Deviations from the classic Gouy-Chapman (GC) model due to the finite size of hydrated counterions were tested for negatively charged Langmuir monolayers with different surface charge densities. Monolayers with the largest charge density (>0.6 C.m(-2)) show an increase of the surface potential for a series of alkali metal cations from Li(+) to Cs(+) by 200-250 mV. The increase is similar for different monolayers and suggests that this effect is independent of the particular type of headgroup. The magnitude of variation is comparable with model estimations of the electrical double layer (EDL) potential implying that the deviation from the GC model is drastic. Deviations from the GC model rapidly vanish with decreasing monolayer charge density and become hardly observable below 0.3 C.m(-2). For monolayers with a high charge density on subphases containing different sized counterions, preferential participation of the smallest ions in the EDL should be favorable in terms of electrostatic free energy because of packing density limitations. This effect was demonstrated for behenyl sulfate (BS) monolayers (0.64 C.m(-2)) with the X-ray reflectivity technique. For the Cs(+)-Li(+) system, the fraction of Cs(+) in the EDL is 50-60% compared with only 10% of Cs(+) in the subphase. Providing high surface charge density, a small univalent Cs(+) is capable to compete even with a bulky divalent Mg(2+). For equal concentrations of Cs(+) and Mg(2+) in the subphase, the Cs(+)/Mg(2+) ratio in EDL of BS monolayer is 1.3 to 2.0 (in contrast to 0.04, predicted by the GC model). All experimental results of this study are described in terms of packing density limitations for hydrated counterions in the EDL.

A simplified total reflection X-ray fluorescence (TRXF) technique is proposed for the study of the electrical double layer (EDL) near charged monolayers at the air-water interface. In contrast to the parent NTEF (near total external reflection X-ray fluorescence) method, TRXF uses a fixed angle of incidence (below the critical one) and abandons both "spatial resolution" (which is poor anyway) and "absolute calibration" with the bulk reference. These modifications reduce both the duration of experiments and the complexity of the data treatment by 1-2 orders of magnitude and turn TRXF into a truly simple tool for elemental analysis within the EDL. A few TRXF experiments appear sufficient to disprove the model of simultaneous binding of alkali earth metal cations and inorganic anions to negatively charged phospholipid monolayers as proposed in literature. Direct experimental support was provided to the essential feature of the EDL near highly charged interfaces: The main amount of counterions is concentrated in the thin inner part of the EDL irrespective of the electrolyte concentration in the bulk. A study of the counterion competition for the participation in the EDL of highly negatively charged behenylsulfate (BS) monolayers (resulting from packing density limitations) revealed that univalent Cs+ is quite competitive with divalent Ca2+ and Ba2+ (which contradicts the classical Gouy-Chapman model). If the univalent/divalent ion ratio in bulk is 9:1, the corresponding ratio in the EDL is ca. 1.5 for Cs+/Ca2+ and 0.5 for Cs+/Ba2+, whereas the model predicts 0.14 only. Bearing in mind packing density limitations, these values are consistent with a series of sizes for hydrated ions: Cs+ < Ba2+ < Ca2+.

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