pubs.acs.org/langmuir © 2009 American Chemical Society
Complex Adsorption Behavior of Rodlike Polyelectrolyte-Surfactant Aggregates Chris S. Hodges,*,† Simon Biggs,† and Lynn Walker‡ †
‡
Institute of Particle Science and Engineering, University of Leeds, Leeds, United Kingdom LS2 9JT and Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received October 10, 2008. Revised Manuscript Received December 19, 2008
A quartz crystal microbalance (QCM) and an optical reflectometer have been used to quantify the long-term adsorption behavior of polyelectrolyte-surfactant aggregates of alkyltrimethylammonium and poly(4-vinylbenzoate) or pCnTVB at the silica-water interface. In solution, these polyelectrolyte-surfactant aggregates exist as weakly anionic semiflexible rodlike structures of several nanometers in radius and hundreds of nanometers in length. The optical reflectivity (OR) data confirmed our earlier proposed model of a two-stage adsorption process (Biggs, S.; Kline, S. R.; Walker, L. M. Langmuir, 2004, 20 (4), 1085-1094) where free CTA+ ions initially adsorb and charge reverse the silica surface, thus allowing the weakly anionic aggregates to adsorb. Combining data from the two techniques allows a distinction to be made between contributions to the measured signal from the bulk and the interface. The isotherm determined by OR showed a clear plateau at higher concentrations, whereas the isotherm obtained by QCM continues to increase across all concentrations tested. This indicates a significant influence of the bulk fluid on the measured signals from the QCM as the concentration is increased. Slow changes in the apparent adsorbed mass observed with the QCM were not reproduced in the OR data, suggesting that these effects were also caused by the bulk and were not a densification of the adsorbed layer. The combination of techniques clarifies the adsorption kinetics and mechanism of adsorption in polyelectrolyte-surfactant aggregate systems.
Introduction Many polymers can be easily spin coated onto a wide variety of surfaces, leading to a relatively low cost and efficient method of film deposition. Furthermore, using selective solvents and block copolymers can provide a route to surface coatings with complex nanoscale features. However, a number of limitations on this technology hinder its widespread application. Among these are the ability to rapidly coat very large surface areas, the use of harmful solvents, and difficulties in employing such films for multistep wet synthesis protocols. There is a need, therefore, to find alternative systems that retain the nanoscale ordering seen in these films but which provide alternative processing and application opportunities, especially aqueous based systems. The capacity of small-molecule surfactant films on surfaces to form nanoscale structures of different types, depending on the surface charge and surfactant concentration, has been widely reported.2,3 These surface structures are related to the structures reported in the bulk, although the exact details of the interplay between the two is not yet clear. The advantages of using such self-ordering layers at a commercial scale are obvious in both the time and cost saved. In addition, such systems offer the possibility to coat complex surface shapes as well as particles. A key disadvantage for such surfactant systems is the requirement for a relatively high bulk concentration to be maintained at all times in the bulk. Also, the fact that the structures formed are strongly dependent on the *To whom correspondence should be addressed. E-mail: c.s.hodges@ leeds.ac.uk. (1) Biggs, S.; Kline, S. R.; Walker, L. M. Langmuir 2004, 20(4), 1085–1094. (2) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103(3), 219–304.
