Thermodynamic properties of some N-alkyl-N-methylpiperidinium chlorides and N-alkylpiperidine hydrochlorides in water

Densities and heat capacities at 25°C were measured for N-octyl-, N-decyl- and N-dodecyl-N-methylpiperidinium chlorides and for N-octyl- and N-dodecylpiperidine hydrochlorides in water as functions of concentration. Enthalpies of dilution at 25°C and osmotic coefficients at 37°C of the N-methyl-N-alkylpiperidinium chlorides were also measured as functions of concentration. The partial molar volumes, heat capacities, relative enthalpies, nonideal Gibbs energies and entropies at 25°C were derived as functions of the surfactant concentration. By increasing the alkyl chain length of the surfactant, both the apparent molar volume vs. concentration curves are shifted toward greater values while the corresponding ones for the heat capacity are moved toward more negative values. These results are consistent with the higher hydrophobicity the longer the alkyl chain of the surfactant is. In the micellar region, the entropy and enthalpy vs. log m/m _cmc curves increase in a parallel manner by decreasing the alkyl chain length of the surfactant. Consequently, the negligible effect of the hydrophobicity of the surfactant on the Gibbs energy vs. log m/m _cmc trends is due to the enthalpy-entropy compensative effect. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. The absolute values of both the volume and heat capacity of micellization increase with an increasing number of carbon atoms in the alkyl chain (n _c ). The enthalpy and entropy of micellization vs. n _c are convex curves. Comparisons are also made between the present data and those of some alkylpyridinium chlorides reported elsewhere.     

Thermodynamic properties of N-octyl-, N-decyl- and N-dodecylpyridinium chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl-, N-decyl- and N-dodecyl-pyridinium chlorides in water over a wide concentration region. Conductivity measurements were performed in order to evaluate the cmc and the degree of counterion dissociation. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived from the experimental data as functions of the surfactant concentration. The changes with concentration of these properties are quite regular with the exception of the heat capacities which display anomalies at about 0.9, 0.25 and 0.12 mol-kg^–1 for the octyl, decyl and dodecyl compounds, respectively. At these concentrations there were also changes in the slopes of the specific conductivity and of the product of the osmotic coefficients and the molality vs. concentration. These peculiarities can be ascribed to micelle structural transitions. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data have been compared to those for alkyltrimethylammonium bromides and alkylnicotinamide chlorides. It is shown that the introduction of the hydrophilic CONH₂ group lowers the hydrophilic character of the pyridinium ring.     

Thermodynamic properties of N-octyl- and N-dodecylnicotinamide chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl- and N-dodecylnicotinamide chlorides in water over an extended concentration region. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived as a function of the surfactant concentration. For both surfactants, plots of volumes, enthalpies and free energies vs. concentration are regular whereas those of heat capacities and entropies present anomalies at about 0.8 and 0.1m for the octyl and dodecyl compounds, respectively. Changes in the slope of a plot of osmotic coefficients times molality vs. molality were also observed at these same concentrations. These peculiarities are ascribed to micelle structural transitions. The nonideal free energies do not seem to depend on the alkyl chain length when they are plotted vs. m/C _cmc . Also, a plot of the nonideal free energy vs. logm/C _cmc is roughly independent of the nature of the surfactant because of the constant activity of surfactants in micellar solutions. Nonideal free energies, enthalpies and entropies have been calculated at 15 and 35°C. At each concentration the nonideal free energy is temperature independent as a result of a compensatory effect between enthalpy and entropy. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data suggest that the nicotinamide group possesses less hydrophilic character than the ammonium group.     

