Excluded Volume Model Research Articles

Molecular Mechanism for the Hofmeister Effect Derived from NMR and DSC Measurements on Barnase.

The effects of sodium thiocyanate, sodium chloride, and sodium sulfate on the ribonuclease barnase were studied using differential scanning calorimetry (DSC) and NMR. Both measurements reveal specific and saturable binding at low anion concentrations (up to 250 mM), which produces localized conformational and energetic effects that are unrelated to the Hofmeister series. The binding of sulfate slows intramolecular motions, as revealed by peak broadening in 13C heteronuclear single quantum coherence spectroscopy. None of the anions shows significant binding to hydrophobic groups. Above 250 mM, the DSC results are consistent with the expected Hofmeister effects in that the chaotropic anion thiocyanate destabilizes barnase. In this higher concentration range, the anions have approximately linear effects on protein NMR chemical shifts, with no evidence for direct interaction of the anions with the protein surface. We conclude that the effects of the anions on barnase are mediated by solvent interactions. The results are not consistent with the predictions of the preferential interaction, preferential hydration, and excluded volume models commonly used to describe Hofmeister effects. Instead, they suggest that the Hofmeister anion effects on both stability and solubility of barnase are due to the way in which the protein interacts with water molecules, and in particular with water dipoles, which are more ordered around sulfate anions and less ordered around thiocyanate anions.

A promising approach to low electrical percolation threshold in PMMA nanocomposites by using MWCNT-PEO predispersions

Electrical conductive poly(methyl methacrylate) (PMMA) nanocomposites with low percolation threshold are very challenging to be prepared. Here, we show that the miscibility between poly(ethylene oxide) (PEO) as matrix for predispersions of multi-walled carbon nanotubes (MWCNTs) and PMMA represents an efficient approach to achieve very low electrical percolation threshold. PMMA/PEO-MWCNTs nanocomposites were prepared by a two-step solution casting method involving pre-mixing of MWCNTs with PEO and then mixing of PEO-MWCNTs with PMMA, resulting in a PMMA/PEO ratio of 80/20wt%. The electrical percolation threshold (EPT) value was determined to be ~0.07wt% which is significantly lower than most of the reported EPT values in the literature for PMMA/CNT composites. The very low electrical percolation threshold was attributed to the effectual role of PEO in self-assembly of secondary structures of nanotubes into an electrically conductive network. This was further confirmed by transmission electron microscopy and by comparing the obtained EPT value with the prediction of the excluded volume model in which statistical percolation threshold is defined based on uniform distribution of high-aspect ratio sticks in a matrix. Moreover, based on UV–Vis measurements and linear viscoelastic rheological measurements, optical and rheological percolation thresholds were obtained at nearly 0.01wt% and 0.5wt%, respectively.

Separation of 1,3-dioxolane, 1,4-dioxane, acetonitrile and tert-butanol from their aqueous solutions by using Good's buffer HEPES-Na as an auxiliary agent

The biological buffer 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid sodium (HEPES-Na) was found that can induce liquid–liquid phase splitting in the aqueous solutions of 1,3-dioxolane, 1,4-dioxane, acetonitrile, and tert-butanol at ambient conditions. To prepare the phase diagrams, we measured the solid–liquid equilibrium (SLE), solid–liquid–liquid equilibrium (SLLE), and liquid–liquid equilibrium (LLE) data for the ternary systems containing water+1,3-dioxolane, 1,4-dioxane, acetonitrile, or tert-butanol+HEPES-Na at 298.2K under atmospheric pressure. The effective excluded volume model (EEV) was used to fit the experimental LLE phase boundary data. The consistency of the experimental LLE tie-line data was confirmed by Othmer–Tobias equation. These tie-line data were also correlated with the NRTL model. By using the HEPES-Na as an auxiliary agent, the maximum concentrations of 1,3-dioxolane (97.7wt%), 1,4-dioxane (97.4wt%), acetonitrile (93.9wt%), and tert-butanol (92.7wt%) in the organic-rich phase are greater than the azeotropic compositions of the corresponding aqueous systems. These results indicate that HEPES-Na can be served as a high efficiency, non-corrosive, and biocompatible green agent for the separation of 1,3-dioxolane, 1,4-dioxane, acetonitrile, and tert-butanol from their aqueous solutions.

