in
ira
o Es
04;
15
ani
pin
e eff
mo
tate
oxidation state of the conducting polymer and the decrease of the amount of water molecule in the solvent. From those results, we have
proposed one straightforward method to eliminate water contamination in the solvent DMSO using ultrasonic waves.
waves with the specific substances in use. It is well known
that propagation of an ultrasonic wave in liquid generates
10,000 atm, respectively [1]. These can modify drastically
the physical and chemical properties of the reaction med-
distribution [9]. Also polyaniline colloids preparation has
been improved [6], where morphologies and conductivity
interesting results has been presented making use of ultra-
sound. Unfortunately, the reasons and the mechanisms for
that improvement have not been satisfactory explained.
In addition, DMSO, is one of the most versatile and effi-
cient solvent used in organic chemistry. However, they suf-
fer from on inconvenient problem, they are extremely
* Corresponding author. Tel.: +55 81 2126 7461; fax: +55 81 2126 8442.
E-mail address: wma@ufpe.br (W.M. de Azevedo).
Ultrasonics Sonochemistry 13the formation of cavitations bubbles, which can grow and
implode under periodic variation of the pressure field. In
solution, implosion and fragmentation of the bubble,
which collapses, are the center of high energy phenomena.
This rapid formation, growth, and implosive collapse of
the gas vacuoles generate short lived (ns), localized
‘‘hot spots’’, whose peak temperature and pressure have
been calculated to reach as high as 10,000 C and
have been modified and improved, respectively. Some
interesting results, using ultrasound, is in the preparation
of organic/inorganic composite and the enhancement of
intercalation of organic molecule in inorganic lattices
[4,5,7,8,10,17], for the former, it was found that polyani-
line/TiO2 nanocomposite has been synthesized and con-
ductivity measurements show improvements when
compared with traditional stirring process. Although many 2005 Elsevier B.V. All rights reserved.
Keywords: Polyaniline; Ultrasonic wave; Solvent purification
1. Introduction
In the past and nowadays, the use of ultrasounds waves
has been used as a common procedure to accelerate solubil-
ity of a substance, or reagents, in a specific solvent. As a
consequence of that procedure, we have often forgotten
to analyze every possible interaction of the ultrasound
ium. Although ultrasound has been known for more than
70 years [2], its application to chemical reactions has not
been investigated thoroughly.
Recently, we have found in the literature, several papers
using ultrasound as a tool to improve conducting polymers
synthesis process [3–17]. As a consequence, polyaniline
nanoparticles have been prepared with more uniform sizeThe effect of ultrasonic waves
W.M. de Azevedo *, A.J.H. de Olive
Departamento de Quı´mica Fundamental, Laborato´rio de Quı´mica d
Received 11 December 20
Available online
Abstract
The effects of ultrasonic wave on the conducting polymer poly
and NMR analysis show that the polymer undergo a redox and do
dissolved into the solvent. The proposed mechanism to explain thes
action with the ultrasonic wave, homolitically dissociates the water
dissolved conducting polymer changing its oxidation and doped s1350-4177/$ - see front matter  2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2005.10.002conducting polymer solution
Luna, E.F.V.B.N. Silva, R.O. Silva
tado So´lido, CCEN-UFPE 50670-901, Recife, Pernambuco, Brazil
accepted 10 October 2005
December 2005
line dissolved in DMSO were observed. The UV–visible, infrared
g transition when the ultrasound wave interacts with the polymer
ects is based on the solvents hygroscopicity properties. The inter-
lecule producing radical species, and these species interact with the
. The resulting effects of that interaction are the modification of
www.elsevier.com/locate/ultsonch
(2006) 433–437

IF 66, respectively. For the UV–visible and infrared char-
acterization, a quartz cell of 5 cm of path length and a
ics Sonochemistry 13 (2006) 433–437hygroscopic [18], and in order to perform a synthesis in an
anhydrous condition, a quite laborious and time consume
purification step must be carried out before hand [19].
Due to its excellent solubility properties, macromolecule,
such as conventional polymer, and more recently conduct-
ing polymer has been processed in this medium. Among
them, polyaniline, one of the most studied polymer, in
the polyemeraldine base form is quite soluble in DMSO
[20–26]. For that reason, this medium has been used to
study the non-linear optical properties of the polymer
and to prepare Semi-interpenetrating polymer network
[27–29].
The aim of this paper is to investigate the effects of ultra-
sounds waves in polyaniline dissolved in DMSO and pro-
pose a mechanism to explain the polymer optical
modification induced by ultrasonic waves. The spectro-
scopic analysis shows that optical and redox properties of
the polyaniline dissolved in DMSO, completely change
when the ultrasound radiation interacts with polymer solu-
tion, and consequently, the amount of water molecule
adsorbed in DMSO decreases.
