Controlling Water Evaporation through Self-Assembly
Orateur : K. Roger(1) & E. Sparr(2)
(1) CNRS, Toulouse Chemical Engineering Laboratory, Université Fédérale de Toulouse
(2) Lund University, Physical Chemistry Division
Résumé : The evaporation of water from a water-rich medium to a water-poor gas phase is a ubiquitous phenomenon in nature. Evaporation can occur freely, from oceans into the air, or be hindered by membranes or barrier films. Land-living organisms face the challenge that the ambient air is much drier - or using a thermodynamic vocabulary - has a much lower water chemical potential than what is found in a living cell.
One strategy to counter this drying-out threat are lipid-based membranes, such as the human skin, which decrease the evaporation rate, while remaining partially permeable to small molecules. However, a passive diffusional barrier would still allow substantial changes in the evaporation rate with changes in the air humidity. Yet, the evaporation rate through the skin is nearly independent of the air humidity : the skin is a responsive membrane.
Using aqueous solutions of amphiphilic molecules, we show that the responsiveness originates from changes in the self-assembly structures along the water concentration gradient that spreads from the air/liquid interface towards the bulk of the solution. We designed a controlled drying setup coupled to a collection of characterization techniques (small-angle X-ray scattering, polarized and infra-red microscopy) to study the interplay between the formation of the gradient due to evaporation and the formation of a structural gradient leading to a gradient of water permeabilities. We show that a dry phase of low permeability forms at the air/liquid interface and adapts its thickness to counter changes in the air humidity, while setting a constant water chemical potential to the rest of the liquid. This responsive shield is a universal feature of systems for which the structure/permeability changes with the water content.
Unveiling this mechanism allows the preparation of responsive ointments/coatings to treat damaged biological external membranes (skin diseases, dry eye syndrome). Our findings also lead to new possibilities to design robust and homogeneous coatings if amphiphilic molecules are present in the formulation.
Date et lieu : le Vendredi 11 Mars 2016 à 11h, salle 250, IUSTI