Excess Heat Capacity Research Articles

Acoustical study of molecular interactions in polymer solutions through various thermodynamical parameters and Flory's theory at 298.15 K

This study intends to portray the nature of polymer solutions by ultrasonic velocity and density data. Acoustical parameters, such as relaxation strength, relative association, molecular constant, fractional free volume, available volume, Beyer's non-linearity parameter, internal pressure, van der Waals’ constants and molecular radius have been computed for the binary mixtures of PEG400 + methanol, PEG400 + ethanol, PPG400 + ethanol and PPG400 + 2-propanol systems at 298.15 K. Flory's theory has been used for computing ultrasonic velocity, surface tension, thermal expansion coefficient, thermal conductivity, excess heat capacity and excess isothermal compressibility. Isothermal compressibilities of mixtures have been theoretically evaluated using different theories. A new relation has been proposed by us for evaluating excess molar volume using ultrasonic velocity of the mixture which gives fairly good result when compared with the experimental values. Excess molar volume has been calculated using Flory's theory and Prigogine–Flory–Patterson theory. The partial molar volume and its excess value at infinite dilution have also been evaluated.

Non-Debye excess heat capacity and boson peak of binary lithium borate glasses

The non-Debye excess heat capacities of binary lithium borate glasses with different Li 2O compositions of x = 8, 14 and 22 (mol%) are investigated to understand origin of the boson peak. The low-temperature heat capacities are measured between 2 and 50 K by a relaxation calorimeter. The experimental non-Debye heat capacities with x = 14 is successfully reproduced using the excess vibrational density of states measured by inelastic neutron scattering. This finding indicates that the non-Debye heat capacities of lithium borate glasses originate from the excess vibrational density of states measureable by inelastic neutron scattering. Moreover, it is demonstrated that all of the excess heat capacity spectra lie on a single master curve by the scaling using boson peak temperature and intensity.

Volumes, Heat Capacities, and Compressibilities of the Mixtures of Acetonitrile with N,N-Dimethylacetamide and Propylene Carbonate

The densities of the mixtures of acetonitrile and N,N-dimethylacetamide or propylene carbonate were measured within the whole composition range of the systems at temperatures (308.15 and 318.15) K, while the isobaric heat capacity and speed of sound in these mixtures were measured within the whole composition range at 298.15 K. The results obtained were used to calculate the excess molar volume and isobaric heat capacity and the partial molar volumes and heat capacities of both components of the mixtures as well as the adiabatic (κS) and isothermal (κT) compressibility and their excess. The internal pressures of the systems were also calculated. The results have been analyzed and discussed.

On the nature of the excess heat capacity of mixing

The excess vibrational entropy (ΔSvibex) of several silicate solid solutions are found to be linearly correlated with the differences in end-member volumes (ΔVi) and end-member bulk moduli (Δκi). If a substitution produces both, larger and elastically stiffer polyhedra, then the substituted ion will find itself in a strong enlarged structure. The frequency of its vibration is decreased because of the increase in bond lengths. Lowering of frequencies produces larger heat capacities, which give rise to positive excess vibrational entropies. If a substitution produces larger but elastically softer polyhedra, then increase and decrease of mean bond lengths may be similar in magnitude and their effect on the vibrational entropy tends to be compensated. The empirical relationship between ΔSvibex, ΔVi and Δκi, as described by ΔSvibex = (ΔVi + mΔκi)f, was calibrated on six silicate solid solutions (analbite–sanidine, pyrope–grossular, forsterite–fayalite, analbite–anorthite, anorthite–sanidine, CaTs–diopside) yielding m = 0.0246 and f = 2.926. It allows the prediction of ΔSvibex behaviour of a solid solution based on its volume and bulk moduli end-member data.

Thermodynamic and Acoustic Properties of Mixtures of 1,6-Dichlorohexane with Heptane from (293 to 313) K

For {xCl(CH2)6Cl + (1 − x) n-C7H16}, densities ρ were measured with a vibrating-tube densimeter at (293.15, 298.15, 303.15, 308.15, and 313.15) K, covering the whole composition range. From the densities the corresponding isobaric expansivities αp and the excess molar volumes VE were calculated. At the same temperatures, the molar heat capacities at constant pressure Cp were also measured over the whole composition range using a differential scanning calorimeter (DSC III from Setaram), and excess molar heat capacities at constant pressure CpE were calculated therefrom. In addition, ultrasound speeds u were determined with a pulse-echo-overlap apparatus. From these results, isentropic compressibilities κS, isothermal compressibilities κT, molar heat capacities at constant volume CV, and internal pressures Π were obtained for this system at all five temperatures. The VE are negative throughout; they are somewhat skewed with minima around x = 0.6, and they become less negative with decreasing temperature. Th...

