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Application of electrochemical impedance spectroscopy: A phase behavior study of babassu biodiesel-based microemulsions
Application of electrochemical impedance spectroscopy: A phase behavior study of babassu biodiesel-based microemulsions Thulio C. Pereira, Carlos A.F. Conceic¸a˜ o, Alamgir Khan, Raquel M.T. Fernandes, Maira S. Ferreira, Edmar P. Marques, Aldal´ea L.B. Marques PII: DOI: Reference:
S1386-1425(16)30286-4 doi: 10.1016/j.saa.2016.05.034 SAA 14455
To appear in: Received date: Revised date: Accepted date:
23 January 2016 19 May 2016 22 May 2016
Please cite this article as: Thulio C. Pereira, Carlos A.F. Concei¸c˜ao, Alamgir Khan, Raquel M.T. Fernandes, Maira S. Ferreira, Edmar P. Marques, Aldal´ea L.B. Marques, Application of electrochemical impedance spectroscopy: A phase behavior study of babassu biodiesel-based microemulsions, (2016), doi: 10.1016/j.saa.2016.05.034
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ACCEPTED MANUSCRIPT APPLICATION OF ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY: A PHASE BEHAVIOR STUDY OF BABASSU BIODIESEL-BASED
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MICROEMULSIONS
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Thulio C. Pereiraa, Carlos A. F. Conceiçãoa, Alamgir Khanb, Raquel M.T. Fernandesb, Maira S. Ferreirac*, Edmar P. Marquesa, Aldaléa L.B. Marquesa
Laboratório de Pesquisa em Química Analítica, Universidade Federal do
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a
b
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Maranhão, 65085-580, São Luís, Maranhão, Brazil. Departamento de Química e Biologia, Universidade Estadual do Maranhão , 65055-970, São Luís, Maranhão, Brazil.
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Coordenação de Ciência e Tecnologia, Universidade Federal do Maranhão,
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c
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65085-580, São Luís, Maranhão, Brazil.
* Corresponding author: [email protected]
ACCEPTED MANUSCRIPT Abstract
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Microemulsions are thermodynamically stable systems of two immiscible
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liquids, one aqueous and other of organic nature, with a surfactant and/or co-
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surfactant adsorbed in the interface between the two phases. Biodiesel-based microemulsion, being consist of alkyl esters of fatty acids, opens a new means of analysis for the application of electroanalytical techniques, and is advantageous as it
microemulsions
was
investigated
through
electrochemical
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biodiesel-based
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eliminates pre-treatment of sample required. In this work, the phase behaviours of
impedance spectroscopy (EIS) technique. Observed that, on increasing the amount
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of biodiesel in the microemulsion formulation increases the resistance to charge
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transfer at the interface. Also, the electrical conductivity measurements revealed that the decrease or increase in electrical properties depends on the amount of biodiesel.
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EIS studies of the biodiesel-based microemulsion samples showed the presence of two capacitive arcs: one high-frequency and other low-frequency. Thus, the
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formulation of microemulsions plays an important role in estimating the electrical properties through electrochemical impedance spectroscopy technique.
Keywords Microemulsions, Nyquist Diagram, biodiesel, electroanalytical, electrochemical impedance spectroscopy
ACCEPTED MANUSCRIPT Introduction Babassu (Orbignya phalerata) plants, belongs to the family of Arecaceae,
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being a native species of South America, is largely cultivated in different states of
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Brazil [1-3]. The cusi oil extracted from the seeds of these palms, which are basically
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consist of saturated fatty acids, such as lauric (48 %), myristic (16 %), palmitic (10 %) of other unsaturated, is an important source for the production of Biodiesel [4]. The Biodiesel and other biofuels from the biomass or vegetable oils are
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considered, as a renewable and biodegradable sources of energy, and offers a
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reduction in air pollution compared to fossil fuels [5]. Since Biodiesel, as a fuel, offers advantages over the fossil fuel, but it also
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presents disadvantages: poor cold flow properties, high viscosity, low chemical and
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thermal stabilities which cause the formation of wax, when exposed to low temperature. And also, its low conductivity, due to its high resistance, hindered the
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applications of electroanalytical techniques, which intends to decrease the interests of its measurement [6]. An alternative, more physically realistic approach is to
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consider the use of surfactant to reduce the interfacial tension and enhance the electrochemical properties (like dipole moments) of micelles. Therefore, the preparation of biodiesel-based microemulsions present, as an alternative mean for the use of analytical methods, avoids the conductivity resistance hindrance of interested
medium
[7].
The
microemulsions
are
clear,
isotropic
and
thermodynamically stable interfacial systems, consisting mainly of three or more constituents, produced spontaneously by self-organization of surfactant and/or cosurfactant molecules in the oil-water interfaces, forming microstructures dispersed in a continuous medium [8].
ACCEPTED MANUSCRIPT Castro
Dantas
and
collaborators,
developed
some
diesel-based
microemulsions with blends of diesel and vegetable oils, to investigate the influences
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of the nature of surfactant (Texapon HBN and Comperlan SCD), cosurfactant and oil
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phases, as well as the surfactant/co-surfactant mass ratio [9]. Although the current
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standards for biodiesel analysis do not include dielectric characterization, there have been an increasing number of works, that explore the use of electrochemical methods to assess properties of biofuels [10-11]. Perini et al. evaluated the of
electrochemical
impedance
spectroscopy
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application
technique
for
the
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characterization of contents of petroleum and water-in-oil emulsions of the refining production process at different stages [12].
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Thus, electrochemical impedance spectroscopy (EIS) has been proposed as a
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simple and non-invasive way to characterize the biodiesel, diesel and their mixture samples. The proposed technique allows us the determination of physical properties,
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i.e. dielectric constant, resistivity and relaxation time or frequency, which have been shown to be sensitive to the inherent characteristics of the fuel. This strongly
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recommended the implementation of EIS measurement of biodiesel and/or diesel samples, to evaluate and monitor the processes of production and quality of biofuel products [13-14].
