Alternating Metastable/Stable Pattern in the Mercuric Iodide Crystal Formation Outside the Ostwald Rule of Stages

Abstract

We report a reaction-diffusion system in which two initially separated electrolytes, mercuric chloride (outer) and potassium iodide (inner), interact in a solid hydrogel media to produce a propagating front of mercuric iodide precipitate. The precipitation process is accompanied by a polymorphic transformation of the kinetically favored (unstable) orange, (metastable) yellow, and (thermodynamically stable) red polymorphs of HgI2. The sequence of crystal transformation is confirmed to agree with the Ostwald Rule of Stages. However, a region is found of initial inner iodide concentration, where a stationary pattern of alternating metastable/stable crystals is formed. A theoretical model based on reaction diffusion coupled to a special nucleation and growth mechanism is proposed. Its numerical solution is shown to reproduce the experimental results.

Self-organization in precipitation reactions far from the equilibrium

“…The crossover from a traveling precipitation band to a periodic pattern suggests mechanistic commonalities between these different behaviors. Furthermore, similar precipitation-dissolution dynamics have been reported for other reaction couples such as Co(OH) 2 /[Co(NH 3 ) 6 ] 2+ , HgI 2 /[HgI 4 ] 2− , and Al(OH) 3 /[Al(OH) 4 ] − ( 158 – 161 ). Additional observations in these systems include the occurrence of dynamic Liesegang patterns, temporal chaos in the number of precipitation bands, and a nontrivial spatial distribution of these bands ( Fig.…”

Targets, ripples and spirals in a precipitation system with anomalous dispersion

“…In our system, HgCl 2 (inner electrolyte) is evenly distributed in the agar gel, and then the KI (outer electrolyte) is poured on to the gel surface, which diffuses into the gel forming HgI 2 precipitates. Specifically, the formation of HgI 2 in gel media 22 in the wake of a diffusion front results in three different polymorphs: orange (tetragonal), b-yellow-HgI 2 (orthorhombic) and a-red-HgI 2 (tetragonal). 23 The orange and yellow forms are the kinetically favored polymorphs, thus appearing at early stages of diffusion, but they are not thermodynamically stable.…”

“…All of the aforementioned systems produce, under no advection, propagating fronts with constant velocities 17,18 , or velocities in conformity with normal diffusive profile leading to a linear relation between the mean square spread of particles with time. Although recent experiments have confirmed the occurrence of superdiffusive chemical waves, such anomalous behavior is exhibited when the system is subjected to external modulations that enhance diffusion [21][22][23][24][25] . Recently, the authors reported the propagation and interaction of three-dimensional superdiffusive target patterns in the absence of external forcing in a novel heterogeneous system, namely the mercuric iodide precipitation system 19 .…”

“…Although recent experiments have confirmed the occurrence of superdiffusive chemical waves, such anomalous behavior is exhibited when the system is subjected to external modulations that enhance diffusion. [21][22][23][24][25] Recently, the authors reported the propagation and interaction of three-dimensional superdiffusive target patterns in the absence of external forces in a novel heterogeneous system, namely the mercuric iodide precipitation system. 19 In this sequel work, we present a full study of the aforementioned mercuric iodide system.…”

Band Propagation, Scaling Laws, and Phase Transition in a Precipitate System. 2. Computational Study

“…The resulting patterns might not be static in space and can exhibit rich spatiotemporal dynamics, especially when other transformations, such as redissolution reactions and polymorphic transitions, occur at the same time in the system [14][15][16][17][18][19][20][21][22][23] .…”