Microporous silica beads preparation

Microporous particles are available in two sizes 20 to 40 jam diameter with longer pores and 5 to 10 fim with short pores (see Figure 3.14B). These are now more widely used than the porous layer beads because they offer greater resolution and faster separations with lower pressures. The micro-porous beads are prepared from alumina, silica, ion-exchanger resins, and chemically bonded phases (see next section). 

In microporous particles, pore depths are decreased by decreasing dp. Particles of 20-40 g diameter with longer pores and of 3-10 g diameter with short pores are available. Microporous beads are prepared from porous materials like silica or alumina. 

The most common supports are insoluble particles of polystyrenes (PS), cross-linked with divinylbenzene (DVB), and silica gel (SG). Soluble polymers have been used as supports and separated by precipitation or by ultrafiltration. Insoluble PS beads are prepared by suspension polymerization. They can be either microporous or macroporous (synonymous with macroretic-ular). " Typical average particle diameters are 50 gm for peptide synthesis and 500 jum for ion exchange. PS beads are used in the form of solvent-swollen gels. The micropores are created by solvent, and removal of solvent collapses the pores. A macroporous polymer retains pores in the dry state and may have as much as 700 m g" of internal surface. The macropores are created during polymerization by a solvent from which the polymer precipitates as it is formed. A macroporous polymer is usually, but not necessarily, highly cross-linked. In a good solvent, the polymer phase of a macroporous PS also becomes a microporous, solvent-swollen gel. 

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