Dielectric spectroscopy study on ionic liquid microemulsion composed of water, TX-100, and BmimPF6.

We report here a broadband dielectric spectroscopy study on an ionic liquid microemulsion (ILM) composed of water, Triton X-100 (TX-100), and 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF(6)). It is found that the phase behavior of this ILM can be easily identified by its dielectric response. The dielectric behavior of the ILM in the GHz range is consistent with that of TX-100∕water mixtures with comparable water-to-TX-100 weight ratio. It consists of the relaxations due to ethylene oxide (EO) unit relaxation, hydration water dynamics, and∕or free water dynamics. The water content dependence of the EO unit relaxation suggests that this relaxation involves dynamics of hydration water molecules. In the IL-in-water microemulsion phase, it is found that bmimPF(6) molecules are preferentially dissolved in water when their concentration in water is lower than the solubility. An additional dielectric relaxation that is absent in the TX-100∕water mixtures is observed in the frequency range of 10(7)-10(8) Hz for this ILM. This low-frequency relaxation is found closely related to the bmimPF(6) molecule and could be attributed to the hopping of its cations∕anions between the anionic∕cationic sites.

Does transparent nematic phase exist in 5CB∕DDAB∕water microemulsions? From the viewpoint of temperature dependent dielectric spectroscopy.

Liquid crystal colloids have received tremendous attention because of its great potential both in the understanding of the liquid crystalline phase and in searching for new application of liquid crystals. Inverse microemulsion composed of 4-cyano-4-n-pentylbiphenyl (5CB), didodecyl dimethyl ammonium bromide, and water was investigated by means of broadband dielectric spectroscopy in this study. Based on the understanding of previous investigations on the same system, the isotropic phase was taken into account to quantitatively characterize the bulklike relaxations after the isotropic-to-nematic phase transition. Analogous results concerning the phase transition and phase composition to other investigations were obtained. In addition to bulklike relaxations, a new relaxation was observed at the frequency range about two orders lower than bulklike relaxations. This new relaxation shows abnormal temperature dependence, suggesting that superstructures composed of water droplets and confined 5CB molecules exist. This superstructure possibly possesses a confined nanoscaled liquid crystal ordering and may correspond to the notion of the transparent nematic phase.

Hydropower generation by transpiration from microporous alumina.

Traditional hydropower generation is one of the most sustainable energy sources; however, the local environmental impact of hydroelectric dams and reservoirs is serious, and hydroelectric power requires high-cost turbines and generators. Because these installations utilize gravitational potential energy of massive volumes of falling water, this sort of hydropower generation is unsuitable for ubiquitous, small-scale energy production. Here, we report that wetting and evaporation of pure water from a tiny block of porous alumina generates electrical current in the direction of water transpiration. The current induced in microporous alumina is associated with mass transport of water accompanying ions that accumulate near the negatively charged surface of alumina pores. Without any pre-treatment or additives, once evaporation commences, a 3 × 3 cm2 piece of alumina can generate an open-circuit voltage as large as 0.27 V. The power generation scheme we propose here is simple, clean, and versatile, and it can be employed anywhere, as it utilizes only spontaneous capillary action of water and Coulombic interaction at the alumina-water interface, without requiring any input of heat or light.

Liquid crystalline phase in ethanol suspension of anisometric 4-cyano-4-n-hexylbiphenyl microcrystals: Witnessed by dielectric spectroscopy.

The suspensions of 4-cyano-4-n-hexylbiphenyl (6CB) anisometric microcrystal were obtained by quenching homogeneous 6CB/ethanol mixtures with different 6CB concentrations. Such suspensions were strongly suggested from the differential scanning calorimetry examinations and the image observations of the samples during the quench and heating processes. The crystallization process for the mixtures with higher 6CB concentration looked like the isotropic-nematic phase transition of bulk liquid crystal. Dielectric measurement was carried out on the mixtures during the heating process after quench. Distinct dielectric relaxation was observed in the frequency range between kHz and 100 kHz, which exhibited obvious dependence on temperature and 6CB concentration. Curve fitting on the complex conductivity spectra indicates that three Debye type relaxations exist in this narrow frequency range. Based on the dependences of relaxation parameters on temperature and 6CB concentration, the possible relaxation mechanisms and the phase conformation of the mixtures were discussed. It was concluded that the relaxations, from high to low relaxation frequency, originate from the Maxwell-Wagner polarization, the rotation of 6CB microcrystal around its long axis, and the reorientation of 6CB microcrystal around its short axis, respectively. It was also confirmed that the quenched 6CB/ethanol mixtures show isotropic-nematic phase transition with the increase of 6CB concentration.

