Influence of the Ionic and Non-Ionic Additives on the Acid Strength of Neutral Red in the Aqueous Solutions of Poly (Sodium 4-Styrenesulfonate)

Abstract

Polyions of poly (sodium 4-styrenesulfonate) in aqueous solution may be considered as colloidal pseudophase. Analogously to micelles, some species can be bind by the charged polyelectrolyte coils. In this paper, the colored acid-base indicator dyes were used for studying the penetration of ionic and non-ionic solution components in polyelectrolyte coils. Such analysis relies on alteration of the acid strength of dyes. The aim of this work was to determine the dependences of the apparent ionization constants of neutral red, app a K , on the ionic strength of solution, I, in different mixed systems containing poly (sodium 4-styrenesulfonate) (NaPSS) and tetraethylammonium bromide, tetra-n-butylammonium iodide, or 1-butanol with NaCl. Neutral red is fixed in the polyelectrolyte coils due to electrostatic and hydrophobic interactions. The polyelectrolyte : dye concentration ratio, P : D, an important parameter for spectophotometrical measurements, equals 100. It was found that tetra-n-alkylammonium salts display a stronger influence on the acid strength of neutral red as compared with that of NaCl. For instance, in the case of tetraethylammonium bromide app a pK = 7.90 ± 0.07 at I = 0.02 M whereas app a pK = 8.45 ± 0.04 with NaCl at the same ionic strength. It can be explained by specifical adsorption tetra-n-alkylammonium cations on polyions 'surface'. Introduction of 1-butanol significantly increases the acid strength of neutral red too, however its influence isn't monotonic with change of ionic strength. The dependences of app a pK on log I were discussed in terms of regularities which are well-known for surfactant micellar systems. All mentioned systems were also examined by dynamic light scattering measurements using Zetasizer Nano ZS Malvern Instruments apparatus. As it turned out, when the ionic strength is 3×10 -3 М or more the size (by number) of polyions in the presence of NaCl is of about 7 nm. Introduction of the additives results in somewhat higher size: in the case of 1-butanol the size of NaPSS is 10 nm.