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Electrodeposition of particles at nickel electrode surface in a laminar flow cell
Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 55 /63 www.elsevier.com/locate/colsurfa
Electrodeposition of particles at nickel electrode surface in a laminar flow cell C. Filiaˆtre a,*, C. Pignolet a, A. Foissy a, M. Zembala b, P. Warszyn´ski b a
b
LCMI, Universite´ de Franche-Comte´, 16 route de Gray, 25030 Besanc¸on cedex, France Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Cracow, Poland
Abstract Electrodeposition of polystyrene particles on a nickel electrode was observed in the laminar flow cell. The efficiency (rate) of the electrodeposition was determined depending on flow intensity, electrode polarization vs. Ag/AgCl electrode and ionic strength of the solution. The negative polystyrene latex particles were turned positive by adsorption of cetyl trimethyl ammonium bromide (CTAB) and experiments were performed at fixed concentration of the surfactant. It was found that at the CTAB concentration 5 /10 5 mol dm 3 mostly single particles were deposited. It was observed that during the experiment, particles were deposited at a constant rate, which well correlated with the electrode polarization and their zeta potential. When the experiments were performed at various flow conditions in the laminar flow cell, the rate of deposition was found only weakly dependent on the flow intensity. That indicated that the hydrodynamic blocking effects were negligible under conditions encountered in the experiment. It was confirmed by the simulation of the dependence of the hydrodynamic blocking on the shear rate in the simple shear flow. At higher concentration of CTAB 5 /10 4 mol dm 3 and for the electrode polarization of /1.5 V (vs. Ag/AgCl), transient aggregation at the electrode surface was observed. Qualitative aspects of this aggregation are presented. When the electrode polarization was removed the aggregates moved away except the initial ones, which were firmly attached to the surface. The observed phenomena can be explained in terms of clustering of particles by electrokinetic flows. # 2003 Elsevier B.V. All rights reserved. Keywords: Colloid particle; Electrodeposition; Laminar flow cell; CTAB; Surfactant; Electroosmotic flow
1. Introduction Electrodeposition of particles on an electrode surface in aqueous medium is encountered in different processes in particular in coatings and separation technologies. Composite coatings are
* Corresponding author. E-mail address: [email protected] Filiaˆtre).
(C.
produced by embedding particles in a metallic matrix during the electrolytic process. Another expanded area in materials processing is the controlled assembly of colloid particles with micrometer and submicrometer patterns. In this case electrodeposition is used to create particle aggregates on an electrode whereas in composite coating single particle must be deposited. The electrodeposition mechanism can be described as consisting of different steps: (a) stabilisation of the particles in the bath, (b) transport of the particles from the
0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00234-6
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C. Filiaˆtre et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 55 /63
solution toward the electrode and (c) adhesion of the particles to the substrate. Surfactants are added to industrial plating bath to prevent particle aggregation. For instance desaggregation of latex particles in nickel electrolytic bath has been attempted by adsorption of sodium dodecyl sulphate or cetyl ammonium bromide onto particles [1]. Moreover surfactants may directly influence electrodeposition by modifying the interfacial properties of both the particles and the electrode [2 /6]. Liu et al. [7] adsorbed surfactants on gold particles to control the morphology of aggregates. In a previous study on the elaboration of composite coatings (nickel/organic particles), we found that the most favourable conditions for particle electrodeposition on a nickel electrode were in a limited range of surfactant concentration [4]. In the present paper we aim to investigate more deeply the interaction mechanism of surfactants in the electrodeposition of particles on the nickel substrate. Therefore, we became interested in understanding the influence of surfactant molecules on the particle electrodeposition on the nickel electrode. A parallel plate flow cell has been designed to observe in situ the electrodeposition of particles onto the electrode surface. In the present work the effects of fluid flow, electrode polarization, ionic strength and addition of cationic surfactant cetyl trimethyl ammonium bromide (CTAB) on the electrodeposition rate will be studied.
2. Experimental set-up A laminar flow cell has been specifically designed for the in-situ investigation of electrodeposition of micronic particles on any metallic electrode [8]. The liquid flows in a rectangular channel with dimensions 2 mm thick, 20 mm wide and 200 mm long, between two parallel PMMA plates. One plate contained a removable plug comprising a circular metallic electrode. The other plate contains a conductive glass, coated with a tin oxide film, acting both as a window and a counter electrode. The electrodes were connected to a power supply. The electric potential of the collecting electrode was measured relative to a silver wire
inserted through the plug and immersed in the solution. Separate measurements of the potential difference between the Ag/AgCl electrode and the silver calomel electrode gave value of /0.02 V. For all experiments the longer cell dimension was on the horizontal axis, the electrodes and the wide side of the rectangular channel were placed vertically. Steady flow in the channel was maintained by gravity. A peristaltic pump allowed recycling in order to maintain constant liquid levels between two reservoirs before and after the channel. The optical setup consisted of a long working distance lens and a microscope allowing observation of 1 to 100 mm particles between the two facing electrodes. Images were stored on a videotape and the surface coverage by deposited particles was determined using an image analysis system (Essilab, Aries). A parallel plate channel has a practical significance since it constitutes one of the technological geometries used for electrodeposition of materials. Moreover it enables very easy, in-situ observation and monitoring of particles migration and deposition. Actually a laminar flow cell is not the most favourable system for convective deposition in comparison with impinging jet cell and rotating disk electrodes. The particle flux to the electrode surface occurs via an external force. In our system, this force is an electrophoretic force. It is directed perpendicular to the hydrodynamic force and to the resultant of gravity and buoyancy forces (Fig. 1). The lifting force close to the electrode surface can be neglected at low Reynolds numbers, which was the case in current experiments. Fransaer [2] presented a general analysis of the forces encoun-
Fig. 1. Forces acting on a particle in the bulk of the cell: the hydrodynamic force Fh, the electrophoretic force Fe, the gravity Fg and buoyancy Fb forces.
