O N T H E M E C H A N I S M OF F O R M A T I O N OF
M ONTMORILLONITE-ACETONE
COMPLEXES
BY RACHAEL GLAESER.
(Translated by D. M. C. MACEWAN).
This work is part of a more general study of interactions between
montmorillonite and organic substances, carried out in connection
with the fundamental results of MacEwan and Mackenzie. The
object of this work was the study of adsorption of acetone in the
vapor phase.
Experimental method.--Samples o f sodium and calcium mont=
morillonites were placed in pyrex tubes provided each with a ground
glass stopper and a tap. Each tube was joined to a vacuum p u m p
through a desiccating column and heated for several hours to 280~
so as to desorb the water contained in the clay and on the glass walls.
At the conclusion of the evacuation, the tube was opened while hot,
so as to introduce a recipient containing acetone in solution in castor
oil (which had been previously degassed in vacuum). The tube was
again closed, re-evacuated, cooled, then isolated from the vacuum
p u m p and put into a thermostat (40~
After eight days, each tube
was opened and the acetone content of the montmorillonite and the
castor oil determined. Thc final concentration of acetone in the oil
allowed the equilibrium vapor pressure to be determined, thanks to
preliminary measurements by Mr Vandon.
A series of identical tubes prepared in the same way contained pre=
parations of montmorillonite for X-ray exposures. These preparaations had been closed off as soon as equilibrium was reached. The
X-ray diagrams were taken with copper Ka radiation given by a
focusing monochromator.
An effort was made to carry out the adsorption in as anhydrous
conditions as possible. Unfortunately, it is not possible to remove all
adsorbed water from montmorillonite without altering it also by the
removal of some of its structural hydroxyls. The evacuation at
250~ in vacuum left 0.5 to 1 per cent. of water. To intercept the
traces of water which could exist in the acetone-oil solution, as well
as to preserve the atmosphere in the interior of the tubes, a buffer of
CaC12 (of mass equal to 40 times that of the montmorillonite) was
placed between the solution and the clay samples. The use of a more
active dehydrating agent was made impossible by the presence of
acetone vapor. It may be estimated that the relative humidity inside
the tubes was of the order of 1 to 2 per cent. The uptake of water
by the montmorillonite was of the order of 1 per cent. in eight days.
Results.--(a) 40~ Adsorption isotherm--The adsorption of acetone
as a function of relative pressure varies according to an S-shaped
curve of the well-known form. The Na-montmorillonite isotherm
differs very little from that of the Ca-montmorillonite. The total
adsorption surface determined by the Brunauer-Emmett-Teller
88
MONTMORILLONITE-ACETONE COMPLEXES
89
method (taking for the density of the adsorbed acetone that of the
liquid) is 350.2 sq.m./gm., the theoretical figure for the total surface of
the silicate sheets is 400.2 sq.m./gm.
(b) X-ray diagrams--The diffraction diagrams were designed to
measure the variation of the d space (in MacEwan's notation),
occupied by the molecules of acetone between the silicate sheets, as a
function of the relative pressure P/Po.
It is found that d is constant for all values of PIPo between 0.1 and 1.
A = 3 . 7 ~ for Na-montmorillonite
/1--3.9 ~ for Ca-montmorillonite
A prolonged immersion in saturated actone vapor gives the same
results. The figures obtained are very close to those observed by
MacEwan with halloysite, and correspond to the formation of a complex with one layer of acetone molecules.
(c) The part played by humidity---Exposing montmorillonite to the
simultaneous action of saturated acetone vapor (P/Po= 1) and water
vapor at 50 per cent. relative humidity, the observed value of A was
8.0 to 8.2 &, as obtained by MacEwan by the action of liquid acetone
on air-dried montmorillonite, and corresponding to the fixation of
two molecular layers.
The simultaneous action of two saturated vapors (water and acetone)
gave A=15.1 ~, which is a little less than the thickness of four molecular layers of acetone, This value of A is much bigger than that
given by saturated water vapor alone (10 ~). The complex formed is
probably not homogeneous and must be made up of a disordered
mixture of intervals with 3 and 4 molecular layers. These complexes
obtained in a moist atmosphere contain significant amounts of water.
It is found, however, that A remains about the same when the water
is removed by boiling in acetone.
To clarify these observations, it may be noted that :
(1) in the case of adsorption in an almost anhydrous medium, the
montmorillonite was slightly hydrated, with a small proportion (of
the order of ~ to ~) of intervals occupied by one layer of water
molecules.
(2) In a relative ht~midity of 50 per cent. the mineral has a variable
proportion (according to the nature of the cation) of intervals with
two molecular layers of water.
(3) In saturated water vapor X-rays show a certain proportion
(~ for Na-montmorillonite, appreciably less for Ca-montmorillonite) of hydrated intervals with 4 layers of water. The action of
the acetone vapor (P/Po~-1) on a hydrate with three layers of water
only gives a complex with two layers of acetone.
It may thus be stated that preliminary hydration is necessary for
the formation of a complex with acetone and that the form of the
complex obtained depends on the degrees of previous hydration.
The mechanism of formation of these complexes seems to be as
follows: (1) hydration; (2) solution of the acetone in the hydrate and
formation of a mixed acetone-water complex of the type described by
90
RACHEL GLAESER
Mackenzie in the case of glycerol (the value of/1 which characterizes
this complex depends on the previous degree of hydration); (3) in
some cases, elimination of water, A remaining constant.
hz.fluence of previous hydrafion in the action of liquid acetone--To
confirm the preceding, complexes w e r e formed using MacEwan's
method (action of a large excess of liquid acetone with or without
boiling):
(a) with montmorillonite which had been dehydrated as far as
possible.
(b) with montmorillonite of maximum hydration.
The results were less constant:
(a) With the almost anhydrous mineral the Ca-montmorillonite
always gave a two-layer complex (A--8.1 ~); the Na-montmorillonite gave in 50 per cent. of the cases A = 3.9 ,~ (one-layer complex),
and in 50 per cent. of the cases 2 = 8 . 1 ~.. No intermediate results
were observed. It seems that these variable results were due to traces
of water which were contained in the acetone and difficult to climinate. This suggestion is in accord with the case of Ca-montmorillonite, which more readily takes up water in the dry state than does
Na-montmorillonite.
(b) Before being treated with liquid acetone the montmorillonite
was hydrated either by long soaking in saturated water vapor or by
direct contact with liquid water. The values obtained were, for
Na-montmoriUonite
A - - 12.5 ~ (3-layer complex)
/1--=-17.6 ~ (irregular 4- and 5-layer complex),
and for Ca-montmorillonite
/1 =~ 15.1 ~ (irregular 3-and 4-layer complex).
In 50 per cent. of the cases the two montmorillonites gave a two-layer
complex (d :-8.1 4).
Apparently one here has two superimposed effects: first of all, the
acetone tends to penetrate into the montmorillonite-water complex
to form a mixture with enlarged A, secondly the liquid acetone acts as
a dehydrating agent and lowers the degree of previous hydration, so
diminishing the number of molecular layers in the mixed complex
formed.
The preponderance of one or the other of these two contrary effects
must depend in a very sensitive way on the rapidity of diffusion of the
acetone into the water surrounding the clay particles. The second
effect does not occur in the vapor phase because the high degree of
hydration is there maintained by the presence of saturated water
vapor.
Ecole de Physique et Chimie Industrielles,
Paris.
References.
D. M. C. MacEwan, 1948. Trans. Faraday So.c., 44, 349-67.
R. C. Mackenzie, 1948. Ibid., 44, 368.