Particles in monolayers and thin liquid films

Small particles can adsorb strongly at fluid interfaces and form monolayers which can be studied using a Langmuir trough. For sufficiently large particles the monolayers can be viewed microscopically. The driving force for particle adsorption is the concomitant removal of fluid/fluid interface. For very small adsorbed particles, the free energy of forming the three-phase contact line around particles (hence the line tension) may also contribute significantly to the free energy of adsorption. Adsorption can be enhanced by having areas of particle surface with different wettability (Janus particles). Monolayers have structures dependent on lateral interactions between particles; for particles at the oil/water interface, electrical repulsion through oil is often the dominant interaction, which can give rise to highly ordered monolayers. Adsorbed particles can either inhibit or facilitate the formation of stable thin liquid films, depending on particle wettability.

Rheology of colloids

Lyophobic colloidal dispersions, aggregated surfactant systems, and polymer solutions, as well as foams and emulsions, can all be deformed by weak external forces; rheology is the study of deformation and flow of materials. Various rheological quantities arising from the response of a material to shear are defined. For liquids the stress, τ‎, applied is related to the rate of deformation, that is, the shear strain rate, γ̇. For Newtonian fluids τ‎ and γ̇ are linearly related and τ‎ / γ̇ is the viscosity, η‎. Other nonlinear relationships correspond to shear thinning and shear thickening fluids and to plastic behaviour in which there is a yield stress. Viscoelastic systems exhibit both viscous and elastic properties; such behaviour is often treated using the simple Maxwell model. Some illustrative experimentally observed rheological behaviour is presented.

Adsorption of surfactants at solid/liquid interfaces

The physical properties of solid/liquid interfaces are more diverse than those of liquid/fluid interfaces, and consequently the interactions giving rise to adsorption of surfactant or polymeric surfactant are more varied. Solid surfaces can be either hydrophilic or hydrophobic, the former being water-wetted and containing polar or ionogenic sites. Electrical charge at the solid surface is neutralized by ions in the inner and outer Helmholtz planes and in the diffuse part of the electrical double layer. Surface charge has a strong influence on adsorption of ionic surfactants. Standard free energies of surfactant adsorption are obtained by use of an appropriate adsorption isotherm such as the Stern–Langmuir equation. Micellar aggregates of various shapes and sizes can also form at solid/liquid interfaces.

Adsorption of surfactants at liquid/vapour and liquid/liquid interfaces

The variation of interfacial tension of a solution with surfactant concentration in bulk can be used, in conjunction with the Gibbs adsorption equation, to probe the behaviour of adsorbed surfactant monolayers. An adsorption isotherm equation expresses the relationship between bulk and surface concentrations of surfactant, and is used to determine thermodynamic quantities of surfactant adsorption. The variation of the surface pressure of a surfactant monolayer with the surface concentration is described by a surface equation of state, which reflects something of the nature of a monolayer. The way in which inorganic electrolytes modify the adsorption and monolayer behaviour of ionic surfactants is explained, and adsorption from surfactant mixtures is also introduced. In the Appendix, the discussion is extended to the treatment of adsorbed monolayers as two-dimensional solutions of surfactant with solvent molecules, rather than as two-dimensional gases.

Direct characterization of adsorbed surfactant layers

Some widely used techniques for the direct physical investigation of the structure of adsorbed surfactant films are introduced. Neutron reflection has yielded very detailed information about adsorbed surfactant films, although it is not readily accessible to many researchers. There are however commercial instruments available for a number of other techniques which are to be found in numerous laboratories. Scanning probe microscopies (STM and AFM) are capable of producing quite remarkable images of surfactant layers on solids and clearly show how surfactants form aggregates at surfaces. Ellipsometry is capable of yielding adsorbed layer thickness and refractive index from which composition with respect to solvent and surfactant can be deduced. The quartz crystal microbalance (QCM) and its variant, QCM-D, can give adsorbed amounts (including hydration in aqueous systems). Brewster angle microscopy (BAM) is a useful tool for the visualization of phase behaviour in surfactant films.