The objective of this study is to evaluate the use of microemulsions, as a means of analysis by providing an easy and stable handling system and decrease the use of organometallic standards and carcinogenic solvents in the pretreatment of analytical samples, to measure the electrical properties through electrochemical impedance spectroscopy.
ACCEPTED MANUSCRIPT Materials and Methods Materials
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The materials used for biodiesel synthesis were: refined Babassu oil
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(commercial, purchased from a local market), ethanol (99.5% purity), and sodium
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hydroxide (97.0% purity).
The materials used for the preparation of microemulsion systems were: a) The ultrapure water with a specific resistivity of 18.2 MΩcm, obtained from a
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Nanopure Master water purification system (Gehaka, USA)
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b) The surfactant Triton® X-100 (Merck) and the alcohols (Merck); ethanol and isopropanol.
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Synthesis of biodiesel
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Maranhão, São Luiz, Brazil.
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c) Biodiesel samples were produced in the laboratory, at the Federal University of
For the production of biodiesel from babassu seed oil, one step catalysed
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process was adopted [15]:
The pre-dried crude oil, at 100oC in the oven for 2h, was esterified with alcohol, in the presence of sodium hydroxide as a catalyst, in a reactor at 60oC with agitation for 1h. After the completion of the reaction, the product mixture was poured into a separating funnel for the decantation of byproducts (glycerine), which subsequently separates the biodiesel from glycerol. Subsequently, the biodiesel was washed with a dilute acid solution to neutralize it. After washings, pure biodiesel was kept in the oven at 100 oC, to reduce the moisture content, finally, the dried biodiesel, identified, was stored in a closed flask.
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Characterization of Biodiesel
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The confirmation of a transesterification reaction of alkyl ester babassu oil was
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performed through the infrared spectroscopic analysis (as shown in figure 1). To
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ensure the quality of the biodiesel, samples were characterized by a number of analytical methods: (A) methods to evaluate the production process, as an aspect, i.e. methanol content and flash point; (B) methods to evaluate the inherent properties
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of the molecular structures, such as density, viscosity and iodine index, and (C)
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methods for monitoring the storage stability of biodiesel, i.e. oxidative stability and
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acid index [16].
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Construction of microemulsion systems The babassu oil samples, after characterizing its parameters, as mentioned
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above, were used as one of the components for the construction of microemulsion systems. Besides the biodiesel, water, surfactant (S) Triton ® X-100, and ethanol as
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co-surfactant (CS) was used as constituents of diagrams. The construction of the phase diagram was made by preparing several samples, of above-mentioned constituents, weighted at different proportions. The samples were homogenized manually by stirring and then kept thermostatically in closed tubes, to avoid any loss of the component, at 25°C for a few days. During this period, changes in their physical appearance (phase separation) were observed that ceased to reach chemical equilibrium. While in some cases, the differentiation in the phases took a week to occur.
Identification of biodiesel-based microemulsions regions in phase diagram
ACCEPTED MANUSCRIPT The identification of regions in the diagram was performed visually. Three regions comprising one phase, two phase and three phase, were being observed.
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The region of one phase, that has a clear appearance and transparent, is identified
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as microemulsion region, depending on its location in the diagram, can be
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distinguished it as a microemulsion of water in oil (region of the diagram with more oil), or as an oil in water microemulsion (region of the diagram with more water). Other regions were of biphasic and triphasic systems, formed by an excess of one or
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more constituents of the system.
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After characterizing the phase diagram, three microemulsions were selected for the Electrochemical Impedance Spectroscopy measurement. The selection of the
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samples was realized by varying the composition of the microemulsion, particularly,
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based on the increase in the concentration of biodiesel sample, and with a
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subsequent decrease in amounts of water.
Electrochemical Impedance Spectroscopic measurements
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The electrochemical impedance measurements were performed in an electrochemical cylindrical cell, containing two platinum electrodes of dimensions (1.2 x 0.4 cm2), with a capacity of 1 ml of sample. To obtain the electrochemical experimental data, an Autolab electrochemical Analyzer (Metrohm) PGSTAT 302 coupled to a computer for data logging, using software programs: FRA-Frequency Response Analyser, was used. The EIS measurements were performed in the electrochemical cell containing 0.5 mL of methyl babassu biodiesel-based microemulsions sample, where the following parameters have been established: frequency range of 0.1 Hz to 105 Hz, amplitude of 15 mV rms, open circuit potential (OCP) time 20s and 10 points per decade/frequency for each perturbation of AC
ACCEPTED MANUSCRIPT potential. The electrical conductivity measurement was performed by using the
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conductivity of Digimed, Model DM 32.
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Results and Discussion
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The production of methyl babassu biodiesel (MBB-100), through methanol route, obtaining approximately 89% yield relative to the amount of crude Babassu palm oil, was identified, from the presence of the characteristic absorption bands of
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alkyl esters functional groups, using the IR spectroscopic technique. The FTIR
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spectra of the biodiesel samples (MBB-100), obtained from Babassu palm oil, is depicted in Figure 1. A strong intense band at 1700 cm-1 and a medium intensity band at 1200 cm-1 have been seen in the spectra, respectively. Which revealed the
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presence of the ester carbonyl group (C=O) and esteric –COC stretching vibration peaks, respectively. The referenced methyl (CH2)n group, in the carbon chain,
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vibration bands are seen at 3000 cm-1 and 720cm-1, respectively. These absorption bands are referred to the axial deformation and angular deformations vibrational
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bands of the C-H bonds, respectively [17]. Thus, the IR-spectrum confirmed the conversion of alkyl esters through the transesterification reaction of babassu oil, satisfactorily.
The characterization of biodiesel samples was performed, by determining the physicochemical properties, following the standards of the Resolution nº 07 of 2008, by the National Agency of the Petroleum, Natural Gas and Biofuels (ANP) [18]. The physicochemical parameters of the biodiesel samples, along with the standard values are shown in Table 1. Observed that, all the measured physicochemical properties of the MBB-100 are all within the established limits set (as given in the Table 1) and according to the norms of National Agency of Petroleum Gas and
ACCEPTED MANUSCRIPT Biofuels (ANP). Thus, characterizations of the MBB 100 confirmed the quality of biodiesel product, and can be appropriately used in the study and the construction of
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the pseudo-ternary diagrams.
