Emulsions stabilized with solid nanoparticles: Pickering emulsions

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Highlights

Abstract

Pickering emulsions are attractive formulations because they are simple and bear strong similarities with the well-known surfactant-based emulsions. Pickering emulsions have been largely ignored since their early disclosure in 1907 and arouse a renewed interest quite recently. Since this unintelligible time gap raises suspicion, the first aim of the present review is giving the simple fundamental rules as an introduction for newcomers in the topic. The basic physical chemistry of Pickering emulsions is explained and the ways to control the parameters of higher relevance with respect to development of applications are given. This first part covers the choice of the solid nanoparticles used as stabilizers and their surface properties, the control of emulsion type, droplet size, and rheology. A second part gives examples of some applications in drug delivery and manufacturing of porous nanomaterials as illustrations of the potential of such emulsions.

Introduction

Pickering emulsions are emulsions of any type, either oil-in-water (o/w), water-in-oil (w/o), or even multiple, stabilized by solid particles in place of surfactants [1], [2], [3] (Fig. 1).

Pickering emulsions retain the basic properties of classical emulsions stabilized by surfactants (emulsifiers), so that a Pickering emulsion can be substituted for a classical emulsion in most applications of emulsions. The stabilization by solid particles brings about specific properties to such emulsions. The high resistance to coalescence is a major benefit of the stabilization by solid particles. The ‘surfactant-free’ character makes them attractive to several applications fields, in particular cosmetic and pharmaceutical applications where surfactants often show adverse effects (irritancy, hemolytic behavior…). Solid stabilizing particles are necessarily smaller than emulsion droplets. Solid particles of nanometric size (or sub-micron, ∼100 nm) allow the stabilization of droplets as small as few micrometers diameter; stabilization of larger droplets is possible as well. Micron-sized solid particles can stabilize larger droplets, the diameter of which possibly reaching few millimeters. The availability of stable millimeter-sized emulsions is a supplementary benefit of Pickering emulsions with respect to classical emulsions; this possibility comes from their high stability against coalescence.

As in the case of surfactants, stabilization of emulsion droplets takes place by means of adsorption of solid particles at the surface of emulsion droplets. The mechanism of adsorption is very different of surfactants however, since the solid particles do not need being amphiphilic. Partial wetting of the surface of the solid particles by water and oil is the origin of the strong anchoring of solid particles at the oil–water interface.

Pickering emulsions are named after S.U. Pickering whose paper [4] is considered as the first report of o/w emulsions stabilized by solid particles adsorbed at the surface of oil droplets. Actually, Ramsden [5] published a paper four years before (cited in the paper by Pickering) reporting the adsorption of solid particles at the air-water interface, its contribution to foaming, and the recovery of solid particles adsorbed as a rigid layer at the surface of liquids. Although it has been mentioned that ‘solid particles’ adsorbed and stabilized interfaces, the paper focused on organic particles called ‘proteids’ (e.g. albumin) which underwent reorganization upon adsorption at the interface. Such particles are not real solids; they might better be considered as ‘soft solids’. Ramsden claimed that such adsorption might also take place at the oil–water interface; but he did not give an experimental proof supporting his speculative idea. The paper by Pickering [4] reported the formation of stable emulsions of paraffin oil stabilized by solid particles adsorbed at the oil droplet surface, and it disclosed the most prominent features of o/w emulsions stabilized by solid particles. He gave evidence for the adsorption of solid particles, and showed the improved stability of such emulsions with respect to surfactant-based emulsions; he reported the evaluation of stabilization by several types of solid particles. Although it was quite clear since the paper by Pickering in 1907 that such emulsions had definite advantages in comparison to surfactant-based emulsions, the later were preferred and showed considerable development in various types of applications; Pickering emulsions were largely ignored for a long period.

Isolated papers of academic bearing appeared [6], [7], [8], [9], [10], [11], [12], [13], [14] till the eighties when several research teams launched long-term projects on Pickering emulsions [15], [16], [17], [18]. Pickering emulsions have been discussed in the Encyclopedia of Emulsion Technology edition of 1983 although they did not pass into common practice [19]. At the same time it has been discovered that water droplets surrounded by a shell of adsorbed silica particles behaved as solid spherical balls though they contained 95% water [20], [21], [22]. Such water droplets surrounded by silica particles may be considered as water-in-air emulsions. The Degussa Company marketed this material in 2004 under the trade name ‘dry water’ [23].

