The aromatic amino acid behaviour in aqueous amide solutions

The enthalpies of solution of L-phenylalanine in the mixtures of water with the protein denaturant urea have been measured in the temperature range of 288.15–318.15 K. Using the results of the present research and literature data of free energies, the standard thermodynamic functions of the solute transfer from water to aqueous urea solutions have been estimated in a wide temperature range. The enthalpic, heat capacity, entropic and free energy parameters of the solute-urea pair and triplet interactions have been computed. The amino acid — amide pair interaction was found to be attractive in the temperature range studied due to the favourable enthalpic term. The triplet interaction being slightly repulsive reveals the enthalpic origin also. The examination of the Savage and Wood additivity-of-groups approach does indicate the inapplicability of this scheme to enthalpies and entropies of interaction. It has been found for the first time that the heat capacity of interaction changes its sign at 303 K, i.e. the temperature dependence of enthalpic and entropic parameters passes through the pronounced extrema near the temperature of the minimum of the heat capacity of pure water.     

粒度对纳米体系化学反应热力学性质的影响

为了研究纳米粒子的粒度对化学反应热力学性质的影响规律, 以球形原子簇来模拟纳米金刚石颗粒, 用量子化学方法对粒度不同的金刚石纳米粒子与氧气反应的热力学性质进行了计算. 结果表明, 粒度对多相反应的标准摩尔反应焓△rH0m、标准摩尔反应熵△rS0m、标准摩尔反应吉布斯函数△rG0m和标准平衡常数K0均有明显的影响, 随着反应物粒径的减小, △rH0m、△rS0m和△rG0m均降低,而K0增大. 这些影响规律与实验结果基本一致.     

Quick method for determination of equilibrium temperature of calcium carbonate dissociation

A quick method for determination of equilibrium temperature of high temperature reactions, which is inexpensive and suitable for small industrial laboratories, is reported. Reaction of high temperature dissociation of calcium carbonate is used as example for quick method application. The method is based on calorimetric measurement, Hess Law, and thermodynamic calculations. The calibrated calorimeter is used to determine enthalpy change for reactions CaCO₃(s)?+?HCl(l), Ca(s)?+?2HCl(l), and CaO(s)?+?2HCl(l). By application of Hess??s energy cycle, enthalpies of formation of calcium carbonate and calcium oxide were determined. Acquired results were used to calculate enthalpy change for carbonate dissociation reaction. Calculated enthalpy change value was used for free energy change in dependence of temperature and also for equilibrium constant in dependence of temperature calculation using equations derived from basic thermodynamic equations. Using this method, equilibrium temperature for calcium carbonate dissociation reaction is found to be equal to 1154.14?K, which confirms accuracy of the method.     

SURFACE ENTHALPY AND ENTROPY AND THE PHYSICO-CHEMICAL NATURE OF HYDROPHOBIC AND HYDROPHILIC INTERACTIONS

The attractive Interactions between typically hydrophobic molecules such as hexane or CCl₄, and the repulsive Interactions between extremely hydrophilic molecules such as poly(ethylene oxide) (PEO), when immersed in water, as well as the interactions between these molecules and water, have been examined from a surface thermodynamic viewpoint, taking the changes in surface free energy into account, as a function of temperature. It was found that attractive hydrophobic Interactions are not, as vas generally believed up to now, invariably entropic. Hydrophobic Interactions can be mainly enthalpic or mainly entropic, or more or less equal mixtures of both, depending on each individual case; however, all hydrophobic interactions are polar (in the sense of Lewis acid-base) in nature. Repulsive hydrophilic interactions are enthalpic, and also polar in nature. The interaction between hydrophobic solutes and water is mainly enthalpic, and is apolar in nature.     

