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Influence of Surfactant Counterions during Electrophoretic Particle Deposition Ce´line Pignolet, Claudine Filiaˆtre,* and Alain Foissy Institut UTINAM UMR 6213, UniVersite´ de Franche-Comte´, 16 Route de Gray, 25030 Besanc¸on Cedex, France ReceiVed April 7, 2008. ReVised Manuscript ReceiVed June 5, 2008 This study demonstrates the influence of a cationic surfactant on colloidal particle electrodeposition (migration and adhesion). Three cetyltrimethylammonium salts (CTA+) with various counterions (bromide, chloride, and hydrogenosulphate) were studied. Particle transport toward the electrode was driven by the electrophoretic force. Once particles reached the electrode, a wide variety of behaviors were observed, depending on surfactant concentration and counterions: particles would stick permanently or slide along the electrode surface, remain or detach upon potential switching, act as nuclei for aggregate growth, or produce a homogeneous particle film. The experimental results also demonstrate the specific influence of surfactant counterions on the deposited film morphology.
Introduction An expanded area in materials processing is the controlled assembly of colloidal particles at the solid/liquid interface. These assemblies of particles can form uniform monolayer (single particles) or patterns in the micrometric or nanometric range (aggregates). The production of these materials is of interest in a large range of applications such as composite coatings,1,2 ceramic shaping,3,4 adhesion of paint and inks,5 and filtration6 in the case of uniform monolayers, but also optical devices,7,8 optoelectronic materials,9,10 biological and chemical sensors11 in the case of patterned structures. The particle deposition mechanism can be described as consisting of two successive steps: the transport of particles toward the surface and particle adhesion onto the surface. The transport of particles may be governed by external forces such as gravity,12,13 capillarity,14,15 and hydrodynamic16,17 or electrophoretic forces.18-21 Particle transport acts upon the flux of * Corresponding author. Tel: +33 (0)3 81 66 65 31. Fax: +33 (0)3 81 66 20 49. E-mail:
[email protected]. (1) Li, J.; Sun, Y.; Qiao, J. Surf. Coat. Technol. 2005, 192, 331–335. (2) Roos, J. R.; Celis, J. P.; Fransaer, J.; Buelens, C. JOM 1990, 42, 60–63. (3) Sarkar, P.; Nicholson, P. S. J. Am. Ceram. Soc. 1996, 79, 1987–2002. (4) Vleugels, J.; Xu, T.; Huang, S.; Kan, Y.; Wang, P.; Li, L.; van der Biest, O. O. J. Eur. Ceram. Soc. 2007, 27, 1339–1343. (5) Hair, M.; Croucher, M. D. Colloids and Surfaces in Reprographic Technology; American Chemical Society: Washington, D.C., 1982. (6) Brown, R. C. Air Filtration; Pergamon Press: New York, 1993. (7) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B.-R.; Go¨rnitz, E. Physica E 2003, 17, 433–435. (8) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Ka¨ll, M.; Bryant, G. W.; Garcia de Abajo, F. J. Phys. ReV. Lett. 2003, 90, 057401. (9) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143149. (10) Busch, K.; John, S. Phys. ReV. Lett. 1999, 83, 967–970. (11) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829–832. (12) Davis, K. E.; Russel, W. B.; Glantschnig, W. J. J. Chem. Soc., Faraday Trans. 1991, 87, 411–424. (13) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Mifsud, A.; Moya, J. S.; Va´zquez, L. Langmuir 1997, 13, 6009–6011. (14) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538–540. (15) Thill, A.; Spalla, O. Langmuir 2002, 18, 4783–4789. (16) Adamczyk, Z.; Siwek, B.; Warszynski, P.; Musial, E. J. Colloid Interface Sci. 2001, 242, 14–24. (17) Go¨ransson, A.; Tra¨gårdh, C. Colloids Surf., A 2002, 211, 133–144. (18) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706–709. (19) Solomentsev, Y.; Bo¨hmer, M.; Anderson, J. L. Langmuir 1997, 13, 6058– 6068. (20) Yeh, S.-R.; Seul, M.; Shraiman, B. I. Nature 1997, 386, 57–59. (21) Golding, R. K.; Lewis, P. C.; Kumacheva, E.; Allard, M.; Sargent, E. H. Langmuir 2004, 20, 1414–1419.
