Tải bản đầy đủ - 0 (trang)
II. Bonding Mechanisms in Clay-Organic Complexes

II. Bonding Mechanisms in Clay-Organic Complexes

Tải bản đầy đủ - 0trang

78



M. M. MORTLAND



for the cation exchange capacity of the mineral:

RNHT + M+-clay ~2

RNHT-clay



+ M+



(1)



where RNHf is some organic cation and M+ another species of cation.

Many organic cations are positively charged because of protonation of an

amine groups as in the case of alkyl amines and amino acids. However,

some compounds such as urea and amides are protonated on the oxygen

of the carbonyl group. Certain properties of the organic cations set them

apart from metal ions and will influence their adsorption on or displacement from clay mineral surfaces. These properties are as follows: (1)

The ionic property is usually pH dependent. (2) Other forces influence

adsorption to the clay surface. These other forces include hydrogen bonding, ion-dipole, and physical forces, and their importance depends upon

such factors as molecular weight, nature of the functional groups present,

and configuration of the molecule. (3) The interaction of the organic ion

with water will be quite variable, depending on the nature of the ion. This

interaction of the organic cation with water may be of crucial importance

in its interrelations with the clay surface and exchangeable metal ions

residing there.

Numerous scientists have contributed to the literature on the interaction of organic cations with clays; the earlier work will be found cited

by Greenland (1 965a). More recently, studies on the replacement of exchangeable sodium and calcium from montmorillonite by various alkylammonium cations were made by Theng et al. (1967). They found that

the affinity of the clay for the organic cation was linearly related to

molecular weight with the exception of the smaller methylammonium

and the larger quaternary ammonium ions. Thus, the more the length of

the alkylammonium chain increases, the greater is the contribution of

physical, noncoulombic forces to adsorption. Within a group of primary,

secondary, and tertiary amines, the affinity of the alkylammonium ions

for the clay decreased in the series R3NH+> R2NH,+ > RINH3+.These

differences were explained in terms of size and shape of the ions. In

general, Theng et al. found that the Na+ was much more easily exchanged

by the alkylammonium ions than was the Ca2+,as would be expected from

previous work on metal ion exchange in clays. In studies in which the

alkylammonium ion is replaced by metal cations, Mortland and Barake

(1964) showed that the order of effectiveness in replacing ethylammonium ion was A13+ > Ca2+ > Li+. In addition, it was noted in X-ray

diffraction studies on partially exchanged systems that the organic and

inorganic cations were not distributed uniformly throughout all the sur-



CLAY-ORGANIC COMPLEXES A N D INTEIMCTIONS



79



faces of montmorillonite, but that a segregation of the two kinds of ions

took place in various layers. This suggests that when the displacement of

ethylammonium ion by the metal ion from one interlamellar position

begins, it is completed before ethylammonium ions from other layers are

exchanged. Barrer and Brummer ( I 963) have made similar observations

and suggest that this segregation of organic and metal cations may be explained on the basis of accompanying water layers in which homoionic

cation layers tend to give regular continuous and stable monolayer or

double-layer arrangements. Mixtures of cations in any one interlamellar

position may render it impossible to form this type’ of geometrically

regular water layer. Thus it appears that, in montomorillonite partially

saturated with organic and metal cations, interstratified layers occur in

which each layer contains mainly one type of cation, a situation which

must be the most thermodynamically stable.

The effect of charge density of the clay mineral on the competitive

adsorption by ion exchange of two divalent organic cations was studied

by Weed and Weber (1 968). The two cations diquat (6,7-dihydrodipyrido( 1,2-a :2’, 1 ’-c)-pyrazidiinium dibromide) and paraquat (1,l’dimethyl-4,4’-dipyridium dichloride) differ in one respect in that the

charge centers of the cations are 3-4 A apart in the former and 7-8

A apart in the latter. The preference for one or the other of the cations by

layer silicates was related to the geometric “fit” between the charges on

the cation and those on the clay in that the cation whose charge centers

could most nearly approach the adsorption sites on the mineral surface

would be preferred. These results indicate that the negative charge on the

layer silicate lattices are discrete and relatively fixed and are not smeared

out as has been suggested by some workers. If the latter were the case,

the differential selectivity by various layer silicates for these two organic

cations would not have been obtained.

