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X-ray reflectivity and fluorescence near total reflection experiments wereperformed to examine the affinities of divalent ions ($\mathrm{Ca^{2+}}$ and$\mathrm{Ba^{2+}}$) from aqueous solution to a charged phosphatidic-acid (PA)surface. A phospholipid (1,2-Dimyristoyl-sn-Glycero-3-Phosphate, DMPA), spreadas a monolayer at the air/water interface, was used to form and control thecharge density at the interface. We find that for solutions of the pure salts(i.e., $\mathrm{CaCl_{2}}$ and $\mathrm{BaCl_{2}}$), the number of bound ionsper DMPA at the interface is saturated at concentrations that exceed$\mathrm{10^{-3}M}$. For a 1:1 $\mathrm{Ca^{2+}/Ba^{2+}}$ mixed solutions, wefind that the bound $\mathrm{Ca^{2+}/Ba^{2+}}$ ratio at the interface is 4:1.If the only property determining charge accumulation near PA were the ioniccharges, the concentration of mixed $\mathrm{Ca^{2+}/Ba^{2+}}$ at the interfacewould equal that of the bulk. Our results show a clear specific affinity of PAfor Ca compared to Ba. We provide some discussion on this issues as well assome implications for biological systems. Although our results indicate anexcess of counterion charge with respect to the surface charge, that is, chargeinversion, the analysis of both reflectivity and fluorescence do not revealexcess of co-ions (namely, $\mathrm{Cl^{-}}$ or $\mathrm{I}^{-}$).

Preferential affinity of calcium ions to charged phosphatidic-acid  surface from a mixed calcium/barium solution: X-ray reflectivity and  fluorescence studies