Local Density Augmentation Research Articles

Solvent density inhomogeneities and solvation free energies in supercritical diatomic fluids: A density functional approach

Density functional theory is used to explore the solvation properties of a spherical solute immersed in a supercritical diatomic fluid. The solute is modeled as a hard core Yukawa particle surrounded by a diatomic Lennard-Jones fluid represented by two fused tangent spheres using an interaction site approximation. The authors' approach is particularly suitable for thoroughly exploring the effect of different interaction parameters, such as solute-solvent interaction strength and range, solvent-solvent long-range interactions, and particle size, on the local solvent structure and the solvation free energy under supercritical conditions. Their results indicate that the behavior of the local coordination number in homonuclear diatomic fluids follows trends similar to those reported in previous studies for monatomic fluids. The local density augmentation is particularly sensitive to changes in solute size and is affected to a lesser degree by variations in the solute-solvent interaction strength and range. The associated solvation free energies exhibit a nonmonotonous behavior as a function of density for systems with weak solute-solvent interactions. The authors' results suggest that solute-solvent interaction anisotropies have a major influence on the nature and extent of local solvent density inhomogeneities and on the value of the solvation free energies in supercritical solutions of heteronuclear molecules.

Local density augmentation and dynamic properties of hydrogen-and non-hydrogen-bonded supercritical fluids: A molecular dynamics study

The local density inhomogeneities in neat supercritical fluids were investigated via canonical molecular dynamics simulations. The selected systems under investigation were the polar and hydrogen-bonded fluid methanol as well as the quadrupolar non-hydrogen-bonded carbon dioxide one. Effective local densities, local density augmentation, and enhancement factors were calculated at state points along an isotherm close to the critical temperature of each system (T(r)=1.03). The results obtained reveal strong influence of the polarity and hydrogen bonding upon the intensity of the local density augmentation. It is found that this effect is sufficiently larger in the case of the polar and associated methanol in comparison to those predicted for carbon dioxide. For both fluids the local density augmentation values are maximized in the bulk density region near 0.7rho(c), a result that is in agreement with experiment. In addition, the local density dynamics of each fluid were investigated in terms of the appropriate time correlation functions. The behavior of these functions reveals that the bulk density dependence of the local density reorganization times is very sensitive to the specific intermolecular interactions and to the size of the local region. Also, the estimated local density reorganization time as a function of bulk density of each fluid was further analyzed and successfully related to two different time-scale relaxation mechanisms. Finally, the results obtained indicate a possible relationship between the single-molecule reorientational dynamics and the local density reorganization ones.

Estimation of Local Density Augmentation and Hydrogen Bonding between Pyridazine and Water under Sub- and Supercritical Conditions Using UV-Vis Spectroscopy

The local density around pyridazine was evaluated by examining the UV-vis spectral shift of pyridazine in a high-pressure liquid state and supercritical water from 25 to 450 degrees C and from 20 to 45 MPa. Augmentation of the local density was observed from 380 to 420 degrees C, and showed the maximum at a lower density than the critical density of water. The degree of hydrogen bonding was estimated in consideration of the local density augmentation. The estimated degree of hydrogen bonding under subcritical conditions without any difference between the local density and the bulk density corresponded to the previously reported results with a UV-vis absorbance spectral shift of quinoline and an NMR proton chemical shift. However, the degree of hydrogen bonding near the critical point of water was larger than that in the case that the local density augmentation was not taken into account. At 380 degrees C and 0.2 g cm(-3) of the bulk density there are 30% as many hydrogen bonds as those under the ambient condition, and it was around 1.5-times that without considering local-density augmentation.

