Available online at ScienceDirect. Procedia Engineering 102 (2015 )

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1 Available online at ScienceDirect Procedia Engineering 1 (15 ) 3 38 The 7th World Congress on Particle Technology (WCPT7) Characterization of Colloidal Particles using Electrical Imedance Sectroscoy in Two-electrode System with Carbon Probe Yanlin hao a,, Mi Wang b, Jun Yao c a Deartment of Thermal Energy Engineering, College of Mechanical and Transortation Engineering, China University of Petroleum (Beijing), Beijing 149, China b Institute of Particle Science and Engineering, School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS 9JT, UK c School of Energy Research, Xiamen University, Xiamen 3615, China Abstract The colloidal articles have an electrical double layer associated with their surfaces when susended in an aqueous medium. Under the influence of an alternating electric field, an induced electrical diole moment can be formed due to the olarization of the electrical double layer. The electrical imedance sectroscoy (EIS) measurement can record the comlex imedance, conductivity, relaxation frequency and hase angle caused by the olarization of the electrical double layer. These imedance arameters are in relation to article characteristics, for examle, the article size. The research about article size effect on electrical imedance sectra was carried out in a four-electrode system and the result indicated that imedance arameters shows a caability for characterizing the article size. This aer reorts the exerimental results from electrical imedance sectroscoy measurements on silica susensions in a two-electrode system with carbon robe. The main aim is to study the article size effect on imedance arameters, esecially the relaxation frequency and hase angle, to comare the data with those obtained from a four-electrode system with stainless steel electrodes and verify the caability for characterizing colloidal articles in different electrode systems. The article size effect on the relaxation frequency and imedance hase angle was studied in two different electrode systems and a similar tendency can be observed. It indicates that the caability of imedance arameters for article characterization is not limited in a four-electrode system, but commonly alicable in different electrode systems The Authors. Published by Elsevier by Elsevier Ltd. This Ltd. is an oen access article under the CC BY-NC-ND license (htt://creativecommons.org/licenses/by-nc-nd/4./). Selection and eer-review under resonsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Selection Academy and of eer-review Sciences (CAS). under resonsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS) Corresonding author. Tel.: ; fax: address: ylzhao@cu.edu.cn The Authors. Published by Elsevier Ltd. This is an oen access article under the CC BY-NC-ND license (htt://creativecommons.org/licenses/by-nc-nd/4./). Selection and eer-review under resonsibility of Chinese Society of Particuology, Institute of Process Engineering, Chinese Academy of Sciences (CAS) doi:1.116/j.roeng

2 Yanlin hao et al. / Procedia Engineering 1 ( 15 ) Keywords: electrical imedance sectroscoy; two-electrode system; article size; colloidal susension; article characterization 1. Introduction Electrical imedance sectroscoy (EIS) is a owerful technique to investigate the electrokinetic roerties of materials and their interfaces [1]. The EIS measurement can be achieved by alying an alternating voltage to the system and measuring the AC current resonse. If an AC electric field is alied to a article susension, a diole moment can be induced on the article surface due to the deformation of the electric double layer[]. The macroscoic dislay of induced diole moment is usually reresented by the imedance arameters, including the imedance real art, imaginary art, hase angle and the relaxation frequency[3]. These quantities are related to the surface roerties of the articles, and therefore can be used to characterize the colloidal articles. The electrode system which is used in electrical imedance sectroscoy measurement usually includes three different forms: -electrode, 3-electrode and 4-electrode systems. In the two-electrode system, two electrodes act as the working electrode (WE) and counter electrode (CE). The two-electrode system is common and easy to use; however the roblem of the electrode olarization (electrode-electrolyte interface effects) usually influences the measurements[4]. Three-electrode system involves a third electrode, called reference electrode, which is used to measure the otential of the working electrode. The three-electrode system could be used to investigate the kinetics and mechanism of the electrode reaction occurring on the working electrode surface. In the four-electrode system, two sensor electrodes are involved to measure the AC current resonse from the susension. The working electrode (WE) and counter electrode (CE) are used to aly the exciting voltage. Since the functions of electrodes are searated, the electrode olarization effect could be decreased in the four-electrode system[5]. The electrical imedance sectroscoy of silica susensions have been studied using four-electrode system[6]. However, due to the comlex on fabrication and installation, the four-electrode system is not suitable for industrial alication. A robe including two-electrode system can be easily ut in a vessel in many industrial rocesses, for examle, the batch reaction rocess, mixing rocess, and batch crystallization rocess, therefore, it could have wide alication in industries. In this study, a two-electrode system with carbon robe was roosed and fabricated. The electrical imedance sectra of silica susensions were measured using this two-electrode robe. The electrical olarization effect was studied and corrected using a constant hase element in the equivalent circuit model. The EIS results were comared with those obtained from four-electrode system. The caability for characterizing colloidal articles by EIS method in different electrode systems was verified.. Exerimental details.1. Exerimental setu Electrical imedance sectroscoy measurements on silica colloidal susensions were carried out using a twoelectrode system with carbon robe as shown in Figure 1. The two-electrode system includes one carbon electrode in the central with surface area of 5.4 mm and a stainless steel electrode with annular shae. The outer shell of the robe was made by stainless steel. The EIS measurement was carried out by alying an alternating excitation voltage (1 volt) with a frequency sectrum from 1 Hz to 3 MHz to the robe. The electrical imedance measurements were taken using a Solartron 16 Imedance/Gain-Phase Analyzer, with smart software... Materials Aqueous silica susensions with different article size (1 nm, 18 nm, 35 nm, 7 nm, nm) which were ordered from Fuso Chemical Co., Ltd. Jaan were used in EIS measurement. The original samles were firstly diluted to different concentration (1.wt%, 5. wt % et al.) using distilled water. Then 4g mixed bed ion exchange resin (Bio-red) was added into 1g diluted susension. The susension was stirred overnight (more than 1 hours) by magnetic stirrer at 15 rm. Finally, the deionized susensions can be obtained by filtrating the resin using funnel

