Molecular Level Interfacial Studies of Nanoemulsions: Surface Structure, Specific Ion Effects and Stability

This thesis is a study on the molecular structure of the interface of nanometer-sized oil droplets dispersed in water, a system known as oil-in-water nanoemulsion. The motivation for this research is the large significance of the interfacial nanostructures both for in industry and biology. Here we apply the nonlinear optical techniques of sum frequency scattering and second harmonic scattering to study the stabilization mechanism of nanoemulsions, as well as specific ion effects at the nanoscale. We begin with the study of the interfacial structure of oil nanodroplets stabilized with a positively charged, a negatively charged and a neutral surfactant, along with the stability of each system. We show that the surface density of charged surfactants on nanodroplets is an order of magnitude lower than on planar interfaces, due to repulsive interactions between like charges on opposing sides on the droplets surface through the oil phase, allowed by the small droplet size. Moreover, we find no experimental correlation between stability and surfactant surface density. Instead droplet stability is found to depend on the relative cooperativity between charge-charge, charge-dipole and hydrogen bonding interactions. Then, we study the effect of the inversion of the oil and the water phases on the droplet stability for nanometer-sized and micrometer-sized droplets. We employ an oil-soluble neutral surfactant, a water-soluble anionic surfactant, and the combination of the two. We find that, while microdroplets and water-in-oil nanodroplets follow the widely accepted empirical rules, and are stabilized only with a surfactant soluble in the continuous phase, nanometer-sized oil-in-water emulsions can be stabilized with an oil-soluble surfactant. Moreover, the structure of the surfactant is different when it approaches the interface from the dispersed or the continuous phase. Next, we study the interaction of four anions (SCN-, NO3-, Cl- and SO4-2) with different molecular structure with the nanointerface of droplets stabilized with a surfactant with a positively charged headgroup (trimethylammonium) for different anionic concentrations. Our results reveal a unique adsorption pattern for each anion that changes with concentration, possibly involving reorientation of the anions and adsorption to different patches of the interface. Interestingly, not only the weakly-hydrated SCN- and NO3 , but also the well-hydrated SO¬4-2 approaches the nanointerface, inducing strong interfacial water ordering. Last, we further study the interaction of SCN- with the positively charged nanointerface employing ab-initio molecular dynamics simulations. We find ion-pairing of SCN- with the interfacial trimethylammonium groups at concentrations as low as 5 millimolar. Moreover, a variety of ion species emerge at different ionic strengths, with differently oriented SCN- groups adsorbed on hydrophilic and/or hydrophobic parts of the surface. This diverse and heterogeneous chemical environment is surprisingly different from the behaviour at extended liquid planar interfaces, where ion pairing is typically detected at molar concentrations.