Local Density Inhomogeneities and Dynamics in Supercritical Water: A Molecular Dynamics Simulation Approach

Molecular dynamics atomistic simulations in the canonical ensemble (NVT-MD) have been used to investigate the "Local Density Inhomogeneities and their Dynamics" in pure supercritical water. The simulations were carried out along a near-critical isotherm (Tr = T/Tc = 1.03) and for a wide range of densities below and above the critical one (0.2 rho(c) - 2.0 rho(c)). The results obtained reveal the existence of significant local density augmentation effects, which are found to be sufficiently larger in comparison to those reported for nonassociated fluids. The time evolution of the local density distribution around each molecule was studied in terms of the appropriate time correlation functions C(Delta)rhol(t). It is found that the shape of these functions changes significantly by increasing the density of the fluid. Finally, the local density reorganization times for the first and second coordination shell derived from these correlations exhibit a decreasing behavior by increasing the density of the system, signifying the density effect upon the dynamics of the local environment around each molecule.

Simulations of solvation free energies and solubilities in supercritical solvents

Computer simulations are used to study solvation free energies and solubilities in supercritical solvents. Solvation free energies are calculated using the particle insertion method. The equilibrium solvent configurations required for these calculations are based on molecular dynamics simulations employing model solvent potentials previously tuned to reproduce liquid-vapor coexistence properties of the fluids Xe, C(2)H(6), CO(2), and CHF(3). Solutes are represented by all-atom potentials based on ab initio calculations and the OPLS-AA parameter set. Without any tuning of the intermolecular potentials, such calculations are found to reproduce the solvation free energies of a variety of typical solid solutes with an average accuracy of +/-2 kJmol. Further calculations on simple model solutes are also used to explore general aspects of solvation free energies in supercritical solvents. Comparisons of solutes in Lennard-Jones and hard-sphere representations of Xe show that solvation free energies and thus solubilities are not significantly influenced by solvent density fluctuations near the critical point. The solvation enthalpy and entropy do couple to these fluctuations and diverge similarly to solute partial molar volumes. Solvation free energies are also found to be little affected by the local density augmentation characteristic of the compressible regime. In contrast to solute-solvent interaction energies, which often provide a direct measure of local solvent densities, solvation free energies are remarkably insensitive to the presence of local density augmentation.

Local density augmentation of excited 1-(dimethylamino)naphthalene in supercritical water

Local density around excited 1-(dimethylamino)naphthalene in supercritical water was measured by peak shift of fluorescence spectrum from 8 to 37 MPa at 380 °C. Local density augmentation was evaluated by the excess density, which was defined as the difference between the local density and bulk density. Local density augmentation was observed to be lower than the critical density ( ρ c = 0.32 g cm −3). The excess density exhibited a maximum at about 0.11 g cm −3, which is lower than the critical density. Our results show that excited-state 1-(dimethylamino)naphthalene has strong interactions with water molecules and that local density augmentation occurs even if the solute life time is very short (ca. ns order).

Solvation of Coumarin 153 in Supercritical Fluoroform

We present a study of local density augmentation around an attractive solute (i.e., giving rise to more attractive interaction with the solvent than solvent-solvent interactions) in supercritical fluoroform. This work is based on molecular dynamics simulations of coumarin 153 in supercritical fluoroform at densities both above and below the critical density, ranging from dilute gas-like to liquid-like, at a reduced temperature (T/T(c)) of 1.03. We focused on studying the structure of the solvation shell and the variation of the solute electronic absorption and emission shifts with density. Quantum calculations at the density functional theory (DFT) level were run on the solute in the ground state, and time-dependent DFT calculations were performed in the solute excited state in order to determine the solute-solvent potential parameters. The results obtained for the Stokes shift are in agreement with the experimental measurements. To evaluate local density augmentation from simulations, we used two different definitions, one based on the solvation number and the other derived from solvatochromic shifts. In the former case, the agreement with experimental results is good, while, in the latter case, better agreement is achieved by perturbatively including the induced-dipole contribution to the solvation energy.

