The molecular nature of the eliminating azeotropy in water–acetonitrile system by ionic liquid entrainer using FTIR and DFT calculations

• The structural properties of IL–water–acetonitrile mixtures are studied. • The mixing process is nonideality, with interactions occurring between components. • Three excess peaks are identified. • Species transformation with the addition of IL was identified. • The nature of the phase equilibrium changes in the azeotrope system was revealed. This study focused on investigating the structure and hydrogen-bonding characteristics of 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc])-water-acetonitrile mixtures aiming at revealing the molecular intrinsic of the azeotropy breaking by ionic liquid entrainer. Our approach combined FTIR with density functional theory (DFT) calculations applying the C N stretching vibration region as the responsive indicator of the microenvironment to discern the structural and hydrogen-bonding properties of the H 2 O‒CH 3 CN and [EMIM][OAc]‒CH 3 CN binary systems and [EMIM][OAc]−H 2 O−CH 3 CN ternary systems. Distinct interaction models were observed between [EMIM][OAc] and CH 3 CN compared to those in water and CH 3 CN system, leading to different influences on the C N stretching vibration region. Excess peaks associated with H 2 O−CH 3 CN, CH 3 CN self-aggregator, and IL−CH 3 CN interaction complexes were identified. The interaction strength between [EMIM][OAc] and CH 3 CN was notably more potent than that between water and CH 3 CN. Consequently, IL had the ability to disrupt the interaction complex of water-acetonitrile with the formation of the IL−CH 3 CN complex. This species transformation during the mixing process elucidated the characteristics of phase equilibrium shift in the azeotrope system. The water-acetonitrile interaction complex broken apart by the ionic liquid−acetonitrile interaction complexes