Surface Rheological Data for a Polymeric Surfactant Using a Pulsed Drop Rheometer

Measurements of dynamic interfacial tension of adsorbed layers of the oil-soluble polymeric surfactant E5 have been made using a pulsed drop rheometer. The pulsed drop rheometer is based on the instantaneous expansion of a water droplet in oil. After perturbation an interfacial relaxation occurs and is followed from the drop profile. The difference in pressure across a curved interface and the interfacial tension are directly related. The decay of pressure change, and hence the interfacial tension decay, is followed as a function of time using a sensitive pressure transducer. Concentrations of E5 above and below the CMC were investigated at the n-decane/water and Isopar M/water interfaces. The interfacial tension decays obtained were fitted to known relaxation mechanisms. Fourier transforms were calculated over a complete frequency spectrum to obtain the dilational elasticity and viscosity. Above the CMC, the interfacial relaxation of E5 at both the n-decane/water and Isopar M/water interfaces was shown to be due to the diffusion of micelles to the interface and the subsequent lowering of the interfacial tension. From the calculated diffusion coefficient and micelle size, the micellar aggregation number could be calculated. Below the CMC, both diffusion and reorientation contribute to the interfacial relaxation. It was not possible to determine the parameters for each process because the characteristic frequencies for the two processes are of similar magnitude.