State Heat Capacity Research Articles

Thermal and structural properties of polyzwitterions: Effects of monomer chemistry and salt addition

Thermal and structural properties of polyzwitterions (PZI)s comprising the sulfobetaine moiety were studied as a function of LiCl addition and variations in monomer chemistry. The polyzwitterions chosen for this study are: poly(sulfobetaine methacrylate), PSBMA; poly(sulfobetaine acrylate), PSBA; and, poly(ethyl sulfobetaine methacrylate), PESBMA. When LiCl salt is added, the Li+and Cl− ions disrupt the dipole-dipole electrostatic interaction of the PZI side groups. Fourier transform infrared spectroscopy shows that the vibrational frequencies of the symmetric and asymmetric S=O bonds increase as LiCl content increases. Structural studies by wide angle X-ray scattering show that all the PZIs are amorphous, and the average interatomic spacing, d, increases in the order: dPSBA(= 0.456±0.006nm) < dPSBMA (= 0.474±0.015nm) < dPESBMA (= 0.493±0.011nm). Upon the addition of LiCl, d-spacing for a given PZI decreases as LiCl content increases. Thermogravimetry showed that PZIs had degradation onset temperatures Td ∼ 300°C. As LiCl content increases, the onset temperature of degradation decreases suggesting that the disruption of dipole-dipole crosslinks destabilizes the polymer. All the PZIs are hydrophilic and hygroscopic, and a drying procedure was developed so that well-defined glass-to-liquid transitions could be observed in the dry solid state. PZIs without LiCl have glass transition temperatures, Tg, increasing in the order: Tg,PSBA(= 200±1°C) < Tg,PESBMA (= 204±1°C) < Tg,PSBMA (= 237±1°C). As the LiCl content increases, Tg decreases, and the liquid state heat capacity increases, in all PZIs. The specific heat capacity increment at Tg was studied for PSBA as a function of LiCl content. In PSBA, the heat capacity increment reached a maximum for molar ZI:LiCl content at and above 1:1, and its value agreed well with predictions based on an empirical bead model of polymers. Thermal data on the dry state of PZI-LiCl complexes suggest an analogy between electrostatically crosslinked, fully amorphous PZIs and semicrystalline polymers. The dipolar crosslinks confine the molecular motion of PZIs just as crystal domains confine the molecular motion of the amorphous phase in semicrystalline polymers.

Thermal Properties of Porous Silicon Nanomaterials.

The thermal properties, including the heat capacity, thermal conductivity, effusivity, diffusivity, and phonon density of states of silicon-based nanomaterials are analyzed using a molecular dynamics calculation. These quantities are calculated in more detail for bulk silicon, porous silicon, and a silicon aerocrystal (aerogel), including the passivation of the porous internal surfaces with hydrogen, hydroxide, and oxygen ions. It is found that the heat capacity of these materials increases monotonically by up to 30% with an increase in the area of the porous inner surface and upon its passivation with these ions. This phenomenon is explained by a shift of the phonon density of states of the materials under study to the low-frequency region. In addition, it is shown that the thermal conductivity of the investigated materials depends on the degree of their porosity and can be changed significantly upon the passivation of their inner surface with different ions. It is demonstrated that, in the various simulated types of porous silicon, the thermal conductivity changes by 1-2 orders of magnitude compared with the value for bulk silicon. At the same time, it is found that the nature of the passivation of the internal nanosilicon surfaces affects the thermal conductivity. For example, the passivation of the surfaces with hydrogen does not significantly change this parameter, whereas a passivation with oxygen ions reduces it by a factor of two on average, and passivation with hydroxyl ions increases the thermal conductivity by a factor of 2-3. Similar trends are observed for the thermal effusivities and diffusivities of all the types of nanoporous silicon under passivation, but, in that case, the changes are weaker (by a factor of 1.5-2). The ways of tuning the thermal properties of the new nanostructured materials are outlined, which is important for their application.

