From Salt-in-Water to Water-in-Salt: How Ion Identity Governs Surfactant Self-Assembly in Salt–Water–Nonionic Surfactant Mixtures

Self-assembled mesostructures derived from nonionic surfactants can be tuned by adjusting parameters, such as the hydrophilic-lipophilic balance or electrolyte content. Such tuning permits control of key functional properties including conductivity, mass transport, and rheology. Although salts have frequently been used to influence the phase behavior of nonionic surfactant mixtures, they are almost always treated as minor additives. In contrast, here we extended the salt concentration range to hydrous melt and even nearly anhydrous conditions in which the salt no longer acts as an additive but replaces water as the solvent component. To elucidate the effect of ion identity on phase behavior, partial pseudoternary phase diagrams of salt-water-C₁₂E₁₀ mixtures were constructed using NaNO₃, Zn(NO₃)₂, Ca(NO₃)₂ and CaCl₂. Each salt represents a distinct set of intermolecular interactions and gives rise to markedly different phase diagrams, demonstrating that salt selection, or using a mixture of salts, can be strategically employed to finely tune self-assembly across the full hydration window.