Previous measurements of concentration gradients formed in long columns of supersaturated glycine solutions indicate that small amounts of impurities, such as valine, can lead to increase in the magnitude of the gradient. This indicates that the impurity enhances cluster formation [1]. The concentration gradients were measured in two supersaturated systems- Potassium dihydrogen phosphate with potassium chloride as the impurity and Benzophenone with Urea as the impurity. Metastable zone widths of the supersaturated solutions were also measured in the presence of impurities. Weight average molecular weight,M_W, Degree of Association, p, Number average molecular weight, M_n , Average number of molecules per cluster, n, and weight average cluster diameter were calculated for Potassium dihydrogen phosphate and Benzophenone supersaturated solutions. The cluster sizes are estimated from the concentration gradient data in the presence and absence of impurities and also changes noted in metastable zone widths. The changes in concentration gradients of supersaturated solutions appear to be a reliable indicator of the effect of impurities on cluster growth and nucleation.

The presence of KCl in KDP caused an increase in the concentration gradient and cluster size compared to pure system. The presence of Urea in Benzophenone caused decrease in the concentration gradient and cluster size compared to pure system. Recently discovered Sankaranarayanan-Ramasamy method is used to grow long crystals of good optical quality [2-3]. Long columns of solution are used in this method. For larger particular column height cluster size may attain critical size. At a particular stage more number of embryos and critical nuclei present in solution may come to the bottom of the solution due to gravitational field. At this stage crystal growth process will be disturbed. Determination of critical height of Potassium dihydrogen phosphate and Benzophenone solutions is in progress.

References

[1] R.M.Ginde, A.S.Myerson, J.Crystal Growth.116 (1992)41.

[2] K. Sankaranarayanan, J.Crystal.Growth.284 (2005)203.

[3] N.Balamurugan and P.Ramasamy, Crystal Growth and Design 6(2006)1642.

[4] J.W.Mullin and C.L.Leci, Phil .Mag.19 (1969)1075.

[5] M.A.Larson and J.Garside, Chem.Eng.Sci.41 (1986)1285.

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