In the electronic ground state, Auramine O has a planar conformation, while in its first singlet excited state, it has a perpendicular conformation. Because of this feature, Auramine O is classified as a molecular rotor. Upon changing the conformation, the emission spectrum shifts bathochromically by about 10 nm. Intuitively, it is expected that this conformational change is medium viscosity dependent, that is, larger the medium viscosity, smaller the shift. In this project, the emission spectrum of Auramine O was measured as a function of temperature in 1-propanol. The emission spectrum maxima were correlated to media viscosity; as a result, a calibration diagram was produced (viscosity vs emission maximum wavenumber), which allows Auramine O to be used as a lumiprobe to assess the microviscosity of polar liquid droplets dispersed in low polarity media. A sample containing 40 wt% water and 60 wt% poly(propylene glycol), PPG, was prepared. Previous research determined that the mixture is nano-heterogeneous; the water droplets dispersed in PPG contained about one thousand molecules. The microviscosity of these water droplets was assessed using Auramine O lumiprobe in the temperature range from 25 0C to -50 0C. The comparison between the rheological properties of water droplets dispersed in PPG matrix and that of bulk water (Frederic Caupin and coworkers, 2015) shows no significant difference between the two, that is, there is no size effect on the water viscosity provided that the sample contains more than one thousand molecules. In the aforementioned reference the viscosity was measured to -38 0C; we expanded the temperature range to -50 0C. Our low temperature viscosity data fell on the extrapolated polynomial fit corresponding to the +25 0C to -38 0C range.

Using auramine O Lumiprobe to study rheological properties of supercooled water droplets dispersed in poly(propylene glycol) matrix

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.