Alkali earth metal binding properties of ionic microgels

Spherical micron-sized (4-10 μm in diameter) poly(methacrylic acid-co-acrylic acid) microgels were synthesized by precipitation polymerization, and their chelation reactions with chloride salts of Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ were investigated by isothermal titration calorimetry (ITC). Ion concentrations obtained by inductively coupled-plasma mass spectrometry (ICP-MS) were used to obtain binding constants and to verify the results obtained by ITC. Although the two methods agreed within 20%, the ITC measurements were experimentally easier to obtain and more accurate. Interference contrast microscopy and micropipet manipulation techniques were used to measure the volume change and corresponding dehydration of the microgels as a function of divalent ion type and concentration. The ITC results showed that the addition of MCl₂ electrolytes, where M represents a divalent metal, with the microgels was an entropy driven reaction in that ΔG approx. -20 kJ/mol ≅ TΔS. These data suggest that the free energy driving the ion exchange (M²⁺ divalent ions for monovalent Na⁺) is the result of the increase in the entropy of the system; this entropy increase is due to (1) water being 'squeezed' from the microgels into the bulk solution and (2) the collapse of the entropy elastic network that accompanies the decrease in the volume of the microgels.