ABSTRACT
In this paper the relaxation dynamics of ionic glass-former acebutolol hydrochloride (ACB-HCl) is studied as a function of temperature and pressure by using dynamic light scattering and broadband dielectric spectroscopy. These unique experimental data provide the first direct evidence that the decoupling between the charge transport and structural relaxation exists in proton conductors over a wide T-P thermodynamic space, with the time scale of structural relaxation being constant at the liquid-glass transition (τα = 1000 s). We demonstrate that the enhanced proton transport, being a combination of intermolecular H+ hopping between cation and anion as well as tautomerization process within amide moiety of ACB molecule, results in a breakdown of the Stokes-Einstein relation at ambient and elevated pressure with the fractional exponent k being pressure dependent. The dT g /dP coefficient, stretching exponent ßKWW and dynamic modulus E a /ΔV # were found to be the same regardless of the relaxation processes studied. This is in contrast to the apparent activation volume parameter that is different when charge transport and structural dynamics are considered. These experimental results together with theoretical considerations create new ideas to design efficient proton conductors for potential electrochemical applications.
ABSTRACT
The influence of interactions between particle surface and host fluids in electrorheological suspensions is explored. It is observed that dispersions of nanosized particles of titania in octanoid acid exhibit an anomalously large electrorheologic effect when compared with a similar dispersion of micrometric particles or with a more conventional colloidal suspension of silica in silicone oil. The effect is interpreted as originated by the formation of a thin layer of octanoid acid molecules with the surface of the titania solid particle. The experimental data are fitted with the outcomes of a modified version of conductive models existing in the literature. It is suggested that anomalous large electrorheological effect is mainly originated by the increasing of the effective radius of the nanometric particles, which results in an increasing of the effective volume fraction of the dispersed phase. It is also shown that the deformation of the soft shell around the solid particles, induced by Coulombic force, plays a not negligible role. Some hints for tailoring electrorheologic fluids suitable for different applications are proposed.
ABSTRACT
We performed a Brillouin scattering experiment on deeply supercooled water and compared the results with similar literature data obtained both at the same and at higher values of the exchanged wave vector. The whole set of available experimental data can be well reproduced with the use of the generalized hydrodynamic model where all the involved thermodynamic parameters are fixed to their literature values. On the contrary, the model based on the memory function approach generates the wrong estimates for measurables when the same values of the thermodynamic parameters are used. This result confirms our recent criticisms against the utilization of models originating from linear response theory [Phys. Rev. E 84, 051202 (2011)]. The inconsistency between models explains apparent discrepancies between the different conclusions on water acoustic behavior which may be found in the literature. We demonstrate that the observed behavior can be explained by assuming only a single relaxation process that is typical of any viscoelastic system. With all thermodynamics quantities fixed, the hydrodynamic description needs only two parameters to model the experimental data, namely, the relaxation time and the high-frequency limit of the sound velocity. The whole body of the experimental data can be well reproduced when the relaxation time behaves in an Arrhenian manner and the difference between the relaxed and not relaxed sound velocities is a constant. The high-frequency sound velocity is never higher than 2200 m/s. We conclude that, at least from experiments performed within the hydrodynamic regime, there is no indication for a fast sound close to the hypersonic velocity observed in ice.
Subject(s)
Models, Chemical , Models, Molecular , Nephelometry and Turbidimetry/methods , Rheology/methods , Water/chemistry , Cold Temperature , Computer Simulation , Light , Scattering, Radiation , ViscosityABSTRACT
The most familiar approaches used to describe the dynamical structure factor from adiabatic density fluctuations in liquids are based on generalized hydrodynamics and on the memory function, respectively. We show that, contrary to the common belief, the two approaches are not fully equivalent. In particular, models based on the memory function of a normalized damped oscillator fail in reproducing the correct experimental spectral profiles of systems close to the relaxation process. The discrepancy is due to misleading interpretation of the theoretical memory-function expressions, producing an unavoidable mixing of spectral contribution at different wave vectors when the theory is forced beyond its limits of validity.
ABSTRACT
Recent phenomenological studies have drawn attention to an appealing effect, observed for the first time in 1893, today known as water-bridge. The phenomenon has been ascribed to unknown properties of water. We report some experimental results showing that, contrary to a widely common belief, the phenomenon is not to be related with water neither with a property of hydrogen bonded networks. Using a very simple model, we show that the liquid bridge phenomenon is originated by electrostatic effects and can be reproduced in any dense fluid with no respect of its peculiar molecular properties. This basic approach is able to reproduce many of the experimentally observed features of the bridge formation. In perspective of future investigations, the possible phenomena responsible of the bridge stability, after its formation, are briefly discussed.
ABSTRACT
The structural relaxation behavior of aqueous solution of poly(ethylene glycol) and methoxy-capped poly(ethylene glycol), both of mean molecular mass 400 g/mol, is investigated by Brillouin scattering experiments. In both cases non-Debye relaxation processes have been detected, proceeding on the picosecond time scale. The average values of the detected relaxation time distributions fail to follow the simple Arrhenius behavior. The temperature evolution of the relaxation time is adequately fitted using the phenomenological Vogel-Fulcher-Tamman (VFT) model. In spite of the different temperature and concentration dependences observed for the two kinds of systems, with the exception of the highest samples concentrations, a unique scaling behavior has been found for the real and imaginary parts of the loss modulus plotted as a function of the reduced inverse temperature, T(0)/T, T(0) being the VFT arrest temperature. The presence of a unique scaling law in aqueous solutions of polymers characterized by different end groups suggests the establishment of similar hydrogen-bonded local structures. Within this scenario, water acts as a stabilizer and plays the main role bridging neighboring polymer chains. The possible physical interpretation of the obtained fit parameters is discussed, and the results are compared with other literature findings.
