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.
WETTING PROPERTIES AND THEIR INDUSTRIAL IMPORTANCE
These methylated groups cannot hydrogen bond, and hence water now does not wet and instead forms beads of high ‘contact angle’ (θ ) droplets, and the glass now appears to be hydrophobic, with water droplets similar to those observed on paraffin wax.
This dramatic macroscopic difference in wetting behaviour is caused by only a thin molecular layer on the surface of glass and clearly demonstrates the importance of surface properties. The same type of effect occurs every day, when dirty fingers transfer natural grease and fats on to a drinking glass! Thorough cleaning of a glass flask (e.g., by washing in concentrated NaOH solution, as used in dishwashers) initially allows only a thin water film to coat (wet) the inside of the flask, producing visible coloured interference patterns (see photo). When contaminants from the air atmosphere slowly leak in via the stopper, this wetting film is displaced by a fine layer of non‐wetting droplets, forming an opaque mist film.
Figure 1.5 A non‐wetting water droplet on the surface of methylated, hydrophobic silica.
Figure 1.6 Clean glass flask with water wetting film and start of mist layer intrusion.
Surface treatments offer a remarkably efficient method for the control of macroscopic properties of materials. When insecticides are sprayed onto plant leaves, it is vital that the liquid wet and spread over the surface, rather than form a mist layer. Another important example is the froth flotation technique, used by industry to separate about a billion tons of ore each year. Whether valuable mineral particles will attach to rising bubbles and be ‘collected’ in the flotation process is determined entirely by the surface properties or surface chemistry of the mineral particle, and this can be controlled by the use of low levels of ‘surface‐active’ materials, which will selectively adsorb and change the surface properties of the mineral particles. Very large quantities of minerals are separated simply by the adjustment of their surface properties.
Although it is relatively easy to understand why some of the macroscopic properties of liquids, especially their shape, can depend on surface properties, it is not so obvious for solids. However, the strength of a solid is determined by the ease with which micro‐cracks propagate when placed under stress, and this depends on its surface energy. This understanding is crucial for the safe design of aircraft. That is, the design must consider the amount of (surface) work required to continue a crack and hence expose new surface. This also has the direct effect that materials are stronger in a vacuum, where their surface energy is not reduced by the adsorption of either gases or liquids typically available under atmospheric conditions. Much lighter structures can be made in space.
Many other industrial examples where colloid and surface chemistry plays a significant role will be discussed later; these include:
latex paint technology
water treatment
cavitation
emulsions and microemulsions
soil science
soaps and detergents
food science
mineral processing
Recommended Resource Books:
| Adamson, A. W. (1990) | Physical Chemistry of Surfaces, 5th edn, New York, Wiley. |
| Birdi, K. S. (ed,) (1997) |
CRC Handbook of Surface and Colloid Chemistry,
|