Commercial nonionic sucrose ester surfactants (SEs) contain varying ratios of monoesters and polyesters. In our previous study, we found that when the mono-to-polyester (MP) ratio is close to 1, SE dispersions can form hydrogels at very low surfactant and electrolyte concentrations. In the current study, we systematically investigate how the MP ratio and particle size affect the salt-induced gelation process. We reveal that SE dispersions form hydrogels only within a certain MP range. At low MP ratios (<0.4), micrometer-sized particles form and sediment, preventing gel formation, whereas at high MP ratios (≳3), particle concentration decreases, confining gelation to a very narrow electrolyte region. Sonication significantly reduces particle size across all dispersions, expanding the gelation domain down to MP ≈ 0.3. The critical salt concentration required for hydrogel formation exhibits a maximum as a function of MP ratio at MP ≈ 0.43, which is explained with the highest measured zeta potential for these particles. The obtained results support the hypothesis that increasing the monoester content enhances surface mobility, suppresses hydroxide ion adsorption, and reduces particle charge, thereby lowering the threshold electrolyte concentration. Finally, particle size reduction enhances overall gel strength and lowers the minimum surfactant concentration required for gelation. These findings enable the rational design of simple, biodegradable hydrogels with tunable properties for future applications in food, pharmaceutical, and cosmetic formulations.

Escin is a triterpenoid saponin with one carboxyl group which is non-ionized at pH 4.7. The major aim of the current study is to determine how the electrolyte concentration affects the properties of concentrated escin solutions (5 wt% and 10 wt%) at pHs of 4, 6, and 8. Ionized escin molecules at pH >4.7 form charged micelles that repel one another when there is no added electrolyte and solutions remain clear and stable for more than a month. Lowering the pH to 4 leads to formation of uncharged micelles. These micelles attract each other and form inter-micellar hydrogen bonds, which enable formation of micrometer aggregates that cause turbidity and phase separation. The addition of background electrolytes to the solutions at pHs of 6 and 8 screens the electrostatic repulsion between micelles, causing partial aggregation of the micelles and gelation of solutions. As the salt concentration increases, the viscosity of the escin solution also increases, reaching a maximum—similar to the behavior observed with conventional surfactants. However, the mechanism behind this viscosity maximum is different. In solutions of conventional surfactants, the maximum is due to the formation of worm-like micelles, whereas the maximum for escin solutions is due to formation of a network of escin aggregates that imparts yield stress and elasticity to the solution. These dispersions remain stable for at least one month at room temperature and can be used as cosmetic and detergent formulations.