Solvation and solvatochromism in CO2-expanded liquids. 2. Experiment-simulation comparisons of preferential solvation in three prototypical mixtures

Electronic absorption and emission spectra of 10-bis(phenylethynyl)anthracene (PEA) and coumarin 153 (C153) are measured as functions of composition along the bubble-point curve at 25 degrees C in CO2-expanded cyclohexane (c-C6H12), acetonitrile (CH3CN), and methanol (CH3OH). The nonlinear dependence of the spectral frequencies on composition suggests substantial preferential solvation of both solutes by the liquid components of these mixtures. Estimates of enrichment factors (local mole fraction of a component divided by its bulk value) based on the assumption that spectral shifts are linearly related to local composition are quite large (approximately 10) in the cases of the C153/CH3CN + CO2 and C153/CH3OH + CO2 systems at high xCO2. Computer simulations of anthracene, the chromophore of PEA, and C153 in these three CO2-expanded liquids are used to clarify the relationship between local composition and spectral shift. A semiempirical model consisting of additive electrostatic and dispersive interactions is able to capture the main features observed experimentally in all six solute/solvent combinations. The simulations show that the commonly used assumption of a linear relation between spectral shifts and local compositions grossly exaggerates the extent of preferential solvation in these mixtures. The collective nature of electrostatic solvation and the composition dependence of the solute's coordination number are shown to be responsible for the breakdown of this assumption.     

Solvation and solvatochromism in CO2-expanded liquids. 1. Simulations of the solvent systems CO2 + cyclohexane, acetonitrile, and methanol

Molecular dynamics simulations of CO(2)-expanded cyclohexane, acetonitrile, and methanol are reported at various compositions along the experimental bubble-point curve at 298 K. Simulated properties include energies, local compositions, viscosities, diffusion coefficients, and dielectric constants and relaxation times. On the basis of the limited comparisons to experimental data currently available, the results indicate that simple intermolecular potential models previously developed for simulating the pure components provide reasonable representations of the energetics and dynamics of these gas-expanded liquids.     

CO2-expanded bio-based liquids as novel solvents for enantioselective biocatalysis

For the first time, CO₂-expanded bio-based liquids were reported as novel and sustainable solvents for biocatalysis. Herein, it was found that by expansion with CO₂, 2-methyltetrahydrofuran (MeTHF), and other bio-based liquids, which were not favorable solvents for immobilized Candida antarctica lipase B (Novozym 435) catalyzed transesterification, were tuned into excellent reaction media. Especially, for the kinetic resolution of challenging bulky secondary substrates such as rac-1-adamantylethanol, the lipase displayed very high activity with excellent enantioselectivity (E value > 200) in CO₂-expanded MeTHF (MeTHF concentration 10% v/v, 6 MPa), whereas there was almost no activity observed in conventional organic solvents.     

Transport properties of CO2-expanded acetonitrile from molecular dynamics simulations

Carbon-dioxide-expanded liquids, which are mixtures of organic liquids and compressed CO2, are novel media used in chemical processing. The authors present a molecular simulation study of the transport properties of liquid mixtures formed by acetonitrile and carbon dioxide, in which the CO2 mole fraction is adjusted by changing the pressure, at a constant temperature of 298 K. They report values of translational diffusion coefficients, rotational correlation times, and shear viscosities of the liquids as function of CO2 mole fraction. The simulation results are in good agreement with the available experimental data for the pure components and provide interesting insights into the largely unknown properties of the mixtures, which are being recognized as important novel materials in chemical operations. We find that the calculated quantities exhibit smooth variation with composition that may be represented by simple model equations. The translational and rotational diffusion rates increase with CO2 mole fraction for both the acetonitrile and carbon dioxide components. The shear viscosity decreases with increasing amount of CO2, varying smoothly between the values of pure acetonitrile and pure carbon dioxide. Our results show that adjusting the amount of CO2 in the mixture allows the variation of transport rates by a factor of 3-4 and liquid viscosity by a factor of 8. Thus, the physical properties of the mixture may be tailored to the desired range by changes in the operating conditions of temperature and pressure.     

