Mutual diffusion coefficient of two alcohols (ethanol, n-butanol) in n-hexane from 298.15 K to 308.15 K

Reliable diffusion data for alcohol–alkane mixtures are still limited, particularly for systems exhibiting strong non-ideal behavior. In this work, mutual diffusion coefficients for ethanol + n-hexane and n-butanol + n-hexane were measured over the entire composition range at 298.15–308.15 K using digital holographic interferometry. The relative standard uncertainty associated with the optical measurement subsystem was estimated to be 0.0118. After propagation of the identified instrumental uncertainty sources, including temperature control, phase-parameter determination, composition preparation, and material purity, the instrumental relative standard uncertainty of the diffusion coefficients was evaluated to be 0.0121, corresponding to an expanded uncertainty of approximately 2.42% (k = 2). Considering the additional variability among independent experimental realizations, the overall expanded relative uncertainty was estimated to be approximately 4.8% (k = 2). For both systems, the mutual diffusion coefficient shows a pronounced U-shaped dependence on composition, with a minimum at intermediate alcohol mass fractions. In the limit of ω 1  → 0, the mutual diffusion coefficient approaches the tracer diffusion coefficient of alcohol in n-hexane, corresponding to diffusion under infinite dilution conditions. The observed behavior reflects strong non-ideal interactions, primarily associated with hydrogen bonding and composition-dependent molecular interactions. Compared with ethanol, the n-butanol + n-hexane system exhibits stronger non-ideality, consistent with the longer alkyl chain of n-butanol. The experimental data were evaluated using the Darken, Vignes, and UNIDIF models. Among them, the UNIDIF model provided the best overall agreement, although deviations remain significant over the full composition range. For ethanol + n-hexane at 298.15 K, the present results show good agreement with available literature data in terms of the overall composition-dependent trend, supporting the reliability of the measurement methodology. The new diffusion data reported here extend the available database for alcohol–alkane systems and provide a consistent basis for improving transport modeling.