On the X-ray Scattering Pre-peak of Linear Mono-ols and the Related Microstructure from Computer Simulations

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 0.3–1 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).

Isomeric effects in structure formation and dielectric dynamics of different octanols

“…Typically the diffraction intensities of monohydroxy alcohols exhibit two prominent features, (i) a main peak reflecting the average particle-particle distance, which is dictated by the size of the methylene group for larger alcohols and (ii) a characteristic pre-peak at lower momentum transfer that is assigned to clusters of hydroxyl groups surrounded by their alkyl tails 40,41 . In a previous study of monohydroxy alcohols 39 , we showed that the intensity of the pre-peak is not directly related to the local cluster microstructure, while its position is correlated to the mean size of the formed meta-objects. Strictly speaking, this structure is reflected in the various atom-atom correlation functions and structure factors, but these are not directly experimentally observed and can only be obtained through computer simulations.…”

“…For a detailed analysis, the intensity, FWHM (Full Width at Half Maximum) and position of both the pre-peak and the main peak were determined by a fit with a Pearson VII function and a linear slope in the corresponding peak area (see Ref. 39 for details). The corresponding error bars were determined by changing the fit ranges.…”

“…The systems were simulated in the isobaric-isothermal ensemble, at the temperature of T = 300 K and pressure p = 1 bar in order to represent the laboratory conditions as closely as possible (see Ref. 39 for more details). To maintain constant temperature, we used the v-rescale thermostat 51 , while the Parrinello-Rahman barostat 52,53 was used to keep the pressure constant.…”

“…Recent dielectric stud-ies of nM3H (n-Methyl-3-Heptanol) with n=3,4,5,6 support this assumption 14 . Although evidence of different types of molecular association and their interchange with pressure and temperature was found by spectroscopy and molecular dynamics simulations, many efforts have been made to trace the evolution of these supramolecular arrangements using direct structural probes such as X-ray and neutron diffraction, often in combination with molecular dynamics simulations [37][38][39] . However, interpreting the results obtained by these experimental approaches need to be critically discussed particularly when compared with calculated diffraction intensities.…”

On the nanoscopic structural heterogeneity of liquid n-alkyl carboxylic acids

“…Such a feature and corresponding behaviour as a function of chain length are uncommon in simple organic liquids, which are usually homogeneous at mid-long scales, presenting flat SAXS patterns below 1 Å À1 . Nevertheless, the pre-peak has been reported in markedly amphiphilic systems such as n-alcohols, [27][28][29][30] and n-aldehydes, 31,32 and also in more complex systems like ionic liquids, [33][34][35][36] deep eutectic solvents, 37 and pseudo-ionic liquids. [38][39][40] The trend of the pre-peak position as a function of the alkyl tail length is shown in Fig.…”