Solvent phase behavior and the interaction of uniform and patterned solutes

Isotropic and anisotropic hypernetted-chain (HNC) integral equation theories are used to obtain the interaction of solutes both near and far from the solvent liquid-vapor coexistence. Spherically symmetrical and chemically patterned (patched) solutes are considered, and the influences of particle and patch sizes are investigated. Solvophilic and solvophobic solutes (or patches) are examined. Near coexistence, in the solvophobic case drying-like behavior occurs for solutes (patches) of sufficient size. This gives rise to relatively long ranged attractive forces that are strongly orientation dependent for the patched solute particles. We also report grand canonical Monte Carlo results for a pair of spherically symmetric solutes. This demonstrates that the anisotropic HNC theory gives qualitatively correct solvent structure in the vicinity of the solutes. Comparison with previous simulations also shows that the solute-solute potentials of mean force given by the anisotropic theory are more accurate (particularly at small separations) than those obtained using the isotropic method.     

Structural and dynamical properties of a core-softened fluid in a supercritical region

We present a theoretical study of the structural, thermodynamic, and transport properties of a supercritical fluid comprising particles interacting via isotropic attractive core-softened potential. The shear viscosity and self-diffusion coefficient are computed on the basis of the mode-coupling theory, with required structural input obtained from the thermodynamically self-consistent integral equation theory. We also consider dilute solutes in a core-softened fluid and use the anisotropic integral equation theory to obtain the solute-solute potential of mean force, which yields the second virial coefficient. We analyze its dependence on the solvent density and solute-solvent interaction strength.     

Phase Transitions and Electrochemical Properties of Ionic Liquids and Ionic Liquid—Solvent Mixtures

Recent advances in studies of ionic liquids (IL) and ionic liquid–solvent mixtures are reviewed. Selected experimental, simulation, and theoretical results for electrochemical, thermodynamical, and structural properties of IL and IL-solvent mixtures are described. Special attention is paid to phenomena that are not predicted by the classical theories of the electrical double layer or disagree strongly with these theories. We focus on structural properties, especially on distribution of ions near electrodes, on electrical double layer capacitance, on effects of confinement, including decay length of a dissjoining pressure between confinig plates, and on demixing phase transition. In particular, effects of the demixing phase transition on electrochemical properties of ionic liquid–solvent mixtures for different degrees of confinement are presented.     

Electrode screening by ionic liquids

In this work we are concerned with the short-range screening provided by the ionic liquid dimethylimidazolium chloride near a charged wall. We study the free energy profiles (or potentials of mean force) for charged and neutral solutes as a function of distance from a charged wall. Four different wall charge densities are used in addition to a wall with zero charge. The highest magnitude of the charge densities is ±1 e nm(-2) which is close to the maximum limit of charge densities accessible in experiments, while the intermediate charges ±0.5 e nm(-2) are in the range of densities typically used in most of the experimental studies. Positively and negatively charged solutes of approximately the size of a BF ion and a Cl(-) ion are used as probes. We find that the ionic liquid provides excellent electrostatic screening at a distance of 1-2 nm. The free energy profiles show minima which are due to layering in the ionic liquid near the electrodes. This indicates that the solute ions tend to displace ionic liquid ions in the layers when approaching the electrode. The important role of non-electrostatic forces is demonstrated by the oscillations in the free energy profiles of uncharged solutes as a function of distance from the wall.     

Absorption anisotropy of cubic or randomly oriented chromophores in anisotropic solvents. Dispersion induced linear dichroism (DILD)

A new mechanism through which cubic or orientationally averaged solutes could gain absorption anisotropy (linear dichroism) in the presence of an anisotropic (oriented) solvent medium is proposed. Transitions of the unoriented species exhibit a dispersion induced linear dichroism (DILD) as a result of dispersive coupling to the transitions of the oriented system. The phenomenon depends on the nature of the angular distribution of solute molecules about a particular solvent species, being maximised for a cylindrical distribution around a polymer, but still yielding a measurable DILD for spherical distributions of the solute. It is also shown that the LD of non-cubic or oriented solutes in anisotropic media should be corrected for a significant DILD contribution.     

