The size, composition, and morphology of KNO3 particles
produced under the conditions of isothermal evaporation crystallization
were found to depend on the concentration ratios of surfactant in
mixed reverse micelles of Tergitol NP-4 + AOT, content of water pseudophase
(w
o = 1.1–4.4), concentration of
salt (0.8–2.5 M), and temperature (20–40 °C). In
optimal conditions (w
o = 2.2, T = 35 °C), powders with micrometer particles of tubular
structure are formed in reverse micellar solutions of Tergitol NP-4
in n-decane. The addition of 0.01–0.05 M AOT
results in the splitting of tubes, a 103-fold increase
in the specific surface area, and formation of whisker-type particles
5–10 μm long and 0.2–0.5 μm thick. We used
photon-correlation spectroscopy and a spinning drop procedure, which
showed that the addition of AOT to Tergitol NP-4 leads to a decrease
in the hydrodynamic diameter of reverse micelles, and interfacial
tension passes through the maximum, which increases with growing temperature.
We have determined
the solubilization and crystallization fields
of KNO3 for simple (Tergitol NP-4 and AOT) and mixed (Tergitol
NP-4 + AOT) microemulsions at 35 °C, depending on water content
and concentration of KNO3. An increase in the content of
AOT in mixed microemulsions leads to widening of the solubilization
field, but the crystallization field narrows up to complete disappearance
(in simple microemulsions of AOT). Fourier transform-infrared spectroscopy,
photon–correlation spectroscopy, and flame photometry were
used to determine the rate of water evaporation (2.6–7.4 μL/min),
hydrodynamic diameter of micelles (4.3–10 nm), and the content
of water and K+ and Na+ ions in the studied
microemulsions at different stages of evaporation crystallization
of KNO3. In simple microemulsions of Tergitol NP-4 crystallization
begins well before that in mixed microemulsions; in mixed microemulsions
the beginning of crystallization requires higher supersaturation (up
to 13 M) compared to simple microemulsions (∼4 M at initial
solubilization capacity of 0.5 vol %). In accordance with the Gibbs–Thomson
(Kelvin) theory, high supersaturations suggest that in mixed micelles
cores of smaller sizes are formed. During crystallization, micelles
with KNO3 solution coexist with KNO3 crystals.
The function of micelles is to supply “building material”
for their growth. Numerical simulation with the help of direct optimization
showed that in mixed micelles at low water contents, Na+ ions are stronger bound with AOT–, owing to which
Na+ ions occur only as impurities in powders.
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