On the driving forces for complexation of methyl orange with polycations.

Hypothesis: Complexation between Methyl orange and polycations involves multiple interactions dictated by molecular structure, composition (D/P), pH and ionic strength. The effect of ionic strength is considered a generic effect. By step-wise construction of complexes, we expect to gain insight in the nature of interactions and whether displacement by competing ions is a generic effect.

Experiments: We step-wise constructed complexes of methyl orange with two model polycations, whilst recording visible light spectra, size and electrophoretic mobility in buffered solution. MO organization was derived from discrete spectral changes, whereas complexes were described in terms of size and zeta-potential data. Spectral data were used to study the effect of competing ions, both potassium halides and polyanions, using a manual titration method.

Findings: Spectral and size data reveal a complex stoichiometry of D/P=2.2 and 4.6 for poly(ethylenimine hydrochloride)(PEI) and poly(di allyldimethyl amine hydrochloride) PDADMAC, respectively. Contrary to PEI-MO, the formation of PDADMAC-MO complexes is driven by hydrophobic rather than electrostatic interactions. Organization of PDADMAC-MO complexes also shows a strong dependency on the order of construction and polycation concentration. Displacement of MO by halides shows no effect of ion size for PEI, whereas Hofmeister series ordering was found for PDADMAC. The displacement by polyanions is shown to be charge-stoichiometric.