Debye-Falkenhagen effect

Debye-Falkenhagen effect physchem The increase in the conductance of an electrolytic solution when the applied voltage has a very high frequency. d3 bT fal-kon,hag-on i,fekt ... 

DEBYE-FALKENHAGEN EFFECT. The variation of the conductance of an electrolytic solution with frequency. This effect, which is noted at high frequencies, is also called the dispersion of conductance,... 

The concentration dependence of ionic mobility at high ion concentrations and also in the melt is still an unsolved problem. A mode coupling theory of ionic mobility has recently been derived which is applicable only to low concentrations [18]. In this latter theory, the solvent was replaced by a dielectric continuum and only the ions were explicitly considered. It was shown that one can describe ion atmosphere relaxation in terms of charge density relaxation and the elctrophoretic effect in terms of charge current density relaxation. This theory could explain not only the concentration dependence of ionic conductivity but also the frequency dependence of conductivity, such as the well-known Debye-Falkenhagen effect [18]. However, because the theory does not treat the solvent molecules explicitly, the detailed coupling between the ion and solvent molecules have not been taken into account. The limitation of this approach is most evident in the calculation of the viscosity. The MCT theory is found to be valid only to very low values of the concentration. 

See - conductance, - conductivity cell, -> conductometry, - Debye-Falkenhagen effect, -> Debye-Huckel-Onsager theory, - electrolyte, -> ion, -> Kohlrausch square root law, - mass transport. 

University in Ithaca. Nobel Prize in 1936 for contributions to the knowledge of molecular structure based on his research on dipole moments, X-ray diffraction (Debye-Scherrer method), and electrons in gases. His investigations of the interaction between ions and electric fields resulted in the - Debye-Huckel theory. See also -> Debye-Falkenhagen effect, - Debye-Huckel limiting law, - Debye-Huckel length, - Debye relaxation time. 

Debye-Falkenhagen effect - Debye and - Falken-hagen predicted, that in - electrolyte solutions the ionic cloud may not be established properly and maintained effectively when the ion and the cloud are exposed to an alternating (AC) electric field in particular of high frequency. Thus the impeding effect of the ion cloud on the ion movement should be diminished somewhat resulting in an increased value of the ionic conductance. Above frequencies of v 107 to 108 s-1 this increase has been observed, see also - Debye relaxation time. 

As the dependency does not include any specific property of the ion (in particular its chemical identity) but only its charge the explanation of this dependency invokes properties of the ionic cloud around the ion. In a similar approach the Debye-Huckel-Onsager theory attempts to explain the observed relationship of the conductivity on c1/2. It takes into account the - electrophoretic effect (interactions between ionic clouds of the oppositely moving ions) and the relaxation effect (the displacement of the central ion with respect to the center of the ionic cloud because of the slightly faster field-induced movement of the central ion, - Debye-Falkenhagen effect). The obtained equation gives the Kohlrausch constant ... 

Debye-Falkenhagen effect. In the absence of a complete and perfectly shaped ionic cloud movement of the ions is less impeded by the ionic cloud, thus electrolytic conductivity should increase. Above frequencies / 107 to 108 s-1 this increase has been observed, accordingly the Debye relaxation time is r 10-8 s. 

If the electrolyte is of the uni-univalent type and has a concentration of 0.001 molar, the Debye-Falkenhagen effect should become evident with high-frequency oscillations of wave length of about 20 meters or less. The higher the valence of the ions and the more concentrated the solution the smaller the wave length, and hence the higher the frequency, of the oscillations required for the effect to become apparent. 

The measurements of the Debye-Falkenhagen effect are generally made with reference to potassium chloride the results for a number of electrolytes of different valence types have been found to be in satisfactory agreement with the theoretical requirements. Increase of temperature and decrease of the dielectric constant of the solvent necessitates the use of shorter wave lengths for the dispersion of conductance to be observed these results are also in accordance with expectation from theory. 

Utilize the results obtained in the preceding problem to calculate the relaxation times of the ionic atmospheres and the approximate minimum frequencies at which the Debye-Falkenhagen effect is to be expected. It may be assumed that Aqtjo has a constant value of 0.6. The viscosities of the solvents are as follows nitrobenzene (0.0183 poise) ethyl alcohol (0.0109) and ethylene dichloride (0.00785). 

Debye-Falkenhagen effect 1.5.60,1.6.6c, 4.111 Debye-Huckel approximation 1.5.19(intr.)... 

There are two experiments which beautifully illustrate the correctness of the idea of an ionic atmosphere and its manifestation in terms of the relaxation and electrophoresis which occur when the ion moves under the influence of an external field. These are the Debye-Falkenhagen effect and the Wien effect. 

The discussion in this section and in the previous one on the Debye-Falkenhagen effect is qualitative, but it does illustrate the physical significance of the Debye-Falkenhagen and the Wien effects. 

The concept of the ion atmosphere is further substantiated by the Wien effect and the Debye-Falkenhagen effect. In very high fields, > 10 V/m, an increase in conductivity is observed (Wien effect), resulting from the fact that a finite time (the relaxation time) is required for the atmosphere to form about an ion. In very high fields the ion moves so quickly that it effectively loses its atmosphere the atmosphere does not have time to form and so cannot slow the ion. The asymmetry effect disappears and the conductance increases. 

For the same reason the conductivity increases at high frequencies, 3 x 10 Hz (Debye-Falkenhagen effect). The ion changes its direction of motion so quickly that the more sluggish atmosphere cannot adjust and follow the motion of the ion. The ion moves as if it had no atmosphere, and the conductivity increases. At high frequencies both the asymmetry and electrophoretic effects are absent. 

The origin of the power law decay is still not clearly understood. Possible origins include (i) a contribution from the ion atmosphere due to the counter ions and (ii) correlated motion of the water molecules along the grooves. The first contribution could be related to the well-known Debye-Falkenhagen effect which arises from correlated ion motion. The second contribution can arise from correlated motion of water molecules between grooves. 

The conductivity increases at high frequency (>3 to 30 MHz, Debye-Falkenhagen effect). It takes approximately 0.1—1 ns to form an ionic atmosphere, and the time is dependent on the ion concentration. The literature is not clear as to the conductivity frequency dependence of electrolytes such as NaCl, but Cooper (1946) found no variations in the concentration range of 1—4 wt% and frequency range of 1—13 MHz. 

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