Single Microgels in Core–Shell Equilibrium: A Novel Method for Limited Volume Studies

The interactions of cationic surfactant dodecyltrimethylammonium bromide and cationic protein cytochrome c with anionic polyacrylate microgels have been investigated in microscopic liquid droplets by means of a micropipette technique at ionic strength 0.01 M and pH 8. Experiments on single microgels in solutions of limited amounts of the surfactant provide the first evidence of microgels in a stable biphasic core–shell state with the surfactant partitioned to the shell. Under the same conditions, the protein is found to distribute uniformly in the microgels. Quantitative data in the form of swelling and binding isotherms are presented and compared with literature data for macrogels and with predictions of a recent gel theory. Theory is found to be in semiquantitative agreement with the experiments. The importance of polyion-mediated attractions between the protein molecules is analyzed theoretically and proposed to explain the continuous but highly cooperative binding isotherms.