This work successfully verified that the addition of a flux (NH 4 F, NH 4 Cl, and H 3 BO 3 ) during synthesis has an impact on the crystallite size and quantum efficiency of submicron-sized particles of CaMgSi 2 O 6 :Eu 2+ phosphors. The addition of NH 4 F or NH 4 Cl increased the crystallite size in the submicron-sized particles, yielding an increase in emission intensity and quantum efficiency. On the other hand, the use of the H 3 BO 3 flux crystallized a secondary phase, SiO 2 , and changed the lattice parameters, which degraded the luminescent properties. In addition, an excessive amount of NH 4 Cl was examined, resulting in nucleation of a secondary phase, CaSiO 3 , which changed the lattice parameters with no improvement in luminescent properties. These results demonstrate that the addition of a flux could be a method to improve the quantum efficiency of submicron-sized particles composed of nanocrystallites; however, a judicious choice of the flux composition and amount has to be carefully considered.
Colloidal
systems, including micellar and reverse micellar mixtures,
are essential for a variety of natural transport processes, such as
the flow of organic and inorganic contaminants in lakes, rivers, and
underground fissures. Thus, an understanding of their structure and
stability is important for prediction of their behavior in complex
environments. Previous experiments have shown that the solvodynamic
diameters (D) of reverse micelles contract linearly
with increased concentrations of salts such as NaBH4, FeSO4, Mg(NO3)2, CuCl2, Al(NO3)3, Fe(NO3)3, and Y(NO3)3. It has also been previously determined that
reverse micelle size is a function of cation valency, through the
Debye screening length (κ–1), and of anion
hydrated radius. Here, we present a new theoretical model for the
aqueous reverse micelle core substructure in water/AOT/isooctane colloidal
systems with added salts. Our model is based on electrical double
layer (EDL) theory and assumes ions are evenly distributed within
the reverse micelle water core. We further analyze reverse micelle
size with respect to ion hydration, reverse micelle water dynamics,
and ion distribution, to propose a mechanism for reverse micelle contraction
and determine the cause of system instability at the critical destabilization
concentration for each salt. We find that destabilization occurs when
the interfacial core water and waters needed for complete ion hydration
exceed the water contained within the reverse micelle at its stable
size. This establishes ion hydration capacity a likely primary mechanism
for reverse micelle destabilization.
We describe the effects of ethanol on the phase behavior of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in nheptane. Using dynamic light scattering (DLS), molecular dynamics (MD) simulations, and nuclear magnetic resonance ( 1 H NMR) spectroscopy, we investigate the aggregation behavior of AOT across a wide range of ethanol/AOT/n-heptane compositions. We conclude that reverse micelles do not form at any of the investigated concentrations. Instead, we observe the formation of other surfactant aggregate morphologies unique to this system, namely, multilayered cylindrical structures and spherical AOT-in-ethanol structures, which vary significantly with changes in ethanol concentration. We also identify mixed-solvent polarity as a driving factor for the surfactant behavior in the system. When the concentration of ethanol is 20 wt % or below, the system is inhomogeneous with varying sizes of AOT, ethanol, and AOT + ethanol aggregates, with the ethanol primarily exhibiting a cosurfactant behavior, almost exclusively binding at the surface of AOT aggregates. With increased ethanol concentration, the ethanol in the system also exhibits solvent-like behaviors in addition to the cosurfactant behaviors. Most significantly, when the ethanol concentration is raised above 35 wt %, the transition to solvent-like behavior allows AOT Na + counterions to dissociate from the headgroups and they are dissolved in the ethanol. We use these results to construct a preliminary phase diagram for the ethanol/AOT/n-heptane system.
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