Introduction

Froth flotation has a long history (over 100 years) of development and widespread applications. Essentially, the process involves the attachment of finely dispersed hydrophobic particles to air bubbles to produce so-called “three-phase froths” on the surface of the flotation cell with the hydrophilic particles remaining dispersed in the suspension. In this way, the particles are separated, based on their differences in surface wettability. Although froth flotation includes several major elementary sub-processes, one of the most important operations involves the interaction of the selected suspended particles with a chemical reagent (a flotation collector) in order to make the surfaces sufficiently hydrophobic and become “targets” for bubbles generated in the cell. The “gangue” particles remain hydrophilic and do not interact with the collector reagent but remain dispersed in the suspension.

The following unit processes are also important:

1. (i) Generation and dispersion of gas bubbles in the presence of a surfactant (frother) in the pulp and the formation of the froth layer.

2. (ii) Collision of hydrophobic particles with gas bubbles.

3. (iii) Adhesion of hydrophobic particles to gas bubbles and the formation of particle–bubble aggregates.

4. (iv) Ascension of particle–bubble aggregates from the pulp into the three-phase froth.

Both the fundamental and practical aspects of froth flotation have been well studied and developed but the process still undergoes modification and advancement.

Solid particles of colloidal dimensions (nm—μm) adsorb at fluid interfaces, either liquid—vapour or liquid—liquid, in many products and processes. Examples include fat crystals around air bubbles in certain foods, particles of sand or clay partially coating water drops in crude oil and the selective attachment of mineral particles to bubbles in froth flotation. The properties of these systems are due in part to the irreversible nature of particle adsorption, and such particles behave in many ways like surfactant molecules. The pioneering work in the area of particle-stabilised foams and emulsions was conducted by Ramsden and Pickering, respectively, early in the 20th century. During the last 10 years or so, there has been a revival of interest in this field, and in the behaviour of particles at planar liquid interfaces, and we felt that it was time to prepare the first book encompassing most of this activity. It is anticipated that this will be the start of a new series in this rapidly evolving field.

Following an introductory chapter to the whole area by the editors, the book is divided into two parts. The first part, dealing with particles at planar interfaces, contains chapters describing simulation and theoretical approaches to the structure, and dynamics of particle monolayers and how particles can assist with the wetting of oils on water.

Introduction

Particulate additives are found in the formulations of a great many high-surface area products in the form of emulsions and foams. Their presence is normally desirable for the purposes of stability. In the case of ice cream foams, tiny fat globules can attach themselves to the surfaces of the air pockets and hinder the process of coarsening by Ostwald ripening. In this case, the particles are a natural ingredient. In other instances, colloidal particles are deliberately added and Pickering emulsions are an important example. The occurrence of particles leading to stabilization can also be unwelcome, as in the case of emulsions formed when seawater and crude oils vigorously mix. This environmental problem can lead to very stable emulsions as a result of particles formed by asphaltenes or clay collecting at the oil–water interface.

The presence of particles at a fluid–fluid interface leads to numerous, profound consequences. Since a very large amount of energy is normally required to remove a particle from an interface (see Chapter 1 for a detailed explanation of this point), particles in these monolayers are normally irreversibly attached. Furthermore, as described in Chapter 2, these systems can be modified by exquisite tuning of interparticle forces, particle chemistry and particle size to create a wide range of morphologies of these “2-D suspensions”.

Introduction

The antifoam effect

Foams appear as an integral part of various technological applications, such as ore and mineral flotation, tertiary oil recovery, production of porous insulating materials, fire fighting and many others. Foams are also encountered in certain types of consumer products, e.g. the mousses and ice-creams as food products, and shaving and styling foams in cosmetics. It has been known for many years that the presence of oil droplets and/or hydrophobic solid particles in the aqueous foaming solutions can strongly reduce the foaminess and foam stability, which might be a problem in various applications. For example, the fat particles in food products and the droplets of silicone oil used in personal care products (such as shampoos and hair/skin conditioners) have a strong antifoam effect, which should be suppressed to achieve an acceptable product quality from a consumer viewpoint.

