Structure of the isotropic and lamellar phase of a solution of tetramethylammonium perfluorononanoate in D2O

A 30% by weight solution of tetramethylammonium perfluorononanoate in D2O (TMAHFN; C8F17COON(CH3)4) is in a lamellar state below 33°C. In this state the repeat distance and the thickness of the perfluoronated part of the micellar lamellae are around 7.11 and 2.14 nm, respectively. In the presence of a strong magnetic field of 2.0 T a slow reduction of the sample temperature from 40°C (isotropic phase) via 34°C (nematic phase) down to 25°C gives a strong alignment of the nematic and lamellar liquid crystalline phases. By small angle X-ray scattering (SAXS) experiments, the mean scattering length density across the lamellae, the structure factor related to the one-dimensionality of the lamellar crystal, and information about the perforation of the lamellae could be extracted. The scattering intensity distribution of the isotropic phase can be described quite satisfactorily assuming disklike micelles, where the perfluoronated part of the micelle has a thickness of 2.4 nm and a radius of about 3.0 nm.