Revealing structural properties of the marine nanolayer from vibrational sum frequency generation spectra

Abstract

Natural nanolayers originating from sea surface and subsurface water samples collected in the Baltic Sea have been investigated using surface‐sensitive vibrational sum frequency generation (VSFG) spectroscopy. Distinct spectral signatures of CH and OH bond stretch vibrations have been detected at wavenumbers ranging from 2700 to 3900 cm−1. Measured water‐air interface spectra as well as observed signal intensity trends are discussed in terms of composition and structure of the natural organic nanolayer. Reasoning was based on the comparison with reference spectra, spectral trends inferred from previous VSFG studies, reported average composition of dissolved organic matter in seawater, and simplified assumption that surfactants can be classified as soluble (wet) and insoluble (dry) surfactants. Wet surfactants have been found to be dominant, and often lipid‐like compounds form a very dense surfactant nanolayer. Supported by comparison spectra of xanthan gum solutions, the observed VSFG spectral signatures were tentatively assigned to lipopolysaccharides or other lipid‐like compounds embedded in colloidal matrices of polymeric material. In addition, VSFG spectra of a polluted harbor water sample and a water sample covered with diesel oil are reported.

Seasonal signatures in SFG vibrational spectra of the sea surface nanolayer at Boknis Eck Time Series Station (SW Baltic Sea)

“…For spectroscopy, the samples were poured into Teflon dishes measuring 14 cm in diameter and given a rest for at least 20 min prior to measurement to allow for equilibration between interface and aqueous phase. Details of the sampling procedure and consequences for the interpretation of the measured SFG signal intensity are outlined in our previous publications Laß and Friedrichs, 2011). Briefly, taking into account the sampling efficiency as well as the ratio of the areas of the field sampler and the spectrometer dish, our specific procedure yielded samples with a theoretical surface concentration of dry surfactants 12.5 times as high as on the original sea surface.…”

“…This study aims to show and interpret seasonal changes in amount and composition of the sea surface nanolayer at a coastal site in the southwestern Baltic Sea. Samples were analyzed with non-linear sum frequency generation spectroscopy, which has been quite recently established as a surface-specific tool for environmental ocean-air interfaces Laß and Friedrichs, 2011). Collected data serve as a step towards a more complete understanding of the state of the sea surface nanolayer in the course of the year, which is an important keystone for an improved parametrization of its role in the context of climate models.…”

Organic Matter in the Surface Microlayer: Insights From a Wind Wave Channel Experiment

“…Duplicate SML samples of about 50∼mL were collected on 8 days, transferred into polypropylene bottles and immediately frozen at −40 • C for transport and stored at −79 • C prior to analysis by vibrational sum-frequency generation (VSFG) spectroscopy using a commercial picosecond VSFG spectrometer (EKSPLA, 532 nm up-conversion) as described elsewhere (Laß and Friedrichs, 2011). The use of VSFG spectroscopy for environmental surfactant analysis has been introduced by Laß et al (2010) and the interpretation of the measured spectra in terms of surfactant abundance has been outlined and discussed in some detail in Laß and Friedrichs (2011) and Laß et al (2013). Non-linear VSFG provides surface-specific infrared spectra of the molecular nanolayer, i.e., the signal arises from the molecules residing directly at the interface.…”

Night-Time Oxidation of a Monolayer Model for the Air–Water Interface of Marine Aerosols—A Study by Simultaneous Neutron Reflectometry and in Situ Infra-Red Reflection Absorption Spectroscopy (IRRAS)

“…PA is commonly used as a proxy for marine aerosols [41]. Lipopolysaccharides have been discovered both in the sea surface microlayer [42] and in atmospheric aerosol [43]. Facchini et al [43] found that sea water microgels, mainly composed of lipopolysaccharides, are preferentially transferred to the submicron aerosol size fraction during bubble bursting.…”