The boson peak of silicate glasses: The role of Si–O, Al–O, and Si–N bonds

The role of Si-O, Al-O, and Si-N bonds on the boson peak of silicate glasses has been investigated from a study of amorphous Si, SiO(2), and two calcium aluminosilicates with 0 (Ca28-O) and 4.4 (Ca28-N) mol % Si(3)N(4). The low-frequency part of the vibrational density of states g(omega) has been calculated from inversion of literature data and new heat capacity measurements. As defined by g(omega)/omega(2), the boson peak correlates with the excess heat capacity observed with respect to Debye T(3) limiting law. That libration of SiO(4) tetrahedra represents the main source of low-frequency excitations in silica glass is illustrated by the strong difference between the anomalies of amorphous Si and SiO(2) glass and the marked decrease observed for SiO(2) phases of increasing density. When Al substitutes for Si, libration of AlO(4) tetrahedra appears hampered by the presence of a charge-compensating cation. Rigidification of the silicate network resulting from substitution of N for O causes the boson peak of Ca28-N to be smaller than that of Ca28-O and shifted toward higher frequencies as increased cross-linking hinders libration of SiO(4) or AlO(4) tetrahedra. In agreement with their universal phenomenology, the calorimetric boson anomalies of Ca28-O and Ca28-N plot on the master curve defined previously by SiO(2) and alkali silicate glasses.

Energetic domains analysis of bovine α-lactalbumin upon interaction with copper and dodecyl trimethylammonium bromide

Domain analysis of the dialyzed form of α-lactalbumin (m-α-LA) with varying concentrations of Cu +2 and DTAB has been carried out by differential scanning calorimetry (DSC), circular dichroism (CD) and resonance Rayleigh scattering (RLS) to elucidate the effect of the ligands on the thermal and structural properties of m-α-LA. The DSC profile displayed two dissimilar temperature-induced heat-absorption peaks as well as two melting points ( T m = 305 K, T m = 333 K). The m-α-LA is not a new form of α-LA, but rather contains a mixture of the apo- and holo-forms of α-LA (i.e., a-α-LA and h-α-LA) at low and high temperatures, respectively. The presence of Cu +2 as the metal ion and DTAB as the non-metal ion altered the two heat-absorption peaks in such a manner that, with the addition of Cu +2 to m-α-LA, the excess molar heat capacity profile showed three sub-peaks, i.e., one sub-peak for a-α-LA at 303.2 K and two other sub-peaks for h-α-LA at 325 K and 334 K. The presence of these peaks was due to the molecular population of the a-α-LA form changing into h-α-LA. Contrarily, when it came to the interaction between DTAB and m-α-LA, the DSC thermogram showed two sub-peaks, i.e., one sub-peak for a-α-LA and another sub-peak for h-α-LA, resulting from the molecular population of the h-α-LA form changing into a-α-LA. The CD experiments on m-α-LA upon interaction with Cu +2 and DTAB demonstrated an increment and a decrement, respectively, of the α-helix content relative to that of the protein in the absence of the ligands. However, the α-helix induced by Cu +2 as a metal ion inspired one energetics domain in m-α-LA, wherefore it could be deduced that the helicity content caused an increment of the energetics content of α-LA. Hence, Cu +2 and DTAB at various concentrations played important roles as good probes for defining the electrostatic moiety for domains of m-α-LA initiated through a dissimilarity with regard to the α-helicity of these domains. The RLS intensity of m-α-LA upon interaction with Cu +2 and DTAB determine the DSC and CD results for inducing h-α-LA and a-α-LA respectively.

Excess heat capacity and entropy of mixing in ternary series of high-structural-state feldspars

Low- and high-temperature heat capacities were measured for ternary series of synthetic, high structural state feldspars in the NaAlSi 3 O 8 –KAlSi 3 O 8 –CaAl 2 Si 2 O 8 system that had been characterised by X-ray diffraction and transmission light microscopy. The compositions of the phases lie in the NaAlSi 3 O 8 -rich part of the system where the stability is less limited by the ternary miscibility gap than in other compositional fields. The heat capacities of the end-members had been measured in previous studies using the same calorimeters as in this study (relaxation calorimeter and differential scanning calorimeter). Below 300 K, all samples showed strong positive excess heat capacities of mixing giving rise to excess entropies of mixing up to Δ S ex = 5.8 ± 1.6 J mol −1 K −1 at T = 298.15 K. Above 300 K, further contributions to the excess entropies of mixing do not appear to be significant. The non-ideal entropic mixing behaviour was described by a ternary asymmetric Margules model, resulting in the ternary interaction parameter W AbOrAn S = 93.9 J mol −1 K −1 .

