Selective retardation of perfume oil evaporation from oil-in-water emulsions stabilized by either surfactant or nanoparticles.

We have used dynamic headspace analysis to investigate the evaporation rates of perfume oils from stirred oil-in-water emulsions into a flowing gas stream. We compare the behavior of an oil of low water solubility (limonene) and one of high water solubility (benzyl acetate). It is shown how the evaporation of an oil of low water solubility is selectively retarded and how the retardation effect depends on the oil volume fraction in the emulsion. We compare how the evaporation retardation depends on the nature of the adsorbed film stabilizing the emulsion. Surfactant films are less effective than adsorbed films of nanoparticles, and the retardation can be further enhanced by compression of the adsorbed nanoparticle films by preshrinking the emulsion drops.

Phase inversion of particle-stabilised perfume oil-water emulsions: experiment and theory.

Phase inversion of fumed silica particle-stabilised emulsions of water and perfume oil can be effected in three ways. The transitional inversion from water-in-oil (w/o) to oil-in-water (o/w) occurs upon increasing the particle hydrophilicity for 9 oils of different polarity and structure. Results are compared for systems in which particles are pre-dispersed in one of the bulk phases and for those in which a novel powdered particle method is used. Using a simple theory involving the surface energies of the various interfaces, the contact angle θ of a particle with the oil-water interface is calculated as a function of the particle hydrophilicity. Assuming phase inversion occurs at θ = 90°, very good agreement is obtained for all oils between the calculated and experimental particle hydrophilicity required for inversion in the case of the powdered particle method. Inversion from o/w to w/o induced by simply increasing the particle concentration is shown to be as a result of changes in the aggregation state of the particles influencing their wettability. Finally, catastrophic phase inversion from w/o to o/w is achieved by increasing the volume fraction of water, and multiple emulsions form around inversion in a system containing only one particle type. Results of the latter two inversion routes are combined to develop an emulsion compositional map allowing a variety of emulsions with different characteristics to be described by varying the relative amounts of the three components.

Drop sizes and particle coverage in emulsions stabilised solely by silica nanoparticles of irregular shape.

We have investigated emulsions stabilised solely by partially-hydrophobised fumed silica particles which consist of a mixture of primary particles and irregularly-shaped fused aggregates and larger agglomerates. The particle wettability is controlled by varying the extent of hydrophobisation of their surfaces. This, in turn, controls the contact angle between the oil-water interface and the particle surface (θ(ow)) which affects the particle adsorption energy and the type of emulsion formed (oil-in-water, o/w or water-in-oil, w/o). Progressive particle hydrophobisation causes transitional phase inversion of the emulsions from o/w to w/o which occurs when θ(ow) = 90° and the energy of particle adsorption to the oil-water interface is maximally favourable. The key problem addressed here is to understand why the emulsion drop size passes through a minimum at the point of emulsion phase inversion. In principle, this effect could be the result of particle desorption, changes in the extent of close-packing of the adsorbed particle film (at constant particle orientation), particle re-orientation or a combination of these processes. Using measurements of emulsion drop size and the extent of particle desorption, we have derived adsorbed particle surface concentrations as a function of the energy of desorption of the particles from the oil-water interface for a range of particle concentrations and different oil-water systems. The main conclusion is that the minimum in emulsion drop size through phase inversion is mainly caused by re-orientation of the particles from a high surface area orientation when the energy of desorption is high to a low surface area orientation when the energy of desorption is low. Some particle desorption occurs but this is a secondary effect.

Compositional ripening of particle- and surfactant-stabilised emulsions: a comparison.

When beta-ionone-in-water emulsions are mixed with squalane-in-water emulsions, the slightly water-soluble, mobile beta-ionone undergoes mass transfer to the drops of highly water-insoluble, immobile oil squalane. We have investigated this compositional ripening process for emulsions stabilised either by particles or by surfactant molecules. For particle-stabilised emulsions, the swelling of the squalane-containing drops triggers droplet coalescence which causes the final swollen droplet radius to be proportional to the swelling ratio to the power of 1. Surfactant-stabilised emulsions swell without coalescence which causes the final droplet radius to be proportional to the swelling ratio to the power 1/3. Addition of excess, non-adsorbed particles to the particle-stabilised emulsions suppresses the swelling-triggered coalescence and causes a switchover from particle to surfactant behaviour.