Molecules or ions which are amphiphilic, that is they contain both a hydrophobic and hydrophilic part, in aqueous solution frequently assemble at interfaces. This property has given them the name surface-active agents which is very often abbreviated to surfactants. The hydrophobic, usually hydrocarbon portion, which can be linear or branched, interacts very weakly with the water molecules in an aqueous environment. This is due to the strong interactions between the water molecules, arising from dispersion forces and hydrogen bonding acting cooperatively to squeeze the hydrocarbon tail out of the water. Hence the tail is usually termed hydrophobic.
There are four main classes of surfactants namely anionic, cationic, non-ionic and zwitterionic. Surfactants are grouped into one of these categories depending on the nature of the head group.
In anionic surfactants the surface active species is the anion, as in Sodium Dodecyl Sulfate, CH3(CH2)11SO4-Na+.
In cationic surfactants the surface active species is positively charged, as in dodecylamine hydrochloride, CH3(CH2)11+NH3 Cl-.
Zwitterionic surfactants have two ionogenic groups producing a cation and an anion. Zwitterionic surfactants can be ampholytic and can behave as either cationic, anionic, or non-ionic species depending on the pH of the solution, an example is N-dodecyl-N,N-dimethyl betaine, C12H25N+(CH3)2CH2COO-.
Non-ionic surfactants have uncharged head groups which are however polar in nature. These head groups are usually based on a polyoxyethylene chain. An example is polyethyleneglycol mono[4-(1,1,3,3-tetramethylbutyl)phenyl]ether. This surfactant is also available commercially under the name TritonX-100.
In these surfactants the head group is usually larger than the hydrocarbon tail. Non-ionic surfactants, though, with small head groups also exist, such as dodecyl sulfinyl ethanol ( C12H25SOCH2CH2OH ).
Besides assembly at interfaces surfactants undergo a process of self-assembly which is more commonly known as micellization. This is another way by which surfactants can sequester their apolar part from contact with the aqueous phase and thus reduce the free energy of surfactant systems.
The concentration of surfactant at which micelles first appear is called the critical micelle concentration (CMC). Below the CMC virtually all the dissolved surfactant exists in the monomer form, whereas above the CMC the surfactant in excess of this critical concentration exists in the micellar form. The variation of many physical properties with concentration of surfactant in solution changes dramatically in excess of the CMC as can be seen from the graph.The surface tension is constant above the CMC, this shows that although more material is being added to the solution, the surfactant's surface activity is remaining constant. This can only be explained if the surface-active molecules are associating to form units with low activity in solution. The near constant conductance of ionic micellar solutions above the CMC shows that the concentration of free surface-active molecules remains fairly constant with added surfactant. The increase in absorbance maxima above the CMC is also indicative of the formation of larger units i.e. micelles in solution. Observations of these physical properties are therefore used in the determination of the CMC of surfactants.
In general micellization is favored by the following factors:
The number of surfactant molecules in a micellar aggregate, or the aggregation number, is usually between 50 to 120 at surfactant concentrations just above the CMC. The aggregation number increases with concentration of surfactant in solution, temperature and electrolyte concentration. The increase in micellar size is often accompanied by a transition in micellar shape from sphere to rod.
