Density enhancement

The two applicational areas above are normally associated with minor quantities (less than 8% by volume) of entrained air. However, using different chemical types of materials, much larger quantities (up to 30% by volume) can be entrained to lower the density, enhance the thermal insulation properties, or to produce lightweight concrete in conjunction with... 

In order to investigate the effects of local density fluctuations on solvation properties, we decided to study two supercritical thermodynamic state points of the same density (5.7 at/nm3) but at different temperatures (295 and 153 K). The low temperature state point, close to the Ar critical point (Tc= 150.8 K, pc= 8.1 at/nm3), is expected to involve significant local density enhancements [5]. 

Previous experimental and theoretical studies have found what appears to be clear evidence for cluster formation, or local density enhancement, in near critical solutions (7t12t42-45). These include experimental optical absorption, fluorescence and partial molar volume measurements as well as theoretical simulation studies. These offer compelling evidence for local solvent density enhancement in near critical binary SCF systems. Theoretical models suggest that local density enhancement should be strongly dependent on the relative size and attractive force interactions strengths of the solute and solvent species as well as on bulk density and temperature (7,44). 

Substantial evidence suggests that in highly asymmetric supercritical mixtures the local and bulk environment of a solute molecule differ appreciably. The concept of a local density enhancement around a solute molecule is supported by spectroscopic, theoretical, and computational investigations of intermolecular interactions in supercritical solutions. Here we make for the first time direct comparison between local density enhancements determined for the system pyrene in CO2 by two very different methods-fluorescence spectroscopy and molecular dynamics simulation. The qualitative agreement is quite satisfactory, and the results show great promise for an improved understanding at a molecular level of supercritical fluid solutions. 

In what follows, we present new fluorescence spectra for pyrene in supercritical carbon dioxide. This is followed by molecular dynamics results on density augmentation in a mixture of Lennard-Jones atoms whose potential parameters were chosen so as to simulate pyrene and carbon dioxide. Finally, we compare the experimental and computational results, thereby obtaining information on the magnitude and extension of the density enhancements suggested by the experiments. 

The resulting local and bulk densities for pyrene in CO2 at Tr=1.02 are given in Table I. Local density enhancements around the pyrene solute, defined as the local density divided by the bulk density, are also included in Table I. These local density enhancements will be used later for direct comparison with simulation results. 

Bulk Density Local Density Density Enhancement... 

Figure 6 shows a comparison between the density enhancements deduced from experiments and those calculated via simulation (the latter at R = 1.94). The reduced temperature in both cases is 1.02. This very good agreement suggests that the density augmentation measured in the fluorescence spectra corresponds to the first solvation shell. 

Local density enhancements, being by definition short-ranged, are not peculiar to the highly compressible near-critical region. Very close to the solute molecule, the local environment differs markedly from the bulk (for example, the local density in the first solvation shell at bulk near-critical conditions is p (R) = 1.43 pc when p = 0.31 and T/Tc = 1.02). However, even this region does not appear to have a liquid-like character, as suggested by other spectroscopic experiments (35-36),... 

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The A operator in this case is just (1 12)5 with P12 the permutation operator. Equation [12] demonstrates that p° is different from pA + pB = loriginal density enhanced by a factor 1/(1 - S2) > 1. The last term in the expression for p° on the last line of Eq. [12] shows that this enhancement is effected by a depletion of density from the overlap region, where both (pA and have sizable values. 

Another class of systems for which the use of the continuum dielectric theory would be unable to capture an essential solvation mechanism are supercritical fluids. In these systems, an essential component of solvation is the local density enhancement [26,33,72], A change in the solute dipole on electronic excitation triggers a change in the extent of solvent clustering around the solute. The dynamics of the resulting density fluctuations is unlikely to be adequately modeled by using the dielectric permittivity as input in the case of dipolar supercritical fluids. 

Now let all the mass be concentrated equally into N small boxes, each of volume V, so that each box contains a mass M = M/N and thus has a density p = M /V. The density enhancement is then 5 = V/NV. The annihilation rate from the whole box is now given by a sum over the N small... 

One aspect of the last set of experiments on W(CO)6 in supercritical ethane that we have not yet discussed involves the possible role of local density enhancements in VER and other experimental observables for near-critical mixtures. The term local density enhancement refers to the anomalously high solvent coordination number around a solute in attractive (where the solute-solvent attraction is stronger than that for the solvent with itself) near-critical mixtures (24,25). Although Fayer and coworkers can fit their data with a theory that does not contain these local density enhancements (10,11) (since in their theory the solute-solvent interaction has no attraction), based on our theory, which is quite sensitive to short-range solute-solvent structure and which does properly include local density enhancements if present, we conclude that local density enhancements do play an important play in VER and other spectroscopic observables (26) in near-critical attractive mixtures. 

The direct hydrocarbon fuel cell has shown a great promise for its simplicity and significant potential for further improvement in the power density. Enhancing the current density for intermediate and low temperature solid oxide fuels requires innovation in both materials development for electrode catalysts as well as fabrication approaches. 

Surface state densities of the order of 10 cm are commonplace for semiconductor electrodes of the sort considered in previous sections of this chapter. These translate to equivalent volume densities of 10 cm for nanocrystalline films. Such high densities enhance light absorption by trapped electrons in surface states, giving rise to photochromic and electrochromic efiects [297-299] (see below). Unusually high photocurrent quantum yields are also observed with sub-band-gap light with these photoelectrode materials. Corresponding sub-band-gap phenomena are rather weak and difficult to detect with single crystal counterparts. 

The dilute supercritical mixtures were examined in the framework of the Kirkwood—Buff theory of solutions. Various expressions were employed for the excess number of aggregated molecules of solvent around individual solute molecules to conclude that at infinite dilution the above mentioned excess is zero. This suggested that the density enhancement observed when small amounts of a solute were added to a solvent near the critical point of the latter may not be caused by the aggregation of the solvent molecules around individual solute molecules as usually considered. Further, comparing experimental results, it was shown that the density enhancement caused by the near critical fluctuations in a pure solvent are almost the same, in a wide range of pressures, as those in dilute supercritical mixtures near the critical point of the solvent. 

In this paper, some recent experimental results regarding the density fluctuations in pure SCF are used to show that the local density enhancement in dilute SCR mixtures is mainly due to the near critical fluctuations in the solvent and an explanation is suggested for the negative partial molar volnme of the solute. This conclusion was also strengthened by a discussion, presented in the following section, based on the Kirkwood—Buff (KB) theory of solution. First, the problem will be examined in the framework of the Kirkwood—Buff theory of solution. Second, nsing experimental results about the near critical fluctuations in pure SCF, it will be shown that the density enhancement in dilnte SCR mixtures is mainly caused by the near critical density fluctuations in pure SCF. 

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