Effect of Specific Anion on Templated Crystal Nucleation at the Liquid Liquid Interface

In this work, we have investigated the effect of potassium salts of different anions upon the crystal nucleation of K2SO4 as interfacially templated by a surfactant monolayer of I-octadecylamine (ODA), in an aqueous microdroplet system bounded by a liquid-liquid interface with I-decanol. The salts used were K2HPO4, KCl, K Br, KI, KNO3, and KSCN, present at an initial concentration of 10 mM within an aqueous microdroplet containing K2SO4 at an initial concentration of 287 mM. Supersaturation and subsequent crystallization were isothermally induced by droplet dissolution into the dehydrating decanol phase. The K2SO4 solute crystallization behavior was studied by measurement of the calculated concentration of solute in the microdroplet at the onset of crystallization, i.e., at the first perceptible microscopic appearance of a solid phase, and by crystal habit. Certain salts, e.g., K2HPO4, had almost no influence on the templating ability of ODA, while the ability of ODA to template nucleation and direct the formation of regular crystal habit of K2SO4 became appreciably disrupted in the presence of more chaotropic anions, such as SCN- or NO3-. The propensity for anions to disrupt crystal templating was clearly seen to follow a Hofmeister trend. For crystallization events induced in the absence of ODA, however, these added salts had no influence on the outcome of the events. Microdroplets bounded by an ODA monolayer were also found to undergo droplet shrinkage into the surrounding dehydrating phase at a rate which generally depended upon the nature of the anion in the droplet, with chaotropic anions having an apparent effect of promoting shrinkage. Our findings suggest that the packing or ordering of an ODA monolayer at a liquid-liquid interface is strongly influenced by an interaction between anions in the aqueous phase and the surfactant monolayer at the liquid-liquid interface, which is manifested in its effect upon the crystal templating behavior. These intriguing results can have important implications for the understanding of biomineralization processes which occur in heterogeneous environments.