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surface charge and pH of the solution requires fine control over deposition conditions. Recently, we have examined the possibility of using an aqueous polyelectrolyte-surfactant (PES) aggregate system formed through the polymerization of wormlike micelles as a surface coating. While generated through polymerization of a surfactant system, the results are general to other preparations of PES aggregates. These aggregates have the potential to provide robust surface structures at relatively low bulk concentrations. Our previous publications1,4,5 showed that these analogues did indeed remain stable even after rinsing and that repeatable structures formed uniformly over a silica or mica surface. The published quartz crystal microbalance (QCM) data showed that these systems readily adsorb to silica and indicated that there may be some longer time rearrangement of the aggregates at the interface. The aggregates studied here are prepared by polymerization of a purified complex salt (CnTVB) made up of a cationic surfactant and anionic polymerizable counterion. The system is polymerized using an aqueous phase initiator to yield the product (pCnTVB), where n denotes the number of carbons in the surfactant alkane tail. Detailed explanations of all steps involved in this synthesis are available in previous publications.6-9 The result of the polymerization is a stable (3) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5(1-2), 88–94. (4) Biggs, S.; Labarre, M.; Hodges, C.; Walker, L. M.; Webber, G. B. Langmuir 2007, 23(15), 8094–8102. (5) Biggs, S.; Walker, L. M.; Kline, S. R. Nano Lett. 2002, 2(12), 1409–1412. (6) Gerber, M. J. The Characterization of Polymerized Worm-like Surfactant Micelles. Ph.D. Thesis, Carnegie Mellon University, Pittsburgh, PA, 2006. (7) Gerber, M. J.; Kline, S. R.; Walker, L. M. Langmuir 2004, 20(20), 8510–8516. (8) Kline, S. R. Langmuir 1999, 15(8), 2726–2732. (9) Kuntz, D. M.; Walker, L. M. J. Phys. Chem. B 2007, 111(23), 6417–6424.
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polyelectrolyte-surfactant aggregate. We have shown that the same structure is generated by mixing premade polyelectrolyte (pVB) and surfactant (cetyltrimethylammonium bromide, CTAB), indicating the robustness of the aggregate structure. The advantage of polymerizing from the complex salt is that the charge groups on the polyelectrolyte and the oppositely charged surfactant are present in a 1:1 molar ratio and there are no other small molecule salts (i.e., NaCl) remaining in solution. In aqueous solution, polyelectrolytes (PEs) and surfactants (S) are driven by a combination of electrostatic, hydrophobic, and steric interactions that associate to form PES aggregates. In many oppositely charged PES systems, these associative interactions lead to the formation of highly nanostructured precipitates,10-14 gel phases,15 and immiscible liquid phases (coacervation)16-22 at the 1:1 molar stoichiometry ratio (with respect to charge) of the complexes. In other systems, the formation of stable “beads-on-string” structures useful in solubilization in aqueous media is observed.13 The PES aggregates used here fall between these limits and form rodlike aggregates that are stable in solution; the aggregates are semiflexible cylinders with diameters defined by the tail length of the surfactant used (4-8 nm) and lengths ranging from tens to hundreds of nanometers, a dimension easily controlled through the polymerization conditions.21 The aggregates maintain amphiphilic behavior,22 can be concentrated into liquid crystalline solutions,23 and have been used to make structured layers at solid-liquid interfaces.1,4,5 The surfactant (CnTA+) in the aggregates is able to partition into the bulk aqueous phase, which has been quantified;24 this partitioning gives the aggregates a weakly negative net charge. Our earlier work4 demonstrated that the PES adsorption increases rapidly to a plateau, whose value depends on the bulk concentration. Rinse-off data from the QCM for the same systems showed that the adsorbed aggregates remained on the surface, even though the bulk was removed. The degree of absorbed water within the polymer surface layer was not investigated in this study. Preliminary evidence for some longer-term effects in the adsorbed layer at higher concentrations was suggested by the QCM data, although this was not investigated in great detail. Comparable AFM data demonstrated that rodlike structures of significant length adsorbed readily onto silica from an aqueous solution and maintained (10) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. 1994, 33(18), 1869– 1870. (11) Antonietti, M.; Conrad, J.; Thunemann, A. Macromolecules 1994, 27 (21), 6007–6011. (12) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15(9), 673–683. (13) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993; p 427. (14) La Mesa, C. J. Colloid Interface Sci. 2005, 286(1), 148–157. (15) Wang, C.; Tam, K. C.; Jenkins, R. D.; Tan, C. B. J. Phys. Chem. B 2003, 107(19), 4667–4675. (16) Bai, G. Y.; Nichifor, M.; Lopes, A.; Bastos, M. J. Phys. Chem. B 2005, 109(1), 518–525. (17) Morishima, Y.; Mizusaki, M.; Yoshida, K.; Dubin, P. L. Colloids Surf., A 1999, 147(1-2), 149–159. (18) Michaeli, I.; Overbeek, J. T. G.; Voorn, M. J. J. Polym. Sci. 1957, 23 (103), 443–450. (19) Mohanty, B.; Bohidar, H. B. Int. J. Biol. Macromol. 2005, 36(1-2), 39–46. (20) Overbeek, J. T.; Voorn, M. J. J. Cell. Physiol. 1957, 49(Suppl 1), 7–22. (21) Gerber, M. J.; Walker, L. M. Langmuir 2006, 22(3), 941–948. (22) Lindsey, M. W. Pyrene and Nile Red Solubilization and Adsolubilization in Polymerized Micelles. Ph.D. Thesis, Carnegie Mellon University, Pittsburgh, PA, 2004. (23) Kuntz, D. M.; Walker, L. M. Soft Matter 2008, 4(2), 286–293. (24) Kuntz, D. M.; Walker, L. M. Ind. Eng. Chem. Res. In press.
