Studies on the behaviour of growth steps on sucrose crystals

ExperimentM observations of growth steps on sucrose crystals are described, together with their behaviour under controlled conditions.The paper is divided into four sections.1./k method is given by which crystal-growth processes can be studied on selected areas of sucrose crystals.When a sucrose solution in contact with silicone grease is used for crystal growth, the crystals can be removed from solution with the growth features preserved.A drop of the same solution placed on the crystal does not spread, and if the solution is supersaturated growth steps on the surface grow out to the boundary of the drop leaving a smooth area.Growth in this area can continue if the termination of a screw dislocation is located within it or by a two-dimensional nucleation process.The method can be used either to grow crystals from selected dislocations or to study two-dimensional nucleation.2. We have previously shown that large growth steps are visible on sucrose crystals which have been nucleated and grown in the presence of dextrose.Further experiments are described showing that when such crystals are transferred to pure sucrose solution there is no alteration in the step height or change in step pattern.The rate of step movement remains as before.When another impurity (raffinose) is added to a pure solution in which crystals are growing normally the growth steps stop moving even though the solution is supersaturated.On replacing this solution by pure sucrose solution, the growth steps resume their normal rate of movement.3. Complex step patterns are common on growing sucrose crystals and these patterns sometimes show a continuous change in detail.This change has been studied and shown to arise close to the growth centre.Under the conditions appertaining in these experiments, mechanisms which have been advanced in the literature for the production and stability of large growth steps do not appear to operate.4. Further information on the growth process for sucrose crystals has been obtained by measurements of the temperature coefficient of the rate of step advance for the range 24.5 °C. to 35"3 °C.From these measurements an activation energy of 16.7 cal.per mole for the process was cMeulated.This value indicates that the rate controlling step is not diffusion in the solution.Measurements at 50 °C.and 70 °C.suggest that this conclusion may also be valid at these temperatures.