Contact Angles and Monolayer Depletion

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Abstract
Contact angles and contact angle hysteresis are very sensitive to surface heterogeneity. The degree of coverage of a surface by organic monolayers can be estimated by using the average of the cosines of advancing and receding angles in the equation of Cassie. This estimate can be refined by using a calibration curve computed from an idealized model of a heterogeneous surface. Adhesion can be significantly influenced by the presence of monolayers and partial monolayers on adherends. These monolayers often control the rate of wetting by an adhesive and also the ultimate contact angle of the system. There is also some speculation that such monolayers might act as weak boundary layers. Since contact angles are so sensitive to coverage by monolayers (1-2), the question arises as to what extent contact angles can be used to measure surface coverage. The work described in this paper was undertaken to answer that question. While the concepts developed here are applicable to any heterogeneous system, they were developed primarily to study the adsorption and depletion of organic monolayers on high energy surfaces.

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