Identification of Structural Relaxation in the Dielectric Response of Water

Abstract

One century ago pioneering dielectric results obtained for water and n-alcohols triggered the advent of molecular rotation diffusion theory considered by Debye to describe the primary dielectric absorption in these liquids. Comparing dielectric, viscoelastic, and light scattering results we unambiguously demonstrate that the structural relaxation appears only as a high-frequency shoulder in the dielectric spectra of water. In contrast, the main dielectric peak is related to a supramolecular structure, analogous to the Debye-like peak observed in mono-alcohols. PACS: 82.30.Rs, 77.22.Gm, 83.60.Bc, 78.35.+c Being in the focus of intensive research for the last few centuries [1], water still presents many challenging scientific puzzles. They include complex phase diagram [2], possible liquid-liquid transition [3] and significant role of quantum effects [4]. Among them is also the anomalously large dielectric constant that makes water an excellent solvent and is exploited on a daily basis in microwave heating. Not only the amplitude, but also the spectral shape of water's dielectric response is rather peculiar. For most liquids the dominating dielectric relaxation process is the structural α-relaxation that has asymmetric spectral shape corresponding to a stretched exponential relaxation in time domain [5,6]. In contrast, the dielectric spectrum in water is dominated by a Debye-like peak (single exponential process I), and has another less intense relaxation feature (process II) at higher frequencies [7,8]. The microscopic mechanism triggering this response remains highly debated with the focus on the main question: Does the Debye process reflect molecular scale structural relaxation or polarization of intrinsic supramolecular structures mediated by H-bonds?In his seminal dielectric work [9] Debye himself argued in favor of the first scenario, based on hydrodynamic estimates of the rotational time for a single H2O molecule that appears close to the time scale of the process I, τI. Several recent studies also assigned process I to reorientation diffusion of single water molecules [10,11,12]. In contrast, other phenomenological works consider process I related with dynamics of H-bonded network [13,14,15]. One major problem is that dielectric spectroscopy lacks microscopic information [5] and standalone cannot clarify the molecular nature of the processes observed for water. Hence information from other techniques needs to be involved.In many aspects dielectric response of water resemble that known for mono-alcohols (MA) [16]. These liquids (e.g. npropanol [17]), also display a bimodal dielectric spectra with dominating low-frequency Debye-like peak. Although Debye again assigned the main peak of n-propanol to rotational diffusion of single alcohol molecules [9], it is known now that this process has strikingly different microscopic origin [16,18] Inspired by recent developments for MA, we pursue in this work the same strategy of combining dielectric, viscoelastic, and light scattering studies to unravel the nature o...

Nature of the Debye-Process in Monohydroxy Alcohols: 5-Methyl-2-Hexanol Investigated by Depolarized Light Scattering and Dielectric Spectroscopy

“…Moreover, it turns out that in DDLS the Debye contribution is broadened: Although the uncertainty in determining the stretching parameter is quite large due to the strong overlap of Debye-like contribution and α-process in DDLS the slowest process is characterized by β DDLS D = 0.65 ± 0.15 for all temperatures in Fig. 2, while the strong Debye contribution in the dielectric data is well described by β BDS D ≈ 1, in accordance with previous findings for Debye-contributions in BDS and DDLS [5,6].…”

“…Initially the Debye model was designed as a model to describe the structural relaxation [2], but later on it turned out that in monohydroxy alcohols the α-process is related to a relaxation faster than the Debye peak, and experiments have provided increasing evidence that the Debye contribution is caused by relaxation of transient supra-molecular structures [1]. In particular the recently proposed model of transient chains [3], which form due to H-bonding and, despite their transient nature, effectively lead to a relaxation of an average endto-end dipole vector, seems promising in that respect.Although it appeared for a long time that only dielectric experiments show evidence of the Debye process, recently it was identified in the shear mechanical response [4] and was also identified in DDLS of several H-bonding liquids [5,6]. However, a recent DDLS investigation of 1-propanol with an improved DDLS setup did not show any sign of a Debye contribution, even down to a level of a few percent of the α-relaxation amplitude [7].…”

Electromagnetic-radiation absorption by water

“…Nevertheless, even for this approach the HFPL still is well revealed as excess intensity in the loss around 1 THz. As mentioned above, in literature broadband spectra of water were fitted by different combinations of relaxation and resonance functions [2,9,15,18,20,21,22,27], with only marginal differences in the agreement of fit and experimental data. 275 K is the lowest temperature covered by the present investigation, where all processes should be maximally separated.…”

“…Its spectral shape can be relatively well fitted by a Debye relaxation-function [3] and it is often ascribed to the so-called  or structural relaxation, reflecting the molecular dynamics that governs, e.g., the viscosity of a liquid [4,5] dependence and for its relevance for the controversially discussed glass temperature of water.) However, very recently earlier ideas [1,7,8] were revived [9] explaining this spectral feature in a completely different way, namely along similar lines as the Debye relaxation known to arise in most monohydroxy alcohols [10,11,12]. This relaxation process is ascribed to the dynamics of clusters formed by hydrogenbonded molecules, which is significantly slower than the  relaxation, essentially arising from single-molecule motions [13].…”

Field induced changes in the ring/chain equilibrium of hydrogen bonded structures: 5-methyl-3-heptanol

“…The dielectric properties of these materials have been studied extensively with the goal of understanding the role of the hydrogen bonds regarding the structure and dynamics in these liquids. 1 Many monohydroxy alcohols, 2,3 various amides, [4][5][6] and possibly water 7,8 belong to a class of liquids for which the prominent dielectric relaxation process does not reflect the typical signatures of structural relaxation, such as the heat capacity step, [9][10][11] shear mechanical relaxation, 12,13 or density-density correlation functions. 14 Another common feature of these liquids is the Debye type nature of its prominent dielectric loss peak, which is retained even at the glass-transition for those cases where crystallization can be avoided.…”