J. Am. Ceram. Soc., 94 [8] 2354–2361 (2011) DOI: 10.1111/j.1551-2916.2011.04401.x r 2011 The American Ceramic Society
Journal
Electrophoretic Deposition of Titania Nanoparticles in Different Alcohols: Kinetics of Deposition Morteza Farrokhi-Radz and Mohammad Ghorbaniw,z,y z
Materials Science and Engineering Department, Sharif University of Technology, Tehran, PO Box 11356–8639, Iran y
Institute For Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran
of EPD is an important parameter and it has been widely investigated in the literature.12,18–21 Controlling the kinetics of deposit formation during EPD is critical in obtaining a deposit with the desired thickness. The aim of the present work was to study the rate of deposit formation from stable alcoholic suspensions (methanol, ethanol, and butanol) and to investigate the effect of various parameters such as applied voltage, solvent type, and deposition time on the EPD kinetics.
The suspension of titania nanoparticles in different alcohols (methanol, ethanol, and butanol) was prepared and triethanolamine (TEA) was used as a dispersant. The suspensions were characterized by different tests such as sedimentation, zeta potential, and Fourier transform infrared spectroscopy. The electrophoretic deposition (EPD) was performed at various voltages and times. EPD from butanolic suspension (0.8 g/L TEA) showed the slowest kinetics, because of the low electrophoretic mobility of titania nanoparticles in it (2.40 105 cm2 . (V . s)1). Also, it was observed that at low voltages (5, 10, and 20 V), the EPD kinetics from methanolic suspension (0 g/L TEA) was the fastest; however, with deposition voltage, the weights of deposits formed from methanolic (0 g/L TEA) and ethanolic (2.4 g/L TEA) suspensions approached each other so that at (30 V, t 240 s), (40 V, t 120 s), and (60 V, t 80 s) the weight of deposits formed from the ethanolic suspension was higher. These interesting observations were interpreted by the values of the electrophoretic mobility of titania nanoparticles in methanolic (7.78 105 cm2 . (V . s)1) and ethanolic (4 105 cm2 . (V . s)1) suspensions as well as the rate of increase in the electrical resistance of the deposits formed from these suspensions.
II. Experimental Procedure (1) Materials Titania nanopowder (99.7%, Sigma–Aldrich Corporation, St. Louis, MO) with an average particles size of 5 nm was used. Methanol (99.9%, Merck Co., Germany), absolute ethanol (99.7%, Bidestan Co., Qazvin, Iran), and butanol (99%, Merck Co., Darmstadt) were used as the solvents. Triethanolamine (TEA, reagent grade, Merck Co.) was used as a dispersant. (2) Suspension Preparation and Characterization For suspension preparation, the various concentrations of TEA (0, 0.8, 1.6, 2.4, 3.2, and 4 g/L) were added into 50 mL of alcohols and 2 g of titania nanopowder was added to them, then stirred for 24 h, and exposed to ultrasonic waves for 30 min (Sonopuls HD 2200, 30 kHz, Bandelin Co., Berlin, Germany). The sedimentation test was used to determine the optimum dosages of TEA in the suspensions. To carry out this, the prepared suspensions with various concentrations of TEA (0, 0.8, 1.6, 2.4, 3.2, and 4 g/L) were poured into 10 mL graduated cylinders and allowed to settle for 30 days. During the test, suspensions were separated into three regions: the supernatant alcohol in the above, the suspension in the middle, and the sediment in the bottom of the cylinders. The zeta potential of particles in the suspensions with various concentrations of TEA was measured using zetameter (Malvern instrument, Worcestershire, U.K.). For zeta potential measurement based on light-scattering methods, the suspensions must be diluted to a particle concentration of less than about 0.1 g/L. To achieve this, the suspensions were centrifuged at a high speed (6000 rpm) for 20 min, followed by removing the supernatant portion and adding one drop of original suspension to them. By this method, the ionic environment of nanoparticles does not change, which results in the negligible errors arising from dilution. The Fourier transform infrared spectroscopy (FTIR) was used to investigate the adsorption of TEA on the titania nanoparticles. The samples for FTIR analysis were prepared by removing small amounts of powder from suspensions without as well as with TEA (at an optimum concentration for ethanolic and butanolic suspensions and 2.4 g/L for methanolic one) and then drying at 1401C overnight. The electrical resistivity of the suspensions was determined by measuring their electrical conductivity (model: RE387TX, with a conductivity cell glass K 5 1 E8071, EDT Company, Kent, U.K.) and reversing the obtained value.
