Guanqing Sun

One-step formation of w/o/w multiple emulsions stabilized by single amphiphilic block copolymers.

Multiple emulsions are complex polydispersed systems in which both oil-in-water (O/W) and water-in-oil (W/O) emulsion exists simultaneously. They are often prepared accroding to a two-step process and commonly stabilized using a combination of hydrophilic and hydrophobic surfactants. Recently, some reports have shown that multiple emulsions can also be produced through one-step method with simultaneous occurrence of catastrophic and transitional phase inversions. However, these reported multiple emulsions need surfactant blends and are usually described as transitory or temporary systems. Herein, we report a one-step phase inversion process to produce water-in-oil-in-water (W/O/W) multiple emulsions stabilized solely by a synthetic diblock copolymer. Unlike the use of small molecule surfactant combinations, block copolymer stabilized multiple emulsions are remarkably stable and show the ability to separately encapsulate both polar and nonpolar cargos. The importance of the conformation of the copolymer surfactant at the interfaces with regards to the stability of the multiple emulsions using the one-step method is discussed.

Porous TiO2 Materials through Pickering High-Internal Phase Emulsion Templating

We report a facile method for preparing porous structured TiO2 materials by templating from Pickering high-internal phase emulsions (HIPEs). A Pickering HIPE with an internal phase of up to 80 vol %, stabilized by poly(N-isopropylacrylamide)-based microgels and TiO2 solid nanoparticles, was first formulated and employed as a template to prepare the porous TiO2 materials with an interconnected structure. The resultant materials were characterized by scanning electron microscopy, X-ray diffraction, and mercury intrusion. Our results showed that the parent emulsion droplets promoted the formation of macropores and interconnecting throats with sizes of ~50 and ~10 μm, respectively, while the interfacially adsorbed microgel stabilizers drove the formation of smaller pores (~100 nm) throughout the macroporous walls after drying and sintering. The interconnected structured network with the bimodal pores could be well preserved after calcinations at 800 °C. In addition, the photocatalytic activity of the fabricated TiO2 was evaluated by measuring the photodegradation of Rhodamine B in water. Our results revealed that the fabricated TiO2 materials are good photocatalysts, showing enhanced activity and stability in photodegrading organic molecules.

Gelatin Particle-Stabilized High Internal Phase Emulsions as Nutraceutical Containers

In this paper, we report for the first time the use of a well-dispersed gelatin particle as a representative of natural and biocompatible materials to be an effective particle stabilizer for high internal phase emulsion (HIPE) formulation. Fairly monodispersed gelatin particles (∼200 nm) were synthesized through a two-step desolvation method and characterized by dynamic light scattering, ζ-potential measurements, scanning electron microscopy, and atomic force microscopy. Those protein latexes were then used as sole emulsifiers to fabricate stable oil-in-water Pickering HIPEs at different concentrations, pH conditions, and homogenization times. Most of the gelatin particles were irreversibly adsorbed at the oil-water interface to hinder droplet coalescence, such that Pickering HIPEs can be formed by a small amount of gelatin particles (as low as 0.5 wt % in the water phase) at pH far away from the isoelectric point of the gelatin particles. In addition, increasing homogenization time led to narrow size distribution of droplets, and high particle concentration resulted in more solidlike Pickering HIPEs. In vitro controlled-release experiments revealed that the release of the encapsulated β-carotene can be tuned by manipulating the concentration of gelatin particles in the formulation, suggesting that the stable and narrow-size-distributed gelatin-stabilized HIPEs had potential in functional food and pharmaceutical applications.

Systematic studies of Pickering emulsions stabilized by uniform-sized PLGA particles: preparation and stabilization mechanism

Pickering emulsions stabilized by solid particles have been widely studied in the past decades due to improved stability and reduced use of small molecular surfactants. Recently, the application of Pickering emulsions in pharmaceutics has been attracting increasing attention but very limited practical use has been demonstrated, because most of the investigated particles possess poor biodegradability, which is inappropriate in pharmaceutics. Some reported biodegradable particles were too hydrophilic to stabilize emulsions, which needs further particle modification or additional surfactants. Fortunately, biodegradable poly(d,l-lactic-co-glycolic acid) (PLGA) with tunable hydrophilicity makes itself a promising material to prepare Pickering emulsions. However, the mechanism of emulsion stabilization still remains unknown. Moreover, fabrication of large amounts of uniform-sized and size-controlled PLGA particles by traditional methods is very difficult, which further increases the difficulty to perform the research. In the present study, we applied Shirasu Porous Glass (SPG) premix membrane emulsification to solve this problem. The stabilization mechanism of Pickering emulsions stabilized by PLGA particles was systematically studied for the first time. The factors including oil type, particle properties, concentration, molecular weight (Mw) and oil-water volume ratio were analyzed through particle wettability and interfacial influence. We found that octanol was an appropriate oil type, and its small particle size, high particle concentration and high Mw were favorable for emulsion stability. By changing the oil-water volume ratio, stable emulsions were also readily achieved. These studies proved that Pickering emulsions stabilized by PLGA particles had wide potential applications in pharmaceutics and tissue engineering.

