Mixed-emulsifier stabilised emulsions: Investigation of the effect of monoolein and hydrophilic silica particle mixtures on the stability against coalescence

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Abstract

The stability against coalescence of vegetable oil-in-water “food grade” emulsions in the presence of both surfactant (monoolein) and colloidal particles (hydrophilic silica) has been studied and compared to the stability of systems where only the low molecular weight surfactant or the colloidal particles act as the emulsifier. No attempt was made to stop the emulsions from creaming and the data presented for coalescence stability is for droplets in the creamed layer. These are severe conditions as the contact time between droplets in such closed packed conditions is very high or even infinite.

These mixed emulsifier systems were found to induce long-term emulsion stability against coalescence via a synergistic “two-part” mechanism in which both the surfactant and colloidal particles components have specific functions. The role of the surfactant is to initially “delay” the re-coalescence phenomena and induce further droplet break-up during emulsification by rapidly covering the new (naked) interface and reducing interfacial tension in order to allow the time for the silica particles to assemble at the oil/water interface and provide long-term stability. This dual manner by which mixed-emulsifier systems induced stability was found to depend on the concentrations of both monoolein and silica particles.

Graphical abstract

“Food-grade” O/W Pickering emulsions stabilised by a mixed-emulsifier system containing surfactants and colloidal particles is investigated. Long-term stability and emulsion droplet size reduction are induced by a synergistic “two-part” mechanism.

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Introduction

The fact that emulsions can be stabilised by finely divided solid particles was first reported over a century ago. The discovery of this type of emulsions is usually ascribed to Pickering (hence the term “Pickering emulsions”) who published the first extensive experimental study on particle-stabilised emulsions for plant sprays applications [1]. Despite their early discovery, interest in this type of emulsions has only increased in the last two decades, mainly due to their potential for enhanced stability, over the more common surfactant stabilised emulsions, and also as a result of the recent achievements in material sciences and nanoparticle technology [2], [3], [4], [5]. Consequently some interesting reviews comparing particles and surfactants, in terms of their ability to stabilise emulsions and foams, have recently been published [6], [7]. Clues and understanding on the behaviour of these systems can be gained from the food science area, since it has been known for some time that in many food emulsions, stabilised primarily by phospholipids or proteins, particles are necessary for the required stabilisation, e.g. fat particles in margarine [8]. Other industrial applications can be encountered in areas including cosmetics, pharmaceutics, oil-recovery and wastewater treatment [7].

The ability of surfactants (and surface-active polymers) to stabilise emulsions has been long appreciated [9]. Due to their amphiphilic character, surfactants adsorb at the water–oil interface and, depending on the size of their hydrophilic and hydrophobic parts (surfactant's “head” and “tail” respectively), will tend to curve it towards the water or oil phase of the emulsion. The most important variable determining this water or oil “liking” tendency, and therefore the type of emulsion the surfactant is more suitable to stabilise, is the hydrophilic–lipophilic balance (HLB). The more hydrophilic surfactants (high HLB) tend to stabilise oil-in-water (O/W) emulsions, as they curve towards the oil phase while the more lipophilic ones (low HLB) tend to stabilise water-in-oil (W/O) emulsions, in the opposite way [10].

Emulsion stabilisation by colloidal particles also occurs as a result of their strong adsorption at the water–oil interface [1], [10], [11]. The exact positioning of a (spherical) particle at the interface is a result of its wettability by the oil and water phases of the emulsion (which phase the particles prefer to be dissolved in) and this preference can be best quantified by the contact angle θ, which is the angle that the particle makes with the water–oil interface. In many ways like the surfactant's HLB, the contact angle determines the type of emulsion that the particle can stabilise [10], [12], [13]. Particles of contact angles (measured through the water phase) θ<90° tend to stabilise O/W emulsions and particles of θ>90° tend to stabilise W/O emulsions. However, pure hydrophilic particles (very small contact angle) or too hydrophobic (very high contact angle) particles are more likely to stay dispersed in the water or the oil phase respectively, and therefore provide emulsions of very poor stability.

