Demixing

The availabihty of spray-dried lactose, microcrystaUine cellulose, and other excipients allows for the use of granular rather than powdered phases. This eliminates some of the problems of particle segregation according to size (demixing) and even flow to the die. Direct compression eventually may be the preferred method of tablet preparation. 

Tablet Press. The main components of a tablet compression machine (press) are the dies, which hold a measured volume of material to be compressed (granulation), the upper punches which exert pressure on the down stroke, and the lower punches which move upward after compaction to eject the tablets from the dies. Mechanical components deflver the necessary pressure. The granulation is fed from a hopper with a feed-frame on rotary-type presses and a feeding shoe on single-punch presses. A smooth and even flow ensures good weight and compression uniformity. Using the proper formulation, demixing in the hopper is minimized.

Rhovanil Pine Mesh, a specially caUbrated extra pure vanillin that avoids demixing with other very fine dry ingredients such as sucrose, flour, and dextrose, provides a faster dissolution rate at lower stirring, at lower temperature, in low acidity medium, or in viscous Hquids. 

M. Dijkstra, D. Frenkel. Evidence for entropy-driven demixing in hard-core fluids. Phys Rev Lett 72 298-300, 1994. 

In 1970 Widom and Rowlinson (WR) introduced an ingeniously simple model for the study of phase transitions in fluids [185]. It consists of two species of particles, A and B, in which the only interaction is a hard core between particles of unlike species i.e., the pair potential v jsir) is inflnite if a P and r < and is zero otherwise. WR assumed an A-B demixing phase transition to occur in dimensions D >2 when the fugacity... 

FIG. 2 Phase diagram in the M-z plane for a square lattice (MC) and for a Bethe lattice q = A). Dashed lines Exact results for the Bethe lattice for the transition lines from the gas phase to the crystal phase, from the gas to the demixed phase and from the crystal to the demixed phase full lines asymptotic expansions. Symbols for MC transition points from the gas phase to the crystal phase (circles), from the gas to the demixed phase (triangles) and from the crystal to the demixed phase (squares). (Reprinted with permission from Ref. 190, Fig. 7. 1995, American Physical Society.)... 

In a somewhat wider sense, one can define amphiphiles as molecules in which chemically very different units are linked together. For example, the structures formed by A B block copolymers in demixed A and/or B homopolymer melts and their phase behavior are very similar to those of classical amphiphiles in water and/or oil [13,14]. Copolymers are used not only to disperse immiscible homopolymer phases in one another, but also to create new, mesoscopically structured materials with unusual and interesting properties [15]. 

Figure 8 represents the system CHC13-H2S-H20, where hydrates occur in the binary systems H2S-H20 and CHC13-H20, both of which are known.41 48 It is assumed that the binary system H2S-CHC13 does not show demixing in the liquid phase so that Lx represents a nonaqueous liquid which changes continuously from pure H2S to pure CHC13. All binary equilibria have been drawn with thin lines in Fig. 8. 

One of the hallmarks of the special behavior of polymers is their tendency toward phase separation. Two phenomena come immediately to mind the demixing of... 

Loudet JC, Poulin P (2003) Monodisperse Aligned Emulsions from Demixing in Bulk Liquid Crystals. 226 173-196... 

The mechanism of formation of morphology structures in iPP-E-plastomers blends via shear-dependent mixing and demixing was investigated by optical microscopy and electron microscopy. A single-phase stmcture is formed under high shear condition in injection machine after injection, namely under zero-shear environments, spinodal decomposition proceeds and leads to the formation of a bicontinuous phase stmcture. The velocity of spinodal decomposition and the phase separation depend on the molecular stmcture of iPP and E-plastomer components. 

In this application the reader will examine the influence of the interrelation between the two liquids on the extent and rate of the demixing process. In Example 5.1, the two liquids have a modest affinity for each other, characterized by rules describing a relatively low breaking probability rule between them. These setup data are found in Parameter setup 5.1. 

Figure 5.2. A cellular automata model of the interface between two immiscible liquids, after the demixing process has reached an equilibrium...

Observe the progress of the demixing. Make a rough estimate of the time required to achieve a clear interface with this set of parameters. The interface seen after a long run is shown in Figure 5.2. 

Studies described in earlier chapters used cellular automata dynamics to model the hydrophobic effect and other solution phenomena such as dissolution, diffusion, micelle formation, and immiscible solvent demixing. In this section we describe several cellular automata models of the influence of the hydropathic state of a surface on water and on solute concentration in an aqueous solution. We first examine the effect of the surface hydropathic state on the accumulation of water near the surface. A second example models the effect of surface hydropathic state on the rate and accumulation of water flowing through a tube. A final example shows the effect of the surface on the concentration of solute molecules within an aqueous solution. 

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