Universidade de São Paulo
Biblioteca Digital da Produção Intelectual - BDPI
Departamento de Física e Ciências Materiais - IFSC/FCM
Artigos e Materiais de Revistas Científicas - IFSC/FCM
2014-03
The role of viscosity in the fluorescence
behavior of the diesel/biodiesel blends
Renewable Energy,Amsterdam : Elsevier,v. 63, p. 388-391, Mar. 2014
http://www.producao.usp.br/handle/BDPI/51297
Downloaded from: Biblioteca Digital da Produção Intelectual - BDPI, Universidade de São Paulo
Renewable Energy 63 (2014) 388e391
Contents lists available at ScienceDirect
Renewable Energy
journal homepage: www.elsevier.com/locate/renene
Technical note
The role of viscosity in the fluorescence behavior of the diesel/
biodiesel blends
Anderson R.L. Caires a, *, Marisa D. Scherer a, José E. De Souza a, Samuel L. Oliveira b,
Jean-Claude M’Peko c
a
b
c
Grupo de Óptica Aplicada, Faculdade de Ciências Exatas e Tecnologia, Universidade Federal da Grande Dourados, CP 364, 79804-970 Dourados, MS, Brazil
Grupo de Óptica e Fotônica, Universidade Federal de Mato Grosso do Sul, CP 549, 79070-900 Campo Grande, MS, Brazil
Grupo Crescimento de Cristais e Materiais Cerâmicos, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 January 2013
Accepted 25 September 2013
Available online
Recently, the potentiality of fluorescence spectroscopy to be used in the quantification of biodiesel
content in diesel/biodiesel blends (DBB) was demonstrated. However, the source of the fluorescence
dependence of the DBB with biodiesel concentration remains unanswered. In the present paper, a close
analysis of the optical properties of the DBB was performed over a wide composition range. The findings
suggest that the alterations in the fluorescence intensity can be accounted for only after taking into
account changes in viscosity as well as absorbance, in a model where the fluorophores were considered
as molecular rotors.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Diesel
Biodiesel
Blend
Fluorescence
Viscosity
1. Introduction
Biodiesel is a biodegradable fuel produced from renewable
sources that can partly or completely replace petroleum-derived
diesel fuel because its properties are quite similar to the ones of
the diesel [1,2]. Consequently, the mandatory use of diesel and
biodiesel blends (DBB) is growing around the world due to their
environmental, economic, and social advantages [3,4]. In this
worldwide scenario, to ensure compliance with legislation, it is
necessary to either develop or improve methods able to determine
the biodiesel content in the DBB. Of course, it is desirable that this
determination be achieved via experimental methods that are easy
to handle, provide rapid and accurate results, and have a low cost
per sample analysis.
In a recent study, our research group proposed an alternative
method based on the fluorescence spectroscopy to quantify the
biodiesel content in DBB [5]. We showed that a linear increase in
the fluorescence intensity of the blends occurs when the biodiesel
content in the DBB increases from 0 to 10% (v/v). In consequence,
this linear relationship can be used as a calibration curve to
determine the biodiesel percentage [5]. This method has the
* Corresponding author. Tel./fax: þ55 67 34102072.
E-mail
addresses:
[email protected],
(A.R.L. Caires).
[email protected]
0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.renene.2013.09.041
advantage of working without a sample pre-preparation and can
potentially be applied directly at the gas station by using a portable device. However, from a theoretical viewpoint, the origin of
the fluorescence increase in these blends when raising the biodiesel concentration is not yet understood and deserves to be
accounted for.
Here, to gain further insights into the fluorescence behavior of
the DBB, a close analysis of the optical properties of these blends
was performed by measuring and analyzing their UVeVis absorption, molecular fluorescence, and viscosity data over a wide
composition range.
2. Materials and methods
Biodiesel was obtained from the transesterification process of
refined soybean oil, using a 6:1 M ratio of methanol/oil. The NaOH
catalyst (0.4 wt.% with respect to oil weight) was dissolved in
methanol and then added to the preheated soybean oil at 60 C. The
solution was stirred for 60 min and then placed in a separating
funnel for 24 h. Two phases were observed, one containing mostly
biodiesel and the other containing glycerol. After that, the biodiesel
was rotary-evaporated under reduced pressure to eliminate excess
methanol. Subsequently, the biodiesel was washed four times using
tap water (30% v/v) at room temperature and intervals of 30 min.