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Construction of the phase diagram
The pseudo-ternary phase diagram (TPD) were constructed with an aim to identify and characterize the microemulsion regions. The Microsoft 2013 software
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was used to normalise and convert the concentrations of surfactant, co-surfactant,
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biodiesel oil and water values into percentages.
The pseudo-ternary phase diagram was constructed by representing a binary
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mixture of co-surfactant to surfactant concentrations at one vertex, the concentration
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of the biodiesel oil on the second vertex, and the concentration of water on the last vertex of the equilateral triangle phase diagram. After, the construction of phase
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diagram, the lateral boundary of the microemulsion region was determined. As mentioned above, in the experimental section, the recognitions of the different
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phases were performed through visual inspection. In the TPD, the one-phase water-in-oil microemulsion region (1ɸ) represents a clear, transparent, and homogeneous appearance. Due to its position and dilution tests in the phases, as shown in the diagram, was characterized as an isotropic region. Oil in water region of microemulsion in the TPD was also not been recognized, when the pseudo-binary system (at TPD vertex of ethanol/Triton®X-100 and MBB) was analyzed. A high solubility of these components has been observed compared to pseudo-binary systems (at TPD vertex of ethanol/Triton®X-100 and water). This result allows us to conclude that although the Triton®X-100 has a hydrophilic portion formed by polyethylene, and is being soluble in ethanol to its
ACCEPTED MANUSCRIPT hydrophobic part has a great affinity for MBB forming reverse micelles, when added to water forming water-in oil microemulsions.
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The border lines that delimit the region in microemulsion systems of two-
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phase (2ɸ) and three-phase (3ɸ) regions, were drawn dashed, representing the
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estimate of boundaries in between the phases, as illustrated in Figure 2. In order to make a comparison and get some better results, in the microemulsion sample preparations, the isopropanol a co-surfactant and different
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ratios of co-surfactant/surfactant (CS/S) (2; 1.5; 1; 0.5 and 0.25) were evaluated.
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From the application of different ratios of the CS/S, it was observed that, the decrease in the ratio from 2 to 0.25 of CS/S increases the microemulsion.
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Consequently, occupying an approximately 50% of diagram for the last CS/S value.
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Since the lower quantity of surfactant, being amphiphilic substances, can not necessarily be found in the interface of the organic/inorganic solvents, and therefore
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the dispersion of surfactant in the medium can cause interference in the electrochemical measurements.
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Thus, it is suggested from the results of the system with ethanol that, a ratio of CS/S =2 having a lower concentrations of surfactant, which is advantageous also in terms of reducing the use of expensive reagents in the preparation of microemulsion systems, was chosen for the EIS measurement. This emphasized that, the results of the present study is very uprighted, and based on the informations of these biodiesel-based microemulsions, it can be used as a means of analysis (as alternative test), of qualifying the quality standards for biodiesel.
Electrochemical Impedance Spectroscopy (EIS)
ACCEPTED MANUSCRIPT After the construction of the phase diagram, the choice of co-surfactant and the CS/S ratio is being studied, three samples within the microemulsion region were
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selected for the EIS measurements: microemulsion A (22% biodiesel, 62% CS/S,
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and 16% water – w/w/w), microemulsion B (35% biodiesel, 51% CS/S, and
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14% water – w/w/w) and microemulsion C (43% biodiesel, 44% CS/S, and 13% water – w/w/w).
The results gathered were validated with the help of Kramers-Kronig test [19-
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22], using software AUTOLAB, acquired for the electrochemical system. In this way,
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the results obtained were considered satisfactory, as they exhibit a magnitude of the order 10-5. This demonstrates that, since the system fulfilled the required conditions
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(linearity, stability and causality) for the impedance measurements. Thus it increases
microemulsions.
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the authors interests and certainty to study the electric properties of Biodiesel
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The Nyquist plots representing the normalized area for the microemulsions A, B and C were investigated. The plots are depicted in Figure 3. Each of Nyquist plot
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consists of an out-of-shaped semicircle alike in the appearance of a half ellipse in high and low-frequency ranges for all the samples. The first arc, at the highfrequency region, is attributed to the capacitive/resistive behaviour of the low conductivity system; in the present case, the system of interest is babassu oil microemulsion. The second arc, observed, in the low-frequency regions, could be assigned to interfacial phenomena, hereby, electrode-microemulsion interfacial interactions [23-24]. Since, in the present work, different compositions of microemulsions with different proportions of biodiesel are studied, and also, the microemulsion C containing a high amount of biodiesel with a lesser amount of water compared to other microemulsion samples. It is suggested that, as represented by
ACCEPTED MANUSCRIPT second intercept at the real axis (Z') of the Nyquist diagram, by increasing the amount of biodiesel, in the formulation of microemulsions (following the order A >
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B > C), increases the charge transfer resistance at the interface (Rtc), values are
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given in Table 2. Therefore, the increasing Rtc values can be related, not only, to the
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high quantity of the most resistive component (biodiesel), but also, with the lower mobility of the water droplets in the continuous phase, hampered the charge transport [14]. The confirmation of these data is represented in the conductivity
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values for the three microemulsions (see Table 2). Where it can be observed that,
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smaller the amount of biodiesel, higher will be the conductivity. Thus, the conductivity values for the microemulsions A > B > C, as confirmed by the literature
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studies, as the diameter of the first arc decreases with the increasing conductivity
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[12].
The Bode diagram of the impedance module for the platinum immersed in the
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microemulsion A, B and C is illustrated in Figure 4. It is observed from the Bode plots that, the microemulsion sample C shows the largest total impedance /Z/ among
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all the studied cases, such behaviour indicates that, the current passing through the system is strenuous compared to the other two systems. Consequently, the decrease in the resistance of the microemulsions is observed, due to an increasing amount of biodiesel oil concentration in the media/system, alternatively, suggested a decrease in the conductivity. Another important factor that, may be contributing to the decrease in the conductivity of the medium, is the formation of a possible insulating film on the surfaces of the plates of platinum electrodes, which can be verified by the formation of the second capacitive arc, in the low frequency range, which is the characteristic of the species, adsorbed on the metal surface and forming an insulating layer [25]. The authors believed that this behaviour can be attributed to
ACCEPTED MANUSCRIPT the addition of biodiesel to the medium since, it is observed that, the addition of biodiesel causes an increase in the value of the adsorption resistance (R ads).
