Large Positive Deviations Research Articles

Why does formation of 1,3-dimethylenecyclobutane-2,4-diyl from 1,3-dimethylenecyclobutane show a large positive deviation from bond enthalpy additivity?

B3LYP/6-31G∗ calculations have been performed in order to better understand the reasons for the large positive deviation from bond enthalpy additivity (BEA), found by Hill and Squires, for formation of triplet 1,3-dimethylenecyclobutane-2,4-diyl (2) from 1,3-dimethylenecyclobutane (3). The calculations find that, in contrast to the formation of diradical 2 from diene 3, formation of 2 from 1-methyl-3-methylenecyclobutene (5) has a negative deviation from BEA of −0.5kcal/mol. Our analysis of the differences between these two pathways to 2 shows that about a third of the large positive deviation from BEA in the formation of diradical 2 from diene 3 comes from the fact that the double bonds in 3 are not conjugated. The other two thirds of the deviation is due to the fact that, although “antiaromatic” resonance structures can be written for diradical 2, the symmetry of one of the singly occupied MOs prevents such structures from making any contribution to the ground state of 2.

Activities and immiscibility in the system Cu-Rh

The thermodynamic activity of rhodium in solid Cu-Rh alloys is measured by the electromotive force method in the temperature range from 1050 to 1325 K with a solid-state cell: Rh, Cu 1 Cu2O//(Y2O3)ZrO2//Cu-Rh 1 Cu2O, Rh The activity of copper and the Gibbs energy, enthalpy, and entropy of mixing of the solid solution are derived. Activities exhibit large positive deviation from Raoult’s law. The mixing properties can be represented by a pseudo-subregular solution model in which excess entropy has the same type of functional dependence on composition as the enthalpy of mixing: DH 5 XRh(1 2 XRh)[41,340 1 12,670 XRh] J mol21 DSE 5 XRh(1 2 XRh)[15.46 1 4.72 XRh] J mol21 K21 The results predict a solid-state miscibility gap with Tc 5 1408 (65) K at XRh 5 0.59 (60.02). The computed critical temperature is 15 K lower than that reported in the literature. The coherent spinodal, calculated using Cahn’s model, is characterized by T8c 5 983 (65) K and X8Rh 5 0.43 (60.02). The large difference between the coherent and chemical spinodal is caused by the relatively large difference in lattice parameters of Cu and Rh and high Young’s modulus of the alloy. The positive enthalpy of mixing obtained in this study contrasts with the negative values predicted by Miedema’s model. Thermodynamic properties of liquid alloys are estimated from the data for solid alloys obtained in this study and the approximate location of the liquidus available in the literature.

Adsorption of Glycols, Sugars, and Related Multiple −OH Compounds onto Activated Carbons. 1. Adsorption Mechanisms

Several different experimental methods were used to understand better the mechanisms of adsorption of glycerol, glycols, and sugars from dilute aqueous solution onto activated carbons. From batch equilibrium experiments, the affinity and capacity (25 °C) of carbons for single and multiple −OH compounds were greatest for solutes having larger positive deviations from solution ideality. Carbons with progressively oxidized surfaces exhibited reduced uptakes for all multiple −OH solutes. Larger isosteric heats of adsorption on carbons were measured for compounds with fewer −OH groups or higher molecular weight. Thermogravimetric analysis showed that the volatility of the adsorbed multiple −OH solute was reduced by higher molecular weight and solute hydrophobicity. Collectively, these findings show that the adsorption mechanism is characterized by: (a) attractive dispersion interactions between surface and solute, (b) solution-phase nonidealities, and (c) secondary, competitive interactions of the solute −OH groups(s) or surface oxygen groups with water.

Molar excess volumes of liquid hydrogen and neon mixtures from path integral simulation

Volumetric properties of liquid mixtures of neon and hydrogen have been calculated using path integral hybrid Monte Carlo simulations. Realistic potentials have been used for the three interactions involved. Molar volumes and excess volumes of these mixtures have been evaluated for various compositions at 29 and 31.14 K, and 30 atm. Significant quantum effects are observed in molar volumes. Quantum simulations agree well with experimental molar volumes. Calculated excess volumes agree qualitatively with experimental values. However, contrary to the existing understanding that large positive deviations from ideal mixtures are caused due to quantum effects in Ne–H2 mixtures, both classical as well as quantum simulations predict the large positive deviations from ideal mixtures. Further investigations using two other Ne–H2 potentials of Lennard–Jones (LJ) type show that excess volumes are very sensitive to the cross-interaction potential. We conclude that the cross-interaction potential employed in our simulations is accurate for volumetric properties. This potential is more repulsive compared to the two LJ potentials tested, which have been obtained by two different combining rules. This repulsion and a comparatively lower potential well depth can explain the positive deviations from ideal mixing.

