Articles | Volume 17, issue 12
Research article
29 Jun 2017
Research article |  | 29 Jun 2017

Surface-charge-induced orientation of interfacial water suppresses heterogeneous ice nucleation on α-alumina (0001)

Ahmed Abdelmonem, Ellen H. G. Backus, Nadine Hoffmann, M. Alejandra Sánchez, Jenée D. Cyran, Alexei Kiselev, and Mischa Bonn

Abstract. Surface charge is one of the surface properties of atmospheric aerosols, which has been linked to heterogeneous ice nucleation and hence cloud formation, microphysics, and optical properties. Despite the importance of surface charge for ice nucleation, many questions remain on the molecular-level mechanisms at work. Here, we combine droplet-freezing assay studies with vibrational sum frequency generation (SFG) spectroscopy to correlate interfacial water structure to surface nucleation strength. We study immersion freezing of aqueous solutions of various pHs on the atmospherically relevant aluminum oxide α-Al2O3 (0001) surface using an isolated droplet on the surface. The high-pH solutions freeze at temperatures higher than that of the low-pH solution, while the neutral pH has the highest freezing temperature. On the molecular level, the SFG spectrum of the interfacial water changes substantially upon freezing. At all pHs, crystallization leads to a reduction of intensity of the 3400 cm−1 water resonance, while the 3200 cm−1 intensity drops for low pH but increases for neutral and high pHs. We find that charge-induced surface templating suppresses nucleation, irrespective of the sign of the surface charge. Heterogeneous nucleation is most efficient for the nominally neutral surface.

Short summary
We report the effect of surface charge on heterogeneous immersion freezing for the atmospherically relevant sapphire surface. Combining linear and nonlinear optical techniques and investigating isolated drops, we find that charge-induced surface templating is detrimental for ice nucleation on α-alumina surface. This study provides new insights into atmospheric processes and can impact various industrial and research branches, particularly climate change and tracing of water in the hydrosphere.
Final-revised paper