Aggregation morphologies of a series of heterogemini surfactants with a hydroxyl head group in aqueous solution

The aggregation behaviors of a series of heterogemini surfactants N,N-dimethyl-N-[3-(alkyloxy)-2-hydroxypropyl]-alkylammonium bromide, abbreviated as CmOhpNCn (m, n = 10, 8; 10, 14; 12, 8; 12, 10; 12, 12; 12, 14; 14, 8 and 14, 10), have been investigated in aqueous solution. Electric conductivity, steady-state and frequency sweep rheological measurements were adopted to study the micellar morphology affected by molecular structure, concentration and temperature, and a close relationship among them was established. Electric conductivity measurement results indicated that Krafft temperatures were mainly related to the total number of carbon atoms, while the CMC values were affected by the combined factors of alkyl chain length, degree of dissymmetry and hydroxyl head group. And some unusual values may come from the poor water solubility induced by the relatively long alkyl chain n. Dynamic rheological results and cryo-TEM images revealed various micellar behaviors of CmOhpNC8 affected by different combined factors. When m value was increased, the formation of quite long rodlike or wormlike micelles derived from the enhanced hydrophobility and hydrogen bonding. And the effect of concentration on aggregation behavior was similar to that of m value except for formation of micellar joints. With temperature increase, the decreased viscosity of C14OhpNC8 was due to formation of micellar joints and weakening of hydrogen bonding, which could be confirmed by dynamic light scattering (DLS). Furthermore, the C14OhpNC8 aggregates formed below the Krafft temperature were more likely to be affected by temperature at high concentrations.