Identity-reaction proton transfers from oxygen acids yielding localized vs. delocalized conjugate bases. An ab initio study

Identity-reaction proton transfers from a series of oxygen acids to the corresponding conjugate bases have been studied by ab initio methods at the MP2/6-31+G*//MP2/6-31+G* level. The acids are H3O+, CH3OH2+, CH2 = OH+, HC(O)OH2+, CH2 = CHOH2+, H2O, CH3OH, HOOH, HOCH2OH, FOH, FCH2OH, HC(O)OH, and CH2 = CHOH. Gas-phase acidities were calculated at the G2(MP2) level in order to have benchmark values for all acidities regardless of whether experimental values were available. Barriers to proton transfer relative to the separated reactants, ΔHTS, show a straight-line relation to acidity for all but two of the neutral acids and for all but one of the cationic acids. Two neutral acids, HOOH and FOH, show negative deviations that can be attributed to polarizability of the atoms attached to the proton donor oxygens. The cationic acid HC(O)OH2+ shows a large positive deviation, which probably arises from substantial heavy-atom reorganization from reactant to TS. Charges provide evidence of a lag in delocalization in the reaction of CH2 = CHOH2+, though it does not show an elevated ΔHTS.Key words: ab initio, oxygen acids, proton transfer, acidity.

Magnetization and Interactions of Single Domain Sm2Co17 Particles Embedded in CaO Matrix

ABSTRACTInvestigation of the interactions in two magnetic composite systems composed of ∼8 vol% isolated Sm2Co17 nanoparticles of different size ranges in a CaO matrix has been made by comparing the isothermal remanent magnetization curves (IRM) with the dc demagnetization curves (DCD) using Wohlfarth remanence relation. The presence of a weak demagnetizing field was evidenced in the sample with smaller magnetic particles (7-90nm), indicating the magnetostatic interaction between the magnetic particles was dominant in this sample. The large positive deviation from Wolhfarth relation in the sample with larger particles (10-250nm) suggests the presence of multigrain particles in which neighboring grains interact strongly with each other. The hysteresis curves of the two composite systems were measured at temperatures between 5 and 350K. A nearly linear temperature dependence of the coercivity was observed in both samples up to 350 K. From these results it is shown that although the sizes of the Sm2Co17 particles are well below the critical size for single domain particle, there is no evidence that the magnetization reversals in these particles are coherent rotation.

Excess Gibbs energies of the ternary system ethanol+ N, N-dimethylformamide+cyclohexane at 298.15 K

Vapour–liquid equilibrium (VLE) for the ternary system ethanol (EtOH)+ N, N-dimethylformamide (DMF)+cyclohexane (Cy) and for the relevant binary mixtures containing DMF have been determined at 298.15 K by headspace gas chromatographic analysis of the vapour phase directly withdrawn from an equilibrium apparatus. Measurements of liquid–liquid equilibria in both binary DMF+Cy and ternary mixtures have been also carried out. The binary VLE data have been described with different correlation equations. The capabilities of different models of either predicting or reproducing the ternary data have been compared. Excess Gibbs energies G E as well as activity coefficients γ i of components have been obtained and briefly discussed. While EtOH+DMF behaves almost ideally with slightly negative G E-values, both EtOH+Cy and DMF+Cy exhibit large positive deviations. The G Es of the ternary system are positive with the exception of a narrow region in dilute Cy. The excess entropy and the temperature dependence of G E and γ i have been calculated in the whole ternary domain from the known excess enthalpy and heat capacity. The predictions by different equations of the effect of temperature on the mutual solubilities of DMF and Cy as well as on the binodal curve of EtOH+DMF+Cy have been compared with experiment.

