Small Volumes and Large Surfaces The World of Colloids

The situation became confused at the beginning of the twentieth century, when it was shown that the crystalloids could have a similar diffusion behaviour to the colloids in certain solvents. It soon became clear that the colloids should not be considered as a special class of chemical substances, but rather that matter could naturally or artificially adopt a colloidal state that corresponded to a certain degree of physical division. From this moment, the size of objects was taken as the relevant parameter in defining the colloidal state. 

Although the International Union of Pure and Applied Chemistry (lUPAC) proposed in 1972 that the word colloid should be reserved for objects in which 

In fact, the colloidal domain is associated with an intermediate scale, lying between the scale of atoms and small molecules and the scale of objects observable using an ordinary microscope. This is a range of sizes we are not familiar with. To give some idea, if we place a small sphere of diameter 10 nm in a glass of diameter 10 cm, and we wish to increase the size of the system until the colloidal object becomes visible to the naked eye, taking it to a diameter of 1 mm, for example, then the container will have diameter 10 km. 

The transition between the macroscopic state and the colloidal state involves a very great increase in the surface-to-volume ratio. Thus, whereas 1 g of sand, in the form of silica beads 1 mm in diameter, possesses the modest surface area of 30 cm, the same quantity of silica, divided up into superfine particles only 10 nm in diameter, will have an enormous total surface area, of the order of 300 m.  

In addition, when the degree of division is increased, the proportion of atoms on the interfaces also grows. To take the example of the silica, whereas the proportion of Si02 constituents in the material at the surface of the particles is infinitesimal for the 1 mm beads, it reaches 20% for the 10 nm beads. The importance of interfaces for the physicochemical properties of colloidal systems is clear. 

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Daniel, J.C. and Audebert, R., Small volumes and large surfaces the world of colloids, in Soft Matter Physics, Daoud, M. and Williams C.E., eds.. Springer Verlag, 1999, p. 320. 

Small Volumes and Large Surfaces The World of Colloids... 

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