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Consequently, a decrease in the capacitance of the film (Cads), as shown in Table 2.
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In addition to these variables, there is also the influence of biodiesel
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concentration on the solution resistance (Rs) and the charge transfer resistance (Rtc), which increases proportionally with the addition of biodiesel in the system. However, it is known that platinum does not react with biodiesel or other microemulsion
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components in situations where it does not apply any specific potential and the
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measurements are performed in open circuit potential. Authors suggested that, since our EIS measurements were performed in an open circuit potential, a simple
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adhesion phenomenon, due to irregularities, on the surface of the metal electrode
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plate took place [26-27].
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Conclusions
The biodiesel-based microemulsions were successfully prepared and were
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found to be stable, representing low viscosity and high conductivity, compared to pure biodiesel samples. Since the high resistance of the biodiesel hinders its direct application in the electroanalytical techniques, the use of biodiesel-based microemulsions is being suggested as an efficient and alternative for these analyses, particularly in the measurement of EIS. All the electrochemical impedance measurements performed on biodiesel-based microemulsion system meets the standard condition of linearity, causality and stability. Thus, the main advantage of the proposed method, use of biodiesel-based microemulsion are: very low cost, alternative, effective and easy handled sampling
ACCEPTED MANUSCRIPT technique to explore the electroanalytical properties of biodiesel. Furthermore, the proposed method has a potential to be extended to the determination of trace
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elements in biodiesel, using for the other types of electroanalytical techniques (such
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as Voltammetry).
Acknowledgements
The authors are grateful to the funding Brazilian agency CAPES and FAPEMA
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for financial support.
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ACCEPTED MANUSCRIPT 8. D. P. Acharya, P. G. Hartley, Current Opinion in Colloid & Interface Science 17 ( 2012) 274-280
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ACCEPTED MANUSCRIPT 22. D. D. Macdonald, Electrochimica Acta 35 (1990) 1509-1525. 23. Gamry
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26. M. E. Orazem, B. Tribollet, Electrochemical Impedance Spectroscopy, John
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ACCEPTED MANUSCRIPT Caption of Figures
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Figure 1. Infrared spectra of the pure methylated biodiesel (MBB-100)
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Figure 2. Phase diagram of biodiesel-MBB-100/water/Triton® X-100 and ethanol
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mixtures at 25 oC. Region I: water-in-oil microemulsion region; region II: water-in-oil microemulsion in coexistence with a water excess phase; region III: the middle-
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phase is the microemulsion phase and two excess-phase of water and oil. In the phase diagram, the circle points (●) represent a composition of samples, that formed
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microemulsions, and the lozenge points (♦) are representing systems within the microemulsion region for the EIS measurements. Quadrate and triangle points (■
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phases, respectively.
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and ▲) represent the composition of samples that formed two phase and three
Figure 3. Nyquist diagrams for the three microemulsions studied by varying the amount of biodiesel with 15 mV rms amplitude in a frequency range from 100 mHz to
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100 kHz, using a cell with two similar platinum electrodes facing each other.
Figure 4. Bode diagrams for the three microemulsions studied by varying the amount of biodiesel.
ACCEPTED MANUSCRIPT Caption of Tables
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Table 1. Physicochemical properties of Babassu oil biodiesel
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the components of the biodiesel-based microemulsions.
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Table 2. Resistance, capacitance and conductivity for three different proportions of
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CE P
TE
D
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Figure 1
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AC
CE P
TE
D
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SC R
IP
T
Figure 2
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AC
CE P
TE
D
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Figure 3
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CE P
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Figure 4
ACCEPTED MANUSCRIPT Table 1 Characteristics
MBB-100
ANP Limits (Method)
0.01%
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Methanol and Ethanol
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Clean and Clean and pollution free (1) without particles
Aspect
< 0.20(1)
0.875 g/cm3
Kinematic Viscosity at 40oC
3.420 mm2/s
3.6-6.0(3)
Flash Point Iodine Index Stability of the Oxidation an 110oC Acid Index
118oC 29.9 mg/100g
100oC(4) <120g I2/100(5)g
25.4 h
6(6)
1.12 mg KOH/g
<0.5 mg KOH/g(1)
1
1(7)
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RANP 07/08 ASTM D1298 ABNT NBR 10441 ASTM D93 EN 14112 ASTM D 130 EN 14112
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Corrosiveness to Copper, 50oC, max. 1. 2. 3. 4. 5. 6.
850-900(2) at 20oC
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Specific Gravity
ACCEPTED MANUSCRIPT Table 2 ME – A
ME - B
ME - C
Rs bulk (Ohm/cm²)
6.0 x 10²
8.8 x 102
9.3 x 102
Rtc (ME/film) (ohm/cm²) Rads film/electrode (ohm/cm²) Bulk capacitance (F/cm²) Film capacitance (Cads) (F/cm²) Conductivity (µS/cm)
4.1 x 104
6.6 x 104
1.2 x 105
2.9 x 105
4.3 x 105
1.8 x 10-10
1.5 x 10-10
8.4 x 10-6
6.1 x 10-6
4.2 x 10-6
9.5
6.6
5.2
7.7 x 105
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Parameter
1.2 x 10-10
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AC
CE P
TE
D
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Graphical abstract
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HIGHLIGHTS
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Biodiesel, as a fuel, offers advantages over the fossil fuel. Preparation of biodiesel-base microemulsion as a mean to measure the electrical properties through electrochemical impedance spectroscopy. The high resistance of the biodiesel hinders its direct application in the electroanalytical techniques, the use of biodiesel-based microemulsions is being suggested as an efficient and alternative for electrochemical analyses.
