Solid-State Organic Chemistry at University of Kentucky

University of Kentucky | College of Pharmacy

Tonglei Li

analyze, compute & design
	solid-state organic/drug chemistry

The surface energy, or, more strictly speaking, surface free energy, is a determining parameter that characterizes the interfacial phenomena such as adsorption, wetting, adhesion, etc. Because the crystallization process is greatly affected by solvent-solute interactions, understanding surface energy at the solvent-crystal interface can provide insightful knowledge regarding polymorphic formation and growth morphology.

The surface energy of solids can be derived by measuring contact angles of a group of carefully selected liquids. At the interface between a solid and a liquid, the function to connect the contact angle with the surface energy is expressed by the Young’s equation. Unlike the surface tension of liquids, the surface energy of solids can not be measured directly because of elastic and viscous restrains of the bulk phase. In order words, there are two unknows in the Young's equation of surface energy, solid-vapor and solid-liquid. Thus, indirect methods are typically used, which typically involve using different types of liquids. At the surface of liquids, the surface tension results from an imbalance of molecular forces. The liquid molecules are attracted to each other and exert a net force pulling themselves together. The stronger molecules interact, the higher the surface tension. Because of the hydrogen bonding, water has a very high value of surface tension. Organic molecules with polar groups (such as iodide and hydroxyl) have a slightly lower surface energy than water. Pure hydrocarbons have even lower values as only the dispersion forces exist. Surface tension values of common liquids are well known. Therefore, using these different solvents, it’s possible to identify the disperse and polar contributions to the surface energy of a solid surface.

Nevertheless, the ideal solid surface (i.e., chemically homogeneous, rigid, and smooth on the atomic scale) has to be assumed in order to satisfy the Young’s equation. The hysteresis of the contact angle is the result of the systems failure to meet the conditions of ideality. For example, surface roughness was shown to affect the contact angle (Zografi 1984). Because of the unavoidable structural defect, the droplet to measure the contact angle needs to be as small as possible until reaching the limit to record the contact angle.

It is even more challenging to use current indirect methods for studying the solvent and additive effects on crystal growth. Using a liquid droplet to measure the contact angle is not sensitive the energy change caused by surface reconstruction due to either solvent or addtive molecules. All these difficulties make the AFM (atomic force microscopy) an attractive tool for the surface energy measurement to study surface energy that is affected by the solvent/additive-induced surface reconstruction.

AFM has been employed in many different areas. The force curves obtained by contact mode AFM have been widely used to study the interactions between two surfaces (Burnham 1993). The adhesion force obtained by AFM is related to the surface energies of interfaces involved during the measurement. Additionally, the AFM measurement is carried out in the liquid environment so that problem associated with the air-liquid-surface interface as occurred in the sessile drop method can be eliminated. The adhesion force is sensitive enough to detect the small energy change due to the surface reconstruction because the size of AFM tips is typically less than 100 nm. The force measured can be as small as a few pico-Newtons, which is close to the strength of a covalent bond!

References:

J. Israelachvili, Adhesion: In Intermolecular & Surface Forces, 2nd ed., San Diego: Academic Press Inc, 312-330, 1991

G. Buckton, Assessment of the wettability of pharmaceutical powders, Journal of Adhesion Science and Technology v. 7, 205-219, 1994

P. Frantz, A. Artsyukhovich, R. W. Carpick and M. Salmeron, Use of capacitance to measure surface forces. 2. Application to the study of contact mechanics, Langmuir v. 13, 5957-5961, 1997

L. D. Gelb and R. M. Lynden-Bell, Effects of atomic-force-microscope tip characteristics on measurement of solvation-force oscillations, Physics Review B v. 49, 2058-2066, 1994

T. Han, J. M. Willliams and T. P. J. Beebe, Chemicl bonds studied with functionalized atomic force microscopy tips, Analytica Chimica Acta, v. 307, 365-376, 1995

K. L. Johnson, K. Kendall and A. D. Roberts, Surface energy and the contact of elastic solids, Proceedings of the Royal Society of London A, v. 324, 301-313, 1971

S. Kidoaki and T. Matsuda, Adhesion forces of the blood plasma proteins on self-assembled monolayer surfaces of alkanethiolates with different functional groups measured by atomic force microscope, Langmuir v. 15, 7639-7646, 1999

L. Kuutti, J. Peltonen, P. Myllarinen, O. Teleman and P. Forssell, AFM in studies of thermoplastic starches during ageing, Carbohydrate Polymers v. 37, 7-12, 1998

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