Face-by-face growth of sucrose crystals from aqueous solutions in the presence of raffinose. I. Experiments and kinetic-adsorption model

Abstract Experimental growth kinetics of {1 0 0}, {1 0 1¯}, {1 1 0} and {1¯ 1¯ 0}, the four most important F (flat) forms of sucrose crystal, has been investigated within a range of low supersaturations ( 0.040 ⩽ σ ⩽ 0.080 ) and under varying raffinose concentrations (0.0 ⩽C raff ⩽8.0% H 2 O). The comparison with growth isotherms determined in pure solution confirms that raffinose dramatically slows down the growth rates of the first three forms, which still go on growing by the screw dislocation mechanism. Further, dead zones occur for both the {1 0 1¯} and {1 1 0} forms. On the contrary the {1¯ 1¯ 0} form is weakly affected, at least up to C raff ≈3% H 2 O, owing to the low structural compatibility between its surface sites and the adsorbed raffinose molecules. Both Cabrera–Vermilyea and Kubota–Mullin models, associated with Langmuir-type equilibrium isotherms, are tested to find the more suitable description of the kinetic behaviour in the presence of raffinose. The Kubota–Mullin model resulting the better one, we concluded that adsorption occurs at kink sites of all the interested forms. The predictive power of α h k l , the effectiveness coefficient of this model, is outlined as well. Finally, the disagreements between the model predictions and the experimental behaviour of the {1 0 0} form are attributed to the peculiar structure of its surface.