High internal phase emulsions stabilized solely by microgel particles.

High internal phase emulsions (HIPEs) are often defined as very concentrated emulsions in which the volume fraction of the internal phase exceeds 0.74. Conventional HIPEs consisting of a continuous organic phase and an internal aqueous phase (water-in-oil, w/o emulsion) are commonly stabilized by large amounts of surfactant. If the continuous phase contains one or more monomeric species that are polymerizable, HIPEs can be used as templates for the fabrication of highly porous materials. Highly porous materials have proven to be useful in a variety of applications, including filtration membranes for molten metals and hot gases, bioreactors, catalyst carriers, and scaffolds for bone replacement and tissue engineering. In addition to surfactants, colloidal particles have also been used to stabilize HIPEs and produce macroporous materials after drying and sintering. 8] The concept of using particle-stabilized, or Pickering, emulsions as templates to manufacture porous materials provides a number of benefits that are not achievable using low-molecular-weight surfactants. Firstly, the particles used as stabilizers in Pickering emulsions are irreversibly adsorbed at the interface of emulsions because of their high energy of attachment, which makes the resultant Pickering emulsions extremely stable with shelf-life stabilities of months or even years. Secondly, the use of Pickering emulsion droplets as templates can functionalize the cell walls of the porous materials with a layer of solid particles, which could, for example, contain functional groups and lead to a variety of further applications. The stability and type of Pickering emulsions depend on the hydrophilicity of the particles. While considerable progress has been made on the fabrication of porous materials by Pickering HIPE templating, one important limitation has still remained. All previous reports on Pickering emulsions deal with emulsions for which the volume fractions of the dispersed phases are less than 0.70. On the basis of the developed thermodynamic model, Kralchevsky et al. have predicted that Pickering emulsions will phase-invert above an internal phase volume fraction of 0.5. In practice, phase inversion is usually observed at a volume fraction of 0.70 after kinetic factors are considered. Binks and co-workers 12] have experimentally demonstrated that Pickering emulsions phase-invert between volume fractions of 0.65 and 0.70, which means that the majority phase becomes the continuous phase. The formation of HIPEs stabilized solely by solid particles seems to be impossible. Menner et al. reported on the first successful preparation of an HIPE stabilized solely by carbon nanotubes with 60% internal phase volume. Akartuna et al. also reported on the preparation of macroporous materials from Pickering HIPE templates stabilized by inorganic particles, but about 35 vol% particles were needed to stabilize the emulsions with internal phase fractions of 72–78%. Recently, using titania or silica particles, stable Pickering HIPEs with an internal phase volume fraction greater than 90% could be prepared. However, the hydrophilicity of the particles should be tailored through prior chemical treatment. Furthermore, to avoid the collapse of the emulsion structure during the production of porous materials, consolidation of the HIPEs by gelation of the continuous phase requires additional strict control over the reaction chemistry, which may be challenging. Hence, the fabrication of porous materials using the Pickering HIPE templating approach that would not need chemical reactions and could be easily extended to a wide variety of chemical compositions is highly desirable. We report herein the stabilization of Pickering hexane-inwater (o/w) HIPEs with volume fractions as high as 0.9 by soft poly(N-isopropylamide)-co-(methacrylic acid) (PNIPAM-coMAA) microgel particles at concentrations as low as 0.05 wt %. Microgel particles are adsorbed at the oil–water interface to hinder extensive droplet coalescence. Excess microgel particles simultaneously form a gel in the continuous phase to trap oil droplets in the gel matrix, which in turn can inhibit creaming of the oil droplets and enhance the emulsion stability. Evaporation in air of the gelled continuous phase of the HIPEs directly leads to porous materials without any chemical reactions. Furthermore, the porosity of the final structure can be tailored by simply changing the microgel particle concentration. Fluorescent PNIPAM-co-MAA microgel particles were synthesized using surfactant-free precipitation polymerization as described before. The details are provided in the Supporting Information (Figures S1–S4). The resultant microgel particles have an average hydrodynamic diameter of 248 nm at room temperature with a solution pH value of 7.5, as determined by dynamic laser light scattering (see the [*] Z. F. Li, Prof. T. Ngai Department of Chemistry, The Chinese University of Hong Kong Shatin, N.T., Hong Kong (China) E-mail: tongai@cuhk.edu. hk Homepage: http://www.chem.cuhk.edu. hk/faculty_ngai_to. htm