What is it about?
When water evaporates from a mixture, it creates a gradient: the material becomes drier near the air and more hydrated deeper inside. We show that this simple and very common situation is enough to spatially separate two phospholipids that are almost chemically identical. Using confocal Raman microscopy together with small- and wide-angle X-ray scattering, we simultaneously mapped water content, local composition, and self-assembled structure along the drying direction. We observed that the saturated lipid (DPPC) accumulates in the driest region close to the air interface, while the unsaturated lipid (DOPC) concentrates in a more hydrated intermediate zone. The local composition can even become the opposite of the initial mixture. This segregation occurs because the lamellar structures formed by the two lipids do not incorporate the same amount of water. Under a hydration gradient, each lipid migrates toward the region where its preferred structure is most stable. Although evaporation drives the system out of equilibrium, the process can be understood as a succession of local thermodynamic equilibria.
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Why is it important?
Hydration gradients are ubiquitous: they exist in skin and lung surfactant layers, in drying coatings, in drug-delivery systems, and in many soft materials exposed to air. Our results show that such gradients do not only change structure — they can also reorganize composition in a strong and predictable way. This provides a general mechanism for spontaneous spatial organization in multicomponent soft matter. It also offers a quantitative framework linking multicomponent diffusion, self-assembly, and thermodynamics under nonequilibrium conditions. Understanding and controlling this effect could help design responsive coatings, improve lipid-based drug delivery, and better describe biological barriers that operate under fluctuating hydration.
Perspectives
This work started from a simple question: can evaporation do more than just concentrate a mixture? What we found is that it can actually “sort” molecules that are almost identical, purely because their self-assembled structures interact differently with water. For me, an exciting aspect is that a globally nonequilibrium process can still be predicted using equilibrium thermodynamics — provided we look at it locally along the gradient. This opens the way to a predictive description of transport and organization in drying multicomponent systems, which is a central challenge for many soft-matter and formulation problems. More broadly, hydration gradients are a very generic driving force. We expect similar segregation phenomena in many other systems and under other perturbations such as osmotic stress, filtration, or centrifugation.
Kevin Roger
Centre National de la Recherche Scientifique
Read the Original
This page is a summary of: Hydration gradients drive lipid self-segregation, Proceedings of the National Academy of Sciences, February 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2518553123.
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