This work provides unambiguous definitions from theoretical simulations of the two interfacial regions named the BIL (binding interfacial layer) and DL (diffuse layer) at charged solid/water and air/water interfaces. The BIL and DL nomenclature follows the pioneering work of Wen et al. [Phys. Rev. Lett. 2016, 116, 016101]. Our definitions are based on the intrinsic structural properties of water only. Knowing the BIL and DL interfacial regions, one is then able to deconvolve the χ(ω) non-linear SFG (sum frequency generation) response into χ(ω) and χ(ω) contributions, thus providing a detailed molecular interpretation of these signals and of the measured total SFG. We furthermore show that the χ(ω) spectrum arises from the χ(ω) non-linear third order contribution of bulk liquid water, here calculated for several charged interfaces and shown to be universal. The χ(ω) contribution therefore has the same origin in terms of molecular normal modes at any charged interface. The molecular interpretation of χ(ω) is hence at the heart of the unambiguous molecular comprehension and interpretation of the measured total SFG signal at any charged interface.

SignificanceSilica, the most abundant mineral on Earth, is exploited in many technologies and naturally occurring geological and atmospheric processes. The physical and chemical interactions between silica and water are the fundamental driving forces for water purification systems, oil extraction, and coatings. Characterizing the silica/water interface is therefore important to improve existing technologies, in particular for silica coatings, which rely on wettability and thermal-resistant properties to remain effective. We investigated the silica/water interface using a mixture of macroscopic and microscopic techniques, including experimental and theoretical surface-specific sum frequency generation spectroscopy and contact angle measurements. Strikingly, we observed the presence of water molecules non–hydrogen bonded to the nominally hydrophilic silica surface.