Heterodyne-detected sum frequency generation of water at surfaces with varying hydrophobicity

Aqueous surfaces mediate many atmospheric, biological, and technological processes. At the interface, the bulk hydrogen-bonding network is terminated and the interfacial water molecules restructure according to the surface chemistry of the interface. Given the complexity of both natural and technical aqueous interfaces, self-assembled monolayers provide a platform for controllably tuning the chemical composition of the surface and thus the water restructuring. Here, we study a hydrophobic monolayer, a hydrophilic monolayer, and a mixed hydrophobic/hydrophilic monolayer in contact with water. Monolayers composed of both hydrophilic and hydrophobic chains mimic the complex and heterogeneous chemical composition of natural and technological surfaces. By employing heterodyne-detected sum frequency generation, the purely absorptive vibrational line shape of interfacial water is measured experimentally. We examined the structure of the interfacial water in contact with each of the monolayers by analyzing the relative dipole moment orientations and fitting the imaginary component of χ⁽²⁾ with a combination of Lorentzian and Gaussian line shapes. For all of the monolayers, the hydrogen-bonded water points toward the monolayer, which is opposite of the orientation of the hydrogen-bonded water at the air-water interface. Additionally, a strongly hydrogen-bonded water species exists for the monolayers containing hydrophilic chains. The spectroscopic results suggest that the microscopic water structure in contact with the mixed monolayer is dominated by the hydrophilic parts of the monolayer, while the contact angle shows that at the macroscopic level the surface properties lie closer to the pure hydrophobic monolayer.