Surface Properties of Saponin-Chitosan Mixtures

Abstract

The surface properties of saponin and saponin-chitosan mixtures were analysed as a function of their bulk mixing ratio using vibrational sum-frequency generation (SFG), surface tensiometry and dilational rheology measurements. Our experiments show that saponin-chitosan mixtures present some remarkable properties, such as a strong amphiphilicity of the saponin and high dilational viscoelasticity. We believe this points to the presence of chitosan in the adsorption layer, despite its complete lack of surface activity. We explain this phenomenon by electrostatic interactions between the saponin as an anionic surfactant and chitosan as a polycation, leading to surface-active saponin-chitosan complexes and aggregates. Analysing the SFG intensity of the O-H stretching bands from interfacial water molecules, we found that in the case of pH 3.4 for a mixture consisting of 0.1 g/L saponin and 0.001 g/L chitosan, the adsorption layer was electrically neutral. This conclusion from SFG spectra is corroborated by results from surface tensiometry showing a significant reduction in surface tension and effects on the dilational surface elasticity strictly at saponin/chitosan ratios, where SFG spectra indicate zero net charge at the air-water interface.

Keywords: bio-compatible systems; biodegradable surfactants; chitosan; saponin; saponin—chitosan interactions.

Figure 1

Vibrational SFG spectra of air–water interfaces modified by saponin–chitosan mixtures as a function of the chitosan concentration, which was as indicated on the right axes of the panel (b). Note that the saponin concentration in the aqueous solution was fixed at 0.1 g/L, while the chitosan concentration was varied. The solution contained 0.1 wt% acetic acid in order to adjust the pH value to 3.4 for all chitosan concentrations. Spectra were offset for better visual inspection. Panel (a) shows a magnified view of the SFG spectra that concentrate on the details of C-H modes from interfacial molecules. Bands assignments to CH₂ (d⁺) and CH₃ (r⁺), the Fermi resonance r⁺_FR, the CH₃ asymmetric stretch (r⁻) and a C-H stretch (C-H) are labelled in (a) and also discussed in the main text. Panel (b) overviews spectra that show both C-H and O-H modes from interfacial molecules, such as the broad O-H stretching modes which are indicated. The dotted line highlights the red shift of the OH frequency as the concentration of chitosan increases. (See details in the main text).

Figure 2

Integrated SFG intensities of O-H stretching bands (3055–3355 cm⁻¹) related to the hydrogen-bonded interfacial water molecules (black squares), which is also dependent on the net charging state of the interface, as discussed in the main text. Integrated SFG intensities (3393–3695 cm⁻¹) from O-H groups at the oligosaccharide glycone of the saponin (red circles) as well as from water molecules close to the hydrophobic parts of the chitosan/saponin mixtures at the interface are also shown as a function of the chitosan concentration. Red and black dashed lines show the SFG intensities of both contributions to the SFG spectra (bands at 3200 and ~3550 cm⁻¹). Solid lines guide the eye.

Figure 3

Zeta-potential for saponin or saponin–chitosan aggregates in mixtures. Fixed saponin concentration of 0.1 g/L in the presence of 1 wt% acetic acid. Chitosan concentrations varied in a range from 0.0005 to 0.5 g/L. Solid and dashed lines guide the eye (solid line show the not filtered aggregates, dashed—filtered).

Figure 4

The dynamic surface tension of saponin solutions and saponin–chitosan mixtures in the presence of 0.1 wt% acetic acid and a solution pH of 3.4. The Saponin concentration was fixed at 0.1 g/L, while the chitosan concentration was varied from 0.0005 to 0.3 g/L as indicated in the figure.

Figure 5

Equilibrium surface tensions of saponin solutions and saponin–chitosan mixtures in the presence of 0.1 wt% acetic acid for pH 3.4 that were measured after 3 h (>10,800 s) of adsorption. The saponin concentration was fixed to 0.1 g/L while we varied the chitosan from 0.0005 to 0.3 g/L.

Figure 6

Dilational viscoelasticity versus frequency measured by the oscillating drop method in the presence of 0.1 wt% acetic acid and a solution pH of 3.4. The saponin concentration was fixed to 0.1 g/L, while the chitosan concentration was varied from 0.0005 to 0.3 g/L as indicated in the figure.

Figure 7

Surface dilational elasticity results as a function of chitosan concentration in aqueous mixtures with saponin 0.1 g/L and acetic acid 0.1 wt% (pH 3.4). Plots for five various oscillation frequencies, which were 0.005, 0.008, 0.02, 0.05 and 0.95 Hz, as indicated in the figure, are shown.