Finally, DBB samples were prepared by using a certified diesel
provided by Petrobras (Petróleo Brasileiro S.A.). Henceforth, DBB
A.R.L. Caires et al. / Renewable Energy 63 (2014) 388e391
389
Fig. 1. Diesel and biodiesel absorbance between 240 and 360 nm; diesel and biodiesel
fluorescence in the 360e720 nm region when excited at 260 nm.
are named BX, where X represents the volume percentage of biodiesel in the blend. This work was conducted on DBB samples with
the composition varying from B0 to B100, in steps of 10%.
The UV absorption spectra were obtained in the 240e360 nm
range using an absorption spectrophotometer (Cary 50, Varian). To
prevent the saturation of the absorption signal in the UV region, the
samples were diluted in n-hexane (1% v/v). All absorption measurements were performed at room temperature, using 1-cm path
length quartz cells. Fluorescence emission spectra were obtained
using a fluorescence spectrophotometer (Cary Eclipse, Varian). The
system is based on CzernyeTurner monochromators, a R928 photomultiplier, and a Xenon pulsed lamp. UV light was used to excite
the samples at 260 nm, while the fluorescence spectra were
recorded in the spectral region between 360 and 720 nm. The
“right-angle” geometry was used to collect the fluorescence spectra
at room temperature, by using 1-cm path length quartz cells.
The plastic viscosity, h, of the samples was determined in a
Brookfield digital rheometer model LVDV-III through the Bingham
math model available in the Rheocalc V2.5 software. A thermostatic
bath was used to ensure a temperature of 25.0 0.5 C. Measurements were made using the coaxial-cylinder geometry with a cylinder of external diameter of 100 mm (reference Spindle SC4-18).
3. Results and discussion
Fig. 1 shows both the absorption and fluorescence spectra of the
diesel (B0) and biodiesel (B100) samples. As pointed out in the
previous section, the UV absorption was measured between 240
and 360 nm, while the fluorescence signal was collected in the
360e720 nm range with a 260 nm excitation light. In particular, B0
shows two fluorescence bands centered at 475 and 585 nm, and
B100 shows two bands with fluorescence peaks at around 440 and
660 nm. Of great significance for this work is the observation that
B0 presents a higher absorbance and fluorescence than B100, a
result ascribed to the aromatic compounds present in B0, which
exhibit a well-defined absorption band at around 260 nm [6]. The
origin of the fluorescence in B100 may be attributed to remaining
fluorophores from vegetable oil used in the biodiesel production,
such as tocopherols and chlorophyll, since biodiesel has an emission spectral profile similar to the one observed in the oil feedstock
[7]. Nevertheless, the contribution of the mono- and polyunsaturated fatty acid methyl esters to the fluorescence may not be
ruled out [7,8].
For the reason given below, we will focus our attention on
analyzing the optical properties of those DBB ranging from B10 to
B90. Absorbance (at 260 nm) and fluorescence (at 475 nm)
Fig. 2. Absorbance at 260 nm (a) and fluorescence at 475 nm (b) as a function of the
biodiesel content in diesel/biodiesel blend.
magnitudes of the blends upon biodiesel content variation are
shown in Fig. 2. The results reveal that, in the one hand, DBB
absorbance presents a linear decrease with biodiesel content
(Fig. 2a); the fitting giving a linear correlation coefficient (R2) of
0.9932. On the other hand, regardless (i) the above monotonous
decrease of absorption and (ii) the fact that the fluorescence intensity of B100 is lower than B0 (refer to Fig. 1), Fig. 2b rather shows
a linear increase of the DBB fluorescence intensity within B10eB60
composition range, reaching a maximum value for B70, after which
the emission decreases. Before proceeding with discussion of these
results, the following comments should be made. The fluorescence
data presented in the current study were all collected from nondiluted samples with the expectation that fluorescence spectroscopy could be used to quantify the biodiesel content in DBB without
any previous sample preparation. Nevertheless, we should point
out that fluorescence measurements performed on diluted samples
(data not shown here) also revealed similar DBB fluorescence
comportment when the biodiesel content is altered.