During the time gap between the early report by Pickering and the recent renewal of interest, several people worked with Pickering emulsions without acknowledging it, as such especially in the field of food emulsions where crystals of solid fats often adsorb at the surface of emulsion droplets [14], [24]. Even quite recently, materials scientists reported on polymer particles decorated at their surface with attached inorganic particles and seemingly missed the connection with Pickering emulsions [25]. Scientific issues pertaining to Pickering emulsions have mainly been addressed with regards to the difficult demulsification of crude oil in the field of oil recovery. Indeed, water is present in crude oil as droplets of a w/o emulsion. Water removal by means of demulsification is a difficult technical issue because the w/o emulsion is stabilized by adsorbed solid particles [18]. Crude oil contains several polyaromatic molecules that act as surfactants; maltenes are such soluble species, and asphaltenes are insoluble species present as colloidal suspension of solid particles. Crude oil often is a Pickering w/o emulsion, which is very efficiently stabilized by adsorption of solid nanoparticles of waxes and asphaltenes at the surface of water droplets [26], [27], [28]. Because of the high resistance of Pickering emulsion against coalescence, the difficult demulsification of water from crude oil deserved an important research effort throughout the world [29], [30], [31], [32], [33], [34].

This paper gives a presentation of the classical picture of Pickering emulsions that is well-established and given in several reviews [1], [2], [3]. This picture is still valid and is sufficient for people who wish to start working on Pickering emulsions. Indeed, new insights and developments have appeared been since 2003, but they addressed specific points that did not question the basic rules governing the physical chemistry of Pickering emulsions. Accordingly, the first part of the review is focused on the basic physical chemistry for beginners, followed by a discussion of the control of properties that mostly matter before going to the development of applications: droplet size, emulsion stability, and rheology. It is intended to show that Pickering emulsions are simple and do not differ so much from classical emulsifier-based emulsions. The late interest to Pickering emulsions makes them somehow mysterious; one wonders why the present developments could not arise sooner. Actually there is no special trick that would be the hidden know-how of specialists. The present review aims at demystifying the field so that people can go to the development of innovative applications using Pickering emulsions. The basic rules are given and the ways the emulsions type, droplet size, emulsion stability can be controlled are specified. The rheology of Pickering emulsions is somehow different of surfactant-based emulsions; in particular, the role of excess solid particles in the continuous phase is featured. The review is restricted to the most fundamental aspects; the reader is referred to previous reviews for entering the topic in more details [1], [2], [3], [35]. Specific applications of Pickering emulsions are currently emerging; the increasing attention paid to them is illustrated with the help of examples of applications taken in the fields of drug delivery and polymer materials.

Section snippets

Physical chemistry of Pickering emulsions

Successful preparation of stable emulsions requires that two main criteria be fulfilled: The emulsions are stable for a long time against any destabilization phenomenon (coagulation, coalescence, Ostwald ripening); the emulsification process is possible. As for classical emulsions, both designs of a suitable formulation and of an efficient process are of chief importance. The long term stability mainly depends on the formulation; but the process also matters since the droplet size is often

Few examples of applications

There are many instances where Pickering emulsions can find application. Any technology where emulsions are used can be switched to Pickering emulsion. There is no well-known marketed product or material where the utilization of Pickering emulsion is claimed. One reason is that Pickering emulsions have only recently been recognized as a technology which could be considered as distinct of an emulsifier-based emulsion. Another reason is that the use of Pickering emulsions has often been

Conclusions

Nanoparticles can be used as emulsifier for the stabilization of Pickering emulsions. The replacement or classical emulsifiers by nanoparticles is quite a direct substitution. Indeed the manufacture process is exactly the same: nanoparticles are dispersed in the water or oil phase in the same way as emulsifier is dissolved in one the phases, and emulsification of the dispersed phase makes use of the same processes as for surfactant-based emulsions. The selection of the solid nanoparticles can

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