Surface sensitive synchrotron X-ray scattering studies were

performed to obtain the distribution of monovalent ions next to a

highly charged interface at room temperature. To control surface

charge density, lipids, dihexadecyl hydrogen-phosphate (DHDP) and

dimysteroyl phosphatidic acid (DMPA), were spread as monolayer

materials at the air/water interface, containing $\mathrm{CsI}$ at

various concentrations.

Five decades in bulk concentrations ($\mathrm{CsI}$) are

investigated, demonstrating that the interfacial distribution is

strongly dependent on bulk concentration. We show that this is due

to the strong binding constant of hydronium $\mathrm{H_{3}O^{+}}$ to

the phosphate group, leading to proton-transfer back to the

phosphate group and to a reduced surface charge. Using anomalous

reflectivity off and at the $L_{3}$ $\mathrm{Cs^{+}}$ resonance, we

provide spatial counterion ($\mathrm{Cs^{+}}$) distributions next to

the negatively charged interfaces. The experimental ion

distributions are in excellent agreement with a renormalized surface

charge Poisson-Boltzmann theory for monovalent ions without fitting

parameters or additional assumptions.

Energy Scans at four fixed momentum transfers under specular

reflectivity conditions near the $\mathrm{Cs^{+}}$ $L_{3}$ resonance

were conducted on $10^{-3}\:\mathrm{M}$ $\mathrm{CsI}$ with DHDP

monolayer materials on the surface. The energy scans exhibit a

periodic dependence on photon momentum transfer. The ion

distributions obtained from the analysis are in excellent agreement

with those obtained from anomalous reflectivity measurements,

providing further confirmation to the validity of the renormalized

surface charge Poisson-Boltzmann theory for monovalent ions.

Moreover, the dispersion corrections $f^{\prime}$ and

$f^{\prime\prime}$ for $\mathrm{Cs^{+}}$ around $L_{3}$ resonance,

revealing the local environment of a $\mathrm{Cs^{+}}$ ion in the

solution at the interface, were extracted simultaneously with output

of ion distributions.

Another independent technique, X-ray fluorescence near total

reflection was used to study ion adsorption at charged surfaces.

Below the critical angle, the X-ray fluorescence spectra are only

surface sensitive, providing the direct evidence of existence of

$\mathrm{Cs^{+}}$ at the surface. Above the critical angle,

combination of fluorescence spectra with and without the presence of

monolayer materials yields the number of accumulated

$\mathrm{Cs^{+}}$ per lipid at the surface. In addition, the

fluorescence spectra collected as a function of incident X-ray

energy near the $L_{3}$ edge provide the dispersion corrections,

consistent with the results from the energy scans.