Concluding remarks for Faraday Discussion on Water at Interfaces

Water at interfaces is a fascinating and multifaceted topic that has garnered signi ﬁ cant attention in various scienti ﬁ c ﬁ elds due to its relevance and implications. This Faraday Discussion explored the complexity of water at di ﬀ erent interfaces. Many of the reports highlight the need for a molecular-level understanding. The Discussion was lively and constructive. In these summarizing remarks, I do not aim to be complete, but will rather try to sketch the status of the ﬁ eld, highlight the progress that we as a community have made, and present eclectic examples of where more work needs to be done.

supercooled water, including in ice nucleation.Finally, density functional theory molecular dynamics (DFT-MD), with the support of machine learning, is the most recent advance, 4 and has enabled, for instance, the predicting of the phase diagram of monolayer nanoconned water. 5hese advances in modeling, along with those in experiments, now allow for a direct, quantitative comparison of complex aqueous systems (e.g., density functional theory and atomic force microscopy on well-dened surfaces in ultrahigh vacuum): theory and experiment have really come together, as evident from several papers in this present issue of Faraday Discussions † that have theory directly connecting to experiments, or vice versa.
A secondheateddebate that has been ongoing revolves around the distribution of ions near or at interfaces in ionic solutions, which is crucial, especially in the context of ocean acidication.The dissolution of excess carbon dioxide from the atmosphere into the oceans can alter the pH and affect the stability of biominerals.The distribution of ions near the interface plays a signicant role in these processes and can impact marine life, including organisms that rely on calcium carbonate to build their shells and skeletons.Likewise, the ion distribution at the water surface is important for (photo-) chemistry occurring on the surface in the atmosphere.
In the 2008 Faraday Discussion, a major discussion emerged about the surface activity of hydrated protons or hydroxyl ions.From the 2008 Spiers Memorial Lecture: 'Therefore, in the topmost layer of water there shall be an excess of hydronium over hydroxide.We denote this situation as an "acidic" surface of water'. 6Yet, the title of the subsequent paper from the same Faraday Discussion reads: "The surface of neat water is basic". 7Meanwhile, this controversy has been resolved by quantitative measurements of proton and hydroxyl propensity at the water-air interface.There is indeed an excess of hydronium at the surface, while hydroxide ions are depleted from the surface.There is an adsorption well for hydronium at the water-air interface, which has been quantied independently by two groups 8,9 as being characterized by an adsorption free energy, DG z −4.5 kJ mol −1 , corresponding to a partitioning coefficient of the surface with respect to bulk water of a factor of ∼5.Such a surface propensity is absent for hydroxyl ions. 10As highlighted in this year's Spiers Memorial Lecture, we now also have a much better understanding of how other ions behave at the water-air interface, 11 and how we can use ion exchange to enable micro swimmers in water (https://doi.org/10.1039/D3FD00098B),for instance.
And now, the sobering awareness that essential unanswered questions persist: Despite the enormous progress that the community has clearly made, one can also identify remaining open questions in areas where more research is needed.Somewhat ironically, an important interface that still raises controversial and open questions is the water-air interface.Despite arguably being the simplest aqueous surface conceivable, there is considerable debate about whether this nominally neutral interface is, in fact, neutral.One intriguing phenomenon is the enhanced chemistry occurring on the water surface, compared to reaction rates in the bulk.Somewhat controversial 23 in this context are the claims of spontaneous dissociation of water molecules at the surface of pure water microdroplets. 24This dissociation is thought to be induced by a large interfacial electric eld leading to the formation of OH radicals.These radicals can subsequently combine to form hydrogen peroxide (H 2 O 2 ) through an associative reaction.Strong electric elds have been reported by indirect measurements, 25 and one of the challenges is clearly to measure these eld strengths more directly, for instance, using nonlinear optical approaches. 26n various environmental, biological, and technological contexts, water interfaces with charged surfaces.Fundamental questions remain about water and aqueous electrolyte solutions at charged interfaces.These questions pertain to concepts like the Debye length, the magnitude of the interfacial electric eld and associated surface potentials, and related to this, fundamental questions about ion-water, ion-ion, and ion-surface interactions.Indeed, much like the situation in 2008, 27 several papers in the 2023 edition of this Faraday Discussion address the behavior of water at charged interfaces.‡

Conclusions
Finally, there was a recurrent discussion during the meeting about the denition of the water interface.That debate reminded me of a quote from Robert M. Pirsig 28 about quality: "Quality.You know what it is, yet you don't know what it is. .But for all practical purposes it really does exist.".Along the same lines, I would argue that the interface most certainly exists, even if we cannot dene it unequivocally.