Density Functional Theory and the Capillary Evaporation of a Liquid in a Slit

Density functional theory is applied to a Lennard-Jones fluid near a single hard wall and in a slit formed by two walls. We use some simplified versions of the Weeks-Chandler-Andersen (WCA) and the Barker-Henderson (BH) theories. Only the most crude mean field version of the WCA theory, in which the hard-sphere correlation function is set equal to unity for all distances, seems useful. Use of the full WCA approximation is impractical because the effective hard-sphere diameter is density dependent. Generally, the best results are obtained using the BH macroscopic compressibility approximation. Our earlier study of "evaporation" of Lennard-Jones molecules in a slit is extended to other densities using the mean field theory. Copyright 2000 Academic Press.     

A quasichemical approach for protein-cluster free energies in dilute solution

Reversible formation of protein oligomers or small clusters is a key step in processes such as protein polymerization, fibril formation, and protein phase separation from dilute solution. A straightforward, statistical mechanical approach to accurately calculate cluster free energies in solution is presented using a cell-based, quasichemical (QC) approximation for the partition function of proteins in an implicit solvent. The inputs to the model are the protein potential of mean force (PMF) and the corresponding subcell degeneracies up to relatively low particle densities. The approach is tested using simple two and three dimensional lattice models in which proteins interact with either isotropic or anisotropic nearest-neighbor attractions. Comparison with direct Monte Carlo simulation shows that cluster probabilities and free energies of oligomer formation (DeltaG(i) (0)) are quantitatively predicted by the QC approach for protein volume fractions approximately 10(-2) (weight/volume concentration approximately 10 g l(-1)) and below. For small clusters, DeltaG(i) (0) depends weakly on the strength of short-ranged attractive interactions for most experimentally relevant values of the normalized osmotic second virial coefficient (b(2) (*)). For larger clusters (i"2), there is a small but non-negligible b(2) (*) dependence. The results suggest that nonspecific, hydrophobic attractions may not significantly stabilize prenuclei in processes such as non-native aggregation. Biased Monte Carlo methods are shown to accurately provide subcell degeneracies that are intractable to obtain analytically or by direct enumeration, and so offer a means to generalize the approach to mixtures and proteins with more complex PMFs.     

Refractometric studies of mesogenic molecules in isotropic solutions

The Lorenz-Lorentz equation for binary isotropic mixtures, consisting of anisotropic molecules, has been derived using the point-dipole approximation taking into account only pair molecular correlations. This permits the calculation of the effect of molecular correlations on the refractive index. Special attention has been paid to the case of infinite dilution in the solvents consisting of isotropic molecules, where an experimental check of the equations is possible. The specific refraction of some solutes with different molecular polarizability anisotropy and polarity has been studied in various solvents. It has been shown, that the main theoretically predicted features are observed in these experiments, but for a quantitative comparison information on the two-particle distribution function is needed.     

Stimuli‐Responsive Shape Switching of Polymer Colloids by Temperature‐Sensitive Absorption of Solvent

The dynamic manipulation of colloidal particle shape offers a novel design mechanism for the creation of advanced responsive materials. To this end, we introduce a versatile new strategy for shape control of anisotropic polymeric colloidal particles. The concept utilizes temperature‐sensitive absorption of a suitable solvent from a binary mixture. Specifically, increasing the temperature in the vicinity of the demixing transition of a binary mixture causes more solvent to be absorbed into the polymeric colloidal particle, which, in turn, lowers the glass transition temperature of the polymer inside the particle, with a concomitant decrease in viscosity. The balance between the internal viscosity and surface tension of the particle is thus disrupted, and the anisotropic shape of the particle shifts to become more spherical. Subsequent rapid temperature quenching can halt the process, leaving the particle with an intermediate anisotropy. The resultant shape anisotropy control provides new routes for studies of the phase transitions of anisotropic colloids and enables the fabrication of unique particles for materials applications.     

On the mechanism of hydrophobic association of nanoscopic solutes

The hydration behavior of two planar nanoscopic hydrophobic solutes in liquid water at normal temperature and pressure is investigated by calculating the potential of mean force between them at constant pressure as a function of the solute-solvent interaction potential. The importance of the effect of weak attractive interactions between the solute atoms and the solvent on the hydration behavior is clearly demonstrated. We focus on the underlying mechanism behind the contrasting results obtained in various recent experimental and computational studies on water near hydrophobic solutes. The length scale where crossover from a solvent separated state to the contact pair state occurs is shown to depend on the solute sizes as well as on details of the solute-solvent interaction. We find the mechanism for attractive mean forces between the plates is very different depending on the nature of the solute-solvent interaction which has implications for the mechanism of the hydrophobic effect for biomolecules.     