I. Introduction
T
nanostructured coatings and thin films have been widely investigated in recent years, due to their potential applications in self-cleaning,1 air and water purification,2,3 fungus and bacterial resistance coatings,4 sensors,5 and photocatalytic coatings.4,6 Several deposition methods, such as chemical vapor deposition,7 physical vapor deposition,8 sol–gel,9 plasma spray,10 tape casting,11 etc. have been used to produce titania coatings. Electrophoretic deposition (EPD) is the another technique for depositing colloidal particles, which has many advantages such as simplicity, low cost, allowing good control on deposit microstructure by adjusting the deposition parameters, etc.12 The EPD is a two-step process: in the first step, under the effect of an applied electrical field, the charged particles in a suspension move toward the electrode with an opposite charge and in the second step they deposit on the electrode and form a relatively dense layer of particles on it.12 Because of its electrolysis at low applied voltages, using water as a solvent for suspension preparation in EPD is limited; water electrolysis results in the formation of hydrogen and oxygen gases on the cathode and the anode, respectively, which causes pin holes in the deposits, poor adherence to substrate, and low homogeneity13; thus in EPD, organic solvents are usually preferred to aqueous ones. Among organic solvents, alcohols are most widely used.14–17 The kinetics ITANIA
R. Moreno—contributing editor
Manuscript No. 28293. Received July 7, 2010; approved December 23, 2010. Based in part on the thesis submitted by M. Farrokhi-rad for the M.Sc. degree in materials engineering, Sharif University of Technology, Tehran, Iran, 2009. w Author to whom correspondence should be addressed. e-mail:
[email protected]
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In a suspension, the current is carried by free ions in the liquid phase as well as by the charged particles in it. Hence, the electrical resistance of a suspension can be obtained by the following equation22: Rsus ¼
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EPD of Titania Nanoparticles in Different Alcohols
1
Rp;sus
þ
1
1
Rl;sus ðL dÞ where Rl;sus ¼ rl;sus A ðL dÞ Rp;sus ¼ mAcQeff
(1)
where Rl,sus is the resistance of the liquid phase in the suspension, Rp,sus is the resistance of the charged particles in the suspension, L and A are the electrode distance and surface area, respectively, d is the deposit thickness, rl,sus is the resistivity of the liquid phase in the suspension, m is the electrophoretic mobility of charged particles, and Qeff is the effective charge on the particles surface.
(3) EPD Plates of 304 stainless-steel with an exposed area of 1 cm2 were used as the substrates. The counter electrode in the electrophoretic cell was a plate from 304 stainless-steel with a surface area of 1.5 cm2. Two electrodes were 1 cm apart. EPD tests were performed at different voltages (5, 10, 20, 30, 40, and 60 V) and times (15, 30, 60, 120, and 240 s) from the suspensions with an optimum concentration of TEA using a laboratory power supply (JPS-302D, Shenzhen Hamtech Electronic Co., Shenzhen city, China). The EPD experimental setup used in this study is schematically shown in Fig. 1. The microstructure of the coatings deposited at 5 and 60 V was investigated using a scanning electron microscope (SEM). The electrical resistance of the deposits at any moment was calculated using the following equation derived from applying the Kirchhoff’s laws on the
equivalent circuit of EPD shown in Fig. 2: Rd ¼
V Rs i
(2)
where V is the applied voltage, i is the current passes through the circuit, and Rs is the resistance of the suspension. Rs was determined using the Ohm’s law by dividing the applied voltage by the current that passes through the circuit at initial seconds of deposition. To determine the values of i in Eq. (2), the current variations during EPD were recorded using a computer-connected multimeter (Fluke, 189 True RMS, Everett, WA). The resistance of a deposit produced by EPD can be calculated using the following equation22: Rdep ¼
1
Rp;dep
þ
1
1
Rl;dep d where Rp;dep ¼ rp;dep pA d Rl;dep ¼ rl;dep ð1 pÞA
(3)
where the Rp,dep is the powder resistance, Rl,dep is the resistance of continuous liquid phase between particles (interparticles liquid), p is the packing fraction, d is the deposit thickness, A is the deposit surface area, rl,dep is the resistivity of the interparticles liquid, and rp,d is the resistivity of the dry powder in the deposit. The weight of deposits was determined by weighing the substrates before and after deposition using a 0.1 mg accuracy balance (Mettler Toledo, Columbus, OH). The rate of deposit formation by EPD follows the equation proposed by Hamaker17: dw ¼ f mEAc dt
Fig. 1. Setup of the electrophoretic cell used in this study.
