Thermodynamic Phase Behavior Research Articles

Salting-Out of Lysozyme and Ovalbumin from Mixtures: Predicting Precipitation Performance from Protein−Protein Interactions

Protein−protein interactions, whether desirable or not, can have direct effects on protein separations. The most obvious of these are interactions between similar molecules, which determine the thermodynamic properties and phase behavior of protein solutions. Interactions between dissimilar molecules also affect the properties and phase behavior of protein mixtures and, therefore, protein separations. Here, we seek to quantify these effects by comparing precipitation behavior from binary solutions with direct measurements of protein−protein interactions using cross-interaction chromatography, which is a variant of affinity chromatography that provides data that can be interpreted in terms of the virial cross-coefficient. First, the effects of pH, ionic strength, different precipitants and the initial protein concentrations and their ratio on the binary precipitation of lysozyme and ovalbumin were investigated. Next, self- and cross-interaction measurements were used to suggest the optimal precipitation conditions for separating lysozyme from ovalbumin. The results show that protein interactions can explain anomalies and inconsistencies that frequently confound the extraction of meaningful general trends in separations analyses.

Structure of molecular liquids: Hard rod-disk mixtures

The structure of hard rod-disk mixtures is studied using Monte Carlo simulations and integral equation theory, for a range of densities in the isotropic phase. By extension of methods used in single component fluids, the pair correlation functions of the molecules are calculated and comparisons between simulation and integral equation theory, using a number of different closure relations, are made. Comparison is also made for thermodynamic data and phase behavior.

Polymer dispersed liquid crystals based on poly(styrene-b-ethylene oxide), poly(bisphenol a carbonate) or poly(methylphenylsiloxane), and 4′-(hexyloxy)-4-biphenyl-carbonitrile: Analysis of phase diagrams and morphologies generated

Thermodynamic phase behavior was studied for polymer dispersed liquid crystal (PDLC) films consisting of blends of different thermoplastics such as poly (bisphenol A carbonate) (PC), poly(methylphenylsiloxane) (PMPLSi), poly(styrene-b-ethylene oxide) (PSEO) block copolymer, and low-molecular weight 4′-(hexyloxy)-4-biphenyl-carbonitrile (HOBC) liquid crystals. The phase diagrams of these PDLC systems were built up using thermal transitions determined by differential scanning calorimetry (DSC), transmission optical microscopy (TOM), and dynamic mechanical analysis (DMA). Final morphology of these PDLC films generated by temperature induced phase separation was monitored by TOM. The final morphology of HOBC blends with PC, PSEO, or PMPLSi could be modified by controlling the relative compositions resulting in PDLC systems with small and narrow size-distribution of nematic droplets, suitable for use in thermo-optical devices. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Electroneutrality and phase behavior of colloidal suspensions

Several statistical mechanical theories predict that colloidal suspensions of highly charged macroions and monovalent microions can exhibit unusual thermodynamic phase behavior when strongly deionized. Density-functional, extended Debye-Hückel, and response theories, within mean-field and linearization approximations, predict a spinodal phase instability of charged colloids below a critical salt concentration. Poisson-Boltzmann cell model studies of suspensions in Donnan equilibrium with a salt reservoir demonstrate that effective interactions and osmotic pressures predicted by such theories can be sensitive to the choice of reference system, e.g., whether the microion density profiles are expanded about the average potential of the suspension or about the reservoir potential. By unifying Poisson-Boltzmann and response theories within a common perturbative framework, it is shown here that the choice of reference system is dictated by the constraint of global electroneutrality. On this basis, bulk suspensions are best modeled by density-dependent effective interactions derived from a closed reference system in which the counterions are confined to the same volume as the macroions. Lower-dimensional systems (e.g., monolayers, clusters), depending on the strength of macroion-counterion correlations, may be governed instead by density-independent effective interactions tied to an open reference system with counterions dispersed throughout the reservoir, possibly explaining the observed structural crossover in colloidal monolayers and anomalous metastability of colloidal crystallites.

