Effect of poly (sodium 4-styrenesulfonate) on the ionization constants of acid-base indicator dyes in aqueous solutions

摘要

Presently, the acid-base equilibria of indicators fixed in surfactant micelles are described thoroughly. In contrast, analogous studies with polyelectrolytes are few in number. As a result, though the interactions of polyelectrolytes with dyes in aqueous solution have been studied for a long time, the research was mainly added up to metachromasy and solvatochromism. The properties of the polyion coils as media for the acid-base interactions of pH-indicators are less elucidated. The present article is aimed to fill this gap to a certain degree. A set of indicator dyes is examined via the spectrophotometric method in aqueous solutions of poly (sodium 4-styrenesulfonate), NaPSS, with molar weight of approximate to 70 x 10(3) g mol(-1). First, this colloidal system is characterized by the dynamic light scattering method. Then, we report the apparent ionization constants, K-a(app), of a series of dyes of different structure and charge types, neutral red, methyl yellow, rhodamine B, pseudoisocyanine, quinaldine red, and pinacyanol. The K-a(app) values of the indicators are substantially influenced by polyelectrolyte concentration and the ionic strength of solutions. The selection of the appropriate polyelectrolyte : dye concentration ratio allows avoiding the interfering influence of the metachromasy of dyes. The shifts of pK(a)(app) as compared with the values in water, pK(a)(w), are different for the dyes studied, and qualitatively agree with those observed for the same compounds in micellar solutions of an anionic surfactant sodium dodecylsulfate, NaDS. The effects are, however, less expressed than in NaDS micelles. This is in line with (i) less negative zeta-potential of the NaPSS coils as compared with that of the NaDS micelles at the same ionic strength of the bulk phase and (ii) the lack of the hydrocarbon core in the case of the pseudophase of this polyelectrolyte. The study of salt effects, i.e., the dependence of pK(a)(app) vs. the ionic strength allows estimating the degree of binding of Na+ ions, beta, by the polyions. In accord with Manning's theory, this parameter appeared to be constant and is within the range of 0.6 <= beta <= 0.8, similar to that in NaDS micelles. Additives of tetraalkylammonium salts display marked influence on both size distribution of the polyelectrolyte species and pK(a)(app) values of indicators.