Interfacial Films and Spontaneous Curvature

The main point established up to now is that amphiphilic molecules tend to form an interface when put into contact with a solvent preferring one of their sequences. This interface segregates the two sequences, thereby producing an interfacial film. The film may have variable curvature and its properties depend on temperature in a way determined by the liquid or solid state of the chains. 

In fact, the predominant forces determining association of amphiphiles in well defined structures (e.g., micellar, cylindrical, lamellar) are the hydrophobic effect, tending to associate chains together, and repulsions between head groups. The latter are of electrostatic origin for ionic surfactants and steric for non-ionic surfactants. These two forces tend respectively to diminish or increase the interfacial area per molecule at the water/chain interface. The result is an optimal interfacial area Uq. 

Once the parameter uq is fixed, packing geometry is determined by  

It has been shown by Israelachvili, J. that the dimensionless parameter v/aolc determines the structure in the following way  

One way of causing the structure to vary is therefore to change Uq by modifying molecular interactions. For example, increasing the ion concentration, electrostatic repulsions between head groups are reduced by screening, uq can then decrease and the curvature can vary, e.g., micelles may transform from spherical to cylindrical shapes. 

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