High-precision gigahertz-to-terahertz spectroscopy of aqueous salt solutions as a probe of the femtosecond-to-picosecond dynamics of liquid water

Abstract

Abstract. Because it is sensitive to fluctuations occurring over femtoseconds to picoseconds, gigahertz-to-terahertz dielectric relaxation spectroscopy can provide a valuable window into water's most rapid intermolecular motions. In response, we have built a vector network analyzer dielectric spectrometer capable of measuring absorbance and index of refraction in this frequency regime with unprecedented precision. Using this to determine the complex dielectric response of water and aqueous salt solutions from 5.9GHz to 1.12 THz (which we provide in the SI), we have obtained strong new constraints on theories of water's collective dynamics. For example, while the salt-dependencies we observe for water's two slower relaxations (8 and 1 ps) are easily reconciled with suggestions that they arise due to rotations of fully and partially hydrogen bonded molecules, respectively, the salt-dependence of the fastest relaxation (180 fs) appears difficult to reconcile with its prior assignment to liberations of single hydrogen bonds.

Terahertz absorption of lysozyme in solution

“…The experimental data 32 for the absorption coefficient of lysozyme were obtained by averaging absorption over the interval of frequencies 0.38 − 0.92 THz. There is therefore a sufficient frequency overlap between the data for bulk water 40 and solutions. 32 The refractive index of the protein, n p ≃ 1.55, is from Ref.…”

“…Figure 3. Real, ǫ ′ (ν), and imaginary, ǫ ′′ (ν), dielectric constants of water 40 and of TIP3P force-field model of water in the THz range of frequencies. MD simulations of TIP3P water at 300 K performed for this study were fitted with two Debye processes with dielectric increments ∆ǫ1 = 101.0 and ∆ǫ2 = 2.2 and relaxation times τ1 = 7.05 ps and τ2 = 0.07 ps.…”

“…In the analysis below, we use the input for the frequency-dependent dielectric constant of water from experiment 40 and model the electronic induced dipole of the protein by its refractive index. The force fields typically used in MD simulations miss polarizability of at least one of these two components.…”

New terahertz dielectric spectroscopy for the study of aqueous solutions

“…Given these attributes, our spectrometer provides unique capabilities for the accurate measurement of even very strongly absorbing materials such as aqueous solutions 9,19 .…”

“…Specifically, the true dielectric response (equivalently the absorbance and refractive index) of a mixture is not the weighted sum of the dielectric responses of its components. 9,16,[19][20][21] Moreover, this problem is particularly acute when the refractive index of the solvent changes very rapidly with frequency, as it does for aqueous solutions below The disadvantage is the steep power roll-off leading to low signal-to-noise ratio for higher frequencies in the terahertz region.…”