Adsorption kinetics of C10E4 at the air–water interface: consider molecular interaction or reorientation

The adsorption kinetics of C10E4 was studied using a video-enhanced pendant bubble tensiometer. Two dynamic processes were studied. The bubble was suddenly created and C10E4 adsorbs onto a freshly created interface. After it had reached the equilibrium, the bubble was then impulsively shrunk and the surface was compressed with some change of area large enough to appreciably enrich the surface concentration and change the surface tension. Two sets of equilibrium data, equilibrium surface tension γ(C∞) and surface equation of state γ(Γ) were measured and utilized for the determination of model parameters. The data were analyzed by either considering the molecular interaction between the adsorbed molecules or assuming that surfactant molecules adsorb at interface in two orientation states. Both models fit the equilibrium γ(C∞) and dynamic γ(t) data well. There exists a shift on the control mechanism for C10E4 onto a freshly created air–water interface: diffusion-control at dilute C∞ and mixed-control at higher C∞. The re-equilibration for C10E4 out of a suddenly compressed surface at C∞=1.0×10−8 mol cm−3 is also a mixed controlled process. An average diffusivity of (7.2±1.0)×10−6 cm2 s−1 and adsorption/desorption rate constants (β1=3.0×107 cm3 (mol s)−1 and α1=5.1×10−2 s)−1 are obtained using the Frumkin model. A discussion on the effect of interfacial curvature of fluid/liquid interface was also provided.