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the nanoscale structure of the aggregates in bulk solution.1,5 It was suggested, therefore, that the slower trends visible in the QCM data might be caused by a densification or conformational rearrangement of the polymer layer at the interface. In this Article, we explore the adsorption of these PES aggregates at the silica-aqueous solution interface in more detail. In particular, we investigate these previously observed slow changes within the adsorbed layer. We examine this behavior by comparing longer time QCM data with optical reflectivity (OR) data. OR is sensitive to only the interfacial adsorption, whereas QCM is also sensitive to changes in the bulk fluid properties as well as the interfacial adsorption,24 which we will show to be important for the long time portion of the adsorption. Previous adsorption studies of surfactant solutions demonstrated the sensitivity of the QCM to an adsorbed monolayer.25-30 In particular, the work by Macakova et al.30 compared the CTAB adsorbed amount with the amount obtained from OR measurements on the same system at a concentration above the cmc. They found that the optical reflectometer always recorded smaller values of adsorbed mass, which is not related to the roughness of the surface caused by the micelles adsorbed onto it, but instead is caused by the amount of absorbed water mechanically trapped within the adsorbed layer as well as the presence of any water of hydration. Optical reflectivity measurements on polymers and surfactant-polymer systems have been performed quite recently by Buron et al.,31 who compared data for poly(dimethylaminoethyl methacrylate chloride) and poly(acrylic acid) with QCM results. They describe difficulties in converting the QCM measurements into adsorbed masses due to the significant quantity of absorbed water in the multilayers. Interestingly, these authors used the QCM data to extract the OR sensitivity factor for their measurements. Velegol and Tilton32 examined the coadsorption of poly-L-lysine with cetyltrimethylammonium bromide (CTAB) on silica using OR and found that in salt the poly-L-lysine inhibited the CTAB adsorption significantly. The optical reflectometer was easily able to distinguish between the presence or absence of poly-Llysine even though the difference was only 0.2 mg/m2. Kovacevic et al.33 adsorbed multilayers of poly(2-vinyl-N-methylpiridinium iodide) (PVP) and bovine serum albumin (BSA) onto silica in different buffers and observed sequential layer build-up as each layer charge reversed the surface. The BSA showed evidence of a slow relaxation process, whereas the PVP absorbed much more quickly. In a similar manner, we expect to uncover more detailed behavior of our aggregates by comparing QCM and OR data to further elucidate the kinetics of these systems. (25) Boschkova, K.; Feiler, A.; Kronberg, B.; Stalgren, J. J. R. Langmuir 2002, 18(21), 7930–7935. (26) Soares, D. M.; Gomes, W. E.; Tenan, M. A. Langmuir 2007, 23(8), 4383–4388. (27) Naderi, A.; Claesson, P. M. Langmuir 2006, 22(18), 7639–7645. (28) Stalgren, J. J. R.; Eriksson, J.; Boschkova, K. J. Colloid Interface Sci. 2002, 253, 190–195. (29) Knag, M.; Sjoblom, J.; Oye, G.; Gulbransen, E. Colloids Surf., A 2004, 250, 269–278. (30) Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23(24), 12436–12444. (31) Buron, C. C.; Filiatre, C.; Membrey, F.; Perrot, H.; Foissy, A. J. Colloid Interface Sci. 2006, 296, 409–418. (32) Velegol, S. B.; Tilton, R. D. Langmuir 2001, 17(1), 219–227. (33) Kovacevic, D.; Glavanovic, S.; Peran, N. Colloids Surf., A 2006, 277, 177–182.
