Direct Measurement of Molecular Contact Area Using Sum Frequency Generation Spectroscopy

The molecular interactions between two solid surfaces at a subnanometer-length scale can control phenomena such as adhesion, friction, and wetting properties. These interactions, which comprise the actual contact area at the molecular scale between two materials, have historically been a challenge to measure. Here, we show the use of surface-sensitive sum frequency generation spectroscopy to probe the molecular contact between two solid surfaces that vary in roughness and modulus. The molecular interactions are measured by a shift in the vibrational energy of surface hydroxyl groups (OH) on sapphire substrates after interacting with rough poly(methyl methacrylate) sheets. We relate the shift of the OH peak to the contact area and find a dramatic drop in the contact area with increases in roughness and modulus. In contrast, softer sheets were able to deform and create a complete mechanical contact. Interestingly, we find that complete mechanical contact alone was insufficient to maximize the molecular contact. Instead, the molecular rearrangement of interfacial molecules was necessary to enhance the interaction energy.