Partial Heat Capacities Research Articles

Hybrid energy-temperature method (HETM): A low-cost apparatus and reliable method for estimating the thermal capacity of solid–liquid phase change material for heat storage system

Estimating the total thermal capacity for phase change material (PCM) as heat storage material is extremely important to set the proper operational parameter of the latent storage tank (LST) unit. However, estimating the total thermal capacity for solid–liquid PCM is relatively complex due to temperature-dependent properties for each phase. Thus, predicting the state of charge (SoC) indicator for the LST unit is technically complex. The common approach is taken by estimating the heat of fusion during the melting process and monitoring the working fluid temperature, which makes the SoC estimation less accurate. The present project proposes a reliable method with an affordable apparatus to estimate the total thermal capacity simultaneously based on the temperature of low-temperature PCM. Moreover, sensible heating during phase transition is also estimated precisely based on the energy balance at a given temperature. The developed apparatus employs widely available components at an affordable cost which is possible for further customization. Validation and characterization are done comprehensibly by comparing the measurement from differential scanning calorimetry and previous studies. The results indicate a suitable estimation for estimating the solid–liquid heat capacity, including partial heat capacity and latent heat of fusion.

Thermodynamics of globular protein native structure

The temperature dependence of the partial heat capacity of the native protein structure in an aqueous solution has been analyzed. It is shown that the strictly linear temperature dependence is due to the contributions of the vibrational and conformational components, which indicates volume consistensy and the absence of conformational transitions up to the main two-state transition. The two-level structural and functional organization of the protein three-dimensional structure are considered in relation to the energy and conformational entropy properties in accordance with the principles of the organization of the protein macromolecule. Communicated by Ramaswamy H. Sarma

Superswelling of Hydrogels Based on the Copolymer of Acrylamide and Methacrylic Acid

Weakly crosslinked hydrogels of copolymers of acrylamide and methacrylic acid are synthesized by radical polymerization in aqueous solution in a wide range of comonomer ratios. It is found that gels containing 20–40 mol % methacrylic acid are characterized by superswelling that exceeds the degree of swelling of homopolymer hydrogels by tens of times. The observed degree of swelling reaches 830, corresponding to an almost completely stretched conformation of subchains between network junctions. For dried hydrogel samples, the equilibrium degree of swelling is also maximal in the mentioned composition range; however, its maximum value is lower than the equilibrium degree of swelling for the initial samples. The possible reason behind the superswelling of hydrogels containing 20–40% units of methacrylic acid is an increase in ionization of network subchains. This hypothesis is proved by an increased sensibility of hydrogels of this composition to a change in the pH of a medium and the lowest values of the effective acidity constant determined by potentiometric titration for methacrylic acid units. The values of partial heat capacity of a gel polymer network are determined by calorimetry. These values are significantly higher than the heat capacities typical of vinyl polymers. This is evidence that the hydrophobic type of hydration predominates in the copolymers of acrylamide and methacrylic acid. The partial heat capacity reaches its maximum within the copolymer composition range corresponding to superswelling.

Energetics of poloxamer micellization at normal and high pressures

Micellization of a poloxamer P105 in diluted aqueous solutions was first investigated by means of high-sensitivity differential scanning calorimetry at different pressures up to ∼160 MPa. The temperature dependence of partial heat capacity of the poloxamer clearly indicates that the unimers have the random coil conformation while the micelles are built of a “dry” PO core and hydrated EO shell. Independently of the polymer concentration, the micellization temperature increases upon increasing pressure while the enthalpy decreases. Based on these dependences, the micellization increments of the partial volume Δtv as well as the coefficients of thermal volume expansion and compressibility, Δtα and Δtχ, of the poloxamer were calculated. They prove that the dehydration of PO hydrophobic blocks makes the main contribution to the micellization energetics. We demonstrate that poloxamer micelles are formed via a nucleation stage of unimer dimerization followed by consecutive energetically favored attachment of unimers to the dimer and further to the growing micelle.

Neutrino luminosities and heat capacities of neutron stars in analytic form

We derive analytic approximations for the neutrino luminosities and the heat capacities of neutron stars with isothernal nucleon cores as functions of the mass and radius of stars. The neutrino luminosities are approximated for the three basic neutrino emission mechanisms, and the heat capacities for the five basic combinations of the partial heat capacities. The approximations are valid for for a wide class of equations of state of dense nucleonmatter. The results significantly simplify the theoretical interpretation of observations of cooling neutron stars as well as of quasistationary thermal states of neutron stars in X-ray transients. For illustration, we present an analysis of the neutrino cooling functions of nine isolated neutron stars taking into account the effects of their magnetic fields and of the presence of light elements in their heat blanketing envelopes. These results allow one to investigate the superfluid properties of neutron star cores.