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tered during the deposition of particles in electrochemical systems. The hydrodynamic force is proportionnal to the particle size, to the fluid viscosity and to the fluid velocity, which follows a parabolic profile in this laminar flow cell [4]. The electrophoretic force depends on the applied field and on the size and zeta potential of the particles. So it will act on charged particle in low ionic strength solution. The resultant of the gravity and buoyancy forces is a function of the difference between the particle and liquid densities. When the particle approaches the electrode, the latter affects both the hydrodynamic and the electrophoretic forces, in addition short range interactions (dispersion, double layer, hydratation, . . .) between particle and electrode take place. The trajectory of the particle in the cell will depend on the sum of all the forces acting on it.
3. Materials and methods Cationic surfactant, CTAB was obtained from Merck. Monodisperse polystyrene latex particles having a density of 1.05 g cm 3 were synthesised at the Institute of Catalysis and Surface Chemistry according to the polymerisation procedure described in [9]. Their mean diameter determined by Coulter Counter was found to be 1.2 mm. The particles were cleaned by successive dilution/filtration. Electrophoretic mobilities were measured with a Mark II apparatus (Rank Brothers, Bottisham) and the zeta potential was calculated using the Smoluchowski equation. Zeta potential particles at pH 4 used in experiments was about /30 mV (Fig. 2a). The electric charge on the PS particles originated from the ionisation of the sulphonate groups formed by the polymerisation initiator. Negative PS particles were turned positive by adsorption of CTAB (Fig. 2a and b). At 5 /10 5 M CTAB concentration the positive charge of particles was not high enough to prevent their aggregation and since particles remained partly hydrophobic. At higher CTAB concentration (5 /10 4 M), the electrostatic repulsion was sufficient to stabilise them, particles no longer aggregated in the solution.
Fig. 2. Zeta potential of polystyrene particles: (a) in pure water (") and in 5 /10 5 M CTAB (') solution as a function of pH; (b) at pH 4 as a function of CTAB concentration.
The experiments were thus performed at two CTAB concentrations (5 /10 5 and 5 /10 4 M), at pH 4, room temperature and at fixed particle concentration (1.2 /107 particles per cm3). This low particle concentration was chosen to have mainly single particles in the suspension even at a CTAB concentration of 5 /10 5 M. Before the experiment, the suspensions were stirred for 12 h. The nickel electrode (99.9% pure) was polished first with SiC emery paper and then with diamond powder down to 1 mm (Struers). Between each step the electrode was sonicated in ethanol. Constant voltage between 0 and /2 V was applied and the current was monitored during the experiment.
4. Results and discussion 4.1. Effect of flow rate The effect of flow rate was studied for solution containing 5 /10 5 M of CTAB. At this surfac-
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tant concentration mostly single particles were deposited on the electrode (Fig. 3). Fig. 4 represents the surface coverage (defined as the fraction of electrode surface area occupied by deposited particles) as a function of time at applied voltage of /1.5 V and for the flow rate of 2 ml min 1. As it can be seen up to ca. 60 min the increase of surface coverage was linear. The deposition was irreversible, particles did not detach neither when the electrode voltage was switched off nor when the flow rate or the ionic strength was increased. In the above experiments, particles and electrode have opposite charges which means that the deposition was carried out in barrierless conditions. At low surface coverage, several authors [10 /13] have already shown the occurrence of linear adsorption regime in barrier-less transport conditions. Since the deposition occurs irreversible all particles that reach the surface remain attached and the deposition rate is controlled only by transport conditions and to the much less extent by attractive particle /surface interactions [11]. The initial particle flux to the electrode surface j Þ (number of deposited particles per unit area per unit time), and the normalised initial particle flux j¯ (particle flux normalised by the bulk concentration of particles), can be determined directly from the experimental coverage rate:
Fig. 4. The kinetics of surface coverage of monodisperse polystyrene particle on nickel electrode observed in the laminar flow cell at an applied voltage ( /1.5 V) and fluid flow (2 ml min 1).
jÞ
NS t
1 U pa2 t
j j¯ Þ nb
(2)
where: NS is the number of particles deposited on an unit electrode area, t the time, a the particle radius, U the surface coverage (U /pa2NS), and nb the particle concentration in the bulk. As it can be seen in Fig. 4, after 100 min, the percentage of the covered surface reached about 35%. At longer deposition time the interactions of the depositing particles with ones already attached to the interface have to be taken into account. In case of stable suspension particle /particle interactions are repulsive and part of the surface becomes unavailable for deposition and the deposition rate decreases. Therefore, the particle flux becomes a function of the surface coverage and can be expressed as [14]: j jÞ (1 C1h UC2h U2 L)
Fig. 3. Picture of particles deposited on the nickel electrode after 60 min of experiments at an applied voltage ( /1.5 V) and fluid flow (3 ml min 1).
