Charge determination of synthetic cationic polyelectrolytes by colloid titration

The stoichiometric interaction between oppositely charged polyelectrolytes forms the basis of the technique of colloid titration. This can be used, for instance, to determine the charge density of a cationic polyelectrolyte, using an anionic polyelectrolyte of known charge density, such as potassium polyvinyl sulphate (PPVS). The technique requires a suitable method of end-point detection and there are several possibilities. In this work three methods have been investigated: visual observation of the colour change of a cationic dye (o-toluidine blue), spectrophotometric determination of the colour change of the same dye, and electrokinetic detection of charge neutralisation, using a streaming current detector. These have been applied to a range of cationic polyelectrolytes, with different charge densities and molecular weights. In all cases, the cationic charge was due to quaternary nitrogen groups. In the case of the cationic dye, it was shown that the sharpness of the colour change depends on the charge density of the cationic polyelectrolyte. With lower charge density materials the binding to PPVS is weaker and binding of the dye to PPVS can occur before all of the polyelectrolyte charge has been neutralised. However, by carrying out titrations at several polyelectrolyte concentrations, good linear relationships were found, from which reliable charge density values could be derived. The streaming current method avoids the problem of competitive interaction of PPVS with o-toluidine blue and gives quite sharp transitions at the isoelectric point, although there was some dependence on polyelectrolyte charge density. This method gave good agreement with charge densities derived from the spectrophotometric technique. Effects of pH and ionic strength were also briefly investigated. For most of the cationic polyelectrolytes, there was some loss of charge at higher pH values, probably as a result of hydrolysis. Increasing ionic strength causes a less distinct colour change of o-toluidine blue, as a result of weaker electrostatic interactions. (C) 1999 Elsevier Science B.V. All rights reserved.