Dielectric relaxation electrolytes

Chemical models of electrolytes take into account local structures of the solution due to the interactions of ions and solvent molecules. The underlying information stems from spectroscopic, kinetic, and electrochemical experiments, as well as from dielectric relaxation spectroscopy. The postulated structures include ion pairs, higher ion aggregates, and solvated and selectively solvated ions. 

During the last two decades, studies on ion solvation and electrolyte solutions have made remarkable progress by the interplay of experiments and theories. Experimentally, X-ray and neutron diffraction methods and sophisticated EXAFS, IR, Raman, NMR and dielectric relaxation spectroscopies have been used successfully to obtain structural and/or dynamic information about ion-solvent and ion-ion interactions. Theoretically, microscopic or molecular approaches to the study of ion solvation and electrolyte solutions were made by Monte Carlo and molecular dynamics calculations/simulations, as well as by improved statistical mechanics treatments. Some topics that are essential to this book, are included in this chapter. For more details of recent progress, see Ref. [1]. 

Static solution permittivity, e(c), and static solvent permittivity, es(c), for solutions of various electrolytes at various concentrations (c) have been obtained by dielectric relaxation spectroscopy [44]. Ion-pairs contribute to permittivity if their lifetime is longer than their relaxation time. However free ions do not contribute to permittivity. Thus,... 

A simplified version of this model, termed the hybrid model (VIG, p. 305) [32-34, 39] (see also Section IV.E) was proposed for the case of a small cone angle p. In this model the rotators move freely over the barrier U0 as if they do not notice the conical surface the librators move in the diametric sections of a cone—that is, they librate. The hybrid model was widely used for investigation of dielectric relaxation in a number of nonassociated and associated liquids, including aqueous electrolyte solutions (VIG, p. 553) [53, 54]. The hat model was recently applied to a nonassociated liquid [3] and to water [7, 12c]. 

As expected, the capacitance of the cell increases when the frequency is decreased (Figure 1.25a) below the knee frequency, the capacitance tends to be less dependent on the frequency and should be constant at lower frequencies. This knee frequency is an important parameter of the EDLC it depends on the type of the porous carbon, the electrolyte as well as the technology used (electrode thickness, stack, etc.) [20], The imaginary part of the capacitance (Figure 1.25b) goes through a maximum at a given frequency noted as/0 that defines a time constant x0 = 1 lf0. This time constant was described earlier by Cole and Cole [33] as the dielectric relaxation time of the system, whereas... 

This short discussion shows that in the case of the non-Debye solvents further work is necessary on both the electrode kinetics and the dielectric relaxation behavior of the solvents in the presence of various electrolytes. There are also significant discrepancies between the results on the relaxation dynamics of these solvents reported by various authors (see [169]). 

Figure 4.39. Dielectric relaxation and conductivity spectrum of a Corynebacterium species (strain 440161. Electrolyte. 10" M KNO3 pH = 6.5, 25°C, to between 0.01 and 0.1. Discussion in the text. Unpublished data by A. van der Wal et at.

The broad features of the dielectric relaxation of electrolyte solutions may be summarized as follows ... 

We have studied a variety of transport properties of several series of 0/W microemulsions containing the nonionic surfactant Tween 60 (ATLAS tradename) and n-pentanol as cosurfactant. Measurements include dielectric relaxation (from 1 MHz to 15.4 GHz), electrical conductivity in the presence of added electrolyte, thermal conductivity, and water self-diffusion coefficient (using pulsed NMR techniques). In addition, similar transport measurements have been performed on concentrated aqueous solutions of poly(ethylene oxide)... 

Results of such an analysis for the MgS04 system are shown in fig. 3.11. The best values of a and for this system are 610 pm and 185LmoP, respectively. On the basis of dielectric relaxation experiments, the permittivity of MgS04 solutions as a function of electrolyte concentration is given by... 