Theory of solvation in polar nematics

We develop a linear response theory of solvation of ionic and dipolar solutes in anisotropic, axially symmetric polar solvents. The theory is applied to solvation in polar nematic liquid crystals. The formal theory constructs the solvation response function from projections of the solvent dipolar susceptibility on rotational invariants. These projections are obtained from Monte Carlo simulations of a fluid of dipolar spherocylinders which can exist both in the isotropic and nematic phases. Based on the properties of the solvent susceptibility from simulations and the formal solution, we have obtained a formula for the solvation free energy which incorporates the experimentally available properties of nematics and the length of correlation between the dipoles in the liquid crystal. The theory provides a quantitative framework for analyzing the steady-state and time-resolved optical spectra and makes several experimentally testable predictions. The equilibrium free energy of solvation, anisotropic in the nematic phase, is given by a quadratic function of cosine of the angle between the solute dipole and the solvent nematic director. The sign of solvation anisotropy is determined by the sign of dielectric anisotropy of the solvent: solvation anisotropy is negative in solvents with positive dielectric anisotropy and vice versa. The solvation free energy is discontinuous at the point of isotropic-nematic phase transition. The amplitude of this discontinuity is strongly affected by the size of the solute becoming less pronounced for larger solutes. The discontinuity itself and the magnitude of the splitting of the solvation free energy in the nematic phase are mostly affected by microscopic dipolar correlations in the nematic solvent. Illustrative calculations are presented for the equilibrium Stokes shift and the Stokes shift time correlation function of coumarin-153 in 4-n-pentyl-4'-cyanobiphenyl and 4,4-n-heptyl-cyanopiphenyl solvents as a function of temperature in both the nematic and isotropic phases.     

Swelling Behaviour of Isotropic Poly(n-butyl acrylate) Networks in Isotropic and Anisotropic Solvents

Summary: The swelling properties of photochemically crosslinked poly(n-butyl acrylate) (PABu) networks in isotropic and anisotropic solvents were investigated experimentally. The purpose of this study was to examine the swelling kinetics of PABu networks in isotropic solvents and to compare the results obtained which those observed in the case of the low molecular weight liquid crystal 4-cyano-4′-n-pentyl-biphenyl known as 5CB. The phase diagrams were established in terms of composition and temperature for isotropic solvents, as toluene, acetone, cyclohexane, and methanol, and 5CB, using the plateau values corresponding to equilibrium states of swelling. The polymer networks were prepared via free radical polymerization/crosslinking processes by ultraviolet (UV) radiation of initial mixtures made up from a monomer, a crosslinker, and a photoinitiator. PABu networks with several crosslinking densities were formed using different quantities of difunctional monomer hexanedioldiacrylate (HDDA). Immersion of these networks in excess solvent allows measuring the solvent uptake by determination of the weight in isotropic solvents and diameter in an anisotropic solvent (5CB). Swelling data were rationalized by calculating weight and diameter ratios considering swollen to dry network states of the samples.     

Thermodynamic study of a liquid crystal as a liquid phase in gas-liquid chromatography. II. A cholesteric liquid crystal.

Gas-liquid chromatography is utilized for the determination of thermodynamic solution parameters for various organic solutes at infinite dilution in the meso- and isotropic phases of cholesteryl palmitate. The thermodynamic data and trends in values of the activity coefficients for the solutes are discussed in relation to their structure and to the orientations of the liquid crystal.     

A molecular dynamics computer simulation study of room-temperature ionic liquids. I. Equilibrium solvation structure and free energetics

Solvation in 1-ethyl-3-methylmidazolium chloride and in 1-ethyl-3-methylimidazolium hexafluorophosphate near equilibrium is investigated via molecular dynamics computer simulations with diatomic and benzenelike molecules employed as probe solutes. It is found that electrostriction plays an important role in both solvation structure and free energetics. The angular and radial distributions of cations and anions become more structured and their densities near the solute become enhanced as the solute charge separation grows. Due to the enhancement in structural rigidity induced by electrostriction, the force constant associated with solvent configuration fluctuations relevant to charge shift and transfer processes is also found to increase. The effective polarity and reorganization free energies of these ionic liquids are analyzed and compared with those of highly polar acetonitrile. Their screening behavior of electric charges is also investigated.     

