Measurements of temporal and spatial sequences of events in periodic precipitation processes

Abstract

A series of new experiments on Liesegang ring (or band) formation is presented which is concerned with the temporal and spatial evolution of the process of structure formation. We have chosen NH4OH and MgSO4 to form rings of Mg(OH)2 precipitate in a gelatin gel, as well as KI and Pb(NO3)2 for periodic precipitation of PbI2 in an agar gel. A temporal sequence of events during the entire period from the start of a Liesegang experiment in a test tube to the completion of the final ring pattern has been determined at many locations in the tube by visual observations and by measurements of transmitted light, of scattered light, of deflection of the transmitted light beam, and of gravity effects. After diffusion of one electrolyte into the gel medium containing the second electrolyte results in an ion product larger than three times the solubility product, at any and all points in space, we observe the onset of homogeneous nucleation of colloidal particles by a steplike increase of the index of refraction. The colloid concentration and the particle number density at the nucleation site are estimated to be 10−2 mol/l and 1015 to 1016 cm−3, respectively. Nucleation is followed by the growth of colloidal particles which gives rise to distinct light scattering (turbidity). Both nucleation and colloid formation take place in space continuously; the fronts of these phenomena move through the system and obey a simple diffusion law. A substantial time interval after their passage, there arises a localized gradient of the index of refraction at the prospective ring positions which indicates onset of structure formation by means of a focusing mechanism. While the localized gradient becomes more pronounced and narrower in space, the turbidity in the regions on either side of the ring location decreases, which indicates a depletion in colloidal material in the neighboring zones. Eventually, a sharp band of visible precipitate appears, which is clearly separated from the preceding ring. We conclude that the ring formation is a postnucleation phenomenon in that structure arises from a spatially continuous region of colloid a long time after nucleation has occurred, and propose that it is associated with the autocatalytic growth of colloidal particles. The location of rings is not determined by the spatial pattern of nucleation and colloid deposition as predicted by the Ostwald–Wagner–Prager theory. Our conclusions are supported by experiments on the influence of gravity on the ring locations, which provide evidence for the existence of colloidal particles of several hundred angstroms in size for a substantial fraction of the time required for the formation of a visible structure.

Obtaining new composite biomaterials by means of mineralization of methacrylate hydrogels using the reaction–diffusion method

“…This is an old research subject and during most of the early period, the main interest was held by the phenomenon of Liesegang rings; however, more recently a new approach was taken in the context of order formation out of chaos and simulation of the process [1][2][3][4][5]. For several decades mineralization of an organic matrix has been an important topic in bone-tissue engineering [6].…”

Spatial patterns in a photobiochemical system

“…That the spatial patterns appeared only in the presence of a concentration gradient of the electron acceptor, that with the present experimental conditions no oscillatory behavior in the reduction of DCIP was detected, and that diffusion is probably involved suggest that the explanation of the observed spatial periodicity lies in the capability of the reaction mixture to have multiple stationary states-i.e., that it has bistability. The coupling of autocatalysis with diffusion has also been postulated to account for periodic precipitation processes (Liesegang rings) in the absence or presence of gradients of substances (31,32).…”

A mesoscopic model of nucleation and Ostwald ripening/stepping: Application to the silica polymorph system

“…With this we conclude that our numerical approach yields well controlled approximations to the PSD problem. 21,22 In Fig. 9, (n 1/3 ) max vs ͱt is plotted for numerical simulations with N F ϭ200, 400, and 800.…”