Effects of Ligand Binding on Relative Stability of Subchain Conformations of Weakly Charged N-Isopropylacrylamide Gels in Swollen and Shrunken States

The thermotropic collapse of the hydrogel of copolymer N-isopropylacrylamide with acrylic acid (NIPA-AAc) upon heating has been studied by high-sensitivity differential scanning calorimetry. Curves of the polymer partial heat capacity have been measured at different pH (from pH 2 to 11) and different concentrations of cationic ligands: CaCl ₂, dodecyltrimethylammonium bromide (DTAB), and cetyltrimethylammonium bromide (CTAB). Dependencies of the main thermodynamic parameters of the gel collapse-temperature, enthalpy, and width of the transition on pH and ligand concentration-have been obtained, It has been found that the collapse is characterized by a negative value of the heat capacity increment Δ _tC _p Its average value determined over all variables studied is Δ _tC _p = -0.62 ± 0.02 J/(g K). This result is in accordance with the observed correlation between the transition enthalpy and temperature of the NIPA-AAc gel. The free energy of the collapse has been calculated as a function of pH and concentration of CaCl ₂, DTAB, and CTAB and analyzed using the model of macromolecular binding on the independent equivalent sites. Parameters characterizing affinity of the gel in swollen and collapsed state with respect to the ionic ligands have been determined. A preferential electrostatic binding of protons, Ca ²⁺, and surfactant ions by subchains in collapsed gel has been evaluated. The results also reveal a weak affinity of the subchains in swollen gel to amphiphilic ligands.