Xiaokong Liu

Cleaning of Oil Fouling with Water Enabled by Zwitterionic Polyelectrolyte Coatings: Overcoming the Imperative Challenge of Oil–Water Separation Membranes

Herein we report a self-cleaning coating derived from zwitterionic poly(2-methacryloyloxylethyl phosphorylcholine) (PMPC) brushes grafted on a solid substrate. The PMPC surface not only exhibits complete oil repellency in a water-wetted state (i.e., underwater superoleophobicity), but also allows effective cleaning of oil fouled on dry surfaces by water alone. The PMPC surface was compared with typical underwater superoleophobic surfaces realized with the aid of surface roughening by applying hydrophilic nanostructures and those realized by applying smooth hydrophilic polyelectrolyte multilayers. We show that underwater superoleophobicity of a surface is not sufficient to enable water to clean up oil fouling on a dry surface, because the latter circumstance demands the surface to be able to strongly bond water not only in its pristine state but also in an oil-wetted state. The PMPC surface is unique with its described self-cleaning performance because the zwitterionic phosphorylcholine groups exhibit exceptional binding affinity to water even when they are already wetted by oil. Further, we show that applying this PMPC coating onto steel meshes produces oil-water separation membranes that are resilient to oil contamination with simply water rinsing. Consequently, we provide an effective solution to the oil contamination issue on the oil-water separation membranes, which is an imperative challenge in this field. Thanks to the self-cleaning effect of the PMPC surface, PMPC-coated steel meshes can not only separate oil from oil-water mixtures in a water-wetted state, but also can lift oil out from oil-water mixtures even in a dry state, which is a very promising technology for practical oil-spill remediation. In contrast, we show that oil contamination on conventional hydrophilic oil-water separation membranes would permanently induce the loss of oil-water separation function, and thus they have to be always used in a completely water-wetted state, which significantly restricts their application in practice.

Polymeric complexes as building blocks for rapid fabrication of layer-by-layer assembled multilayer films and their application as superhydrophobic coatings

A facile method for rapid fabrication of micrometre-thick films with hierarchical micro- and nanostructures was developed by layer-by-layer (LbL) deposition of hydrogen-bonded complexes of poly(vinylpyrrolidione) (PVPON) and poly(acrylic acid) (PAA) (denoted PVPON&PAA) with poly(methacrylic acid) (PMAA). FT-IR spectroscopy confirmed that hydrogen-bonding interactions between the PVPON of PVPON&PAA complexes and PMAA was the driving force for the successful construction of the LbL assembled PVPON&PAA/PMAA films. A non-drying film preparative process was critically important to realize the rapid fabrication of PVPON&PAA/PMAA films with hierarchical micro- and nanostructures because the structure of the adsorbed spherical PVPON&PAA complexes can be well preserved during film fabrication which led to an exponential growth of the PVPON&PAA/PMAA films. After chemical vapor deposition of a layer of fluoroalkylsilane on top of the as-prepared PVPON&PAA/PMAA films with hierarchical micro- and nanostructures, superhydrophobic coatings were conveniently fabricated. The use of polymeric complexes as building blocks for LbL film fabrication not only provides a facile method for the rapid fabrication of micrometre-thick films, but also enables the convenient tailoring of film structures because of the structural diversity of polymeric complexes in solution.

Layer-by-Layer-Assembled Multilayer Films of Polyelectrolyte-Stabilized Surfactant Micelles for the Incorporation of Noncharged Organic Dyes

Noncharged pyrene molecules were incorporated into multilayer films by first loading pyrene into poly(acrylic acid) (PAA)-stabilized cetyltrimethylammonium bromide (CTAB) micelles (noted as PAA&(Py@CTAB)) and then layer-by-layer (LbL) assembled with poly(diallyldimethylammonium chloride) (PDDA). The stable incorporation of pyrene into multilayer films was confirmed by quartz crystal microbalance (QCM) measurements and UV-vis absorption spectroscopy. The resultant PAA&(Py@CTAB)/PDDA multilayer films show an exponential growth behavior because of the increased surface roughness with increasing number of film deposition cycles. The present study will open a general and cost-effective avenue for the incorporation of noncharged species, such as organic molecules, nanoparticles, and so forth, into LbL-assembled multilayer films by using polyelectrolyte-stabilized surfactant micelles as carriers.

