Phase behavior of self-associating fluids with weaker dispersion interactions between bonded particles

In this study, we explore the global phase behavior of a simple model for self-associating fluids where association reduces the strength of the dispersion interactions between bonded particles. Recent research shows that this type of behavior likely explains the thermodynamic properties of strongly polar fluids and certain micellar solutions. Based on Wertheim's theory of associating liquids [M. S. Wertheim, J. Stat. Phys. 42, 459 (1986); 42, 477 (1986)], our model takes into account the effect that dissimilar particle interactions have on the equilibrium constant for self-association in the system. We find that weaker interactions between bonded molecules tend to favor the dissociation of chains at any temperature and density. This effect stabilizes a monomeric liquid phase at high densities, enriching the global phase behavior of the system. In particular, for systems in which the energy of mixing between bonded and unbonded species is positive, we find a triple point involving a vapor, a dense phase of chain aggregates, and a monomeric liquid. Phase coexistence between the vapor and the monomeric fluid is always more stable at temperatures above the triple point, but a highly associated fluid may exist as a metastable phase under these conditions. The presence of this metastable phase may explain the characteristic nucleation behavior of the liquid phase in strongly dipolar fluids.     

Nucleation versus spinodal decomposition in confined binary solutions

Basic features of spinodal decomposition, on one side, and nucleation, on the other side, and the transition between both mechanisms are analyzed within the framework of a generalized thermodynamic cluster model based on the generalized Gibbs approach. Hereby the clusters, representing the density or composition variations in the system, may change with time both in size and in their intensive state parameters (density and composition, for example). In the first part of the analysis, we consider phase separation processes in dependence on the initial state of the system for the case when changes of the state parameters of the ambient system due to the evolution of the clusters can be neglected as this is the case for cluster formation in an infinite system. As a next step, the effect of changes of the state parameters on cluster evolution is analyzed. Such depletion effects are of importance both for the analysis of phase formation in confined systems and for the understanding of the evolution of ensembles of clusters in large (in the limit infinite) systems. The results of the thermodynamic analysis are employed in both cases to exhibit the effect of thermodynamic constraints on the dynamics of phase separation processes.     

Wetting and non-wetting fluids in surface-functionalised activated carbons

Pore filling by non-polar and polar molecules is investigated by small-angle X-ray scattering in activated carbons that had been surface-functionalised to different degrees by oxidation. A pseudo-binary model for the scattering response is used to trace the filling characteristics of water and n-hexane in the pore structure. While the pores are uniformly filled by n-hexane, pore filling by water is only partial. In the latter case, a significant contribution from liquid–vapour interfaces appears and the system becomes fully ternary. This feature is direct evidence of the development of water droplets, which form even in the most oxidised carbon. The appreciable differences between the carbons illustrate the influence on the small-angle X-ray scattering response of surface chemistry and of the polarity of the adsorbed molecules.     

Influence of Particle Nucleation in Pressure Sensitive Adhesive Properties: Miniemulsion versus Emulsion Polymerization

Miniemulsion polymerization is a promising approach to produce and tailor pressure sensitive adhesives (PSAs). In this paper, a systematic comparison of the adhesive properties of latexes produced by miniemulsion and conventional emulsion polymerization is presented. Specifically, the influence of the total surfactant concentration, chain transfer agent concentration and chemical composition on the final adhesive properties of the polymer 2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid was discerned using a 2³ factorial design for each polymerization method. In addition to the adhesive properties (i.e., loop tack, peel strength and shear strength), molecular weight distribution, particle size distribution (PSD) and glass transition temperature were analyzed. The results show that under the conditions used in this work, it is possible to produce PSAs using miniemulsion polymerization, a process wherein monomer droplet nucleation is the dominant particle nucleation mechanism. The use of a miniemulsion polymerization process, as opposed to the conventional emulsion technique, produced several differences such as larger particles sizes and narrower molecular weight distributions. Focusing on the PSA films that exhibited adhesive rather than cohesive failure, the PSA films generated via miniemulsion polymerization displayed higher values of loop tack and peel strength compared to those produced via conventional emulsion polymerization. Shear strength results were strongly dependent on the amount of gel content and sol molecular weight for both cases.     

