Structures of Water Molecules at the Interfaces of Aqueous Salt Solutions and Silica: Cation Effects

Abstract

Structures of water molecules at water/silica interfaces, in the presence of alkali chloride, were investigated using infrared-visible sum frequency vibrational spectroscopy. Significant perturbations of the interfacial water structure were observed on silica surfaces with the NaCl concentration as low as 1 × 10 -4 M. The cations, which interact with the silica surface via electrostatic interaction, play key roles in perturbing the hydrogen-bond network of water molecules at the water/silica interface. This cation effect becomes saturated at concentrations around 10 -2 to 10 -1 M, where the sum frequency generation peaks at 3200 and 3400 cm -1 decrease by 75%. Different alkali cation species (Li + , Na + , and K + ) produce different magnitudes of perturbation, with K + > Li + > Na + . This order can be explained by considering the effective ionic radii of the hydrated cations and the electrostatic interactions between the hydrated cations and silica surfaces. The interfacial water structure associated with the 3200 cm -1 band is more vulnerable to the cation perturbation, suggesting that the more ordered water structure on silica is likely associated with the vincinal silanol groups, which create a higher local surface electrical field on silica.

Na+ and Ca2+ Effect on the Hydration and Orientation of the Phosphate Group of DPPC at Air−Water and Air−Hydrated Silica Interfaces

“…100 Induced orientational order of water molecules near the silica surface in the presence of NaCl has been reported at the pH of these studies. 101,102 As discussed above, the headgroup dipole potential leads to an ordering of the solvating water molecules. 66 As a result, the trapped thin aqueous layer between the silica surface and the headgroup of the DPPC monolayer is likely to be structured as described for the case in the absence of salts.…”

Experimental Correlation Between Interfacial Water Structure and Mineral Reactivity

“…This is in agreement with previous observations, albeit performed at a single pH, that the 3200 cm −1 feature experiences larger perturbation with increasing ionic strength. 33 We speculate that the overall salt effect on the vSFG signal comes mainly from the response of water molecules in strongly hydrogen bonded environments.…”

“…At all pHs above 2, addition of 0.1 M NaCl leads to an overall decrease in the vSFG intensity ( Figure 1), consistent with previous observations. 25,32,33 The reduction in the vSFG signal ( Figure 1) due to salt strongly depends on the pH and hence the extent of silica surface ionization.…”

“…However, it is unclear why electrolytes would increase molecular ordering of water at the interface with a simultaneous decrease in the vSFG spectral intensity as observed in many vSFG reports. 22,25,33 The vSFG signal at a charged interface originates from water molecules in a noncentrosymmetric environment induced by interfacial molecular ordering and/or the surface electric field (E 0 ). The resonant vSFG intensity consists of two terms: where χ (2) and χ (3) are the second-and third-order susceptibilities, respectively.…”

Determination of Surface Potential and Electrical Double-Layer Structure at the Aqueous Electrolyte-Nanoparticle Interface

“…Moreover, x-ray reflectivity measurements indicate no structuring beyond the first layer of adsorbed water molecules at a similar silica surface [47], and sum-frequency spectroscopy results show the structure of water at fused silica interfaces is independent of cation (Li þ , Na þ , K þ ) in 50-mM chloride electrolytes [48].…”