Measuring the activation energy of the structural evolution in vesicle formation with combined spectroscopic methods and revealing the different ionic effects from Na+ and Ca2+

It is not easy to in-situ probe the structural evolution in assembly systems in solutions. Determining the activation energy of the molecular processes in vesicle formation, which is important in revealing the mechanism in the transformation of vesicles, is also challenging. In this work, with the combined second harmonic generation (SHG) and two-photon fluorescence (TPF) techniques, we determined that the activation energies of the ag-gregation of micelles, the evolution of aggregated micelles to vesicles and the penetration of an anticancer drug (doxorubicin, DOX) on the surface of vesicles composed of DOX and a surfactant bis (2-ethylhexyl) sodium sulfosuccinate (AOT) were 48.6 kJ/mol, 49.4 kJ/mol and 20.5 kJ/mol, respectively. In addition, we investigated the effect of different types of salts on the self-assembly behavior of DOX and AOT molecules. It was observed that compared with Na+, Ca2+ significantly reduced the activation energy in the transformation of aggregated micelles to vesicles at a much lower concentration, which indicated a strong association ability between Ca2+ and AOT head group.