Concentration-dependent Change in Aggregation States among Monomer, Micelles and Vesicles Observed in Aqueous Solution of Didodecyldimethylammonium Bromide

Abstract

Aggregation states of didodecyldimethylammonium bromide (DDAB) in aqueous solution have been investigated as a function of DDAB concentration by conductivity, dynamic light scattering, spin labelling and turbidity methods. The concentration of DDAB was increased by a stepwise addition of a concentrated vesicle solution prepared by ultrasonic irradiation into water at 25 °C. It was suggested that the aggregation state of DDAB changes as follows with the increase in the concentration. ( i ) In the extremely low concentration below 0.1 mmol kg^-1, the added DDAB vesicles are completely destroyed to produce the surfactant monomer. ( ii ) In the concentration range between 0.1 and 0.7 mmol kg^-1, the vesicles are also completely destroyed, and micelles are formed, the size of which increases with the in-. crease in the concentration. ( iii ) In the concentration range between 0.7 and 1 mmol kg^-1, the destruction of the vesicles is weakened, and rather long-lived vesicles exist. (iv) In the concentration above 1mmol kg^-1, the vesicles are not destroyed and coexist with the micelles and the mononer. This characteristic concentration (1 mmol kg^-1) with respect to the vesicle destruction is regarded as a critical vesicle concentration (CVC). It was found that the CVC increased monotonically with the increase in ternperature, indicating that the DDAB vesicles are destroyed more easily at higher temperature.