Electric transport properties of alkali polymethacrylates in alkali bromide solutions

Abstract

Electric mobilities of polyions, bromide ions, and alkali ions have been determined in solutions of Li, Na, and K salts of polymethacrylic acid (PMA) in aqueous solutions of the corresponding bromide of concentrations varying from 0.001 to 0.1 M. The Hittorf method was used for the determination of the mobilities of the PMA ion and of the Br − ion. The mobilities of the alkali ions followed from these two mobilities and the conductivity. A few moving boundary experiments have been carried out with tetramethylammonium PMA. The mobility of PMA was independent of the PMA concentration and of the kinds of cations and changed only slowly with the degree of neutralization. The bromide mobility decreased slowly with increasing PMA concentration. The interpretation was based upon a separation of the contributions from the polyelectrolyte salt and those of the supporting electrolytes. The mobilities of the counterions that neutralized the polyions were very low and in several cases even negative. Electrophoretic retardations varied from about 20 to 70 Ω −1 cm 2 eq −1. By using the relaxation correction as found from conductivity data we could interpret the mobility of the PMA ion as that of a cylinder of 0.35-nm radius surrounded by a solvation layer of 0.2- to 0.3-nm thickness. For bromide concentrations up to 0.03 M the agreement was excellent; at a 0.1 M bromide concentration binding of about 25% of the counterions to the polyions had to be assumed to reconcile the model calculations with the experiments. Intrinsic viscosities of NaPMA in NaBr solutions have been used to obtain an estimate of the overall size of the polyelectrolyte coil and to justify the cylinder model.