The Diverse Nature of Ion Speciation at the Nanoscale Hydrophobic/Water Interface

Many biological systems are composed of nanoscale structures having hydrophobic and hydrophilic groups adjacent to one another and in contact with aqueous electrolyte solution. The interaction of ions with such structures is of fundamental importance. Although many studies have focused on characterizing planar extended (often air/water) interfaces, little is known about ion speciation at complex nanoscale biological systems. To start understanding the complex mechanisms involved, we use a hexadecane nanodroplet system, stabilized with a dilute monolayer of positively charged dodecyltrimethylammonium cations (DTA(+)) groups in contact with an electrolyte solution (NaSCN). Using vibrational sum frequency scattering, second harmonic scattering, zeta-potential measurements, and quantum density functional theory, we find DTA(+)-SCN- ion pairing at concentrations as low as 5 mM. A variety of ion species emerge at different ionic strengths, with differently oriented SCN- groups adsorbed on hydrophilic or hydrophobic parts of the surface. This diverse and heterogeneous chemical environment is surprisingly different from the behavior at extended liquid planar interfaces, where ion pairing is typically detected at molar concentrations and nanoscale system stability is no requirement.