Ethyltrimethylammonium iodide is primarily used as a phase transfer catalyst in organic synthesis, facilitating the transfer of reactants between immiscible phases, such as aqueous and organic layers, to enhance reaction efficiency. It is also employed in the preparation of quaternary ammonium compounds, which are widely utilized in the production of surfactants, disinfectants, and fabric softeners. Additionally, this compound finds application in the synthesis of ionic liquids, which are used as solvents in various chemical processes due to their low volatility and high thermal stability. In biochemistry, it serves as a reagent for modifying proteins and studying enzyme mechanisms. Furthermore, it is used in the development of certain pharmaceuticals and in the stabilization of nanoparticles for advanced material applications.