Imbibition and dewetting of silica colloidal crystals: An NMR relaxometry study.

HYPOTHESIS: The wettability of the inner surfaces in a porous network is challenging to be accessed but essential to understand the complex performance of e.g. particulate systems. EXPERIMENTS: Here we investigate the water behavior in the macroporous (50-100 nm) voids of dense particle packings by performing low-field 1H NMR relaxometry in solid silica colloid crystals. The systems chosen guarantee a regular, known void size distribution with controllable water affinity (through thermal annealing), where the NMR experiment clearly discriminates the void water from micropore or bound water contributions, allowing separate monitoring. FINDINGS: Analysis of the saturated state indicates that the interparticle voids are completely filled after imbibition, even for less hydrophilic spheres. Due to the interaction with the silica surface, proton relaxation in void water is up to 100 times faster than that in bulk water, serving for assessment of the hydrophilicity within the sample. The relaxation time evolution upon dewetting provides an empirical measurement of the wettability inside the ensemble, revealing a progressively inhomogeneous wetting of increasingly hydrophobic surfaces. Our results provide insight into the imbibition state and dewetting performance in meso- and macroporous systems, with emphasis in the marked influence of the surface nature on the pore wetting distribution.

Surface influence on the rotational and translational dynamics of molecules confined inside a mesoporous carbon xerogel.

Low-field nuclear magnetic resonance techniques are employed to extract information about the effects introduced by the interaction with the surface on the rotational and translational dynamics of molecules confined inside a mesoporous carbon xerogel. The molecules under study were water, cyclohexane, and hexane. They were chosen due to their different interaction strength with the carbonaceous matrix. Frequency dependent longitudinal relaxation measurements, using the fast field cycling technique, allowed extraction of the fractal dimension of the carbon xerogel surface. It was observed that the measured value is influenced by the molecule affinity to the surface. Diffusion measurements, using the pulse field gradient technique, have revealed that the stronger interaction with the surface of cyclohexane and hexane molecules leads to an increased diffusive tortuosity, as compared with water.