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Langmuir 2008, 24, 4806-4816
Experimental Studies on the Adsorption of Two Surfactants on Solid-Aqueous Interfaces: Adsorption Isotherms and Kinetics Camille Gutig, Brian P. Grady, and Alberto Striolo* School of Chemical, Biological and Materials Engineering, UniVersity of Oklahoma, Norman, Oklahoma 73019 ReceiVed January 5, 2008. In Final Form: February 19, 2008 A quartz crystal microbalance with dissipation (QCM-D) was used to measure the adsorption from aqueous solutions of CTAB (cationic) and C12E6 (nonionic) surfactants on gold and silica surfaces. QCM-D allows for the determination of adsorption isotherms and also the monitoring of the dynamics of adsorption in real time. By considering the atomic-scale roughness of the solid surfaces and the surface area per head group at the air/water interface, our experiments indicate that at bulk concentrations above the critical micelle concentration adsorbed C12E6 forms a monolayer-like structure on both surfaces and CTAB yields a bilayer-like structure. Although our measurements do not allow us to discriminate between the morphology of the aggregates (i.e., between flat monolayers, hemicylinders, or hemispheres in the case of C12E6 and between flat bilayers, cylinders, or spheres in the case of CTAB), these results are particularly significant when compared to recent QCM-D data reported by Macakova et al. (Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23, 12436). These authors reported that QCM-D overestimates the amount of CTAB adsorbed on silica by as much as 30-40% as a result of entrapped water. Our analysis suggests that the effect of entrapped solvent is not as important as previously assumed and, in fact, QCM-D may not overestimate the amount of CTAB adsorbed when roughness is considered. Results for the kinetics of adsorption suggest that the aggregate structure as well as whether micelles are present may influence the adsorption mechanism. We discuss our results in the perspective of molecular theories for both the equilibrium and kinetics of surfactant adsorption.
1. Introduction Surfactants, amphiphilic molecules composed of hydrophobic tails and hydrophilic heads, are used in industrial processes such as detergency, mineral flotation, corrosion inhibition, colloidal dispersion, and oil recovery.1 Surfactant adsorption at the solidliquid interface has been studied, for example, by atomic force microscopy,2-9 ellipsometry,10-17 neutron reflectivity,18-22 and * To whom all correspondence should be addressed. Phone: 1 415 325 5716. Fax: 1 415 325 5813. E-mail:
[email protected]. (1) Paria, S.; Khilar, K. C. AdV. Colloid Interface Sci. 2004, 110, 75-95. (2) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (3) Liu, J. F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895-4903. (4) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294. (5) Liu, J. F.; Ducker, W. A. J. Phys. Chem. B 1999, 103, 8558-8567. (6) Zhang, J. H.; Yoon, R. H.; Mao, M.; Ducker, W. A. Langmuir 2005, 21, 5831-5841. (7) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (8) Patrick, H. N.; Warr, G. G.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349-4356. (9) Jaschke, M.; Butt, H. J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381-1384. (10) Tiberg, F.; Jonsson, B.; Lindman, B. Langmuir 1994, 10, 3714-3722. (11) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 22942300. (12) Harwigsson, I.; Tiberg, F.; Chevalier, Y. J. Colloid Interface Sci. 1996, 183, 380-387. (13) Brinck, J.; Tiberg, F. Langmuir 1996, 12, 5042-5047. (14) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531-538. (15) Brinck, J.; Jonsson, B.; Tiberg, F. Langmuir 1998, 14, 1058-1071. (16) Brinck, J.; Jonsson, B.; Tiberg, F. Langmuir 1998, 14, 5863-5876. (17) Brinck, J.; Jonsson, B.; Tiberg, F. Langmuir 1999, 15, 7719-7724. (18) Mcdermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204-1210. (19) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1992, 96, 1373-1382. (20) Lu, J. R.; Hromadova, M.; Simister, E.; Thomas, R. K.; Penfold, J. Physica B 1994, 198, 120-126. (21) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496-2503. (22) Penfold, J.; Staples, E.; Tucker, I.; Creeth, A.; Hines, J.; Thompson, L.; Cummins, P.; Thomas, R. K.; Warren, N. Colloids Surf., A 1997, 128, 107-117.
surface plasmon resonance.23 The mechanism of adsorption of nonionic surfactants from aqueous solution has been described, among others, by Clunie and Ingram,24 whereas that of ionic surfactants was discussed by Hough and Rendall.25 Atkin et al. recently provided a detailed review of the current understanding of cationic surfactant adsorption at solid-aqueous interfaces.26 Clunie’s work shows that the orientation of adsorbed nonionic surfactants depends on the surface structure. Clunie also showed that different adsorption isotherms are obtained depending on the surfactant orientation on the surface. It is generally accepted that adsorption isotherms of nonionic surfactants are Langmuirian, of either the L2 or L4 type. For ionic surfactants, the electric double-layer theory is employed to explain the experimental adsorption isotherms.1,26 Whereas equilibrium adsorption isotherms have been reported in the literature going back more than 40 years, less is known about the kinetics of adsorption. According to Atkin et al.,27 this lack of information is due to the relatively fast adsorption typical of surfactants and the fact that adsorption isotherms are often measured by depletion measurements in systems that contain solid materials of relatively high surface area. Studies conducted using ellipsometry and quartz crystal microbalances (QCM) are filling this gap. For example, Tiberg et al. studied adsorption and desorption kinetics of various polyethylene glycol monoalkyl ethers at the silica-water interface using ellipsometry.10 Their results show that the rate of adsorption increases as the bulk surfactant (23) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546-1552. (24) Clunie, J. S.; Ingram, B. T. Adsorption of Nonionic Surfactants. In Adsorption from Solution at the Solid/Liquid Interface; Rochester, G. D. P. a. C. H., Ed. Academic Press: 1983; pp 105-152. (25) Rendall, H. M.; Houg, D. B. Adsorption of Ionic Surfactants. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; pp 247-319. (26) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. AdV. Colloid Interface Sci. 2003, 103, 219-304. (27) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2000, 16, 9374-9380.
