Thermodynamics, reaction Dynamics, and Temperature-Dependent Mechanistic insights into Surfactant-Free and Surfactant-Mediated formation of selenium nanoparticles in aqueous media

Abstract

This study investigates the thermodynamics of selenium nanoparticles (Se NPs) formation through the reduction of Se(IV) using natural lemon juice as a reducing and stabilizing agent, within the temperature range of 283 – 333 K, both in the absence and presence of cationic and anionic surfactants. Spectroscopic monitoring of the reaction revealed an optimal temperature of 323 K, as indicated by maximum surface plasmon resonance (SPR) intensity. The formed Se NPs were characterized by Transmission electron microscopy and UV–visible spectroscopy. Thermodynamic and kinetic parameters, including activation energy, enthalpy (ΔH^≠), entropy (ΔS^≠), and Gibbs free energy (ΔG^≠), were determined using Arrhenius and Eyring equations. The positive ΔH^≠ and ΔG^≠ values, coupled with a negative ΔS^≠, suggest an endothermic, non-spontaneous, and multi-step reduction process primarily driven by ascorbic acid present in lemon juice. The study highlights the degradation of citric acid at higher temperatures, resulting in reduced stabilization efficiency, which is effectively mitigated by the introduction of surfactants. These surfactants, including sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), enhance nanoparticle stability by acting as additional capping agents without participating in the rate-determining step. The findings provide molecular-level insights into the thermodynamics of nano selenium formation, emphasizing the role of temperature and surfactant interactions in optimizing green synthesis pathways for applications in diverse temperature environments.