Spatial distribution of polyelectrolytes in thin free-standing aqueous films resolved with fluorescence spectroscopy

Pyrene labels are used as a molecular probe to investigate the rearrangement of polyelectrolytes confined in a free-standing aqueous film of progressively decreasing thickness. Monitoring the spectroscopic changes of covalently labeled poly(acrylic acid) in a surfactant-stabilized film, we followed the effects of confinement down to a film thickness of 15 nm. Already upon film formation at thicknesses of 50 nm, a marked increase of the excimer/monomer ratio was found irrespective of the degree of charge of the polymer. Hence, the majority of polyelectrolytes is located within regions of locally elevated polymer density. Surface tension measurements showed these regions to be located at the interfaces in case of uncharged polyelectrolytes. In charged polyelectrolytes, stratification of the film is observed and accompanied by an additional rise of the excimer/monomer ratio, suggesting that charged polyelectrolytes reside in a few distinct sites within the film.