Heat capacity and index of refraction of polyzwitterions

Specific heat capacities and indices of refraction were measured for a series of six sulfobetaine zwitterionic polymers and compared to topological and group contribution models. Poly(sulfobetaine methacrylate) (PSBMA), poly(sulfobetaine acrylate) (PSBA), poly(ethyl sulfobetaine methacrylate) (PESBMA), poly(sulfobetaine methacrylamide) (PSBMAm), poly(sulfobetaine 2-vinylpyridine) (PSB2VP) and poly(sulfobetaine 4-vinylpyridine) (PSB4VP) were specially synthesized with changes of chemical structure in order to study effects of either the polymer backbone or the side group on their properties. Temperature modulated differential scanning calorimetry (TMDSC) as well as quasi-isothermal TMDSC were used to determine the temperature dependent specific heat capacity, cp(T) from 30 to 200 °C. The solid state specific heat capacities were measured for all the polyzwitterions, while liquid state heat capacity measurements were limited to PSBA and PESBMA. Ellipsometry at wavelengths from 300 nm to 3000 nm was used to measure the index of refraction for polyzwitterion thin films at room temperature. Theoretical room temperature values of cp and n were calculated for the polyzwitterions using topological and group contribution methods and compared to the measured values. The measured index of refraction values agree well with predictions for the polyzwitterions. However, for the solid state heat capacity, the model produces accurate prediction only for PSB2VP and PSB4VP. The discrepancy between the measured and predicted cp values is most likely due to the reduction of vibrational degrees of motion in the solid state for some of the polyzwitterions due to physical crosslinking.

Analysis of the heat transfer time in rubber aggregate concrete as a function of humidity percentage at very high temperature

This study deals with the analysis of the thermal behavior of concrete based on rubber aggregates. The objective is to study the effect of humidity and the role of rubber aggregates on the thermal behavior of concrete, especially the time of heat transmission within the material at very high temperatures such as during a fire. The study is carried out according to two approaches, an experimental approach that allows us to determine the characteristics of concrete in the initial state (density, heat capacity and thermal conductivity), and a numerical approach to characterize the behavior of recycled concrete at very high temperature and determine the time of heat transmission. The results obtained showed that recycled concrete based on rubber aggregates has several advantages when the concrete is exposed to high temperatures such as during a fire. Indeed, with a humidity of 1.5% and a percentage of 10%, 20% and 30% of rubber aggregates, we can gain 23.8%, 41% and 63.7% on the time of heat transmission compared to the reference concrete, and with a humidity of 3% and a percentage of 10%, 20% and 30% of rubber aggregates we can gain 22. 5%, 40% and 63.3% compared to the reference concrete, and with a humidity of 10% and a percentage of 10%, 20% and 30% of rubber aggregates we can gain 23.6%, 40.7% and 63.8% still compared to the reference concrete. However, a high percentage of moisture in the concrete generates an increase in pressure and favors the increase of the thermal gradient due to the energy necessary for the vaporization of the liquid water, and according to the literature it is recommended not to exceed a percentage of moisture 3% in order to avoid the thermal stability as well as the spalling of concrete.

Heat capacity anomalies of the molecular crystal 1-fluoro-adamantane at low temperatures

Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T = 180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.

J-O study of Nd-doped 8Y-ZrO2 transparent ceramic and its potential application in infrared laser

Transparent 1 at% Nd3+:8Y-ZrO2 ceramics were fabricated using the solid-state sintering method. Using Judd-Ofelt analysis, the J–O intensity parameters were calculated based on the absorption spectrum, and Ω2, Ω4 and Ω6 were calculated to be 0.46 × 10−20 cm2, 0.29 × 10−20 cm2, and 0.22 × 10−20 cm2, respectively. The radiation transition probability, fluorescence branch ratio, and radiation lifetime were determined using the parameters. The branching ratios for the 4F3/2→4I9/2 and 4F3/2→4I11/2 transitions were 47.69% and 43.87%, respectively. The fluorescence lifetime of the 4F3/2 level was approximately 442 μs, and the quantum efficiency was 19%. Combined with its high temperature resistance and good mechanical strength, Nd3+:8Y-ZrO2 transparent ceramics can have potential applications in solid state heat capacity laser.

