Ca/Na Montmorillonite: Structure, Forces and Swelling Properties.

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Bibtex

@article{d6640ac9cd4649e58d10c92882d748a1,
title = "Ca/Na Montmorillonite: Structure, Forces and Swelling Properties.",
abstract = "Ca/Na montmorillonite and natural Wyoming bentonite (MX-80) have been studied experimentally and theoretically. For a clay system in equilibrium with pure water, Monte Carlo simulations predict a large swelling when the clay counterions are monovalent, while in presence of divalent counterions a limited swelling is obtained with an aqueous layer between the clay platelets of about 10 A. This latter result is in excellent agreement with X-ray scattering data, while dialysis experiments give a significantly larger swelling for Ca montmorillonite in pure water. Obviously, there is one {"}intra-lamellar{"} and a second {"}extra-lamellar{"} swelling. Montmorillonite in contact with a salt reservoir containing both Na(+) and Ca(2+) counterions will only show a modest swelling unless the Na(+) concentration in the bulk is several orders of magnitude larger than the Ca(2+) concentration. The limited swelling of clay in presence of divalent counterions is a consequence of ion-ion correlations, which reduce the entropic repulsion as well as give rise to an attractive component in the total osmotic pressure. Ion-ion correlations also favor divalent counterions in a situation with a competition with monovalent ones. A more fundamental result of ion-ion correlations is that the osmotic pressure as a function of clay sheet separation becomes nonmonotonic, which indicates the possibility of a phase separation into a concentrated and a dilute clay phase, which would correspond to the {"}extra-lamellar{"} swelling found in dialysis experiments. This idea also finds support in the X-ray scattering spectra, where sometimes two peaks corresponding to different lamellar spacings appear.",
author = "Mo Segad and Bo J{\"o}nsson and Torbj{\"o}rn {\AA}kesson and B. Cabane",
note = "The information about affiliations in this record was updated in December 2015. The record was previously connected to the following departments: Theoretical Chemistry (S) (011001039)",
year = "2010",
doi = "10.1021/la9036293",
language = "English",
volume = "26",
pages = "5782--5790",
journal = "Langmuir",
issn = "0743-7463",
publisher = "The American Chemical Society (ACS)",
number = "8",

}