International Science Community Association Research Journal of Engineering Sciences 15 1 2026 01 26 Characterization of Non-Ideal Behavior in Acetonitrile-Based Binary liquid mixtures via Excess Viscosity and Viscosity Measurements at Different Temperatures 16 24 EN Quipo June C. 1 College of Engineering, Nueva Vizcaya State University, Bayombong, Nueva Vizcaya, 3700, Philippines Jagtap Viplove Kumar 1 Department of Civil Engineering, Sushila Devi Bansal College of Engineering, Indore, MP, India Golghate Kavita 2 Department of Civil Engineering, Sushila Devi Bansal College of Engineering, Indore, MP, India Gupta Vikesh 3 Department of Applied Science & Humanities, Sushila Devi Bansal College of Engineering, Indore, MP, India Awasthi Naveen 1 Department of Chemistry, Janta College Bakewar (206124), Etawah, India > Research Article 2025 9 16 2026 01 26 This paper is concerned with the excess viscosity and molecular interactions in binary liquid mixtures which are essential for understanding non-ideal solution behavior and optimizing industrial processes. This work investigates the binary systems of Acetonitrile + Formamide and Acetonitrile + N-Methylacetamide at temperatures ranging from 293.15 K to 313.15 K. Experimental measurements of density, viscosity, and reduced molar volume were performed over the full concentration range at atmospheric pressure. Excess viscosity (ηᴱ) and theoretical viscosities were calculated using the Prigogine-Flory-Patterson model and compared with experimental results. Significant non-ideal behavior was observed, characterized by negative excess viscosity for the Acetonitrile + Formamide system and predominantly positive excess viscosity for Acetonitrile + N-Methylacetamide. Theoretical models showed large deviations from experimental values, particularly at higher temperatures, suggesting inadequacies in conventional predictive approaches. The findings highlight the need for improved models capable of accounting for hydrogen bonding and dipolar interactions. Copyright@ International Science Community Association