Specific Ion Effects at the Interface of Nanometer-Sized Droplets in Water: Structure and Stability

The interaction of ions with interfaces influences numerous processes in chemistry, physics, and biology, while different ion species behave differently at hydrophobic or hydrophilic interfaces. Despite its importance, ion specificity and its relation to stability of such interfaces are not understood on the molecular interfacial level, in particular when the interfacial dimension approaches the submicron length scale. To study the molecular mechanism involved in interfacial nanoscale ion specificity, we use a hexadecane nanodroplet system stabilized with a dilute monolayer of positively charged (dodecyltrimethylammonium, DTA+) groups and bring it in contact with aqueous electrolyte solutions containing Na2SO4, NaCl, NaNO3, and NaSCN salts. Using vibrational sum frequency scattering, second harmonic scattering, and ζ-potential measurements, we determine the ionic structure, the interfacial water orientation, and the electrokinetic mobility of nanodroplets for each anion as a function of bulk electrolyte concentration. We observe diverse ion–water interface structuring behavior consisting of ion pairing, the reorientation of anions, and the adsorption to chemically different patches of the interface. Employing an adopted DLVO theory for our experimentally studied emulsion systems, we find that nanoemulsion stability is preserved far beyond the crucial coagulation concentrations estimated by DLVO. This discrepancy is due to the diverse interfacial structuring that occurs on the nanodroplets, which is not included in the averaged and the uniform ion–surface interaction employed in the models.