Influence of Fluorosubstitution on the Heat Capacity of Aliphatic Alcohols

Heat capacity of controlled amounts of water in Vycor's 2 nm radius pores has been determined in real time during the course of water's isothermal nanoconfinement from bulk state at 358 K, by using temperature-modulated calorimetry. As water transfers from bulk to nanopores via the vapor phase, its heat capacity per molecule increases asymptotically toward a limiting value of 1.4 times the heat capacity of bulk water for 1.8 wt % water in Vycor and 1.04 times for 10.0 wt %. The observations indicate that vibrational and configurational contributions to the heat capacity are highest when the amount of water is insufficient to completely cover the pore wall, and they decrease as more water is present in the nanopores and water clusters form. The heat capacity of water in completely filled nanopores approaches the value for bulk water, thus indicating that the heat capacity varies with the water molecules' position in the nanopores.

Hydrogen bond strength depends on both temperature and pressure. The gradient for hydrogen bond strength with temperature, or pressure, depends upon the hydrogen bonded structure. These features create an intimate connection between quantum mechanics and thermodynamics in the structure of liquid water. The equilibrium structural model of liquid water developed from analysis of the heat capacity at constant pressure is complex. The model is based on the assumptions that: (i) the hydrogen bond length and molecular packing density of water both vary with temperature; (ii) the number of different geometries for hydrogen bonding is limited to a small set; (iii) water molecules that possess these hydrogen bonding geometries are in equilibrium with each other under static conditions; (iv) significant changes in the slope of the heat capacity, Cp, and to a lesser extent other properties of the liquid, reflect the onset of significant changes in the chemical structure of the liquid; (v) the partial molal enthalpies and entropies of the different water arrays generated from these building blocks differ from each other in their dependence upon temperature; and (vi) the structure of the liquid is a random structural network of the structural components. The equilibrium structural model for liquid water uses four structural components and the assumptions listed above. At the extrapolated-homogeneous nucleation temperature, 221 K, a disordered hexagonal-diamond lattice (tetrahedrally hydrogen bonded water clusters) is the structure of liquid water. At the homogeneous nucleation temperature, ∼238 K, liquid water is a mixture of disordered tetrahedral water arrays and pentagonal water arrays. The abundance of tetrahedral water structures at this temperature causes the system to self-nucleate. As the temperature increases to 266 K the proportion of disordered pentagonal water clusters in the equilibrium mixture increases. At 256 K, the temperature of the previously unrecognized maximum in the heat of fusion of water, “planar”-hexagonal water arrays appear in the liquid. At 273 K the concentration of tetrahedral hydrogen bonded water approaches zero. At the temperature of maximum density, 277 K, the liquid consists of a disordered dodecahedral-water lattice. The equivalence point between pentagonal and “planar”-hexagonal water arrays occurs near 291 K, the approximate temperature of minimum solubility of large hydrocarbons in water. At temperatures above 307.6 K, the minimum in Cp, square water arrays first appear in significant concentrations. Pentagonal water arrays become insignificant in the liquid at the temperature of minimum isothermal compressibility, ∼319 K. The equilibrium point between “planar”-hexagonal and square water arrays occurs near 337 K. As the temperature increases the liquid structure becomes dominated by disordered cubic arrays of water molecules. Structures with fewer than four hydrogen bonds per water molecule appear in the liquid near 433 K. “Planar”-hexagonal clusters are no longer present in the liquid at the temperature of the maximum dissociation constant for water, 513 K. These views are certainly oversimplified. Simple models for density are introduced. A model for viscoscosity based on the variation of hydrogen bond strength with temperature is introduced. Attempts to model density, heat capacity, or other thermodynamic properties of liquid water, using only two functions will not capture the subtle complexity of the equilibrium process. The equilibrium structural model of water has the potential to provide a basis for quantitative descriptions of the liquid’s seeming anomalies.

Isobaric heat capacity for water shows a rather strong anomalous behavior, especially at low temperature. However, almost all experimental studies supporting this statement have been carried out at low pressure; very few experimental data were reported above 100 MPa. In order to explore the behavior of this magnitude for water up to 500 MPa, a new heat flux calorimeter was developed. With the aim of testing the experimental methodology and comparing with water results, isobaric heat capacity was also measured for methanol and hexane. Good agreement with indirect heat capacity estimations from the literature was obtained for the three liquids. Experimental results show large anomalies in water heat capacity. This is especially true as regards its temperature dependence, qualitatively different from that observed for other liquids. Heat capacity versus temperature curves show minima for most studied isobars, whose location decreases with the pressure up to around 100 MPa but increases at higher pressures.

