Local density enhancement

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]. [Pg.254]

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). [Pg.31]

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. [Pg.64]

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. [Pg.67]

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),... [Pg.72]

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. [Pg.383]

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. [Pg.701]

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. [Pg.76]

K. I. Saitow, K. Otake, H. Nakayama, K. Ishii, and K. Nishikawa, Local density enhancement in neat supercritical fluid due to attractive intermoleeular interaetions. Chem. Phys. Lett., 368 (2003), 209-214. [Pg.322]

Tucker (these proceedings, and [13]) reasons that in addition to the local density enhancement due to the direct inlermolecular interaction, an additional enhancement must be due to the critical-fluctuation induced long-range inhomogeneity of the fluid. [Pg.12]

The environment of a solute molecule or reactant is probed by spectroscopic studies of light emitted or absorbed by the solute or by reacting species (See Johnston, these proceedings). Much experimental evidence has been published of local density enhancement around such solutes or species. [Pg.12]

Simulations of chains grafted to interfaces in vacuum and in liquid solvents have been reviewed[73]. The repulsive force of interaction between surfaces coated with grafted athermal chains (good solvent conditions) has been calculated with lattice MC[74] and continuum molecular dynamics[75] methods. The first simulations of interfaces in supercritical fluids considered the adsorption of pure solvent (no chains) in a flat-wall pore[76]. Near the solvent critical temperature (7 ) a maximum in adsorbed amount was observed at densities slightly below the solvent critical density (pc) The maximum in adsorbed amount was attributed to local density enhancement of solvent in the pore. [Pg.218]

The origin of local solvent density enhancements in a compressible SCF can be understood from two different viewpoints. The more recently proposed viewpoint [10,12] ties the existence of local density enhancements directly to the presence of the solvent s critical, correlated density fluctuations, while the more common viewpoint bases the existence of local density enhancements upon the attractiveness of the solute-solvent interaction potential and the compressibility of the fluid [2,10,17,22,23,29-32]. These two viewpoints are described below. (Note that the effect of local density enhancements can also be understood within a purely thermodynamic framework through the Krichevskii parameter, lirrix- Q dP/dx)v r where x is the solute mole fraction. See Refs. [27], [28] and [33].)... [Pg.397]

Effect of Local Density Enhancements on Solute Reaction Rates... [Pg.400]

While mean local density enhancements (LDE) increase the favorable free energy of solvation, over that expected in the absence of compression, i, e. for an... [Pg.402]

In addition to altering the activation barrier height, reaction path dependent local density enhancements can alter the location of the transition state. This type of behavior... [Pg.404]

Because the importance of local density enhancement effects depends upon the compressibility of the fluid, these effects have an unusual bulk density (pressure) dependence. Below the critical density [65] the local density enhancement effects increase with increasing bulk-density, whereas at densities greater than the critical density, these effects will decrease with further increases in the bulk-density. This is in contrast to the bulk solvent effects, which increase monatonically with increasing bulk-density as the solvent properties, e. g. the dielectric constant, vary from their gas-like to liquid-like values. Hence, the bulk-density dependences of the activation barriers, AG(T5), on... [Pg.405]

As mentioned earlier, this type of 3-regime behavior with bulk density — rapid change followed by a near invariance (or lesser change) followed by subsequent, more rapid change — is the characteristic signature of local density enhancement effects which has been observed in numerous spectroscopic studies [13,21,22,26,66-68]. Thus, the question naturally arises as to whether the observed vibrational relaxation behavior on the near-critical isotherm is a consequence of local density enhancements. The additional experimental evidence of such plateau behavior in infrared [68,76] and electronic [75] spectroscopic shifts for these same vibrational systems on the near-critical isotherm supports this conjecture, but is not conclusive, because no direct relationship has been shown between the presence of local density enhancements and vibrational relaxation lifetimes, or between local density enhancements and infrared spectroscopic shifts (although such a relationship has been demonstrated for electronic spectral shifts) [77]. [Pg.407]

In order to ascertain whether the 3-regime behavior observed in the experimental vibrational lifetimes is indeed a result of local density enhancements, Goodyear and Tucker [12] computed both vibrational lifetimes and local density enhancements from molecular dynamics simulation for a model solute-solvent SCF solution. These authors considered a diatomic solute in a 2-dimensional supercritical Lennard-Jones fluid of 1150 atoms (Fig. 1). In this model, each of the solute atoms was designated as a Lennard-Jones site, and the Lennard-Jones parameters between solute and solvent atoms were taken to be the same as those between solvent atoms. The vibrational lifetimes were computed using the standard, classical Landau-Teller expression [69,70,72,73,78], i.e. [Pg.407]

Figure 11. Local density pt) (with rt = 1.78

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