Extrathermodynamic Study of Retention Equilibrium in RP-LC Using a C18-Silica Monolithic Stationary Phase

This study is concerned with the explanation of some thermodynamic properties of the retention equilibrium on a C₁₈-silica monolithic column. Pulse response experiments were carried out in a reversed-phase liquid chromatography system using a methanol/water mixture (70/30, v/v) and n-alkylbenzene homologs as the mobile phase and sample compounds, respectively, in the temperature range between 278 and 318 K. The retention equilibrium constant (K _a) was calculated from the first absolute moment of elution peaks. The dependence of K _a on the column temperature was analyzed using the modified van??t Hoff plot proposed by Krug et al. to derive the changes of the Gibbs free energy, the enthalpy and the entropy concerning the retention behavior. First, the presence of a real enthalpy?Centropy compensation (EEC) for the retention equilibrium was demonstrated. Then, a thermodynamic model based on the real EEC was developed to explain the temperature dependence of the linear free energy relationship (LFER) of the retention equilibrium. The model indicates how the slope and intercept of the LFER are correlated with the compensation temperatures and several molecular thermodynamic parameters. The model was effective for explaining the thermodynamic properties of the retention equilibrium of the C₁₈-silica monolithic stationary phase.     

ON THE PHENOMENOLOGICAL THERMODYNAMICS OF HYDROPHOBIC BONDING

This paper is a review of the points of view of Frank and Evans, Shinoda and the author regarding the hydrophobic hydration of hydrocarbon molecules, with the emphasis on the contribution of the author. It is demonstrated that the enthalpic free energy change, due to the interactions between the hydrocarbon molecules and water, is compensated by the entropic free energy change, due to the ordering caused by the hydrocarbon molecules in the neighboring water molecules. Further, it is shown that the free energy change due to the iceberg formation is negative. Some simplifying assumptions make it possible to conclude that the absolute value of the free energy for iceberg formation can be as large as 1/3 of the free energy change associated with the formation of a cavity. The thermodynamic approach employed can also explain the existence of a minimum in the temperature dependence of the hydrocarbon solubility in water.     

Enthalpy-entropy contributions to the potential of mean force of nanoscopic hydrophobic solutes

Entropic and enthalpic contributions to the hydrophobic interaction between nanoscopic hydrophobic solutes, modeled as graphene plates in water, have been calculated using molecular dynamics simulations in the isothermal-isobaric (NPT) ensemble with free energy perturbation methodology. We find the stabilizing contribution to the free energy of association (contact pair formation) to be the favorable entropic part, the enthalpic contribution being highly unfavorable. The desolvation barrier is dominated by the unfavorable enthalpic contribution, despite a fairly large favorable entropic compensation. The enthalpic contributions, incorporating the Lennard-Jones solute-solvent terms, largely determine the stability of the solvent separated configuration. We decompose the enthalpy into a direct solute-solute term, the solute-solvent interactions, and the remainder that contains pressure-volume work as well as contributions due to solvent reorganization. The enthalpic contribution due to changes in water-water interactions arising from solvent reorganization around the solute molecules is shown to have major contribution in the solvent induced enthalpy change.     

Heat capacities and volumes of interaction between mixtures of alcohols in water at 298.15 K

The heat capacities and densities of mixtures of aqueous solutions of normal alcohols (methanol to n-butanol) and t-butanol were measured at 298.15 K at low molalities. The results were used to calculate the thermodynamic pair and triplet interaction parameters between solutes for heat capacities and volumes. The pair parameters are approximately a linear function of the total number of carbon atoms of the two solutes. The enthalpic pair and triplet interaction parameters for (ROH + H₂O) are also reported. The temperature dependence of the pair parameters for Gibbs free energies, enthalpies, entropies, heat capacities, and volumes are discussed in terms of structural changes in the aqueous solutions.     