On the other hand, excessive foaming might create serious problems in many industrial processes. Typical examples are during fermentation in drug and food manufacturing, the processing of drug emulsions and suspensions, pulp and paper production, industrial water purification, beverage production and packaging, textile dyeing, oil rectification and many others. That is why special additives called “antifoams” or “defoamers” are widely used in these and other industrial applications to suppress foam formation or to destroy already formed foam.

Introduction

An emulsion is a system of dispersed droplets of one immiscible liquid in another. Simple emulsions are either oil-in-water (o/w) or water-in-oil (w/o). Emulsions can be defined as colloidal systems, although emulsion droplets are usually larger than the range specified for a colloidal system, i.e. diameter > 1 μm. Emulsions are encountered in many industries and scientific disciplines. Multidisciplinary study is required for a better understanding of emulsion behaviour and better control over industrial emulsions. In this review, solids-stabilized emulsions are reviewed and they are defined as an emulsion that is stabilized by fine solid particles. Some finely divided solids assist in the emulsion formation, and/or improve its stability. These types of emulsions have widespread applications in industrial settings and have a history of being studied, dating back to 1903.

Objective of review

Over the last decade, solids-stabilized emulsion experimentations are becoming increasingly more sophisticated and focused on microscopic level understanding. Some recent work has been performed to study the structure of particles at the droplet interfaces. In this review we will summarize important experimental and theoretical studies related to solids-stabilized emulsions. Considering the vast literature on solids-stabilized emulsions, this review aims at a selective, not comprehensive, overview of the progress in the field, with emphasis on key factors affecting the stability of solids-stabilized emulsions and the structure of emulsion drop interfaces.

Introduction

The structure displayed by assemblies of colloidal particles, whether in three dimensions (3-D) or in two dimensions (2-D), is an important aspect in many industrial processes and products, e.g. waste-water treatment, paints and ceramics, but also for the assembly of new materials. Colloidal particles can be made to organise into ordered arrays or to attain heterogeneous structures with different degrees of disorder. The control of colloidal structure formation starts with the particle interactions (attractive or repulsive) and colloidal dynamics. These interactions balance against thermal forces and external influences such as gravity and applied force fields to determine what configurations the particles will adopt, e.g. network-like, random or ordered configurations (see Figure 2.1).

These colloidal structures acquire interesting and useful properties not only from their constituent materials, but also from the spontaneous emergence of mesoscopic order that characterises their internal structure. Ordered arrays of colloidal particles with lattice constants ranging from a few nanometres to a few microns have potential applications as optical computing elements and chemical sensors, and templates for fabricating quantum electronic systems. Restricting the colloidal array to 2-D has particular relevance to sensor and membrane applications. Disordered systems can be used as ceramic membranes.

The construction of photonic crystals emphasises the importance of the correlation between structural features and the material properties.

Introduction

Many food colloids are stabilized, at least in part, by the presence of particulate material that accumulates at oil–water or air–water interfaces. As applied to emulsion droplets, this type of stabilization mechanism is commonly referred to as Pickering stabilization. Some examples of particles involved in Pickering stabilization in food emulsions are casein micelles (in homogenized milk), egg-yolk lipoprotein granules (in mayonnaise) and fat crystals (in spreads and margarine). In addition, dairy-type foams such as whipped creams and toppings are stabilized by a protective layer of partially aggregated emulsion droplets (or fat crystals) which adhere to the air bubbles during whipping.

This chapter reviews recent progress in the making and stabilization of food emulsions and foams using solid particles. To put the topic into context, we need to make direct comparison with the interfacial and stabilizing roles of the key molecular species – emulsifiers, proteins and hydrocolloids. It will be assumed that the reader is familiar with the basic physico-chemical principles of surfactant and polymer adsorption, with the meaning of terms commonly used in colloid science like “flocculation” and “coalescence”, and with the essence of the established theories of stabilization (and destabilization) of food emulsions and foams, as described in existing texts.

Introduction

Liquid foams are collections of gas bubbles uniformly dispersed in fluids and separated from each other by self-standing thin films. If the distance between bubbles is comparable to the bubble size one prefers to speak of bubble dispersions. In foams, bubble arrangements are usually disordered and gas volume fractions are high. If a liquid foam is solidified, a solid foam is obtained. Solid foams show many interesting properties which is the reason for their wide use, e.g. in civil engineering, chemistry or the food industry.

