Unexpected composition dependence of the first sharp diffraction peak in an alcohol-aldehyde liquid mixture: n-pentanol and pentanal

The total scattering structure factors of pure liquid n-pentanol, pentanal, and 5 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 Q range above 1 ${\AA}^{-1}$, but the absolute values of the intensities of the small-angle scattering maximum ('pre-peak', 'first sharp diffraction peak'), around 0.6 ${\AA}^{-1}$, change in an unexpected fashion, non-linearly with the composition. MD simulations are not able to reproduce this low-Q behavior; on the other hand, they do reproduce the experimental diffraction data above 1 ${\AA}^{-1}$ 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.