Atomic force microscopy studies of surface morphology and growth kinetics in thaumatin crystallization

Surface morphology and growth kinetics of thaumatin crystals in the supersaturation range sigma = 0-1.65 were investigated using in situ atomic force microscopy (AFM). At sigma < 0.7 growth steps were produced both by screw dislocations and two-dimensional nuclei. At higher supersaturations growth proceeded exclusively by two-dimensional nucleation, Adsorption of three dimensional clusters on the surfaces of growing crystals that resulted in the formation of multilayer stacks which then grew tangentially was observed. Strong directional kinetic anisotropy in the tangential step growth rates in different directions was also seen. From the supersaturation dependence of tangential step rates, the kinetic coefficient of the steps beta((011)), was determined to be 2 x 10(-4) cm/s. The super-saturation dependence of the rate of two-dimensional nucleation was measured, and this yielded the supersaturation dependence of the normal growth rate. From these data the surface free energy of the step edge was calculated to be alpha approximate to 0.4 erg/cm(2) for sigma > 1.2 and 0.2 erg/cm(2) for sigma < 1.2. It was suggested that, at sigma < 1.2, two-dimensional nucleation was not homogeneous and occurred at sites more active than normal, e.g., at defects or sites of impurity incorporation. Values of alpha in the range of 0.4-0.7 erg/cm(2) were also obtained on the basis of the sizes of critical nuclei, both for growth on dislocations and for growth by two-dimensional nucleation.