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.     

Sedimentation equilibrium of suspensions of colloidal particles at finite concentrations

Equilibrium concentration profiles of non-dilute colloidal suspensions are calculated by means of the Carnahan-Starling expression for the osmotic compressibility of hard sphere liquids. The profiles depend on the average volume fraction of the suspension, , and on the field interaction parameter, ₀ (reciprocal of the Péclet number at infinite dilution). Profiles are computed for values of and ₀ typical of those encountered in sedimentation field-flow fractionation experiments. It is found that, in most cases, the volume fraction at the depletion wall is negligibly small and that the volume fraction at the accumulation wall ₀, depends on the ratio <>₀/₀ only. An inflexion point is found in the concentration profile if ₀ is larger than 0.13 whatever the value of ₀.     

Mathematical structure of chromatographic optimization based on information theory: II. Analytical role of chromatographic variables

Summary The influence of variables Z (mobile phase composition X, column length L, etc.) on chromatographic analysis is studied in terms of the two separate stages of the whole optimization process. In the first stage, peak separation is the central concern and the resolution R_s can be related to Shannon information _j of peak where C is a coefficient (>0). In the second stage, only chromatograms with no peak overlap are considered and R_s is not connected with _j. The analytical role of Z is evaluated by comparing the signs and magnitudes of the derivatives of the information, _j, and its transmission rate, _j, with respect to Z in both stages.     

The dependence of build-up233U,232U,233Pa and fission products from ThO2 irradiated in HFETR on integral thermal neutron fluxes and neutron spectra

In this paper the dependence of build-up²³³U,²³²U,²³³Pa and fission products from ThO₂ irradiated in HFETR on integral thermal neutron fluxes and neutron spectra have been investigated. The yields of all above nuclides in ThO₂ increase with the increase of integral thermal neutron fluxes at different neutron spectra. The values of²³³U/²³²Th increase with the increases of _th and decreases with the increase of fast/thermal neutron ratios (_f/_th). The values of²³²U/²³³U increase with the increase of both _th and _f/_th ratio. The amount of fission products relative to original irradiated thorium decreases with the increase of _f/_th ratios. These results could be used to evaluate the behaviour of thorium-based nuclear fuel in reactor.     

3D Monte Carlo Simulations of Diffusion Limited Cluster Aggregation up to the Sol-Gel Transition: Structure and Kinetics

Three-dimensional (3D) Monte Carlo simulations of diffusion limited cluster aggregation at different concentrations () show a crossover from a flocculation regime at short times to a percolation regime close to the gel time (t_g). Contrary to suggestions in the literature t_g is independent of the system size (L) for large L. The structural and temporal crossovers between flocculation and percolation take place at characteristic values of the cluster mass (m_c) and the time (t_c) which depend on . After normalisation by these characteristic values the crossovers are independent of except for very small clusters and at short times. The concentration dependence of m_c and t_c indicates that the crossover takes place at a given cumulated volume fraction of the clusters independent of . At low concentrations the -dependence of t_g is determined by the cluster growth in the flocculation regime.     

Etude par viscosité et autodiffusion de solutions aqueuses de divers n-alkyl-benzènesulfonates

Resumé Les propriétés physico-chimiques de solutions aqueuses de différents alkyl-p-benzènesulfonates de pureté contrôlée ont été déterminées par diverses méthodes: solubilité, viscosité, autodiffusions. Les micelles formées par len-octyl-p-benzènesulfonate de sodium (C₈ SO₃ Na) et len-dodécyl-p-benzènesul-fornate de sodium (C₁₂ Na) sont globulaires dans tout le domaine de concetration exploité; les micelles (C₈ SO₃ Na + C₁₂ SO₃ Na) sont de taille proche de celles de C₈ SO₃ Na.La détermination du nombre de contre-ions liés aux micelles de C₈ SO₃ Na confirme cette structure.

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Summary Various methods, i. e. solubility, viscosity and self-diffusion have been employed to determine the physico chemical parameters of aqueous solutions of puren-alkyl-p-benzene-sulfonates. Micelles of sodiumn-octyl-p-benzene sulfonate (C₈ SO₃ Na) are of globular shape; mixed micelles formed by these compounds (C₈ SO₃ Na + C₁₂ SO₃ Na) show a very slight swelling compared with C₈ S03 Na micelles.The degree of counter-ion association of C₈ SO₃ Na micelles is in good accordance with a globular shape of the micelles.