3 34 Yanlin hao et al. / Procedia Engineering 1 ( 15 ) 3 38 and water um. Carbon electrode Stainless steel electrode with thickness of 1mm Fig. 1. hotograhs of the robe with two-electrode sensor. 3. Results and discussion 3.1. EIS of colloidal silica susensions The electrical imedance sectra for the silica susensions (5. wt%) measured using two different electrode systems are shown in Figure. Two features can be observed in Figure (a): several straight lines occurring at low frequency range (1 Hz 1Hz) and five suressed arcs occurring at high frequency range, which shows the dielectric resonse of the susensions under the external electric field. The linear section at low frequency ranges results from the electrode olarization (EP) at the surface of late electrodes. The EP henomenon occurs due to formation of electric double layer (EDL) caacitances by the free charges that build u at the interface between the electrolyte and the electrode surface. In Figure (b), no straight lines can be observed at low frequency range, but only five arcs can be found in the whole frequency range, which means the electrode olarization was decreased significantly in the four-electrode system. The suressed arc in electrical imedance sectra is a characteristic of dielectric disersion in the silica susension, which shows the olarization of charged silica articles under the alied electric field. Over the frequency range of hertz to a few megahertz, the mechanism for an imedance resonse is the erturbation of charges at the solution-solid interface. Usually, two relaxation mechanisms are: (i) olarization of the counter ions in the diffuse double layer (DDL) on the surface of colloidal articles, (ii) the Maxwell-Wagner (MW) effect that arises from olarization of the large created by contact of two hases with different ermittivity [7-8]. 3.. Electrode olarization correction The electrode olarization contribution to imedance in two-electrode system can be described by a constant hase angle (CPA) element in the equivalent circuit model. The imedance function is given by equation (1): (T, P) T( j ) P (1) CPA Where T is a frequency-indeendent arameter and P is a number between and 1 related to the hysical texture of the electrode. For a smooth surface P = 1, and for large ores P =.5 [9]. The electrical imedance sectra measured in two-electrode system can be analyzed using an equivalent circuit model as shown in Figure 3. The comlex imedance is given by ( R ) ( R, C ) ( T, P) () s s

