For emulsions containing equal volumes of toluene and water stabilized by nanometer-size silica particles alone, we demonstrate that transitional inversion, from oil-in-water (o/w) to water-in-oil ...(w/o) and vice versa, can be achieved using mixtures of two particle types of different wettability. Using conductivity, stability, light diffraction, and optical microscopy measurements, we detail three main ways of effecting this which include addition of hydrophilic silica to w/o emulsions stabilized by hydrophobic silica, addition of hydrophobic silica to o/w emulsions stabilized by hydrophilic silica, and varying the weight fraction of one of the particle types at constant total particle concentration. Although both types of emulsion are very stable to coalescence, the stability to creaming of o/w emulsions and to sedimentation of w/o emulsions is least around conditions of inversion, consistent with the average drop sizes displaying a maximum. An increase in the proportion of charged hydrophilic silica results in deflocculation of the drops in w/o emulsions.
2-(Dimethylamino)ethyl methacrylate (DMA) was block copolymerized with methyl methacrylate (MMA) using group transfer polymerization to give four AB diblock, ABA triblock, and BAB triblock copolymers ...of low polydispersity (M w/M n < 1.20). In addition, a near-monodisperse styrene-functionalized DMA-based macromonomer was synthesized via oxyanionic polymerization using a potassium 4-vinylbenzyl alcoholate initiator. These five well-defined, tertiary amine methacrylate-based copolymers were evaluated as steric stabilizers for the synthesis of polystyrene latexes via emulsion and dispersion polymerization. The most efficient steric stabilizers proved to be the DMA−MMA diblock copolymer and the DMA-based macromonomer. The polystyrene latexes were characterized in terms of their particle size and morphology, stabilizer content, surface charge, and surface activity using dynamic light scattering, scanning electron microscopy, 1H NMR spectroscopy, aqueous electrophoresis measurements, and surface tensiometry, respectively. The pH-dependent surface activity exhibited by selected latexes suggests potential applications as stimulus-responsive particulate emulsifiers for oil-in-water emulsions.
The rapid phase separation of oil-in-water (o/w) emulsions prepared from non-polar oil and alkaline dispersions of hydrophilic silica nanoparticles alone is described. A study of the roles of the ...surface chemistry of the particles and the type and composition of the oil and aqueous phases in improving the stability of these emulsions is then reported. Alteration of the particle charge and flocculation by control of pH and addition of divalent electrolyte causes temporary improvements in emulsion stability, while the addition of cationic surfactant results in the preparation of stable emulsions. Increasing the oil phase polarity is found to improve emulsion stability and this is attributed to adsorption of polar solvent molecules to the silica surface changing the particle wettability. Attempts to reduce the affinity of the particles for the aqueous phase by adding non-aqueous solvents to that phase are unsuccessful, perhaps due to the extreme hydrophilicity of the particles. For the stable emulsions, the effect of silica particle size on the emulsion drop size is also investigated.
Oil-in-water (o/w) emulsions stabilized by aggregates of completely hydrophilic silica nanoparticles alone were prepared by the microchannel emulsification method. The emulsification conditions and ...mechanism in a cross-flow type microchannel setup were investigated. Increasing the flow rate of the continuous phase along the exits of microchannels prevented the coalescence of droplets. The emulsions obtained are stable over several months with a much reduced polydispersity than those prepared with a homogenizer. The silica aggregates form shell-like layers around the droplets with a thickness in agreement with the size of the aggregates in bulk, indicating that silica aggregates form a monolayer on droplet surfaces. On the other hand, the adsorbed layer of silica aggregates prepared with a homogenizer was thinner than that prepared with microchannel emulsification, indicating the breakup of the aggregates during homogenization. Results reveal that microchannel emulsification offers advantages over the conventional mechanical emulsification in the preparation of fine particle-stabilized emulsions since it involves no extensive mechanical shearing.
We have investigated the mechanism of the spontaneous growth of a gold nanoparticle film on a container wall when an aqueous dispersion of gold nanoparticles is shaken with an oil phase containing ...octadecylamine, as first described by Mayya and Sastry (Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902.). Experimental evidence is described, which shows that the film growth is driven by the coalescence of particle-coated emulsion drops with the flat oil−water interface separating the oil and water phases.
Aqueous dispersions of lightly cross-linked poly(4-vinylpyridine)/silica nanocomposite microgel particles are used as a sole emulsifier of methyl myristate and water (1:1 by volume) at various pH ...values and salt concentrations at 20 °C. These particles become swollen at low pH with the hydrodynamic diameter increasing from 250 nm at pH 8.8 to 630 nm at pH 2.7. For batch emulsions prepared at pH 3.4, oil-in-water (o/w) emulsions are formed that are stable to coalescence but exhibit creaming. Below pH 3.3, however, these emulsions are very unstable to coalescence and rapid phase separation occurs just after homogenization (pH-dependent). The pH for 50% ionization of the pyridine groups in the particles in the bulk (pK a) was determined to be 3.4 by acid titration measurements of the aqueous dispersion. Thus, the charged swollen particles no longer adsorb at the oil−water interface. For continuous emulsions (prepared at high pH with the pH then decreased abruptly or progressively), demulsification takes place rapidly below pH 3.3, implying that particles adsorbed at the oil−water interface can become charged (protonated) and detached from the interface in situ (pH-responsive). Furthermore, at a fixed pH of 4.0, addition of sodium chloride to the aqueous dispersion increases the degree of ionization of the particles and batch emulsions are significantly unstable to coalescence at a salt concentration of 0.24 mol kg-1. The degree of ionization of such microgel particles is a critical factor in controlling the coalescence stability of o/w emulsions stabilized by them.
Getting the right mix: Controlling the extent of the charge on the surfaces of carboxy‐coated latex nanoparticles enables ultrastable oil‐in‐water and water‐in‐oil emulsions to be prepared. Emulsion ...inversion is simply achieved by changing the pH value (see picture) or salt concentration.
A change for the wetter: The effect of temperature T on the position of a sterically stabilized polystyrene latex particle adsorbed at an oil–water interface is described. Emulsions stabilized by ...such particle monolayers are oil‐in‐water at low T and water‐in‐oil at high T (see scheme). The results are also in line with the predicted change in the wettability of the particles in emulsions.
Emulsions of equal volumes of a cyclic silicone oil and water stabilized by fumed silica nanoparticles alone can be inverted from oil-in-water (o/w) to water-in-oil (w/o) by simply increasing the ...concentration of particles. The phenomenon is found to be crucially dependent both on the inherent hydrophobicity of the particles and on their initial location. Inversion only occurs in systems with particles of intermediate hydrophobicity when dispersed in oil; emulsions prepared from the same particles but initially dispersed in water remain o/w at all particle concentrations. The stability and drop size distributions in the different emulsions are compared. Various hypotheses are put forward and argued to explain this novel inversion route including adsorption of oil onto particle surfaces, hysteresis of contact angle affecting particle wettability in situ, and the structure of particle dispersions in oil or water prior to emulsification inferred from rheology and light scattering measurements. We propose that the tendency for particles to behave more hydrophobically at higher concentrations in oil is due to the reduction in the effective silanol content at their surfaces as a result of gel formation via silanol−silanol hydrogen bonds. In water, solvation of particle surfaces prevents this from occurring and particles behave as hydrophilic ones at all concentrations. A concentration-induced change in particle wettability is thus advanced.
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