Determination of Kamlet–Taft solvent parameters π* of high pressure and supercritical water by the UV-Vis absorption spectral shift of 4-nitroanisole

Kamlet-Taft solvent parameters, pi*, of high pressure and supercritical water were determined from 16-420 degrees C based on solvatochromic measurements of 4-nitroanisole. For the measurements, an optical cell that could be used at high temperatures and pressures was developed with the specification of minimal dead space. The low dead space cell allowed us to measure the absorption spectra of 4-nitroanisole at high temperature conditions before appreciable decomposition occurred. The behavior of pi* in terms of water density (pi* = 1.77rho- 0.71) was found to be linear, except in the near critical region, in which deviations were observed that could be attributed to local density augmentation. Excess density, which was defined as the difference between local density and bulk density, showed a maximum near the critical density of water. The frequencies of UV-Vis spectra of 4-(dimethylamino)benzonitrile and N,N-dimethyl-4-nitroaniline were correlated with pi* based on a linear solvation energy relationship (LSER) theory. Local density augmentation around 4-nitroanisole and that around 4-(dimethylamino)benzonitrile were similar but the augmentation observed around N,N-dimethyl-4-nitroaniline was larger.

極短パルスレーザー分光法による超臨界流体中のダイナミクス

Solvation dynamics and rotational relaxation in supercritical fluids have been examined using the techniques of the subpicosecond laser spectroscopy. The fluorescence up-conversion technique has been employed to detect the fluorescence anisotropy decay of coumarin 153 in supercritical CO2 (310 K) and CHF3 (302, 310 K) over a wide density range. The rotational correlation times in supercritical CHF3 exhibit the remarkable increase at 0.6-0.7 times ρc, which implies that the local solvent density augmentation slows down the rotational dynamics in supercritical fluids. This conclusion is supported by computer simulations based on a 2-site model of fluoroform. The solvation dynamics of the probe 4-dimethylamino-4'-cyanostilbene have been measured in supercritical CHF3 at 310 K using the optical Kerr-gated emission technique, which appears to have captured most (∼90%) of the dynamics occurring over the accessible density range between 1.4-2.0ρc. The measured spectral response exhibits a bimodal decay with ∼0.6 ps and ∼9 ps components and only a modest density dependence.

Temperature Dependence of Local Density Augmentation for Acetophenone N,N,N‘,N‘-Tetramethylbenzidine Exciplex in Supercritical Water

Local density augmentation around exciplex between acetophenone and N,N,N',N'-tetramethylbenzidine in supercritical water was measured by observing the peak shift of transient absorption spectrum at temperatures from 380 to 410 degrees C and at pressures from 6 to 37 MPa. Large local density augmentation was observed at lower solvent densities. Local density augmentation was evaluated by the excess density, which was defined as the difference between local density and bulk density, and the density enhancement factor, which was defined by the ratio of the local density to the bulk density. The number of solvating molecules was estimated with a Langmuir adsorption model. The excess density was found to exhibit a maximum at approximately 0.15 g cm(-3), which decreased with increasing temperature. The density enhancement factor was found to decrease with increasing temperature; however, its value was much greater than unity at 410 degrees C, which provides evidence that exciplex-water interactions still exist at these conditions. The temperature dependence of local density augmentation around the exciplex in supercritical water was comparable with that in supercritical carbon dioxide, which suggests that the ratios of the solute-solvent and solvent-solvent interactions are comparable between these two systems.

Local Solvent Density Augmentation around a Solute in Supercritical Solvent Bath: 1. A Mechanism Explanation and a New Phenomenon