Charge density of the clay mineral may also affect the orientation of

adsorbed organic cations through steric effects. Thus, Serratosa ( I 966)

showed by infrared absorption technique that in pyridinium-montmorillonite the organic cation assumed an orientation where the plane of the

pyridine ring was parallel with the platelets of the clay mineral and a

resulting 00 1 spacing of 12.5 A.On the other hand, pyridinium-vermiculite has the pyridinium cations vertically positioned with respect to the

clay platelets and a 001 spacing of 13.8 A.Apparently the close proximity

of the cation exchange sites one to another prevents the pyridinium from

assuming the parallel position because of the restricted area permitted

for each pyridinium. Where neutral but polar organic molecules are bound

to the clay surface by other mechanisms, such as ion-dipole interaction,



80



M. M. MORTLAND



charge density would also be expected to affect their orientation within

the interlamellar regions of swelling clay minerals.

Certain organic cations such as butylammonium, when placed on the

cation exchange sites of vermiculite have been shown by Walker and

Garrett (1 967) to cause gross one-dimensional swelling of vermiculite

crystals in water. Hundreds of angstroms may separate the individual

platelets. These workers have shown dispersions of these crystals to be

stable over long periods in distilled water, but very sensitive to electrolytes which cause their flocculation. The macro swelling of vermiculite

is apparently activated by those organic ions which form clathrate structures with water.



2. Protonation of Organic Molecules at Clay Surfaces

In addition to adsorption of organic compounds by ion exchange of

organic with inorganic cations on the exchange complex of clays, many

compounds may become cationic after adsorption at the clay surface

through protonation. The sources of the protons for such a reaction are:

(1) exchangeable H+ occupying cation exchange sites, (2) water associated with metal cations at the exchange sites, or (3) proton transfer from

another cationic species already at the clay surface. It is thus quite obvious that the existence of an organic compound in cationic or molecular

form is dependent upon the acidity or proton-supplying power of the clay

surface. The reaction of a compound with exchangeable H+ to form the

cationic species is quite straightforward:



where R is some alkyl group. This reaction goes to completion because

it is essentially one between a strong acid and a relatively strong base to

give a salt. It is therefore characterized by high heats of reaction and a

high degree of irreversibility. Considerable energy must be put into the

system to drive the reaction back to the left. It has been shown calorimetrically by Mortland et al. (1963) that when R N H 2 is N H 3 , the heat of

reaction (enthalpy) is about 35 kcal per mole. Similar heats of reaction

would also result when R is an organic group of some kind. Where the

hydrogen is part of a hydroxyl group at the edges of clay minerals or on

amorphous material, the ability to protonate organic bases would be very

much pH dependent.

The second process by which organic molecules may become protonated is by proton donation from water at the clay mineral surface.



CLAY-ORGANIC



COMPLEXES A N D INTERACTIONS



81



Ordinary water is not likely to be acidic enough to protonate many

organic molecules. However, when water is associated with metal

cations, hydrolysis of this complex produces more or less H+, depending

upon the properties of the metal ion involved. The more electronegative

the metal cation, the more acidic will be the complex with water. Thus

aqueous solutions of AI3+are quite acidic, and those of cations like Na+

are much less so. When such hydrated metal ions are present on the cation

exchange sites of clay minerals, they impart differential proton-donating

powers to that mineral surface. The hydrolysis of such hydrated metal

cations can be described in the following equation:

[M (H*O)s]+" S [MOH ( H ~ O ) ~ - I ] + +

" -H

' i

Acid

Conjugate base



(3)



where M is the metal cation in question. The degree to ,whichthis reaction

goes to the right is described by the equilibrium constant and the acid

properties of alarge number of hydrated metal ions have been summarized

by Hunt ( 1 963). It would be expected then that the ability of a clay surface to protonate compounds would be dependent upon the nature of the

metal cations saturating the exchange sites on the clay, and this has been

shown for NH3 by Russell (1965) and Mortland and Raman (1968). The

overall reaction involved when an organic molecule is protonated by such

a process is:



where M is the exchangeable metal cation and B is the base in question.