The effects of solute-solvent electrostatic interactions on solvation dynamics in supercritical CO2

We present here the results of molecular-dynamics simulation of solvation dynamics in supercritical CO(2) at a temperature of about 1.05T(c), where T(c) is the critical temperature, and at a series of densities ranging from 0.4 to 2.0 of the critical density rho(c). We focus on electrostatic solvation dynamics, representing the electronic excitation of the chromophore as a change in its charge distribution from a quadrupolar-symmetry ground state to a dipolar excited state. Two perturbations are considered, corresponding to different magnitudes of solute excited-state dipoles, denoted as d5 and d8. The d8 solute is more attractive, leading to a larger enhancement in CO(2) clustering upon solute electronic excitation. This has a large impact on solvation dynamics, especially at densities below rho(c). At these densities, solvation dynamics is much slower for the d8 than for the d5 solute. For both solutes, solvation dynamics becomes faster at densities above rho(c) at which solvent clustering diminishes. We show that the slowest solvation time scale is associated with solvent clustering and we relate it to solute-solvent mutual translational diffusion and the extent of change in effective local density resulting from solute electronic excitation.     

Solvation of uranium hexachloro complexes in room-temperature ionic liquids. A molecular dynamics investigation in two liquids

We report a molecular dynamics study of the solvation of UCl(6)(-), UCl(6)(2-), and UCl(6)(3-) complexes in the [BMI][Tf(2)N] and [MeBu(3)N][Tf(2)N] ionic liquid cations based on the same anion (bis(trifluoromethylsulfonyl)imide (Tf(2)N-)) and the butyl-3-methyl-imidazolium+ (BMI+) or methyl-tri-n-butyl-ammonium (MeBu(3)N+) cation, respectively. The comparison of two electrostatic models of the complexes (ionic model with -1 charged halides versus quantum mechanically derived charges) yields similar solvation features of a given solute. In the two liquids, the first solvation shell of the complexes is positively charged and evolves from purely cationic in the case of UCl(6)(3-) to a mixture of cations and anions in the case of UCl(6)(-). UCl(6)(3-) is exclusively "coordinated" to BMI+ or MeBu(3)N+ solvent cations that mainly interact via their CH aromatic protons or their N-Me group, respectively. Around the less charged UCl(6)(-) complex, the cations interact via the less polar moieties (butyl chains of BMI+ or MeBu(3)N+) and the anions display nonspecific interactions. In no case does the uranium atom further coordinate solvent ions. According to an energy components analysis, UCl(6)(3-) interacts more attractively with the [BMI][Tf(2)N] liquid than with [MeBu(3)N][Tf(2)N], while UCl(6)(-) does not show any preference, suggesting a significant solvation effect of the redox properties of uranium, also supported by free energy perturbation simulations. The effect of ionic liquid (IL) humidity is investigated by simulating the three complexes in 1:8 water/IL mixtures. In contrast to the case of "naked" ions (e.g., lanthanide(3+), UO2(2+), alkali, or halides), water has little influence on the solvation of the UCl(6)(n-) complexes in the two simulated ILs, as indicated by structural and energy analysis. This is in full agreement with the experimental observations (Nikitenko, S. I.; et al. Inorg. Chem. 2005, 44, 9497).     

Solvation dynamics through Raman spectroscopy: hydration of Br2 and Br3(-), and solvation of Br2 in liquid bromine

Raman spectroscopy of bromine in the liquid phase and in water illustrates uncommon principles and yields insights regarding hydration. In liquid Br(2), resonant excitation over the B((3)Π(0u)(+)) ← X((1)Σ(g)(+)) valence transition at 532 nm produces a weak resonant Raman (RR) progression accompanied by a five-fold stronger non-resonant (NR) scattering. The latter is assigned to pre-resonance with the C-state, which in turn must be strongly mixed with inter-molecular charge transfer states. Despite the electronic resonance, RR of Br(2) in water is quenched. At 532 nm, the homogeneously broadened fundamental is observed, as in the NR case at 785 nm. The implications of the quenching of RR scattering are analyzed in a simple, semi-quantitative model, to conclude that the inertial evolution of the Raman packet in aqueous Br(2) occurs along multiple equivalent water-Br(2) coordinates. In distinct contrast with hydrophilic hydration in small clusters and hydrophobic hydration in clathrates, it is concluded that the hydration shell of bromine in water consists of dynamically equivalent fluxional water molecules. At 405 nm, the RR progression of Br(3)(-) is observed, accompanied by difference transitions between the breathing of the hydration shell and the symmetric stretch of the ion. The RR scattering process in this case can be regarded as the coherent photo-induced electron transfer to the solvent and its radiative back-transfer.     

Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.     

A comparative study of solvation dynamics in room-temperature ionic liquids

The solvation dynamics of ionic liquids have been the subject of many experimental and theoretical studies but remain poorly understood. We analyze these dynamics by modeling the time-resolved fluorescence response of coumarin 153 in two room-temperature ionic liquids: 1-butyl-1-methylpyrrolidinium bromide and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Our results demonstrate that phenomena such as electrostatic screening operate significantly differently in the two liquids, and the relative importance of translational and rovibrational components of the ionic response depends significantly on the character of the ions involved. However, collective motion dominates the response of both ionic liquids, and the qualitative features of this collective behavior are strikingly similar in both cases.     

Phase behavior of CO2-expanded fluorinated microemulsions

The formation of CO2-expanded, fluorinated reverse microemulsions is demonstrated for the system of perfluoropolyether (PFPE) surfactant (ClPFPE-NH4, MW = 632) and PFPE oil (PFPE, MW = 580). The phase behavior of this system is examined as a function of temperature (25-45 degrees C), pressure, CO2 concentration, and water to surfactant molar ratios (W0 = 10 and 20). Visual observations of one-phase behavior consistent with reverse microemulsion formation are further supported by spectroscopic measurements that establish the existence of a bulk water environment within the aqueous core. Microemulsion formation is not observed in the absence of CO2 for this PFPE surfactant/PFPE oil system, and a CO2 content greater than 70 mol % is required to induce microemulsion formation. Over the range of water loadings and temperatures investigated, the lowest cloud point pressure is observed at 46 bar (5 wt % ClPFPE-NH4 in PFPE oil, W0 = 20, xCO2 = 0.7, T = 25 degrees C). In the regions where one-phase behavior is observed, the cloud point pressures increase with temperature, water loadings, and CO2 content. The driving forces of microemulsion formation in the CO2-expanded fluorinated solvent are discussed relative to traditional reverse microemulsions and CO2-continuous microemulsions.     

Femtosecond molecular motions and implications for electron solvation dynamics in liquids

Molecular motions at femtosecond times are observed experimentally, via nonlinear laser spectroscopy, as responses to the imposition of a sudden, intense laser field in the optical Kerr effect. The electronic, librational, collision-induced translational and reorientational diffusional motions identified through the nonlinear polarizability responses are linked to the probable responses of the disordered medium to an excess electron undergoing multiple scattering at times <10^-12s.     

Uranyl and strontium salt solvation in room-temperature ionic liquids. A molecular dynamics investigation

Using molecular dynamics simulations, we compare the solvation of uranyl and strontium nitrates and uranyl chlorides in two room-temperature ionic liquids (ILs): [BMI][PF(6)] based on 1-butyl-3-methylimidazolium(+),PF(6)(-) and [EMI][TCA] based on 1-ethyl-3-methylimidazolium(+),AlCl(4)(-). Both dissociated M(2+),2NO(3)(-) and associated M(NO(3))(2) states of the salts are considered for the two cations, as well as the UO(2)Cl(2) and UO(2)Cl(4)(2)(-) uranyl complexes. In a [BMI][PF(6)] solution, the "naked" UO(2)(2+) and Sr(2+) ions are surrounded by 5.8 and 10.1 F atoms, respectively. The first-shell PF(6)(-) anions rotate markedly during the dynamics and are coordinated, on the average, monodentate to UO(2)(2+) and bidentate to Sr(2+). In an [EMI][TCA] solution, UO(2)(2+) and Sr(2+) coordinate 5.0 and 7.4 Cl atoms of AlCl(4)(-), respectively, which display more restricted motions. Four Cl atoms sit on a least motion pathway of transfer to uranyl, to form the UO(2)Cl(4)(2)(-) complex. The free NO(3)(-) anions and the UO(2)Cl(4)(2)(-) complex are surrounded by imidazolium(+) cations ( approximately 4 and 6-9, respectively). The first shell of the M(NO(3))(2) and UO(2)Cl(2) neutral complexes is mostly completed by the anionic components of the IL, with different contributions depending on the solvent, the M(2+) cation, and its counterions. Insights into energy components of solvation are given for the different systems.     