A recently proposed partitioned density functional (DF) approximation (Phys. Rev. E 2003, 68, 061201) and an adjustable parameter-free version of a Lagrangian theorem-based DF approximation (LTDFA: Phys. Lett. A 2003, 319, 279) are combined to propose a DF approximation for nonuniform Lennard-Jones (LJ) fluid. Predictions of the present DF approximation for local LJ solvent density inhomogeneity around a large LJ solute particle or hard core Yukawa particle are in good agreement with existing simulation data. An extensive investigation about the effect of solvent bath temperature, solvent-solute interaction range, solvent-solute interaction magnitude, and solute size on the local solvent density inhomogeneity is carried out with the present DF approximation. It is found that a plateau of solvent accumulation number as a function of solvent bath bulk density is due to a coupling between the solvent-solute interaction and solvent correlation whose mathematical expression is a convolution integral appearing in the density profile equation of the DF theory formalism. The coupling becomes stronger as the increasing of the whole solvent-solute interaction strength, solute size relative to solvent size, and the closeness to the critical density and temperature of the solvent bath. When the attractive solvent-solute interaction becomes large enough and the bulk state moves close enough to the critical temperature of the solvent bath, the maximum solvent accumulation number as a function of solvent bath bulk density appears near the solvent bath critical density; the appearance of this maximum is in contrast with a conclusion drawn by a previous investigation based on an inhomogeneous version of Ornstein-Zernike integral equation carried out only for a smaller parameter space than that in the present paper. Advantage of the DFT approach over the integral equation is discussed.

Solvation of purine alkaloids in supercritical CO 2 by molecular dynamics simulations

A molecular dynamics (MD) computer simulation study is presented for the solvation of purine alkaloids (caffeine, theophylline, and theobromine) in supercritical CO 2 at 313 K and density in the range 0.117–0.94 g/cm 3 using force fields that treat all atoms explicitly. The work discusses the local density augmentation effect in these systems and provides molecular-level details for the solvent density maps around each solute for different pressures.

Ionic solvation in polar supercritical fluids: an integral equation study

A theoretical study of ion solvation in a polar supercritical solvent is reported. The solute–solvent microstructure is computed from the nonlocal integral equation theory and is shown to be in good agreement with simulation results. The nonlinear behavior of the solvation free energy of the ion in a highly compressible supercritical regime is rationalized in terms of a strong electrostriction effect. It is shown that the accuracy of the Born expression for the solvation energy can be substantially improved by taking into account the local solvent density augmentation around the ionic solute.

Local density augmentation around acetophenone N,N,N ′,N ′-tetramethylbenzidine exciplex in supercritical water

Local density augmentation around exciplex between acetophenone and N,N, N ′, N ′-tetramethylbenzidine (TMB) in supercritical water was measured by peak shift of transient absorption spectrum at 380 °C and at pressures from 6 to 37 MPa. Large local density augmentation was observed at lower densities. The excess density, defined by the difference between local density and bulk density, exhibited a maximum at approximately 0.15 g cm −3. Our results show that the degree of local density augmentation that exists in supercritical water is comparable to that observed in supercritical carbon dioxide in some system.

Temperature dependence of local density augmentation around exciplex in supercritical carbon dioxide

To clarify the temperature dependence of local density augmentation around short-lived species, the pressure dependence of the formation and decay processes of exciplex between two neutral species, acetophenone (AP) and N, N, N′, N′-tetramethylbenzidine (TMB), in supercritical carbon dioxide was investigated by a transient absorption technique at 40, 55, and 70 °C. In the high-density (>0.6 g cm −3) region, the decay rate constant could be described by the Kirkwood equation. However, in the low-density (0.35–0.6 g cm −3) region, the exciplex was more stable than that predicted by Kirkwood analysis, which means that strong influence of local density augmentation around the exciplex occurred. The local density augmentation can be described in terms of an excess density which is defined as a difference between local and bulk density. The excess density was derived from the difference between experimental results and Kirkwood analysis and exhibited a maximum at near-critical density. The excess density decreased with increasing temperature and became negligible at high temperatures and high bulk densities.

Temperature Dependence of Local Density Augmentation around Exciplex in Supercritical Water

Temperature Dependence of Local Density Augmentation around Exciplex in Supercritical Water

Local density augmentation in neat supercritical fluids: the role of electrostatic interactions

Motivated by a recent study of Saitow et al. [Chem. Phys. Lett. 368 (2003) 209–214 ], computer simulations and dielectric analyses are used to explore the effect electrical interactions have on density augmentation in supercritical fluoroform. Simulations of 2-site models of fluoroform and ethane show that electrical interactions increase local densities but not dramatically. The density augmentation simulated in both fluids (Δ ρ max=0.12 ρ c and 0.15 ρ c) is much smaller than the value Δ ρ max≅0.5 ρ c deduced by Saitow et al. from Raman measurements. This discrepancy can be reconciled if the observed vibrational shifts reflect mainly dipolar interactions, an interpretation justified by ab initio calculations.