Prediction of thermodynamic properties of polymer solutions by a group-contribution method

The Flory–Huggins lattice-theory expression for solvent activity in a polymer-solution is commonly used to calculate the thermodynamic interaction parameter χ with the aid of experimental data from vapor pressure osmometry. This expression assumes that χ is independent of composition. However, experimental data for a variety of polymer-solvent mixtures indicate that χ exhibits an appreciable concentration dependence. A group contribution method, UNIFAC (UNIQUAC Functional-Group Activity Coefficients) incorporating the free-volume correction of Oishi and Prausnitz is used to predict the dependence of χ on solvent concentration. Agreement with previously reported experimental data is within 15%. Calculated values of χ obtained from the Flory–Huggins expression for solvent activity and from the corresponding Gibbs free energy of mixing (which does not assume that χ is independent of composition) are compared. Calculations based on the Gibbs free energy of mixing predict a somewhat larger value of χ relative to those based on solvent activity. The specific Gibbs free energy of mixing for polystyrene-solvent mixtures is calculated using the UNIFAC model, and is found to represent qualitatively the phase equilibrium behavior. Quantitative discrepancies are observed, however, for the polystyrene-acetone system in light of the actual experimental solubility reported by Suh and Clark (20). Most of the thermodynamic predictions for polymer-solvent systems investigated herein are correlated qualitatively with the relative mismatch between solubility parameters of both components.     

Thermodynamics of water sorption in acrylic homonetworks and IPNs

The equilibrium thermodynamic properties of poly(hydroxyethyl acrylate) and poly(ethyl acrylate)-i-poly(hydroxyethyl acrylate) hydrogels are investigated starting from the water sorption isotherms of the systems. Partial enthalpy and entropy of the sorbed water in the gel differ markedly from the values of pure water at the lowest water contents, and tend to those of liquid water as saturation is approached. The residual mixing free energy is calculated, as a means of assessing the intensity of the water-polymer interaction. Its small positive magnitude shows that water-polymer hydrogen bonds are labile compared to water-water and polymer-polymer hydrogen bonds, and thus the stability of the gel state is still mainly due to the combinatorial entropic contribution to the mixing free energy. An equation correctly describing the sorption isotherms, when combined with the thermodynamic equations, can deliver the true water-polymer interaction parameter and its dependence on the polymer volume fraction in the gel.     

Estimating the temperature dependence of peptide folding entropies and free enthalpies from total energies in molecular dynamics simulations

The temperature dependence of thermodynamic quantities, such as heat capacity, entropy and free enthalpy, may be obtained by using well-known equations that relate these quantities to the enthalpy of the molecular system of interest at a range of temperatures. In turn, the enthalpy of a molecular system can be estimated from molecular dynamics simulations of an appropriate model. To demonstrate this, we have investigated the temperature dependence of the enthalpy, heat capacity, entropy and free enthalpy of a system that consists of a beta-heptapeptide in methanol and have used the statistical mechanics relationships to describe the thermodynamics of the folding/unfolding equilibrium of the peptide. The results illustrate the power of current molecular simulation force fields and techniques in establishing the link between thermodynamic quantities and conformational distributions.     

Kinetic and thermodynamic assessments of the mediator-template assembly of nanoparticles

The understanding of kinetic and thermodynamic factors governing the assembly of nanoparticles is important for the design and control of functional nanostructures. This paper describes a study of the kinetic and thermodynamic factors governing the mediator-template assembly of gold nanoparticles into spherical assemblies in solutions. The study is based on spectrophotometric measurements of the surface plasmon (SP) resonance optical property. Gold nanoparticle cores ( approximately 5 nm) encapsulated with tetraoctylammonium bromide shells were studied as a model system. The mediator-template assembly involves a thioether-based multidentate ligand (e.g., MeSi(CH2SMe)3) which functions as a mediator, whereas the tetraoctylammonium bromide capping molecules function as template agents. On the basis of the temperature dependence of the SP optical property in the mediator-template assembly process, the kinetic and thermodynamic parameters such as the reaction rate constant and reaction enthalpy have been determined. The results led to two important findings. First, the mediator-template assembly of nanoparticles is an enthalpy-driven process. Second, the enthalpy change (-1.3 kcal/mol) is close to the magnitude of the van der Waals interaction energy for alkyl chains and the condensation energy of hydrocarbons. Implications of the findings to the understanding of the interparticle interactions have also been discussed.     

