glycine and its peptides on Camontmorillonite that as molecular weight increased, the entropy factor

became more favorable for adsorption. The desorption of several water

molecules accompanies the adsorption of the polymer thus leading to

a favorable entropy effect for adsorption. For proteins of large molecular

weight, the entropy factor must be important in promoting adsorption.

Uncoiling and shape alterations as the protein becomes adsorbed at the

clay mineral surface will have entropy effects. A number of workers

referred to in Greenland’s review (1 965a) have shown that globular proteins may or may not uncoil upon adsorption in montmorillonite. Very

large amounts of proteins may be adsorbed by montmorillonite resulting

in interlamellar spacings of tens of angstroms.

The adsorption of purines, pyrimidines, and nucleosides by montmorillonite have been studied by Lailach et al. ( 1968a,b)and Lailach and

Brindley (1969). They investigated the relationships of adsorption to the

exchangable cations on the clay and the pH of the medium. Generally it

was found that adsorption takes place primarily by a cation exchange

reaction between the inorganic cations on the clay and protonated organic

molecules when the ambient pH was near the pK, value for the organic

compound, thus under acidic conditions. For the alkali metal and alkaline

earth cation saturated clays the effects of the metal cation on adsorption

were considered to be secondary except for the nucleosides, which exhibited considerable differences. The greater adsorption of the nucleosides on the alkali metal-saturated clays was thought to result from their

greater dispersion and therefore a greater accessibility of the internal

surfacesof the clay mineral. Where transition-type metal cations saturated

the exchange capacity of the clay, adsorption of the organic compounds

at low pH took place by cation exchange processes after protonation of

the organic compound. As pH was increased, complex formation with the

inorganic cations became increasingly important. Considerable differences between various compounds were observed, and thus the original

work should be consulted by the interested reader. It was observed by

Lailach and Brindley (1969) that thymine and uracil were not adsorbed

from aqueous solutions by Na- and Ca-montmorillonitein a pH range of

1-6. However, when such a compound as adenine was present too,

appreciable adsorption occurred. Lailach and Brindley attributed this



CLAY-ORGANIC COMPLEXES A N D INTERACTIONS



99



coadsorption to hydrogen bond formation between the two species of

molecules, the protonated one being adsorbed by cation exchange at the

clay surface. This agrees with the infrared absorption observations of

Mortland ( 1 968b) on pyridinium-ethyl N,N-di-n-propylthiolcarbamatemontmorillonite complexes, and of Doner and Mortland ( 1 969) on trimethylammonium-dialykl amide-montmorillonite complexes.



B. INTERACTION



OF



ORGANIC

PESTICIDES

WITH CLAYS



The subject of clay interactions with organic pesticides has been reviewed by Bailey and White (1964) and again in 1970. The interested

reader should consult these excellent reviews for detailed information.

The discussion here will therefore be limited in scope.

Most, or probably all, of the specific bonding mechanisms discussed in

Section I 1 apply to organic pesticides, which are extremely diversified

in their structures and properties. Some pesticides are cationic and so

may be adsorbed by clays by ion exchange processes. Examples of this

kind are the herbicides diquat and paraquat, which are quaternary

ammonium compounds and therefore strong bases. Their salts ar.e very

soluble in water and are completely ionized. Weber et al. ( 1 965) studied

their adsorption by montmorillonite and found them to be preferentially

adsorbed by the clays up to their cation exchange capacities. Knight and

Tomlinson ( I 967) studied the interaction of paraquat with a number of

mineral soils and found it to be strongly adsorbed. Weed and Weber

( 1969) found that the kind of exchangeable cation markedly affected

adsorption of diquat and paraquat by vermiculite but had much less effect

on adsorption by montmorillonite. Once adsorbed, the two organic

herbicides were much more difficult to exchange with salt solutions from

montmorillonite than from vermiculite. Bioassay studies (Weber and

Scott, 1966; Weber et al., 1969) revealed that paraquat adsorbed by

montmorillonite exhibited very little phytotoxicity while that adsorbed

by kaolinite and vermiculite became available for bioactivity with time.

