Density profiles and solvation forces for a Yukawa fluid in a slit pore

The effect of varying wall-particle and particle-particle interactions on the density profiles near a single wall and the solvation forces between two walls immersed in a fluid of particles is investigated by grand canonical Monte Carlo simulations. Attractive and repulsive particle-particle and particle-wall interactions are modeled by a versatile hard-core Yukawa form. These simulation results are compared to theoretical calculations using the hypernetted chain integral equation technique, as well as with fundamental measure density functional theory (DFT), where particle-particle interactions are either treated as a first order perturbation using the radial distribution function or else with a DFT based on the direct-correlation function. All three theoretical approaches reproduce the main trends fairly well, but exhibit inconsistent accuracy, particularly for attractive particle-particle interactions. We show that the wall-particle and particle-particle attractions can couple together to induce a nonlinear enhancement of the adsorption and a related "repulsion through attraction" effect for the effective wall-wall forces. We also investigate the phenomenon of bridging, where an attractive wall-particle interaction induces strongly attractive solvation forces.     

Role of solvation forces in the gelation of fumed silica-alcohol suspensions

Aggregation and gelation kinetics of fumed silica were investigated by altering the solvent-surface interactions. Native and surface-modified (hydrophobic) fumed silica particles were dispersed in short-chain linear alcohols. Based on the kinetics of aggregation and gelation, we show that the solvent-surface interactions have a tremendous impact on the bulk suspension properties. The gelation kinetics were qualitatively similar in all of the fumed silica-alcohol samples, and the gel times for all the alcohols were captured on a master curve requiring two parameters. The two parameters, the stability ratio and critical volume fraction, describe the two regimes of gelation. At low concentrations, gelation occurs due to aggregation of the particles diffusing over a potential barrier (15-25 kT). The rate of aggregation and time to gelation then scales with the stability ratio. At high particle loadings, gelation occurs at a critical volume fraction due to localization in a secondary minimum with a depth of 3-4 kT. These observations are supported by evidence of hydrogen bonding between the solvent and the particle, creating oscillatory solvation forces that govern the magnitude of these two parameters.     

Microscopic structure and solvation in dry and wet octanol

Octanol-water partition coefficients are extraordinarily successful for correlating and predicting numerous processes of pharmacological and environmental importance. The amphiphilic nature of the 1-octanol molecules allows the octanol phase to mimic the complexities of many different environments ranging from biomembranes to soil. However, the structural details of the octanol phase and whether its structure is altered upon water saturation are not yet fully understood. Configurational-bias Monte Carlo simulations in the Gibbs ensemble demonstrate that a diverse spectrum of hydrogen-bonded aggregates exists in dry and wet 1-octanol, and that water saturation substantially alters the 1-octanol environment from predominantly linear aggregates in dry octanol to larger cylindrical micelles with water cores in wet octanol. These simulation results are able to reconcile the conflicting views (chain-like or water-centered aggregates vs spherical micelles) of the 1-octanol structure inferred from thermodynamic arguments, spectroscopic measurements, and diffraction experiments. Calculated partition constants quantify the influence of water saturation on the solubility characteristics of the dry and wet octanol phases.     

Oxidative properties of FeO2+: electronic structure and solvation effects

An electronic structure analysis is provided of the action of solvated FeO(2+), [FeO(H(2)O)(5)](2+), as a hydroxylation catalyst. It is emphasized that the oxo end of FeO(2+) does not form hydrogen bonds (as electron donor and H-bond acceptor) with H-bond donors nor with aliphatic C-H bonds, but it activates C-H bonds as an electron acceptor. It is extremely electrophilic, to the extent that it can activate even such poor electron donors as aliphatic C-H bonds, the C-H bond orbital acting as electron donor in a charge transfer type of interaction. Lower lying O-H bonding orbitals are less easily activated. The primary electron accepting orbital in a water environment is the 3sigma*alpha orbital, an antibonding combination of Fe-3d(z(2)) and O-2p(z), which is very low-lying relative to the pi*alpha compared with, for example, the sigma* orbital in O(2) relative to its pi*. This is ascribed to relatively small Fe-3d(z(2)) with O-2p(z) overlap, due to the nodal structure of the 3d(z(2)).The H-abstraction barrier is very low in the gas phase, but it is considerably enhanced in water solvent. This is shown to be due to strong screening effects of the dielectric medium, leading to relative destabilization of the levels of the charged [FeO(H(2)O)(5)](2+) species compared to those of the neutral substrate molecules, making it a less effective electron acceptor. The solvent directly affects the orbital interactions responsible for the catalytic reaction.     