4 Yanlin hao et al. / Procedia Engineering 1 ( 15 ) where, s is the solution imedance, is the comlex imedance due to the electrode olarization. Using the resistance and caacitance in the equivalent circuit to exress P is the comlex imedance due to the articles and their double layers, yields: 1 ( ) P Rs j j T 1 ( RC ) 1 ( RC ) R R C (3) From equation (3), searating real and imaginary arts yields 1 P R T 1 ( RC) P Re( )=' Rs cos( ) (4) 1 P RC T 1 ( RC) P Im( ) " sin( ) (5) The equation (3) can be used to fit to the exerimental data, and obtain the values of T and P. Therefore, the electrode olarization correction can be made by simly removing the term in the above equations from the exerimental imedance data. The fitting result of silica susension (5. wt % with 7 nm article size) was shown in the Figure 4 (a). It can be seen that the equivalent circuit model fits well and the electrode olarization effect can be eliminated by removing the term. The correction result after eliminating the electrode olarization effect was showed in Figure 4(b). It can be seen that the EP causes significant effect on the low frequency range (around 1-1 Hz). For the frequencies higher than 1 Hz, the EP effect can be neglected nm silica 5. wt% 9nm silica 5. wt% 7nm silica 5. wt% 35nm silica 5. wt% 1nm silica 5. wt% 1Hz -1-8 nm silica 5. wt% 9nm silica 5. wt% 7nm silica 5. wt% 35nm silica 5. wt% 1nm silica 5. wt% - -6 "(ohm) -15 "(ohm) kHz '(ohm) '(ohm) (a) (b) Fig.. electrical imedance sectra for silica susensions (5. wt %) with different article size measured using (a) two-electrode system, and (b) four-electrode system Fig. 3. the equivalent circuit for EIS measurement in two-electrode system

5 36 Yanlin hao et al. / Procedia Engineering 1 ( 15 ) Exerimental data Fitting result -5 Exerimental data EP correction data z"(ohm) -1 "(ohm) Hz '(ohm) (a) '(ohm) (b) Fig. 4. electrical imedance sectra for silica susensions (5. wt %, 7 nm) measured using two-electrode system: (a) fitting result using equivalent circuit; (b) the correction data after eliminating the electrode olarization effect Particle size deendence of relaxation frequency and hase angle The effect of article size on EIS was studied in colloidal silica susensions with concentration of 5. wt %. The exerimental results of the electrical imedance sectra measured using two-electrode system for the colloidal silica susensions with various article sizes (1 nm, 18 nm, 35 nm, 7 nm, nm) are showed in Figure 5. The relaxation due to the olarization of the counter ions in the diffuse double layer (DDL) on the surface of colloidal articles usually occurs at the khz range. From Schwartz s study [1], the relaxation frequency is related with the ath length for the movement of the ion swarm, as shown by the equation: f relaxation D (6) a where, a is the article radius, and D is the diffusion coefficient of the counter ions. From Figure 5 (a), it can be found that the relaxation frequencies, obtained from the eak osition on the lots of imedance imaginary arts, decrease with increasing article size. From Figure 5(b), it can be observed that at a fixed frequency (for examle 8 khz), the absolute values of the hase angle increase with increasing article size. It can be also noticed that at the MHz range, a relaxation occurs, evidenced by the beginning of an uturned curve at about 4MHz in Figure 5(b). This relaxation is due to the Maxwell-Wagner (MW) effect. In order to comare EIS measurement using different electrode systems, the article size effect on the relaxation frequency and imedance hase angle was studied using two-electrode system and four-electrode system and the results are shown in Figure 6 and Figure 7. From Figure 6, it can be found that the relaxation frequencies change inversely with the square of the article size according to equation (6) in both two-electrode and four-electrode systems. The intercet and sloe obtained by linear fitting results are different in different electrode systems. The ossible reason could be the different materials which were used in the fabrication of electrode system and the different dimensions of electrodes. In addition, it can be noticed that deviation exists between the exerimental data and the linear fitting results based on equation (6). The main ossible reason could be the unexected aggregation of the colloidal articles in susensions. The silica susensions which are tested in our exeriments are not monodisersed samles, and may have aggregation esecially when the article size becomes smaller. From Figure 7, it can be found that the absolute values of imedance hase angle increase with increasing the article size in both two-electrode and four-electrode systems, which is very clear at 8 khz. The ossible reasons is that the time need in the diffusion rocess increases with article radius, and therefore the induced diole moment lags behind the change of alied electric field to cause a large hase angle shift. The line in Figure 7 is just for the connection