Studies of relationship between polymer structure and hydration environment in amphiphilic polytartaramides

ABSTRACTThe relationships between the chemical structures and hydration environment of the polymers can provide significant insight into the water‐amphiphilic polymer interactions. Here, the hydrophobicity of amphiphilic block copolymers poly(ethylene tartaramide‐b‐alkyl isocyanate) is gradually tuned by using of a series of pendant alkyl (isopropyl, n‐butyl, cyclopentyl, and cyclohexyl) groups. Dynamics of hydration probed by low‐field NMR relaxometry exhibits a heterogeneous environment of water molecules, corresponding to tightly bound water with slow re‐orientational mobility and loosely bound water with fast re‐orientational mobility. Progressively larger amounts of bound water are present in the copolymers, ongoing from pendant isopropyl, n‐butyl, cyclopentyl, and finally to cyclohexyl group. Water in the copolymer bearing the cyclohexyl group has a significantly high partial specific heat capacity. Therefore, hydrophobic interaction between the polymer and water is enhanced when the hydrophobicity of the polymer is increased, resulting in considerable hydrophobic hydration with decreased mobility of the bound water. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 138–145

Water anomalous thermodynamics, attraction, repulsion, and hydrophobic hydration.

A model composed of van der Waals-like and hydrogen bonding contributions that simulates the low-temperature anomalous thermodynamics of pure water while exhibiting a second, liquid-liquid critical point [P. H. Poole et al., Phys. Rev. Lett. 73, 1632 (1994)] is extended to dilute solutions of nonionic species. Critical lines emanating from such second critical point are calculated. While one infers that the smallness of the water molecule may be a relevant factor for those critical lines to move towards experimentally accessible regions, attention is mainly focused on the picture our model draws for the hydration thermodynamics of purely hydrophobic and amphiphilic non-electrolyte solutes. We first focus on differentiating solvation at constant volume from the corresponding isobaric process. Both processes provide the same viewpoint for the low solubility of hydrophobic solutes: it originates from the combination of weak solute-solvent attractive interactions and the specific excluded-volume effects associated with the small molecular size of water. However, a sharp distinction is found when exploring the temperature dependence of hydration phenomena since, in contrast to the situation for the constant-V process, the properties of pure water play a crucial role at isobaric conditions. Specifically, the solubility minimum as well as enthalpy and entropy convergence phenomena, exclusively ascribed to isobaric solvation, are closely related to water's density maximum. Furthermore, the behavior of the partial molecular volume and the partial molecular isobaric heat capacity highlights the interplay between water anomalies, attraction, and repulsion. The overall picture presented here is supported by experimental observations, simulations, and previous theoretical results.

High-pressure differential scanning microcalorimeter.

A differential scanning microcalorimeter for studying thermotropic conformational transitions of biopolymers at high pressure has been designed. The calorimeter allows taking measurements of partial heat capacity of biopolymer solutions vs. temperature at pressures up to 3000 atm. The principles of operation of the device, methods of its calibration, as well as possible applications are discussed.

Thermodynamics and structure of aqueous electrolyte solutions of glycine at different temperatures

The enthalpies of dissolution of glycine in aqueous electrolyte solutions are measured by calorimetry at temperatures of 288 K, 298 K, and 313 K in the molality range from 0.25 mol/kg to 6 mol/kg NaCl and to 4.5 mol/kg KCl. The enthalpy and heat capacity parameters are determined for the pair and triple interactions of glycine with electrolyte in water. Special (singular) points are discovered on the concentration dependences of the heats of dissolution of glycine in aqueous electrolyte solutions and in urea solutions at different temperatures. At these points, such a structurally sensitive property as the entropy of dissolution is temperature independent and the partial heat capacity of glycine is equal to that of the crystalline amino acid. The position of the singular point on the concentration axis is identical for NaCl and KCl solutions; in the case of urea solutions, the singular point is achieved at a much higher concentration of urea.

