Then, SHG fringes are collected in fourfold replicates for each solution condition. After each set of four fringes, the next solution is introduced to the system and is flowed for at least 30 minutes at 2 mL min -1 . Due to well established hysteresis effects, fringes are recorded in order of increasing ion concentration. For the SHG phase referencing, we use a freshly prepared pH 2.5, 500 mM NaCl aqueous solution each day (vide infra).