Gregory D. Botsaris

Fundamental studies in secondary nucleation from solution

Abstract The nucleation behavior of seeded sodium chlorate solutions was studied experimentally. Because sodium chlorate crystallizes in two easily distinguished enantiomorphic forms, the origin of the secondary nuclei (whether the seed crystal or the solution) could be easily identified. This work shows that secondary nucleation does not involve a singular mechanism but that different kinds of secondary nucleation can take place under different conditions of supersaturation, liquid velocity, and impurity concentration. It was found that under certain conditions the resulting nuclei were all of the same enantiomorphic form as the seed. It was then concluded that these nuclei resulted from the growth and detachment of surface irregularities such as dendrites from the seed crystal. In two other ranges of conditions, however, secondary nucleation produced both left and right handed crystals. In one case it was shown that the nucleation could have taken place in an impurity concentration gradient resulting from the rapid incorporation of dissolved impurities in the growing seed crystal. To explain the second case, a different mechanism (largely speculative) was proposed. This mechanism hypothesized that secondary nucleation could occur because of changes in the liquid layer surrounding the crystal. These changes in the liquid layer could be the result of an ordering of the solvent water molecules at the crystal-solution interface.

A new mechanism for nuclei formation in suspension crystallizers : the role of interparticle forces

Abstract This paper brings together the current nucleation models and the theories describing the behavior of collodial suspensions. In particular this study explores the interparticle attractive forces acting between the embryos in the solution-whose existence is postulated by the classical nucleation theory-and most importantly between the embryos and the much larger crystal seeds in a suspension crystallizer. The paper shows that these forces are too important in most cases to be neglected. They actually lead to a high concentration of embryos at the immediate vicinity of the crystal seeds. This high concentration near the crystal seed surfaces promotes a rapid coagulation of the embryos to produce particles larger than the critical nuclei. The proposed nucleation model, termed Embryos Coagulation Secondary Nucleation (ECSN), provides an explanation for an experimentally observed type of secondary nucleation.

Nuclei breeding from a chiral crystal seed of NaClO3

Abstract Secondary nucleation by seeding in stagnant and agitated aqueous sodium chlorate solutions was investigated with the purpose of determining the origin of nuclei — i.e. the crystal seed or the superstaurated solution — and elucidating the mechanism of nuclei breeding. Sodium chlorate crystallizes in two enantiomeric (chiral) forms. Seeds of one chirality were used and the chirality of the secondary nuclei was determined. At low supercooling all nuclei were of the same chirality as the seed pointing to the seed as their origin. At relatively high, but still lower than the critical value for spontaneous nucleation, supercoolings many nuclei with opposite chirality to that of the seed were formed. This phenomenon points to the solution around the seed as the origin of the nuclei and it was predicted by the Embryos Coagulation Secondary Nucleation (ECSN) mechanism previously proposed by the authors. This mechanism explains the above findings as well as data obtained in non-seeded nucleation and in heterogeneous nucleations on Teflon shavings. It also accounts for the observed random fluctuation, from run to run, in the total number of nuclei and in the percentage of nuclei with chirality opposite to that of the seed.

Secondary Nucleation — A Review

Crystallizing systems, that is supersaturated systems in which crystals are already present, can form new nuclei at conditions under which primary (spontaneous) nucleation would not occur. This phenomenon leads to the definitions which will be followed in this paper: a) Secondary is the nucleation which occurs, irrespectively of its mechanism, only because of the presence of crystals of the material being crystallized. When no crystals are present no nucleation occurs. b) The terms primary or spontaneous nucleation will be used to characterize any nucleation that is not secondary in nature. This includes both the classical homogeneous and the heterogeneous nucleation.

Mechanism of contact nucleation

Abstract Experiments were conducted in which a sodium chlorate seed crystal immersed in a supersaturated solution was either tapped with a stainless steel rod or allowed to slide about the bottom of a crystallization flask. Because sodium chlorate crystallizes in two easily distinguished enantiomorphic forms, it was possible to determine whether the secondary nuclei produced by the contacts and the sliding came from the parent seed crystal or from the solution. These experiments showed that essentially all of the nuclei came from the parent seed crystal.

The effect of coal particle aggregation on the sedimentation and rheology of coal—oil mixtures

Abstract The degree of coal particle aggregation in model oil suspension was assessed by particle size distribution using a sedimentation balance. Various statistical parameters were calculated for three suspensions each prepared with Sewell coal, model oil, and (1) calcium petroleum sulfonate (2) no additive and (3) an ethoxylated quartenary ammonium salt. These formulations represented suspensions which were (1) aggregatively stable (2) slightly aggregated and (3) highly aggregated, respectively. Batch settling tests were also conducted with the three formulations at coal concentrations of 10, 35 and 50%. These results indicated that the clarity of the supernatant, the initial settling velocity and the terminal sediment volume increased as the degree of aggregation increased. Rheological data indicated that the aggregatively stable suspensions were Newtonian but that pseudoplasticity and yield stresses were observed in the other formulations to an extent which increased as the degree of aggregation increased.

Mechanism of potassium alum crystal growth from solution

Abstract This paper examines crystal growth models that have been proposed to describe the crystallization of potassium alum within narrow supersaturation ranges in order to determine whether or not these models can correlate recent growth rate data taken over broad supersaturation ranges.

Simulation of nucleation kinetics—an application to secondary nucleation

Abstract In order to study the effect of a disturbance on a supersaturated system (as occurs during secondary nucleation), the kinetics of the nucleation process has been simulated. During the process of secondary nucleation the currently accepted survival theory predicts that only clusters whose sizes are greater than the critical size will survive. This study has shown, however, that the critical size does not establish a sharp cutoff point. Rather, certain conditions can cause subcritical clusters (embryos) to survive and supercritical clusters (nuclei) to dissolve.

An investigation of the hydrophilic/oleophilic nature of various coals

Abstract An investigation of the relative wettability of various coals in contact with water and oil was undertaken to improve the understanding of the effect of additives on the sedimentation and the rheological properties of coal—oil mixtures (COM). Coals were classified as hydrophilic or oleophilic in an oil/water system by investigating their fate in a mixture of equal volumes of kerosene and water. Subsequently, Differences in the relative hydrophilic/oteophilic nature of the various coals were quantified by a test based on the spherical agglomeration process.

The effect of nonionic surface-active agents on the stability and coagulation of ferric hydroxide sol

Abstract The effect of twenty nonionic surfactants on the aggregative stability of a colloidal dispersion of ferric hydroxide was investigated. This ionically stabilized sol was chosen for the intermediate hydrophobic/hydrophilic character of its particles. The obtained results were essentially different from those of previous studies on the effect of nonionic surfactants on ionically stabilized sols with really hydrophobic colloidal particles. Many of the surfactants produced coagulation with no added electrolyte and with no reversal of electric charge of colloidal particles. A qualitative model is proposed to explain the observed results which is based on the way the surfactant is adsorbed on the surface of the ferric hydroxide particles.