Contact Ion Pair Formation Is Not Necessarily Stronger than Solvent Shared Ion Pairing.

Vibrational sum frequency spectroscopy (VSFS) and pressure-area Langmuir trough measurements were used to investigate the binding of alkali metal cations to eicosyl sulfate (ESO) surfactants in monolayers at the air/water interface. The number density of sulfate groups could be tuned by mixing the anionic surfactant with eicosanol. The equilibrium dissociation constant for K to the fatty sulfate interface showed 10 times greater affinity than for Li and approximately 3 times greater than for Na. All three cations formed solvent shared ion pairs when the mole fraction of ESO was 0.33 or lower. Above this threshold charge density, Li formed contact ion pairs with the sulfate headgroups, presumably via bridging structures. By contrast, K only bound to the sulfate moieties in solvent shared ion pairing configurations. The behavior for Na was intermediate. These results demonstrate that there is not necessarily a correlation between contact ion pair formation and stronger binding affinity.