Adaptability of monoglyceride-induced crystallization of K2SO4: effect of various anions and lipid chain splay.

Understanding the interaction of a surfactant assembly with surrounding ionic compositions is critical in controlling the nanostructure formed by amphiphilic systems. In this study, we investigated the influence of specific anions upon the character of the crystallization process of a model inorganic salt (K2SO4) in an aqueous microdroplet (10-100 μm) in the presence of monolayers of monoelaidin (ME), monovaccenin (MV), monoolein (MO), and monolinolein (ML) assembled at a water-decanol interface. In particular, control monolayers of each monoglyceride at the water-oil interface did not accelerate nucleation (i.e., act as a template) of the crystallization of K2SO4. However, in the presence of 1-10 mM exogenous anions, certain types of monoglycerides did induce nucleation. The influence of the anions was consistent with the Hofmeister series. Monoglyceride monolayers in combination with chaotropic anions, such as SCN(-), imparted a euhedral crystal habit with a relatively lower C(onset), while kosmotropic anions showed less notable changes in the crystallization characteristics. However, this specific anion effect was seen only for monolayers of ME, MV, and MO but not for ML. This enhancement of nucleation-inducing ability upon addition of chaotropic anions appears to be related to the degree of chain splay: the larger the chain splay, the less anion efficiency in enhancing the templating capability. Our findings indicate that the improved nucleation-enhancing capabilities may originate from mutual interactions between anions and the monolayer, allowing the monoglyceride monolayers to adapt to a modulated self-assembled structure resulting in the greatest degree of control over K2SO4 crystallization.