Cubic phases phase behaviour

High-resolution dilatometric measurements have revealed the appearance of anisotropy in the cubic-phase thermal strain in the precursive temperature region for the soft-mode martensitic transformations in VaSi/ Ni-Al, In-Tl/ and SrTiOa In the case of Ni-Al and SiTiOa, the onset temperatures for the strain anisotropy are close to those at which the appearance of central peak behaviour occurs. 

In this work we will focus on the use of the cubic phase as a delivery system for oligopeptides - Desmopressin, Lysine Vasopressin, Somatostatin and the Renin inhibitor H214/03. The amino acid sequences of these peptides are given in Table I. The work focuses on the cubic phase as a subcutaneous or intramuscular depot for extended release of peptide drugs, and as a vehicle for peptide uptake in the Gl-tract. Several examples of how the peptide drugs interact with this lipid-water system will be given in terms of phase behaviour, peptide self-diffusion, in vitro and in vivo release kinetics, and the ability of the cubic phase to protect peptides from enzymatic degradation in vitro. Part of this work has been described elsewhere (4-6). 

MAS has been applied to a highly viscous cubic phase of a lyotropic LC formed by 1-monooleolyl-rac-glycerol and water in order to obtain liquid-like and 13C spectra.330 Deuterium, sodium, and fluorine NMR spectroscopy have been applied to study the phase behaviour of several dilute lamellar systems formed by low concentrations of an ra-hexadecylpyridinium salt, a sodium salt (e.g., NaBr, NaCl, or sodium trifluoroacetate), 1-hexanol, and D20.331 The 2H, 19F, and 23Na splittings were used to monitor the phase equilibria. The last two studies are motivated by the search of new lyotropic LC for the alignment of biomolecules. 

The phase behaviour of binary polymer - supercritical fluid systems can be modelled with an equation of state model. In general, non-cubic equations of state are used, mainly from the PHCT and SAFT families. Lattice-fluid equations of state are also commonly used for the... 

The phase behaviour established for concentrated aqueous solutions of PEO-PPO-PEO copolymers has its counterpart in PEO/PBO copolymer solutions. A phase diagram for PE058PB0i7PE0M based on tube inversion experiments is shown in Fig. 4.14 (Luo et al. 1992). The hard gel is isotropic under the polarizing microscope and can be characterized as a cubic phase formed from spherical micelles of a similar size to those in the dilute micellar solution. 

The phase behaviour of blends of PS-PI or PS-PB diblocks with PS homopolymer was summarized by Winey et al. (19926). Regions of stability of lamellar, bicontinuous cubic, hexagonal-packed cylindrical and cubic-packed spherical structures were mapped out as a function of homopolymer molecular weight, copolymer composition and homopolymer concentration. All blends were... 

A Statistical-Mechanics based Lattice-Model Equation of state (EOS) for modelling the phase behaviour of polymer-supercritical fluid mixtures is presented. The EOS can reproduce qualitatively all experimental trends observed, using a single, adjustable mixture parameter and in this aspect is better than classical cubic EOS. Simple mixtures of small molecules can also be quantitatively modelled, in most cases, with the use of a single, temperature independent adjustable parameter. 

The present paper gives an overview of results on high-pressure phase equilibria in the ternary system carbon dioxide-water-1-propanol, which has been investigated at temperatures between 288 and 333 K and pressures up to 16 MPa. Furthermore, pressure-temperature data on critical lines, which bound the region where multiphase equilibria are oberserved were taken. This study continues the series of previous investigations on ternary systems with the polar solvents acetone [2], isopropanol [3] and propionic add [4], A classification of the different types of phase behaviour and thermodynamic methods to model the complex phase behaviour with cubic equations of state are discussed. 

MODELING THE PHASE BEHAVIOUR OF THE CARBON DIOXIDE-WATER-1-PROPANOL SYSTEM WITH CUBIC EQUATIONS OF STATE... 

The phase transformations mechanism depicted in reaction (6.1) is a general behaviour for the Zr02-containing ceramics in that by doping the monoclinic (/n) phase, tetragonal (/) and cubic (c) phases can be stabilised, also at room temperature (RT). 

Besides the asymmetry between monolayers in cytomembranes, two of the more obvious differences between cubic phases and membranes are the unit cell size and the water activity. It has been argued that tire latter must control the topology of the cubic membranes [15], and hence tiiat the cubic membrane structures must be of the reversed type (in the accepted nomenclature of equilibrium phase behaviour discussed in Chapters 4 and 5 type II) rather than normal (type I). All known lipid-water and lipid-protein-water systems that exhibit phases in equilibrium with excess water are of the reversed type. Thus, water activity alone cannot determine the topology of cubic membranes. Cubic phases have recently been observed with very high water activity (75-90 wt.%), in mixtures of lipids [127], in lipid-protein systems [56], in lipid-poloxamer systems [128], and in lipid A and similar lipopolysaccharides [129,130]. 

What are the functions, if any, of the cubic membranes It may be that cubic membranes are but an inevitable self-assembled product of the complex molecular soup of lipids and proteins the result of molecular packing considerations and inter-molecular interactions. This would be in analogy with known phase behaviour in equilibrium systems. Even though this is a very appealing solution to the long and unresolved debate about "non-lamellar" lipids in conjunction with cell membranes, we rather believe that these structural organisations have been chosen to fulfil a purpose (see, e.g. [134] and references therein for current theories, and [4] for a more comprehensive discussion), and the formation cannot be rationalised solely by molecular packing. 

Salamacha and coworkers304 306 have carried out a series of studies on Lennard-Jones fluids confined to nanoscopic slit pores made from parallel planes of face centred cubic crystals. Grand canonical and canonical ensemble MC simulations have been used to determine the structure and phase behaviour as the width of the pore and the strength of the fluid-wall interactions were varied. The pore widths were small accommodating 2 to 5 layers of fluid molecules.304,305 The strength of the fluid-wall interaction is linked to the degree of corrugation of the surface, and it is found that the structure of the... 

Crystallization-induced phase separation can occur for concentrated solutions (gels) of diblocks [58,59]. SAXS/WAXS experiments on short PM-PEO [PM=poly(methylene) i.e. alkyl chain] diblocks revealed that crystallization of PEO occurs at low temperature in sufficiently concentrated gels (>ca. 50% copolymer). This led to a semicrystalline lamellar structure coexisting with the cubic micellar phase which can be supercooled from high temperatures where PEO is molten. These experiments on oligomeric amphiphilic diblocks establish a connection to the crystallization behaviour of related nonionic surfactants. 

TiBe2 has the C15 cubic Laves phase structure and is an itinerant antiferromagnet with a Neel temperature near 10 K and a magnetic moment of 1.64jub. The Laves phase can be maintained when Cu is substituted for Be over the range 0 < x < 0.5 and a transition to ferromagnetism is observed at xcrit 0.115 (Acker et al. 1981) near which the SEW behaviour, i.e. the Landau parameter in the magnetic equation of state... 

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