(1)
(3)
As it was demonstrated in [14], the decrease of the deposition rate is due to blocking of electrode surface by formerly deposited charged particles. In a laminar wall shear flow an electro-hydrodynamic shadow is formed behind the deposited particle [14,15] and the masked area depends on the shear flow, the size and the charge of the particle. Size of this area can be evaluated theoretically by Brownian dynamics simulations [14]. Fig. 5 illustrates the dependence of the hydrodynamic blocking
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Fig. 5. The blocking coefficient C1h (ratio of the size of the electro-hydrodynamic shadow by the particle cross-sectional area) in a laminar flow cell as a function of the shear Peclet number obtained from Brownian dynamics simulations.
coefficient C1h, which is numerically equal to the size of the shadow normalised by particle crosssectional area, on the Peclet number. The Peclet number is defined for wall shear flow as Pe// / Ga2/D with G, shear rate at the surface of electrode; a, particle radius (0.6 mm); and D, particle coefficient diffusion (3.6 /10 9 cm2 s 1). As the results of simulations shown in Fig. 5 suggest, for the experimental conditions encountered in the laminar flow cell, the Peclet number ranged from 1.9 to 7.5 so the blocking coefficient is low (between 4 and 5) and only weakly dependent on the flow intensity. Therefore, the surface coverage found in experiment after 75 min (Fig. 6) of deposition when the surface coverage started to saturate was very weakly dependent on the flow conditions. The experimentally obtained limiting value of surface coverage (35%) corresponds to one predicted for the ionic strength of 5 /10 5 M [13]. 4.2. Effect of polarization and ionic strength Deposition experiments were performed as a function of the electrode polarization setting the fluid flow at 3 ml min 1 and starting the applied potential difference at /0.8 V (Fig. 7). At lower
Fig. 6. The dependence of surface coverage on the fluid flow after 75 min of experiment.
potential values the electrophoretic force was insufficient to carry the particles towards the electrode. Fig. 7 shows that the particle flux increased linearly with the polarization of the electrode up to /1.5 V, at higher values, dihydrogen bubbles formed on the cathode altering the hydrodynamic flow of the solution which caused the decrease of surface coverage. Keeping the flow rate at 3 ml min 1 and the potential difference at /1.5 V, the influence of the ionic strength was investigated by addition of KCl. Fig. 8 reveals that after 60 min experiments the
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Fig. 7. The dependence of surface coverage on the nickel electrode polarisation (vs. the Ag/AgCl reference electrode) for a fluid flow of 3 ml min 1.
Fig. 9 shows that the normal particle flux to the electrode increases linearly with the increase of the potential difference. The observed decrease of particle flux after 75 min of deposition was caused by geometrical blocking of electrode surface described above. The current varied from about 500 mA cm 2 at the beginning of the experiment to about 200 mA cm 2 after 75 min but it was too noisy and its variation could not be measured with sufficient accuracy to calculate the electric field from the current density and the specific conductivity (58 mS cm 1). However, assuming that the electric field remains proportional to the polarization, the linear dependence of the normal flux with the increase of the polarisation suggests that the particle deposition is driven by electrophoretic transport (migration) to the electrode. In this case, the normalised particle flux is given by: j¯ uE
(4)
surface coverage decreased upon salt addition and that the phenomenon was well correlated with the decrease of the zeta potential.
where u is the electrophoretic mobility and E is the electric field. A linear dependence of the normalised particle flux versus the electrophoretic mobility (determined separately) is shown in Fig. 10 for an applied voltage of /1.5 V and after 75 min of experiment. From the slope of the curve the electric field amounts to about 340 V m 1, i.e. 680 mV of potential drop over the cell width. For the sake of comparison the convective / diffusion flux of particles has been calculated in the same experimental conditions. We followed the calculations of Adamczyk and van de Ven [16] for
Fig. 9. The dependence of the normal particle flux on the electrode polarisation for a fluid flow of 3 ml min 1.
Fig. 10. The dependence of the normalised particle flux on the electrophoretic mobility of the particles for an applied voltage ( /1.5 V) and fluid flow (3 ml min 1).
Fig. 8. Variations of the electrode surface coverage and zeta potential of the particle with the increase of the ionic strength (log I).
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particle deposition rate in a plane-parallel channel, based on the solution of convective diffusion equation for particle flux to the channel walls. The resulting value of the normalised deposition flux (6 /10 3 mm s 1) was at least two orders of magnitude lower than the one found here for electrophoretic flux. So one can conclude the electrophoretic migration to the electrode surface is the prevailing mechanism of particle adsorption in our experimental conditions. 4.3. Effect of surfactant We observed entirely different behaviour of the particles when the CTAB concentration was increased to 5 /10 4 M. Upon application of the potential single particles were seen to migrate in the bulk solution and to slow down when approaching the electrode. Most of them did not attach to the electrode. It is possible that at higher surfactant concentration CTAB forms a bilayer on the electrode with positive head group pointing towards the solution. An electrostatic repulsion arises between the positive particles and the adsorbed surfactant layer on the electrode that prevents adsorption. Instead of adsorbing particles were seen to keep moving along the electrode at a very slow rate (about 1 mm s 1). During their motion they approached each other to form aggregates. The particles attraction leading to aggregation extended over length scale of a few particle diameters. At a certain stage of size the aggregates immobilised on the electrode, presumably due to some inhomogeneities of the electrode caused by surface roughness. At these primary aggregates further aggregation was observed. Initially their growth (Fig. 11a) was two-dimensional (parallel to the electrode surface). A few minutes after the beginning of the experiment, other particles coming from inside the cell attached to the aggregates and they started to grow in three dimensions (Fig. 11b). Finally (after about 100 min) they produced a very high surface coverage. It is very interesting to note that the aggregation process was fully reversible. When the polarization was switched off, desaggregation occurred and particles moved away from the electrode except the first deposited ones.
Fig. 11. Pictures of particles deposited on the nickel electrode in a 5 /10 4 M CTAB solution at an applied voltage ( /1.5 V) and fluid flow (3 ml min 1) after: (a) 10 min; (b) 60 min of experiment.
