Aggregation of Benzopurpurin 4B
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2009
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Swarthmore College. Dept. of Physics & Astronomy
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Thesis (B.A.)
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Abstract
Benzopurpurin 4B (BPP) is a textile dye that has been studied in the Liquid Crystal Laboratory
at Swarthmore College over the last two years. Previous investigations have utilized the
commercially available impure material. One of the goals of this study was to use a purified
sample of BPP to establish a phase diagram. The impurity was previously determined to be
salt, and so purification was achieved through dialysis. Once purified, the liquid crystal transition
of different concentrations of BPP in water was examined by monitoring the optical
retardation of the sample as the temperature was increased from below room temperature
to up to 95°C. The other portion of the work involved determining the dependence of the
absorption coefficient on concentration and imaging the aggregates in the system with a confocal
microscope. Absorption coefficient measurements were taken on very dilute samples of
BPP using a spectrophotometer.
The phase diagram produced showed that the liquid crystal phase forms at much lower
concentrations than in other chromonic systems. This suggests that the aggregates of BPP
may form differently than those of most other chromonic liquid crystals. The absorption
spectrum showed a smooth development with concentration, indicative of a gradual and
continuous aggregation process as is characteristic of chromonic liquid crystals. Images taken
with the confocal microscope over several weeks suggest that the aggregation process occurs
very slowly. Using these images, the length of BPP aggregates was estimated to be between
3 and 5 microns initially, but after a longer period of time the aggregates reached lengths
over 10 microns. The diameter of the aggregates was estimated to be no greater than 140
nanometers. A more precise measurement could not be achieved since the aggregate diameter
lies well below the resolution of the confocal microscope.