Langmuir 2007, 23, 8789-8797
8789
Enhanced Pearl-Chain Formation by Electrokinetic Interaction with the Bottom Surface of Vessel Satoshi Nishimura,* Hideo Matsumura, Katsunori Kosuge, and Tomohiko Yamaguchi Nanotechnology Research Institute, National Institute of AdVanced Industrial Science & Technology (AIST), Tsukuba, Higashi1-1-1, Tsukuba, Ibaraki 305-8565, Japan ReceiVed February 26, 2007. In Final Form: April 26, 2007 Counterions in an electric double layer (EDL) around a colloidal particle accumulate on one side of the EDL and are deficient on the other side under an electric field, resulting in an imbalance of ionic concentration in the EDL, that is to say, the ionic polarization of EDL. It is well known that the ionic polarization of EDL induces electric dipole moments whereby the alignments of colloidal particles (e.g., pearl chains) are formed under alternating electric fields. In this study, we focus on the effect of the frequency of applied electric fields (100 Hz-1 kHz) on the alignment of silica particles settling at the bottom of a silica glass vessel. In digital imaging analyses for pearl chains of silica particles, it is confirmed that surface distances between two neighboring particles decrease but the number of particles in a pearl chain increases as the frequency of the applied electric field is lowered from 1 kHz to 100 Hz. More interestingly, electrical conductance measurements suggest that the induced ionic polarization of EDL around silica particles at the bottom of the silica vessel is enhanced as the frequency is lowered from 1 kHz to 100 Hz, whereas the ionic polarization around isolated silica particles in uniform dispersions is alleviated by the relaxation of ionic concentration in the EDL as a result of the diffusion of counterions. This curious phenomenon can be explained by considering that the ionic polarization of EDL of silica particles at the bottom of a vessel is affected by the electroosmosis of the silica surface at the bottom of the vessel.
Introduction It is known that counterions in an electrical double layer (EDL) around a colloidal particle accumulate on one side of the EDL and are deficient on the other side when an electric field is applied to a dispersion.1 This is referred to as the ionic polarization of EDL. The ionic polarization of EDL induces electrical dipole moments that cause the pearl chain formation and dielectricphoresis under nonuniform electric fields.2-4 Technological applications of these phenomena, for example, cell fusion,5 separation,6 and laboratories-on-tips,7 are rapidly expanding. However, the details of these phenomena have not been fully understood. One reason is that the long-range interaction of colloidal particles is dominated by many complicated subprocesses including electrohydrodynamics besides the ionic polarization of EDL.8-11 Electrohydrodynamic interactions exist because of electro-osmotic flow around particles and their alignments in the vicinity of walls of a vessel and/or electrodes whereby the convection of the solution gives rise to various types of colloid particle assemblies.12-15 The other reason is that * Corresponding author. E-mail:
[email protected]. (1) Dukhin, S. S.; Shilov, V. N. AdV. Colloid Interface Sci. 1980, 13, 153. (2) Muth, E.; Kolloid, Z. 1927, 41, 97. Kruyt, R.; Vogel, J. G. Kolloid Z. 1941, 95, 2. (3) Matsumura, H. Colloids Surf., A 1995, 104, 343. (4) Pohl, A. H.; Hawk, I. Science 1996, 272, 706. Matsue, T.; Matsumoto, N.; Uchida, A. Electrochim. Acta 1997, 42, 3251. Velev, O. D.; Laler, E. W. Langmuir 1999, 15, 3693. (5) Pohl, A. H.; Pollock J. K. In Method of Cell Separation; Castimopoolas, N., Ed.; Plenum Press; New York, 1978; Vol. 1, p 67. Pohl, A. H.; Pollock, J. K. In Modern Bioelectrochemistry; Gutmann, F., Keyzer, H., Eds.; Plenum Press: New York, 1986; Chapter 12, p 329. (6) Zimmermann, U. Biochim. Biophys. Acta 1982, 694, 227. (7) Hughes, M. P. Electrophoresis 2002, 23, 2569. (8) Dukhin, A. S.; Murtsovkin, V. A. Kolloidn. Zh. 1986, 48, 203. (9) Gamajunov, N. I.; Murtsovkin, V. A.; Dukhin, A. S. Kolloidn. Zh. 1986, 48, 197. (10) Malkin, E. S.; Dukhin, A. S. Kolloidn. Zh. 1982, 44, 801. (11) Shimonova, T. S.; Shilov, V. N. Kolloidn. Zh. 1982, 78, 81. (12) Jennings, B. R. In Abstracts of 8th International Conference of the Chemistry of Solid/Liquid Interfaces; 1989, p 27. (13) Gong, T.; Wu, D. T.; Marr, W. M. Langmuir 2002, 18, 10064.
