Surface Viscosity and Elasticity Theory

The adsorbed surfactant film is assumed to control the medianical-dynamical properties of the surface layers by virtue of its surface viscosity and elastidty. This concept may be true for thick films ( 100 nm) whereby intermolecular forces are less dominant (i.e. foam stability under dynamic conditions). Surface viscosity reflects the speed of the relaxation process that restores the equilibrium in the system after imposing a stress on it. Surface elastidty is a measure of the energy stored in the surface layer as a result of an external stress. 

The viscoelastic properties of the surface layer are an important parameter. Surface scattering methods are the most useful techniques for studying the viscoelastic properties of surfactant monolayers. When transversal ripples occur, periodic dilation and compression of the monolayer arises and this can be accurately measured. This enables one to obtain the viscoelastic behaviour of monolayers under equilibrium and non-equilibrium conditions, without disturbing the original state of the adsorbed layer. 

Some correlations have been found between surface viscosity and elasticity and foam stability, e.g. when adding lauryl alcohol to sodium lauryl sulphate, which tends to increase the surface viscosity and elastidty [10]. 

The Gibbs coefficient of elasticity, e, was introduced as a variable resistance to surface deformation during thinning  

The main deficiency of the early studies on Gibbs elasticity was that they were applied to thin films and diffusion from the bulk solution was neglected. In other words, the Gibbs theory applies to the case where there is insufficient surfactant molecules in the film to diffuse to the surface and lower the surface tension. This 

In addition to the Laplace capillary pressure, three additional forces can operate at surfactant concentration below the cmc, namely electrostatic double layer repulsion Tj1, van der Waals attractions and steric (short-range) forces 

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The polymer-surfactant complex has high surfeice viscosity and elasticity (i.e. surfeice viscoelasticity), both will enhance the foam stability (see below). The amphoteric surfactants such as betaines and the phospholipid surfeictants when used in conjunction with alkyl sulfeites or alkyl ether sulfeites can also enhance the foam stability. All these molecules strengthen the film of surfactant at the air/water interface, thus modifying the lather from a loose lacy structure to a rich, dense, small bubble size, luxurious foam. Several foam boosters have been suggested and these include fatty acid alkanolamide, amine oxides. Fatty alcohol and fatty acids can also act as foam boosters when used at levels of 0.25-0.5 %. Several approaches have been considered to explain foam stability (a) Surface viscosity and elasticity theory The adsorbed surfeictant film is assumed to control the mechanical-dynamical properties of the surface layers by virtue of its surface viscosity and elasticity. This may be true for thick films (> 100 nm) whereby intermolecular forces are less dominant. Some correlations... 

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