Particles 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. 

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... 

The rheo cells can easily be replaced by various types of mixers, propellers or paddles (Figure 2.1.10). It is then possible to analyze the temporal evolution of chemical/physical reactions of mixing, demixing and sedimentation of materials in process engineering, e.g., during the mash process or fermentation [28, 29]. The stirring mechanics and speed can be optimized for various materials of different particle sizes and viscosity. 

At high polymer concentrations, one may also have what is known as depletion stabilization. The polymer-depleted regions between the particles can only be created by demixing the polymer chains and solvent. In good solvents the demixing process is thermodynamically unfavorable, and under such conditions one can have depletion stabilization. 

Does extensive blending cause demixing and segregation of components This is important, especially if particle size/density of the powder/granula-tion vary widely. 

Van den Dries and Vromans41 studied three different particle sizes of lactose with a low-dose formulation containing micronized estradiol at a drug load of 0.1%. A demixing potential was calculated which related to the nonuniform distribution of the dmg in the different sized granules. It was found that a large demixing potential... 

When particles to be mixed have the same physical properties, then a random mixture will be obtained if the particles are mixed for long enough. However, if the properties are different (in particular, particle size and particle density), then the particles may demix or segregate where particles with same physical property separate out and collect in one part of the mixture. The major property difference causing segregation is difference in particle size, although differences in particle density can also cause segregation. 

FIGURE 2.1 Cross-section of a pile of binary powder showing demixing of larger black particles ( 1 mm) to the periphery and finer white particles (—0.2 mm) to the center of the pfle. Taken from Figure 1.11 in Allen [1]. 

According to Cimmino et al. [25], OM measurements show that in the melt (340 °C) demixing of PVME occurs at all compositions, and during crystallization segregated PVME particles remain occluded in the interspherulitic region of sPS crystals. At the end of the crystallization process, an increase in the spherulite dimensions is observed, indicating that PVME addition reduces the nucleation ability of sPS. 

To examine the behavior of mixing measures, it is useful to begin by considering systems free of experimental uncertainties. Particle dynamic simulations such as those discussed in the sections Mixing mechanism in Three Dimensional Tumblers and Demixing, represent such ideal systems the presence and locations of all particles are known and are free of sampling errors (discussed in the section on Sampling Techniques ). 

Donald, M.B. Roseman, B. Mixing and demixing of solid particles, part 1. Mechanisms in a horizontal drum mixer. British Chemical Engineering 7, 749-753. 

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