ABSTRACT
Brillouin scattering experiments have been carried out on some mixtures of molecular liquids. From the measurement of the hypersonic velocities we have evaluated the adiabatic compressibility as a function of the volume fraction. We show how the quadratic form of the excess compressibility dependence on the solute volume fraction can be derived by simple statistical effects and does not imply any interaction among the components of the system other than excluded volume effects. This idea is supported by the comparison of the experimental results with a well-established prototype model, consisting of a binary mixture of hard spheres with a nonadditive interaction potential. This naive model turns out to be able to produce a very wide spectrum of structural and thermodynamic features depending on values of its parameters. An attempt has made to understand what kind of structural information can be gained through the analysis of the volume fraction dependence of the compressibility.
ABSTRACT
Aqueous solutions of poly(ethylene glycol) (PEG) of mean molecular mass of 600 g/mol (PEG600) are investigated by Brillouin scattering technique. At high PEG content, a relaxation phenomenon is observed, which is related to a local rearrangement of the polymer structure where the interaction, via hydrogen bonding, with the solvent molecules plays a role. The obtained values of the relaxation times match the literature data very well for a fast relaxation time revealed by dielectric relaxation measurements in very similar mixtures. The calculated concentration behaviors of the excess adiabatic compressibility turns out in good agreement with the previous findings from ultrasonic measurements at 3 MHz. The observed minimum in the adiabatic compressibility is interpreted as the result of the interaction between water and the EO units of the PEG chain, which results in a structure tighter then that typical of bulk water and of pure PEG600. Such a hypothesis is supported by the observation that volume fraction value of about 0.3 coincides with the concentration value at which full hydration of EO units takes place. The observation that at the same concentration, the polymer coils start to overlap each other further supports the idea that the adiabatic compressibility behavior is monitoring the structural evolution of the mixture. However, similar results are obtained for largely different binary mixture which suggests caution in taking this conclusion too literally. In particular, the hypothesis that the occurrence of an extreme in the excess adiabatic compressibility could be simply originated by statistical effects and that further work is required for disentangling entropic contribution from effects of hetero-association and self-aggregation of one or both the components.
ABSTRACT
The concentration dependence of the hypersonic properties of solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in benzene and toluene has been investigated by Brillouin scattering. The two solvents are very similar in structure and chemical properties, but while benzene is nonpolar, toluene possess a modest dipole. In both solvents a high-frequency relaxation process has been observed at high concentrations which has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. In both cases, the concentration dependence of the adiabatic compressibility deviates significantly from linearity, indicating the existence of nonideal mixing phenomena driven by aggregation processes taking place in the systems. However, there is no temperature dependence for solutions of PEG600 in benzene; on the contrary, the results obtained for solutions of PEG600 in toluene are noticeably dependent on the temperature. The comparison of the experimental data with the results of previous experiments on similar systems allows a general picture for weakly interacting mixtures of hydrogen-bonded systems and organic solvents to be developed. In particular, in the presence of a nonpolar solvent molecule the local structure of the mixture is dominated by solute self-association processes and any resulting solute-solvent correlation is barely induced by excluded volume effects. At high enough dilution the self-aggregation of solute molecules produces a variety of new local topologies that cannot be observed in bulk solute, and as a consequence, the concentration evolution of the system is too rich to be described in terms of a linear combination of a few components over the whole concentration range. The situation seems to be simpler for the polar toluene solvent molecules, where a three-component model seems able to fit the experimental concentration dependence of the hypersonic velocity. This result is interpreted to imply that the interaction between the solvent dipoles and the active sites of the solute produces a relatively stable heterocoordination, while the relevance of self-association is partially reduced.
ABSTRACT
We present results of a Brillouin scattering experiment on solutions of poly(ethylene glycol) of mean molecular mass 600 g/mol (PEG600) in CCl4. The relaxation process detected has been assigned to conformational rearrangements of the polymeric chains, triggered by reorientation of the side groups. The concentration dependencies of the hypersound velocity and normalized absorption are compared against the indications from several models proposed in the literature. The concentration evolution of the system is described in terms of two distinct regimes. At high polymer content, the system is dominated by the structure of the dense polymer, where polymer-polymer interactions, together with excluded volume effects, induce the existence of a preferred local arrangement resulting in a narrow distribution of the relaxation times, with the average value of the relaxation time following a simple Arrhenius temperature dependence. As the concentration decreases, the original structure of the hydrogen bonded polymer network is destroyed, and a number of different local configuration coexist, giving rise to a wider distribution of relaxation times or to a multiple relaxation. At low concentrations, the experimental data are well fitted assuming a Vogel-Fulker-Tammon behavior for the average relaxation time. In addition, the observed deviation from the ideal behavior for the refractive index and the density suggests that CCl4 does not behave as an inert solvent, and due to polarization effects, it can develop local hetero-associated structures via electrostatic interaction with the O-H end groups of the polymeric chains. The hypothesis has been successfully tested by fitting the concentration behavior of the hypersonic velocity to a recent three-component model, suitable to describe the concentration dependence of sound velocity in moderately interacting fluids. The indication of the model furnishes a very high value for the association constant of the PEG600, confirming the literature indication that, in polymeric systems capable of developing long liner aggregates via hydrogen bonding interaction, the Brillouin probe is insensitive to the true length of the polymeric chains. The Brillouin scattering experiment just sees an effective hydrogen bonded aggregate that is huge relative to the length of the single polymeric chain and becomes sensitive only to the density fluctuations of the local segmental motions.
