Deuterium Nuclear Magnetic Resonance Studies of Water Molecules Restrained by their Proximity to a Clay Surface

Jean Grandjean1 and Pierre Laszlo2
1 Laboratoire de Chimie Fine aux Interfaces, Institut de Chimie B6 Université de Liège, Sart Tilman par 4000 Liège, Belgium
2 Laboratoire de Chimie Fine, Biomimétique et aux Interfaces Ecole Polytechnique, 91128 Palaiseau, France

Abstract: Observation of quadrupolar splittings for 2H and 17O nuclei from D2O solvent molecules in suspensions of the Ecca Gum BP bentonite points unambiguously to the ordering of water molecules with respect to a static magnetic field. The application of the magnetic field apparently orients the clay platelets as tactoids. Some water molecules adhere to the platelets and are oriented. The magnitude of the quadrupolar splitting depends strongly on the nature of the interstitial cations. The sign of the splitting changes as the ratio of divalent to monovalent cation increases, reflecting a switch in the microdynamics of water molecules next to the plates. With the entry of divalent cations, the clay becomes more cohesive due to the vertical electrostatic attraction of the counterions to the charged plates. More of these hydrated divalent ions compared with monovalent ions condense onto the charged sheet, thereby increasing metal coordination to those water molecules in contact with the clay surface. The dominant reorientation mode of the water molecules switches from rotation of water molecules around hydrogen bonds to the charged sheets to rotation around the electrostatic bond to the divalent metal cation. The effect is spectacular and important in understanding the chemical properties of cation-exchanged montmorillonites.

Key Words: Nuclear magnetic resonance • Orientation • Quadrupolar splitting • Smectite • Surface acidity • Water

Clays and Clay Minerals; October 1989 v. 37; no. 5; p. 403-408; DOI: 10.1346/CCMN.1989.0370503
© 1989, The Clay Minerals Society
Clay Minerals Society (