Re-entrant melting and freezing in a model system of charged colloids

Abstract

We studied the phase behavior of charged and sterically stabilized colloids using confocal microscopy in a low polarity solvent ͑dielectric constant 5.4͒. Upon increasing the colloid volume fraction we found a transition from a fluid to a body centered cubic crystal at 0.0415Ϯ0.0005, followed by reentrant melting at 0.1165Ϯ0.0015. A second crystal of different symmetry, random hexagonal close packed, was formed at a volume fraction around 0.5, similar to that of hard spheres. We attribute the intriguing phase behavior to the particle interactions that depend strongly on volume fraction, mainly due to the changes in the colloid charge. In this low polarity system the colloids acquire charge through ion adsorption. The low ionic strength leads to fewer ions per colloid at elevated volume fractions and consequently a density-dependent colloid charge.

Crystallization and reentrant melting of charged colloids in nonpolar solvents

“…Instead, x-ray scattering studies [23] showed a fascinating phase behavior of Wigner crystals, including a body-centered-cubic (bcc) phase at low concentration, and a solid-solid transition to a face-centered-cubic (fcc) phase at higher densities [15][16][17][18]24,25]. It is also possible to induce charge on particles in nonaqueous solvents through the addition of charge control agents [21,[26][27][28]. However, in this case, there is strong coupling between the charge on the particle surface and the ions in solution.…”

“…Typically, the colloids used for these studies are stericallystabilized polymeric particles in nonaqueous solvents, which can match both the density ρ and refractive index n of the particles, enabling confocal microscopy to be used for these investigations. Even earlier studies focused on charged particles, where crystallization is driven by strong long-range repulsive interactions arising from Coulombic charges on the particles [1,[10][11][12][13][14][15][16][17][18][19][20][21][22]. These studies were performed on particles in aqueous solvents, which makes charge effects much easier to induce, but precludes index matching, limiting the use of optical techniques except at very low densities.…”

Freezing lines of colloidal Yukawa spheres. I. A Rogers-Young integral equation study

“…In the work by Royall et al, 6 a similar reentrant fluidbcc−fluid transition has been observed experimentally for charged PMMA spheres in a solvent mixture as the volume fraction was increased. The unusual part in this sequence is the reentrant melting at a larger φ. Royall et al explain this sequence essentially by the strong φ dependence of the effective colloid charge which, in the considered concentration range, decreases with an increasing φ.…”

“…Well-studied experimental systems which fall into this category are (sulfonate) polystyrene latex spheres in water 8,9 or in an ethanol-water mixture, 3,4 silica and polymethylacrylate (PMMA) spheres in an organic solvent (mixture), 6,10,11 and to some extent also charged globular proteins in water such as apoferritin 12,13 or bovine serum albumin (BSA). 14,15 The pair potential in Eq.…”

“…A wide class of industrially and biologically relevant dispersions of charge-stabilized colloidal particles of globular shape [1][2][3][4][5][6] can be described by a repulsive Yukawa-type effective pair energy of the form 7 βu(r) = L B Z 2 e κa 1 + κa 2 e −κr r , r > σ = 2a (1) in combination with an excluded-volume interaction characterized by the hard-sphere diameter σ . Here, β = 1/(k B T) with Boltzmann constant k B and temperature T, r is the centerto-center separation of two spheres, L B = e 2 /(ε k B T) is the Bjerrum length of the suspending fluid of dielectric constant ε, and Z is the effective number of elementary charges, e, on a colloidal particle.…”