The solution morphology of polycationic brushes was next characterised. Different models were examined first (Supplementary Figure ), using data recorded in D2O, as it gave strongest fringes that could be used to evaluate the quality of the fits, in addition to correlation coefficients. All models presented at least a layer of silicon oxide (considering its importance to fit dry brush data) in addition to two brush layers. With 2-4 layers, fits remained relatively poor, in particular at high Q values, and failed to capture the detail of the weak fringes observed in the range of 0.007-0.05 Å -1 . A slight improvement was observed when introducing a gradient in scattering length density along the brush height (corresponding to neutral brushes in the first layer and increasing gradually to the SLD of a protonated brush in the upper layer). This was proposed as a potential model to take into account the protonation gradient proposed to establish in weak polyelectrolyte brushes in deionised water . However, the correlation coefficient remained high. It was also potentially difficult to distinguish contributions stemming from an increased SLD due to protonation, from increased hydration within each layer. Therefore, gradient models were not explored further to reduce the number of parameters introduced in our fitting. With a fifth brush layer, a better fit was observed, with improved correlation coefficients, although residual fringes were too weak in the neutron reflectometry data to allow resolving.