(4)
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Fig. 2. Equivalent circuit of electrophoretic deposition.
where m is the electrophoretic mobility, E is the electrical field, A is the surface area of the electrode, c is the particles mass concentration in the suspension, and f is a coefficient that takes into account that not all particles brought to the substrate electrode are incorporated into the deposit (fr1). The electrophoretic mobility of particles can also be calculated using the following equation23: m¼
e0 er z Z
(5)
Where e0 is the vacuum permittivity (D8.854 1012 F/m), er is the relative dielectric constant of medium, z is the zeta potential of particles, and Z is the viscosity of medium.
III. Results and Discussion (1) Suspensions Properties (A) Sedimentation Test: The suspension volume for alcoholic suspensions with various TEA concentrations after 5 min and 30 days of sedimentation is shown in Figs. 3(a) and (b), respectively. As can be seen in Fig. 3(a), in contrast to ethanolic and butanolic suspensions, the addition of TEA into methanolic one, even at very low concentrations, results in the accelerated settling of particles in it; therefore, TEA is not an effective dispersant for stabilizing the titania nanoparticles in methanol (optimum concentration of 0 g/L). As shown in Fig. 3(b), suspension volume at the TEA concentrations of 0, 0.8, and 2.4 g/L was the highest for methanolic, ethanolic, and butanolic suspensions, respectively. These concentrations were selected as the optimum concentrations and used throughout the EPD experiments. (B) Zeta Potential: The results of zeta potential measurements for alcoholic suspensions with various TEA concentrations are shown in Fig. 4. As shown in the figure, with an addition of TEA, the zeta potential gradually changes from negative values to positive. The surface of the oxide particles is hydrated and depending on the pH of the alcohol, they can acquire negative or positive surface charge.24 The pH at which the particle surface charge is zero is called the point of zero charge, which is about 5.8 for titania.25 At pH 45.8, titania nanoparticles acquire a negative surface charge by donating the proton to the alcohol molecules, while at pH o5.8, they acquire a positive one by accepting the proton from alcohol molecules. In this work, the pH of all three alcohols was 45.8 (7.3, 7.6, and 7.8 for methanolic, ethanolic, and butanolic suspensions, respectively), so in the absence of TEA, by donating the proton to the alcohol molecules, the titania nanoparticles acquired the negative surface charge. Each TEA molecule has a nitrogen atom with a lone pair of electrons, so that TEA behaves as a weak organic base and can take proton from alcohol molecules. TEA takes the proton from methanol more easily than both ethanol and butanol (regarding that the equilibrium constant for the reaction of ROH þ 17.2 ROH , RO þ ROHþ , 1018.88, and 1021.56 for 2 are 10 methanol, ethanol, and butanol, respectively26). In contrast to the methanolic suspension, these protonated TEA species are efficiently adsorbed on the titania nanoparticles in the ethanolic
Fig. 3. Suspension volume for alcoholic suspensions with various triethanolamine (TEA) concentrations after (a) 5 min and (b) 30 days of sedimentation.
and butanolic suspensions (see Fig. 5), so that they gradually convert their surface charge into a positive one and enhance their stability by the electrostatic stabilization mechanism. (C) FTIR Analysis: The results of FTIR analysis obtained for powder removed from different suspensions are shown in Fig. 5. The powder removed from the methanolic suspension with 2.4 g/L TEA only shows two peaks at 1635 and
Fig. 4. Zeta potential of titania nanoparticles in different alcoholic suspensions with various triethanolamine (TEA) concentrations.
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rate of resistance increase decreases with deposition time; this is due to the decrease in the deposition kinetics with EPD time, as will be discussed later. It is seen from Fig. 7 that at the same deposition voltages and times, among the deposits, those deposited from butanolic suspension have the highest resistance, followed by those deposited from methanolic and ethanolic ones. The resistivity of the dry titania powder is very high ( 1012 O cm), so according to the Eq. (3): Rdep ffi Rl;dep ¼ rl;dep
Fig. 5. Spectra of Fourier transform infrared spectroscopy analysis obtained for powder removed from different alcoholic suspensions.