Table 1. Particle Zeta Potential, Observations on Deposition, and Aggregate Structures as a Function of Solution solution water 5 × 10-5 5 × 10-4 5 × 10-5 5 × 10-4 5 × 10-5 5 × 10-4
M M M M M M
CTAB CTAB CTAC CTAC CTAHS CTAHS
ζ (mV) -34.6 ( 0.4 24.2 ( 0.8 35.2 ( 1.2 22.0 ( 0.7 57.5 ( 2.9 14.9 ( 1.4 23.6 ( 1.7
observations aggregate structure no deposition single particle aggregation aggregation aggregation single particle aggregation
dense open open dense
particles toward the surface and therefore on the surface coverage by deposited particles. The adhesion of particles is controlled by several interactions comprising electrostatic double-layer and dispersion forces described by the DLVO theory but also specific interactions such as hydrophobic22,23 and hydratation22,24 forces. These interactions, which modify particles adhesion, influence the deposit morphologies. In this article, the electrophoretic deposition of colloidal particles onto an electrode surface in an aqueous medium was investigated using a specific cell designed for that purpose in our laboratory.25,26 This cell allows the continuous laminar flow of the solution containing the particle parallel to the electrode. In the present case, the particle transport toward the electrode is governed both by hydrodynamic and electrophoretic forces. In previous work, we found that the flux of particles toward the electrode was completely determined by the electrophoretic force.27 Our interest in the present work is to investigate the influence of surfactant molecules and, more specifically, the influence of their counterions on particle electrodeposition. Indeed, because surfactant molecules are usually added to the bath to stabilize colloidal suspensions, there is a lack of data as to their specific effects on the electrophoretic deposition of particles. (22) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1991. (23) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390–392. (24) Pashley, R. M. AdV. Colloid Interface Sci. 1982, 16, 57–62. (25) Filiaˆtre, C. Analyse et Modelisation de la Microbalance a` Quartz en Phase Liquide en Vue de L’E´tude de De´poˆts E´pais. Ph.D. Thesis, Universite´ de Franche-Comte´, 1994. (26) Filiaˆtre, C.; Towarnicki, L.; Mange, F.; Foissy, A. J. Appl. Electrochem. 1999, 29, 1393–1400. (27) Filiaˆtre, C.; Pignolet, C.; Foissy, A.; Zembala, M.; Warszynski, P. Colloids Surf., A 2003, 222, 55–63.
10.1021/la8010794 CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
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Figure 1. Schematic diagram of the EPD cell (upper view). Figure 2. AFM image of the nickel electrode topography after polishing.
Experimental Section Preparation of Particle/Surfactant Dispersion. The polystyrene (PS) spheres (average diameter 1.5 µm) used in this study were synthesized at the Institute of Catalysis and Surface Chemistry (Krakow, Poland) according to the polymerization procedure described by Goodwin et al.28 The particles were washed by successive centrifugation and dispersed in ultrapure water (resistivity 18.2 MΩ · cm, Millipore) to form a concentrated aqueous suspension. The PS particles, initially negatively charged (as a result of residual sulfonate groups on the surface), were made positive by cationic surfactant adsorption. Three cetyltrimethylammonium-based cationic surfactants (CTA+) with different anions, bromide (CTAB, >99% pure, Merck), chloride (CTAC, >98% pure, Fluka), and hydrogenosulphate (CTAHS, 97% pure, Fluka), were studied. The critical micelle concentration (cmc) values for CTAB, CTAC, and CTAHS were, respectively, 9.4 × 10-4 M,29 1.3-1.6 × 10-4 M,30 and 4 × 10-4 M.31 All surfactants were used as received without further purification. In the case of CTAHS, in aqueous solution the hydrogenosulphate anion (HSO4-) undergoes dissociation into the divalent sulfate ion (SO42-). Because the dissociation constant is equal to K ) 1.2 × 10-2 mol · dm-3,32 there are mainly divalent anions in experimental solutions. The dispersions were prepared by adding a concentrated suspension of particles to 250 mL of a surfactant solution (either 5 × 10-5 or 5 × 10-4 M) to obtain a final concentration of 1.25 × 107 particles per cm3. This small particle concentration was chosen to avoid the aggregation of particles in suspensions and not to perturb the hydrodynamic flow. Indeed, the influence of particles on the colloid flow can be neglected if the volume fraction is CO32- > HPO42- > F- > Cl- > Br- > NO3- > I- > ClO4- > SCN-
Figure 7. Pictures of particles deposited on the electrode in 5 × 10-5 M (a) and 5 × 10-4 M (b) CTAC solutions after 75 min of experiment.
Figure 8. SEM observation of the electrode after particle deposition (5 × 10-4 M CTAB) and fixation. Table 2. Zeta Potential of Mica Sheets as a Function of Solution solution water 5 × 10-5 5 × 10-4 5 × 10-5 5 × 10-4 5 × 10-5 5 × 10-4
M M M M M M
CTAB CTAB CTAC CTAC CTAHS CTAHS
ζ (mV) -53.4 ( 2.2 100.0 ( 8.5 131.7 ( 2.7 122.8( 12.0 155.7 ( 3.7 85.0 ( 2.7 77.7 ( 7.4
anionic lyotropic series (so-called Hofmeister series42) based on the difference in the hydration of anions.43 According to Filankembo et al.44 and Salomaki et al.,45 the series for anions is as follows: (41) Zembala, M.; Adamczyk, Z.; Warszynski, P. Colloids Surf., A 2003, 222, 329–339. (42) Collins, K. D.; Washabaugh, M. W. Q. ReV. Biophys. 1985, 18, 323–422. (43) Burgess, J., Ions in Solution: Basic Principles of Chemical Interactions; Horwood Publishing: Chichester, U.K., 1999. (44) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865–5868. (45) Salomaki, M.; Tervasmaki, P.; Areva, S.; Kankare, J. Langmuir 2004, 20, 3679–3683.
