Experiments. Details of our HD-SHG spectrometer have been previously described. Solutions were flowed through a home-built Teflon sample cell at a rate of 5 mL min -1 for ten minutes for a complete exchange of the sample cell contents. This process was monitored using homodyne-detected SHG. We have previously found the sample cell exchange time to be around three to four minutes. Interference fringes were collected by translating an a-quartz crystal 30 equidistant positions along a 100 mm translational stage, with an acquisition time of approximately 3 minutes per fringe. Sets of five replicate interference fringes were collected for each ionic strength/salt concentration condition, over a period of 20 minutes. We control for phase drift by clamping in the sample cell two to three hours prior to the experiment, after which the spectrometer's phase is stable for up to six hours. The salt titrations were performed in a stepwise fashion, with five replicate measurements acquired for each aqueous phase condition. Each salt titration was repeated once, and our analysis is from the duplicate isotherms. Starting at ultrapure water (Millipore-Sigma 18.2MΩ cm, pH 5.8, strength of 100 mM, after which we return the ionic strength in a stepwise fashion to the starting condition of ultrapure water at 2 µM. Using the procedure described, the phase drift during the duration of the experiment is negligible (<1°).