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

Abstract

Abstract--Observation of quadrupolar splittings for 2H and 170 nuclei from D20 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.

Octahedral or Tetrahedral Isomorphous Substitution: How it Affects Water and Acetonitrile Interaction with a Clay, as Seen by NMR

“…Indeed, the magnitude and the sign of D are both highly sensitive to the content of exchangeable divalent cations (Ca 2+, Mg 2+ ) when a typical montmorillonite (Wyoming) is suspended in heavy water. We have interpreted this key initial observation in terms of fast exchange of water molecules between the bulk (no splitting) and two interfacial sites having residual anisotropies A of opposite signs (Grandjean and Laszlo 1989;Delville et al 1991). At the clay interface, water toolCopyright 9 1994, The Clay Minerals Society ecules either form hydrogen bonds to the negatively charged surface or interact with exchangeable cations.…”

“…The basis of the determination is to suspend the clay particles in heavy water and to insert the sample in the probe of a routine high-resolution nuclear magnetic resonance (NMR) spectrometer. In the several tesla-strong magnetic field thus provided, the clay platelets orient themselves within a few minutes (Grandjean and Laszlo 1989;Grandjean and Laszlo 1990). This puts those water molecules that are attached to the clay platelets in an anisotropic environment which in turn endows the quadrupolar ZH and 170 nuclei with non-degenerate transitions.…”

“…This puts those water molecules that are attached to the clay platelets in an anisotropic environment which in turn endows the quadrupolar ZH and 170 nuclei with non-degenerate transitions. Since the quadrupolar interaction is very large (of the order of magnitude of 0.1-10 MHz), both these nuclei display residual quadrupolar splittings (Grandjean and Laszlo 1989;Grandjean and Laszlo 1990;Delville et al 1991), even in the presence of fast exchange between the interfacial water molecules and the bulk solution. The measured splitting D is the product of three terms (Halle and Wennerstrom 1981).…”

“…At the clay interface, water toolCopyright 9 1994, The Clay Minerals Society ecules either form hydrogen bonds to the negatively charged surface or interact with exchangeable cations. The presence of divalent cations favors the latter cationic site (Grandjean and Laszlo 1989;Delville et al 1991). When clay minerals having isomorphous substitution confined to a single site, either octahedral or tetrahedral, are suspended in water, the resulting residual quadrupolar splittings are nearly-invariant to the relative amounts of univalent and divalent interlamellar counterions: with hectorite, saponite, and another montmorillonite (from Gonzales County, Texas), the water deuteron splitting is only weakly affected by the presence of exchangeable cations (Grandjean and Laszlo 1994a).…”

“…In typical experiments, 5000 transients were acquired in 16 K points, with a spectral width of 1000 Hz, and were transformed with 32 K points (digital resolution = 60 mHz). Deconvolution procedures assuming all lineshapes to be Lorentzian, were applied if the doublet was insufficiently resolved (Grandjean and Laszlo 1989;Delville et al 1991). Hectorite (from Hector, California), purified according to standard procedure (Fripiat et al 1982), is affected by isomorphous cationic substitution in the octahedral layer only (Xu and Harsh 1992).…”