Interfacial displacement of nanoparticles by surfactant molecules in emulsions

Abstract

The remarkable stability of nanoparticles attached to oil–water interfaces in macroemulsions hinders controlled detachment of these particles from emulsions. In this work it is shown that adding surfactant molecules which preferentially adsorb at the oil–water interface displaces nanoparticles from the interface. Surfactant adsorption at the oil–water interface is energetically favoured and readily occurs on mixing nanoparticle-stabilised oil-in-water emulsions with surfactant solutions. Depending on the surfactant concentration, there is a significant reduction in the interfacial tension. Hence there is substantial fragmentation of the oil droplets and foaming of the emulsion during mixing. Surfactant concentrations above the critical micelle concentration are required to achieve complete interfacial displacement and hence recovery of the nanoparticles from the emulsions. The effects of surfactant addition have important implications for tailoring the interfacial composition of emulsions.

Introduction

Nanoparticles attach to the surfaces of drops and bubbles and impart remarkable stability to the interface [1]. Nanoparticles are trapped at fluid interfaces, providing there are attractive interactions between the particles and drops [2]. Particle wettability dictates the equilibrium position of the particles at liquid interfaces [3] and hence the interfacial curvature [4]. The trapped particles form compact networks, due to strong lateral attractions, that make the drop or bubble surfaces highly rigid and resistant to coalescence [5]. The presence of the rigid interfacial layer can significantly slow interfacial transfer of molecular species [6]. The attached nanoparticle layer alters drop and bubble flow behaviour [7], [8].

Particle attachment to drop or bubble surfaces is a route for preparing hollow capsules [9] of controlled permeability and elasticity [10] that are biocompatible [11] and robust bicontinuous structures [12]. Particles stabilize metallic foams, lightweight materials with unique mechanical, thermal and electrical properties [13]. The selective attachment of hydrophobic particles to bubbles is used to separate valuable minerals from gangue in froth flotation [2]. The remarkable stability of particles attached to interfaces can be a problem. Particle-stabilised emulsions that form during the extraction of bitumen from oil sands reduce the volume of oil recovered and generate waste problems [14]. Strategies for recovering bioparticles selectively separated at liquid–liquid interfaces [15] may be limited by the stability of the particle–drop aggregates. The efficient use of particles for the encapsulation and removal of non-aqueous phase contaminants from soils [16] requires displacement of the particles from the interface so they can be recycled.

Displacing particles from interfaces in emulsions is a challenge. Yan and Masliyah [17] mixed solid-stabilized oil-in-water emulsions with large volumes of oil to induce macroscopic separation of the oil and water. Fujii et al. [18] demonstrated that pH-responsive particles form emulsions where a variation in the pH can induce separation of the oil and water. Detaching particles trapped at liquid–liquid interfaces is, however, rarely achieved efficiently. Separation of particles in biphasic solutions is typically hindered by particles remaining attached to the interface even after separation of the two liquid phases [19]. Particle coatings on water (or oil) drops in oil (water) buckle as the drops are deflated and remain attached rather than dispersing into the liquid [20]. We show here that mixing emulsions of nanoparticle-stabilised drops with surfactant which competitively adsorbs at the oil–water interface causes interfacial displacement of the nanoparticles.

Typically surfactant molecules are mixed with nanoparticles in emulsions to enhance particle attachment [21], [22], [23]. Surfactant adsorption onto the particle surfaces alters the interactions between drops and nanoparticles. Tambe and Sharma [24] and Schulman and Leja [25] first linked the influence of surfactant adsorption on the oil–water contact angle of the particles to the type of emulsion formed. Lucassen-Reynders and van den Tempel [26] and Hassander et al. [27] linked improvements in emulsion stability (which depends on the extent of particle attachment) to changes in the extent of particle flocculation. Ravera et al. [28] found that surfactant adsorption affects the dynamics of particle transport to the interface. Wang et al. [29] observed that at very high surfactant concentrations, surfactant molecules can dominate emulsion formation and particle attachment does not occur.

In contrast, we show how surfactant molecules mixed with nanoparticle-stabilised emulsions cause particle detachment. This alternative way of mixing surfactant molecules and particles favours competitive adsorption of the surfactant molecules at the oil–water interface. The macroscopic and microscopic changes to emulsion structure caused by mixing particle-stabilised emulsions with surfactant solutions reveal the extent to which the surfactant molecules adsorb at, and displace particles from, the interface. The novel outcome is a method for recovering particles strongly attached to liquid interfaces.