Molecular dynamic study of the anomalous behavior of heat capacity in a methanol-water mixture

The molecular dynamic (MD) method is used to study the anomalous behavior of heat capacity in the range of small concentrations of methanol-water solutions. The behavior of the concentration dependence of heat capacity as calculated by the MD method qualitatively coincides with the experimental values. The calculation of contributions from different types of interaction to heat capacity showed that the greatest contribution is made by the interaction between the methanol molecules. The reason for the anomalous behavior of heat capacity is discussed based on the calculation of the mean force potential, radial distribution functions, and hydrogen bond network parameters. Translated fromZhurnal Strukturnoi Khimii, Vol.40, No. 2, pp. 304–313, March–April, 1999.     

Study on nonlinear phase‐separation for PMMA/α‐MSAN blends by dynamic rheological and small angle light scattering measurements

The phase‐separation behavior of poly(methyl methacrylate)/poly(α‐methyl styrene‐co‐acrylonitrile) (PMMA/α‐MSAN) blends upon heating was studied through dynamic rheological measurements and time‐resolved small angle light scattering, as a function of temperatures and heating rates. The spinodal temperatures could be obtained by an examination of the anomalous critical viscoelastic properties in the vicinity of phase‐separation induced by the enhanced concentration fluctuation on the basis of the mean field theory. It is found that the dependence of the critical temperatures determined by dynamic rheological measurements and small angle light scattering on heating rates both deviates obviously from the linearity, even at the very low heating rates. Furthermore, the cloud‐point curves decrease gradually with the decrease of heating rates and present the trend of approaching T_gs of the blends. The nonlinear dependence is in consistence with that extracted from the isothermal phase‐separation behavior as reported in our previous paper. It is suggested that the equilibrium phase‐separation temperature could be hardly established by the linear extrapolating to zero in the plotting of cloud points versus heating rates. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1547–1555, 2006     

Enhancement of Structural fluctuation in the region connecting two kinds of critical points in temperature-pressure-composition three-dimensional phase diagram: Raman studies of benzene/CO2 binary systems up to supercritical region

Pressure dependence of Raman spectra of benzene/CO2 two-component systems was systematically studied at different temperatures and compositions. We estimated the magnitude of inhomogeneous component in Raman bandwidth to get information on the structural fluctuation in the system. It was found that the inhomogeneous bandwidth attains a maximum on an isothermal plane in the temperature-pressure-composition three-dimensional phase diagram when the state point crosses the line connecting the region where the density fluctuation is large (the vicinity of the critical point of neat CO2) and the region where the concentration fluctuation in a binary system is enhanced (the vicinity of the critical solution point). By accumulating such data, we found that the points of large structural fluctuation comprise a sheet that includes the extension line of the gas-liquid equilibrium line in the phase diagram of neat CO2 and the line connecting critical solution points of the two-component system at different temperatures. Interaction between benzene and CO2 molecules in the supercritical region is briefly discussed.     

Thermodynamic properties of an alternating copolymer of bicyclo[2.2.1]hepta-2,5-diene and carbon monoxide in a region from <Emphasis Type="Italic">T</Emphasis> → 0 to 550 K

The temperature dependence of the heat capacity of an alternating copolymer of bicyclo[2.2.1]hepta-2,5-diene and carbon monoxide in the temperature range 6–550 K (with an error of 0.2–0.5% at 6–350 K and 0.5–1.5% at 330–550 K) was studied by the adiabatic vacuum and dynamic calorimetry. Physical transformations of the copolymer in the studied temperature region were identified, and their thermodynamic characteristics were determined. The combustion energy of the copolymer at 298.15 K was measured in a calorimeter with a static bomb and isothermal jacket. The thermodynamic functions for a region of 0–550 K, enthalpy of combustion, and thermodynamic parameters of copolymer formation from simple substances at T = 298.15 K and p = 101.325 kPa were calculated from the obtained experimental data. The new results and earlier published data were used for the calculation of the thermodynamic characteristics of the alternate copolymerization of bicyclo[2.2.1]hepta-2,5-diene and CO under standard pressure for a region of 0–350 K for the bulk reaction. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1483–1487, June, 2005.     

