Determining the Phase Diagram and Aggregate Size of a Chromonic Liquid Crystal
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2006
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Swarthmore College. Dept. of Physics & Astronomy
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Thesis (B.A.)
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Abstract
Although the most recent studies of Sunset Yellow FCF, dis odium chromoglycate,
and several other chromonic liquid crystals suggest that chromonic liquid crystals form
rod-shaped aggregates with a distribution of sizes that shifts towards larger aggregates as
the concentration increases, limited studies of another chromonic liquid crystal,
Benzopurpurin 4B (BPP 4B), suggest that the aggregation process is very different for BPP
4B. These studies found that unlike other chromonic liquid crystals, BPP 4B solutions
scatter visible light and form a liquid crystal phase at significantly lower concentrations,
which implies that their aggregates are much larger. To extend this research, both the
phase diagram in water and the aggregate size of BPP 4B were investigated. To
determine the phase diagram, the temperatures marking the beginning and end of the
coexistence region between the liquid crystal and isotropic liquid phases were measured
optically. The results suggest that BPP 4B forms aggregates with a distribution of sizes
and has a liquid crystal phase at significantly lower concentrations and volume fractions
than other chromonic liquid crystals. Additionally, measurements of the hydrodynamic
and optical radii, the relative scattering intensity, and the absorption coefficient suggest
that the size distribution does not change with concentration. One possible explanation is
that BPP 4B forms micelles with a distribution of sizes. However, the presence of an
impurity much larger than the aggregates might also explain these results. Although the
exact aggregate structure of BPP 4B remains largely uncertain, the results of these
experiments suggest that it is very different from the simple, rod-like structures of other
chromonic liquid crystals.
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