10.1021/la800035w CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
Surfactant Adsorption on Solid-Aqueous Interfaces
concentration increases, even above the bulk cmc. At all concentrations, the adsorption rate was fast at low observation times (attributed to the diffusion of monomers and micelles from the bulk to the surface) followed by slower adsorption rates at longer observation times (presumably reflecting saturation of the surface). Eskilsson and Yaminsky28 studied the kinetics of cetyltrimethylammonium bromide (CTAB) adsorption on silica and found that up to 2 h was required to reach equilibrium for bulk surfactant concentrations below 0.1(cmc) whereas adsorption was complete within minutes at concentrations approaching or exceeding the cmc. Interestingly, desorption was fast and complete at all concentrations tested. In a series of papers, Biggs et al.27,29,30 used optical reflectometry to investigate the role of micelles in the kinetics of CTAB adsorption on silica. Evidence of cooperative adsorption was found, and this group proposed that at bulk concentrations above the cmc micelles directly adsorb on the surface. This type of cooperative adsorption was found by the same group for other cationic surfactants as well.29,31 However, for a narrow concentration range, evidence suggests that surfactant adsorption can be extremely slow (i.e., CTAB adsorption on pyrogenic silica from a bulk concentration of 0.66(cmc) requires approximately 12 000 s to complete).27 This “slow adsorption region” was interpreted as a consequence of a stochastic process that determines how surfactants fill the available surface adsorption sites coupled with the fact that the structures on the surface have morphological order. These findings suggest that surfactant aggregates at a surface may in some cases be kinetically trapped in local minima of the free-energy landscape. The quartz crystal microbalance with dissipation (QCM-D) allows the measurement of both the “apparent equilibrium” and kinetics of surfactant adsorption from liquid systems onto solid surfaces. We introduce the term apparent equilibrium because, as discussed later in the text, the total amount of surfactant adsorption obtained as the bulk concentration increases from zero to above the cmc in small steps is different from that observed when the bulk concentration is increased suddenly from zero to above the bulk cmc. A nonequilibrium hysteresis-type effect has been seen previously for CTAB on silica using optical reflectometry. As reported in the review paper by Wanless et al.,26 the amount of surfactant adsorbed using a sequential increase in bulk concentration was different from that observed using a sequential decrease. In our case, we believe adsorbed levels obtained via sequential addition are closer to equilibrium than those obtained with bulk addition. Hence, in the remainder of this article we use the term “equilibrium” to indicate results obtained when the bulk surfactant concentration is increased gradually. This assignment is made even though Wanless et al.26 claims that desorption experiments are more accurate in presenting true equilibrium adsorption; our position is that it is not clear whether adsorption or desorption yields true equilibrium adsorption. QCM-D can be used to measure adsorption from aqueous or nonaqueous solution32 and can detect an amount adsorbed of 0.5 ng/cm2 from water.33 Sta¨lgren, Eriksson, and Boschkova34 compared adsorption isotherms obtained from QCM-D and (28) Eskilsson, K.; Yaminsky, V. V. Langmuir 1998, 14, 2444-2450. (29) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. J. Colloid Interface Sci. 2003, 266, 236-244. (30) Fleming, B. D.; Biggs, S.; Wanless, E. J. J. Phys. Chem. B 2001, 105, 9537-9540. (31) Atkin, R.; Craig, V. S. J.; Biggs, S. Langmuir 2001, 17, 6155-6163. (32) Caruso, F.; Rinia, H. A.; Furlong, D. N. Langmuir 1996, 12, 2145-2152. (33) Q-Sense E4 Operator Manual, 2005. (34) Stalgren, J. J. R.; Eriksson, J.; Boschkova, K. J. Colloid Interface Sci. 2002, 253, 190-195.
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ellipsometry. The surfactant considered was C14E6; surfaces considered were both hydrophobic and hydrophilic. The results suggested that QCM-D overestimates the adsorbed mass because, in addition to the amount of adsorbed surfactant, it also measures trapped water and hydration water near surfactant heads. Similarly, Macakova et al.35 recently compared experimental results obtained from QCM-D and optical reflectometry for the adsorption of didodecyltrimethylammonium bromide (DDAB), CTAB, and dodecyltrimethylammonium bromide (DTAB) on silica surfaces. The results indicate that QCM-D and reflectometry yield comparable estimates in the case of DDAB but that QCM-D measures larger adsorbed amounts of both CTAB and DTAB. The authors suggest that the nonflat morphology of CTAB and DTAB aggregates on silica promotes the presence of trapped water, which in turn is responsible for the larger adsorbed amount estimated by QCM-D. However, the authors did not use a surface area that took into account the roughness of the dry silica supports. The roughness is considerably more pronounced on the substrates used for the QCM-D experiments than on those used for reflectometry as shown by their atomic force microscopy (AFM) images. Because a typical QCM-D crystal has substantial roughness, the amount/area adsorbed must be considered on the actual roughness-corrected surface area. In the present work, surfactant adsorption isotherms at solidwater interfaces were measured using the QCM-D. AFM and X-ray photoelectron spectroscopy (XPS) were employed to determine the morphology and the chemical composition of the bare surfaces, respectively. The surfactants used were the nonionic hexaethylene glycol monododecyl ether CH3(CH2)11(OCH2CH2)6OH, abbreviated as C12E6, and the cationic cetyltrimethylammonium bromide CH3(CH2)15N(CH3)3Br, abbreviated as CTAB. The surfaces studied are silicon dioxide and gold. Gold should provide a hydrophobic surface whereas silica is expected to be partially hydrophilic. In fact, the gold surface in this study has substantial hydrophilic character, which has been noted before with QCM-D gold surfaces.34 By studying both the equilibrium and kinetics of adsorption for the two surfactants on two surfaces, we will assess quantitatively how the surface properties affect the adsorption mechanism. Of particular interest is to assess the importance of surface roughness on QCM-D measurements for surfactant systems. The remainder of the article is organized as follows: In section 2, we discuss experimental procedures. In section 3, we present equilibrium adsorption isotherms and kinetics of adsorption. To describe the kinetics of adsorption, we employed in most cases a two-step first-order model used previously by Biswas and Chattoraj,36 although in some circumstances a linear model was more suitable. In section 4, we briefly summarize our conclusions. 2. Materials and Methods Materials. CTAB was purchased from Sigma-Aldrich (cetyltrimethyl-ammonium bromide, minimum 99%, batch no. 026K0185). It was recrystallized three times in acetone-ethanol (10:1 v/v) before use.37 A 22.5 mM stock solution was prepared with the purified surfactant. C12E6 was purchased from Sigma-Aldrich (hexaethylene glycol monododecyl ether, BioChemika, g98.0% (TLC), batch no. 1102090) and used as received. A 1.5563 mM stock solution was prepared. On the basis of literature data, the cmc for CTAB in water is 0.9 mM,38 and that for C12E6 is 0.087 mM.15 (35) Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23, 1243612444. (36) Biswas, S. C.; Chattoraj, D. K. J. Colloid Interface Sci. 1998, 205, 1220. (37) Berr, S. S. J. Phys. Chem. 1987, 91, 4760-4765. (38) Pagac, E. S.; Prieve, D. C.; Tilton, R. D. Langmuir 1998, 14, 2333-2342.