The rheological injectability of N-succinyl-chitosan solutions

The viscosity of a set of N-succinyl-chitosan (NSC) solutions was characterized near the 0.2Pas rheological injectability limit. This is believed to be the first such report in the open literature. Viscosity was characterized at physiological pH and ionic strength as a function of NSC degree of substitution, NSC concentration, temperature, and shear rate. NSC was synthesized via Yamaguci's method and characterized using H-NMR, membrane osmometry, TGA and isothermal vacuum drying. NSC synthesis results were shown to fit a reproducible log-linear correlation and both optimum drying temperature and thermal decomposition temperature were found to be a function of NSC degree of substitution. Viscosity results were explained using Katchalsky's full model for polyampholyte ionization combined with a charge induced excluded volume model proposed by Higgs. The model predicted a polyelectrolyte/polyampholyte transition which agreed well with experimental data. For minimally injectable formulations a maximum in primary amine concentration is expected at 32sub% amine NSC.

Is the manifestation of the local dynamics in the spin-lattice NMR relaxation in dendrimers sensitive to excluded volume interactions?

The effect of excluded volume (EV) interactions on the manifestation of the local dynamics in the spin-lattice NMR relaxation in dendrimers has been studied by using Brownian dynamics simulations. The study was motivated by the theory developed by Markelov et al., [J. Chem. Phys., 2014, 140, 244904] for a Gaussian dendrimer model without EV interactions. The theory connects the experimentally observed dependence of the spin-lattice relaxation rate 1/T(1)H on the location of NMR active groups with the restricted flexibility (semiflexibility) of dendrimers. Semiflexibility was introduced through the correlations between the orientations of different segments. However, these correlations exist even in flexible dendrimer models with EV interactions. We have simulated coarse-grained flexible and semiflexible dendrimer models with and without EV interactions. Every dendrimer segment consisted of two rigid bonds. Semiflexibility was introduced through a potential which restricts the fluctuations of angles between neighboring bonds but does not change orientational correlations in the EV model as compared to the flexible case. The frequency dependence of the reduced 1/T(1)H(ωH) for segments and bonds belonging to different dendrimer shells was calculated. It was shown that the main effect of EV interactions consists of a much stronger contribution of the overall dendrimer rotation to the dynamics of dendrimer segments as compared to phantom models. After the exclusion of this contribution the manifestation of internal dynamics in spin-lattice NMR relaxation appears to be practically insensitive to EV interactions. For the flexible models, the position ωmax of the peak of the modified 1/T(1)H(ωH) does not depend on the shell number. For semiflexible models, the maximum of 1/T(1)H(ωH) for internal segments or bonds shifts to lower frequencies as compared to outer ones. The dependence of ωmax on the number of dendrimer shells appears to be universal for segments and bonds in dendrimer models with and without EV interactions.

Mean-field approach in the multi-component gas of interacting particles applied to relativistic heavy-ion collisions

A generalized mean-field approach for the thermodynamic description of relativistic single- and multi-component gas in the grand canonical ensemble is formulated. In the framework of the proposed approach, different phenomenological excluded-volume procedures are presented and compared to the existing ones. The mean-field approach is then used to effectively include hard-core repulsion in hadron-resonance gas model for description of chemical freeze-out in heavy-ion collisions. We calculate the collision energy dependence of several quantities for different values of hard-core hadron radius and for different excluded-volume procedures such as the van der Waals and Carnahan–Starling models. It is shown that a choice of the excluded-volume model becomes important for large particle densities. For large enough values of hadron radii ( fm) there can be a sizable difference between different excluded-volume procedures used to describe the chemical freeze-out in heavy-ion collisions. At the same time, for the smaller and more commonly used values of hard-core hadron radii ( fm), the precision of the van der Waals excluded-volume procedure is shown to be sufficient.