Study of heat of micellization and phase separation for Pluronic aqueous solutions by using a high sensitivity differential scanning calorimetry

Heat of micellization and phase separation temperature (known as cloud point) for the poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (abbreviated by PEO–PPO–PEO) triblock copolymers, the Pluronics F108, F98, F88, F68, F38, P65, and L62, in water are carefully determined by using a high sensitivity differential scanning calorimeter. It is interesting to find out that there exists a maximum heat of micellization for all these Pluronics. In this study, the heat of micellization of all of the Pluronics decreases as the temperature increases, as expected, at high temperature region (low Pluronic concentration region). However, the enthalpy change has a surprisingly positive relationship with temperature at low temperature region (high Pluronic concentration region). The critical micelle temperature consistently decreases as the Pluronic concentration increases. This unexpected behavior of the positive heat capacity changes of Pluronic aqueous solutions at higher concentration region is somewhat related to the variation of water accessible polar (PEO groups) and non-polar (PPO groups) surface areas in the micellization process. Especially, the removal of polar surface area from water may dominate the contribution to the positive heat capacity change upon micellization. In addition, the cloud points of Pluronic solutions are also discussed. The enthalpy–entropy compensation phenomenon for the micellization of Pluronics is discussed, and the enthalpy–entropy compensation temperature is calculated.     

Van der Waals supercritical fluid: exact formulas for special lines

In the framework of the van der Waals model, analytical expressions for the locus of extrema (ridges) for heat capacity, thermal expansion coefficient, compressibility, density fluctuation, and sound velocity in the supercritical region have been obtained. It was found that the ridges for different thermodynamic values virtually merge into single Widom line only at T < 1.07T(c), P < 1.25P(c) and become smeared at T < 2T(c), P < 5P(c), where T(c) and P(c) are the critical temperature and pressure. The behavior of the Batschinski lines and the pseudo-Gruneisen parameter γ of a van der Waals fluid were analyzed. In the critical point, the van der Waals fluid has γ = 8/3, corresponding to a soft sphere particle system with exponent n = 14.     

Rheological properties of carboxymethyl cellulose (CMC) solutions

In this study, we investigated the way of predicting two critical concentrations of sodium carboxymethyl cellulose (CMC) solutions using simple experimental procedures with a rotational rheometer. It was found that, above a critical shear rate, all CMC solutions (0.2 to 7 wt.%) exhibit shear-thinning behavior and the flow curves could be described by the Cross model. A first critical CMC concentration c*, transition to semidilute network solution, was determined using the following methods (1) study of the flow curve shapes, (2) Cross model parameters, (3) plot of the specific viscosity vs the overlap parameter, and (4) empirical structure–properties relationships. Furthermore, both creep and frequency-sweep measurements showed that the solutions behaved as viscoelastic materials above a second critical CMC concentration c** (transition to concentrated solution). The characterization of CMC solutions was completed with a time-dependent viscosity study that showed that the CMC solutions exhibited strong thixotropic behavior, especially at the highest CMC concentrations.     

Equilibrium adsorption of binary mixtures of gases by zeolites and the state of the adsorption phase

A method for analyzing the state of the adsorption phase was developed on the basis of statistical thermodynamics for the case of equilibrium adsorption of binary gaseous mixtures. The procedure for treating experimental data to determine the Helmholtz energy and other thermodynamic functions of a mixture of molecules occluded within zeolite cavities was proposed. A measure of ideal behavior of a mixture of a small number of molecules in the micropore was formulated; in the asymptotic limit such a behavior leads to the Raoult law and to assumption of the validity of the Raoult law when moving along the line of constant value of the Gibbs integral in the ideal adsorption solution theory. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1927–1932, October, 1998.     

Soft X-ray radiation-induced electron emission from the surface of polarized ferroelectrics

Additional electron emission that reflects specific properties of ferroelectrics was detected after the negatively charged surface of these preliminarily polarized materials was exposed to soft X-radiation (hv) ≤3 kV). The properties of this emission, which is referred to as anomalous electron emission (AEE) and is excited in single-crystal and ceramic samples, are studied with a standard X-ray photoelectron spectrometer. It is shown that the parameters of AEE spectra correlate with the characteristics of the materials. The conclusions of the phenomenological theory of the phenomenon are in satisfactory agreement with the experimental results. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1031–1036, November–December, 1998     

Microphase segregation in bridging polymeric brushes: Regular and singular phase diagrams

A mean-field theory of deformation-induced microphase segregation in bridging polymeric brushes anchored to two parallel surfaces is presented. Models with isotropic and orientation-dependent liquid-crystalline interactions between segments are considered. For the first model, the problem is similar to that of classical liquid-vapor phase separation, and the phase diagram in the P-T plane has a line of first-order transitions terminating at the critical point. We show that the critical pressure is negative implying that a free brush tethered only to one surface always exists at supercritical conditions and hence cannot undergo the collapse phase transition. In the second model, the free energy density depends on two coupled order parameters, one related to segment density and the other to the orientational order, which strongly modifies the phase behavior. Depending on the grafting density the system is described by a phase diagram of a regular or a singular type. In the regular phase diagram the first-order transition line terminates at the critical point. In a singular diagram, the first-order transition line extends to infinity; the critical point corresponds to infinite pressure so that the system undergoes the phase transition at arbitrary external pressures. Regular phase diagrams correspond to dense grafting, and singular ones to sparse grafting. The change from a regular phase behavior to another occurs at a certain marginal value of the grafting density. On approaching this value the critical point on the regular diagram moves to infinity, logarithmically with the deviation from the critical grafting density. We relate the analytical properties of the free energy density as a function of the segment concentration to the type of the phase diagram and the shape of the coexistence curve in the temperature- concentration plane.     