Partial molar volume and solvation structure of naphthalene in supercritical carbon dioxide: a Monte Carlo simulation study

Monte Carlo simulations were used to investigate the solvation of naphthalene in supercritical carbon dioxide at a temperature of 308.38 K just above the solvent's critical temperature and at pressures of 74.6, 79.7, 87.8, and 310.2 bar covering a range from just below to far above the solvent's critical pressure and at a slightly elevated temperature of 318.15 K and a pressure of 93.0 bar. The Monte Carlo simulations were carried out in the isobaric-isothermal ensemble and employed the transferable potentials for phase equilibria (TraPPE) force field. Systems containing 2000 carbon dioxide molecules and from 0 to 4 solute molecules were used for all five state points, and additional simulations with 16 000 solvent molecules were carried out at the lower temperature and p = 79.7 bar. In agreement with experiment, the simulations yield large, negative partial molar volumes of naphthalene near the critical pressure at 79.7 bar, with values of -4340 +/- 750 and -3400 +/- 620 cm(3) mol(-1) for the 2000 and 16 000 molecule systems, respectively. Structural analysis through radial distribution functions and the corresponding number integrals yields good agreement with neutron diffraction data and shows evidence for a long-range density enhancement around solutes but lacking any specific solute-solvent clustering. Solvatochromic shifts estimated from the local solvent structure correlate well with the experimental data over the entire pressure range.     

Electronic structure and solvation of copper and silver ions: a theoretical picture of a model aqueous redox reaction

Electronic states and solvation of Cu and Ag aqua ions are investigated by comparing the Cu(2+) + e(-)--> Cu(+) and Ag(2+) + e(-)--> Ag(+) redox reactions using density functional-based computational methods. The coordination number of aqueous Cu(2+) is found to fluctuate between 5 and 6 and reduces to 2 for Cu(+), which forms a tightly bound linear dihydrate. Reduction of Ag(2+) changes the coordination number from 5 to 4. The energetics of the oxidation reactions is analyzed by comparing vertical ionization potentials, relaxation energies, and vertical electron affinities. The model is validated by a computation of the free energy of the full redox reaction Ag(2+) + Cu(+) --> Ag(+) + Cu(2+). Investigation of the one-electron states shows that the redox active frontier orbitals are confined to the energy gap between occupied and empty states of the pure solvent and localized on the metal ion hydration complex. The effect of solvent fluctuations on the electronic states is highlighted in a computation of the UV absorption spectrum of Cu(+) and Ag(+).     

Role of water structure in thermodynamics of nonpolar solvation

The SSOZ (site-site Ornstein-Zernike) equation with an original closure condition for liquid molecular systems is used to calculate thermodynamic functions of noble gas solvation in water. Water is modeled by two close sets of atom-atomic potential functions. The calculations indicate that the chemical solvation potential is strongly sensitive to water structure. A comparison with experiment is given. Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 736–741, July–August, 1996. Translated by L. Smolina     

Intramolecular Diels-Alder reaction in ionic liquids: effect of ion-specific solvent friction

