Simulation of scattering and phase behavior around the isotropic–nematic transition of discotic particles

Abstract

The detailed study of the isotropic-nematic phase transition in a system of discotic particles of aspect ratios L/D≤0.1 presented here is relevant to a broad range of colloidal suspensions of chemically modified clay particles. Using Monte Carlo simulation techniques the equation of state, radial distribution functions, structure factors and normalized scattering intensities are calculated for each phase. The results are interpreted and related to previously reported free energy calculations [Fartaria and Sweatman, Chem. Phys. Lett. 478 (2009) 150], suggesting a nearly continuous isotropic-nematic transition for lower aspect ratios. Given this behavior we examined the structural information for each phase to determine how experimental scattering data might be used to distinguish the two phases. The radial distribution functions in each phase depend strongly on aspect ratio, and for larger aspect ratios a dramatic increase in the local ordering of discotic particles (represented here as cut-spheres) is observed just before the phase transition. However, this nearest-neighbor ordering seen in g(r) around r/D=0.1 would hardly be discernible in experimental scattering data subject to usual statistical errors. The structure factors and scattering intensities were calculated for L/D=0.1, 0.04 and 0.01 for the isotropic and nematic phases at and away from the isotropic-nematic transition. While the isotropic-nematic phase transition can be detected from the height and shape of the first scattering peak around 7QD for larger aspect ratios, this feature becomes much less discriminatory with decreasing aspect ratio. Instead, scattering intensities at low scattering vector amplitudes (Q→0) can be used for detection of the phase transition at low aspect ratios. These results provide useful insight to guide interpretation of X-ray and light scattering measurements for colloidal dispersions of thin platelets undergoing isotropic-nematic transitions.

Aqueous Graphene Dispersions–Optical Properties and Stimuli-Responsive Phase Transfer

“…This kind of shear alignment is referred to as an isotropic to nematic transition, where the “nematic” orientation produces constructive reflectance. While it is known that shear fields align platelets, − we have not succeeded in finding a reference reporting being able to detect this phenomenon visually in room light as is illustrated in Figure . A rationalization for this paucity of earlier reports may be that dispersions of clays and other disklike materials will tend to be milky and opaque to the eye if the platelets are big enough to scatter visible light, and the marginal increase of scattering on alignment may not be so obvious, as it is in this case where the constructive reflectance is cast against an actinic (black) background.…”

Plastic deformation mechanism of calcium-silicate hydrates determined by deviatoric-stress Raman spectroscopy

“…The third notable feature is the broad peak in I(q)/F 2 (q) around q ~ 0.04 Å -1 . This is a grain-grain correlation, as it is present at roughly q = 2π/2T [43], and corresponds to grains stacked on top of one another.…”

“…The compaction at 740 MPa imparts enough stress on the lamellae to break them apart and disrupts the stable 70 Å spacing structure from the powder. The heterogeneity of the compaction prevents intensity from building up at any characteristic q [43]. Further experiments based on X-ray diffraction patterns of compacted C-S-H may yield more detailed insights to the compaction mechanism.…”