Wetting by Solutions Containing Surfactants

If the test liquid is not a pure liquid but a solution containing a compound that adsorbs at the LG and/or SL interface, the contact angle is affected by the adsorbed amount E which, in turn, is related to the concentration of that surface active compound in solution. Hence, a pure liquid that wets the surface poorly (cos 0 1) may change into a good wetting liquid by adding a surface active compound and vice versa. 

According to Gibbs law (Equation 3.88), adsorption leads to a lowering of the interfacial tension. Reduction of y and at constant y (no surfactant adsorp- 

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