The present work aims at understanding the role of viscosity or solvent friction in ionic liquids for an intramolecular Diels-Alder (IMDA) reaction of (E)-1-phenyl-4-[2-(3-methyl-2-butenyloxy)benzylidene]-5-pyrazolone (1). The results have been analyzed on the basis of the current theoretical models, and their failure to account for the observed trends is discussed in terms of "effective" viscosity or microviscosity. The rates of the reaction decrease with the increasing viscosity of the ionic liquids. As evident from the anionic effect, the solute-solvent specific interactions play a role in governing the kinetics of the reaction. The lower viscosities of the bistrifluoromethanesulfonimide [NTf2](-) based ionic liquids as compared to those based on tetrafluoroborate [BF4](-) anion fail to result in a corresponding acceleration in the rates of the reaction. These contradictory results indicate that solvent microviscosity, rather than the bulk macroscopic viscosity, should be the criteria for selecting the ionic liquids as reaction media.     

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.     

Ions in a binary asymmetric dipolar mixture: mole fraction dependent Born energy of solvation and partial solvent polarization structure

Mean spherical approximation (MSA) for electrolyte solution has been extended to investigate the role of partial solvent polarization densities around an ion in a completely asymmetric binary dipolar mixture. The differences in solvent diameters, dipole moments, and ionic size are incorporated systematically within the MSA framework in the present theory for the first time. In addition to the contributions due to difference in dipole moments, the solvent-solvent and ion-solvent size ratios are found to significantly affect the nonideality in binary dipolar mixtures. Subsequently, the theory is used to investigate the role of ion-solvent and solvent-solvent size ratios in determining the nonideality in Born free energy of solvation of a unipositive rigid ion in alcohol-water and dimethyl sulfoxide-acetonitrile mixtures, where the solvent components are represented only by their molecular diameters and dipole moments. Nonideality in Born free energy of solvation in such simplified mixtures is found to be stronger for smaller ions. The slope of the nonideality for smaller alkali metal ions in methanol-water mixture is found to be opposite to that for larger ion, such as quaternary tertiary butyl ammonium ion. For ethanol-water mixtures, the slopes are in the same direction for all the ions studied here. These results are in qualitative agreement with experiments, which is surprising as the present MSA approach does not include the hydrogen bonding and hydrophobic interactions present in the real mixtures. The calculated partial polarization densities around a unipositive ion also show the characteristic deviation from ideality and reveal the microscopic origin of the ion and solvent size dependent preferential solvation. Also, the excess free energy of mixing (in the absence of any ion) for these binary mixtures has been calculated and a good agreement between theory and experiment has been found.     

热改质过程中渣油结构及其重组分溶剂化变化规律研究

以委内瑞拉减压渣油为原料,采用微型反应釜,研究了其在410℃、2.0 MPa氮气初压下,不同反应停留时间的热改质过程生成油的化学结构组成及其重组分溶剂化变化规律。通过1H-NM R技术研究了热改质过程生成油中沥青质和重胶质不同化学位移归属氢的转化路径;并结合改进的Brown-Ladner法分析了热改质过程生成油中沥青质和重胶质的平均分子结构参数变化;采用蒸汽压渗透法考察了热改质过程生成油中沥青质和重胶质在甲苯溶液中所形成的复合超分子结构的平均相对分子质量。结果表明,随着热改质程度的加深,沥青质和重胶质的H/C原子比减小,供氢能力逐渐下降,沥青质和重胶质的芳香环共轭程度和fA在体系生焦后(45 min)显著提高;沥青质的聚集趋势相关值在热改质15 min前变化不大,15 min后显著增强,而重胶质在整个热改质过程中,其聚集趋势相关值的增势较为缓和;沥青质和重胶质的聚集趋势相关值差异逐渐增大,15 min时增加了1.5%、25 min时增加了50.8%、45 min时增加了142.3%,表明沥青质和重胶质的结构差异越来越明显;重胶质溶剂化沥青质的能力逐步减弱,体系的溶剂化参数从0时的32.9%逐步降到15 min时的29.5%、25 min时的14.1%和45 min时的9.6%;热改质生成油的斑点实验等级逐渐增加,体系的胶体稳定性逐渐降低。     

