Investigating Bénard-Marangoni migration at the air-water interface in the time domain using sum frequency generation (SFG) spectroscopy of palmitic acid monolayers.
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Authors
Fellows, AP
Casford, MTL
Davies, PB
Publication Date
2022-04-28Journal Title
J Chem Phys
ISSN
0021-9606
Publisher
AIP Publishing
Type
Article
This Version
AM
Metadata
Show full item recordCitation
Fellows, A., Casford, M., & Davies, P. (2022). Investigating Bénard-Marangoni migration at the air-water interface in the time domain using sum frequency generation (SFG) spectroscopy of palmitic acid monolayers.. J Chem Phys https://doi.org/10.1063/5.0090532
Abstract
Sum-frequency generation (SFG) spectroscopy is frequently used to investigate the structure of monolayer films of long-chain fatty acids at the air-water interface. Although labeled a non-invasive technique, introducing intense SFG lasers onto liquid interfaces has the potential to perturb them. In the present work, narrowband picosecond SFG is used to study the structural changes that occur in palmitic acid and per-deuterated palmitic acid monolayers at the air-water interface in response to the high field strengths inherent to SFG spectroscopy. In order to determine structural changes and identify measurement artifacts, the changes in specific resonance intensities were measured in real-time and over a broad range of surface concentrations from films spread onto a stationary Langmuir trough. Using narrowband instead of broadband SFG minimizes the overlap of the incident infrared beam in the lipid C-H stretching region with resonances from the water sub-phase. Nevertheless, narrowband SFG still generates a thermal gradient at the surface, which produces a significant decrease in local concentration in the area of the laser spot caused by Bérnard-Marangoni convection originating in the sub-phase. The decrease in concentration results in an increase in the conformational disorder and a decrease in the tilt angle of lipid tails. Crucially, it is shown that, even at the highest monolayer concentrations, this gives rise to a measurement effect, which manifests itself as a dependence on the spectral acquisition time. This effect should be taken into account when interpreting the structure of monolayer films on liquid surfaces deduced from their SFG spectra.
Keywords
Sum-Frequency Generation, Benard-Marangoni Convection, Laser-Induced Heating, Palmitic Acid, Langmuir Monolayer
Sponsorship
Unilever
Funder references
EPSRC (2110577)
Engineering and Physical Sciences Research Council (EP/R511870/1)
Identifiers
External DOI: https://doi.org/10.1063/5.0090532
This record's URL: https://www.repository.cam.ac.uk/handle/1810/335849
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