Riechers Birte, Maes Florine, Akoury Elias, Semin Benoît, Gruner Philipp, Baret Jean-Christophe
Soft Microsystems, Centre de Recherche Paul Pascal, Unité Propre de Recherche 8641, CNRS, University of Bordeaux, 33600 Pessac, France; Droplets, Membranes and Interfaces, Max Planck Institute for Dynamics and Self-Organization, 37077 Goettingen, Germany.
Department of Biochemistry, Ludwig Maximilians University, 81377 Munich, Germany; Gene Center, Ludwig Maximilians University, 81377 Munich, Germany; Department for NMR-based Structural Biology, Max Planck Institute for Biophysical Chemistry, 37077 Goettingen, Germany.
Proc Natl Acad Sci U S A. 2016 Oct 11;113(41):11465-11470. doi: 10.1073/pnas.1604307113. Epub 2016 Sep 29.
Emulsions are metastable dispersions. Their lifetimes are directly related to the dynamics of surfactants. We design a microfluidic method to measure the kinetics of adsorption of surfactants to the droplet interface, a key process involved in foaming, emulsification, and droplet coarsening. The method is based on the pH decay in the droplet as a direct measurement of the adsorption of a carboxylic acid surfactant to the interface. From the kinetic measurement of the bulk equilibration of the pH, we fully determine the adsorption process of the surfactant. The small droplet size and the convection during the droplet flow ensure that the transport of surfactant through the bulk is not limiting the kinetics of adsorption. To validate our measurements, we show that the adsorption process determines the timescale required to stabilize droplets against coalescence, and we show that the interface should be covered at more than [Formula: see text] to prevent coalescence. We therefore quantitatively link the process of adsorption/desorption, the stabilization of emulsions, and the kinetics of solute partitioning-here through ion exchange-unraveling the timescales governing these processes. Our method can be further generalized to other surfactants, including nonionic surfactants, by making use of fluorophore-surfactant interactions.
乳液是亚稳分散体系。它们的寿命与表面活性剂的动力学直接相关。我们设计了一种微流控方法来测量表面活性剂在液滴界面的吸附动力学，这是泡沫形成、乳化和液滴粗化过程中的一个关键过程。该方法基于液滴中的pH值衰减，作为羧酸表面活性剂在界面吸附的直接测量方法。通过对pH值整体平衡的动力学测量，我们全面确定了表面活性剂的吸附过程。小液滴尺寸和液滴流动过程中的对流确保了表面活性剂在主体中的传输不会限制吸附动力学。为了验证我们的测量结果，我们表明吸附过程决定了稳定液滴防止聚结所需的时间尺度，并且我们表明界面覆盖率应超过[公式：见正文]以防止聚结。因此，我们通过离子交换定量地将吸附/解吸过程、乳液的稳定化以及溶质分配动力学联系起来，揭示了控制这些过程的时间尺度。通过利用荧光团 - 表面活性剂相互作用，我们的方法可以进一步推广到其他表面活性剂，包括非离子表面活性剂。
