Hydration gradients drive lipid self-segregation

Evaporation from a multicomponent aqueous mixture not only establishes a hydration gradient but may also lead to composition gradients in the other components. Here, we show that such a gradient induces a strong segregation between two phospholipids that differ only in the saturation of their acyl chains. Using high-resolution confocal Raman microscopy combined with small- and wide-angle X-ray scattering, we simultaneously resolve local composition and structure along the evaporation direction. We find that the saturated phospholipid, dipalmitoyl phosphatidylcholine (DPPC), accumulates near the air-liquid interface, while the unsaturated phospholipid, dioleoyl phosphocholine (DOPC), is displaced toward a more hydrated intermediate region, resulting in a complete inversion of their initial proportions. This nonmonotonic lipid gradient reflects the different water swelling capacities of the L_β' and L_α lamellar phases favored by DPPC and DOPC, respectively. Despite the system being out of equilibrium, the segregation is quantitatively captured by a description based on local equilibrium chemical potentials and multicomponent diffusion with cross-coupling. Our findings identify hydration gradients as a robust driver of lipid segregation and establish a general framework for predicting transport and organization in evaporating soft-matter systems.