Neutron spectroscopy of water dynamics in NaX and NaA zeolites

We have investigated the dynamics of water molecules in zeolites NaA and NaX by high-resolution quasielastic neutron scattering methods. Between 260 and 310 K, the local translational diffusive motion of water in the zeolites is one to two orders of magnitude slower than in bulk water. The $Q$ dependence of the scattering shows effects of confinement and the presence of both relatively mobile and immobile molecules. The speed of the diffusive motion depends strongly on hydration level. Comparison with other hydrated siliceous materials indicates that the host charge per water molecule is a major factor in determining the time scale of diffusion.

Figure 1

Fixed-window elastic scans for three hydrated aluminosilicate systems. From top to bottom, the curves are for hydrated NaA and NaX zeolites, and a synthetic layered silicate (fluormica), heating (dashed line) and cooling (solid line). The hydration levels correspond to fractional mass gains of 0.20 for the NaA sample, 0.22 for the NaX sample, and 0.27 for the fluormica sample.

Figure 2

Quasielastic scattering from water in NaA zeolite at 300 K, $Q=1.32{\text{Å}}^{-1}$. The curves are, from top to bottom, the total fitted intensity, the Lorentzian component broadened by resolution, and the shape of the resolution function. Uncertainties due to counting statistics are approximately the size of the symbols.

Figure 3

Wave-vector dependence of the apparent elastic fraction for scattering from water in NaA zeolite. Fits are to a constant plus a Gaussian centered at $Q=0$. The hydration level corresponds to a fractional mass gain of 0.20 over the dry zeolite.

Figure 4

Estimated confinement diameters (see text) for the mobile water fractions in NaX (open symbols) and NaA zeolite (filled symbols). For NaA, the hydration level corresponds to a fractional mass gain of 0.20. For NaX, the triangles are for a lower hydration level (0.22 fractional mass gain) than the squares (0.28 fractional mass gain). The error bars are intended to be standard deviations.

Figure 5

Quasielastic widths for water in NaA zeolite. Fits are to Eq. (3), as described in the text. The hydration level corresponds to a fractional mass gain of 0.20 over the dry zeolite.

Figure 6

Wave vector dependence of the apparent elastic fraction for scattering from water in NaX zeolite. Fits are to a constant plus a Gaussian centered at $Q=0$. Similar data and fitted curves were obtained at 310 and 290 K. The hydration level corresponds to a fractional mass gain of 0.22 over the dry zeolite.

Figure 7

Quasielastic widths for water in NaX zeolite. Similar data sets and fitted curves were obtained at 310, 290, and 265 K. The hydration level corresponds to a fractional mass gain of 0.20 over the dry zeolite.

Figure 8

Quasielastic widths at 300 K for water in NaX at two hydration levels.

Figure 9

Temperature dependence of immobile water fraction for water in NaA and NaX zeolites. The error bars are intended to be standard deviations. The hydration levels correspond to fractional mass gains of 0.20 for the NaA sample, and 0.22 for the NaX sample.

Figure 10

Librational mode spectra for water at 12 K in NaA zeolite at three hydration levels. The lower, middle, upper, and middle spectra have been shifted upward by 1500, 2000, and 2500 counts, respectively. The data for ice (on an arbitrarily adjusted vertical scale) are from Ref. [43].

Figure 11

The width of QENS line shapes at $Q=0.8{\text{Å}}^{-1}$, $T=300\text{K}$ in (from right to left) hydrated NaA, hydrated NaX, fluormica in the one-water layer hydration state (1-wlhs), fluormica in the two-water layer state (2-wlhs), aqueous solutions of NaCl and NaI, and bulk water. The horizontal axis of the semilogarithmic plot is the charge per water molecule due to the host material or ions in solution.