John H. Clint

Emulsions stabilised solely by colloidal particles

Abstract The preparation and properties of emulsions, stabilised solely by the adsorption of solid particles at the oil–water interface, are reviewed especially in the light of our own work with particles of well-controlled surface properties. Where appropriate, comparison is made with the behaviour of surfactant-stabilised emulsions. Hydrophilic particles tend to form oil-in-water (o/w) emulsions whereas hydrophobic particles form water-in-oil (w/o) emulsions. Many of the properties can be attributed to the very large free energy of adsorption for particles of intermediate wettability (contact angle at the oil–water interface, say, between 50 and 130°). This effectively irreversible adsorption leads to extreme stability for certain emulsions and is in contrast to the behaviour of surfactant molecules which are usually in rapid dynamic equilibrium between the oil–water interface and the bulk phases. There is evidence that, in some systems, weak flocculation of the particles improves the emulsion stability. Phase inversion from w/o to o/w can be brought about by increasing the volume fraction of water. Emulsions close to this inversion point tend to be the most stable, again in contrast to surfactant systems. The volume fraction needed for inversion depends on the particle wettability and the nature of the oil and these effects have been rationalised in terms of surface energy components. Stable multiple emulsions (w/o/w and o/w/o) can be made using two types of particles with slightly different wettability. Similar multiple emulsions prepared with two types of surfactant tend to be much less stable. The possibility of preparing novel solid materials by evaporating solid-stabilised emulsions is also proposed. Finally we report on some extensions to the work of Levine et al. who obtained expressions for the free energy of formation of emulsion drops covered with close-packed monolayers of monodisperse spherical particles. In particular in the light of the observations that nanoparticles can act as excellent emulsion stabilisers, we have considered potential effects on the free energy of emulsion formation of the action of small (physically realistic) positive and negative line tensions in the 3-phase contact lines skirting particles adsorbed at the droplet interfaces. We also explore the possibility that curvature properties of close-packed particle monolayers can affect emulsion properties in much the same way that surfactant monolayer properties influence emulsion type and stability.

Adhesion and components of solid surface energies

Abstract Contact angle data for sets of probe liquids allow the determination of components of solid surface energies which in turn can be used to calculate the work of adhesion of other materials to the solid surface. There is much debate currently about the correct choice of the acid–base components for the probe liquids. For many systems, the strength of adhesion measured independently correlates well with the calculated work of adhesion. Recent trends in this area include adhesion under water and the adhesion of bacterial and other cells to immersed solids.

Adhesion under water: surface energy considerations

Abstract Contact angles for a set of probe liquids on solid surfaces have been used to determine the components of the solid surface energies. By the use of suitable combining rules, such data have then been used to calculate the work of adhesion of oil onto various solid surfaces under water. A surprising outcome of such studies is that the order of strengths of adhesion of ‘liquid’ to solid surfaces under water is opposite to that for adhesion in air. For example, oily materials do not adhere well to ‘non-stick’ surfaces such as PTFE in air but adhere strongly under water. Such considerations are extended to the adhesion of bacteria to solid surfaces under water and the possible role of adsorbed protein layers in the adhesion process. In these systems, the predictions are born out by direct observations of bacterial fouling taken from the recent literature.

Particle wettability and line tension

For a spherical solid particle with a uniform surface at rest in a fluid/fluid interface, it is shown that if the line tension associated with the three-phase contact line around the particle is increased (conceptually), the contact angle between the particle and interface does not change continuously to 0 or 180°. Rather, when a critical value of line tension is attained a discontinuous wetting–drying transition can occur. Such a transition corresponds to the condition where the free energy of the particle at local equilibrium in the interface is equal to that of the particle completely immersed in one of the bulk phases. However, local equilibrium in the interface is possible for line tensions greater than that required for the transition. This means that an ‘activation’ energy is required for the irreversible removal of the particle into one of the bulk phases. Literature values of line tensions span at least five orders of magnitude. If the larger of the reported values operate in systems with the geometry of interest here, interesting wetting phenomena should be apparent for particles with diameters of the order of 1 mm.

Aspects of the stabilisation of emulsions by solid particles: Effects of line tension and monolayer curvature energy

