Particle mobility, contact-induced

In acid media (pH 2) magnetite crystals ca. 10 nm across transform topotactically to maghemite via an adsorption reaction which traps mobile electrons from the bulk material and reduces interfacial Fe the Fe ions that form are selectively leached into solution (Jolivet Tronc, 1988). Electron delocalization also induces ferrihydrite in contact with small magnetite particles to transform into a spinel layer (Belleville etal., 1992). 

In GC the gaseous mobile phase must be confined in a column, so that a pressure gradient can cause it to flow past the stationary phase and eventually elute the separated bands out the effluent end of the column. This is inherently a ID separation, along the column, from one end to the other. This dimensionality applies even should the column be coiled to fit in a GC oven rather than vertically straight, like Tswett s gravity flow liquid mobile phase column. However, unlike gases, liquids as mobile phases do not always require confinement to move in a desired direction or retain their volume. If they are in contact with porous beds of small particles or fiber mats, surface forces (capillary attraction) can often induce them to flow. Thus it is possible to carry out the LC process on a stationary phase arrayed as a thin surface layer, usually a planar, 2D surface. Examples include the matted cellulose fibers of a sheet of paper, or a thin layer of silica gel or alumina particles on a planar support (e.g., a pane of glass). 

Under certain conditions, particles of two different types of solid materials may react. This can be as a result of a more favorable match of character according to the hard-soft interaction principle. In order to make the structural units (usually ions) mobile, some energy source is normally apphed. The most common way to bring about the reaction between two sohds is by heating them in close contact. The application of pressure may also provide added inducement for the reaction to occur. 

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