The isobaric molar heat capacities as a function of temperature and pressure are, for the first time, reported for a set of imidazolium-based ionic liquids. The selected compounds belong to the 1-alkyl-3-methylimidazolium series, concretely, [Emim][BF4], [Bmim][BF4], [Hmim][BF4], and [Omim][BF4]. Isobaric heat capacity were determined in the temperature and pressure intervals of (283.15 to 323.15) K and (0.1 to 60) MPa using a micro DSCII calorimeter recently adapted to work at high pressure. The data at atmospheric pressure were compared with literature data; as a rule, good results were obtained for all liquids. Isobaric molar heat capacities for the studied ionic liquids show quite a different behavior than that obtained for usual organic solvents, which could be understood as these compounds behave like highly compressed molecular fluids. This is consistent with the behavior of other previously reported thermodynamic properties.

The heat capacity and the permittivity of multiferroics Bi1 − xGdxFeO3 (x = 0, 0.05, 0.10, 0.15, 0.20) have been studied in the temperature range 130–800 K. It has been found that insignificant substitution of gadolinium for bismuth markedly shifts the temperature of antiferromagnetic phase transition and increases the heat capacity over a wide temperature range. It has been shown that the temperature dependence of the excess heat capacity is due to the manifestation of three-level states. Additional anomalies characteristic of the phase transitions have been revealed in the temperature dependences of the heat capacity for the compositions with x = 0.1 and 0.15 at T ≈ 680 K and T ≈ 430 K, respectively. The results of studies of the heat capacity have been discussed simultaneously with the data of structural studies.

The heat capacity and the permittivity of multiferroics Bi1 – xErxFeO3 (x = 0, 0.10, and 0.15) have been studied in the temperature range 130–800 K. It is found that an insignificant substitution of erbium for bismuth significantly increases the heat capacity in a wide temperature range T > 300 K. The temperature dependence of the excess heat capacity is shown to be due to manifestation of the three-level states. An additional anomaly characteristic of a phase transition has been revealed in the temperature dependences of the heat capacity and the permittivity for the compositions with x = 0.15 at T = 587 K. The results of studies are discussed in combination of the data of structural studies.

The heat capacity and the permittivity of multiferroics Bi_1 – x Er_ x FeO_3 ( x = 0, 0.10, and 0.15) have been studied in the temperature range 130–800 K. It is found that an insignificant substitution of erbium for bismuth significantly increases the heat capacity in a wide temperature range T > 300 K. The temperature dependence of the excess heat capacity is shown to be due to manifestation of the three-level states. An additional anomaly characteristic of a phase transition has been revealed in the temperature dependences of the heat capacity and the permittivity for the compositions with x = 0.15 at T = 587 K. The results of studies are discussed in combination of the data of structural studies.

The temperature dependence of the molar heat capacity of erbium tetraboride in the range 2-300 K has been experimentally studied. Anomalies were found on the temperature dependence of the heat capacity due to the processes of transition to antiferromagnetic state at ТN=15.2 K. Using the received data of temperature dependence of heat capacity, temperature changes of magnetic, lattice, electronic and Schottky contributions to the heat capacity of ErB4 have been determined and analyzed. The lattice contribution to the heat capacity of erbium tetraboride was calculated by the correspondence method by comparison with the lattice heat capacity of LuB4; the temperature dependence of the excess component of the heat capacity Δc(T) was obtained by subtracting the lattice and electron contributions from the total heat capacity. In our opinion, the value of Δc(T) together with the magnetic contribution contains one more component - the Schottky contribution to the heat capacity. The temperature dependences of the magnetic component and the Schottky contribution to the heat capacity extracted from the total heat capacity allowed us to determine the value of the exchange integral J (or the exchange parameter J/k).

Modeling heat capacity of saturated hydrocarbon in liquid phase over a wide range of temperature and pressure

The isobaric and isochoric heat capacities of seven 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides, two 1-alkyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imides, and two bis(1-alkyl-3-methylimidazolium) tetrathiocyanatocobaltates were determined at atmospheric pressure in the temperature range from 293.15 to 323.15 K. The isobaric heat capacities were determined by means of differential scanning calorimetry, whereas isochoric heat capacities were determined along with isothermal compressibilities indirectly by means of the acoustic method from the speed of sound and density measurements. Based on the experimental data, as expected, the isobaric heat capacity increases linearly with increasing alkyl chain length in the cation of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides and no odd and even carbon number effect is observed. After critical comparison of the obtained data with the available literature data, the most reliable values are recommended. It has been also...

