A facile synthesis of dispersable NaCl nanocrystals

Article abstract of DOI:10.1039/b911047j

During the classical malonic ester synthesis, sodium chloride is eliminated. Sodium diethyl malonate was reacted either with phenacyl chloride or acetyl chloride, both in toluene solution and without solvent. NaCl nanocrystals with a size of 100-300 nm were obtained that could be easily redispersed in organic solvents if phenacyl chloride was used as reagent. The dispersion in dichloromethane was stable for at least two weeks without sedimentation or agglomeration. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b911047j

Dalton
Transactions
An international journal of inorganic chemistry
www.rsc.org/dalton
Number 44 | 28 November 2009 | Pages 9661–9936
ISSN 1477-9226
PAPER
COMMUNICATION
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Annen and Epple
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nanocrystals
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PAPER
www.rsc.org/dalton | Dalton Transactions
A facile synthesis of dispersable NaCl nanocrystals
Thomas Annen and Matthias Epple*
Received 5th June 2009, Accepted 15th August 2009
First published as an Advance Article on the web 25th August 2009
DOI: 10.1039/b911047j
During the classical malonic ester synthesis, sodium chloride is eliminated. Sodium diethyl malonate
was reacted either with phenacyl chloride or acetyl chloride, both in toluene solution and without
solvent. NaCl nanocrystals with a size of 100–300 nm were obtained that could be easily redispersed in
organic solvents if phenacyl chloride was used as reagent. The dispersion in dichloromethane was stable
for at least two weeks without sedimentation or agglomeration.
Introduction
The preparation of nanoparticles can be accomplished by “top-
down” (physical) and by “bottom-up” (chemical) syntheses.1,2 In
chemical syntheses, a typical approach is the formation of water-
insoluble compounds like metals (e.g. Au, Ag) or semiconductors
(e.g. ZnS, CdS) by a suitable chemical reaction. However, this
is difficult for easily water-soluble compounds like NaCl due to
their high solubility. On the other hand, the grinding of sodium
chloride microcrystals to nanocrystals has not been reported so
far, probably because it is difficult due to the necessity for the
exclusion of water.
However, NaCl nanoparticles can be prepared by rapid evap-
oration of water from aerosols or by shock-cooling of NaCl
vapour. Typically, such salt nanoparticles are studied with respect
to their size-dependent deliquescence and efflorescence behaviour.
For instance, Biskos et al. prepared NaCl nanoparticles with a
size between 6 and 60 nm by an aerosol technique.3 La¨hde et al.
prepared nanoscopic NaCl crystals with a diameter of 60–120 nm
by spray-drying in the presence of leucine and lactose.4 Park et al.
prepared NaCl nanocrystals in a furnace reactor and also in an
atomizer with particle sizes of 8 and 100 nm.5 Earlier, we prepared
metal halides, including NaCl, with a size of a few 100 nm by
thermally-induced solid-state reaction from halogenoacetates and
halogenopropionates.6–8 The thermolysis of halogenobutyrates led
to metal halide crystals with a diameter of a few mm.9
Fig. 1 The reaction of sodium diethylmalonate with phenacyl chloride
(A) or acetyl chloride (B).
We have carried out this synthesis with a special emphasis on
the metal halide and found that it occurs as sub-micrometer
crystals, an observation which was made as early as 1908 by
Paal and Ku¨hn.10 This is interesting because this allows the facile
preparation of NaCl nanocrystals which can be easily redispersed
in organic solvents due to their generation in such a solvent. These
nanocrystals may therefore serve as water-soluble nano-templates
or porogens within polymeric or organic structures.
Materials and methods
X-Ray powder diffraction (XRD) was carried out with a Siemens
˚
D500 diffractometer (Cu-Ka radiation, l = 1.54 A) in Bragg–
Brentano mode. Dynamic light scattering (DLS) of the colloidal
dispersions was performed with a Malvern Zetasizer (Nano
ZS, 633 nm laser; Smoluchowsky method). Scanning electron
microscopy (SEM) was performed with a FEI Quanta 400
ESEM instrument after gold–palladium alloy sputtering. ATR-IR
spectroscopy was carried out with a Varian 3100 FT-IR Excalibur
Series Spectrometer equipped with a miRacle sample holder from
PIKE.
If sodium chloride nanocrystals are to be used as templates,
e.g. for nanoporous polymers, they must be dispersable in organic
solvents, i.e. their surface must be compatible with the hydrophobic
solvent. Therefore, we used a classical organic reaction in which
sodium chloride occurs as by-product.
Malonic ester syntheses represent a well established method in
synthetic organic chemistry. Here, a carbanion is reacted with an
alkyl halogenide to give C–C coupling under elimination of NaCl
(Fig. 1).
Acetyl chloride (CH3COCl; Fluka), phenacyl chloride (C6H5-
CO-CH2Cl; Fluka), diethyl malonate (Fluka), and sodium (Fluka)
were all used in p.a. quality. All syntheses were carried out under
strict exclusion of water, i.e. with thoroughly dried reagents and
under inert gas atmosphere (argon).
Of course, the organic reaction product is the focus of the
synthesis, and the eliminated metal halide is usually ignored.
Toluene was used in p.a. quality (J. T. Baker) and thoroughly
dried over sodium. Dichloromethane was used in p.a. quality
(Acros) and thoroughly dried over phosphorus pentoxide. Ethanol
was used in p.a. quality (Sigma Aldrich) and thoroughly dried over
sodium with the addition of 30 g L-1 of diethylphthalate.
Inorganic Chemistry and Center for Nanointegration Duisburg-Essen
(CeNIDE), University of Duisburg-Essen, Universitaetsstrasse 5-7, 45117
Essen, Germany. E-mail: matthias.epple@uni-due.de; Fax: +49 201 183-
2621; Tel: +49 201 183-2413
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 9731–9734 | 9731
©

