A Small-Angle Neutron Scattering Investigation of the TDMAO Micelle System at High Hydrostatic Pressure

The phase behaviour and phase transition of the tetradecyldimethylaminoxide micellar system TDMAO/ D₂O have been investigated by small-angle neutron scattering (SANS) at temperatures T between 287 and 325 K, pressures up to P = 842 MPa and at TDMAO concentrations of 55 and 102 mmol l⁻¹. A pressure-induced structural (shape) phase transition was observed for the whole pressure versus temperature range, with a slope of the transition line of dT_m/dP ≃ 0.31° MPa⁻¹. The reversibility and reproducibility of this phase change were tested. The SANS experiments showed a pressure-induced structural phase transition from a low-pressure phase (i), where rod-like micelles with a radius of R = 1.94 ± 0.03 nm were present, to a high-pressure phase (ii). An accurate determination of the structure of phase (ii) has not been possible up to now. In phase (ii), a discrete correlation peak in the Q range 1.8–2.2 nm⁻¹ was found.