Surface tension of aqueous electrolyte solutions. Thermodynamics

A thermodynamic theory is developed for obtaining the enthalpic and entropic contributions to the surface excess Gibbs energy of electrolyte solutions from the dependence of the surface tension on concentration and temperature. For elaboration, accurate activity coefficients in solution as functions of concentration and temperature are required. The theory is elaborated for (1-1) electrolytes and applied to HClO(4), HNO(3), NaCl, NaBr, and LiCl, of which the first two adsorb positively and the other three negatively. One of the conspicuous outcomes is that in all cases, the surface excess entropies slightly decrease with electrolyte activity but remain close to that of pure water, whereas the enthalpy is different from that. The implication is that the driving force for positive or negative adsorption must have an enthalpic origin. This finding can be useful in developing and evaluating theoretical models for the interpretation of surface tensions of electrolyte solutions.     

Surface thermodynamics reveals selective structural information storage capacity of c-Fos-phospholipid interactions

The surface properties of c-Fos, a regulator of normal and pathologic cell growth and a modulator of phospholipid metabolism, suggest that it has the potential to transduce information through molecular reorganization, placing the nature of its interaction with phospholipids at the basis of its possible effects at the membrane level. Previous studies established that c-Fos induces condensation and depolarization of PIP2 films and expansion and hyperpolarization of PC. We have now explored more in depth the thermodynamic aspects of these lipid-protein interactions, finding that the mixtures have associated hysteresis. The analysis of the excess thermodynamic functions provides evidence of entropic-enthalpic compensations that result in a favorable enthalpic contribution derived from the interaction of c-Fos with PIP2, which exceeds the unfavorable configurational entropy. On the contrary, favorable entropy terms dominate the interaction of c-Fos with PC over the unfavorable enthalpy. The free energy of hysteresis is stored as excess free energy. A shift in molecular packing-dependent surface reorganization, compared to that of ideally mixed films, indicates a gain in information content at the lipid-protein interface in mixed films of c-Fos with PIP2 but not with PC. It is postulated that the free energy stored in these mixtures could act as a bidirectional structural information transducer for dynamic compression-expansion processes occurring on the membrane surface.     

Perturbation of solute transport at a liquid-liquid interface by polyethylene glycol (PEG): implications for PEG-induced biomembrane fusion

The effect of polyethylene glycol 400 (PEG) dissolved at various concentrations (0-40% v/v) in water, on the interfacial transport of methyl nicotinate across an aqueous-isopropyl myristate interface was investigated with a rotating diffusion cell. At four temperatures studied (20-37 °C), the presence of PEG decreased the rate of solute transfer both into and out of the organic phase in a concentration-dependent fashion. The bulk partition coefficient of the solute (organic/aqueous) increased with increasing PEG in the aqueous phase. Analysis of the temperature dependence of the interfacial transfer kinetics allowed thermodynamic activation energy parameters for the phase transport process to be determined. Although the free energy of activation (ΔG(≠)) for transfer was not affected by PEG, the relative enthalpic and entropic contributions were dramatically altered. At PEG concentrations of 10-40% v/v the enthalpic portion of ΔG(≠) was decreased by about a factor of two, while the entropic contribution (which is large and positively favorable in the absence of PEG) was reduced considerable such that it was totally eliminated at higher PEG levels. These observations suggest novel and direct experimental evidence for the concept that high PEG concentrations substantially alter water structure at an aqueous solution-organic liquid biomembrane model interface. The results support the hypothesis that the critically important function of PEG in inducing cell-cell and liposome-liposome fusion is to remove the hydration layer that impedes the close apposition of converging phospholipid bilayers.     

