Small Negative Deviations Research Articles

Thermodynamic properties of liquid Al-Mg alloys measured by the Emf method

The thermodynamic properties of liquid Al-Mg alloys containing 0.027 to 95.50 at. Pct Mg were determined by measuring the emf, between 973 and 1073 K, of a magnesium concentration cell of the type Mg(l)|MgCl2-CaCl2 (eutectic melt, l)|Mg (in Al, l). Special attention has been given to low-magnesium aluminum alloys which are most commonly used in industry and for which definitive thermodynamic data are not reported in the literature. Alloys containing up to 12 at. Pct Mg follow Henry’s law, and the magnesium activity is given by the relation aMG = 0.88 XMG at 1073 K. Above 12 at. Pct Mg, the magnesium activity shows a small negative deviation from the ideal solution behavior. The activity of aluminum, however, closely follows the ideal solution behavior up to 75 at. Pct Mg and thereafter it shows a small negative deviation. The emf data have also been used to determine the free energy, entropy, enthalpy, and excess free energy for liquid Al-Mg alloys.

Excess Gibbs free energies of (tetrachloromethane + an alcohol or a cyclic ether) at 298.15 K

(Vapour + liquid) equilibria for tetrachloromethane + methanol, + ethanol, + 1-propanol, + 1-butanol, + 2-methyl-2-propanol, + tetrahydrofuran, and + 1,4-dioxane were determined at 298.15 K by head-space g.c. analysis of the vapour phase directly withdrawn from an equilibration apparatus. Measurements on the test mixture (cyclohexane + ethanol) were also made. Excess molar Gibbs free energies G m E for the examined mixtures were obtained by a least-squares treatment of the equilibrium results. G m E values are positive for (tetrachloromethane + an alcohol), and decrease with increasing molar mass of the alcohol. The mixtures containing an ether behave almost ideally, with small negative deviations shown by those containing tetrahydrofuran. The behaviour of the activity coefficients at infinite dilution as well as of the free energies of solvation of alcohols are briefly discussed.

Concentration dependence of electrokinetic transport coefficients of non-aqueous binary mixtures through weakly charged porous plugs

An experimental investigation of the concentration dependence of electrokinetic transport coefficients of methanol + ethanol mixtures through a quartz plug is described. Electro-osmotic flow and streaming-potential measurements have been carried out. The Onsager reciprocity relations have been experimentally checked for all compositions of the binary mixture used. The form of the fluidity curve indicates that the ethanol + methanol mixture is a quasi-ideal system with a small negative deviation. The hydrodynamic permeability of systems comprising methanol + ethanol mixtures and a porous plug and the fluidity of the mixture vary with the composition in a similar way. The variation of the cross-phenomenological coefficient with composition is due to an analogous variation of the term Dζ/η with the mole fraction of methanol. On the other hand, the ratio of streaming potential to pressure difference (in the interval 0–20 cmHg) decreases as the pressure drop increases. This result agrees with the theoretical findings of Rutgers and Boumans concerning the effect of turbulence on the streaming potential. The concentration dependence of the second-order coefficients has been partially explained on the basis of Jha's equations.

Chromium-aluminum mixing in garnet: A thermochemical study

The enthalpies of mixing of synthetic grossular (Ca 3Al 2Si 3O 12)-uvarovite (Ca 3Cr 2Si 3O 12) solid solutions have been determined by oxide melt calorimetry at 970 K. The results indicate that the solid solution exhibits a very small negative deviation from Raoult's law. In terms of the regular solution model a chromium-aluminum interaction parameter W H Cr-Al of between 0 and −1000 cal/gm atom is consistent with the data.

X-ray and density measurements on “early-late” transition metal glasses: consistency with “soft-sphere” structural models

X-ray measurements on glassy alloys (T E) 1−x(T L) x formed between transition metals occurring “early” (T E) and “late” (T L) in the Periodic Table show that (a) the first-broad-peak scattering vector Q p increases approximately linearly with atomic concentration x, (b) the very approximate average nearest-neighbor separation d 1 as simply d 1≾4π Q p −1 increases approximately linearly with 2<R G⪢ (R G ≡ Goldschmidt radius). Both (a) and (b) appear consistent with structural models based on the dense random packing of hard spheres. On the other hand, measured densities indicate (1) high R G-based packing fractions within 2% of the ideal close-packed crystalline value 0.74, (2) small negative deviations from Zen's Law. Both (1) and (2) suggest a “soft-sphere” structural model compatible with charge transfer and/or chemical short-range order.

Thermodynamic properties of solid rhodium-nickel alloys

The thermodynamic properties of the solid solutions formed by rhodium and nickel were investigated between 1100 and 1400K, using oxygen concentration cells with a ZrO 2CaO solid electrolyte. The activities of nickel show positive deviations from Raoult's law in the rhodium-rich alloys and small negative deviations in the nickel-rich alloys. The integral enthalpy of mixing and the excess integral entropy of mixing are positive. The thermodynamic data indicate that a miscibility gap may be expected at low temperatures.