Phase behavior of dipolar associating fluids from the SAFT-VR+D equation of state

The statistical associating fluid theory for potentials of variable range plus dipole (SAFT-VR+D) is extended to study associating dipolar fluids. In the SAFT-VR+D approach dipolar interactions are taken into account through the use of the generalized mean spherical approximation to describe a reference fluid of dipolar square-well segments. This enables the effect of the dipolar interactions on the thermodynamics and structure of fluids to be explicitly described. Predictions for the thermodynamic properties and phase behavior of dipolar associating square-well monomers with one, two, and four association sites are considered and compared with new isothermal-isobaric and Gibbs ensemble Monte Carlo simulation data. The results show that the SAFT-VR+D equation provides a good description of the phase behavior of dipolar associating fluids. Additionally we have applied the new theoretical approach to study the vapor pressure and saturated liquid density of water.

Thermodynamic Phase Behavior of Ionic Liquids

The phase diagrams of the following imidazolium ionic liquids, {1-ethyl-3-methylimidazolium tosylate [EMIM][TOS] + hexane or cycloheptane} and {1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4] + hexane or benzene}, have been measured. The (solid + liquid) (SLE) and (liquid + liquid) (LLE) phase equilibria of binary mixtures containing {hexyl(2-hydroxyethyl)dimethylammonium bromide (C6Br) + hexane or benzene or an alcohol (ethanol or dodecan-1-ol)} and the salt with a different anion {hexyl(2-hydroxyethyl)dimethylammonium tetrafluoroborate (C6BF4) + hexane or benzene or dodecan-1-ol} have been measured. The effect of anion type (Br- vs BF4-) on the thermophysical properties of the IL and on the phase behavior is dependent on the strength of the cation−anion interaction in the investigated ILs and is noticeable. The phase diagrams for some novel ammonium ionic liquids, (2-decanoyloxyethyl)dimethylpentyloxymethylammonium trifluoro-acetate [C10OOEtC5OMMMN][CF3COO] or (2-methyloyoxyethyl)dimethylpentylox...

Systematic characterization of thermodynamic and dynamical phase behavior in systems with short-ranged attraction

In this paper we demonstrate the feasibility and utility of an augmented version of the Gibbs ensemble Monte Carlo method for computing the phase behavior of systems with strong, extremely short-ranged attractions. For generic potential shapes, this approach allows for the investigation of narrower attractive widths than those previously reported. Direct comparison to previous self-consistent Ornstein-Zernike approximation calculations is made. A preliminary investigation of out-of-equilibrium behavior is also performed. Our results suggest that the recent observations of stable cluster phases in systems without long-ranged repulsions are intimately related to gas-crystal and metastable gas-liquid phase separation.

Molecular simulation and theoretical modeling of polyhedral oligomeric silsesquioxanes

Polyhedral oligomeric silsesquioxane (POSS) molecules are unique nanometre-size inorganic/organic hybrid structures based on a (SiO1.5)8 core. Depending on the functionalization of the POSS cages, the resulting systems can be solid or liquid, or, upon crosslinking, turned into a network. While much is known experimentally about the chemical synthesis of POSS systems, very little theoretical understanding exists at the molecular level or beyond. Of particular interest is the study of the effect of tether groups on the thermodynamic properties and phase behaviour of POSS fluids, which is addressed in this work through both molecular simulation and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). In this application of SAFT-VR to POSS molecules the hetero-SAFT-VR approach is used. The POSS cage is represented as a single sphere and the tether groups by one or more smaller spheres, depending on chain length. Since very limited experimental data is available for the thermodynamic properties of POSS systems, molecular dynamics simulations are used to obtain pseudo-experimental data with which to validate the theoretical approach. Good agreement is obtained between the simulation data and the theoretical results and predictions are made for systems yet to be studied by simulation or experiment.

