Simultaneous determination of interfacial molarities of an alcohol, bromide ion, and water during an alcohol induced microstructural transition: the difference between medium and long chain alcohols

The transitions between surfactant aggregate structures are triggered by changes in chemical or physical stimulations, including addition of additives. Effects of added alcohols on aggregate morphologies correlate strongly with alcohol chain length. The local molarities of alcohol, water, and counterions in the interfacial regions play an important role in controlling the aggregate morphologies. However, direct experimental estimates of changes of interfacial alcohol molarities during alcohol induced micelle-to-vesicle transitions have never been reported. Ellipsoidal-wormlike micelle-vesicle transitions in CTAB/KBr aqueous solutions in the presence of long-chain octanol were characterized by using combined rheological, dynamic light scattering (DLS), transmission electron microscopy (TEM) and turbidity measurements. However, the transitions are absent with added butanol. The chemical trapping method (CT) was employed to understand the differences between medium- and long-chain alcohols in determining aggregate morphology. The CT method was used to estimate interfacial water, alcohol, and counterion molarities with increasing stoichiometric alcohol concentrations. With 55 mM alcohol added, the interfacial octanol molarity is 0.9 M, which is three times higher than that for butanol. With added octanol, the ellipsoidal-wormlike micelle-vesicle transition is accompanied by a concurrent sharp increase of interfacial water molarities and a decrease of interfacial counterion molarity, which is not observed with added butanol. The CT data was also employed to estimate the changes of Israelachvili's packing parameter with increasing added alcohol concentration. Our result provides critical molecular level information for understanding the morphological transitions of CTAB/additives.