Bulk Contributions Modulate the Sum-Frequency Generation Spectra of Water on Model Sea-Spray Aerosols

Abstract

Sea-spray aerosol particles have major yet poorly understood influence on the state of the atmosphere. Although non-linear vibrational spectroscopy is a reliable technique for probing the nature of aerosol interfaces, resolving the spectral features into specific structural and dynamical properties of the interface poses substantial difficulties. Here, computer simulations are used to disentangle strictly surface-sensitive contributions from bulk-dependent effects at a model sea-spray aerosol, which allows for a detailed, molecular-level characterization of the interfacial properties.

Perturbation of Hydrogen-Bonding Networks over Supported Lipid Bilayers by Poly(allylamine hydrochloride)

“…42 Because the ionic strength of 1 mM PAH (0.3 M) is~3x higher than our original buffer solution (0.1 M), a control experiment was carried out to determine whether the changes in the H-bonding network was due to the PAH interaction or ionic strength. Figure 4D shows only negligible changes in the OH stretching region when the salt concentration is raised from 0.1 to 0.3 M. This outcomes is consistent with negligible absorptive-dispersive mixing between the second-order (χ (2) ) and the surface potential-dependent third-order contribution (χ (3) Φ(0)) to the SFG signal generation process, according to [27][28]33,[54][55] χ total 2 = ! (!)…”

Identifying Eigen-like hydrated protons at negatively charged interfaces

“…At higher frequencies, between 3100 cm −1 and 3650 cm −1 , the OH stretching bands from water in the interfacial region are apparent (the OH stretch from the hydrogen bonded carboxylic acid gives rise to a weak broad band centered at~2900-2950 cm −1 30 ). The signal primarily originates from water molecules directly interacting with the carboxylic acid headgroup 26,31 , with only a small contribution, if any, from water molecules in the diffuse double layer. At 1 μM the Debye length (i.e.,~300 nm) is significantly longer than the non-linear coherence length of the experimental geometry (Δk z −1 ≈ 50 nm), and most of the signal generated within the diffuse double layer cancels due to destructive interference 32,33 .…”

Specifics about Specific Ion Adsorption from Heterodyne-Detected Second Harmonic Generation

“…Compounding the complexity of the problem is the observation from electrokinetic measurements that the interfacial charge density of certain oxides varies with ionic strength, at least for the ionic strengths accessible in those experiments. [42][43][44]117 Despite these challenges, recent years have seen some significant progress: For instance, ion identity (element and charge), electrolyte concentration, and bulk solution pH are some of the factors that are now known to control surface charge density and the structure of interfacial water molecules and the hydrogen-bond networks they establish within the electrical double layer. [12][13][14][15][16][17] Moreover, potentiometric measurements and calculations with surface complexation models have established that cations and anions influence charge densities at silica/water interfaces via their adsorption affinities.…”