Characterization of micelles of the thermo-responsive zwitterionic surfactant, 3-(-nonyl N,N-dimethylammonio)-propyl sulfate, present in isotropic solutions above its upper critical solution temperature

Tanji, Tamao*; Kusunoki, Yuto*; Nakagawa, Taichi  ; Takase, Tsugiko* ; Ueda, Yuki   ; Motokawa, Ryuhei   ; Hinze, W. L.*; Takagai, Yoshitaka*

The thermosensitive zwitterionic surfactant, CAPSO, 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 was examined using surface tension and small-angle neutron scattering (SANS) measurements. The critical micelle concentration (CMC) of CAPSO was determined in both HO and DO at 40 and 70C as well as the cloud point temperature determined over a surfactant concentration range of 0.05-0.70 M. The CMC values were in the range of 48-55 mM (40C) and 61-65 mM (70C), respectively. The SANS data was analyzed using a core-shell model in which the CAPSO is represented by a nonyl hydrophobic core and polar [(N,N-dimethylammonio)-propylsulfate] headgroup shell. Results revealed that the CAPSO micelle aggregates are spherical in shape with a core diameter of 2.56 nm and shell thickness of 2.28 nm and an aggregation number of 30 at 40C. The data at 70C indicated that the aggregates were slightly larger in size, i.e., core diameter of 2.70 nm and shell thickness of 2.52 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 40C.