Chemical Speciation of Nicotinamide in Aqueous Solution: Ionic Strength and Medium Effects and Data Analysis

Despite its widespread use in pharmaceuticals, cosmetics, and animal nutrition, the environmental implications of nicotinamide remain largely underexplored. Given its high solubility and prevalent presence in wastewaters, nicotinamide has been proposed as a potential environmental marker and emerging contaminant. This study focuses on the evaluation and modeling of medium and ionic strength effects on the acid-base behavior of nicotinamide in aqueous solution. To this aim, the protonation constants of nicotinamide were determined in aqueous media containing various supporting electrolytes (sodium chloride, tetramethylammonium chloride, tetraethylammonium iodide) at T = 298.15 K and different ionic strengths. The medium and ionic strength dependencies were described through well-established thermodynamic models, including the Extended Debye–Hückel equation, the Specific ion Interaction Theory, and the Pitzer approach. Additionally, the Setschenow constant of nicotinamide was determined through distribution measurements between 1-octanol and NaCl(aq) solutions. Furthermore, a critical data analysis was performed to assess the reliability of the proposed models and provided interaction parameters. Overall, this work thoroughly describes the acid-base behavior of nicotinamide in different aqueous media and its distribution between aqueous and organic phases, offering fundamental insights for the assessment of its chemical speciation in real systems, which is crucial for future applications in environmental monitoring and remediation strategies.