Aggregation Properties of the Chromonic Liquid Crystal Benzopurpurin 4B
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2007
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Swarthmore College. Dept. of Physics & Astronomy
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Thesis (B.A.)
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Abstract
Unlike solids, which have both positional and orientation order, and liquids, which have no molecular organization, liquid crystals have have a degree of orientation order while still flowing like liquids. This unusual combination of properties makes them especially useful in technological applications. In certain lyotropic liquid crystals, molecular aggregates forming in solution orient along the same axis rather than individual molecules themselves. We studied this aggregation process in the chromonic liquid crystal Benzopurpurin 4B (BPP), a textile dye. In order to study the process of aggregation, we used dynamic light scattering on a range of different solutions to determine aggregate size and shape, the concentration needed for aggregate formation, and their response when heated. Results indicate that large anisotropic aggregates form in solution starting in the 0.11-0.12 wt% concentration range, but then grow in size as the concentration is increased. This type of aggregation behavior differs from other chromonic liquid crystals in that aggregation only begins above a certain concentration. On the other hand, amphiphillic aggregates, which also form only above certain concentration, do not grow in size. Therefore, BPP behaves differently than both more typical chromonic aggregates and amphiphillic systems. Since BPP comes with several different salt impurities, we developed a purification process using dialysis to obtain solutions of pure BPP. By reintroducing sodium salts in known concentrations and making phase measurements at different temperatures, we determined the salt effects on the formation of aggregates. Results for BPP paralelled a similar study [12] on disodium chromogylcate, with the addition of salt increasing the temperatures at which the liquid crystal phase can exist. Given the differences in aggregation behavior between these two compounds, these findings contribute to a more general understanding of how salt effects chromonic aggregation.