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Silica particles have proved to be a good candidate for these systems, because the surface silanol groups present on hydrophilic particles can be controllably converted to more hydrophobic groups by reaction with silane reagents

Phase inversion of particle-stabilized materials from foams to dry water

NATURE MATERIALS, no. 11 (2006): 865.0-869

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摘要

Small particles attached to liquid surfaces arise in many products and processes, including crude-oil emulsions and food foams and in flotation, and there is a revival of interest in studying their behaviour. Colloidal particles of suitable wettability adsorb strongly to liquid-liquid and liquid vapour interfaces(1), and can be sole stabi...更多

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简介
  • Solid particles of size between a few nanometres and a few micrometres can be strongly attached to oil–water or air–water interfaces[1].
  • Following the pioneering work of Ramsden[9] and Pickering[10], emulsions of oil and water[2] and foams of air and water[3,11] of long-term stability to coalescence have been prepared recently using particles alone.
  • If the particle is relatively hydrophilic, θ < 90◦ and oil-in-water emulsions are preferred in mixtures of equal volumes of the two liquids.
重点内容
  • Solid particles of size between a few nanometres and a few micrometres can be strongly attached to oil–water or air–water interfaces[1]
  • In particle-stabilized emulsions, a key parameter influencing the type of emulsion formed is the wettability of the particle, quantified in terms of the contact angle θ it makes with the oil–water interface
  • The inversion from one emulsion type to the other can be achieved in two ways in a batch process
  • Silica particles have proved to be a good candidate for these systems, because the surface silanol (SiOH) groups present on hydrophilic particles can be controllably converted to more hydrophobic groups by reaction with silane reagents
  • This possible inversion means a transition from air bubbles dispersed in water, as in normal foams, to water drops dispersed in air (Fig. 1a—lower quadrants)
  • The powder looks like the original fluffy fumed silica and the scanning electron microscopy (SEM) image in Fig. 3c reveals a porous structure of agglomerated silica particles composed of aggregates of 100 nm in size
结果
  • A series of fumed silica powders, composed of aggregated particles of primary diameter ≈20–30 nm, ranging from completely hydrophilic (100% SiOH) to very hydrophobic (≤20% SiOH), using dichlorodimethylsilane (DCDMS), was used to effect both types of emulsion inversion[7,8].
  • Air + water b water drops together in a unified picture, by demonstrating both transitional and catastrophic phase inversion of foams to powders as a function of silica-particle wettability and air/water ratio, respectively, in one system.
  • The contact angle of a water drop in air on a pressed tablet of DCDMS-coated fumed silica particles[17] provides an estimate and varies from 20◦ for the most hydrophilic (100% SiOH) to 130◦ for the most hydrophobic (14% SiOH).
  • At a fixed volume fraction of water, φw, equal to 0.056, mixing air, pure water and fumed silica powders results in three types of dispersed phase, depending on the particle hydrophobicity (Fig. 2a).
  • SiOH ≤ 20%, virtually all the water is taken up as stable drops in air into a free-flowing powder.
  • Photographs of the appearances of the two very different materials formed either side of phase inversion are given in Fig. 2b for the powder and in Fig. 2c for the foam.
  • As particle-stabilized interfaces are of high surface tension, and their monolayers are rigid compared with those containing adsorbed surfactant, the authors do not expect this phase to be bicontinuous in both air and water.
  • The powder looks like the original fluffy fumed silica and the SEM image in Fig. 3c reveals a porous structure of agglomerated silica particles composed of aggregates of 100 nm in size.
结论
  • It is noted that the aggregate size is smaller than in the original silica, as expected, given the high shear input in preparing the water-containing powder in the first place.
  • SEM, Fig. 3e, of the inner surface of the material shows air cells of size comparable to the original bubbles, separated by fused particle aggregates.
  • To prepare the dispersed phases[14], a mass of water was poured into a blender (Braun, Glass Jug PowerBlend MX2050, 1.7 l with lid) and an amount of powdered particles was placed on top.
基金
  • We thank the EPSRC, UK, for a postdoctoral grant to R.M., T
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