Charge-Selective Adsorption of Organic Molecules in Microfluidic Mg(OH)2 Precipitate Membranes

The porous mineral structures of alkaline hydrothermal vent chimneys have been proposed as natural concentrators of dissolved organic molecules, potentially addressing the longstanding dilution problem in deep-sea origin-of-life hypotheses. Here, we use microfluidic chemical garden precipitates composed of a primary mineral phase in white smoker chimneys, Mg(OH)₂, to evaluate the adsorption and diffusion behavior of organic dye molecules as a function of their charge state. Pregrown precipitate membranes are exposed to anionic (methyl orange, fluorescein), zwitterionic (rhodamine B), and cationic (crystal violet, malachite green) dye solutions under controlled flow conditions. UV–visible spectroscopy of digested membranes reveals millimolar loading concentrations for anionic dyes after 20 h, limited uptake of the zwitterionic dye over the same period, and negligible adsorption of cationic species. Light-intensity analyses of time-lapse microscopy confirm that anionic dyes permeate the full membrane width within 4 h, while zwitterionic and cationic dyes remain confined near the injection interface. Steady-state and time-resolved emission measurements indicate increased environmental heterogeneity for membrane-bound fluorophores, consistent with adsorption across a distribution of pore sites. Raman spectroscopy corroborates dye-mineral interactions. These results demonstrate that monomineral Mg(OH)₂ precipitates have pronounced charge-selective permeability, favoring anion transport in a manner analogous to the preferential hydroxide diffusion observed during membrane growth. This selectivity implies that negatively charged prebiotic molecules may have been preferentially concentrated within alkaline vent chimney walls, offering a mineral-mediated mechanism to locally elevate reactant concentrations in an otherwise dilute deep-sea environment.