Structural definition of the BIL and DL: a new universal methodology to rationalize non-linear chi((2))(omega) SFG signals at charged interfaces, including chi((3))(omega) contributions

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 chi((2))(omega) non-linear SFG (sum frequency generation) response into chi((2))(BIL)(omega) and chi((2))(DL)(omega) contributions, thus providing a detailed molecular interpretation of these signals and of the measured total SFG. We furthermore show that the chi((2))(DL)(omega) spectrum arises from the chi((3))(omega) non-linear third order contribution of bulk liquid water, here calculated for several charged interfaces and shown to be universal. The chi((2))(DL)(omega) contribution therefore has the same origin in terms of molecular normal modes at any charged interface. The molecular interpretation of w (2) chi((2))(BIL)(omega) is hence at the heart of the unambiguous molecular comprehension and interpretation of the measured total SFG signal at any charged interface.