Equilibrium of two liquid phases and critical phenomena in a ternary system of cyclohexane-pyridine-acetic acid in the 10–55°C range

A visual polythermal method is used to study the mutual solubility of components and critical phenomena in a ternary system of cyclohexane-pyridine-acetic acid in the 10.0–55.0°C range. It is noted that the isothermal solubility diagrams of the system in the 10.0–52.5°C range is characterized by the occurrence of a closed separation region. A temperature dependence of the mixture composition, which corresponds to the critical point of solubility, is determined. It is found that with the increase in the temperature a two-liquid phase region disappears through a non-critical point.     

Widom line and the liquid-liquid critical point for the TIP4P/2005 water model

The Widom line and the liquid-liquid critical point of water in the deeply supercooled region are investigated via computer simulation of the TIP4P/2005 model. The Widom line has been calculated as the locus of compressibility maxima. It is quite close to the experimental homogeneous nucleation line and, in the region studied, it is almost parallel to the curve of temperatures of maximum density at fixed pressure. The critical temperature is determined by examining which isotherm has a region with flat slope. An interpolation in the Widom line gives the rest of the critical parameters. The computed critical parameters are T(c)=193 K, p(c)=1350 bar, and ρ(c)=1.012 g/cm(3). Given the performance of the model for the anomalous properties of water and for the properties of ice phases, the calculated critical parameters are probably close to those of real water.     

Isothermal compressibility and correlation radius near the critical point: Numerical analysis of the Ornstein-Zernike equation

Solution of the Ornstein-Zernike equation is analyzed numerically in the Percus-Yevick and hyperchain approximations for a system of Lennard-Jones particles in a critical region. The temperature dependences of correlation functions, isothermal compressibility η, and correlation radius of density fluctuations ζ are investigated at a critical density; the corresponding critical indices are determined. It is shown that the Percus-Yevick approximation yields satisfactory results when the correlation functions are calculated within a range corresponding to approximately 50 atomic (molecular) diameters. In this case, with ≈5% deviations from the critical temperature, the calculated and experimental values of η and critical indices are in good agreement. Tver State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 5, pp. 799–807, September–October, 1995. Translated by I. Izvekova     

Thermodynamics of alternating copolymer of propylene and CO in the 0–550 K region

The temperature dependences of the heat capacity and the temperatures and enthalpies of physical transitions of the alternating copolymer of propylene and CO were studied in the 5–550 K region by adiabatic vacuum and dynamic calorimetry techniques. The heat of combustion of the copolymer was measured at 298.15 K in a calorimeter with a static bomb and an isothermal shield. The thermodynamic parameters of glass transition and fusing were estimated. The thermodynamic functions in the 0–450 K region and the thermodynamic characteristics of the formation of the copolymer from simple substances atT=298.15 K andp=101.325 kPa were calculated. The thermodynamic parameters of the alternating copolymerization of bulk propylene and CO were calculated in the 0–450 K region at standard pressure.     

Simulation of first- and second-order transitions in asymmetric polymer mixtures

The critical properties of dense asymmetric binary polymer mixtures are studied by grand canonical simulations within the framework of the 3-dimensional bond fluctuation lattice model. The monomers interact with each other via a potential ranging over the entire first peak of the pair distribution. An asymmetry is realized by giving the ratio of interactions λ = ∈_AA/∈_BB between monomers of the A-species and of the B-species a value different from 1. Using multiple histogram extrapolation techniques for the data analysis, the two phase region, which is a line of first-order transitions driven by the chemical potential difference, and the critical point are determined for a mixture of chains with 32 monomers each. At a critical potential difference Δμ_c unmixing occurs below a critical temperature T_c. It is found that Δμ_c is proportional to the asymmetry (1 - λ) and that the quantity 4k_BT_c/(3 + λ)∈ is independent of the asymmetry, consistent with the prediction of the Flory theory.     