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Materials and Methods All water for these experiments was from a Milli-Q Elix RiOs Synthesis A10 system (measured resistivity was 18.2 MΩ cm). The silica substrates used for the QCM work were cleaned as described previously.4 The QCM used here was a Q-Sense D300 (Scientific and Medical Instruments Ltd., U.K.). This QCM has a sample cell with a required liquid volume of 80 μL and is capable of measuring both the frequency of the silica-coated quartz crystal at four harmonics as well as the dissipation at each harmonic. The QCM frequency shifts observed in this paper were converted into adsorbed masses using the Sauerbrey relation34 Δm = -(C/n)Δf, where Δm is the mass change from adsorption, Δf is the frequency change measured, n is the overtone number, and C is the crystal sensitivity given by (Fqνq)/(2f20), where Fq and νq are the density and shear modulus of the quartz crystal and f0 is the fundamental resonant frequency of the unloaded crystal. In our case, C = 18 ng cm-2 Hz-1. The optical reflectometer (purchased from the Laboratory of Physical Chemistry and Colloid Science at the Wageningen University, Netherlands) has a polarized red He-Ne laser operating at 632 nm that is incident onto a silicon wafer near to the Brewster angle. The laser is reflected back toward a pair of photodetectors mounted at right angles to each other. If the surface is modified in any way, for example, by an adsorbed layer, the polarization of the reflected laser beam will be modified and this change will be picked up by the photodetectors. Perpendicularly to the silicon wafer, a flow of liquid enters the cell. Once the flow has been established, a stable cone of liquid forms near the silicon surface, within which there exists a stagnant point where the fluid motion is diffusion limited. The silicon wafers for the optical reflectometry work (Silicon Valley Microelectronics Inc.) are cut to show the 100 crystal face and had a 115 nm thermally deposited oxide layer on the surface. A piece of wafer approximately 1 cm in width and 3 cm in length was cut from this wafer for experiments. These pieces were then treated with a UV/ozone cleaner for 15 min and thoroughly rinsed with water before being inserted into the optical reflectometer. The chamber of the optical reflectometer was cleaned by repeatedly filling the cell with Decon 90 solution and then thoroughly rinsing the cell with water. This process was carried out four times. The voltage changes recorded from the optical reflectometer were converted into adsorbed masses using Γ = (ΔS/ S)As, where ΔS is the recorded voltage change in the initial set voltage, S, and S = Ip/Is, that is the ratio of the parallel to the perpendicular polarized components of the reflected laser light. As is a sensitivity coefficient calculated from a four-layer optical model,35 and for our instrument this was found to be 38.8 mg m-2. The surfactant/counterion pair, alkyltrimethylammonium 4-vinylbenzoate (CnTVB), used for this work is synthesized from commercially available cetyltrimethylammonium bromide (C16TAB (98% purity, BDH Limited, Poole, U.K.) or from octadecyltrimethylammonium chloride (C18TAC, Arquad 18-50, donated from Akzo-Nobel, Chicago, IL) via two counterion exchange steps. After separation and purification, the pure complex salt CnTVB is polymerized using an aqueous phase initiator to yield the pCnTVB product. Detailed explanations of all steps involved in this synthesis are available in previous publications.6-9 The dimensions of the aggregates used in this work are determined via scattering. The diameters are found using small-angle neutron scattering (SANS; experiments performed at the NIST Center for Neutron Research, Gaithersburg, MD) and are reported as 4.1 nm for the n = 16 surfactant and 4.5 nm for the n = 18 surfactant.26 Lengths are determined from a combination of static light scattering (Brookhaven BI-200SM, Huntsville, NY) and SANS; the average aggregate length is roughly 165 nm for both the n = 16 and n = 18 aggregates, measured using light scattering and assuming that the aggregate is a rigid rod. The aggregates are semiflexible, and the persistence (34) Sauerbrey, G. Z. Phys. 1959, 155(2), 206–222. (35) Dijt, J. C.; Stuart, M. A. C.; Fleer, G. J. Adv. Colloid Interface Sci. 1994, 50, 79–101.
