Hydrocarbon Interactions: Determination of Interfacial Energy through Contact Angle Measurements

Hydrophobic interaction is known as a strong attraction between nonpolar molecules in water. This interaction plays an important role for survival and function of most biological substances, like phospholipid bilayer membrane. However, the question on the energy of hydrophobic interaction has not been answered. In this project, we examine the changes of hydrophobic interaction through contact angle measurement to compliment the forces observed in Surface Force Apparatus. We use Young's law to help us determine the interfacial energy at the hydrocarbon air-water interface. The hydrophobic surface was prepared by attaching hydrocarbon chain on gold surface. We deposited a hydrocarbon layer of 18 (C-18) and 11(C-11) carbon chain length on a molecularly smooth gold surface by using octadecanethiol and undecanethiol respectively. Dynamic contact angle measurements were performed by advancing and receding DI water, 5mM NaOH, and 5mM HNO3 on the surfaces at a constant rate of 2 μL/min. Static contact angle was also obtained by contacting the same solutions on the C-11 and C-18 hydrocarbon surfaces for 6-7 hours. The contact angle did not change during advancing; however, it decreased by 20 degrees during receding contact angle measurements.  The measured static contact angle decreased from about 110O to 95O for DI water, NaOH and HNO3 on both, the C-18 and C-11 hydrocarbon surfaces.