Thermodynamic parameters of Rhodamine B in powder and nanofilms forms

We report the use of UV–Vis absorbance spectroscopy in determining thermodynamic properties of materials based on their absorbance in the visible region of the electromagnetic spectrum. Rhodamine B (RhB) dye which has always been considered to be a non-volatile material, was chosen for the validation of this method because of its well-defined and strong absorbance in this region. Continuous films of RhB suitable for absorbance spectroscopic study were prepared by spin coating in a 0.2 M ethanol solution to determine the sublimation rates and the activation energy of RhB in the temperature range 65–100 °C. For comparison, the same thermodynamic parameters of powder RhB were also determined using thermogravimetric analysis, at higher temperatures (100–180 °C). The sublimation of RhB was determined to be a first-order process by both methods. The activation energies of sublimation calculated by both techniques are in very good agreement. However, the sublimation rates calculated using UV–Vis absorbance spectroscopy are two orders of magnitude smaller than those calculated using TG which is explained by the inaccurate calculation of the powder surface area due to its small needle-like morphology. The results indicate that RhB sublimates at relatively low temperatures as well as the ability of UV–Vis absorbance spectroscopy to determine such parameters, accurately, in the nanometer scale at relatively lower temperatures.