Abstract
Stepwise dilution of lipid-surfactant mixed micelles first results in extraction of surfactant from the mixed micelles into the aqueous medium. Subsequently mixed micelles transform into vesicles, within a range of compositions that corresponds to equilibrium coexistence between these two types of aggregates. Further dilution results in extraction of surfactant from the resultant mixed vesicles. In the present study, we have investigated the heat evolution of these processes, as they occur in mixed systems composed of egg phosphatidylcholine (PC) and the nonionic surfactant octylglucoside (OG). A combined use of isothermal titration calorimetry (ITC) and photon correlation spectroscopy (PCS), capable of monitoring phase transformations, revealed that 1) The sum of all of the studied processes (i.e., extraction of OG from mixed micelles and vesicles and the phase transformation) is isocaloric at approximately 40 degrees C throughout the whole dilution. At lower temperatures, all of the dilution steps are exothermic, whereas at higher temperatures all of them are endothermic. 2) At all temperatures, the absolute value of the heat associated with each dilution step within the range of coexistence of micelles and vesicles is almost constant and larger than in either the micellar or the vesicular range. We give an interpretation of these calorimetric data in terms of the relationship between the composition of the mixed aggregates Re and the aqueous concentration of surfactant monomers Dw. Assuming that the main contribution to the heat evolution is due to extraction of surfactant from mixed aggregates to the aqueous solution, we deduce the relationship Dw(Re) characterizing the system over the whole range of compositions. We find that, in accord with thermodynamic expectations, Dw is almost constant throughout the range of coexistence of mixed micelles and vesicles.