Experimental confirmation of the oscillating bubble technique with comparison to the pendant bubble method: The adsorption dynamics of 1-decanol

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 theta between the radial and the pressure oscillations and the amplitude ratio of these two quantities are measured as a function of forcing frequency omega' and concentration C-(o)'. These data are analyzed according to a linear analysis presented in part I of this research (J. Colloid Interface Sci, 168, 21, 1993) 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 omega'less than or equal to 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 elevated C-(0)' and omega'greater than or equal to 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 fields a value for the desorption kinetic constant of alpha=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. (C) 1996 Academic Press, Inc.