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Aqueous surfactant solutions were prepared by dissolving the desired amounts of surfactants in nanopure water (resistivity 17.9 MΩ cm) obtained by filtering tap water with a Barnstead NANOpure II instrument. No additional compounds (e.g., electrolytes) were added to the solutions. Adsorption Isotherms and Kinetics. QCM-D measures the variation in resonance frequency of a sensor quartz crystal. All crystals used had a nominal resonance frequency of 4.95 MHz. When molecules adsorb on the crystal, the resonance frequency decreases according to the Sauerbrey relation ∆m )
-K∆f n
(1)
In eq 1, ∆m is the amount adsorbed, ∆f is the difference in resonance frequency due to adsorption, K is the mass sensitivity constant (K ) 17.7 ng/cm2 Hz at 5 MHz), and n is the overtone number.34 Equation 1 is valid provided that the mass adsorbed is small compared to the crystal mass. Different surfaces can be prepared by casting various materials on the quartz crystal. All crystals used were purchased from Q-Sense AB. The crystals used were gold 50 nm (QSX 301, batch nos. QP 06275 and QP 0521) and silicon dioxide 50 nm (QSX 303, batch nos. 060612-2 and 051102). The silica surfaces are stoichiometric and microcrystalline. The surface density of hydroxyl groups depends on the solution pH, which in all of our surfactant-adsorption experiments was ∼7. Measurements were performed using a Q-Sense E4 instrument composed of three parts: the flow modules, which contain the sensor crystals, the chamber platform, which holds the modules, and the electronics unit. These parts are all connected to a computer to monitor the different signals (resonance, oscillation frequency, and temperature). The chamber platform is equipped with precise temperature control. All results reported here were obtained at 25 °C with typical temperature variations in the range of (0.05 °C. A peristaltic pump forces the surfactant solution through the chambers that contain the crystals. The flow rate used in our experiments was 0.267 mL/min. All parts are connected with Teflon tubing. The internal volume of the tubing plus the measurement chamber was ∼1 mL per module. The bulk surfactant solution was contained within a Teflon beaker and continuously stirred. Every measurement started by first obtaining a baseline for each crystal in contact with nanopure water. A reliable experiment can be obtained only when the baseline does not drift by more than 0.5 Hz for at least 4 h. Sometimes it was necessary to leave the crystals overnight in their modules before the baseline was sufficiently stable. Before starting an experiment, the solid surfaces on the sensing elements must be thoroughly cleaned. Surface treatment can influence the surface composition and consequently the adsorption of surfactants from solution.39 Thus, it is not surprising that reproducible results can be obtained only when solid surfaces have been “conditioned” following strictly controlled procedures. The following cleaning procedure was employed. The crystals were first placed for 10 min within the plasma cleaner (PDC-32G from Harrick plasma) at a medium rf level. Then they were immersed for 10 min in an ammonia-peroxide mixture (APM) (5:1:1 ultrapure water, ammonia peroxide 25%, and hydrogen peroxide 30%, respectively)40 at 75 °C to remove organics. Finally, the crystals were rinsed with nanopure water and dried with flowing N2 and again placed in the plasma cleaner for 5 min at a low rf level. The cleaning procedure was essential for obtaining reproducible measurements. After conditioning, the crystals were inserted into the flow modules. Nanopure water was fluxed through the modules using the peristaltic pump until a stable baseline was obtained. Once the baseline was sufficiently stable, the experiments began. Concentrated surfactant solutions were injected into the beaker that contains the bulk surfactant solution which, in turn, was pumped to the cell. The bulk surfactant concentration was increased from zero to a maximum concentration of approximately 1.6(cmc) in steps of ∼0.2(cmc) to 0.1(cmc); smaller (39) Penfold, J.; Staples, E.; Tucker, I. Langmuir 2002, 18, 2967-2970. (40) Q-Sense Methods - Methods and Protocols, 2006.
steps were taken near the bulk cmc. At each concentration, the surfactant solution flowed through the flow modules (which contain the sensor crystals) at a constant flow rate for 1 h, during which time surfactants adsorbed on the crystals. After the pump stopped, we waited for a stable signal, which was recorded as the plateau measurement, before increasing the surfactant concentration further. The wait time can be as short as 20 min and as long as 2 to 3 h, depending on the surfactant concentration and the amount adsorbed on the crystals at the beginning of the measurement. No attempt was made to control the surface properties during the adsorption experiments (e.g., by adding electrolytes or by altering the solution pH). However, the solution pH was monitored for the surfactant solutions before and after the adsorption measurements. In all cases tested, the solution pH was approximately neutral, thus suggesting that the surface properties did not mutate widely during the course of each experiment. Because the resonance oscillation frequency is monitored in real time with our instrument, it is possible to study the kinetics of adsorption by recording the oscillation resonance frequency as a function of time when the bulk surfactant concentration is increased. During the kinetics experiments, the peristaltic pump is turned on, but the induced vibrations did not seem to affect the results. Because we are interested in understanding the role of micelles in the adsorption mechanism, we compared the kinetics of adsorption below and above the cmc on clean surfaces (i.e., no surfactant was adsorbed on the surfaces when the kinetics experiments started). To monitor the adsorption kinetics of pure monomers on a clean surface, we considered the kinetics of adsorption when the bulk surfactant concentration was increased from zero to a very small concentration (0.1(cmc)) as well as a large concentration (1.4(cmc)). With a bulk concentration above the cmc, it is possible that micelles adsorb and/or single surfactant molecules adsorb. If the former mechanism dominates, because one micelle is formed by several surfactants, we expect the kinetics of adsorption above the bulk cmc to be faster (at least at short observation times). If adsorption kinetics observed for bulk surfactant concentrations below and above the cmc are similar, then the second mechanism likely dominates. However, to interpret the kinetic results it is important to consider the bulk self-diffusion coefficient for both surfactants when they are single monomers as opposed to when they are associated in micelles. The surfactants chosen for our investigation show comparable self-diffusion coefficients in water. The self-diffusion coefficient reported for C12E6 by Tiberg et al.10 is 4 × 10-10 m2/s for the monomers and 1 × 10-10 m2/s for the micelles. That for CTAB, reported by Atkin et al.,27 is 2 × 10-10 m2/s for monomers and 0.836 × 10-10 m2/s for micelles. The self-diffusion coefficient for micelles is ∼1/4 that for monomers in the case of C12E6 and ∼1/2 in the case of CTAB. Surface Characterization: AFM and XPS. Surfaces were characterized under “dry” conditions (i.e., when the surfaces are not in contact with the aqueous surfactant solutions). Once immersed in aqueous solutions, it is possible that the surface structure changes at the molecular level; quantifying such effects is beyond the scope of the present manuscript. To quantify the surface roughness, a NanoScope III AFM from Digital Instruments was employed in tapping mode. MikroMasch Ultrasharp NSC15 silicon nitride cantilevers were used with a backside aluminum coating and a typical resonance frequency of 325 kHz and a force constant of 40 N/m. All images were captured at 512 samples per line for clearer pictures. By knowing the surface roughness, it is possible to estimate the effective surface area on which adsorption occurs. Surface roughness may affect the amount of surfactant adsorbed not only by increasing the available surface area but also by repressing, or favoring, the development of long-range ordered structures.26 In Figure 1, we report AFM section analysis for two crystals used in our experiments. The crystals considered are gold (Figure 1A,B) and silica (Figure 1C,D). To assess whether the cleaning procedure affects the surface roughness, we performed the AFM analysis before (Figure 1A,C) and after a complete cleaning cycle (Figure 1B,D). The profiles in Figure 1 indicate that before the cleaning procedure gold is the smoother of the two surfaces, whereas silica shows significant roughness. However, after the cleaning procedure, silica becomes
