Multiphase coexistence in colloidal dispersions of positively charged layered double hydroxides

Colloidal dispersions of Mg-Al (Mg/Al molar ratio 2:1) layered double hydroxide (LDH) have previously been found to exhibit an isotropic-nematic (I-N) phase transition [S.Y. Liu, J. Zhang, N. Wang, W.R. Liu, C.G. Zhang, D.J. Sun, Chem. Mater. 15 (2003) 3240-3241]. In the present paper, we show that an effective attraction induced by the addition of low molecular weight polyethylene glycol (PEG) enriches the phase behavior of Mg-Al LDH dispersions. At certain concentrations of LDH and PEG, multiphase coexistence is observed. The addition of PEG leads to the appearance of a four-phase equilibrium and even a five-phase equilibrium. We explain the experimental phenomena by the interplay between PEG-induced effective attraction, LDH particle polydispersity and sedimentation.