Alkali earth metal binding properties of ionic microgels

Spherical micron-sized (4-10 mu m in diameter) poly(methacrylic acid-co-acr ylic acid) microgels were synthesized by precipitation polymerization, and their chelation reactions with chloride salts of Mg-2+(,) Ca2+, Sr2+, and B a2+ were investigated by isothermal titration calorimetry (ITC). Ion concen trations 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 m icroscopy and micropipet manipulation techniques were used to measure the v olume change and corresponding dehydration of the microgels as a function o f divalent ion type and concentration. The ITC results showed that the addi tion of MCl2 electrolytes, where M represents a divalent metal, with the mi crogels was an entropy driven reaction in that Delta G similar to -20 kJ/mo l congruent to T Delta S. These data suggest that the free energy driving t he 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) t he collapse of the entropy elastic network that accompanies the decrease in the volume of the microgels.