Novel oil-in-water (O/W) emulsions are prepared which are stabilised by a cationic surfactant in combination with similarly charged alumina nanoparticles at concentrations as low as 10 m and 10 wt %, respectively. The surfactant molecules adsorb at the oil-water interface to reduce the interfacial tension and endow droplets with charge ensuring electrical repulsion between them, whereas the charged particles are dispersed in the aqueous films between droplets retaining thick lamellae, reducing water drainage and hindering flocculation and coalescence of droplets. This stabilization mechanism is universal as it occurs with different oils (alkanes, aromatic hydrocarbons and triglycerides) and in mixtures of anionic surfactant and negatively charged nanoparticles. Further, such emulsions can be switched between stable and unstable by addition of an equimolar amount of oppositely charged surfactant which forms ion pairs with the original surfactant destroying the repulsion between droplets.
A novel
CO2/N2 switchable n-decane-in-water
emulsion was prepared, which is stabilized by a
CO2/N2 switchable surfactant [N′-dodecyl-N,N-dimethylacetamidine
(DDMA)] in cationic form in combination with positively charged alumina
nanoparticles at concentrations as low as 0.01 mM and 0.001 wt %,
respectively. The particles do not adsorb at the oil–water
interface but remain dispersed in the aqueous phase between surfactant-coated
droplets. A critical zeta potential of the particles of ca. +18 mV
is necessary for the stabilization of the novel emulsions, suggesting
that the electrical double-layer repulsions between particles and
between particles and oil droplets are responsible for their stability.
By bubbling N2 into the emulsions, demulsification occurs
following transformation of DDMA molecules from the surface-active
cationic form to the surface-inactive neutral form and desorption
from the oil–water interface. Bubbling CO2 into
the demulsified mixtures, cationic DDMA molecules are re-formed, which
adsorb to the droplet interfaces, ensuring stable emulsions after
homogenization. Compared with Pickering emulsions and traditional
emulsions, the amount of switchable surfactant and number of like-charged
particles required for stabilization are significantly reduced, which
is economically and environmentally benign for practical applications.
Novel oil-in-water (O/W) emulsions are prepared which are stabilised by ac ationic surfactant in combination with similarly charged alumina nanoparticles at concentrations as low as 10 À5 m and 10 À4 wt %, respectively.T he surfactant molecules adsorb at the oil-water interface to reduce the interfacial tension and endowd roplets with charge ensuring electrical repulsion between them, whereas the charged particles are dispersed in the aqueous films between droplets retaining thickl amellae,r educing water drainage and hindering flocculation and coalescence of droplets.This stabilization mechanism is universal as it occurs with different oils (alkanes, aromatic hydrocarbons and triglycerides) and in mixtures of anionic surfactant and negatively charged nanoparticles. Further,s uch emulsions can be switched between stable and unstable by addition of an equimolar amount of oppositely charged surfactant whichf orms ion pairs with the original surfactant destroying the repulsion between droplets.
It is very challenging to achieve efficient and deep desulfurization, especially in flue gases with an extremely low SO 2 concentration. Herein, we report a microporous metal−organic framework (ELM-12) with specific polar sites and proper pore size for the highly efficient SO 2 removal from flue gas and other SO 2 -containing gases. A high SO 2 capacity of 61.2 cm 3 •g −1 combined with exceptionally outstanding selectivity of SO 2 /CO 2 (30), SO 2 /CH 4 (871), and SO 2 /N 2 (4064) under ambient conditions (i.e., 10:90 mixture at 298 K and 1 bar) was achieved. Notably, the SO 2 /N 2 selectivity is unprecedented among ever reported values of porous materials. Moreover, the dispersion-corrected density functional theory calculations illustrated the superior SO 2 capture ability and selectivity arise from the high-density SO 2 binding sites of the CF 3 SO 3 − group in the pore cavity (S δ+ •••O δ− interactions) and aromatic linkers in the pore walls (H δ+ •••O δ− interactions). Dynamic breakthrough experiments confirm the regeneration stability and excellent separation performance. Furthermore, ELM-12 is also stable after exposure to SO 2 , water vapor, and organic solvents.
The
transition between a novel oil-in-dispersion emulsion and an
oil-in-water (O/W) Pickering emulsion triggered by pH was achieved
using alumina nanoparticles in combination with a cationic surfactant.
In acidic and neutral aqueous media, positively charged particles
and the surfactant both at very low concentrations costabilize an
oil-in-dispersion emulsion with the surfactant adsorbed at droplet
interfaces and particles dispersed in the aqueous phase between the
droplets. In alkaline media, however, particles become negatively
charged and are hydrophobized in situ by adsorption
of the surfactant to become surface-active and stabilize an O/W Pickering
emulsion. The transition between the two is also possible by lowering
the pH. The transformation can be achieved several times in a mixture
of 0.1 wt % nanoparticles and 0.01 mM surfactant. This transition
is significant, since particles can be made to either adsorb at the
oil–water interface, which is beneficial for applications like
biphasic catalysis, or remain dispersed in the aqueous phase, which
is favorable for their recovery and reuse.
Stable n-decane-in-water Pickering emulsions were prepared using positively charged alumina nanoparticles in combination with a trace amount of the anionic surfactant sodium dodecyl sulfate (SDS) as a stabilizer. Particles were hydrophobized in situ by adsorption of surfactant enhancing their surface activity. Emulsions can be readily demulsified by addition of an equal amount of a switchable surfactant,, which can be transformed between a surface-active amidinium/cationic form and a surface-inactive amidine/neutral form by bubbling CO 2 or N 2 ,respectively. Following addition of cationic DDAA which prefers to form ion pairs with SDS, desorption of SDS from particle surfaces occurs and alumina particles are rendered hydrophilic resulting in demulsification of the emulsion. However, by bubbling N 2 into the demulsified mixture, DDAA molecules are converted to the amidine/neutral form leading to collapse of the ion pairs and re-establishment of the in situ hydrophobization of particles. Stable Pickering emulsions can be prepared again following homogenization. This simple demulsification/re-stabilization cycle can be repeated several times.Experimental evidence including measurement of the adsorption isotherm, zeta potentials, extent of particle adsorption at droplet interfaces in emulsions and microscopy is given to support the postulated mechanisms.
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