Crystals and liquids of ionic microgel particles: Osmotic pressure and phase coexistence

We quantify the phase behavior of suspensions comprised of swollen, ionic microgels while measuring the system osmotic pressure. Surprisingly, the osmotic pressure of dilute suspensions is much larger than that expected for an ideal gas. Furthermore, we find that the width of the liquid-crystal phase coexistence region increases as the microgels are made softer; this is true in terms of a generalized volume fraction, $\zeta =$\textit{nV}$_{0}$, with $n$ the particle density and $V_{0}$ the dilute microgel volume. We will discuss the role played by the ions in our observations and compare with expectations based on computer simulations.