Obviously, the dieleetrie behavior of systems with ion pairing is much more complex than those without it. The presence of the ion pair gives the solution a higher permittivity at lower frequencies than it otherwise would have. This feature is important in understanding the equilibrium properties of these solutions. The permittivity data for the low-frequency process may be used to determine the ion pairing equilibrium constant and the rate constants for formation and breakup of this species. Thus, dielectric relaxation experiments in electrolytes provide valuable information about ion association equilibria. 

Dielectric relaxation and dielectric losses of pure liquids, ionic solutions, solids, polymers and colloids will be discussed. Effect of electrolytes, relaxation of defects within crystals lattices, adsorbed phases, interfacial relaxation, space charge polarization, and the Maxwell-Wagner effect will be analyzed. Next, a brief overview of... 

If the aqueous phase contains electrolytes, a relaxation due to the Maxwell-Wagner-Sillars effect will be observed. Since the electrolyte is not incorporated in the clathrate structures, an increased electrolyte concentration in the remaining free water will result, thus changing the dielectric relaxation mode. In Fig. 42 we note that the relaxation time r decreases from the initial 1000 100 ps to a final level of 200 20 ps during hydrate formation. The experimental value of 200 ps corresponds roughly to a 3% (w/v) NaCl solution, as compared with the initial salt concentration of 1% (w/v). 

Armunanto R, Schwenk CF, Tran HT, Rode BM (2004) Structure and dynamics of Au+ ion in aqueous solution ab initio QM/MM MD simulations. J Am Chem Soc 126 2582-2587 Asaki MLT, Redondo A, Zawodzinski TA, Taylor AJ (2002) Dielectric relaxation of electrolyte solutions using terahertz transmission spectroscopy. J Chem Phys 116 8469-8482 Azam SS, Hofer TS, Randolf BR, Rode BM (2009a) Hydration of sodium(l) and potassium(l) revisited a comparative QM/MM and QMCF MD simulation study of weakly hydrated ions. J Phys Chem A 113 1827-1834... 

Balbuena PB, Johnston KP, Rossky PJ, Hyun J-K (1998) Aqueous ion transport properties and water reorientation dynamics from ambient to supercritical conditions. J Phys Chem B 102 3806-3814 Barthel J, Buchner R, Eberspacher P-N, Miinsterer M, 8tauber J, Wurm B (1998) Dielectric relaxation spectroscopy of electrolyte solutions, recent developments and prospects. J Mol Liq 78 83-109... 

Studies of dielectric relaxation processes [Le 74] are more promising than classical dielectrometry. By this means, phenomena with characteristic durations of 10 10-10" s can be examined even in electrolyte solutions. With this method, for instance, it is possible to follow the effects of the ions in a solution on the motion of the solvent molecules. Thus, the dielectric examinations permit conclusions to be drawn about the solvation numbers of ions [Ba 67, Ba 71], and they may also give information on other orientation processes. 

Bioimmittance is frequency dependent. In dielectric or electrolytic models there is a choice between a step (relaxational) and sinusoidal (single-frequency) waveform excitation. As long as the step response waveform is exponential and linear conditions prevail, the information gathered is the same. At high voltage and current levels, the system is nonlinear, and models and parameters must be chosen with care. Results obtained with one variable cannot necessarily be recalculated to other forms. In some cases, one single pulse may be the best waveform because it limits heat and sample destruction. 

Progress in the understanding of superionic conduction is due to the use of various advanced techniques (X-ray (neutron) diffuse scattering, Raman spectroscopy and a.c.-impedance spectroscopy) and-in the particular case of protons - neutron scattering, nuclear magnetic resonance, infrared spectroscopy and microwave dielectric relaxation appear to be the most powerful methods. A number of books about solid electrolytes published since 1976 hardly mention proton conductors and relatively few review papers, limited in scope, have appeared on this subject. Proton transfer across biological membranes has received considerable attention but is not considered here (see references for more details). 

S. Greenbaum, J. J. Wilson, M. C. Wintersgill, and J. J. FontaneUi [1990] Dielectric Relaxation Studies on Polymer Electrolytes, in Second Intern. Symp. On Polymer Electrolytes, B. Scrosati (Ed.), Elsevier, London. 

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