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Zusammenfassung Löslichkeit, Viskosität und Selbst-Diffusion wurden benutzt, um physikalisch-chemische Parameter von wäßrigen Lösungen reinern-Alkyl-p-benzenesulfonate zu bestimmen. Mizellen von Natriumn-Oktylpbenzenesulfonar (C₈ SO₃ Na) haben eine globulare Form; Mischmizelle von C₈ SO₃ Na und C₁₂ SO₃ Na zeigen eine sehr geringe Quellung im Vergleich mit Mizellen von C₈ SO₃ Na.Der Grad von Gegenionenassoziation zu C₈ SO₃-Mizellen ist in guter Übereinstimmung mit einer globularen Form der Mizellen.

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Avec 6 figures et 7 tableaux

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The Lund Institute of Technology, Lund (Suède).

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Groupe de Dynamique des phases condensées, USTL.     

Monte Carlo simulation of a lattice model for ternary polymer mixtures

Monte Carlo studies of symmetrical polymer mixturesAB, modelled by selfavoiding walks withN _A =N _B =N steps on a simple cubic lattice, are presented for arbitrary concentrations of vacancies _v in the range from _v =0.2 to _v =0.8 and chain lengthsN64. We obtained the phase diagrams and the equation of state for three choices of the ratio / _AB ( being the energy between monomers of the same kind, _AB being the energy between different monomers). Flory-Huggins theory provides only a qualitative understanding of these results. If the equation of state is fitted with an effective Flory-Huggins parameter _eff , the latter turns out to be strongly dependent on both concentration and temperature.Contributed paper delivered at the Tagung der Deutschen Physikalischen Gesellschaft, Fachausschuß Polymerphysik, Berlin, March 30–April 3, 1987.     

Basis extension in the Roothaan method

Quantum-chemical calculations show that in the Roothaan method, even when the Hartree-Fock energy limit is achieved, the basis set extension can significantly influence other molecular characteristics. In this work we study the influence of an additional (orthogonal to an initial basis set) orbital on one-electron levels, total energy, and quantities that are calculated as average values of one-electron operators. We assume that the initial M-dimensional basis is sufficiently good, so that adding orbital does not lead to the redistribution of occupied and virtual levels. Then, each occupied (M+1)-dimensional MO _i is approximately the same as the corresponding M-dimensional MO _i ⁰ and differs from it only by admixture of the other orbitals _k ⁰ and the function . The coefficients of _k ⁰ and in _i are considered to be small parameters, and molecular characteristics of interest are expanded in power series with respect to them. It has been established that a decrease in the total energy during the basis extension corresponds to the quadratic in these parameters terms while quantities calculated as average values of one-electron operators change linearly during the basis extension. A shift of one-electron levels _i during the basis extension is examined. It is shown that the MacDonald rule is satisfied if integrals containing the energy shift operator for electron interaction are small with respect to integrals <F⁰_j ⁰> where F⁰ is the Fock operator in the initial basis. An an illustration, results of calculations for H₂ and HeH₊ molecules in an ellipsoidal basis are given.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 22, No. 1, pp. 3–11, January–February, 1986.     

Tris(pyrazol-1-yl)-s-triazine (TPT): X-ray crystallographic,computational, and gas-phase electron diffraction studies

The conformation of the TPT molecule has been analyzed using experimental and computational techniques. The solid-state molecular structure shows similar conformational features to those in the 2-pyrimidine and phenyl derivatives although a different pattern of bond angles in the triazine ring was observed. The AM1 calculations predicted two conformations of comparable stability (E=1.8 kcal/mol) differing in the orientation of one pyrazole ring. While the minimum energy conformation corresponds to a model displayingC _3h symmetry ( ₁= ₂= ₃=0°), the other minimum ( ₁= ₂=0°, ₃=180°) is close to that observed in the solid state. The electron diffraction results are consistent with a planar or nearly planar conformation in agreement with the preceding studies.On leave from the Depto. Química. Universidade Federal Rural do Rio de Janeiro. Itaguai (RJ) 23851 Brazil.     

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.     

PVT properties of concentrated electrolytes. VI. The speed of sound and apparent molal compressibilities of NaCl,Na2SO4, MgCl2, and MgSO4 solutions from 0 to 100°C

The sound velocities of aqueous NaCl, Na₂SO₄, MgCl₂, and MgSO₄ solutions were measured from 25 to 95°C in 10^o intervals from dilute to saturated solutions. The results were combined with our earlier data and fitted to functions of molality and temperature to within ±0.4 m-sec^–1. The adiabatic compressibilities _S were calculated from sound speeds and used to calculate the adiabatic apparent molal compressibilities . Isothermal compressibilities and isothermal apparent molal compressibilities were calculated from _S using literature values for the expansibilities and heat capacities. The values of were extrapolated to infinite dilution using the Debye-Huckel limiting law to determine partial molal compressibilities. The apparent molal compressibilities were fitted to Pitzer's equations. The Pitzer parameters for the concentration dependence of were determined as a function of temperature. Correlations of and V at various temperatures were found for the electrolytes.     

Diffusion–Migration Currents on Microelectrodes: Comparing Two Approximate Approaches

Two electroneutrality and constant-field solutions of the diffusion–migration problem in steady-state conditions on microelectrodes, where both approximations admit analytical solutions, are compared. Analytical equations for the potential drop across the diffusion layer ₀ in terms of RT/F and the migration factor Y are obtained for three- and four-component systems containing two and three types of electroactive ions, respectively, and one type of ions that takes no part in the reaction. Both methods yield virtually identical ₀. The migration coefficients at large absolute values of ₀ noticeably differ. The Y vs. ₀ dependences in the two approaches different. The Y values yielded by these methods are close only at |₀| 1. For real electrochemical reactions considered, the electroneutrality condition at the limiting current is fulfilled at electrode radii >1 m and electrolyte concentrations >0.1 mM.     

Voltammetric Determination of Trace Amounts of Fenitrothion on a Novel Nano-TiO2 Polymer Film Electrode

A novel nano-TiO₂ polymer modified glassy carbon (GC) electrode was developed for the determination of an organophosphorous pesticide, fenitrothion (-NO₂), in citrate buffer solution. The electrochemical behavior of fenitrothion was characterized by using cyclic voltammetry. An irreversible form, -NO₂, was transformed into a reversible redox couple (-NHOH/-NO), and it can be used to determine trace fenitrothion by square wave voltammetry. The experimental parameters, such as film thickness, pH value, accumulation potential and time were optimized. Interestingly, a cyclic voltammetric scan was observed to be more effective than a constant potential for the accumulation of fenitrothion. A linear response over a fenitrothion concentration of 2.5×10^–8 to 1.0×10^–5M was exhibited, with a detection limit of 1.0×10^–8M (S/N=3). The high sensitivity and selectivity of this film electrode was demonstrated by its practical application to the determination of trace amounts of fenitrothion in lake water and apple samples.     

Isentropic compressibilities of alcohol-water mixtures at 25°C

The sound velocities of aqueous solutions of methanol, ethanol, 2-propanol, tertbutanol and 2-butoxyethanol (BE) were measured over the whole mole fraction range at 25°C. The isentropic apparent and partial molar compressibilities, _K,S and _S,A were derived from these data. In the case ofBE, the isothermal partial molar compressibilities were also calculated. _K,S and _S,A for all alcohols except BE initially decrease slightly with the mole fraction and then increase sharply, especially with the higher members, to the value of the pure liquid. In the case of BE, _K,S and _S,A do not go through an initial minimum and the latter goes through a sharp maximum. The compressibilities of water in these mixtures are significantly lower than those of pure water itself. These data can be correlated with other properties and are consistent with the existence of microphase transitions in these aqueous organic mixtures.     

An Alternative Method of Introducing Fugacity

In current textbooks fugacity is introduced according to its differential or integral mathematical formulation. In this article an alternative method of explanation is offered. It is suggested that the real state of a pure gas can be described by comparing it to a hypothetical idealized state. The differences between these two states can then be expressed in terms of a function, , defined as (T,P) = _real(T,P) - _ideal(T,P) where _real and _ideal are the chemical potentials of the gas in its real and ideal states, respectively. The function is a molar excess quantity and is expressed as (T,P) = RT1n where is the fugacity coefficient. This approach introduces fugacity deductively through the function, which leads to , the fugacity coefficient. This method is also appropriate for introducing the activity of solution components and the fugacity of a real gas in gaseous mixtures.     