Quite a similar result was obtained by Poortinga et al. [17,18] in the case of the adsorption of bacteria on an electrode in a laminar flow cell. Bacteria interact with each other over a distance of several cell diameters, forming clusters where the inter-bacteria distance was controlled both by the ionic strength and the DC field. Liu et al. [7] also showed that the structure of gold particle aggregates changed with the amount of surfactant and that inter-particles distance in the aggregates was 1/2 nm. The decrease of the fluid flow from 3 to 1.5 ml min 1 resulted in different morphology of the aggregates which were smaller but more numerous (cf. Fig. 12a and b). The formation of 2D aggregates at an ITO glass electrode was formerly observed by Richetti et al. [19], Trau et al. [20], Bohmer [21] and Yeh et al. [22]. However, their experiments were done with-
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electrode and distortion of ionic currents by particles ion currents. However, both theoretical models do not take into account the external flow or the presence of surfactant at the electrode and do not consider formation of three-dimensional aggregates.
5. Conclusion
Fig. 12. Pictures of particles deposited on the nickel electrode in a 5 /10 4 M CTAB solution at an applied voltage ( /1.5 V) and fluid flow (1.5 ml min 1) after: (a) 10 min; (b) 60 min of experiment.
out external (tangential) flow, which has obviously a determining influence in the morphology and the aggregation kinetics in the present investigation (cf. Figs. 11 and 12). A possible explanation of the surface aggregation described above could be the mechanism proposed by Solomentzev et al. [23]. The electric field of an electrode induces the migration ionic flux in the particle double layer. This flux causes electroosmotic flow with the characteristic circular pattern ranging for the distances of several particle radii. Another particle can migrate in this flow towards the first one and they can form a transient aggregate, which can be easily destroyed when the electrode polarisation is switched off. Another mechanism of aggregate formation was proposed by Trau et al. [24]. They consider migration due to electrohydrodynamic flows associated with polarization layer at the
The measurements of electrodeposition of polystyrene particles at nickel electrode in the laminar flow cell showed that at low CTAB concentration (5 /10 5 M) particle deposition is weakly affected by the fluid flow and that the mean factor determining the rate of adsorption is the electrophoretic migration. In these conditions the particles do not desorb at higher ionic strength (KCl solution) or when the potential is switched off. We have shown that at this surfactant concentration the particles have turned positive but remained partly hydrophobic which provides with an adsorption potential since the electrode was covered with a single layer of CTAB pointing their hydrocarbon tails towards the solution [25]. At higher CTAB concentration (5 /10 4 M), particles are fully covered with CTAB monolayer [1] and the surfactant molecules form a bilayer or hemi-micelles on the electrode with positive head group directed towards the solution. In these conditions the electrostatic repulsion between particles and electrode prevents adsorption of single particles. When approaching the electrode the particles roll and hit each other. They formed two-dimensional clusters, parallel to the electrode surface which has been already explained by the electroosmotic or electrohydrodynamic flows as already suggested by other authors [21 /24]. However, the proposed mechanisms in the literature did not take into account the modification of the electrode surface by adsorption of a surfactant. The reversible formation of 3D aggregates was observed. It was attributed to the superposition of particle migration in these, electric field induced, flows and convective transport of particles by externally imposed flow, since particles clearly and progressively detach when the electric field is cut off. However, the theoretical description of 3D
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aggregation occurring in our experimental conditions is very complex. The electrode surface (oxide layer, roughness) can play a role in the adsorption of both the surfactant and the particles. To study the effect of the roughness of the electrode, electrodeposition experiments were carried out with strips (1 mm wide and 0.1 mm deep) parallel and perpendicular to the fluid flow direction. We did not observe preferential deposition on the strips. On the other hand, after the polishing step, an oxide layer is formed in air. It can induce surface heterogeneities, which modify locally the electric field and the surfactant adsorption. Electrochemical study of the electrode is in progress to follow the nickel electrode behaviour when it is cathodically polarised for rather long time (2 h). To better understand the mechanisms of the electrodeposition phenomena, it is important to know the effect of surfactant on the electrode properties. The quantity of adsorbed CTAB on the electrode can be measured by reflectometry and the zeta potential can be determined by streaming potential measurements. This will be the objective of our future work. Also, for sake of comparison, experiments on electrodeposition of cationic particles without addition of surfactant and the effect of the electrode substrate (zinc, steel, gold) are in progress.
Acknowledgements This research was supported by the European Community in the framework of the COPERNICUS program, contract COPELFLOW-ERBI C 15 98 01 21.