For most other organic pesticides which are weaker bases, their existence as cations and therefore their ability to exchange with metal ions

on the clay will depend upon their ability to accept a proton from the

medium, which in turn is determined by the pH. Thus, the surface acidity

of clay minerals may provide the source of H+for protonating pesticides.

As described in an earlier section, these protons may exist at exchange

sites on the clay mineral or be generated from water associated with exchangeable metal cations. Some s-triazines were shown to become pro-



100



M. M . MORTLAND



tonated at clay mineral surfaces by Russell et al. ( 1 968b) utilizing infrared absorption. Weber ( 1966) demonstrated for a series of s-triazine compounds that the maximum adsorption on montmorillonite occurred at a

pH in the vicinity of the pKo value of each compound, that is, the pH at

which the compound became protonated. A further lowering of the pH

resulted in some desorption of the s-triazine compounds which was attributed to competition of the protonated species with H+. An alternate

explanation would be that the low pH released A13+from the clay lattice

which would be a much better competitor than would H+ to displace the

protonated organic cation from the exchange complex. In studying the

adsorption of 3-aminotriazole (a herbicide) by montmorillonite Russell

et al. ( I 968a) found by infrared absorption that it would protonate to

form the 3-aminotriazolium cation. In the case of montmorillonite

saturated with polyvalent cations (Ca2+,Cu2+,Na+, A13+) protonation

was thought to be due to highly polarized water molecules in direct coordination with these cations. The decreasing order of extent of protonation (Ca < Mg < Al) reflects the order of decreasing polarizing power of

the cations.

Infrared absorption results of Russell et al. (1 968a) indicated coordination of 3-aminotriazole to Ni2+ and Cu2+ cations on montmorillonite;

thus binding of a pesticide to the clay mineral surface by coordination

interaction is established. Mortland and Meggitt ( 1966) showed that

ethyl N,N-di-n-propylthiolcarbamate(EPTC) complexes to montmorillonite by ion-dipole interaction between the carbonyl of EPTC and the

exchangeable metal cation on the clay. The decrease in CO stretching and

increase in the CN frequency was related to the electron affinity of the

cation. EPTC was also shown by Mortland (1968b) to be capable of

being bonded to montmorillonite through a hydrogen bond provided by an

organic cation on the exchange complex. Such organic-organic complexes at clay surfaces as demonstrated by this model system undoubtedly

exist in nature.

Complexation of pesticides with clays must affect their bioactivity to a

greater or lesser extent depending upon the energy of adsorption and the

ease of displacement. An example of the effect of exchangeable cation on

montmorillonite on the relative ease of release of the adsorbed pesticide,

are some results of the author in Fig. 3, where the differential release of

3-aminotriazole from Ca-, Cu-, and Al-montmorillonite is shown. As indicated in the figure, the Cu-clay-pesticide complex was very stable, showing only a gradual release, while the Ca-clay-pesticide system was almost

immediately completely released, the A1 system being intermediate in

release properties. Table I1 shows the relative phytotoxicity of these



CLAY-ORGANIC



COMPLEXES A N D INTERACTIONS



101



Number of washes



FIG.3. Release of 3-aminotriazole cornplexed with homoionic montmorillonites (Ca, At,

Cu) upon extraction with water (dashed curves) and with 0.01 N MgCL (solid curves).



TABLE I I

Yield of Ryegrass in Pots Treated with 5 ppm

of 3-Aminotriazole, Free and Complexed with 3

Different Homoionic Montmorillonite Clays"



Treatment

Control

3 AMT

Ca-clay 3 AMT

AI-clay 3 AMT

Cu-clay + 3 AMT



+



+



Yield

(dry weight, rng/pot)*

522

191



234

256

545



"Bioassay by courtesy of G. R. Stephenson and S. K. Ries,

Horticulture Department, Michigan State University.

"Average of 3 replicates.



102



M.



M. MORTLAND



complexes to ryegrass in comparison with the uncomplexed compound.

