Permittivity change

Debye and Falkenhagen [92] also predicted that the permittivity of electrolyte solutions should increase as c,/2 where c is the ionic concentration. According to Hasted [105], such an effect has not been demonstrated experimentally, probably because the high conductivity of such solutions can mask permittivity changes. On the contrary, the permittivity of electrolyte solutions decreases with concentration [106] by 25—50% at lmoldm-3. This is probably associated with the binding of dipolar solvent molecules to ions, thus reducing the solvent orientational contri-butional to the permittivity (dielectric saturation). 

If all other variables are kept constant, then a decrease in er will cause a decrease in k if zaZb is positive, and will cause an increase if za zu is negative. However, this assumes that r/ does not change as the permittivity changes. This is probably not justifiable. 

The first attempt to use these ideas in epoxy cure was by Fisch and Hofmann 66), but their assignment of permittivity changes to changes in polar group concentrations was marred by what we interpret as electrode polarization effects. Blyakhman et al. 51 52), examined the post-cure dielectric permittivity and loss tangent of anhydride-... 

The mean hydration number hg is calculated by assuming that the calculated relative permittivity change of any water molecule is a measiure of its partial immobilization [equation (24)], where is the number of... 

An alternative experiment is one in which permittivity changes are studied at constant electrical displacement D, for example, for constant charge on the plates of a capacitor. The resulting relaxation phenomena are then characterized by a... 

The relative permittivity was obtained from a comparison of Cm and Co. The results of this comparison are shown in Fig. 4 for the various types. It can be seen that the relative permittivity changes from about 2 on the kaolin side to about 5 on the titanium oxide side. 

Solvating ability of mixed solvent differs from solvating ability of individual components. In addition to the permittivity change and the correspondent electrostatic interaction energy change, this is also caused by a number of reasons, the most important of which are discussed in the chapter. 

Let the resolvation process proceeds at substantial abundance of the component A in mixed solvent and initial concentrations HA (HA" ) and B to be equal. The output of the process can be calculated from the equation similar to equation [9.66]. The large value of K s in all considered processes of proton resolvation indicates the effect of permittivity change on the yield of complex HB formation. The output of resolvated proton in process [9.104] proceeding in methanol equals 100%, whereas in the same process in low polarity solvent (e.g., methanol-hexane), with abundance of the second component, the equilibrium is shifted to the left, resulting in solvate output of less than 0.1%. K s values in single alcohol solvents are large, thus the output of reaction does not depend on solvent exchange. 

The thermodynamic characteristics of a chemical process, calculated from equilibrium constant polytherms, are integral characteristics (AH and AS), i.e., they consist of terms related to process itself (AH and AS ) and terms depended on permittivity change with temperature changing (AHg and AS) ... 

Pig. 51. Permittivity change measured over a range of frequencies during the isothermal cure at 137°C of an epoxy—amine system (DGEBA/DDS) (120). 

The dynamics of lubricant interaction with added water is even more complicated. In addition to high-frequency permittivity changes described by Eq. 7-7, the entire impedance spectrum undergoes a complicated pattern of time-dependent changes. The low-frequency impedance changes caused by water will be discussed in a later section of this chapter. In the bulk solution a complex kinetics of water-oil interactions occurs, combining several mutually dependent processes of emulsification of free nonbound water, formation of inverse micelles, and evaporation of free and micellated water [21]. 

From low frequencies with a value of q to high frequencies with a value of Soo permittivity decreases through several transitions. In the transition region the total permittivity changes and an energy dissipation occurs. This transition and the frequency dependence of permittivity can be described by a relaxation model. Debye relaxation and Cole-Cole relaxation are the two fundamental models used to describe the frequency dependence of dielectric constant ... 

Conductometric relies on the measurement of either conductivity or resistivity. This type of sensor comprises a capacitor that changes its capacitance when exposed to the desired analyte. The capacitance of the sensor changes due to a selectively absorbing material such as polymers or other insulators. These absorbing materials serve as the dielectric layer of the capacitor and their permittivity changes with exposure to the analyte. These sensors are commonly used to detect humidity as well as carbon dioxide and volatile organic compounds (Patel et al., 2003). In the humidity sensor case, the dielectric layer comprises a water-sensitive polymer (Lazarus and Fedder, 2011). 

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