Ultraslow Dynamics of Water in Organic Molecular Solids

Abstract

The relaxation dynamics of water in hygroscopic molecular solids is probed by broadband dielectric spectroscopy in the temperature range from 200 to 450 K. Evidence is found for three types of dynamic processes. The intermediate process is common to all probed materials and is associated with the reorientation of bound water molecules that are attached directly onto organic molecules and counterions. A faster process is observed in rhodamine chloride and fullerol, which is the dynamic signature of water in higher hydration layers, either at grain boundaries (rhodamine) or in interstitial clusters (fullerol). All these processes are observed near room temperature and exhibit nonmonotonic temperature dependence and decreasing spectral strength upon heating. In fullerol a third, ultraslow relaxation is observed at high temperature, which may be due to the reorientation of water–fullerol complexes or to the diffusion of water vapor through intermolecular voids.

Water-Triggered Conduction Mediated by Proton Exchange in a Hygroscopic Fulleride and Its Hydrate

“…Such normalization is equivalent to rescaling the spectra to the relative fullerene content. 10 The presence of water in the as-stored powder is confirmed by the observation of intense bands (indicated by arrows in Figure 1a) at 3465 cm -1 (stretching vibration of the O-H bonds of water) and 1690 cm -1 (bending mode), whose intensity is significantly reduced after heating to 423 K.…”

“…14 The non-monotonic temperature-dependence of σ dc and of the frequency of the loss maximum observable in Figure 2 may be at first surprising, as one expects both the conductivity and the characteristic loss frequency to increase with temperature. 18 However, such behavior is not uncommon in water containing porous systems near the water desorption temperature, 1,2,10,[23][24][25][26] as will be discussed in Section 3.b. Given the complexity of the results and the presence of two modalities of hydration in the as-stored powder (Figure 1c), we have probed separately the effect of the structural and surface hydration water.…”

“…The observation of well-defined diffraction peaks indicates that the as-stored material is in fact a crystalline hydrate, whose stoichiometry was shown to be C 60 (ONa) 24 · 16 H 2 O in a previous study. 10 We will hereafter refer to this phase as the hydrate salt. The diffraction pattern of the pure material, obtained by heating the hydrate, exhibits a much higher scattering background and significantly broader peaks, suggesting only partial ordered and smaller grain size in the pure material than in the hydrate.…”

“…10 Although fullerols have been suggested to act as proton conductors both in powder form, inside membranes and in aqueous solution, [11][12][13] no evidence for proton conductivity was reported in a recent studies on the pure C 60 (OH) 24 and C 60 (ONa) 24 materials. 10,14 In fact, pure C 60 (ONa) 24 exhibits low intrinsic conductivity stemming from the hopping of electronic charge carriers, and no evidence of ionic conductivity at least up to 575 K. 14 We show here that exposure of pure C 60 (ONa) 24 to ambient air leads to a dc conductivity increase by four orders of magnitude, which arises from the charge transport through the hydration water layers at the grains' surface. In the hydrate, the dc conductivity is strongly temperature-dependent, and it is found to be higher than that of the pure material by almost two orders of magnitude around 350 K. We argue that both conductivity enhancements are due to a hydrogen-bond exchange (proton shuttling) mechanism.…”

Collective relaxation dynamics and crystallization kinetics of the amorphous Biclotymol antiseptic

“…This is also confirmed by the fact that the activation energy (slope) of the secondary relaxation does not change below T g (see Figure 2), as it is instead customary for Johari-Goldstein relaxations (Ngai, 1998;Capaccioli et al, 2007). Judging from its relatively high activation energy (75.5±0.7 kJ mol -1 ), it is likely that this intramolecular secondary dynamics involves the hydrogen bonds formed by the hydroxyl groups (and possibly the chlorine atoms) of Biclotymol, as found in other organic small-molecule glass formers (Hensel-Bielowka et al, 2005), given that hydrogen bonds have a relatively high cleavage energy leading to a high activation energy of associated secondary processes (Macovez et al, 2014). 15 (Böhmer et al, 1993).…”