Surface Energy Evaluation with Atomic Force Microscopy

  • Li, Tonglei (PI)

Grants and Contracts Details


The aim of this proposal is to investigate how the solvent and additive/impurity molecules affect the surface energy of pharmaceutical crystalline materials by using atomic force microscopy (AFM). It has been found in our laboratory that during the dissolution process of acetaminophen single crystals, the etching pattern is regular and solvent-dependent. The pattern can be affected also by socalled tailor-made or structurally similar additives such as 4-methylacetanilide. It has been hypothesized that the adsorption of solvent or additive molecules on the crystal surface may trigger or stabilize the surface reconstruction of host molecules on the surface. It is further believed that the surface reconstruction or relaxation may playa key role in the polymorph formation as well as growth morphology. What is proposed here will provide a way to test this hypothesis. The anticipated study is to use AFM to measure the surface energy in situ of a model crystal surface upon dissolving in selected solvents or solutions of tailor-made additives. Using AFM makes it possible to measure the adhesion force between an AFM tip and a crystal surface. Based on the Johnson-Kendall-Roberts OKR) model, the adhesion force can be related to the adhesion work that is a function of surface energies including the one of the crystal-solution interface. Contact-angle measurement or sessile drop method will also be carried out to determine other surface energies involved. With the help of combining both measurements, the surface energy of the crystal-solution interface can be derived. Thus, the hypothesis can be tested by using solutions of a tailor-made additive on the model surface and monitoring the surface energy change with concentrations varied. This study may provide a pivotal contribution to the crystal engineering as well as pharmaceutical sciences. The lack of consideration of the surface structure and energy hinders the development of the polymorph prediction and growth morphology calculation. This study will make it possible to measure the surface energy of a solid-solution interface, and, furthermore, it may allow us to explore how the surface reconstruction affects the surface energy upon using different solvents or additive/impurity solutions. Such an understanding may open the door for the rational design of the internal structure and external shape of molecular crystals. This study may also show a novel way for the surface energy measurement. It is well know that the contact angle measurement requires a homogenous surface but most of time, due to the size of the droplet, it is difficult to achieve. With AFM, it is possible to measure an area as small as 20x20 nm2. Thus, the "true" surface energy can be obtained of almost every pharmaceutical crystalline material.
Effective start/end date7/1/036/30/05


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