Adsorption of Counterions to a Stearate Monolayer Spread at the Water-Air Interface: A Synchrotron X-Ray Study

The near-total-external-fluorescence technique was used to measure in situ the adsorption of a metal ion from a subphase solution to the liquid-air interface. For a monolayer formed by the spreading of stearic acid on a ${10}^{-3\mathrm{}}$-mole/l solution of Mn${\mathrm{Cl}}_2$, the ratio of Mn ions segregated to the interface to the number of stearate molecules on the surface was determined as 0.6 ± 0.2. Surface extended x-ray-absorption fine-structure experiments revealed local order of Mn ions at the surface at the condensed phase but no order showed in the expanded phase. We explain these findings using a self-consistent Poisson-Boltzmann calculation.