Evanescent wave spectroscopy: application to the study of the spatial distribution of charged groups on an adsorbed polyelectrolyte at the silica/water interface
A new evanescent wave experimental technique with the capacity to determine simultaneously the spatial distribution of several chromophoric species located near a reflecting interface is reported. In a first step, to illustrate the capacity of this technique, the absorption of a model polyelectrolyte polymer (polyacrylamide/diacetone acrylamide copolymer grafted with an ionizable acridine chromophore)(EPI-26) onto the silica/aqueous solution interface has been studied. For this system, it has been demonstrated that variable angle of incidence evanescent wave spectroscopy may be used to determine quantitatively both the surface excess, Γ, and the mean separation distance from the interface z, of charged and uncharged segments attached to the adsorbed polyelectrolyte. The technique has also been used to measure the kinetics of adsorption, as well as changes in Γ, z and degree of ionization of the adsorbed layer as a function of surrounding solution conditions (e.g. pH or salt concentration). It was found that increasing the ionic strength resulted in a larger surface excess of the polyelectrolyte, and decreasing the pH, which further ionizes the polyelectrolyte, reduced the surface excess and caused the polyelectrolyte layer to expand. Both of these trends are in accordance with what is expected from simple electrostatic considerations and further show the sensitivity of the evanescent wave technique to the microscopic structure of such adsorbed layers.