Aspect-ratio-dependent phase transitions and concentration fluctuations in aqueous colloidal dispersions of charged platelike particles

Abstract

Phase transitions of aqueous colloidal dispersions of charged platelike particles of niobate nanosheets were investigated as a function of the aspect ratio (${r}_{\mathrm{asp}}$) and particle volume concentration (${\ensuremath{\varphi}}_{\mathrm{p}}$) by means of small-angle neutron scattering and small-angle x-ray scattering. The results elucidated the following three pieces of evidence: (1) the macroscopic phase separation of the dispersions into an isotropic phase and a liquid crystalline (LC) phase under the conditions of (a) varying ${r}_{\mathrm{asp}}$ (1.3$\ifmmode\times\else\texttimes\fi{}$10${}^{\ensuremath{-}4}$ \ensuremath{\le} ${r}_{\mathrm{asp}}$ \ensuremath{\le} 2.5$\ifmmode\times\else\texttimes\fi{}$10${}^{\ensuremath{-}3}$) at a constant ${\ensuremath{\varphi}}_{\mathrm{p}}$ $=$ 0.01 and (b) varying ${\ensuremath{\varphi}}_{\mathrm{p}}$ (0.01 \ensuremath{\le} ${\ensuremath{\varphi}}_{\mathrm{p}}$ \ensuremath{\le} 0.025) at a constant ${r}_{\mathrm{asp}}$ $=$ 2.5$\ifmmode\times\else\texttimes\fi{}$10${}^{\ensuremath{-}3}$, a mechanism of which is proposed in the text, where ${r}_{\mathrm{asp}}$ \ensuremath{\equiv} $d$/$\overline{L}$, with $d$ and $\overline{L}$ being thickness and the average lateral size of the plates, respectively; (2) the ${r}_{\mathrm{asp}}$-induced phase transition of the LC phase from a nematic phase to a highly periodic layered phase, the line shapes of the scattering peaks of which were examined by Caill\'e's analysis, upon increasing ${r}_{\mathrm{asp}}$ under the condition (a); (3) the LC phase having remarkable concentration fluctuations of the particles which are totally unexpected for the conventional lyotropic molecular LC but which are anticipated to be general for the platelike colloidal particles.