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Foulant-foulant and foulant-membrane interfacial interactions play an important role in dictating the extent of fouling. In order to understand the impact of the surface energy of particulate foulants on the fouling extent, the direct observation through the membrane (DOTM) technique was used to characterize critical flux and also assess the initial evolution of foulant deposition. Polystyrene and glass with diameters approximating 10 μm were used as foulants because (i) the similar particle diameter eliminated the differences due to particle back-transport, and (ii) the Gibbs free energies (ΔG) of foulant-membrane and foulant-foulant interactions were both negative (i.e., attractive) for polystyrene and both positive (i.e., repulsive) for glass. Results indicate that: (1) because of the attractive and repulsive foulant-membrane interactions of the polystyrene and glass, respectively, (i) critical flux was lower for polystyrene than glass, (ii) the phenomenon of a flowing particle layer was observed at a lower cross-flow velocity (CFV) for glass than polystyrene, and (iii) relaxation was more effective for glass than polystyrene; (2) because of the attractive and repulsive foulant-foulant interactions of the polystyrene and glass, respectively, clustering was observed for the former but not latter; (3) out of the four particle deposition mechanisms (namely, convective, entrapment, dead zone and clustering) distinguished, the first three occurred for both attractive and repulsive interfacial interactions, but the forth (i.e., clustering) only occurred when foulant-foulant interactions were attractive.

The ability to finely tune the amphiphilic balance of Janus nanoparticles (JNPs) could represent a step forward toward creating the next generation of solid-state amphiphiles with significant potential for applications. The inherent amphiphilicity of JNPs stemming from an intrinsic polarity contrast between two surface regions is well-acknowledged, but remained difficult to demonstrate experimentally in the absence of surfactants and stabilizers. We have designed two homologous series of surfactant-free polymeric JNPs starting from polystyrene (PS) seed nanoparticles (NPs) on which we grew Janus lobes of different sizes via seed polymerization and phase separation of the 3-(triethoxysilyl)propyl-methacrylate (3-TSPM) monomer. The two series differ only by the radical initiator used in the seed polymerization: polar ionic ammonium persulfate (APS) vs nonpolar oil-soluble 2,2'-azobis(2-methylpropionitrile) (AIBN). To compare the two series, we employed them in the emulsification of water with heptane or molten paraffin wax. A polarity reversal of the JNPs within AIBN-JNP series could be observed from the catastrophic and transitional emulsion phase inversions and occurred when the more polar lobe was larger than the nonpolar seed PS lobe. Furthermore, the AIBN-JNPs appeared to be amphiphilic and adopt preferred orientation within the monolayer at the oil/water interface. We therefore demonstrated that in the absence of surfactants the amphiphilicity of the JNPs depends not only on the relative size of the lobes, but also on the surface polarity contrast, which can be tuned by changing the nature of radical initiator.

The synthesis of sub-100 nm polymeric nanoparticles in a surfactant-free form is currently very challenging due to the oil nanoemulsion instability in polar solvents and in the absence of stabilizers. Here we report for the first time the surfactant-free synthesis method of poly(styrene-co-divinylbenzene) nanoparticles in an aqueous environment using ultrasonic radiation. This method involves emulsification of the monomers mixture in water, followed by free-radical polymerization under pulsed or continuous acoustic fields. The local energy produced in water by cavitation effects was sufficient to: (1) generate and stabilize the monomer nanoemulsion due to mechanical forces, and (2) drive the radical polymerization due to the heat generated. The average size of the final polymer nanoparticles obtained depended: (i) inversely on the monomer/water interfacial energy, emulsification power, and (ii) directly on temperature, amount of initiator and monomer solubility. The polymer nanoparticle size distribution and shape was considerably improved upon the addition of a co-polymerizable surfactant.

Functionalized nanoparticles (NPs) are critical to a wide-ranging of applications. The more versatile use of the NPs hinges on the capability to readily modify them to perform different functions. For example functionalized polymeric nanomaterials bearing ligands can be employed in metal ion extraction and recovery. However, obtaining ion-imprinted particles via ligand copolymerization can be difficult to scale-up. We therefore present an alternative method to obtain ligand-carrying polymeric nanoparticles (LC-NPs) for metal ion absorption. Branched polyethyleneimine (b-PEI) was used to modify the negatively charged surfactant-free polystyrene (PS) NPs obtained via surfactant-free emulsion polymerization by employing “surfmers”. The obtained PEI-PS NPs were further cross-linked with phthaldialdehyde to form cross-linked PEI-PS (CPEI-PS). Both PEI-PS and CPEI-PS were capable of absorbing chromium, nickel, cobalt and copper ions, as evident in the color change upon absorption. Furthermore, the ion absorption capacity of both types of particles was comparable, but with a greater selectivity toward copper ions. The selectivity towards copper ions of CPEI-PS increased with the degree of dialdehyde cross-linking. Repeated absorption-desorption cycles showed no loss in the absorption capacity of CPEI-PS as compared to the non-crosslinked PEI-PS. Finally, both were further reacted with two different aldehydes, which ascertained that the primary amine groups were present for further reaction with aldehydes and thereby enabling further functionalization of these particles.

The synthesis of sub-100 nm polymeric nanoparticles in a surfactant-free form is currently very challenging due to the oil nanoemulsion instability in polar solvents and in the absence of stabilizers. Here we report for the first time the surfactant-free synthesis method of poly(styrene-co-divinylbenzene) nanoparticles in an aqueous environment using ultrasonic radiation. This method involves emulsification of the monomers mixture in water, followed by free-radical polymerization under pulsed or continuous acoustic fields. The local energy produced in water by cavitation effects was sufficient to: (1) generate and stabilize the monomer nanoemulsion due to mechanical forces, and (2) drive the radical polymerization due to the heat generated. The average size of the final polymer nanoparticles obtained depended: (i) inversely on the monomer/water interfacial energy, emulsification power, and (ii) directly on temperature, amount of initiator and monomer solubility. The polymer nanoparticle size distribution and shape was considerably improved upon the addition of a co-polymerizable surfactant.

The ability to finely tune the amphiphilic balance of Janus nanoparticles (JNPs) could represent a step forward toward creating the next generation of solid-state amphiphiles with significant potential for applications. The inherent amphiphilicity of JNPs stemming from an intrinsic polarity contrast between two surface regions is well-acknowledged, but remained difficult to demonstrate experimentally in the absence of surfactants and stabilizers. We have designed two homologous series of surfactant-free polymeric JNPs starting from polystyrene (PS) seed nanoparticles (NPs) on which we grew Janus lobes of different sizes via seed polymerization and phase separation of the 3-(triethoxysilyl)propyl-methacrylate (3-TSPM) monomer. The two series differ only by the radical initiator used in the seed polymerization: polar ionic ammonium persulfate (APS) vs nonpolar oil-soluble 2,2'-azobis(2-methylpropionitrile) (AIBN). To compare the two series, we employed them in the emulsification of water with heptane or molten paraffin wax. A polarity reversal of the JNPs within AIBN-JNP series could be observed from the catastrophic and transitional emulsion phase inversions and occurred when the more polar lobe was larger than the nonpolar seed PS lobe. Furthermore, the AIBN-JNPs appeared to be amphiphilic and adopt preferred orientation within the monolayer at the oil/water interface. We therefore demonstrated that in the absence of surfactants the amphiphilicity of the JNPs depends not only on the relative size of the lobes, but also on the surface polarity contrast, which can be tuned by changing the nature of radical initiator.