3430 cm1 belonging to the H–O–H bending vibration and O–H stretching vibration, respectively.27,28 This proves that in the methanolic suspension, the protonated TEA species are not effectively adsorbed on the titania nanoparticles and rather increase the ionic strength of the suspension, resulting in the strong loss of suspension stability. On the other hand, the spectra of the powders picked up from ethanolic and butanolic suspensions, show two other peaks in about 1400 and 1070 cm1, which are attributed to C–N and C–H stretching vibrations, respectively,29 proving the adsorption of the protonated TEA species on the titania nanoparticles in these suspensions. Methanol has relatively a small molecular size, thus the solvation interaction between its molecules and protonated TEA spices is very strong so that the releasing of the protonated TEA ions from the shield of surrounding methanol molecules and then their adsorption on the titania nanoparticles is not energetically favorable. In the cases of ethanol and especially butanol, because of their relatively large molecular size, the solvation interaction is weaker so that the protonated TEA can release from the shield of surrounding alcohol molecules and be adsorbed on the titania nanoparticles.
(2) EPD (A) Microstructure Examination: The SEM images of the deposits produced from various alcoholic suspensions with an optimum concentration of TEA at deposition voltages of 5 and 60 V and EPD time of 60 s are shown in Fig. 6. With increasing deposition voltage from 5 to 60 V, the microstructure of the deposits does not change considerably. However, the deposits which are produced at 60 V have slightly coarser particles, more pores as well as lower microstructure homogeneity than those deposited at 5 V. This can be due to the fact that at high deposition voltages, nanoparticles move so fast toward the electrode with an opposite charge that they cannot obtain enough time to sit in their best positions in the deposit to form a homogenous close-packed microstructure.12 At very high applied electrical fields (>100 V/cm), the deterioration in the deposition quality is more appreciable.30 (B) Resistance of Deposits: Figure 7 shows the changes in the electrical resistance of the deposits formed from methanolic (0 g/L TEA), ethanolic (2.4 g/L TEA), and butanolic (0.8 g/L TEA), suspensions against EPD time. As can be seen from Fig. 7, for all three suspensions and at all applied voltages, the resistance of deposits increases with deposition time, which is due to the increase in the deposit thickness (d in Eq. (3)) with deposition time; also, the higher the deposition voltage, the higher the magnitude of the increase in the deposit resistance; according to the Eq. (4), this is due to the higher rates of deposit formation at higher voltages. Also it is seen from Fig. 7 that the
d and rdep r1;dep ð1 pÞA
(6)
Differentiating Eq. (6) with respect to time leads to (in constant voltage EPD, the variations in the deposits packing density with deposition time is small,31 so that we can assume it as a constant parameter; A and rl,dep are also constant values): rl;dep dd dRdep ¼ dt Að1 pÞ dt
(7)
According to the Eq. (7), although, among the deposits, those deposited from the butanolic suspension (0.8 g/L TEA) have the slowest rates of deposition (see Fig. 9) and hence the lowest dd dt values, the rate of increase in resistance is the highest for them, due to their very large value of (rdep)but (rl,dep)but (the subscript ‘‘but’’ is the abbreviation of butanol). Because of their higher mobility, the majority of the current in a suspension is carried by the free ions rather than the charged particles,32 so according to Eq. (1): Rsus Rl;sus ¼ rl;sus
ðL dÞ and rsus r1;sus A
(8)
