Microemulstion synthesis of powders of water-soluble energy-saturated salts

Article abstract of DOI:10.1134/S003602361206006X

The feasibility of preparing energy-saturated salts (NH₄NO ₃, KNO₃, and NaBH₄) in powders with various particle sizes in microemulsion systems based on oxyethylated surfactant Tergitol NP-4 has been demonstrated. Powders were isolated by destroying microemulsions with acetone. The regions of micellar synthesis have been determined depending on the solubilization capacities and concentrations of the reagents and salts at a fixed Tergitol NP-4 concentration (0.25 mol/L). The morphologies and particle sizes of the thus-prepared salt powders were characterized by scanning electron microscopy (SEM), X-ray powder diffraction, differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravimetry; the hydrodynamic radii of microemulsions were characterized by photon-correlation spectroscopy. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S003602361206006X

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 6, pp. 769–776. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © A.I. Bulavchenko, M.G. Demidova, T.Yu. Podlipskaya, V.V. Tatarchuk, I.A. Druzhinina, A.V. Alekseev, V.A. Logvinenko, V.A. Drebushchak, 2012, published
in Zhurnal Neorganicheskoi Khimii, 2012, Vol. 57, No. 6, pp. 839–846.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Microemulstion Synthesis of Powders of WaterꢀSoluble
EnergyꢀSaturated Salts
A. I. Bulavchenko, M. G. Demidova, T. Yu. Podlipskaya, V. V. Tatarchuk, I. A. Druzhinina,
A. V. Alekseev, V. A. Logvinenko, and V. A. Drebushchak
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 3, Novosibirsk, 630090 Russia
Received March 15, 2011
Abstract—The feasibility of preparing energyꢀsaturated salts (NH4NO3, KNO3, and NaBH4) in powders
with various particle sizes in microemulsion systems based on oxyethylated surfactant Tergitol NPꢀ4 has been
demonstrated. Powders were isolated by destroying microemulsions with acetone. The regions of micellar
synthesis have been determined depending on the solubilization capacities and concentrations of the reagents
and salts at a fixed Tergitol NPꢀ4 concentration (0.25 mol/L). The morphologies and particle sizes of the
thusꢀprepared salt powders were characterized by scanning electron microscopy (SEM), Xꢀray powder difꢀ
fraction, differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravimeꢀ
try; the hydrodynamic radii of microemulsions were characterized by photonꢀcorrelation spectroscopy.
DOI: 10.1134/S003602361206006X
Microemulsion (micellar) synthesis is a popular
method and is widely used for preparing nanoparticles
of metals [1], oxides [2], and sparingly soluble salts [3,
4]. The advantages of this method include: (1) the ease
of synthesis; (2) a narrow particleꢀsize distribution
function of the product; (3) a feasibility of preparing
particles of complex composition, multilayered,
hybrid, and other types of particles [5]; and (4) a good
knowledge of the micellar and microemulsion strucꢀ
ture. Unfortunately, there only few studies directed to
the preparation of nanoparticles of waterꢀsoluble salts
by the microemulsion method. For example,
Co(NO3)2, CaCl2, Na2HPO4, and Cu(NO3)2 nanoparꢀ
EXPERIMENTAL
Reagents
The micelleꢀforming surfactant used was oxyethyꢀ
lated nonylphenol with an average oxyethylation
number of four, namely, Tergitol NPꢀ4 (from Dow
Chemical). A micellar solution contained 0.25 mol/L
Tergitol NPꢀ4 in
were pure grade
n
n
ꢀdecane. The organic solvents used
ꢀdecane and ꢀdimethylformaꢀ
N,N
mide (DMF) and high purity grade heptane and aceꢀ
tone. The initial reagents used were concentrated nitric
acid (high purity grade, 16.2 mol/L), concentrated
aqueous ammonia (high purity grade, 10.8 mol/L),
KOH, and NaBH4 (both of pure grade), NH4NO3
(pure for analysis grade), and KNO3 (chemically pure
ticles have been prepared in a solidꢀphase concentrate grade).
in an AOT matrix (sodium di(2ꢀethylhexyl)sulfosucciꢀ
nate) via distilling off the solvent (heptane) and water
Structural Study of Microemulsions and Powders
The limiting solubilization capacity Vs Vo (the volꢀ
[6, 7].