Thermal and volumetric properties of chloroform + triethylamine mixtures and the ideal associated solution model of complex formation

In the ideal associated solution model, activity coefficients of all species (labelled A, B, and AB here) are taken to be unity at all mole fractions and all temperatures, with several derivative consequences that have not previously been investigated. We have applied this model to an analysis of the thermodynamic properties (vapor pressures, excess volumes, excess enthalpies, partial molar enthalpies of solution, and excess heat capacities) of the chloroform + triethylamine system in terms of K, V, H, and C _p for the equilibrium represented by A+B=AB. It is shown that there is good consistency between the relatively simple chemical ideal associated solution model and all available thermodynamic data except heat capacities, for which the consistency between model and data is less good. Some limitation of the ideal associated solution model are discussed, along with the relationship of this model to hydrogen bonding in the AB complex and to spectroscopic investigations. New thermal (partial molar enthalpies of solution and excess heat capacities) and volumetric data are presented.     

Thermodynamics ofN,N,N-octylpentyldimethyl-ammonium chloride in water-urea mixtures

Specific conductivities, densities, heat capacities, and enthalpies of dilution at 25‡C were measured forN,N,N-octylpentyldimethylammonium chloride (OPAC) in water-urea mixtures at various urea concentrations m_u as functions of the surfactant concentration m_s. From conductivity data, the cmc and the degree of the counterion dissociation Β of the OPAC micelles were calculated. The cmc increases linearly with increasingm _u while Βvs. m_u is a smooth concave curve. From the experimental thermodynamic data, the apparentY _Φ and partialY ₂ molar properties (volumes, heat capacities, and relative enthalpies) are derived as functions of m_u andm _s . The effect of urea on the dependences of the different properties on m_s are discussed. From data in the premicellar region the standard partial molar volumesV ₂ ⁰ and heat capacitiesC _p2 ⁰ were evaluated. It was observed thatV ₂ ⁰ increases linearly withm _u whileC _p2 ⁰ decreases. The properties of OPAC in the dispersed and micellized forms at the cmc were obtained and, therefore, the thermodynamic functions of micellization were calculated on the basis of the pseudo-phase transition model.     

Thermal and volumetric properties of chloroform+benzene mixtures and the ideal associated solution model of complex formation

In the ideal associated solution model, activity coefficients of all species (labelled A, B, and AB here) at equilibrium are taken to be unity at all compositions and temperatures. We have applied this model to an analysis of thermodynamic properties (vapor pressures, excess enthalpies, partial molar enthalpies of solution, excess heat capacities, and excess volumes) of the chloroform+benzene system in terms of K, H, C _p , and V for the equilibrium represented by A+B=AB. It is demonstrated that there is reasonably good consistency between this simple model and all of the thermodynamic data, which shows that the model is realistic enough to be useful in assessing the properties of the not-very-stable AB complex in the chloroform+benzene system. New thermal (partial molar enthalpies of solution and excess heat capacities) and volumetric properties of the chloroform+benzene system have been measured, with results presented here.     

Thermodynamic and19F NMR studies of antimony trifluoride in water

Densities, specific heat capacities per unit volume and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for antimony trifluoride in water as functions of concentration. From the first three properties the apparent and partial molar volumes, heat capacities and relative enthalpies were derived. As well, pH measurements in water at 25°C and¹⁹F NMR spectra in water and methanol at 33°C were also carried out. All the thermodynamic properties together with the chemical shifts abruptly change in the very dilute concentration region (<0.1m) and, then, tend to a constant value. These trends have been rationalized through a simple model based on an equilibrium of dissociation of SbF₃ into two ionic species. From the simulation of all the data it is derived that two concomitant equilibria are present in solution: the hydrolysis process of SbF₃ which explains the pH values and the ionic dissociation of SbF₃ which accounts for the¹⁹F NMR data.     

Enthalpies of dilution of aqueous electrolytes: The NaCl/H2O system

Analysis of enthalpy of dilution data for the system NaCl/H₂O at 298.15°K with correlating equations presently in widespread use shows that these do not satisfactorily fit the data at the lowest available concentrations. A new approach based on the extended Debye-Hückel theory is suggested. It is shown that a plot of the ratio of the apparent molar enthalpy of dilution to the change in ionic strength, L/I, versus (I _i ^1/2 –I _f ^1/2 )/I, should give a straight line at low enough concentration with a slope 2S _H/3. The intercept is related to the Debye distance parameterA and the coefficient of the first virial correction term. The quantityS _H is the limiting Debye slope as calculated from the properties of the pure solvent. These expectations are substantiated by the fits. Values of the parameters are compared with older estimates, and it is concluded that the choiceA=0 is reasonable.     