the electrical resistivity of the butanolic suspension with 0.8 g/L TEA is about (rsus)but 5 10 MO cm, so according to Eq. (8) (rsus rl,sus)but 10 MO cm; although as will be discussed below, the resistivity of the interparticles butanol in the deposit is slightly lower than that of the liquid phase butanol in the butanolic suspension with 0.8 g/L TEA ((rl,dep)buto((rl,sus)but 5 10 MO cm)). Also, in comparison with ethanolic suspension (2.4 g/L TEA), the higher rates of resistance increase for the coatings deposited from the methanolic suspension (0 g/L TEA) are due to their higher (rdep rl,dep)met value (the subscript ‘‘met’’ is the abbreviation of methanol) as well as faster deposition kinetics (see Fig. 9) and so higher dd dt values. As mentioned previously, in ethanolic and butanolic suspensions, the protonated TEA species are adsorbed on the titania nanoparticles; therefore, in contrast to the interparticles methanol, the interparticles ethanol and butanol have a high concentration of protons (Fig. 8); hence, although the electrical resistivities of the methanolic suspension without TEA ((rsus rl,sus)met 0.06 MO cm) is lower than that of ethanolic one with 2.4 g/L TEA ((rsus rl,sus)et 0.1 MO cm, the subscript ‘‘et’’ is the abbreviation of ethanol), the resistivity of the interparticles methanol is higher than that of interparticles ethanol. The interparticles methanol in the deposit and the liquid phase methanol in the methanolic suspension (0 g/L TEA) have nearly the same concentrations of free ions, thus (rsus rl,sus rdep rl,dep)met 5 0.06 MO cm. Although similar to the interparticle ethanol, the interparticle butanol has also a high concentration of protons (Fig. 8), and according to Eq. (6) and Fig. 7 it can be concluded that the coatings deposited from butanolic suspension (0.8 g/L TEA) have a considerably higher rdep rl,dep value than those deposited from the methanolic one (0 g/L TEA) (note that the deposition rate (and so d values) from methanolic suspension (0 g/L TEA) is considerably higher than that of the butanolic one (0.8 g/L TEA), see Fig. 9); this is due to the very large difference between the resistivity of the butanolic
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Fig. 6. Scanning electron microscopic images of coatings deposited from different alcoholic suspensions (a and b: methanolic without triethanolamine (TEA)), (c and d: ethanolic with 2.4 g/L TEA), and (e and f: butanolic with 0.8 g/L TEA), and different applied voltages (a, c, and e: 5 V) and (b, d, and f: 60 V) and constant deposition time of 60 s.
suspension with 0.8 g/L TEA ((rsus rl,sus)but 10 MO cm) and the methanolic one without TEA ((rsus rl,sus)met 5 0.06 MO cm), so that even the high concentration of protons in the interparticles butanol cannot considerably reduce this resistivity difference. Briefly, it can be stated that the following
relation is established between the different resistivities: ðrdep rl;dep Þet < ððrdep rl;dep rsus rl;sus Þmet 0:06 MO cmÞ < ððrsus rl;sus Þet 0:1 MO cmÞ < < ðrdep rl;dep Þbut < ððrsus rl;sus Þbut 10 MO cmÞ
(9)
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2359
Fig. 7. Changes in the electrical resistance of the deposits formed from different alcoholic suspension at various deposition voltages with deposition time.
Fig. 8. Schematic illustration showing the titania nanoparticles and interparticles alcohol in the deposits formed from (a) methanolic without triethanolamine (TEA) and (b) ethanolic with 2.4 g/L TEA, and butanolic with 0.8 g/L TEA suspensions.
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Fig. 9. Deposition weight against time for different suspensions (methanolic with 0 g/L triethanolamine (TEA), ethanolic with 2.4 g/L TEA and butanolic with 0.8 g/L TEA) at different applied voltages (a) 5 V, (b) 10 V, (c) 20 V, (d) 30 V, (e) 40 V, and (f) 60 V.