Loading effect on swelling of nematic elastomers

Externally imposed loading has substantially different effects on the swelling of nematic elastomers in the high-temperature isotropic and low-temperature nematic states. In the isotropic state, the stretching drives a considerably large degree of further swelling, whereas the stretching-induced volume change in the nematic state is significantly suppressed. In the isotropic phase that favors the less anisotropic state, the further swelling occurs to reduce the shape anisotropy caused by the imposed elongation. In the nematic phase, no significant swelling is induced because further swelling decreases the nematic order enhanced by the applied stretching. These different loading effects in the isotropic and nematic states observed in the experiments are qualitatively described by a mean field theory.     

Salting out near the critical point

The salting out of solid solutes near the critical point of the solvent is investigated using the results of a previous paper (J. Phys. Chem. B 2006, 110, 24077) on the fluctuation theory of salting out. It is found that the salting out coefficient at infinite dilution of cosolvent is approximately proportional to the compressibility of the solvent and is consequently quite large near the critical point. No estimate is given for the range of cosolvent concentrations over which the infinite dilution slope might be a good approximation. Far from the critical point, it is known to be a good approximation over a considerable cosolvent concentration range.     

The effect of shear flow on the conformation of polyelectrolyte brushes

The behavior of flat polyelectrolyte brushes under the action of a lateral force or flow was studied. Special attention was focused on the case when a lateral force acts on a brush that occurs near the point of phase transition from the swollen state to the collapsed state. The difference between phase transitions in a brush induced by isotropic and anisotropic interactions is analyzed. As examples of such transitions, the collapse of a polyelectrolyte brush upon cooling and the nematic collapse of an anisotropic brush are considered. It was shown that lateral force (flow), exerting a marked effect on the nematic collapse of an anisotropic brush, has no practical effect on the collapse of a brush with isotropic interactions.     

Molecular density functional theory of solvation: from polar solvents to water

A classical density functional theory approach to solvation in molecular solvent is presented. The solvation properties of an arbitrary solute in a given solvent, both described by a molecular force field, can be obtained by minimization of a position and orientation-dependent free-energy density functional. In the homogeneous reference fluid approximation, limited to two-body correlations, the unknown excess term of the functional approximated by the angular-dependent direct correlation function of the pure solvent. We show that this function can be extracted from a preliminary MD simulation of the pure solvent by computing the angular-dependent pair distribution function and solving subsequently the molecular Ornstein-Zernike equation using a discrete angular representation. The corresponding functional can then be minimized in the presence of an arbitrary solute on a three-dimensional cubic grid for positions and Gauss-Legendre angular grid for orientations to provide the solvation structure and free-energy. This two-step procedure is proved to be much more efficient than direct molecular dynamics simulations combined to thermodynamic integration schemes. The approach is shown to be relevant and accurate for prototype polar solvents such as the Stockmayer solvent or acetonitrile. For water, although correct for neutral or moderately charged solute, it tends to underestimate the tetrahedral solvation structure around H-bonded solutes, such as spherical ions. This can be corrected by introducing suitable three-body correlation terms that restore both an accurate hydration structure and a satisfactory energetics.     

Molecular dynamics simulations of the effect of the composition of calcium alumino-silicate intergranular films on alumina grain growth

Molecular dynamics simulations were performed to study the effect of the composition of the intergranular film (IGF) on anisotropic and isotropic grain growth in alpha-Al2O3. In the simulations, the IGF is formed while in contact with two differently oriented alumina crystals, with the alumina (0001) basal plane on one side and the (110) prism plane on the other. Five different compositions in the IGFs were studied. Results show preferential growth along the [110] of the (110) surface in comparison to growth along the [0001] direction on the (0001) surface for compositions near a Ca/Al ratio of 0.5. Such preferential growth is consistent with anisotropic grain growth in alumina, where platelets form because of faster growth of the prism orientations than the basal orientation. The simulations also show the mechanism by which Ca ions in the IGF inhibit growth on the basal surface. At compositions with high or low Ca/Al ratios, growth along each surface normal is equivalent, indicating isotropic grain growth, although the attachment rates are quite different, which may indicate differences between normal grain growth and abnormal, but isotropic, grain growth. The simulations provide an atomistic view of attachment onto crystal surfaces, affecting grain growth in alumina.