CO₂/N₂ triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant

Abstract

Stable n-decane-in-water Pickering emulsions were prepared using positively charged alumina nanoparticles in combination with a trace amount of the anionic surfactant sodium dodecyl sulfate (SDS) as a stabilizer. Particles were hydrophobized in situ by adsorption of surfactant enhancing their surface activity. Emulsions can be readily demulsified by addition of an equal amount of a switchable surfactant,, which can be transformed between a surface-active amidinium/cationic form and a surface-inactive amidine/neutral form by bubbling CO 2 or N 2 ,respectively. Following addition of cationic DDAA which prefers to form ion pairs with SDS, desorption of SDS from particle surfaces occurs and alumina particles are rendered hydrophilic resulting in demulsification of the emulsion. However, by bubbling N 2 into the demulsified mixture, DDAA molecules are converted to the amidine/neutral form leading to collapse of the ion pairs and re-establishment of the in situ hydrophobization of particles. Stable Pickering emulsions can be prepared again following homogenization. This simple demulsification/re-stabilization cycle can be repeated several times.Experimental evidence including measurement of the adsorption isotherm, zeta potentials, extent of particle adsorption at droplet interfaces in emulsions and microscopy is given to support the postulated mechanisms.

Novel Oil‐in‐Water Emulsions Stabilised by Ionic Surfactant and Similarly Charged Nanoparticles at Very Low Concentrations

“…[18] Some of these particle types are switchable between being surfaceactive and surface-inactive using stimuli such as pH, temperature and CO 2 /N 2 addition. Arab et al have just given areview on this topic, [19] and up to now systems involving ionic surfactants and oppositely charged inorganic particles have been widely studied, [19][20][21][22][23][24][25][26][27][28] in which hydrophilic particles like silica, [19,21,23,24] laponite, [20] or calcium carbonate [15,22] can be made surface-active by adsorbing oppositely charged ionic surfactant so as to stabilize aP ickering emulsion. Arab et al have just given areview on this topic, [19] and up to now systems involving ionic surfactants and oppositely charged inorganic particles have been widely studied, [19][20][21][22][23][24][25][26][27][28] in which hydrophilic particles like silica, [19,21,23,24] laponite, [20] or calcium carbonate [15,22] can be made surface-active by adsorbing oppositely charged ionic surfactant so as to stabilize aP ickering emulsion.…”

“…Arab et al have just given areview on this topic, [19] and up to now systems involving ionic surfactants and oppositely charged inorganic particles have been widely studied, [19][20][21][22][23][24][25][26][27][28] in which hydrophilic particles like silica, [19,21,23,24] laponite, [20] or calcium carbonate [15,22] can be made surface-active by adsorbing oppositely charged ionic surfactant so as to stabilize aP ickering emulsion. [26][27][28][29] On the other hand, emulsion systems involving similarly charged ionic surfactant and particles are much less studied. [26][27][28][29] On the other hand, emulsion systems involving similarly charged ionic surfactant and particles are much less studied.…”

“…We reported recently that stable n-decane-in-water Pickering emulsions with much larger droplet size (50-200 mm) can be prepared by using cationic alumina nanoparticles plus an anionic surfactant SDS (0.02-0.2 mm)a se mulsifier, [26] as shown in Figure S6, where the alumina nanoparticles are hydrophobized in situ by adsorption of SDS molecules and are then capable of adsorbing at the oil-water interface.I ti s natural to ask what is the microstructure of emulsions stabilised by alumina nanoparticles in combination with at race amount of CTAB?A re they classical Pickering emulsions or conventional emulsions stabilised solely by surfactant?I ft hey are conventional emulsions,h ow do the particles enhance emulsion stability?…”

“…Depending on the type and adsorbed amount of surfactant, the Pickering emulsions can undergo as ingle or double phase inversion [15,22,23] and even be made switchable [25] or stimuli-responsive. [26][27][28][29] On the other hand, emulsion systems involving similarly charged ionic surfactant and particles are much less studied. [19,[30][31][32] Herein we report for the first time novel O/W emulsions which can be stabilised by ac ationic surfactant (cetyltrimethyl ammonium bromide,C TAB) in combination with positively charged alumina nanoparticles at extremely low concentration (ca.…”

“…[1] Surfactant behaves as the emulsifier whereas particles behave as an additional stabilizer.B yc ontrast, the zeta potential of the particles in SDS solutions decreases significantly and changes sign with increasing SDS concentration, in line with adsorption of SDS at the particle-water interface and subsequent stabilization of Pickering emulsions. [26] Theexcellent stability of emulsions at very low (< 0.6 mm) CTAB concentration can then be attributed to the presence of non-adsorbed particles.Bymeasuring the lifetime of asingle n-decane droplet (2 mL) at aplanar n-decane-water interface containing an adsorbed CTAB monolayer (a good measure of droplet stability), it is found that in the absence of particles the droplet lifetime is very short (< 10 s) at CTAB concentrations below 0.06 mm and increases to several min by 0.3 mm.However,inthe presence of 0.5 wt %particles in the aqueous phase,t he lifetime increases dramatically at CTAB concentrations beyond 0.003 mm,asshown in Figure 5a.Ifwe take the CTAB concentration at the break point of the curves as C min,CTAB ,i tdecreases from 0.06 mm in the absence of alumina particles to 0.001 mm in the presence of 0.5 wt % alumina particles.I nr eality,C min,CTAB falls from 0.6 mm for CTAB alone ( Figure S4B) to 0.001 mm in the presence of 0.5 wt %p articles ( Figure 1A and Figure 3), in good agreement with droplet lifetime measurements.A tafixed CTAB concentration of 0.01 mm,the droplet lifetime increases with increasing particle concentration, giving ac ritical particle concentration of 2 10 À5 wt %, Figure 5b,b eyond which the droplet lifetime increases dramatically and emulsions are stabilised ( Figure 2).…”