Ionization Behavior Of Polyelectrolyte In Solution

The paper being aimed at ionization behavior of polyelectrolyte, the concentration dependence of the degree of ionization of polyelectrolyte in solution was studied taking aqueous solution of polyacrylic acid (PAAH) as an example. The results were compared with those of weak electrolyte solution of small molecule. Thermodynamics was taken into consideration to describe the physical nature of both polyelectrolyte solution and weak electrolyte solution of small molecule. An ionization equation which was applicable for the both was established. The main contents and conclusions were as follows.Firstly, PAAHs with different molecular weights, as 2-Azobisisobutyronitrile (AIBN) for initiator, were synthesized by the precipitation polymerization. The concentration dependence of the degree of ionization of PAAH and AAH was studied in a wide concentration range (1～10-/mol/L). The results showed that PAAH and AAH have different ionization behaviors in aqueous solution. At higher concentration region, the degree of ionization of PAAH was agree with AAH at the same concentration of structural units and followed the theory of the equilibrium constant. However, at low concentration, the degrees of ionization of PAAHs were different with AAH at the same concentration of structural units and both of them were deviated from the theory of the equilibrium constant. Additionally, the degrees of ionization of PAAHs with different molecular weights were nearly consistent.Secondly, the thermodynamics parameters of polyelectrolyte solution and small molecule weak electrolyte solution in the ionization process were studied. An ionization equilibrium equation which was applicable for both polyelectrolyte solution and small molecule weak electrolyte solution covering the entire concentration range was deduced. αφ2ΔE+RT[(1-φ2+φ2/x)ln1/1+αφ2+αφ2lnα2φ2/(1-α)(1+αφ2)+φ2ln(1-α)]=0The equation showed that when temperature and solution system are fixed, that is to say ΔE is a constant, the degree of ionization of small molecule weak electrolyte in solution was determined only by concentration, but for polyelectrolyte the degree of ionization was also related to the degree of polymerization x. However, when the polymerization degree exceeded a critical value, it would not affect the degree of ionization any more.Comparing the equation with the ionization equilibrium constant theory, the results showed that the ionization equilibrium constant formula can be derived at the condition of higher concentration by approximate treatment for our formula, which indicated that the ionization equilibrium constant theory is applicable just for solution concentration exceeding a certain threshold. In addition, a clear physical meaning of ionization equilibrium constant is shown in the comparison results.Thirdly, the relationship between acidity and concentration of mixed aqueous solution of polyacrylic acid and sodium polyacrylate (PAAH-PAANa) with different degree of neutralization was studied. The quantitative relationship between acidity and concentration was fitted. Two sorts of buffer solutions were obtained by using the quantitative relationship.One of the buffer solutions had such feature:the degree of neutralization r= 0.27, pH= 6.45, existing in lower concentration range(C<0.001 mol/L); the other buffer solutions:the degree of neutralization r=0.96, pH=9.64, existing in higer concentration range(C>0.001 mol/L).