Particle-Stabilized Emulsions and Colloids: Formation and Applications: Volume 3 (Soft Matter Series) - Hardcover

Hardcover

ISBN 10: 1849738815 ISBN 13: 9781849738811

Publisher: Royal Society of Chemistry, 2014

Synopsis

Particle-Stabilized Emulsions and Colloids looks at recent studies on the behaviour of particles at liquid interfaces. With contributions from leading experts in these fields, this book will provide a background to academic researchers, engineers, and graduate students in chemistry, physics and materials science.

About this title

About the Authors

To Ngai is an associate professor in Chemistry department at the Chinese University of Hong Kong (CUHK). In recent years, he has mainly focused on the study of synthetic strategies for hydrogel colloidal particles, as well as their physicochemical properties, such as their behavior as dispersions and their assembly at interfaces. In particular, he has been interested in using microgel particles to stabilize emulsions and has successfully demonstrated that the soft and porous microgels give rise to special dynamic properties of interfacial layers, packing structure and emulsion stability. As a result, he has published a number of papers on this topic and extended such microgel-stabilized emulsions for the fabrication of novel materials.

Stefan A. F. Bon is an associate professor in the Department of Chemistry at the University of Warwick. He has studied chemical engineering at the Eindhoven University of Technology (TUe) in the Netherlands (cum laude, 1989-1993). He did his Ph.D. (1993-1998) in the polymer chemistry group of prof.dr.ir. Anton L. German at TUe, after which he worked as a post-doctoral research assistant in the group of prof. David M. Haddleton at the University of Warwick (1998-2000). He was appointed as Unilever Lecturer in Polymer Chemistry at the University of Warwick in January 2001. During this period of research he focussed on the mechanistic aspects of living radical polymerisation in both homogeneous and heterogeneous systems, including the first ever living radical polymerization performed in emulsion. From 2005 Stefan Bon shifted his research interests from living radical polymerization to supracolloidal polymer chemistry. Current research focusses on the design of assembled supracolloidal structures and the synthesis of their colloidal and macromolecular building blocks through combination of polymer chemistry, colloid science, soft matter physics, and chemical engineering. Stefan Bon is a Fellow of the International Union of Pure and Applied Chemistry (IUPAC), a board member of the UK Polymer Colloids Forum (UKPCF), and an invited member and current vice-chair of the International Polymer Colloids Group (IPCG).

From the Back Cover

There has been much scientific interest in the behaviour of colloidal particles at liquid interfaces. From a research aspect they provide model systems for fundamental studies of condensed matter physics. From a commercial aspect they provide applications for making new materials in the cosmetics, food and paint industries.

In many cases of colloidal particles at interfaces, the mechanism of particle interactions is still unknown. Particle-Stabilized Emulsions and Colloids looks at recent studies on the behaviour of particles at liquid interfaces. The book first introduces the basic concepts and principles of colloidal particles at liquid-liquid interfaces including the interactions and conformations. The book then discusses the latest advances in emulsions and bicontinuous emulsions stabilized by both solid and soft particles and finally the book covers applications in food science and oil extraction.

With contributions from leading experts in these fields, this book will provide a background to academic researchers, engineers, and graduate students in chemistry, physics and material science. The commercial aspects will also be of interest to those working in the cosmetics, food and oil industry.

Excerpt. © Reprinted by permission. All rights reserved.

Particle-Stabilized Emulsions and Colloids

Formation and Applications

By To Ngai, Stefan A. F. Bon

The Royal Society of Chemistry

Copyright © 2015 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-881-1

Contents

Chapter 1 The Phenomenon of Pickering Stabilization: A Basic Introduction Stefan A. F. Bon, 1, 
Chapter 2 Interactions and Conformations of Particles at Fluid-Fluid Interfaces Bum Jun Park, Daeyeon Lee and Eric M. Furst, 8, 
Chapter 3 Polymer Colloidal Particles Prepared by Pickering Emulsion Polymerization or Self-Assembly Method Hanying Zhao and Jia Tian, 45, 
Chapter 4 Pickering Suspension, Mini-Emulsion and Emulsion Polymerization Stefan A. F. Bon, 65, 
Chapter 5 Emulsions Stabilized by Soft Microgel Particles Zifu Li and To Ngai, 93, 
Chapter 6 Bicontinuous Emulsions Stabilized by Colloidal Particles Joe W. Tavacoli, Job H. J. Thijssen and Paul S. Clegg, 129, 
Chapter 7 Complex Pickering Emulsions Yoshimune Nonomura, 169, 
Chapter 8 Multiple Pickering Emulsions for Functional Materials Yu Yang, Yin Ning, Zhen Tong and Chaoyang Wang, 180, 
Chapter 9 Particle-Stabilized Emulsions as Templates for Hollow Spheres and Microcapsules Simon Biggs and Olivier Cayre, 228, 
Chapter 10 Particle-Stabilized Food Emulsions R. Pichot, L. Duffus, I. Zafeiri, F. Spyropoulos and I. T. Norton, 247, 
Chapter 11 Particle-Stabilized Emulsions in Heavy Oil Processing David Harbottle, Chen Liang, Nayef El-Thaher, Qingxia Liu, Jacob Masliyah and Zhenghe Xu, 283, 
Subject Index, 317, 

