The superior stability of Pickering emulsions can be detrimental in several applications. There is therefore growing interest in using stimuli responsive particle emulsifiers that can considerably reduce the cost of demulsification. Among the possible triggers for the remote control of emulsion stability, pH sensitive systems are advantageous because of the simplicity and ease of implementation. Herein, the destabilization mechanism of pH-switchable emulsions stabilized solely by particles of different shapes and surface properties-namely-hematite, silica and polystyrene-are discussed. The surface activity of all these particles can be tuned by controlling the surface charge density via dispersing them in aqueous solutions of different pH. Hematite particles of cuboidal, spherocylindrical, peanut and ellipsoidal shapes stabilized O/W emulsions at pH 6.5, but they were completely destabilized when the pH of the continuous phase was adjusted to either 2 or 12. Similarly, the O/W emulsions stabilized by silica rods and spheres at pH 2 were destabilized upon adjusting the pH to 6.5 and 12. The detachment of particles from the droplet surfaces resulted in partially covered drops that coalesced, leading to the destabilization of emulsions as confirmed by direct visualization via optical microscopy. The destabilization mechanism of the Pickering emulsions stabilized solely by pH responsive particles was observed to be general, and was mediated by the detachment of particles because of the pH induced wettability change. Furthermore, the pH responsive behaviour was found to be reversible. Since stable emulsions are formed in several particle-water-oil systems, the possibility of conveniently changing the pH of the continuous phase can be exploited to tune the particle wettability and thus the destabilization in applications that demand controlled demulsification. © 2017 the Owner Societies.

Madivala Gurapa Basavaraja

Department Of Chemical Engineering

Thriveni G. Anjali

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Physical Chemistry Chemical Physics

Research in the 1980s demonstrated the formation of monolayers of particles achieved by interfacial particle trapping as a model system for investigating colloids in two dimensions. Since then, microscopy visualization of two-dimensional particle monolayers and quantification of the microstructure have led to significant fundamental understanding of a number of phenomena such as crystallization, freezing and melting transitions, dislocation dynamics, aggregation kinetics, and others. On the application front, particles at curved interfaces, as often the case in particle-stabilized emulsions and foams, have received considerable attention in the last few decades. The growing interest in the search for novel particles and new strategies to effect emulsion stabilization stems from their application in several disciplines. Moreover, particle-stabilized Pickering emulsions and foams can also be used to derive a number of advanced functional materials. Compared to several accounts of research on spherical colloids at fluid-fluid interfaces, investigations of the behavior of shape-anisotropic particles at interfaces, albeit receiving considerable attention in recent years, are still in a nascent stage. The objective of this feature article is to highlight our recent work in this area. In particular, the adsorption of shape-anisotropic particles to interfaces, wetting behavior, interfacial self-assembly, the response of nonspherical-particle-coated interfaces to compression and shear, and their ability to stabilize emulsions are discussed. © 2018 American Chemical Society.

Langmuir

Shape-Anisotropic Colloids at Interfaces

Solid-stabilized emulsions commonly known as Pickering emulsions offer unique benefits such as superior stability and controlled permeability compared to conventional surfactant stabilized emulsions. In this article, the effect of pH, the electrolyte and particle concentration, homogenization speed, and volume fraction of oil on the formation, stability, and the microstructure of emulsion droplets stabilized by micron-size peanut-shaped hematite particles are investigated. The influence of surface charge of particles on emulsification is studied by varying the pH of the dispersing medium, the addition of an electrolyte or a combination of both. Stable O/W emulsions are formed only when the aqueous dispersions at intermediate pH between 4 and 11, and decane (2:1 volume ratio) are vigorously mixed. However, emulsions are not formed when the particles are highly charged that is, at pH 2 and 12. The presence of monovalent salt or high-speed homogenization assists the emulsion formation at pH 3, whereas their combination helps in emulsification at pH 2. However, neither the addition of an electrolyte nor the high-speed homogenization or their combination facilitates the formation of emulsions at pH 12. We show that the image-charge repulsion and the surface charge induced wettability change can explain the influence of both pH and salt concentrations on the formation of Pickering emulsions. Although oil-in-water emulsions typically cream because of the density difference, microscopy observations revealed the presence of a large number of small particle-covered oil droplets in the sediments of the emulsified samples. These drops are observed to be entrapped in dense-particle networks. This leads to a considerable reduction in the number of particles available for the stabilization of floating emulsion droplets and thus influences their size and surface coverage. The possibility of tailoring the stability, droplet size and, the surface coverage discussed in this article can play a crucial role in situations that demand controlled release of active components. © 2018 American Chemical Society.