Motifs for molecular recognition exploiting hydrophobic enclosure in protein-ligand binding.

The thermodynamic properties and phase behavior of water in confined regions can vary significantly from that observed in the bulk. This is particularly true for systems in which the confinement is on the molecular-length scale. In this study, we use molecular dynamics simulations and a powerful solvent analysis technique based on inhomogenous solvation theory to investigate the properties of water molecules that solvate the confined regions of protein active sites. Our simulations and analysis indicate that the solvation of protein active sites that are characterized by hydrophobic enclosure and correlated hydrogen bonds induce atypical entropic and enthalpic penalties of hydration. These penalties apparently stabilize the protein-ligand complex with respect to the independently solvated ligand and protein, which leads to enhanced binding affinities. Our analysis elucidates several challenging cases, including the super affinity of the streptavidin-biotin system.

THE PHYSICS OF LIQUID PARA-HYDROGEN

Macroscopic systems of hydrogen molecules exhibit a rich thermodynamic phase behavior. Due to the simplicity of the molecular constituents a detailed exploration of the thermal properties of these boson systems at low temperatures is of fundamental interest. Here, we report theoretical and experimental results on various spatial correlation functions and corresponding distributions in momentum space of liquid para-hydrogen close to the triple point. They characterize the structure of the correlated liquid and provide information on quantum effects present in this Bose fluid. Numerical calculations employ Correlated Density-Matrix (CDM) theory and Path-Integral Monte-Carlo(PIMC)simulations. A comparison of these theoretical results demonstrates the accuracy of CDM theory. This algorithm therefore permits a fast and efficient quantitative analysis of the normal phase of liquid para-hydrogen. We compare and discuss the theoretical results with available experimental data.

Phase behavior of dipolar fluids from a modified statistical associating fluid theory for potentials of variable range

A statistical associating fluid theory for potentials of variable range to model dipolar fluids is presented. The new theory, termed the SAFT-VR+D equation (the statistical associating fluid theory for potentials of variable range plus dipole), explicitly accounts for dipolar interactions and their effect on the structure of the fluid. This is achieved through the use of the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. Isothermal-isobaric and Gibbs ensemble Monte Carlo simulations have been performed in order to test the new theoretical approach. Predictions for the thermodynamic properties and phase behavior of dipolar square-well monomer and chain fluids, in which one or more segments are dipolar, are considered and compared with new computer simulation data. The results show that the equation of state in general provides a good description of the phase behavior of dipolar monomer and chain fluids. Some deviations are seen between the simulation data and theoretical predictions for monomer fluids with large dipole moments and for molecules composed of segments with dipoles in different orientations. This is addressed through the replacement of the GMSA by the linearized exponential approximation.

Thermophysical properties and thermodynamic phase behavior of ionic liquids

The thermal stability of many tested ionic liquids (ILs) was investigated by the TGA and DTA curves over the wide temperature range from 200 to 780 K. The TGA curves have mainly a sigmoid shape, which can be split into three segments. The thermal decomposition of the samples was higher than 500 K. For the ammonium salts, C 2BF 4, or C 2PF 6, or C 2N(CN) 2, or C 4Br, the temperatures of the decompositions were 583.5, 556.1, 545.1 and 525.3 K, respectively. Generally, it was found that the temperature of decomposition of investigated ionic liquid is strongly depended on the type of cation and the anion. Phase equilibria and thermophysical constants were measured also for the dialkoxy-imidazolium ILs, [(C 4H 9OCH 2) 2IM][BF 4], [(C 8H 17OCH 2) 2IM][Tf 2N], [(C 10H 21OCH 2) 2IM][Tf 2N] and for pyridinium IL, [Pyr][BF 4]. The characterization and purity of the compounds were obtained by the elemental analysis, water content (Fisher method) and differential scanning microcalorimetry (DSC) analysis. From (DSC) method, the melting points, the enthalpies of fusion, the temperatures and enthalpies of solid–solid phase transitions and the half C p temperatures of glass transition of all investigated ionic liquids were measured. The phase equilibria of these salts with common popular solvents: water, or alcohols or n-alkanes, or aromatic hydrocarbons have been measured by a dynamic method from 290 K to the melting point of IL, or to the boiling point of the solvent in the whole mole fraction range, x from 0 to 1. These salts mainly exhibit simple eutectic systems with immiscibility in the liquid phase with upper critical solution temperatures (UCST), not only with aromatic hydrocarbons, cycloalkanes and n-alkanes but also with longer chain alcohols. For example the C 2BF 4 salt show simple eutectic system with water and simple eutectic systems with immiscibility in the liquid phase with upper critical solution temperature with alcohols. The solid–liquid phase equilibria, SLE curves were correlated by means of the different G Ex models utilizing parameters derived from the SLE. The root-mean-square deviations of the solubility temperatures for all calculated data depend on the particular system and the equation used.

