rXXXX American Chemical Society A dx.doi.org/10.1021/la104192y | Langmuir XXXX, XXX, 000–000 ARTICLE pubs.acs.org/Langmuir American Chemical Society Chemical-Garden Formation, Morphology, and Composition. I. Effect of the Nature of the Cations Julyan H. E. Cartwright,* Bruno Escribano,* ,† and C. Ignacio Sainz-Díaz* Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Facultad de Ciencias, E-18071 Granada, Spain b S Supporting Information ABSTRACT: We have grown chemical gardens in different sodium silicate solutions from several metal-ion salts—calcium chloride, manganese chloride, cobalt chloride, and nickel sulfate—with cations from period 4 of the periodic table. We have studied their formation process using photography, examined the morphologies produced using scanning electron microscopy (SEM), and analyzed chemical compositions using X-ray powder diffraction (XRD) and energy dispersive X-ray analysis (EDX) to understand better the physical and chemical processes involved in the chemical-garden reaction. We have identified different growth regimes in these salts that are dependent on the concentration of silicate solution and the nature of the cations involved. ’ INTRODUCTION Chemical gardens are the plantlike structures produced by a variety of different metal-ion salts when immersed in a solution of silicate or some other anions. 1 When the salt crystal begins to dissolve in the silicate, it forms a gel around itself. This gel acts as a semipermeable membrane, allowing water from the silicate solution to flow in toward the salt crystal driven by osmotic pres- sure. As the water flows in and the crystal continues to dissolve, the pressure inside the membrane rises. At some point, the membrane ruptures, forming a hole through which the metal-ion solution from the inside flows out. This metal-ion solution reacts with the silicate outside, forming a tube around the flow by precipitation. Hence, its morphology is a product of forced convection driven by osmotic pressure through the semiperme- able membrane and free convection due to buoyancy, since the ejected solution is generally lighter than the external silicate. The final result is generally a combination of tubes of different sizes and shapes, resembling a tree or a garden. The main interest in them recently has been for educational purposes, but chemical gardens also have some fairly unexplored applications in processes of industrial importance that involve precipitation across a colloidal gel membrane separating two different aqueous solutions, for example, in the hydration of Portland cement 2 and the corrosion of metals. 3 It has been speculated that the membranes of chemical gardens produced in submarine vents may be an ideal site for the origin of life. 4,5 There is not a great deal of published work that includes a thorough study of their morphology, chemical composition, and micro- structure. 6-8 Different regimes of tube growth have been identified upon injecting a copper sulfate solution into sodium silicate. 9-11 At lower concentrations of copper sulfate, there are found thin tubes that grow steadily: a regime termed jetting. At higher concentra- tions, growing tubes show pulsating behavior, called budding, in which the tip of the tube develops a bubble that inflates and bursts, nucleating a new droplet at the tube’s end. The resulting morphol- ogy is one of wider globular tubes. We will use this jetting and budding terminology throughout this work. The purpose of this article is to study the formation, morphology, and composition of tubes grown with several metal-ion salts, CaCl 2 , MnCl 2 , CoCl 2 , and NiSO 4 , with cations from period 4 of the periodic table, using different sodium silicate concentrations. The morphol- ogies were studied using SEM imaging, and the compositional analysis was performed with EDX and XRD, and some interesting di fferences between the cations are noted. A better understanding of the physics and chemistry of the processes of formation of chemical gardens should allow us to improve the control of the relative solution densities, osmosis, and di ffusion, which is a step toward being able to fabricate microtubes with a desired morphology. This work complements both a systematic investigation that we reported recently of the behavior of cations of group 2 in the Received: October 18, 2010 Revised: January 9, 2011