Comparative adsorption studies of indigo carmine dye on chitin and chitosan

The adsorption of indigo carmine dye onto chitin and chitosan from aqueous solutions was followed in a batch system. The ability of these materials to adsorb indigo carmine dye from aqueous solution was followed through a series of adsorption isotherms adjusted to a modified Langmuir equation. The maximum number of moles adsorbed was 1.24 +/- 0.16 x 10(-5) and 1.54 +/- 0.03 x 10(-4) mol g(-1) for chitin and chitosan, respectively. The same interactions were calorimetrically followed and the thermodynamic data showed exothermic enthalpic values of -40.12 +/- 3.52 and -29.25 +/- 1.93 kJ mol(-1) for chitin and chitosan, respectively. Gibbs free energies for the two adsorption processes of indigo carmine dye presented a positive value for chitin and a negative one for chitosan, reflecting that dye/surface interactions are thermodynamic favorable for chitosan and nonspontaneous for chitin at 298.15 K. The interaction processes were accompanied by an increase of entropy value for chitosan (90 +/- 6 J mol(-1)K(-1)) and a decrease for chitin (-145 +/- 13 J mol(-1)K(-1)). Thus, dye/chitosan interaction showed favorable enthalpic and entropic processes, reflecting thermodynamic stability of the formed complex, while dye/chitin interaction showed an exothermic enthalpic value and a highly nonfavorable entropic effect, resulting in a nonspontaneous thermodynamic system.     

Theoretical Studies on the Molecular Structures and Thermodynamic Properties of Polychlorinated Acenaphthylenes

Geometric structures of 135 polychlorinated acenaphthylene (PCAC) molecules were optimized using density functional theory (DFT) at the B3LYP/6-311G** level and some thermodynamic properties of them in the ideal gas state were calculated. The relations of these thermodynamic properties with the number and position of chlorine atoms were also explored, from which the relative stability of PCAC congeners was theoretically proposed according to the magnitude of the relative standard Gibbs free energy of formation (△r,fGθ). The results show that all PCAC isomers have planar geometric configuration. There exists intramolecular Cl···Cl weak interaction in some PCAC molecules. The change of △fHθ and fGθ of most stable PCAC isomers with increasing the number of chlorine atoms is different from that in the least stable PCAC congeners. The values of fHθ and fGθ for PCAC isomers with the same number of chlorine atoms show a strong dependence on the position of chlorine atoms and the relative stability of PCAC congeners has close relation with the intramolecular Cl···Cl nuclear repulsive interaction.     

Spectroscopic and kinetic studies on the interaction of ketoconazole and povidone drugs with DDQ

The kinetics and mechanism of the interaction between 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and ketoconazole and povidone drugs has been investigated spectroscopically. In the presence of large excess of donor, the 1:1 CT complex is transformed into a final product, which has been isolated and characterized by FT-IR and GC-MS techniques. The rate of formation of product has been measured as a function of time in different solvents at three temperatures. The thermodynamic parameters, viz. activation energy, enthalpy, entropy and free energy of activation were computed from temperature dependence of rate constants. Based on the spectro-kinetic results a plausible mechanism for the formation of the complex and its transformation into final product is presented and discussed.     

Conductometric studies of hexadecyltrimethylammonium bromide in aqueous solutions of ethanol and ethylene glycol

The effect of cosolvent on micellization of hexadecyltrimethyl ammonium bromide (CTAB) in aqueous solutions was studied. The conductivity of a mixture (cosolvent + water) as function of CTAB concentration was measured at different temperatures. Ethylene glycol and ethanol were used as a cosolvent. The conductivity data were used to determine the critical micelle concentration (CMC) and the effective degree of counterion dissociation of micelle in the temperature range 303.2 to 313.2 K. In all the cases studied, a linear relationship between log([CMC]_mix/mol dm⁻³) and the mass fraction of cosolvent in solvent mixture has been observed. The free energy (ΔG _mic ⁰ ), enthalpy (ΔH _mic ⁰ ), and entropy (ΔS _mic ⁰ ) of micellization were determined using the temperature dependence of CMC. The dependence of these thermodynamic parameters on solvent composition was determined. The standard free energy of micellization was found to be negative in all cases and becomes less negative as the cosolvent content increases. The enthalpy and entropy of micellization are independent of temperature in pure water, while ΔH _mic ⁰ and ΔS _mic ⁰ decrease dramatically with temperature in mixed cosolvents. Furthermore, the entropic contribution is larger than the enthalpic one in pure water, while in the mixed solvents, the enthalpic contribution predominates. The text was submitted by the authors in English.