SOLID SURFACES GENERAL PROPERTIES

Limited mobility of the molecules of solids. The essential difference between liquids and solids is that the particles of liquids can move, easily, long distances, while those of solids are practically fixed in position. The effects of this limited mobility, on their surface properties, are of two principal kinds. In the first place, those properties of liquid surfaces which are due to the free motion of the particles are absent, or very much less conspicuous, in solids. Thus solid surfaces do not contract spontaneously as a general rule, and it will be seen in Chapter VI that liquids do not spread over the surface of solids to form surface films, nearly as easily as they do on liquids, even when the attraction of the liquid for the solid is great enough for such a film, once spread, to be very stable. 

In the second place, the non-mobility of the surface particles of a solid results in the surface being often extremely uneven, unless special means such as polishing have been employed to smooth out the irregularities. The atoms in a solid surface stay where they are placed when the surface is formed, and this may result in no two adjacent atoms or molecules having the same properties. 

The heterogeneous character of solid surfaces is a matter of the greatest importance to Chemistry, as it is on the exceptional state of strain in certain atoms in the surface that the catalytic properties of surfaces usually depend. This subject will be dealt with in Chapter VII in this chapter, those properties which can be averaged over considerable areas of the solid surface, such as their power of being wetted by liquids, will be considered. The average properties of solids are often of very great industrial importance. 

---