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S1386-1425(16)30286-4 doi: 10.1016/j.saa.2016.05.034 SAA 14455
To appear in: Received date: Revised date: Accepted date:
23 January 2016 19 May 2016 22 May 2016
Please cite this article as: Thulio C. Pereira, Carlos A.F. Concei¸c˜ao, Alamgir Khan, Raquel M.T. Fernandes, Maira S. Ferreira, Edmar P. Marques, Aldal´ea L.B. Marques, Application of electrochemical impedance spectroscopy: A phase behavior study of babassu biodiesel-based microemulsions, (2016), doi: 10.1016/j.saa.2016.05.034
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ACCEPTED MANUSCRIPT APPLICATION OF ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY: A PHASE BEHAVIOR STUDY OF BABASSU BIODIESEL-BASED
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MICROEMULSIONS
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Thulio C. Pereiraa, Carlos A. F. Conceiçãoa, Alamgir Khanb, Raquel M.T. Fernandesb, Maira S. Ferreirac*, Edmar P. Marquesa, Aldaléa L.B. Marquesa
Laboratório de Pesquisa em Química Analítica, Universidade Federal do
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b
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Maranhão, 65085-580, São Luís, Maranhão, Brazil. Departamento de Química e Biologia, Universidade Estadual do Maranhão , 65055-970, São Luís, Maranhão, Brazil.
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Coordenação de Ciência e Tecnologia, Universidade Federal do Maranhão,
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65085-580, São Luís, Maranhão, Brazil.
* Corresponding author: [email protected]
ACCEPTED MANUSCRIPT Abstract
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Microemulsions are thermodynamically stable systems of two immiscible
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liquids, one aqueous and other of organic nature, with a surfactant and/or co-
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surfactant adsorbed in the interface between the two phases. Biodiesel-based microemulsion, being consist of alkyl esters of fatty acids, opens a new means of analysis for the application of electroanalytical techniques, and is advantageous as it
microemulsions
was
investigated
through
electrochemical
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biodiesel-based
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eliminates pre-treatment of sample required. In this work, the phase behaviours of
impedance spectroscopy (EIS) technique. Observed that, on increasing the amount
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of biodiesel in the microemulsion formulation increases the resistance to charge
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transfer at the interface. Also, the electrical conductivity measurements revealed that the decrease or increase in electrical properties depends on the amount of biodiesel.
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EIS studies of the biodiesel-based microemulsion samples showed the presence of two capacitive arcs: one high-frequency and other low-frequency. Thus, the
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formulation of microemulsions plays an important role in estimating the electrical properties through electrochemical impedance spectroscopy technique.
Keywords Microemulsions, Nyquist Diagram, biodiesel, electroanalytical, electrochemical impedance spectroscopy
ACCEPTED MANUSCRIPT Introduction Babassu (Orbignya phalerata) plants, belongs to the family of Arecaceae,
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being a native species of South America, is largely cultivated in different states of
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Brazil [1-3]. The cusi oil extracted from the seeds of these palms, which are basically
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consist of saturated fatty acids, such as lauric (48 %), myristic (16 %), palmitic (10 %) of other unsaturated, is an important source for the production of Biodiesel [4]. The Biodiesel and other biofuels from the biomass or vegetable oils are
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considered, as a renewable and biodegradable sources of energy, and offers a
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reduction in air pollution compared to fossil fuels [5]. Since Biodiesel, as a fuel, offers advantages over the fossil fuel, but it also
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presents disadvantages: poor cold flow properties, high viscosity, low chemical and
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thermal stabilities which cause the formation of wax, when exposed to low temperature. And also, its low conductivity, due to its high resistance, hindered the
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applications of electroanalytical techniques, which intends to decrease the interests of its measurement [6]. An alternative, more physically realistic approach is to
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consider the use of surfactant to reduce the interfacial tension and enhance the electrochemical properties (like dipole moments) of micelles. Therefore, the preparation of biodiesel-based microemulsions present, as an alternative mean for the use of analytical methods, avoids the conductivity resistance hindrance of interested
medium
[7].
The
microemulsions
are
clear,
isotropic
and
thermodynamically stable interfacial systems, consisting mainly of three or more constituents, produced spontaneously by self-organization of surfactant and/or cosurfactant molecules in the oil-water interfaces, forming microstructures dispersed in a continuous medium [8].
ACCEPTED MANUSCRIPT Castro
Dantas
and
collaborators,
developed
some
diesel-based
microemulsions with blends of diesel and vegetable oils, to investigate the influences
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of the nature of surfactant (Texapon HBN and Comperlan SCD), cosurfactant and oil
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phases, as well as the surfactant/co-surfactant mass ratio [9]. Although the current
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standards for biodiesel analysis do not include dielectric characterization, there have been an increasing number of works, that explore the use of electrochemical methods to assess properties of biofuels [10-11]. Perini et al. evaluated the of
electrochemical
impedance
spectroscopy
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application
technique
for
the
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characterization of contents of petroleum and water-in-oil emulsions of the refining production process at different stages [12].
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Thus, electrochemical impedance spectroscopy (EIS) has been proposed as a
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simple and non-invasive way to characterize the biodiesel, diesel and their mixture samples. The proposed technique allows us the determination of physical properties,
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i.e. dielectric constant, resistivity and relaxation time or frequency, which have been shown to be sensitive to the inherent characteristics of the fuel. This strongly
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recommended the implementation of EIS measurement of biodiesel and/or diesel samples, to evaluate and monitor the processes of production and quality of biofuel products [13-14].
The objective of this study is to evaluate the use of microemulsions, as a means of analysis by providing an easy and stable handling system and decrease the use of organometallic standards and carcinogenic solvents in the pretreatment of analytical samples, to measure the electrical properties through electrochemical impedance spectroscopy.
ACCEPTED MANUSCRIPT Materials and Methods Materials
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The materials used for biodiesel synthesis were: refined Babassu oil
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(commercial, purchased from a local market), ethanol (99.5% purity), and sodium
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hydroxide (97.0% purity).
The materials used for the preparation of microemulsion systems were: a) The ultrapure water with a specific resistivity of 18.2 MΩcm, obtained from a
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Nanopure Master water purification system (Gehaka, USA)
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b) The surfactant Triton® X-100 (Merck) and the alcohols (Merck); ethanol and isopropanol.