Ultraslow dielectric relaxation process in supercooled polyhydric alcohols.

Complex permittivity was obtained on glycerol, xylitol, sorbitol and sorbitol-xylitol mixtures in the supercooled liquid state in the frequency range between 10 microHz and 500 MHz at temperatures near and above the glass transition temperature. For all the materials, a dielectric relaxation process was observed in addition to the well-known structural alpha and Johari-Goldstein beta relaxation process [G. P. Johari and M. Goldstein, J. Chem. Phys. 53, 2372 (1970)]. The relaxation time for the new process is always larger than that for the alpha process. The relaxation time shows non-Arrhenius temperature dependence with correlation to the behavior of the alpha process and it depends on the molecular size systematically. The dielectric relaxation strength for the new process shows the effect of thermal history and decreases exponentially with time at a constant temperature. It can be considered that a nonequilibrium dynamics causes the new process.

Difference and similarity of dielectric relaxation processes among polyols.

Complex permittivity measurements were performed on sorbitol, xylitol, and sorbitol-xylitol mixture in the supercooled liquid state in an extremely wide frequency range from 10 microHz to 500 MHz at temperatures near and above the glass transition temperature. We determined detailed behavior of the relaxation parameters such as relaxation frequency and broadening against temperature not only for the alpha process but also for the beta process above the glass transition temperature, to the best of our knowledge, for the first time. Since supercooled liquids are in the quasi-equilibrium state, the behavior of all the relaxation parameters for the beta process can be compared among the polyols as well as those for the alpha process. The relaxation frequencies of the alpha processes follow the Vogel-Fulcher-Tammann manner and the loci in the Arrhenius diagram are different corresponding to the difference of the glass transition temperatures. On the other hand, the relaxation frequencies of the beta processes, which are often called as the Johari-Goldstein processes, follow the Arrhenius-type temperature dependence. The relaxation parameters for the beta process are quite similar among the polyols at temperatures below the alphabeta merging temperature, T(M). However, they show anomalous behavior near T(M), which depends on the molecular size of materials. These results suggest that the origin of the beta process is essentially the same among the polyols.

Systematic study of the glass transition in polyhydric alcohols.

We have investigated the glass transitions of trihydric alcohols using broadband dielectric spectroscopy, and compare the results with those previously reported for sugar alcohols. Although a systematic glass transition feature related to molecular size has been reported for sugar alcohols, the essential factor governing this feature is still unclear because the number of carbon atoms (N(C)) and the number of OH groups (N(OH)) per molecule are identical in sugar alcohols. By examining trihydric alcohols (N(C)≠N(OH)), we conclude that N(OH) is dominant for the characteristics of the slow dynamics, such as fragility and glass transition temperature. This result suggests that the topological structure of the hydrogen-bonding network (coordination number) plays an important role in the glass transition of polyhydric alcohols. Furthermore, the orientational correlation factor evaluated using the Kirkwood-Fröhlich theory reveals a similarity in hydrogen bond formation among a variety of polyhydric alcohols. Based on these two experimental results, we discuss a possible physical picture of the glass transition of polyhydric alcohols.

Time-dependent phase behavior of 5CB-DDAB-water microemulsions monitored by means of dielectric spectroscopy.

