Structure investigation of a Laponite-NaCl dispersion induced by desiccation in a confined geometry using SAXS

Abstract

The drying of colloidal dispersions is a dynamic area that has many practical applications across a wide range of sectors. While most research focuses on the drying patterns formed after desiccation and the physical-chemical parameters that influence this process, the structure formed as a result of particle accumulation has received comparatively far less attention. In this work, an investigation of the structure formed after the drying of a 4% w/w Laponite - NaCl dispersion, with ionic force I=10−3M, in a confined geometry between two hydrophilic mica windows was performed. The desiccation was characterized by measuring the mass of the dispersion over time, until no further mass loss was detected. The sample shrank to the middle of the sample holder, and SAXS/WAXS experiments were conducted after complete drying to better understand the structure formed. Such experiments revealed the formation of clusters with various sizes, shapes, and anisotropic orientations, as well as the formation of structures with different crystalline orders that were not present in the initial sample. These structures were all quantified and interpreted on the basis of Laponite interparticle interactions, possible structured water that remained inside the dried sample even after drying ceased, and NaCl crystallization. Finally, the macroscopic consequence of the microstructure formed was observed in photographs using polarized optical microscopy. The results provided a better comprehension of the structure formed in concentrated Lap dispersions before as well as after drying, showing that X-ray scattering techniques can be useful to investigate the drying of a variety of other colloidal systems.