Tuning Colloid-Interface Interactions by Salt Partitioning

Abstract

We show that the interaction of an oil-dispersed colloidal particle with an oil-water interface is highly tunable from attractive to repulsive, either by varying the sign of the colloidal charge via charge regulation, or by varying the difference in hydrophilicity between the dissolved cations and anions. In addition, we investigate the yet unexplored interplay between the self-regulated colloidal surface charge distribution with the planar double layer across the oil-water interface and the spherical one around the colloid. Our findings explain recent experiments and have direct relevance for tunable Pickering emulsions.PACS numbers: 82.70. Kj, 68.05.Gh Colloidal particles experience a deep potential well when they intersect fluid-fluid interfaces. They therefore adsorb strongly to such interfaces, self-assembling into structures such as two-dimensional monolayers [1] or particle-laden droplets in Pickering emulsions [2,3]. The free energy gain caused by the reduction of the fluid-fluid surface area is γπa 2 (1 + cos θ) 2 10 3 − 10 7 k B T , where γ is the fluid-fluid surface tension, the particle radius a is typically between 10 − 10 3 nm, θ is the three-phase contact angle and k B T is the thermal energy [1]. With the exception of nanoparticles [4,5], the binding is essentially irreversible and hardly prone to physicochemical modifications such as the pH or salt concentration. Only strong mechanical agitation is able to detach micron-sized particles from the interface [6]. However, the recovery of particles from fluid-fluid interfaces is an essential step for the realization of applications, such as in biofuel upgrade [7], "dry water" catalysis [8] and gas storage [9].An alternative route to overcome the difficulties associated with strong adsorption was offered in Refs.[3, 10], which show that charged poly(methylmethacrylate) (PMMA) particles that stabilize water-cyclochexylbromide (CHB) Pickering emulsions are (almost) non-wetting (cos θ → −1), such that the colloidal particles reside essentially in the oil phase. The crucial ingredient of this system is the relatively "polar" oil which solvates a small but significant amount of charge that stabilizes the colloids [3,11]. Within a modified Poisson-Boltzmann theory, qualitative agreement was found with the experimental out-of-plane structure of the particles, provided a small degree of wetting was assumed [10,12].Very recent experiments on the same system by Elbers et al. [13] revealed that the colloidal particles in fact do not penetrate the water-CHB interface, but are trapped at a finite ∼ nanometer distance [13,14], completely circumventing the irreversible wetting effects de- * j.c.everts@uu.nl Figure 1. (a) Geometry of a colloidal sphere with radius a = 1 µm and a dielectric constant c = 2.6 in cylindrical coordinates (r, z) at a distance d from an oil-water interface, with the position on the particle surface indicated by the polar angle ϑ. The oil and water are characterized by dielectric constants o = 7.92 and w = 80, respectively. The self-ener...

Electric Double Layer Composed of an Antagonistic Salt in an Aqueous Mixture: Local Charge Separation and Surface Phase Transition

“…(2), so we set v 1 /v 0 = 0.5 for small cations [1, 36,37] and v 2 /v 0 = 5 for large anions [18]. The bulk free energy density is given by [4][5][6][7][8][9][10][11][12][13][14]…”

Colloid–oil–water-interface interactions in the presence of multiple salts: charge regulation and dynamics

“…In addition to this force balance, we have recently shown in Ref. [27] that the dissolved ions play an important role in the emulsion stability. In addition to the usual screening and charge regulation, ions can redistribute among the oil and water phase according to their solvability and hence generate a charged oil-water interface that consists of a back-to-back electric double arXiv:1703.08892v2 [cond-mat.soft] 7 Jun 2017 layer.…”

“…Within a single-particle picture, this ion partitioning can be shown to modify the interaction between the colloidal particle and the oil-water interface. For a non-touching colloidal particle, the interaction is tunable from attractive to repulsive for large enough separations, by changing the sign of the product Zφ D [27], where Ze is the particle charge and k B T φ D /e the Donnan potential between oil and water due to ion partitioning, with e the elementary charge. The tunability of colloid-ion forces is a central theme of this work, in which we will explore how the quantities Z and φ D can be rationally tuned.…”

“…This motivates us to extend the formalism of Ref. [27] by including at least three ionic species which are all known to be present in the experimental system of interest that we will discuss in this paper. Including a second salt compound with an ionic species common to the two salts, allows us to independently vary the ionic strength and the particle charge.…”

Charge regulation with fixed and mobile charged macromolecules

“…The Poisson-Boltzmann theory with CR surfaces has been studied in the past for uniform charge distributions of dissociable groups, in contact with an electrolyte solution [9,33,34]. Other studies involved modeling of a single CR colloid in solution, in the proximity of another charged surface [35]. Most of previous calculations employed linearized CR boundary conditions or a linearized version of the PB equation itself (known as the Debye-Hückel limit) [36][37][38][39].…”