Determination Of Energy At Hydrophobic Interfaces Through Contact Angle Measurements

Hydrophobic interactions have been under investigation for decades and questions regarding the energies between these molecules continue unanswered. These interactions cannot be explained through existing force laws. We did surface force measurements using the Surface Force Apparatus (SFA) and developed a force law to explain the nature of hydrophobic interactions. We need to determine the surface energy term, a parameter in the proposed hydrophobic force law for the interface between the 18-carbon chain length and air-water interface. The hydrophobic surface was mimicked by covalently attaching 18-carbon chain length on mica-templated gold surface using thiol chemistry. Dynamic contact angle experiments were done by advancing and receding a drop of 5mM HNO3/NaOH/KNO3 solution, as well as DI water on the surface, equilibrium contact angle was also measured by allowing the drop to sit on the surface for several hours. Control experiments were preformed on pure mica-templated gold surface with the same solutions. The equilibrium value for the contact angle between the 18-carbon chain length and air-water interface was observed to relax from 110 degrees down to 95 degrees for a variety of solutions. From this equilibrium value, the interfacial energy was calculated to be 31 mJ/m2 and was independent of the surface history.