Composition Inversion to Form Calcium Carbonate Mixtures Crystengcomm
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Volume 19 Number 26 14 July 2017 Pages 3529–3710 CrystEngComm rsc.li/crystengcomm PAPER Eddie Peh, Klaus Tauer et al. Composition inversion to form calcium carbonate mixtures CrystEngComm View Article Online PAPER View Journal | View Issue Composition inversion to form calcium carbonate † Cite this: CrystEngComm,2017,19, mixtures 3573 Eddie Peh, *a Clemens Liedel, a Andreas Taubertb and Klaus Tauer*a Composition inversion takes place in equimolar solid mixtures of sodium or ammonium carbonate and cal- cium chloride with respect to the combination of anions and cations leading to the corresponding chloride and calcite in complete conversion. The transformation takes place spontaneously under a variety of differ- Received 1st March 2017, ent situations, even in a powdery mixture resting under ambient conditions. Powder X-ray diffraction data Accepted 20th April 2017 and scanning electron microscopy micrographs are presented to describe the course of the reaction and to characterize the reaction products. The incomplete reaction in the interspace between two compressed DOI: 10.1039/c7ce00433h tablets of pure starting materials leads to an electric potential due to the presence of uncompensated rsc.li/crystengcomm charges. Creative Commons Attribution 3.0 Unported Licence. 1. Introduction enormous number of publications dealing with the proper- ties and crystallization of calcium carbonate. A SciFinder The state of the art of formation of minerals encompasses the search of the topic ‘calcium carbonate’ leads to almost combination of nucleation and growth processes from ini- 200 000 hits indeed, a gigantic amount of knowledge but im- tially homogeneous conditions (solution or melt), ending possible to completely overlook in detail. Fortunately, the cal- however in a heterogeneous state as a suspension in either a cium carbonate system has been well documented in several – liquid or solid continuous phase. The experimental condi- comprehensive works.12 17 tions during this kind of reaction crystallization of minerals Any chemical transformation goes in the direction of min- This article is licensed under a are controlled in a way that the desired product precipitates imizing free energy which means the evolution of the system from the reaction mixture and unwanted by-products are left toward the composition possessing the lower chemical poten- behind in the mother liquor.1 This holds not only for indus- tial of all components on either side. The reaction stops Open Access Article. Published on 02 May 2017. Downloaded 10/4/2021 8:28:25 AM. trial crystallization but also for the many biogenic and non- when the difference in the chemical potential ceases to exist. biogenic mineralization processes. Considering isothermal conditions, differences in the chemi- Calcium carbonate (CC) is the core of probably the most cal potentials initiate diffusion of matter over a certain spa- important biogeochemical cycle on earth2,3 and it is abun- tial distance.18 For nucleation and growth processes, the dant in living and dead matter. In living nature, extremely ef- interaction of solutes with interfaces plays a crucial role as ficient ways are present to develop calcium carbonate- nucleation sites and after that for crystal growth and forma- containing structures with superior functionalities, for exam- tion of crystal habits. This is mainly due to the minimization – ple the mechanical properties of nacre.4 7 In our modern life, of the free energy for both processes.1,19 However, for solid- we use calcium carbonate as an additive in personal care state reactions (SSR), that is, the initial components are pres- products and cosmetics, and as a filler in a variety of poly- ent as solids (either crystalline or amorphous), the situation mer–mineral composites.8,9 However, dissolved calcium is is different because a chemical transformation is possible also the main source of water hardness and scale forma- only at the phase boundaries, more precisely, at the direct tion.10,11 The formation of such deposits leads to reductions contact spots between the grains of the different starting ma- in the efficiency, increases in energy costs, and even damage terials. In general, SSR are less common in mineralization to industrial plants. All these facts are the reasons for the chemistry, with the important exception in the production of metal alloys, ceramics, bricks, pottery, cement, and glasses. a Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany. Frequently, SSR require high temperatures in order to accel- – E-mail: [email protected], [email protected] erate diffusion of the reactants.20 23 b Institute of Chemistry, The University of Potsdam, D-14476 Potsdam-Golm, Reactions in powdery blends came to the fore in chemistry Germany † Electronic supplementary information (ESI) available: XRD, DSC, SEM and in the 19th century and it is among the merits of van't Hoff EDX of the SSR products, chemical potentials, water uptake measurements, and to establish the