Vesicle Formation by Chemical Reactions: Spontaneous Vesicle Formation in Mixtures of Zwitterionic and Catanionic Surfactants

摘要

The influence of shear on the vesicle formation in surfactant systems was studied by preparing the liquid crystalline phases in the surfactant system tetradecyldimethylamine oxide (TDMAO)/sodium-3-hydroxy-2-naphthoate (SHNC)/formic acid (HCO_2H)/water with and without shear. The transition between the different liquid crystalline states in this system is controlled by the degree of protonation of the zwitterionic surfactant TDMAO and thus by the concentration of HCO_2H. Using the corresponding ester methyl formiate (MF) instead of HCO_2H, the transition between the different liquid crystalline phases can be studied without application of any shear. The phase transitions are induced by the change in the degree of charging during the hydrolysis of the ester that takes place on a time scale of several hours. We started from a micellar solution of 100 mM TDMAO and 25 mM SHNC and added 100 mM MF. With time we observed the formation of a lamellar L_α phase, a vesicular L_v phase, and a vesicular precipitate. This precipitate dissolved to form a vesicular phase, an L_α phase, and finally again a micellar phase. The hydrolysis of the MF and the resulting phase transitions were observed by measurements of pH, conductivity, turbidity, and by means of freeze-fracture transmission electron microscopy. The microstructures of the phases obtained were compared with the microstructures of the corresponding phases when increasing amounts of HCO_2H were mixed with solutions of 100 mM TDMAO and 25 mM SHNC.