Thermosensitive hard spheres

We demonstrate an extension of a UV–Vis spectroscopy method to determine the phase boundaries for thermosensitive colloids as an alternative to the time-consuming sedimentation method. The Bragg attenuation peak from colloidal crystallites was monitored during the quasi-equilibrium colloidal crystal melting. The melting and freezing boundaries of the coexistence region were determined via a blue shift of Bragg's peak and the disappearance of peak area. We confirm this method using poly(N-isopropylacrylamide) (PNIPAM) particles at different charge densities and temperatures far below the lower critical solution temperature. At low pH, the particles behave as thermosensitive hard spheres.

Graphical abstractPhase diagram of the PNIPAM-co-acrylic acid microgel dispersions at pH 2.8 measured via UV–Vis spectroscopy. At room temperature, the fluid–crystal transition width normalized to the freezing transition concentration is approximately 7%.Figure optionsDownload full-size imageDownload as PowerPoint slide