Excess Heats Research Articles

Thermodynamics of 1-ethyl-3-methylimidazolium methylsulfate co-solvent mixtures using molecular dynamics simulation

In the present work, isothermal-isobaric (NPT) and canonical (NVT) ensemble molecular dynamics (MD) simulations have been used to calculate the binary mixture properties (excess molar heats and volumes) of 1-ethyl-3-methylimidazolium methyl sulfate [EMIM][MeSO4] with six polar covalent molecules (acetic acid, acetone, chloroform, dimethyl sulfoxide, isopropyl alcohol and methanol) over the entire composition range. Excess partial molar properties as a function of mole fraction of co-solvent were calculated by using a Redlich-Kister expansion equation. The excess molar heats and volumes for [EMIM][MeSO4] with all the six co-solvents were all negative over the entire composition range. The size and structure of these molecules, as well as the nature of interaction between the mixture components seems to play a major role in the observed effects. These results have been interpreted in terms of hydrogen bonding and intermolecular interactions between ionic liquid ions and co-solvent molecules via radial distribution functions. The results showed that hydrogen bond interactions between ionic liquid and co-solvents play a major role in the volumetric and energetic properties of the mixtures. Further, radial distribution functions suggest that ionic liquid ions aggregate at higher concentrations of co-solvents.

Thermodynamics of Ionic Liquid Cosolvent Mixtures Using Molecular Dynamics Simulation: 1-Ethyl-3-methylimidazolium Acetate

Molecular dynamics (MD) simulations were conducted to calculate the binary mixture properties (excess molar heats and volumes) of 1-ethyl-3-methylimidazolium acetate [EMIM][Ac] with six polar covalent molecules (acetic acid, acetone, chloroform, dimethyl sulfoxide, isopropyl alcohol, and methanol) over the entire composition range. All of the binary mixtures of [EMIM][Ac] with cosolvents reported negative volumes of mixing over the entire composition range while releasing heat (exothermic heats of mixing), except for the acetone mixture, which had miscibility issues at higher concentrations (xacetone ≥ 0.6). These results were attributed to the nature, size, and structure of the molecules as well as the nature of the interaction between the mixture components. Further, aggregation of the ionic liquid (IL) at higher concentrations of the cosolvent added to the above observed effects, except for the acetic acid mixture. These results have been interpreted in terms of hydrogen-bonding and intermolecular inte...

Fluctuation theorems for excess and housekeeping heat for underdamped Langevin systems

We present a simple derivation of the integral fluctuation theorems for excess housekeeping heat for an underdamped Langevin system, without using the concept of dual dynamics. In conformity with the earlier results, we find that the fluctuation theorem for housekeeping heat holds when the steady state distributions are symmetric in velocity, whereas there is no such requirement for the excess heat. We first prove the integral fluctuation theorem for the excess heat, and then show that it naturally leads to the integral fluctuation theorem for housekeeping heat. We also derive the modified detailed fluctuation theorems for the excess and housekeeping heats.

Thermal contact through a diathermal wall: a solvable toy model

A diathermal wall between two heat baths at different temperatures can be mimicked by a layer of independent spin pairs with some internal energy and where each spin σa is flipped by thermostat a (a = 1,2). The transition rates are determined from the modified detailed balance. Generalized heat capacities, excess heats, the housekeeping entropy flow and the thermal conductivity in the steady state are calculated. The joint probability distribution of the heat cumulated exchanges at any time is computed explicitly. We obtain the large deviation function of heat transfer via a variety of approaches. In particular, by a saddle-point method performed accurately, we obtain the explicit expressions not only of the large deviation function, but also of the amplitude prefactor, in the long-time probability density for the heat current. The following physical properties are discussed: the effects of typical time scales of the mesoscopic dynamics which do not appear in equilibrium statistical averages and the limit of strict energy dissipation towards a thermostat when its temperature goes to zero.

Calorimetric and conductometric titrations of nanostructures of water molecules in iteratively filtered water

This work continue the study of the physico-chemical properties of samples of pure, twice distilled water, when subject to a procedure of iterative filtrations through Pyrex glass filters (Buchner funnels). After the filtrations, electrical conductivity and heat of mixing with NaOH and HCl solutions increase. The hypothesis is that the iterative filtration procedure, that involves a flux of energy and material in an open system, is able to induce the formation of “dissipative structures” or nanostructures of water molecules (WNS). Water exhibits an extraordinary auto-organization potentiality triggered by several kinds of perturbations, including mechanical ones. We measured the heats of mixing of acid or basic solutions with such iterated filtered waters (IFW) and their electrical conductivity, comparing with the analogous heats of mixing, electrical conductivity of the solvent. We found some relevant exothermic excess heats of mixing and higher conductivity than those of the untreated solvent. The heats of mixing and electrical conductivity of IFW show a good correlation, underlining a single cause for the behavior of the samples.