Layer-by-Layer Assembly of Salt-Containing Polyelectrolyte Complexes for the Fabrication of Dewetting-Induced Porous Coatings

The layer-by-layer (LbL) assembly of salt-containing nonstoichiometric polyelectrolyte complexes (PECs) with oppositely charged uncomplexed polyelectrolyte for the fabrication of dewetting-induced porous polymeric films has been systematically investigated. Salt-containing poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) complexes (noted as PAH-PAA) with a molar excess of PAH were LbL assembled with polyanion poly(sodium 4-styrenesulfonate) (PSS) to produce PSS/PAH-PAA films. The structure of the PAH-PAA complexes is dependent on the concentration of NaCl added to their aqueous dispersions, which can be used to tailor the structure of the LbL-assembled PSS/PAH-PAA films. Porous PSS/PAH-PAA films are fabricated when salt-containing PAH-PAA complexes with a large amount of added NaCl are used for LbL assembly with PSS. In-situ and ex-situ atomic force microscopy measurements disclose that the dewetting process composed of pore nucleation and pore growth steps leads to the formation of pores in the LbL-assembled PSS/PAH-PAA films. The present study provides a facile way to fabricate porous polymeric films by dewetting LbL-assembled polymeric films comprising salt-containing PECs.

Electrostatic Repulsion-Controlled Formation of Polydopamine− Gold Janus Particles

Polydopamine (PDA)-Au Janus particles were obtained by simply adding HAuCl(4) to a PDA particle suspension, prepared via self-polymerization of dopamine in basic solution at room temperature. The structures of the PDA-Au particles are readily controlled by electrostatic repulsion between the constituent particles, which can be realized simply via adjusting the environmental pH. PDA-Au Janus particles are formed only in a narrow pH range of 2.5-3.0 due to the properly enhanced electrostatic repulsion between the Au particles growing on as-prepared PDA particles and between the Au and PDA particles. The obtained PDA-Au Janus particles can become interfacially active and self-assemble at oil/water interfaces as a result of spatially well-separated hydrophilic (PDA) and hydrophobic (Au) domains on the surfaces, reminiscent of amphiphilic molecules.

Confined Flocculation of Ionic Pollutants by Poly(l-dopa)-Based Polyelectrolyte Complexes in Hydrogel Beads for Three-Dimensional, Quantitative, Efficient Water Decontamination

The development of simple and recyclable adsorbents with high adsorption capacity is a technical imperative for water treatment. In this work, we have successfully developed new adsorbents for the removal of ionic pollutants from water via encapsulation of polyelectrolyte complexes (PECs) made from positively charged poly(allylamine hydrochloride) (PAH) and negatively charged poly(l-3,4-dihydroxyphenylalanine) (PDopa), obtained via the self-polymerization of l-3,4-dihydroxyphenylalanine (l-Dopa). Given the outstanding mass transport through the hydrogel host matrixes, the PDopa-PAH PEC guests loaded inside can effectively and efficiently remove various ionic pollutants, including heavy metal ions and ionic organic dyes, from water. The adsorption efficiency of the PDopa-PAH PECs can be quantitatively correlated to and tailored by the PDopa-to-PAH molar ratio. Because PDopa embodies one catechol group, one carboxyl group, and one amino group in each repeating unit, the resulting PDopa-PAH PECs exhibit the largest capacity of adsorption of heavy metal ions compared to available adsorbents. Because both PDopa and PAH are pH-sensitive, the PDopa-PAH PEC-loaded agarose hydrogel beads can be easily and completely recovered after the adsorption of ionic pollutants by adjusting the pH of the surrounding media. The present strategy is similar to the conventional process of using PECs to flocculate ionic pollutants from water, while in our system flocculation is confined to the agarose hydrogel beads, thus allowing easy separation of the resulting adsorbents from water.

Ion-specific oil repellency of polyelectrolyte multilayers in water: molecular insights into the hydrophilicity of charged surfaces.