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References [1] F. Mange, Ph.D. thesis, University of Franche-Comte´, Besanc¸on, France, 1996. [2] J. Fransaer, Ph.D. thesis, Katholieke Universiteit, Leuven, Belgium, 1994. [3] A. Hovestad, R.J.C.H.L. Heesen, L.J.J. Janssen, J. Appl. Electrochem. 29 (1999) 331. [4] C. Filiaˆtre, L. Towarnicki, F. Mange, A. Foissy, J. Appl. Electrochem. 29 (1999) 1393. [5] V. Terzieva, J. Fransaer, J.-P. Celis, J. Electrochem. Soc. 147 (2000) 198. [6] Z. Serhal, J. Morvan, P. Berc¸ot, M. Rezrazi, J. Pagetti, Surf. Coat. Technol. 145 (2001) 233. [7] J. Liu, W.Y. Shih, M. Sarakaya, I.A. Aksay, Phys. Rev. A 41 (1990) 3206. [8] C. Filiaˆtre, F. Mange, A. Foissy, F. Membrey, JADH’95, La Colle sur Loup, 15 /20 Octobre 1995. [9] J.W. Goodwin, J. Hearn, C.C. Ho, R.H. Ottewill, Colloid Polym. Sci. 252 (1974) 464. [10] T. Dabros, T.G.M. van de Ven, Colloid Polym. Sci. 261 (1983) 694. [11] Z. Adamczyk, M. Zembala, B. Siwek, J. Czarnecki, J. Colloid Interf. Sci. 10 (1986) 188. [12] C.A. Johnson, A.M. Lenhoff, J. Colloid Interf. Sci. 179 (1996) 342. [13] Z. Adamczyk, P. Warszynski, Adv. Colloid Interf. Sci. 63 (1996) 41. [14] P. Warszynski, Adv. Colloid Interf. Sci. 84 (2000) 47. [15] Z. Adamczyk, B. Siwek, P. Warszynski, J. Colloid Interf. Sci. 248 (2002) 244. [16] Z. Adamczyk, T.G.M. van de Ven, J. Colloid Interf. Sci. 80 (1981) 340. [17] A.T. Poortinga, R. Bos, H.J. Busscher, Biotechnol. Bioeng. 67 (2000) 117. [18] A.T. Poortinga, R.J. Smit, H.C. van der Mei, H.J. Busscher, Biotechnol. Bioeng. 676 (2001) 397. [19] P. Richetti, J. Prost, P. Barois, J. Physique Lett. 45 (1984) 1137. [20] M. Trau, D.A. Saville, I.A. Aksay, Science 272 (1996) 706. [21] M. Bo¨hmer, Langmuir 12 (24) (1997) 5747. [22] S.R. Yeh, M. Seul, B.I. Sraiman, Nature 386 (1997) 57. [23] Y. Solomentsev, M. Bo¨hmer, J.L. Anderson, Langmuir 13 (1997) 6058. [24] M. Trau, D.A. Saville, I.A. Aksay, Langmuir 13 (1997) 6375. [25] M. Kaisheva, S. Usui, Q. Dai, Colloids Surf. 29 (1988) 147.
Electrodeposition of particles at nickel electrode surface in a laminar flow cell C. Filiaˆtre a,*, C. Pignolet a, A. Foissy a, M. Zembala b, P. Warszyn´ski b a
b
LCMI, Universite´ de Franche-Comte´, 16 route de Gray, 25030 Besanc¸on cedex, France Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Cracow, Poland
Abstract Electrodeposition of polystyrene particles on a nickel electrode was observed in the laminar flow cell. The efficiency (rate) of the electrodeposition was determined depending on flow intensity, electrode polarization vs. Ag/AgCl electrode and ionic strength of the solution. The negative polystyrene latex particles were turned positive by adsorption of cetyl trimethyl ammonium bromide (CTAB) and experiments were performed at fixed concentration of the surfactant. It was found that at the CTAB concentration 5 /10 5 mol dm 3 mostly single particles were deposited. It was observed that during the experiment, particles were deposited at a constant rate, which well correlated with the electrode polarization and their zeta potential. When the experiments were performed at various flow conditions in the laminar flow cell, the rate of deposition was found only weakly dependent on the flow intensity. That indicated that the hydrodynamic blocking effects were negligible under conditions encountered in the experiment. It was confirmed by the simulation of the dependence of the hydrodynamic blocking on the shear rate in the simple shear flow. At higher concentration of CTAB 5 /10 4 mol dm 3 and for the electrode polarization of /1.5 V (vs. Ag/AgCl), transient aggregation at the electrode surface was observed. Qualitative aspects of this aggregation are presented. When the electrode polarization was removed the aggregates moved away except the initial ones, which were firmly attached to the surface. The observed phenomena can be explained in terms of clustering of particles by electrokinetic flows. # 2003 Elsevier B.V. All rights reserved. Keywords: Colloid particle; Electrodeposition; Laminar flow cell; CTAB; Surfactant; Electroosmotic flow
1. Introduction Electrodeposition of particles on an electrode surface in aqueous medium is encountered in different processes in particular in coatings and separation technologies. Composite coatings are
* Corresponding author. E-mail address: [email protected] Filiaˆtre).
(C.
produced by embedding particles in a metallic matrix during the electrolytic process. Another expanded area in materials processing is the controlled assembly of colloid particles with micrometer and submicrometer patterns. In this case electrodeposition is used to create particle aggregates on an electrode whereas in composite coating single particle must be deposited. The electrodeposition mechanism can be described as consisting of different steps: (a) stabilisation of the particles in the bath, (b) transport of the particles from the
0927-7757/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0927-7757(03)00234-6
56
C. Filiaˆtre et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 222 (2003) 55 /63
solution toward the electrode and (c) adhesion of the particles to the substrate. Surfactants are added to industrial plating bath to prevent particle aggregation. For instance desaggregation of latex particles in nickel electrolytic bath has been attempted by adsorption of sodium dodecyl sulphate or cetyl ammonium bromide onto particles [1]. Moreover surfactants may directly influence electrodeposition by modifying the interfacial properties of both the particles and the electrode [2 /6]. Liu et al. [7] adsorbed surfactants on gold particles to control the morphology of aggregates. In a previous study on the elaboration of composite coatings (nickel/organic particles), we found that the most favourable conditions for particle electrodeposition on a nickel electrode were in a limited range of surfactant concentration [4]. In the present paper we aim to investigate more deeply the interaction mechanism of surfactants in the electrodeposition of particles on the nickel substrate. Therefore, we became interested in understanding the influence of surfactant molecules on the particle electrodeposition on the nickel electrode. A parallel plate flow cell has been designed to observe in situ the electrodeposition of particles onto the electrode surface. In the present work the effects of fluid flow, electrode polarization, ionic strength and addition of cationic surfactant cetyl trimethyl ammonium bromide (CTAB) on the electrodeposition rate will be studied.
