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 surfa ctant-mass-transfer kinetics and surface viscosities. In this paper, a theoretical framework for measuring these parameters by analyzing for ced radial oscillations of a spherical pendant bubble about an equilib rium, quiescent base state is developed. Because of the Gibbs-Marangon i elasticity caused by hindered surfactant mass transfer, and the surf ace viscosities, oscillations in the gas phase pressure and bubble rad ius are out of phase. Using a linear analysis of the governing fluid m echanical and mass-transfer equations, the phase lag (theta) and the a mplitude ratio of these two quantities (Lambda) are derived. Three cas es are considered for the surfactant mass transfer: a mixed-controlled model in which diffusion and sorption kinetics play a role, along wit h the limiting cases of diffusion-control and sorption-control, respec tively. Both theta and Lambda depend upon the bulk diffusivity, the eq uilibrium physicochemical constants, and two unknowns: the sorption ki netic constant and the surface dilatational viscosity. In this paper, by varying these unknowns, theoretical families of curves for both the ta and Lambda vs forcing frequency, omega', are generated using values for the bulk diffusivity and the equilibrium physicochemical constant s for decanol at aqueous-air interfaces from Lin et al. (Langmuir 7, 1 055, 1991). These curves indicate the potential of the oscillating bub ble as a measurement tool, i.e., that experiments in which theta and L ambda are measured vs omega' can be used to determine the adsorption-d esorption kinetic constants and the surface dilatational viscosity and to differentiate them from each other. (C) 1994 Academic Press,Inc.