Thermodynamic properties and conductivities of some dodecylsurfactants in water

Densities, heat capacities, enthalpies of dilution, osmotic coefficients and conductivities are reported for dodecylamine hydrochloride, dodecyldimethylammonium and dodecyltrimethylammonium chloride in water over a wide range of concentration. The last two properties were also measured for dodecyltrimethylammonium bromide. From the thermodynamic data partial molar volumes, heat capacities and relative enthalpies and nonideal free energies and entropies were derived as a function of the surfactant concentration. The cmc's and degree of counterion dissociation were also calculated from the transport properties. It is shown that the trends of volumes, enthalpies, free energies and entropies are quite regular whereas heat capacities present maxima and minima at concentrations which depend on the nature of surfactants. Corresponding changes were observed in the osmotic coefficients and specific conductivities. The thermodynamic functions of micellization were evaluated on the basis of the pseudo-phase transition model. Finally, the effects of the introduction of methyl groups in the hydrophilic moiety of the surfactant and of the nature of the counterion on the thermodynamic properties of monomers and micelles are examined.     

Thermal and volumetric properties of chloroform + dimethylsulfoxide: Thermodynamic analysis using the ideal associated solution model

We have made new measurements of partial molar enthalpies of solution, excess heat capacities, and excess volumes of mixtures of chloroform and dimethylsulfoxide. In combination with published vapor pressures and excess enthalpies the results of these measurements have been analyzed within the context of the ideal associated solution model to yield C _p and V for the formation of AB and A₂B complexes. It is shown that the ideal associated solution model is consistent with all of the available thermodynamic data for this system, indicating that nearly all of the deviations of this system from ideal solution behavior can be accounted for in terms of chemical interactions of the two components.     

Partial molal heat capacities of tetraalkylammonium bromides in methanol from 10 to 50°C

Enthalpies of solution habe beenmeasured for tetraethyl-, tetra-n-propyl-, and tetra-n-butylammonium bromides in anhydrous methanol at several temperatures ranging from 5 to 55°C. The data were extrapolated to infinite dilution by an extended Debye-Hückel equation to obtain standard enthalpies of solution ΔH ₃ ⁰ , and the integral heat method was employed to obtain partial molal heat capacities , of the corresponding salts from 10 to 50°C. These data, along with similar data for tetramethylammonium bromide previously reported, were used to assign an, absolute heat capacity to the bromide ion in anhydrous methanol. Comparison of the absolute heat capacities of the bromide and tetraalkylammonium ions in water and methanol suggests that the processes occurring in the two solvents are quite dissimilar.     

Thermodynamic Properties of Aqueous Dimethylamine and Dimethylammonium Chloride at Temperatures from 283 K to 523 K: Apparent Molar Volumes, Heat Capacities, and Temperature Dependence of Ionization

Data for the apparent molar volumes of aqueous dimethylamine and dimethylammonium chloride have been determined with platinum vibrating tube densimeters at temperatures 283.15 K T 523.15 K and at different pressures. Apparent molar heat capacities were measured with a Picker flow microcalorimeter over the temperature range 283.15 K T 343.15 K at 1 bar. At high temperatures and steam saturation pressures, the standard partial molar volumes of dimethylamine and dimethylammonium chloride deviate towards positive and negative discontinuities at the critical temperature and pressure, as is typical for many neutral and ionic species. The revised Helgeson-Kirkham-Flowers (HKF) model and fitting equations based on the appropriate derivatives of solvent density have been used to represent the temperature and pressure dependence of the standard partial molar properties. The standard partial molar heat capacities of dimethylamine ionization , calculated from both models, are consistent with literature data obtained by calorimetric measurements at T 398 K to within experimental error. At temperatures below 523 K, the standard partial molar volumes of dimethylamine ionization agree with those of morpholine to within 12 cm³-mol^-1, suggesting that the ionization of secondary amine groups in each molecule is very similar. The extrapolated value for of dimethylamine above 523 K is very different from the values measured for morpholine at higher temperature. The difference is undoubtedly due to the lower critical temperature and pressure of (CH₃)₂NH(aq).     