The effect of temperature and molar mass on the (liquid + liquid) equilibria of (poly ethylene glycol dimethyl ether + di-sodium hydrogen citrate + water) systems: Experimental and correlation

The (liquid+liquid) equilibrium for the {polyethylene glycol dimethyl ether 2000 (PEGDME2000)+di-sodium hydrogen citrate+H2O} system was studied at T=(298.15, 308.15 and 318.15)K and atmospheric pressure (≈85kPa). The free energies, enthalpies and entropies of cloud points were calculated in order to investigate the driving force formation of this two-phase system. To investigate the effect of molar mass of the polymer on the binodals and tie-lines, similar measurements were also made at T=298.15K on this two-phase system consisting of the PEGDME with molar masses of 500 and 250g⋅mol−1. The effective excluded volume model was used for representation of the phase-forming ability in PEGDME systems. An empirical and the Merchuck equations with the temperature dependency were used to correlate the binodal curves. The Othmer–Tobias and Bancraft and Setschenow equations, the osmotic virial and the extended NRTL models were used to fit the tie-line data.

Thermal Stabilization of Proteins by Mono- and Oligosaccharides: Measurement and Analysis in the Context of an Excluded Volume Model.

The reversible thermal denaturation of apo α-lactalbumin and lysozyme was monitored via measurement of changes in absorbance and ellipticity in the presence of varying concentrations of seven mono- and oligosaccharides: glucose, galactose, fructose, sucrose, trehalose, raffinose, and stachyose. The temperature dependence of the unfolding curves was quantitatively accounted for by a two-state model, according to which the free energy of unfolding is increased by an amount that is independent of temperature and depends linearly upon the concentration of added saccharide. The increment of added unfolding free energy per mole of added saccharide was found to depend approximately linearly upon the extent of oligomerization of the saccharide. The relative strength of stabilization of different saccharide oligomers could be accounted for by a simplified statistical-thermodynamic model attributing the stabilization effect to volume exclusion deriving from steric repulsion between protein and saccharide molecules.

Equation of state and sound velocity of a hadronic gas with a hard-core interaction

Thermodynamic properties of hot and dense hadronic systems with a hard-sphere interaction are calculated in the Boltzmann approximation. Two parametrizations of pressure as a function of density are considered: the first one, used in the excluded-volume model and the second one, suggested earlier by Carnahan and Starling. The results are given for one-component systems containing only nucleons or pions, as well as for chemically equilibrated mixtures of different hadronic species. It is shown that the Carnahan--Starling approach can be used in a much broader range of hadronic densities as compared to the excluded-volume model. In this case the superluminal sound velocities appear only at very high densities, in the region where the deconfinement effects should be already important.

Hadron resonance gas equation of state from lattice QCD

The Monte Carlo results in lattice QCD for the pressure and energy density at small temperature $T<155$ MeV and zero baryonic chemical potential are analyzed within the hadron resonance gas model. Two extensions of the ideal hadron resonance gas are considered: the excluded volume model, which describes a repulsion of hadrons at short distances, and the Hagedorn model with an exponential mass spectrum. Considering both of these models we do not find conclusive evidence in favor of either of them. The controversial results appear because of rather different sensitivities of the pressure and energy density to both excluded volume and Hagedorn mass spectrum effects. On the other hand, we have found clear evidence for a simultaneous presence of both of them. They lead to rather essential contributions: suppression effects for thermodynamical functions of the hadron resonance gas due to the excluded volume effects and enhancement due to the Hagedorn mass spectrum.