An improved renormalization group theory for real fluids

On the basis of White's theory, an improved renormalization group (RG) theory is developed for chain bonding fluids inside the critical region. Outside the critical region, the statistical associating fluid theory based on the first-order mean sphere approximation [Fluid Phase Equilibria 171, 27 (2000)] is adopted and all the microscopic parameters are taken directly from its earlier application of real fluids. Inside the critical region, the RG transformation for long-range density fluctuation is derived in the k space, which illustrates explicitly the contributions from the mean-field term, the local density fluctuation, and the nonlocal density fluctuation. The RG theory is applied to describe physical behavior of ten n alkanes (C1-C10) both near to and far from the critical point. With no additional parameters for chain bonding fluids, good results are obtained for critical specific heat and phase coexistence curves and the resulting critical exponents are in good agreement with the reported nonclassic values.     

Hard-sphere perturbation theory for a model of liquid Ga

Investigating thermodynamic properties of a model for liquid Ga, we have extended the application of the hard-sphere (HS) perturbation theory to an interatomic pair potential that possesses a soft repulsive core and a long-range oscillatory part. The model is interesting for displaying a discontinuous jump on the main-peak position of the radial distribution function at some critical density. At densities less than this critical value, the effective HS diameter of the model, estimated by the variational HS perturbation theory, has a substantial reduction with increasing density. Thus, the density dependence of the packing fraction of the HS reference fluid has an anomalous behavior, with a negative slope, within a density region below the critical density. By adding a correction term originally proposed by Mon to remedy the inherent deficiency of the HS perturbation theory, the extended Mansoori-Canfield/Rasaiah-Stell theory [J. Chem. Phys. 120, 4844 (2004)] very accurately predicts the Helmholtz free energy and entropy of the model, including an excess entropy anomaly. Almost occurring in the same density region, the excess entropy anomaly is found to be associated with the anomalous packing faction of the HS fluid.     

A theoretical charge density study on nitrogen-rich 4,4′,5,5′-tetranitro-2,2′-bi-1H-imidazole (TNBI) energetic molecule

The bond topological and electrostatic properties of nitrogen-rich 4,4′,5,5′-tetranitro-2,2′-bi-1H-imidazole (TNBI) energetic molecule have been calculated from the DFT method with the basis set 6-311G** and the AIM theory. The optimized geometry of this molecule is almost matched with the experimental geometric parameters. The electron density at the bond critical point and the Laplacian of electron density of C–NO₂ bonds are not equal, one of them is much weaker than the other. Similar trend exists in the C–N bonds of the imidazole ring of the molecule. The ratio of the bond dissociation energy (BDE) of the weakest bond to the molecular total energy exhibits nearly a linear correlation with the impact sensitivity; its h ₅₀% value is ~32.01 cm. The electrostatic potential around both the nitro groups are found unequal; the NO₂ group of weakest C–NO₂ bond exhibits an extended electronegative region.     

Concepts against mathematics: self-inconsistency in thermodynamic evaluations

Various ways of thermodynamic evaluations can yield different results, contradicting to one another. Such a case is considered a paradox, with the attempts to solve it. This is because thermodynamics is thought to be the science established on solid conceptual ground, with accurate mathematical evaluations. Recently, we faced similar problem, when the relationship between heat capacity and pressure is described by two different equations, predicting opposite behavior. Both equations are free of evident errors. In searching for the reason of the discrepancy, we found out that it is common practice in thermodynamic evaluations to tune the mathematical operations in order to receive the necessary result. It is typical of empirical science, but inappropriate for the fundamental knowledge based on axiomatic background. Short historical survey on experimental data and theoretical concepts dealing with the relation between heat capacity and pressure proves that the thermodynamics is very flexible and effective tool for the solution of the problems in the field of relationships among P–V–T parameters and thermophysical properties of matter. One should not consider the solutions as the universal laws.     

Molecular theory of phase equilibria in model and real associated mixtures III. Binary solutions of inert gases and n-alkanes in ammonia and methanol

Using new molecular models of ammonia and methanol and thermodynamic perturbation theory, the global phase diagrams of model mixtures of these compounds with a van der Waals fluid, representing a simple nonpolar fluid, have been calculated. The global phase diagram of these mixtures is much richer than that of corresponding aqueous mixtures. More types of critical line behavior are found, including the presence of van Laar points and a small region where the mixtures exhibit a closed liquid-liquid immiscibility loop (Type VI phase behavior). The individual mixture components are characterized by two molecular parameters, which can be adjusted to their critical temperature and critical volume; the mixture model itself contains no adjustable parameters. It is shown that the theory gives qualitatively correct predietions of mixtures with n-alkanes. This includes the prediction of Type III critical line behavior for small and large values of the ratio of the critical temperatures of the components, and Type II over a large range of conditions, including the presence or absence of absolute or limited azeotropy, and temperature and pressure extrema of critical lines and their dependence on the number of carbon atoms.