Excess free energies of interaction of chlorophyll a with monogalactosyldiacylglycerol and phytol a mixed monolayer study

The surface pressure isotherms of chlorophyll a, monogalactosyldiacylglycerol and phytol at the air-water interface were studied on a Langmuir trough at 20.0±0.5° C. The subphase was a phosphate buffer, 10 −3 M at pH 8.0. The extrapolated limiting areas per molecule are 115, 82 and 38 Å 2/molecule, respectively. The isotherms of eight mixtures of chlorophyll a with monogalactosyldiacylglycerol and eight mixtures of chlorophyll a with phytol, covering in both cases the whole range of molar fractions have also been measured. The results for the mixed monolayers were analysed in terms of the additivity rule. They show that a small negative deviation with respect to ideality is observed upon mixing chlorophyll a with monogalactosyldiacylglycerol. However, chlorophyll a forms an ideal two-dimensional solution when mixed with phytol. The excess free energies of mixing of chlorophyll a with monogalactosyldiacylglycerol as a function of concentration were calculated from the surface pressure isotherms at 10, 15, 20 and 25 mN·m −1. The values are negative, reflecting the interaction prevailing between these components in the monolayers. For the four surface pressures studied, the excess free energies are symmetrical with respect to the mode fraction. The values for an equimolar mixture range from −300 to −540 J·mol −1 at 10 and 25 mN·m −1, respectively. A comparison between the thermodynamics of mixing of chlorophyll a with monogalactosyldiacylglycerol and phytol suggests that the polar head of monogalactosyldiacylglycerol together with the polar groups of chlorophyll a are probably involved in the interaction. However, this does not completely rule out the possibility that structural effects due to a different packing of chlorophyll a with monogalactosyldiacylglycerol and phytol may also be involved. Furthermore it is shown that the small interactions between these constituents are not inconsistent with the specific coupling existing between the apoprotein of the chlorophyll a-protein complexes and chlorophyll a.

Refractivity of Molten Nitrates and Chlorides: Binary Mixtures Containing Cesium Ions

Abstract By using an interferometric technique, the refractive index of some molten salt mixtures containing Cs+ ions was measured with high accuracy: (Li-Cs)NO3, (Na-Cs)NO3, (Ag-Cs)NO3, (Li-Cs)Cl, and also pure RbCl. The isotherms of molar refractivity show a small negative deviation from additivity in the (Li-Cs)NO3 and (Li-Cs)Cl systems and a positive deviation in the (Ag-Cs)NO3 mixture. A tentative attempt was made to relate the excess molar refractivities with the absorption bands of the ions.

Thermal expansion of mixed-alkali silicate glasses

Thermal-expansion measurements were made for five varieties of mixed-alkali silicate glasses (Na-K, Na-Rb, Na-Cs, K-Rb, and K-Cs), with special emphasis on the Na-K silicates. Negative deviations from additivity were observed in the glass transformation temperature in every case, with the deviations increasing with total alkali-oxide concentration. Positive deviations from additivity were noted in the thermal-expansion coefficients in all but one case (Na-Cs), where a small negative deviation was observed. The deviations in both properties were maximized when the radius ratio of the two alkali ions was approximately 1.4, i.e., for the Na-K glasses. These results are quite similar to those reported for mixed-alkali germanate glasses which suggests that they are due to ion-ion interactions rather than to gross alterations of the glass network.

Thermodynamic investigation of carbon tetrachloride + thiophene and toluene + chlorobenzene

Isothermal vapor-liquid equilibrium results are presented for the binary mixtures carbon tetrachloride + thiophene and toluene + chlorobenzene at 343.15 K. The results are thermodynamically consistent and are correlated by the Redlich-Kister equation. An attempt is made to interpret, qualitatively, the small negative deviations from ideal behavior shown by toluene + chlorobenzene and the relatively larger positive deviations from ideality shown by carbon tetrachloride + thiophene in terms of the complex-formation ability of the solution constituents.

Equilibrium adsorption of mixtures of nitrogen with oxygen on NaA zeolite

1. The adsorption of mixtures of O2 + N2 on NaA zeolite was studied at 195.1, 214.6, and 225.7°K. A method was proposed for finding the partial pressures according to the set values of the adsorption of the components with the aid of a graph of the isobars of the adsorption of oxygen and nitrogen for the indicated temperatures. For the system studied, there is a Henry region in the adsorption of the mixtures, in which proportionality between the adsorption of the component and its partial pressure is preserved. In this region the adsorption solutions are ideal. 2. The activity coefficientsγ1 andγ2 in the adsorption solution at temperatures exceeding the critical temperatures of the components are very close to one, and only as Tcr is approached, and the degree of filling of the adsorption volume of the zeolite is increased, small negative deviations from Raoult's law are observed.