CHAPTER 1

The Phenomenon of Pickering Stabilization: A Basic Introduction

STEFAN A. F. BON

1.1 A Brief Historic Perspective on Pickering Stabilization

The ability of solid particles to adhere to soft deformable interfaces, for example to the surface of emulsion droplets or bubbles, is currently the subject of renewed interest in material science. The phenomenon that solid particles can reside at the interface of droplets and bubbles, thereby providing them with resistance against coalescence or fusion, and (debatable) coarsening or Ostwald ripening, is known as Pickering stabilization and named after Spencer Umfreville Pickering. Food science and flotation technology show a steady stream of research over the 20th century using Pickering stabilization in, for example, table spread/margarine formulations, where fat crystals sit on the surface of water droplets dispersed into the oil matrix. Interestingly, the origins of Pickering stabilization in the area of (froth) flotation lie further back than the cited works by Pickering (1907) and Ramsden (1903). Patents by William Haynes (1860) and the Bessel brothers (1877) clearly reported the phenomenon, the latter patent interestingly illustrating the concept with graphite flakes attached to bubbles. In the area of polymer chemistry the idea of using solid particles as stabilizers for the fabrication of polymer beads by suspension polymerization was explored to some extent from the 1930s to the 1950s. A revival of the concept of using solid particles as stabilizers in heterogeneous polymerizations did not emerge until 50 years later with the development of Pickering mini-emulsion polymerization and Pickering emulsion polymerization. The idea of using Pickering stabilization as a way of assembling colloidal particles into intricate supracolloidal structures drew attention from the soft matter physics crowd initiated by the works of Velev et al. and Dinsmore and coworkers, the latter coining the term 'colloidosomes' for the semi-permeable hollow structures made by assembly of particles onto droplets. Not only does the fabrication of supracolloidal structures receive great attention, but also the underlying physics is studied and discussed widely, for example looking at why particles adhere to a liquid–liquid interface, how strong the interaction energy is, and what the interplay between particles at the droplet surface is.

1.2 A Basic Physical Understanding of Pickering Stabilization

This short introductory chapter does not aim to provide a thorough literature review of the underlying physics of Pickering stabilization, but merely to give the reader a basic understanding. The question of why a particle would prefer to sit at the interface of an emulsion droplet instead of being dispersed in either the water or oil phase has already been raised and discussed by, for example, Hildebrand and coworkers in 1923. They said that for solid particles to adhere to and be collected at the surface of emulsion droplets, the powder had to be wetted by both liquids. They stated that in general particles have a preference for one of the two liquids, which meant that the particles would reside for longer in that liquid. They described how the assembly of particles onto the oil–water interface will cause the interface to bend in the direction of the more poorly wetting liquid, thereby facilitating its emulsification into droplets. They concluded that the type of emulsion, i.e. oil-in-water or water-in-oil, could be predicted on the basis of this wettability, and thus on the basis of the contact angle of the interface with the solid.

To describe the behaviour of a single particle at the liquid–liquid interface we often see the following expression for the adhesion energy as a function of the contact angle:

ΔE = πR2σ12(1 [+ or -] cosθ12)2

Care must be taken to define the right contact angle and whether to use a plus or minus sign within the brackets to calculate the escape energy needed to remove the particle from the interface and place it into either phase 1 or 2. This can lead to confusion. An approach that circumvents this issue is to calculate the energy well completely, as reported by Pieranski in 1980. In his work, he studied the adhesion of polystyrene spheres at the air–water interface.

Imagine a thermodynamic type of experiment as shown in Figure 1.1. We take a perfectly smooth spherical particle that we disperse in a liquid phase, which we call phase 1. We ignore all dynamics (kinetics) and external force fields, such as gravitational, electrical, optical and magnetic. We also do not consider any surface roughness of the particle, and we ignore any ionic (Coulombic) interactions, dielectric effects and thus van der Waals interactions. The question we pose is what would happen to the free energy if we move the particle, which is dispersed in phase 1, all the way to phase 2, and thus through the interface?