Adsorption of chain molecules into a thin film structure and solvation interaction versus molecular flexibility

The influence of molecular flexibility on the properties of thin fluid films formed by linear chain molecules is studied by means of a singlet level of inhomogeneous integral equation theory. The considered m-mer chain molecules are formed through the polymerization of m hard-sphere beads with two sticky bonds randomly placed inside each bead core. Different molecular flexibility, from totally flexible up to almost completely rigid is reached by varying the interbead bonding length. The homogeneous properties of the same model that is necessary input to the singlet approach are extracted from the Wertheim’s theory of polymerization. The adsorption, local density distribution, disjoining pressure and solvation force of the chain molecule films confined by attractive and repulsive surfaces are analyzed. The obtained results indicate significant influence of the molecular flexibility on the film layering that is the origin of oscillations of solvation interaction arising between film surfaces. The oscillations of solvation pressure and force become more pronounced with restriction of molecular flexibility and with increase of bulk volume fraction of chain molecules. The decay of the oscillations across the film depends on the chain length and on the physical nature of the film surfaces, i.e. whether they are lyophilic or lyophobic. The partitioning of chain molecules from the bulk into the film strongly depends on the chain flexibility and this effect is more pronounced for the lyophilic surfaces.     

Polar solvation dynamics in supercritical fluids: a mode-coupling treatment

A mode-coupling treatment of polar solvation dynamics in supercritical fluids is presented. The equilibrium solvation time correlation function for the solute fluctuating transition frequency is obtained from the mode-coupling theory method and from molecular-dynamics simulations. The theory is shown to be in good agreement with the simulation. The solvation time correlation function exhibits three distinct time scales, with rapid initial decay, followed by a recurrence at intermediate times, and a slowly decaying long-time tail. Our theoretical analysis shows that the short-time decay arises from the coupling of the solute energy gap to the solvent polarization modes, the recurrence at intermediate times is due to the energy modes, while the slow long-time decay reflects the coupling to the number density modes.     

Consequences of strong coupling between solvation and electronic structure in the excited state of a betaine dye

The electronic ground and excited-state structures of the betaine dye molecule pyridinium- N-phenoxide [4-(1-pyridinio)phenolate] are investigated both in the gas phase and in aqueous solution, using the reference interaction site model self-consistent-field (RISM-SCF) procedure within a CASSCF framework. We obtain the total free energy profiles in both the ground and excited states with respect to variation in the torsion angle between the phenoxide and pyridinium rings. We analyze the effect of solvent on the variation of the solute dipole moment and on the charge transfer character in the excited state. In the gas phase, it is shown that the potential energy profile in the excited-state decreases monotonically toward a perpendicular ring orientation and the dipole moment decreases along with decreasing charge localization. In water, the free energy surface for twisting is better characterized as nearly flat along the same coordinate for sterically accessible angles. These results are analyzed in terms of contributions of the solvation free energy, the solute electronic energy, and their coupling. Correspondingly, the dependence of the charge transfer character on solute geometry and solvation are analyzed, and the important roles in the excitation and subsequent relaxation processes for the betaine dye are discussed. It is found that there is considerable solute electronic reorganization associated with the evolution of solvation in the excited state, and it is suggested that this reorganization may contribute significantly to the early time evolution of transient spectra following photoexcitation.     

Erratic ions: self-assembly and coassembly of ions of nanometer size and of irregular structure

The solution behavior of some ions resembles that of surfactants. The aim of this review is to contextualize the behavior of ions with uneven structure and/or hydration shell, such as nanoions, intrinsically amphiphilic ions, and ions with irregular chemical composition. Hints regarding ionspecificity and other peculiar features of less conventional ions are discussed. Such ions can exhibit ion pairing and like-charge self-assembly, thus resembling amphiphilic molecules. Irregular ions and nanoions are in some cases surface active that reminds classical surfactants. Coassembly of several ions with synthetic polymers and proteins cannot be described by simple salting-in and host–guest models pointing to the approach, in which ions are not used as solutes but as building nanoblocks.     

