ARTICLE IN PRESS YJCIS:13022 Please cite this article as: V.N. Paunov et al., Emulsions stabilised by food colloid particles: Role of particle adsorption and wettability at the liquid interface, J. Colloid Interface Sci. (2007), doi:10.1016/j.jcis.2007.03.031 JID:YJCIS AID:13022 /FLA [m5+; v 1.67; Prn:19/04/2007; 8:14] P.1 (1-9) Journal of Colloid and Interface Science ••• (••••) •••–••• www.elsevier.com/locate/jcis Emulsions stabilised by food colloid particles: Role of particle adsorption and wettability at the liquid interface Vesselin N. Paunov a,∗ , Olivier J. Cayre b,∗ , Paul F. Noble a , Simeon D. Stoyanov c , Krassimir P. Velikov c , Matt Golding c a Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, UK b Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA c Food Structural Design/UFHRI, Unilever Research, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands Received 24 October 2006; accepted 9 March 2007 Abstract We study the effect of the particle wettability on the preferred type of emulsion stabilised solely by food colloid particles. We present results obtained with the recently developed gel trapping technique (GTT) for characterisation of wettability and surface structuring of individual food colloid particles adsorbed at air–water and oil–water interfaces. This method allows us to replicate a particle monolayer onto the surface of polydimethylsiloxane (PDMS) without altering the position of the particles. By observing the polymer surface with scanning electron microscopy (SEM), we are able to determine the contact angle of the individual particles at the initial liquid interface. We demonstrate that the GTT can be applied to fat crystal particles, calcium carbonate particles coated with stearic acid and spray-dried soy protein/calcium phosphate particles at air–water and oil–water interfaces. Subsequently, we prepare emulsions of decane and water stabilised by the same food colloid particles and correlate the wettability data obtained for these particles to the preferred type of emulsions they stabilise.  2007 Elsevier Inc. All rights reserved. Keywords: Food particles; Solid-stabilised emulsions; Fat crystals; Wettability; Contact angle 1. Introduction The wettability of colloid particles and powder materials and their structuring at liquid surfaces is of crucial importance for a number of food processing technologies, cosmetic and phar- maceutical products [1–4]. It has long been recognised that nanoparticles and microparticles can be used as efficient emul- sifying agents [3–5]. Recently, the type and stability of emul- sions stabilised by hydrophobised silica nanoparticles [6] were studied for different fractions of –SiOH groups on the particle surface. It was found that the type of emulsion formed de- pends very much on the wettability (contact angle θ , measured through the water phase) of the individual particles when ad- sorbed at the liquid interface. Hydrophobic particles tend to sta- * Corresponding authors: Faxes: +44 (0) 1133 432377; +44 (0) 1482 466410. E-mail addresses: v.n.paunov@hull.ac.uk (V.N. Paunov), o.j.cayre@leeds.ac.uk (O.J. Cayre). bilise water-in-oil emulsions while hydrophilic particles form oil-in-water emulsion [7]. Inversion of the type of emulsion has also been observed as a function of the volume fraction of the dispersed phase for model silica nanoparticles of intermediate hydrophobicity [8]. Hence, characterisation of the wettability of individual col- loid particles is a valuable information when preparing solid- stabilised emulsions. Various techniques have been developed for characterisation of the wettability of powders, which treat the powder as a porous media. The Washburn method [9–11] is based on measurements of the liquid penetration rate in a compressed powder bed and produces averaged estimates of the particle contact angle which often depends on the degree of compression of the powder [12]. An alternative method re- lies on measuring the contact angle of a liquid drop on the surface of a powder tablet [13] which suffers from the same drawbacks. Direct observation [14] of particles attached to the liquid interface by an optical microscope has also been used to 0021-9797/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.03.031