2. Experimental
Aniline (Nuclear) was distilled twice under atmospheric
pressure, and stored in the dark at low temperature prior to
synthesis. DMSO (Merck), ammonium persulfate
(Aldrich), formic acid (Merck), and all other reagents were
used without further purification. All aqueous solutions
were prepared using distilled and deionized water. The
method used to prepare polyaniline was adapted from the
procedure described by Cao et al. [30]. Shortly, a solution
of 0.8 M of ammonium persulfate was added, dropwise,
to an aqueous solution of 0.5 M of aniline dissolved in
1 M of aqueous HCl, the mixture was vigorously stirred
at 5 C for one day. After that, the precipitate emeraldine
salt, was filtered and washed in 1 M of aqueous HCl acid
until the filtrate was clear, to remove low molecular weight
and monomeric residue. To obtain the emeraldine base
form of polyaniline the product was treated with ammo-
nium hydroxide 1 M for 10 h at room temperature. The
polymer was subsequently, washed with water and acetoni-
trile, and dried under dynamic vacuum at room tempera-
ture for 24 h. Stock solution of emeraldine base polymer
was prepared dissolving 1 · 103 g of emeraldine base in
100 ml of DMSO. Part of this solution was set aside in a
glass test tube and placed inside of a commercially ultra-
sonic bath, Branson model 2210 (47 kHz, 35 W cm2),
for 1 h in air atmosphere at 25 C where the temperature
was maintained by adding small amount of the ice directed
to the ultrasound bath in order to avoid the temperature
increase with the time. At regularly time interval, an ali-
quot of the solution was taken for analysis. The spectro-
scopic characterization in the UV–visible and infrared
region of the solution before and after reaction was per-
434 W.M. de Azevedo et al. / Ultrasonformed using a Perkin–Elmer spectrophotometer model
Lambda-6 and a Bruker FTIR spectrophotometer modelKBr pellet was used, respectively. The 1H NMR spectra
was recorded on a Varian unity plus 300 MHz spectrome-
ter, where we first measure the proton relaxation time T1 in
DMSO 2 s, and H2O 0.8 s and after that, we use the
pulse sequence of 90 with relaxation delay d1 = 6.256 s
and acquisition time of at = 3.744 s, for the quantitative
water analysis measurements.
3. Results and discussion
Soon after the solution is set aside to react with the
ultrasound radiation, the polymer solution starts to change
its color, from blue to pale green.1 Fig. 1 shows that the
blue color of undoped polyaniline characterized by the
absorption bands at 625 nm [30], starts to decrease its
intensity, and completely disappear. At the end of the reac-
tion, only a small absorption band can be seen at 440 nm,
which corresponds to the radical cation, i.e., polaron band
produced at the polyaniline matrix. Comparing these
results with the spectroelectrochemical results of polyani-
line [31], we may conclude that after reacting with the ultra-
sound, the polymer presents a spectrum similar to the
partially reduced form of polyaniline [32], showing one
strong absorption band at 310 nm, and a small absorption
band at 450 nm. It is interesting to mention here that a sim-
ilar behavior happens when a solution of polyaniline in
DMSO interacts with ionization radiation, a reduction of
the oxidation state of the polymer is the resulting effect
of the interaction [33]. In addition we can use the fact that,
for water, the H atoms and OH radicals which escape
recombination in the gas phase and in the interfacial zone
react with solutes in the bulk of the solution at ambient
temperature to form products similar to those observed
in the radiation chemistry of the same solutions [34]. Using
these experimental evidence we may conclude that a similar
process is observed in ours experiments. In order to prove
that a reduction reaction is taking place with the polyemer-
aldine base, some different polyaniline structures have been
tested. Figs. 2 and 3 show that the same effect also happens
with the polyemeraldine salts and the pernigraniline form,
the doped and the oxidized form of polyaniline, respec-
tively. From these results, we can see that for the doped
form, Fig. 2, before the reaction, presents three character-
istic absorption bands, one at 350 nm, the second at
430 nm and the last at 800 nm, which can be assigned to
the p–p* absorption band, polaron absorption band, and
the dication absorption band, (bipolaron form), respec-
tively [31]. As the reaction proceeds, the absorption band
at 800 nm decreases its intensity, remaining only the
absorption band at 320 and 420 nm corresponding to the
partially reduced form of polymer. For the oxidized form1 For interpretation of color in Figs. 1–6 the reader is referred to the web
version of this article.