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.     

Partial molal volumes of ions in organic solvents. IV. Ethylene glycol

Apparent molal volumes have been measured for several electrolytes in ethylene glycol (EG) and the standard state partial molal volumes, V₂°, evaluated. Ultrasonic vibration potentials (uvp) have also been measured for most of the alkali metal halides in EG, and these employed to evaluate ionic partial molal volumes, V° (ion). The results show unambiguously that the uvp is essentially independent of solvent viscosity. The V₂° data have also been divided into ionic components by four other techniques including, the method of Mukerjee, the use of Ph₄AsBPh₄, the correspondence method and an extrapolation of V₂° for a series of tetraalkylammonium bromides as a funtion of cation molecular weight. With the exception of the latter technique, all methods used give 30±2 cm³-mol^–1 for V° (ion), although the uvp leads to the largest value for V° (ion). The divisions have been analyzed also with the aid of Hepler's equation, and the results suggest that the uvp method gives a more accurate division and that the EG dipole is more hindered than the dipole in ethanol.     

Group additivity for partial molal heat capacities and volumes of alkanes in methanol at 25°C

A flow heat capacity calorimeter and a flow vibrating tube densimeter have been used to measure the apparent molal heat capacities and volumes of 14 linear and branched alkanes in methanol at 25°C. These quantities have been extrapolated to infinite dilution to obtain the standard partial molal heat capacities and volumes. The C _p2 ^o and V ₂ ^o data can be expressed by equations having the general form: Y=A_Y+ N_k Y_k+(steric factors), where A_Y is solute independent and the Y_k terms are the individual group contributions. A rationale for use of the above equation is presented.     

Group additivity for the standard partial molal heat capacities and volumes of polar compounds in methanol at 25°C

A flow heat capcity calorimeter and a flow vibrating tube densimeter have been used to measure the apparent molal heat capacities and volumes of benzene and 25 polar compounds in methanol at 25°C. These quantities have been extrapolated to infinite dilution to obtain the standard partial molal heat capacities and volumes. The and data have been used in conjunction with an additivity scheme previously determined for alkanes. Group contributions were evaluatd for –OH, –NH₂, –COOH, –C₆H₅, C=O, –COO–, –CONH–, –O–, –S–, and –S₂–. The concentration dependences of _cp and _v of nonelectrolytes in methanol are qualitatively similar but much smaller than in water.     

Apparent molal volumes of aqueous solutions of bis-tetraalkylammonium salts at 25°C: Methylene-group contribution to the limiting molal volume

Densities of aqueous solutions of a series of polymethonium chloride and bromide salts (CH₃)₃–N–(CH₂)_n–N–(CH₃)₃X₂ have been measured at 25°C. Apparent molal volumes have been calculated, and methylene-group contributions to the limiting apparent molal volumes °_v have been estimated. Constant values of the methylene-group contribution of 16.5 and 17.0 cm³-mole^–1 were obtained for the bromide and chloride salts, respectively. These values are consistent with methylene-group contributions reported for other series of organic electrolytes.     

Aqueous solutions of azoniaspiroalkane halides. VI. Apparent molal volumes and apparent molal heat capacities of chlorides and iodides

Densities and heat capacities of aqueous solutions of azoniaspiroalkane halides, (CH₂) _n N⁺ (CH₂) _n X^– (where X=Cl, I andn=5,6), have been measured at 25°C using a flow densimeter and a flow microcalorimeter. The limiting apparent molal volumes (ø _v ) and apparent molal heat capacities (ø _cp ) obtained from these data are compared with those of the azoniaspiroalkane bromides and the corresponding tetraalkylammonium halides. The concentration dependence of ø _v and ø_cp are examined for clues on the influence of solute hydration, structure, and conformational flexibility on the excess functions of quaternary ammonium halides.     

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.     

Partial molal heat capacities of sodium perchlorate in N,N-dimethylformamide from 10 to 75°C

Enthalpies of solution of NaClO₄ have been measured calorimetrically in the aprotic solvent N,N-dimethylformamide at several temperatures ranging from 5 to 80°C. The data have been extrapolated to infinite dilution to obtain standard enthalpies of solution. The integral enthalpy of solution method was used to evaluate the standard partial molal heat capacity c _p2 ^° of NaClO₄ in N,N-dimethylformamide as a function of temperature. This function is almost temperature invariant in N,N-dimethylformamide, in contrast to its complex behavior in aqueous and methanolic solutions. This suggests that ionic heat capacities are extremely sensitive to the structure of solutions and that this function can be used as a probe for studying the structure of electrolytic solutions. The complex temperature dependence of c _p2 ^° in water and methanol can be explained in terms of the decreased hydrogen bonding and dielectric constant of the solvents at the higher temperatures. The data show that one must be cautious in interpreting single-temperature heat capacities of transfer between solvents.This paper was taken from the work submitted by Shuya Chang to the Graduate School of the University of Miami, in partial fulfillment of the requirement for the Master of Science Degree.     

