  X-ray reflectivity and fluorescence near total reflection experiments were
performed 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), spread
as a monolayer at the air/water interface, was used to form and control the
charge 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 ions
per 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, we
find 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 ionic
charges, the concentration of mixed $\mathrm{Ca^{2+}/Ba^{2+}}$ at the interface
would equal that of the bulk. Our results show a clear specific affinity of PA
for Ca compared to Ba. We provide some discussion on this issues as well as
some implications for biological systems. Although our results indicate an
excess of counterion charge with respect to the surface charge, that is, charge
inversion, the analysis of both reflectivity and fluorescence do not reveal
excess of co-ions (namely, $\mathrm{Cl^{-}}$ or $\mathrm{I}^{-}$).
