J. Phys. Chem. C 2007, 111, 12289-12304
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Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: I. Cetyl Pyridinium Chloride on Hydrophilic Silica Thomas D. Curwen,† James A. Warner,† Colin D. Bain,*,† Richard G. Compton,† and Jemimah K. Eve‡ Department of Chemistry, UniVersity of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, United Kingdom, and Syngenta Limited, P.O. Box A38, Leeds Road, Huddersfield, HD2 1FF, United Kingdom ReceiVed: April 14, 2007; In Final Form: June 18, 2007
A dual-inlet channel flow cell has been developed for the study of the adsorption kinetics of surfactants to solid-liquid interfaces under hydrodynamic control. This cell, with ellipsometric detection of the adsorbed surfactant, has been used to study the adsorption kinetics of cetyl pyridinium chloride (CPC) to hydrophilic silica in 0.1 M KCl and in pure water. The methodology provides detailed insight into the kinetic parameters and casts light on the adsorption mechanisms. The convection-diffusion behavior in the cell was calculated numerically using the backward implicit finite difference (BIFD) method: the CPC monomer and micelle populations were modeled with constant diffusion coefficients and assumed to equilibrate quickly on the time scale of the experiment. A Frumkin model was used to describe the adsorption behavior at the silica surface: the fitting parameters were determined from the equilibrium adsorption isotherm and kinetically limited desorption measurements. The adsorption kinetics in the cell were then modeled with no free parameters. Adsorption was in the mixed diffusion-kinetic regime under the mass transport conditions of the channel flow cell (adsorption times on the order of tens of seconds). The Frumkin model described well the adsorption of CPC in 0.1 M KCl, but in pure water the fit to the equilibrium adsorption isotherm and the adsorption kinetics was poor. In the presence of 0.1 M KCl, the kinetic parameters suggest a late transition state in the adsorption process. In the absence of salt, both the adsorption and desorption rate constants increase with surface coverage, possibly suggesting a change in mechanism. The experimental methodology can be used to study alternative surfaces and mixed surfactant/polymer systems without modification and can be adapted to study faster kinetics and to incorporate different detection methods.
1. Introduction Surfactants play an essential role in modifying the physical and chemical properties of solid-liquid interfaces in a wide range of applications. Consequently, the adsorption of surfactants at the solid-liquid interface has been the subject of a large number of experimental studies. The current status of our understanding is presented in a number of recent reviews.1-4 Until the 1990s, surfactant adsorption was investigated primarily by depletion studies: finely divided powders with a known surface area were stirred with a surfactant solution, and the extent of adsorption was determined by monitoring the depletion of the surfactant in the solution. Such studies provide valuable information on equilibrium adsorption isotherms, but useful kinetic data is limited to adsorption processes occurring on the time scale of minutes to hours.5-8 The relevant time scale for surfactant adsorption onto solid-liquid interfaces in applications such as wet milling, dispersion of powders, wetting and coating of solid surfaces, and detergency is secondssor less. To address adsorption on shorter timescales, several groups have developed alternative approaches based on flat interfaces and optical detection of surfactant adsorption. Tiberg and co-workers used * Corresponding author. Current address: Department of Chemistry, Durham University, South Road, Durham DH1 3EA, U.K.; e-mail:
[email protected]. † Oxford University. ‡ Syngenta Limited.
ellipsometry to study the kinetics of adsorption and desorption of nonionic surfactants on hydrophilic and hydrophobic silicon wafers.9-11 The silicon wafers were mounted in the center of a quartz cuvette, and the solutions were exchanged with constant stirring. Adsorption and desorption were modeled as two consecutive steps: the diffusion of the surfactant monomers and micelles across a stagnant layer of solution to the subsurface and the passage of the surfactant into the adsorbed layer. During these adsorption processes, the surfactant monomer and micelle populations were equilibrated continuously.12,13 The modeling showed that the adsorption and desorption were predominantly diffusion-controlled. The method used by Tiberg and co-workers to exchange the solutions leads to ill-defined mass transport, which makes it difficult to isolate the kinetic processes occurring on a molecular scale, such as adsorption to the solid surface or monomer-micelle interconversion, from diffusion and convection in the bulk. This limitation can be overcome by use of experimental designs with well-defined hydrodynamics (and therefore mass-transport): Tilton and co-workers14-16 and Stroeve and co-workers17 used channel flow cells to study the adsorption of cationic and anionic surfactants, respectively, whereas Cohen Stuart and co-workers18 and Biggs and coworkers19-23 used wall jet stagnation point flow cells to study the adsorption of nonionic surfactants onto polystyrene and cationic surfactants onto silica and plasma polymers, respectively. To analyze their data, Tilton and co-workers, Cohen
