GROWTH-MECHANISM AND TWINNING OF SUCROSE EFFECTED BY SURFACE-ADSORPTION PROCESSES

Growth experiments with sucrose dependent on temperature and supersaturation of solutions were carried out to find the conditions for appearance of the three different growth types. Measurements of growth rates of {110} and {110BAR} faces at 50-degrees-C and supersaturation c(s) = 10% resulted in the known feature that in pure solutions a crystal grows faster in positive b-direction than in the negative one. Addition of raffinose stopped growth of {110}. In an electric field generated by 20 kV direct voltage (crystal orientation: negative pole of b-axis pointed towards the negative electric pole) the crystal grew in negative b-direction three times faster than in the positive one. In the presence of raffinose the growth rates in both directions become equal. If the crystal is turned by 180-degrees in the solution (pure solution and in the presence of raffinose) the growth behaviour is the same as without an electric field. These results can be interpreted in such way that in pure solutions sucrose molecules adsorb to {110BAR}, (bonds exist between the fructose rings of molecules in liquid and solid phases) and that in presence of raffinose these molecules adsorb to {110} (bonds exist between glucose (solid) and galactose rings (liquid phases)). By the electric field the adsorption is hindered. The theory is in accordance with the fact that crystals situated in a suspension of the solution can intergrow with a host crystal in twin positions (in pure solutions on {110BAR} and in the presence of raffinose on {110}).