/
ume ratio of an aqueous solubilizate solution to a solꢀ
ubilizateꢀcontaining micellar solution) of a microꢀ
emulsion system with respect to aqueous solutions of
reagents and salts was determined by serial injections
of small volumes of the aqueous phase. The cloud
point was detected visually and spectrophotometriꢀ
cally by an abrupt increase in the absorbance of the
solution. Measurements were carried out on a Shiꢀ
madzu 1700 spectrophotometer using quartz cells (l =
1 cm); the reference solution was a “dry” micellar
solution.
Microemulsions with initial reagents were structurꢀ
ally studied by photonꢀcorrelation spectroscopy and
Meanwhile, studies of microemulsion synthesis as
applied to prepare powders and liquid hydrophobic
concentrates of nanoparticles of waterꢀsoluble energyꢀ
intensive salts seem to be very promising for both sciꢀ
ence and application. For example, reverse emulsions
of supersaturated ammonium nitrate solutions [8, 9]
in diesel fuel have almost substituted for TNTꢀconꢀ
taining industrial explosives [10–13].
Our goal here was to study the possibility of preparꢀ
ing powders of waterꢀsoluble salts in reverse microꢀ
emulsions of oxyethylated surfactant Tergitol NPꢀ4.
769

770
BULAVCHENKO et al.
The scattered light intensity was determined at a
° angle using a 90Plus spectrometer as the count rate
(the scattered photon number per millisecond).
The morphology and particle size of ultrafine powꢀ
ders were studied with a JEOL JSMꢀ6460LV scanning
electron microscope on a graphite substrate.
Xꢀray powder diffraction studies of polycrystalline
samples were carried out on a DRONꢀ3M automated
A
2.5
2.0
1.5
1.0
0.5
0
(а)
A
90
3
2.0
2
1.5
1.0
0.5
1
600 800
400
300
500 700
λ
, nm
powder diffractometer (
Ni filter, a scintillation detector with amplitude disꢀ
crimination, 2.5 Soller slits on the primary and
reflected beams) in a range of –70 with scan
steps of 0.03 and exposure time per point of 1 s.
R = 192 mm, CuKα radiation,
°
2θ
5°
°
°
0
1
2
3
4
o, vol %
In some cases (for small samples or strong preferred
orientation of crystallites), patterns were recorded in
Debye–Scherrer geometry on a Bruker X8 APEX sinꢀ
gleꢀcrystal diffractometer (MoKα radiation, graphite
monochromator) as described in [16]. Diffraction patꢀ
terns were recorded with a CCD area detector which
was positioned normal to the primary beam (detector
Vs/
V
r
h, nm
(b)
I
7
6
5
4
3
2
1
400
300
resolution: 1024
× 1024 pixels; sampleꢀtoꢀdetector
distance: 50 mm; exposure time: 15 min).
Xꢀray phase analysis was carried out in an autoꢀ
mated mode with reference to the PDFꢀ2 power data
file [17]. Crystallite sizes were derived from the inteꢀ
grated widths of single diffraction lines using the
Scherrer relationship and with a correction applied for
instrumental line broadening (the reference was
corundum).
200
100
0
0
1
2
3
4
o, vol %
Vs/
V
Calorimetric measurements were carried out on a
DSCꢀ204 Netzsch differential scanning calorimeter
Fig. 1. (a) Absorbance
A
at
λ
= 800 nm and (b) hydrodyꢀ
vs solubiliꢀ
for the soluꢀ
using standard (40ꢀμL) aluminum crucibles in a dry
namic radius and light scattering intensity
r
I
argon flow (10 mL/min) at a heating rate of 6 K/min.
Ammonium nitrate sample sizes were 12.50 mg (for
the commercially available product) and 5.07 mg (for
the synthesized samples).
h
zation capacity of the micellar solution
V /V
s o
bilization of water. In the inset: absorption spectra of
micellar solutions with different water contents: ( ) 4.2,
) 4.3, and ( ) 4.5 vol %.
