On the X-ray scattering pre-peak of linear mono-ols and the related micro-structure from computer simulations

TL;DR

This study combines experiments and computer simulations to analyze the X-ray scattering pre-peak in linear alkanols, revealing how molecular clustering and model parameters influence the scattering features and micro-structure.

Contribution

It provides a detailed comparison of experimental and simulated X-ray scattering data, elucidating the origin of the pre-peak and the impact of force field parameters on micro-structural features.

Findings

Pre-peak position decreases with chain length n.

Pre-peak amplitude peaks at n=4, showing model dependence.

Hydroxyl head-group clustering relates to the pre-peak features.

Abstract

The X-ray scattering intensities I(k) of linear alkanols OH(CH2)n-1CH3, obtained from experiments (methanol to 1-undecanol) and computer simulations (methanol to 1-nonanol) of different force field models, are comparatively studied, particularly in order to explain the origin and the properties of the scattering pre-peak in the k-vector range 0.3A^{-1}-1A^{-1}. The experimental I(k) show two apparent features: the pre-peak position kP decreases with increasing n, and more intriguingly, the amplitude AP goes through a maximum at 1-butanol (n=4). The first feature is well reproduced by all force field models, while the second shows a strong model dependence. The simulations reveal various shapes of clusters of the hydroxyl head-group, from n>2. kP is directly related to the size of the \emph{meta-objects} corresponding to such clusters surrounded by their alkyl tails. The explanation of the Ap turnover at n=4 is more involved, in terms of cancellations of atom-atom S(k) contributions related to domain ordering. The flexibility of the alkyl tails tend to reduce the cross contributions, thus revealing the crucial importance of this parameter in the models. Force fields with all-atom representation are less successful in reproducing the pre-peak features for smaller alkanols n<6, possibly because they blur the charge ordering process since all atoms bear partial charges. The analysis clearly shows that it is not possible to obtain a model free explanation of the features of I(k)