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

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Ca/Na Montmorillonite: Structure, Forces and Swelling Properties. / Segad, Mo; Jönsson, Bo; Åkesson, Torbjörn; Cabane, B.

In: Langmuir, Vol. 26, No. 8, 2010, p. 5782-5790.

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Segad, Mo ; Jönsson, Bo ; Åkesson, Torbjörn ; Cabane, B. / Ca/Na Montmorillonite: Structure, Forces and Swelling Properties. In: Langmuir. 2010 ; Vol. 26, No. 8. pp. 5782-5790.

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TY - JOUR

T1 - Ca/Na Montmorillonite: Structure, Forces and Swelling Properties.

AU - Segad, Mo

AU - Jönsson, Bo

AU - Åkesson, Torbjörn

AU - Cabane, B.

N1 - 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)

PY - 2010

Y1 - 2010

N2 - 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.

AB - 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.

U2 - 10.1021/la9036293

DO - 10.1021/la9036293

M3 - Article

VL - 26

SP - 5782

EP - 5790

JO - Langmuir

JF - Langmuir

SN - 0743-7463

IS - 8

ER -