Single Molecular, Self-Assembly and Adsorption Properties of N-Tetradecyl-N-Methylmorpholinium Bromide in Aqueous Solutions

This study explores the single molecular, self-assembly, and adsorption properties of a synthesized morpholinium-based surface active ionic liquid N-tetradecyl-N-methylmorpholinium bromide (TMB) in aqueous solutions. It utilizes the recently developed ORCA 6.0.1—an ab initio, Density Functional Theory, and semiempirical Self Consistent Field-Molecular Orbital package—along with the Solvent Model based on Density for optimization of the structure of TMB in its ground state. The micellization and adsorption properties of TMB were investigated by conductometry, tensiometry, and spectrofluorimetry. A confirmation of the cmc estimated by these techniques has also been obtained by a method suggested by the Moulik group (Basu Ray et al. 2006), which remained almost overlooked, based on the UV absorption of pyrene. Quantification of the thermodynamic parameters of micellization and interfacial parameters has been done. The aggregation number of TMB micelles has been computed from theoretical considerations and compared with that estimated using fluorimetry with pyrene as a probe. Results demonstrate distinctive features of the two hydrocarbon chains of TMB in contrast to the previous report. This study proves unequivocally that while the longer tetradecyl group (except its methylene group attached to the polar head) enters the micellar core, the methyl group attached to the quaternary nitrogen atom lies within the hydration sphere of the polar head. The micelles formed have been inferred to be spherical in shape both from theoretical analysis and FETEM. This study is expected to provide new knowledge towards the design and development of novel surface active ionic liquids with target specific applications.