Thermodynamic study on vesicle formation and adsorption of decyltrimethylammonium decyl sulfate

The surface tension of an aqueous solution of decyltrimethylammonium decyl sulfate (DeTADeS) was measured as a function of temperature T at various mo lalities (m) over cap under atmospheric pressure. DeTADeS has been found to form equilibrium multilamellar vesicles (MLV) spontaneously. The surface d ensity, the entropies of adsorption, and the entropy of vesicle formation a re evaluated. The mechanism of formation of equilibrium vesicles is investi gated from the standpoint of thermodynamics and from the comparison of the results with those of the micelle-forming systems. From the relatively smal l change of the surface density (Gamma)over-cap(H) on T at a given (m) over cap, the adsorbed film is implied to be tightly packed due to the strong e lectrostatic attraction between the polar headgroups. The energy change ass ociated with adsorption from the vesicular state per mole of surfactant Del ta U-H(V) is positive in the entire temperature range; thus, the curved bil ayer in MLV is energetically more favorable than the planar adsorbed him. F rom the negative values of the entropy of vesicle formation Delta S-V(W), i t is concluded that vesicle formation is driven by enthalpy whereas micelle formation is mostly entropy driven. (C) 2001 Academic Press.