Non-equilibrium Composition of Mixed Metal Hydroxide Membranes Grown in Flow Systems

Multi-compound precipitates are important in various processes, from recovering resource-limited metals to forming chimney structures at deep-sea hydrothermal vents. Their growth often occurs rapidly, involving steep concentration gradients, fluid flow in the surrounding solution, and transient reaction conditions. Our study systematically controls these complex factors using a microfluidic device that produces co-flowing laminar streams of NaOH and (Ni,Mg)Cl₂ solutions. The resulting precipitate membranes thicken exclusively in the direction of the mixed metal solution and consist of Ni(OH)₂ and Mg(OH)₂. Energy-dispersive spectroscopy (EDS) and mass spectrometry show that, for the investigated metal solution ratios and a constant growth time, the average [Mg²⁺]/[Ni²⁺] product ratio is proportional to the [Mg²⁺]/[Ni²⁺] reactant ratio. The Mg levels in the membrane are high overall. Over the initial 1.5 h, the membrane width is proportional to the square root of time, yielding effective diffusion coefficients that increase with increasing [Mg²⁺]/[Ni²⁺] reactant ratios. EDS maps also show a strong compositional gradient across the membrane, where early formed layers contain the highest levels of Mg. These results demonstrate strong deviations from equilibrium behavior and are discussed in the context of kinetic factors favoring Mg(OH)₂ precipitation. A simple model is proposed to explain the observed compositional gradient, attributing it to a cation-exchange process that becomes more effective during the slow late growth of the membrane.