pH effects on the molecular structure and charging state of β-Escin biosurfactants at the air-water interface

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

“…The of pH on the surface tension of escin solutions has already been well described. [26] Surface tension of the escin solutions by varying the concentrations at pH 6.0 is shown in Figure S2, and these data are in good agreement with previous experiments. [19] The differences in orientation angles of the hydrophobic aglycone moieties depending on pH may be related to the charging state of the interface.…”

“…The spectra attributed to the OH bands derived from the water at the interface are in good agreement with the SFG spectra of previously reported. [ 26 ] The SFG spectra of escin aqueous solution surface in OH stretching region are shown in Figure S1. The SFG spectra in Figure 3 are attributed to the CH mode from 2800 to 3050 cm −1 .…”

“…Despite the approximate orientation analysis based on SFG spectra and optimized geometry based on ab initio calculations, the orientation angles of hydrophobic moieties at air-water interface are in qualitatively good agreement with predictions from MD simulations [21,23] and with previously reported SFG results. [26]…”

“…The relationship between interfacial charging behavior and the water structure on the escin aqueous solution surfaces has been clarified recently by Glikman et al using sum frequency generation (SFG) measurements. [ 26 ] They reported that the escin aqueous interfaces at pH > 4.7 led to form the electric double layer and the changes in the intensities of the OH stretching bands of the SFG was a function of bulk pH. [ 26 ] They also provide evidence for the pronounced behavior of surface tension and surface rheology are changed by the pH of escin aqueous solutions.…”

“…[ 26 ] They reported that the escin aqueous interfaces at pH > 4.7 led to form the electric double layer and the changes in the intensities of the OH stretching bands of the SFG was a function of bulk pH. [ 26 ] They also provide evidence for the pronounced behavior of surface tension and surface rheology are changed by the pH of escin aqueous solutions. In addition to the charging of the surface properties, the interfacial molecular orientation of the escin may also be a function of solution pH, as predicted by changes in the CH bands.…”