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Figure 1. AFM images (top) and section analysis (bottom) showing the surface structure before (A and C) and after (B and D) the cleaning procedure. Results are shown for gold (A and B) and silica (C and D). AFM images (top panels) are for sections of 1 µm length. much smoother, and gold becomes significantly rougher. Using a line profile analysis, we estimated the actual surface areas of the two substrates by multiplying the nominal area of one crystal by the square of the ratio between the profile line in Figure 1B,D and the nominal horizontal length (i.e., the line integral/nominal length). The estimated roughness-corrected actual surface areas for gold and silica crystals were 1.39 and 0.99 cm2, respectively, and the nominal surface area in both cases was 0.79 cm2. We recognize that this procedure could overestimate the roughness-corrected actual surface area in our crystals; for example, the ratio between the actual and nominal surface area in a simple pattern consisting of a series of radial steps yields a value ∼40% of the way between the ratio and the square of the ratio between integral and nominal lengths. However, we also did not attempt to de-
convolute the roughness from the smoothing caused by the finite tip size. Such deconvolution can only increase the line integral, so the square is likely a reasonable estimate of the actual surface area ratio. XPS (Physical Electronics ESCA System PHI 5800) was used to characterize the chemical nature of the surfaces. Results of one typical XPS experiment performed on a gold surface after the cleaning procedure are shown in Figure 2. The collective data for sample surfaces are reported in Table 1. XPS measurements were performed on the different surfaces before and after cleaning to assess whether the treatment affects the surface composition. We notice that in both gold and silica surfaces carbon atoms are present in the amount of ∼20-40% before the cleaning procedure is performed. The presence of carbon is the signature of organic impurities deposited on the
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Gutig et al. and silica to identify the crystal surfaces used in our experiments, despite the results shown in Table 1.
3. Results and Discussion
Figure 2. Experimental XPS analysis for one gold crystal surface after the cleaning procedure. Table 1. Surface Atomic Composition from XPS Analysis for Two Representative Surfaces (Gold and Silica) before (Top) and after (Bottom) the Cleaning Procedurea as received %
O 1s
C 1s
Si 2p
silica gold
40.9 26.9
36.3 34.9
16.4
%
O 1s
C 1s
Si 2p
silica gold
62.5 36.3
8.9 26.3
27.8
Au 4f
Sn 3d5
33.9
0.1 4.3
Au 4f
Sn 3d5
26.6
0.2 10.9
after the cleaning procedure
a
Average values for three different crystals are given.
surfaces. Completing the cleaning cycle has the effect of removing these impurities, although not completely (carbon atoms represent ∼10-20% of the atomic composition even after the cleaning procedure has been completed). Unfortunately, longer and harsher cleaning procedures were not advisable because the appearance of Sn atoms after the cleaning cycle suggests that the plasma cleaner contaminates the surfaces. Zn and Cu can also appear in small amounts after the cleaning procedure. XPS measurements performed on several crystals indicate that tin is always present in cleaned crystals and that its concentration is consistently larger on gold than on silica. The relative amounts of gold, carbon, oxygen, and silica remain approximately constant for all of the crystals tested, although they come from different batches. We finally notice that in the gold crystals more than 40% of the surface atoms are actually oxygen and fewer than 30% are gold. No attempt was made to characterize the chemical nature of the crystal surfaces further, but the presence of oxygenated sites renders the gold surfaces used more hydrophilic than pure gold surfaces. To corroborate the XPS results, contact angles for nanopure water on silica and gold surfaces were measured before and after cleaning using the sessile drop technique. Average contact angles of ∼40 ( 2° and ∼70 ( 2° on silica and gold crystals were measured before cleaning, suggesting rather hydrophobic substrates. After cleaning, the contact angle decreased to ∼5 ( 2° and ∼10 ( 2° for silica and gold substrates, respectively. These latter values are representative of hydrophilic surfaces, corroborating the XPS analysis. Although XPS results indicate that the differences in composition before and after cleaning are not significant (other than the decreasing carbon content), AFM data shown in Figure 1 suggest that cleaning etches the surfaces. Interestingly, this etching is different on the different substrates. We do not attempt to understand these phenomena herein because the scope of this work is to understand surfactant adsorption, as measured by the QCM-D, rather than solid surface modifications. However, it is possible to reuse the Q-Sense crystals only three times before the results become inconsistent. For convenience, in the remainder of this article we will refer to gold
Adsorption Isotherms. Adsorption isotherms are shown in Figure 3 for C12E6 and for CTAB on both gold and silica surfaces. In panels A and B, we report the amount of surfactant adsorbed per unit of nominal surface area. Results shown in Figure 3A agree reasonably well with those reported by others for CTAB adsorption on silica obtained using ellipsometry27 or QCM-D.41 Literature results on gold are often smaller than ours.41,42 Slight discrepancies between the results shown in Figure 3 and those reported in the literature could be explained by the variety of treatments that are currently used to condition the surfaces before measuring surfactant adsorption. Given the surface characterization results (Figures 1 and 2 and Table 1), we point out that the gold surfaces employed in our experiments contain a large number of oxygen atoms. Thus, they can hardly be considered hydrophobic, and it may as well be that CTAB yields bilayer-like structures on our gold surfaces and would not on pure gold. In panels C and D, we report the amount adsorbed per actual unit surface area obtained when the surface roughness shown in Figure 1 is taken into consideration. To better compare results for the two surfactants, the bulk surfactant concentration (x axis) is shown in Figure 3 as the ratio of the actual concentration to the critical micelle concentration for the surfactants in bulk aqueous solution. In the case of C12E6 on silica (squares in Figure 3B,D), the amount adsorbed is very small at low bulk concentrations and increases rapidly at bulk concentrations approximately equal to 0.75(cmc), leveling off at the bulk cmc. As expected for nonionic surfactants, this adsorption isotherm is an L4 isotherm in the classification discussed by Clunie and Ingram.43 The adsorption of C12E6 on gold (triangles in Figure 3B,D) is more gradual as the bulk concentration increases. When the concentration increases above 1.1(cmc), the experimental data seem to indicate that the amount of surfactant adsorbed increases. Macakova et al.35 found also that in the case of DTAB adsorption on silica, QCM senses an amount adsorbed that keeps increasing as the bulk surfactant concentration increases above the cmc. They elegantly explained how that increase is due to changes in the viscoelastic properties of the bulk liquid rather than to an actual increase of the adsorbed mass of surfactants. Such an effect may be the cause of the apparent rise in the adsorbed amount above the cmc in our results, but we observe that the error bars are quite pronounced, suggesting that there is no increase (real or measured by the QCM-D) in the mass adsorbed above the cmc. Experimental adsorption isotherms measured for CTAB (Figure 3A,C) manifest a number of differences compared to C12E6. First, adsorption measured at low bulk surfactant concentrations is quite significant and increases continuously up to bulk concentrations of approximately 0.9(cmc). At these bulk concentrations, the experimental uncertainty is the largest for this surfactant-surface system. Furthermore, the amount of surfactant adsorbed as measured by QCM-D increases very little as the bulk concentration increases above the cmc. However, we point out that the error bars are larger in the case of C12E6 at large bulk surfactant concentrations than they are for CTAB. Thus, it is entirely possible that the increased adsorbed amounts measured (41) DissipatiVe QCM, Application Example. (42) Knag, M.; Sjoblom, J.; Gulbrandsen, E. J. Dispersion Sci. Technol. 2005, 26, 207-215. (43) Ingram, J. S.; Clunie, B. T. Adsorption of Nonionic Surfactants. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; pp 105-152.