10.1021/jp0729213 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/02/2007
12290 J. Phys. Chem. C, Vol. 111, No. 33, 2007 Stuart and co-workers, and Biggs and co-workers compared the initial rate of adsorption to the theoretical flux of surfactant to the interface for a range of different injection concentrations. Care is needed in this approach because the steady-state equations used to determine the flux of surfactant to the interface are not valid during the early stages of adsorption while the diffusion layer is being established.24 In all of these studies, the introduction of the surfactant was achieved by manual switching of a flow upstream of the flow cell. This approach limits the reliability of data on shorter timescales (18.2 MΩ cm, TOC e 5 ppb). 1-Hexadecylpyridinium chloride (monodydrate 99.0%) and KCl (99.99+%) were purchased from Sigma-Aldrich. The 1-hexadecylpyridinium Chloride (CPC) was recrystallized twice from ethanol. Elemental analysis showed that the water of crystallization was still present after the recrystallization process. The KCl was baked for 24 h at 600 °C before use to remove organic contaminants. All bulk-flushing solutions were made up by dilution of a concentrated stock solution. The glassware used was cleaned with Hellmanex II cleaning solution (Hellma) and then copiously rinsed (∼50×) with UHP water. Surface tension measurements of CPC solutions in 0.1 M KCl at 20 °C exhibited a sharp break at the critical micelle concentration, 0.045 mM, indicating the absence of contaminants. This evaluation of the cmc is in good agreement with the literature.51 Silicon wafers (Prime grade, , n-type, phosphorus doped, resistivity 0.1-20 Ω cm) were purchased from Compart Technology Ltd. The native oxide layer on the wafers was measured by ellipsometer to be ca. 2.5 nm thick (before and after cleaning). The wafers were first cut to size (10 × 50 mm2) and then thoroughly cleaned before use in the flow cell. Cleaning was achieved in four steps: (1) sonication in methanol for 5 min; (2) cleaning in piranha solution [3:1 volume ratio mixture of concd H2SO4 and H2O2 (30% in water)] for 30 min at 90 °C and cooling to room temperature; (3) irradiation under a UV light for 15 min; (4) cleaning in SC2 solution [1:1:6 volume ratio mixture of HCl (37%), H2O2 (30%) and H2O] for 15 min at 75 °C. The wafers were rinsed thoroughly with UHP water between each step and then finally once more at the end of the process before being stored under UHP water before use. Wafers were stored for between 1 day and a week. All chemicals used
I. Cetyl Pyridinium Chloride on Hydrophilic Silica in the cleaning process were TraceSelect grade from Fluka. Cleaned wafers were readily wet by water. Caution: piranha solution is highly corrosiVe and can react explosiVely with organic materials. Glassware should be dried thoroughly after rinsing with organic solVents prior to exposure to piranha solution. (ii) Methodology. Syringe pumps (Harvard Apparatus Model 11plus and Razel Scientific Instruments Model A99) were used to control the fluid flow into the flow cell. During bulk-flushing measurements both the blank and surfactant solutions were delivered from 50 mL gastight glass syringes (SGE Analytical Science) at 135 mL h-1. This flow rate corresponds to maximum fluid velocity in the center of the cell of 0.014 m s-1 and a Reynolds number of 3.5. During measurements of adsorption kinetics, the blank solution was delivered at the same flow rate as in the bulk-flushing experiments and the surfactant solution was delivered from a 2.5-mL gas-tight glass syringe (SGE Analytical Science) at 1.35 or 5.40 mL h-1. The syringes were connected to the flow cell using Teflon tubing (Omnifit Ltd). Prior to setting up the experiment the flow cell, Teflon tubing and syringes were also cleaned thoroughly using Hellmanex II cleaning solution (Hellma) followed by copious rinsing with UPW. In 0.1 M KCl, we measured the adsorption kinetics at five different CPC injection fluxes, 1.3, 1.9, 3.2, 13 and 51 µmol h-1. The first four injection fluxes were achieved by injecting 0.95, 1.425, 2.375, and 9.5 mM CPC solutions respectively, at 1.35 mL h-1. The maximum injection flux is limited by the physical properties of the surfactant solution and the maximum injection flow rate achievable with the syringe pump. In 0.1 M KCl, CPC solutions of approximately 10 mM and higher are very turbid (see the discussion on Krafft temperature below). The highest injection flux was, therefore, achieved by injecting a 9.5 mM solution at 5.4 mL h-1 rather than a 38 mM solution at 1.35 mL h-1. Similarly, in pure water the adsorption kinetics were also measured at five different CPC injection fluxes, 13, 32, 58, 130, and 510 µmol h-1. The injection fluxes were achieved by exactly the same method used in 0.1 M KCl but using 9.5, 23.75, 42.75, and 95 mM CPC solutions. (iii) Environment. All experiments were carried out in a temperature-controlled room kept at 20 ( 0.25 °C. No