Abstract--Methods previously used to distinguish between water adsorbed on external surfaces and in the interlamellar space of Na-montmorillonite during adsorption and desorption of water vapor have been extended to a set of homoionic Li-, Na-, K-, Rb-and Cs-montmorillonite. The textural and structural features have been investigated at different stages of hydration and dehydration using controlled-rate thermal analysis, nitrogen adsorption volumetry, water adsorption gravimetry, immersion microcalorimetry and X-ray powder diffraction under controlled humidity conditions. During hydration, the size of the quasi-crystals decreases from 33 layers to 8 layers for Na-montmorillonite and from 25 layers to 10 layers for K-montmorillonite, but remains stable around 8-11 layers for Cs-montmorillonite. Each homoionic species leads to a one-layer hydrate, which starts forming at specific values of water vapor relative pressure. Li-, Na-and K-montmorillonite can form a two-layer hydrate. By comparing experimental X-ray diffraction patterns with theoretically simulated ones, the evolution of structural characteristics of montmorillonites during hydration or desorption can be described. Using structural and textural data, it is shown that during adsorption: (1) the rate of filling of interlamellar space of the one layer hydrate increases with the relative pressure but decreases with the size of the cations; and (2) the different hydrated states are never homogeneous.
Due to their crystallochemical properties, clay minerals feature
different types of structural surfaces
which have their own adsorption energy distribution. To study that
type of surface heterogeneity, the DIS
(derivative isotherm summation) method has been developed by us.
Now, a modified version of the DIS
method is derived by using the Jagiełło-Rudziński approach
and assuming that the local energy distributions
are represented by the Dubinin−Asthakov distributions. Two
different types of isotherms equations are
used, one to describe adsorption in micropores and another one for
describing adsorption on external
surfaces. The derivatives of experimental adsorption isotherms
with regard to ln(p/p
s) are
simulated by
combinations of the derivatives of corresponding local adsorption
isotherms. This best fit provides
information on adsorption capacity of the local existing domains, on
the symmetry of their energy distribution
function, and on the parameters characterizing the lateral interactions
in each adsorption domain. Using
the new equations for the local isotherm derivatives allows now to
simulate very accurately derivatives
of experimental adsorption isotherms, obtained by using our
high-resolution quasi-equilibrium volumetric
technique. This was proven in the case for three different well
characterized clay minerals: a structural
microporous one (palygorskite) and two nonporous lamellar ones
(kaolinites). The obtained parameters
allow a description of the adsorption energy distribution of their
different surfaces, their textural parameters,
as well as the energy distrubution in micropores for the microporous
samples. In addition to the experimental
adsorption isotherms, the related experimental heats of adsorption were
employed as a second independent
source of information about the energetic heterogeneity of the studied
clay minerals. Using the parameters
determined from adsorption isotherms, the corresponding isosteric heats
of adsorption were calculated
and compared with experimental values. The simultaneous good fit
of the experimental isotherm derivatives
and of the experimental heats of adsorption was a solid check for the
correctness of the determined parameters
characterizing the adsorption energy distributions and the lateral
interactions between the adsorbed
molecules.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.