1
(2
3
TGA and DTA curves were obtained with a Netzsch
TG 209 F1 calorimeter (sample sizes: 1 to 10 mg; heating
rate: 9.6 K/min).
static light scattering. Aqueous electrolyte solutions
were introduced into micellar solutions of Tergitol
NPꢀ4 by injection. All microemulsions obtained in
this manner and the initial “dry” micellar solution
were dedusted by fiveꢀ to tenfold cyclic filtering through
no. 5 glass Schott filter (with a pore size of 5.1 μm) in a
closed box. Strongly opalescent and clouded systems
with largeꢀsize particles were not dedusted. Measureꢀ
Preparation of Powders
Microemulsions of NH4NO3 and KNO3 aqueous
solutions were prepared in two routes: (1) from the
reagents by a neutralization reaction and (2) from
commercially available salts.
According to the first route, to a microemulsion
with a solubilized ammonia solution, nitric acid was
additionally injected. In order to prepare HNO3, the
order in which reagents (HNO3 and KOH) were added
was opposite. The reagent ratio was stoichiometric, or
a multiple excess of one reagent was created. Accordꢀ
ing to the second route, an aqueous solution of a salt
ments were carried out at a 90° angle in quartz cells
(
l
= 1 cm) on a 90Plus Brookhaven Inst. spectrometer.
A Lasermax solidꢀstate laser had a power of 35 mW;
the wavelength was 658 nm; scattered photons were
accumulated by a highꢀsensitivity APD detector (Perꢀ
kinꢀElmer). The zꢀaverage hydrodynamic radius [14]
was calculated as the average of 10 measurements
using the Stokes–Einstein relationship for spherical
particles [15]. The photon accumulation time was 10 s.
Temperature was maintained with an accuracy of 0.1 K;
the measurement error in hydrodynamic radius did
not exceed 5%.
(
NH4NO3 or KNO3) was solubilized into micelles.
Powders were isolated from microemulsions by
destroying the microemulsions by acetone. The parꢀ
ticular conditions for preparing microemulsions and
for isolating ultrafine salt powders from them were as
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 6 2012

MICROEMULSTION SYNTHESIS OF POWDERS
771
follows (for the samples that were characterized by
scanning electron microscopy).
r
h, nm
(а)
I
8
6
4
2
First synthesis route. Concentrated aqueous ammoꢀ
nia was introduced into a micellar solution by injection
solubilization to a concentration of up to 1.5 vol %; into
the resulting solution, concentrated nitric acid was
then solubilized so that the ratio NH3 : HNO3 = 1.5 : 1
was held.
Second synthesis route. In order to obtain precurꢀ
sor microemulsions, an aqueous solution of 2 M
KNO3 was injected in two aliquots into the micellar
solution under stirring for 2 and 10 min. The concenꢀ
tration of the solubilized aqueous phase was 2 vol %.
From the microemulsions prepared in this way,
NH4NO3 and KNO3 powders were isolated by diluting
them with acetone in the volume ratio 1 : 1. The isoꢀ
lated powders were washed with acetone and then
three times with heptane; then, they were dried in air
and stored in glass weighing bottles inside a desiccator
over P2O5. Products yields were 35–75%.
A specific situation occurred with NaBH4, which is
vigorously decomposed by acids, and hydrolyzes in
water and aqueous (even concentrated) ammonia
solutions and in aqueous alkalis with concentrations
less than 2 mol/L. Of the nonaqueous polar solvents,
DMF is one of the best for NaBH4 because of providꢀ
ing solubilities of about 16% and stable solutions [18];
we used it in studying solubilization. Having low soluꢀ
bilization capacities, DMF solutions of borohydride
were unsuitable for preparing microemulsions with
considerable NaBH4 concentrations. For this reason,
ultrafine NaBH4 powders were prepared by means of
strongly reducing the polarity of the medium and rapꢀ
idly crystallizing the salt in the presence of a surfactant
in acetone. For this purpose, we mixed equal volumes
of the saturated solution of NaBH4 in DMF and a
solution of 0.5 M Tergitol NPꢀ4 in acetone; acetone is
completely miscible with DMF but does not dissolve
borohydride. The resulting suspension was centriꢀ
fuged; the mother solution was decanted; the precipiꢀ
tate was washed several times with acetone and hepꢀ
tane (also by decantation) and stored under heptane.
In order to study solid product samples, the solvent
was removed by unforced evaporation at room temꢀ
perature. The product yield was 80%.