The syntheses were carried out as follows: 0.3 g sodium
(13 mmol) were dissolved in 10 mL ethanol at room temperature.
2.08 g diethyl malonate (13 mmol) were added under stirring.
10 mL of toluene were added. 13 mmol of phenacyl chloride
(dissolved in 20 mL toluene) or acetyl chloride (as pure liquid) were
rapidly added under stirring at room temperature. The reaction
immediately occurred as visible by the change of the colour of
the reaction mixture from colourless to yellow/orange. Sodium
chloride was isolated by centrifugation at 2500 g and washed with
20 mL ethanol. This was repeated until the NaCl was colourless
(about five times). NaCl was then redispersed in dichloromethane
for dynamic light scattering and scanning electron microscopy
or dried in vacuum. A typical yield was 0.58 g (10 mmol; 76%)
for phenacyl chloride and 0.7 g (12 mmol; 92%) for acetyl
chloride.
For solvent-free syntheses, solid sodium diethyl malonate was
isolated by removing the solvent ethanol in oil-pump vacuum.
2.37 g (13 mmol) of sodium diethyl malonate were reacted with
2.01 g (13 mmol) phenacyl chloride at room temperature after
thorough mechanical mixing with a glass rod and storage at
room temperature for 15 h. In the case of acetyl chloride, 2.37 g
(13 mmol) of sodium diethyl malonate were cooled to -196 ◦C in
a Schlenk tube under argon atmosphere. Then 1.2 g (15 mmol)
of acetyl chloride (at room temperature) were added over 5 min
with the Schlenk tube still immersed in liquid nitrogen. Then the
Schlenk tube was taken out of the liquid nitrogen bath, and the
initially solid mixture was allowed to warm up freely to room
temperature over 20 min, thereby undergoing melting. If the
reaction was carried out at room temperature, i.e. without pre-
cooled diethyl malonate, the reaction was very vigorous (and
would be potentially dangerous in an scaled-up process). The
reaction mixtures were taken up in ethanol, and sodium chloride
was isolated by centrifugation as described above.
Results and discussion
Fig. 2 shows the sodium chloride crystals that were obtained under
different conditions with phenacyl chloride as chloride donor. If
the reaction was carried out in toluene at room temperature, a
uniform crystal population with a particle size of about 200 nm
was obtained. The crystals were more or less spherical, but in some
cases sharp edges occurred. If the reaction was carried out without
solvent at room temperature, the particle size distribution was less
uniform, but now crystals with a size between 100 and 300 nm were
obtained. In this case, the initially solid reaction mixture rapidly
liquefied and turned into a yellowish oil.
The particles were redispersed in dichloromethane (which was
possible without ultrasonication), and the dispersions were studied
by dynamic light scattering. Note that the sedimentation of NaCl
(density 2.16 g cm-3) was considerably slowed compared to an
ethanolic dispersion by the higher density of CH2Cl2 (1.33 g cm-3).
The dispersions were stable for at least two weeks without visible
sedimentation. Older dispersions with sedimented NaCl could be
easily restored by shaking. Dynamic light scattering confirmed the
disperse character of colloidal NaCl with an average particle size
of 150–200 nm (Fig. 3). Equivalent results were obtained for the
NaCl dispersions which resulted from solvent-free syntheses. The
reason for this good dispersability is probably the adsorption of a
thin layer of the organic reaction product on the NaCl surface. It is
reasonable to assume that the phenyl groups are pointing towards
the organic solvent.
Fig. 2 Sodium chloride as product of the reaction between sodium diethylmalonate and phenacyl chloride in two different magnifications. The particle
diameter was about 200 nm in both cases. (a) and (b): Reaction in toluene solution at room temperature. (c) and (d): Reaction without solvent at room
temperature.
9732 | Dalton Trans., 2009, 9731–9734
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The Royal Society of Chemistry 2009
©

