ATOMIC-FORCE MICROSCOPY STUDIES OF SURFACE-MORPHOLOGY AND GROWTH-KINETICS IN THAUMATIN CRYSTALLIZATION

Surface morphology and growth kinetics of thaumatin crystals in the su persaturation range sigma = 0-1.65 were investigated using in situ ato mic force microscopy (AFM). At sigma < 0.7 growth steps were produced both by screw dislocations and two-dimensional nuclei. At higher super saturations 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 the n grew tangentially was observed. Strong directional kinetic anisotrop y in the tangential step growth rates in different directions was also seen. From the supersaturation dependence of tangential step rates, t he 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-di mensional nucleation was measured, and this yielded the supersaturatio n dependence of the normal growth rate. From these data the surface fr ee 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 ho mogeneous and occurred at sites more active than normal, e.g., at defe cts or sites of impurity incorporation. Values of alpha in the range o f 0.4-0.7 erg/cm(2) were also obtained on the basis of the sizes of cr itical nuclei, both for growth on dislocations and for growth by two-d imensional nucleation.