A Kirkwood-Buff Derived Force Field for Aqueous Alkali Halides

A classical nonpolarizable force field is presented for the simulation of aqueous alkali halide solutions (MX), where M = Li(+), Na(+), K(+), Rb(+), Cs(+) and X = F(-), Cl(-), Br(-), I(-), and their interactions with biomolecules. The models are specifically designed to reproduce the experimental Kirkwood-Buff integrals, and thereby the solution salt activities, as a function of salt concentration. Additionally, we demonstrate that these models reasonably reproduce other experimental properties including ion diffusion constants, dielectric decrements, and the excess heats of mixing. The parameters are developed by considering the properties of aqueous NaX and MCl solutions using a previously established model for NaCl. Transferability of the parameters to other salts is then established by the successful simulation of additional aqueous salt solutions, KI and CsBr, not originally included in the parameterization procedure.

Conductometric and calorimetric studies of the serially diluted and agitated solutions

We systematically analysed the experimental data related to the specific conductivities and heats in excess of several serially diluted and agitated solutions (SDA for short). For all of the analysed samples, we found that both the excess conductivity, χE (μS cm−1), and excess heat, QmixE (J kg−1), varied with the age of the sample (up to 2 years of ageing). Furthermore, we found that after a certain period of ageing, small volume samples exhibited a much higher excess than large volume ones. The results we report in this paper are the product of a systematic study, during which we operated on known and constant volumes across the life of the samples. The incidence of volume on χE and QmixE turned out to be overwhelming when compared with that of time. The temporal evolution of the smaller samples was found significantly higher than that of the larger volume ones. A careful numerical analysis of the results uncovered an extraordinary and unexpected correlation, of exponential kind, between the excess parameters and the volume of the solution in the container. As for the temporal evolution of these systems, we found that the measured excess heats and conductivity often reach a maximum. That led us to the conclusion that the temporal evolution of the physico-chemical parameters is not caused by the slow process of equilibrium attainment; on the contrary, these systems are far from equilibrium systems, dissipative structures, whose experimental behaviour is certainly due to the variation of the super-molecular structure of the solvent, water. The agitation phase during the preparation could be the trigger for the formation of dissipative structures and the emergence of the novel behaviour. We put forth a simple rationalizing hypothesis, based on the general idea of water as an auto-organizing system that, when elicited by even small perturbations, can enter a far from equilibrium state, sustained by the dissipation of the electromagnetic energy coming from the environment. (Dissipative Structures).

On the stability of extremely diluted aqueous solutions at high ionic strength

An extensive study has been carried out on extremely diluted aqueous solutions (EDS). These solutions revealed a really intriguing physico-chemical behaviour, characterized by multiple independent variables. Because of their behaviour, EDS can be described as far-from-equilibrium systems, capable of self-organization as a consequence of little perturbations. In this paper we investigate the stability of the calorimetric behaviour of EDS with a high ionic force, due to the presence of the sodium chloride electrolyte. We measured the excess heats of mixing of EDS with basic solutions, both with and without a high concentration of NaCl, and compared the results. In particular, we explored these concentrations: 0.5 and 1Mmol kg−1). The analysis of the experimental results shows that the calorimetric response of the EDS is stable when they are in a concentrated solution of NaCl. That is of great relevance for the eventual pharmacological action of these solutions, since it involves the interaction with fluids of complex chemical composition and high concentration.

Determination of the solubility parameter of cellulose acrylate using inverse gas chromatography

The solubility parameters of cellulose acrylate substituted degree 2.12 (CEA) have been calculated from the measured retention data by inverse gas chromatography at various temperatures. The weight fraction activity coefficients of the solvents at infinite dilution (Θ 1 ∞ ), the Flory-Huggins thermodynamic interaction parameters between CEA and solvents (χ 12 ∞ ), the excess molar heats of mixing (ΔH 1 s ), the partial molar heats of mixing at infinite dilution (ΔH 1 ∞ ), the solubility parameters of solvent (δ 1), and the solubility parameters of CEA (δ 2), were calculated at various temperatures. The δ 2 of the CEA was 17.32, 18.00, 18.13, 18.54, 19.39 at 55, 60, 65, 70 and 75°C, respectively.

