LATTICE RESOLUTION AND SOLUTION KINETICS ON SURFACES OF AMINO-ACID CRYSTALS - AN ATOMIC FORCE MICROSCOPE STUDY

We report atomic force microscopy (AFM) results on six amino acid crys tal surfaces: glycine, L-aspartic acid, L-valine, L-isoleucine, L-leuc ine, and L-phenylalanine. Samples were grown by slow evaporation of co ncentrated aqueous solutions. All samples contained crystalline areas where the AFM showed extended molecularly flat sheets (up to hundreds of nm in size) separated by steps a single molecule thick. The ordered lattice of each amino acid could be imaged on the sheets. Images reve aled periodicities corresponding to bulk terminations in most cases, a s well as other periodicities which probably correspond to molecular s tructure within the unit cell. Step motion kinetics were also imaged i n situ during dissolution of L-leucine in flowing propanol. Steps orie nted along the [010] direction traveled with speeds that were independ ent of both interstep distance and solvent flow rate for flow rates ab ove 20 mul/s, indicating a reaction rate limited process. Orthogonal b ends along the [001] direction moved at speeds one to ten times that o f steps, with narrow bends moving faster than wide. We speculate that these speed differences were caused by anisotropy in reaction kinetics coupled with partially saturated boundary layers near wide bends.