Download Clay Minerals (1976) 11, 331. NOTE COMMENTS ON THE PAPER

Clay Minerals (1976) 11, 331.
NOTE
C O M M E N T S ON T H E PAPER ' S U R F A C E ACIDITY OF I M O G O L I T E A N D
ALLOPHANE'
INTRODUCTION
Very recently Henmi & Wada (1974) measured the surface acidity of montmorillonite
by using Hammett indicators. As the Hammett acidity function, Ho, includes the
activity of surface hydrogen (Br/3nsted) acids only (Benesi, 1956), three fundamental
conditions must be fulfilled for the measurement of surface acidity of montmorillonite
with Hammett indicators; (i) montmorillonite surfaces must act as proton donors,
(ii) the proton transfer on clay surfaces must strictly only be a function of relative
acidities/basicities of montmorillonite surfaces and the indicator base, and (iii) the
phenomenon of acid colour development must be only due to protonation. Unfortunately, none of the three conditions is fulfilled in montmorillonite and consequently the measurement of its surface acidity by Hammett indicators is questionable.
Surfaces of montmorillonite are basic. The centres of positive charge deficiency
in montmorillonite are associated either with replacements in the octahedral layer
or with replacements in the tetrahedral layer. In the first case the negative charge is
distributed over at least the ten surface oxygens of the four silicon-oxygen tetrahedra
whose apices are linked to a site of octahedral substitution, and, in the second, over
at least the three surface oxygens coordinated to tetrahedral A13+. Consequently,
the surface oxygens of montmorillonite are electron donors and are capable of
hydrogen bonding with interlamellarly adsorbed polar molecules. In fact, in montmorillonite the negatively charged lattice groups are basic and act as proton acceptors.
Because of the basicity of lattice groups there is a competition for the proton between
the interlayer material(s) and the clay sheets. Therefore, it should not be surprising
to note that on dehydration protons from H-montmorillonites (H3 O+, pKa = - 1.7)
(Russell & Fraser, 1971), protonated urea (pKb = 13.82) (Mortland, 1966), anilinium
ions (pKa = 4.6) (Yariv & Heller, 1973), and NH4+ (pKa = 9.25) (Russell &
Farmer, 1964) migrate towards the lattice. If the observation of Johnson & Rumon
(1965) (who found that proton transfer occurred between the acid and base only
when the ApKa was about 3.75 or greater) holds true, then, these observations
suggest that under dehydrated conditions montmorillonite lattices should have a
pKa value of about 13. Whether this is true or not, strong bases like cyclohexylammonium ions (pKa = 10.64) (Yariv & Heller, 1973), ethylenediammonium ions
(pKa = 10.65) (Cloos & Laura, 1972), and ethylammonium ions (pKa = 10-75)
(Chaussidon & Calvet, 1965; Durand, Fripiat & Pelet, 1972) do not lose their
protons even on heating under vacuum.
The acidity of proton sources on montmorillonite surfaces is well known. In
montmorillonite used by Henmi & Wada (1974) there are three sources of protons,
e.g. hydrogen, NH4+- and interlamellar water. Theoretically NH4-montmorillonite
332
Note
should transfer its proton only to bases having pKa much larger than 9-25. However,
it has been observed that on montmorillonite the surface N H ~ (pKa = 9-25)
transfers its proton to 3-aminotriazole (pKa = 4.17) (Russell, Cruz & White, 1968)
and pyridine (pKa = 5.23) (Raman & Mortland, 1969). These observations have
been explained by a mass effect. Since indicators are always applied in minimum
possible amounts they can be protonated by a large concentration of protons even
from sources with lower acidity, e.g. with higher pKa values. This fact jeopardizes
the very concept of measurement of pH by obse;ving the colour of a protonated
base on clay surfaces. H-montmorillonite should not protonate a base having a
pKa less than - 1 . 7 . Moreover, it is worth considering that even on H-montmorillonite, Mortland (1966) could not observe protonation of N H 2 groups of urea
(pKa = 13-82). The observation of Yariv, Heller & Kaufherr (1969) with aniline
(pKa = 4-6) was similar.
According to the prevailing view, the water associated with the exchangeable
cations on montmorillonite surfaces acts as a proton source and its proton donating
ability increases with increasing ionic potential (valence/ionic radius) of the central
metal cation. Unfortunately, this is a misleading oversimplification of the many
phenomena which occur simultaneously when organic bases are adsorbed on clay
surfaces. In fact, reactions of metal cations with water are reactions of complex
cation formation. Complex cation formation involves competition among the water
molecules, the anions and the complexing agent (e.g. ligand) in the coordination
sphere of metal cations. Water molecules are attached to a metal cation by donation
of the unshared electron pair on the oxygen atom and in the absence of anions and
complexing agent aquo-complexes (or hydrated cations) are formed. The exchangeable
cations on clay surfaces form the pure aquo-complexes due to the absence of buffering
anions in the adsorbed layer, the negative charge being inherent to the clay phase.
