(161b) Microtensiometer to Directly Probe Kinetic Adsorption Constants

Prediction of dynamic properties of interfaces requires detailed knowledge of the rates of diffusion and adsorption/desorption for surfactants at these interfaces. The coefficients of diffusion, adsorption, and desorption should be independent of the measurement technique. However, the approaches currently used to obtain these parameters are highly context dependent and can lead to unphysical trends such as a concentration-dependent diffusion coefficient and large mismatches between predicted and observed dynamic behavior. In pendant drop studies, one is restricted to conditions in which diffusion limits mass transport, or cases in which timescales for diffusion and sorption kinetics are comparable. Recently, theoretical and numerical work has demonstrated that a characteristic lengthscale governs the transition from diffusion-limited to kinetic-limited mass transfer for spherical interfaces [1]. This theory predicts that if the spherical interface has a radius much smaller than the intrinsic length scale, the mass transport process is kinetically limited. To experimentally validate this predicted transition to kinetic-limited mass transport, we have developed a micro-tensiometer to measure the dynamic surface tension at 10 to 100 micron diameter spherical interfaces. Using this new technique, we have shown that a transition from diffusion-limited to kinetic-limited dynamics is achieved as the radius decreases for a series of alkyl polyethylene oxide (CiEj) surfactants. We describe a new scaling for the diffusion timescale which better captures the physics of diffusion limited adsorption of surfactants onto spherical interfaces. Finally, using experiments, numerics, and scaling analyses, we demonstrate that the microtensiometer enables the direct measurement of sorption kinetic constants for these surfactants.

[1] Jin, F., Balasubramaniam, R., and Stebe, K.J., J. Adhesion. 80 (2004) 773.