Short communication Why foams containing colloidal hydrophilic particles are unstable

AbstractIn this work, properties from colloidal silica containing foam systems were analyzed and correlated to the fundamental concepts ofthe particle adsorption at the gas–liquid interface, specifically for the adsorption energy and its relationship to the thermal energy of theparticle. Based on the presented data, it can be pointed out that hydrophilic colloidal particles should give rise to unstable foams due totheir low adsorption energy, which is below the minimum threshold required evaluated for the particle containing foam stabilization.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Particle stabilized foams; Silica; Contact angle; Adsorption energy 1. IntroductionFoaming containing particles can result in systemshaving outstanding stability [1] or no foam formationdue to the action of highly hydrophobic materials [2]. Thisopposing trend can also be attained with the same sort ofparticle depending on the physicochemical state of theirsurface, which can be modified by various techniques [3,4](e.g., by using water-soluble amphiphilic molecules thatcan adsorb at the metal oxide surface and modify itsaqueous wettability or by changing the morphology of thesolid surface [5]).The literature presents many studies related to thefoaming behavior of colloidal silica suspensions. However,due to the various data related to the physicochemicalproperties of colloidal silica foams presented by Binks andHorozov [6], Blute et al. [7] and Gonzembach et al. [4]these works were selected to be explored and furtheranalyzed in this paper.Blute et al [7] evaluated hydrophilic (unmodified) col-loidal silica suspensions (10 wt%) and analyzed the foamstability of these systems. The selected experimental pro-cedure consisted of introducing air into the colloidalsuspensions and measuring the time required for the foamto collapse (the stability time). Fig. 1 presents the stabilitytime of foams evaluated by Blute et al. [6] as a function ofthe particle size (nm) of colloidal silica.Fig. 1 shows a general trend where the smaller theparticle, the higher the stability time, which is related tothe higher viscosity of the suspensions containing tinyparticles [7]. However, although the stability time changesare significant among those systems evaluated by Bluteet al. [7], they are almost irrelevant when compared tohighly stable ones [8], as the latter displays stability timevalues ranging from many days to years (which makesthem some million to billion times more stable than thoseshown in Fig. 1).In order to increase the stability time and also thesystem’s foaminess, Binks et al. [6] reported a method toproduce colloidal silica foams by changing the chemicalnature of the particles in suspension. They carried out asilanization by using dichlorodimethylsilane in alcoholicmedium, which resulted in the silica’s contact angle (y)increase, making a less hydrophilic particle surface. Aim-ing to quantify this effect, some measurements of thesurface relative SiOH content were carried out. Becausethe SiOH chemical group is commonly present at the silicasurface, this compound should be an indicative of itshighly hydrophilic feature (contact angleE5–101 [9]).Furthermore, by the silanization, Binks and Horozov [6]