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Synthesis of biodiesel
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Maranhão, São Luiz, Brazil.
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c) Biodiesel samples were produced in the laboratory, at the Federal University of
For the production of biodiesel from babassu seed oil, one step catalysed
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process was adopted [15]:
The pre-dried crude oil, at 100oC in the oven for 2h, was esterified with alcohol, in the presence of sodium hydroxide as a catalyst, in a reactor at 60oC with agitation for 1h. After the completion of the reaction, the product mixture was poured into a separating funnel for the decantation of byproducts (glycerine), which subsequently separates the biodiesel from glycerol. Subsequently, the biodiesel was washed with a dilute acid solution to neutralize it. After washings, pure biodiesel was kept in the oven at 100 oC, to reduce the moisture content, finally, the dried biodiesel, identified, was stored in a closed flask.
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Characterization of Biodiesel
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The confirmation of a transesterification reaction of alkyl ester babassu oil was
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performed through the infrared spectroscopic analysis (as shown in figure 1). To
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ensure the quality of the biodiesel, samples were characterized by a number of analytical methods: (A) methods to evaluate the production process, as an aspect, i.e. methanol content and flash point; (B) methods to evaluate the inherent properties
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of the molecular structures, such as density, viscosity and iodine index, and (C)
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methods for monitoring the storage stability of biodiesel, i.e. oxidative stability and
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acid index [16].
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Construction of microemulsion systems The babassu oil samples, after characterizing its parameters, as mentioned
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above, were used as one of the components for the construction of microemulsion systems. Besides the biodiesel, water, surfactant (S) Triton ® X-100, and ethanol as
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co-surfactant (CS) was used as constituents of diagrams. The construction of the phase diagram was made by preparing several samples, of above-mentioned constituents, weighted at different proportions. The samples were homogenized manually by stirring and then kept thermostatically in closed tubes, to avoid any loss of the component, at 25°C for a few days. During this period, changes in their physical appearance (phase separation) were observed that ceased to reach chemical equilibrium. While in some cases, the differentiation in the phases took a week to occur.
Identification of biodiesel-based microemulsions regions in phase diagram
ACCEPTED MANUSCRIPT The identification of regions in the diagram was performed visually. Three regions comprising one phase, two phase and three phase, were being observed.
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The region of one phase, that has a clear appearance and transparent, is identified
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as microemulsion region, depending on its location in the diagram, can be
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distinguished it as a microemulsion of water in oil (region of the diagram with more oil), or as an oil in water microemulsion (region of the diagram with more water). Other regions were of biphasic and triphasic systems, formed by an excess of one or
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more constituents of the system.
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After characterizing the phase diagram, three microemulsions were selected for the Electrochemical Impedance Spectroscopy measurement. The selection of the
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samples was realized by varying the composition of the microemulsion, particularly,
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based on the increase in the concentration of biodiesel sample, and with a
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subsequent decrease in amounts of water.
Electrochemical Impedance Spectroscopic measurements
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The electrochemical impedance measurements were performed in an electrochemical cylindrical cell, containing two platinum electrodes of dimensions (1.2 x 0.4 cm2), with a capacity of 1 ml of sample. To obtain the electrochemical experimental data, an Autolab electrochemical Analyzer (Metrohm) PGSTAT 302 coupled to a computer for data logging, using software programs: FRA-Frequency Response Analyser, was used. The EIS measurements were performed in the electrochemical cell containing 0.5 mL of methyl babassu biodiesel-based microemulsions sample, where the following parameters have been established: frequency range of 0.1 Hz to 105 Hz, amplitude of 15 mV rms, open circuit potential (OCP) time 20s and 10 points per decade/frequency for each perturbation of AC
ACCEPTED MANUSCRIPT potential. The electrical conductivity measurement was performed by using the
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conductivity of Digimed, Model DM 32.
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Results and Discussion
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The production of methyl babassu biodiesel (MBB-100), through methanol route, obtaining approximately 89% yield relative to the amount of crude Babassu palm oil, was identified, from the presence of the characteristic absorption bands of
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alkyl esters functional groups, using the IR spectroscopic technique. The FTIR
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spectra of the biodiesel samples (MBB-100), obtained from Babassu palm oil, is depicted in Figure 1. A strong intense band at 1700 cm-1 and a medium intensity band at 1200 cm-1 have been seen in the spectra, respectively. Which revealed the
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presence of the ester carbonyl group (C=O) and esteric –COC stretching vibration peaks, respectively. The referenced methyl (CH2)n group, in the carbon chain,
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vibration bands are seen at 3000 cm-1 and 720cm-1, respectively. These absorption bands are referred to the axial deformation and angular deformations vibrational
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bands of the C-H bonds, respectively [17]. Thus, the IR-spectrum confirmed the conversion of alkyl esters through the transesterification reaction of babassu oil, satisfactorily.
The characterization of biodiesel samples was performed, by determining the physicochemical properties, following the standards of the Resolution nº 07 of 2008, by the National Agency of the Petroleum, Natural Gas and Biofuels (ANP) [18]. The physicochemical parameters of the biodiesel samples, along with the standard values are shown in Table 1. Observed that, all the measured physicochemical properties of the MBB-100 are all within the established limits set (as given in the Table 1) and according to the norms of National Agency of Petroleum Gas and
ACCEPTED MANUSCRIPT Biofuels (ANP). Thus, characterizations of the MBB 100 confirmed the quality of biodiesel product, and can be appropriately used in the study and the construction of
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the pseudo-ternary diagrams.
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Construction of the phase diagram
The pseudo-ternary phase diagram (TPD) were constructed with an aim to identify and characterize the microemulsion regions. The Microsoft 2013 software
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was used to normalise and convert the concentrations of surfactant, co-surfactant,
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biodiesel oil and water values into percentages.
