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Abstract

Surfactants are molecules with polar and non-polar regions that exhibit a phenomenon called the critical micelle concentration (CMC). At and above this concentration, the molecules arrange into spherical formations called micelles. Each surfactant has a unique critical concentration based on its structure, chemical properties, and interactions with its environment. Distinct variables that are known to change a surfactant’s CMC are temperature and ionic strength of the media. The aim of this project was to determine the CMCs of two surfactants, trimethyltetradecylammonium bromide (cationic) and sodium dodecyl sulfate (anionic), using UV-visible spectroscopy, conductivity, and by investigating the transition of the ketone tautomer to the enol tautomer as the CMC was reached. To investigate the CMC of surfactants, several techniques and instruments have been used. One of the reported techniques, used a probe molecule,1-phenyl-1,3-butanedione, which is a β-Diketone, to find the CMC. Ketones are a specific type of organic compound that contain a carbonyl functional group (>C=O). This research project incorporated a broad study of β-Diketones as probe molecules including: 1- phenyl-1,3-butanedione, 1,3-di-(2-pyridyl)-1,3-propanedione, 1,3-cyclohexanedione, 1,3-cyclopentanedione, ethyl acetoacetate, 4,4,4-trifluoro-1-phenyl-1,3-butanedione, and 1-(3,4-dimethoxyphenyl)-1,3-butanedione. In theory, the introduction of several structurally diverse β-Diketones could interrupt the aggregation of micelles, and therefore alter the corresponding CMC range. This was suggested because the enol tautomer is known to energetically associate itself with the nonpolar tail groups of surfactants. When micelles are formed at the CMC, the enol tautomer is enclosed in the center of the spherical formation. However, we observed that the measured concentration range of the surfactants was not drastically shifted by the structural interference of the β-Diketone probes, and that the determined ranges align with values 10 previously reported within the literature. For trimethyltetradecylammonium bromide (TTABr) the determined CMC range was 3 mM to 8 mM, and for sodium dodecyl sulfate (SDS) 6 mM to 14 mM. Using the observed CMC ranges and β-Diketone responses, the future of this project can investigate enol tautomer reactivity within a water-micelle environment, which acts as a green solvent system. Using the green solvent system (deionized water and trimethyltetradecylammonium bromide), a condensation reaction between 1-phenyl-1,3-butanedione (the enol isomer) and 2-aminothiphenol was performed successfully and produced 2-Benzoyl-3-methyl-4H-1,4-benzothiazine. The resulting product underwent an oxidative ring expansion to form the final product, 1-(3-Phenyl-4H-1,4-benzothiazin-2-yl)ethanone, which was deep orange in color. The reaction mixture identity was confirmed by UV-visible spectroscopy and high-performance liquid chromatography mass spectrometry (HPLC-MS). Further reactions that can be facilitated using a micelle-water solvent are: Michael Addition, Diels Alder, Amination, and Alkylation reactions.

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