Freezing of water next to solid surfaces probed by infrared-visible sum frequency generation spectroscopy

Ice formation next to solid surfaces is important in many biological, materials, and geological phenomena and may be a factor in how they impact various technologies. We have used sum frequency generation (SFG) spectroscopy to study the structure of ice as well as the freezing and melting transition temperatures of water in contact with sapphire substrates. We have observed that the structure of ice and water are a function of pH and the surface charge of the sapphire substrate. At low pH, we observed an increase in the SFG signal subsequent to ice formation. Contrary to expectations, at pH 9.8, corresponding to a negatively charged surface, the intensity of the ice SFG signal is about 10 times lower than that of water. Recent simulation studies have suggested that charge transfer is important for the high intensity of the ice peak at the ice-air interface. We believe that the segregation of sodium ions next to the negatively charged sapphire substrate may be responsible for disrupting the charge transfer and stitching bilayer at high pH, providing a plausible explanation for the experimental observations. Even though the structure of water and ice are affected by pH, the freezing and melting transition temperatures are independent of the surface charge. This report offers a unique insight on how ions next to solid surfaces could influence the structure of ice.