Thermodynamic properties of a liquid–vapor interface in a two-component system

We report a complete set of thermodynamic properties of the interface layer between liquid and vapor two-component mixtures, using molecular dynamics. The mixtures consist of particles which have identical masses and diameters and interact with a long-range Lennard-Jones spline potential. The potential depths in dimensionless units for like interactions is 1 (for component 1) and 0.8 (for component 2). The surface excess entropy decreases when the temperature increases, so the surface has a negative excess heat capacity. This is a consequence of the fact that the surface tension decreases to zero at the critical point, proportional to ( T C , i − T ) 2 ν . The surface entropy decreases also as the excess concentration of component 2 increases, at a given temperature. The surface becomes less able to store energy as the more volatile component accumulates. We show that the more volatile component 2 has a maximum in its excess concentration in the interfacial region when the density of component 1 starts to go down. The vapor is described with the Soave–Redlich–Kwong equation of state, and the liquid activity coefficients are described by the three-suffix Margules equation. The observed densities and surface tension variations are fitted with good accuracy, with the critical exponents β = 0.32 and 2 ν = 1.26 , respectively. The results for the critical temperature of mixtures can be fitted to a second order polynomial, with significant deviations from Kay's rule.

Molar heat capacities of the (water + acetonitrile) mixtures at T = (283.15, 298.15, 313.15, and 328.15) K

The molar heat capacities of the (water + acetonitrile) mixtures have been measured at T = (283.15, 298.15, 313.15, and 328.15) K as a function of mole fraction. The excess molar heat capacities, C p E , limiting partial molar heat capacities of water and acetonitrile C ¯ p ∘ have been calculated. The excess molar heat capacities are positive over the whole mole fraction range and increase with increasing temperatures from (283.15 to 328.15) K. The excess molar heat capacities have been fitted to the Redlich–Kister equation.

Measurements of Density and Heat Capacity for Binary Mixtures {x Benzonitrile + (1 −x) (Octane or Nonane)}

Densities and isobaric heat capacities per unit volume have been measured for binary mixtures {x benzonitrile + (1 − x) octane} and {x benzonitrile + (1 − x) nonane} in the temperature ranges of (283.15 to 318.15) K and (283.15 to 313.15) K, respectively. Measurements covered the whole composition range at the ambient pressure. From these measured data, the excess molar volumes, excess isobaric thermal expansivities, and excess isobaric molar heat capacities were calculated and fitted with the Redlich−Kister equation. The excess molar volumes, excess isobaric thermal expansivities, and excess isobaric molar heat capacities of both systems were found to exhibit the S-shaped curves against the composition and the feature of the nonrandomness mixing. The dependences of these properties on the temperature and composition were analyzed and discussed in terms of the natures of the two pure components and the molecular interactions in the mixtures.

Glass transition and fragility in the simple molecular glassformer CS2 from CS2–S2Cl2 solution studies

With an interest in finding the fragility for a simple, single component, molecular glassformer, we have determined the dielectric relaxation and glass transition behavior for a series of glasses in the CS(2)-S(2)Cl(2) and CS(2)-toluene systems. Crystallization of CS(2) can be completely avoided down to the composition 20 mol% second component, and the fragility proves almost independent of CS(2) content in each system. Since the glass temperature T(g) obtained from both thermal studies and from dielectric relaxation (using T(g,diel)=T(tau=100 s)) is quite linear over the whole composition range in each system, and since relaxation time data for pure CS(2) fall on the same master plot when scaled by the linearly extrapolated T(g) value, we deduce that pure CS(2) has the same high fragility as the binary solutions. The value is m=86, as for ortho-terphenyl (OTP). Based on observations of independent studies for the vibrational density of states (VDoS) (of inherent structures for OTP and instantaneous, at-temperature structures for CS(2)), we attribute the high fragility to an excess vibrational heat capacity (defined by C(p) (vib, excess)=dS(vib, excess)/d ln T) originating in the behavior of the low frequency modes of the VDoS (the boson peak modes). Both low frequency DoS and anharmonicity increase with increasing temperature, augmenting the configurational entropy drive to the top of the system energy landscape. The surprising implication is that fragility is determined in the vibrational, not configurational, manifold of microstates.

