Effect of polymer admixtures on the growth habit of ionic crystals. Study on crystal growth kinetics of potassium dihydrogen phosphate in water/polyol mixtures

The impact of polyol admixtures (10-55% w/w) on crystal growth of potassium dihydrogen phosphate (KDP) is investigated by means of single crystal growth experiments at 25 °C. In contrast to tiny concentrations of organic additive which have shown growth-promotion in the past, larger amounts show growth retardation of the crystal faces. When compared to low-molecular organic admixtures, growth retardation of the polyol is significantly stronger, reducing overall growth rates and broadening the dead zone of crystal growth. The prismatic (1 0 0)-faces have shown to be much stronger affected by the presence of polyol than the bipyramid (1 0 1)-faces, which leads to needle-like crystals for higher polyol-contents. The growth regime switches from dislocation control to surface nucleation control due to the presence of polyol. However, this change takes place for much lower polyol-concentrations for the (1 0 0) than for the (1 0 1)-face. Growth-rate experiments for a ternary system are laborious and sometimes experimentally restricted as in case of the KDP/water/polyol system in this work. This is why the capability of predictive models for regime determination and growth-rate approximation are investigated in this work. Regime predictions have shown to work qualitatively well. Overall growth rates based on predictive BCF and Birth and Spread models were in agreement with experimental data for the (1 0 1)-face. Growth-rate predictions, however, deviated significantly from the experimental observations for the (1 0 0)-face. This is attributed to the different surface-structure of the (1 0 0)-face allowing for strong preferential adsorption of the polyol, hence retarding growth units.