Predicting the High-Pressure Phase Equilibria of Binary Mixtures of Perfluoro-n-alkanes + n-Alkanes Using the SAFT-VR Approach

The phase behavior of perfluoro-(n)-alkane + (n)-alkane binary mixtures is of particular interest given their unexpected large positive deviations from Raoult's law. Binary mixtures of perfluoromethane + (n)-alkanes from methane to heptane are studied here, illustrating the continuous change in phase behavior from type II to type III that these systems exhibit. Some symmetrical systems that display heteroazeotropy are also examined. Using the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR),we predict the high-pressure phase diagram for each binary mixture studied. The perfluoro-n-alkane and n-alkane molecules are treated as attractive spherical segments tangentially bonded together to form chains. We use simple empirical relationships between the number of carbon atoms and the number of segments in each chain. The attractive interactions are included as a square-well potential of depth ε and range λ. The pure component parameters are obtained by fitting to experimental vapor pressure and saturated liquid density data from the triple to the critical point. These optimized parameters are rescaled by the respective experimental critical points and used to determine the critical lines and phase behavior of the mixtures. The critical lines predicted by SAFT-VR for the perfluoro-n-alkane + n-alkane mixtures are in excellent agreement with the experimental data and improve significantly the results obtained with the simpler SAFT-HS approach, where the attractive interactions are treated at the mean-field level. This is particularly gratifying as the unlike dispersion interaction between the perfluoroalkane and alkane segments is obtained from a single mixture (perfluorobutane + n-butane) and then transferred to the other systems.

Interfacial properties of liquid alloys: An experimental study on Ga‐Bi and Ga‐Ge

AbstractInterfacial phase transitions like wetting and prewetting transitions are of considerable interest in physics and chemistry of condensed matter since they represent phase transitions in reduced dimensionality. Besides this interfacial properties are of profound practical and technological interest. Most systems studied experimentally in this respect are characterized by Van der Waals intermolecular interactions. However, in the last few years it was shown that Coulomb liquids like liquid alloys or metal molten salt solutions exhibit interfacial phase transitions similar to those known in Van der Waals systems.In order to get more insight into these phenomena the fluid‐vapor interface of two different alloy systems have been studied using ellipsometry. Gallium‐bismuth is a binary alloy with large positive deviations from Raoult's law, exhibiting a distinct miscibility gap. Approaching liquid‐liquid coexistence a Bi‐rich film completely wets the fluid‐vapor interface. As can be estimated from the ellipsometric results the film thickness jumps at the monotectic temperature to a value of about 50 Å. In contrast, gallium‐germanium shows continuous miscibility and deviates much less from ideal mixing. As the Ge concentration in liquid Ga increases along the solid‐liquid coexistence curve the optical properties at the surface also vary continuously, which can be modelled within a simple effective medium approach.

Effects of the polyamide molecular structure on the performance of reverse osmosis membranes

Thin-film composite reverse osmosis membranes of polyamides were prepared by interfacial polymerization. Various benzenediamines and poly(aminostyrene) were interfacially reacted with various acyl chlorides to prepare a skin layer of composite membranes. Among the membranes prepared from the structural isomeric monomers of benzenediamines and acyl chlorides, i.e., the same chemical composition but different in the position of functional groups on the aromatic ring, the membrane with the best salt rejection was obtained when the reacting groups forming amide are located at the same position on the aromatic ring. Membranes prepared by interfacially reacting various diamines with trimesoyl chloride revealed that the salt rejection depends on the linear chain structure of polyamides and network formed by crosslinking. Membranes obtained by interfacial polymerization of poly(aminostyrene) with trimesoyl chloride showed higher water flux but lower salt rejection than those obtained by interfacial polymerization of various benzenediamines with trimesoyl chloride. Membranes obtained here showed the typical trade-off behavior between salt rejection and water flux. However, membranes prepared by interfacially reacting trimesoyl chloride with a mixture of poly(aminostyrene) and m-phenylenediamine or a mixture of poly(aminostyrene), m-phenylenediamine, and diaminobenzoic acid showed a performance advantage over usual membranes, i.e., a large positive deviation from the usual trade-off trend. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1821–1830, 1998

Excess molar enthalpies and excess molar heat capacities of (2-methoxyethanol + tetrahydrofuran + cyclohexane) at the temperature 298.15 K

Excess molar enthalpies and excess molar heat capacities atT=298.15 K were determined for {x1CH3O(CH2)2OH+(1−x1)c-(CH2)4O, orc-C6H12} and for {x1CH3O(CH2)2OH+x2c-(CH2)4O+(1−x1−x2)c-C6H12}. The experimental data were described through a Redlich–Kister type equation. The ternary mixtures exhibit large positive deviations for both properties investigated. TernaryHmEandCp, mEdata were poorly predicted by rules which empirically combine the same data of the pertinent binary mixtures. TheHmEdata were instead satisfactorily predicted by semi-empirical equations such as UNIQUAC and Wang, but not by UNIFAC.

