The role of counterion on the thermotropic phase behavior of DODAB and DODAC vesicles

Dioctadecyldimethylammonium bromide and chloride surfactants (DODAX, X representing Br− or Cl− counterions) assemble in water, above their melting temperatures (Tm), as cationic unilamellar vesicles at the typical surfactant concentration of 1.0 mM. The larger Tm of DODAC (49 °C) relative to DODAB (45 °C) has been attributed to the differing affinity and binding specificity of the counterions to the vesicle interfaces. In this communication it is reported differential scanning calorimetry (DSC), conductimetry and dynamic light scattering (DLS) data for mixtures of DODAB and DODAC in water at 1.0 mM total surfactant concentration and varying surfactant concentration, to investigate the effect of counterion on the pre-, main- and post-transition temperatures (Ts, Tm and Tp), and the data compared to the neat surfactants in water. Accordingly, Tm increases sigmoidally from 45.8 to 48.9 °C when DODAC molar fraction (xDODAC) is varied from 0 to 1. Neat DODAB exhibits in addition to Tm, Ts and Tp that are inhibited by DODAC. The main peak width ΔT1/2 does not depend on the surfactant molar fraction but the melting enthalpy change ΔH is smaller for DODAB-rich dispersions due to the stronger affinity of Br−. The conductivity and the apparent hydrodynamic diameter as well do not vary much with xDODAB, indicating that the surface charge density is similar for DODAB and DODAC, evidencing the role of the counterion binding specificity and affinity on the properties of DODAX vesicles.