It has been observed by a number of workers ming spectroscopic

methods (Mortland et al., 1963; Fripiat et al., 1965; Russell, 1965;

Mortland, 1966; Mortland and Raman, 1968; Harter and Ahlrichs,

1967; Swoboda and Kunze, 1968) that the acidity or proton-donating

properties of the clay surface are greater than would be expected from

pH measurements of the clay in water. This conclusion has been reached

on the basis of infrared absorption studies, where the protonated condition of a compound can be differentiated from the neutral form. For example, when NH3 is adsorbed on Ca2+and even Na+ saturated montmorillonite films, considerable quantities of N H t were observed to be formed

(Mortland et al., 1963; Russell, 1965; Mortland and Raman, 1968).

Urea and various amides, which are extremely weak bases, have been observed by Mortland ( 1 966) and Tahaun and Mortlaiid ( 1 966a) to become protonated at montmorillonite surfaces where the exchangeable



82



M. M. MORTLAND



cation was H+, AI3+,or Fe3+,but not in Na+ or Ca2+saturated clays. On

the other hand, some substituted ureas were not observed to protonate on

montmorillonite surfaces (Kim and Weed, 1968; W. J. Farmer and

Ahlrichs, 1969). Ordinarily a pH of less than 0.5 is required for a molecule

like acetamide to become protonated. These excessive acidic effects have

been ascribed to increased dissociation of the water associated with the

exchangeable metal cations above and beyond that predictable from their

hydrolysis constants. An important point in proof that the water associated with exchangeable cation is involved is that the proton donating

process of the clay surface is greatly dependent upon the kind of exchangeable cation present. Another factor is the water content itself.

The surface acidity of the clay increases as the water content decreases.

This has been explained by Mortland and Raman (1 968) on the basis that

when a great deal of water is present, polarization forces of the exchangeable cation may be said to be distributed among a large number of water

molecules. However, as the water content decreases, these polarization

effects become more concentrated on the fewer remaining water molecules, causing an increase in hydrolysis and so in their proton-donating

capabilities. An example of this process was shown by V. C. Farmer and

Mortland ( 1 966); they found that pyridine when adsorbed by hydrated

Mg-montmorillonite was coordinated to the Mg2+ by bridging through

directly coordinated water molecules. When this system was dehydrated,

pyridinium ions were observed, suggesting an increase in the acidity of

the clay surface. In other work, Mortland and Raman (1968) found that

when there was 40.7% water (oven-dry basis) in Ca2+montmorillonite,

only 16 meq of N H i per 100 g of clay was formed in the presence of NH,.

On the other hand, when the water content was 5.9% the NHi formed

was 80 meq per 100 g, thus showing in a dramatic way the effect of water

content on the proton-donating properties of the clay surface.

The third mechanism by which organic compounds may become protonated after adsorption at a clay mineral surface is by proton transfer

from a protonated species already present. The general reaction is:

AH+



+ B % BH++ A



(5)



where AH+ represents the protonated species (proton donor) on the

surface of the clay, and B represents the base (proton acceptor) with

which it reacts. The degree to which Eq. ( 5 ) will go to the right will depend largely on two factors: the relative magnitude of the dissociation

constants of the two interacting species, and the relative concentration

or activities of the reactants and products. The protonated species can



CLAY-ORGANIC



COMPLEXES A N D INTERACTIONS



83



be considered an acid capable of donating a proton while the uncharged

species is a base capable of accepting a proton. Thus, Russell, Cruz, and

White ( I 968a) showed by infrared spectroscopy the formation of 3aminotriazolium cation when NH2 montmorillonite was treated with 3aminotriazole. As the infrared spectrum for the 3-aminotriazolium cation

appeared, that of NH2 tended to decrease in intensity thus establishing

the proton transfer process. Raman and Mortland (1 969) have confirmed

the above reaction and in addition have observed proton transfer between a number of organic compounds on montmorillonite surfaces. The

systems in which proton transfer was observed are contained in Table I,

the degree of transfer being quite variable but generally in accordance

with the relative basicities of the interacting species and the concentrations of reactants and products.