Sigma-Profile Database for Using COSMO-Based Thermodynamic Methods

Thermodynamic methods based on conductor-like screening models (COSMO) originated from the use of solvation thermodynamics and computational quantum mechanics. These methods rely on sigma profiles specific to each molecule. A sigma profile is the probability distribution of a molecular surface segment having a specific charge density. Two COSMO-based thermodynamic models are COSMO-RS (realistic solvation) developed by Klamt and his colleagues, and COSMO-SAC (segment activity coefficient) published by Lin and Sandler. Quantum mechanical calculations for generating the sigma profiles represent the most time-consuming and computationally expensive aspect of using COSMO-based methods. A growing number of scientists and engineers are interested in the COSMO-based thermodynamic models but are intimidated by the complexity of performing quantum mechanical calculations. This paper presents the first free, web-based sigma profile database of 1432 compounds. We describe the procedure for sigma profile generation, and we have validated our database by comparing COSMO-based predictions of activity coefficients, normal boiling point and solubility with experimental data and thermodynamic property database. We discuss improvements which include using supplemental geometry optimization software packages to provide good initial guesses for molecular conformations as a precursor to the COSMO calculation. Finally, this paper provides a FORTRAN program and a procedure to generate additional sigma profiles, as well as a FORTRAN program to generate binary phase-equilibrium predictions using the COSMO-SAC model. Our sigma profile database will facilitate predictions of thermodynamic properties and phase behaviors from COSMO-based thermodynamic models.

Solvent effects on phase behavior with false UCST in blends of PVPh with aliphatic polyesters

Solvent effects on phase behavior in blends were examined. Misinterpretation might be resulted if effects of solvent were not properly evaluated and avoided. Solvent effects were found to influence to widely different levels on blend systems comprised of poly(4-vinyl phenol) (PVPh) and a homologous series of polyesters of different CH 2/CO ratios. Confusion and misinterpretation in phase behavior between miscibility and immiscibility with upper-critical-solution-temperature (UCST) could be resulted if solvent effects were not fully excluded from true thermodynamic phase behavior in the blends. However, apparent solvent effects on interfering true phase behavior were found in the blends of PVPh with several polyesters whose structures led to borderline miscibility. Superficially, the blends prepared by solvent casting behaved similarly to phase-separated blends with UCST. By comparison, regardless of methods of blend preparation, the miscibility was straightforwardly proven for the blends of PVPh with poly(hexamethylene adipate) (PHA) that possesses an average CH 2/CO ratio = 5.0 located on the center of window, and the phase behavior of PVPh/PHA blends is largely free of solvent interference. Care must be exercised in interpretation of phase behavior by excluding the solvent effects, especially for those blends with borderline miscibility with weak interactions.