Ionic enthalpies of transfer from water to water-sulfolane mixtures

NaCl, NaBr, NaI, NaClO₄, KCl, KClO₄, NaBPh₄, and Ph₄PBr solution enthalpies were measured in water-sulfolane mixtures at 30°C. Ionic enthalpies of transfer from water to mixed solvents were calculated on the basis of the assumption H _s ^o (BPh ₄ ^– )=H _s ^o (Ph₄P⁺). The variation of the ionic enthalpies of transfer with solvent composition is discussed in terms of ion-solvent interactions and of the effects caused by sulfolane on the structure of water.     

Enthalpies of solution,partial molal heat capacities and apparent molal volumes of sugars and polyols in water

Integral enthalpies of solution of some sugars and polyols in water at low concentrations have been determined calorimetrically at 25 and 35°C. These data have been used to derive heat capacities of solution C°_p at 30°C. Partial molal heat capacities C°_p,2 have been obtained by combining C°_p with C _p,2 ^* , the heat capacity of pure solid compounds. Apparent molal volumes have been obtained from density data. The sugars as well as polyols show significantly high positive C°_p and C°_p,2 values. The results have been explained in terms of a specific hydration model. The effect of substitution of-OH by glycosidic-OCH₃ and of-CHOH by deoxy-CH₂ are also discussed.     

Volumes and heat capacities of water andN-methylformamide in mixtures of these solvents

The densities of mixtures ofN-methylformamide (NMF) and water (W) have been measured at 5, 15, 25, 35, and 45°C, and the heat capacities of the same system at 25°C, both over the whole mole-fraction range. From the experimental data the apparent molar volumes (_v) and heat capacities (_c) of NMF and of water are evaluated. The relatively small difference between the partial molar volumes or heat capacities at infinite dilution and the corresponding molar volumes or heat capacities of the pure liquids for both NMF and water suggests that with regard to these quantities replacement of a NMF molecule by a water molecule or vice versa produces no drastic changes. The partial molar volume of water at infinite dilution in NMF is smaller than the molar volume of pure water, but the corresponding partial molar heat capacity is unexpectedly high.     

Quartz Microbalance Microcalorimetry: A new method for studying polymer-solvent thermodynamics

We have developed a sensitive method of determining enthalpy changes for gas-surface interactions: quartz microbalance microcalorimetry. We mount in an isoperibol environment both sample and reference combinations of a quartz crystal microbalance (QCM) in intimate thermal contact with a heat flow sensor. We coat the sample QCM with a thin (1 µm) polymer film. By exposing the film to ethanol vapor, we measure simultaneously the change in mass per unit area (to ±0.25 ng cm^–2) and the resulting heat flows (to ±50 nW) when the polymer adsorbs or desorbs ethanol. The molar enthalpies of sorption of ethanol vapor in Tecoflex, an aliphatic polyurethane elastomer, are _adsorptionH= –53±8 kJ mol^–1 and _desorptionH=52±3 kJ mol^–1.This revised version was published online in November 2005 with corrections to the Cover Date.     

