Effects of Surfactant Structure on the Phase Inversion of Emulsions Stabilized by Mixtures of Silica Nanoparticles and Cationic Surfactant

Abstract

Silica nanoparticles without any surface modification are not surface active at the toluene-water interface due to their extreme hydrophilicity but can be surface activated in situ by adsorbing cationic surfactant from water. This work investigates the effects of the molecular structure of water-soluble cationic surfactant on the surface activation of the nanoparticles by emulsion characterization, adsorption and zeta potential measurements, dispersion stability experiments, and determination of relevant contact angles. The results show that an adsorbed cationic surfactant monolayer on particle surfaces is responsible for the wettability modification of the particles. In the presence of a trace amount of cationic surfactant, the hydrophobicity of the particles increases, leading to the formation of stable oil-in-water O/W(1) emulsions. At high surfactant concentration (>cmc) the particle surface is retransformed to hydrophilic due to double-layer or hemimicelle formation, and the concentration of the free surfactant in the aqueous phase is high enough to stabilize emulsions alone. O/W(2) emulsions, probably costabilized by free surfactant and particles, are then formed. The monolayer adsorption seems to be charge-site dependent. Thus, using single-chain trimethylammonium bromide surfactants or a double-head gemini cationic surfactant, the hydrophobicity of the particles achieved is not sufficient to stabilize water-in-oil (W/O) emulsions, and no phase inversion is induced. However, using a double-chain cationic surfactant, the chain density on the particle surfaces endows them with a hydrophobicity high enough to stabilize W/O emulsions, and double phase inversion, O/W(1) --> W/O --> O/W(2), can then be achieved by increasing the surfactant concentration.

Electromagnetophoretic Migration Velocity of Organic Microdroplets with Surfactants Using Permanent Magnets

“…20 The apparent conductivity of organic droplets was governed by their surface conductivity and the obtained value was reasonable as the surface conductivity of organic droplets, because the zeta potential of polystyrene latex in 0.1 M KCl aqueous solution, which was reported to be about -25 mV, was comparable with those of organic emulsions. 21 Our study thus suggests that organic droplets in the aqueous solution could be characterized by electromagnetophoretic migration velocimetry.…”

“…21 Therefore, we tried to evaluate the surface property of the droplets from their electromagnetophoretic migrations. Figure 4 shows the current dependency of the measured electromagnetophoretic velocity normalized to the unit radius of 2-fluorotoluene droplets with a diameter of 10 -16 μm dispersed in a 1.0 M KCl solution in the presence of CTAB with concentration range of 10 -6 -10 -4 M. Generally, the migration velocity of organic droplets decreased as the concentration of CTAB increased in these measurements.…”

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

“…Then 7 mL of oil (n-decane) was added and the mixture was homogenized using an ultraturrax homogenizer (IKA T18 basic, S18N-10G head) at 11 000 rpm for 2 min, with the emulsion type being identied by the drop test. 39 The concentration of alumina and surfactant is expressed as weight percentage (wt%) and moles per liter (mol L À1 ) in the aqueous phase respectively. Emulsions were allowed to stand at room temperature to observe their stability by taking photographs of the vessels and micrographs of the droplets using a VHX-1000 microscope system (Keyence Co.).…”

Smart and green interfaces: From single bubbles/drops to industrial environmental and biomedical applications

“…The large development of nanomaterials fostered during the last decade the use of micro/nanoparticles as additives in the formulation and for the control of the stability of emulsions and foams [23,24,25]. The subject presents several "smart" and "green" aspects.…”