These results are in accord with the extractions in that the toxicity is

directly related to the ease of extraction. Similar results have been

obtained by the author for a number of other pesticides, and bioassay

data on the complexes are generally in accord with extraction results.

Since it is possible to make specific cation-clay-pesticide complexes

which exhibit differential release properties, it would seem a fruitful area

for pesticide formulation for controlling pesticide release where such is

desirable.

Many pesticides are not adsorbed from water onto the clay mineral

surface. These are molecules that are negatively charged (anionic) or

electrically neutral and not polar enough to compete with water for

adsorption sites on the clay. Anionic pesticides are often negatively

adsorbed; that is, the concentration of the organic compound in the

solution is increased in the presence of clay due to the repulsion of the

negatively charged clay surface. On the other hand, many of these

materials may be complexed on the clay surface when water is removed

merely by drying in the air. Evidence for these complexations has come

from infrared adsorption studies, i.e., benzoic acid (Yariv et al., (1966),

and EPTC (Mortland and Meggitt, 1966). It was further shown that when

EPTC-clay complexes were reintroduced to water, the herbicide was

quantitatively displaced from the clay surface to the solution. The above

observations suggest that water content of the system is a very important

factor in determining whether or not some clay-pesticide interaction

takes place and that conclusions drawn from suspension systems may not

be valid at low water contents. Dry as well as wet conditions exist in

nature, and the condition of the pesticide in both environments must be

taken into account.

A N D SURFACTANTS

WITH CLAYS

C. INTERACTIONOF POLYMERS

All the binding mechanisms described earlier apply to polymer adsorption on clays if the appropriate functional groups are present. However,

because of their large size and shape properties, entropy effects may play

an important role in their adsorption by clays, as indicated by Parfitt

(1969). Changes in shape of the molecules on adsorption as well as

solvent-polymers and solvent-clay relationships will have their contributions to entropy effects. The adsorption of polymers is usually characterized by slow adsorption rates and a relatively small effect of temperature on adsorption. The nature of the solvent is of importance in adsorption

of the polymer, being more strongly adsorbed by a surface from poor

solvents and less strongly from better solvents.



CLAY-ORGANIC COMPLEXES A N D INTERACTIONS



103



In the study of the adsorption of polyoxyethylated alcohols of molecular weight around 1000 on montmorillonite, Schott (1964) found stronger

adsorption on Ca2+than on Na+ clay. Rapid coverage of the Ca2+clay

up to a monolayer on each silicate surface was observed. Direct interaction between the ether oxygens and Ca2+or through a bridging water

molecule may have been involved. Adsorption of polyethylene glycols

from water was found by Howard and McConnell ( 1967) to take place on

silica. A plateau in the adsorption curve occurred at a value corresponding to slightly more than a monolayer for polymers ranging in molecular

weight from 1400 to 18,000.

Clapp et af. ( I 968) investigated the adsorption of a bacterial polysaccharide on montmorillonite. They found that the material was adsorbed

within the interlamellar regions of the clay giving a maximum 001 spacing

of 16.9 A. Adsorption depended upon the degree of dispersion of the clay,

which in turn was a function of the saturating cation and the salt concentration. The polymer could be removed by washing with NaZS04 solutions.

Since the amount of release was a function of the salt concentration, it

was suggested that displacement of the polymer from the interlamellar

positions of Na-montmorillonite was caused by a salt-controlled reduction in double-layer swelling. Periodate oxidation studies indicated that

the polymer held in interlamellar positions was partially shielded from

periodate reaction. Apparently the release of the polymer from the clay

with salt solutions results from contractive forces between the clay layers

overcoming the binding forces between the polymer and clay surface and

the polymer is squeezed out. In this connection it would be interesting

to compare polymer-clay complexes prepared with various exchangeable cations for the purpose of comparing release properties with the

salt solution. If direct ion-dipole interactions were predominant, considerable differences in release properties would result, while if only

water bridges to metal ions and hydrogen bonding to the silicate surface

were the chief binding mechanisms, much less effect of exchangeable

cation could be observed. Also, the effects of dehydration on the stability

of the clay-polymer systems would be very interesting since it often results in much more stable clay-organic complexes.