It is well-documented that solid particles can be effective stabilisers of emulsions, and that the type of the emulsion formed is related, inter alia, to the particle wettability. In general it is to be expected that the free energy change accompanying the formation of a solid-stabilised emulsion will be positive and that emulsion stability will therefore be kinetic in nature. Recently work has appeared showing that very small solid particles, with radius around say 15 nm, can be excellent stabilisers of emulsions. This raises the possibility that line tension acting in the three-phase contact lines around adsorbed particles could have an effect on particle adsorption and hence on their effectiveness as emulsion stabilisers. We explore the effects of both positive and negative line tension on the free energy of adsorption of spherical particles at spherically curved liquid interfaces and on the free energy of drop formation. For small particles it is shown that physically realistic values of positive line tension can lead to exclusion of particles from drop interfaces, either by rendering the adsorption free energy positive or by creating energy barriers to adsorption. We also consider the effects of lateral interactions between adsorbed particles on particle adsorption, particularly strong Coulombic repulsion mediated through the oil phase. It is known that the preferred type of emulsions stabilised by surfactants can be accounted for by the existence of a curvature energy possessed by close-packed surfactant monolayers. We show here that close-packed monolayers of spherical particles at a liquid surface also possess curvature energies, and we calculate bending elastic moduli (κ) as a function of particle size, oil/water interfacial tension, line tension and contact angle. For a monolayer of (hypothetical) particles with radius 0.5 nm (similar to that of a low molar mass surfactant), the value of κ is expected to be of the order of kT, just as for surfactant monolayers. It has been assumed in our work that equal spherical particles are hexagonally close-packed around spherical drops. It is well known however that such packing is not possible, but we show in an Appendix that the assumption of hexagonal packing leads to only small errors (in the context) in calculated free energies of emulsion formation and in the calculated bending elastic moduli of particle monolayers.

Evaporation rates of pure liquids measured using a gravimetric technique

We describe a gravimetric method for the determination of evaporation rates. The liquid sample is held in a partially filled, cylindrical open-topped tube within a vertically flowing gas stream. A simple model appropriate to this geometry is found to account for the variation of rate with liquid height within the sample tube and gas flow rate. Evaporation rates for a range of pure liquids with vapour pressures ranging from 0.1 to 500 Torr were determined and showed reasonable agreement with theoretical values estimated using literature values of the vapour pressures and vapour diffusion coefficients in air.

Liquid droplets and solid particles at surfactant solution interfaces

We consider the behaviour of liquid droplets at the liquid/vapour interface. We discuss the feasibility of drop entry into the interface in terms of the various interfacial tension in equilibrated systems, noting the thermodynamic constraints on equilibrium tensions; we also note the important possibility of the formation of metastable thin films between drop and interface. Dispersed oil droplets are often effective antifoam agents, and the discussion is extended to a consideration of the rupture of thin liquid films (and foams) by dispersed liquid droplets.Attachment of solid particles to liquid interfaces is determined by the solid wettability, i.e. the contact angle of the liquid interface with the particle. As with liquid droplets, solid particles can bridge and rupture thin liquid films. We analyse this process for symmetrical (gas/liquid/gas and liquid/liquid/liquid) films, which is central to the effects of solid particles on the stability of foams and emulsions. Some preliminary work on the bridging of asymmetrical (liquid/liquid/gas) films is also briefly discussed.A recent method for the direct determination of the contact angle of a liquid interface with small (diameter ⩽3 µm say) monodisperse spherical particles is explained and it is shown how this method can be extended to the measurement of line tensions of solid/liquid/gas contact lines. We point to important effects on particle wettability at interfaces (wetting transitions) which could result from the existence of high line tensions.

Size-dependent lens angles for small oil lenses on water

Line tension is the excess energy associated with unit length of a three-phase contact line and it has long been of interest, in part because if sufficiently large, it can affect various processes of industrial and biological importance. Most recently, interest has centred on the magnitude and sign of experimentally determined values. Reported line tensions in systems with liquid alkanes in contact with aqueous phases include values from about +10−10 to 10−9 N on the one hand, and −10−6 N on the other. If the actual values of line tension lie close to the lower end of the spectrum quoted above, the influence on many systems of interest will be negligible. The higher values, however, could lead to pronounced effects. A study to determine line tension in the three-phase contact line around lenses of dodecane resting on a water subphase is presented. The method involves measuring, by interferometry, the variation of lens angle with the contact line radius. In order to bring the angles into a convenient range for measurement (around 2°), small amounts of dodecanol (ca. 2 mmol dm−3) have been added to the dodecane. The line tension is found to be +1.6±0.3×10−11 N. The magnitude and sign of the line tension is discussed in terms of surface forces.

Evaporation rates of structured and non-structured liquid mixtures

We have used a gravimetric technique to measure the rate of evaporation of a volatile liquid in mixtures with a second, involatile component under conditions of controlled gas flow. A range of non-structured and structured mixtures were investigated in order to examine whether the rate limiting step for evaporation may switch from vapour diffusion across the stagnant gas layer above the liquid to mass transfer within the liquid mixture. Evaporation rates of pentane and hexane from mixtures with squalane (involatile) show excellent agreement with rates calculated on the basis that vapour diffusion across a stagnant gas layer is rate limiting and that mass transfer within the liquid mixture is fast. Hexane gelled by the addition of silica particles is found to evaporate at a rate very similar to that for un-gelled hexane because the equilibrium vapour pressure of hexane is unaffected by silica particle addition. Water evaporation rates from mixtures with the non-ionic surfactant n-dodecyl hexaoxyethylene glycol ether (C12E6) were found to be up to 10 times slower than calculated vapour space diffusion controlled rates owing to the slow development of concentration gradients within these highly structured liquid mixtures.

Retardation of oil drop evaporation from oil-in-water emulsions

The rate of evaporation of volatile oils from oil-in-water emulsions can be strongly retarded by using a polymeric emulsion stabiliser instead of a low molar mass surfactant.