As the DBB fluorescence behavior cannot be totally attributed to
the absorption changes, we looked for further insights into this
issue by evaluating whether the fluorescence increase (in the B0e
B70 composition range) might be associated with changes in the
viscosity of the blends. The results are presented in Fig. 3, and show
a linear increase of the DBB viscosity upon variation of the biodiesel
content, with a high linear correlation coefficient (R2) of 0.9922.
This result is predictable as biodiesel is well known to be more
viscous than diesel, because the former is slightly more polar due to
the presence of oxygen in the structure. As a consequence, the increase in the viscosity may lead to an increase of the fluorescence
quantum yield, FF, due to the potential dependence of fluorescence
with respect to the viscosity of the medium in which the fluorophores are immersed [9]. In the following, aiming to evaluate
whether the observed fluorescence behavior can be explained by
considering both the absorption and viscosity changes in the blend,
the relationship between VF and the emission spectra as well as the
viscosity dependence of VF will be presented and discussed.
390
A.R.L. Caires et al. / Renewable Energy 63 (2014) 388e391
Furthermore, integrating both sides of equation (6) with respect
to lF, the following relationships are verified:
ZN
Fl ðlF ÞdlF ¼
0
ZN
0
FF ¼
IF ðlE ; lF Þ
dl
2:3kI0 ðlE ÞAðlE Þ F
1
ZN
(7)
IF ðlE ; lF ÞdlF
(8)
IF ðlE ; lF ÞdlF ¼ 2:3kI0 ðlE ÞFF AðlE Þ:
(9)
2:3kI0 ðlE ÞAðlE Þ
0
and
ZN
0
In addition, by using the molecular rotor approach, the viscosity
dependence of VF is given by Ref. [10]:
FF
Fig. 3. Changes on the blend viscosity with respect of the biodiesel concentration.
1 FF
For the steady-state fluorescence, it is well established that VF
can be conveniently expressed in terms of emission spectrum as:
ZN
Fl ðlF ÞdlF ¼ FF
(1)
0
where Fl(lF) represents the fluorescence spectrum, i.e., the distribution of the probability of the transitions from the lowest vibrational level of the S1 state (first excited electronic level) to the
vibrational levels of the S0 state (ground electronic level). In practical situations, the fluorescence intensity IF(lF) measured at a
wavelength lF is proportional to Fl(lF) and to the absorptionrelated intensity IA(lE) at the excitation wavelength lE, where
IA(lE) is defined as the difference between the intensity of incident
light, I0(lE), and the transmitted light, IT(lE). Therefore, the fluorescence intensity can be written as:
IF ðlE ; lF Þ ¼ kFl ðlF ÞIA lE
(2)
IF ðlE ; lF Þ ¼ kFl ðlF Þ½I0 ðlE Þ IT ðlE Þ
(3)
in which k is a constant that depends on several experimental parameters (i.e., the bandwidth of the excitation and emission
monochromators, voltage of the photomultiplier tube, solid angle
of fluorescence collection, etc.). The transmitted light can be evaluated by the BeereLambert law IT(lE) ¼ I0(lE)e2.3ε(l)lc
E , where ε(lE)
represents the molar absorption coefficient at lE, l the optical path
and c the concentration. Furthermore, the product ε(lE)lc provides
the absorbance A(lE) at the wavelength lE. Consequently, the
fluorescence intensity can be expressed by the relationship:
i
h
IF ðlE ; lF Þ ¼ kFl ðlF ÞI0 ðlE Þ 1 e2:3AðlE Þ :
(4)
In fact, for highly diluted solutions, the equation (4) can be
rewritten to obtain a linear relationship between IF(lE) and A(lE) by
expanding the ½1 e2:3AðlE Þ term, and keeping only the first term.
The result of this operation is shown in equation (5).
