Aqueous interfaces are ubiquitous in nature and technological applications. Numerous physical, chemical and engineering processes occur at interfaces involving water and hydrophobic surfaces. Despite long study, many issues still remain highly controversial regarding the properties of water at hydrophobic interfaces. One particular interface in this regard that has received a lot of attention is the air-water interface. In particular, the origins of the surface charge of water, remains highly controversial [1-5].
In this work we investigate the electronic properties of the air-water interface using state-of-the-art linear scaling DFT methods  allowing us to model systems consisting of over thousands of atoms. We examine how the dipole and quadrupole moments of water change near the interface as well as how charge transfer between layers is rooted in topological properties of the water network such as coordination defects and water wires. The role of both classical electrostatics and quantum mechanical effects like charge transfer and exchange, are examined with respect to the affinity of protons and hydroxide ions for the surface of water.
Finally these results are discussed within the broader framework of water near hydrophobic interfaces such as proteins, solid surfaces and hydrophobic polymers.
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