On the Question of the Method for Determining the Critical pH Value during the Formation of Complexes between Nonionic Polymers and Polyacid in Aqueous Solutions

A new method is proposed for the quantitative determination of critical pH values during the formation of complexes between nonionic polymers and polyacids in aqueous solutions. As an indicator reflecting the pH value, where there is a transition from a solution containing noninteracting macromolecules to a solution in which the formation of an interpolymer complex occurs, the pH value should be taken with a yield of the complex formation reaction 1/2 of the maximum. The expediency of using just such an indicator is due to the fact that there is a fairly extended pH boundary region with the coexistence of noninteracting macromolecules and the reaction product between them. It is shown that the indicator determined in this way can be conveniently to calculated using the phase portrait method. This technique was tested by analyzing families of experimentally obtained curves describing the effect of potassium chloride additives on the complexation process between a copolymer of 2-hydroxyethyl acrylate and butyl acrylate and poly(acrylic acid) in an aqueous solution. It was found that for this system, the nature of the dependence of the effect of ionic strength on the formation of the polycomplex significantly depends on the composition of the copolymer, while the increase of only 4.9 mol % in the content of hydrophobic butyl acrylate units in the copolymer is accompanied by the transition of this system from a weak complex to strong complex.