Communication: Slow supramolecular mode in amine and thiol derivatives of 2-ethyl-1-hexanol revealed by combined dielectric and shear-mechanical studies

In this paper, we present results of dielectric and shear-mechanical studies for amine (2-ethyl-1-hexylamine) and thiol (2-ethyl-1-hexanethiol) derivatives of the monohydroxy alcohol, 2-ethyl-1-hexanol. The amine and thiol can form hydrogen bonds weaker in strength than those of the alcohol. The combination of dielectric and shear-mechanical data enables us to reveal the presence of a relaxation mode slower than the α-relaxation. This mode is analogous to the Debye mode seen in monohydroxy alcohols and demonstrates that supramolecular structures are present for systems with lower hydrogen bonding strength. We report some key features accompanying the decrease in the strength of the hydrogen bonding interactions on the relaxation dynamics close to the glass-transition. This includes changes (i) in the amplitude of the Debye and α-relaxations and (ii) the separation between primary and secondary modes. Among different types of the intermolecular interactions, hydrogen bonds play an exceptional role in biology and practical applications. They are responsible for unique chemical and physical properties of polar substances such as water, alcohols, polyols, sugars, or their aqueous solutions. 1-3 The behaviour of hydrogen bonding liquids is often more complex and distinct from liquids with non-hydrogen bonding interactions. Their structure and dynamics are affected by the formation/reorganization of the extended hydrogen bonding networks of different sizes and geometries. In the highly viscous regime, this can bring about a breakdown of the otherwise general dynamic scaling rules (i.e., isochronal superposition or density scaling) and a variety of anomalous behaviour with decreasing temperature and increasing pressure. 11-13 This is a slow mode which is often very intense in the dielectric signal sometimes completely covering the signal of the α-relaxation. 18-21 It was earlier believed that the Debye relaxation a) Electronic address: [email protected] was only seen in the dielectric signal but with improved experiments and data analysis it has over the last 10-15 years been detected also by other experimental techniques, such as mechanical spectroscopy, dynamic light scattering, and nuclear magnetic resonance. 22-26 The origin of the Debye relaxation in monohydroxy alcohols has been hotly discussed and intensively upgraded throughout the last century. 27 Today the consensus is that the process is due to slow dynamics in supramolecular hydrogen bonded structures. However, there is still controversy about the mechanisms for the relaxation. The prevailing picture today is that the Debye relaxation can be viewed as a sequential rearrangement of the chain structures, analogous to what happens for polymeric aggregates, 28 either by selfrestructuring transient chain-like dynamics (end-to-end vector migration) 29 The size and the architecture of the intermolecular hydrogen bonding network depend on the molecular structure. It is believed that terminal hydroxyl group facilitates long chain-like structures, whereas intermediate attachment of the hydroxyl group to the alkyl chain creates steric hindrance and favors ring-like morphology

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