Effect of the Structures of Ionic Liquids and Alkylbenzene-Derived Amphiphiles on the Inhibition of Asphaltene Precipitation from CO2-Injected Reservoir Oils

The inhibition of asphaltene precipitation from high-pressure, CO(2)-injected reservoir oils by ionic and nonionic amphiphiles, the ionic liquids based on p-alkylpyridinium ([C(n)()py](+)) and N-butylisoquinolinium ([C(4)iql](+)) cations, and the alkylbenzene-derived amphiphiles p-alkylphenol (C(n)()phol), p-alkylbenzenesulfonic acid (C(n)()bsa), and sodium p-alkylbenzenesulfonate (C(n)()bsNa) was investigated for the first time. The influences of the structures of these compounds and the effect of the combination of their cations and anions were studied. The results show that the inhibition abilities of the alkylbenzene-derived amphiphiles first increase when n = 2-8 and then remain almost constant when n >/=8 and that the effectiveness follows the order C(n)()phol < C(n)()bsa approximately C(n)()bsNa. The inverse trend is observed for the ionic liquids [C(n)()py][Cl]; that is, their inhibition abilities decrease as n increases from 4 to 8 to 12. [C(4)iql][Cl] is more effective than [C(4)py][Cl], but [C(n)()py][BF(4)] and [C(n)()py][PF(6)] have almost no effect on the stabilization of asphaltenes. It was found that the effectiveness of an alkylbenzene-derived amphiphile on the inhibition of asphaltene precipitation from reservoir oils relies on its ability to form a stable steric-stabilization layer around asphaltenes, which is controlled by the polarity of its headgroup and the length of its alkyl tail. The novel mechanism of inhibiting asphaltene precipitation using the ionic liquids [C(n)()py] ([Cl], [BF(4)], and [PF(6)]) and [C(4)iql][Cl] was proposed. The mechanism states that the ionic liquids can effectively prevent asphaltene precipitation from the reservoir oils by breaking the asphaltene associations, which are due to the local nonneutrality of the charge densities of the cation and the anion. The ionic liquids that are based on an anion with high charge density, in connection with cations with sufficiently low charge densities, can effectively inhibit asphaltene precipitation from the reservoir oils. This mechanism is also important for studying the thermodynamic properties and phase behavior of the ionic liquids.

A new model for predicting the thermodynamic phase behavior of electrolyte in aqueous solutions

In the present study, the phase behavior of aqueous electrolyte solutions has been correlated with a new three-parameter model based on the Debye–Hückel model. The new model was evaluated by the estimation of the osmotic and mean ionic activity coefficients of 123 electrolytes in aqueous solutions. The adjustable parameters of the new model were obtained by using non-linear regression between the experimental data and the results of the new model. Using adjustable parameters, the osmotic coefficients of the aqueous solutions were calculated. The results of the standard deviations of the osmotic and mean ionic activity coefficients were in a good agreement.

Thermodynamic phase behavior of fluoropolymer mixtures with supercritical fluid solvents

Cloud-point experimental data were presented at temperatures up to 260 °C and pressures up to 2,500 bar for poly(vinylidene fluoride) (PVDF) [Mw=180,000 and 275,000] in CO2, CHF3, CHClF2, and dimethyl ether (DME). Also, phase behavior curves were shown for the PVDF [Mw= 180,000] in CO2 with 1-propanol and 1-pentanol. Poly(vinyl fluoride) does not dissolve in CO2 even at 215 °C and 2,500 bar, while poly(vinylidene fluoride) does dissolve in CO2 at 220 °C and pressures of 1,700 bar. To dissolve poly(vinylidene fluoride) in dimethyl ether, pressures in excess of 200 bar and temperatures in excess of 165 °C are needed. The cloud-point behavior between the ternary PVDF [Mw=180,000]-CO2-DME mixture shows a temperature range of 100–195 °C and pressure range of 350–1,360 bar by adding the 11, 29, and 60 wt% DME in cosolvent. The phase behavior for PVDF [Mw=180,000]-CHF3-9, 19, and 40 wt% CHClF2 system shows a temperature at below 200 °C and pressure up to 1,700 bar. The effect of CHClF2 as a cosolvent for PVDF [Mw=275,000] in CO2 decreased with the pressure increasing 10, 26, 40 and 61 wt% of cosolvent (CHClF2) contents. Also, the effect of 15 and 40 wt% DME for the PVDF [Mw=275,000]-CHF3 mixture shows temperature ranges of 120–206 °C and pressures up to 1,570 bar.

