Carbon dioxide permittivity

The most common sc-fluid for industrial processing and benchtop research is supercritical carbon dioxide, chosen because of its moderate and easily attained critical temperature and pressure and its non-toxicity. Reactions in SC-CO2 produce similar results as reactions in nonpolar organic solvents. Its solvent polarity, empirically determined using solvatochromic dyes as polarity indicators (see Section 7.4), corresponds to that of hydrocarbons such as cyclohexane [221, 222]. Carbon dioxide has no dipole moment and only a small quadrupole moment, a small polarizabihty volume, and a low relative permittivity (er = 1.4-1.6 at 40 °C and 108-300 bar) [221, 223]. Thus, SC-CO2 is only suitable as a solvent for nonpolar substances, which unfortunately imposes considerable limitations on its practical applications. To overcome this limitation, more polar co-solvents (modifiers) such as methanol can be added to SC-CO2. 

When carbon dioxide is heated beyond its critical point, with a critical temperature of tc = 31.0 °C, a critical pressure of pc = 7.38 MPa, and a critical density of Pc = 0.47 g cm , the gaseous and the liquid phase merge into a single supercritical phase (SC-CO2) with particular new physical properties very low surface tension, low viscosity, high diffusion rates, pressure-dependent adjustable density and solvation capability ( solvation power ), and miscibility with many reaction gases (H2, O2, etc.). It can dissolve solids and liquids. The relative permittivity of an sc-fluid varies linearly with density, e.g. for SC-CO2 at 40 °C, r = 1.4 1.6 on going from 108 to 300 bar. This... 

In Fig. 2, the relative permittivities (static dielectric numbers) e of carbon dioxide [23], argon [24], and liquid pentane [25] are plotted against pressure p up to 200 MPa. Even at the highest pressures corresponding to liquid-like densities, e (CO2) is smaller than 1.8, and thus nearly equal to that of a liquid alkane (such as pentane). Since CO2 molecules do not have any permanent electrical dipole moment, the polarization is more or less restricted to the contributions of the electrons and the nuclei. Therefore, typical solvation effects are normally less important, and the intermolecular interactions are predominantly of van-der-Waals type with some higher electrostatic such as quadrupolar interactions. 

Figure 2. Relative permittivities (static dielectric numbers) e of pure carbon dioxide [23], argon [24], and pentane [25] as a function of pressure p (see also [4]).

Moriyoshi, T., T. Kita, and Y. Uosaki. 1993. Static Relative Permittivity of Carbon Dioxide and Nitrous Oxide up to 30 Mpa. Berichte der Bunsengesellschaft fur physikalische Chemie 97 (4) 589-596. 

The polarity of the molecules is usually considered to be measured on a gross scale by the relative permittivity and on a molecular scale by the electrical dipole and higher moments. Molecules lacking a dipole moment (carbon dioxide, for example) may still exert short-range effects due to quadrupole, and so on, moments. Dipolar bonds that are well separated in a molecule may act almost independently on neighboring molecules Hildebrand and Carter (1930) showed that the three isomeric dinitrobenzenes, in their binary solutions in benzene, exhibit nearly identical deviations from Raoult s law, though their dipole moments are different. The part of the electrical influence of a solvent on solute molecules that arises from the polarizability of the solvent molecules may be represented by the refractive index, n, or by functions of n such as the volume polarization, R, given by ... 

An example for permittivity measurements is given in Figure 11. It shows the real part of the complex capacity (C = C(f, T)) as a function of the frequency (f) of the (weak) oscillating electric field applied to the capacitor for the zeolite DAY-carbon dioxide (CO2) system at 298 K. The lowest line refers to vacuum, the upper line to the maximum gas pressure of 1.9924 MPa. Note that all curves are shifted monotonously to higher capacity values as the pressure of the gas aud thus the amount of CO2 adsorbed increases. 

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). 

In contrast, dipolar aprotic solvents possess large relative permittivities (sr > 15), sizeable dipole moments p > 8.3 10 ° Cm = 2.5 D), and average C.f values of 0.3 to 0.5. These solvents do not act as hydrogen-bond donors since their C—H bonds are not sufficiently polarized. However, they are usually good EPD solvents and hence cation sol-vators due to the presence of lone electron pairs. Among the most important dipolar aprotic solvents are acetone, acetonitrile [75], benzonitrile, A,A-dimethylacetamide [76, 77], A,A-dimethylformamide [76-78], dimethylsulfone [79], dimethyl sulfoxide [80-84], hex-amethylphosphoric triamide [85], 1-methylpyrrolidin-2-one [86], nitrobenzene, nitro-methane [87], cyclic carbonates such as propylene carbonate (4-methyl-l,3-dioxol-2-one) [88], sulfolane (tetrahydrothiophene-1,1-dioxide) [89, 90, 90a], 1,1,3,3-tetramethylurea [91, 91a] and tetrasubstituted cyclic ureas such as 3,4,5,6-tetrahydro-l,3-dimethyl-pyr-imidin-2-(l//)-one (dimethyl propylene urea, DMPU) [133]. The latter is a suitable substitute for the carcinogenic hexamethylphosphoric triamide cf. Table A-14) [134]. 

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