Any liquid matter should be foamable and so is liquid metal. The prospect of being able to make light durable metallic foams already triggered research more than half a century ago. In 1943 Benjamin Sosnick attempted to foam aluminium with mercury. He first melted a mix of Al and Hg in a closed chamber under high pressure. The pressure was released, leading to vaporisation of the mercury at the melting temperature of aluminium and to the formation of a foam. Less hazardous processes were developed in the mid-1950s when it was realised that liquid metals could be more easily foamed if they were pre-treated to modify their properties. This could be done by oxidising the melt or by adding solid particles.

Introduction

Wetting and de-wetting of surfaces by a liquid are fascinating phenomena of great importance for scientific and technological problems including the self-protection of living organisms, the production of microstructures, as well as the integrity and uniformity of decorative, lubricating and protective coatings and the prevention of fogging. Wetting is influenced by short-range forces, such as hydrogen bonding and donor/acceptor interactions, and by long-range dispersion forces. Depending on the relative strength of these forces, one can observe de-wetting, complete wetting or partial wetting. In the first case, the liquid forms lenses co-existing with the bare surface. In the second case, any amount of liquid applied to the surface will spread out as an even layer with a thickness given by the volume of the applied liquid per area of the surface. In the last case, the competition between favourable short-range forces and unfavourable long-range forces gives rise to the formation of a wetting layer of limited thickness, which often is a monolayer but may in principle have any thickness, that co-exists with lenses formed by excess of the liquid. While most of us are familiar with the technological importance of wetting of solid surfaces, it is worth noting that the wetting of liquid surfaces is technologically important as well, e.g. for the production of thin uniform sheets of material in float cast processes or in the context of slowing down the evaporation of water from open reservoirs in arid regions.

Introduction

Functional materials

The research in novel nano- and microstructured materials with custom designed properties, composition and structure (symmetry) has been driven by the potential of using such materials in forthcoming advanced technologies. There is specific strong interest in complex one dimensional (1-D) and periodic two dimensional (2-D) and three dimensional (3-D) structures from colloidal particles. Complex 1-D assemblies have been considered as novel building blocks for materials with advanced hierarchical structure. Periodic 2-D and 3-D structures from colloidal particles display several unique and potentially usable properties resulting from the existence of a long-range order. The formation of such materials is dependent on the interactions of particles with liquid—solid or liquid—liquid interfaces and/or their confinement in thin liquid films. The need to understand the forces involved in the assembly of structures from colloidal particles has led to important fundamental insights in the field of colloidal forces and self-organization.

The materials formed by organization and assembly of colloidal particles have a number of unique and potentially utilizable features. Coatings and surface patterning with colloidal particles allow for modification of properties like wetting and reflectivity. The long-range organization of particles in coatings brings a number of significant advantages. Particle arrays often display strong interaction with light and other electromagnetic radiation.

In this chapter, we summarize and expand somewhat on those results from quantum mechanics and spectroscopy most germane to our study of statistical thermodynamics. We then prepare for revisiting intensive properties in Chapter 9 by considering the nature of thermodynamic calculations before the advent of quantum mechanics. From a pedagogical point of view, the previous three chapters have focused on the properties of a single atom or molecule. For our purposes, the most important such properties are the allowed energy levels and degeneracies corresponding to the translational, rotational, vibrational, and electronic energy modes of an independent particle. Exploiting this knowledge, we proceed to a macroscopic assembly of atoms or molecules, with a focus on calculations of thermodynamic properties for any pure ideal gas. Assemblies composed of different particle types subsequently permit the evaluation of properties for both nonreacting and reacting gaseous mixtures, including equilibrium constants for various chemical reactions. Finally, re-applying spectroscopy to such mixtures, we examine the utility of statistical thermodynamics for experimentally determining temperature or concentrations in realistic gaseous mixtures at high temperatures and pressures.

Energy and Degeneracy

Our foray into quantum mechanics and spectroscopy has led to relations giving the energy and degeneracy for all four energy modes – translation, rotation, vibration, and electronic. If we insist on mode independence, any consideration of diatomic molecules also mandates the simplex model, which presumes a combined rigid rotor and harmonic oscillator.