Solutions of 2NapFF (Figure ) were prepared at pH 10.5. Changing the counterion of the base used to deprotonate the 2NapFF at low concentration and high pH leads to different selfassembled structures in solution. Here we focus on two examples, the sodium salt and the tetrabutylammonium salt (2NapFF-Na and 2NapFF-TBA respectively). Solutions were prepared at concentrations of 5 to 100 mg/mL (5-75 mg/mL for 2NapFF-TBA). It is necessary to carefully control the shear history during formulation to prepare reproducible samples (Section 2, Supporting Information). 1D SAXS data were collected on solutions of increasing concentrations of 2NapFF-Na and 2NapFF-TBA. The data for the 2NapFF-Na can be fitted to a flexible cylinder form factor with a radius of ~4.2 nm (Figure , intensity offset for clarity; Figure and fitting parameters in Table , Supporting Information). The exceptions to this are the highest concentrations studied (60, 75, 100 mg/mL), where the flexibility has been lost and the fit is now to a rigid cylinder form factor. The length of the cylinders is outside of the fittable Q range (i.e., > 100 nm). These data demonstrate that the overall structure and dimensions of the cylinder are unchanged with concentration. The structures are actually hollow tubes; 7 however, using SAXS, the contrast from the hollow core cannot be detected and so a fit to a flexible cylinder is most appropriate. Examination of the scale parameter in the model fit, indicating the volume fraction of scatterers in solution, shows that this value increases with increasing concentration and as such, there are a greater number of cylinders formed. The reduced c 2 value of the fit increases with increasing concentration as the samples can no longer be considered to be dilute. scaling; photographs of (e) 10 mg/mL and (f) 75 mg/mL 2NapFF-Na (left) and 2NapFF-TBA (right). The scale bar for (e) and (f) represents 1 cm. The bubbles are due to the stirring protocol just prior to photographing.