There is however good evidence that sorption of Br⁻ on silica-gel, if any, is very weak and can be neglected. Indeed, Br⁻ is a large anion that could form outer sphere complexes only on positively charged surfaces, but the extensive review of shows that all silica surfaces, including silica-gel, have very low points of zero charge (PZC), usually at pH < 3-4. The highest reported PZC for a silica-gel in NaCl is 5.7-6.4, however this singular value is highly uncertain . Therefore, it can be safely assumed that the silica-gel surface is negatively charged, and no Br⁻ sorption occurred during our experiment. Moreover, substitution of Br⁻ for O 2-in the chain network structure of the silica gel can be excluded based on the large ionic size of Br⁻. Consequently, the fitted diffusion coefficient can be set equal to 𝐷 (𝐷 = 𝐷 ) and 𝐾 = 0, so that Eq. ( ) simplifies to