Densities and apparent molal volumes of aqueous CaCl2 and MgCl2 solutions

The Pitzer ion interaction model has been used to evaluate literature data for the densities of aqueous CaCl₂ and MgCl₂ solutions between 0 and 100°C. The selected data can be adequately fitted by setting ^(1),v equal to zero. The variations of ^(0),v and C ^v with temperature have been found to be linearly correlated. The uncertainty in the calculated density is lower than 50 ppm below 1M but raises to 300 ppm at high concentrations. When plotted vs. the square root of the molality, the apparent molal volume of MgCl₂ shows a change at a concentration where a transition in the speed of sound has already been reported by Millero, et al.     

Apparent molal heat capacities and volumes of pyridine and methyl-substituted pyridines in aqueous solution at 25°C

We have made calorimetric measurements leading to apparent molal heat capacities of pyridine and four methyl-substituted pyridines in aqueous solution at 25.0°C. Measurements of densities of the same solutions have led to apparent molal volumes. The results are as follows: pyridine, _C ^° = 305.7 J–°K^–1-mole^–1 and _V ^° = 77.5 cm³-mole^–1; 2-methylpyridine, _C ^° = 370.0 J–°K^–1-mole^–1 and _V ^° = 94.3 cm³-mole^–1; 3-methylpyridine, _C ^° = 380.2 J–°K^–1-mole^–1 and _V ^° = 93.7 cm³-mole^–1; 4-methylpyridine, _C ^° = 378.9 J–°K^–1-mole^–1 and _V ^° = 94.3 cm³-mole^–1; 2,6-dimethylpyridine, _C ^° = 441.8 J–°K^–1-mole^–1 and _V ^° = 109.9 cm³-mole^–1. These _C ^° and _V ^° values are discussed in terms of effects of substitution of CH₃-for H– in the various solute molecules.The research reported here was carried out in the Department of Chemistry, University of Lethbridge, Lethbridge, Alberta, Canada T1K 3M4.     

Partial molal volumes of ions in organic solvents from ultrasonic vibration potential and density measurements. III. Dimethylsulfoxide

The partial molal volumes of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Cl^–, Br^–, I^–, and NO ₃ ^- in DMSO at 25°C have been determined from ultrasonic vibration potential data and density data for solutions of uni-univalent electrolytes. Hepler's semiemprirical equation has been used to split ionic partial molal volumes into geometric and electrostrictive contributions. The results obtained in this work confirm the conclusion of our previous studies, namely, that the contribution of electrostriction is essentially determined by the properties of that layer of atoms, 0.3 to 0.4 nm thick, in contact with the ion and by the degree of steric hindrance of the poles of the dipole of the solvent molecule. On the other hand, the geometric contribution depends on the size of the solvent molecule and also on the arrangement of the solvent molecules about the ions. It is shown that the geometric contribution to the partial molal volume of ions is largely increased when ions cannot come close enough to the poles of the solvent-molecule dipole, owing to steric hindrance.     

Solution thermodynamics of first row transition elements. 3. Apparent molal volumes of aqueous ZnCl2 and Zn(CIO4)2 from 15 to 55°C and an examination of solute-solute and solute-solvent interactions

The apparent molal volumes of aqueous ZnCl₂ and Zn(ClO₄)₂ solutions have been measured from 15–55°C. The dilute solution data are extrapolated to infinite dilution using the Redlich-Meyer equation. The full concentration range data are fitted with the Pitzer formalism. The data are then compared with the data on the previously measured salts of Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺. The effect of complex ion formation is easily seen in the Cu²⁺ and Zn²⁺ salt data. A new approach to single ion volumes from salt volumes is proposed. The calculated ionic volumes at infinite dilution are compared, and it is clear that crystal field effects must be considered in any quantitative theory of transition element volumes.     

Densities and apparent molal volumes of aqueous concentrated calcium chloride solutions from 50 to 200°C at 20.27 bar

Densities of aqueous calcium chloride solutions are reported for molalities up to 6.4 mol-kg^–1 at temperatures from 50 to 200°C and at 20.27 bar. Apparent molar volumes calculated from experimental densities were fitted to the equations of Rogers and Pitzer, and the temperature dependence of the Pitzer parameters were obtained. The standard deviation of fit for the apparent molar volumes is 0.21 cm³-mol^–1 from 50 to 200°C at 20.27 bar.     