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Figure 3. Experimental adsorption isotherms for CTAB (A and C) and C12E6 (B and D) surfactants on gold (2) and silica (9) as a function of the bulk surfactant concentration at 25 ( 0.05 °C. Panels A and B are obtained when the nominal surface area is considered, showing the error associated with our experiments. Panels C and D are obtained using the roughness-corrected actual surface area of the crystals. For clarity, we do not report error bars in C and D.
by QCM-D above the C12E6 cmc, but not above the CTAB cmc, are due to experimental uncertainty. Using the nominal surface area (Figure 3A,B), smaller adsorption is found on silica surfaces. However, to compare the amount adsorbed on each surface quantitatively, it is necessary to consider the actual surface area available for adsorption (Figure 3C,D). Our results show that in the case of CTAB the amount adsorbed on silica is approximately the same as that adsorbed on gold when the bulk surfactant concentration is above the cmc. Some differences appear at low bulk concentrations; adsorption on gold is larger than that on silica. For C12E6 (Figure 3D), differences observed at low bulk concentration are not as large, but again the adsorption on gold is larger. Results obtained above the cmc also coincide within the uncertainty of our measurements. These results, especially in the case of CTAB, are quite surprising. In fact, one would expect that on a hydrophilic surface (silica) CTAB would form bilayer-like structures and on a hydrophobic surface (gold) CTAB would form monolayer-like structures. On the contrary, our results indicate that the aggregate characteristics are identical on the two surfaces considered, further confirming contact angle and XPS measurements indicating that the gold surface is hydrophilic. Additional details can be inferred from a geometric argument. The surface area per head group available from the literature44 corresponds to ∼0.46 nm2 for CTAB and ∼0.52 nm2 for C12E6. Although these areas per head group were obtained at the waterair interface, they allow us to calculate the total number of moles adsorbed on each crystal, given the estimated actual surface area (Table 2). Within the accuracy of our measurements, our data indicate the formation of a bilayer-like structure for CTAB and a monolayer-like one for C12E6 on both surfaces. We use the terms bilayer-like and monolayer-like because our measurements do not allow us to discriminate between the morphology of the (44) Rosen, M. Surfactants and Interfacial Phenomena, 3rd Ed.; Wiley Interscience: New York, 2004; pp 61-80.
Table 2. Adsorbed Surfactant (nmol) Calculated and from Experimenta gold silica
C12E6 (calcd/exptl)
CTAB(calcd/exptl)
0.444/0.547 0.317/0.332
0.502/1.08 0.357/0.670
a The calculated values correspond to one monolayer formed on the total crystal surface exposed to the surfactant solution in the QCM-D experiments shown in Figure 3.
aggregates (i.e., between flat monolayers, hemicylinders, or hemispheres in the case of C12E6 and between flat bilayers, cylinders, or spheres in the case of CTAB). Despite this deficiency, our data are quite interesting, especially in light of the recent data reported by Macakova et al.35 on CTAB adsorption. In fact, because of the good agreement between the calculated and the experimental values shown in Table 2 as well as the fact that the amount adsorbed does not depend on the substrate, it is almost certain that QCM-D does not overestimate the amount of surfactant adsorbed on a surface for these two surfactants. Stalgren et. al.34 and Macakova et al.35 attributed the apparent overestimation of the adsorbed mass obtained from QCM compared to ellipsometry to the presence of solvent entrapped within the adsorbed layers. Because the crystals used for optical reflectometry experiments were atomically smooth whereas those used for the QCM-D experiments were rough at the molecular level (Figure 8 in ref 35), QCM-D experiments should yield larger amounts of surfactants adsorbed if nominal surface areas are used in the calculation. In fact, looking at the results for CTAB adsorption on silica, the 30-40% overestimation reported by Macakova et al. is roughly equivalent to the difference in surface area as obtained by the admittedly imprecise visual inspection of AFM images. However, our explanation also must consider the case provided by Macakova et al. where it is shown that DDAB adsorption on silica coincides in the case of reflectometry and QCM-D using the nominal surface area for both. In other words, if the roughness
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Figure 4. Measured energy dissipation for C12E6 (A) and CTAB (B) on gold (2) and silica (9) corresponding to the adsorption isotherms shown in Figure 3.