400
300
200
100
0
0
0.5
1.0
1.5
(b)
2.0
2.5
12
10
8
400
300
200
100
0
6
4
2
0
0.5
1.0
1.5
(c)
2.0
V
2.5
Vs o, vol %
/
80
60
400
300
200
100
0
40
20
0
0.5
1.0
1.5
2.0
Vs
2.5
o, vol %
/
V
RESULTS AND DISCUSSION
Fig. 2. Light scattering intensity
I
V /V for the solubilization of (a) 2 M KNO , (b) 8 M
s o
and hydrodynamic
radius vs solubilization capacity of the micellar solution
Preparation and Characterization of Precursor
Microemulsions for Synthesis
r
h
3
NH NO , and (c) 4 M HNO . The dotted line marks the
4
3
3
Solubilization was studied both for the initial
reagents (acids and bases) and synthesis products
(salts). This was necessitated by the following: first, the
need to determine the optimal reagent concentrations
to provide a maximal product yield; and second, a
relationship between the solubilization capacity, on
one the hand, and micelle sizes [19] and, accordingly,
limiting solubilization capacity determined spectrophotoꢀ
metrically.
nonionic oxyethylated surfactant (Tergitol NPꢀ4) and
solvent ( ꢀdecane) to be used in synthesis, we were
guided by the greater universality of and tolerance of
n
the resulting particle sizes, on the other. In choosing solutions of nonionic surfactants in saturated hydroꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 6 2012

772
BULAVCHENKO et al.
apparent absorbance (at λ = 800 nm) with the absorpꢀ
(а)
Vs/
Vo, vol %
tion spectrum (in the inset) as a function of solubilizaꢀ
tion capacity for water. The cloud point was detected
as the strong increase in absorbance of the solution.
Similar curves were obtained for all systems under
study. Noticeable is good convergence between the
spectrophotometric data and the results of visual
detection. The hydrodynamic radius and scattered
light intensity are shown in Fig. 1b as a function of solꢀ
ubilization capacity for water as the initial base system.
Noticeable are weak maxima at 3.0% aqueous pseudoꢀ
phase. For solubilization capacities greater than 4.0%,
the hydrodynamic radius abruptly increases at the
cloud point to 150 nm (these data are not shown in
Fig. 1b).
For solubilization of salt solutions, similar curves
were obtained (Figs. 2a, 2b). The curve for nitric acid
has a slightly different trend (Fig. 2c). The hydrodyꢀ
namic radius starts to rapidly increase before reaching
the cloud point due to the formation of strongly
extended micelles. The curves we obtained agree with
those obtained earlier [19, 20]. Noticeable is a poor
precision of the results (especially in regard of light
scattering intensity) in the vicinity of the cloud point,
which is because of the systems being unstable. The
visual detection and spectrophotometric determinaꢀ
tion of the cloud point give more precise results comꢀ
pared to photon correlation spectroscopy and static
light scattering. Therefore, these methods were used to
determine limiting solubilization capacities (Fig. 3).
3
6
4
5
4
NH4NO3
2
2
1
KNO3
0
4
8
12
16
, mol/L
с
(b)
с
, mol/L
0.4
0.3
0.2
0.1
5
4
3
2
1
0
4
8
12
16
, mol/L
с
The limiting solubilization capacities of micellar
solutions upon injection solubilization were shown to
decrease, as concentration increases, in the following
Fig. 3. (a) Solubilization capacity and (b) concentration in
the micellar solution of Tergitol NPꢀ4 vs electrolyte conꢀ
centration in the aqueous pseudoꢀphase: (
1) KOH,
4
order: acids > bases
~ salts (Fig. 3a). The least solubiꢀ
(2
) KNO , ( ) HNO , ( ) NH NO , and ( ) NH OH.
3
4
5
3
3
4
3
lization was noticed for KOH. For HNO3, the solubiꢀ
lization has a wellꢀdefined maximum (7 vol %) for a
concentration of 2 mol/L. We had established similar
trends for HCl and H2SO4 [20]. Noticeable are abnorꢀ
mally high capacities (4–5 vol %) for NH4NO3 and
NH4OH; at low concentrations, they are close to the
solubilization capacity for acid, and at high concenꢀ
trations, even exceed it.
carbons toward electrolytes compared to ionic surfacꢀ
tants [20].