Despite the similarities between colloidal particles and surfactants, in terms of their behaviour at the water–oil interface and therefore the stability they impart upon emulsions, there are also a number of important differences. For example particles absorb more strongly at the interface and, once they do, it is less likely for them to be removed (e.g. displaced by other amphiphiles or removed during intensive processing). Furthermore interfaces of particle-stabilised emulsions are more rigid [14], than surfactant stabilised ones, and therefore emulsion droplets are less prone to coalescence once the surface is fully covered, however they are more prone to coalescence at partial coverage [15]. On the other hand, surfactants, due to their smaller size, adsorb at the water–oil interface much faster than particles. This can prove extremely advantageous in those instances when the surface area of the dispersed phase is increased (e.g. by the stretching of the interface or by droplet break-up) and emulsifier from the bulk needs to adsorb and stabilise the developed “naked” interface segments, before coalescence occurs. Moreover the positioning of surfactants at the water–oil interface (dictated by their HLB value) can be more easily manipulated (e.g. by adjusting the size of the surfactant's hydrophilic “head” or hydrophobic “tail”) than that of particles, which instead would require the introduction, by usually a rather ill-controlled process, of a level of hydrophobicity (or hydrophilicity) onto the surface of the hydrophilic (or hydrophobic respectively) particle.

Understandably the need arises to make use of the advantages of both surfactant and particle-stabilised emulsions in order to eliminate the disadvantages associated with their individual use as sole emulsifiers. Nonetheless emulsions where surfactants and particles act as a mixed-emulsifier system have received little to no attention [16], [17], [18], [19]. Gelot et al. [18] reported on the emulsion stability of oil–water systems in the presence of combinations of anionic or cationic surfactants and hydrophilic or hydrophobic particles. The authors concluded that stability in the mixed systems depends on the interaction between the two emulsifiers as well as on the effect of each of them individually. For example it was deduced that the enhanced stability in the studied water–toluene system when clay particles and sodium dodecyl sulphate (SDS) were combined was probably a result of both an increase in the electrical repulsion between the emulsion droplets (ascribed to the clay particles) and a decrease in interfacial tension, ascribed to SDS [18]. In another study Tambe and Sharma [14] showed that small concentrations of stearic acid improved the stability of water-in-decane emulsions stabilised by either hydrophilic or hydrophobic particles. The authors came to the conclusion that the effectiveness of the solid particles was greatly influenced by the presence of the surfactant. The use of small emulsifiers to control the wettability of the particles and the way they sit at the interface (the contact angle) is of growing interest.

However, and in light of the great potential of these systems in food applications, what seems to be absent from the limited mixed-emulsifiers studies, and also, to some extent, from those reporting on solely particle-stabilised emulsions, is an in-depth investigation into more “food-grade” systems. The available literature is mainly concerned with water–oil emulsions containing non-“food-grade” oil-phases, i.e. toluene, hexane, n-decane, silicone oil, etc., or stabilised by non-“food-grade” surfactants/emulsifiers, i.e. SDS, etc. In the present study there was a conscious effort to move towards more “food-grade” systems by using commercially available pure vegetable oil as the oil-phase and also monoglycerides (naturally occurring surface active compounds) as the surfactant/emulsifier component of the formed emulsions.

The aim of the work reported herein was to investigate the stability against coalescence of O/W emulsions in the presence of both surfactants (monoolein) and colloidal (hydrophilic silica) particles. Systems stabilised solely by either surfactants or colloidal particles are initially considered. Emulsions in the presence of the mixed-emulsifier system are then extensively discussed and compared, in terms of their stability, to those where only surfactants or colloidal particles act as sole emulsifiers.

Section snippets

Materials

The oil used in this study was commercially available pure vegetable oil. Monoolein (monooleate-1-glycerin, HLB = 3.8), used as the surfactant component in this study, was provided by Fluka AG Buchs SG (Switzerland). Hydrophilic silica particles (Aerosil 200), with a primary particle diameter of 12 nm and a surface area of 200 m2/g, were kindly provided by Degussa in powder form. The water used in all experiments was passed through a reverse osmosis unit and then a Milli-Q water system. All

Surfactant-stabilised emulsions

The stability of O/W emulsions, adjusted to pH 2, against coalescence in the presence of monoolein, acting as the sole emulsifier, was initially investigated. Although the presence of oil droplets in the investigated systems was detected by light microscopy, shortly after emulsification, these O/W emulsions were found to be very unstable against coalescence. More specifically complete phase separation into the oil and aqueous phases was found to occur within 2–3 h after emulsification and

Conclusions

The stability against coalescence of O/W emulsions in the presence of mixed-emulsifier systems, containing both monoolein and colloidal particles, was investigated. The emulsions produced are “food grade” as pure vegetable oil was used as the oil-phase in all investigated systems combined with the use of a “food-grade” emulsifier/surfactant and hydrophilic silica particles, regularly used as food additives [23]. By investigating such “real” food emulsion systems we believe that this work

Acknowledgments

The authors would like to acknowledge financial support from EPSRC.

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