Is a dissociation process underlying the molecular origin of the Debye process in monohydroxy alcohols?

Herein, we investigated the molecular dynamics as well as intra-molecular interactions in two primary monohydroxy alcohols (MA), 2-ethyl-1-hexanol (2EHOH) and n-butanol (nBOH), by means of broad-band dielectric (BDS) and Fourier transform infrared (FTIR) spectroscopy. The modeling data obtained from dielectric studies within the Rubinstein approach [Macromolecules 2013, 46, 7525−7541] originally developed to describe the dynamical properties of self-assembling macromolecules allowed us to calculate the energy barrier (Ea)of dissociation from the temperature dependences of relaxation times of Debye and structural processes. We found Ea ∼ 19.4 ± 0.8 and 5.3 ± 0.4 kJ/mol for the former and latter systems, respectively. On the other hand, FTIR data analyzed within the van’tHoff relationship yielded the energy barriers for dissociation Ea ∼ 20.3 ± 2.1 and 12.4 ± 1.6 kJ/mol for 2EHOH and nBOH, respectively. Hence, there was almost a perfect agreement between the values of Ea estimated from dielectric and FTIR studies for the 2EHOH, while some notable discrepancy was noted for the second alcohol. A quite significant difference in the activation barrier of dissociation indicates that there are probably supramolecular clusters of varying geometry or a ring-chain-like equilibrium is strongly affected in both alcohols. Nevertheless, our analysis showed that the association/dissociation processes undergoing within nanoassociates are one of the main factors underlying the molecular origin of the Debye process, supporting the transient chain model

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

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

Aromaticity Effect on Supramolecular Aggregation. Aromatic vs. Cyclic Monohydroxy Alcohols

In this paper, the steric hindrance effect related to the presence of either an aromatic or cyclic ring on the self-association process in the series of monohydroxy alcohols (MAs), from cyclohexanemethanol to 4-cyclohexyl-1-butanol and from benzyl alcohol to 4-phenyl-1- butanol, was studied using X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared (FTIR) spectroscopy, Broadband Dielectric Spectroscopy (BDS) and the Pendant Drop (PD) methods. Based on FTIR results, it was shown that phenyl alcohol (PhA) and cyclohexyl alcohol (CA) derivatives reveal substantial differences in the association degree, the activation energy of dissociation, and the homogeneous distribution of supramolecular nanoassociates suggesting that the phenyl ring exerts a stronger steric impact on the self-assembling of molecules than cyclohexyl one. Additionally, XRD data revealed that phenyl moiety introduces more heterogeneity in the organization of molecules compared to the cyclic one. The changes in the self-association process of alcohols were also reflected in differences in the molecular dynamics of the H-bonded aggregates, as well as in the Kirkwood factor, defining the long-range correlation between dipoles, which were slightly higher for CAs with respect to those determined for PhAs. Unexpectedly it was also found that the surface layers of PhAs were more organized than those formed by CAs. Thus, these findings provided insight into the impact of aromaticity on the self–assembly process, Hbonding pattern, supramolecular structure, and intermolecular dynamics of the studied alcohols