TABLE I

Proton Transfer Reactions on Montmorillonite Observed by

Raman and Mortland ( I 969)

Exchangeable cation on clay



Molecule observed to accept a proton



NH i

Pyridinium

Ethylammonium

Methylammonium

(NH&COH+ (urea)



Pyridine, methylamine, 3-aminotriazole

NH3, methylamine, 3-aminotriazole

N Ha, pyridine, 3-aminotriazole

NH3, pyridine, 3-aminotriazole

NH3, pyridine



While relatively simple clay-organic systems have been described

above, it is reasonable to expect that similar proton transfer would occur

in other clay-organic systems, such as soil organic matter, and that this

is an important reaction to be recognized in natural systems. An example

might be reactions that occur when ammonia is applied to soils. In the

zone of ammonia injection there is a tremendous sink for protons. Ammonia would accept protons from available sources in organic matter

rendering those sites no longer electopositive. This would change the

bonding characteristics of the organic matter within the organic colloids

themselves as well as with the inorganic colloids.

3. Hem isal t Forma tion



When the amount of an adsorbed base (B) on a clay exceeds the number

of protons available for cation formation, one of the following situations

occurs: (1) the protonated molecule retains its proton against attraction



84



M. M . MORTLAND



by the nonprotonated molecule; (2) two molecules compete for the proton

on an equal basis, and it does not identify with either one but belongs

equally to both, forming a strong symmetrical hydrogen bond and a

cation of the [Bz- H]+ type. Many organic bases form symmetrical

hydrogen bonds or hemisalt type complexes which are well documented

in the chemical literature. Examples of these complexes observed in

clays are ethylammonium-ethylamine montmorillonite by V. C. Farmer

and Mortland (1969, pyrindinium-pyridine by V. C. Farmer and Mortland ( 1 966), urea-montmorillonite by Mortland ( 1966) and various

amide-montmorillonitecomplexes by Tahoun and Mortland ( 1 966a).

Hemisalt formation has quite a striking effect upon the infrared absorption spectra of the organic cation, so that if excess base is present

beyond the number of protons available there might be a mistaken conclusion that no protonated species is present. For example, in ethylammonium-montmorillonite a strong infrared absorption band appears at

1510 cm-' in spectrum 1 (Fig. I), which is the symmetric deformation

vibration of NH;. When ethylamine is adsorbed on this system, the 15 10

cm-l band disappears and strong, broad, featureless absorption appears

from 3300 to at least 1200 cm-l as shown in spectrum 2, which has the

Wavenumbers (Cm-')



0

c

u)



.-



:

e



k



FIG. I . Infrared spectra o f ( I ) ethylammonium-saturated montrnorillonite, degassed 20

minutes; (2) ethylammonium-saturated montmorillonite after adsorbing ethylamine (50 cm

pressure) for 15 minutes and then degassed for 45 minutes; (3) copper-saturated montmorillonite, after adsorbing ethylamine (50 cm pressure) for 2 hours and then degassed for 30

minutes. Curves 1 and 2 have the same baseline; curve 3 is displaced. (Reprinted from J .

Phys. Chem. 69,684. Copyright (1965) by American Chemical Society. Reprinted by permission of the copyright owner.)



CLAY-ORGANIC



COMPLEXES A N D INTERACTIONS



85



same baseline as spectrum 1. The ethylammonium-ethylamine complex

is stable in a vacuum, but it breaks up and ethylamine is lost when exposed to water in the atmosphere with the concomitant reappearance of

the infrared spectrum of ethylammonium-montmorillonite.The formation of the B2H+ type of cations has been observed in the experience of

the author to be very common rather than the exception in clay-organic

systems containing excess organic base and must be considered in interpretations of infrared spectra.



