Chapter 28 Phase Diagram of Ionic Colloidal Crystals T. Okubo
Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 11, 2017 | http://pubs.acs.org Publication Date: December 13, 1993 | doi: 10.1021/bk-1994-0548.ch028
Department of Polymer Chemistry, Kyoto University, Kyoto 606-01, Japan
Very large single crystals(3 to 8 mm) are observed with the naked eye in the highly deionized colloidal suspensions of monodisperse polystyrene and silica spheres. Deionization is carefully made with the mixed beds of ion-exchange resins more than three weeks. Two kinds of single crystals,i.e., block-like crystals from homogeneous nucleation in the bulk phase far from the cell wall and the pillar-like ones from the heterogeneous nucleation along the cell wall are observed. Size of the single crystals increases sharply as the concentration of spheres decreases, and is largest at the concentration slightly higher than the critical concentration of melting(Φ).Φ and the melting temperature(T ) have been measured againforthe completely deionized suspensions. NewΦ values are very small compared with the previous data, whereas T values are high. Important role of the long Debye-screening length around spheres and the intersphere repulsion is strongly supported. c
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Ionic colloidal suspensions display the extraordinary structures in particle distribution, such as gas-like, liquid-like and solid-iike[crystal-like and amorphous solid-like(or glass-like)] distributions, especially in deionized state(7-4). The suspensions showing the crystal-like structures are ideal systemsformodel studies of metals, since the colloidal structures are analyzed with the optical techniques and theirforcesare readily manipulated by controlling the composition of the suspension. Furthermore, phase transition phenomena such as crystallization and melting occur sharply. A study of the extraordinary structures of colloidal particles is also helpful in understanding fundamental properties of states of substances and electrostatic interactions of macro-ionic systems. The two essentially important factors in the characteristic properties for colloidal systems are an electrostatic interparticle repulsion and an expanded electrical double layers around the particle in the deionized state. These factors are quite different from the features of real metals. Observation with the naked eye of the iridescent colors and single crystals(or crystallites) is beautiful and impressive! Colloidal crystals are surrounded by grain boundaries and are quite similar to the morphology of metals. Iridescent colors of the crystal-like suspension, which are ascribed to the Bragg diffraction of visible 0097-6156/94/0548-0364$06.00/0 © 1994 American Chemical Society
Schmitz; Macro-ion Characterization ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 11, 2017 | http://pubs.acs.org Publication Date: December 13, 1993 | doi: 10.1021/bk-1994-0548.ch028
28.
OKUBO
Phase Diagram of Ionic Colloidal Crystals
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light by the arrayed lattice planes. Lattice spacing of the colloidal crystals is very long by several thousands times compared with that of metals and in the range of light wavelenghts. The colors change from reddish to bluish as the concentration of colloidal particles increases in many cases. In thisreportwe deionized the colloidal suspensions as completely as possible with the Bio-Rad mixed beds of cation- and anion-exchange resins coexisted more than three weeks. Very big colloidal single crystals, 3 to 8 mm in size have been observed with the naked eyes, for thefirsttime, for the completely deionized suspensions in a test tube of 13 mm in outer diameter! Furthermore, the critical concentrations of melting for the completely deionized colloidal suspensions was significantly low compared with thereferencevalues hitherto, and the melting temperature was quite high. Importance of the complete deionization on the appearance of the colloidal single crystals and their phase equilibria is discussed in this report Experimental Section Materials. D1C25, D1C27, D1B76, D1K88, D1B72 and D1B28 were polystyrene spheres purchased from Dow Chemical Co. Colloidal silica spheres of CS-81 and CS-91 were gift from Catalyst & Chemicals Ind. Co.(Tokyo). Diameters(d), standard deviation^*/) are listed in Table I with other characteristics. The values of d and 5 were determinedfroman electron microscope. The charge densities of the spheres were determined by conductometric titration with a WayneKerr autobalance precision bridge, model B331, mark II(Bognor Regis, Sussex). Strongly acidic and weakly acidic groups coexisted. All these spheres were carefully purified several times using an ultrafiltration cell(model 202, membrane: DiafloXM300, Amicon Co.). Then the samples were treated on a mixed bed of cation- and anion-exchangeresins[Bio-Rad,AG501-X8(D), 20-50 mesh] for at least one month. Water used for the purification and for suspension preparation was deionized by using cation- and anion-exchangeresins[Puric-R,type 10, Organo Co., Bedford, MA), and further treated with the ion-exchange resins of Bio-Rad. Colloidal suspension was prepared in a test tube(disposable culture tube, borosilicate glass, Corning Glass Works, Corning, N.Y.. 11 and 13 mm, inside and outside diameters) shielded tightly with Parafilm(American Can Co., Greenwich, CT). The sample suspensions were treated with a small amount of Bio-Radresinsmore than three weeks with up-and-down mixing several times a day.
Sphere D1C25 D1C27 D1B76 D1K88 D1B72 D1B28 CS-81 CS-91
Table I. Characteristics of Colloidal Spheres Used diameter b/d charge density (pC/cm )
