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 alpha-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 angstrom, much less than the all-trans length of the hexadecyl chains. The headgroup layer thickness is approximately 8 angstrom. Both thicknesses 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.