Biogeochemical Cycles & Geobiology

Charge-Selective Adsorption Of Organic Molecules In Microfluidic Mg(OH)2 Precipitate Membranes

By Keith Cowing

Status Report

chemrxiv.org

May 27, 2026

Filed under biochemistry, chemrxiv.org, geochemistry, hydrothermal vent, hydrothermal vent chimney, microscopy, Oceanography, organic chemistry, origin of life, Raman spectroscopy, Sample Return, Spectroscopy, white smokers

Charge-Selective Adsorption Of Organic Molecules In Microfluidic Mg(OH)2 Precipitate Membranes — Comparison of precipitate membranes following either fluorescein or methyl orange dye flow. (a) and (b) show two separate precipitate membranes following injection of (a) 10-4 M and (b) 2.5×10-4 M fluorescein for 20 h at a pump rate of 0.10 mL/h per syringe. Examples of large, embedded structures observed in various precipitate membranes following methyl orange dye flow are shown in (c-f). (c) and (e) are optical micrographs taken at different locations along the same membrane after injection of 10-4 M methyl orange for 20 h at a pump rate of 0.10 mL/h per syringe. (d) and (f) represent two different precipitate membranes after injection of 2.5×10-4 M methyl orange for 20 h at a pump rate of 0.10 mL/h per syringe. Scale bars: 250 μm in (a-d); 50 μm in (e) and (f). — chemrxiv

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.

re-grown 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 timelapse 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 and ATR-IR spectroscopy corroborate 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.

Charge-Selective Adsorption Of Organic Molecules In Microfluidic Mg(OH)2 Precipitate Membranes, chemrxiv.org

Astrobiology, Astrochemistry

Keith Cowing

Biologist, Explorers Club Fellow, ex-NASA Space Biologist and Payload integrator, Editor of NASAWatch.com and Astrobiology.com, Lapsed climber, Explorer, Synaesthete, Former Challenger Center board member 🖖🏻

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