Partial molal enthalpies,excess partial molal heat capacities,and structural effects ofn-Am4NBr in the water-dioxane system

Integral heats of solution of tetra-n-pentylammonium bromide, n-Am₄NBr, in aqueous binary mixtures of dioxane are reported at 25° and 35°C from 0.0 to 0.3 mole fraction of dioxane. The excess partial molal heat capacity c _p ^° at 30°C is derived from the integral heats of solution at infinite dilution at 25° and 35°C. The partial molal enthalpies of n-Am₄NBr exhibit a rather flat maxima at 0.2 mole fraction dioxane. The c _p ^° values suggest a structure-breaking role for dioxane. The results obtained in this study are compared with those obtained with n-Bu₄NBr in the same system in an earlier study.     

Thermodynamic properties of alkali halides. II. Enthalpies of dilution and heat capacities in water at 25°C

The enthalpies of dilution and volumetric specific heats of most alkali halides were measured in water at 25°C with flow microcalorimeters in the concentration range 0.01 to 1m. Apparent molal relative enthalpies _L, derived from the enthalpies of dilution, can be represented by a parametric equation in molality. Combining _L with osmotic data, excess entropies can be calculated. Excess free energies, enthalpies, and entropies are compared at 0.5m, and the observed trends are consistent with a model of structural interactions in aqueous alkali halide solutions. The apparent molal heat capacities _C were fitted with the equation _C= _C ^° +A_C(d₀m)^1/2+B _C m. The _C ^° are, in general, additive to better than 1 J-K^–1-mole^–1 and reflect mostly the structural part of ion-solvent interactions. Taking _C ^° (H⁺)=0, conventional ionic _C ^° are obtained. The parameterB _C for different pairs of ions follows approximately the same trends as the corresponding parameterB _V for apparent molal volumes and seems to reflect structural interactions between the ions.     

Enthalpies of Dilution of Cobalt–Amine-Type Salts in Aqueous Solutions at 25°C

The enthalpies of dilution for aqueous solutions of [Co(en)₃]Br₃, [Co(pn)₃]Cl₃, and [Co(tn)₃]Cl₃ (where en = 1,2-diaminoethane, pn = 1,2-diaminopropane, and tn = 1,3-diaminopropane) have been measured at 25°C, and up to m = 1 mol-kg^–1, using an isoperibol calorimeter by the long-jump method. Relative apparent molar enthalpies L have been extracted as an empirical equation relating L and m. Previously reported experimental data and theoretical predictions in the restricted primitive model (RPM) for 3:1 and 1:3 aqueous electrolytes are shown together with the new experimental material.     

Application of the Bjerrum Association Model to Electrolyte Solutions. II. Enthalpies of Dilution in Water and Nonaqueous Solvents

The association theory based on the Bjerrum model, which had been developed for the treatment of apparent and partial molar volumes of electrolyte solutions, is extended to apparent molar relative enthalpies L _2,. Experimental enthalpies of dilution for tetrabutylammonium bromide in acetonitrile, propylene carbonate, and -butyrolactone and for lithium perchlorate and sodium thiocyanate in acetonitrile were obtained and analyzed with this model. Literature data for various electrolytes in water, acetonitrile, and n-propanol were also reanalyzed. Through the Bjerrum equations, enthalpies of dilution can be extrapolated to infinite dilution and reliable L _2, obtained for associated electrolytes. The model can be used to estimate the association constant K _A of the electrolyte and these K _A are compared with literature values (generally obtained from conductivity). Considering the difference in the concentration ranges investigated in L _2, and measurements, K _A extracted from L _2, generally fall within an expected range of deviation from the ones obtained by conductivity, provided that no specific interactions are present in solution.     