2. Experimental set-up A laminar flow cell has been specifically designed for the in-situ investigation of electrodeposition of micronic particles on any metallic electrode [8]. The liquid flows in a rectangular channel with dimensions 2 mm thick, 20 mm wide and 200 mm long, between two parallel PMMA plates. One plate contained a removable plug comprising a circular metallic electrode. The other plate contains a conductive glass, coated with a tin oxide film, acting both as a window and a counter electrode. The electrodes were connected to a power supply. The electric potential of the collecting electrode was measured relative to a silver wire
inserted through the plug and immersed in the solution. Separate measurements of the potential difference between the Ag/AgCl electrode and the silver calomel electrode gave value of /0.02 V. For all experiments the longer cell dimension was on the horizontal axis, the electrodes and the wide side of the rectangular channel were placed vertically. Steady flow in the channel was maintained by gravity. A peristaltic pump allowed recycling in order to maintain constant liquid levels between two reservoirs before and after the channel. The optical setup consisted of a long working distance lens and a microscope allowing observation of 1 to 100 mm particles between the two facing electrodes. Images were stored on a videotape and the surface coverage by deposited particles was determined using an image analysis system (Essilab, Aries). A parallel plate channel has a practical significance since it constitutes one of the technological geometries used for electrodeposition of materials. Moreover it enables very easy, in-situ observation and monitoring of particles migration and deposition. Actually a laminar flow cell is not the most favourable system for convective deposition in comparison with impinging jet cell and rotating disk electrodes. The particle flux to the electrode surface occurs via an external force. In our system, this force is an electrophoretic force. It is directed perpendicular to the hydrodynamic force and to the resultant of gravity and buoyancy forces (Fig. 1). The lifting force close to the electrode surface can be neglected at low Reynolds numbers, which was the case in current experiments. Fransaer [2] presented a general analysis of the forces encoun-
Fig. 1. Forces acting on a particle in the bulk of the cell: the hydrodynamic force Fh, the electrophoretic force Fe, the gravity Fg and buoyancy Fb forces.
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tered during the deposition of particles in electrochemical systems. The hydrodynamic force is proportionnal to the particle size, to the fluid viscosity and to the fluid velocity, which follows a parabolic profile in this laminar flow cell [4]. The electrophoretic force depends on the applied field and on the size and zeta potential of the particles. So it will act on charged particle in low ionic strength solution. The resultant of the gravity and buoyancy forces is a function of the difference between the particle and liquid densities. When the particle approaches the electrode, the latter affects both the hydrodynamic and the electrophoretic forces, in addition short range interactions (dispersion, double layer, hydratation, . . .) between particle and electrode take place. The trajectory of the particle in the cell will depend on the sum of all the forces acting on it.
3. Materials and methods Cationic surfactant, CTAB was obtained from Merck. Monodisperse polystyrene latex particles having a density of 1.05 g cm 3 were synthesised at the Institute of Catalysis and Surface Chemistry according to the polymerisation procedure described in [9]. Their mean diameter determined by Coulter Counter was found to be 1.2 mm. The particles were cleaned by successive dilution/filtration. Electrophoretic mobilities were measured with a Mark II apparatus (Rank Brothers, Bottisham) and the zeta potential was calculated using the Smoluchowski equation. Zeta potential particles at pH 4 used in experiments was about /30 mV (Fig. 2a). The electric charge on the PS particles originated from the ionisation of the sulphonate groups formed by the polymerisation initiator. Negative PS particles were turned positive by adsorption of CTAB (Fig. 2a and b). At 5 /10 5 M CTAB concentration the positive charge of particles was not high enough to prevent their aggregation and since particles remained partly hydrophobic. At higher CTAB concentration (5 /10 4 M), the electrostatic repulsion was sufficient to stabilise them, particles no longer aggregated in the solution.
Fig. 2. Zeta potential of polystyrene particles: (a) in pure water (") and in 5 /10 5 M CTAB (') solution as a function of pH; (b) at pH 4 as a function of CTAB concentration.
The experiments were thus performed at two CTAB concentrations (5 /10 5 and 5 /10 4 M), at pH 4, room temperature and at fixed particle concentration (1.2 /107 particles per cm3). This low particle concentration was chosen to have mainly single particles in the suspension even at a CTAB concentration of 5 /10 5 M. Before the experiment, the suspensions were stirred for 12 h. The nickel electrode (99.9% pure) was polished first with SiC emery paper and then with diamond powder down to 1 mm (Struers). Between each step the electrode was sonicated in ethanol. Constant voltage between 0 and /2 V was applied and the current was monitored during the experiment.
4. Results and discussion 4.1. Effect of flow rate The effect of flow rate was studied for solution containing 5 /10 5 M of CTAB. At this surfac-
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tant concentration mostly single particles were deposited on the electrode (Fig. 3). Fig. 4 represents the surface coverage (defined as the fraction of electrode surface area occupied by deposited particles) as a function of time at applied voltage of /1.5 V and for the flow rate of 2 ml min 1. As it can be seen up to ca. 60 min the increase of surface coverage was linear. The deposition was irreversible, particles did not detach neither when the electrode voltage was switched off nor when the flow rate or the ionic strength was increased. In the above experiments, particles and electrode have opposite charges which means that the deposition was carried out in barrierless conditions. At low surface coverage, several authors [10 /13] have already shown the occurrence of linear adsorption regime in barrier-less transport conditions. Since the deposition occurs irreversible all particles that reach the surface remain attached and the deposition rate is controlled only by transport conditions and to the much less extent by attractive particle /surface interactions [11]. The initial particle flux to the electrode surface j Þ (number of deposited particles per unit area per unit time), and the normalised initial particle flux j¯ (particle flux normalised by the bulk concentration of particles), can be determined directly from the experimental coverage rate:
Fig. 4. The kinetics of surface coverage of monodisperse polystyrene particle on nickel electrode observed in the laminar flow cell at an applied voltage ( /1.5 V) and fluid flow (2 ml min 1).
jÞ
NS t
1 U pa2 t
j j¯ Þ nb
(2)
where: NS is the number of particles deposited on an unit electrode area, t the time, a the particle radius, U the surface coverage (U /pa2NS), and nb the particle concentration in the bulk. As it can be seen in Fig. 4, after 100 min, the percentage of the covered surface reached about 35%. At longer deposition time the interactions of the depositing particles with ones already attached to the interface have to be taken into account. In case of stable suspension particle /particle interactions are repulsive and part of the surface becomes unavailable for deposition and the deposition rate decreases. Therefore, the particle flux becomes a function of the surface coverage and can be expressed as [14]: j jÞ (1 C1h UC2h U2 L)
Fig. 3. Picture of particles deposited on the nickel electrode after 60 min of experiments at an applied voltage ( /1.5 V) and fluid flow (3 ml min 1).
