Multimodal character of shear viscosity response in hydrogen bonded liquids

Abstract

The impact of supramolecular aggregate formation on the shear viscosity response of hydrogen bonded liquids was investigated. In particular, we study the shear mechanical response of several monoalcohols showing exponential and non-exponential shape dielectric Debye-like relaxation. In addition to the structural relaxation, distinctive of the glass transition, and the terminal crossover to pure viscous flow, characteristic of simple liquid flow, systematic analysis of complex viscosity curves evidences the presence of an additional intermediate process between those two. While the recovery of pure viscous flow would reflect the complete relaxation of the hydrogen bonded aggregates the intermediate process correlates with the rotational dynamics of hydroxyl groups, potentially caused by the breaking of individual hydrogen bonds.

Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

“…[28] It should also be noted that similar slow, sub-structural relaxations are also observed in some other mesoscopically ordered liquids, most notably monohydroxy alcohols, where they are also attributed to a supramolecular origin. [44][45][46][47][48][49] Despite the apparent similarities in the dielectric and dynamic-mechanical spectra, these techniques are sensitive to distinctly different correlations within the bulk liquid, i.e. polarization and the stress tensor, respectively.…”

“…[28] It should also be noted that similar slow, sub-structural relaxations are also observed in some other mesoscopically ordered liquids, most notably monohydroxy alcohols, where they are also attributed to a supramolecular origin. [44][45][46][47][48][49] Several factors could potentially influence the mesoscale aggregate dynamics such as the composition-dependent volume fraction, shape, and size of the aggregated non-polar domains as well as any alteration in ion-ion interactions at the polar/non-polar interfaces. By our choice of cation and anion for the two ionic liquid components we have attempted to minimize the latter effect and will not consider it further.…”

On the non-exponentiality of the dielectric Debye-like relaxation of monoalcohols

“…On the one hand, the high frequency peak in η′′(ω) matches that of the dielectric α relaxation, confirming its assignment to the structural relaxation. On the other hand, the characteristic time where the liquid recovers pure viscous flow (crossover to η′′(ω) ∝ ω) correlates well with that of the dielectric Debye-like relaxation in many MAs [17,19,21,22], supporting the idea that the dielectric Debye-like relaxation reflects the complete relaxation of hydrogen-bonded aggregates. The intermediate process observed in the shear response, however, seems not to have a clear dielectric analogue.…”

“…In order to avoid the extra level of complexity introduced by the dynamics of a second liquid we have studied in this work the evolution of the mechanic and dielectric response of 2E1H upon LiCl salt addition. Among different MA the choice of 2E1H results convenient due to the relatively large timescale separation between the structural and the Debye-like relaxation in this MA and its clear multimodal shear response [17,19]. Solutions of LiCl in 2E1H were prepared at 1/30, 1/20, 1/15, and 1/10 LiCl/2E1H molar ratios (between 1% and 3% weight percentage).…”

“…In addition to test the effect of the addition of a 'structure breaker' on the dynamics of 2E1H, in this work we also aimed at modifying the relative contribution of the slowest and intermediate processes to the response obtained by different techniques. In particular, one of the objectives was to make more evident the very subtle sign of the intermediate process in the dielectric relaxation [9,19,23] previously proposed by us. For pure 2E1H the width of the slowest dielectric relaxation slightly increases as temperature increases, or otherwise, as intensity decreases.…”