The cause of accelerated desorption of sparingly soluble dodecanol monolayers: Convection or leakage?
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2021-11-20
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Abstract
The dissolution of sparingly soluble surfactants from spread monolayers is a complex multi-staged process. The desorption of dodecanol from the surface of water follows mixed barrier/diffusion kinetics only in the first stages of the dissolution. Significant acceleration of the desorption has been observed experimentally after this initial period, which has been hypothesized to be due to onset of convective diffusion; the source of convection, however, has never been identified. The goal of this work is to investigate the question through desorption experiments under controlled convection and respective modeling of the process under mixed barrier/convective diffusion control. Several hypotheses for the cause of the accelerated desorption have been tested. The analysis has shown that natural convection, Marangoni convection, convection due to the motion of the mechanical barrier of the Langmuir trough, and artificial convection caused by an electromagnetic stirrer cannot produce desorption rates of the observed magnitude. These findings convincingly prove for the first time that the convective diffusion has less of a role in the transport process than previously thought. The most likely reason for the acceleration is identified as leakage through the movable barrier. The rate of this leakage is estimated form the experimental data. Implications for the use of adsorption isobars to study desorption kinetics are discussed.