*2.4. Stability Studies*

*Stability studies in solution.* Mixtures of 9ETADE and oxalic acid in 1:1 ratio and using Milli-Q water or acetonitrile as a solvent were conducted at room temperature at different times (2, 24 and 48 h) to follow phase evolution.

*Physical stability in solid state.* The physical stability of the two anhydrous salts was carried out by exposing the solids to the certain range of relative humidity (RH). The humidity inside the chamber/desiccator was maintained at 75% RH using a sodium chloride saturated salt solution [27]. The samples were exposed for one week at 40 ◦C and 1 month at room temperature and subsequentially analyzed by PXRD.
