Thiocyanate Ions Form Antiparallel Populations at the Concentrated Electrolyte / Charged Surfactant Interface

Anions, such as nitrate and thiocyanate, play significant roles in the separation of lanthanides and actinides. The molecular-scale details of how these anions behave at aqueous interfaces are not well understood. This is especially true at high ionic strengths, where classical mean field theories fail. Since most separation processes are conducted at high ionic strength conditions, a thorough experimental and theoretical understanding of these systems are necessary. Here, we describe the interfacial structure of thiocyanate anions at a soft charged interface up to 5 M bulk concentration with combined phase-sensitive vibrational sum frequency generation (PS-VSFG) spectroscopy and molecular dynamics (MD) simulations. At low concentrations thiocyanate ions are mostly oriented with their sulfur end pointing towards the charged surfactants. At high concentrations a new thiocyanate population emerges with their sulfur end pointing towards the bulk liquid. The emergence of the new population happens after an apparent saturation of the interface and a plateau in concentration dependent VSFG signal. The -CN stretch frequency is different for up and down oriented SCN- ions, which indicates that they are in, on average, different coordination environments. MD simulations confirm the emergence of oppositely oriented SCN- ions. Together, these results demonstrate the formation of a completely new interfacial SCN- profile with significantly different orientational ordering above 1 M bulk concentration, providing the key molecular-level insights for the interfacial behavior of complex anions in highly concentrated solutions.