Semicrystalline morphology in thin films of poly(3-hexylthiophene)

The thermodynamic phase behavior and the morphology in thin films of poly(3-hexylthiophene) (P3HT) has been studied using calorimetry, X-ray scattering, and scanning force microscopy (AFM). Around 225 °C a phase transition from the crystalline state to a layered, liquid crystalline structure occurs in regioregular P3HT, while the regiorandom counterpart material is disordered at all temperatures and displays a glass transition temperature T g ≈−3 °C. Regioregular P3HT is semicrystalline and forms needle or plate like crystallites which in solution cast thin films are oriented with respect to the substrate. Films produced by spin coating display a non-equilibrium structure with reduced order and orientation. Annealing of these films in the liquid crystalline state leads to the formation of a morphology similar to the one observed in solution cast films.

Adsorption and two-dimensional phases of a large polar molecule: Sub-phthalocyanine on Ag(111)

The adsorption and the two-dimensional (2D) ordering of chloro[subphthalocyaninato]boron(III) (SubPc) on Ag(111) has been studied in detail by combined scanning tunneling microscopy and photoelectron spectroscopy at room temperature. SubPc is a polar, highly symmetric molecule, consisting of an extended aromatic system and a central B-Cl bond. When growing on Ag(111) an interesting phase behavior is observed for the first molecular layer of SubPc. At low coverage, below $\ensuremath{\approx}0.2$ monolayer (ML), a 2D lattice gas is present, whereas at medium coverage (on the order of 0.2--0.5 ML), 2D condensed molecular islands are observed in coexistence with the 2D lattice gas. In these condensed islands, the molecules assemble into a well-ordered honeycomb pattern. At higher coverage (approximately 0.5--0.9 ML) the molecules organize into a 2D hexagonal close-packed (hcp) pattern, in equilibrium with a dense 2D gas phase. In the honeycomb and in the hcp pattern, individual molecules are imaged with submolecular resolution, giving information on their orientation. For both the honeycomb and hcp patterns, islands with two different orientations of the superstructures with respect to the Ag(111) substrate are observed. In case of the honeycomb pattern, the two superstructures are enantiomorphic. The chirality of these layers originates in the loss of the symmetry of the metal surface upon adsorption of SubPc, while the molecules alone are intrinsically achiral. Based on different photoelectron spectroscopy experiments we conclude that the SubPc molecule is adsorbed on Ag(111) with its Cl atom towards the substrate and that the molecule remains intact. Finally, several aspects of the observed 2D condensed phases and the thermodynamic phase behavior are discussed with respect to the charge distribution and the adsorption physics and chemistry of the SubPc molecules.

Quantum mechanics: a new tool for engineering thermodynamics

Computational quantum mechanics is leading to new, theoretically based methods for the prediction of thermodynamic properties and phase behavior of interest to engineers. Three such methods we have been working on are reviewed here. In the most direct and computational intensive form, computational quantum mechanics is used to obtain information on the multidimensional potential energy surface between molecules, which is then used in computer simulation to predict thermodynamic properties and phase equilibria. At present, this method is limited to the study of small molecules due to the computational resources available. The second method is much less computationally intensive and provides a way to improve group-contribution methods by introducing corrections based on the charge and dipole moment of each functional group that is unique to the molecule in which it appears. The final method we consider is based on the polarizable continuum model, in which the free energy of transferring a molecule from an ideal gas to a liquid solution is computed, leading directly to values of activity coefficients and phase equilibrium calculations.