We will first examine how soaps function as detergents and emulsifying agents; the more complex synthetic surfactants act in an identical manner.
When a small amount of a soap, such as sodium stearate, is added to water in a beaker, the first step is dissociation into sodium and stearate ions. The latter then accumulate at the water–air and water–glass boundaries. This is the phenomenon of surface activity, but why does it occur? We know that the molecules have a long, unbranched, hydrophobic alkyl chain that does not interact with water molecules. In fact, the alkyl chain causes the surrounding water molecules to pull back from it and aggregate together by hydrogen bonding, forming a structured cage wall with the alkyl chain of the soap inside, minimising its contact with the water. The soap is soluble because the terminal anionic carboxylate group is hydrophilic and strongly solvated by polar water molecules. At the boundary between water and a more hydrophobic surface such as glass, air or oil, the soap molecules arrange themselves so that the long alkyl chain orients itself into the hydrophobic surface away from the water, with the carboxylate ion remaining in the water. A single layer of such oriented molecules occupies the surface between the two phases.
The available surface between the two phases becomes saturated with soap molecules at quite low soap concentrations and they then begin to accumulate in the bulk of the water. This unfavourable situation, in which the hydrophobic alkyl chains and their surrounding water cages are quite incompatible, is remedied by the soap molecules accumulating together to form molecular aggregates called micelles. The solution is colloidal since the micelles are large enough to scatter a light ray. The hydrophobic alkyl chains come together in the centre of the micelle, excluding any water molecules. In this way, the alkyl chains interact only with each other and avoid contact with water molecules. The carboxylate ion ‘heads’ of each soap molecule are at the outer surface of the micelle oriented into the water, thus keeping it in solution (Figure 9.3). The micelle may incorporate a number of sodium ions, but in smaller numbers than the carboxylate ions so that the micelle is anionic overall.
Surface adsorption and micelle formation are governed by the laws of thermodynamics. The ordering of the soap molecules at a phase boundary, or in a micelle, represents the most stable state of these molecules in the solution. When a soap molecule passes from the aqueous solution into a micelle, the structured cage of hydrogen-bonded water molecules surrounding the alkyl chain collapses.
Thus, hydrogen bonds will be broken but the water molecules gain considerable
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freedom of movement. The alkyl group of the soap molecule, which had no bonding interaction with the water molecules, now forms weak bonds with other alkyl groups in the micelle but the molecule has lost much of its freedom of movement. For micelle formation, DH0 = +2 kJ mol–1, indicating that the bond energy liberated from the hydrophobic interaction of the alkyl chains with each other in the micelle almost offsets that needed to break the hydrogen bonds when the structured water cage around the alkyl groups collapses. Micelle formation is, however, favourable because DS0 = +77 J mol–1 K–1, a substantial increase in entropy. The soap molecules give up a considerable degree of freedom on aggregating in the micelle. The entropy increase for the process is, however, dominated by the collapse of the structured cage around each alkyl group of a soap molecule in solution, when it goes into a micelle. Many water molecules that were bonded into the cage wall are therefore free and more mobile. This is the driving force for micelle formation.
Micelle
Molecules at the surface
Isolated molecule Key
CH3(CH2)16—CO2–
Figure 9.3 Surface activity and micelle formation in an aqueous soap solution SURFACEACTIVITYOFDETERGENTS
The formation of micelles is an important facet of surface activity. It occurs quite suddenly once the phase boundaries are saturated with a monolayer of surfactant molecules and the total concentration in solution increases above a critical value called the critical micelle concentration or CMC. The CMC is a characteristic property of a given surfactant. Its value is often lower than 1.0 g l–1. The formation of micelles depends on:
(1) the preference of the surfactant alkyl groups to interact with each other, in their own hydrophobic environment, rather than remain exposed to the water;
(2) the freeing of water molecules from the hydrogen-bonded cage around the hydrophobic groups when the surfactant molecules aggregate together.
With increasing surfactant concentration above the CMC, the numbers of micelles, and the average number of molecules in a micelle, both increase until eventually precipitation occurs. At the CMC, the change in the arrangement of surfactant molecules in the solution causes abrupt variations in various physical properties of the solution. These include the osmotic pressure, the molar conductivity and the air–solution surface tension, as well as the detergent power of the solution (Figure 9.4).
Figure 9.4 Variation of some physical properties of detergent solutions close to the CMC (critical micelle concentration)
C A
C
B
CMC
Property
Detergent concentration
Key
A Molar conductivity B Surface tension C Detergent action
161 In the interior of liquid water, each molecule is surrounded uniformly and equally attracted by neighbouring molecules in all directions. The surface tension originates from the unbalanced attraction of water molecules at the air–liquid interface towards neighbouring molecules in the bulk of the water pulling them inwards away from the air. These intermolecular attractions are mainly hydrogen bonds. If a liquid such as water has little attraction for molecules in the interface with which it is in contact, such as air, the surface tension will act to reduce the liquid surface area in contact with that interface. This is why water droplets in air, or on a wax surface, are approximately spherical. It is exactly the same phenomenon that causes water molecules to become more structured around the hydrophobic alkyl chain of a soap molecule. The air–water surface tension of a surfactant solution decreases rapidly as the concentration increases because surfactant molecules replace water molecules at the interface, with their alkyl chains and carboxylate groups oriented away from and towards the water, respectively. The surface tension increases somewhat, however, once the CMC has been exceeded (Figure 9.4).
Surface activity and micelle formation are dynamic effects and surfactant molecules are undergoing constant interchange between the interfacial monolayers, micelles and solution. Micelles may have different shapes and sizes depending on the surfactant, its concentration and the temperature. The effectiveness of a surfactant in emulsifying oils and fats depends very much on the micellar composition of the solution and therefore on the detergent concentration and temperature. Once the CMC has been reached, the detergent action increases only slowly and the use of a large excess of surfactant is wasteful (Figure 9.4).
It is not uncommon for a combination of two different surfactants to produce an effect that is greater than the combined individual effects of the two components. This is called a synergistic effect. It probably comes about from the formation of mixed micelles, containing molecules of both surfactants, that are more effective than micelles of the individual surfactants. These are complex chemical systems. Many products have special formulations for a particular purpose and the best combinations and concentrations for a particular process are often determined by trial and error.