Electrochemistry of Inorganic Precipitates Formed in Self-Organizing Chemical Systems

Abstract

The interfacing of two solutions, each containing a reactive ion (e.g. Fe+2 / HS-), produces a self-assembling mineral film or membrane at the solution interface. When one solution is injected into the other, the membrane forms as a self-organizing “chemical garden” structure that grows vertically upward according to buoyancy and internal fluid pressure. When the solutions are instead interfaced across a porous template, such as parchment paper or dialysis tubing, the precipitate forms as a mineral film on the template. In either case, the formation of self-assembling precipitates in steep chemical/pH gradients at solution interfaces produces inorganic membranes that exhibit organized structure at both macro and micro scales. Chemical garden membranes can grow as bulbs, tubes, or gelatinous ‘chimney’-like structures, and the membranes themselves are permeable to some ions, and exhibit morphological gradients from interior to exterior (e.g. a crystalline side and a smoother side). Inorganic membranes often exhibit compositional gradients from interior to exterior as well, reflecting the chemical gradients in which they precipitated. These are dynamic chemical systems and in recent work we have studied the ability of mineral membranes to generate and maintain electrical potentials (Barge et al. 2012). Inorganic membranes of iron sulfides produced on dialysis tubing templates between contrasting solutions were also found to be partially bipolar and capable of preferential ion exchange, as well as electrically conductive (Barge et al. 2014). In some ways, the self-assembling inorganic precipitates formed in solution interface experiments are similar to the ion-exchange membranes and gas diffusion layers in commercial as well as experimental fuel cells. This has led to some preliminary work in using commercially available electrode / GDL material as a template to precipitate chemical membranes that can then be used in fuel cell experiments (Barge et al. 2014) to determine if these are capable of acting as redox catalysts. Interfacial precipitates can be grown in many different reactant systems and depending on the chemistry can be useful for various materials / electrochemical applications. Natural analogs to these self-assembling membranes include the growth of chimneys at hydrothermal vents, which are formed as minerals precipitate in the chemical / pH gradient where subsurface hydrothermal fluid feeds into the chemically contrasting seawater. Hydrothermal chimneys are natural chemical gardens, and it has even been proposed that life on Earth emerged in such a system, so we can also use electrochemical studies of out-of-equilibrium chemical precipitates to understand the mechanisms that might operate in geological settings.L. M. Barge, T. P. Kee, I. J. Doloboff, J. M. P. Hampton, M. Ismail, M. Pourkashanian, J. Zeytounian, M. M. Baum, J. Moss, C.-K. Lin, R. D. Kidd, I. Kanik (2014) The Fuel Cell Model of Abiogenesis: A New Approach to Origin-of-Life Simulations. Astrobiology, 14(3):254-70.L.M. Barge, I. J. Doloboff, L. M. White, G. D. Stucky, M. J. Russell, I. Kanik. (2012) Characterization of Iron-Phosphate-Silicate Chemical Garden Structures. Langmuir, 28 (8), pp 3714-3721.