Counter-ion specificity in colloidal ‘crystals’ and ‘liquids’ as studied by transmitted-light spectroscopy

Abstract

Crystal-like and liquid-like distributions of colloidal spheres, monodisperse polystyrene (D1C27 and D1B76) and colloidal silica spheres (Cataloid-45), have been observed, for the first time, for suspensions in which the counter-ions are sodium, tetramethylammonium (TMA⁺) and tetra-n-butylammonium ions (TBA⁺). These suspensions of the colloidal salts display iridescence and several samples even display crystallites. Peak profiles of the transmitted-light spectra of polystyrene spheres show the phase transition from crystal-like to liquid-like structures as the degree of neutralization (α) against strongly acidic groups of the spheres increase above unity. Intersphere distance (D) decreases during melting. D increases in the order H⁺ < Na⁺ < TMA⁺ < TBA⁺ at the same sphere concentration. All the suspensions of Cataloid-45 in which the gegenions are H⁺, Na⁺, TMA⁺ and TBA⁺ display the liquid-like nature in their peak profiles of difference spectra of transmitted-light, i.e. a broad single peak appears. The suspension structure and intersphere spacing are insensitive to the nature of the counter-ions. From these results it is clear that Na⁺, TMA⁺ and TBA⁺ ions from thick electrical double layers around colloidal spheres as H⁺ ions do in the deionized suspension, and electrostatic intersphere repulsion is essential to explain the appearance of crystal-like and liquid-like structures.