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Investigations of polyelectrolyte adsorption at the solid/liquid interface by sum frequency spectroscopy: evidence for long-range macromolecular alignment at highly charged quartz/water interfaces.

IR-visible sum frequency spectroscopy (SFS) was employed to investigate the molecular level details of the adsorption of the positively charged polyelectrolyte, polydiallyldimethylammonium chloride (PDDA), at the quartz/water interface. Below pH 9.0, signal from the interfacial water structure was visible, but none from the adsorbed polymer could be detected. This indicated that the PDDA was not well enough aligned at the interface under these conditions to elicit a sum frequency response. At more basic pH values (>or=9.6), however, adsorbed PDDA molecules became well-ordered as indicated by the presence of CH stretch peaks from methylene and methyl groups. The intensities of the CH stretch modes were independent of the adsorbed amount of PDDA at pH 12.3 but decreased as the pH of the bulk solution was lowered. The conditions for polymer alignment fell outside the parameters where layer-by-layer growth of oppositely charged polyelectrolytes was possible because the net charge on the surface under high pH conditions remained negative.