Infra-red detection of depolymerization in Tl2O-GeO2-SiO2 glasses

Three series of glasses containing 15, 26, and 35 mol % Tl2O were prepared in the Tl2O-GeO2-SiO2 system and their infra-red spectra, densities, and refractive indices determined. Refraction decreases as SiO2 substitutes for GeO2 and is additive with respect to the two binary series, except for slight positive deviations with the low SiO2content 35 mol % Tl2O glasses. All glass series exhibit small negative molar volume deviations from additivity at low SiO2 contents and small positive deviations from additivity at higher SiO2 contents. The strong compositional dependence of the main infra-red stretching frequency values (1100 cm−1 for SiO2 and 875 cm−1for GeO2) are shown to be relatively sensitive indicators of changes in the polymeric makeup of these ternary oxide glasses. Loosening and/or depolymerization of either pure or damaged MO4 networks is accompanied byv Si-O and/orv Ge-O shifts to smaller frequencies. The frequency changes that accompany the progressive replacement of GeO2 by SiO2 suggest that (a) SiO4 tetrahedra initially replace GeO4 tetrahedra, (b) GeO6 octahedra subsequently transform to GeO4 tetrahedra, and (c) GeO4 tetrahedra finally take part in repair of damaged SiO4 networks. Use of a previously developed infra-red isofrequency contour technique revealed similar ternary trends for the two vibrational modes of interest. This emphasizes the ability of such contour trends to reflect the overall degree of network crosslinking present in ternary oxide glasses.

Pure and mixed monomolecular films of 12-nitroxide stearate

Pure and mixed monomolecular films of 12-nitroxide stearic acid (12-nitroxide stearate) have been studied at the air water interface and both surface pressures and surface potentials obtained as a function of area/molecule. The isotherm for the pure compound shows an inflection at approx. 70 Å 2/molecule and a collapse at 39 Å 2/molecule. These observations indicate that close-packed films occur with both polar groups (carboxyl and nitroxide) and then with one polar group (carboxyl) located at the air-water interface. The pure film shows a slow deterioration with time at both pH 2 and 6 due to an attack on the nitroxide group. In mixed films positive deviations from ideality occur at higher pressures with a trend toward small negative deviations at low pressure. Under membrane-like conditions the free energy of chain-chain interactions is reduced. The probe proved to be partially immiscible in a liquid condensed film and immiscible in a solid condensed film, an observation in accordance with a similar segregation observed below the liquid crystalline-gel transition in liposomes and membranes.

Oxygen and Metal Activities of the Iron‐Nickel‐Oxygen System at 1000°C

The equilibrium oxygen pressures for alloys coexisting with their oxides have been determined at 1000°C using an electrochemical cell with a calcia‐zirconia electrolyte. These pressures were related to compositional determinations of the solid phases and the isotherm at 1000°C. Oxygen pressure over the wustite‐alloy phase field increases from for equilibrium to for the invariant three‐phase field of wustite containing 0.51 a/o (atom per cent) Ni, spinel containing 0.6 a/o Ni, and an alloy containing 79.6 a/o Ni. The oxygen pressure subsequently increased through the spinel‐alloy phase field to , that of the invariant phase field composed of spinel containing a calculated composition of 11.4 a/o Ni, nickel oxide containing 42.5 a/o Ni, and an alloy containing 99.6 a/o Ni. Iron behaved ideally up to the alloy content of 40 a/o Ni, but then exhibited small negative deviations, whereas nickel deviated negatively at alloy compositions greater than 10 a/o Fe. The spinel‐alloy equilibria have been described by a Gibbs‐Duhem calculation and by a structural model for the spinel based on a lattice point defect model.

A thermodynamic study of solid Pd-Pt alloys

The vapor pressure of palladium over nine Pd-Pt alloys was measured by the torsion-effusion method. Calorimetric heat-of-solution measurements were carried out on specimens from the same alloys to derive the enthalpy of formation. The activities and partial and integral free energies at 1600°K, computed from the vapor-pressure measurements, were combined with the integral enthalpies at 298°K to determine the integral entropies, assuming Neumann-Kopp behavior. The activities and the free energies of formation exhibit very small negative deviations from ideal behavior. The enthalpies of formation are moderately negative over the entire composition range. The entropies of formation are positive but smaller than the ideal entropies at 1600°K. The thermodynamic results provide additional evidence for the existence of short-range order in the Pd-Pt system.

Gas chromatographic studies of the solvent extraction systems — II: Diphenyl methyl phosphate

The activity coefficients and excess thermodynamic functions at infinite dilution were evaluated from gas chromatographic measurements for a number of binary diphenyl methyl phosphate-diluent systems. The interactions between the phenyl groups of DPMP and a diluent are predominant whereas the central phosphoryl group plays only a secondary role. A weak hydrogen bonded complete is formed between the hydrogen of chloroform and π-electrons of benzene rings. Hydrocarbons and water exhibit strong positive deviations from ideality which are sufficient to produce the phase separation. Solutions containing alcohols, toluene and carbon tetrachloride are characterized by moderate positive deviations, and ketones by small negative deviations. It was found that the DPMPC 6H 6 system is ideal, and that the undiluted DPMP is a complex, highly associated, viscous liquid.