Matching excluded-volume hadron-resonance gas models and perturbative QCD to lattice calculations

We match three hadronic equations of state at low energy densities to a perturbatively computed equation of state of quarks and gluons at high energy densities. One of them includes all known hadrons treated as point particles, which approximates attractive interactions among hadrons. The other two include, in addition, repulsive interactions in the form of excluded volumes occupied by the hadrons. A switching function is employed to make the crossover transition from one phase to another without introducing a thermodynamic phase transition. A chi-square fit to accurate lattice calculations with temperature $100 < T < 1000$ MeV determines the parameters. These parameters quantify the behavior of the QCD running gauge coupling and the hard core radius of protons and neutrons, which turns out to be $0.62 \pm 0.04$ fm. The most physically reasonable models include the excluded volume effect. Not only do they include the effects of attractive and repulsive interactions among hadrons, but they also achieve better agreement with lattice QCD calculations of the equation of state. The equations of state constructed in this paper do not result in a phase transition, at least not for the temperatures and baryon chemical potentials investigated. It remains to be seen how well these equations of state will represent experimental data on high energy heavy ion collisions when implemented in hydrodynamic simulations.

The effect of time-dependent macromolecular crowding on the kinetics of protein aggregation: a simple model for the onset of age-related neurodegenerative disease

A linear increase in the concentration of "inert" macromolecules with time is incorporated into simple excluded volume models for protein condensation or fibrillation. Such models predict a long latent period during which no significant amount of protein aggregates, followed by a steep increase in the total amount of aggregate. The elapsed time at which these models predict half-conversion of model protein to aggregate varies by less than a factor of two when the intrinsic rate constant for condensation or fibril growth of the protein is varied over many orders of magnitude. It is suggested that this concept can explain why the symptoms of neurodegenerative diseases associated with the aggregation of very different proteins and peptides appear at approximately the same advanced age in humans.

Production of Strange, Non-strange particles and Hypernuclei in an Excluded-Volume Model

We present a systematic study of production of strange and non-strange hadron yields and their ratios obtained in various experiments using our thermodynamically consistent excluded-volume model. We also analyze the production of light nuclei, hypernuclei and their antinuclei in terms of our excluded-volume model over a broad energy range starting from Alternating Gradient Synchrotron (AGS) to Large Hadron Collider (LHC) energies. Further, we extend our model for studying rapidity spectra of hadrons produced in heavy-ion collisions.

Excluded Volume Model for the Reduction of Polymer Diffusion into Nanocomposites

An analytic model for the slowing down of polymer chain diffusion in nanocomposites attributable to excluded volume effects is presented. The nanocomposite is modeled as an ensemble of cylinders through which the polymer chains diffuse. The reduction of polymer diffusion in each cylinder is equated with the reduction of diffusion for a sphere through a cylinder. The distribution of cylinder diameters within the ensemble is determined from statistical mechanical theories based on the packing of spherical particles. For low loadings of spherical particles in nanocomposites, this model results in a master curve for the reduced diffusion coefficient. With no adjustable parameters, the model agrees with recent data for tracer diffusion measurements in polymer nanocomposites at low loading.

STRONGLY INTERACTING MATTER AT FINITE CHEMICAL POTENTIAL: HYBRID MODEL APPROACH

Search for a proper and realistic equation of state (EOS) for strongly interacting matter used in the study of the QCD phase diagram still appears as a challenging problem. Recently, we constructed a hybrid model description for the quark–gluon plasma (QGP) as well as hadron gas (HG) phases where we used an excluded volume model for HG and a thermodynamically consistent quasiparticle model for the QGP phase. The hybrid model suitably describes the recent lattice results of various thermodynamical as well as transport properties of the QCD matter at zero baryon chemical potential (μB). In this paper, we extend our investigations further in obtaining the properties of QCD matter at finite value of μB and compare our results with the most recent results of lattice QCD calculation.

