OSCILLATING BUBBLE TENSIOMETRY - A METHOD FOR MEASURING THE SURFACTANT ADSORPTIVE-DESORPTIVE KINETICS AND THE SURFACE DILATATIONAL VISCOSITY

Mobilities of surfactant laden interfaces are determined by both surfactant-mass-transfer kinetics and surface viscosities. In this paper, a theoretical framework for measuring these parameters by analyzing forced radial oscillations of a spherical pendant bubble about an equilibrium, quiescent base state is developed. Because of the Gibbs-Marangoni elasticity caused by hindered surfactant mass transfer, and the surface viscosities, oscillations in the gas phase pressure and bubble radius are out of phase. Using a linear analysis of the governing fluid mechanical and mass-transfer equations, the phase lag (theta) and the amplitude ratio of these two quantities (Lambda) are derived. Three cases are considered for the surfactant mass transfer: a mixed-controlled model in which diffusion and sorption kinetics play a role, along with the limiting cases of diffusion-control and sorption-control, respectively. Both theta and Lambda depend upon the bulk diffusivity, the equilibrium physicochemical constants, and two unknowns: the sorption kinetic constant and the surface dilatational viscosity. In this paper, by varying these unknowns, theoretical families of curves for both theta and Lambda vs forcing frequency, omega', are generated using values for the bulk diffusivity and the equilibrium physicochemical constants for decanol at aqueous-air interfaces from Lin et al. (Langmuir 7, 1055, 1991). These curves indicate the potential of the oscillating bubble as a measurement tool, i.e., that experiments in which theta and Lambda are measured vs omega' can be used to determine the adsorption-desorption kinetic constants and the surface dilatational viscosity and to differentiate them from each other. (C) 1994 Academic Press,Inc.