First, we studied the solubilization capacity of a
solution of Tergitol NPꢀ4 in decane for pure water,
which is the major solubilizer. Figure 1a shows the
The different solubilization characters of the
reagents arise from their different natures and the
ampholitic properties of Tergitol NPꢀ4. Due to having
oxyethyl groups and a hydroxide group, the surfactant
has noticeable basic properties (the protonation conꢀ
Composition of the microemulsions and KNO3 yields (the
second synthesis route by the scheme Tergitol NPꢀ4
HNO3 KOH + acetone (1 : 1)) at fixed (nos. 1–4) and
variable (nos. 5–7) aqueous pseudoꢀphase contents
stant
tion of the hydroxide group, it behaves as a weak acid
Kа = 10–11) [22]. The solubilized compounds are
strong electrolytes (salts, nitric acid, and potassium
hydroxide), a weak base (ammonia; b = 1.8
10–5),
and a weak cationic acid (NH4+ 10–10) [23].
a = 5.5
K
H = 10–2 to 10–3) [21], and due to the dissociaꢀ
Microemulsion
KNO3
yield, %
KNO3 concentraꢀ
tion in microemulꢀ
sion, mol/L
No.
(
aqueous pseudoꢀ
phase content, vol %
K
×
1
2
3
4
5
6
7
2
2
2
2
0.5
1
2
0.005
0.01
0.02
0.04
0.01
0.02
0.04
0
26
45
73
36
63
73
;
K
×
The other conditions being equal, evidently, the soluꢀ
bilization capacity for a solution of a strong base is
lower than for a weak one. The preferred interaction of
ammonium ions with the oxyethyl groups of the surꢀ
factant (compared to potassium ions) had been shown
in [24].
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 6 2012

MICROEMULSTION SYNTHESIS OF POWDERS
773
For monitoring the synthesis output, it is necessary
Fraction, %
to know the overall salt content in the organic phase
(Fig. 3b). Once the solubilization capacity and overall
salt concentration are known, one can choose both the
optimal reagent composition for synthesis and the
micelle sizes. In preparing ammonium nitrate by the
second route, the same high salt contents can be proꢀ
vided by adding micelles with 3 vol % of 5 M NH4NO3
solution or 2 vol % of a 7.5 M solution.
30
20
10
0
80 120160 200 240
d, nm
The solubilization of aqueous NaBH4 solutions is
accompanied with H2 evolution because of hydrolysis
of the salt. Stable aqueous solutions of NaBH4 were
obtained only on the basis of 2 M NaOH. In solutions
based on concentrated NH4OH solutions and 0.1 M
NaOH, sodium borohydride slowly decomposes
despite of their alkalinity. For solutions of ~3 mol/L
NaBH4 in concentrated NH4OH, the solubilization
capacity of Tergitol NPꢀ4 solution was on the order of
1 vol %. This dictated the need for finding alternative
solvents for NaBH4. Stable NaBH4 solutions with conꢀ
centrations of up to 1.6 mol/L were prepared in DMF.
The solubilization capacity of a decane solution of
Tergitol NPꢀ4 for DMF solutions of NaBH4 was
almost independent of borohydride concentration (up
to 1.6 mol/L) and was 1.5 vol %, as for DMF,
1 μm
(а)
Fraction, %
30
20
10
0
1.0 1.8 2.6
d, μm
1 μm
(b)
In this way, microemulsions with different concenꢀ
trations of the reagents and aqueous pseudoꢀphase and
with different polarities of micellar cores and sizes of
polar cores were provided for the synthesis.
Fraction, %
40
30
20
10
0
Separation of Powders from Microemulsions.
Spontaneous separation of salts from microemulsions
was not observed during synthesis. Noticeably, the
opposite situation is more frequently observed in
reduction of metals: it is quite problematic to obtain
stable microemulsions with nanoparticles, because
coagulation frequently starts during synthesis [25].
The situation with waterꢀsoluble salts is more comꢀ
plex: in microemulsions, we have not salt nanopartiꢀ
cles; rather, we have their solutions. Therefore, for
separating powders from microemulsions containing
aqueous salt solutions, the following is necessary: (1)
to obtain nanoparticles by decreasing the polarity of
aqueous micellar cores and thereby reducing salt soluꢀ
bilities; (2) to induce coagulation of nanoparticles by
destroying micelles; and (3) to retain micellar water in
the organic phase in order to keep the separated salt
from being dissolved by desolubilized water. For isolatꢀ
ing metal nanoparticles in similar cases, polar organic
solvents are used [26] where surfactants dissolve better
than in saturated hydrocarbons and do not form
micelles. As a result, nanoparticles are deprived of the
protective surfactant layer, and osmotic repulsion
strongly decreases. The potential well depth becomes
greater due to the fact that particles can come to small
distances to each other and coagulation starts in the
system.
