Molecular Organization of Gemini Surfactants in Cylindrical Micelles:  An Infrared Dichroism Spectroscopy and Molecular Dynamics Study

The organization of cationic dimeric (gemini) surfactants assembled in cylindrical micelles was investigated using polarization modulation infrared linear dichroism (PM-IRLD) and molecular dynamics simulations. These molecules consist of two quaternary ammonium amphiphiles connected at the headgroup by an ethylene spacer, resulting in an elongated polar head. PM-IRLD measurements and molecular dynamics simulations indicate that the long axis of the surfactant headgroup may adopt a preferred orientation of ca. 30−50° with respect to the micelle cylinder axis. This may be interpreted as the result of the competition between the charge repulsion between headgroups which favors an orientation of the molecules perpendicular to the cylinder axis (high curvature) and the steric repulsion between hydrophobic tails which favors an orientation of the molecules parallel with the cylinder axis (flat curvature). Experiment and calculation both show a water layer a few angstroms thick with a specific orientation at the micelle interface.