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lengths were determined to be 110 nm for the pC16TVB and 349 nm for the pC18TVB, so the rigid rod assumption is reasonable for these samples.
Results and Discussion 1,4,5
Previously, we investigated the adsorption behavior of pCnTVB on silica using a QCM, and isotherms for two different aggregate solutions (pC16TVB and pC18TVB) were presented. The primary difference between these two is the tail length of the surfactant in the aggregates: the additional two carbons in the surfactant template of pC18TVB increase the aggregate diameter. It was demonstrated that after removing the bulk solution and rinsing with water only a small amount of the adsorbate was removed, even after multiple successive rinses. We also reported evidence for slower adsorption and/ or conformational changes. Here, we present a more detailed study of these longer time effects, across a wider range of aggregate concentrations. Figure 1 shows the isotherms obtained by the QCM for (a) pC16TVB and (b) pC18TVB solutions. These isotherms for the aggregates were allowed to settle for a longer time period than that in our previous work,4 permitting examination of any reorganization at the surface, something that we expected to be important in these systems. A two-stage isotherm is observed for both aggregates. In the case of the pC16TVB aggregates, a rapid increase in adsorbed mass up with concentration is seen up to a small plateau, followed by another significant increase in adsorbed mass with concentrations above 0.2 mg/mL. It is probable that this second increase in the isotherm is due to bulk property changes of the aggregate solution rather than any further adsorption occurring. This hypothesis is confirmed by examining the rinse-off data also shown in Figure 1a. Below 0.1 mg/mL, there is virtually no mass removed after rinsing, whereas above this concentration the amount removed increases in proportion to the apparent increase in mass shown in the isotherm, settling at a value of about 2.5 mg/m2. This tends to suggest that below 0.1 mg/mL a monolayer of aggregates forms on the silica surface that cannot be removed easily by rinsing with water alone. Once the silica surface is completely covered by a monolayer of pC16TVB aggregates, any excess aggregates reside above this layer but are not bound to it, possibly forming a loose multilayer of aggregates. Thus when water is passed through a pC16TVB solution at a concentration higher than 0.2 mg/mL, the excess material lying above the monolayer of aggregates is easily washed away. Even at lower concentrations where only partial surface coverage occurs, the bond between the silica and the pC16TVB is much stronger than the driving force to break it created by diluting the bulk solution. Similar results are seen for the pC18TVB solutions shown in Figure 1b. The isotherm again shows two stages of different behavior, with complete coverage of the silica surface occurring at a concentration of 0.025 mg/mL, a similar value to 0.05 mg/mL observed for pC16TVB. The amount of material adsorbed in a monolayer of aggregates (the mass that remains above 0.025 mg/mL but after rinsing) is 3 mg/m2, approximately the same as that observed in the case of pC16TVB. In both cases, the rinse-off steps are reported in number of rinses, as this is a batch process in the QCM with each step representing a replacement of 30 cell volumes of bulk solution with clean water. The corresponding dissipation data for each of these polymers was also recorded (not shown), and the data showed very similar shaped responses to the frequency. Langmuir 2009, 25(8), 4484–4489
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Figure 1. Isotherms obtained from the QCM at 25 °C for (a) pC16TVB and (b) pC18TVB solutions Also shown are the QCM rinse-off data where water was successively passed through the QCM cell.
Figure 2. OR and QCM isotherms comparing (a) pC16TVB and (b) pC18TVB. Also included are the QCM data after the fourth rinse with water (triangles).