B. ANIONIC

While anions are normally expected to be repelled from the surface of

the negatively charged clay minerals, their presence at the clay surface

has been observed by Yariv et al. (1966) utilizing infrared absorption. In

studying interactions of benzoic acid with montmorillonite, they observed benzoate anion formation in relatively dry clay films as a result of

the following reaction:

Mii+-clay+ nHOB,+ nH+-clay+ M(OB,),,



(6)



The amount of benzoate anion present depended greatly on the kind of

exchangeable metal ion (M) present being greatest for the polyvalent

cations. These observations were made on systems where benzoic acid

was adsorbed from the vapor phase on relatively dry clay or where

aqueous solutions of benzoic acid had been utilized and the water evaporated away. Probably little benzoate anion would have been adsorbed

from an aqueous solution, and points up the differences in surface chemistry of clay minerals between aqueous suspensions and air-dry environments, both of which are possible in nature.

c .



ION-DIPOLE

AND



COORDINATION



The classical view of adsorption of polar but nonionic organic molecules by clay minerals has been to attribute a major function to the oxygen

atoms or hydroxyl groups of the silicate surface. This interaction has

been said to be one of hydrogen bonding between them and functional

groups on organic molecules. This idea has developed mainly as an extension from earlier concepts of the mode of water adsorption at clay

mineral surfaces as being mainly one of hydrogen bonding between oxygen atoms of the silicate surface and the water molecules. Such concepts

arose from indirect evidence obtained many ways and from knowledge

regarding the chemical characteristics of the compounds in question.



86



M. M. MORTLAND



With the advent of rigorous infrared absorption techniques, however, it

has often been possible to view the condition of the adsorbed molecule

directly and to sometimes draw relatively unambiguous conclusions regarding the mechanisms by which they are held at the clay mineral surface. Thus, Russell and Farmer (1964) were able to distinguish water

which is directly coordinated to the exchangeable cations from more

labile water in outer spheres of coordination.

From the studies of a large number of polar molecules adsorbed on clay

minerals, it is quite evident that the nature of the saturating cation on

the exchange complex plays a decisive role in the adsorption process.

This was evident in the preceding section on protonation, where it was

shown that the kind qf exchangeable cation with its associated water

molecules determined the acidity of the clay surface and therefore protonation processes. So also they serve as adsorption sites for polar nonionic molecules by ion-dipole or coordination types of interaction. An

example is given in Fig. 1, where the infrared absorption spectrum of

ethylamine on Cu-montmorillonite is shown in curve 3. This particular

complex had an intense blue color, was stable in the air, and the proportion of amine to copper was 4 : 1 indicating a square-planar complex.

The greater affinity the exchangeable cations have for electrons, the

greater will be the energy of interaction with polar groups of organic

molecules capable of donating electrons. Thus, transition metal cations

on the exchange complex having unfilled d orbitals will interact strongly

with electron supplying groups. In the case of molecules such as water and

ammonia, the solvation of the exchangeable cations on the clay surface

is the most energetic and therefore the primary mechanism of adsorption. Where there is not an exchangeable cation in the interlamellar

regions to solvate, there is no expansion of 2: 1 type minerals, i.e., talc

and pyrophyllite. In accordance with this the heats of wetting of clay

minerals are generally in relation to the solvation energy of the exchangeable cation (Keay and Wild, 196 1;Kijne, 1969). The preceding discussion

does not exclude other types of interaction with the silicate surface as

additional mechanisms of adsorption, but it is apparent that they are

generally weaker and come about after ligand positions with exchangeable cations are occupied. These weaker interactions are of greatest importance in clay systems where ions of relatively low solvation energy

are on the exchange complex.

The classical view of the adsorption of alcohols by clay minerals has

been one of hydrogen bonding to the oxygens of the silicate surface. In

fact, the use of ethylene glycol and glycerol for the measurement of

specific surface of clays was predicated on this mode of interaction. Some



CLAY-ORGANIC



COMPLEXES A N D INTERACTIONS



87



earlier workers, however, have recognized the possible influence of the

exchangeable metal ions, as for example, Glaeser ( 1 954), who observed

retention of methanol and ethanol by clays to be a function of the exchangeable cation. McNeal ( 1 964) in working with ethylene glycol

showed significant effects of exchangeable cation on the retention by

clay minerals. So also Bissada etal. (1967) showed the effect of exchangeable cation on clay interactions with alcohol. Direct observation of

alcohol-clay complexes with infrared absorption were made by Dowdy

and Mortland ( 1967,1968), Ovcharenko etal. ( I967), and Tarasevich etal.

