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.

When the electronic many-body problem is solved at any level of theory, its solution provides energies and atomic forces that allow for the determination of structural and thermodynamic properties. Forces also open the road to molecular dynamics simulation and the computation of dynamical properties. In addition, and at variance with classical force fields, any such theory would give access to electronic properties such as electronic energy levels or bands, electronic density, charge transfer, and, in principle, also optical and electronic transport properties.

It is clear that the simpler the treatment of the electronic variables, the more efficient the calculation. Therefore, larger systems can be studied, and longer MD simulations permit us to accumulate more reliable statistics and give access to dynamical phenomena occurring on longer time scales. If the information arising from the electronic degrees of freedom is not strictly necessary, then a strategy that removes the electrons altogether out of the picture by means of effective interatomic potentials (classical force fields) is a winning strategy. However, as long as electrons are treated explicitly, the determination of interatomic potentials requires the solution of the Schrödinger equation, and this is far more expensive than just replacing distances and angles in an explicit formula.

Linear scaling with the system size in the solution of Hartree–Fock or Kohn– Sham equations can only be achieved for very large systems. But even in that limit, when using an atom-centered basis set, the calculation of the matrix elements and the diagonalization of the Hamiltonian matrix require a major computational effort; the latter scales as N3.

The wave functions for free electrons in a periodic crystal can be expanded in plane waves (PWs). If the potential due to the atoms is neglected, then PWs are the exact solution. If the potential is reasonably smooth, then it can be treated as a perturbation, thus leading to the so-called nearly-free electron model (see Section 6.2). The potential originated in the atomic nuclei, however, is far from smooth. In the simplest case of hydrogen the potential is –1/r, which diverges at the origin. The 1s wave function does not diverge, but it exhibits a cusp at the origin, and decays exponentially with distance. For heavier atoms the wave functions associated with core states are even steeper. Therefore, a PW expansion of the wave functions in a real crystal is a rather hopeless task, because the number of PW components required to represent such steep wave functions is huge. However, it would be desirable to retain the simplicity of the PW approach. Slater (1937) suggested a possible solution to this problem, where the PW expansion was augmented with the solutions of the atomic problem in spherical regions around the atoms, and the potential was assumed to be spherically symmetric inside the spheres, and zero outside (APW method).

In order to overcome this shape approximation of the potential, Herring (1940) proposed an alternative method consisting of constructing the valence wave functions as a linear combination of PW and core wave functions. By choosing appropriately the coefficients of the expansion, this wave function turns out to be orthogonal to the core states. Hence the name of orthogonalized plane wave (OPW) method.