Insoluble monolayers of a preformed polymer containing carboxylic acid hydrophilic groups - cadmium ion binding to monolayers and ionization in multilayers

We have investigated the properties of insoluble monolayers and Langmuir-Blodgett (LB) multilayers of a preformed derivative of an α-olefin maleic anhydride copolymer. The polymer repeat unit contained a hexadecyl hydrophobic group and two carboxylic acid moieties in the hydrophilic headgroup. Using X-ray reflectivity we have determined the floating monolayer structure and the adsorption of cadmium ions from the subphase as a function of pH at a fixed surface pressure. The tailgroup thickness is 14 Å, much less than the all-trans length of the hexadecyl chains. The headgroup layer thickness is approximately 8 Å. Both thickness are independent of pH. Cadmium ion binding to the carboxylic acid headgroups occurs with a 1:2 stoichiometry and a pK of 5.7. Using attenuated total reflectance IR spectroscopy, the ionization of the corresponding LB multilayers deposited on germanium was also measured. The pH of 50% ionization is approximately 5, in reasonable agreement with the pK for cadmium ion binding to the floating monolayer.