Thermal denaturation of a blue-copper laccase: Formation of a compact denatured state with residual structure linked to pH changes in the region of histidine protonation

The partial (absolute) heat capacity of a laccase enzyme from Myceliophthora thermophila (MtL) was determined from calorimetric scans in the 4.5–10.0 pH range. Above pH 7.5, the heat capacity of the thermally denatured state (CpD) of this blue-copper glycoprotein is consistent with that for an unfolded, fully solvated polypeptide chain, if its carbohydrate content is taken into account. Below pH 7.5, CpD decreases and eventually levels off within the 5.5–4.5 pH region, where a compact, partially solvated denatured state is formed. In the compact state, denatured MtL is an oligomer, and exhibits considerable native-like secondary structure and a perturbed environment of its copper atoms. Analysis of the pH dependence of CpD and the content of secondary structure gives results implying that His residues play an important role in the stability of the compact denatured state.

Thermodynamique des systèmes micellaires Partie III : Capacités calorifiques apparente et partielle des alcanoates de sodium

Thermodynamics of micellar systems Part III : Apparent and partial heat capacities of sodium alkanoates To obtain the heat capacities of the aqueous solutions of the sodium carboxylates, we carry out microcalorimetric heat capacities measurements. The aim of this work is to calculate the quantities of the apparent and partial molar heat capacities, and also to define the critical micellar concentrations. The variations of the partial molar heat capacities by increment -CH2 allows us to give prove for products supposed to form true micelles and products do not form micelles. To discuss the solute-solvent and solutesolute interactions we used the standard apparent molar heat capacities.

Energetics of phase separation in aqueous solutions of poly(N-isopropylacrylamide)

The energetics of phase separation in dilute aqueous solutions of poly(N-isopropylacrylamide) is studied by high-sensitivity differential scanning calorimetry. The temperature dependences of the partial heat capacity of the polymer are obtained. The effect of the heating rate on their shape is examined. The concentration dependences of thermodynamic parameters of phase transition are determined. After phase separation of the system, the partial heat capacity of the polymer is much smaller than its partial heat capacity in the state of a swollen random coil. This finding indicates the occurrence of the polymer hydrophobic structure in the concentrated phase of the system probably in the form of clusters of monomer units. The profile of transition is described by the Schroder-van Laar equation with the van’t Hoff enthalpy independent of polymer concentration. The size of the cooperative unit of the ordered hydrate structure of the polymer is estimated and found to be coincident with the size of the Kuhn segment. High-velocity sedimentation measurements of the polymer are conducted at various temperatures both below and above the binodal curve of the system. It is shown that the sizes of poly(N-isopropylacrylamide) macromolecules before the phase transition temperature and in the diluted phase after phase separation of the system coincide. Thus, in the diluted phase, macromolecules retain the coil conformation.

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.

Properties and structure of aqueous urea up to the singular temperature of overcooled water: Isotopy effects

In equations for the partial volume, dilatability, heat capacity, and adiabatic compressibility of aqueous urea, the properties are “fitted” to the singular temperature Ts of overcooled water Ts = 227.15 K (R. J. Speedy, J. Phys. Chem., 91, 3354 (1987)). The equations for the characteristics of D2O-(ND2)2CO and T2O-(NT2)2CO systems were obtained by scaling the temperature to 6 and 9.4 K, respectively. The properties of infinitely dilute solutions did not reveal the sign reversal of the isotope effect. The isoconcentrates of the partial volumes and dilatability form wagging lines in the region of low temperatures. The temperature dependences of the properties have extrema and inflection points, while the isotope effects in solutions of finite concentrations experience sign reversal. The maximum density temperature (MDT) of the solution decreases as the urea concentration increases, and also on passing from D2O-(ND2)2CO to H2O-(NH2)2CO. Thus, for the 8m solution, the MDT is 259.6 K (protium system) and 266.7 K (deuterium system). The difference is almost equal to the shift of the MDT after the H2O → D2O transition (7.2 K).

Macromolecular Crowding Induces a Molten Globule State in the C-Terminal Domain of Histone H1

We studied the secondary structure of the C-terminal domains of the histone H1 subtypes H1° (C-H1°) and H1t (C-H1t) in the presence of macromolecular crowding agents (Ficoll 70 and PEG 6000) by IR spectroscopy. The carboxyl-terminal domain has little structure in aqueous solution but became extensively folded in the presence of crowding agents. In 30% PEG, C-H1° contained 19% α-helix, 28% β-sheet, 16% turns, and 31% open loops. Similar proportions were observed in 30% Ficoll 70 and for C-H1t in both crowding agents. The proportions of secondary structure motifs were comparable to those of the DNA-bound domain. Kratky plots of the small-angle x-ray scattering showed that in crowding agents the C-terminus had the compaction of a globular state. Progressive dissipation of the secondary structure and a linear increase in partial heat capacity with temperature together with increased binding of ANS indicated that the C-terminus is not cooperatively folded in crowded conditions. Native-like secondary structure and compactness in absence of folding cooperativity indicate that the C-terminus in crowding agents is in a molten globule state. Folding of the C-terminus in crowded conditions may increase the rate of the transition toward the DNA-bound state and facilitate H1 diffusion inside cell nuclei.

