The effects are described of supersaturation, temperature, and relative motion of solution and crystal on the rate of sucrose crystal growth from aqueous solutions containing a wide range of impurities. The results are consistent with the conclusion, drawn from data in pure sucrose solution, that two main steps, neither of which are clearly rate-controlling under the experimental conditions, are important in crystal growth kinetics. Impurities can be classified broadly as those which have their main effect on the mass transfer of sucrose from the bulk of the solution to the site of incorporation on the sucrose crystal, and those which exert their main effect by surface adsorption, hindering the incorporation of sucrose into the crystal lattice.

Graphical projections of sucrose crystal structure along the three principal crystallographic axes are presented. The packing of sucrose molecules at crystal surfaces shown by these projections is important in determining the relative growth rates of the various faces and the manner in which these are influenced by impurities. Relative growth rates of eight principal forms of the sucrose crystal in pure sucrose solutions are compared with those in the presence of impurities. This shows that the dominant factor in the influence of impurities on the relative growth rates of the faces of the sucrose crystal, and the concomitant effect on sucrose crystal shape, is the stereospecific adsorption of these impurities at kink sites on the growing crystal surface.

A method is described for the measurement of rate of growth of sucrose crystals from aqueous solutions under controlled conditions of temperature, super-saturation, and relative velocity of the solution with respect to the crystal. The effects of these parameters on sucrose crystal growth rates in pure sucrose solutions are reported. The results can be explained qualitatively in terms of a two-step rate process, the two steps of which are interdependent and neither solely rate-controlling under the conditions studied.

Le addition de phosphates exerceva nulle influentia super le rapiditate del dissolution. Sub conditiones de un vigorose convection fortiate, le emanation de iones ab sitos de defectos superficial pareva restringer le intensitate del dissolution. Alora le addition de phosphates organic e inorganic reduceva le rapiditate del dissolution per un factor de inter 2 e 8, e iste phenomenon esseva attribute a absorption a sitos de defectos superfical.Phenomenologically, tooth decay is simply the dissolution of enamel in saliva. The inorganic structural component of human dental enamel, which constitutes about 95 per cent by weight of this layer, is predominantly hydroxyapatite.J Thus, the various process consists essentially of the dissolution of hydroxyapatite in an aqueous solvent. The influences exerted by different substances on this dissolution step are relevant to the consideration of possible mechanisms whereby cariostatic agents exert their activity.Studies of the rate of dissolution of human dental enamel in acidic solutions, both in the presence and in the absence of additives, have previously been reported. 2-Interpretation of the results of these studies was, however, complicated by three factors: by the presence of the organic matrix1 in the enamel; by the failure to differentiate clearly between changes in equilibrium solubility and changes in dissolution rate; and by the performance of experiments in poorly defined hydrodynamic situations.Less equivocal information7 can be derived from studies of the dissolution kinetics of synthetic hydroxyapatite pellets both in the presence and in the absence of additives. Such studies, which are themselves of intrinsic interest, have not apparently been reported in detail previously. Materials and MethodsAll chemicals were of A.R. grade unless otherwise stated, and all solutions were prepared in double-distilled water.Hydroxyapatite was prepared by a modification of the method of McCann.5 To an alkaline boiling solution of 1.5M ammonium nitrate (1.5 1.), solutions of 0.20M calcium acetate (500 ml.) and 0. 12M ammonium dihydrogen phosphate (500 ml.) were added separately, but at equal rates, over a period of 3 hours. The resulting slurry was refluxed for a further 24 hours, the product being then separated, washed, and dried. The yield was about 95 per cent.

No abstract