IF ðlE ; lF Þy2:3kFl ðlF ÞI0 ðlE ÞAðlE Þ
(5)
By using this approach, the fluorescence spectrum Fl(lF) can be
expressed as:
IF ðlE ; lF Þ
Fl ðlF Þ ¼
2:3kI0 ðlE ÞAðlE Þ
¼ ahx
(10)
where h is the viscosity of the medium in which the molecular
rotors are present, a is a constant, and x is the viscosity sensitivity of
the molecular rotor [11]. Therefore, for a molecular rotor, the
dependence of the fluorescence intensity upon molecular absorption and viscosity should be expressed as:
ZN
IF ðlE ; lF ÞdlF ¼ 2:3kI0 ðlE ÞAðlE Þ
0
hx
(11)
a1 þ h
Assuming as a good approach that the molecules forming the
DBB are molecular rotors, it is clear from equation (11) that changes
in the absorption and viscosity of the DBB with varying biodiesel
Z fluorescence
content may induce alterations in the
intensity. This
N
relationship is very useful because
IF ðlE ; lF ÞdlF is easily obtained by calculating the area under0 the fluorescence intensity
curve of each blend so that the fluorescence intensity dependence
with respect to the biodiesel content in the DBB can be evaluated if
the absorbance and viscosity are known for each blend. In the
present case, according to Figs. 2(a) and 3, the following relationships apply for the absorbance and viscosity:
AðcÞ ¼ aA þ bA c and hðcÞ ¼ ah þ bh c;
(12)
where c represents the biodiesel content in DBB, aA and ah represent the absorbance and viscosity linear coefficients, respectively,
while bA and bh are the corresponding angular coefficients.
Therefore, to evaluate whether the complex behavior of fluorescence observed in this work may be accounted for by considering
changes in the absorption as well as viscosity in the blend, equations
(11) and (12) were combined to give the equation (13), in which the
factor 2.3kI0(lE) has been simply set as constant Q. In addition, a
correction factor given by 100:5AðlE Þ was introduced in this equation
(13), considering that the fluorescence spectra of the DBB were
collected from non-diluted samples in a “right-angle” geometry, and
so the inner filter effect cannot be ruled out [12].
Z720
"
IF ðlE ; lF ÞdlF ¼ Q ðaA þ bA cÞ10
0:5ðaA þbA cÞ
360
x #
ah þ bh c
x :
a1 þ ah þ bh c
(13)
Z
720
Fig. 4 shows the behavior of
360
(6)
:
x
IF ðlE ; lF ÞdlF , obtained by
calculating the area under each fluorescence curve in the 360e
720 nm range, upon changes in biodiesel content. A good
A.R.L. Caires et al. / Renewable Energy 63 (2014) 388e391
391
on the changes in the absorption of the blends because the absorption decreases monotonically as a function of biodiesel concentration. Here, it is demonstrated that the success in accounting
for this complex behavior requires considering not only the
absorbance but also the viscosity changes in the DBB, in a model
where the fluorophores are assumed as molecular rotors.
Acknowledgments
The authors are grateful for financial support from Conselho
Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and
Fundação de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia do Estado de Mato Grosso do Sul (FUNDECT). This work was
performed under the auspices of the National Institute of Science
and Technology of Optics and Photonics (INOF). One of the authors
(S.L.O) also thanks the support provided by National Institute of
Science and Technology of Photonics/CNPq. We also thank Valter
Souza Lima for technical support.
Fig. 4. Fluorescence of the diesel/biodiesel blend as a function of the biodiesel content.
agreement between experimental data and theoretical fitting based
on equation (13) is found. These results show that the complex
behavior of fluorescence observed in this study for the DBB, while
varying the biodiesel content, can fairly be accounted for by taking
into consideration both viscosity and absorbance alterations in the
blend, in a model where the fluorophores are considered to be
molecular rotors. During the fitting of the data in Fig. 4, the constants aA,bA,ah, and bh, experimentally determined from Figs. 2a
and 3, i.e.: A(c) ¼ 0.94 0.009c and h(c) ¼ 3.39 þ 0.013c, were
used as fixed parameters. The Q, a, and x constants were used as
free fitting parameters. The obtained values for these constants
were 9.07 107, 5.37 106, and 0.85, respectively. It is worth
pointing out that our fitting result is in accordance with the expected value for x, as usually found to be less than 1 in most investigations performed on intermediate or high viscosity media
[10].
4. Conclusions
This study has shown that the fluorescence intensity behavior of
the DBB reveals complex in nature, with an increase in the B10 to
B60 composition range, followed by a drop above B70. This
behavior could not be exclusively explained in terms of the fluorescence intensity of B0 and B100 since the B0 fluorescence is
higher than the B100 one as well as it could not be justified based
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