Influence of pH and Salt Concentration on Pickering Emulsions Stabilized by Colloidal Peanuts

The three phase contact angle of particles, a measure of its wettability, is an important factor that greatly influences their behaviour at interfaces. It is one of the principal design parameters for potential applications of particles as emulsion/foam stabilizers, functional coatings and other novel materials. In the present work, the effect of size, shape and surface chemistry of particles on their contact angle is investigated using the gel trapping technique, which facilitates the direct visualization of the equilibrium position of particles at interfaces. The contact angle of hematite particles of spherocylindrical, peanut and cuboidal shapes, hematite-silica core-shell and silica shells is reported at a single particle level. The spherocylindrical and peanut shaped particles are always positioned with their major axis parallel to the interface. However, for cuboidal particles at air-water as well as decane-water interfaces, different orientations namely - face-up, edge-up and the vertex-up - are observed. The influence of gravity on the equilibrium position of the colloidal particles at the interface is studied using the hematite-silica core-shell particles and the silica shells. The measured contact angle values are utilized in the calculations of the detachment and surface energies of the hematite particles adsorbed at the interface. © 2016 Elsevier Inc.

Journal of Colloid and Interface Science

Contact angle and detachment energy of shape anisotropic particles at fluid-fluid interfaces

In this contribution, we discuss the role of surface charge on the adsorption of shape anisotropic particles to fluid-fluid interfaces in the context of their application in particle-stabilized emulsions. Starting with a pendent aqueous drop containing nano-ellipsoids of known surface charge density suspended in an oil medium, we study the kinetics of adsorption of the ellipsoids to the water-decane interface using pendant drop tensiometry. The interfacial tension of the drop is recorded as a function of time by analyzing the shape of the drop. We show that the particles that are weakly charged readily adsorb to the water-decane interface and the adsorption behavior is influenced by the particle surface charge density. Furthermore, as the area available for the particles deposited at the interface is reduced, the interface populated with self-assembled ellipsoids shows wrinkles indicating buckling of the particle-laden interface under compression. However, the buckling is not observed if nano-ellipsoids are highly charged confirming that the particles do not adsorb to the interface when they are highly charged. This suggests that in several examples where the particles at interfaces concept is exploited, the repulsive energy barrier due to the particle surface charge plays a key role in the adsorption of particles to the interfaces. However, once the particles are adsorbed, the interfacial properties of the monolayer depend on the particle-particle interactions. Thus a combination of these interactions determines the concentration of particles at the interface, their microstructure and interfacial properties. The effect of these interactions on the quantity and size of the emulsion drops stabilized by ellipsoidal particles is also explored. © The Royal Society of Chemistry 2016.

Faraday Discussions

Nano ellipsoids at the fluid-fluid interface: Effect of surface charge on adsorption, buckling and emulsification

Liquid drops containing insoluble solutes when dried on solid substrates leave distinct ring-like deposits at the periphery or along the three-phase contact line-a phenomena popularly known as the coffee-ring or the coffee stain effect. The formation of such rings as well as their suppression is shown to have applications in particle separation and disease diagnostics. We present an experimental study of the evaporation of sessile drops containing silica rods to elucidate the structural arrangement of particles in the ring, an effect of the addition of surfactant and salt. To this end, the evaporation of aqueous sessile drops containing model rod-like silica particles of aspect ratio ranging from ∼4 to 15 on a glass slide is studied. We first show that when the conditions such as (1) solvent evaporation, (2) nonzero contact angle, (3) contact line pinning, (4) no surface tension gradient driven flow, and (5) repulsive particle-particle/particle-substrate interactions, that are necessary for the formation of the coffee-ring are met, the suspension drops containing silica rods upon evaporation leave a ring-like deposit. A closer examination of the ring deposits reveals that several layers of silica rods close to the edge of the drop are ordered such that the major axis of the rods are oriented parallel to the contact line. After the first few layers of ordered arrangement of particles, a random arrangement of particles in the drop interior is observed indicating an order-disorder transition in the ring. We monitor the evolution of the ring width and particle velocity during evaporation to elucidate the mechanism of the order-disorder transition. Moreover, when the evaporation rate is lowered, the ordering of silica rods is observed to extend over large areas. We demonstrate that the nature of the deposit can be tuned by the addition of a small quantity of surfactant or salt. © 2014 American Chemical Society.

Journal of Physical Chemistry B

Evaporation of sessile drops containing colloidal rods: Coffee-ring and order-disorder transition

A control over the nature of deposit pattern obtained after the evaporation of solvent from a sessile drop containing dispersed materials has been demonstrated to have applications in materials engineering, separation technology, printing technology, manufacture of printed circuit boards, biology, and agriculture. In this article, we report an experimental investigation of the effect of particle shape and DLVO (Derjaguin-Landau-Verwey-Overbeek) interactions on evaporation-driven pattern formation in sessile drops. The use of a model system containing monodisperse particles where particle aspect ratio and surface charge can be adjusted reveals that a control over the nature of deposit pattern can be achieved by tuning the particle-particle and particle-substrate interactions. A clear coffee-ring formation is observed when the strength of particle-particle repulsion is higher than the particle-substrate attraction. However, complete suppression of ringlike deposits leading to a uniform film is achieved when particle-substrate and particle-particle interactions are attractive. Results illustrate that for the system of submicron ellipsoids that are hydrophilic, the nature of deposit patterns obtained after evaporation depends on the nature of interactions and not on particle shape. © 2014 American Chemical Society.

Journal	Data powered by TypesetPhysical Chemistry Chemical Physics
Publisher	Data powered by TypesetRoyal Society of Chemistry
ISSN	14639076
Open Access	No