Thermodynamics of the Enantiotropic Pharmaceutical Compound Benzocaine and Solubility in Pure Organic Solvents

The thermodynamic relationship between FI and FII of ethyl 4-aminobenzoate (benzocaine) has been investigated. Slurry conversion experiments show that the transition temperature below which FI is stable is located between 302 K–303 K (29 °C–30 °C). The polymorphs FI and FII have been characterised by infrared spectroscopy (IR), Raman spectroscopy, transmission powder X-ray diffraction (XRPD) and differential scanning calorimetry (DSC). The isobaric solid state heat capacities have been measured by DSC. The quantitative thermodynamic stability relationship has been determined in a comprehensive thermodynamic analysis of the calorimetric data. The solubility of both polymorphs has been determined in eight pure organic solvents over the temperature range 278 K–323 K by a gravimetric method. The mole fraction solubility of benzocaine decreases in the order: 1,4-dioxane, acetone, ethyl acetate, chloroform, acetonitrile, methanol, n-butanol and toluene. Comparison with the determined activity of solid benzocaine forms shows that negative deviation from Raoult's law ideality is found in dioxane, acetone and ethyl acetate solutions, and positive deviation in solutions of the other investigated solvents.

A Simple Method of Finding New Dry and Isentropic Working Fluids for Organic Rankine Cycle

One of the most crucial challenges of sustainable development is the use of low-temperature heat sources (60–200 °C), such as thermal solar, geothermal, biomass, or waste heat, for electricity production. Since conventional water-based thermodynamic cycles are not suitable in this temperature range or at least operate with very low efficiency, other working fluids need to be applied. Organic Rankine Cycle (ORC) uses organic working fluids, which results in higher thermal efficiency for low-temperature heat sources. Traditionally, new working fluids are found using a trial-and-error procedure through experience among chemically similar materials. This approach, however, carries a high risk of excluding the ideal working fluid. Therefore, a new method and a simple rule of thumb—based on a correlation related to molar isochoric specific heat capacity of saturated vapor states—were developed. With the application of this thumb rule, novel isentropic and dry working fluids can be found applicable for given low-temperature heat sources. Additionally, the importance of molar quantities—usually ignored by energy engineers—was demonstrated.

Thermochemical properties of 1,2,3,4-tetraphenylnaphthalene and 1,3,5-triphenylbenzene in crystalline and liquid states studied by solution and fast scanning calorimetry

Significant deviations were found between the solution enthalpy in benzene at 298.15 K and the fusion enthalpy at the melting temperature of 1,2,3,4-tetraphenylnaphthalene (3.9 ± 0.5 kJ mol−1 and 23.6 ± 0.6 kJ mol−1 (470.8 K), respectively) and 1,3,5-triphenylbenzene (20.1 ± 0.2 kJ mol−1 and 32.4 ± 1.2 kJ mol−1 (447.3 K)). These aromatic compounds are structurally related to benzene and their solution enthalpies in hypothetical liquid state at 298.15 K in benzene should be close to zero, so the differences between the solution enthalpy of crystal and the fusion enthalpy at the melting temperature should be attributed to the temperature dependence of the fusion enthalpy.Direct adjustment of the fusion enthalpy to 298.15 K requires knowledge of the supercooled liquid state heat capacity, which is hardly attainable by conventional methods due to fast crystallization. The heat capacities of 1,2,3,4-tetraphenylnaphthalene and 1,3,5-triphenylbenzene in liquid, deeply supercooled liquid, glassy and crystalline states were obtained for the first time by using fast scanning calorimetry in wide temperature ranges. Crystalline 1,2,3,4-tetraphenylnaphthalene heat capacity was measured over the temperature range from 323 to 410 K by conventional DSC. The temperature dependences of the heat capacities of the studied compounds in supercooled liquid state were fitted with the linear equations. The experimental results obtained by fast scanning calorimetry are in excellent agreement with the data obtained by solution calorimetry and conventional DSC.