of the substrate were considered, then the actual surface excess as sensed by QCM-D would be less than that sensed by reflectometry. As pointed out by the authors, DDAB yields a flat bilayer structure on a smooth surface, and CTAB forms a rougher cylindrical structure. We note that the roughness typical of the crystals used in our experiments occurs on a length scale comparable to that characteristic of adsorbed surfactant aggregates. Therefore, it is not unreasonable to think that the roughness of a substrate perturbs a flat bilayer structure more significantly than a cylindrical structure with wormlike undulations and fairly small aspect ratios. In fact, solution AFM data shown for DDAB on the molecularly smooth reflectometry silica substrates (Figure 2B in ref 35) suggest the formation of aggregates with large-scale undulations, which are difficult to obtain on a rough substrate. Surface roughness alone cannot be used to explain DTAB adsorption, for which the results of Macakova et al. show that QCM measures ∼3 times the amount adsorbed as measured by reflectometry. Trapped water within the adsorbed aggregates could influence the results in this case, although it is not clear why trapped water would be an issue with adsorbed spherical aggregates but not with cylindrical ones. Another plausible explanation is that roughness causes the amount of adsorbed surfactant to be larger than expected when the adsorbed aggregates are spherical. Clearly, more studies are necessary to clarify how surface roughness affects surfactant adsorption. A previous neutron reflectivity study seems to corroborate parts of the above discussion.45 In this study, CTAB adsorption was considered on silica surfaces of different roughnesses. The amount of adsorbed surfactant (which seems to have been corrected for surface roughness, although it is difficult to tell from reading the paper) was found to drop 10% upon increasing the roughness (from 6 to 14 Å; the paper does not specify whether the average roughness or root-mean-square roughness is used). Using sophisticated deuterium labeling procedures, the authors attributed the decrease in adsorption to less adsorption of the bottom layer in a bilayer-type structure. Hence, the idea of roughness decreasing the amount adsorbed is not new. In addition to adsorbed mass, QCM-D measures dissipation. These measurements give an idea of the change in the flexibility (45) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 6036-6043.
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of the surface aggregates but also depend on the solution viscoelastic properties. Dissipation is larger for a flexible surface aggregate than for a rigid one. In Figure 4, the average dissipation is shown as a function of bulk surfactant concentration for the two surfaces and the two surfactants considered above. In all cases, the dissipation is low at low bulk concentration, consistent with the fact that the adsorbed aggregates are not fully formed on either surface. In all cases except for CTAB on silica, the dissipation increases as the bulk surfactant concentration approaches the bulk cmc. Dissipation on gold surfaces for concentrations at or near the bulk cmc is always larger than that on silica surfaces. The difference between gold and silica is likely due to roughness; a soft phase (i.e., a surfactant aggregate) that is adsorbed to a part of the surface that does not lie in the plane of the nominal surface would likely cause more dissipation. Dissipation on gold, especially in the case of C12E6 surfactants, increases even above the bulk cmc, consistent with the large apparent increase in the amount adsorbed as estimated by QCM-D. Again, the higher roughness is likely the cause of the difference. It is very interesting that the dissipation measured for CTAB on silica is never very pronounced and, in particular, is less above the cmc than below it. This result probably indicates that the surface structure formed by CTAB on this surface provides very uniform, rigid coverage. Dynamics of Adsorption. Dynamic measurements were performed to help us understand the mechanism of surfactant adsorption. The fluid-dynamic pattern within each flow module has not been assessed, preventing the attainment of a more complete understanding of the limiting steps in adsorption (e.g., diffusion-limited vs adsorption-limited mechanisms). However, because kinetic data obtained in flow modules of the same geometry are compared, our results allow the assessment of the effect of micelles on the kinetics of adsorption. In particular, we would like to test the likelihood of whole micelle adsorption on surfaces at a bulk surfactant concentration above the cmc. At the flow rate of the bulk solution (0.267 mL/min), the Reynolds number is ∼0.65 in the flow module, which corresponds to laminar flow in contact with the crystal. The dynamics of adsorption was studied by plotting the percentage of the amount adsorbed compared to the equilibrium adsorption amount as a function of time. To assess the effect of micelles on the adsorption kinetics, adsorption from a solution well below the cmc (0.1(cmc)) was compared to that from well above the cmc (1.4(cmc)). In Figures 5-7, experimental results expressed as a percentage of the equilibrium adsorbed amount at each bulk surfactant concentration are reported as a function of observation time. From visual inspection, multiple sections are noticeable in the experimental data. During the first few (∼3) minutes, no adsorption is detected; this time is required to pump the surfactant solution from the bulk reservoir to the chambers. After the initial 3 min, two types of behavior are observed: one in which the amount of surfactant adsorbed increases according to a fast first step, which in turn is followed by a slow second step, and one in which the amount of surfactant adsorbed increases at a more or less constant rate. In both cases, the adsorbed amount reaches a plateau. Once the plateau was reached, these measurements were interrupted. The whole process typically occurred in 1 h. The behavior with two distinct adsorption rates may be representative of two competitive adsorption mechanisms. For example, it is possible that at short times individual surfactant molecules adsorb on the solid surface without yielding selfassembled supramolecular structures, whereas at longer times the amount of surfactant adsorbed is such that additional surfactant adsorption requires the rearrangement of the adsorbed surfactants.
Surfactant Adsorption on Solid-Aqueous Interfaces
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Figure 5. Percentage of surfactant adsorbed from an aqueous solution on gold surfaces as a function of time, with 100% adsorption corresponding to the maximum adsorption amount with respect to the bulk surfactant concentrations used in the experiments. Results are reported for C12E6 (left) and CTAB (right) surfactants. Bulk concentrations considered are either above or below the bulk cmc, as indicated by the notes within the panels.
Figure 6. Percentage of surfactant adsorbed from an aqueous solution on solid surfaces as a function of time, with 100% adsorption corresponding to the equilibrium adsorption amount with respect to the bulk surfactant concentrations used in the experiments. Results are reported for C12E6 (left) and CTAB (right) surfactants. In all cases considered here, bulk concentrations considered are below the bulk cmc. Surfaces considered are either gold or silica, as indicated within the panels.
Figure 7. Percentage of surfactant adsorbed from aqueous solution on gold surfaces as a function of time, with 100% adsorption corresponding to the maximum adsorption amount with respect to the bulk surfactant concentrations used in the experiments. Results are reported for C12E6 and CTAB surfactants. Bulk concentrations considered are above the bulk cmc. The left panel is for results on gold surfaces, and the right panel is for those on silica.