Fig. 3 Dynamic light scattering data of sodium chloride as product of the
reaction between sodium diethylmalonate and phenacyl chloride, reacted
in toluene, and redispersed in dichloromethane. The particle diameter
was 190 nm (maximum by intensity) and 160 nm (maximum by number)
(polydispersity index; PDI = 0.037).
The identity of the product NaCl was clearly shown by X-ray
powder diffraction for syntheses in toluene as well as for solvent-
free syntheses (Fig. 4).
Fig. 4 X-Ray powder diffractogram of sodium chloride as product of
the reaction between sodium diethylmalonate and phenacyl chloride in
toluene. Only peaks of NaCl are visible.
The reaction was then transposed to acetyl chloride as a more
reactive and liquid chlorine donor. If the reaction was carried out
at room temperature in toluene, well-developed cubic crystals with
a diameter of about 1–3 mm were obtained (Fig. 5). The reaction
was vigorous, strongly exothermic and led to microcrystals. If the
reaction was carried out without solvent, the sodium malonate had
to be cooled to -196 ◦C to calm down the vigorous reaction (see
Materials and methods for details). In this case, much smaller
NaCl crystals with a diameter of 100–200 nm were obtained.
However, the cubic shape of the crystals had almost vanished.
X-Ray diffraction clearly showed the presence of NaCl (Fig. 6).
In this case, dynamic light scattering of the dispersed NaCl
crystals showed strongly agglomerated particles (mm-sized), i.e.
a quantitative evaluation of the DLS data was impossible. In
comparison to the organic product from phenacyl chloride, the
product from acetyl chloride contains a methyl group instead
of a phenyl group. This is not hydrophobic enough to turn
to the organic solvent, and obviously the colloidal stabilization
is not sufficient to prevent the agglomeration of the NaCl
crystals.
Fig. 5 Sodium chloride as product of the reaction between sodium diethylmalonate and acetyl chloride in two different magnifications. (a) and (b): The
reaction in toluene solution at room temperature led to micrometer-sized crystals. (c) and (d): The reaction of the pure compounds at -196 ◦C led to
agglomerated crystals with a diameter of about 100–200 nm.
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 9731–9734 | 9733
©

Fig. 7 Infrared spectra of NaCl crystals from the reaction of sodium
diethylmalonate and acetyl chloride without solvent at -196 ◦C (top) and
phenacyl chloride in toluene at room temperature (bottom), showing the
bands of the organic condensation product.
Fig. 6 X-ray powder diffractogram of sodium chloride as product of the
reaction between sodium diethylmalonate and acetyl chloride in toluene
at room temperature. Only peaks of NaCl are visible.
is the first synthesis of uniform and dispersable sub-micrometre
sodium chloride crystals. A potential application could be
easily water-soluble nano-templates for polymeric or organic
structures.
The NaCl crystals are covered by a thin layer of organic material
as visible in the IR spectra (Fig. 7). The bands correspond to those
expected for the malonic ester condensation products.
References
Conclusion
1 Nanoparticles. From Theory to Application, ed. G. Schmid, Wiley-VCH,
Weinheim, 2004.
2 G. Ozin and A. Arsenault, Nanochemistry. A Chemical Approach to
Nanomaterials, Springer, New York, 2005.
3 G. Biskos, A. Malinowski, L. M. Russell, P. R. Buseck and S. T. Martin,
Aerosol Sci. Technol., 2006, 40, 97.
4 A. La¨hde, J. Raula and E. I. Kauppinen, J. Nanopart. Res., 2008, 10,
121.
5 K. Park, J. S. Kim and A. Miller, J. Nanopart. Res., 2009, 11,
175.
6 M. Siedler, S. J. Kitchin, K. D. M. Harris, A. L. C. Lagoa, H. P. Diogo,
M. E. M. da Piedade and M. Epple, J. Chem. Soc., Dalton Trans., 2001,
3140.
7 O. Herzberg and M. Epple, Eur. J. Inorg. Chem., 2001, 1395.
8 M. Epple and L. Tro¨ger, J. Chem. Soc., Dalton Trans., 1996, 11.
9 T. Annen and M. Epple, Thermochim. Acta, 2008, 473, 19.
10 C. Paal and G. Ku¨hn, Ber. Dtsch. Chem. Ges., 1908, 41, 51.
We found that the reaction between sodium diethyl malonate
and phenacyl chloride or acetyl chloride, respectively, occurred
rapidly in a vigorous, exothermic way, and led to the elimination
of NaCl crystals. In solvent-free syntheses, smaller crystals were
obtained, probably due to the higher rate of nucleation in the
absence of a solvent. In contrast, the presence of a solvent led to a
slower crystallization and to larger, cubically-shaped crystals. The
particles can be easily separated from the organic reaction product,
isolated and redispersed in organic solvents if phenacyl chloride is
used as reagent, provided that all operations are carried out under
strict exclusion of water. We assume that a thin layer of the organic
product is coating the salt crystals, thereby acting like a surfactant
and stabilizing the NaCl nanocrystals. To our knowledge, this
9734 | Dalton Trans., 2009, 9731–9734
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The Royal Society of Chemistry 2009
©