Neutron diffraction study of aqueous Laponite suspensions at the NIMROD diffractometer

The process of dynamical arrest, leading to formation of different arrested states such as glasses and gels, along with the closely related process of aging, is central for both basic research and technology. Here we report on a study of the time-dependent structural evolution of two aqueous Laponite clay suspensions at different weight concentrations. Neutron diffraction experiments have been performed with the near and intermediate range order diffractometer (NIMROD) that allows studies of the structure of liquids and disordered materials over a continuous length scale ranging from 1 to 300 Å, i.e., from the atomistic to the mesoscopic scales. NIMROD is presently a unique diffractometer, bridging the length scales traditionally investigated by small angle neutron scattering or small angle x-ray scattering with that accessible by traditional diffractometers for liquids. Interestingly, we have unveiled a signature of aging of both suspensions in the length scale region of NIMROD. This phenomenon, ascribed to sporadic contacts between Laponite platelets at long times, has been observed with the sample arrested as gel or as repulsive glass. Moreover, water molecules within the layers closest to Laponite platelets surface show orientational and translational order, which maps into the crystalline structure of Laponite.

Figure 1

Differential cross-section of Laponite suspension in ${\mathrm{D}}_2\mathrm{O}$ ($C_w=1.5\%$, “old” sample) measured at four different detector groups (diffraction angle increases with the detector group number, labeled as “no. xx”) before correction of inelasticity effects and multiple scattering events. The arrows show structures in the raw data due to multiple scattering events. At odds with the genuine DCS structures, which are visible at $Q\sim 1.9{\AA }^{-1},\sim 4 {\AA }^{-1}$, and $\sim 8 {\AA }^{-1}$ at all detector banks, structures due to multiple scattering events move in $Q$ going from low- to high-diffraction angles. The slope of the data at low-$Q$ contains an angle-dependent contribution, superimposed to the genuine low-$Q$ features of the sample, due to the presence of inelasticity effects. Detector groups at the lowest diffraction angles extend over a shorter $Q$ range, exhibiting lower inelasticity effects and higher noise.

Figure 2

Differential cross-section of pure water (averaged over all detector groups), after subtraction of the silica sample container contribution, before correction for inelasticity and multiple scattering effects. We notice that the inelasticity effects increase with increasing H content and that, in particular, the data of the ${\mathrm{H}}_2\mathrm{O}$ sample exhibit a spurious broad bump centered at about $20 {\AA }^{-1}$ and a slope at low $Q$. This spurious bump is less visible in the HDO sample and absent in the deuterated one. All datasets show the spurious structures due to multiple contributions at low-$Q$ values.

Figure 3

DCS data for heavy water at two selected diffraction angles (offset for clarity and labeled as “no. xx”), as a function of $Q$ (A) and as a function of $\lambda$ (B). The arrow indicates the contributions of multiple scattering events: these appear as broad peaks at low $Q$, in panel (a), and as a bump, centered at $\lambda \sim$ 4.7 Å, in panel (b). The dashed lines are fits of the inelasticity contribution at each angle. The black solid line represents the average multiple scattering contribution. Data are reported as symbol plus errors (vertical bar).

Figure 4

Raw (thin line) and corrected (thick line) DCS data, for ${\mathrm{D}}_2\mathrm{O}$ [(a) and (d)], HDO [(b) and (e)], and ${\mathrm{H}}_2\mathrm{O}$ [(c) and (f)].

Figure 5

Raw DCS data for the “old” $C_w=1.5\%$ Laponite suspension in ${\mathrm{D}}_2\mathrm{O}$ (blue) for a selected detector group, compared with the raw DCS of pure water (red) at the same diffraction angle. The statistical error is reported as a vertical bar.

Figure 6

Comparison between raw (dashed line) and corrected (solid line) DCS data in the case of the “old” $C_w=1.5\%$ Laponite suspension in ${\mathrm{D}}_2\mathrm{O}$.

Figure 7

Comparison between raw (dashed line) and corrected (solid line) DCS data in the case of the “old” $C_w=1.5\%$ Laponite suspension in ${\mathrm{H}}_2\mathrm{O}$.

Figure 8

Comparison between raw (dashed line) and corrected (solid line) DCS data in the case of the of the “old” $C_w=1.5\%$ Laponite suspension in HDO.

Figure 9

(a) Comparison of the experimental (red points) and fitted (black solid lines) $F\left(Q\right)$ functions [Eq. (1)] of the “young” Laponite suspension at $C_w=$ $3.0\%$. Deuterated, hydrogenated samples and their equimolar mixture are reported from bottom to top. (b) Zoom of panel (a) in the region of the main diffraction peak. Data have been offset for clarity.

Figure 10

The low-$Q$ region of the DCS of all five samples, showing the signatures of aging. Notice that at $3.0\%$ weight concentration the DCS of the “young” (green) and “old” (blue) samples are coincident within the experimental error (reported as vertical bar), while the DCS of the “very old” sample (orange) shows a very broad feature centered at $0.4 {\AA }^{-1}$, similar to the signature of aging shown by the “old” sample at $c_w=1.5\%$ (red).

Figure 11

Density profile of water atoms (oxygens, black squares; hydrogens, red circles) along the direction normal to the Laponite surface, for the $C_w=1.5\%$ “old” sample. The profiles for all other samples are similar.

Figure 12

Radial distribution function of the ${\mathrm{O}}_s\text{−}{\mathrm{H}}_w$ and ${\mathrm{O}}_s\text{−}{\mathrm{O}}_w$ pairs, for samples at different Laponite concentration and age. No differences can be appreciated due to Laponite concentration and age. Interestingly, the $g_{{\text{O}}_s{\text{O}}_w}\left(r\right)$ functions show a clear periodicity, due to the interaction of water with the crystalline surface of Laponite platelets. Conversely, the $g_{{\text{O}}_s{\text{H}}_w}\left(r\right)$ functions suggest that the arrangement of the hydrogen sites is orientationally disordered and that water does not form H bonds with Laponite (unless very weak ones). Notice also that the ${\mathrm{O}}_s\text{−}{\mathrm{H}}_w$ minimum approach distance (at $\sim 2.7$ Å) is shorter than that of the ${\mathrm{O}}_s\text{−}{\mathrm{O}}_w$ pairs (at $\sim 3.2$ Å).

Figure 13

(a) A snapshot of the central part of the EPSR simulation box, showing the laponite crystal in the middle (${\text{O}}_s$ atoms in red) and the first layers of water molecules (${\text{O}}_w$ in red, ${\text{H}}_w$ in white). Notice that all water molecules in the Laponite first-neighbor layer orient their hydrogens toward the Laponite surface. (b) The red circles represent the ${\text{O}}_s$ spatial arrangement on the Laponite surface; the regions where the probability of finding a water molecule exceeds the $20\%$ are highlighted in gray.

Figure 14

Water-water radial distribution functions at $C_w=1.5\%$ and $C_w=3.0\%$ for “young” and “old” samples, respectively, compared with those of pure bulk water (magenta line).