The pseudo-ternary phase diagram was constructed by representing a binary
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mixture of co-surfactant to surfactant concentrations at one vertex, the concentration
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of the biodiesel oil on the second vertex, and the concentration of water on the last vertex of the equilateral triangle phase diagram. After, the construction of phase
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diagram, the lateral boundary of the microemulsion region was determined. As mentioned above, in the experimental section, the recognitions of the different
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phases were performed through visual inspection. In the TPD, the one-phase water-in-oil microemulsion region (1ɸ) represents a clear, transparent, and homogeneous appearance. Due to its position and dilution tests in the phases, as shown in the diagram, was characterized as an isotropic region. Oil in water region of microemulsion in the TPD was also not been recognized, when the pseudo-binary system (at TPD vertex of ethanol/Triton®X-100 and MBB) was analyzed. A high solubility of these components has been observed compared to pseudo-binary systems (at TPD vertex of ethanol/Triton®X-100 and water). This result allows us to conclude that although the Triton®X-100 has a hydrophilic portion formed by polyethylene, and is being soluble in ethanol to its
ACCEPTED MANUSCRIPT hydrophobic part has a great affinity for MBB forming reverse micelles, when added to water forming water-in oil microemulsions.
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The border lines that delimit the region in microemulsion systems of two-
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phase (2ɸ) and three-phase (3ɸ) regions, were drawn dashed, representing the
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estimate of boundaries in between the phases, as illustrated in Figure 2. In order to make a comparison and get some better results, in the microemulsion sample preparations, the isopropanol a co-surfactant and different
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ratios of co-surfactant/surfactant (CS/S) (2; 1.5; 1; 0.5 and 0.25) were evaluated.
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From the application of different ratios of the CS/S, it was observed that, the decrease in the ratio from 2 to 0.25 of CS/S increases the microemulsion.
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Consequently, occupying an approximately 50% of diagram for the last CS/S value.
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Since the lower quantity of surfactant, being amphiphilic substances, can not necessarily be found in the interface of the organic/inorganic solvents, and therefore
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the dispersion of surfactant in the medium can cause interference in the electrochemical measurements.
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Thus, it is suggested from the results of the system with ethanol that, a ratio of CS/S =2 having a lower concentrations of surfactant, which is advantageous also in terms of reducing the use of expensive reagents in the preparation of microemulsion systems, was chosen for the EIS measurement. This emphasized that, the results of the present study is very uprighted, and based on the informations of these biodiesel-based microemulsions, it can be used as a means of analysis (as alternative test), of qualifying the quality standards for biodiesel.
Electrochemical Impedance Spectroscopy (EIS)
ACCEPTED MANUSCRIPT After the construction of the phase diagram, the choice of co-surfactant and the CS/S ratio is being studied, three samples within the microemulsion region were
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selected for the EIS measurements: microemulsion A (22% biodiesel, 62% CS/S,
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and 16% water – w/w/w), microemulsion B (35% biodiesel, 51% CS/S, and
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14% water – w/w/w) and microemulsion C (43% biodiesel, 44% CS/S, and 13% water – w/w/w).
The results gathered were validated with the help of Kramers-Kronig test [19-
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22], using software AUTOLAB, acquired for the electrochemical system. In this way,
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the results obtained were considered satisfactory, as they exhibit a magnitude of the order 10-5. This demonstrates that, since the system fulfilled the required conditions
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(linearity, stability and causality) for the impedance measurements. Thus it increases
microemulsions.
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the authors interests and certainty to study the electric properties of Biodiesel
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The Nyquist plots representing the normalized area for the microemulsions A, B and C were investigated. The plots are depicted in Figure 3. Each of Nyquist plot
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consists of an out-of-shaped semicircle alike in the appearance of a half ellipse in high and low-frequency ranges for all the samples. The first arc, at the highfrequency region, is attributed to the capacitive/resistive behaviour of the low conductivity system; in the present case, the system of interest is babassu oil microemulsion. The second arc, observed, in the low-frequency regions, could be assigned to interfacial phenomena, hereby, electrode-microemulsion interfacial interactions [23-24]. Since, in the present work, different compositions of microemulsions with different proportions of biodiesel are studied, and also, the microemulsion C containing a high amount of biodiesel with a lesser amount of water compared to other microemulsion samples. It is suggested that, as represented by
ACCEPTED MANUSCRIPT second intercept at the real axis (Z') of the Nyquist diagram, by increasing the amount of biodiesel, in the formulation of microemulsions (following the order A >
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B > C), increases the charge transfer resistance at the interface (Rtc), values are
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given in Table 2. Therefore, the increasing Rtc values can be related, not only, to the
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high quantity of the most resistive component (biodiesel), but also, with the lower mobility of the water droplets in the continuous phase, hampered the charge transport [14]. The confirmation of these data is represented in the conductivity
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values for the three microemulsions (see Table 2). Where it can be observed that,
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smaller the amount of biodiesel, higher will be the conductivity. Thus, the conductivity values for the microemulsions A > B > C, as confirmed by the literature
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studies, as the diameter of the first arc decreases with the increasing conductivity
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[12].
The Bode diagram of the impedance module for the platinum immersed in the
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microemulsion A, B and C is illustrated in Figure 4. It is observed from the Bode plots that, the microemulsion sample C shows the largest total impedance /Z/ among
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all the studied cases, such behaviour indicates that, the current passing through the system is strenuous compared to the other two systems. Consequently, the decrease in the resistance of the microemulsions is observed, due to an increasing amount of biodiesel oil concentration in the media/system, alternatively, suggested a decrease in the conductivity. Another important factor that, may be contributing to the decrease in the conductivity of the medium, is the formation of a possible insulating film on the surfaces of the plates of platinum electrodes, which can be verified by the formation of the second capacitive arc, in the low frequency range, which is the characteristic of the species, adsorbed on the metal surface and forming an insulating layer [25]. The authors believed that this behaviour can be attributed to
ACCEPTED MANUSCRIPT the addition of biodiesel to the medium since, it is observed that, the addition of biodiesel causes an increase in the value of the adsorption resistance (R ads).
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Consequently, a decrease in the capacitance of the film (Cads), as shown in Table 2.