Parfitt ( 1 969) has studied the adsorption of a number of polymers and

polysaccharides by montmorillonite. In addition to speciffc bond interactions described earlier, he emphasized the point that conformational

changes in the polymers are important. A polymer which is coiled in the

solvent may become uncoiled or the coils compressed upon adsorption

at the clay surface, which can result in large entropy effects. Parfitt

(1969) was able to show adsorption of high molecular weight dextran,



104



M . M . MORTLAND



amylose, soil polysaccharide by montmorillonite. An aminopolysaccharide was only slightly adsorbed by Ca-montmorillonite. Work of

Schott ( 1 964) and Parlitt ( I 969) suggests salting-out effects as being of

possible importance in the adsorption of certain polymers: that is, the

polymer may be less soluble in the vicinity of the clay-water interface

than in the bulk of the solvent due to the presence of the exchangeable

cations. Other recent workers showing adsorption of polysaccharides

by montmorillonite are Finch et al. (1966) and Swincer ( 1968).

The binding of polyethyelene to clay surfaces through the intermediation of ionizing radiation has been studied by Nahin (1966). Many polymerization reactions may be induced through effects of ionizing radiation,

and Nahin was able to promote cross-linking between polyethylene and

clay surface by such a process. He proposed that the radiation caused

momentarily positively charged polyethylene radicals to form, which then

exchanged with H+ ions at exchange sites. Such a treatment resulted in

a much more mechanically stable clay compared with samples that were

not irradiated. In addition, in the irradiated samples, much less organic

material could be extracted with toluene than in samples that were not

irradiated. Both gamma and beta radiation were employed on kaolinite

and montmorillonite clays, and a number of other organics in combination with the clay besides polyethylene were utilized as well. The results generally indicated that polyethylene could be directly bonded to

the clay surface and that it was more effective if the clay surface already

contained an organic such as polyvinyl alcohol than if the surface were

completely inorganic. These results suggest that, over geologic time,

similar bonding in natural systems between saturated and unsaturated

hydrocarbons and clay materials may come to pass.

The adsorption of amine-terminated polystyrene by kaolinite and

montmorillonite was investigated by Dekking (1964). He found that adsorption on the clay could be accomplished by ion exchange processes if

the salt of the polystyrene-amine was used or by protonation by acid-base

reaction if the polystyrene-amine was reacted directly with an acid clay.

The clay-polymer complexes so formed had greatly different properties

from the inorganic clays as would be expected, the complexes being

hydrophobic and organophilic. X-ray diffraction data indicated a polystyrene one layer thick in the montmorillonite. Van Olphen (1967) prepared a number of clay gels in combination with polyelectrolytes (macromolecular chains of polyions) and found an increase in strength of the

clay gels probably because of bridging between the clay particles. The

increase in strength permitted various forms of the clay gel-polyelectro-



CLAY-ORGANIC COMPLEXES A N D INTERACTIONS



I05



lyte to be created. For example, uniform thin sheets could be prepared.

It was further shown that such complexes could be used for chromatographic separations. As an example, ortho, meta, and para xylenes could

easily be separated by use of one of the clay-organic gels in the column

of a chromatograph.

Surface-active chemicals (surfactants) have come into broad use for

agricultural applications. They may be generally classified as ionic and

nonionic; the ionic may be subdivided into cationic and anionic. Law and

Kunze ( 1 966) have studied the adsorption of compounds representing all

above categories on montmorillonite and kaolinite. They concluded that

the cationic species were adsorbed on the clay as cations by ion exchange

and that the anionic varieties were not appreciably adsorbed. Electrically

neutral but polar surfactants containing hydroxyl groups were said to be

adsorbed by hydrogen bonding to oxygen atoms of the silicate surface.

