Colloidal particles at liquid interfaces

Although much of the behaviour of colloidal particles in bulk dispersions is well known, their interactions and monolayer structure when adsorbed/spread at liquid interfaces is much less studied. Due partly to their ability to stabilise droplets in emulsions and bubbles in foams when present alone, there has been a substantial increase in interest in this field during the last 10 years or so and this Themed Issue provides a snapshot of some of this activity. Micro- and nanoparticles can adsorb very strongly to both liquid/vapour^1–10 and liquid/liquid^11–22 interfaces and their presence at such interfaces is important in e.g. flotation, mineral processing and refining crude oil. In fact, it is now believed that the superior stability of mayonnaise (an oil-in-water emulsion first formulated in 1756) may be due in part to the fact that the mustard particles coat a fraction of the droplet interfaces! In addition to investigating their properties at interfaces per se, particle-containing interfaces are being utilised as templates for the preparation of novel materials.

The packing of equal-sized particles on a spherical interface has relevance to the stabilisation of foams and emulsions, and defects such as disclinations and dislocations are common.¹ It is well documented that the wettability of particles at a liquid interface is important in determining some of their behaviour but measuring the contact angle that a particle makes with an interface is not trivial. Two new methods are proposed for nanoparticles at a planar air/water surface, one involving scanning-angle reflectometry² and the other employing surface pressure/area isotherms with and without added surfactant.³ The surface rheological properties of planar particle monolayers at both air/water and oil/water interfaces have been determined and compared with those of their three-dimensional counterparts.⁴ It is suggested that the response observed in the two-dimensional particle layers could provide insight into the rheological properties of concentrated colloidal dispersions. Particle monolayers can be used in the same way that insoluble monolayers of molecules have been used to build up Langmuir–Blodgett multilayers on solid substrates which have interesting optical properties.⁵ The importance of the surface groups on the particles to the monolayer and multilayer structure is discussed. The air/water surface can also be used as a template during the preparation of anisotropic particles formed by spreading a solution of an appropriate wax onto it followed by controlled cooling.⁶ Lens-shaped, doughnut-shaped and hexagonal microparticles can be formed which should display different interfacial behaviour from those of spherical geometry. Since many industrial foams occur in systems containing both particles and surfactant, understanding the interaction between the two surface-active agents is paramount. It is found that, for air bubbles in water stabilised initially with particles, addition of surfactant causes the gas to dissolve and, in certain cases, colloidosomes (or capsules made of particles) are formed.⁷ It may be that the change in curvature of the air/water surface between adsorbed particles is implicated in such shrinkage. Recently, particles have also been shown to be effective stabilisers of metal foams, i.e. air-in-molten metal, which give rise on cooling to porous, lightweight solid foams of use in the automotive industry. Studying the processes like drainage and coalescence of isolated air–molten-metal–air films represents a simpler system for understanding the stability of bulk metal foams.⁸ Particles like those of silicon carbide which can withstand the very high processing temperatures are excellent candidates in such cases. A novel composite photocatalyst has been synthesised in which the particles have a core(TiO₂)–shell(SiO₂) morphology and in which the pore size of the silica shell can be varied to allow small substrate molecules to diffuse inside and prevent larger ones from doing so.⁹ By suitably treating the outer surface of such particles, they can be made to adsorb at the air/water surface and such floating particles can be efficient photocatalysts of chemical reactions in water. Their potential use in water purification is intriguing.

In the case of liquid–liquid mixtures, the study of planar interfaces^11–15 sheds light on the behaviour of particle-coated curved interfaces in emulsions.^16–22 These interfaces have been used to prepare thin-shelled microcapsules which are mechanically stable and well suited to applications such as enzyme therapy.¹¹ The self-assembly of true nanoparticles (< ca. 10 nm) at oil/water interfaces has been investigated in two directions.^12,13 Since such particles are not held very strongly at the interface, the kinetics of the replacement of small particles by larger ones allow the energy barrier for adsorption to be determined.¹² Also, an ingenious use of specific DNA-base pairing is employed to build up trilayer particle films of CdTe–Au–Ag by anchoring the central Au particles to the interface.¹³ The importance of the charge of the adsorbed particles is investigated theoretically and experimentally.^14,15 Both the attraction between like-charged particles¹⁴ and the Coulombic repulsion through the oil phase for hydrophobic particles¹⁵ are verified. The effects of salt concentration,^16,17 pH¹⁷ and (added) surfactant concentration¹⁸ on the stability and structure of oil-in-water emulsions stabilised by silica or latex particles is discussed and the findings are linked to the behaviour of the particle dispersions prior to emulsification. Liquids other than oil or water can be used to prepare silica particle-stabilised emulsions as is demonstrated in the case of water-in-carbon-dioxide systems¹⁹ and systems comprising room-temperature ionic liquids.²⁰ Stabilisation of the former is notoriously difficult with surfactant molecules, whilst the emulsion type in the latter is correlated with the contact angles at the liquid–liquid–solid interface. Curved and planar particle-laden interfaces are compared by determining the pressure–volume relationship of particle-coated drops and the surface pressure–area dependence of particle monolayers.²¹ In the fluid and jammed compression states, the bulk pressure enables the interfacial tension to be deduced, whereas in the high compression buckled state the drop pressure is zero, as is the interfacial tension. Finally, a study of the incorporation of virus particles and nanoparticles of different wettability into vesicles, which are water-in-water systems, concludes that hydrophobic particles can be embedded into the bilayer membrane whereas hydrophilic ones distribute evenly between the inner and outer aqueous compartments.²² These composite vesicles have potential uses in pharmaceutical applications as drug-delivery vehicles.

It is clear from this small number of papers that the field is inter-disciplinary and continues to evolve. Many thanks to all the authors and to the staff of the journal at the RSC. Enjoy the read!

B. P. Binks, University of Hull, UK.

Papers in this issue
1	M. J. Bowick et al., DOI: 10.1039/b710773k
2	A. Déak et al., DOI: 10.1039/b702937n
3	D. O. Grigoriev et al., DOI: 10.1039/b711732a
4	S. Reynaert et al., DOI: 10.1039/b710825g
5	S. Reculusa et al., DOI: 10.1039/b711038n
6	H. A. Wege et al., DOI: 10.1039/b705046a
7	A. B. Subramaniam et al., DOI: 10.1039/b712172e
8	G. S. Vinod Kumar et al., DOI: 10.1039/b710497a
9	S. Ikeda et al., DOI: 10.1039/b709891j
10	J. Yang et al., DOI: 10.1039/b709624k
11	K. D. Hermanson et al., DOI: 10.1039/b709808a
12	S. Kutuzov et al., DOI: 10.1039/b710060b
13	B. Wang et al., DOI: 10.1039/b705094a
14	M. P. Boneva et al., DOI: 10.1039/b709123k
15	M. E. Leunissen et al., DOI: 10.1039/b711300e
16	T. S. Horozov et al., DOI: 10.1039/b709807n
17	F. Gautier et al., DOI: 10.1039/b710226g
18	N. G. Eskandar et al., DOI: 10.1039/b710256a
19	S. S. Adkins et al., DOI: 10.1039/b711195a
20	B. P. Binks et al., DOI: 10.1039/b711174f
21	C. Monteux et al., DOI: 10.1039/b708962g
22	W. H. Binder et al., DOI: 10.1039/b711470m

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