The MgSO 4 water solution was prepared by dissolving MgSO 4 •7H 2 O (Sigma-Aldrich, Gillingham, UK) in Milli-Q water (IQ 7000, Merck, Darmstadt, Germany, resistivity 18.2 MΩ cm) to the desired concentration in a beaker. The mixture was placed on a magnetic stirrer for constant stirring until the sample was fully dissolved. It was sometimes heated to a slightly higher temperature (in the range of 25 °C to 35 °C) when preparing solutions with high concentrations to facilitate dissolution. EDTA solution was used for cleaning and purging the crystallisation cell after each measurement. For the cleaning solution, commercial EDTA solution (pH = 8; Fisher Scientific, Loughborough, UK) was mixed with NaOH solution (Reagecon Diagnostics, Shannon, Ireland) to adjust the pH to 10, in which MgSO 4 •7H 2 O has a better solubility.