Charged colloidal dispersions undergo crystallization with increasing magnitude of an electrostatic interparticle interaction. Major experimental variables which determine the interaction, and thus the state of the dispersions, are the effective surface charge density of the particles #sigma#_e, particle volume fraction #phi#, and salt concentration, C_s. We have determined a phase diagram of the crystallization as a function of these three parameters. Aqueous dispersions of colloidal silica (SiO_2) particles have been used, whose surfaces were covered by weakly acidic silanol groups (Si-OH). The #sigma#_e value could be continuously tuned by varying the degree of dissociation of the silanols by adding a strong base, such as NaOH (2,3).
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