(1)
(3)
As it was demonstrated in [14], the decrease of the deposition rate is due to blocking of electrode surface by formerly deposited charged particles. In a laminar wall shear flow an electro-hydrodynamic shadow is formed behind the deposited particle [14,15] and the masked area depends on the shear flow, the size and the charge of the particle. Size of this area can be evaluated theoretically by Brownian dynamics simulations [14]. Fig. 5 illustrates the dependence of the hydrodynamic blocking
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Fig. 5. The blocking coefficient C1h (ratio of the size of the electro-hydrodynamic shadow by the particle cross-sectional area) in a laminar flow cell as a function of the shear Peclet number obtained from Brownian dynamics simulations.
coefficient C1h, which is numerically equal to the size of the shadow normalised by particle crosssectional area, on the Peclet number. The Peclet number is defined for wall shear flow as Pe// / Ga2/D with G, shear rate at the surface of electrode; a, particle radius (0.6 mm); and D, particle coefficient diffusion (3.6 /10 9 cm2 s 1). As the results of simulations shown in Fig. 5 suggest, for the experimental conditions encountered in the laminar flow cell, the Peclet number ranged from 1.9 to 7.5 so the blocking coefficient is low (between 4 and 5) and only weakly dependent on the flow intensity. Therefore, the surface coverage found in experiment after 75 min (Fig. 6) of deposition when the surface coverage started to saturate was very weakly dependent on the flow conditions. The experimentally obtained limiting value of surface coverage (35%) corresponds to one predicted for the ionic strength of 5 /10 5 M [13]. 4.2. Effect of polarization and ionic strength Deposition experiments were performed as a function of the electrode polarization setting the fluid flow at 3 ml min 1 and starting the applied potential difference at /0.8 V (Fig. 7). At lower
Fig. 6. The dependence of surface coverage on the fluid flow after 75 min of experiment.
potential values the electrophoretic force was insufficient to carry the particles towards the electrode. Fig. 7 shows that the particle flux increased linearly with the polarization of the electrode up to /1.5 V, at higher values, dihydrogen bubbles formed on the cathode altering the hydrodynamic flow of the solution which caused the decrease of surface coverage. Keeping the flow rate at 3 ml min 1 and the potential difference at /1.5 V, the influence of the ionic strength was investigated by addition of KCl. Fig. 8 reveals that after 60 min experiments the
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Fig. 7. The dependence of surface coverage on the nickel electrode polarisation (vs. the Ag/AgCl reference electrode) for a fluid flow of 3 ml min 1.
Fig. 9 shows that the normal particle flux to the electrode increases linearly with the increase of the potential difference. The observed decrease of particle flux after 75 min of deposition was caused by geometrical blocking of electrode surface described above. The current varied from about 500 mA cm 2 at the beginning of the experiment to about 200 mA cm 2 after 75 min but it was too noisy and its variation could not be measured with sufficient accuracy to calculate the electric field from the current density and the specific conductivity (58 mS cm 1). However, assuming that the electric field remains proportional to the polarization, the linear dependence of the normal flux with the increase of the polarisation suggests that the particle deposition is driven by electrophoretic transport (migration) to the electrode. In this case, the normalised particle flux is given by: j¯ uE
(4)
surface coverage decreased upon salt addition and that the phenomenon was well correlated with the decrease of the zeta potential.
where u is the electrophoretic mobility and E is the electric field. A linear dependence of the normalised particle flux versus the electrophoretic mobility (determined separately) is shown in Fig. 10 for an applied voltage of /1.5 V and after 75 min of experiment. From the slope of the curve the electric field amounts to about 340 V m 1, i.e. 680 mV of potential drop over the cell width. For the sake of comparison the convective / diffusion flux of particles has been calculated in the same experimental conditions. We followed the calculations of Adamczyk and van de Ven [16] for
Fig. 9. The dependence of the normal particle flux on the electrode polarisation for a fluid flow of 3 ml min 1.
Fig. 10. The dependence of the normalised particle flux on the electrophoretic mobility of the particles for an applied voltage ( /1.5 V) and fluid flow (3 ml min 1).
Fig. 8. Variations of the electrode surface coverage and zeta potential of the particle with the increase of the ionic strength (log I).
