Surface Charge Affects the Structure of Interfacial Ice

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.