Influence of Ag⁺ Interaction on 1D Droplet Array Spacing and the Repulsive Forces between Stimuli-Responsive Nanoemulsion Droplets

This paper reports results on the effect of interaction of Ag⁺ on 1D droplet array spacing and the repulsive forces between stimuli-responsive nanoemulsion droplets, stabilized with an anionic surfactantsodium dodecyl sulfateand a diblock polymerpoly­(vinyl alcohol)–vinyl acetate. The repulsive interaction is probed by measuring the in-situ equilibrium force–distance in the presence of Ag⁺ using the magnetic chaining technique. At a constant static magnetic field, emulsion droplets form 1D array that diffract visible light. A large blue-shift in the diffracted light is observed in the presence of interacting Ag⁺ because of the reduction in the interdroplet spacing within the 1D array. The in-situ equilibrium force–distance measurement results show that the onset of repulsions and magnitude of repulsive forces are strongly influenced by the presence of Ag⁺ in ppb levels. This suggests that the Ag⁺ ions screen the surface charges through the formation of both Stern and diffuse electric double layer and produces a dramatic blue-shift in surfactant-stabilized emulsion, whereas a dramatic conformational change in the adsorbed polymer layer causes a reduction in the 1D array spacing in the diblock polymer stabilized emulsion. The force–distance results are compared with the predictions of electrical double-layer and repulsive steric forces. The droplet array shows an excellent selectivity to Ag⁺ due to the strong interaction of Ag⁺ with the stabilizing moieties at the oil–water interface. The possible mechanisms of interaction of Ag⁺ with surfactant and polymer are discussed. The dramatic decrease in the 1D array spacing in the presence of Ag⁺ may find promising practical applications in the development of optical sensors for selective detection of cations with ultrahigh sensitivity.