Dielectric properties of gases

The dielectric properties of gases are closely related to collision-induced absorption. It is well known that collisions modify molecular properties. Specifically, we are here interested in the dipole moments induced by collisions (Chapter 4) which manifest themselves not only in collision-induced absorption, but also in the dielectric virial properties of gases. 

Dielectric Properties of Gases, Permittivity and Breakdown Voltage... 

Recent advances in basic research, especially in electron-and ion-collision physics, have resulted in improved understanding of the dielectric properties of gases. This understanding, in turn, has aided efforts to identify, improve, and tailor new dielectric gases for a variety of electrical insulation needs. 

The Hamaker summation treats the form of interaction as though the dielectric responses of all media had the properties of gases. That is, the dielectric response is assumed to be proportional to the number density of component atoms or molecules. 

Review Articles.— Dielectric measurements up to 1956 have been discussed by Nelson, whereas de Broucker6 and Mandel have reviewed the results on dielectric properties of dilute polymer solutions. Work on the pemuttivity of gases as a function of density and pressure is discussed by Cole and Smyth and by Beaume et al., who moreover report some simple theories of the second didectric virial coefiident. Dielectric relaxation phenomena (dispersion and absorption) are discussed in the artides by Cole and Smyth and by Davies and Boudoiuis. A recent artide by Grant deals with stutfles of biological molecules by dielectric methods. 

The term dielectric properties in its broad usage refers to those responses other than the simple ohmic currents which develop in a medium on the application of an electric potential. Thus an ideal dielectric medium shows no direct current when a static electric potential is applied, but as only gases approach this condition it is usual to speak of the dielectric properties of what may also be a semi-conductor. 

For pure liquids, the Debye equation suggests that the molar polarization should be a linear function of the reciprocal temperature. Furthermore, one should be able to analyze relative permittivity data for a polar liquid like water as a function of temperature to obtain the dipole moment and polarizability from the slope and intercept, respectively. In fact, if one constructs such a plot using data for a polar solvent, one obtains results which are unreasonable on the basis of known values of p and ocp from gas phase measurements. The reason for the failure of the Debye model in liquids is the fact that it neglects the field due to dipoles in the immediate vicinity of a given molecule. However, it provides a reasonable description of the dielectric properties of dilute polar gases. In liquids, relatively strong forces, both electrostatic and chemical, determine the relative orientation of the molecules in the system, and lead to an error in the estimation of the orientational component of the molar polarization. 

German physicist. Clausius is best remembered as one of the founders of thermodynamics. In papers published in the 1850s he stated the second law of thermodynamics and he coined the word entropy in 1865. Clausius made other contributions to thermodynamics and also to the development of the kinetic theory of gases. In electrochemistry he pioneered the idea of dissociation of substances into ions in solution. He also studied the dielectric properties of materials. 

Table 19.10. Dielectric properties of selected common gases at 293.15 K and 101.325 kPa...

Accurate ab initio calculations for the hehum dimer and trimer can be used in several other ways in thermal physics. One can use theoretical predictions to calibrate instruments designed to measure the density and dielectric virial coefficients, viscosity, thermal conductivity, speed of sound, and other properties of gases based on comparisons of theory with experiment for helium. 

HF is a colourless volatile liquid and an oligomeric H-bonded gas (HF), whereas the heavier HX are colourless diatomic gases at room temperature. Some molecular and bulk physical properties are summarized in Table 17.10. The influence of H bonding on the (low) vapour pressure, (long) liquid range and (high) dielectric constant of HF have already been discussed... 

This definition of electrochemistry disregards systems in which nonequilibrium charged species are produced by external action in insulators for example, by electric discharge in the gas phase (electrochemistry of gases) or upon irradiation of liquid and sohd dielectrics (radiation chemistry). At the same time, electrochemistry deals with certain problems often associated with other fields of science, such as the structure and properties of sohd electrolytes and the kinetics of ioific reactions in solutions. 

Argon, krypton, and xenon have polarizabilities of 16.5, 25.4, and 41.3 X 10 26 cm.3, respectively. McDonald found that these gases produce shifts of 8, 16, and 19 cm.-1. Nitrogen, oxygen, and methane, which have polarizabilities of 17.6, 16.0, and 26.0 X 10-26 cm.3, produce shifts of 24, 12, and 32 cm.-1, respectively. McDonald interpreted these results as showing that the polarizability is not the only factor involved and that the frequency shifts depend on an additional factor related to the chemical nature of the adsorbed molecules. He concluded that the frequency shifts cannot be completely explained in terms of macroscopic dielectric properties. 

It is of practical as well as ideological importance that the modern theory of van der Waals forces reduces to the older forms derived for the interaction of individual small molecules in dilute gases. The modern approach can in fact be used to derive new expressions for the interaction between pairs of solutes in dilute solutions. The essential property of e in the dilute-gas or dilute-solution limit is that the dielectric response is strictly proportional to the number density of gas or solute molecules. That is, an electric field applied to a dilute gas or solution acts on each dilute species without distortion of the field by other gas or solute molecules. 

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