Magnetic and thermal characteristics of armchair graphene nanoribbons in the two-band Harrison model

In this study, within a two-band tight-binding Harrison Hamiltonian and the Green’s function approach, the influences of both localized σ and delocalized π electrons on the Pauli paramagnetic susceptibility, heat capacity and density of states of armchair graphene nanoribbons with different widths are investigated. Temperature dependence of the paramagnetic susceptibility and electronic heat capacity are compared for s and p orbitals of every width. In the density of states, an extension in the bandwidth and the extra number of Van-Hove singularities are observed. More importantly, a paramagnetic behavior associated with the π electrons could take place. Finally, a Schottky anomaly for the heat capacity is reported. Overall, in this paper, it was observed that both the σ and the π electrons contribute significantly to the aforementioned quantities.

Prediction of thermodynamic properties of aqueous electrolyte solutions using equation of state

In this study, a predictive model is presented for estimation of second order thermodynamic properties of electrolyte solutions. In order to provide a comprehensive understanding, the capability of modified electrolyte PC‐SAFT up to high pressure and temperature has been studied. In addition to the first order derivative thermodynamic properties, the Gibbs free energy, enthalpy and heat capacity of aqueous electrolyte solutions at infinite dilution are predicted. Using new methodology, the dielectric constant is modified to keep the pressure, temperature, and ionic strength dependency. Our results show that the Born term has a significant contribution on prediction of second order derivative properties. Meanwhile the impact of temperature‐dependent solution dielectric constant on standard state heat capacity is studied. Finally, the isobaric heat capacity at various salt concentrations is predicted without any adjustable parameters. The results of this work indicate an acceptable agreement with experimental data especially at high pressure and temperature. © 2017 American Institute of Chemical Engineers AIChE J, 2017

The influence of flow and thermal properties on injection pressure and cooling time prediction

Thermoplastic materials properties play an important role in mould filling and cooling analysis of injection moulding. Among the many, the melt's viscosity, heat capacity and thermal conductivity may be the critical ones. An experimental and computational case study to determine the injection moulding window of a rectangular ABS plate is presented. When apparent viscosity of the material was adopted for cavity filling simulations, it was found that the computed injection pressure was overestimated in contrast to experimental data. Shear stress and rate corrections applied to apparent viscosity as well as the no-flow temperature (NFT) set to Tg (glass-transition temperature) helped to achieve more accurate pressure estimation. The heat capacity and thermal conductivity were both measured separately in solid and molten states and it was found that the best estimation for pressure and cooling time was achieved when molten state heat capacity was adopted for computation. The effect of thermal conductivity was negligible on pressure prediction, although the most accurate prediction for cooling time was attained when both molten state heat capacity and thermal conductivity were utilised.Therefore, the quality of material data input was found to be a critical factor in achieving reliable flow properties. Accurate data available for mould and process design purposes may help to generate less production waste and save costs, making a step towards sustainable manufacturing.

Using flash DSC for determining the liquid state heat capacity of silk fibroin

In this technical note, we report a study of the heat capacity of amorphous silk fibroin protein evaluated using the Flash DSC1 for fast scanning calorimetry. Nineteen amorphous thin films were obtained either after casting directly from water solutions, or after melting of previously crystalline films. Fibroin films were mounted onto Flash DSC1 sensors, dried free of bound water, relaxed just above the glass transition, Tg, then scanned in heating and cooling at ±2000K/s. The heat flow rate data were analyzed by finding the symmetry line between the heating and cooling scans, and by subtraction of the empty sensor heat flow rate, similarly corrected for its symmetry line. To evaluate the sample mass, two approaches were compared. First, the mass was obtained from known solid state heat capacity, Cpsolid(T), at a temperature below Tg. Second, the mass was obtained from the known heat capacity increment at the glass transition, ΔCp(Tg). The Cpsolid(T) and ΔCp(Tg) had been obtained previously from slow scanning differential scanning calorimetry. The use of either method for mass determination necessitated additional corrections to the heat capacity data to bring them into agreement with the literature values. After these corrections, the heat capacity of silk fibroin in the liquid state was evaluated over a wide temperature range above Tg. We find Cpliquid(T)=(1.98±0.06)J/gK+T·(6.82±1.4)×10−4J/gK2 in the temperature interval from 510 to 570K with an uncertainty of about ±5%.

