GROWTH AND ETCHING KINETICS OF TETRAGONAL LYSOZYME

The morphology and kinetics of the growth and dissolution of lysozyme in aqueous solutions with 2.5% NaCl and at pH = 4.5 were studied in-situ with a depth resolution of 300 angstrom (4 unit cells) by high resolution optical microscopy and digital image processing. The bulk super- or undersaturation, sigma, of the solution inside a closed growth cell was controlled by temperature. The growth habit was bound by (110) and (101) faces that grew through layer spreading, although with different growth rate dependencies on supersaturation/temperature. At sigma < 10 (obtained at higher temperatures) growth was purely kinetically controlled, with impurity effects (macrostep formation and kinetic hindrance) becoming significant for sigma < 2. At sigma > 10 (lower temperatures), anisotropies in the interfacial kinetics were more pronounced, with interfacial kinetics and bulk transport becoming equally important to the growth morphology. Growth rates were growth-history dependent. The formation of striations (layers of irregularly incorporated solution) was unambiguously correlated with growth temperature variations. Etching exposed dislocations and various high-index faces whose growth morphologies were studied during return to the steady-state growth form. Growth steps were observed to originate from two-dimensional nuclei or from outcrops of growth striations, and from dislocations that preferentially formed in growth sector boundaries.