EVANESCENT-WAVE SPECTROSCOPY - APPLICATION TO THE STUDY OF THE SPATIAL-DISTRIBUTION OF CHARGED GROUPS ON AN ADSORBED POLYELECTROLYTE AT THESILICA WATER INTERFACE

A new evanescent wave experimental technique with the capacity to dete rmine simultaneously the spatial distribution of several chromophoric species located near a reflecting interface is reported. In a first st ep, to illustrate the capacity of this technique, the absorption of a model polyelectrolyte polymer (polyacrylamide/diacetone acrylamide cop olymer grafted with an ionizable acridine chromophore) (EPI-26) onto t he silica/aqueous solution interface has been studied. For this sytem, it has been demonstrated that variable angle of incidence evanescent wave spectroscopy may be used to determine quantitatively both the sur face excess, GAMMA, and the mean separation distance from the interfac e zBAR, of charged and uncharged segments attached to the adsorbed pol yelectrolyte. The technique has also been used to measure the kinetics of adsorption, as well as changes in GAMMA, zBAR and degree of ioniza tion of the adsorbed layer as a function of surrounding solution condi tions (e.g. pH or salt concentration). It was found that increasing th e ionic strength resulted in a larger surface excess of the polyelectr olyte, and decreasing the pH, which further ionizes the polyelectrolyt e, reduced the surface excess and caused the polyelectrolyte layer to expand. Both of these trends are in accordance with what is expected f rom simple electrostatic considerations and further show the sensitivi ty of the evanescent wave technique to the microscopic structure of su ch adsorbed layers.