Surface stratification determines the interfacial water structure of simple electrolyte solutions

Abstract

The distribution of ions at the air/water interface plays a decisive role in many natural processes. Several studies have reported that larger ions tend to be surface-active, implying ions are located on top of the water surface, thereby inducing electric fields that determine the interfacial water structure. Here we challenge this view by combining surface-specific heterodyne-detected vibrational sum-frequency generation with neural network-assisted ab initio molecular dynamics simulations. Our results show that ions in typical electrolyte solutions are, in fact, located in a subsurface region, leading to a stratification of such interfaces into two distinctive water layers. The outermost surface is ion-depleted, and the subsurface layer is ion-enriched. This surface stratification is a key element in explaining the ion-induced water reorganization at the outermost air/water interface.

Spontaneous Appearance of Triiodide Covering the Topmost Layer of the Iodide Solution Interface Without Photo-Oxidation

“… 46 The 2730 cm –1 peak, the 2650 cm –1 shoulder peak, and the 2550 cm –1 band originate from free O–D groups, the antisymmetric O–D stretch mode of the D 2 O molecules donating two hydrogen bonds, and hydrogen-bonded O–D groups, respectively. 47 − 49 Upon the addition of NaI, the Imχ eff,ssp (2) amplitude decreases above 2560 cm –1 , while it increases below 2560 cm –1 , consistent with refs ( 29 and 50 ). The Imχ eff,ppp (2) spectra commonly show a 2730 cm –1 positive peak, a 2650 cm –1 negative band, and a broad 2550 cm –1 positive band.…”

“…A closer look at the peak amplitude extracted from the fits (Figure e) reveals a highly nonlinear behavior, also apparent from the raw data: the Imχ yyz (2) and Imχ zzz (2) amplitudes of the free O–D peak ( a yyz and a zzz , respectively) only start to decrease above [NaI] = ∼2 M. The surface pressure data of the NaI solution also shows a similar nonlinear behavior above ∼2 M (see Supporting Information). In contrast, we could not see such a nonlinear trend for other halide ions, such as Cl – , as displayed in Supporting Information …”

“…In contrast, we could not see such a nonlinear trend for other halide ions, such as Cl − , as displayed in Supporting Information. 50 Since the SFG peak amplitude of the free O−D peak is determined not only by the surface density of the free O−D stretch chromophores (N S ) but also by the orientation of the free O−D group, 51,55 one needs to disentangle these contributions to access N S . To do so, we extracted the free O−D peak area in the Imχ yyz (2) and Imχ zzz (2) spectra (denoted as A yyz and A zzz , respectively).…”

“…In contrast, we could not see such a nonlinear trend for other halide ions, such as Cl – , as displayed in Supporting Information . 50 …”

Double-Layer Distribution of Hydronium and Hydroxide Ions in the Air–Water Interface

“…Efforts should be made to address these limitations in future simulations in order to conduct a more comprehensive examination of the air–water interface. Fortunately, recent high-level heterodyne-detected vibrational SFG results for electrolyte solutions may have partly corroborated our findings, providing indirect experimental evidence of ionic double-layer distribution, but the challenge of directly detecting the ionic preference at a pure air–water interface should be addressed in the future experiments.…”