Role of Gravity in the Formation of Liesegang Patterns J. M. Garcı ´a-Ruiz,* D. Rondo ´ n, A. Garcı ´a-Romero, and F. Ota ´ lora Laboratorio de Estudios Cristalogra ´ ficos, IACT, CSIC-UniVersidad de Granada, AV. FuentenueVa s/n, Granada 18002, Spain ReceiVed: NoVember 14, 1995; In Final Form: February 26, 1996 X We report the results obtained in four different kinds of experiments designed to test the effect of gravity on the formation of Liesegang patterns. Both reacting solutions (KI and Pb(NO 3 ) 2 ) were gelled with agarose L. The position of the PbI 2 precipitates was determined by image analysis, and the kinetic coefficients k m ) (X n+1 - X n )/X n and k p ) (X n /A) 1/n were obtained at different relative orientations of the gravitational field with respect to the direction of the advance of the precipitation front. We conclude that there is not an apparent influence of the gravitational forces on the kinetics of the pattern formation when it is obtained in gelled media at an agarose concentration of 1% (w/v). When the experiments were performed with agarose at 0.5% (w/v) or when one of the reacting solutions was ungelled, our tests show clearly the effect of gravity. I. Introduction The Liesegang phenomenon is an intermittent precipitation that occurs when two reacting solutions at concentrations far from stoichiometry are allowed to counterdiffuse. 1,2 The fact that diffusion is the mass transport mechanism linked to this phenomenon has been realized since the earlier years of this century, because two gravity-related phenomena (convective flow and sedimentation of the solid phase) are known to pro- voke the breaking of the laws governing the formation of the patterns. This is the reason that Liesegang patterns are usually obtained in gelled media. The tridimensional network of the solid polymeric phase forming the gel leaves a fractal distribution of pores filled by the liquid phase. Due to the small average pore size, the gel structure prevents convective mass flow while allowing the Brownian motion of ions and small clusters through the intraporous fluid phase. These phenomena may occur in other porous media, such as sedi- mentary rocks or biological materials, a circumstance that increases the interest in revealing the origin and properties of these structures. To determine whether gravity alters the location and the time of formation of the successive bands or rings is of interest from a theoretical viewpoint. As pointed out by Bewersdorff et al. 3 , since the effect of the gravity field on ions can be neglected, the existence of a gravitational shift must be due to the presence of clusters of colloidal size (0.1 μm < r < 10 μm), which could support the explanation of Liesegang patterns as a post- nucleation event. 4,5 The first experimental results on the influence of gravity on the formation of Liesegang rings were reported by Davies, 6 who claims that the kinetics of the pattern formation is slower when operating against gravity. As quoted by Hedges, 7 this effect might be counterbalanced by hydrostatic pressure. More recently, Kai et al. 8 have considered this problem and proposed an explanation as to why such an effect could be active. They measured the dependence of the ring position on the relative directions of the advance of the precipitation front with respect to the gravitational field. Their experiments were carried out using the reaction between KI and Pb(NO 3 ) 2 to precipitate PbI 2 and between MgSO 4 and NH 4 OH to produce MgOH. However, quantitative relations were only reported for the first case due to experimental problems that appear when gelling NH 4 OH solutions. Their experimental protocol consisted of three identical experiments in three test tubes which were arranged parallel, perpendicular, and antipar- allel to the g vector. It was found that, for the same ring number (n), the distance X n+1 - X n increases in the sequence antiparallel, perpendicular, and parallel to the gravity field. This effect was observed to be clear for large values of n and also when the pattern formation is slowed down by the use of more dilute solutions. The same kind of experimental proof has been recently described by Das et al. 9,10 for the case of HgI 2 and CuCrO 4 , but in none of these cases were statistical results reported. Finally, Zrı `nyi et al. 11 reported no gravity effect on the Cr(OH) 3 Liesegang system. The main problem in proving the existence of a gravitational influence comes from the difficulty of performing reproducible Liesegang experiments. There are several internal parameters which modify the spatial geometry and the time relations displayed by Liesegang patterns. The initial concentrations of reactants, the surface of the reaction front, the geometry (shape, volume, and arrangement) of the reactant sources, the nature and viscosity of the gelled media, etc., are variables which change the spatiotemporal distribution of precipitate. There are other external parameters which have been claimed to influence the geometry of the Liesegang patterns. The intensity, periodic- ity, and wavelength of the illumination clearly affect the pattern of silver and mercury salts, 10 and the temperature has a clear effect on the precipitation behavior of other salts such as PbI 2 . The effect of the type of glass reservoir and cleaning procedures has also been pointed out. Furthermore, there are factors which are much more difficult to evaluate because they vary as precipitation progresses, such as the pH of the medium and the concentration of the by-products, the age of the gel, 12 the tensional effect on the gel structures, 13 the electric field, 14,15 and the presence of impurities. How these factors affect the complex behavior of the Liesegang patterns is still a matter of research. The aim of this work was to study the effect of the gravitational field on the kinetics of Liesegang bands on the basis that the sensitivity of these patterns to internal and external experimental conditions could mask the experimental results. We present the results of four different tests. * Author to whom all the correspondence should be addressed. Phone: +34-58-243360. Fax: +34-58-243384. Email: jmgruiz@goliat.ugr.es X Abstract published in AdVance ACS Abstracts, May 1, 1996. 8854 J. Phys. Chem. 1996, 100, 8854-8860 S0022-3654(95)03351-X CCC: $12.00 © 1996 American Chemical Society