Reappearance of structure in colloidal suspensions

Abstract

Abstract. -Static structure factors S(q) of deionized aqueous suspensions of charged polystyrene particles with similar radii but strongly differing bare charges have been measured for volume fractions 3.5 × 10 −4 ≤ ϕ ≤ 1.55 × 10 −2 , using a cross-correlation light scattering technique which allows for the extraction of single scattered light from highly turbid samples. Measurement of absolute intensities allowed to determine unambiguously normalized values of S(q). With decreasing volume fraction, the amplitude of the first peak of S(q) reflecting the order of the suspension does not monotonically decrease, but rather shows a distinct minimum at ϕ ∼ (5 − 8) × 10 −3 . This reappearance of structure is compared with theoretical predictions on the basis of a jellium model for the effective interparticle potential U eff (r).The forces governing the structure and phase behavior of suspensions of charged colloidal particles are of prime importance for many technological applications, such as ceramic processing, drug delivery or the stability control of paints. In the standard picture of Derjaguin, Landau, Verwey, and Overbeek (DLVO) [1], the electrostatic interaction between two charged colloidal particles in an electrolyte arises from the repulsion of the electric double layers that form due to the competition between configurational entropy of the small ions and their potential energy in the Coulomb field of the macroions with negative charge −Ze 0 , e 0 being the elementary charge. When the bare charge Z is large, a part of the surrounding counterions is strongly bound to the particle and thus reduces the bare charge to an effective chargeZ < Z which then governs the ion distribution at distances comparable to the Debye screening length κ −1 [2]. In contrast to the situation in simple liquids, however, the strength of the effective interparticle potential U eff (r) determined by the effective chargeZ and its range κ −1 does not only depend on the bare charge Z but also on the concentrations of co-and counterions and the particle volume fraction ϕ [3]. This density dependence of the effective potential is reflected by the fact that even at volume fractions as low as 10 −4 strongly charged particles show considerable order, as reflected by the strong first peak in the colloid-colloid structure factor S(q) measured in light scattering experiments from deionized suspensions [4,5]. When, at higher volume fractions, the number of counterions balancing the particle charge becomes larger than the number of excess salt ions, the screening will become dominated by the former,

Effective charge saturation in colloidal suspensions

“…The competition between these two effects might be a key point in the understanding of these systems. It appears therefore interesting to reconsider the phase stability of macro-ion suspension in light of these results (see also [68] and more recently [69]). …”

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

“…13 In addition, when the counterions form a significant fraction of the ionic strength, the colloid-colloid interactions depend strongly on the volume fraction, decreasing the volume fraction reduces the ionic strength, increases the Debye length and can enhance the structure at low densities. 14 Moreover, when the range of the interaction exceeds the mean interparticle separation, the assumption of pairwise addivity becomes questionable. 15,16 Effective attractions due to nonpairwise additivity 17,18 may help explain a range of recent experimental observations such as superheated crystals, 19 and "liquid-gas" phase separation, 20 which are not expected for purely repulsive interactions.…”

Pair structure of the hard-sphere Yukawa fluid: An improved analytic method versus simulations, Rogers-Young scheme, and experiment

“…In addition to atomic systems, the HSY model has found its most widespread application in the study of the equiliba) Electronic mail: m.heinen@fz-juelich.de. rium structure, dynamics and phase behavior of macromolecular systems, including suspensions of large charge-stabilized colloidal spheres, [6][7][8][9][10][11][12][13][14][15][16][17] and solutions of globular charged proteins, 18,19 micelles 20, 21 and short DNA fragments. 22 In the standard Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal charge stabilization, 23 the screened Coulomb repulsion of two charged colloidal particles (macroions) caused by the overlap of their diffuse microionic clouds, is represented by Eq.…”