The non-random distribution of free volume in fluids: polydisperse polymer systems

A new equation-of-state model is presented that accounts for the non-random distribution of free volume in multicomponent fluid mixtures. The classical quasi-chemical approach is used in connection with the Lattice Fluid (LF) model. The model is extended via continuous thermodynamics to polydisperse polymer systems. The algorithm for the application of the model to phase equilibrium calculations is also presented. The model is applied to both solvent—polydisperse polymer and to polydisperse polymer—polydisperse polymer systems. Besides phase equilibrium calculations, the distributions of the polymeric species in principal and conjugate phases are obtained. The comparison with available experimental data is satisfactory.     

Sequential isomerism of activated complexes: Interfering versus noninterfering intermediate

A unimolecular process is considered that was treated experimentally as a concerted one; however, subsequent quantum-chemical analysis has proved that the process is realized via an intermediate (double sequential isomerism of activated complexes). The intermediate is supposed to contribute directly to values of the primary observed data (e.g., spectral absorbances). Within this approach of interfering intermediate any degree of this interference is allowed including the limit case of noninterfering intermediate serving as a reference standard. A technique is suggested enabling correct comparison of the quantum-chemical outputs with the experimental overall values for rate constants and activation enthalpy and entropy. The technique employs mere amount of information currently obtainable from quantum-chemical study of a rate process (i.e., representation of potential energy hypersurface by its stationary points and activated-complex theory). Properties of the derived formulae are illustrated with model examples. The results are important for meaningful comparison of experimental and theoretical data in the case of processes with sequential isomerism of activated complexes disclosed only theoretically.     

Rapid calculation of partition functions and free energies of fluids

The partition function (Q) is a central quantity in statistical mechanics. All the thermodynamic properties can be derived from it. Here we show how the partition function of fluids can be calculated directly from simulations; this allows us to obtain the Helmholtz free energy (F) via F = -k(B)T ln Q. In our approach, we divide the density of states, assigning half of the configurations found in a simulation to a high-energy partition and half to a low-energy partition. By recursively dividing the low-energy partition into halves, we map out the complete density of states for a continuous system. The result allows free energy to be calculated directly as a function of temperature. We illustrate our method in the context of the free energy of water.     

Nucleation in Emulsion Polymerization: Another Step towards Non-Micellar Nucleation Theory

Summary : Comprehensive experimental results of the nucleation stage of styrene emulsion polymerization in the absence as well as in the presence of emulsifier at different concentrations are presented. In addition, the influence of initiator type and presence of seed particles are studied. The nucleation mechanism is verified by means of on-line monitoring of the optical transmission and the conductivity of the aqueous phase. Results prove that micelles do not alter the nucleation mechanism which comprises the initiation of water soluble oligomers in the aqueous phase followed by their aggregation into colloidally stable latex particles. Surfactants assist with nucleation as they lower the activation free energy of particle formation. Contrary, in the presence of seed particles above a critical volume fraction the formation of new particles can be suppressed.     

Equilibrium sampling of self-associating polymer solutions: a parallel selective tempering approach

We present a novel simulation algorithm based on tempering a fraction of relaxation-limiting interactions to accelerate the process of obtaining uncorrelated equilibrium configurations of self-associating polymer solutions. This approach consists of tempering (turning off) the attractive interactions for a fraction of self-associating groups determined by a biasing field h. A number of independent configurations (replicas) with overlapping Hamiltonian distributions in the expanded (NVTh) ensemble with constant NVT but different biasing fields, forming a chain of Hamiltonians, were simulated in parallel with occasional attempts to exchange the replicas associated with adjacent fields. Each field had an associated distribution of tempered interactions, average fraction of tempered interactions, and structural decorrelation time. Tempering parameters (number of replicas, fields, and exchange frequencies) were chosen to obtain the highest efficiency in sampling equilibrium configurations of a self-association polymer solution based on short serial simulation runs and a statistical model. Depending on the strength of the relaxation-limiting interactions, system size, and thermodynamic conditions, the algorithm can be orders of magnitude more efficient than conventional canonical simulation and is superior to conventional temperature parallel tempering.     

