Unexpected Composition Dependence of the First Sharp Diffraction Peak in an Alcohol–Aldehyde Liquid Mixture: n‐Pentanol and Pentanal

Abstract

The total scattering structure factors of pure liquid n‐pentanol, pentanal, and five of their mixtures, as determined by high energy synchrotron X‐ray diffraction experiments, are presented. For the interpretation of measured data, molecular dynamics computer simulations are performed, utilizing “all‐atom” type force fields. The diffraction signals in general resemble each other over most of the monitored scattering variable, Q, range above 1 Å−1, but the absolute values of the intensities of the small‐angle scattering maximum (“pre‐peak,” “first sharp diffraction peak”), around 0.6 Å−1, change in an unexpected fashion, non‐linearly with the composition. Molecular dynamics simulations are not able to reproduce this low‐Q behavior; on the other hand, they do reproduce the experimental diffraction data above 1 Å−1 rather accurately. Partial radial distribution functions are calculated based on the atomic coordinates in the simulated configurations. Inspection of the various OO and OH partial radial distribution functions clearly shows that both the alcoholic and the aldehydic oxygens form hydrogen bonds with the hydrogen atoms of the alcoholic OH‐group.