Dynamic surface tension and Gibbs elasticity of perfluoropolyether surfactant films

Axisymmetric drop shape analysis has been applied to provide a first insight into the adsorption kinetics and surface viscoelasticity of perfluoropolyether carboxylic salts at the water-air interface. The two surfactants Cl(C3F6O)(n)CF2COONH4, with two or three perfluoroisopropoxy units, n, have been examined in aqueous solution at molar concentrations well below their critical micelle concentration (cmc), first to compare, at constant reduced concentration [logc-log(cmc)], the rate of surface tension reduction with time up to 2,500 s, typically sufficient to attain equilibrium conditions. Then the surface area of each drop was varied with a sinusoidal profile at a frequency in the range 0.016-0.2 s(-1), to obtain the Gibbs elasticity, epsilon(0), and the relaxation frequency, coo. The adsorbed films of both perfluoropolyether surfactants displayed Eo decreasing upon concentration increase, while epsilon(0) increased with the surfactant chain length.