Abnormal Ionization Behavior Of Dilute Polyacrylic Acid Solution And Double Equilibrium Constant Theory

The ionization property of poly electrolyte aqueous solution has an important impact on the application of poly electrolyte in industry and life,but the essence of poly electrolyte ionization in aqueous solution is still unclear.In order to explore the abnormal ionization behavior of poly electrolyte and small molecule weak electrolyte in dilute solution,this paper mainly studies the ionization process of polyacrylic acid and acrylic acid in aqueous solution from the perspective of thermodynamics,establishes the ionization equilibrium equation suitable for both,and comprehensively analyzes the ionization behavior and conductivity behavior of polyacrylic acid and acrylic acid aqueous solution in a wide range of concentration.The main research work is as follows:Firstly,a series of polyacrylic acids with different polymerization degrees are prepared by precipitation polymerization,and the relationship between ionization degree and concentration of polyacrylic acid and acrylic acid in a wide range of concentration is studied.The results show that the ionization behavior of polyacrylic acid has little dependence on molecular weight.The ionization behavior of polyacrylic acid and acrylic acid is in the high concentration region(C>2×10-3 mol/L)and low concentration area(C<1×10-5 mol/L)is basically the same,but when the concentration range is between the high and low concentration regions,the ionization degree of the two is quite different.Compared with the theoretical pH and ionization degree calculated by the equilibrium constant,the measured pH and ionization degree of both small molecule electrolytes and polyacrylic acid deviate,with the small molecule electrolytes deviating in the range of 10-7～10-4 mol/L and the polyacrylic acid deviating in the range of 10-7～2×10-3 mol/L.Secondly,the dissociation behavior of small molecule weak electrolyte and poly electrolyte in aqueous solution is studied from the perspective of thermodynamics.The entropy change,enthalpy charge,and Gibbs free energy change of the dissociation proces are calculated by the lattice model.Through the further derivation of the formula,the dissociation equilibrium equation which can unify the whole concentration range of these two kinds of electrolytes is obtained.By further analyzing the dissociation equilibrium equation,another calculation formula of the traditional ionization equilibrium equation is inversely deduced:The physical meaning of traditional ionization equilibrium is obtained:the relative strength of ionization hydration interaction in the dissociation process of electrolyte.Through the analysis of x term in the dissociation equilibrium equation(x represents the polymerization degree of polyacrylic acid),it is confirmed that the dissociation behavior of electrolyte has little relationship with molecular weight.Through mathematical derivation,it is found that there is an ionization hydration energy in both small molecule weak electrolyte and polyelectrolyte in high concentration region and low concentration region,which explains why the dissociation behavior of electrolyte is different in high concentration region and low concentration region.Two groups of buffer solutions of polyacrylic acid sodium polyacrylate are found through experimental research,which demonstrated that there are two ionization equilibrium constants of polyacrylic acid in the whole concentration range(10-7 mol/L-2 mol/L).Finally,the conductivity behavior of polyacrylic acid and acrylic acid is studied,and the conductivity behavior is compared with the ionization behavior.The results show that the conductivity behavior of polyelectrolytes is similar to that of small molecule weak electrolytes,and there is a concentration transition region of conductivity behavior,which is located in the same concentration range as the concentration transition region of ionization behavior.The limiting molar conductivity of polyelectrolytes and small molecule weak electrolytes show different "step" distributions in the high and low concentration regions,indicating that the microscopic state of conductive particles is significantly different in different concentration regions,mainly due to the different hydration states of polyelectrolyte solutions and small molecule electrolyte solutions in the high and low concentration regions,which also leads to the difference of ionization hydration energy in different concentration regions.