Surface wetting on polyelectrolyte multilayers (PEMs), prepared by alternating deposition of polydiallyldimethylammonium chloride (PDDA) and poly(styrene sulfonate) (PSS), was investigated mainly in water-solid-oil systems. The surface-wetting behavior of as-prepared PEMs was well correlated to the molecular structures of the uncompensated ionic groups on the PEMs as revealed by sum frequency generation vibrational and X-ray photoelectron spectroscopies. The orientation change of the benzenesulfonate groups on the PSS-capped surfaces causes poor water wetting in oil or air and negligible oil wetting in water, while the orientation change of the quaternized pyrrolidine rings on the PDDA-capped surfaces hardly affects their wetting behavior. The underwater oil repellency of PSS-capped PEMs was successfully harnessed to manufacture highly efficient filters for oil-water separation at high flux.

Exponential growth of layer-by-layer assembled coatings with well-dispersed ultrafine nanofillers: a facile route to scratch-resistant and transparent hybrid coatings

We report an innovative and straightforward method to well-disperse a low loading content of inorganic nanofillers of extremely small size in exponentially growing layer-by-layer (LbL) assembled micrometre-thick polymeric coatings. Complexes of poly(acrylic acid) (PAA) and in situ synthesized CaCO3 nanoparticles (noted as PAA-CaCO3) were alternately deposited with poly(allylamine hydrochloride) (PAH) to fabricate exponentially growing PAA-CaCO3/PAH coatings. The ultrafine CaCO3 nanofillers with a size of ∼2 nm were homogeneously dispersed in the hybrid PAA-CaCO3/PAH coatings because of the strong interaction of CaCO3 nanofillers with PAA and the “in-and-out” diffusion of the polyelectrolytes during the LbL assembly process. Thermogravimetric analysis indicates that the PAA-CaCO3/PAH coatings have a loading content of ∼4.2 wt% CaCO3 nanofillers. The thermally cross-linked PAA-CaCO3/PAH coatings, which have greatly enhanced hardness and Youngs elastic modulus because of the well-dispersed CaCO3 nanofillers, are highly transparent and scratch-resistant. The transparent and scratch-resistant PAA-CaCO3/PAH coatings are further proved to be highly useful as scratch-protection layers of other functional film materials. The present study provides a convenient and rapid method to prepare mechanically robust and transparent coatings for various applications.

Ultrafast colorimetric humidity-sensitive polyelectrolyte coating for touchless control

Herein we report the visible colorimetric humidity-sensitive properties of the layer-by-layer assembled poly(diallyldimethylammonium) (PDDA)/poly(styrenesulfonate) (PSS) polyelectrolyte coatings and their application for a touchless control system that can be accessed by humidity change. The as-developed touchless control system enables humidity signals, such as those imparted by human breath or a close-proximity fingertip, to switch a light-emitting diode and trigger a computer to implement commands (e.g., making phone calls and playing music). The PDDA/PSS coatings exhibit different colors at different thicknesses, which results from the interference of visible light in the coating. Notably, they can undergo vivid and reversible color changes that span the whole visible spectrum at an ultrafast speed (ca. 35 ms) according to the local humidity change. Exploiting the ability of the coating to modulate light, the output optical signal (i.e., color change) was converted into an electrical signal by irradiating the coating with a beam of monochromatic light and collecting the reflected modulated light using a photodetector. The obtained electrical signal was subsequently processed and used as an input signal for touchless control that is accessible by a humidity signal. The as-developed touchless control system is applicable for the touchless interface of electronic devices, smart switches, automotive, smart buildings, and so forth.

Ultra-fast Hygrometer based on U-shaped Optical Microfiber with Nanoporous Polyelectrolyte Coating

Real-time measurement of the relative humidity of air has applications ranging from process control to safety. By using a microfiber form-factor, we demonstrate a miniature and fast-response hygrometer with the shortest-ever response time (3 ms). The sensor head consists of an optical microfiber of 10 µm diameter and 2 mm length configured to form a compact U-shaped probe, and functionalized with a polyelectrolyte multilayer coating of 1.0 bilayer. The sensing mechanism is primarily water-absorption-based optical loss. We have measured a response time of 3 ms and a recovery time of 36 ms. The sensitivity is as high as 0.4%/%RH, and the detection limit is as low as 1.6%RH. The maximum relative humidity is 99%RH, before reaching a recoverable dew-point.