2.2. Solubility Studies

Solubility studies of DNa in EtOH and distilled water as the main solvents, as well as in PG and PEG as viscosity and surface tension modifiers, were conducted to optimize the ink composition. In detail, 10 mL of each solvent was placed in airtight glass vials, and preweighted amounts of DNa were gradually added until a cloudy mixture was produced, indicative of exceeding the saturation solubility. The vials were kept under stirring (200 rpm) for 24 h. Aliquots (5 mL) of each mixture were withdrawn, centrifuged at 4000 rcf for 30 min, and filtered through a 0.45 μm Whatman Nylon filter to remove excess amounts of the API, and the supernatant was collected. The saturation solubility of DNa in each solvent was quantified by high-performance liquid chromatography (HPLC).