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The Effect of Nanoparticle Hydrophobicity on the Rheology of Highly Concentrated Emulsions

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The Effect of Nanoparticle Hydrophobicity on the Rheology of Highly Concentrated Emulsions The Effect of Nanoparticle Hydrophobicity on the Rheology of Highly Concentrated Emulsions N.N. Tshilumbu*, I. Masalova Department of Civil Engineering, Faculty of Engineering, Cape Peninsula University of Technology, PO Box 652, Cape Town 8000, Republic of South Africa * Corresponding author: [email protected] Fax: x27.21.4603990 Received: 18.2.2013, Final version: 7.8.2013 Abstract: A series of fumed silica nanoparticles were used as an additional emulsifier for highly concentrated (HC) water- in-oil (W/O) emulsions. These nanoparticles, with different hydrophobicity index (HI) in the 0.60 – 1.34 and HI >3 range, were mixed with the conventional low molecular weight surfactant, sorbitan monooleate (SMO), in the oil phase prior to the emulsification process. The rheological properties of these emulsions were measured and compared with the properties of emulsions stabilized with SMO alone. In the mixed emulsifier system, the changes in rheological parameters were clearly expressed as a function of HI. The mixture of silica nanoparti - cles and SMO significantly increases the yield stress and plateau modulus of fresh emulsion, compared to the SMO only system. The effect was found to be more pronounced with a decrease in the HI. This is probably relat - ed to the reduction in micelle content with the decrease in HI, owing to a concomitant increase in the amount of SMO adsorbed onto the particle surface. Then, interestingly, the Foudazi-Masalova model recently developed for surfactant-stabilized highly concentrated emulsions (HCE) was found to describe successfully the rheologi - cal behavior of emulsions in the presence of a mixture of surfactant and fumed nanosilica. Zusammenfassung: Mehrere pyrogene Silika-Nanopartikel wurden als zusätzliche Emulgierungsmittel für hochkonzentrierte (HC) Wasser-in-Öl (W/O)-Emulsionen verwendet. Diese Nanopartikel, die unterschiedliche Hydrophobizitätsindizes (HI) im Bereich von 0.60 bis 1.34 und HI > 3 besitzen, wurden mit einem konventionellen, niedermolekularen Sur - faktanten (Sorbitanmonooleat, SMO) in die Ölphase vor dem Emulsifikationsprozess gemischt. Die rheologi - schen Eigenschaften dieser Emulsionen wurden gemessen und mit den Eigenschaften der mit dem SMO allei - ne stabilisierten Emulsionen verglichen. Bei den gemischten Emulgierungsmitteln waren die rheologischen Eigenschaften eindeutig vom HI-Wert abhängig. Die Mischungen der Silika-Nanopartikel und dem SMO erhöh - ten in signifikanter Weise die Fließspannung und den Plateau-Modul der frischen Emulsion im Vergleich zu dem System, das mit SMO alleine stabilisiert wurde. Der Effekt trat deutlicher bei einem geringeren HI-Wert auf. Dies steht wahrscheinlich mit der Abnahme des Mizellengehaltes aufgrund der als Begleiterscheinung auftretenden Zunahme des SMO-Gehalts in Zusammenhang, das auf der Partikeloberfläche adsorbiert ist. Weiterhin beschreibt interessanterweise das Foudazi-Masalova-Modell, das kürzlich für mit durch Surfaktanten-stabili - sierte hochkonzentrierte Emulsionen (HCE) entwickelt wurde, das rheologische Verhalten der Emulsionen mit Surfaktanten und pyrogenen Nanosilikapartikeln. Résumé: Une série de nanoparticules de fumée de silice a été utilisée comme émulsifiant additionnel d’émulsions eau- dans-huile (W/O) concentrées. Ces nanoparticules, possédant différents index d’hydrophobicité (HI) dans la gam - me 0.6 – 1.34 et HI > 3, ont été mélangées avec du surfactant conventionnel de bas poids moléculaire, du mono oléate de sorbitane (SMO), dans une phase d’huile, avant le procédé émulsifiant. Les propriétés rhéologiques de ces émulsions ont été mesurées et comparées avec celles des émulsions stabilisées seulement par le SMO. Dans le système d’émulsifiant mixte, les changements des paramètres rhéologiques avec le HI sont clairement révé - lés. La mixture de nanoparticules et de SMO augmente significativement la contrainte seuil et le module pla - teau de l’émulsion fraîche, comparé au système avec le SMO seul. L’effet s’est avéré plus prononcé lorsque le HI décroît. Ceci est probablement lié à la réduction du nombre de micelles avec la baisse du HI, du à l’augmenta - tion correspondante de la quantité de SMO adsorbé sur la surface de la particule. Ensuite, de manière intéres - sante, le modèle Foudazi-Masalova récemment développé pour les émulsions concentrées stabilisées par du sur - factant (HCE), décrit avec succès le comportement rhéologique en présence du mélange de surfactant et de fumée de silice. Key words: emulsions, rheology, fumed nano-silica, surfactant © Appl. Rheol. 