Solute-solvent interactions in water-tert-butyl alcohol mixtures. X. Partial molal volumes of alkali and halide lons

Ultrasonic vibration potentials for five alkali metal chlorides and five bromides in water-tert-butanol (t-BuOH) mixtures (t-BuOH mole percent X=2.5; 5.0; 7.5; 10.0 and 13.1%) have been measured as a function of concentration. From these data it has been possible to obtain for the first time the partial molal volumes of the individual ions in solvent mixtures. The partial molal volumes of Cl^– and Br^– ions go through a shallow minimum while those for alkali ions first incease or level off and then decrease as X is increased. A rapid decrease in the partial molal volumes occurs for H⁺ and Li⁺ in a narrow range of composition and these two ions appear to be essentially solvated by t-BuOH at X=13%. The modified Hepler's plots yield a single straight line for anions and cations up to X=5.0%, whereas two distinct lines are found at X7.5%. This change of behavior appears to be related to the maximum structuring effect of water upon addition of t-BuOH, which occurs in the same range.     

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.     

Volumes and heat capacities of benzene derivatives in water at 25°C: Group additivity of the standard partial molal quantities

Differences in densities and heat capacities per unit volume were measured in water with a flow densimeter and a flow microcalorimeter for a series of substituted benzene compounds in water at 25°C. Apparent molal volumes and heat capacities were calculated from the data, and, by extrapolation to infinite dilution, the standard partial molal quantities were dervied. An additivity scheme is proposed to obtain the group contribution to these systems. The standard partial molal volumes and heat capacities of rather complex aromatic molecules in water can be predicted inside about 1 cm³ mol^?1 and 10 J K^?1 mol^?1. The values of the volumes and heat capacities of groups adjacent to an aromatic ring are not the same as those on an aliphatic molecule but a simple relation exists between both sets.     

The effect of pressure on the ionization of water at various temperatures from molal-volume data1

The apparent molal volumes of dilute (0.002 to 1.0m) aqueous HCl and NaOH solutions have been determined at 0, 25, and 50°C and NaCl solutions at 50°C. The partial molal volumes ( ) of HCl, NaOH, and NaCl solutions have been determined from these apparent molal volumes and other reliable data from the literature. The partial-molal-volume changes ( ) for the ionization of water, H₂OH⁺+OH^–, have been determined from 0 to 50°C and 0 to 1m ionic strength from the partial molal volumes of HCl, NaOH, NaCl, and H₂O. The partial molal compressibilities ( for HCl, NaOH, NaCl, and H₂O have been estimated from data in the literature and used to determine the partial molal compressibility changes ( ) for the ionization of water from 0 to 50°C and 0 to 1m ionic strength. The effect of pressure on the ionization constant of water has been estimated from partial-molal-volume and compressibility changes using the relation from 0 to 50°C and 0 to 2000 bars. The results agree very well with the directly measured values.Contribution Number 1548 from the University of Miami.     

Partial molal heat capacities of aqueous tetraalkylammonium bromides as functions of temperature

Enthalpies of solution have been measured from 5 to 85°C for aqueous tetraethyl- and tetrapropylammonium bromides, and the integral heat method is employed to evaluate for these electrolytes over a wide temperature range. Data taken from the literature have been used to evaluate for aqueous Bu₄NBr over a similar temperature range. These data, along with similar data for Me₄NBr, previously reported, have been used to evaluate absolute ionic heat capacities. While the absolute values agree only qualitatively with two other methods of division, the temperature dependences of the three methods essentially agree up to 65°C. Heat capacities due to structural effects on the solvent, obtained by subtracting the inherent heat capacities of the ions, are extraordinarily positive for all four tetraalkylammonium ions and have negative temperature coefficients, indicating that all four ions, including the tetramethylammonium ion, are structure-making ions.     

Apparent molal volumes of phosphonium halides at 15, 25 and 35°C

The densities of aqueous solutions of the phosphonium halides, Bu _4-n Ph _n PX(n=0–4), some of which were synthesized from the phosphines, were measured at 15, 25 and 35°C. Partial molal volumes at infinite dilution, , and B _v coefficients for the apparent molal volumes were determined at each temperature. For the first four cations varies little with n. For all salts B _v are negative but become less negative with increasing n. The temperature dependence of B _v is positive for butyl-rich salts (n<2) but negative for phenyl-rich salts (n>1). Also it appears that is relatively large for phenyl-rich cations in comparison with that for butyl-rich cations.     

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.