Enthalpies of dilution for 1∶3 and 3∶1 electrolytes aqueous solutions

The enthalpies of dilution for aqueous solutions of [Co(cn)₃)]Cl₃, [Co(pn)₃)]Cl₃ and [Co(tn)₃]Cl₃, (whereen=1,2-diaminoethane,pn=1,2-diaminopropane, andtn=1,3-diaminopropane) have been measured at 298.15 K, and up to 1 mol kg^–1+, using a new large isoperibol calorimeter by the long-jump method. Relative apparent molar enthalpies,_291-01, have been extracted as an empirical equation relatingL andm. We have compared our complex salts with the 31 and 13 aqueous systems found in the literature. Theoretical predictions for 13 and 31 electrolytes in the Restricted Primitive Model are also shown.This work was supported in part by CICYT (M.E.C., Spain) under Research Project No.: PB92-0553.     

Thermodynamic properties of dilution of chondroitin 4-sulfate having tetraalkylammonium counterions

The dilution enthalpies, _dil H, of tetraalkylammonium chondroitin 4-sulfate (R₄NChSA) and polyacrylate (R₄NPAA) having methyl (Me), ethyl (Et), propyl (Pr) and butyl (Bu) as alkyl groups (R) were measured to elucidate the effects of hydrophobic counterions. The _dil H of R₄NChS-A and R₄NPAA were negative (exothermic) and decreased as the carbon number of tetraalkylammonium cation (R₄N⁺) increased. These results were found to possess the same effects on _dil H as these of bromides. The non-electrostatic terms of dilution enthalpies _dil H ⁿ of R₄NChS and R₄NPAA were evaluated by using the electrostatic terms of _dil H obtained from Manning's theory. The _dil H ⁿ of R₄NChS-A and R₄NPAA decreased, i.e., their exothermic tendencies increased as the carbon number of R₄N⁺ increased, and the rate of decrease of the R₄NPAA was more than that of the R₄NChS-A. These results are discussed in relation to the increase of the hydrated water structure around R₄N⁺ on dilution.     

Thermal properties of <Emphasis Type="Italic">n</Emphasis>-R<Subscript>4</Subscript>NBr solutions in binary solvents containing formamide

The enthalpies of solution of tetraethyl- and tetra-n-hexylammonium bromides have been measured in mixtures of formamide with ethylene glycol at 298.15 and 313.15 K in the whole mole fraction range by the calorimetric method. The standard enthalpies of solution in binary mixtures have been calculated with Redlich–Rosenfeld–Meyer type equation. The enthalpy and heat capacity parameters of pair interaction of organic electrolytes with EG in FA and with FA in EG have been computed and discussed. The enthalpy interaction parameters of single ions with EG in FA medium have been evaluated and compared with those for ion–water and ion–MeOH interaction in FA. The standard heat capacities of solution have been evaluated. The excess enthalpies of solution, Δ_sol H ^E, of Et₄NBr, Bu₄NBr, and Hex₄NBr have been determined. The Δ_sol H ^E values are positive for Et₄NBr and negative for Bu₄NBr and Hex₄NBr and become more negative from Bu₄NBr to Hex₄NBr.     

Thermodynamic characteristics of solvation of individual ions in ethanol at −50 to 55°C

Experimentally determined are the enthalpies of solution of 12 electrolytes (LiBr, LiI, NaBr, NaI, NaBPh₄, Et₄NCl, Et₄NBr, Pr₄Br, Bu₄NBr, Am₄NBr, Ph₄PCl, Ph₄PBr) in ethanol at –50 to 55°C. _sH^o values obtained on the basis of four different extrapolation equations are analyzed. The effect of temperature changes on the thermodynamic parameters of solvation indindividual ions are calculated using thermodynamic data for the salt crystals (lattice) with the assumption that _solvC _p ^o (Ph₄P⁺)=_solvC _p ^o (Ph₄P^-).     

