Pure ethanol

The sol-gel method was used to prepare SiO₂/PEG/CGA hybrid materials. Tetraethyl orthosilicate (5.6 mL) (TEOS; Si(OC₂H₅)₄; Sigma-Aldrich, St. Louis, MO, USA), used as metal alkoxide precursor, was added in a solution of HNO₃ (1.0 mL) (≥65%, Sigma-Aldrich), distilled water (2.2 mL) and pure ethanol (8.8 mL) (99.8% Sigma-Aldrich) to obtain the silica matrix. The microstructural properties of the inorganic matrix are affected by acid pH, as well as the H₂O/alkoxide molar ratio [52 (link)].
To the silica sol, a solution of PEG (MW = 400, Sigma-Aldrich) (C = 6 wt%) solubilized in 3.5 mL of ethanol (99.8%, Sigma-Aldrich) was added and the mixture stirred for 15 min. Subsequently, solutions of CGA (95%, Sigma-Aldrich) of different concentrations (C = 5, 10 wt%) dissolved in 3.5 mL of ethanol (99.8%, Sigma-Aldrich) was slowly added to the silica and PEG solution while stirring and kept stirring for 15 min. After 20 min the various gels were air-dried at 40 °C for 24 h to obtain a dry powders and to remove the residual solvent to prevent the thermal degradation of both polymer and drug. In the gels the molar ratios of the reagents are: EtOH/TEOS = 6.2 TEOS/HNO₃ = 1.7, H₂O/TEOS = 6. A flow chart of the sol-gel procedure used is shown in Figure 9.

Sodium chloride crystals were prepared as detailed by Marshall [18] . Briefly, a saturated solution of sodium chloride (Sigma-Aldrich) was prepared and a 5% additional volume of reverse osmosis (RO) water was added. Four 25 mL aliquots of this solution were frozen in dry ice, then broken apart and shaken vigorously in 2 L pure ethanol at − 20 °C (Sigma-Aldrich). Once the frozen salt was completely melted, the precipitate was collected by vacuum filtration, prior to lyophilisation. This product is referred to in this paper as microcrystalline sodium chloride.
For comparison, dry sodium chloride crystals (Sigma-Aldrich) were thoroughly ground in a pestle and mortar.
Sodium chloride crystal face lengths were assessed by analysing light micrographs, obtained using an IX51 microscope (Olympus). Face lengths were measured using Cell^P software (Olympus).