However, the water in aquo-complexes cannot act as a proton source under all
circumstances. When the complexing agent forms the stronger bond with the metal
cation, the water molecules are replaced by the complexing agent and a real coordination or chelation complex is formed. On montmorillonite surfaces a variety of
organic bases form coordination or chelation complexes with transition, alkaline
earth and alkali metal cations depending on the degree of hydration of the system
and basicity of the organic molecule (Laura & Cloos, 1970, 1975a, 1975b; Yariv
et al., 1968 ; Yariv, Heller & Kaufherr, 1969; Mortland, 1966; Farmer & Mortland,
1965, 1966; Heller & Yariv, 1969).
N o doubt the aquo-complexes are acids, but, the hydrolysis of aquo-complexes
is not a 'spontaneous' ionization. Rather, the H + is 'pulled off' by an approaching
base (Reutov, 1967). Therefore, the hydrolysis of an aquo-complex by an approaching
base can be formulated as follows:
[M(H 2O),] +" + n B ~ M(OH),, + nHB +
Unfortunately, whenever there is a question of hydrolysis of aquo-complexes under
the influence of a base most of the workers consider only the left hand side of the
equation. But the reaction will also proceed further to the right on clay surfaces if
Note
333
the hydroxide, M(OH)n, is removed as insoluble precipitate. This is especially true
for trivalents and divalents and has been confirmed by Russell (1965) and Laura &
Cloos (1975b). Consequently, the degree and extent of protonation of bases on
montmorillonite saturated with trivalents and divalents might not be a true index of
the acidity of the water coordinated to these cations.
The development of acid colour occurs on basic surfaces too. Morton & Bolton
(1953) observed that on organo-sodium reagents, potassium and sodium hydroxide
surfaces, those azo indicators which have a replaceable hydrogen will give the acid
colour, probably due to the replacement of the hydrogen of the azo indicator by the
alkali-metal cation. In the experiments of Henmi & Wada (1974) this phenomenon
cannot be completely excluded. Moreover, on montmorillonite saturated with alkali
metal cations there is no protonation of NH 3 (Russell, 1965) and ethylenediamine
(Laura & Cloos, 1975a). All the indicator bases used by Henmi & Wada (1974)
have pKa lower than that of NH4+ and consequently are not expected to protonate
on montmorillonite saturated with alkali metal cations.
The number of acid sites in montmorillonite should be equal to its cation exchange
capacity. The estimation of the number of acid sites by n-butylamine titration method
involves the protonation of amine by proton sources on montmorillonite surfaces.
n-Butylamine has a pKa = 10.61 and is expected to take all protons from H- and
NH4-montmorillonite. Cloos, Laura & Badot (1975) observed a stoichiometric
transfer of protons from NH4-montmorillonite to ethylenediamine (pKa = 10.65)
even in aqueous suspension. In case of trivalent ions also there might be a supply of
protons equal to cation exchange capacity, because in Al-montmorillonite AI(OH)3
is precipitated quantitatively (Russell, 1965) and the same could be expected for
Fe-montmorillonite. However, from the pKa of aquo-complexes of divalent and
monovalent ions (e.g. [Mg(H20)x] 2+ = 11.4; [Ca(H20)x] 2+ = 12-7; [Na(H20)x] +
= 14-6) it is not expected that montmorillonites saturated with these cations could
protonate n-butylamine. Any adsorption of protonated amine in these clays could
be only due to preferential adsorption of the organic cations in interlamellar space
as argued by Farmer & Russell (1967) and confirmed by Laura & Cloos (1975a, b).
Moreover, Benesi (1957) found only 4 and 9 ~ adsorption of amine (of the total
cation exchange capacity) on Na- and Ca-montmorillonite respectively. It would
also have been very interesting if Henmi & Wada (1974) had determined the number
of acid sites on Cu-montmorillonite by the amine titration method.
Moreover, even if there is no precipitation of hydroxides on hydrolysis of aquocomplexes, hydroxyl will form inner-sphere complexes.
HO
OH 2
H20
OH 2
HO
OH 2
H20
OH 2
\ /
H20--A1--OH
/ \
\ /
HO--Ca--OH
/ \
H20
OH 2
H20
OH 2
\ /
HEO--Na---OH
/ \
In inner-sphere complexes, the negative ion (anion) is immediately adjacent to the
cation. Thus, the charge of the cation is mainly neutralized by the anion itself and
334
Note
the r e m a i n i n g water molecules are little affected irrespective o f the n a t u r e o f the
central m e t a l cation. Consequently, it can be said t h a t the different exchangeable
cations m a y affect differently only the degree o f dissociation o f c o o r d i n a t e d water
b u t n o t the extent. Since the charge o f the exchangeable cations is i n v a r i a b l y equal
to the c a t i o n exchange c a p a c i t y o f the clay, the available p r o t o n w o u l d never exceed
t h a t limit even if every t y p e o f cation increases the degree o f dissociation o f coo r d i n a t e d water on clay surfaces.
The p h e n o m e n o n o f p r o t o n a t i o n o f organic bases is very i m p o r t a n t to chemists,
clay mineralogists a n d soil scientists. Short b u t informative reviews on this subject
have been r e p o r t e d b y L a u r a (1974, 1975).
R. D. LAURA
D e p a r t m e n t o f Chemistry,
University o f Georgia,
Athens, G e o r g i a 30602, U.S.A.
2 April, 1976
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