23 (2013) 62835 DOI: 10.3933/ApplRheol-23-62835 Applied Rheology 62835 -1 Volume 23 · Issue 6 1 INTRODUCTION [25, 28]. In addition, there is no data on the stabili - ty or rheological properties of such emulsions, espe - Highly concentrated emulsions (HCE) represent cially when the internal phase is an oversaturated a rather special class of colloids. As with any oth - aqueous salt solution. We attempted to prepare HC er emulsion, they are a mixture of two incom - W/O Pickering emulsions (90 vol.%) using fumed patible liquids usually stabilized with a surfac - nanosilica particles with different hydro phobicity tant. But in contrast to standard emulsions, the index (HI) as emulsifiers. Our preliminary experi - concentration of the dispersed phase in HCE ments showed that nanoparticles with intermedi - exceeds the limit of the closest packing of regu - ate HI (0.97 £ HI £ 1.34) can form W/O emulsions lar spheres (app. 0.74), which is possible because with aqueous phase concentration of up to 77 – 79 of the ‘compressed’ state of droplets that have a vol.% only, and the resulting emulsions are charac - polyhedral shape. The subject of this investiga - terized by high instability under shear [14]. Then we tion is water-in-oil super-concentrated emul - attempted to increase the dispersed phase volume sions (90 vol.%), with oversaturated aqueous salt fraction by mixing silica nanoparticles with a typi - solution as dispersed phase, as in liquid explo - cal low-molecular weight emulsifier-sorbitan sives. The determination of the rheological char - mono oleate (SMO), as it is well known that SMO acteristics of such emulsions is crucial, as they are alone can form stable HC W/O emulsion up to 90 used as the base for transport and technological vol.% [34 – 36]. Additional interest in this kind of applications of these systems. Moreover, in some mixtures is related to the possibility of modifying of their technological applications, a high yield the particle hydrophobicity in a wide range, and stress is usually desirable, as it enables the emul - consequently varying its interaction with SMO and sion to resist water intrusion, retain its stability both phases of emulsions. Tambe and Sharma and be able to resist flow under the action of focused on idealized systems of naturally occurring gravity [29]. oil-brine emulsions, using barium sulphate, calci - The rheology of these emulsions was stud - um carbonate and silica [40 – 42]. Stearic acid was ied and described by the research group in sev - used as a surfactant emulsifier. It was found that eral publications [1 – 9]. However, in all previous surfactant addition to particle mixtures led to studies, standard surfactants usually used in emulsion stability far greater than found with technology – derivatives of poly (isobutylene) either particles or surfactant alone. This synergy succinic anhydride – were used as emulsifiers. In was attributed to the surfactant’s ability to increase this case high yield stresses are usually obtained the particle contact angle on adsorption. In a simi - by subjecting the emulsion to a process referred lar approach, Gosa and Uricanu found that systems to as homogenization, whereby the droplet size of silica and PEO–PPO–PEO block copolymers is significantly reduced. The latter process, how - caused synergistic stability [50]. They attributed the ever, results in an increasing tendency of the synergy to principally increased flocculation of sili - oversaturated aqueous salt solution in the dis - ca with surfactant adsorption. Midmore furthered persed phase of the emulsion to crystallize, these findings, in systems of silica and both PEO [43] because of the high shear conditions. Thus, for a and HPC [38, 39] surfactants. Again clear synergy given composition, there is a practical limit to the was found between silica and each of surfactants. degree of homogenization that can occur before There was evidence for increased flocculation of sil - crystallization of the oversaturated aqueous salt ica with surfactant addition, and in creased contact solution in the dispersed droplets becomes unac - angle. Recently Binks and co- wor kers looked at the ceptable [29]. synergy between moderate concentrations of Nanosilica as an emulsifier in Pickering emul - nanosilica (20 nm) and non-ionic PEO type surfac - sions is well known in applications to dilute or con - tants [37]. For systems with high stability clear evi - centrated emulsions and the resulting emulsions dence of small aggregate formation was present, are reported to demonstrate outstanding stability, confirmed with bulk rheology tests showing particularly with regard to coalescence and unusu - increases in viscosity linked to this flocculation. al rheological behavior [10 – 13, 25, 44, 48, 49]. How - Despite some activity in this area, system - ever, only few studies have reported the possibility atic studies of stability and rheology of HC W/O of forming high internal volume fraction Pickering emulsions using mixtures
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