How then does the surface activity of a surfactant influence removal of dirt from a fabric and its emulsification in solution? The emulsification of oil droplets in water
SURFACEACTIVITYOFDETERGENTS
using a surfactant is an appropriate starting point. As the two immiscible liquids are vigorously stirred, the oil breaks up into small droplets dispersed in the water.
The oil–water interface has a high surface tension. The molecules in the bulk of the hydrophobic oil pull in those at the surface in contact with the water. This minimises the surface and the interaction with the surrounding water and the oil droplets become spherical. If stirring is halted, the small oil drops will coalesce and the liquids will separate into two layers, with the oil floating on top of the water. In the presence of an anionic surfactant, such as sodium stearate, at its critical micelle concentration, each tiny oil droplet becomes surrounded by surfactant molecules, with their alkyl chains oriented into the oil surface and the anionic
‘heads’ extending into the water. The tiny droplet is like a giant micelle with an oily centre. Once agitation stops, the tiny oil droplets cannot now coalesce as the negatively charged monolayer of surfactant molecules at the oil–water interface causes mutual repulsion of the drops. The oil remains emulsified. The droplets are very small and they scatter light effectively, giving the emulsion a milky appearance.
The ability of surfactant molecules to adsorb on surfaces and orient themselves so that the ionic group is in contact with the water, and the alkyl chain is oriented away from it, is crucial in the wetting of fibres. This involves the spreading of water all over their surfaces. There must be strong molecular interaction between the water and the fibre to be wetted. This is aided by the accumulation and orientation of surfactant molecules at the water–air and water–fibre interfaces.
These effects are usually illustrated by means of the contact angle at the fibre–
water–air interface (Figure 9.5). The surfactant in the water decreases the water–
air and water–fibre surface tensions, but the unchanged fibre–air surface tension
Figure 9.5 Contact angles at the fibre surface for wetting and non-wetting
For wetting For non_wetting
Fibre Air Water
> + cos < + cos
Key
Contact angle
Surface tension for the fibre_air ( water_fibre ( ) and water_air ( ) interfaces
=
= ),
163 causes extension of the droplet with a decrease in the contact angle. The water drop therefore spreads out over the surface.
Complete wetting of fibre surfaces is crucial in all wet processes for textiles, particularly for dyeing. Uneven fibre wetting in dyeing invariably leads to uneven dye absorption. Wetting agents are widely used in dyeing processes to ensure good penetration of the dye liquor into the fibre mass. The chemicals used must be compatible with the dyeing conditions and be effective wetting agents over a range of temperatures, at various pH values and in the presence of salts.
The action of a detergent involves similar principles. Many types of dirt on a textile surface are hydrophobic in character. During washing, the surface of the dirt on the fabric becomes surrounded by surfactant molecules, with their hydrophobic ‘tails’ oriented into the dirt. The surfactant molecules also saturate the fibre–water interface in a similar fashion. This decreases the fibre–water and oil–water surface tensions. The unchanged oil–fibre surface tension, and the repulsion between the negative charges of the fibre–water and dirt–water interfaces, cause the dirt to reduce its surface area by rolling up. The effect of the surfactant in the washing solution is to decrease the contact angle, the reverse effect of that in wetting (Figure 9.6). The contact angle must be greater than 90°
to remove oil from a fibre. Eventually, the dirt begins to lift from the fibre and is removed into the solution. It is held in suspension by electrostatic repulsion, as in the case of emulsified oil droplets. Emulsified dirt is not likely to be redeposited on the fibre surface because of mutual repulsion of the negatively charged layers of adsorbed surfactant molecules on the dirt–water and fibre–water interfaces.
Detergent solution
Hydrophobic dirt
Fibre Hydrophilic
dirt
Figure 9.6 Surfactants removing hydrophobic and hydrophilic dirt from a fibre surface SURFACEACTIVITYOFDETERGENTS
A similar mechanism applies for eliminating insoluble hydrophilic polar dirt.
Firstly, surfactant molecules adsorb onto the surface of the hydrophilic dirt, but, in this case, the anionic ‘heads’ orient into the surface of the polar dirt. To avoid contact of the hydrophobic ‘tails’ with the water, a second layer of surfactant molecules forms with their hydrocarbon tails interacting with those extending outwards from the first layer. This second layer of molecules has its anionic groups exposed towards the water. Such dirt therefore rolls up and lifts from the surface and is held in suspension, as before (Figure 9.6).
Although many surface-active chemicals can act as both wetting agents and detergents, this is not always true. Wetting depends mainly on reducing the surface tension of the wetting liquid, whereas detergency depends on micelle formation and the ability of the surfactant to keep dirt in suspension. The surfactant alkyl chain must be at least six carbon atoms long for surface activity to occur but at least twelve for even minimal detergent action.
Having established the principles of surface activity and micelle formation, and their role in wetting, emulsification and detergency by means of simple soap molecules, we will now examine the chemical nature of the many types of synthetic surfactant.