Nucleation free energy of pore formation in an amphiphilic bilayer studied by molecular dynamics simulations

The formation of a pore in a membrane requires a considerable rearrangement of the amphiphilic molecules about to form the bilayer edge surrounding the pore, and hence is accompanied by a steep increase of the free energy. Recent rupture and conductance experiments suggest that this reshuffling process is also responsible for a small energy barrier that stabilizes "prepores" with diameters of less than 1 nm, rendering both the opening and closing of pores an activated process. We use the potential of mean constraint force method to study this free energy profile, as a function of pore radius, in a coarse grained bilayer model. The calculations show that the free energy rises by (15-20) kT during pore opening, making it an extremely rare nucleation event. Although we do not observe a barrier to pore closure, the results do make the existence of such a barrier plausible. For larger pores we find a smooth transition to Litster's model, from which a line tension coefficient of about 3.7 x 10(-11) J m(-1) is deduced.     

Nucleation of Hematite: A Nonclassical Mechanism

Hematite (α-Fe₂O₃) is thermodynamically stable under ambient conditions, of vast geological importance, and widely used in applications, for example, as corrosion protection and as a pigment. It forms at elevated temperatures, whereas room-temperature reactions typically yield metastable akaganéite or ferrihydrite. The mechanistic key changes underlying this observation were explored in the present study. The entropic contribution to the prenucleation hydrolysis reaction categorically implies the presence of prenucleation clusters (PNCs) as fundamental precursors. The formation of hematite is then due to a change in the reaction mechanism above approximately 50 °C, whereby the reaction limitation towards oxolation in phase-separated clusters is overcome. A model that rationalizes the occurrence of hematite, akaganéite, and ferrihydrite based on the chemistry of olation PNCs is proposed. Supersaturation and the temperature dependence of olation and oxolation rates from monomeric precursors are irrelevant in this nonclassical mechanism.     

Theory of correlated two-particle activated glassy dynamics: general formulation and heterogeneous structural relaxation in hard sphere fluids

We generalize the nonlinear Langevin equation theory of activated single particle dynamics to describe the correlated motion of two tagged spherical particles in a glass- or gel-forming fluid as a function of their initial separation. The theory is built on the concept of a two-dimensional dynamic free energy surface which quantifies the forces on two particles moving in a cooperative manner. For the hard sphere fluid, above a threshold volume fraction we generically find two relaxation channels corresponding largely, but not exclusively, to a center-of-mass-like displacement and a radial separation of the two tagged particles. The entropic barriers and mean first passage times are computed and found to systematically vary with volume fraction and initial particle separation; both oscillate as a function of the latter in a manner related to the equilibrium pair correlation function. A dynamic correlation length is estimated as the length scale beyond which the two-particle activated dynamics becomes uncorrelated in space and time, and is found to modestly grow with increasing mean relaxation time. The theory is also applied to a simplified model of cage escape, the elementary step of structural relaxation. Predictions for characteristic relaxation times, translation-relaxation decoupling, and stretched-exponential decay of time correlation functions are obtained. A novel mechanism for understanding why strong decoupling emerges in the activated regime, but stretched nonexponential time correlation functions do not change shape as the mean relaxation time grows, is presented and favorably compared with experiment. The theory may serve as a starting point for constructing a predictive model of multiple correlated caging and hopping (forward and backward) events of a pair of tagged particles.     

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

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

Influence of coagulant and free stabilizer on formation of aggregates in magnetic fluids

The influence of coagulant (isopropanol) and free oleic acid on the formation of quasi-spherical aggregates in a colloidal magnetite solution in kerosene is studied experimentally. It is revealed that the mean sizes of these aggregates fit the 60–90-nm range and are independent of the concentrations of coagulant and free oleic acid provided that these concentrations do not exceed 10–12 vol %. The independence of the mean sizes of aggregates on temperature and disperse composition of particles is considered as a supplementary argument in favor of hypothesis that the main reason for their formation are the defects of protective layers, whereas magnetodipole interactions play a secondary role. It is shown that an excess of oleic acid causes a disproportional decrease in the initial susceptibility of magnetic fluid that is interpreted as a result of the formation of droplet aggregates with characteristic sizes of a few micrometers or more. Apparently, isopropanol leads to an analogous effect, but only at low temperatures.     

