Here we try to answer the following two questions

1. Why do surfactants tend to go to the surface/interface of a medium ?
2. How do surfactants reduce the interfacial tension ?

The surface of a liquid is defined as the boundary between two bulk phases, usually between the liquid and air.
An interface is formed between two "immiscible" liquids when they are saturated with each other.

To explain why amphiphilic molecules tend to migrate to the surface/interface of a system - we should understand that in general any system tends to evolve such that its lowers it potential energy and hence reach a stable state. An amphiphilic molecule at the interface places itself such that the head and tail are in different mediums according to their liking - this configuration is the one with lowest potential energy .

A stable interface (neglecting gravity) requires a positive value of the interfacial tension -- this implies the energy must be increased if the area of the interface is to be increased. If this were not the case then even in the absence of an external agent to apply the energy the area would keep increasing as this would lead to a decrease of the energy (remember: a system tends to a state of lower energy). This would continue until the interface became so convoluted that the two immiscible phases had dissolved into each other to become miscible.(schematic diagram of this process) But we know that this could not be true for a stable interface. If the value of the interface tension can be reduced to very small values (typically 0.0000001 Newtons/meter or even negative values) then mixing by this process becomes possible - actually this is what is required to make oil and water mix.

It can be stated in general terms that solutes that concentrate at the interface (i.e get adsorbed) tend to decrease the interfacial tension .
The interfacial tension is a function of

• number of molecules adsorbed at the interface ,
• the concentration of the bulk phases (i.e the fluids) and
• temperature.

So we see that amphiphilic molecules tend to migrate to the interface and lower the interfacial tension.

Thus an easy way to decrease the value of the interfacial tension to low enough values that the two fluids can mix (become miscible) is to add surfactants that get adsorbed at the interface.
For a system of fluids under given conditions we can reduce to nearly zero the interfacial tension by making an appropriate choice of a surfactant and amount of surfactant added.