Temperature and precipitation anomalies over Brazil during the 1995 winter season: atmospheric and oceanic characteristics

O inverno de 1995 nas Regiões Sul, Sudeste e Centro Oeste do Brasil foi atípico, pois as temperaturas médias mensais registradas neste período apresentaram grandes desvios positivos e a precipitação ficou abaixo da média. Neste estudo foi feita uma análise dos campos atmosféricos, oceânicos e de superfície com o objetivo de se diagnosticar essas anomalias observadas. Nas anomalias de função de corrente em 200 hPa, foram identificados bloqueios na atmosfera que impediram a penetração de sistemas frontais pelo interior do continente. Foi também identificada uma configuração de "trem de ondas" com circulações anticiclônicas sobre o Pacífico sudeste e sobre a América do Sul. Usando-se séries temporais de anomalias de Temperatura da Superfície do Mar (TSM) observou-se que as áreas 3 (no Oceano Atlântico Sul) e 4 (no Oceano Pacífico Sul) estiveram em oposição de fases. A área 3 apresentou um contínuo resfriamento associado a uma circulação ciclônica em baixos níveis e a área 4 apresentou um aquecimento contínuo associado a uma circulação anticiclônica subtropical.

Thermoanalytical and spectroscopic study of poly(vinyl pyrrolidone)/poly(styrene-co-vinyl phenol) blends

In the present study a series of poly(vinyl pyrrolidone) (PVP) blends with a series of styrene-vinyl phenol copolymers (SVP) have been prepared and studied. Copolymers with a vinyl phenol content greater than 4 mol% show complete miscibility with PVP over the whole blend composition range. Copolymers containing more than 25 mol% vinyl phenol form complexes with PVP, which precipitate in solution with a common solvent. The miscibility was verified both by visual inspection of the blends and by differential scanning calorimetry measurements, which showed a single glass transition temperature ( T g) throughout the whole composition range. Unusually large positive deviations from linearity were observed in the T g versus composition diagrams of some blends. The extensive hydrogen bonding interactions between the mixed polymers were studied by FTi.r. spectroscopy.

Precise Measurement of the Activity/Composition Relations of H 2O-N 2 and H 2O-CO 2 Fluids at 500°C, 500 Bars

The activity/composition relations of H 2O-N 2 and H 2O-CO 2 fluids have been measured at 500°C, 500 bars. The results are more accurate, and much more precise, than any currently available, especially for H 2O-poor compositions. Samples were reacted at fixed water activities (0.062 ≤ a H 2O ≤ 0.777), using Cu 3N as the source of N 2, and Ag-oxalate as the source of CO 2. After each experiment the masses of water and gas in the samples were analyzed manometrically. Results depend on the value used for the hydrogen fugacity for pure H 2O in equilibrium with the oxide buffer, but using a newly measured value for Ni-NiO and fitting the data to a two-parameter Margules equation yields: for H 2O-N 2 fluids: W G,H 2O = 3455.0 J/mol, W G,N 2 = 1990.1 J/mol; and for H 2O-CO 2 fluids: W G,H 2O = 3882.5 J/mol, W G,CO 2 = 4902.6 J/mol. As uncertainties are not orthogonal, standard errors cannot be given. The data suggest that H 2O-CO 2 fluids exhibit large positive deviations from ideality at 500°C, 500 bars, in marked contrast to values predicted by available equations of state. These results indicate that more accurate models for both H 2O-N 2 and H 2O-CO 2 fluids are needed.

Binary AIII nitride solid solutions: Estimation of the excess Gibbs energy

Empirical correlations between the excess Gibbs energy interaction parameters and the molar volume or the nearest neighbour distance differences were used to estimate the excess thermodynamic properties of the mixed A III nitrides. Using the two coefficient Redhlich-Kister equation the values of a symmetric parameter L 0 obtained by the correlations are almost the same. On the other hand the value of an asymmetric parameter L 1 for the molar volume differences correlation is always greater than in the case of the nearest neighbour distance differences. The estimated values of interaction parameters indicate that the AIN-GaN binary system is nearly ideal while indium containing mixed nitrides exhibit large positive deviations from ideal behaviour.

