Experimental Confirmation of the Oscillating Bubble Technique with Comparison to the Pendant Bubble Method: The Adsorption Dynamics of 1-Decanol

Abstract

A new oscillating bubble method is used to measure surfactant mass transfer kinetics at liquid–gas interfaces. A spherical bubble is formed, equilibrated, and oscillated radially with a small amplitude. The radial oscillations cause the gas-phase pressure to cycle about its equilibrium because of the periodic changes in bubble curvature and surface tension. The phase angle θ between the radial and the pressure oscillations and the amplitude ratio of these two quantities are measured as a function of forcing frequency ω′ and concentrationC′(0). These data are analyzed according to a linear analysis presented in part I of this research (J. Colloid Interface Sci.168,21, 1994) to find surfactant diffusivities and adsorption/desorption coefficients. The required input data are the equilibrium adsorption isotherm and the corresponding surface equation of state. For 1-decanol at the air–aqueous interface, equilibrium surface tension data are obtained by video-enhanced pendant bubble tensiometry and fitted to the generalized Frumkin model. The oscillating bubble method is then used to determine the mass transfer kinetics of 1-decanol. For ω′ ≤ 1 rad/s, the mass transfer is diffusion-controlled. Diffusivities found from the oscillating bubble data are in agreement with those obtained from pendant bubble relaxation data. For elevatedC′(0)and ω′ ≥ 1.0 rad/s, the mass transfer is controlled by both diffusion and the kinetics of adsorption–desorption. A mixed diffusion–kinetic model applied to these data yields a value for the desorption kinetic constant of α = 2.7 s−1. These results are consistent with the shift in controlling mechanism from pure diffusion control at dilute concentrations to mixed diffusion–kinetic control at elevated concentrations.