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Determination of Degree of Ionization of Poly(allylamine hydrochloride) (PAH) and Poly[1-[4-(3-carboxy‑4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) in Layer-by-Layer Films using Vacuum Photoabsorption Spectroscopy

Ferreira, Q. ; Gomes, P. ; Ribeiro, P. A. R. ; Jones, N. C. J. ; Hoffmann, S.V. ; , ; Oliveira, O. O. ; Raposo, M. R.

Langmuir Vol. 29, Nº 1, pp. 448 - 455, January, 2013.

ISSN (print): 1520-5827
ISSN (online): 0743-7463

Scimago Journal Ranking: 1,90 (in 2013)

Digital Object Identifier: 10.1021/la304036h

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Electrostatic and hydrophobic interactions govern most
of the properties of supramolecular systems, which is the reason
determining the degree of ionization of macromolecules has become crucial for many applications. In this paper, we show that highresolution ultraviolet spectroscopy (VUV) can be used to determine the degree of ionization and its effect on the electronic excitation energies of layer-by-layer (LbL) films of poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4 hydroxyphenylazo)-benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). A full assignment of the VUV peaks of these polyelectrolytes in solution and in cast or LbL films could be made, with their pH dependence allowing us to determine the pKa using the Henderson-Hasselbach equation.
The pKa for PAZO increased from ca. 6 in solution to ca. 7.3 in LbL films owing to the charge transfer from PAH. Significantly, even using solutions at a fixed pH for PAH, the amount adsorbed on the LbL films still varied with the pH of the PAZO solutions due to these molecular-level interactions. Therefore, the procedure based on a comparison of VUV spectra from solutions and films obtained under distinct conditions is useful to determine the degree of dissociation of macromolecules, in addition to permitting interrogation of interface effects in multilayer films.