Interface structure of escin at air–water interface probed by sum frequency generation spectroscopy

Abstract

Saponins are natural amphiphilic surfactants with high surface activity and unique surface properties at the air-water interfaces. We have applied sum frequency generation (SFG) vibrational spectroscopy to study escin, a kind of saponins that is extracted from chestnut (Aesculus hippocastanum), which adsorbed at water surfaces as a function of the solution concentrations, and have characterized the net orientation of the hydrophobic aglycone moieties at the water interfaces by varying the solution concentration of the escin. SFG results of escin adsorbed water surfaces demonstrate that the hydrophobic aglycone moieties show nearly standing-up configuration above the critical micelle concentration (cmc), while its preferred orientation is nearly parallel to the surface below cmc. In addition to the changes in the CH stretching band intensities from the escin adsorbed interfaces, we found that the structural arrangement of the escin molecules at the air-water interface is also a function of solution pH and clarified the correlation between the ionic state of the solution and the structure.

Surface Properties of Saponin—Chitosan Mixtures

“…Figure 1 a shows a detailed view of the C-H region, where the band assignments are also indicated in the figure, and the symmetric stretching modes of methylene (d + ) and methyl (r + ) groups, the fermi resonance of the methyl (r + FR ) and the methyl asymmetric stretch (r − ) give rise to vibrational bands at frequencies of ~2855, ~2890, ~2927 and ~2952 cm −1 , respectively [ 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. Chitosan shows an additional C-H band at ~2994 cm −1 , which is observed in the SFG spectra of the pristine air–saponin interface [ 22 , 30 , 31 , 32 , 33 , 34 , 35 ] (see also our results, Figure S2a ). Acetic acid shows no SFG spectra in studied concentration 0.1 g/L [ 36 ] (see also Figure S3 ).…”

“…We have now incorporated complementary information from in situ nonlinear optical spectroscopy–vibrational sum-frequency generation spectroscopy (SFG), which allows direct identification and analysis of compounds adsorbed in the interfacial layer [ 21 , 22 ]. This analysis targets specific functional groups such as -methylene, -methyl, -hydroxy groups and their molecular structure at the interface, while the interfacial charge can be qualitatively addressed by SFG spectroscopy using O-H stretching modes from interfacial water molecules.…”

Editorial on the special issue of JRS on vibrational spectroscopy of water

“…Azam and Hore [16] reviewed novel two-dimensional Raman-SFG correlation spectroscopy that allows for reliable interpretation of surface-selective vibrational spectra. Kagiyama and Miyamae [17] reported on a natural amphiphilic surfactant at the air/water interface using SFG spectroscopy with particular emphasis on molecular orientation. DOI: 10.1002/jrs.6451 Sarkar et al [18] reviewed their SFG spectroscopic studies on biomolecules adsorbed at lipid/water interfaces.…”