Effects of selenium atom on the solution properties of N-alkyl-N-methylpyrrolidinium bromide

Abstract

Selenium-containing surfactants have recently attracted considerable interest, but the influences of a selenium (Se) atom and its site on the solution properties are still a challenge. Herein, we systematically studied the micellization behavior and foam/emulsion properties of ionic liquid surfactants (CmSeCnMPB, m = 14,12,10, m + n = 16, MPB = N-methylpyrrolidinium bromide) bearing a Se atom at different sites of the hydrophobic tail, and compared with N-cetyl-N-methylpyrrolidinium bromide (C16MPB). The results showed that the micellization behavior and foam/emulsion properties of CmSeCnMPB are closely related to the site of the Se atom in the hydrophobic tail. With increasing the spacer between Se atom and headgroup, the critical micelle concentration (cmc) and minimum area per surfactant molecule at the air–water interface (Amin) all increased, whether for the reduced or oxidized form, and the pC20 values, standard Gibbs free energy of micelle formation (ΔGmo) decreased accompanied by an increasing contribution of the Se atom, reflecting a shortening in the effective length of the hydrophobic chain. Upon increasing temperature, the micellization of CmSeCnMPB was transformed from entropy-driven mode to enthalpy-driven mode, and the transformation temperature decreased with increasing the spacer length. The longer the spacer, the worse the foam/emulsion performance, and the greater the oxidation-induced changes in cmc, Amin, pC20, and foam/emulsion properties. Oxidation can completely switch off the foam/emulsion properties of C10SeC6MPB, but only impair those of C12SeC4MPB or C14SeC2MPB. By comparison with C16MPB, it is confirmed that the Se atom brings two competing effects: enhancing hydrophobicity akin to one methylene and decreasing hydrophobicity by disturbing the regularity. The former prevails over the latter for C14SeC2MPB, but the latter surpasses the former for C10SeC6MPB.