30 52 73 95
Thickness, nm
1 μm
(c)
Fig. 4. SEM images and particleꢀsize distribution funcꢀ
tions for (a) sodium borohydride, (b) potassium nitrate,
and (c) ammonium nitrate powders.
NPꢀ4 microemulsions, ultrafine crystalline powders
can be separated only within certain ranges of reagent
concentrations (and reagent ratios) and aqueous
pseudoꢀphase contents (the first synthesis route). In
preparing ammonium nitrate from HNO3 and
NH4OH, product separation was observed only when
the aqueous pseudoꢀphase content was 1.3 to 2 vol %
(for concentrated reagent solutions) with the stoichioꢀ
metric reagent ratio and up to a twofold excess of
NH4OH. In preparing KNO3 by the neutralization
reaction, the opposite situation was observed: the salt
was isolated only with the stoichiometric reagent ratio
or in an excess of HNO3 (up to a fourfold excess). Salt
Of the polar solvents used to isolate powders, aceꢀ
tone was most efficient with acetoneꢀtoꢀmicroemulꢀ
sion phase ratios of at least 1 : 1 (vol/vol). For Tergitol separation by acetone occurred only when salt conꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 6 2012

774
BULAVCHENKO et al.
DSC, W/g
I
, pulses/s
(а)
(а)
1
0.8
0.6
0.4
0.2
2
4000
3
2
1
0
3000
2000
1000
0
2
0
50
25
1
10
20
30
2θ
, deg/МоKα
(b)
0
50
100
T, °C
150
200
I
, pulses/s
(b)
DTA, К
0.5
TG, %
100
6000
4000
2000
80
60
40
20
0
0
–0.5
–1.0
–1.5
–2.0
–2.5
1
2
2
1
2
2
1
0
20
30
, deg/CuKα
40
50
2θ
I
, pulses/s
(c)
100
300
500
700
6000
T, °C
Fig. 6. (a) DSC and (b) DTA and TG data for NH NO
4
3
samples: (1) a reference and (2) a sample prepared by
4000
micellar synthesis. In inset (a): the lowꢀtemperature DSC
range.
1
2000
0
separated when concentrations were higher than
0.5 mol/L and aqueous pseudoꢀphase contents were
0.8–4 vol % (the regions for separating NH4NO3 and
KNO3 powders are shown in Fig. 3a schematically).
Thus, the salt separation regions are bounded from
above by the solubilization curve, and from the left and
from below, by some minimal concentrations and solꢀ
ubilization capacities as specified above. In order for
powders to be separated, it is evidently necessary that
both the salt content and water content in the microꢀ
emulsion be optimal. In moving to the “left” from the
aforementioned regions, addition of acetone gives rise
to the separation of microemulsions into two liquid
phases of comparable volumes (their compositions
and structures have not been studied). In moving
“down,” neither phase separation nor powder isolaꢀ
tion occurs.
2
20
30
40
50
2θ, deg/CuKα
Fig. 5. Xꢀray diffraction patterns for (a) KNO , (b)
3
NH NO , and (c) KBH : (
1
) the initial salts and (
pared samples. Vertical lines show peak positions for the
reference samples of KNO (PDF no. 71ꢀ1558), NH NO
3
2) preꢀ
4
3
4
3
4
(PDF no. 74ꢀ1891), and KBH (PDF no. 74ꢀ1891).
4
centrations in solubilized solutions were higher than
0.5 mol/L. The acid could enhance the solubility of
NH4NO3 and suppress the solubility of KNO3 in the
complex multicomponent system water(salt–
acid(base))–decane–acetone –Tergitol NPꢀ4.