Local density effects on the hyperfine splitting constant and line width of TEMPO radical in gaseous and supercritical carbon dioxide

The density dependences of the nitrogen hyperfine coupling constant ( a N) and line width (Δ H pp) of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) free radical have been investigated at near-infinite dilution in gaseous and supercritical carbon dioxide using steady-state EPR spectroscopy. The local density augmentation around the radical is quantified based on the density dependences of the a N. The viscosity dependences of the Δ H pp are reproduced by the modified Stokes–Einstein equation, suggesting the local solute-fluid density augmentation.

Contribution of local density augmentation in the vicinity of 9-cyanoanthracene and dynamic fluorescence quenching by oxygen in liquid and supercritical carbon dioxide

The local density augmentation in the vicinity of 9-cyanoanthracene (CNA) and the dynamic fluorescence quenching of CNA by oxygen is described in order to study the contribution of local composition enhancement near the fluorophore molecule to the quenching in supercritical fluid (SCF) CO2 at 35 °C. For comparison, the fluorescence quenching of CNA by oxygen in liquid CO2 and n-hexane at 25 °C was also investigated. Local density augmentation determined by conventional spectral shifts in absorption and fluorescence spectra was observed at pressures below about 10 MPa in SCF CO2 at 35 °C, in good agreement with the results reported by other workers. The pressure dependence on the quenching rate constant, kq, showed a monotonic increase, a maximum and a minimum in liquid CO2, n-hexane and SCF CO2, respectively. The apparent activation volumes for kq were evaluated to be −220 ± 10 (6.65–8.15 MPa) and −28 ± 2 (9.17–35.24 MPa) in liquid CO2, −6.7 ± 1.4 (0.1–75 MPa) and 4.5 ± 0.2 cm3 mol−1 (100–500 MPa) in n-hexane and 820 ± 50 (7.11–8.25 MPa) and −27 ± 2 (8.53–35.17 MPa) in SCF CO2. For liquid CO2 and n-hexane, the pressure dependence of kq was successfully interpreted by using the quenching model we reported previously. However, for SCF CO2, this model failed at pressures below about 10 MPa, but held in the higher-pressure region where the local density augmentation was not observed. From these results, the contribution of the local composition enhancement in the vicinity of the fluorophore molecule to the quenching was discussed.

Solvation and Friction in Supercritical Fluids: Simulation−Experiment Comparisons in Diphenyl Polyene/CO2Systems

Computer simulations of realistic models of the two solutes diphenylbutadiene (“DPB”) and hydroxymethylstilbene (“HMS”) in supercritical CO2 (310 K, 0.25−2.0ρc) are used to explore the interplay between local density augmentation and friction in supercritical solvents. Good agreement is found between the extent of local density augmentation observed in these simulations and that deduced from experimental measurements of electronic spectral shifts (also reported here). To test the accuracy of the solvent model for treating solute friction, the viscosity and self-diffusion constants of the neat solvent were simulated and compared to experiment and good agreement was also found. The rotation times of these solutes are compared to results of prior experimental measurements made by Anderton and Kauffman (J. Phys. Chem. 1995, 99, 13 759). In the case of DPB, the simulated times are larger than experimental estimates by as much as 30% at higher densities (ρ ≈ 1.8ρc). The origins of this disagreement are presently unknown. The density dependence of the simulated friction on both rotational and translational motions is curious in that it appears insensitive to the considerable density augmentation present in these systems. This insensitivity is shown to result from a cancellation of static and dynamic aspects of the friction, both of which are individually responsive to the excess density in the neighborhood of the solute.