(C) Deposition Kinetics: Figure 9 shows the rate of deposit formation from methanolic (0 g/L TEA), ethanolic (2.4 g/L TEA), and butanolic (0.8 g/L TEA) suspensions at various applied voltages. As expected from Hamaker equation (Eq. (4)), in all cases, at the same deposition time, the larger the deposition voltage, the larger is the deposition rate. Also at the constant voltage, with the deposition time, the deposition rate decreases, which is more obvious at higher voltages. Also among the suspensions, at the same deposition voltage, the reduction in the deposition rate with time is considerably higher for the methanolic suspension (0 g/L TEA). In the EPD process, the formation of an insulator ceramic layer on the substrate electrode results in the voltage drop across the deposit; hence, the electric field (effective electric field) is present across the suspension and induces the particles movement toward the electrode with an opposite charge, and thus the deposition rate declines.33 Sarkar and Nicholson18 also stated that in order to prevent from electric field reduction during EPD, the process should be performed at constant current density rather than constant voltage. At the same deposition time, the higher the deposition voltage, the thicker the deposit, resulting in higher drops in the effective electric field and so deposition rate. As can be seen in Fig. 9, at all deposition voltages and same deposition times, the weight of deposit formed from butanolic suspension (0.8 g/L TEA) is the lowest. Also with deposition
voltage and time, the changes in the weight of deposits produced from ethanolic and methanolic suspensions show an interesting trend. At low voltages (5 V), the weight of deposits produced from methanolic suspension (0 g/L TEA) is higher than that of those deposited from the ethanolic one (2.4 g/L TEA), while with the increase in the deposition voltage and time, the difference between them decreases, so that at a deposition voltage of 30 V and deposition time of 240 s, the weight of the two deposits becomes equal. With a further increase in the deposition voltage, the weight of deposits becomes equal at shorter times, and at times longer than this time, the weight of deposits formed from the ethanolic suspension (2.4 g/L TEA) is higher than those deposited from the methanolic one (0 g/L TEA). The values of the electrophoretic mobility, calculated using Eq. (5) are listed in Table I. According to Eq. (4), the low rates of deposit formation during EPD from butanolic suspension are due to the low electrophoretic mobility of titania nanoparticles in it. At low deposition voltages (5, 10, and 20 V) and at all times (up to 240 s), the effect of higher electrophoretic mobility of titania nanoparticles in methanolic suspension (0 g/L TEA) is dominating and therefore the weight of deposit formed from it is the highest. However, according to the Fig. 7, at higher applied voltages (r30 V) the electrical resistance of the deposits and therefore the voltage drop across them becomes more important, which results in the higher deposition weight from ethanolic suspension (2.4 g/L TEA).
Table I. Results for Electrophoretic Mobility Calculated Using Eq. (5) Suspension
Methanolic (without TEA) Ethanolic (with 2.4 g/L TEA) Butanolic (with 0.8 g/L TEA)
Viscosity of alcohol (cP)
Relative dielectric constant of alcohol
Zeta potential (mV)
Electrophoretic mobility (cm2 (V s))
0.557 1.0885 2.5875
32.63 24.55 17.51
15 20 40
7.78 105 4 105 2.40 105
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At the deposition time and voltage where the weights of deposits formed from methanolic and ethanolic suspensions are equal, the thicknesses of the deposits are also approximately equal. But, from Fig. 7 it is seen that at (30 V, 240 s), (40 V, 120 s), and (60 V, 80 s), the resistance of the coatings deposited from the methanolic suspension is still higher than that of those deposited from the ethanolic one, which according to Eq. (6) is another evidence for this claim that the coating deposited from the methanolic suspension (0 g/L TEA) has higher rdep rl,dep than those deposited from the ethanolic one (2.4 g/L TEA).
IV. Conclusion Alcoholic suspensions of titania nanoparticles were prepared and TEA was used as a dispersant. It was found that TEA acts as an effective dispersant for the ethanolic (optimum concentration of 2.4 g/L) and butanolic suspensions (optimum concentration of 0.8 g/L), while it intensively decreases the stability of methanolic suspension (optimum concentration of 0 g/L). It was proved that in contrast to the methanolic suspension, in ethanolic, and butanolic suspensions the protonated TEA molecules are adsorbed on the titania nanoparticles and thereby increasing the suspension stability by an electrostatic stabilization mechanism. It was observed that the resistivity of the coatings deposited from butanolic suspension is the highest followed by that of those deposited from methanolic suspension without TEA and the ethanolic suspension with 2.4 g/L TEA. Because of the low electrophoretic mobility (2.40 105 cm2 (V s)) of titania nanoparticles in butanolic suspension with 0.8 g/L TEA, the EPD from this suspension has the slowest rate. The fastest kinetics from methanolic suspension without TEA at low voltages ( 30 V) and times ( 240 s) is due to the higher electrophoretic mobility of titania nanoparticles in this suspension (7.78 105 cm2 (V s) than that of those in ethanolic suspension with 2.4 g/L TEA (4 105 cm2 (V s)). However, as the coating thickness increases, the effect of deposit resistance becomes more important; so that at (30 V and t4240 s), (40 V and t4120 s), and (60 V and t480 s), the deposition weight from the ethanolic suspension with 2.4 g/L TEA is higher than that from the methanolic one without TEA.
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