Читать книгу Applied Colloid and Surface Chemistry - Richard M. Pashley - Страница 16

The link between colloids and surfaces follows naturally from the fact that particulate matter has a high surface area to mass ratio. The surface area of a 1 cm diameter sphere (4πr²) is 3.14 cm², whereas the surface area of the same amount of material but in the form of 0.1 micron diameter spheres (i.e., the size of the particles in latex paint) is 314,000 cm². The enormous difference in surface area is one of the reasons why the properties of the surface become very important for colloidal solutions. One everyday example is that organic dye molecules or pollutants can be effectively removed from water by adsorption onto particulate or granular activated charcoal because of its high surface area. This process is widely used for water purification and in the oral treatment of poison victims.

Although it is easy to see that surface properties will determine the stability of colloidal dispersions, it is not so obvious why this can also be the case for some properties of macroscopic objects. As one important illustration, consider the interface between a liquid and its vapour:

Figure 1.2 Schematic diagram to illustrate the complete bonding of liquid molecules in the bulk phase but not at the surface.

Table 1.2

Liquid	Surface Energy in mJm−2 (at 20 °C)	Type of Intermolecular Bonding
Mercury	485	metallic
Water	72.8	hydrogen bonding + vdw
n‐Octanol	27.5	hydrogen bonding + vdw
n‐Hexane	18.4	vdw
Perfluoro‐octane	12	weak vdw

Molecules in the bulk of the liquid can interact via attractive forces (e.g., van der Waals) with a larger number of nearest neighbours than those at the surface. The molecules at the surface must therefore have a higher energy than those in bulk, since they are partially freed from bonding with neighbouring molecules. Thus, work must be done to take fully interacting molecules from the bulk of the liquid to create a new surface. This work gives rise to the surface energy or tension of a liquid. Hence, the stronger the intermolecular forces between the liquid molecules, the greater this work will be, as is illustrated in the table.

The influence of this surface energy can also be clearly seen on the macroscopic shape of liquid droplets, which in the absence of all other forces will always form a shape of minimum surface area – that is, a sphere in a gravity‐free system. This is the reason why small mercury droplets are always spherical. Note that the term ‘interface’ is often used where the surface is formed between two different materials.

Although a liquid will always try to form a minimum surface area shape, if no other forces are involved, it can also interact with other macroscopic objects to reduce its surface tension via molecular bonding to another material, such as a suitable solid. Indeed, it may be energetically favorable for the liquid to interact and ‘wet’ another material. The wetting properties of a liquid on a particular solid are very important in many everyday activities and are determined solely by surface properties, which are derived from intermolecular forces. One important and common example is that of the behaviour of water on clean glass. Water wets clean glass because of the favourable hydrogen bond interaction between the surface silanol groups on glass and adjacent water molecules, as illustrated below.

However, exposure of glass to Me₃SiCl vapour rapidly produces a 0.5 nm layer of methyl groups on the surface:

Figure 1.3 Water molecules form hydrogen bonds with the silanol groups at the surface of clean glass.

Figure 1.4 Water molecules can only weakly interact (by vdw forces) with a methylated glass surface.