The heat capacities of water swollen poly[2‐(2‐hydroxyethoxy)ethyl methacrylate] were determined in a DSC‐2 calorimeter within the temperature range 220–350K for concentrations from 0 to 1,2g of water per 1g of the polymer. At temperatures above 0°C the partial specific heat capacity of water in gel is concentration independent and equal to the specific heat capacity of pure liquid water. It seems, therefore, that water does not form stable icelike structures near polymer chains. To analyze phase transformations of water in gel below 0°C, general thermodynamic equations were derived and used as a basis for suggesting criteria which allow to decide whether in a given experiment the phase transformation proceeded in an equilibrium way. In measurements below the melting point of ice the conditions for an equilibrium process consist in the preceding heating of the frozen sample to a temperature close to the melting point, followed by cooling to 220K. The assessed composition dependence of the melting point depression is consistent with the dependence of activity on concentration obtained from measurements of water vapour sorption at 35°C. Analysis of data on the heat capacity below 0°C led to the conclusion that at a water content of 0,4g/g and lower, or at temperatures below 250K the crystallization of water from gel was inhibited by kinetic factors originating probably in the reduced diffusivity of water in gel, due to the reduced mobility of polymer chains. Hence, non‐freezing water need not be identical with “strongly bound” water; in the study of water structure in polymers based on heat capacities, preference should be given to data obtained at usual and elevated temperatures.

Isobaric heat capacity and structure of water and heavy water in the liquid state

Design and optimization of various thermodynamic processes that are used in chemical industries such as low- temperature refrigeration and cryogenic cycles, requires computer software to simulate the thermodynamic cycle and examine the effect of various parameters on the performance of the cycle. So the existence of an equation of state to predict thermodynamic properties of working fluids in a wide range of temperatures and pressures is essential. Despite the greater complexity than the other equations, the fundamental equations of state have higher accuracy in the calculation of the thermodynamic properties and the main advantage of them is that other thermodynamic properties can be obtained with high accuracy by differentiating them. In this study, the thermodynamic properties of Helium and Neon as working fluids in cryogenic processes including density, internal energy, enthalpy, entropy, isobaric specific heat capacity, and isochoric heat capacity, have been calculated based on fundamental equations of state using a developed computer code in MATLAB software. By comparing the results with the valid reference data, a range of temperature and pressure in which the fundamental equations of state can be used with high accuracy, have been presented. In this high accuracy region, the maximum error is related to the isobaric heat capacity that is 3.37 % and 1.1 % respectively for Helium and Neon.

Measurements for isobaric specific heat capacity of ethyl fluoride (HFC-161) in liquid and vapor phase

We examine the SPCE [H. J. C. Berendsen et al., J. Chem. Phys. 91, 6269 (1987)] and TIP5P [M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys 112, 8910 (2000)] water models using a temperature series of molecular-dynamics simulations in order to study heat-capacity effects associated with the hydrophobic hydration and interaction of xenon particles. The temperature interval between 275 and 375 K along the 0.1-MPa isobar is studied. For all investigated models and state points we calculate the excess chemical potential for xenon employing the Widom particle insertion technique. The solvation enthalpy and excess heat capacity is obtained from the temperature dependence of the chemical potentials and, alternatively, directly by Ewald summation, as well as a reaction field based method. All three methods provide consistent results. In addition, the reaction field technique allows a separation of the solvation enthalpy into solute/solvent and solvent/solvent parts. We find that the solvent/solvent contribution to the excess heat capacity is dominating, being about one order of magnitude larger than the solute/solvent part. This observation is attributed to the enlarged heat capacity of the water molecules in the hydration shell. A detailed spatial analysis of the heat capacity of the water molecules around a pair of xenon particles at different separations reveals that even more enhanced heat capacity of the water located in the bisector plane between two adjacent xenon atoms is responsible for the maximum of the heat capacity found for the desolvation barrier distance, recently reported by Shimizu and Chan [J. Am. Chem. Soc. 123, 2083 (2001)]. The about 60% enlarged heat capacity of water in the concave part of the joint xenon-xenon hydration shell is the result of a counterplay of strengthened hydrogen bonds and an enhanced breaking of hydrogen bonds with increasing temperature. Differences between the two models with respect to the heat capacity in the xenon-xenon contact state are attributed to the different water model bulk heat capacities, and to the different spatial extension of the structure effect introduced by the hydrophobic particles. Similarities between the different states of water in the joint xenon-xenon hydration shell and the properties of stretched water are discussed.