Electrolytic conductivity and molar heat capacity of two aqueous solutions of ionic liquids at room-temperature: Measurements and correlations

As part of our systematic study on physicochemical characterization of ionic liquids, in this work, we report new measurements of electrolytic conductivity and molar heat capacity for aqueous solutions of two 1-ethyl-3-methylimidazolium-based ionic liquids, namely: 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate, at normal atmospheric condition and for temperatures up to 353.2 K. The electrolytic conductivity and molar heat capacity were measured by a commercial conductivity meter and a differential scanning calorimeter (DSC), respectively. The estimated experimental uncertainties for the electrolytic conductivity and molar heat capacity measurements were ±1% and ±2%, respectively. The property data are reported as functions of temperature and composition. A modified empirical equation from another researcher [1] was used to correlate the temperature and composition dependence of the our electrolytic conductivity results. An excess molar heat capacity expression derived using a Redlich–Kister type equation was used to represent the temperature and composition dependence of the measured molar heat capacity and calculated excess molar heat capacity of the solvent systems considered. The correlations applied represent the our measurements satisfactorily as shown by an acceptable overall average deviation of 6.4% and 0.1%, respectively, for electrolytic conductivity and molar heat capacity.

Excess heat capacity and entropy of mixing along the chlorapatite–fluorapatite binary join

The heat capacity at constant pressure, C p, of chlorapatite [Ca5(PO4)3Cl – ClAp], and fluorapatite [Ca5(PO4)3F – FAp], as well as of 12 compositions along the chlorapatite–fluorapatite join have been measured using relaxation calorimetry [heat capacity option of the physical properties measurement system (PPMS)] and differential scanning calorimetry (DSC) in the temperature range 5–764 K. The chlor-fluorapatites were synthesized at 1,375–1,220°C from Ca3(PO4)2 using the CaF2–CaCl2 flux method. Most of the chlor-fluorapatite compositions could be measured directly as single crystals using the PPMS such that they were attached to the sample platform of the calorimeter by a crystal face. However, the crystals were too small for the crystal face to be polished. In such cases, where the sample coupling was not optimal, an empirical procedure was developed to smoothly connect the PPMS to the DSC heat capacities around ambient T. The heat capacity of the end-members above 298 K can be represented by the polynomials: C p ClAp = 613.21 − 2,313.90T −0.5 − 1.87964 × 107 T −2 + 2.79925 × 109 T −3 and C p FAp = 681.24 − 4,621.73 × T −0.5 − 6.38134 × 106 T −2 + 7.38088 × 108 T −3 (units, J mol−1 K−1). Their standard third-law entropy, derived from the low-temperature heat capacity measurements, is S° = 400.6 ± 1.6 J mol−1 K−1 for chlorapatite and S° = 383.2 ± 1.5 J mol−1 K−1 for fluorapatite. Positive excess heat capacities of mixing, ΔC p ex , occur in the chlorapatite–fluorapatite solid solution around 80 K (and to a lesser degree at 200 K) and are asymmetrically distributed over the join reaching a maximum of 1.3 ± 0.3 J mol−1 K−1 for F-rich compositions. They are significant at these conditions exceeding the 2σ-uncertainty of the data. The excess entropy of mixing, ΔS ex, at 298 K reaches positive values of 3–4 J mol−1 K−1 in the F-rich portion of the binary, is, however, not significantly different from zero across the join within its 2σ-uncertainty.

Heat Capacities of Binary Mixtures of Acetic Acid with Acetic Anhydride and Methenamine at Different Temperatures

Heat capacities of binary mixtures of acetic acid with acetic anhydride and methenamine were measured by a cooling method with a simple calorimeter from (308.15 to 333.15) K at atmospheric pressure with different compositions. The excess molar heat capacities of the binary mixture of acetic acid with acetic anhydride were calculated from the experimental data and were fitted to the Redlich−Kister equation.

Measurements of the Molar Heat Capacities and Excess Molar Heat Capacities for Water + Organic Solvents Mixtures at 288.15 K to 303.15 K and Atmospheric Pressure

Experimental molar heat capacity data (Cpm) and excess molar heat capacity data (\(\mathit{Cp}^{\mathrm{E}}_{\mathrm{m}}\)) of binary mixtures containing water + (formamide or N,N-dimethylformamide or dimethylsulfoxide or N,N-dimethylacetamide or 1,4-dioxane) at several compositions, in the temperature range 288.15 K to 303.15 K and atmospheric pressure, have been determined using a modified 1455 PAAR solution calorimeter. The excess heat capacities are positive for aqueous solutions containing 1,4-dioxane, N,N-dimethylformamide or dimethylsulfoxide, negative for solutions containing water + formamide and show a sigmoid behavior for mixtures containing water + N,N-dimethylacetamide, over the whole composition range. The experimental excess molar heat capacities are discussed in terms of the influence of temperature and of the organic solvent type present in the binary aqueous mixtures, as well as in terms of the existing molecular interactions and the organic solvent’s molecular size and structure.

