Surface compaction versus stretching in Pickering emulsions stabilised by microgels

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Highlights

  • Review of Pickering emulsions stabilised by soft deformable microgels

  • Controlled deformation of adsorbed microgels at the oil–water interface

  • Link between interfacial microgel organisation and corresponding emulsion properties

  • Microgel and process parameters governing the microgel packing at the interface

  • New opportunities in the field of materials obtained from microgel-stabilised emulsions

Abstract

Colloidal gel particles called microgels have shown their ability to adsorb at an oil–water interface and stabilise emulsion named Pickering emulsions. Such particles are soft, deformable, and porous, and they can swell or contract under the action of an external stimulus. These specificities make them emulsifiers of special interest as they offer a large versatility to emulsions and materials elaborated thereof. This modularity is in counterpart at the origin of an abundant and often contradictory literature. The aim of this paper is to review recent advances in the emulsion stabilisation mechanism, particularly focusing on the microgel conformation at the interface in relation with the mechanical interface behaviour and the emulsion macroscopic stability. A sum up of the unambiguous knowledge is also proposed as well as few central questions that remain to be answered to in the domain.

Introduction

Pickering [1••] or Ramsden [2] emulsions, also called solid-stabilised emulsions consist in a class of emulsions stabilised by colloidal species [3•]. Although they were discovered more than a century ago, nowadays they benefit from a renewed interest mainly due to both the will of decreasing the use of non-eco-friendly surfactants and the abundance of particles able to adsorb at an oil–water interface. These particles may be of organic or inorganic nature, naturally occurring like viruses, spores, clays or synthesised owing to the improved knowledge in chemistry and surface functionalisation. Most often, the considered particles are solid and non-deformable as lattices, silica particles…. They usually densely pack at the interface providing a solid shell to the drops and conferring a good protection against coalescence and Oswald ripening [4]. Due to lateral interfacial interactions between adsorbed particles, the interface exhibits a plastic behaviour at the origin of the outstanding stability of Pickering emulsions [5•]. Such stability is of large interest for emulsion storage but may become a disadvantage for applications. For this reason, in the last years many works have evolved towards the use of stimuli-sensitive emulsions for which the particles interactions at the interface may be tuned in situ and consequently the emulsion destabilisation can be switched on demand just by modifying the pH, salt concentration, temperature, electric or magnetic field….

Solid and non-deformable particles are not representative of all the possible emulsions stabilisers and many studies have recently focused on the use of deformable particles. Among them, the most widely spread stabilisers consist in microgels. Microgels are colloidal particles made of poorly cross-linked polymer swollen by a solvent. Their swelling state is ruled by several thermodynamic parameters, which can serve as triggers to tune their swelling properties. In particular, external parameters such as temperature, solvent composition or an external field can be used as a stimulus (Fig. 1). A few years ago, these materials started to be studied as emulsion stabilisers and brought examples of responsive emulsions. However, the origin of emulsion responsiveness was obscure, mainly because the mechanism of emulsion stabilisation by microgels was not understood. Besides their responsiveness, these particles differ from hard particles by their softness and deformability. Indeed, microgels are known to deform in a large extend since they may pass through pores more than ten time smaller than their sizes [6]. Microgels have already shown their interest in different application areas, such as permeability [6] and material development [7], [8] where their deformability has been reported at solid interfaces. The question that hence arises is the consequence of such deformability on the liquid interface stabilisation mechanism. Hydrogel particles are physical materials with a defined volume, but they do not have a defined interface with the surrounding medium in the swollen state contrarily to hard particles. Furthermore, from a material point of view, a microgel is the sum of the polymeric network and its solvent. This situation is easy to define when they are dispersed in single solvent but becomes more complicated when they are dispersed in the presence of another immiscible solvent; the microgels can change their swollen state, as for example by incorporating partially or completely the second solvent, and might become different objects from the initial ones. Because of such a rich and complex behaviour, it becomes relevant to wonder whether microgels should be considered as classical particles or pieces of cross-linked polymers, i.e. linear polymers with a few constraints. In this review, we will address these fundamental questions by giving an overview of the progress made in the field. Also, we will describe the most recent results which aimed at understanding the differences between solid and soft particles, the microgel organisation at oil–water interfaces and its consequence on emulsion properties. In particular the possible microgel stretching or compaction at the interface will be examined in detail and the role of the microgel deformability will be highlighted and demonstrated as the source of their versatility.

Section snippets

Description of Pickering emulsion stabilised by hard colloids

The main originality of Pickering emulsions compared to more classical surfactant-stabilised emulsion is the absence of emulsifier dynamical exchange between the interface and the bulk. Indeed, from geometrical considerations of the three phase interface, the desorption energy E to remove an undeformable rigid particle from the interface can be expressed through the following relation:E=γπrp21cosα2where γ is the naked oil–water interfacial tension, rp is the particle radius and α, the contact

Microgel description

Microgel swelling obeys the same thermodynamic rules as their macroscopic analogues. The equilibrium gel-swelling volume is a balance between the inner and outer osmotic pressures of the polymer network. In the case of a neutral polymer, it is governed by polymer–solvent interactions, and the elasticity of the network. If the polymer is charged, an electrostatic contribution has to be taken into account: it arises from the Coulomb repulsions between charges and from the osmotic pressure exerted

Organisation of microgels at the oil–water interface

As it was demonstrated that microgels adsorb, it became important to qualify and quantify this adsorption. In a theoretical approach on gel adsorption, Vilgis and Stapper [23], using scaling theory, predicted that gels with weakly coupled cross-links and low number of cross-links should adsorb much better than hard gels with many cross-links because sufficiently cross-linked gels cannot interpenetrate each other. The most straightforward technique was the measurement of surface tension as

Link between microgel interfacial adsorption and emulsion properties: possible levers for the formulator

As described in [29], the way microgels adsorb at the interface directly impact emulsions stability and resistance towards mechanical stresses. Indeed emulsions stabilised by microgels could be either very resistant or very fragile depending on the microgel cross-linking density. Emulsions could also flow freely as liquids or be much flocculated exhibiting plug flows (Fig. 4). Destribats et al. [38••] showed that the flocculation of emulsions was a consequence of the adhesion between adjacent

Formulating emulsions with responsive microgels

After drawing a picture of the organisation of microgel at the oil–water interface and its impact on emulsion properties, it is now of interest to give an overview of the range of emulsions which can be produced using such stabilisers. We will now better focus on the emulsion composition, such as microgel amount and type, oil nature and volume of dispersed phase.

As already mentioned, oil-in-water and water-in-oil emulsions can be produced using pNIPAM-based microgels as stabilisers. The first

Conclusion

From the set of papers presented in this review, focussing mainly on the class of pNIPAM based microgels, some unambiguous results that make consensus in the community can be highlighted:

  • Microgels adsorb at the interface without any need of charges.

  • Emulsions stabilisation is not provided by charges and electrostatic interactions between microgels.

  • Due to their soft nature, microgels deform at the interface and interconnect. The interconnections are responsible for an interfacial elasticity.

  • The

Acknowledgements

The authors would like to thank Mathieu Destribats for fruitful discussion and careful reading of the manuscript.

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