Measurements of Line Tension for Solid−Liquid−Vapor Systems Using Drop Size Dependence of Contact Angles and Its Correlation with Solid−Liquid Interfacial Tension

According to the modified Young equation, one of the direct ways of measuring line tension for solid−liquid−vapor systems takes advantage of the dependence of the contact angle on the radius of curvature of the three-phase line. This approach was used to determine line tension measurements for six organic liquids on two different self-assembled monolayer (SAM) surfaces (mixed monolayers of methyl and carboxylic acid terminated alkanethiol on gold at two different composition levels). Low-rate advancing contact angles were measured using Axisymmetric Drop Shape Analysis-Profile (ADSA-P). The general trend observed for each system was that the contact angle decreases as the radius of the three-phase line for the sessile drop increases from approximately 1 to 5 mm. It was found that the drop size dependence of contact angles can be interpreted as being due to a positive line tension. The line tension values are all in the order of 10 - 6 J/m. Also a correlation was observed between the line tension and the solid−liquid interfacial tension for a series of liquids on each of the two surfaces. This observation conforms to the predictions from a phase rule discussion for capillary systems.