Available online 9 July 2008.
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The interaction between unmodified CaCO3 nanoparticles and sodium dodecyl sulphate (SDS) in aqueous solution and the subsequent emulsification of the mixed dispersions are studied by means of zeta potential measurements, adsorption and emulsion characterization. Using toluene or octane as oil phase, hydrophilic CaCO3 nanoparticles stabilize only temporarily oil-in-water, O/W, emulsions. Upon the addition of trace amounts of SDS in the aqueous phase, the stability of the emulsions is greatly improved. Further increasing the SDS concentration results in a double phase inversion, O/W(1) to water-in-oil, W/O to O/W(2). It is suggested that the adsorption of SDS molecules on particle surfaces rendering them hydrophobic induces the first inversion. When the adsorption of SDS reaches a maximum, the surface of the particles are re-converted to hydrophilic due to a surfactant bilayer, and the equilibrium concentration of SDS in the continuous phase is high enough to stabilize a water continuous emulsion, promoting the second phase inversion. The O/W(2) emulsions however, are in general unstable, due to flocculation and sedimentation of the particles.
Keywords: CaCO3 nanoparticles; SDS; Pickering emulsion; Phase inversion; Adsorption
Fig. 1. SEM images of 0.5 wt.% CaCO3 nanoparticles dispersed in (a) pure water and (b) 10 mM aqueous SDS.
Fig. 2. Size distributions of 0.5 wt.% CaCO3 nanoparticles dispersed in pure water and 10 mM aqueous SDS using an ultrasound probe.
Fig. 3. Zeta potentials of 2 wt.% CaCO3 nanoparticles dispersed in water of different pH.
Fig. 4. Digital photograph of toluene-in-water emulsions stabilized solely by CaCO3 nanoparticles at different concentrations, from left to right; 0.1, 0.5, 1, 2 and 3 wt.%, taken 7 days after preparation.
Fig. 5. Micrographs of emulsion droplets stabilized solely by CaCO3 nanoparticles taken 24 h after preparation. The scale bar is 500 μm for (a)–(c) and 50 μm for (d). The particle concentration is relative to the water phase. (a) Toluene-in-water, 2 wt.% CaCO3; (b) toluene-in-water, 3 wt.% CaCO3; (c) octane-in-water, 2 wt.% CaCO3; (d) octane-in-water, 3 wt.% CaCO3.
Fig. 6. Digital photograph of the toluene–water emulsions indicating O/W(1) → W/O → O/W(2) double phase inversion taken 24 h after preparation. The emulsions were stabilized by 2 wt.% CaCO3 nanoparticles plus different initial concentrations of SDS, from left to right: 0, 0.1, 0.6, 1, 3, 6 and 100 mM.
Fig. 7. Micrographs of toluene–water emulsions stabilized by 2 wt.% CaCO3 nanoparticles with (a) 0.3 mM SDS, O/W(1); (b) 2 mM SDS, W/O; (c) 6 mM SDS, O/W(2). (d) An O/W emulsion stabilized by 6 mM SDS alone. (a and b) Taken 24 h after preparation, scale bar denotes 500 μm; (c and d) taken immediately after preparation, scale bar denotes 20 μm.
Fig. 8. Digital photographs of toluene-in-water emulsions stabilized solely by SDS at different initial concentrations, from left to right; 0.01, 0.1, 0.6, 1 and 3 mM, taken (a) 24 h and (b) 1 week after preparation.
Fig. 9. Adsorption isotherm of SDS on CaCO3 nanoparticles in water at 25 °C measured by spectrophotometry (●), two-phase titration (Δ) and based on the difference in SDS concentration in the continuous phase of the toluene–water emulsions stabilized solely by SDS and by CaCO3 nanoparticles plus SDS (■).
Fig. 10. Zeta potential of 2 wt.% CaCO3 nanoparticles dispersed in aqueous SDS solutions. Dotted lines delineate different emulsion types in systems containing toluene.
Fig. 13. Appearance of 2 wt.% CaCO3 nanoparticles ultrasonically dispersed in aqueous solutions of SDS at different initial concentrations, from left to right: 3, 6, 10, 30 mM, taken (a) 24 h and (b) 1 week after preparation.
Fig. 11. Appearance of the toluene–water emulsions (a) 2 h and (b) 7 days after preparation stabilized by 2 wt.% CaCO3 nanoparticles plus different concentrations of SDS, from left to right: 3 (W/O), 6, 10, 30 and 100 (all O/W(2)) mM.
Fig. 12. Time dependence of the phase volume fraction for toluene–water O/W(2) emulsions stabilized by 2 wt.% CaCO3 nanoparticles plus different concentrations of SDS given in the label. Lower three lines represent the volume fraction of the water phase, upper three lines represent the volume fraction of the water phase plus emulsion phase.