When ammonium nitrate was separated from the
For KNO3, it was shown that the salt yield in the
microemulsion obtained by the injection solubilizaꢀ separation region is not constant; rather, it is proporꢀ
tion of NH4NO3 (the second synthesis route), a preꢀ tional to the salt concentration in the reverseꢀmicellar
cipitate appeared only when NH4NO3 concentrations solution (table). Measurements were carried out with
were higher than 5 mol/L and aqueous pseudoꢀphase a fixed aqueous pseudoꢀphase content by varying salt
contents were 0.8–3.1 vol %. Potassium nitrate was concentration (samples 1–4) or with a fixed salt conꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 6 2012

MICROEMULSTION SYNTHESIS OF POWDERS
775
DTG, %/min
DTA, К
0
TG, %
TG, %
(а)
100
2
100
80
60
40
20
0
1
80
1
0
–0.5
2
60
40
20
2
–0.5
–1.0
–1.5
–1.0
–1.5
–2.0
1
1
100
300
500
700
2
T, °C
0
200
600
800
400
T, °C
(b)
DSC, W/g
0.7
Fig. 7. DTA and TG data for KNO samples: (
1
) a referꢀ
3
0.6
0.5
0.4
0.3
ence and (
2) a sample prepared by micellar synthesis.
centration in the concentrate by varying aqueous
pseudoꢀphase content (samples 5–7). In the former
case, the water content in microemulsions was fixed
and in the latter was not. The closeness of values sugꢀ
gests that the water content in the synthesis region is
not the decisive factor; rather, the salt concentration in
the organic phase is. Although, naturally, the water
content should influence the particle size and morꢀ
phology of the resulting powder.
2
1
0.2
0
50
100
T, °C
150
200
Fig. 8. (a) DTA and TG and (b) DSC data for NaBH samꢀ
4
ples: (
1) a reference and (2) a sample prepared by micellar
Characterization of Powders
synthesis.
NH4NO3, KNO3, and NaBH4 powder samples preꢀ
pared by the routes described in the Experimental secꢀ
tion have been characterized by scanning electron
microscopy (Fig. 4). Noticeable are different morꢀ
phologies and particle sizes. For sodium borohydride
(Fig. 4a), particle shapes were more close to spherical
ones; for potassium nitrate particles (Fig. 4b), faceting
was characteristic. The ammonium nitrate powder
(Fig. 4c) isolated from microemulsion consisted of
thin needles. Needles grow together by their ends to
form extended dendriteꢀlike structures that resembled
twigs or snowflakes when viewed under small magnifiꢀ
cations. Attempts to take higher resolution images and
Xꢀray powder diffraction showed that the samples
were single phases and contained, respectively, KNO3
(PDF no. 71ꢀ1558, space group Pmcn
= 9.1659 = 6.4309 Å) and NH4NO3 (PDF
no. 47ꢀ867, space group Pnmm = 4.9288
5.4408 = 5.7529 Å; phase IV). Unlike the two preꢀ
, a = 5.4142 Å,
b
Å, c
,
a
Å, b =
Å, c
ceding salts, the sodium borohydride powder was
Xꢀray amorphous (Fig. 5).
The DSC and DTA endotherms correspond to
phase transitions, polymorphic transitions, and
decomposition of ammonium nitrate. Commercially
available ammonium nitrate (the reference) experiꢀ
to redisperse sample ultrasonically failed because of ence polymorphic transitions (Fig. 6) in the vicinity of
the following temperatures: –18
(IV III), 52 (III II), and 126.7
phase transitions correspond to the melting of crystal
water at , melting of ammonium nitrate at
169.1 , and sublimation–decomposition of HA at
200 С < < 300 . The results obtained on the referꢀ
°
С (V
→
IV), –9
→
°С
the particles melting (decomposing) under the elecꢀ
tron beam. Transmission electron microscopy failed
for the same reasons. The particleꢀsize distribution
diagrams of the salts displayed in Fig. 4 were conꢀ
structed using the data set obtained for 40 to 140 parꢀ
ticles under various magnifications. The average partiꢀ
→
°
С
→
°С
(II I). The
0°С
°С
°
T
°С
ence sample agree with the reported data.
cle size (
thickness for NH4NO3 with the monodispersity value
avg) were, respectively, 150 nm (0.20), 1.8
davg) for NaBH4 and KNO3 and the needle
In the sample prepared by micellar synthesis, there
are no the first two lowꢀtemperature polymorphic
transitions. This proves the absence of lowꢀtemperaꢀ
ture polymorphs (phases V and IV), which make a lot
of troubles in the use of ammonium nitrate. The meltꢀ
(s/d
μm
(0.21), and 48 nm (0.31). At the current (initial) step
of the study, ultrafine powders have been prepared only
for sodium borohydride.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 6 2012

776
BULAVCHENKO et al.