Our results indicate that the second mechanism is slower than the first, resulting in slower kinetics of adsorption. Clearly, the appearance of each mechanism and the observed kinetics of adsorption depend on the chemical nature of both surfactant and surface. The results shown in Figure 5 indicate that in the case of CTAB the rate of adsorption is similar irrespectively of the bulk surfactant concentration. In the case of C12E6, adsorption at bulk concentrations below the cmc is faster than at concentrations above the cmc. The latter result indicates that the presence of micelles slows down the adsorption process, suggesting the
possibility that C12E6 micelles first adsorb on gold and then rearrange. The decrease in the rate of adsorption above the cmc could be due to the fact that the bulk self-diffusion coefficient for C12E6 micelles is ∼1/4 that of C12E6 monomers and/or the fact that surfactant rearrangement from micelles to adsorbed aggregates is slow. On the contrary, despite the differences observed between the respective bulk self-diffusion coefficients, solutions containing only CTAB monomers adsorb at the same rate on gold as solutions containing both micelles and monomers (Figure 5, right panel), suggesting that the rate of adsorption is governed by CTAB monomers. However, we also recognize that the
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one step: qt ) qo + kot
(2)
two step: qt ) qe - A1e-k1t - A2e-k2t
(3)
The two-step model was previously used to describe the kinetics of adsorption for cationic surfactants on silica.36 In eqs 2 and 3, qo, qt, and qe are a constant, the amount of surfactant adsorbed at time t, and that adsorbed at equilibrium, respectively. Ai are pre-exponential terms; ki are rate constants. The values of the rate constants are related to the specific surfactant-surface system considered. For the one-step case, a straight line can be fit to the data shown in Figures 6 and 7 (specifically those for C12E6 adsorption on silica both below and above the cmc and on gold at concentrations above the cmc), and in the two-step case, a logarithmic plot was used. As an example of the latter, the plot for CTAB adsorption on gold (filled circles) and silica (empty circles) is shown in Figure 8. The rate constants obtained for all the systems considered are reported in Table 3. These rate constants contain information related to the diffusion of surfactants and micelles from the bulk solution to the surface and to the reorganization of the adsorbed surfactant molecules but also to the perturbation provided by the pump on the transport to the surface. No attempt was made to isolate the effect of the pump. The results shown in Table 3 indicate clearly that the rate of adsorption for the second CTAB step does not vary significantly when either the surfaces or the bulk concentration changes, except for the case of adsorption on silica above the cmc. Agreement is expected if the second step corresponds to the reorganization of surfactant molecules adsorbed on the surface to accommodate other molecules that are adsorbing from the bulk solution; the source of the difference for C > cmc on gold is not known. Also, the rate constants typical of the first adsorption step are 1 order of magnitude larger than the rate constants typical of the second step. Despite the large uncertainty associated to these data, k1 seems larger on gold than on silica (see Figure 6, right panel), which is likely related to the larger equilibrium adsorption measured at low bulk concentration on gold (Figure 3C). In the case of C12E6, the most interesting result is that the kinetics of adsorption on gold can be described by two consecutive firstorder steps at concentrations below the cmc but becomes linear at concentrations above the cmc. In all other cases, C12E6 adsorption appears to follow slow linear kinetics, which can be fit by very similar rate constants, thus suggesting that the nature of the surfactant is the most important parameter in determining the kinetics of adsorption. We can offer no good explanation as to why C12E6 shows two-step kinetics in some cases and onestep kinetics in other cases. Bulk vs Step-by-Step Adsorption. As mentioned earlier, Wanless et al. reported that different adsorbed amounts were obtained if adsorption was allowed to occur in an increasing step-by-step fashion or a decreasing step-by-step fashion.26 We chose to examine nonequilibrium phenomena differently: is the amount of adsorbed surfactant obtained when a clean surface is exposed to surfactant solutions at concentrations above the cmc
Figure 8. Analysis of the kinetics of adsorption for CTAB adsorption on gold (b) and silica (O). In both cases, the bulk surfactant concentration is below the bulk cmc. The raw experimental data are shown in Figure 7.
different adsorption kinetics could be related to the structure of the surface aggregates (i.e., a difference in aggregate rearrangement kinetics on the surface). Table 2 suggests that C12E6 forms monolayer-like aggregates on both gold and silica. Thus, when a C12E6 micelle adsorbs on one of these surfaces it has to rearrange to yield the monolayer-like structures and vice versa: when a CTAB micelle adsorbs, it is not necessary for the surfactants to rearrange as much because CTAB yields bilayer-like structures on both gold and silica. The results in Figure 6 are shown for bulk concentrations below the cmc. Interestingly, the adsorption of both surfactants on gold can be represented by two consecutive steps, whereas the kinetics of adsorption on silica is qualitatively different for the two surfactants. C12E6 adsorbs following a linear mechanism, and CTAB shows a two-step mechanism. Interestingly, for both surfactants the initial adsorption is faster on gold than on silica, reflecting a stronger surface-surfactant attraction that is consistent with the larger amount of surfactant adsorbed at low bulk concentrations as manifested by the equilibrium adsorption isotherms shown in Figure 3 (panels C and D for CTAB and C12E6, respectively). The results shown in Figure 7 are for concentrations above the cmc. The adsorption of CTAB on either silica or gold surfaces follows very similar processes, except that the first adsorption step on gold is slightly faster than that on silica. On both surfaces, the adsorption of CTAB appears to be faster than that of C12E6, and it is characterized by two consecutive adsorption steps. C12E6 adsorption is always slow and linear as time elapses, although on silica there might be some small curvature. The slower adsorption observed for C12E6 may be a consequence of the fact that the cmc, and hence the actual concentrations, are more than 1 order of magnitude less than those of CTAB. To quantify the kinetics results just discussed, the data were fit with either a one-step zeroth-order or a two-step first-order model:
Table 3. Rate Constants Obtained by Fitting the Adsorption Models of Equations 2 and 3 to the Experimental Adsorption Data Shown in Figures 5-7a CTAB
C12E6
C < cmc gold
C > cmc silica
gold
C < cmc silica
gold
C > cmc silica
gold
silica
k0 (% min-1) N/A N/A N/A N/A N/A 0.018 ( 0.004 0.018 ( 0.002 0.016 ( 0.005 k1 (min-1) 0.64 ( 0.11 0.46 ( 0.26 0.82 ( 0.45 0.48 ( 0.02 0.25 ( 0.05 N/A N/A N/A k2 (min-1) 0.031 ( 0.006 0.035 ( 0.011 0.035 ( 0.003 0.051 ( 0.004 0.047 ( 0.004 N/A N/A N/A a
N/A stands for not applicable.
Surfactant Adsorption on Solid-Aqueous Interfaces
Figure 9. Comparison between adsorption amounts observed as the final value of a step-by-step adsorption isotherm (black) as opposed to that obtained from adding surfactant solution of concentration well above the cmc to a clean surface (white). The nominal surface area was used to calculate these adsorbed amounts; using actual surface areas eliminates differences between different surfaces for a given surfactant.
in one discrete step the same as that obtained when a clean surface is exposed to solutions of increasing surfactant concentration in a series of steps, with sufficient time allowed between each step to reach apparent equilibrium? To address this issue, the amount of C12E6 and CTAB surfactant adsorbed on gold and silica surfaces at the end of the adsorption isotherms (1.6(cmc)) shown in Figure 3 is compared in Figure 9 to the amount adsorbed when a clean solid surface (either gold or silica) is exposed to bulk surfactant solutions of concentration larger than the bulk cmc (1.4(cmc)). Our results indicate that the amount of surfactant adsorbed at the end of a step-by-step adsorption isotherm is larger than that adsorbed on clean surfaces from surfactant solutions above the bulk cmc (sometimes by as much as a factor of 2). Because QCM-D-estimated adsorbed amounts increase as the bulk concentration increases above the cmc (Figure 3), it is expected that at the end of an adsorption isotherm, when the bulk concentration is 1.6(cmc), the adsorption is larger than when the bulk concentration is 1.4(cmc). Thus, that differences are shown in Figure 9 is not unexpected. However, the magnitude of the differences is; in other words, the small difference in bulk concentration cannot explain the difference in the adsorbed amount. This result provides further evidence for the possibility that adsorbed surfactant aggregates are often observed in correspondence to kinetically trapped local minima of the free-energy landscape. It is surprising that, to our knowledge, such behavior has not been noticed in ellipsometry experiments, and the fact that this has not been observed suggests that surface roughness may have some influence on the observed difference. Additional studies are clearly necessary to clarify these observations. The total amount adsorbed is determined by a number of surface phenomena, including the rearrangement of adsorbed surfactants and possible templating effects provided by those surfactants adsorbed at low bulk concentrations for other surfactants that adsorb as the bulk concentration increases. These phenomena are likely to occur freely when the adsorbed amount increases slowly. Another possibility is that micelles are adsorbing directly onto the substrate, and the most likely explanation for the differences shown in Figure 9 is that the adsorbed micelles are not able to rearrange into the preferred equilibrium morphology on the time scale of the experiment. However, there is a definite inconsistency with kinetic results. The fact that the rate of adsorption for CTAB is the same above and below the cmc suggests that micelles do not adsorb directly onto the substrate. This inconsistency does not absolutely rule out the adsorbed
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Figure 10. Ratio between experimental dissipation and changes in the resonance frequency (d/f) as a function of time monitored during the adsorption of C12E6 and CTAB on gold from solutions at concentrations above the bulk cmc.
micelle explanation; however, we are unable to offer a definite cause for these results at the present time. To investigate how the rigidity of the surfactant aggregates changes as a function of time, in Figure 10 d/f is plotted versus time for the adsorption of C12E6 and CTAB on gold from aqueous solutions of concentrations larger than the bulk cmc. Not surprisingly, the results indicate that as the fractional surface coverage of surfactant increases the surface becomes more rigid. One observation from Figure 10 is that the structure of the C12E6 surface aggregates is less rigid than that of CTAB, which is in agreement with the step-by-step dissipation results shown in Figure 4. From a molecular point of view, this result could be a consequence of the fact that the head group of C12E6 is not as spherical as the head group of CTAB, thus C12E6 aggregates are necessarily less compact (and more flexible) than CTAB ones, or the difference in rigidity could be a consequence of a difference in morphology. A fast decrease in the d/f ratio occurs for CTAB adsorption during the time in which the fast adsorption step is observed in the kinetics experiments (less than 500-1000 s; Figure 7, left panel). The decrease in the d/f ratio during the time corresponding to the slow adsorption regime is almost negligible, perhaps indicating that the main aggregate structure forms during the fast adsorption step and the slow adsorption step corresponds to individual surfactants filling in where needed. For C12E6, in which case the kinetics of adsorption is slow and linear (Figure 7, left panel), the d/f ratio decreases slowly during the whole observation time, although again as the amount adsorbed increases the d/f decrease becomes slower. Hence, in both cases it seems as if the morphological structure that controls rigidity is set long before all of the surfactant is adsorbed.
4. Conclusions Adsorption isotherms are presented for aqueous solutions of C12E6 (nonionic) and CTAB (ionic) surfactants on gold and silica surfaces. AFM was employed to assess surface roughness, and XPS was employed to assess the chemical composition of the surfaces. XPS analysis indicates that on the gold surfaces significant numbers of oxygen atoms are always present, rendering these surfaces hydrophilic. Contact angle measurements confirmed these findings. When the atomic-scale roughness of the surfaces is considered, the equilibrium adsorption isotherms, measured at 25 °C and pH 7, are consistent with the formation of bilayer-like structures for CTAB and monolayer-like structures for C12E6 on both surfaces when the bulk surfactant concentration exceeds the critical micelle concentration, although our measurements do not allow us to discriminate between the formation of monolayers, hemicylindres, or hemispheres (thus the term monolayer-like aggregates) nor
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between bilayers, cylinders, or spheres (thus the term bilayerlike). Our results suggest that when the surface roughness is considered, QCM-D data do not significantly overestimate the adsorbed amount of surfactant as others have reported for CTAB. Thus, the effect of trapped solvent, often invoked to explain the differences between QCM and ellipsometry results, is not as significant as previously assumed in the case of surfactant adsorption. Our interpretation, when extended to results presented by others, suggests that the roughness might increase, decrease, or have no affect on the amount adsorbed; which one of the three possibilities occurs depends on the morphology of the aggregates. The QCM-D instrument also allowed a qualitative assessment of the kinetics of surfactant adsorption on the two surfaces. Two types of experiments were performed; one in which the clean surface is exposed to a solution with a concentration above the bulk cmc and another in which the clean surface is exposed to a solution with a concentration below the bulk cmc. By fitting the kinetics data with an analytical model, we identified two sequential steps that characterize the adsorption of CTAB. The first, fast, first-order step is followed by a slower one that we attribute to the reorganization of the surfactant molecules on the surfaces. This second slower step is necessary to create room for additional surfactant molecules that are adsorbing on the solid substrates. Differences in the first step for the different surfaces were consistent with differences in the amount adsorbed below the cmc. For C12E6, the kinetics of adsorption on gold slows significantly when the bulk concentration increases from below
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to above the critical micelle concentration, indicating that the presence of the micelles slows adsorption. The adsorption of C12E6 on silica was identical for the two concentrations. Finally, our results show that when adsorption occurs on a clean surface from bulk concentrations above the cmc, the amount adsorbed after 1 h of exposure to the surfactant solution is much less than that obtained by gradually increasing the bulk surfactant concentration and waiting for the apparent equilibrium adsorption at the surface at each intermediate concentration. This result indicates that the structure of surface aggregates may be kinetically trapped in some local minima of the free-energy landscape. The rigidity of structures from both surfactants evolves quickly and then settles, slowly, to some value. This result suggests that the morphological features that determine rigidity are set long before surfactant adsorption is complete. Acknowledgment. This work was financially supported, in part, by the Oklahoma State Regents for Higher Education and by the Vice President for Research at the University of Oklahoma, Norman. We thankfully acknowledge Jeff Harwell, Lubica Macakova, Guruswamy Kumaraswamy, Scott Perry, and Robin Curtis for fruitful discussions; Paolo Solda`, Leann Johnson, Sam Noor-Mohammadi, Nina Wright, Andrea Dal Cin, and Chong Liang for their help in setting up the adsorption experiments; Manuel Ghezzi for performing contact angle measurements; and Yongqiang Tan for conducting AFM measurements. LA800035W
