Index header
Intermediate Membranes

Amphiphiles as Surfactants

Definitions

We know that amphiphilic molecules possess a special molecular structure -- contain both a water loving polar part (hydrophilic) and a water hating non-polar part (hydrophobic). This property turns out to be very important in determining the special structures formed by amphiphilic molecules. The formation of these special structures is responsible for the use of amphiphiles in a large number of applications ranging from oil recovery to efficient delivery of drugs at a desired site in the body to their use in soaps and detergents.

Amphiphiles are a special class of surface active molecules called surfactants . They are called surface active because they have the unique properties of getting adsorbed at various interfaces(e.g air-water, oil-water etc) and altering the properties of the interface. The hydrophilic part of an amphiphilic molecule (usually called the head) is a polar molecule whereas the hydrophobic part (called the tail) is a hydrocarbon chain 8-16 Carbon atoms in length.

Let us first try to understand how (or why?) special structures are formed.
When placed in a solvent (say water -an ionic solvent) the hydrophilic head and the hydrophobic tail behave differently. The hydrophobic parts tend to arrange themselves as close as possible to each other (in order to minimize contact with water) whereas the hydrophilic part try to separate from each other as far as possible. This can be explained by


Depending upon the relative strength of these forces different structures tend to form . If the interaction between the polar heads is strong relative to the interaction between the non-polar parts than the molecules tend to form micelles (from the Latin 'micella' - meaning small bit) .
If the interaction between the polar heads is not strong relative to the non-polar part then the molecules tend to form spherical, double-layered shells called vesicles. The formation of these structures can also be understood by realizing that they are the lowest energy configuration possible (i.e. energetically favorable) at different concentrations and given system parameters.

If the surfactants were placed in oil (or any other non-polar liquid) then the corresponding inverted structures form.Can you explain why ?

The stability of the micelles formed also depends upon whether the concentration of molecules is greater than or less than a critical micelle concentration. On changing the concentration of the amphiphilic molecules the resultant structures formed changes. For example the lamellar phase forms at high concentrations (this is also known as the smectic phase of lyotropic liquid crystals.) .

The property of amphiphilic molecules that invariably plays a role in all applications is their ability to reduce the interfacial tension of the interfaces.

A simple consequence of the property of lowering the interface tension is their ability to act as emulsifiers - that is the phenomenon of otherwise immiscible fluids becoming miscible.
The surface activity of the molecules encourages their migration to the surface (interface) of the two fluids (say oil and water). By lowering the interfacial tension the energy required for the two fluids to mix (by creating more surfaces) is reduced .Consequently the interface that separated the two fluids gets broken up into many many smaller closed interfaces - such that the two fluid have effectively mixed - they are now miscible.

Having understood how interfacial tension is reduced and that different structures are formed at different concentrations let us explain some applications of these structures. We will deal with