The phase behavior of an inverse microemulsion composed of 4-cyano-4-n-pentylbiphenyl (5CB), didodecyl dimethyl ammonium bromide, and water was intensively studied recently, but controversy still remains on the existence of a "transparent nematic" (TN) phase. Previously, we studied the temperature-dependent dielectric behavior of this microemulsion [Z. Chen and R. Nozaki, J. Chem. Phys. 134, 034505 (2011)]. The results suggested that a superstructure composed of water droplets and confined 5CB molecules exists in the coexistence phase region, which may correspond to the notion of a TN phase. In this study, the time-dependent dielectric behaviors of this microemulsion in the isotropic, coexistence, and supercooled phase regions were investigated. In the coexistence and supercooled phase regions, the dielectric behaviors showed strong dependence on time, due to the hysteresis of phase transition and the upward diffusion of the water-droplet-rich isotropic phase. After the isotropic phase has been totally excluded out of the effective measurement area, the slow process was still observed in these two phase regions, which suggests that water droplets can stably exist in the liquid crystalline phase. A local microscopic phase evolution was observed in the coexistence phase region, which is similar to the temperature-induced phase evolution previously proposed by us. The dielectric behavior of the slow process during the phase evolution also suggests the existence of the superstructure. In addition, the mechanism of the slow process in different phase regions proposed in our previous study is also confirmed.

Model of the cooperative rearranging region for polyhydric alcohols.

A simplified model of a hydrogen-bonding network is proposed in order to clarify the microscopic structure of the cooperative rearranging region (CRR) in Adam-Gibbs theory [G. Adam and J. H. Gibbs, J. Chem. Phys. 43, 139 (1965)]. Our model can be solved analytically, and it successfully explains the reported systematic features of the glass transition of polyhydric alcohols. In this model, hydrogen bonding is formulated based on binding free energy. Assuming a cluster of molecules connected by double hydrogen bonds is a CRR and approximating the hydrogen-bonding network as a Bethe lattice in percolation theory, the temperature dependence of the structural relaxation time can be obtained analytically. Reported data on relaxation times are well described by the obtained equation. By taking the lower limit of the binding free energy with this equation, the Vogel-Fulcher-Tammann equation can be derived. Consequently, the fragility index and glass transition temperature can be expressed as functions of the number of OH groups in a molecule, and this relation agrees well with the reported experimental data.

Dynamics and structure of hydrogen-bonding glass formers: comparison between hexanetriol and sugar alcohols based on dielectric relaxation.

Broadband dielectric spectra of supercooled 1,2,6-hexanetriol are presented in order to reveal physical picture behind a glass transition of polyhydric alcohols. It has been reported so far that temperature dependences of alpha relaxation time for sugar alcohols exhibit systematic trend against number of carbon atoms or OH groups per molecule. However, because each molecule is composed of equal number of carbon atoms and OH groups in the case of the reported sugar alcohols, the more dominant parameter to govern the alpha relaxation dynamics has not been discussed. By using a chemical structure of the hexanetriol composed of the deferent number of carbon and OH, it is possible to determine the dominant parameter. From temperature dependence of alpha relaxation times, it is strongly supported that the number of OH groups is the dominant parameter. Furthermore, from an analysis of static dielectric constant, it is suggested that local hydrogen-bonding structure is similar among all polyhydric alcohols. From these two results, a simple picture of the origin of the systematic character is proposed.

New sample cell configuration for wide-frequency dielectric spectroscopy: DC to radio frequencies.

A new configuration for the sample cell to be used in broadband dielectric spectroscopy is presented. A coaxial structure with a parallel plate capacitor (outward parallel plate cell: OPPC) has made it possible to extend the frequency range significantly in comparison with the frequency range of the conventional configuration. In the proposed configuration, stray inductance is significantly decreased; consequently, the upper bound of the frequency range is improved by two orders of magnitude from the upper limit of conventional parallel plate capacitor (1 MHz). Furthermore, the value of capacitance is kept high by using a parallel plate configuration. Therefore, the precision of the capacitance measurement in the lower frequency range remains sufficiently high. Finally, OPPC can cover a wide frequency range (100 Hz-1 GHz) with an appropriate admittance measuring apparatus such as an impedance or network analyzer. The OPPC and the conventional dielectric cell are compared by examining the frequency dependence of the complex permittivity for several polar liquids and polymeric films.