The effect of flow on hadronic spectra in an excluded-volume model

Recently, we proposed a thermodynamically consistent excluded-volume model for the HG fireball and we noticed that our model gives a suitable description of various properties of multiparticle production and their ratios in the entire range of temperatures and baryon densities. Our aim in this paper is to obtain the variations of freeze-out volume in a slice of unit rapidity, i.e. dV/dy, as well as total volume of the fireball with respect to center-of-mass energy , and to compare our model calculations with the corresponding thermal freeze-out volume obtained from the Hanbury–Brown–Twiss pion interferometry method. We also test the validity of our model in extracting the total multiplicities as well as the central rapidity densities of various hadrons and comparing them with the recent results. We further calculate the rapidity and transverse momentum spectra of various particles produced in different heavy-ion collider experiments in order to examine the role of flow by matching our predictions with the available experimental results. Finally, we extend our analysis for the production of light nuclei, hypernuclei and their antinuclei over a broad energy range from alternating gradient synchrotron to large hadron collider energies.

Particle Production in Ultrarelativistic Heavy-Ion Collisions: A Statistical-Thermal Model Review

The current status of various thermal and statistical descriptions of particle production in the ultrarelativistic heavy-ion collisions experiments is presented in detail. We discuss the formulation of various types of thermal models of a hot and dense hadron gas (HG) and the methods incorporated in the implementing of the interactions between hadrons. It includes our new excluded-volume model which is thermodynamically consistent. The results of the above models together with the experimental results for various ratios of the produced hadrons are compared. We derive some new universal conditions emerging at the chemical freeze-out of HG fireball showing independence with respect to the energy as well as the structure of the nuclei used in the collision. Further, we calculate various transport properties of HG such as the ratio of shear viscosity-to-entropy using our thermal model and compare with the results of other models. We also show the rapidity as well as transverse mass spectra of various hadrons in the thermal HG model in order to outline the presence of flow in the fluid formed in the collision. The purpose of this review article is to organize and summarize the experimental data obtained in various experiments with heavy-ion collisions and then to examine and analyze them using thermal models so that a firm conclusion regarding the formation of quark-gluon plasma (QGP) can be obtained.

Examination of the thermodynamical consistency of excluded-volume hadron gas models

The new excluded volume hadron gas model by Singh et al. [1-7] is critically discussed. We demonstrate that in this model the results obtained from relations between thermodynamical quantities disagree with the corresponding results obtained by statistical ensemble averaging. Thus, the model does not satisfy the requirements of thermodynamical consistency.

Hydration properties of small hydrophobic molecules by Brillouin light scattering

We study the relaxation of water molecules next to hydrophobic solutes with different functional groups by Brillouin light scattering. Evidence is given for (i) water activation energy in trimethylamine-N-oxide, proline and t-butyl alcohol diluted solutions which is comparable to that of neat water, almost independent from solute mole fraction and (ii) moderate slowdown of relaxation time of proximal water compared to the bulk, which is consistent with excluded volume models. Assuming that the main contribution to viscosity comes from bulk and hydration water, a rationale is given of the phenomenological Arrhenius' laws for the viscosity of diluted aqueous solutions.

Hybrid model for QCD deconfining phase boundary

Intensive search for a proper and realistic equations of state (EOS) is still continued for studying the phase diagram existing between quark gluon plasma (QGP) and hadron gas (HG) phases. Lattice calculations provide such EOS for the strongly interacting matter at finite temperature ($T$) and vanishing baryon chemical potential (${\ensuremath{\mu}}_{B}$). These calculations are of limited use at finite ${\ensuremath{\mu}}_{B}$ due to the appearance of notorious sign problem. In the recent past, we had constructed a hybrid model description for the QGP as well as HG phases where we make use of a new excluded-volume model for HG and a thermodynamically-consistent quasiparticle model for the QGP phase and used them further to get QCD phase boundary and a critical point. Since then many lattice calculations have appeared showing various thermal and transport properties of QCD matter at finite $T$ and ${\ensuremath{\mu}}_{B}=0$. We test our hybrid model by reproducing the entire data for strongly interacting matter and predict our results at finite ${\ensuremath{\mu}}_{B}$ so that they can be tested in future. Finally we demonstrate the utility of the model in fixing the precise location, the order of the phase transition and the nature of $CP$ existing on the QCD phase diagram. We thus emphasize the suitability of the hybrid model as formulated here in providing a realistic EOS for the strongly interacting matter.