Characterization of Micelles of the Thermo-Responsive Zwitterionic Surfactant, 3-(<i>n</i>-Nonyl-<i>N</i>,<i>N</i>-Dimethylammonio)-Propyl Sulfate, Present in Isotropic Solutions Above Its Upper Critical Solution Temperature

The thermosensitive zwitterionic surfactant, 3-(<i>n</i>-nonyl-<i>N</i>,<i>N</i>-dimethylammonio) propyl sulfate (C₉-APSO₄), in solution displays a liquid-liquid miscibility gap exhibiting an upper critical solution temperature (UCST) in contrast to the lower critical temperature, as is characteristic of nonionic surfactants. In this study, the micellar and aggregational properties of this surfactant in its homogeneous isotropic one phase region above the UCST were examined using surface tension and small-angle neutron scattering (SANS) measurements. The critical micelle concentration (CMC) of C₉-APSO₄ was determined in both H₂O and D₂O at 40 and 70 °C as well as the cloud point (CP) temperature determined over a surfactant concentration range of 0.07-0.70 M. The CMC values were in the range of 48-55 mM (40 °C) and 61-65 mM (70 °C), respectively. The SANS data were analyzed using a core-shell model in which the C₉-APSO₄ is represented by a C-9 nonyl hydrophobic core and polar [(<i>N</i>,<i>N</i>-dimethylammonio)-propylsulfate] headgroup shell. Results revealed that the C₉-APSO₄ micelle aggregates are spherical in shape with a core diameter of 2.56 nm and shell thickness of 2.28 nm (total diameter 4.84 nm) and an aggregation number of 30 ± 1 at 40 °C. The data at 70 °C indicated that the aggregates were slightly larger in size, i.e., core diameter of 2.70 nm and shell thickness of 2.52 nm (total diameter 5.22 nm) with an aggregation number of 35. At a fixed surfactant concentration, the aggregation number and size slightly decreased with a decrease in temperature from 70 to 40 °C (as one approaches the CP and two-phase regime). These results were compared with that reported for other zwitterionic surfactant micellar systems.