Self-Organization of Layered Inorganic Membranes in Microfluidic Devices

Inorganic precipitate membranes play an important role in chemobrionics and origin of life research. They can involve a range of catalytic materials, affect crystal habits, and show complex permeabilities. We produce such membranes in a microfluidic device at the reactive interface between laminar streams of hydroxide and Co­(II) solutions. The resulting linear membranes show striking color bands that, over time, expand in the direction of the Co­(II) solution. The cumulative layer thicknesses (here up to 600 μm) obey square root laws, indicating diffusion control. The effective diffusion coefficients are proportional to the hydroxide concentration, but the membrane growth slows down with increasing concentrations of Co­(II). On the basis of spatially resolved Raman spectra and other techniques, we present chemical assignments of the involved materials. Electron microscopy reveals that the important constituent β-Co­(OH)<sub>2</sub> crystallizes as thin hexagonal microplatelets. Under drying, the membrane curls into spirals, revealing mechanical differences between the layers.