Dynamic aspects of liquid interfaces

Following the rapid formation of the surface of a surfactant so′′lution, the dynamic interfacial tension falls with time as a result of the finite time needed for surfactant adsorption. Surfaces can either be sheared (involving shape change) or dilated (area is changed), and both these processes can give a viscous and/or elastic response. Usually, surfaces of surfactant solutions exhibit a combination of the two and are viscoelastic. If small sinusoidal area changes are imposed on the surface, changes in tension and area are out of phase because surfactant adsorption is relatively slow. The responses to area change are frequency dependent. The complex dilational viscoelastic modulus, ε‎*, has real (elastic) and imaginary (viscous) parts, ε‎′ and ε‎′′, respectively, whose variation with frequency provides insights into relaxation processes occurring at the surface. The way in which dynamic tensions can give insights into the kinetics of surfactant adsorption is explained.

Capillarity, wetting, and surface (interfacial) tension

Capillarity reflects the action of interfacial tension and has been central to understanding intermolecular forces. When a liquid meets a solid surface (with contact angle θ‎) it forms a meniscus which is associated with the rise/depression of liquid in a capillary tube, hence the term capillarity. Interfacial tensions also determine how a liquid wets and adheres to a solid or another liquid. Liquid menisci are curved, and Young, Laplace, and Kelvin have all thrown light upon the properties of curved liquid surfaces. The Young–Laplace equation relates the pressure difference across a curved liquid interface to both the interfacial tension and curvature of the interface. Interfacial tension also gives rise to a dependence of the vapour pressure (and solubility) of a liquid on the curvature of its surface (e.g. drop radius), as expressed in the Kelvin equation. Common methods for measurement of interfacial tensions are described in an Appendix.

Oil and water do not mix—hydrophobic hydration

Many surfactants contain hydrocarbon moieties that are removed from their aqueous environment (‘dehydrated’) in, for example, adsorption and micelle formation. Hydrophobic hydration relates to the interactions between individual nonpolar solute molecules and water, and can be probed using thermodynamic quantities for the dissolution of dilute hydrocarbon vapours to form dilute aqueous solutions. Contrary to the simple expectation that the entropy of hydration of a nonpolar moiety should be positive (due to disruption of water structure), it is large and negative, giving a large positive contribution to the free energy of hydration. The hydration of nonpolar molecules in water leads to an attraction between the molecules in close proximity, which is termed hydrophobic bonding. Although the free energy of hydration of nonpolar groups in bulk aqueous solution is positive, the interaction free energy of nonpolar molecules/groups with interfacial water at an air/water interface is negative.

Wetting

Wetting of one liquid by another can be understood in terms of the spreading coefficient; the relevance of surface forces to wetting is also explained. If a small liquid drop does not spread, it forms a lens whose shape is determined by the various interfacial tensions. The wetting of solids is characterized by the contact angle θ‎ of the liquid with the solid surface; θ‎ usually depends on how a configuration is reached and advancing and receding contact angles are defined. It is often useful notionally to split solid/liquid tensions into polar and nonpolar contributions in the treatment of wetting. Effects of surfactant on the wetting of both hydrophobic and hydrophilic solids by water are explored. Surface topology can greatly influence wettability, and superhydrophobic solid surfaces exist widely in nature. Finally some dynamic aspects of wetting of solid surfaces by surfactant solutions are described briefly.

Surfactants in systems with oil and water, including microemulsions

The solubilization of small amounts of nonpolar oil by surfactant micelles was described in Chapter 9. In the present chapter surfactant behaviour in systems (such as emulsions) that contain comparable amounts of water and oil are considered. Above the critical aggregation concentration surfactant distribution between the oil and water is partly determined by the preferred curvature of the aggregates present, although surfactant ‘monomer’ distribution is unrelated to where the aggregates reside. The formation of microemulsion droplets in so-called Winsor systems is described, as are the concomitant changes in oil/water interfacial tension, which become ultralow and pass through a minimum. Microemulsion droplets, like micelles, are dynamic structures, and aspects of drop dynamics are introduced. In the Appendix some methods for the determination of the size of microemulsion droplets, including conventional and dynamic light scattering and small angle neutron scattering, are described.