Systematic studies on the dynamics, intermolecular interactions and local structure in the alkyl and phenyl substituted butanol isomers

Abstract

In this paper, we have studied local structure, interactions scheme and molecular dynamics of series of aliphatic butanol (AB) isomers: n-butanol, iso-butanol, sec-butanol, and their phenyl counterparts (PhB): 4-phenyl-1-butanol, 2-methyl-3-phenyl-1-propanol, and 4-phenyl-2-butanol by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR), and broadband dielectric spectroscopy (BDS) methods. XRD demonstrated that aside from the main peak related to the nearest-neighbour intermolecular correlations, there is a strong pre-peak at low scattering vector range for ABs, while for PhBs, this diffraction feature was weakly visible or not detected at all. At first sight, it suggests that molecules in aliphatic alcohols tend to associate and form medium-range order, while PhBs can be considered as disordered, simple liquids. However, further thorough FTIR and BDS spectroscopy investigations have shown that the phenyl moiety affects only slightly the degree of association and does not influence the strength of H-bonds in aromatic alcohols. What is more, PhBs are characterized by a similar Kirkwood factor (gk 1) to the ABs. 4-phenyl-2-butanol is characterized by the greatest gk ~ 3.7 among all studied herein alcohols, indicating a strong correlation between dipole moments and the formation of nanoassociates of chain-like topology in its structure. Combining results obtained from different experimental techniques, we pointed out that there are clear differences in dynamic and static properties between primary and secondary alcohols, including medium- and short-range order, variation in the strength of H-bonds and distribution of these types of interactions, the enthalpy of dissociation process, the glass transition temperature, and Kirkwood factor, irrespective of the presence of steric hindrance posed by the phenyl moiety. Results discussed in this paper clearly demonstrated that a superficial analysis of standard diffraction patterns, which are often the first step to probe the structure of materials, may lead to wrong conclusions. That is why complementary techniques must be applied together to understand the structure and behavior of assembling liquids.