Influence of Water on Protein Transitions: Thermal Analysis

We have developed a methodology using advanced thermal analysis to characterize the role of water in a specially synthesized family of recombinant spider silk-like block copolymers. These proteins were inspired by the genetic sequences found in the dragline silk of Nephila clavipes, comprising an alanine-rich hydrophobic block, A; a glycine-rich hydrophilic block, B; and a C-terminus or a His-tag, H. This family of proteins serves as a model system in which the hydrophobicity is controlled by A and B block lengths, allowing systematic comparison of water effects within the family. Temperature-modulated differential scanning calorimetry and thermogravimetric analyses were employed to capture the glass to rubber transition, Tg, in water-cast protein films. Modeling of the solid and liquid state heat capacity baselines allows us to determine the critical role played by bound water which plasticizes and stabilizes the protein through interchain bonding. In samples containing bound water, two sequential glass transitions, Tg(1) and Tg(2), were observed during heating. The lower temperature glass transition, Tg(1), is related to conformational change induced by bound water removal, the hydrophobicity of the protein sequences, and the crystallinity of the protein. The higher temperature glass transition, Tg(2), is characteristic of the dry protein. The binding energy of water to protein compares favorably to ligand–water binding affinities. The energy absorbed by evaporating water depends upon the volume fraction of the hydrophilic B-block.

On the use of departure function correlations for hydrocarbon isobaric liquid phase heat capacity calculation

Constant pressure heat capacities for pure liquids and mixtures are by default evaluated indirectly, in process simulators and in general purpose calculations, using ideal gas isobaric heat capacity values to which equation of state based departure functions are added. As ideal gas heat capacities are known or can be calculated from theory with small uncertainties and typically comprise 75% of liquid heat capacity values, the large relative deviations present in departure function calculations appear to be tolerated or ignored because deviations between indirectly calculated and measured constant pressure liquid heat capacities are typically less than 15% and large deviations are uncommon. For hydrocarbon and petroleum applications, non-cubic equations of state have a limited range of application and a cubic equation of state departure function calculations possesses a systematic skew with respect to absolute and relative temperature. This provides application windows for stand alone departure function correlations: by Tyagi that requires the input properties (Tc, Pc, ω, M) at high reduced and absolute temperatures; and a difference calculation based on correlations by Dadgostar and Shaw (for hydrocarbon liquids) and Laštovka and Shaw (for ideal gases) which extend the range of fluids for which liquid state heat capacity departure functions can be evaluated to include poorly defined (Tc and M are available) individual hydrocarbons or mixtures or ill-defined (only elemental analysis is available) hydrocarbon mixtures. However, none of these approaches provides consistently low deviations from measurements and consequently, direct calculation of liquid phase heat capacities is recommended for detailed engineering calculations.

Explaining the heat capacity of wood constituents by molecular vibrations

The heat capacity of wood and its constituents is important for the correct evaluation of many of their thermodynamic properties, including heat exchange involved in sorption of water. In this study, the dry state heat capacity of cellulose, hemicelluloses and lignin are mathematically described by fundamental physical theories relating heat capacity with molecular vibrations. Based on knowledge about chemical structure and molecular vibrations derived from infrared and Raman spectroscopy, heat capacities are calculated and compared with experimental data from literature for a range of bio- and wood polymers in the temperature range 5–370 K. A very close correspondence between experimental and calculated results is observed, illustrating the possibility of linking macroscopic thermodynamic properties with their physical nano-scale origin.

