Assumption Of Ideal Mixing Research Articles

Phase Transitions in Bilayer Membranes of Dioleoyl-Phosphatidylcholine/Dipalmitoyl-Phosphatidylcholine

Phase diagram has been determined with X-rays for bilayer membranes of mixed lipids of dioleoyl-phosphatidylcholine/dipalmitoyl-phosphatidylcholine in water. Observed liquidus lines were reproduced well by calculation on the assumption of ideal mixing of the two lipids. In the fluid state there seems to be a transition temperature where a small discontinuous structural change takes place. Obtained X-Ray data suggest that the direction of tilting of the lipid hydrocarbon chains becomes indefinite with regard to the longitudinal angle keeping the latitudinal angle constant in the intermediate phase of the membranes of dipalmitoyl-phosphatidylcholine between 35°C and 42.5°C.

Crystallization and Fractionation Trends in the System Andesite-H2O-CO2-O2 at Pressures to 10 Kb

Phase relations of a Mount Hood andesite, which has the composition of an average orogenic andesite, have been determined as a function of O 2 fugacity at 1 atm and of H 2 O fugacity to pressures of 10 kb, at O 2 fugacities of the quartz-fayalite-magnetite (QFM) buffer. All runs contained either a H 2 O or H 2 O–CO 2 fluid phase; melts in runs with a H 2 O–CO 2 fluid phase were H 2 O undersaturated. The H 2 O contents of the melts and H 2 O fugacities were calculated from NaAlSi 3 O 8 –H 2 O thermo-dynamic data on the assumption of ideal mixing in the system H 2 O–CO 2 . One-atmosphere runs show that melting relations of silicates are little affected by f o 2 but that both ilmenite- and magnetite-out temperatures are raised by higher f o 2 . Ilmenite precipitates at higher temperature than magnetite. In these runs and in all runs at high pressure with H 2 O and H 2 O–CO 2 fluid phases, oxides were not stable at temperatures of the silicate liquidus. Oxides might be stable on the silicate liquidus if f o 2 rose two or more log units above the Ni–NiO (NNO) buffer. However, calculations indicate that in natural magmas, those processes which might change f o 2 —crystal-liquid equilibria or exchange of H 2 , or H 2 and H 2 O with the wall rocks—cannot raise f o 2 by that magnitude. Because differentiation of basalt melts to andesite must involve iron-rich oxide phase subtraction, such fractionation models appear unreasonable. For the Mount Hood andesite composition, plagioclase is the liquidus phase under H 2 O–saturated conditions to 5 kb and under H 2 O–undersaturated conditions at 10 kb when the H 2 O content of the melt is less than 4.7 wt percent. For higher H 2 O contents, either orthopyroxene or, at H 2 O saturation at pressure greater than 8 kb, amphibole assumes the liquidus. In all cases, clinopyroxene crystallizes at lower temperature than orthopyroxene. Melting curves in the H 2 O–under-saturated region may be contoured either as percent H 2 O in melt or as P e H 2 O ; in either case, the topology of the various silicate melting curves is different from the case of H 2 O–saturated melting. Therefore, melting relations determined at H 2 O–saturated conditions cannot be used successfully to predict melting relations in the H 2 O–undersaturated region. Amphibole melting relations were studied isobarically at 5 kb as a function of temperature and fluid-phase composition. Amphibole has a maximum stability temperature of 940 ± 15°C for fluid compositions of 100 to 44 mole percent H 2 O; for fluids containing more CO 2 than 56 percent (or, equivalently, less than 4.4 wt percent H 2 O in melt), the melting temperature is lower. The same relations would be seen if CO 2 were not present and the melt were H 2 O undersaturated. These rather low melting temperatures, relative to other silicate phases, preclude andesite generation by basalt fractionation involving amphibole at pressures less than 10 kb.

Scale‐up criteria for stirred tank reactors

AbstractA method is derived for the design of stirred tank reactors for homogeneous reactions. A simple mixing model proposed previously by Curl (4) is used to compute the effects of finite mixing time on complex chemical reactions. It is also shown how the parameters of the model can be obtained by tracer experiments, or estimated theoretically by the assumption of isotropic turbulence. It is shown that in many practical cases the assumption of ideal mixing is a good approximation. However, the effects of imperfect mixing are more likely to be felt in a large reactor than in a pilot plant. Some quantitative examples are discussed. Methods are derived to compute the average outlet concentration for complex systems such as autothermic reactions, polymerization, crystallization, etc.

Studies in Molecular Dynamics. III. A Mixture of Hard Spheres

For an equimolar mixture of hard spheres differing in radius by a factor of 3 it is shown that the numerical method gives the thermodynamic properties within 1% accuracy in the fluid region but generates metastable states at solid densities. The Percus—Yevick theory for mixtures is shown to be as accurate as for pure systems, which makes it plausible that no two-fluid phase region exists for mixtures of spheres. The volume of the mixture is found to be always smaller than the volume of the two pure fluid components, however, at high pressures when a solid and fluid must be mixed to give a fluid mixture, the excess volume is positive. The excess properties are small so that the assumption of ideal mixing (no excess volume) leads to an accurate prediction of the properties of hard sphere mixtures. Furthermore, the conclusion can be drawn that the excess properties in mixing ordinary liquids are primarily determined by differences in the attractive potentials rather than solely by the molecular sizes of the pure components.

Evidence from Model Peptides relating to the Denaturation of Proteins by Lithium Salts

IN an earlier communication1 we reported that mixtures of N-methylacetamide (NMA) or N,N-dimethylacetamide (DMA) with strong aqueous lithium bromide show viscosities much higher than those calculated on the assumption of ideal mixing. This was termed ‘relative viscosity’, which is different from the usual use of this term. It was proposed that the high viscosity is caused by aggregation arising from carbonyl–lithium (I) and carbonyl–water–lithium (II) associations. Dots indicate hydrogen bonds, and dashes ion-dipole interactions. These associations were also considered to occur in protein denaturation. We now report evidence for the existence of both types of association, as well as peptide-anion associations.