Investigation on Thermodynamic Properties of a Water-Based Hematite Nanofluid

A friendly environmental method was used to prepare a water-based hematite (α-Fe2O3) nanofluid through a simple biomolecule-assisted hydrothermal process. The molar heat capacities of the obtained nanofluids, base fluids, and hematite nanoparticles were measured by a high-precision automatic adiabatic calorimeter over the temperature range of (290 to 335) K, respectively. Polynomial equations of the molar heat capacities as a function of temperature were fitted by a least-squares method for the solid and nanofluid samples. Smoothed heat capacities and thermodynamic functions of the obtained samples, such as H(T/K) − H(298.15 K) and S(T/K) − S(298.15 K), were calculated on the basis of the fitted polynomials and the relationships of the thermodynamic functions. On the basis of the as-obtained molar heat capacities, the excess heat capacities of the nanofluids were calculated. These excess heat capacities reveal that the stable hematite nanofluids exhibit unique properties compared with the unstable one.

Surface Debye Temperatures and Specific Heat of Nanocrystals

The present study deals with the contribution of the surface atomic layer to the specific heat of nanocrystals. In the case of small phases with comparable number of bulk and surface atoms, the variation of heat capacity with respect to that of infinitely large phase is significant. To evaluate this variation, in the framework of the classical Debye model, we introduce surface excess heat capacity that accounts for the surface layer contribution. On that physical background, we draw a functional dependence of the specific heat of nano clusters on the number of their bulk and surface atoms and corresponding bulk, , and surface, , Debye temperatures. Since depends on the crystallographic orientation of the surface, the presented model also accounts for the symmetry and atomic density of the respective crystal faces of the nanocrystal.

Quantifying the degree of nanofiller dispersion by advanced thermal analysis: application to polyester nanocomposites prepared by various elaboration methods

An innovative thermal analysis methodology is applied for the characterization of poly(e-caprolactone) (PCL) nanocomposites containing layered silicates, needle-like sepiolite or polyhedral oligomeric silsesquioxane (POSS) nano-cages, aiming at assessing the key factors affecting nanofiller dispersion and nanocomposite properties. This methodology takes benefit of the fact that—for a given nanofiller aspect ratio—the magnitude of the excess heat capacity recorded during quasi-isothermal crystallization is directly related to the occurrence of pronounced changes to the PCL crystalline morphology. The extent of these changes, in turn, directly depends on the amount of matrix/filler interface and can therefore be considered a reliable measure for the degree of nanofiller dispersion, as supported by complementary morphological characterization. The importance of processing parameters is demonstrated in a comparative study using various melt processing conditions, evidencing the need for high shear to effectively exfoliate and disperse individual nanoparticles throughout the polymer matrix. Furthermore, the choice of the nanocomposite elaboration method is shown to profoundly affect the final morphology, as illustrated in a comparison between nanocomposites prepared by melt mixing, by in situ polymerization and by a masterbatch approach. Grafting PCL onto the filler strongly enhances its dispersion quality as compared to conventional melt mixing; subsequently further dispersing such grafted nanohybrids into the polymer matrix through a masterbatch approach provides a highly efficient method for the elaboration of well-dispersed nanocomposites. Finally, the crucial issue of interfacial compatibility is addressed in a comparison between various surface-treated layered silicates, showing that high degrees of filler dispersion in a PCL matrix can only be achieved upon polar modification of the silicate.

Thermodynamic study of sodium decyl sulfate solutions in the region the second critical micellization concentration: 1. Volumetric and heat capacity properties

Densitometry and precision adiabatic scanning calorimetry are used to reveal a number of volumetric and heat capacity properties of sodium decyl sulfate in the region of the second critical micelle concentration. Heat capacities are established in a wide temperature range. The coefficients of thermal expansion, apparent and partial mole expandabilities, volume, heat capacities, and excess partial mole heat capacities are calculated. The analysis of variations in these thermodynamic properties occurring upon variations in concentration and temperature makes it possible to identify the structural variations in sodium decyl sulfate micelles that are relevant to the transition of micelles from globular to cylindrical forms.