To answer this question we need to take into account the interfacial energies upon placing the particle at various heights, z. These are the energy between the particle and phase 1, EP1, the energy between the particle and phase 2, EP2, and the energy between phase 1 and phase 2, E12. For this we need to know the interfacial tensions between the particle and phase 1, σP1, the particle and phase 2, σP2, and the interfacial tension between phase 1 and phase 2, σP1, and multiply these by the respective contact areas.

z0 = z/R; ST = 4πR2; AT = πR2

EP1 = σP1 ST (1 + z0)/2

EP2 = σP2 ST (1 - z)/2

E12 = -σ12 AT (1 - z02)

The sum of the three energies and its division by kBT leads to the following quadratic expression for the energy well (see also Figure 1.2):

E0 = EP1 + EP2 + E12/kBT = [πR2σ12/kBT] (z02 + 2(σP1 - σP2)/σ12z0 + 2(σP1 + σP2/σ12 - 1)

The equilibrium position of the particle can be easily found from dE0/dz0 = 0, which yields:

zmin0 = σP2 - σP1/σ12

For values of zmin0 between -1 and 1 the particle adheres to the liquid–liquid interface. The energy it will take to remove the particle from the interface into either the bulk of phase 1 or phase 2 can easily be obtained from:

δE1 = E0(z0 = 1) E0(zmin0)

δE2 = E0(z0 = 1) E0(zmin0)

An example for a polystyrene sphere of radius 100 nm at the water–hexadecane interface (water = phase 1) using σP1 = 32 mN m-1, σP21 = 14.6 mN m-1 and σ12 = 53.5 mN m-1 yields values needed for the particle to escape the well and enter the water and oil phase of 7.2 x 105 kBT and 1.9 x 105 kBT, respectively.

This continuous model gives a good first approximation for the magnitude of the energy well in which the particles are trapped at the interface, and an idea of what energy is needed for a particle to escape from the interface into one of the two liquid phases. It is, however, a rather crude estimation of reality, not least because of the number of assumptions made under which the model is valid (remember no gravity, no charges, no dynamics, etc.), and because we assume that the particle is a smooth perfect sphere.

In addition to these restrictions the careful observer can also see from Figure 1.1 that in this 2D picture we have overlooked two points at which three phases are in direct contact with each other. This three-phase interaction in 3D is a contact line in the form of a circle between the two liquids and the particle. Gibbs already pointed out that qualitatively this three-phase contact line could be seen as a one-dimensional equivalent of the surface tension (for the latter multiplying with contact area gives interfacial energy), and is referred to as line tension, τ (multiplying by the circumference of the circle gives energy). Aveyard and Clint included line tension in the Pieranski model by providing this extra term:

Eline = 2πRτ/kBT [square root of (term)] (1 - z20

1.2.1 Pickering Stabilizers of Nanoscale Dimensions

Line tension becomes important for smaller spherical particles as it scales linearly with the radius of the particle, whereas surface tension scales quadratically. The experimental difficulty that remains is to determine qualitative values for line tension. Questions that arise when we shrink our particle to nanoscale dimensions are: can we still assume that the liquid–liquid interface is flat and can we still assume continuous wetting? Cheung and Bon carried out molecular simulations and showed that these assumptions break down for small particles and that capillary waves on the liquid–liquid interface and discrete wetting of the particle by individual molecules needs to be taken into account. The non-flat nature of the liquid–liquid interface widens the interaction distance and thus broadens the energy well in comparison the Pieranski model, whereas the combined effects of line tension with discrete wetting led to deeper energy wells, with deviations of up to 50% in adhesion energy, showing that nanoparticles stick considerably better to interfaces than can be predicted on the basis of the Pieranski model.

1.2.2 Pickering Stabilizers with a Rough Surface

When we add roughness to a spherical Pickering stabilizer we can see that the total contact area between the particle and the liquid phase(s) increases dramatically, whereas the 'circular' area that get taken away by placing the particle at the liquid–liquid interface remains relatively unchanged (see it as noise over a circle that cancels out). In systems where we ignore the effects of line tension we can see that the energy levels for dispersing the particle in phase 1 and 2 have considerably higher values. The net effect is that the energy needed for the particle to escape the interface drops considerably. This means that rough particles do not adhere strongly to the interface. This effect is nicely observed experimentally in work by Ballard and Bon for Lycopodium spores that were decorated in a mesh of interpenetrating polymer nanoparticles and used as patchy Pickering stabilizers. Theoretical comparison between smooth and rough particles (modelled with a buckyball type mesh) indeed led to the behaviour explained above (see Figure 1.3).