Group contributions to the thermodynamic properties of non-ionic organic solutes in dilute aqueous solution

The thermodynamic properties G _h ^o,H _h ^o, and C _p,h ^oassociated with the transfer of non-ionic organic compounds from gas to dilute aqueous solution and the limiting partial molar properties C _p ^o ,2 and V₂ ² of these compounds in water are described through a simple scheme of group contributions. A distinction is made between groups made only of carbon and hydrogen, and functional groups i.e. groups containing at least one atom different from carbon and hydrogen. Each group is assigned a contribution, for each property, through a least squares procedure which utilizes only molecules containing at most one functional group. Finally, for compounds containing more than one functional group, correction parameters are evaluated as the differences between the experimental values and those calculated by means of the group contributions. The different behavior of hydrophilic compared with hydrophobic groups is discussed for the various properties. A rationale for the correction parameters, i.e. for the effects of the interactions among hydrophilic groups on the thermodynamic properties, is attempted.     

Heat-capacity changes and partial molal heat capacities of several amino acids in water

Integral enthalpies of solution of several amino acids in water at low concentrations have been determined at 25 and 35°C. These data have been used to derive the heat-capacity change C _p ^o on dissolution at 30°C. Partial molal heat capacities C _p2 ^o have been obtained by combining C _p ^o with C _p2 ^o (heat capacity of pure solid amino acids). The results indicate that the increments in C _p ^o and C _p2 ^o values per CH₂ group increment in the homologous series of -amino acids are constant and in agreement with those found for other homologous series of compounds containing monofunctional groups. However, this is not the case with amino acids having the NH ₃ ⁺ group at the terminal position. The present work also indicates that, as the NH ₃ ⁺ group is shifted away from the COO^– group, hydrophobic hydration decreases, as indicated by a decrease in C _p ^o and C _p2 ^o . the results on various isomers of amino acids show that branching of alkyl groups has no effect on C _p ^o and C _p2 ^o , indicating that hydrophobic hydration is unaffected by branching. The effect of substitution of H by OH and of CH₃ by groups in some amino acids has also been studied and discussed.     

Modeling Heat Capacities of High Valence-Type Electrolyte Solutions with Pitzer's Equations

Apparent molar heat capacities C _p, for 71 rare earth chlorides, nitrates, and perchlorates, alkaline earth and transition metal chlorides, nitrates, and perchlorates, and alkali metal carbonates and sulfates have been fitted to the Pitzer equation for heat capacities. The apparent molar heat capacities at infinite dilution (equal to the standard partial molar heat capacity, ) were used to evaluate a set of best ionic heat capacities, from which improved values of for the electrolytes were calculated. These were then used in the Pitzer equation to reevaluate the higher Pitzer coefficients. The Pitzer coefficients so evaluated can express, in most cases, the behavior of C _p, within experimental error from infinite dilution to the upper limit of the data. Ionic heat capacities have been correlated with the absolute entropies of the ions by statistically assigning the ionic heat capacities to obtain the best linear fit.     

Apparent molar heat capacities and volumes of electrolytes and ions in acetonitrile-water mixtures

Apparent molar volumes V and heat capacities C_p, of NaCl, KCl, KNO₃, AgNO₃, KI, NaBPh₄ and Ph₄PCl have been measured in acetonitrile (AN)-water mixtures up to x_AN=0.25 by flow densitometry and flow microcalorimetry. Limited data have also been obtained for NaF, LiCl and KBr up to x _AN =0.15. Single ion volumes and heat capacities of transfer were obtained using the assumption _tX(PH₄P⁺) = _tX(BPh₄-) where X=V or C _p and _tX is the change in X for a species on transfer from H₂O to AN-H₂O mixtures. Volumes and heat capacities for simple salts show relatively little dependence on solvent composition. However, _tX for simple ions show more pronounced variations, exhibiting at least one extremum. These extrema are similar to but much less pronounced than those derived previously for ions in t-butanol-water mixtures. Surprisingly little correlation is found between the present data and other thermodynamic transfer functions. This is attributed to the predominance of ion-solvent over solvent-solvent interactions in AN-H₂O solutions. _tV and _tC_p, for the silver ion differ markedly from those of the alkali metal ions as a result of the well-known specific interaction between Ag⁺ and AN.