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 0.3–1 Å–1. The experimental I(k) values show two apparent features: the pre-peak position k P decreases with increasing n, and more intriguingly, the amplitude A P goes through a maximum at 1-butanol (n = 4). The first feature is well reproduced by all force-field models, while the second shows strong model dependence. The simulations reveal various shapes of clusters of the hydroxyl head-group from n>2. k P is directly related to the size of the meta-objects corresponding to such clusters surrounded by their alkyl tails. The explanation of the A P turnover at n = 4 is more involved in terms of cancellations of atom–atom structure factor S(k) contributions related to domain ordering. The flexibility of the alkyl tails tends 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).

Ethanol is a hydrogen bonding liquid. When mixed in small concentrations with water or alkanes, it forms aggregate structures reminiscent of, respectively, the direct and inverse micellar aggregates found in emulsions, albeit at much smaller sizes. At higher concentrations, micro-heterogeneous mixing with segregated domains is found. We examine how different statistical methods, namely correlation function analysis, structure factor analysis and cluster distribution analysis, can describe efficiently these morphological changes in these mixtures. In particular, we explain how the neat alcohol pre-peak of the structure factor evolves into the domain pre-peak under mixing conditions, and how this evolution differs whether the co-solvent is water or alkane. This study clearly establishes the heuristic superiority of the correlation function/structure factor analysis to study the micro-heterogeneity, since cluster distribution analysis is insensitive to domain segregation. Correlation functions detect the domains, with a clear structure factor pre-peak signature, while the cluster techniques detect the cluster hierarchy within domains. The main conclusion is that, in micro-segregated mixtures, the domain structure is a more fundamental statistical entity than the underlying cluster structures. These findings could help better understand comparatively the radiation scattering experiments, which are sensitive to domains, versus the spectroscopy-NMR experiments, which are sensitive to clusters.

Substituting benzene for water in computer simulations of binary mixtures allows one to study the various forms of disorder, without the complications often encountered in aqueous mixtures. In particular, we study the relationship between the local order generated by different types of molecular interactions and the nature of the global disorder, by analyzing the relationship between the concentration fluctuations and the correlation functions and the associated structure factors. Alkane-benzene mixtures are very close to ideal mixtures, despite appreciable short range shape mismatch interactions, acetone-benzene mixtures appear as a good example of regular mixtures, and ethanol-benzene mixtures show large micro-segregation. In the latter case, we can unambiguously demonstrate, unlike in the case of water, the appearance of domain-domain correlations, both in the correlation functions and the structure factor calculated in computer simulations. This finding helps to confirm the existence of a pre-peak in the structure factor associated with the micro-heterogeneity, which was speculated from several of our previous simulations of aqueous-alcohol mixtures. The fact that benzene as a solvent allows us to solve some of the problems that could not be solved with water points towards some of the particularities of water as a solvent, which we discuss herein. The concept of molecular emulsion put forward in our earlier work is useful in formulating these differences between water and benzene through the analogy with direct and inverse micellar aggregates.