Journal of Colloid and Interface Science, Vol.488, 166-179, 2017
New sensitive micro-measurements of dynamic surface tension and diffusion coefficients: Validated and tested for the adsorption of 1-Octanol at a microscopic air-water interface and its dissolution into water
Currently available dynamic surface tension (DST) measurement methods, such as Wilhelmy plate, droplet- or bubble-based methods, still have various experimental limitations such as the large size of the interface, convection in the solution, or a certain "dead time" at initial measurement. These limitations create inconsistencies for the kinetic analysis of surfactant adsorption/desorption, especially significant for ionic surfactants. Here, the "micropipette interfacial area-expansion method" was introduced and validated as a new DST measurement having a high enough sensitivity to detect diffusion controlled molecular adsorption at the air-water interfaces. To validate the new technique, the diffusion coefficient of 1-Octanol in water was investigated with existing models: the Ward Tordai model for the long time adsorption regime (1-100 s), and the Langmuir and Frumkin adsorption isotherm models for surface excess concentration. We found that the measured diffusion coefficient of 1-Octanol, 7.2 +/- 0.8 x 10(-6) cm(2)/s, showed excellent agreement with the result from an alternative method, "single microdroplet catching method", to measure the diffusion coefficient from diffusion-controlled micro droplet dissolution, 7.3 +/- 0.1 x 10(-6) cm(2)/s. These new techniques for determining adsorption and diffusion coefficients can apply for a range of surface active molecules, especially the less-characterized ionic surfactants, and biological compounds such as lipids, peptides, and proteins. (C) 2016 Elsevier Inc. All rights reserved.
Keywords:Micropipette interfacial area-expansion method;Single microdroplet catching method;Dynamic surface tension;1-Octanol;Diffusion coefficient;Frumkin isotherm;Ward-Tordai model