Heat capacity of aqueous 2-(2-hexyloxyethoxy)-ethanol solutions by DSC

The specific heat capacities of aqueous solutions of 2-(hexyloxyethoxy)ethanol (C6E2) have been measured from 2 to 55°C within the whole available composition range by DSC. Changes of specific, partial and apparent molar heat capacities of investigated system were analysed and considered as an effect of structural transformations. The two-point scaling theory was applied to the description of the binary mixtures properties in the pseudophases coexistence region.

Energetics of Cooperative Transitions of N‐Vinylcaprolactam Polymers in Aqueous Solutions

AbstractSummary: Phase and conformational behavior of poly(N‐vinylcaprolactam) was studied as a function of polymer concentration, concentration of NaCl, and concentration of sodium dodecyl sulfate (SDS). Precise measurements of the partial heat capacity of the polymer were first carried out within the temperature range 10–125 °C. Thermodynamic parameters of the phase separation transition (transition temperature, enthalpy, and heat capacity increment) do not change with variations in polymer concentration. In the presence of salt the transition temperature decreases, the enthalpy increases, and the heat capacity increment does not vary significantly. Beginning with the SDS concentration of 0.005 M, the phase separation is suppressed and the heat capacity peak splits into two peaks. Upon further increase in surfactant concentration the amplitude of the low‐temperature peak increases while that of the high‐temperature peak vanishes. Phase separation transition in aqueous solutions of N‐vinylcaprolactam‐methacrylic acid copolymers of different compositions were studied at different polymer concentrations and pH (4–12). The transition parameters do not depend on polymer concentration and pH.“Macroglobule” formed spontaneously as a result of the phase separation of a poly(N‐vinylcaprolactam) solution.image“Macroglobule” formed spontaneously as a result of the phase separation of a poly(N‐vinylcaprolactam) solution.

Partial energy fluctuations and negative heat capacities

We proceed to a critical examination of the method used in nuclear fragmentation to exhibit signals of negative heat capacity. We show that this method leads to unsatisfactory results when applied to a simple and well controlled model. These discrepancies are due to an incomplete evaluation of the internal potential energy of the fragments and to the neglect of their mutual interaction. This persists even at quite low densities.

Impact of protein denaturants and stabilizers on water structure.

It is of great interest to determine how solutes such as urea, sugars, guanidinium salts, and trimethylamine N-oxide affect the stability, solubility, and solvation of globular proteins. A key hypothesis in this field states that solutes affect protein stability indirectly by making or breaking water structure. We used a new technique, pressure perturbation calorimetry, to measure the temperature dependence of a solute's partial compressibility. Using fundamental thermodynamic relations, we converted these data to the pressure dependence of the partial heat capacity to examine the impact of protein stabilizing and denaturing solutes on water structure by applying the classic two-state mixture model for water. Contrary to widely held expectations, we found no correlation between a solute's impact on water structure and its effect on protein stability. Our results indicate that efforts to explain solute effects should focus on other hypotheses, including those based on preferential interaction and excluded volume.

Effects of Ligand Binding on Relative Stability of Subchain Conformations of Weakly Charged N-Isopropylacrylamide Gels in Swollen and Shrunken States

The thermotropic collapse of the hydrogel of copolymer N-isopropylacrylamide with acrylic acid (NIPA−AAc) upon heating has been studied by high-sensitivity differential scanning calorimetry. Curves of the polymer partial heat capacity have been measured at different pH (from pH 2 to 11) and different concentrations of cationic ligands: CaCl2, dodecyltrimethylammonium bromide (DTAB), and cetyltrimethylammonium bromide (CTAB). Dependencies of the main thermodynamic parameters of the gel collapsetemperature, enthalpy, and width of the transition on pH and ligand concentrationhave been obtained. It has been found that the collapse is characterized by a negative value of the heat capacity increment Δtcp. Its average value determined over all variables studied is Δtcp = −0.62 ± 0.02 J/(g K). This result is in accordance with the observed correlation between the transition enthalpy and temperature of the NIPA−AAc gel. The free energy of the collapse has been calculated as a function of pH and concentration of C...