Thermal characteristics in a nanometer scale

As examples of studies on thermal characteristics of materials with a nanometer scale two topics are discussed. One is heat capacity and thermal conductivity of small materials at low temperatures. It based upon the recent findings that heat capacity depends on the limited number of the phonon modes in low angular frequency region and the distinct characteristic is the appearance of quantized thermal conductance in heat transfer through a narrow wire with hundreds nm. The other is the thermophysical properties at the ordinary interface. The disordered structure appearing in the interfacial region with a width of a few nm is discussed, which is comparable to the phonon mean free path, should be taken into account to reveal the characteristic thermal behavior at room temperature. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of forces and structure between fluid interfaces

The forces and structures that develop at and between fluid interfaces are responsible for the stability of foam, emulsion and wetting films. Although these forces are less studied than the interactions occurring between solid surfaces, recent quantitative studies of films created between fluid interfaces are providing new information that both complements previous findings obtained with solid surfaces, and reveals unique and important differences for films confined between fluid interfaces. Noteworthy is that fluid interfaces can be much more mobile, thus fluctuations and interfacial boundary conditions can produce a rich variety of phenomena at these interfaces.     

Molecular structure encodes nanoscale assemblies: understanding driving forces in electrostatic self-assembly

Supramolecular nanoparticles represent a key field in recent research as their synthesis through self-assembly is straightforward and they often can respond to external triggers. A fundamental understanding of structure-directing factors is highly desirable for a targeted structure design. This contribution demonstrates a quantitative relation between the size of supramolecular self-assembled nanoparticles and the free energy of association. Nanoparticles are prepared by electrostatic self-assembly of cationic polyelectrolyte dendrimers as model macroions and oppositely charged di- and trivalent organic dye molecules relying on the combination of electrostatic and π-π-interactions. A systematic set of sulfonate-group carrying azo-dyes was synthesized. Light scattering and ζ-potential measurements on the resulting nanoparticles yield hydrodynamic radii between 20 nm < R(H) < 50 nm and positive ζ-potential values indicating a positive particle charge. Studies on dye self-aggregation and dendrimer-dye association by isothermal titration calorimetry (ITC) and UV-vis spectroscopy allow for the correlation of the thermodynamic parameters of dendrimer-dye association with the size of the particles, showing that at least a free energy gain of ΔG ≈ - 32 kJ mol(-1) is necessary to induce dendrimer interconnection. Structural features of the azo dyes causing these to favor or prevent nanoparticle formation have been identified. The dye-dye-interaction was found to be the key factor in particle size control. A simple model yields a quantitative relation between the free energy and the particle sizes, allowing for predicting the latter based on thermodynamic measurements. Hence, a set of different molecular "building bricks" can be defined where the choice of building block determines the resulting assembly size.     

Effect of ion solvation on the diffuse layer structure in mixed electrolytes

A “solvionic” model of a multicomponent electrochemical system (mixed electrolyte) is considered. An ion in the solution is considered as a point charge rigidly fixed inside its solvation shell. The corresponding equations for the diffuse layer on an ideally polarizable electrode are derived, and an effective method of their numerical solution is formulated. The calculations are performed in order to follow the changes in the diffuse layer structure with variations in the electrode charge and electrolyte composition. Far from the zerocharge potential of solution, the dependences of distributions of solution components over the diffuse layer on the electrode charge radically differ from those within the classic Gouy-Chapman theory. Analytical equations (asymptotics at large electrode charges) for concentrations of solvated ions in the plane of their maximum approach and for their “surface excesses” (diffuse adsorption) are determined. Results of numerical calculations for a 0.2 M LiCl + 0.05 M BaCl₂ solution are plotted.