Growth kinetics, adsorption and morphology of sucrose crystals from aqueous solutions in the presence of raffinose

Experimental growth kinetics of the most important F forms of sucrose crystal, {100}, {110}, {101 ($) over bar} and {1 ($) over bar1 ($) over bar0}, has been investigated at T = 40 degrees C within a range of supersaturations (sigma = 0.040 and 0.080), in the presence of varying raffinose concentrations (0.0 <= C-raff <= 8.0% H2O). Raffinose dramatically slows down the growth rate of the first three forms, modifies the shape of their growth isotherms and can induce dead zones as well, the effects increasing with its concentration. However, the screw dislocation mechanism maintains. The comparison between Kubota-Mullin and Cabrera-Vermilyea kinetic models shows that the first one better describes our experimental data. This allows to deduce that the raffinose adsorption mainly occurs at the kinks sites of all the interested forms. Moreover, the values of the effectiveness coefficient of KM model shows the combined effects of supersaturation and raffinose concentration in the growth solutions. On the contrary, the {1 ($) over bar1 ($) over bar0} form is weakly affected by raffinose, owing to the low structural compatibility between its kinks and the additive molecules. Finally, the changes of the crystal morphology, induced by the highly anisotropic kinetic effects, were studied as well. Increasing raffinose concentration and supersaturation produces conflicting effects on the morphology: the first one promotes crystals elongated in the [010] direction and reduces the number of forms on the crystals, while supersaturation acts in the opposite sense. Ratios among the main dimensions of crystals are critically discussed to evaluate their final shapes. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim