Impact of the Stereochemical Structure on the Thermal Phase Behavior of a Cationic Gemini Surfactant

Abstract

Differential scanning calorimetry (DSC) revealed the stereochemical structure of the headgroup to be an important determinant for the thermal phase behavior of aqueous dispersions of a cationic gemini surfactant. More specifically, the meso form (2S,3R)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (abbreviated SR-1) exhibited complex pattern of multi-peak endotherms with two more pronounced peaks at approximately 29 °C and 39 °C. Different behaviors were evident for the stereochemically pure enantiomers (2S,3S)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (SS-1) and (2R,3R)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (RR-1), which both had major endotherms at approximately 36 °C and 41 °C. The heating scans recorded for both RR-1 and SS-1 immediately after a heating-cooling cycle were smooth while SR-I demonstrated a simple endotherm at 29 °C. These results suggest that the geometry of the headgroup of the above gemini surfactants is an important determinant for their lateral packing and organization in an aqueous solution reflected as pronounced effects on both thermal phase behavior and relaxation kinetics. In keeping with this interpretation, including either SS-1 or RR-1 at a mole fraction of 0.10 into SR-1 suspensions resulted in lower transition temperatures and enthalpies, thus demonstrating perturbation of the packing of SR-1 by the presence of the stereoisomeric impurities.