Atomic-Scale Imaging of Surface and Hydration Structures of Stable and Metastable Acetaminophen Crystals by Frequency Modulation Atomic Force Microscopy

摘要

In this paper, we have investigated hydration structures of the stable and the metastable acetaminophen crystals (Form I and Form II, respectively) by frequency modulation atomic force microscopy with a function of threedimensional (3D) force distribution measurement. The 3D force distribution image measured on the (100) surface of From I crystal showed that water molecules form four hydration layers while that measured on the (001) surface of Form II showed that they form only one hydration layer. The water molecule at the first layer of Form I is more strongly bound to the surface than that of Form II. This rigid and multilayered hydration structure of Form I indicates that the energy to form a hydration structure on the (100) surface of Form I is lower than that on the (001) surface of Form II. This is most likely related to the lower solubility of the stable Form I than that of the metastable Form II.