Water Structure and Dynamics in Smectites: X-ray Diffraction and ²H NMR Spectroscopy of Mg–, Ca–, Sr–, Na–, Cs–, and Pb–Hectorite

Abstract

Variable-temperature X-ray diffraction and 2 H NMR spectroscopy of the smectite mineral, hectorite, containing interlayer Na + , K + , Cs + , Mg 2+ , Ca 2+ , Sr 2+ , and Pb 2+ equilibrated at 43% relative humidity (RH) and mixed with 2 H 2 O to form a paste provide a comprehensive picture of the structural environments and dynamics of interlayer 2 H 2 O and the relationships of these properties to interlayer hydration state, the hydration energy and polarizability of the cation, temperature, and the formation of ice-1h in the interparticle pores. The variation in basal spacing shown by the XRD data correlates well with the 2 H NMR behavior, and the XRD data show for the first time in hectorites that crystallization of interparticle ice-1h causes a decrease in the interlayer spacing, likely due to removal of interlayer 2 H 2 O. The variation of the 2 H NMR behavior of all the samples with decreasing temperature reflects decreasing frequencies of motion for the rotation of the 2 H 2 O molecules around their dipoles, reorientation of the 2 H 2 O molecules, and exchange of the 2 H 2 O molecules between interlayer sites coordinated to and not coordinated to the cations.

Temperature dependent structure and dynamics in smectite interlayers: 23Na MAS NMR spectroscopy of Na-hectorite

“…This hectorite develops its permanent structural charge from Li + for Mg 2+ substitution in the octahedral sheet of the T-O-T layers and contains only a trace amount of paramagnetic iron, making it useful for the NMR studies. 1,9,44 The charge balancing cation in the as-received sample is predominantly Na + , with lesser amounts of K + , Mg 2+ , and Sr 2+ . It's permanent structural layer charge is À0.…”

“…37,[39][40][41][42] Because the ions and molecules in the interlayer galleries of smectite clays are structurally and dynamically disordered, their molecular scale behavior is difficult to study with diffraction methods, and in recent years experimental spectroscopic methods and computational molecular modeling have been widely employed. 1,2,5,[7][8][9][10][11][12]43 This paper reports the results of an experimental, variable temperature 23 Na nuclear magnetic resonance (NMR) and computational molecular dynamics (MD) modeling study of the structural and dynamical behavior of Na + in the widely investigated smectite clay, hectorite. The 23 Na NMR behavior of hectorite has been studied previously, 1 but the results here provide more comprehensive understanding, because the spectra were obtained at a higher H 0 eld strength (850 MHz 1 H frequency) and over a wider range of temperatures and water contents.…”

“…Hectorite is commonly used in NMR studies of smectites, because its low Fe content greatly reduces paramagnetic line broadening compared to other natural smectites such as montmorillonite. 44 Hectorite containing a wide variety of exchangeable species has been studied by NMR methods, 1,8,9,22,23,44 X-ray diffraction (XRD), 1,[45][46][47][48] thermogravimetric analysis (TGA), 1,44,46 and quasi-elastic neutron scattering (QENS). [24][25][26][49][50][51] In parallel, there have also been several computational molecular modeling studies of it using principally MD methods.…”

Competitive Adsorption of H2O and CO2 in 2-Dimensional Nanoconfinement: GCMD Simulations of Cs- and Ca-Hectorites

“…Recent experimental and computational modeling studies have clearly shown that the clay swelling behavior depends strongly on the nature of the charge balancing cation (characterized by the CO2 and H2O solvation energies) and on the composition and location of the permanent structural charge of the clay. [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] Experimental and computational modeling studies have shown that at reservoir conditions the amount of intercalated CO2 and its structural, dynamical, and energetic properties depend on the basal spacing and the solvation energies of the charge-balancing cation with H2O and CO2 in smectite interlayers under supercritical conditions (scCO2, Tc~31ºC, Pc~73 bar). X-ray diffraction (XRD) studies at pressurized CO2 conditions by Giesting et al 18,19 showed that increase (expansion) of the basal spacing of the common smectite, montmorillonite, depends on the initial interlayer H2O content.…”

Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanoclay Materials

“…The Br-Hec and I-Hec have a d001 spacing of 14.30 Å (equivalent to 6.2° in 2θ), indicating two hydrate layers in their interlayers when recorded at room temperature [28]. The spacing falls in the middle of the range of 10-20 Å predicted for the basal spacing of smectites [29].…”