Probing the Molecular Dynamics of Aqueous Binary Solutions with THz Time-Domain Ellipsometry

Abstract

Using a customized time-domain ellipsometer operating in the THz range, the molecular dynamics of a liquid binary solution based on water and isopropyl alcohol (2-propanol) is investigated. The setup is capable of detecting small changes in the optical properties of the mixture within a single measurement. The complex dielectric response of samples with different concentrations is studied through the direct measurement of the ellipsometric parameters. The results are described using an effective Debye model, from which the relaxation parameters associated with different activation energies can be consistently extracted. Significant deviations between experimental data and the theoretical expectations at an intermediate volume percentage of 2-propanol in water are observed and interpreted as produced by competing effects: the creation/destruction of hydrogen bonding on the one hand, and the presence of cluster/aggregation between water and alcohol molecules on the other.

Dielectric Response of Different Alcohols in Water-Rich Binary Mixtures from THz Ellipsometry

“…Moreover, because of the amphiphilic nature of alcohol and diol molecules, such a mixture behaves differently at various concentrations of the solute. In a previous work [15], we have already shown that when increasing the alcohol concentration in the mixture, the effective Debye model (describing fully interacting liquids) fails in predicting the observed dielectric response because inhomogeneous mixing affects the long-range collective H-bonding dynamics of water. This gives rise to a reduction in the rotational movements of molecules and therefore in the value of τ 1 .…”

“…The fit procedure is based on a nonlinear regression method, simultaneously applied to Re( ε) and Im( ε). All dielectric strengths ϵ i are kept fixed to the values reported in the literature [15,22,27,33], while ϵ ∞ is set considering the value extrapolated from the experimental data. Relaxation times τ 2 and τ 3 are only considered free parameters.…”

“…We developed a highly flexible custom system with an in-house calibration routine that allows us to probe the polarized response of solids and liquids with ease. Compared to a previous setup [15], the region used for the spectral investigation presently ranges from 0.1 THz to 1.6 THz, spanning more than a frequency decade and improving the bandwidth by a factor of 2. Polimeric TPX (polymethylpentene) lenses and conducting wire grid polarizers (WGPs) are used to collimate and focus the THz pulse and to control and select the polarization state of the signal, respectively.…”

“…Many spectroscopic methods have been used to investigate the unusual behavior of such mixtures such as neutron diffraction [11], nuclear magnetic resonance [12], Raman [13] and infrared spectroscopy [14], and recently time-domain THz spectroscopy [6] and ellipsometry [15], to name a few. In particular, the last two experimental techniques can provide useful information on the energy transitions and dynamics of aqueous mixtures, directly probing the H-bond mechanisms.…”