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particle deposition rate in a plane-parallel channel, based on the solution of convective diffusion equation for particle flux to the channel walls. The resulting value of the normalised deposition flux (6 /10 3 mm s 1) was at least two orders of magnitude lower than the one found here for electrophoretic flux. So one can conclude the electrophoretic migration to the electrode surface is the prevailing mechanism of particle adsorption in our experimental conditions. 4.3. Effect of surfactant We observed entirely different behaviour of the particles when the CTAB concentration was increased to 5 /10 4 M. Upon application of the potential single particles were seen to migrate in the bulk solution and to slow down when approaching the electrode. Most of them did not attach to the electrode. It is possible that at higher surfactant concentration CTAB forms a bilayer on the electrode with positive head group pointing towards the solution. An electrostatic repulsion arises between the positive particles and the adsorbed surfactant layer on the electrode that prevents adsorption. Instead of adsorbing particles were seen to keep moving along the electrode at a very slow rate (about 1 mm s 1). During their motion they approached each other to form aggregates. The particles attraction leading to aggregation extended over length scale of a few particle diameters. At a certain stage of size the aggregates immobilised on the electrode, presumably due to some inhomogeneities of the electrode caused by surface roughness. At these primary aggregates further aggregation was observed. Initially their growth (Fig. 11a) was two-dimensional (parallel to the electrode surface). A few minutes after the beginning of the experiment, other particles coming from inside the cell attached to the aggregates and they started to grow in three dimensions (Fig. 11b). Finally (after about 100 min) they produced a very high surface coverage. It is very interesting to note that the aggregation process was fully reversible. When the polarization was switched off, desaggregation occurred and particles moved away from the electrode except the first deposited ones.
Fig. 11. Pictures of particles deposited on the nickel electrode in a 5 /10 4 M CTAB solution at an applied voltage ( /1.5 V) and fluid flow (3 ml min 1) after: (a) 10 min; (b) 60 min of experiment.
Quite a similar result was obtained by Poortinga et al. [17,18] in the case of the adsorption of bacteria on an electrode in a laminar flow cell. Bacteria interact with each other over a distance of several cell diameters, forming clusters where the inter-bacteria distance was controlled both by the ionic strength and the DC field. Liu et al. [7] also showed that the structure of gold particle aggregates changed with the amount of surfactant and that inter-particles distance in the aggregates was 1/2 nm. The decrease of the fluid flow from 3 to 1.5 ml min 1 resulted in different morphology of the aggregates which were smaller but more numerous (cf. Fig. 12a and b). The formation of 2D aggregates at an ITO glass electrode was formerly observed by Richetti et al. [19], Trau et al. [20], Bohmer [21] and Yeh et al. [22]. However, their experiments were done with-
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electrode and distortion of ionic currents by particles ion currents. However, both theoretical models do not take into account the external flow or the presence of surfactant at the electrode and do not consider formation of three-dimensional aggregates.
5. Conclusion
Fig. 12. Pictures of particles deposited on the nickel electrode in a 5 /10 4 M CTAB solution at an applied voltage ( /1.5 V) and fluid flow (1.5 ml min 1) after: (a) 10 min; (b) 60 min of experiment.
out external (tangential) flow, which has obviously a determining influence in the morphology and the aggregation kinetics in the present investigation (cf. Figs. 11 and 12). A possible explanation of the surface aggregation described above could be the mechanism proposed by Solomentzev et al. [23]. The electric field of an electrode induces the migration ionic flux in the particle double layer. This flux causes electroosmotic flow with the characteristic circular pattern ranging for the distances of several particle radii. Another particle can migrate in this flow towards the first one and they can form a transient aggregate, which can be easily destroyed when the electrode polarisation is switched off. Another mechanism of aggregate formation was proposed by Trau et al. [24]. They consider migration due to electrohydrodynamic flows associated with polarization layer at the
The measurements of electrodeposition of polystyrene particles at nickel electrode in the laminar flow cell showed that at low CTAB concentration (5 /10 5 M) particle deposition is weakly affected by the fluid flow and that the mean factor determining the rate of adsorption is the electrophoretic migration. In these conditions the particles do not desorb at higher ionic strength (KCl solution) or when the potential is switched off. We have shown that at this surfactant concentration the particles have turned positive but remained partly hydrophobic which provides with an adsorption potential since the electrode was covered with a single layer of CTAB pointing their hydrocarbon tails towards the solution [25]. At higher CTAB concentration (5 /10 4 M), particles are fully covered with CTAB monolayer [1] and the surfactant molecules form a bilayer or hemi-micelles on the electrode with positive head group directed towards the solution. In these conditions the electrostatic repulsion between particles and electrode prevents adsorption of single particles. When approaching the electrode the particles roll and hit each other. They formed two-dimensional clusters, parallel to the electrode surface which has been already explained by the electroosmotic or electrohydrodynamic flows as already suggested by other authors [21 /24]. However, the proposed mechanisms in the literature did not take into account the modification of the electrode surface by adsorption of a surfactant. The reversible formation of 3D aggregates was observed. It was attributed to the superposition of particle migration in these, electric field induced, flows and convective transport of particles by externally imposed flow, since particles clearly and progressively detach when the electric field is cut off. However, the theoretical description of 3D
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aggregation occurring in our experimental conditions is very complex. The electrode surface (oxide layer, roughness) can play a role in the adsorption of both the surfactant and the particles. To study the effect of the roughness of the electrode, electrodeposition experiments were carried out with strips (1 mm wide and 0.1 mm deep) parallel and perpendicular to the fluid flow direction. We did not observe preferential deposition on the strips. On the other hand, after the polishing step, an oxide layer is formed in air. It can induce surface heterogeneities, which modify locally the electric field and the surfactant adsorption. Electrochemical study of the electrode is in progress to follow the nickel electrode behaviour when it is cathodically polarised for rather long time (2 h). To better understand the mechanisms of the electrodeposition phenomena, it is important to know the effect of surfactant on the electrode properties. The quantity of adsorbed CTAB on the electrode can be measured by reflectometry and the zeta potential can be determined by streaming potential measurements. This will be the objective of our future work. Also, for sake of comparison, experiments on electrodeposition of cationic particles without addition of surfactant and the effect of the electrode substrate (zinc, steel, gold) are in progress.
Acknowledgements This research was supported by the European Community in the framework of the COPERNICUS program, contract COPELFLOW-ERBI C 15 98 01 21.
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