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.

The water/graphene interface has received considerable attention in the past decade due to its relevance in various potential applications including energy storage, sensing, desalination, and catalysis. Most of our knowledge about the interfacial water structure next to graphene stems from simulations, which use experimentally measured water contact angles (WCAs) on graphene (or graphite) to estimate the water-graphene interaction strength. However, the existence of a wide spectrum of reported WCAs on supported graphene and graphitic surfaces makes it difficult to interpret the water-graphene interactions. Here, we have used surface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy to probe the interfacial water structure next to graphene supported on a sapphire substrate. In addition, the ice nucleation properties of graphene have been explored by performing in situ freezing experiments as graphitic surfaces are considered good ice nucleators. For graphene supported on sapphire, we observed a strong SFG peak associated with highly coordinated, ordered water next to graphene. Similar ordering was not detected next to bare sapphire, implying that the observed ordering of water molecules in the former case is a consequence of the presence of graphene. Our analysis indicates that graphene behaves like a hydrophobic (or negatively charged) surface, leading to enhanced ordering of water molecules. Although liquid water orders next to graphene, the ice formed is proton disordered. This research sheds light on water-graphene interactions relevant in optimizing the performance of graphene in various applications.

We have conducted studies on the freezing of water molecules next to charged surfaces to elucidate the effect of water orientation on the structure of ice using sum frequency generation spectroscopy. We observed that when water is frozen next to a positively charged sapphire surface, the signal intensity of ice is higher than that of liquid water as expected from previous theoretical studies. However, when water is frozen next to a negatively charged sapphire surface (using NaOH as pH adjuster), the signal intensity decreases. The same signal attenuation upon freezing is obtained when cesium hydroxide (CsOH) and tetramethylammonium hydroxide (N­(CH3)4OH) are used as pH adjusters. Since Na+, Cs+, and N­(CH3)4 + ions have different hydration properties, the cation specific effect for this attenuation in signal intensity for ice is ruled out. Experiments on a mica surface (inherently negatively charged) also showed similar attenuation in signal intensity for ice as negatively charged sapphire surface. We conclude that the orientation of the water molecules next to a surface plays an important role in the structure of ice. These results have important implications in understanding the strength of ice nucleation and strength of ice adhesion next to charged surfaces.

Previous studies have found that superhydrophobic surfaces are effective in delaying freezing of water droplets. However, the freezing process of water droplets on superhydrophobic surfaces depends on factors such as droplet size, surface area, roughness, and cooling rate. The role of surface energy, independent of any other parameters, in delaying freezing of water is not understood. Here, we have used infrared-visible sum frequency generation spectroscopy (SFG) to study the freezing of water next to solid substrates with water contact angles varying from 5° to 110°. We find that the freezing temperature of water decreases with increasing surface hydrophobicity only when the sample volume is small (∼10 μL). For a larger volume of water (∼300 μL), the freezing temperature is independent of surface energy. For water next to the surfaces with contact angle ≥54°, we observe a strong SFG peak associated with highly coordinated water. This research sheds new light on understanding the key factors in designing new anti-icing coatings.