Adsorption of multivalent ions on charged Langmuir monolayers

We have investigated the interaction between a Langmuir monolayer of C20H41NH2, a long-chain fatty amine and large, multivalent, heteropolyanions (phosphotungstic acid, PTA for short) dissolved in the aqueous subphase. By using X-ray reflectivity, surface pressure and surface potential measurements, we show that the adsorption is controlled by the electrostatic interaction between the PTA molecules and the amine film. At basic pH, the amine groups are not charged and no adsorption occurs. On the contrary, at acidic pH, the amines are positively charged. The surface pressure isotherms are markedly different in presence of PTA and the surface potential measurements reveal a reversal of the surface polarity. X-ray reflectivity gives a microscopic picture of the interfacial layer. The PTA anions are adsorbed onto the charged amine head groups as a monolayer or at most a bilayer. Changing the surface density of amines allows to control the total amount of PTA adsorbed and to vary the inorganic layer thickness.