Adsorption-driven self-assembly of nanoparticles at fluid interfaces is a promising bottom-up approach for the preparation of advanced functional materials and devices. Full realization of its potential requires quantitative understanding of the parameters controlling the self-assembly, the structure of nanoparticles at the interface, the barrier properties of the assembly, and the rate of particle attachment. We argue that models of dynamic surface or interfacial tension (DST) appropriate for molecular species break down when the adsorption energy greatly exceeds the mean energy of thermal fluctuations and validate alternative models extending the application of generalized random sequential adsorption theory to nanoparticle adsorption at fluid interfaces. Using a model colloidal system of hydrophobic, charge-stabilized ethyl cellulose nanoparticles at neutral pH, we demonstrate the potential of DST measurements to reveal information on the energy of adsorption, the adsorption rate constant, and the energy of particle-interface interaction at different degrees of nanoparticle coverage of the interface. These findings have significant implications for the quantitative description of nanoparticle adsorption at fluid interfaces.
The global demand for clean and safe water will continue to grow well into the 21st century. Moving forward, the lack of access to clean water, which threatens human health and strains precious energy resources, will worsen as the climate changes. Therefore, future innovations that produce potable water from contaminated sources must be sustainable. Inspired by nature, a solar absorber gel (SAG) is developed to purify water from contaminated sources using only natural sunlight. The SAG is composed of an elastic thermoresponsive poly(N‐isopropylacrylamide) (PNIPAm) hydrogel, a photothermal polydopamine (PDA) layer, and a sodium alginate (SA) network. Production of the SAG is facile; all processing is aqueous‐based and occurs at room temperature. Remarkably, the SAG can purify water from various harmful reservoirs containing small molecules, oils, metals, and pathogens, using only sunlight. The SAG relies on solar energy to drive a hydrophilic/hydrophobic phase transformation at the lower critical solution temperature. Since the purification mechanism does not require water evaporation, an energy‐intensive process, the passive solar water‐purification rate is the highest reported. This discovery can be transformative in the sustainable production of clean water to improve the quality of human life.
Nanoparticle attachment at a fluid interface is a process that often takes place concurrently with nanoparticle aggregation in the bulk of the suspension. Here we investigate systematically the coupling of these processes with reference to the adsorption of aqueous suspensions of ethyl cellulose (EC) nanoparticles at the air-water interface. The suspension stability is optimal at neutral pH and in the absence of salt, conditions under which the electrostatic repulsion among EC nanoparticles is maximized. Nonetheless, hydrophobic attraction dominates particle-interface interactions, resulting in the irreversible adsorption of EC nanoparticles at the air-water interface. The addition of salt weakens the particle-particle and particle-interface repulsive electrostatic forces. This leads to destabilization of the suspension at ionic strengths of 0.05 M or greater but does not affect nanoparticle adsorption. The energy of adsorption, the surface tension and interface coverage at steady state, and the particle contact angle at the interface all remain unchanged by the addition of salt. These findings contribute to the fundamental understanding of colloidal systems and inform the utilization of EC nanocolloids, in particular for the stabilization of foams and emulsions.
Diverse processes—e.g., environmental pollution, groundwater remediation, oil recovery, filtration, and drug delivery—involve the transport of colloidal particles in porous media. Using confocal microscopy, we directly visualize this process in situ and thereby identify the fundamental mechanisms by which particles are distributed throughout a medium. At high injection pressures, hydrodynamic stresses cause particles to be continually deposited on and eroded from the solid matrix—notably, forcing them to be distributed throughout the entire medium. By contrast, at low injection pressures, the relative influence of erosion is suppressed, causing particles to localize near the inlet of the medium. Unexpectedly, these macroscopic distribution behaviors depend on imposed pressure in similar ways for particles of different charges, although the pore-scale distribution of deposition is sensitive to particle charge. These results reveal how the multiscale interactions between fluid, particles, and the solid matrix control how colloids are distributed in a porous medium.
To overcome the current scarcity of fresh water sustainably, new technologies will be required that produce potable water from a range of sources, including seawater and moisture from the atmosphere. Moreover, we must recover and reuse water from wastewater streams to reduce our global water footprint. To date, there remain significant concerns about the environmental/ecological impact, high energy consumption, and extensive maintenance costs of current technologies that might prevent their transition to more sustainable routes of potable water generation. One class of material that can enable low-energy water production is thermoresponsive polymers. Due to their unique phase behavior, production flexibility, and biocompatibility, these materials may allow for sustainable routes to fresh water in current and new technologies. In this Perspective, we specifically summarize the design and application of poly(N-isopropylacrylamide)-(PNIPAm-) based thermoresponsive microgels and hydrogels. In particular, we show how these materials have been used for water purification, including wastewater treatment, seawater desalination, and moisture harvesting from the atmosphere. Finally, we discuss the opportunities and challenges of transforming current thermoresponsive materials into practical water-related technologies.
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