New Physico-Chemical Properties of Extremely Diluted Aqueous Solutions

The extremely diluted solutions are anomalous solutions obtained through the iteration of two processes: a dilution 1:100 in mass and a succussion. The iteration is repeated until extreme dilutions are reached (less than 1·10-5mol kg-1) to the point that we may call the resulting solution an extremely diluted solution, namely the composition of the solution is identical to that of the solvent used (e.g. twice distilled water). We conducted thermodynamic and transport measurements of the solutions and of the interaction of those solutions with acids or bases. The purpose of this study is to obtain information about the influence of successive dilutions and succussions on the water structure of the solutions under study. We measured the heats of mixing of acid or basic solutions with such extremely diluted solutions, their electrical conductivity and pH, comparing with the analogous heats of mixing, electrical conductivity and pH of the solvent. We found some relevant exothermic excess heats of mixing, higher electrical conductivity and pH than those of the untreated solvent. The measurements show a good correlation between independent physico-chemical parameters. Care was taken to take into account the effect of chemical impurities deriving from the glass containers. Here we thus show that successive dilutions and succussions can permanently alter the physico-chemical properties of the water solvent. The nature of the phenomena here described still remains unexplained, nevertheless some significant experimental results were obtained.

Density functional theory calculations of local ordering of hydroxy groups and fluoride ions in hydroxyapatite

Electronic structure calculations based on density functional theory are employed to investigate the effect of substitutional fluoride ions on the local ordering of hydroxy groups in hydroxyapatite. The calculated structural parameters of the two end-member minerals are in good agreement with the experimental hexagonal structures. Electronic density contour plots show the apatite structure to be an ionic crystal, where the phosphate and hydroxy groups behave like polyanions. The calculations on hydroxyapatite identified the oxygen and hydrogen positions of the hydroxyl groups in the crystal structure to be well-defined, alternating in a column in the c-direction. Any disorder within the OH− columns carries an energetic cost of 0.22 eV per OH− but comparison with the fully ordered monoclinic phase of hydroxyapatite shows that ordering between the columns has no energetic advantage. We therefore predict that the experimentally found oxygen and hydrogen disorder is due to the presence in the crystal of differently oriented locally ordered domains, giving rise to the average partial occupancy which is found in crystallographic studies. Replacement of OH− groups in hydroxyapatite by F− ions from fluorapatite, is exo-thermic (−0.10 eV per OH− for a 33% replacement and −0.07 eV per OH− for a 50% substitution compared to the lowest energy hydroxyapatite structure). The 33% substitution of OH− groups by F− ions leads to a structure with reversion of the OH− alignment within the column and the presence of F− hence stabilises randomisation of the hydroxy groups in the columns. The negative calculated excess heats of solid solution of fluor- and hydroxyapatite (−7 kJ mol−1) are in agreement with the uptake of F− ions in hydroxyapatite in dental enamel.

Density Functional Theory Calculations of Solid Solutions of Fluor- and Chlorapatites

Electronic structure calculations based on density functional theory are employed to investigate the uptake and distribution of chloride ions into the fluorapatite structure. The calculated structural parameters of the two minerals are in good agreement with the experimental hexagonal structures. Electron density contour plots show the apatite structure to be an ionic crystal, where the phosphate groups behave as polyanions. The replacement of F- ions in fluorapatite by Cl- from gaseous Cl-2 is a highly endothermic process at all partial concentrations (similar to250 kJ mol(-1)), which is in line with the high stability and dominance of the fluorapatite form of these materials. The replacement of F- ions from chlorapatite with the formation of fluorapatite is much less endothermic (<8 kJ mol-1). The most stable structure with a F--to-Cl- ratio of 1:1 contains complete columns of F- and Cl- ions in the c direction. The low calculated excess heats of solid solution of chlor- and fluorapatite (<8 kJ mol-1) are in agreement with the widespread occurrence of Cl- in both geological and biological apatites, as well as with the ease of synthesis of mixed chlorfluorapatites, and indicate that Cl- could easily be absorbed into fluorapatite when both materials are present.