7. V. Marciano, A. Minore, and L. V. Turco, Colloid
ing temperature (166.8°С) was slightly lower than the
Polym. Sci. 278, 250 (2000).
reference value. A slightly unusual feature of the
ultrafine powder was the appearance of a new phase
transition (which is in addition a double one (see the
inset in Fig. 6a), with peaks temperatures of approxiꢀ
8. I. Masalova and A. Ya. Malkin, Kolloidn. Zh. 69, 206
(2007).
9. I. Masalova and A. Ya. Malkin, Kolloidn. Zh. 69, 220
mately 41 and 44°С).
(2007).
From thermoanalytical data, we may infer that the
reference KNO3 sample experiences a polymorphic
10. L. V. Dubnov, N. S. Bakharevich, and A. I. Romanov,
Industrial Explosives (Nedra, Moscow, 1988) [in Rusꢀ
sian].
transition at 132
°
С
(phase II
→
phase I), melting at
) decomposition
333 , and subsequent (at >400°С
°С
11. V. L. Baron and V. Kh. Kantor, Engineering and Techꢀ
nology of Blasting in the USA (Nedra, Moscow, 1989) [in
Russian].
consecutively to KNO2 and K2O. The behavior of the
sample prepared by micellar synthesis is similar, but
has some specific features, in particular, a greater
weight loss, the polymorphic transition temperature
12. M. A. Cook, The Science of Industrial Explosives
(IRECO Chemicals, Salt Lake City, 1974; Nedra, Mosꢀ
cow, 1974).
increased to 149°С, the melting temperature decreased
to 313 , and broadened DTA peaks (Fig. 7).
°
С
13. E. V. Zotov, Electric Spark Initiation of Liquid Exploꢀ
sives, Ed. by A. L. Mikhailov (Russian Federal Nuclear
Center, Sarov) [in Russian].
Thermogravimetry (Fig. 8) shows that the thermal
decomposition kinetics of the samples was almost
identical to that of the references for NH4NO3 and
KNO3 and considerably different from the reference
for NaBH4. In the last case, the ultrafine powder had a
14. S. R. Rafikov, S. A. Pavlova, and I. I. Tverdokhlebova,
Determination of Molecular Weights and Polydispersity
for Macromolecular Compounds (Izd–vo Akad. Nauk
SSSR, Moscow, 1963) [in Russian].
great thermal stability up to 230°С; at higher temperꢀ
atures, it decomposed with higher intensity than the 15. P. J. Missel, N. A. Mazer, G. B. Benedek, and M. C. Carey,
J. Phys. Chem. 87, 1294 (1983).
reference (Fig. 8a).
In summary, this study has demonstrated the feasiꢀ
bility of preparing powders of waterꢀsoluble energyꢀ
intensive salts using oxyethylated surfactants and their
microemulsions.
16. A. V. Alekseev and S. A. Gromilov, Zh. Strukt. Khim.
51 (4), 772 (2010).
17. Powder Diffraction File: Inorganic Phases. Alphabetiꢀ
cal Index (ICDD, 1995), p. 994
18. N. N. Mal’tseva and V. S. Khain, Sodium Borohydride
(Nauka, Moscow, 1985).
ACKNOWLEDGMENTS
19. A. I. Bulavchenko, E. K. Batishcheva, T. Yu. Podlipskaya,
and V. G. Torgov, Kolloidn. Zh. 60, 173 (1998).
20. A. I. Bulavchenko, E. K. Batishcheva, T. Yu. Podlipskaya,
This study was supported by the Russian Foundaꢀ
tion for Basic Research (project no. 09ꢀ03ꢀ00511).
and V. G. Torgov, Kolloidn. Zh. 58, 163 (1996).
21. P. Bonvicini, A. Levi, V. Lucchini, et al., J. Anal. Chem.
Soc. 95, 5960 (1973).
22. M. G. Demidova, A. I. Bulavchenko, and A. V. Alekꢀ
seev, Russ. J. Inorg. Chem. 53, 1446 (2008).
23. V. F. Volynets and M. P. Volynets, The Analytical Chemꢀ
istry of Nitrogen (Nauka, Moscow, 1977) [in Russian].
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 6 2012