Stabilization of Colloidal Suspensions: Competing Effects of Nanoparticle Halos and Depletion Mechanism

Bimodal colloidal mixtures of nanoparticles and microparticles may show different phase behaviors depending upon the interparticle interaction of both species. In the present work, we examined the stabilization of spherical microparticles using highly charged, spherical nanoparticles. Total internal reflection microscopy (TIRM) was used to measure the interaction forces between a charged microparticle and flat glass substrate in aqueous solutions at varying volume fractions of nanoparticles of the same sign. We found that, in the system containing of highly charged nanoparticles, microparticle, and glass substrate, non-adsorbing charged nanoparticles in solution did not lead to depletion attraction. Instead, the addition of nanoparticles was to consistently create a repulsive force between the microparticle and glass substrate even at a very low nanoparticle volume fraction. This result might attributed to the formation of thin shells (halos) with a high local nanoparticle volume fraction to the region near the glass surface, resulting in electrostatic repulsion between the decorated surfaces. This study demonstrates that nanoparticle halos can also arise in binary systems of mutually but highly repulsive microparticle/nanoparticle dispersions.

Dopamine Polymerization in Liquid Marbles: A General Route to Janus Particle Synthesis

Coating a liquid with a particle shell not only renders a droplet superhydrophobic but also isolates a well-confined microenvironment for miniaturized chemical processes. Previously, we have demonstrated that particles at the liquid marble interface provide an ideal platform for the site-selective modification of superhydrophobic particles. However, the need for a special chemical reaction limits their potential use for the fabrication of Janus particles with various properties. Herein, we combine the employment of liquid marbles as microreactors with the remarkable adhesive ability of polydopamine to develop a general route for the synthesis of Janus particles from micrometer-sized superhydrophobic particles. We demonstrate that dopamine polymerization and deposition inside liquid marbles could be used for the selective surface modification of microsized silica particles, resulting in the formation of Janus particles. Moreover, it is possible to manipulate the Janus balance of the particles via the addition of surfactants and/or organic solvents to tune the interfacial energy. More importantly, owing to the many functional groups in polydopamine, we show that versatile strategies could be introduced to use these partially polydopamine-coated silica particles as platforms for further modification, including nanoparticle immobilization, metal ion chelation and reduction, as well as for chemical reactions. Given the flexibility in the choice of cores and the modification strategies, this developed method is distinctive in its high universality, good controllability, and great practicability.

Preparation of Uniform Particle-Stabilized Emulsions Using SPG Membrane Emulsification

Various aspects of particle-stabilized emulsions (or so-called Pickering emulsions) have been extensively investigated during the last two decades, but the preparation of uniform Pickering emulsion droplets via a simple and scalable method has been sparingly realized. We report the preparation of uniform Pickering emulsions by Shirasu porous glass (SPG) membrane emulsification. The size of the emulsion droplets ranging from 10-50 μm can be precisely controlled by the size of the membrane pore. The emulsion droplets have a high monodispersity with coefficients of variation (CV) lower than 15% in all of the investigated systems. We further demonstrate the feasibility of locking the assembled particles at the interface, and emulsion droplets have been shown to be excellent templates for the preparation of monodisperse colloidosomes that are necessary in drug-delivery systems.

Liquid marbles stabilized by charged polymer latexes: how does the drying of the latex particles affect the properties of liquid marbles?

The coating of solid particles on the surface of liquid in air makes liquid marbles a promising approach in the transportation of a small amount of liquid. The stabilization of liquid marbles by polymeric latex particles imparts extra triggers such as pH and temperature, leading to the remote manipulation of droplets for many potential applications. Because the functionalized polymeric latexes can exist either as colloidally stable latex or as flocculated latex in a dispersion, the drying of latex dispersions under different conditions may play a significant role in the stabilization of subsequent liquid marbles. This article presents the investigation of liquid marbles stabilized by poly(styrene-co-methacrylic acid) (PS-co-MAA) particles drying under varied conditions. Protonation of the particles before freeze drying makes the particles excellent liquid marble stabilizers, but it is hard to stabilize liquid marbles for particles dried in their deprotonated states. The static properties of liquid marbles with increasing concentrations of protonating reagent revealed that the liquid marbles are gradually undermined by protonating the stabilizers. Furthermore, the liquid marbles stabilized by different particles showed distinct behaviors in separation and merging manipulated by tweezers. This study shows that the initial state of the particles should be carefully taken into account in formulating liquid marbles.