Article pubs.acs.org/JPCC
Dynamics of Water Confined in Clay Minerals S. Le Caer̈ ,*,† M. Lima,‡ D. Gosset,§ D. Simeone,§ F. Bergaya,∥ S. Pommeret,† J.-Ph. Renault,† and R. Righini‡ †
Institut Rayonnement Matière de Saclay, Service Interdisciplinaire sur les Systèmes Moléculaires et les Matériaux, UMR 3299 CNRS/CEA SIS2M Laboratoire de Radiolyse, Bâtiment 546, F-91191 Gif-sur-Yvette Cedex, France ‡ University of Florence, Polo Scientifico, Via Nello Carrara 1, I-50019 Sesto Fiorentino, Italy § CEA/DEN/SRMA/LA2M, Matériaux Fonctionnels pour l’Energie, Laboratoire de Recherche Commun CEA-CNRS-ECP, CEN Saclay, F-91191 Gif sur Yvette, France ∥ CNRS-Université d’Orléans, CRMD 1b rue de la Férollerie F-45100, Orléans, France S Supporting Information *
ABSTRACT: Ultrafast infrared spectroscopy of the O−D stretching mode of dilute HOD in H2O probes the local environment and the hydrogen bond network of confined water. The dynamics of water molecules confined in the interlayer space of montmorillonites (Mt) and in interaction with two types of cations (Li+ and Ca2+) but also with the negatively charged siloxane surface are studied. The results evidence that the OD vibrational dynamics is significantly slowed down in confined media: it goes from 1.7 ps in neat water to 2.6 ps in the case of Li+ cations with two water pseudolayers (2.2−2.3 ps in the case of Ca2+ cations) and to 4.7 ps in the case of Li+ cations with one water pseudolayer. No significant difference between the two cations is noticed. In this 2D confined geometry (the interlayer space being about 0.6 nm for two water pseudolayers), the relaxation time constants obtained are comparable to the ones measured in analogous concentrated salt solutions. Nevertheless, and in strong opposition to the observations performed in the liquid phase, anisotropy experiments evidence the absence of rotational motions on a 5 ps time scale, proving that the hydrogen bond network in the interlayer space of the clay mineral is locked at this time scale.
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composition of the layers.12,13 The structure but also the dynamics of interlayer water in Mt has been studied using static IR spectroscopy,14,15 quasi-elastic neutron scattering,9 nuclear magnetic resonance,16 X-ray diffraction (XRD),17−19 and computer simulations.20 The results evidence that interlayer water diffuses more slowly than bulk water and that the water structure in the interlayer space is controlled by the nature of the cation, depending on its hydration energy. Mt provides a unique 2D confining system, with a negatively charged surface, a cation whose nature can be controlled in the interlayer space, and the ability to adjust the number of water molecules in the interlayer space by controlling the relative humidity of the sample environment, and hence, the distance of the interlayer space. The fundamental question addressed in this work is: are the dynamics of water in these heterogeneous systems different from those obtained in analogous liquid concentrated salt solutions; that is, has the presence of the interface an influence on water dynamics? Moreover, the possible influence of the nature of the cation on water dynamics
INTRODUCTION When water is confined in a nanoscale space, its structural and dynamical properties are modified with respect to those of bulk water.1−3 Nevertheless, much work on confined water focuses on biological or soft matter confining systems.4,5 Other systems of interest include for their fundamental interest and practical relevance, oxides and clays.6−9 The water retention and ion exchange properties of clay minerals are responsible for many technological applications, including for instance catalysis and radioactive waste disposal.10,11 Among clay minerals, montmorillonite (Mt) due to its swelling and sealing ability is an important component in clay barriers, used to minimize transport of contaminants from waste disposal sites. Mt belongs to the group of smectites and exhibit a layered structure. These layers are composed of two tetrahedral sheets (SiO 4 ) which sandwich a third sheet of octahedrally coordinated cations. Isomorphic substitution in the octahedral or tetrahedral layers creates a negative charge which is balanced by hydrated interlayer cations (Na+, Li+, or Ca2+, etc.) (Scheme 1). When a Mt is exposed to water, water molecules penetrate between the layers and force them apart leading to a swelling. The extent of hydration varies greatly and depends on many factors related to the nature of interlayer cations and the © 2012 American Chemical Society
Received: March 15, 2012 Revised: May 12, 2012 Published: May 15, 2012 12916
dx.doi.org/10.1021/jp302520t | J. Phys. Chem. C 2012, 116, 12916−12925
The Journal of Physical Chemistry C
Article
Scheme 1. Idealized Picture of a Montmorillonite (Mt) Structurea
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The red tetrahedra (resp. blue octahedra) consist of silicon (resp. aluminium) atoms surrounded by oxygen atoms. In the interlayer space of the Mt, cations (in green) are roughly surrounded by three water pseudo-layers. With one water pseudo-layer, less water molecules are around the cations in the first solvation shell.
The CEC (cation exchange capacity) of the Ca2+−Mt is 119 meq/100 g (on a calcined clay mineral basis23,24), and the CEC of the Li+−Mt is 116 meq/100 g. As the CEC are very close, both Mt can be confidently compared. Self-supporting films of these powdered Mt were then prepared to carry out the time-resolved infrared experiments. The films obtained were uniform and yellow-brown, with a thickness of approximately 100 μm. Chemicals. D2O and the different salts (CsF, LiCl, LiI, NaI, and NaCl) were purchased from Sigma-Aldrich and used as received. Aqueous salt solution was prepared by directly dissolving the salt in water or in the isotopically mixed water solution. Controlling the Relative Humidity. To determine the appropriate relative humidity (RH) conditions to perform the time-resolved infrared experiments, XRD diagrams were collected. Mt powders, instead of self-supporting films, were used as it was more convenient to carry out the XRD experiments under a controlled relative humidity using powders. The Mt powders were placed in an airtight chamber with an environment of a known relative humidity created by a saturated salt solution, until the equilibrium is reached. At room temperature, CsF, LiCl, LiI, NaI, and NaCl salts ensure a relative RH of 4, 11, 19, 40, and 75%, respectively. X-ray Diffraction. To avoid any modification of the RH during the measurements, the Mt powders were sealed in glass capillaries under a constant RH. The diffraction patterns of Mt were collected on a D8 Advance Bruker setup in transmission geometry using a Cu Kα radiation (λ = 1.5405 Å). The diffraction diagrams were collected on a 2θ image varying from 2 to 120°. A polynomial correction of the background and a stripping of the Cu Kα radiation were performed on all collected diffraction diagrams. These clay minerals exhibit a highly disordered lamellar structure. Diffraction peaks are thus highly asymmetric. It was then not possible to perform a full pattern matching analysis of these diffraction diagrams.
was investigated by working with monovalent (Li+) and divalent (Ca2+) cations in the interlayer space. IR spectroscopy is a very versatile tool, enabling to work on a broad range of systems, even natural ones which can be difficult to study because of their intrinsic complexity. Moreover, the infrared (IR) stretching band is very broad, corresponding to a wide variety of H bonding, and IR spectroscopy is then an appropriate tool to study the structure of water.8 In this study, the dynamical behavior of water interacting with cations in the interlayer space of Mt is followed by means of time-resolved IR spectroscopy as this technique provides useful information on the structure and dynamics of water in different environments21 and can also be applied to the study of confined media.7 However, the OH stretching mode in liquid H2O is not the ideal probe due to intra- and intermolecular couplings. Therefore D2O diluted in H2O, which enables to decouple the OD stretch from the other stretching modes, was used.
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EXPERIMENTAL SECTION Materials. Homoionic samples of the