Viscosities of Some Tetraalkylammonium and Alkali-Metal Salts in N,N-Dimethylacetamide at 25°C

The viscosities of solutions of tetrapropylammonium bromide (Pr₄NBr), tetrabutylammonium bromide (Bu₄NBr), tetrapentylammonium bromide (Pen₄NBr), tetrahexylammonium bromide (Hex₄NBr), tetraheptylammonium bromide (Hep₄NBr), tetraoctylammonium bromide (Oct₄NBr), tetrabutylammonium tetraphenylborate (Bu₄NBPh₄), sodium tetraphenylborate (NaBPh₄), and potassium tetraphenylborate (KBPh₄) in N,N-dimethylacetamide are reported at 25^°C. The viscosity data havebeen analyzed by the Jones-Dole equation for associated electrolytes to evaluate the viscosity B coefficients of the electrolytes. These data have also been analyzed by the transition-state theory to obtain the contribution of the solutes to the free energy of activation for viscous flow of the solution. The ionic contribution to the viscosity B coefficient and the free energy of activation for viscous flow have been estimated using of the reference electrolyte Bu₄NBPh₄. The bromide, tetraphenylborate, and tetraalkylammonium ions are found to be weakly solvated in N,N-dimethylacetamide, whereas significant solvation has been detected for sodium and potassium ions. The viscosity of the solvent is greatly modified by the presence of all the ions investigated here with the exception of the bromide ion.     

Apparent molar heat capacity and relative enthalpy of aqueous NaOH between 323 and 523K

The apparent molar heat capacity, C _p,, of aqueous NaOH has been measured at temperatures between 50 and 250°C and molalities from 0.05 to 1.5 mol-kg^–1. Enthalpies of dilution L were also determined at 99°C and apparent molar relative enthalpies L were calculated up to 1.9 m. Measurements were performed by means of a flow calorimetric apparatus constructed in our laboratory and standardized for C _p, and L with aqueous Na₂SO₄ and with the formation of water from its ions, respectively. Characteristics and performance of this calorimeter are described in detail. Pitzer's semiempirical equations are used for the representation of the results and a general fitting of C _p, data is reported using also recent literature values measured between 4 and 55°C. The fitted parameters are finally utilized, through an integration procedure, to derive a general equation to calculate L at any temperature between 4 and 250°C.     

Apparent molal volumes and heat capacities of some 1:1 electrolytes in anhydrous methanol at 25°C

A flow calorimeter and flow densimeter have been used to measure volume specific heats and densities of solutions of LiCl, LiBr, NaCl, NaBr, KF, KBr, Kl, CsF, and Bu₄NBr in anhydrous methanol at 25°C. The concentrations ranged from approximately 0.01m to close to saturation in some cases. Apparent molal heat capacities _cp and volumes _v have been evaluated and extrapolated to infinite dilution to obtain _cp ^o and _v ^o . Nearly all the heat capacities in methanol are negative. However, with the exception of the lithium halides and Bu₄NBr they are more positive than heat capacities of the corresponding salts in water. The dependence of the heat capacities on ionic radii is generally opposite in methanol solutions from that observed for aqueous solutions. In agreement with others, the _v ^o data indicate that electrostriction in methanol solutions is greater than in aqueous solutions.     

Crystal structure and thermochemical properties of 1-dodecylamine hydrochloride (C12H28NCl)(s)

Crystal structure of 1-dodecylamine hydrochloride (C₁₂H₂₈NCl)(s) has been determined by an X-ray crystallography. Lattice potential energy and the molar volumes of the solid compound and its cation were respectively obtained. The enthalpy of dissolution of the compound was measured by an isoperibol solution-reaction calorimeter at 298.15 K. The molar enthalpy of dissolution at infinite dilution was determined to be , and relative apparent molar enthalpies (Φ_L), relative partial molar enthalpies (L₂) of the compound and relative partial molar enthalpies (L₁) of the solvent (double distilled water) at different concentrations m (mol kg⁻¹) were obtained through fitted multiple regression equation by means of Pitzer's theory. Finally, hydration enthalpies of the substance and its cation were calculated by designing a thermochemical cycle in accordance with lattice potential energy and the molar enthalpy of dissolution at infinite dilution .     