Identification of free phosphopeptides in different biological fluids by a mass spectrometry approach

Human body fluids have been rediscovered in the post-genomic era as a great source of biological markers and perhaps as source of potential biomarkers of disease. Recently, it has been found that not only proteins but also peptides and their modifications can be indicators of early pathogenic processes. This paper reports the identification of free phosphopeptides in human fluids using an improved IMAC strategy coupled to iterative mass spectrometry-based scanning techniques (neutral loss, precursor ion, multiple reaction monitoring). Many peptides were detected in the enriched extract samples when submitted to the MS-integrated strategy, whereas they were not detected in the initial extract samples. The combination of the IMAC-modified protocol with selective "precursor ion" and constant "neutral loss" triple quadrupole scan modes confers a high sensitivity on the analysis, allowing rapid phosphopeptide identification and characterization, even at low concentrations. To the best of our knowledge this work represents the first report exclusively focused on the detection of free phosphorylated peptides in biological fluids.     

The Cubic-Two-State Equation of State: Cross-associating mixtures and Monte Carlo study of self-associating prototypes

A new equation of state for associating fluids has recently been presented by Medeiros and Tellez-Arredondo, the Cubic-Two-State Equation of State (CTS EoS) [Ind. Eng. Chem. Res. 47 (2008) 5723]. This equation arises from the coupling of the Soave–Redlich–Kwong EoS (SRK) with an association term from a two-state association model. The CTS EoS is polynomial in volume and it is able to describe vapor pressures and molar volume of associating fluids such as water, alcohol and phenol, among others. The equation is also able to describe the liquid–vapor equilibria of their mixtures with alkanes. In this paper, the physical and thermodynamic foundations of the CTS EoS are further investigated. In order to verify its applicability for cross-associating systems, the equation was employed in the prediction of phase equilibria behavior of binary alcohol–alcohol and water–alcohol mixtures. Very good agreement between predictions and experimental phase equilibria data was obtained with very simple combining rules and only one adjustable binary parameter. No additional parameters were necessary to describe ternary systems. With the purpose of checking the model's hypothesis and limitations, the two-state association term was coupled with the hard sphere Carnahan–Starling EoS, forming the CS-TS equation and the association characteristic parameters were determined theoretically for prototype association fluids. Monte Carlo NPT simulations of such fluids were performed and the results were compared with the equation's predictions. The CS-TS was able to describe qualitatively the pvT$pvT$ behavior of the prototype; nevertheless, it is not as accurate as those predictions obtained from the combination CS with Wertheim's association term. It seems that, when adjusting parameters of the CTS EoS to real substances, the discrepancies between the predicted and the real association contribution are dissipated among other adjustable parameters, specially on the dispersive term of the SRK equation. Finally, it is shown that CTS EoS isotherms can only have one or three real bigger roots than the co-volume for positive pressures, similar to cubic equations of state, and then it has the desirable form to describe vapor–liquid phase equilibria of associating compounds mixtures.     

Electron transfer dissociation versus collisionally activated dissociation of cationized biodegradable polyesters

Biodegradable polyesters were ionized by electrospray ionization and characterized by tandem mass spectrometry using collisionally activated dissociation (CAD) and electron transfer dissociation (ETD) as activation methods. The compounds studied include one homopolymer, polylactide and two copolymers, poly(ethylene adipate) and poly(butylene adipate). CAD of [M+2Na]²⁺ ions from these polyesters proceeds via charge‐remote 1,5‐H rearrangements over the ester groups, leading to cleavages at the (CO)O–alkyl bonds. ETD of the same precursor ions creates a radical anion at the site of electron attachment, which fragments by radical‐induced cleavage of the (CO)O–alkyl bonds and by intramolecular nucleophilic substitution at the (CO)–O bonds. In contrast to CAD, ETD produces fragments in one charge state only and does not cause consecutive fragmentations, which simplifies spectral interpretation and permits conclusive identification of the correct end groups. The radical‐site reactions occurring during ETD are very similar with those reported for ETD of protonated peptides. Unlike multiply protonated species, multiply sodiated precursors form ion pairs (salt bridges) after electron transfer, thereby promoting dissociations via nucleophilic displacement in addition to the radical‐site dissociations typical in ETD. Copyright © 2012 John Wiley & Sons, Ltd.