( 1967). The complex formed between ethylene glycol and exchangeable

Cu(I1) ions in montmorillonite showed OH stretching vibrations of the

glycol displaced from around 3360 cm-I in the liquid to 2650 and 2750

cm-l (Dowdy and Mortland, 1968). These bonds result from direct coordination of the glycol to the Cu(I1) through the oxygen atoms. The result of this interaction is to lower the force constant between the 0 and H

with the resulting lowering of OH stretching frequencies. The degree to

which OH stretching vibrations were lowered was found to be related to

the solvation energy of the cation. These results prove the importance

of ion-dipole reactions for alcohols on clay surfaces since if hydrogen

bonding were the adsorption mechanism, the infrared spectrum of the

adsorbed alcohol should remain the same regardless of the exchangeable

cation. In looking at the interaction between water and alcohols on clay

surfaces it was shown by Dowdy and Mortland ( 1 967, 1968) that they

both compete for ligand positions around the cation. Ethanol or ethylene

glycol can completely dehydrate the clay mineral or on the other hand,

water can displace the alcohol according to the mass action requirements:



where M is the exchangeable cation and ROH is some alcohol. Thus

while water has been shown to be retained on clays to high temperatures

especially by the more highly polarizing cations (Fripiat et af., 1960;

Russell and Farmer, 1964), the clay surface can easily be dehydrated at

low temperatures by introducing a polar molecule which competes with

water for ligand sites around the exchangeable cation.

Ion-dipole or coordination types of interaction on clays have been

noted for a wide group of other polar molecules, i.e.: NH3 (James and

Harward, 1962; Cloos and Mortland, 1964; Russell, 1965); ketones

(Rios and Rodrigues, 196 I ; Bissada et af., 1967; Parlitt and Mortland,

1968); urea and amides (Mortland, 1966; Tahoun and Mortland, 1966b;

W. J. Farmer and Ahlrichs, 1969); pyridine (V. C. Farmer and Mort-



88



M. M. MORTLAND



land, 1966); nitrobenzene (Yariv et al., 1966); amino acids (Fripiat et al.,

1966); amines (V. C. Farmer and Mortland, 1965); 3-aminotriazole

(Russell et al., 1968a); ethyl N,N-di-n-propylthiolcarbamate(Mortland

and Meggitt, 1966). For molecules of the amide or urea type, there are

two most likely sites of interaction with an exchangeable cation, the oxygen of the carbonyl group and the amide nitrogen. It has been possible

with infrared absorption to distinguish between these two bonding sites.

The structure of these compounds involves resonance between the following forms:

0



0-



When the formation of oxygen-to-metal bond occurs, the contribution of

structure (I) will decrease, and this will result in more double-bond character for the CN bond and more single-bond character for the CO bond.

The result of this is to decrease the CO stretching frequency and to raise

the CN stretching frequency. On the other hand, when coordination

occurs through the nitrogen the contribution of structure (11) will decrease with resulting increase in the double-bond character of the C O

bond and increasing single-bond character of the CN linkage. The chief

interaction for this group of compounds seems to be through the oxygen

of the carbonyl, although for urea there is some indication that when

alkali metal or alkaline earth cations saturate the exchange complex of

the clay, interaction may be through the nitrosen, but definitely through

the carbonyl for transition metal cations. The amount of decrease in the

CO stretching frequency is usually in proportion to the electrophilic

nature of the cat'ion. Thus, greatest shifts occur when the molecules interact with transition metal cations and least for alkali metal and intermediate for alkaline earth types.

Water has its influence on the coordination of urea and amide type

molecules at clay surfaces. Usually little or none of these compounds

will be adsorbed on clay minerals from a water suspension. However, as

the amount of water is reduced in the system, the polar molecules can

compete with water for ligand positions around the metal cation, first

through a water bridge (to be discussed in a following section), then in

many cases a direct metal-organic interaction. Thus it is obvious that the

chemistry of clay surfaces can be quite different in water suspensions as



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

II. Bonding Mechanisms in Clay-Organic Complexes

Tải bản đầy đủ ngay(0 tr)

×