In our previous study (Mustan et al. 2021) we showed that foams formed from two oil-soluble nonionic surfactants (Span 60 and Brij 72) can remain stable for more than 10 days at room temperature at high sugar concentration. The major aim of the current study is to reveal the interrelation between the surfactant structure and foam stability by investigating 6 polyoxyethelene alkyl ethers and 12 fatty acid esters with a wide variety of hydrophobic chain lengths (C12; C16; C18 and C18:1) and hydrophilic head-groups (sorbitol, glycerol, sucrose). Foams stable for more than 100 days at room temperature are obtained when sucrose palmitate or stearate (P1670 or S1670) are used as surfactants. This exceptional foam stability is related to the gelation of the aqueous phase and to the formation of solid adsorption layer with zero surface tension upon compression, thus preventing water drainage and decelerating the bubble Ostwald ripening. The foam stability decreases with (i) increasing the number of EO groups in polyoxyethylene alkyl ethers and in fatty acid sorbitan esters; (ii) decreasing the number of C-atoms in the surfactant tail for all studied surfactants; (iii) addition of double bond in the surfactant tail. The lower foam stability in all three cases is related to the worse packing of the surfactant molecules within the adsorption layer, leading to faster Ostwald ripening and subsequent bubble coalescence. The diesters present as admixture in the fatty acid esters play an important role in the foam stabilization by further compacting the adsorption layers and lowering the rate of Ostwald ripening. These conclusions can be used as a predictive tool for surfactant selection in the development of food or pharmaceutical foam concentrates that can be diluted before final use.

The surface and foam properties of two oil-soluble surfactants, Span 60 and Brij 72, and two water-soluble surfactants, Tween 60 and Brij S20, are compared. Aqueous surfactant solutions containing sugar at high concentration (63 wt%) are also studied in the context of sweet food-related foams. The experimental results show that foams with very small bubbles of ca. 5 µm are formed and remain stable for more than 10 days at room temperature when oil-soluble surfactants are dispersed in the sugar-rich solutions. The excellent stability of these foams is related to (1) The multilamellar vesicles present in the respective surfactant dispersions which increase the viscosity and decrease the rate of water drainage from the foam; (2) The very slow exchange of surfactant molecules between the interface and the adjacent aqueous solution that leads to very significant difference in the surface tension of shrinking and expanding bubbles, which in turn decreases the rate of bubble Ostwald ripening. The foams formed from Span 60 are more stable as compared to the foams formed from Brij 72, due to the higher viscosity and the slower interface-solution exchange. The foams formed from Brij S20 solutions are least stable – they are completely destroyed after 1 day at room temperature, even in the presence of sugar. The main conclusion of this study is that oil soluble surfactants can form condense adsorption layers on the bubble surfaces after adsorption from aqueous solutions with and without sugar in it. The formed layers, which present a better molecular packing, reduce the gas permeability and decelerate the Ostwald ripening disproportionation of the foam This is obtained by a relatively low interfacial tension imparted by continuous shrinkage and expansion of bubbles during foam generation, ultimately ensuring long-term stabilization of formed foams.

The studied anionic surfactants linear alkyl benzene sulfonate (LAS) and sodium lauryl ether sulfate (SLES) are widely used key ingredients in many home and personal care products. These two surfactants are known to react very differently with multivalent counterions, including Ca2+. This is explained by a stronger interaction of the calcium cation with the LAS molecules, compared to SLES. The molecular origin of this difference in the interactions remains unclear. In the current study, we conduct classical atomistic molecular dynamics simulations to compare the ion interactions with the adsorption layers of these two surfactants, formed at the vacuum−water interface. Trajectories of 150 ns are generated to characterize the adsorption layer structure and the binding of Na+ and Ca2+ ions. We found that both surfactants behave similarly in the presence of Na+ ions. However, when Ca2+ is added, Na+ ions are completely displaced from the surface with adsorbed LAS molecules, while this displacement occurs only partially for SLES. The simulations show that the preference of Ca2+ to the LAS molecules is due to a strong specific attraction with the sulfonate head-group, besides the electrostatic one. This specific attraction involves significant reduction of the hydration shells of the interacting calcium cation and sulfonate group, which couple directly and form surface clusters of LAS molecules, coordinated around the adsorbed Ca2+ ions. In contrast, SLES molecules do not exhibit such specific interaction because the hydration shell around the sulfate anion is more stable, due to the extra oxygen atom in the sulfate group, thus precluding substantial dehydration and direct coupling with any of the cations studied.

This study aims to clarify and explain the similarities and differences in the behavior of adsorption layers of native egg yolk (EY) and enzymatically modified egg yolk (MEY) at a soybean oil-water interface. For this purpose, the interfacial tension and the surface dilatational modulus of EY and MEY solutions are measured and compared. The interactions between two adsorption layers, formed from these solutions on an oil-water or air-water interface, are also studied by optical observations of thin foam and emulsion films, formed in a capillary cell. The chemical composition, the electrophoretic mobility of the molecular aggregates, and the rheological properties of the egg yolk solutions are also characterized. Adsorption layers formed from MEY solutions display a faster rate of adsorption, lower dilatational surface moduli and higher equilibrium surface tension. The enzymatic modification of egg yolk also leads to formation of much thinner foam and emulsion films and to faster film thinning. The observed differences between EY and MEY are explained by assuming that the interfacial properties of MEY are governed mostly by the lysophospholipids and oleic acid, which appear as reaction products of the enzymatic modification of EY. The latter assumption is unambiguously proven by chemical analysis of the MEY solutions and by deliberate addition of lysophospholipids and oleic acid to the non-modified EY solutions. Even at relatively low concentrations, the lysophospholipids and oleic acid change the interfacial and film properties of the EY solutions, making them very similar to those of the enzymatically modified egg yolk.

Bubble Ostwald ripening (OR) leads to a gradual increase of the mean bubble size in foams with time. The rate of OR can be reduced significantly or even arrested completely using appropriate solid particles and/or surfactants as foam stabilizers. In the current paper, we show that saponins, a widespread class of natural surfactants, can reduce significantly the rate of OR in foams. To reveal the reasons for the reduced rate of OR in saponin-stabilized foams, we performed measurements of the rate of bubble diminishing, for single air bubbles placed below a solution surface, with a series of saponin bio-surfactants. These saponin surfactants form adsorption layers with surface elasticity, spanning a very wide range – from almost zero up to several thousand mN/m. The measured rate of bubble OR showed no correlation with the surface elastic modulus (dilatational or shear), as measured at 0.1 Hz frequency of surface oscillations. A reasonable correlation was observed only with the surface stress (deviation from the equilibrium surface tension), measured at very slow rate of surface deformation, which mimics much better the actual processes of bubble OR in foams – higher surface stress corresponds to lower OR rate. New theoretical expression, accounting for the out-of- equilibrium surface tension during bubble shrinkage and for the gas flux across the meniscus regions surrounding the foam films, was derived and used to calculate theoretically the rate of bubble diminishing. The comparison of the theoretical predictions with the experimental data shows clearly that the main reason for the reduced rate of OR in the studied systems is the high resistance to gas transfer of the saponin adsorption layers. The deviations from the equilibrium surface tension, although noticeable, have smaller effect. The complementary experiments with actual foams showed that the rate of OR is even lower (compared to the rate measured with single bubbles) which is explained with the thicker non-equilibrium foam films, formed between the neighboring bubbles in saponin-stabilized foams.