Thermodynamics of ionization of aqueous sulfamic acid,an almost-strong electrolyte

We have measured enthalpies of dilution of aqueous sulfamic acid solutions at 25°C and used results of these measurements to calculate the standard enthalpy of ionization of sulfamic acid. The average H _1on ⁰ obtained in this work was 735±200 J-mol^–1. We have also measured enthalpies of solution of crystalline sulfamic acid in water at 25°C. The results from both measurements are combined with some earlier results from Wu and Hepler to obtain a best value for the standard enthalpy of solution, H _S ⁰ =19.2±0.2 kJ-mol^–1.     

Enthalpies of dilution of aqueous Li2B4O7 solutions at 298.15 K and application of ion-interaction model

The enthalpies of dilution have been measured for aqueous Li₂B₄O₇ solutions from 0.0212 to 2.1530 mol kg⁻¹ at 298.15 K. The relative apparent molar enthalpies, L_?, and relative partial molar enthalpies of the solvent and solute, and were calculated. The thermodynamic properties of the complex aqueous solutions were represented with a modified Pitzer ion-interaction model.     

Enthalpies of dilution of alkaline and alkaline earthp-toluenesulfonates

Enthalpies of dilution (H_D, c0.002 mol-dm^–3) of aqueous solutions of lithium, sodium, potassium, cesium, calcium, magnesium, and strontium salts of p-toluenesulfonic acid were measured at several temperatures between 0° and 40°C, and over a broad concentration range. For monovalent ions the following results were obtained: (i) the enthalpy of dilution for lithium p-toluenesulfonate became more exothermic on increasing the concentration, while those of sodium, potassium, and cesium salts exhibited a maximum, (ii) enthalpies of dilution were strongly dependent on the ionic radius of the cation, and the-H_D values increase in the order, Li>Na>K>Cs. This parallels the behavior of the same quantity in alkaline poly(styrenesulfonate) solutions. For divalent ions the situation was different; Mg, Ca, and Sr p-toluenesulfonates showed little temperature dependence and almost no dependence on the nature of the ion. In contrast with poly(styrenesulfonate) solutions, the enthalpies of dilution were exothermic for all temperatures and salts.     

Apparent molal heat capacities of transfer from H2O to D2O of tetraalkylammonium bromides

The volumetric specific heats (in J-K^–1-cm^–3) of tetraalkylammonium bromides (R ₄ NBr) have been measured at 25°C in the concentration range 0.02 to 0.4 aquamolal in H ₂ O and D ₂ O with a differential flow microcalorimeter. The apparent molal heat capacities _c, calculated from the specific heats and known densities, were fitted with the equation _c= _c ^o +A_cm^1/2+B_cm whereA _c is the Debye-Hückel limiting slope andB _c is an adjustable parameter.The standard heat capacity of transfer C _ptr ^o = _c ^o (D ₂ O- _c ^o (H ₂ O) of R ₄ NBr is positive forR equal ton-propyl andn-butyl and negative for methyl and ethyl. Except for Me ₄ NBr in H ₂ O, allB _c are negative and become more so as the size of the cation increases;B _c is usually more negative in D ₂ O. These results can be interpreted with a two-state model for water and show that a positive C _ptr ^o is evidence that the solute is an overall structure maker, while a negative value indicates a net structure breaker. The negativeB _c is consistent with the existence of strong solute-solute structural (mostly hydrophobic-hydrophobic and hydrophobic-hydrophilic) interactions in the solution.     