Figure 2 compares isotherms determined using the QCM (same data shown in Figure 1) and OR. The OR isotherm data in Figure 2 either match the QCM data or lie well below them for both aggregates solutions. The separation between the OR and the QCM data occurs at approximately the same points described earlier. Again here in Figure 2 we notice that at the higher concentrations the OR data are more or less flat for both aggregates, whereas the QCM data behave quite differently for each system. The greatest difference between the two techniques is seen at higher concentrations where we argue that either multilayers have formed or the QCM signal is sensitive to the bulk properties (viscosity and density) of the solution. The OR data are insensitive to these bulk changes, and hence, once the entire silica surface is covered, no further change in the OR signal occurs. A more direct comparison between the two techniques may be made by comparing the QCM data after the fourth rinse with water. After these rinses, the QCM data for both aggregate systems take on the general shape of the OR isotherm. This is strong evidence for the QCM being influenced by the bulk properties of the solution. Also, even after multiple rinses, the QCM data still do not agree with the OR data at the higher concentrations, suggesting that there is some entrapped water within the adsorbed layer. If we assume that this is the case, the percentage of entrapped water may be estimated by taking a simple ratio of the OR data at high concentration to the QCM data at the same concentration. Thus, we estimate that a complete film of pC16TVB can hold 28% entrapped water, compared with up to 40% entrapped water for pC18TVB. The data presented in Figures 1 and 2 were taken after the adsorbed film had stabilized to a fixed value. To investigate the adsorption kinetics, we examine the behavior of each Langmuir 2009, 25(8), 4484–4489
concentration with time. This is demonstrated for pC16TVB in Figure 3 for both QCM and OR data. Figure 3 shows the QCM data for the pC16TVB compared with the corresponding OR data. Figure 3a-d are in the first stage of the isotherm, while Figure 3 d and e are at concentrations in the second stage of the isotherm shown in Figure 1a. At 0.01 mg/mL, both the OR and QCM data indicate very little adsorption. This suggests that the concentration of CTA+ ions in solution is too low to effectively charge reverse the surface, and so no adsorption of the anionic aggregates can occur. At both 0.03 and 0.05 mg/mL, the OR data show a slow increase over approximately a 30 min period to the same value of adsorbed mass as detected from the QCM. Both the QCM and OR data then show no further change in the detected mass until the rinse-off is carried out, where both techniques give a similar degree of mass removal from the interface. At higher concentrations, significant differences between the OR and QCM measurements are apparent. It is significant that the OR data do not show any increase in surface excess above 0.05 mg/mL. From these data, we may conclude that surface coverage for pC16TVB is complete at this concentration. Note that while the OR data at high concentrations remain stable, the QCM signal behaves differently at each concentration, showing a gradual increase with time at 0.075 mg/mL, a stable value at 0.1 mg/mL, and a gradual decrease at 0.5 mg/mL. As suggested above, this is likely to be due to the QCM probing both absorbed water within the adsorbed aggregate layer as well as possible bulk solution property changes. Since these traces are only detected by the QCM, it is likely that they represent different stages of bulk behavior as the aggregate concentration is increased, or that the coupling between the bulk and the interface is being altered. The fact that the DOI: 10.1021/la8033534
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Figure 3. QCM adsorption data, compared with OR data for pC16TVB on silica at (a) 0.01 mg/mL, (b) 0.03 mg/mL, (c) 0.05 mg/mL, (d) 0.075 mg/mL, (e) 0.1 mg/mL, and (f) 0.5 mg/mL.
Figure 4. QCM adsorption data compared with OR surface excess data for pC18TVB at (a) 0.0025 mg/mL, (b) 0.005 mg/mL, (c) 0.01 mg/mL, (d) 0.03 mg/mL, (e) 0.1 mg/mL, and (f) 0.2 mg/mL.