Perfluoromethacrylate-styrene statistical copolymers synthesized in CO2-expanded monomers

Statistical copolymers of perfluoroalkyl ethyl methacrylate (Zonyl-TM) and styrene (S) were synthesized in CO₂-expanded monomer medium at a low initial pressure of 25 MPa. Different Zonyl-TM/S feed ratios were used during copolymerizations, and it was determined that the increase in the Zonyl-TM content and decrease of the CO₂ amount in the comonomer feed resulted in a decrease of the molecular weights of copolymers due to earlier precipitation of copolymers giving shorter chains. The cloudy CO₂-expanded liquid monomer phase was found to be the main loci of copolymerization. In addition, the increase in the Zonyl-TM feed ratio resulted in an increase in the critical degree of the polymerization time (J _crit) as the time when the copolymer chains start to precipitate. The higher the Zonyl-TM content used in the feed, the higher the J _crit time and the lower the weight-average molecular weight (M _w) of the copolymer obtained. Thermal analysis results of the copolymer indicated that the copolymers are stable up to 387–403 °C.     

A molecular dynamics computer simulation study of room-temperature ionic liquids. II. Equilibrium and nonequilibrium solvation dynamics

The molecular dynamics (MD) simulation study of solvation structure and free energetics in 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in the preceding article [Y. Shim, M. Y. Choi and H. J. Kim, J. Chem. Phys. 122, 044510 (2005)] is extended to investigate dynamic properties of these liquids. Solvent fluctuation dynamics near equilibrium are studied via MD and associated time-dependent friction is analyzed via the generalized Langevin equation. Nonequilibrium solvent relaxation following an instantaneous change in the solute charge distribution and accompanying solvent structure reorganization are also investigated. Both equilibrium and nonequilibrium solvation dynamics are characterized by at least two vastly different time scales--a subpicosecond inertial regime followed by a slow diffusive regime. Solvent regions contributing to the subpicosecond nonequilibrium relaxation are found to vary significantly with initial solvation configurations, especially near the solute. If the solvent density near the solute is sufficiently high at the outset of the relaxation, subpicosecond dynamics are mainly governed by the motions of a few ions close to the solute. By contrast, in the case of a low local density, solvent ions located not only close to but also relatively far from the solute participate in the subpicosecond relaxation. Despite this difference, linear response holds reasonably well in both ionic liquids.     

Synthesis and extraction of beta-D-glucose-stabilized Au nanoparticles processed into low-defect, wide-area thin films and ordered arrays using CO2-expanded liquids

This letter presents a straightforward and economic strategy to create aqueous Au nanoparticle dispersions using small amounts of beta-D-glucose as a "green" alternative to the conventional phase-transfer catalyst approach. Furthermore, this new process provides for efficient extraction of these monodisperse Au nanoparticles into an organic phase that was successfully processed into wide-area, locally ordered nanoparticle arrays and thin films using a precisely controlled CO2-expanded liquid particle deposition technique. This CO2-based technique allows for pressure-tunable particle deposition while eliminating the detrimental surface tension and dewetting effects common to normal solvent-evaporation techniques.     

Dielectric friction and solvation dynamics: Novel results on relaxation in dipolar liquids

In this article we present a new, general but simple, microscopic expression for time-dependent solvation energy of an ion. This expression is surprisingly similar to the expression for the time-dependent dielectric friction on a moving ion. We show that both the Chandra-Bagchi and the Fried-Mukamel formulations of solvation dynamics can be easily derived from this expression. This expression leads to an almost perfect agreement of the theory with all the available computer simulation results. Second, we show here for the first time that the mobility of a light solute ion can significantly accelerate its own solvation, specially in the underdamped limit. The latter result is also in excellent agreement with the computer simulations.     

Solvation and rotational dynamics of coumarin 153 in ionic liquids: comparisons to conventional solvents

