Hexadecanol monolayer at aqueous surface diminishes the effects of iodide ions on interfacial water: A theoretical vibrational sum frequency generation spectroscopic study

Abstract

We have employed molecular dynamics simulations and vibrational sum frequency generation (VSFG) spectral calculations to investigate how a hexadecanol (HDC) monolayer at the surface of an aqueous NaI solution influences the effects of iodide ions. When the aqueous NaI surface is exposed to air, the presence of iodide ions causes a significant change in the orientation of O–H groups at the interface compared to the neat water/vapor interface. Three distinct regions have been observed in the VSFG spectrum of the water–NaI/vapor interfacial system: O–H groups hydrogen-bonded to other water molecules (water-HB), O–H groups hydrogen-bonded to negative ions (ion-HB), and dangling O–H groups without any hydrogen bonds. Overall, O–H groups in each region are oriented upward. When the aqueous NaI surface is covered with HDC monolayers, two distinct regions, ion-HB and water-HB, are found in the VSFG spectrum. In the water-HB region, the observed VSFG spectrum is due to the cancellation of the intensity of up-oriented O–H groups hydrogen-bonded to HDC and down-oriented O–H groups hydrogen-bonded to water. No dangling O–H peak is observed since HDC monolayers completely cover the aqueous surface. The ion-HB peak has the opposite orientation when HDC monolayers are present on the surface in the water–NaI/HDC interfacial system compared to the water–NaI/vapor interfacial system. In stark contrast to the water–vapor interface, the spectral contribution from O–H groups hydrogen-bonded to I− is drastically reduced when alcohol is present at the surface, directly reflecting the diminished interfacial ion propensity.