|University Federico II
of Naples, Italy
Schematic representation of a micelle in aqueous solution
In aqueous solution, molecules having both polar or charged groups and non polar regions (amphiphilic molecules) form aggregates called micelles. In a micelle, polar or ionic heads form an outer shell in contact with water, while non polar tails are sequestered in the interior. Hence, the core of a micelle, being formed of long non polar tails, resembles an oil or gasoline drop. The length of the non polar tail, the nature and size of the polar or ionic head, the acidity of the solution, the temperature, and the presence of added salts are the most important factors determining the kind of the obtained aggregate. If those parameters are changed, it is possible to change shape and size of the micelles. The number of amphiphilic molecules forming the aggregate is called aggregation number; it is a way to describe the size of the micelle.
Micelles are widely used in industrial and biological fields for their ability to dissolve and move non polar substances through an aqueous medium, or to carry drugs which are, often, scarcely soluble in water. The carrying ability of micelles can be altered if parameters determining their size and shape are changed.
Left: the concentration of the amphiphilic molecule is less than cmc
Center: The concentration is just the cmc: micellization begins
Right: The number of micelles increases, but the concentration of free molecules does not change
Micelle aggregates form only when the concentration of the amphiphilic molecule reaches a given concentration called critical micelle concentration (cmc). That condition is monitored by the sudden change in the chemical and physical properties of the solution. On the contrary, below cmc micelles are completely absent.
Hydrophobic interaction between two non polar molecules in water.
(a) Two separated molecules, each one encircled by its own cage of water molecules (blue colour).
(b) The two molecules aggregate, forming a cage having less water molecules. The aggregation of non polar molecules is stabilized by a higher freedom of movement (increased disorder) of water molecules.
Micellization depends on the balance of two main effects: the tendency of the non polar tails to avoid contacts with water and the repulsion among the polar or charged heads, a destabilizing effect on the aggregation process. Hydrocarbon tails avoid contacts with the solvent molecules pointing toward the aggregate interior, which lacks of water. Instead, the repulsion among the charged heads on the surface of the micelle is attenuated by the presence of oppositely charged ions (counter-ions). The favourable association among the non polar tails in the interior of the micelle occurs through the hydrophobic interaction, which is the prevailing effect in the formation process of these aggregates.
A reverse micelle. Polar heads point toward the interior, while the non polar tails are exposed to the non polar solvent. The arrow indicates water "sequestered" in the interior.
Amphiphilic molecules can form micelles not only in water, but also in non polar organic solvents. In such cases, micelle aggregates are called inverse micelles because the situation is inverted as respect to water. In fact, hydrocarbon tails are exposed to the solvent, while the polar heads point toward the interior of the aggregate to escape the contacts with the solvent.
Reverse micelles are able to hold relatively large amounts of water in their interior. In that way, a "pocket" is formed which is particularly suited for the dissolution and transportation of polar solutes through a non polar solvent.