However, as noted in earlier discussion, infrared data indicates that

hydrogen bonding to surface oxygens by alcoholic groups is very weak

and more important interactions are undoubtedly ion-dipole type directly

with exchangeable metal cations or indirect coordination through bridging water molecules of the primary hydration shell of the cations. Valoras

et al. (1969) reported adsorption of several nonionic surfactants by

montmorillonite, vermiculite, and kaolinite as well as by soil and its

constituents.



D. CATALYSIS

REACTIONS

Clays and clay minerals have been shown to catalyze reactions of

various kinds in molecules adsorbed at their surfaces. Most of the work

on catalytic reactions at surfaces of silica, silica-alumina, alumina,

zeolites, and clay minerals has been on systems extensively -dehydrated

and at high temperatures. More recent work, however, has demonstrated

catalytic reactions at relatively low temperatures and sometimes at

appreciable hydration levels, and it is this kind of environment that is of

principal interest in soil clays and in sediments.

McAuliffe and Coleman ( 1955) demonstrated the catalytic effect of

acid clays on the hydrolysis of ethyl acetate and the inversion of sucrose.

They found that Al:’+ saturated clay had less catalytic effect than did clay

which was predominately H saturated. In other hydrolysis studies, Mortland and Raman ( I 967) found that Cu(I1) was very effective in catalyzing

the breakdown of several organic phosphates. When Wyoming bentonite

had Cu( 11) as the exchangeable cation, catalytic activity was very great,



106



M. M. MORTLAND



while Cu(I1) saturated nontronite, vermiculite, and beidellite were much

less active, and Cu(I1)-organic soil had no catalytic effect at all. These

results demonstrate the marked influence of the exchanger on the Cu(I1)

activity and in consequence its ability to catalyze the hydrolysis of the

organic phosphates.

In studies of other reactions at clay mineral surfaces, Chaussidon and

Calvet (1965) noted that alkylammonium cations on the exchange sites

of montmorillonite would decompose at a relatively low temperature to

give ammonium ion plus some hydrocarbons of various chain lengths.

In similar work with protonated lysine on the cation exchange sites of

montmorillonite, Mortland (unpublished) found decomposition to ammonium ion, but at a relatively high temperature of 250°C. Fripiat et al.

( 1966) reported that when glycine and p-alanine adsorbed on montmorillonite was dehydrated and heated, amide linkages were observed to

form. Weiss ( 1963) reported that proteins adsorbed on highly charged

mica surfaces with H 3 0 + were cleaved into peptides and amino acids.

Mortland (1966) observed the catalytic decomposition of urea to ammonium on montmorillonite films in air-dry condition and at 20°C when

the exchangeable cation was Cu(1I). Ni(I1) and Mn(I1) clays showed

smaller amounts of urea decomposition but alkali and alkaline earth

saturated clays showed none at all. Russell et al. (1968a) observed the

formation of hydroxy atrazine when atrazine was adsorbed on Hmontmorillonite. This reaction involves the substitution of a hydroxyl

group for a chlorine atom on the ring structure of the triazine and is an

example of a nonbiological mechanism of degradation of a pesticide.

Skipper (1 970) confirmed this observation and showed that another acid

clay material, allophane, was not able to catalyze this reaction. This

undoubtedly is because the surface acidity of the allophane is not as low

as that of the clay mineral montmorillonite.

Catalytic decomposition of a polyalcohol, glycerol, by layer silicates

has been reported by Walker (1967a). Carbon was apparently the product

and was observed when the clays were immersed in boiling glycerol. His

conclusions were that: ( 1) the glycerol decomposition occurs only when

two silicate surfaces are in simultaneous contact with the molecule; and

(2) small and highly charged cations in the exchange complex greatly

enhance the effect. Other work which shows the effect of metal cations

on catalytic reactions at clay surfaces is that of Solomon (1 968), who

observed that aluminum at crystal edges and transition metals in the

higher valency state on the exchange complex act as electron acceptor

sites, while transition metals in the lower valency states could act as

electron doner sites. Thus, as required for a given polymerization reac-



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