measurements show a different level of mass adsorbed after rinse-off for the three highest concentrations is also interesting, since it probably shows the relative amount of entrapped water within the remaining adsorbed aggregates. The corresponding data for pC18TVB are shown in Figure 4. The OR data for pC18TVB show a similar trend to that shown for pC16TVB in Figure 3. Above 0.03 mg/mL, no further change in the OR signal size is observed, in agreement with the onset of the plateau of the isotherm for this aggregate shown in Figure 1b. These OR data are also very stable over several hours. By comparison, the QCM data rise to a steady plateau and remain there over the duration of the experiment up to 0.03 mg/mL where the QCM data exhibit more 4488
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complicated behavior, in a similar way to pC16TVB. The relaxation behavior seen at and above 0.03 mg/mL in the QCM data appears to act more slowly as the concentration is increased. As suggested for pC16TVB, these slow changes in the QCM data at high concentrations occur over the plateau region of the isotherm (compare Figure 1), and may therefore represent bulk property changes and/or a difference in the coupling between the interface and the bulk. In Figure 5a, the relative behavior of each of the different adsorption experiments becomes clear. In each case, the system is allowed to reach a stable state and then fresh water is flowed through the cell at several injections to characterize the rinse-off behavior. It is more obvious that each of the three Langmuir 2009, 25(8), 4484–4489
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Figure 5. (a) QCM kinetic adsorption data for pC16TVB at 0.01 mg/mL (black), 0.05 mg/mL (red), 0.1 mg/mL (green), 0.2 mg/mL (blue), 0.3 mg/ mL (cyan), and 0.5 mg/mL (magenta). (b) The square root of the difference between the first and the seventh harmonics for the QCM data after stabilization at each concentration is plotted in (a).
Figure 6. (a) QCM adsorption for pC18TVB at 0.0025 mg/mL (black), 0.01 mg/mL (red), 0.02 mg/mL (green), 0.025 mg/mL (blue), 0.03 mg/mL (cyan), and 0.05 mg/mL (magenta). (b) The square root of the difference between the first and the seventh harmonics for the QCM data after stabilization at each concentration is plotted in (a). highest concentrations rinses back to exactly the same value of Sauerbrey mass of 3 mg/m2 and that the amount of rinse-off is much larger than the lower concentrations demonstrate. The reason for this is shown in Figure 5b, where the square root of the difference between the first and the seventh harmonics of the QCM data after stabilization is plotted at each concentration. If there exists any deviation from the thin film assumption of Sauerbrey, then fn/n - fm/m will be nonzero, where fn and fm are raw frequencies at harmonics n and m, respectively. When these data are plotted, a parabolic curve shape is obtained because the response of the bulk liquid on the QCM frequency is proportional to (Fηn)1/2 from the theory put forward by Kanazawa and Gordon.36 If instead one plots (fn/n - fm/m)1/2, a linear response will be seen if the QCM is only responding to the bulk polymer solution properties. This is observed for the three highest concentrations plotted. Figure 6 shows the results of similar experiments for the pC18TVB. In the same way as the QCM data for pC16TVB, the higher concentration pC18TVB data show a rinse-off to the slightly higher value of 3.5 mg/m2. The slower relaxation data presented in Figure 6a also appear to be largely from bulk liquid changes, as is clear from Figure 6b. Note that the gradient for the fitted linear portion of Figure 6b is much steeper (71 Hz-0.5 mL mg-1) than that found for the fitted portion of Figure 5b for pC16TVB (4 Hz-0.5 mL mg-1). This indicates either that the density and/or viscosity of the (36) Kanazawa, K. K.; Gordon, J. G. Anal. Chim. Acta 1985, 175, 99–105.
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bulk is significantly different for these two aggregate solutions or that the degree of coupling with the interface is different.
Conclusions The slow adsorption kinetics of pC16TVB and pC18TVB has been examined with both quartz crystal microbalance (QCM) and optical reflectivity (OR). The results of this work confirmed our earlier work, showing a two-stage adsorption process. The rapid adsorption of the CTA+ ions occurs so quickly that even the OR data show an almost immediate rise in surface excess, followed by a slower rise that we assume is from the slower diffusive adsorption of the aggregates. The OR data presented here demonstrate that the slow changes in the QCM data at higher concentrations are from bulk behavior, rather than conformational rearrangement at the interface itself. In fact, the OR data show that once these films of rodlike aggregates have been formed, they are stable over at least several hours. The isotherms presented here confirm more clearly that any excess aggregates may be easily rinsed off to a level consistent with a monolayer of aggregates at the interface. This is also confirmed by the OR data, but the relative adsorbed amounts differ since the QCM is sensitive to the significant amount of water (up to about 40% of the QCM signal) included into the adsorbed layer. These aggregates provide a different approach to generating nanostructured surfaces, since the structure of the aggregates is controlled in the bulk and not at the surface. This offers a robust method for forming structured layers without concern for complex control of bulk solution properties. DOI: 10.1021/la8033534
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