An Optimization Approach for the Production of High-Purity Vitamin C-Nicotinamide Cocrystals by the Gas Antisolvent (GAS) Technique with CO ₂ and Ethanol

Vitamin C (l-ascorbic acid, ASC) is a powerful antioxidant nutrient with diverse metabolic functions, regenerative properties, and anticancer potential. However, it is a highly unstable molecule. ASC can form a cocrystal with the amide of vitamin B₃ (nicotinamide, NIC) through self-complementary hydrogen bonding, therefore improving its physical stability. Pressurized carbon dioxide (CO₂), via the gas antisolvent (GAS) method, makes an excellent medium for cocrystallizing vitamins, particularly from ethanolic solutions. However, the controllable variables of the GAS method should be optimized for a feasible process. The production of the ASC:NIC cocrystal was optimized using a Box-Behnken experimental design (BBD) at 90 bar and with ethanol as the solvent while varying the temperature, CO₂ flow rate, and ASC:NIC molar ratio. The final ASC and NIC contents in the cocrystals were determined by derivative spectrophotometry and supported by HPLC and elemental analysis. PXRD and DSC confirmed that high-purity (>99%) cocrystals can be produced by setting a proper initial molar ratio of starting compounds. The maximum cocrystal yield by GAS (85.2%) was attained at the optimized condition using a lower pressure (80 bar) due to higher supersaturation of the system. Purest cocrystals exhibited a needle-like morphology, fine particle size, and thermal stability while preserving the antioxidant power of ASC with high crystallinity and displaying no cytotoxicity to healthy epithelial cells up to 0.5 mM. GAS with CO₂/ethanol could be optimized to overcome the solubility discrepancies between ASC and NIC in ethanol, producing vitamin C cocrystals at higher yields with a marked potential for nutritional and pharmaceutical applications.