The Influence of Sodium Iodide Salt on the Interfacial Properties of Aqueous Methanol Solution by a Combined Molecular Simulation and Sum Frequency Generation Vibrational Spectroscopy Study

Abstract

Understanding the influence of salt ions on the microscopic properties of liquid interfaces is of both fundamental and practical importance. A large number of previous experimental and theoretical investigations have explored the salt effects on the surfaces of either pure water or neat organic liquid. However, how the salt ions affect the interfacial structures of water/organic liquid mixtures has rarely been studied. Here, the molecular dynamics (MD) simulations and sum frequency generation vibrational spectroscopy (SFG-VS) were carried out to investigate the influence of sodium iodide (NaI) on the air/liquid interfaces of the methanol–water mixtures. The SFG-VS spectral intensities were discovered to increase with the addition of 3 M NaI, while the center frequencies of the C–H stretching vibrations at high methanol concentrations showed a ∼2 cm–1 blue shift compared with those obtained before adding NaI. The MD results indicated that Na+ and I– can only affect Part I (near the bulk phase) but not Part II (near the gas phase) of the interfacial region. It was also found that the average orientations of interfacial methyl groups were constant and not effectively disturbed by the changes of methanol concentrations or the addition of NaI. It is therefore concluded that the changes of the SFG-VS intensities upon the addition of NaI salts were mainly caused by the increasing number of interfacial methanol molecules. Further analysis showed that the existence of NaI affects the surface tensions more for the interfaces with higher bulk methanol concentrations, which is in agreement with the SFG-VS results. It is noteworthy that the maximum number density of methanol molecules with the net nonzero orientations is reached near the Gibbs dividing surface, the reasons of which are worth further investigating.

Liquid/Vapor Interface of Dimethyl Carbonate–Methanol Binary Mixtures Investigated by Sum Frequency Generation Vibrational Spectroscopy and Molecular Dynamics Simulation

“…The SFG-VS spectra of the pure liquid methanol has already been extensively studied in the literature. − , In Figure , two apparent SFG-VS peaks located at ∼2837 and 2949 cm –1 and a shoulder at 2921 cm –1 are observed for the vapor/pure methanol interface (marked as x M = 1.00, with x M defined as the bulk mole fraction of methanol), which are in good agreement with the previous reports. − , These peaks are much more intense under the ssp polarization than the ppp polarization (see Figure S1 in the Supporting Information), indicating that the SFG-VS peaks observed here are from the symmetric vibrational modes of the methanol molecules . The most dominant peak at ∼2837 cm –1 is commonly assigned as the methyl symmetric stretching mode (r + of methanol, CH 3 -ss mode). − , The less intense peak at ∼2949 cm –1 is associated with the Fermi resonance states between the symmetric stretching and the overtone of the bending mode of the methyl group (r + Fermi ). − , The shoulder at 2921 cm –1 belongs to another Fermi resonance peak according to the previous SFG-VS and Raman assignments. − …”

“…SFG-VS has been extensively employed to study the liquid/vapor interfaces of water, pure organic liquids, water–organic binary mixtures, water–ionic liquid mixtures, etc. ,− These efforts have revealed important physicochemical properties of the liquid surfaces, such as the molecular orientations within the interfacial regions and surface adsorption free energies, providing detailed experimental evidence that can be compared with the results from the MD simulations or other theoretical methods. ,− But as for the organic–organic binary mixtures, the existing investigations are mostly limited to few examples of the solid/liquid interfaces. − The explorations of the competitive adsorption and molecular structures at the liquid/vapor interfaces of multicomponent organic mixtures have still been scarce, both experimentally and theoretically.…”

Investigation of the Atmospheric Moisture Effect on the Molecular Behavior of an Isocyanate-Based Primer Surface

“…A picosecond SFG system was used for experiments in this study (EKSPLA, Lithuania, model PL2231-20-RS232). SFG vibrational spectroscopy was extensively used to probe the surfaces and interfaces of the materials due to its intrinsic surface sensitivity. − SFG theories, setups, and instrumentation will thus not be stated in detail. Briefly, two input beams, a visible beam with a fixed wavelength at 532 nm and a frequency-tunable infrared (IR) beam, were overlapped spatially and temporally on the surfaces of the samples.…”

“…Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a suitable tool to understand the change of molecular structures of surfaces and interfaces. − SFG enables precisely measuring the molecular structure/behavior of surfaces/interfaces by its surface/interface-sensitive nature. − SFG has been extensively applied to study the surface and interfacial molecular structures of polymers, including various polymer adhesives. − ,− In this study, the atmospheric moisture effect on the surfaces of isocyanate-based primers was probed by SFG, and the molecular behaviors of such surfaces were correlated to the macroscopic properties to investigate the relationship. Different conditions in the experiments were set up in a variation of humidity levels and exposure time to understand the influences of their effects on the primer surfaces.…”

Probing Molecular Behavior of Carbonyl Groups at Buried Nylon/Polyolefin Interfaces in Situ

“…SFG theory and spectral data analysis methodology have been extensively published and will not be repeated here. − According to the selection rule governing an SFG process, under the electric-dipole approximation, SFG signals can only be generated from a medium with no inversion symmetry. Most bulk materials have inversion symmetry, therefore, no SFG signal can be generated.…”