Floating nematic phase in colloidal platelet-sphere mixtures.

The phase behaviour of colloidal dispersions is interesting for fundamental reasons and for technological applications such as photonic crystals and electronic paper. Sedimentation, which in everyday life is relevant from blood analysis to the shelf life of paint, is a means to determine phase boundaries by observing distinct layers in samples that are in sedimentation-diffusion equilibrium. However, disentangling the effects due to interparticle interactions, which generate the bulk phase diagram, from those due to gravity is a complex task. Here we show that a line in the space of chemical potentials µ(i), where i labels the species, represents a sedimented sample and that each crossing of this sedimentation path with a binodal generates an interface under gravity. Complex phase stacks can result, such as the sandwich of a floating nematic layer between top and bottom isotropic phases that we observed in a mixture of silica spheres and gibbsite platelets.

Composites of kaolin and polydimethylsiloxane.

Kaolin particles were surface-treated with isobutyltrimethoxysilane (IBTMS), hydrogenated tallow (HT), and a polyisobutyl chain-based stabilizer (SAP) to make composites with polydimethylsiloxane (PDMS). IBTMS did not cover the strong acid sites on the kaolin surface and as a result a cross-linking reaction occurred for silanol-terminated PDMS. The polyisobutyl chain of SAP was found to be incompatible with PDMS and this caused aggregation of the kaolin particles. HT was the most effective at dispersing the particles into silanol-terminated PDMS. The aggregation state of the composites was characterized using rheology and microscopy. Both showed the HT-treated particles were well-dispersed in low molecular weight silanol-terminated PDMS, and they were weakly flocculated in higher molecular weight silanol-terminated PDMS. However, the same particles aggregated when dispersed in methyl-terminated PDMS. It appears the silanol-terminated PDMS acted as costabilizer through interaction with the kaolin surface. Transverse relaxation NMR was used to probe mobility of the PDMS chains in the composites. This showed little dependence on surface treatment, aggregation state, or polymer end groups. For all samples, chain mobility decreased with increasing kaolin concentration.

Nonaqueous suspensions of surface-modified kaolin.

A range of different stabilizers have been used to render natural kaolin clay particles hydrophobic and dispersible in nonpolar solvents such as heptane. Both silanol and aluminol groups are known to be present at the kaolin surface. Use of a Hammett indicator showed that silanes would not neutralize the acidic aluminol sites, whereas amines would neutralize these sites. Both types of stabilizer adsorbed chemically onto the clay. In addition, a combined silane + amine treatment and a polyisobutylene-based stabilizer with a succinimide/amine head group (SAP230) were also considered. Both would neutralize the acid sites. The final sediment density after settling under gravity was used to gauge suspension stability, which varied with the kaolin surface treatment as silanes < amines < silane + amine < SAP230. This behavior was very similar for suspensions in heptane and in a higher molecular weight branched alkane, polydecene. This trend of increasing stability correlated very well with an increase in surface coverage of the stabilizing moieties, a decrease in particle size found using small-angle light scattering, and a decrease in Bingham yield stress obtained by fitting rheological data.