Excess Thermodynamic Properties of Chainlike Mixtures. 1. Predictions from the Soft−SAFT Equation of State and Molecular Simulation

Monte Carlo simulation and theory are used to calculate excess thermodynamic properties of binary mixtures of Lennard-Jones chains. Chainlike molecules are formed by Lennard-Jones spherical sites that are tangentially bonded. This molecular model accounts explictly for the most important microscopic features of real chainlike molecules, such as n-alkanes: repulsive and attractive forces between chemical groups and the connectivity of segments to make up the chain. A version of the statistical associating fluid theory, the so-called Soft−SAFT equation of state, is used to check the theory's ability to predict this kind of property. Predictions from the theory are directly compared to NPT Monte Carlo simulation results obtained in the present work. The influence of segment size, dispersive energy, and chain length on excess properties is studied using simulation and theory, and results are analyzed and discussed. The equation of state is then used to predict the general trends of some excess thermodynamic properties of real n-alkane binary mixtures, such as excess volumes and heats. In particular, the temperature and chain-length dependence of these properties is studied. The Soft−SAFT theory is found to be able to correctly describe the most important features of excess thermodynamic properties of n-alkane models.

Heat capacities of Tschermak substituted Fe-biotite

Six members of the annite–siderophyllite join were synthesized in a three step process – crystallization of biotite from gels, decomposition of the fine-grained biotite under oxidizing conditions and resynthesis of Fe-Al biotite with planned compositions from these products – producing biotite crystals with thicknesses of up to 10 μm. The biotite was characterized by microprobe, electron microscopy and X-ray diffraction. Heat capacities of these biotites were measured with a DSC (differential scanning calorimeter) over the temperature range 143 to 623 K. Using a least-squares technique, the data were fitted to a cp-polynomial, cp=k0+k1T−0.5+ k2T−2+k3T−3. In the temperature range 143 to 250 K, heat capacities of the different annite–siderophyllite members decrease linearly with increasing Al content. At higher temperatures, however, the cp-polynomial of biotites with intermediate composition (except Ann79Sid21) exhibit a steeper slope than those of other biotites. This produces positive excess heat capacities in the annite–siderophyllite join at higher temperatures. The activity-composition data of the same binary derived from phase equilibrium experiments (Benisek et al. 1996) and the data of this study suggest two compositional regions along this join, with different extent of deviation from ideality. One at XSid 0.3 showing a higher nonideality, resulting in a discontinuity visible at this composition. Powder IR-spectra of these solid solutions were measured with a FTIR-spectrometer and used to calculate heat capacities according to the vibrational model of Kieffer (1979). The comparison of the vibrational function with the cp-polynomials shows that the vibrational function reproduces well the DSC-data of the siderophyllite-poor and -rich members, but deviates for intermediate compositions, where the excess heats of mixing occur. With increasing Tschermak vector, the tetrahedral rotation angle α increases from 0 to 13° for annite to siderophyllite, respectively. At the composition of the discontinuity, this rotation angle α reaches a value of ∼8∘. The processing of ∼300 chemical data of natural biotites indicates that over 90% of them have a tetrahedral rotation angle that lies between 7 and 9°. It would appear that biotites with these structural characteristics are most stable.

Excess Specific Heats in Miscible Binary Blends with Specific Interactions

Excess specific heats of different binary blends involving different levels of relatively strong specific interactions have been determined from specific heats of the pure components and those of the blends. A heat-flux (Calvet) calorimeter and a step-by-step methodology have been used. Blends of poly(hydroxy ether of bisphenol A) (phenoxy resin) with polyesters and polyethers, where specific interactions are supposed to play a role in miscibility, and blends with stronger hydrogen-bond interactions, such as poly(vinyl phenol)/poly(methyl methacrylate) (PVPh/PMMA), have been included in the study. Only the experimental excess specific heat for the PVPh/PMMA blend is negative, whereas those relative to phenoxy blends are all positive. These results have been analyzed on the basis of an association model specifically designed for polymer blends in which miscibility is mainly caused by hydrogen bonding. The model predicts, in all cases, a continuous decrease of the enthalpy of mixing with temperature, i.e., a negative value of ΔCp. Equation of state effects have been included in the theoretical simulations, giving modified trends of the enthalpy of mixing with temperature in a more reasonable agreement with the experimental results.

Excess molar enthalpies of eight binary mixtures containing 1,2-epoxybutane + ethyl alkanoates at 298.15 K

Excess molar enthalpies, H E m, have been measured using a flow microcalorimeter at 298.15 K and at atmospheric pressure for the eight mixtures containing 1,2-epoxybutane with ethyl acetate, ethyl propanoate, ethyl butyrate, ethyl pentanoate, ethyl hexanoate, ethyl heptanoate, ethyl octanoate and ethyl decanoate. Excess heats of mixing, H E m, are positive for all the mixtures and increase with the chain length of the alkanoate, with the exception of ethyl acetate whose H E m values were between those of ethyl pentanoate and ethyl hexanoate. Results have been correlated in terms of the Redlich–Kister equation and qualitatively discussed in terms of molecular interactions.

