Effects of solid particle content on properties of o/w Pickering emulsions Justyna Frelichowska, Marie-Alexandrine Bolzinger, Yves Chevalier * Université de Lyon, UMR CNRS 5007, Laboratoire d’Automatique et de Génie des Procédés, Université de Lyon 1, 43 bd 11 Novembre, 69622 Villeurbanne, France article info Article history: Received 21 April 2010 Accepted 5 August 2010 Available online 10 August 2010 Keywords: Emulsions Pickering emulsions Silica Particle size distribution Granulometry abstract The control of droplet size and stability of oil-in-water Pickering emulsions stabilized by hydrophobized fumed silica was investigated. Three regimes were observed according to the silica content: instability at low silica content, stable emulsions with droplet size controlled by the silica content, and emulsions of constant size set by the emulsification process at high silica concentrations. The oil-to-silica ratio was the relevant parameter of the size control in the medium concentration regime. Centrifugation experi- ments and particle size distribution measurements gave evidence of the presence of excess silica present as dispersion in the aqueous phase in the high silica content regime. Adsorption of silica to the droplet surface did not follow adsorption equilibrium; strong adsorption prevailed. Lastly, aggregation of silica particles appeared a crucial parameter. Oil adsorption and capillary condensation of oil within the silica aggregates provided a supplementary mechanism of silica aggregation that contributed to the stability of emulsions. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Emulsions made without use of an emulsifier have attracted particular attention for applications where the surfactants cause adverse effects such as air entrapment, foaming, irritancy, interac- tions with living matter. An elegant way is substituting emulsifiers by solid particles as stabilizing agents of droplets. So-called Picker- ing emulsions are stabilized with solid particles adsorbed at the oil–water interface [1–7]. Adsorption of particles requires partial wetting of their surface by both oil and water. Preferred wetting by water such that the contact angle in water, h ow , is lower than 90°, favors oil-in-water type emulsions; the reverse type for h ow < 90° [6,7]. An o/w type emulsion is formed when oil is emul- sified into an aqueous colloidal suspension of solid particles; w/o type is obtained when solid particles are present in the oil phase. For concentrated emulsions, phase inversion was observed when the fabricated emulsion type was not the preferential one with re- spect to wetting conditions [8–11]. Several types of solid particles have been used among which hydrophobically modified fumed sil- ica particles are the most popular solid used as stabilizing particles [5–11]. Such fumed silica grades are commercially available as thickeners for cosmetic formulations [12]. They consist in aggre- gates of elementary particles of 10–30 nm diameters which adsorb at oil/water interface during the emulsification process. As for surfactant-stabilized emulsions, it is expected that the amount of stabilizing material would control the size of droplets because the extent of stabilized interfacial area is proportional to the amount of stabilizer. The mean droplet diameter, D, is related to the interfacial area per unit volume of emulsion, A/V, and vol- ume fraction of dispersed phase, / v D ¼ 6/ v V A ð1Þ Since the interfacial area is related to the amount of stabilizing par- ticles, this geometrical relationship teaches that the diameter of oil- in-water emulsion droplets is given by the silica-to-oil ratio. It has indeed been shown that a larger amount of stabilizing silica parti- cles allowed the formation of smaller droplets of Pickering emulsion [4,6,13]. There is consensus regarding the control of the droplet size by the amount of silica particles. The claim stating that larger silica content allows stabilizing a larger interfacial area is a general idea that holds at a qualitative level. The geometrical relationship be- tween D and A/V (Eq. (1)) does not link the diameter D with the sil- ica content in a quantitative manner however. There are several open questions regarding the quantitative relationship between silica content and droplet size. Thus it is often stated that the inter- facial area is proportional to the mass of silica because full silica content gets adsorbed at the oil–water interface. The very large affinity of silica particles for the interface was rationalized in terms of wetting behavior [6,14]. On another hand, measured adsorption isotherms of silica particles at the water–silicone oil interface have shown that adsorption equilibrium takes place [14–18]; therefore only part of the silica is adsorbed at the interface. The presence of particles in the bulk phase of emulsion has been reported in case where the droplet surface was not fully covered [14,19]. Particles in excess could be found in the external phase for the emulsions 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.08.019 * Corresponding author. Fax: +33 4 72 43 16 82. E-mail address: chevalier@lagep.univ-lyon1.fr (Y. Chevalier). Journal of Colloid and Interface Science 351 (2010) 348–356 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis