J. Phys. Chem. C 2007, 111, 12305-12314
12305
Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: II. Cetyl Pyridinium Chloride on Methyl and Methyl Ether Surfaces Thomas D. Curwen,† Colin D. Bain,*.† 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
The adsorption of cetyl pyridinium chloride (CPC) in 0.1 M KCl to methyl- and methyl ether-terminated organic monolayers has been studied using a dual-inlet channel flow cell. The surfaces were formed by reaction of H-terminated Si(111) with dodecene and 11-methoxy-1-undecene, respectively. The equilibrium adsorption isotherm, the desorption kinetics, and the adsorption kinetics can be explained self-consistently on the basis of the Frumkin model of adsorption. The equilibrium adsorption isotherms on both surfaces were very close to Langmuirian, indicating that the driving force for adsorption remains roughly constant with coverage. Adsorption was in the mixed diffusion-kinetic regime under the mass transport conditions of the channel flow cell (adsorption times of the order of tens of seconds). Most of the difference in the adsorption energies of the two surfaces appears to be reflected in the adsorption rate constant rather than the desorption rate constant, suggesting a late transition state. Comparing the Frumkin model parameters for the methyl and methyl ether surfaces with those for hydrophilic silica helps clarify and quantify the difference between the surfaces. The nature of the surface has a very-significant effect on the equilibration time for the same initial surfactant distribution because the adsorption rate affects the subsurface concentration and consequently the flux of surfactant to the interface itself. The organic self-assembled monolayers (SAMs) formed by reaction of H-terminated Si(111) with alkenes make robust and versatile substrates for surfactant adsorption experiments. A combination of SAMs with the dual-inlet channel flow cell offers an opportunity for the systematic and quantitative study of the effect of surface chemistry on the adsorption kinetics of surfactants to organic surfaces.
1. Introduction Very often, surfactants are used to modify the properties of hydrophobic and intermediate hydrophobic/hydrophilic surfaces. One such major application is the wet milling of crystalline organic active ingredients in the agrochemical and pharmaceutical industries. Surfactants are used to aid wetting during milling and stabilize the resulting suspensions. The active ingredients are commonly highly functionalized organic molecules, so the crystals formed have varied surface chemistry. Properties such as the water contact angle may vary significantly from face to face and are controlled by a range of different functional groups on the surface.1 Despite obvious practical interest, only a few systematic studies have been carried out on surfactant adsorption at organic surfaces. These studies have been restricted principally to the measurement of the equilibrium adsorption properties, usually above the cmc, and have used either chlorosilane monolayers on silica or thiol monolayers on gold. Using neutrons, Thomas and co-workers studied the adsorption of a series of CnEm surfactants to undecenyltrichlorosilane-modified silica, its hydroxylated analogue, and bare silica.2 The adsorption of a similar series of CnEm surfactants to diethyloctylchlorosilane-modified silica, graphite, and bare silica has also been studied by Ducker * Corresponding author. Current address: Department of Chemistry, Durham University, South Road, Durham DH1 3EA, U.K.; e-mail:
[email protected]. † Oxford University. ‡ Syngenta Limited.
and co-workers using AFM.3 Tiberg and co-workers4 and Boschkova and co-workers5 studied the adsorption of C12E8 to mixed monolayers of hexadecanethiol and 16-hydroxyhexadecanethiol by atomic force microscopy (AFM) and quartz crystal microbalance (QCM), respectively. With both techniques, the equilibrium adsorption of the surfactant was studied on monolayers prepared from solutions containing 0%, 25%, 50%, 75%, and 100% hexadecanethiol. These four studies revealed significant differences in the structures formed by surfactants on different organic surfaces. On the most hydrophobic surfaces amorphous monolayers were observed, whereas on the more hydrophilic surfaces the surfactant adsorbed as micellar structures/ defective bilayers. The importance of specific interactions was highlighted by Thomas and co-workers. On the hydroxylated undecenyltrichlorosilane modified silica surface, the CnEm surfactants adsorbed flat to the interface, which was thought to be due to hydrogen bonds formed with the surface hydroxyl groups.2 Ducker and co-workers used mixed monolayers of 11mercapto-1-undecanol and N,N,N-trimethyl(11-mercaptoundecyl)ammonium chloride on gold to observe the effect of surface charge on the adsorption of SDS using surface plasmon resonance (SPR) to measure the amount of adsorbed surfactant.6 Self-assembled monolayers (SAMs), either of a pure component or of mixtures, also present an opportunity for systematic studies of the influence of surface hydrophobicity and functional groups on the kinetics of adsorption of surfactants to organic surfaces. Tiberg and co-workers compared the adsorption kinetics of C14E6 on hydrophilic silica and silica modified by
10.1021/jp072922v CCC: $37.00 © 2007 American Chemical Society Published on Web 07/28/2007
12306 J. Phys. Chem. C, Vol. 111, No. 33, 2007 adsorption of dimethyloctylchlorosilane.7 Stroeve and coworkers studied the adsorption kinetics of SDS to monolayers of undecanethiol and 2-aminoethanethiol SAMs on gold by SPR.8 Both authors observed that at a particular surfactant concentration (below the cmc) initial adsorption rates were faster at hydrophobic interfaces than hydrophilic interfaces. In Part I of this pair of papers, we have shown how the dualinlet channel flow cell can be used to obtain detailed kinetic information on the adsorption and desorption of a surfactant from hydrophilic silica.9 By first setting up a steady-state surfactant distribution under convection and then allowing the surfactant to diffuse to the surface in the absence of flow, we were able to eliminate uncertainties associated with the initial surfactant distribution and hence to model accurately the mass transport to the surface with a finite-difference simulation. In this paper, we extend this methodology to organic surfaces formed by reaction of H-terminated Si(111) with alkenes. This method for making SAMs was developed by Chidsey and coworkers in the mid-1990s10,11 and is a rapidly developing area of research.12 Although such SAMs have not been used previously in adsorption studies, we have found that they provide robust and reproducible surfaces for surfactant adsorption. With our Beaglehole ellipsometer, they generate a more stable baseline (when used in the channel flow cell) than SAMs on gold. To permit comparison with the work in Part I on hydrophilic silica, we have used the same cationic surfactant in this work, cetyl pyridinium chloride (CPC), in 0.1 M KCl. Dodecene and 11-methoxy-1-undecene were allowed to react with Si(111)-H surfaces to create organic surfaces terminated by methyl groups and methyl ether groups, respectively. The equilibrium adsorption isotherm and the adsorption and desorption kinetics are described with the Frumkin model. In the Experimental section, we describe the functionalization of the silicon wafers’ surfaces and the experimental conditions employed in the flow cell. In the Results section, we describe the characterization of the modified wafers before presenting data on the equilibrium adsorption isotherm and on the desorption and adsorption kinetics of CPC at the two surfaces. In the Discussion section, we compare adsorption of CPC at the methyl and methyl ether surfaces with adsorption at bare silica. We conclude with comments of the scope of the dual-inlet channel flow cell combined with the functionalization methodology. 2. Experimental Section (i) Materials. Chemicals were used as received unless otherwise stated. Ultrapure water (UPW) was produced by a Milli-Q Synthesis system fed from a Milli-Q Elix system (Millipore, resistivity >18.2 MΩ cm, TOC e 5 ppb). 1-dodecene (95%), sodium methoxide (25 wt % in methanol), hexane (99%), hexadecane (99%), and KCl (99.99+%) were purchased from Sigma-Aldrich. 11-bromo-1-undecene (Purum, g95%), ammonium fluoride (∼40% in water, Purum), 1,1,1-trichloroethane (Puriss, g99%), hydrogen peroxide (30%, TraceSelect), sulfuric acid (>95%, TraceSelect), hydrochloric acid (37%, TraceSelect), 1-hexadecylpyridinium chloride (CPC, as monohydrate 99.0%), and ethanol (absolute, Puriss) were purchased from Fluka. Ammonium solution (25 wt % as NH3, Suprapur) was purchased from Merck. The CPC was recrystallized twice from ethanol. The KCl was baked for 24 h at 600 °C to remove organic contaminants. The purity of the CPC and KCl was confirmed by surface-tension measurements as reported in Part I. (ii) Synthesis of 11-Methoxy-1-undecene. 11-Bromo-1undecene (1 equiv) was added to sodium methoxide (25 wt % in methanol) (5 equiv) and refluxed overnight with stirring. The
Curwen et al. reaction mixture was allowed to cool before it was quenched by addition of HCl. Most of the methanol (75%) was then removed by rotary evaporation before the reaction was worked up. First, water was added to the reaction mixture until all of the NaBr was dissolved. The organic phase was then extracted 3 times with ether. The combined organic extracts were washed twice with 1 M HCl solution and twice with water. The organic phase was dried with saturated NaCl solution and Na2SO4 before the ether was removed by rotary evaporation. The target product (pure by NMR) was recovered as a pale yellow oil in 90+ % yield. 1H NMR (CDCl3, 500 MHz) δ 5.8 (m 1H), 4.9-5.0 (m 2H), 3.35 (t, 2H), 3.31 (s, 3H), 2.05 (m, 2H), 1.55 (m, 2H), 1.2-1.4 (m, 12H). (iii) Etching of Silicon Substrates. The etching methodology was developed with reference to work by Chabal and coworkers,13,14 Chidsey and co-workers,15 Allongue and coworkers,16 and Henzler and co-workers.17 Silicon wafers (Prime grade, specified as being cut at an angle of 0.5° ( 0.25° to the (111) face in the direction (to within (0.5°), singleside polished, p-type, boron-doped, resistivity 1-20 Ω cm) were purchased from Compart Technology Ltd. UK. The wafers were first cut to size (10 × 50 mm2). Cleaning was achieved in four steps: (1) sonication in methanol for 5 min; (2) cleaning in piranha solution [3:1 v/v mixture of H2SO4 (>95%) and H2O2 (30%)] for 30 min at 90 °C; (3) cleaning in SC1 solution (0.25: 1:5 v/v mixture of ammonium solution, H2O2, and UPW) for 15 min at 75 °C; and (4) cleaning in SC2 solution (1:1:6 v/v mixture of HCl (37%), H2O2 (30%) and UPW) for 15 min at 75 °C. The wafers were rinsed thoroughly with UPW between each step and at the end of the process. The cleaned wafers were etched for 15 min in unstirred ammonium fluoride solution (deoxygenated for g2 h by sparging with argon gas with stirring) to remove the native oxide and obtain a terraced hydrogen-terminated silicon surface. The etched wafers were washed briefly with UPW and then blown dry with argon before being transferred to a quartz Schlenk tube, which was immediately deoxygenated and placed under argon. Caution: piranha solution is highly corrosiVe and can react explosiVely with organic materials. Glassware should be thoroughly dried after rinsing with organic solVents prior to exposure to piranha solution. (iv) Formation of Alkyl Monolayers on Silicon. The method was developed from the work of Wagner and co-workers,18 Effenberger and co-workers,19 Wayner and co-workers,20,21 Smith and co-workers,22 Bo¨cking and co-workers,23 and Yu and co-workers.24 The alkenes (1-dodecene and 11-methoxy-1undecene) were dried with sodium and distilled under reduced pressure (∼15 mbar) before being put through six freezepump-thaw cycles to thoroughly deoxygenate them. The deoxygenated alkene was then transferred by syringe to the Schlenk tube containing the freshly etched wafer. The Schlenk tube was positioned next to (20 mW cm-2 at 1 in.) and irradiated for 5 h. The modified silicon wafers were rinsed with hexane and ethanol to remove physisorbed material, sonicated for 5 min in 1,1,1-trichloroethane, and finally rinsed briefly with ethanol and blown dry in a stream of argon. The wafers were stored under argon in the fridge until used. (v) Surface Characterization. All stages of the surface modification process were monitored by AFM, ellipsometry, and contact-angle measurements. AFM images were recorded using a Digital Instruments Multimode AFM fitted with a
II. On Methyl and Methyl Ether Surfaces
Figure 1. Contact-mode 1 × 1 µm2 AFM images of (A) a freshly etched Si(111) wafer and (B) dodecene-functionalized (methylterminated) wafer. The images have been flattened but not leveled in order to be able to view the terraces.
Nanoscope IIIa controller. Images were acquired in air in contact mode with sharpened silicon nitride tips (Veeco, NanoProbe NP-20, 0.12 N m-1) at a constant force. Ellipsometric measurements were carried out in air on a Picometer Ellipsometer (Beaglehole Instruments, Wellington, NZ) fitted with a HeNe light source (Melles Griot, Germany). Analyzer modulation was employed to eliminate the effect of stray birefringence. All thicknesses were determined by fitting data from angle scans between 50° and 80°. The following refractive indices were used in the modeling: silicon 3.881 + 0.01945i;25 silicon oxide 1.46;25 organic layers 1.45. At least three measurements were made on each wafer. The variation on any given wafer was within (2 Å. Water and hexadecane static contact angles were measured by drop shape analysis (IT Concept, Longessaigne, France) at ambient humidity and temperature. A drop of liquid (∼10 µL) was placed on the surface, and the contact angle was measured immediately. At least 4 measurements were made on each wafer. Measurements were generally within (2° of the reported averages. (vi) Flow Cell. The design and operation of the dual-inlet channel flow cell have been explained in detail in Part I. Briefly, the cell consists of a shallow rectangular channel (50-mm long
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12307 × 8-mm wide × 0.5-mm deep), the bottom of which is defined by the surface of interestsin this case a functionalized silicon wafer. The top surface is a transparent fused silica window that permits optical access to the substrate. The cell has two inlets and a single outlet. The first inlet, known as the blank inlet, delivers a flow of background electrolyte solution into the channel. The second inlet is a narrow slit that spans the width of the top of the channel, downstream of the blank inlet and upstream of the window, and is used to inject a concentrated surfactant solution into the cell. The flow rates are chosen so that the surfactant is swept away downstream before it is able to diffuse across the channel and adsorb to the silicon wafer. The steady-state surfactant concentration profile that develops in the channel defines the initial condition for the adsorption experiment and is modeled by a finite difference simulation. The adsorption experiment is initiated by switching off both flows simultaneously. The flow comes to a standstill within milliseconds and the surfactant then diffuses across the channel and adsorbs to the functionalized silicon wafer. The diffusionadsorption process is modeled with the relevant time-dependent differential equations. The theoretical description (Frumkin model) of the surfactant adsorption process is incorporated in the boundary condition chosen for the sample surface. A HeNe laser beam passing through the window and reflected from the sample is used to monitor the adsorption of the surfactant to the substrate 35 mm downstream of the second (surfactant) inlet by ellipsometry. Characterization of the adsorption and desorption kinetics is carried out in 3 steps. First, the equilibrium adsorption isotherm is obtained by sealing the second inlet (known as bulk-flushing mode) and replacing the blank solution with surfactant solutions of varying concentrations. Second, the kinetically limited desorption rate is measured by increasing the flushing rate until the desorption of a preadsorbed surfactant layer is independent of the flow rate. Third, the adsorption kinetics are measured and compared to a numerical simulation with no variable parameters. (vii) Surfactant Solutions and Flow Conditions. All solutions were prepared with UPW. Glassware, Teflon tubing, and the flow cell were cleaned with Hellmanex II cleaning solution (Hellma) and then rinsed copiously (∼50×) with UPW. Syringe pumps (Harvard Apparatus Model 11plus and Razel Scientific Instruments Model A99) controlled the fluid flow. During bulkflushing measurements, both the blank and surfactant solutions were delivered from 50-mL gas-tight glass syringes (SGE Analytical Science) at 135 mL h-1. For measurements of adsorption kinetics, the blank flow rate was 135 mL h-1 and the surfactant solution was delivered from a 2.5 mL gastight glass syringe (SGE Analytical Science) at 1.35 or 5.4 mL h-1. The syringes were connected to the flow cell using Teflon tubing (Omnifit). All experiments were carried out in a temperaturecontrolled room kept at 20 ( 0.25 °C. On both SAMs, the adsorption kinetics were measured at five different CPC injection fluxes, 0.4, 0.7, 1.0, 13, and 51 µmol h-1. The first four injection fluxes were achieved by injection of 0.3, 0.5, 0.75, and 9.5 mM CPC solutions, respectively, at 1.35 mL h-1. For reasons explained in Part I, the highest injection flux was achieved by injection of a 9.5 mM solution at 5.4 mL h-1. (viii) Measurement of Adsorbed Amount. Ellipsometry was employed as above, except that the arms were fixed at 60° and the analyzer was not modulated to enable more-rapid data collection (one reading per second). Measured changes in the imaginary part of the complex reflectivity ratio r ) rp/rs were converted to surface coverages using the optical model described
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TABLE 1: Water and Hexadecane Contact Angles on Methyl and Methyl Ether Wafers Used in Flow Cell water hexadecane
methyl 1
methyl 2
methyl 3
ether 1
ether 2
ether 3
106 ( 2 spreads
107 ( 1 spreads
109 ( 1 spreads
80 ( 1.5 spreads
80 ( 1.5 spreads
81 ( 1 spreads
in Part I, with the silicon oxide layer replaced by the selfassembled monolayer and the adsorbed surfactant layer thickness reduced to 20 Å. The assumption that each adsorbed CP+ ion is associated with a Cl- counterion is justified as the SAMs are uncharged. (ix) Numerical Modeling and the Frumkin Isotherm. We have employed the backward implicit finite difference, BIFD, method to compute numerically both the steady-state surfactant concentration profile and the time-dependent diffusion-adsorption behavior. The BIFD method is well-described in the literature,26-29 and its application to the flow cell was described in detail in Part I. The surfactant is modeled as existing as monomers or as monodisperse micelles. Equilibration between the two species is assumed to be fast on the time scale of the experiment, and the diffusion coefficients of the two species are assumed to be concentration-independent. In the presence of salt, where migration fields are eliminated by the added electrolyte, the assumptions hold well. The adsorption of CPC was modeled with a Frumkin boundary condition. We write the Frumkin isotherm in the form
KFc Γ ) Γ∞ 1 + KFc
(1)
where the Frumkin parameter KF is given by
KF ) KLeωΓ/Γ∞
(2)
KL is the Langmuir constant, which is equal to the quotient of the adsorption and desorption rate constants, ka and kd. ω is an energy parameter that characterizes the pairwise interaction between nearest neighbors. The energy barriers to adsorption and desorption are assumed to be proportional to the surface coverage:
dΓ ) kaeβΓ/Γ∞c(Γ∞ - Γ) - kdeRΓ/Γ∞Γ dt
(3)
ω)β-R
(4)
where
Fitting the equilibrium isotherm to eq 1 yields values of Γ∞, KL, and ω. Under kinetically controlled desorption conditions, the first term in eq 3 may be neglected, leaving
dΓ ) -kdΓeRΓ/Γ∞ dt
(5)
so fitting the desorption curve yields kd and R. The relationship between KL, ka, and kd and eq 4 then give ka and β, which fully determines the boundary condition on the surfactant flux at the surface in the adsorption experiments, eq 3. The surfactant cmc and the monomer and micelle diffusion coefficients can be obtained from the literature leaving no unknown parameters in the model for the adsorption kinetics. Any deviation of the model is indicative of limitations in the theoretical model used to model the adsorption process. 3. Results (i) Characterization of Methyl and Methyl Ether Surfaces. AFM was used to optimize the etching process. Conditions were
varied until broad terraces were formed across the whole sample (Figure 1A). The steps in the AFM image are either single or double silicon bilayers. The jagged terrace structure shows that the wafers are not cut as specified (i.e., toward the ) but rather in exactly the opposite direction,17 most probably because the wafers were polished on the wrong side. AFM was also used to check that the functionalized wafers were covered with a uniform monolayer and not a patchy structure. Figure 1B of a dodecene-functionalized (methyl-terminated) wafer shows that the monolayer is uniform. The monolayers were also characterized by ellipsometry and contact angle measurements. Ellipsometric measurements of the etched wafers confirmed the removal of the silicon oxide. Measurements on the three methyl-terminated wafers were modeled by a 20-26 Å organic layer sandwiched between semiinfinite silicon and air. This thickness is greater than the fully extended length of dodocene and is twice the thickness measured by Chidsey and co-workers following the thermal reaction of dodecene with Si(111).11 There are two possible explanations for the discrepancy. First, the UV initiated-reaction may yield a higher surface coverage, reducing the tilt angle of the chains and, therefore, increasing the thickness. This hypothesis is not supported by the hexadecane contact-angle measurements (Table 1), which indicated that the layers are less-ordered than those produced by Chidsey and co-workers. Second, and more likely, some oxidation of the silicon may have occurred below the dodecene layer. Geometric constraints prevent all of the Si sites from being capped with dodecane chains, leaving sites that can be attacked by oxygen. Silica has a very similar refractive index to the organic layer, so one cannot distinguish the two layers by ellipsometry. Contact-angle measurements (Table 1) indicate that any polar silicon oxide groups are screened from the aqueous phase by the alkyl chains of the SAM. Measurements on the three methyl ether-terminated wafers were closely modeled by a 18-19 Å organic layer. The lower thickness compared to the dodecene functionalized wafers is most likely due to a lower packing density. Table 1 records the water and hexadecane contact angles on the three methyl- and three methyl ether-terminated wafers used in the flow cell experiments. There is excellent reproducibility between the three wafers in each set. Some wafers were not wet by hexadecane (θ ) 20-30°), indicating a more densely packed monolayer, but it was difficult to produce nonwettable surfaces reproducibly, so these were not used in the flow cell. The contact angle of water measured on the methyl-terminated surfaces is in good agreement with that measured by Yu and co-workers on wafers modified by a very similar procedure.24 There is no literature precedent for the methyl ether-modified wafers, but the water contact angles measured are in agreement with those observed on methyl ether-terminated thiol monolayers on gold.30 The thiol monolayers on gold were not wet by hexadecane, indicating that the monolayers on silicon are lessordered than those on gold. In summary, covalent modification of Si(111) wafers gives robust, reproducible, readily functionalized monolayers and provides substrates for the flow cell that are optically almost identical to silicon wafers with their native oxide. (ii) Equilibrium Adsorption Isotherms.The equilibrium adsorption isotherms for CPC in 0.1 M KCl on the methyl- and
II. On Methyl and Methyl Ether Surfaces
Figure 2. Equilibrium adsorption isotherms for CPC in 0.1 M KCl on methyl- and methyl ether-terminated surfaces. The adsorption isotherm for CPC in 0.1 M KCl on silica is shown for comparison. Solid lines are the best-fit Frumkin isotherms. The dashed vertical line indicates the cmc.
Figure 3. Kinetically limited desorption of CPC from methyl and methyl ether surfaces into 0.1 M KCl. Solid lines are measured data, and dashed lines are fits to eq 5.
Figure 4. Desorption curves from the methyl-terminated surface for a range of surface coverages. The labels on the desorption curves refer to the surfactant injection flux (µmol h-1) used in the preceding dualinlet adsorption experiment.
methyl ether-terminated surfaces are shown in Figure 2. Each isotherm is the average of measurements made on three wafers functionalized by the same method. The equilibrium surface coverage at each concentration on each wafer was determined from two on-off cycles. The intrawafer variation was comparable to the interwafer variation, so the error bars are determined from the variance calculated from all six measurements and
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12309
Figure 5. Desorption curves from the methyl ether-terminated surface for a range of surface coverages. The labels on the desorption curves refer to the surfactant injection flux (µmol h-1) used in the preceding dual-inlet adsorption experiment.
represent (1σˆ (where σˆ is the standard error in the mean). Best fits to the Frumkin isotherm are also shown in Figure 2, and the fitting parameters (Γ∞, KL and ω) are tabulated in columns 2-4 of Table 2. The parameters determined for adsorption to silica are shown for comparison. The best fit to the data sets and the associated errors were determined by the least-squares method described in Part I. The two parameters are again quite strongly correlated: a positive deviation in KL is correlated with negative deviation in ω. The Frumkin model is in excellent agreement with the two isotherms. The small values of ω indicate that the behavior is very close to Langmuirian. There is little literature precedent for equilibrium adsorption isotherms of cationic surfactants on amorphous hydrophobic interfaces. The shape observed for both isotherms (a gentle s curve spread over >2 orders of concentration on a log-concentration plot) is very similar to that observed by Tiberg and co-workers for C14E6 adsorption on dimethyloctylchlorosilane-modified silica,7 which indicates that the background electrolyte is effectively shielding the headgroup repulsions. The limiting surface coverages recorded for the methyl and methyl ether surfaces are ∼50% and ∼60%, respectively, of that observed on silica. These coverages are consistent with the formation of monolayers or hemiaggregates. (iii) Kinetically Limited Desorption. Figure 3 shows the kinetically limited desorption traces for a saturated film of CPC in 0.1 M KCl for the methyl- and methyl ether-terminated interfaces, together with the least-squares best fits to eq 5. The experimental curves are the average of three independent desorption measurements. The kinetic parameters determined from the fits are recorded in columns 5 and 6 of Table 2. The errors were determined by a method very similar to that used to fit the isotherms. Again, the two parameters are quite strongly correlated: a positive deviation in kd is correlated with negative deviation in R. The Frumkin model is an excellent fit to the data in both cases. Figures 4 and 5 plot the desorption curves from a range of different surface coverages on the methyl- and methyl ether-terminated surfaces, respectively. The desorption curves from the methyl ether surface can be superimposed, suggesting that the desorption pathway proceeds through a series of equilibrium structures formed by adsorption of partial monolayers on the organic surface. For the methyl surface, the quality of data is not as high, but the desorption curves again superimpose to within the measurement noise in the curves. (iv) Adsorption Kinetics. Figure 6A-E and Figure 7A-E plot the surface coverage, determined by ellipsometry, as a function of time for five different injection concentrations on
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Figure 6. CPC adsorption kinetics on methyl-terminated surface. (A-E) Grey: experimentally measured surface coverage. Solid black: calculated surface coverage. Dashed black: calculated surfactant concentration immediately above the surface. CPC injection fluxes are (A) 0.4, (B) 0.7, (C) 1.0, (D) 13, and (E) 51 µmol h-1. (F) Five experimental curves plotted on a common scale for comparison. Labels on lines refer to CPC injection flux (µmol h-1).
TABLE 2: Frumkin Parameters for Adsorption of CPC in 0.1 M KCl to Methyl- and Methyl Ether-Terminated Interfacesa surface methyl methyl ether silica a
Γ (µmol m-2) 2.9 4.0 6.25
KL (m3 mol-1)
ω
kd (s-1)
R
ka (m3 mol-1 s-1)
β
240 ( 40 120 ( 20 6.5 ( 0.5
-0.4 ( 0.4 -0.5 ( 0.4 +3.8 ( 0.2
0.030 ( 0.005 0.035 ( 0.01 0.065 ( 0.01
+0.2 ( 0.4 -0.1 ( 0.7 -1.0 ( 0.2
7(2 4(2 0.4 ( 0.1
-0.2 ( 0.8 -0.6 ( 1.0 +2.8 ( 0.4
Parameters for silica are shown for comparison.
the methyl and methyl ether surfaces, respectively. The concentrations were chosen to achieve a range of mass-transport rates and limiting surface coverages. Figures 6 and 7 F show the five experimental curves on a common scale for comparison. The experimental curves are the average of three independent measurements made on one wafer. Reproducibility between measurements was as good as that seen on silica. The solid black line in Figures 6A-E and 7A-E is the predicted surface coverage, calculated using the kinetic parameters in Table 2 and the cmc, monomer, and micelle diffusion coefficients tabulated in column 2 of Table 3. The critical micelle concentration was measured by surface tensiometry. The diffusion coefficients for the monomers and micelles were taken from the literature. When we allowed the diffusion coefficients to
vary in the model, the best fits (columns 3and 4 in Table 3) were in good agreement with each other, with those determined by fitting the adsorption of CPC in 0.1 M KCl on silica (column 5 Table 3),9 and with the literature values. The dashed black line in Figures 6A-E and 7A-E shows the concentration of CPC in the simulation cell just above the surface. In parts A-C, the bulk concentration at the surface does not reach the cmc, whereas in parts D and E it far exceeds the cmc at longer times. The kinetic model is mostly in good agreement with the experimental data, showing that it describes both the diffusion and adsorption behavior of the surfactant accurately. In Figures 6D and 7C, the model overestimates the steepness of the adsorption profile, but because in each case the agreement is
II. On Methyl and Methyl Ether Surfaces
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12311
Figure 7. CPC adsorption kinetics on the methyl ether-terminated surface. (A-E) Grey: experimentally measured surface coverage. Solid black: calculated surface coverage. Dashed black: calculated surfactant concentration immediately above the surface. CPC injection fluxes are (A) 0.4, (B) 0.7, (C) 1.0, (D) 13, and (E) 51 µmol h-1. F) Five experimental curves plotted on a common scale for comparison. Labels on lines refer to CPC injection flux (µmol h-1).
TABLE 3: Diffusion Coefficients and cmc’s Used in Modeling
c
parameter
model
methyl best fit
methyl ether best fit
silica best fit
cmc (mM) Dmon (×10-10 m2 s-1) Dmic (×10-10 m2 s-1)
0.045a 4.5b 0.65c
4.3 0.79
4.3 0.72
4.4d 0.69d
a Determined by tensiometry at 293 K.9 Measured at 25 °C by cyclic voltammetry.32
b d
Value for CTAB.31 Values from Part I.9
excellent for the higher and lower concentrations, these discrepancies are probably experimentally related. The extent of adsorption on hydrophobic surfaces is lower than that on hydrophilic surfaces, which impairs the signal-to-noise ratio and means that any external perturbations (which affect the baseline from which the extent of adsorption is measured) have a larger impact on the experimental data. Adsorption to the hydrophilic silica surface, described in Part I, was shown to be under mixed diffusion/kinetic control. The rapid increase in surface coverage over a narrow concentration
range on silica led to a plateau in the concentration at the interface during the adsorption process. The more gradual isotherms on the methyl and methyl ether surfaces mean that there is no similar plateau in the surface concentration. The adsorption is under a degree of kinetic control, as shown in Figure 8. For the methyl-terminated surface, we have plotted the adsorption curves for the best-fit parameters (Table 1, solid lines) and the curves for values of ka and kd an order of magnitude larger (dashed lines). The diffusion limit is, in fact, reached if ka and kd are increased only fivefold. The behavior on the methyl ether-terminated interface is very similar. 4. Discussion The equilibrium adsorption isotherm, the desorption kinetics, and the adsorption kinetics of CPC in 0.1 M KCl on methyland methyl ether-terminated surfaces can be explained selfconsistently on the basis of the Frumkin model of adsorption. Comparing the Frumkin model parameters reveals a number of similarities and differences between the two hydrophobic
12312 J. Phys. Chem. C, Vol. 111, No. 33, 2007
Figure 8. Model output for the adsorption of CPC in 0.1 M KCl to the methyl-terminated surface for all five injection fluxes. Solid lines: ka ) 7.0 m3 mol-1 s-1 and kd ) 0.035 s-1. Dashed line: ka ) 70 m3 mol-1 s-1 and kd ) 0.35 s-1.
surfaces. The Langmuir constant, KL, for the methyl surface is twice that for the methyl ether surface. This difference may be ascribed to the stronger interaction of water with the methyl ether surface than with the methyl interface. Sum-frequency experiments on methyl ether-terminated SAMs of thiols on gold have shown that the oxygen of the ether interacts with water.33,34 If we ascribe the lowering of the contact angle of water on the methyl ether surface principally to the interaction between the methyl ether and water, then the strength of this interaction amounts to γlv∆ cos θ ) - 0.034 J m-2. If all of the interfacial water molecules were displaced by CP+ molecules (with a surface coverage of 4 µmol m-2), then the free energy of adsorption of CP+ ions to the methyl ether surface would be raised by 8 kJ mol-1 compared to the methyl surface. The difference in KL corresponds to an adsorption energy difference ∆G ∼ RT ln 2 ∼ 2 kJ mol-1. The difference between these two values of ∆G has two plausible explanations. Either a significant number of water molecules remain bound to the ether surface after adsorption of CP+, hydrogen bonding to the ether oxygen, or the dipole of the methyl ether interacts favorably with the charge distribution on the CP+ ions. It would be interesting to use in situ sum-frequency spectroscopy to observe whether the water molecules were displaced from the interface by the adsorbed surfactant. The maximum surface coverage of CPC in 0.1 M KCl at the air-water interface and on the methyl-terminated wafers are approximately the same (2.9 µmol m-2), suggesting a similarity of structures at the two surfacessprobably a laterally homogeneous monolayer Despite the weaker interaction of CP+ with the methyl ether surface, the limiting coverage, Γ∞, is 40% higher on the methyl ether surface (Γ∞ ) 4.0 µmol m-2 with a plateau coverage of ∼3.4 µmol m-2) One possible explanation for the increased adsorption is that interactions between the CPC headgroup and the methyl ether surface favor the adsorption of surfactants lying flat on the surface, which template the formation of hemispheres or hemicylinders. This possibility has some literature support: Manne and co-workers found by AFM that the cationic surfactant dodecyltrimethylammonium bromide formed globular and short rod-like aggregates on a trimethylchorosilane-modified silica surface, which had a similar water contact angle to the methyl ether surface (80°).35 The interaction parameter, ω, is small and negative for both interfaces: the isotherms are nearly Langmuirian. The small, negative ω indicates that the hydrophobic attractions and the electrostatic repulsions between the adsorbed surfactants are well-balanced with the latter just dominating. The absence of
Curwen et al.
Figure 9. Schematic diagrams of the energy profile along the adsorption coordinate for the adsorption of CPC in 0.1 M KCl: (A) silica and (B) methyl (black) and methyl ether (gray). In part A, the solid line represents low surface coverage, and the dashed line represents high surface coverage. The dashed line has been offset in order to superimpose the two curves for the free molecule (in effect, by neglecting the configurational entropy in solution). In partB, the differences between the free-energy curves at high and low surface coverage are, within experimental error, zero.
strong interactions between the surfactants means that surfacesurfactant interactions control the adsorption rather than surfactant-surfactant interactions. The experimental precision does not allow us to identify unambiguous differences between the kinetic parameters on the methyl and methyl ether surfaces. Correlations between KL and ω and between kd and R prevent the two surfaces from being more clearly distinguished, a problem intrinsic to the Frumkin model. Most of the difference in the adsorption energies of the surfactants on the two surfaces appears to be reflected in the adsorption rate constant rather than the desorption rate constant, suggesting a late transition state as was observed previously on hydrophilic silica. Figure 9 plots the free-energy curves for adsorption of CPC on the methyl- and methyl ether-terminated silica and on silica, all in 0.1 M KCl. The relative positions of the adsorbed states and transition states are defined by the best-fit parameters in Table 2. Comparing the Frumkin model parameters for the methyl and methyl ether surfaces with those for silica helps clarify and quantify the difference between the surfaces. The values of KL are 20-40 times larger on the hydrophobic surfaces than on hydrophilic silica, primarily because of the stronger interaction of silica with water (through dipole interactions and hydrogenbonding), which makes it less-favorable for the surfactant to displace the water and adsorb. The Γ∞ values for the methyl and methyl ether surfaces are approximately 50% lower than those on silica, which is consistent with micellar/fragmented bilayer structures on silica and monolayer/hemiaggregates on the methyl and methyl ether surfaces. The large positive ω value on silica is indicative of highly cooperative adsorption. The adsorption rate constant increases and the desorption rate constant decreases with increasing coverage because adsorption is driven by surfactant-surfactant interactions. In contrast, adsorption to the hydrophobic interfaces is not cooperative, but instead controlled by surfactant-surface interactions and neither the adsorption or desorption rate constants change significantly with surface coverage. In the limit of low coverages, the desorption rate constants are a factor of 2 slower on hydrophobic than hydrophilic surfaces, but the adsorption rate constants are 1 or 2 orders of magnitude faster. If the rate-determining step in adsorption is indeed the displacement of water molecules, then this process will be faster on the hydrophobic surfaces than on the hydrophilic surface because of the stronger dipole-dipole and H-bonding interactions of the water with hydrophilic silica.
II. On Methyl and Methyl Ether Surfaces
Figure 10. Measured adsorption profiles (bottom) and calculated nearsurface concentrations (top) for 13 µmol h-1 injection fluxes on methyland methyl ether-terminated surfaces and silica.
As a consequence of the different values of ka and KL, adsorption on hydrophobic surfaces occurs at much-lower bulk concentrations and is closer to diffusion-controlled than on hydrophilic surfaces. Figure 10 plots the measured adsorption profile and the calculated subsurface concentration in the flow cell for the methyl, methyl ether, and silica surfaces at the same injection flux (13 µmol h-1). The nature of the surface has a verysignificant effect on the adsorption profile for the same initial surfactant distribution. The adsorption rate in turn affects the subsurface concentration and, therefore, the flux of surfactant to the interface itself. For example, at early times the concentration above the hydrophobic surface is lower than that above the hydrophilic surface, which increases the mass transport to the hydrophobic surfaces. The higher equilibrium surface coverage on the silica surface requires more surfactant to be delivered to the surface, yet the rate of delivery is slower. The consequence is that the subsurface concentration of CPC at a hydrophilic silica surface reaches the cmc more than 50 s later than that on the hydrophobic surfaces. This delayed adsorption is not just due to the higher adsorbed amount. If we run the simulations with the same value of Γ∞ but with the other parameters unchanged, then the hydrophilic surface still takes around 30 s longer to equilibrate than the hydrophobic surfaces: the shape of the isotherm has a significant effect on the equilibration time. 5. Conclusions We have applied the dual-inlet channel flow cell to study the adsorption of CPC in 0.1 M KCl to methyl- and methyl ether-terminated organic monolayers. The surfaces were formed by reaction of H-terminated Si(111) with alkenes. The equilibrium adsorption isotherm, the desorption kinetics, and the adsorption kinetics can be explained self-consistently on the
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12313 basis of the Frumkin model of adsorption. Comparing the Frumkin model parameters for the methyl and methyl ether surfaces with those for silica helps clarify and quantify the difference between the surfaces. The equilibrium adsorption isotherms on both functionalized surfaces were very close to Langmuirian, indicating that the driving force for adsorption remains roughly constant with coverage. The small, negative value of the interaction parameter, ω, indicates that the hydrophobic attractions and the electrostatic repulsions between the adsorbed surfactants are well-balanced with the latter just dominating. The Langmuir constant, KL, for the methyl surface is twice that for the methyl ether surface. The corresponding difference in adsorption free energy is only ∼1/4 of the difference between the water-surface interaction energies estimated from the contact angles of water. This discrepancy suggests that either a significant number of water molecules remain bound to the ether surface after adsorption of CP+ or the dipole of the methyl ether interacts favorably with the charge distribution on the CP+ ions. The values of KL are 20-40 times larger on the hydrophobic surfaces than on hydrophilic silica, probably because of the stronger interaction of silica with water molecules that are displaced by the adsorption of surfactant. The limiting coverage, Γ∞, is 40% higher on the methyl ether surface than on the methyl surface, which may suggest the formation of hemiaggregates on the methyl ether surface, though this possibility would need to be confirmed by imaging or scattering methods. On both functionalized surfaces, the 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 experimental precision does not allow us to identify unambiguous differences between the kinetic parameters on the methyl and methyl ether surfaces. Most of the difference in the adsorption energies of the two surfaces appears to be reflected in the adsorption rate constant rather than the desorption rate constant, suggesting a late transition state as was observed previously on hydrophilic silica. In the limit of low coverages, the desorption rate constants are a factor of 2 slower on hydrophobic than hydrophilic surfaces, but the adsorption rate constants are 1 or 2 orders of magnitude faster. As a consequence, adsorption on hydrophobic surfaces occurs at much-lower bulk concentrations and is closer to diffusioncontrolled than that on hydrophilic surfaces. The nature of the surface has a very-significant effect on the equilibration time for the same initial surfactant distribution because the adsorption rate affects the subsurface concentration and consequently the flux of surfactant to the interface itself. The methyl- and methyl ether-terminated organic monolayers used in the experiments make excellent substrates for surfactant adsorption experiments. They are easy to prepare reproducibly and are reasonably robust. In this study, we used only surfaces modified with a single molecule. The functionalization methodology is, however, adapted easily to form mixed monolayers.24 The method, therefore, represents a highly versatile approach to forming model interfaces, characterized by a wide range of compositions and functional groups. A combination of SAMs with the dual-inlet channel flow cell offers an opportunity for the systematic and quantitative study of the effect of surface chemistry on the adsorption kinetics of surfactants to organic surfaces. Acknowledgment. T.D.C. thanks Dr. Robert Jacobs of the Surface Analysis Facility within the Department of Chemistry at Oxford for useful discussions and practical advice. This work
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