Magainin 2 amide interaction with lipid membranes: calorimetric detection of peptide binding and pore formation.

The interaction of the antibiotic magainin 2 amide (M2a) with lipid bilayers was investigated with high-sensitivity titration calorimetry. The enthalpy of transfer of the cationic M2a to negatively charged small unilamellar vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) (75:25, mol/mol) was measured as delta H = -17.0 +/- 1 kcal/mol of peptide. The adsorption isotherm was determined by injecting lipid vesicles into peptide solutions at low peptide concentrations (cPo < 7 microM). The apparent partition coefficient was Kapp approximately 1.2 x 10(4) M-1 at a peptide equilibrium concentration of 1 microM but decreased with increasing peptide concentration. The hydrophobic partitioning of M2a into the lipid membrane is modulated by electrostatic effects that arise from the attraction of the positively charged peptide to the negatively charged membrane. Using the Gouy-Chapman theory to correct for electrostatic attraction, the experimental binding isotherms can be explained with an intrinsic (hydrophobic) partition coefficient of K = 55 +/- 5 M-1 and an effective peptide charge of z = 3.7-3.8. The free energy of binding is delta G = -4.8 kcal/mol. At peptide concentrations cPo > approximately 7 microM, a second effect comes into play, and the titration enthalpies can no longer be explained exclusively by peptide partitioning. The first few injections produce enthalpies of reaction which are distinctly smaller than expected from a pure partition equilibrium, followed by a series of injections with reaction heats larger than expected. After subtracting the enthalpic contribution due to partitioning, the residual enthalpies are endothermic for the first few injections, and exothermic for the consecutive steps. Furthermore, the endothermic excess heat is compensated exactly by the exothermic excess heat; i.e., the excess heat consumed in the first part of the titration experiment is returned during the second part. Endothermic excess enthalpies are observed for total molar peptide-to-lipid ratios of P/L > approximately 3.0%, whereas exothermic excess heats were seen for 0.7% < P/L < 3.0%. Below P/L < approximately 0.7%, the binding follows the partition equilibrium. Based on earlier spectroscopic evidence, it is suggested that magainin 2 amide binds to the lipid membrane and forms pores at high peptide-to-lipid ratio, this process being characterized by an endothermic reaction enthalpy. Pore formation is reversed with increasing lipid concentration, and the peptide pores disintegrate. The limiting peptide-to-lipid ratio deduced from titration calorimetry for M2a pore formation is in excellent agreement with spectroscopic methods. The enthalpy of pore formation amounts to delta H = +6.2 +/- 1.6 kcal/mol peptide or delta H approximately 25-45 kcal/mol pore if the pore is comprised of 4-7 peptide molecules.

Surface Evaluation of Alumina Powders by Cryogenic Specific Heat Measurements

Cryogenic specific heats are measured for the evaluation of the surface state of alumina powders. Specific heats of commercial powders are measured in a temperature range between 15K and 50K. For specific heat measurements, powder particles are covered by Apiezon N grease to increase the thermal conduction of powder compacted bodies. Excess specific heats are observed below 30K for each sample. For the commercial powders, the total entropy becomes higher as the specific surface area increases. The specific surface entropies of commercial powders are higher than those of porous samples. A possible reason of the decrease of entropy upon sintering process is the phase transformation of hydroxide alumina on the surface of commercial powders to other phases like the α-phase by sintering process. The weight change by the heat treatment at 1200°C is in agreement with the calculated weight change for the phase transformation, assuming that the surface thickness is about 1nm. The high entropy of commercial powders may be due to the presence of hydroxide alumina which exists on the surface of the alumina crystals and is not detectable by X-ray diffraction.

Adsorption phenomena at high pressures and temperatures 5. Heats of excess and total adsorption of krypton on zeolite NaA

Thermodynamic principles for the calculation of differential heats of excess and absolute adsorption were considered. A set of isosteres of excess adsorption of krypton on zeolite NaA are presented, from which the coverage and temperature dependences of the heats of excess adsorption are calculated and analyzed. The reasons for infinitely high values of the excess heats at finite values of adsorption are discussed. The problems of recalculation of the excess adsorption to absolute adsorption are considered.

Excess heat of mixing of pyridine bases with toluene

The excess heats of mixing of pyridine bases with toluene at 298.15 K were measured, and the Prigogine-Flory-Patterson theory was applied to describe the experimental results.