6 Yanlin hao et al. / Procedia Engineering 1 ( 15 ) between two oints without any other meanings. The changing of imedance hase angle with article size shows a similar tendency in two-electrode system and four-electrode system. However, the exerimental data measured in the same susensions using different electrode system are different, which might be caused by the different material and dimensions of the electrode nm silica 5. wt% 9nm silica 5. wt% 7nm silica 5. wt% 35nm silica 5. wt% 1nm silica 5. wt% "(ohm) nm silica 5.wt% 9nm silica 5.wt% -16 7nm silica 5.wt% nm silica 5.wt% 1nm silica 5.wt% k 1k 1k 1M 1M Frequency (Hz) hase angle (degree) k 1k 1k 1M 1M Frequency (Hz) Fig. 5. electrical imedance sectra for silica susensions (5. wt %) with different article size measured using two-electrode system: (a) imedance imaginary art vs. frequency; (b) hase angle vs. frequency Equation y = a + bx Adj. R-Square Value Standard Error exerimental data linear fitting B Intercet B Sloe x Equation y = a + bx.836 Adj. R-Square Value Standard Error A Intercet A Sloe exerimental data linear fitting 9x1 4 Relaxation frequency (Hz) 1 5 Relaxation frequency (Hz) 8x1 4 7x1 4 6x1 4 5x1 4 4x E-3 1E-4 1E-5 1/a (nm - ) 3x1 4 1E-3 1E-4 1E-5 1/a (nm - ) (a) (b) Fig.6. the relaxation frequencies change as function of 1/a (a is the article diameter): (a) two-electrode system; (b) four-electrode system.

7 38 Yanlin hao et al. / Procedia Engineering 1 ( 15 ) 3 38 two-electrode system four-electrode system -1 imedance hase angle (degree) article diameter (nm) Fig. 7. imedance hase angles at 8 khz change as function of article size for silica susensions (5. wt %) measured in two-electrode and four-electrode systems resectively. 4. Conclusions Electrical imedance sectra of silica susensions were measured using a two-electrode system with carbon robe. Several straight lines can be observed at low frequency range (1 Hz- 1Hz), which is the feature of electrode olarization effect. For imedance sectra measured using a four-electrode system, the electrode olarization effect can t be observed even though at low frequency range. The electrode olarization can be simulated by a constant hase angle (CPA) element and be corrected by removing the term from the total comlex imedance. The article size effect on the relaxation frequency and imedance hase angle was studied in two different electrode systems and a similar tendency can be observed. It indicates that the caability of imedance arameters for article characterization is not limited in a four-electrode system, but commonly alicable in different electrode systems. Acknowledgements This work was suorted by National Natural Science Foundation of China (No ); Science Foundation of China University of Petroleum, Beijing (No. 4613YJRC3); National Natural Science Foundation of China (No ) and University of Leeds. References [1] J.R. Macdonald, Imedance sectroscoy, emhasizing solid materials and systems, A Wiley-Interscience ublication, [] K.S. hao and K.J. He, Dielectric relaxation of susensions of nanoscale articles surrounded by a thick electric double layer, Phys. Rev. B: Condens. Matter Mater. Phys. 74 (6) 1-1. [3] R.W. O'Brien, The resonse of a colloidal susension to an alternating electric-field, Adv. Colloid Interface Sci. 16 (198) [4] Y. Feldman, E. Polygalov, I. Ermolina, Y. Polevaya and B. Tsentsier, Electrode olarization correction in time domain dielectric sectroscoy, Meas. Sci. Technol. 1(8) (1) [5] B.A. Mazzeo and A.J. Flewitt, Two- and four-electrode, wide-bandwidth, dielectric sectrometer for conductive liquids: Theory, limitations, and exeriment, J. Al. Phys. 1 (7) [6] Y. hao, M. Wang, R.B. Hammond, Characterisation of nano-articles in colloids: relationshi between article size and electrical imedance sectra, J Nanosci Nanotechno. 13 (13) [7] L.M. Dudley, S. Bialkowski, D. Or, C. Junkermeier, Low frequency imedance behaviour of montmorillonite susensions: olarization mechanisms in the low frequency domain, Soil Sci. Soc. Am. J. 67 (3) [8] C. Grosse and A.V. Delgado, Dielectric disersion in aqueous colloidal systems, Curr. Oin. Colloid Interface Sci. 15(3) (1) [9] R. Roldan-Toro and J.D. Solier, Wide-frequency-range dielectric resonse of olystyrene latex disersions, J. Colloid Interface Sci.74 (4) [1] H.P. Schwan, G. Schwarz, J. Maczuk, H. Pauly, On low-frequency dielectric disersion of colloidal articles in electrolyte solution, J. Phys. Chem. 66 (1) (196)