1.2.3 Non-Spherical Pickering Stabilizers

The take-home message is that non-spherical particles have the potential to adopt multiple orientations once adhered to a liquid–liquid interface. In order to understand and verify experimentally observed orientations it is important to calculate the entire energy landscape because, in addition to thermodynamic minima, kinetically trapped states (unexpected orientations) can occur. In our work we showed that an isolated microscopic haematite particle of super-ellipsoidal shape adopts three orientations when adhered to a hexadecane–water interface.

Two of the orientations, and estimates of their relative populations, can be assigned to two thermodynamic minima on the energy landscape. This was determined by both free-energy minimization and particle trajectory simulations. The third orientation was found to correspond to a kinetically trapped state, existing on certain particle trajectories in a region of a negligible gradient in free energy.

CHAPTER 2

Interactions and Conformations of Particles at Fluid-Fluid Interfaces

BUM JUN PARK, DAEYEON LEE AND ERIC M. FURST

2.1 Introduction

Colloidal particles will typically adsorb strongly and irreversibly to fluid–fluid interfaces. Adsorption enables colloids to stabilize interfaces, opening up new potential applications for conventional colloids as alternatives for expensive and potentially environmentally hazardous molecular amphiphiles (i.e. surfactants). An emulsion system in which solid particles are used as surface active additives is called a Pickering–Ramsden emulsion. The presence of solid particles leads to lowering of the interfacial tension between the immiscible fluid phases and imparts kinetic stability to the emulsion droplets, impeding their flocculation, coalescence and consequent creaming.

There are several advantages of studying two-dimensional (2D) colloidal systems. First, 2D colloids are excellent models for well-defined three-phase systems and enable one to evaluate particle behaviours at multi-phasic fluid interfaces. Second, the strong adsorption of colloids to 2D interfaces suppresses their out-of-plane movements to allow the direct visualization of microstructure and rheology. Third, the interactions between particles and the microstructures that follow (including two-dimensional crystals, gels and fluid-like phases) can be easily manipulated by the addition of additives, such as electrolytes and surfactants. This in turn alters the particle wettability, the interfacial tension and the rheological properties of the interface. This chapter aims to describe and understand the interactions, configurations, microstructures and micromechanics of various types of colloidal particles confined at fluid–fluid interfaces. This chapter also connects small-scale measurements between individual particles to their macroscopic phase behaviour and rheological properties.

2.2 Theoretical Background

2.2.1 Particle Wettability and Attachment Energy

2.2.1.1 Homogeneous Particles

For a colloidal particle that is a few micrometres or less in diameter, its attachment energy to a fluid–fluid interface can be calculated by considering the surface area (S) exposed to each fluid phase, the volume (V) confined in each fluid, and the corresponding surface tension (γ). In this case, the interface remains flat because the Bond number (Bo = ρga2/γ where g is the gravitational acceleration, a is the particle radius, ρ is the density of fluid), which is the ratio of the gravitational force and the surface tension, is sufficiently small. For a particle that adsorbs to the oil–water interface from the aqueous phase, the attachment energy is given by the difference in the energy of the system before and after the adsorption (Figure 2.1)

δEIw = EI - Ew (2.1)

The first term on the right-hand side (EI) is the energy when the particle is located at the interface and can be expressed as:

EI = Swγsw + Soγso + SI(2) γow + VwPw + VoPo (2.2)

The second term (EW) indicates the energy when the particle is completely immersed in the water phase:

Ew = Stotγsw + SI(1) γow + VtotPw (2.3)

where the total surface area of the particle is Stot = Sw + So. Note that SI(2) and SI(1) are the surface area of the oil–water interface with and without the particle. The displaced area, which is the cross-sectional area displaced by the presence of the particle at the interface, is defined as SI = SI(1) - SI(2). Therefore, by combining Equation (2.1) with the Young's equation, γowcosθc = γgsw, where θc is the three-phase contact angle, the attachment energy can be expressed as:

δEIw = So (-γsw + γso) - SIγow + Vo(Po - Pw) = γow(So cos θc - SI - 2Vo/RD) (2.4)

(Continues...)

Excerpted from Particle-Stabilized Emulsions and Colloids by To Ngai, Stefan A. F. Bon. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2014
• Language: English
• ISBN 10: 1849738815
• ISBN 13: 9781849738811
• Binding: Hardcover
• Number of pages: 336
• Editor: To Ngai