The enthalpies of dilution of phosphate solutions at 30°C

The enthalpies of dilution of aqueous solutions of HCl, H₃PO₄, NaOH, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ in the molality range 0.1 to 1.0 mole-kg^–1 have been determined at 30°C. The relative apparent molal enthalpies _L of HCl, NaOH, NaH₂PO₄ and Na₂HPO₄ have been determined with the aid of an extended form of the Debye-Hückel limiting law. The relative apparent molal enthalpies for Na₃PO₄ solutions have been corrected for hydrolysis. A value of H _H ^o =9525±150 cal-mole^–1 was determined for the heat of hydrolysis of PO ₄ ^–3 . This value gives H ₃ ^o =3815±150 cal-mole^–1 for the ionization of H₂PO ₄ ^– , which is in good agreement with the value of H ₃ ^o =3500±500 cal-mole^–1 determined directly by Pitzer at 25°C. The relative apparent molal enthalpies for H₃PO₄ solutions have been corrected for ionization. A value of H ₁ ^o =–1900±150 cal-mole^–1 was obtained for the heat of ionization of H₃PO₄ to H⁺+H₂PO ₄ ^– . This value is in good agreement with the value of H ₁ ^o =–2031 cal-mole^–1 at 30°C determined by Harned and Owen from the temperature coefficient of the equilibrium constant and H ₁ ^o =–1950±80 cal-mole^–1 at 25°C determined from calorimetry by Pitzer.     

Energetics of sodium dodecylsulfate-dodecyldimethylamine oxide mixed micelle formation

Enthalpies of dilution and osmotic coefficients of the sodium dodecyl-sulfate (NaDS)-dodecyldimethylamine oxide (DDAO) mixtures in water have been measured at 25 and 37°C, respectively. From the enthalpies of dilution the apparent molar relative enthalpies L were calculated. The change of the L vs. total molality m_t profiles with the mole fraction reflects the variation of the ionic character of the mixed micelles. From the osmotic coefficients the nonideal free energy G ₂ ⁿⁱ were calculated. By combining G ₂ ⁿⁱ with the partial molar relative enthalpies, the nonideal entropies TS ₂ ⁿⁱ were determined. At a given mole fraction, G ₂ ⁿⁱ and TS ₂ ⁿⁱ values are decreasing and increasing respectively, tending to become constant at high m_t. The excess properties for the mixed micelle formation were evaluated as a function of the mixture composition at some m_t. The profiles are compared with those obtained from thermodynamics of binary liquid mixtures and the regular solution theory.     

Heat properties of Bu<Subscript>4</Subscript>NBr solutions in binary mixtures based on formamide

Heat effects of tetrabutylammonium bromide dissolution in mixtures of formamide with methanol and ethylene glycol at 25°C were determined. Partial molar enthalpies of the components of formamide-ethylene glycol mixtures at 25°C were measured by the calorimetric method, and the mixing enthalpies of this system were determined. Within the limits of the second approximation of the Debye-Hueckel theory the standard enthalpies of dissolution Δ_dis H ⁰ were calculated, and the enthalpies of Bu₄NBr transfer from formamide in its mixtures with water methanol and ethylene glycol were found on this basis. The enthalpy parameters of Bu₄NBr pair interactions with the components of the formamide-water, formamide-methanol, and formamide-ethylene glycol mixtures were calculated. The results obtained were compared with the data for the systems containing N-methylformamide and N,N-dimethylformamide.     

Thermodynamic studies of octyltrimethylammonium chloride in water

Densities, heat capacities, enthalpies of dilution at 298 K and osmotic coefficients at 310 K of octyltrimethylammonium chloride were measured as functions of concentration. From the experimental data, the partial molar volumes, heat capacities, relative enthalpies, nonideal free energies and entropies at 298 K were derived as functions of concentration. A comparison between the above data and those of dodecyltrimethylammonium chloride reported in the literature shows that the increase of the alkyl chain length shifts the apparent molar volumevs. concentration curves towards greater values and the heat capacity, relative enthalpy and free energyvs. concentration curves towards smaller values. By assuming the pseudo-phase transition model the properties of micellization (Y_m) were graphically evaluated. TheY_m values of OTAC compared with those of DTAC are consistent with the increase of the hydrophobicity by increasing the alkyl chain length.The authors are grateful to the National Research Council of Italy (CNR, Progetto Finalizzato Chimica Fine II) and to the Ministry of University and of Scientific and Technological Research (MURST) for financial support.