Steady-state and time-resolved emission spectroscopy with 25 ps resolution are used to measure equilibrium and dynamic aspects of the solvation of coumarin 153 (C153) in a diverse collection of 21 room-temperature ionic liquids. The ionic liquids studied here include several phosphonium and imidazolium liquids previously reported as well as 12 new ionic liquids that incorporate two homologous series of ammonium and pyrrolidinium cations. Steady-state absorption and emission spectra are used to extract solvation free energies and reorganization energies associated with the S0 <--> S1 transition of C153. These quantities, especially the solvation free energy, vary relatively little in ionic liquids compared to conventional solvents. Some correlation is found between these quantities and the mean separation between ions (or molar volume). Time-resolved anisotropies are used to observe solute rotation. Rotation times measured in ionic liquids correlate with solvent viscosity in much the same way that they do in conventional polar solvents. No special frictional coupling between the C153 and the ionic liquid solvents is indicated by these times. But, in contrast to what is observed in most low-viscosity conventional solvents, rotational correlation functions in ionic liquids are nonexponential. Time-resolved Stokes shift measurements are used to characterize solvation dynamics. The solvation response functions in ionic liquids are also nonexponential and can be reasonably represented by stretched-exponential functions of time. The solvation times observed are correlated with the solvent viscosity, and the much slower solvation in ionic liquids compared to dipolar solvents can be attributed to their much larger viscosities. Solvation times of the majority of ionic liquids studied appear to follow a single correlation with solvent viscosity. Only liquids incorporating the largest phosphonium cation appear to follow a distinctly different correlation.     

Solvation of hydrophobes in water and simple liquids

The solvation of nonpolar molecules in water and that in simple liquids are compared and contrasted. First, solvation thermodynamics is reviewed in a way that focuses on how the enthalpy and entropy of solvation depend on the choice of microscopic volume change v in the solvation process--including special choices v being zero (fixed-volume condition) and v being the partial molecular volume of a solute molecule (fixed-pressure condition)--and how the solvation quantities are related with temperature derivatives of the solvation free energy. Second, the solvation free energy and the solvation enthalpy of a Lennard-Jones (LJ) atom in model water are calculated in the parameter space representing the solute size and the strength of the solute-solvent interaction, and the results are compared with those for an LJ atom in the LJ solvent. The solvation diagrams showing domains of different types of solvation in the parameter space are obtained both for the constant-volume condition and for the constant-pressure condition. Similarities between water and the simple liquid are found when the constant-volume solvation is considered while a significant difference manifests itself in the fixed-pressure solvation. The domain of solvation of hydrophobic character in the parameter space is large in the constant-volume solvation both for water and for the simple liquid. When switched to the constant-pressure condition accompanying a microscopic volume change, the hydrophobic domain remains large in water but it becomes significantly small in the simple liquid. The contrasting results are due to the smallness of the thermal pressure coefficient of water at low temperatures.     

Solvation structure and dynamics for passivated Au nanoparticle in supercritical CO2: a molecular dynamic simulation

All-atomic molecular dynamics simulations have been performed to study the interfacial structural and dynamical properties of passivated gold nanoparticles in supercritical carbon dioxide (scCO(2)). Simulations were conducted for a 55-atom gold nanocore with thiolated perfluoropolyether as the packing ligands. The effect of solvent density and surface coverage on the structural and dynamical properties of the self-assembly monolayer (SAM) has been discussed. The simulation results demonstrate that the interface between nanoparticle and scCO(2) solvent shows a depletion region due to the preclusion of SAM. The presence of scCO(2) solvent around the passivated Au nanoparticle can lead to an enhanced extension of the surface SAM. Under full coverage, the structure and conformation of SAM are insensitive to the density change of scCO(2) fluid. This simulation results clarify the microscopic solvation mechanism of passivated nanoparticles in supercritical fluid medium and is expected to be helpful in understanding the scCO(2)-based nanoparticle dispersion behavior.     

Polar solvation and solvation dynamics in supercritical CHF3: Results from experiment and simulation

Solvation dynamics of the probe trans-4-(dimethylamino)-4'-cyanostilbene (DCS) have been measured in supercritical fluoroform at 310 K (1.04 Tc) and solvent densities over the range 1.4-2.0 rho(c) using optical Kerr-gated emission spectroscopy. Steady-state measurements and computer simulations of this and the related system coumarin 153 (C153) in fluoroform are used to help interpret the observed dynamics. The solvent contribution to the Stokes shift of DCS is estimated to be 2300 +/- 400 cm(-1) and nearly density independent over the range (0.7-2.0)rho(c). Spectral response functions are bimodal and can be fit to biexponential functions having time constants of approximately 0.5 ps (85%) and 3-10 ps (15%) over the observable range ((1.4-2.0)rho(c)). Computer simulations based on a 2-site model of fluoroform and assuming an electrostatic solvation mechanism appear to properly account for the magnitude and weak density dependence of the Stokes shifts but predict much faster solvation than is observed. Possible reasons for the discrepancy are discussed.