Apparent molar heat capacities and volumes of electrolytes and ions int-butanol-water mixtures

The apparent molar volumes V and heat capacities C _p, of NaCl, LiCl, NaF, KI, NaBPh₄ and Ph₄PCl have been determined in solutions of H₂O containing up to 40 mass% t-butyl alcohol (TBA) by flow densitometry and flow microcalorimetry. Combination of these results with literature data allows calculation of V and C _p, for 16 ions in these mixtures using the assumption that _tX(Ph₄P⁺) = _tX(BPh ₄ ^– ) where X=V or C _p and _tX is the change in X for a species on transfer from H₂O to TBA-H₂O mixtures. These are the first reported single ion values for C _p, in a mixed solvent. While whole electrolyte volumes and heat capacities show relatively smooth changes with solvent composition, _tX(ion) exhibit two well-developed extrema at around 10 and 25 mass% TBA. The shape of the _tX(ion) curves shows considerable uniformity among the alkali metal cations and the halide ions but the extrema become more pronounced with increasing size among the tetraalkylammonium ions. These extrema are analogous to those observed in aqueous organic mixtures of surfactants and are probably indicative of microphase transitions in these strongly interacting solvent mixtures.     

Thermal analysis of manganese dioxide in controlled atmospheres

TG and DTA curves of-MnO₂ have been obtained in nitrogen, air and oxygen. The reactions MnO₂Mn₂O₃ and Mn₂O₃ Mn₃O₄ were observed in the ranges 450° to 600° and 750° to 1100° respectively, and the decomposition temperatures are affected by the partial pressure of oxygen. The endotherm at 1200° is unaffected by the atmosphere and is not accompanied by weight loss. It is, therefore, due to a polymorphic transformation of Mn₃O₄ rather than formation of MnO. Reaction to form MnO was observed by TG in nitrogen above 1400°, but did not occur on heating in oxygen to 1500°.

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Zusammenfassung Die TG und DTA Kurven von-MnO₂ wurden in Stickstoff, Luft und Sauerstoff ermittelt. Die Umwandlungen MnO₂ Mn₂O₃ und Mn₂O₃ Mn₃O₄ wurden im Temperaturbereich von 450° bis 600° bzw. 750° bis 1100° beobachtet. Die Zersetzungstemperaturen werden durch den partiellen Sauerstoffdruck beeinflußt. Der endendothermische Vorgang von 1200° bleibt von der Atmosphäre unbeeinflußt, ist mit keinem Gewichtsverlust verbunden. Er ist eher einer polymorphen Umwandlung von Mn₃O₄ als der Bildung von MnO zuzuschreiben. Die Bildung von MnO wurde thermogravimetrisch in Stickstoff oberhalb 1400° beobachtet, nicht jedoch in Sauerstoff bis zu 1500°.

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Résumé Etude de-MnO₂ par TG et par ATD, dans l'azote, dans l'air et dans l'oxygène. Entre 450 et 600° on observe la réaction MnO₂Mn₂O₃ et entre 750 et 1100° Mn₂O₃ Mn₃O₄. La pression partielle de l'oxygène influence la température de la décomposition. A 1200°, il apparaît un phénomène endothermique undépendant de la nature de l'atmosphère et qui ne s'accompagne d'aucune variation de poids. C'est pourquoi il est attribué à une transformation polymorphique de Mn₃O₄ plutôt qu'à la formation de MnO. Celle-ci s'observe en TG au-dessus de 1400°, dans l'azote, mais ne se produit pas par chauffage dans l'oxygène jusqu'à 1500°.

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, -MnO₂. , MnO₂ Mn₂O₃ Mn₂O₃ Mn₃O₄ 450°–600° 750°–1100°, , . 1200° . , n₃₄, MnO. MnO 1400°, , 1500° .

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The authors wish to thank Dr. D. Dollimore and Dr. R. C. Mackenzie for suggesting the problem, and the S.R.C. for a research assistantship to D.M.T.     

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.     

Enthalpy of dilution and heat capacity of aqueous solutions of tetrabutylammonium butyrate as a function of temperature

Enthalpies of dilution of tetrabutylammonium butyrate are reported as a function of temperature between 10° and 50°C. Heat capacities of aqueous solutions of tetrabutylammonium butyrate were measured in order to obtain values of _cp at 15°, 25°, and 35°C. These data were combined with the enthalpy of dilution data to obtain _cp as a function of molality and temperature. The apparent molal heat content _L decreased with increasing temperature in concentrated solutions but increased with increasing temperatures in dilute solutions (below0.7 m). Over the temperature range studied _cp shows a maximum as a function of molality at approximately 0.5m. The decrease in _cp with increasing concentration of hydrophobic solute is consistent with the view that the hydrophobic hydration cages, formed under the influence of the tetrabutylammonium and butyrate ions, are saturated at about 0.5m, and that at higher molalities increased overlap of the hydration cages occurs.