Escin belongs to a large class of natural biosurfactants, called saponins, that are present in more than 500 plant species. Saponins are applied in the pharmaceutical, cosmetics, and food and beverage industries due to their variously expressed bioactivity and surface activity. In particular, escin adsorption layers at the air-water interface exhibit an unusually high surface elastic modulus (>1100 mN/m) and a high surface viscosity (ca. 130 N·s/m). The molecular origin of these unusual surface rheological properties is still unclear. We performed classical atomistic dynamics simulations of adsorbed neutral and ionized escin molecules to clarify their orientation and interactions on the water surface. The orientation and position of the escin molecules with respect to the interface, the intermolecular interactions, and the kinetics of molecular aggregation into surface clusters are characterized in detail. Significant differences in the behavior of the neutral and the charged escin molecules are observed. The neutral escin rapidly assembles in a compact and stable surface cluster. This process is explained by the action of long-range attraction between the hydrophobic aglycones, combined with intermediate dipole-dipole attraction and short-range hydrogen bonds between the sugar residues in escin molecules. The same interactions are expected to control the viscoelastic properties of escin adsorption layers.

We study the effects of (1) drop size; (2) surfactant chain-length and concentration; and (3) viscosity of the oil phase on the stability of water drops, pressed by gravity towards planar oilwater interface. The experimental results show that at low surfactant concentrations (around and below the CMC) the drop lifetime is controlled by the drainage time of the oily film, viz. by the time for reaching the critical film thickness at which this film ruptures. The small drops coalesce before the formation of a planar film with the large interface and, as a consequence, their lifetime rapidly decreases with the increase of drop diameter. In contrast, the bigger drops coalesce after formation of a plane-parallel film and their lifetime increases with the drop diameter. Therefore, the drop lifetime passes through a minimum when varying the drop diameter. The obtained results at low surfactant concentrations and for small drops are described well by theoretical models, available in the literature. The lifetime of the respective larger drops is shorter, as compared to the theoretical predictions, due to the faster thinning of the oil films which are with uneven thickness. The increase of surfactant concentration above the CMC leads to a significant increase in the stability of the small drops. Interestingly, the stability of the intermediate in size drops remains low for all surfactants and oils studied. At higher surfactant concentrations, the stability of the large water drops increases significantly when Span 80 is used, while it remains rather low for drops stabilized by Span 20. The results obtained with single water drops are in a good agreement with the stability of the respective batch water-in-oil emulsions.

HypothesisMicrometer sized alkane-in-water emulsion drops, stabilized by appropriate long-chain surfactants, spontaneously break symmetry upon cooling and transform consecutively into series of regular shapes (Denkov et al., Nature 2015, 528, 392). Two mechanisms were proposed to explain this phenomenon of drop "self-shaping". One of these mechanisms assumes that thin layers of plastic rotator phase form at the drop surface around the freezing temperature of the oil.This mechanism has been supported by several indirect experimental findings but direct structural characterization has not been reported so far. ExperimentsWe combine small-and wide-angle X-ray scattering (SAXS/WAXS) with optical microscopy and DSC measurements of self-shaping drops in emulsions. FindingsIn the emulsions exhibiting drop self-shaping, the scattering spectra reveal the formation of intermediate, metastable rotator phases in the alkane drops before their crystallization. In addition, shells of rotator phase were observed to form in hexadecane drops, stabilized by C16EO10 surfactant. This rotator phase melts at ca. 16.6°C which is significantly lower than the melting temperature of crystalline hexadecane, 18°C. The scattering results are in a very good agreement with the complementary optical observations and DSC measurements.

HypothesisSaponins are natural surfactants which can provide highly viscoelastic interfaces. This property can be used to quantify precisely the effect of interfacial dilatational elasticity on the various rheological properties of bulk emulsions. ExperimentsWe measured the interfacial dilatational elasticity of adsorption layers from four saponins (Quillaja, Escin, Berry, Tea) adsorbed on hexadecane-water and sunflower oil-water interfaces. In parallel, the rheological properties under steady and oscillatory shear deformations were measured for bulk emulsions, stabilized by the same saponins (oil volume fraction between 75 and 85 %). FindingsQuillaja saponin and Berry saponin formed solid adsorption layers (shells) on the SFO-water interface. As a consequence, the respective emulsions contained non-spherical drops. For the other systems, the interfacial elasticities varied between 2 mN/m and 500 mN/m. We found that this interfacial elasticity has very significant impact on the emulsion shear elasticity, moderate effect on the dynamic yield stress, and no effect on the viscous stress of the respective steadily sheared emulsions. The last conclusion is not trivial, because the dilatational surface elasticity is known to have strong impact on the viscous stress of steadily sheared foams. Mechanistic explanations of all observed effects are described.3 1. Introduction.

Saponins are natural surfactants with high surface activity and unique surface properties. Escin is a triterpenoid saponin which has unusually high surface viscoelasticity [Golemanov et al. Sof t Matter 2013, 9, 5738] and low permittivity to molecular gas diffusion of its adsorption layers. In our previous study [Tsibranska et al. Langmuir 2017, 33, 8330], we investigated the molecular origin of this unconventional behavior and found that escin molecules rapidly assemble in a compact and stable surface cluster. This behavior was explained with long-range attraction between the hydrophobic aglycones combined with intermediate dipole−dipole attraction and strong short-range hydrogen bonds between the sugar residues in the adsorbed escin molecules. In this study, we performed atomistic molecular simulations of escin molecules in dense adsorption layers with two different areas per molecule. The results show that the surfactant molecules in these systems are much less submerged in water and adopt a more upright position compared to the dilute layers studied previously. A significant number of trapped water molecules are located around the hydrophilic groups placed above the water equimolecular surface to solvate them in the dense layer. To maintain the preferred orientation of the escin molecules with respect to the interface, the most compact adsorption layer acquires a significant spontaneous curvature. The substantial elasticity of the neutral escin layers, as in our previous study, is explained with the presence of a specific interaction, which is intermediate between hydrogen bonding and dipole−dipole attraction (populated lengths in the range 0.16 to >0.35 nm), supplemented by substantial flexibility of the surfactant heads, optimal curvature of the interface, and significant normal displacement of the molecules to allow their tight surface packing. The simulations reveal long-range order within the layers, which signifies the role of the collective behavior of the saponin molecules in such dense adsorption layers.

We found experimentally that the elasticity of sunflower oil-in-water emulsions (SFO-in-W) stabilized by Yucca Schidigera Roezl saponin extract, is by > 50 times higher as compared to the elasticity of common emulsions. We revealed that strong specific interactions between the phytosterols from the non-purified oil and the saponins from the Yucca extract lead to the formation of nanostructured adsorption layers which are responsible for the very high elasticity of the oil-water interface and of the respective bulk emulsions. Remarkably, this extra high emulsion elasticity inhibits the emulsion syneresis even at 65 vol % of the oil drops -these emulsions remain homogeneous and stable even after 30 days of shelf-storage. These results demonstrate that the combination of saponin and phytosterols is a powerful new approach to structure oil-in-water emulsions with potential applications for formulating healthier functional food.3 1. Introduction.

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