Conformational Analysis of (Phenylenedimethylene)bis(<i>n</i>-octylammonium)dibromides in Aqueous Solution. Conformational Change upon Micellization

Abstract

Three gemini surfactants, in which two quaternary ammonium species (CH 3 (CH 2 ) 7 N + (CH 3 ) 2 ) are linked at the polar headgroups by o -, m -, or p -phenylenedimethylene spacers, have been synthesized. The critical micelle concentration (cmc) of these surfactants in aqueous solutions was determined by electrical conductivity. Selective-decoupling 13 C NMR and 1 H NMR spectra were measured at various concentrations below and above the cmc. The selective-decoupling 13 C NMR results revealed that the specific rotational isomers about the CH 2 -aromatic carbon single bonds for the gemini surfactant having a m -phenylenedimethylene spacer are preferentially stabilized upon micellization, while for the gemini surfactant having an o -phenylenedimethylene spacer, the presence of only the conformation in which the aromatic ring is sandwiched between two n -octyl chains was confirmed. Furthermore, it was found that variation in the stacking pattern of the aromatic rings after micellization of the surfactants is reflected in the 1 H spectral features of the aromatic protons. Comparison is made with the conformations of bis(quaternaryammonium) bromides with flexible spacer chains ((CH 2 ) s ) and of the corresponding n -alkylammonium bromide monomeric surfactants.