NONMONOTONIC EFFECT OF IONIC-STRENGTH ON SURFACE DYE EXTRACTION DURING DYE-POLYELECTROLYTE MULTILAYER FORMATION

Multilayer formation between organic dyes and polyions is complicated by the possible competition between dye extraction from the surface by the polymer solution instead of polymer adsorption. We study the effe cts of salt concentration (NaCl, Na2SO4, MgCl2, and MgSO4) on this pro cess and note that significant fractions of the dye are removed from t he surface at intermediate salt concentration but that at lower or hig her salt concentration less surface dye extraction occurs. Polymer sol utions with intermediate NaCl concentrations show more than 60% dye ex traction, while those of MgCl2, Na2SO4, and MgSO4 show approximately 3 0%, 25%, and 20% dye extraction, respectively. The maximum in dye extr action for all of these salts occurs when the solution Debye length is approximately 2 Angstrom. The reduced efficiency of dye complexation away from the maximum is interpreted in terms of the polymer's expecte d increase in persistence length (low salt concentration) and the redu ced electrostatic interactions between the charged dye and polymer (hi gh salt concentration). The absorbance of poly(styrenesulfonate) remai ns roughly constant or slightly increases for the 1:1 and 2:2 salts bu t decreases with increasing 2:1 salt concentration. We propose that th is decrease is due to charge reversal on the polymer or surface by the salt. Small shifts in dye peak positions suggest changes in the polar ity of the surrounding matrix.