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In addition to these variables, there is also the influence of biodiesel
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concentration on the solution resistance (Rs) and the charge transfer resistance (Rtc), which increases proportionally with the addition of biodiesel in the system. However, it is known that platinum does not react with biodiesel or other microemulsion
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components in situations where it does not apply any specific potential and the
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measurements are performed in open circuit potential. Authors suggested that, since our EIS measurements were performed in an open circuit potential, a simple
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adhesion phenomenon, due to irregularities, on the surface of the metal electrode
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plate took place [26-27].
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Conclusions
The biodiesel-based microemulsions were successfully prepared and were
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found to be stable, representing low viscosity and high conductivity, compared to pure biodiesel samples. Since the high resistance of the biodiesel hinders its direct application in the electroanalytical techniques, the use of biodiesel-based microemulsions is being suggested as an efficient and alternative for these analyses, particularly in the measurement of EIS. All the electrochemical impedance measurements performed on biodiesel-based microemulsion system meets the standard condition of linearity, causality and stability. Thus, the main advantage of the proposed method, use of biodiesel-based microemulsion are: very low cost, alternative, effective and easy handled sampling
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elements in biodiesel, using for the other types of electroanalytical techniques (such
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as Voltammetry).
Acknowledgements
The authors are grateful to the funding Brazilian agency CAPES and FAPEMA
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for financial support.
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1. E. C. Reipert, D. Anton, C. E. C. Rodrigues, A. J. A. Meirelles J. Chem.
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ACCEPTED MANUSCRIPT 8. D. P. Acharya, P. G. Hartley, Current Opinion in Colloid & Interface Science 17 ( 2012) 274-280
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9. T. N. C Dantas, A. A. D. Neto, A. C. Silva, Fuel 80 (2001) 75-81
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11. A. H. Akitaa, C. S. Fugivaraa, I. V. Aokib, A. V. Benedettia, ECS Transactions 43 (2012) 71-77
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16. I. P. Lôbo, S. L.C Ferreira, R. S. Cruz, Quim. Nova 32 (2009) 1596-1608 17. I. P. Soares, T. F. Rezende, R. C. Silva, E. V. R. Castro, I. C. P. Fortes Energy & Fuel 22 (2008) 2079-2083 18. Agencia Nacional do Petroleo, Gas natural e Biocombustivel see. http://www.anp.gov.br, accessed January 2016. 19. R. de L. Kronig, J. Opt. Soc. Am. 12 (1926), 547-557. 20. H. A. Kramers, Z. Phys. 30 (1929) 521 21. H. Shin, F. Mansfeld, Corrosion Science 28 (1988) 933-938.
ACCEPTED MANUSCRIPT 22. D. D. Macdonald, Electrochimica Acta 35 (1990) 1509-1525. 23. Gamry
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26. M. E. Orazem, B. Tribollet, Electrochemical Impedance Spectroscopy, John
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ACCEPTED MANUSCRIPT Caption of Figures
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Figure 1. Infrared spectra of the pure methylated biodiesel (MBB-100)
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Figure 2. Phase diagram of biodiesel-MBB-100/water/Triton® X-100 and ethanol
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mixtures at 25 oC. Region I: water-in-oil microemulsion region; region II: water-in-oil microemulsion in coexistence with a water excess phase; region III: the middle-
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phase is the microemulsion phase and two excess-phase of water and oil. In the phase diagram, the circle points (●) represent a composition of samples, that formed
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microemulsions, and the lozenge points (♦) are representing systems within the microemulsion region for the EIS measurements. Quadrate and triangle points (■
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phases, respectively.
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and ▲) represent the composition of samples that formed two phase and three
Figure 3. Nyquist diagrams for the three microemulsions studied by varying the amount of biodiesel with 15 mV rms amplitude in a frequency range from 100 mHz to
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100 kHz, using a cell with two similar platinum electrodes facing each other.
Figure 4. Bode diagrams for the three microemulsions studied by varying the amount of biodiesel.
ACCEPTED MANUSCRIPT Caption of Tables
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Table 1. Physicochemical properties of Babassu oil biodiesel
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the components of the biodiesel-based microemulsions.
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Table 2. Resistance, capacitance and conductivity for three different proportions of
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Figure 1
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Figure 2
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Figure 3
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Figure 4
ACCEPTED MANUSCRIPT Table 1 Characteristics
MBB-100
ANP Limits (Method)
0.01%
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Methanol and Ethanol
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Clean and Clean and pollution free (1) without particles
Aspect
< 0.20(1)
0.875 g/cm3
Kinematic Viscosity at 40oC
3.420 mm2/s
3.6-6.0(3)
Flash Point Iodine Index Stability of the Oxidation an 110oC Acid Index
118oC 29.9 mg/100g
100oC(4) <120g I2/100(5)g
25.4 h
6(6)
1.12 mg KOH/g
<0.5 mg KOH/g(1)
1
1(7)
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RANP 07/08 ASTM D1298 ABNT NBR 10441 ASTM D93 EN 14112 ASTM D 130 EN 14112
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Corrosiveness to Copper, 50oC, max. 1. 2. 3. 4. 5. 6.
850-900(2) at 20oC
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Specific Gravity
ACCEPTED MANUSCRIPT Table 2 ME – A
ME - B
ME - C
Rs bulk (Ohm/cm²)
6.0 x 10²
8.8 x 102
9.3 x 102
Rtc (ME/film) (ohm/cm²) Rads film/electrode (ohm/cm²) Bulk capacitance (F/cm²) Film capacitance (Cads) (F/cm²) Conductivity (µS/cm)
4.1 x 104
6.6 x 104
1.2 x 105
2.9 x 105
4.3 x 105
1.8 x 10-10
1.5 x 10-10
8.4 x 10-6
6.1 x 10-6
4.2 x 10-6
9.5
6.6
5.2
7.7 x 105
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Parameter
1.2 x 10-10
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Biodiesel, as a fuel, offers advantages over the fossil fuel. Preparation of biodiesel-base microemulsion as a mean to measure the electrical properties through electrochemical impedance spectroscopy. The high resistance of the biodiesel hinders its direct application in the electroanalytical techniques, the use of biodiesel-based microemulsions is being suggested as an efficient and alternative for electrochemical analyses.
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