Back and forth: The CO2 /N2 trigger of a switchable surfactant (neutral amidine/cationic amidinium) was transferred to mineral nanoparticles through in situ hydrophobization in water. Switchable oil-in-water Pickering emulsions that entail a CO2 /N2 trigger were obtained by using negatively charged silica nanoparticles and a trace amount of the switchable surfactant as the stabilizer.

A stable oil-in-water Pickering emulsion stabilized by negatively charged silica nanoparticles hydrophobized in situ with a trace amount of a conventional cationic surfactant can be rendered unstable on addition of an equimolar amount of an anionic surfactant. The emulsion can be subsequently restabilized by adding a similar trace amount of cationic surfactant along with rehomogenization. This destabilization-stabilization behavior can be cycled many times, demonstrating that the Pickering emulsion is switchable. The trigger is the stronger electrostatic interaction between the oppositely charged ionic surfactants compared with that between the cationic surfactant and the (initially) negatively charged particle surfaces. The cationic surfactant prefers to form ion pairs with the added anionic surfactant and thus desorbs from particle surfaces rendering them surface-inactive. This access to switchable Pickering emulsions is easier than those employing switchable surfactants, polymers, or surface-active particles, avoiding both the complicated synthesis and the stringent switching conditions.

Hin und her: Der CO2/N2‐Trigger eines schaltbaren Tensids (neutrales Amidin/kationisches Amidinium) wurde durch In‐situ‐Hydrophobisierung in Wasser auf mineralische Nanopartikel übertragen. Schaltbare Öl‐in‐Wasser‐Pickering‐Emulsionen wurde durch die Verwendung negativ geladener Siliciumdioxid‐Nanopartikel und einer geringen Menge des schaltbaren Tensids erhalten.

We put forward a simple protocol to prepare thermoresponsive Pickering emulsions. Using hydrophilic silica nanoparticles in combination with a low concentration of alkyl polyoxyethylene monododecyl ether (CE) nonionic surfactant as emulsifier, oil-in-water (o/w) emulsions can be obtained, which are stable at room temperature but demulsified at elevated temperature. The stabilization can be restored once the separated mixture is cooled and rehomogenized, and this stabilization-destabilization behavior can be cycled many times. It is found that the adsorption of nonionic surfactant at the silica nanoparticle-water interface via hydrogen bonding between the oxygen atoms in the polyoxyethylene headgroup and the SiOH groups on particle surfaces at low temperature is responsible for the in situ hydrophobization of the particles rendering them surface-active. Dehydrophobization can be achieved at elevated temperature due to weakening or loss of this hydrogen bonding. The time required for demulsification decreases with increasing temperature, and the temperature interval between stabilization and destabilization of the emulsions is affected by the surfactant headgroup length. Experimental evidence including microscopy, adsorption isotherms, and three-phase contact angles is provided to support the mechanism.

In the recent past there has been a growing interest in switchable surfactants and stimuli-responsive surface-active particles, since both have surface activity which is either switchable or controllable and they can be recovered and re-used afterwards. Among various triggers the CO2/N2 trigger is particularly environmentally benign. In this paper a facile protocol to obtain switchable surface-active silica nanoparticles using a CO2/N2 trigger is proposed and their utilization in producing responsive aqueous foams with the same trigger is examined. Using a switchable surfactant, N'-dodecyl-N,N-dimethylacetamidinium bicarbonate, which can be switched between a cationic surfactant and a surface-inactive neutral form by bubbling with CO2 and N2 respectively, bare silica nanoparticles can be hydrophobised in situ to become surface-active nanoparticles and the switch of the surfactant can thus be transferred to the particles. Thus responsive particle-stabilised aqueous foams can be prepared. Compared with foams stabilised by specially synthesized switchable or stimuli-responsive particles, the method reported here is much easier, whereas compared with those stabilised by switchable or stimuli-responsive surfactants the method here requires a relatively low concentration.

In the recent past, switchable surfactants and switchable/stimulus-responsive surface-active particles have been of great interest. Both can be transformed between surface-active and surface-inactive states via several triggers, making them recoverable and reusable afterward. However, the synthesis of these materials is complicated. In this paper we report a facile protocol to obtain responsive surface-active nanoparticles and their use in preparing responsive particle-stabilized foams. Hydrophilic silica nanoparticles are initially hydrophobized in situ with a trace amount of a conventional cationic surfactant in water, rendering them surface-active such that they stabilize aqueous foams. The latter can then be destabilized by adding equal moles of an anionic surfactant, and restabilized by adding another trace amount of the cationic surfactant followed by shaking. The stabilization-destabilization of the foams can be cycled many times at room temperature. The trigger is the stronger electrostatic interaction between the oppositely charged surfactants than that between the cationic surfactant and the negatively charged particles. The added anionic surfactant tends to form ion pairs with the cationic surfactant, leading to desorption of the latter from particle surfaces and dehydrophobization of the particles. Upon addition of another trace amount of cationic surfactant, the particles are rehydrophobized in situ and can then stabilize foams again. This principle makes it possible to obtain responsive surface-active particles using commercially available inorganic nanoparticles and conventional surfactants.

Injection current and temperature dependence of electroluminescence (EL) is investigated in AlInGaN deep untraviolet light-emitting diodes. Two EL bands with different behaviors are observed. The high-energy band (P1) shows a monotonous redshift and an amazing increase of intensity with increasing current, however, a “U”-shaped shift and a saturation of intensity at high current are measured for the low-energy band (P2). Accordingly, P1 and P2 are attributed to emissions from quantum-well and localized states, respectively, with P1 dominant at high current and high temperature and P2 the main emission mechanism under low temperature and low current. Modeled data based on the theory of random population for localized states in quantum wells taking into account self-heating effect agree well with the experimental results.

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