experimental evidence other than microscopic images are insufficient to explain the phenomena. However, some studies have attempted to explain microscopic images on the basis of mathematical models of electrohydrodaynamics.15-18 The ionic polarization and electro-osmotic flow affect the alignment of colloidal particles in electric fields. Undoubtedly, they are dominated by the mobility of counterions, that is to say, surface conduction. Measurements of electrical conductance and/ or the dielectric response of dispersion are indispensable to our understanding of mechanisms for particle alignment and surface conduction around colloidal particles in electric fields in combination with theoretical considerations.19-24 Most studies on electrical conductance measurements have been limited to the case for uniform dispersions in which each particle must be isolated and have no interaction with other particles as a result of negligibly small ionic polarization under low electric field (14) Abe, M.; Orita, M.; Yamazaki, H.; Tsukamoto, S.; Teshima, Y.; Sakai, T.; Ohkubo, T.; Momozawa, N.; Sakai, H. Langmuir 2004, 20, 5046. (15) Trau, M.; Saville, D. A.; Aksay, I. A. Langmuir 1997, 13, 6375. Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (16) Green, N. G.; Ramos, A.; Gonzarez, A.; Morgan, H.; Castellanos, A. Phys. ReV. E 2000, 61, 4011. Gonzarez, A.; Ramos, A.; Green, N. G.; Castellanos, A.; Morgan, H. Phys. ReV. E 2000, 61, 4019. (17) Solomenstev, Y.; Bohmer, M.; Anderson, J. L. Langmuir 1997, 13, 6058. (18) Guelcher, S. A.; Solomenstev, Y.; Anderson, J. L. Powder Tech. 2000, 110. (19) van der Put, A. G.; Bijsterbosch, B. H. J. Colloid Interface Sci. 1980, 75, 512. (20) Zukoski, C. F., IV; Saville, D. A. J. Colloid Interface Sci. 1986, 114, 32. Zukoski, C. F., IV; Saville, D. A. J. Colloid Interface Sci. 1986, 114, 45. Rosen, L. A.; Saville, D. A. Langmuir 1991, 7, 36. (21) Kijlstra, J.; van Leeuwen, H. P.; Lyklema, J. J. Chem. Soc., Faraday Trans. 1992, 88, 3441. Kijlstra, J.; van Leeuwen, H. P.; Lyklema, J. Langmuir 1993, 9, 1625. Kijlstra, J.; Wegh, R. A. J.; van Leeuwen, H. P.; Lyklema, J. J. Electroanal. Chem. 1994, 366, 37. (22) Mangelsdorf, C. F.; White, L. R. J. Chem. Soc., Faraday Trans. 1990, 86, 2859. (23) Dukhin, S. S.; Shilov, V. N. In Interfacial Electrokinetics and Electrophoresis; Delgado, A. V., Ed.; Marcel Dekker: New York, 2002; Chapter 2, p 55. (24) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: New York, 1995; Vol. II, Chapter 4.
10.1021/la7005524 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007
8790 Langmuir, Vol. 23, No. 17, 2007
Figure 1. Schematic diagrams of the apparatus for conductance measurements.
strength. However, the strength of the applied electric field required for the alignment of particles is at least one order higher than those for electrical conductance measurements for uniform dispersions, and particles are not isolated as a result of the local condensation of particles due to the formation of particle assemblies in the vicinity of a vessel wall and/or electrodes. In a previous paper, we tackled in-situ electrical conductance measurements under microscopic observation to elucidate the relationship between surface conductivity and the alignment of silica particles, and we reported that in-situ electrical conductance measurements enabled us to detect the formation of pearl chains and their bands.25 In this study, we have investigated the dependence of the alignment of silica particles on the frequency of an applied electric field (100 Hz-1 kHz). In particular, we focus on the fact that the ionic polarization around silica particles settling at the bottom of a silica vessel is enhanced as the frequency of the applied electric field is changed from 1 kHz to 100 Hz. Experimental Section Materials. Monodisperse spherical silica particles with an average diameter of 1.62 µm were supplied from Nippon Shokubai Co., Ltd. Prior to use for experiments, the silica particles were dispersed in distilled water ultrasonically, followed by rinsing with distilled water repeatedly until the conductance of the supernatant reached that of distilled water. Sodium chloride (analytical grade) was used as a supporting electrolyte without further purification. Distilled water (