Heat capacity prediction of complex molecules by mass connectivity index

Heat capacity prediction and estimation methods of solid organic compounds in terms of temperature are limited, particularly concerning complex molecules with functional groups such as active principles and intermediaries used in pharmaceutical field.Recently a correlation between heat capacity at constant pressure (Cp), temperature and a new concept named mass connectivity index (MCI), for ionic liquids, was published [1-3]. In this predictive method, heat capacity can be calculated at different temperatures, if standard heat capacity at 298.15 K is known. The effect of molecular structure on heat capacity is accounted for in this model by the mass connectivity index, a molecular descriptor, which differentiates between compounds. The Valderrama generalized correlation admits, in addition, two universal coefficients, which are obtained from experimental data regression.In the present work, a similar approach is used to predict solid state heat capacity of organics and pharmaceutical products. In order to find model parameters, a database was grouped comprising (104) different compounds and a set of more than 5,791 experimental values of solid state Cps obtained from literature. These collected data were used in multiple linear regression to find model parameters. It was found that the values of predicted heat capacities of compounds non-included in the database were good; they are quite close to the ones presented in the literature. Moreover, this method is simple to use, since only molecular structure of the component and its solid state heat capacity at 298.15 K should be known.

Thermochemistry of radicals formed by hydrogen abstraction from 1-butanol, 2-methyl-1-propanol, and butanal

We calculate the standard state entropy, heat capacity, enthalpy, and Gibbs free energy for 13 radicals important for the combustion chemistry of biofuels. These thermochemical quantities are calculated from recently proposed methods for calculating partition functions of complex molecules by taking into account their multiple conformational structures and torsional anharmonicity. The radicals considered in this study are those obtained by hydrogen abstraction from 1-butanol, 2-methyl-1-propanol, and butanal. Electronic structure calculations for all conformers of the radicals were carried out using both density functional theory and explicitly correlated coupled cluster theory with quasipertubative inclusion of connected triple excitations. The heat capacity and entropy results are compared with sparsely available group additivity data, and trends in enthalpy and free energy as a function of radical center are discussed for the isomeric radicals.

Thermal analysis of spider silk inspired di-block copolymers in the glass transition region by TMDSC

We used advanced thermal analysis methods to characterize a new family of A-B di-block copolymers based on the amino acid sequences of Nephila clavipes major ampulate dragline spider silk. Using temperature modulated differential scanning calorimetry with a thermal cycling method and thermogravimetry, we captured the effect of bound water acting as a plasticizer for spider silk-like biopolymer films which had been cast from water solution and then dried. A low glass transition because of bound water removal was observed in the first heating cycle, after which, a shift of glass transition was observed in A-block film due to crystallization and annealing, and in BA film due to annealing. No shift of glass transition after bound water removal was observed in B-block film. The reversing heat capacities, C p, for temperatures below and above the glass transition were measured and compared to the calculated values. The solid state heat capacity was modeled below T g, based on the vibrational motions of the constituent poly(amino acid)s, heat capacities of which are known from the ATHAS Data Bank. Excellent agreement was found between the measured and calculated values of the heat capacity, showing that this model can serve as a standard method to predict the solid state C p for other biologically inspired block-copolymers. We also calculated the liquid state heat capacities of the 100% amorphous biopolymer at T g, and this predicted value can be use to determined the crystallinity of protein-based materials.

A new approach to the exact and approximate anharmonic vibrational partition function of diatomic and polyatomic molecules utilizing Morse and Rosen-Morse oscillators

Exact closed forms of the equilibrium partition functions in terms Jacobi elliptic functions are derived for a particle in a box and Rosen–Morse (Poschl–Teller) oscillator (perfect for modeling bending vibrational modes). An exact form of the equilibrium partition function of Morse oscillator is reported. Three other approximate forms of Morse partition function are presented. Having an exact closed-form for the vibrational partition function can be very helpful in evaluating thermodynamic state functions, e.g., entropy, internal energy, enthalpy, and heat capacity. Moreover, the herein presented closed forms of the vibrational partition function can be used for obtaining spectroscopic and dynamical information through evaluating the two- and four-point dipole moment time correlation functions in anharmonic media. Finally, a closed exact form of the rotational partition function of a particle on a ring in terms of the first kind of complete elliptic integral is derived. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011