Oppositely Charged Ions at Water-Air and Water-Oil Interfaces: Contrasting the Molecular Picture with Thermodynamics

The surface-active ions tetraphenylarsonium (Ph₄As⁺) and tetraphenylboron (Ph₄B^-) have a similar structure but opposite charge. At the solution-air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph₄As⁺ compared with Ph₄B^-, in addition to opposite alignment of interfacial water molecules. At the water-oil interface, the interfacial tension is 7 mN/m lower for Ph₄As⁺ than for Ph₄B^- salts, but this can be fully accounted for by the different bulk solubility of these ions in the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measurements.