foundations of reaction kinetics in such sys- potential measurements of tablets. See DOI: 10.1039/c7ce00433h tems. He concluded ‘the incompatibility of condensed This journal is © The Royal Society of Chemistry 2017 CrystEngComm,2017,19,3573–3583 | 3573 View Article Online Paper CrystEngComm systems’ which means that ‘at equilibrium’ only one of the that this is the theoretical stoichiometric ratio which very systems (either the reactants or the products) can be present. likely is not the real one because both salts are hygroscopic This holds for systems in which reactants and products do and easily take up water under ambient conditions (cf. Table – not mix.24 27 Here, we consider a particular type of SSR for SI-3†). The mixture was then hand-shaken for several mi- which the name double decomposition or double transforma- nutes to ensure sufficient mixing of the two powders. The ini- tion was coined,25,28 as illustrated by reaction (1) where C1, tial mixture was then subjected to various treatments. Subse- C2 and A1,A2 are two chemically different cations and an- quently, the mixtures were kept under the various conditions ions, respectively. This reaction describes what we consider listed below, allowing the SSR to take place. Reactions with as composition inversion. Na2CO3 and (NH4)2CO3 are labelled ‘SC’ and ‘AC’, respec- tively, in addition to a number characterizing the particular C1A1 +C2A2 → C1A2 +C2A1 (1) procedure. SC-1, AC-1. The initial mixture was additionally pounded Interestingly among those many activities in the field of for several minutes using a mortar and pestle under ambient CC mineralization and minerals, there is, to the best of our conditions to decrease the grain size further. It was subse- knowledge, not a single work describing the synthesis of CC quently transferred into a glass vial in which it was stored un- via SSR. Here, we report the results of an initial study der ambient conditions. For XRD measurements which were concerning the reaction between sodium carbonate and cal- carried out periodically, a portion of the sample was trans- cium chloride solids under ambient conditions. The ap- ferred into the specimen holder and covered with a PMMA proach is very simple but versatile regarding the technical dome-shaped screw cap. configuration. The transformation starts shortly after SC-2. The initial mixture was placed in a PTFE Petri dish establishing contact between the reactant grains and pro- and covered with aluminum foil. The sample was then trans- ceeds to complete conversion. ferred into a vacuum oven and kept at 80 °C under vacuum Creative Commons Attribution 3.0 Unported Licence. There is another feature of SSR which is worthy to be (1.5 mbar). Periodically, a small amount of sample was trans- mentioned specifically in comparison with mineralization re- ferred into an XRD specimen holder and covered with a actions via nucleation and growth. SSR do not require the PMMA dome-shaped screw cap where XRD measurements generation of supersaturation to control, at least in a certain were then performed. way, the difference in the chemical potential between reac- SC-3. The reaction mixture was pressed with a weight tants and products and in this way crystal nucleation and of 10 tons into a tablet with equipment routinely used for growth rates. On the contrary, in SSR it is the difference in the preparation of tablets for IR spectroscopy (tablet diam- the molar chemical potentials (i.e. the difference in a given eter of 13 mm, thickness depending on the mass between intensive materials' property comparable with density, molar 1 and 3 mm). The tablet was left under this load for five This article is licensed under a heat, or similar properties) that essentially determines the days. The tablets were stored in a capped glass vial for 10 process. days. Compared to solution-based processes of calcium car- SC-4. The reaction mixture was transferred into a soft sili- Open Access Article. Published on 02 May 2017. Downloaded 10/4/2021 8:28:25 AM. bonate mineralization, solid-state synthesis leads to a con tube which was fitted to a peristaltic pump and its ends product mixture which might be a drawback with respect were short-circuited. The SSR took place while the mixture to product purity but offers additional possibilities for de- was (at least partly) moved inside the tube for variable time. signing composite materials with specific morphologies SC-5. The SSR was carried out inside weighing glasses un- and properties. der magnetic stirring at either room temperature or 80 °C for several days. 2. Experimental information SC-6. The reaction mixture was placed in a covered alumi- 2.1 Materials num crucible and the SSR was monitored in a DSC oven (cf. below for details). Anhydrous 96% calcium chloride (CaCl2, Acros), anhydrous SC-7. The initial mixture was spread on a ceramic cruci- sodium carbonate (Na2CO3, Fluka), ammonium carbonate ble and placed in a chamber tube furnace (Carbolite) > ((NH4)2CO3, Sigma-Aldrich) and sodium chloride 99.8% where heat treatment was conducted under a nitrogen at- (NaCl, Carl Roth) were used as-received.