This series follows the variation in the tendency of ions to adsorb at the interface. Concerning the adsorption of surfactant on particle and electrode surfaces, the nature of the counterions modifies the amount of surfactant adsorbed and also the morphology of the surfactant layer. The difference in the amount of surfactant adsorbed on an electrode as a result of the nature of the counterion was demonstrated by Hamdi et al.46 using an electrocapillarity experiment. They found an important increase in CTA+ affinity for the mercury electrode with the expected trend Cl- < Br- < I-. Indeed, for an electrode with a charge density of -10 µC · cm-2 in 5 × 10-5 M CTA+ solutions, the adsorbed amount was about 30% higher for Br- than for Cl-. Assuming that the surfactant adsorption is the same on the nickel electrode, we infer that there are fewer surfactant molecules on the electrode in the presence of CTAC than in the presence of CTAB. Under these conditions, the attractive hydrophobic interactions between particles and the electrode is weaker, hence the particles do not attach and move along the electrode surface. These ion specificities have also been demonstrated in other colloidal features such as the micellization of surfactant.47 Indeed, the more the counterion binds closely to the CTA+ groups, the more the surfactant charge is screened. The reduction of repulsive interactions between positive head groups improves the formation of micelles and, therefore, the decrease in the cmc value in the bulk and the critical aggregation concentration (cac) value on solid surfaces. For monovalent halide ions, the screening of CTA+ increases in the order Cl-< Br- < I-. The effect of counterions on the structure of the adsorbed surfactant layer on a hydrophobic or hydrophilic substrate has been widely investigated. Reviews of the main results of these studies, in particular, the results obtained from atomic force microscopy imaging may be found in refs 48 and 49. However, mainly because of the limit of AFM, most of these studies have focused on ideally smooth surfaces (silica, mica, and graphite) and concentrations near or above the cmc, revealing dense structures ranging from cylindrical stripes to hexagonally packed spheres.50-53 Moreover, the nature of counterions will also affect the electroosmotic and electrohydrodynamic flux. Indeed, with these effects being based on ion migration, the presence of the counterions in the hydration layer near the electrode must be taken into account. The nature of the counterions will modify electrokinetic effects according to their affinity for the surfactant positive head groups. Following the lyotropic series, Cl- anions were more strongly hydrated than Br- anions and so bind less to the CTA+ head groups. Under these conditions, the migration of Cl- anions induced by the electric field was easier, and the aggregation of particles, due to the electrokinetic effects (electrohydrodynamic and electroosmotic flow), was observed at lower concentrations for CTAC than for CTAB. (46) Hamdi, M.; Bennes, R.; Schuhmann, D.; Vanel, P. J. Electroanal. Chem. 1980, 108, 255–270. (47) Overbeek, J. T. G. Pure Appl. Chem. 1980, 52, 1151–1161. (48) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5, 88–94. (49) Paria, S.; Khilar, K. C. AdV. Colloid Interface Sci. 2004, 110, 75–95. (50) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160–168. (51) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1999, 15, 1685–1692. (52) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548–2556. (53) Subramanian, V.; Ducker, W. A. Langmuir 2000, 16, 4447–4454.
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Conclusions In this work, we have shown that the nature of surfactant counterions influences the rate of deposition of particles by modifying their charge and that these results are consistent with the control of their transport by the electrophoretic force. Moreover, during electrodeposition experiments two behaviors that are dependent on surfactant concentration and counterions were observed: whether the particles attach on the electrode leading to the irreversible deposition of single particles or whether they move along the electrode surface carried by fluid flow and form transient aggregates. Using this latter phenomenon, we obtained the total coverage of the electrode surface, which is difficult to have in a laminar flow,54 and by changing the experimental conditions after the particle deposition, particle attachment on the electrode was permanent (Figure 8). In this work, we also demonstrate that the nature of surfactant counterions induces drastic changes in the morphology of particle aggregates. (54) Pignolet, C. Etude des Interactions Entre Particules Colloı¨dales et E´lectrode en Vue de L′E´laboration de De´poˆts Structure´s. Ph.D. Thesis, Universite´ de Franche-Comte´, 2005.
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We suggest that the existence of specific interactions (such as hydrophobic and hydratation interactions, ionic correlation, etc.) could explain the strong influence of the nature of counterions on particle deposition. To understand the role of surfactant counterions in particle deposition morphology better, the study of the mechanism of aggregate formation and particle velocity is under investigation. Acknowledgment. We thank Dr. Maria Zembala from the Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Cracow, Poland, for the measurements of streaming potential and Dr. Claudine Bainier from the Institut FEMTOST, Universite´ de Franche-Comte´, Besanc¸on, France, for AFM imaging. This work was supported by the Conseil Re´gional de Franche-Comte´. Supporting Information Available: Video clip showing the formation (yellow particles) and growth (red particle) of particle aggregates in 5 × 10-4 M CTAB solution. This material is available free of charge via the Internet at http://pubs.acs.org. LA8010794
