Abstract
In drop profile analysis tensiometry, the ratio of drop surfaces area S to volume V is large, i.e., S/V >> 1. In such a case, the concentration of a surfactant within the drop bulk decreases due to adsorption at the drop surface. In contrast, in bubble profile analysis tensiometry, we have S/V << 1 so that depletion due to adsorption is negligible. A protocol is presented to determine the correct adsorption parameters of surfactants from surface tension data measured by bubble and drop profile analysis tensiometry. The procedure is applied to experimental data measured for selected surfactants of different adsorption activities: C10OH, CTAB, Tween 20, and the equimolar mixture SDS + DoTAB. The results show that for surfactants with higher surface activity, the differences between the surface tensions measured with the drops and bubbles profile analysis tensiometry, respectively, are larger, while for less surface-active surfactants, such as SDS, the results obtained from drop and bubble profile experiments are very close. The correction procedure is based on the same set of adsorption parameters used to fit both the experimental data obtained from drop-based measurements (which involve the depletion effects) and those data measured in a way that depletion effects are negligible.
Highlights
Bubble and drop profile analysis tensiometry is presently the most frequently used technique for studies of adsorption layer properties of aqueous solutions of surfactants, polymers, and their mixtures as summarised in [1]
This protocol takes into account the mass balance of the surfactant in the drop bulk and the amount adsorbed at the drop surface
An algorithm is developed for the determination of the characteristic adsorption parameters from experimental data measured by drop profile analysis tensiometry
Summary
Bubble and drop profile analysis tensiometry is presently the most frequently used technique for studies of adsorption layer properties of aqueous solutions of surfactants, polymers, and their mixtures as summarised in [1]. Due to the possibility of accurately controlling the size of a drop, i.e., of its volume and surface area, this method can be applied to measure the dilational visco-elasticity of interfacial layers [12] It was shown, that the results obtained from the analysis of a drop profile and a corresponding bubble profile can deviate from each other significantly, in both cases the surface tension of the liquid/air interface is the object of investigation. A general overview on this so-called depletion effect has been summarised in [14] From these findings, it becomes clear that for surfactant or protein solutions with concentrations below a certain limit, a respective loss in the bulk of a single drop happens due to the adsorption of the surface active species. A surface tension isotherm constructed in this way, i.e., using data from drop profile analysis tensiometry without considering and correcting possible depletion effects, would lead to incorrect adsorption parameters and even to wrong adsorption models
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