Aqueous solution of poly (hexamethylene guanidine hydrochloride) and poly (diethylenamine guanidine hydrochloride) as studied with acid-base indicators

Abstract

In this paper, the properties of cationic polyelectrolytes as tools for governing the protolytic equilibrium of acid-base indicators in water were examined. For this purpose, water-soluble and pH-dependent poly (hexamethylene guanidine hydrochloride), PHMG, and poly (diethylenamine guanidine hydrochloride), PDEG, were studied. As molecular probes, a set of anionic indicator dyes were used; the key parameter is the so-called apparent ionization constant, K_a^app. The electrokinetic potential of the above polycationic species in the acidic pH region is substantially positive. As a rule, the polyelectrolytes display marked influence on the absorption spectra and state of the acid-base equilibrium of the anionic dyes at pH < 7, especially in the case of PHMG. Both effects resemble those known for the same dyes in aqueous solutions of cationic surfactants but are less expressed. Normally, the acid-base equilibria were studied at polyelectrolyte : dye ratio of 150 : 1, at ionic strength 0.05 M, and 25 ^oC. The decrease in the pK_a^app (≡ –logK_a^app) value on going from water to the PHMG solution is most expressed for bromocresol green (HB^– ↔ B^2– + H⁺): pK_a,2^app – pK_a,2^w = –1.93. For bromophenol blue, bromocresol purple, and sulfonefluorescein, the shift of the equilibrium is less expressed. Some kinds of specific interactions with the polyelectrolytes were revealed for methyl orange and bromophenol blue. Also, the dependence of pK_a^app on logarithm of ionic strength allows estimating the degree of counterion binding by the polycation: β = 0.40.1.