Rheological behaviour of metallocene catalysed high density polytheylene blends

Dynamic and steady state flow measurements and shrinkage experiments have been carried out to investigate the influence of blend composition on viscous and elastic properties of a new type of polyolefin: high density polyethylenes of high and low molecular weight obtained via metallocene catalysts. The polyethylenes are characterized by narrow molecular weight distribution and absence of branching. A model based on the additivity of molecular weight and free volume has been used to fit the complex and steady state viscosities vs composition data, at constant frequency or shear rate, which show a large positive deviation from linearity. The single correlation between G′ and G″ and the small effect of blend composition on dimensional stability correspond to the behaviour expected for a miscible blend. ‘Sharkskin’ and ‘slip-stick’ effects observed in extrusion process of the high molecular weight sample disappear for blends of 50% and higher content of low molecular weight polyethylene.

Nonideality of Mixtures of Pure Nonionic Surfactants Both in Solution and at Silica/Water Interfaces

The behavior of two pure alkylethoxylated nonionic surfactants, both as single components and as binary mixtures of various composition, has been studied in solution and at silica/water interfaces. In solution, the variation of cmc with surfactant composition showed large negative and positive deviations from thermodynamic ideality. These results, as obtained from surface tension measurements, may be interpreted by a partial demixing of the micelles into segregated aggregates in the C10E6-rich composition range and by an increased stability of the aggregates in the C14E6-rich domain. Careful determination of individual isotherms of the surfactants either as single components or as components of mixtures was performed at a nonporous silica/water interface using a reversed phase HPLC technique. The pure nonionic surfactants displayed non superimposable isotherms when sufficiently dilute solution can be attained: a single adsorption plateau is observed with the more hydrophobic component, and a two-plateau behavior is obtained with the more hydrophilic compound. Some experiments with the intermediate surfactant C12E6were also performed showing a single-plateau profile. Experimental confirmation of the demixing process at the silica/water interface was found in the C10E6-rich end of the diagram. However, the larger than usual favorable interaction between the surfactants in the C14E6-end of the diagram could not be deduced directly from the shape of the individual isotherms. The various isotherms were discussed making use of the known properties of these surfactants in solution as micelle-type aggregates and as lamellar phases.

Theory of lipid monolayers comprised of mixtures of flexible and stiff amphiphiles in athermal solvents: Fluid phase coexistence

Statistical thermodynamic theory developed recently to study self-assembled fluid monolayer films of short diblock copolymeric surfactants at the oil/water interface is applied to spread monolayers comprised of mixtures of amphiphiles that differ only in the flexibility of their chains. Although the oil and water solvents are assumed to be athermal for the two blocks of the amphiphiles, we predict large positive deviations from the ideal free energy of mixing that result in phase separation for sufficiently long chains. Pressure–composition phase diagrams and representative pressure–area isotherms are predicted for monolayer films over a range of chain lengths. By comparing predicted structural properties to the contributions to the mixing free energy, we explore the physical origins of the demixing, and consider its implications for experimental systems such as cholesterol/lipid mixtures in monolayers and bilayers.

Melt blends of ?SAN with a phenoxy

Melt blends of poly(α-methyl styrene-co-acrylonitrile) (αSAN) with a phenoxy were prepared using a Brabender Plasticorder. Morphological, thermal, rheological, and mechanical properties of the blends were studied. DSC (differential scanning calorimetry) measurements showed two T g (glass transition temperature) of the blends. T g of αSAN decreased by 5 to 7°C, whereas that of phenoxy increased slightly. Melt viscosity measured using an RDS (Rheometrics dynamic spectrometer) showed a small negative and relatively large positive deviation in aSAN- and phenoxy-rich blends, respectively. Transverse views of SEM (scanning electron microscopy) micrographs of the fractured surfaces again showed two phases, regardless of composition. Traces of full-out upon fracture were seen in αSAN-rich but not in phenoxy-rich blends, indicating stronger interfacial adhesion in phenoxy-rich blends. Longitudinal views of the SEM micrographs showed highly elongated structure of disperse phases (30 to 70% of phenoxy), especially when phenoxy forms dispersed phase, resulting in finer and well developed phenoxy fibrils. Tensile modulus and strengths showed a negative deviation in αSAN and positive deviation (strength) or additivity (modulus) in phenoxy-rich blends.

Flow of liquid dairy foods in small gaps

The viscosities of liquid dairy foods were measured in a slit rheometer with gap spacings as small as 34 μm. Large positive deviations from the true flow curve and an approach to a constant stress were observed below a critical rate in each material studied. This behavior is indicative of probable agglomeration and ‘‘bridging,’’ most likely in the entrance to the dye. In all cases mean particle sizes were an order of magnitude smaller than the slit.