further temperature control was employed. The Krafft temperature of CPC in 0.1 M KCl is approximately 22 °C, which is above the temperature of the room. The solutions were made up above the Krafft temperature in a preparation laboratory. The supersaturated state was observed to be metastable for at least 10 h once the solutions were moved into the experimental room, even for the high concentration injection solutions. During and after each experiment, we checked that crystallization had not occurred. (iv) Ellipsometry. We have used a Beaglehole Picometer Ellipsometer fitted with a HeNe laser (632.8 nm) to make the ellipsometric measurements. The Beaglehole model is a phasemodulated ellipsometer with a 50 kHz modulation frequency and is particularly well suited to kinetic measurements on account of its very-high time resolution. Phase-modulated ellipsometry was first developed by Jasperson and Schnatterly57 and uses a piezoelectric birefringence modulator to continuously, periodically vary the polarization of the incident light. The amplitude of the oscillation of the birefringence modulator is chosen such that information about the polarization properties of the reflected beam may be obtained from the magnitudes of its intensity modulations at 50 and 100 kHz. These signals are extracted using single-frequency lock-in amplifiers. The ellip-
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12303 someter is capable of making readings every 1 ms; however, in this study we averaged the signal over 1 s because the kinetics did not demand millisecond time resolution. Stray birefringence from, for example, the silica window, can be eliminated by double demodulation, where a polarizing prism in front of the analyzer is rotated at subHertz frequency. For fast measurements, it is not possible to modulate the analyzer polarizing prism, which means that the absolute value of Im(r) is offset. This offset is not of major concern because the surface excess is determined from changes in Im(r) and not its absolute value. The laser beam was incident on the window at 60° from the normal. This corresponds to an angle of incidence of approximately 40° at the silicon-water interface. Although the Beaglohole ellipsometer works best at the Brewster angle (∼70°), there is still ample sensitivity at the chosen angle of incidence, which is limited by other design constraints of the cell. Finally, we note the importance of minimizing stress birefringence in the window of the cell. Transient stresses, caused by temperature gradients and uneven tensions in the screws holding the cell together, and pressure-induced strains lead to baseline fluctuations that interfere with kinetic measurements. Acknowledgment. TDC thanks Dr. Robert Jacobs of the Surface Analysis Facility within the Department of Chemistry at Oxford for useful discussions and practical advice. This work was supported by the EPSRC under Grant GR/S15778 and Syngenta via a CASE Award. References and Notes (1) Somasundaran, P.; Huang, L. AdV. Colloid Interface Sci. 2000, 88, 179. (2) Tiberg, F.; Brinck, J.; Grant, L. Curr. Opin. Colloid Interface Sci. 2000, 4, 411. (3) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. AdV. Colloid Interface Sci. 2003, 103, 219. (4) Paria, S.; Khilar, K. C. AdV. Colloid Interface Sci. 2004, 110, 75. (5) Meader, A. L.; Jr.; Fries, B. A. J. Ind. Eng. Chem. (Washington, D.C.) 1952, 44, 1636. (6) Fava, A.; Eyring, H. J. Phys. Chem. 1956, 60, 890. (7) Biswas, S. C.; Chattoraj, D. K. J. Colloid Interface Sci. 1998, 205, 12. (8) Paria, S.; Manohar, C.; Khilar, K. C. Ind. Eng. Chem. Res. 2005, 44, 3091. (9) Tiberg, F.; Jo¨nsson, B.; Lindman, B. Langmuir 1994, 10, 3714. (10) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531. (11) Brinck, J.; Tiberg, F. Langmuir 1996, 12, 5042. (12) Brinck, J.; Jo¨nsson, B.; Tiberg, F. Langmuir 1998, 14, 5863. (13) Brinck, J.; Jo¨nsson, B.; Tiberg, F. Langmuir 1998, 14, 1058. (14) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566. (15) Pagac, E. S.; Prieve, D. C.; Tilton, R. D. Langmuir 1998, 14, 2333. (16) Velegol, S. B.; Fleming, B. D.; Biggs, S.; Wanless, E. J.; Tilton, R. D. Langmuir 2000, 16, 2548. (17) Levchenko, A. A.; Argo, B. P.; Vidu, R.; Talroze, R. V.; Stroeve, P. Langmuir 2002, 18, 8464. (18) Geffroy, C.; Cohen-Stuart, M. A.; Wong, K.; Cabane, B.; Bergeron, V. Langmuir 2000, 16, 6422. (19) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2000, 16, 9374. (20) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2001, 17, 6155. (21) Atkin, R.; Craig, V. S. J.; Hartley, P. G.; Wanless, E. J.; Biggs, S. Langmuir 2003, 19, 4222. (22) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Colloid Interface Sci. 2003, 266, 236. (23) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Phys. Chem. B 2003, 107, 2978. (24) Lok, B. K.; Cheng, Y. L.; Robertson, C. R. J. Colloid Interface Sci. 1983, 91, 104. (25) Rees, N. V.; Dryfe, R. A. W.; Cooper, J. A.; Coles, B. A.; Compton, R. G.; Davies, S. G.; McCarthy, T. D. J. Phys. Chem. 1995, 99, 7096. (26) Goloub, T. P.; Koopal, L. K. Langmuir 1997, 13, 673. (27) Goloub, T. P.; Koopal, L. K.; Bijsterbosch, B. H.; Sidorova, M. P. Langmuir 1996, 12, 3188.
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