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Adsorption of a Cationic Polyelectrolyte followed by Surfactant-Induced Swelling, Studied with a Quartz Crystal Microbalance Mark A. Plunkett,* Per M. Claesson, and Mark W. Rutland Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44, and The Institute for Surface Chemistry, PO Box 5607, SE-11486 Stockholm, Sweden Received September 13, 2001. In Final Form: November 21, 2001 The adsorption and subsequent surfactant-induced swelling of a 10% charged cationic polyelectrolyte (AM-MAPTAC-10), on a gold surface, was monitored by means of a quartz crystal microbalance with dissipation (QCM-D). This instrument gives information on the total adsorbed amount including any adsorbed solvent and on the manner of adsorption. In this case the total adsorbed amount from a 20 ppm AM-MAPTAC-10 solution registered by the QCM-D device was approximately 0.6 µg cm-2. X-ray photoelectron spectroscopy results showed that the polyelectrolyte adsorbed mass was 0.16 µg cm-2; thus the water trapped within the polyelectrolyte layer constitutes about 70% of the mass measured by the quartz crystal microbalance. The adsorption process was found to be rather complex, though the time evolution of the adsorbed mass indicated that the majority of the process was diffusion controlled. Toward the end of the adsorption process, the rate of adsorption drops off and the dissipation rate increases, indicating that as the surface becomes crowded the layer extends further in the direction normal to the surface. The effect of addition of sodium dodecyl sulfate (SDS) to a preadsorped AM-MAPTAC-10 layer was also investigated. It was found that some swelling of the preadsorbed layer occurred once the bulk surfactant concentration reached 20% of the critical micelle concentration (cmc). Between 60% of the cmc and twice the cmc, the adsorbed layer swelled significantly and desorption started to occur. Rinsing the surface with the surfactant-free electrolyte solution results in a rapid decrease in dissipation and adsorbed mass indicating the removal of the surfactant but not the polyelectrolyte.
Introduction Polyelectrolytes and oppositely charged surfactants associate readily in bulk aqueous solutions, displaying a wide range of structural, physical, and chemical properties (see for example ref 1). This richness can be utilized for many applications, including, for example, rheological modifiers, emulsifiers, and wash agents. The interaction between the polyelectrolyte and surfactant is mainly driven by the combination of electrostatic and hydrophobic interactions.2 Additional parameters such as the polyeletrolyte chain stiffness3 may also affect polymer conformations. Interactions between polyelectrolyte-surfactant complexes and surfaces are even more complex since they also depend on the chemical nature of the interface. In this area, the scientific literature is still rather limited and at this stage it is not possible to formulate detailed rules for how the adsorbed layer properties depend on polyelectrolyte and surfactant structure, solution concentration of inorganic salt, and surface charge and hydrophobicity. However, some information can be found in a recent review.4 Measurements of surface interactions in systems containing polyelectrolyte/surfactant mixtures have been conducted which show the effect of addition of surfactants to the structure of preadsorbed polyelectrolyte layers5-7 * To whom correspondence may be addressed. E-mail:
[email protected]. (1) Goddard, E. D.; Ananthapadmanabhan K. P. Interactions of surfactants with polymers and proteins; CRC Press: Boca Raton, FL, 1993. (2) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. W. Phys. Rev. Lett. 1998, 80, 5357-5360. (3) Gong, J. P.; Osada, Y. J. Phys. Chem. 1995, 99, 10971-10975. (4) Claesson, P. M.; Dedinaite, A.; Poptoshev, E. In Physical chemistry of polyelectrolytes; Radeva, T., Ed.; Marcel Dekker: New York, 2001; Vol. 99, p 881.
and of adsorbing preformed polyelectrolyte-surfactant complexes. In particular it is shown that preadsorbed polyelectrolyte layers can be spontaneously swelled with the addition of surfactant, resulting in a dramatically increased range of the surface forces. Recent bulk smallangle neutron scattering (SANS) measurements8 and X-ray scattering measurements (unpublished data) have shown that the internal structures of the complexes formed in bulk solution are highly dependent on the polyelectrolyte charge density and polyelectrolyte/surfactant ratio. Highly charged cationic polyelectrolytes and sodium dodecyl sulfate (SDS) form complexes with internal hexagonal or lamellar structures when the polyelectrolyte/surfactant ratio is close to 1. On the other hand, the structures formed by SDS and AM-MAPTAC-10 (a low charge density polyelectrolyte) cannot be described by such an internal structure. For highly charged cationic polyelectrolytes, the characteristic distance found in the SANS measurements is in good agreement with the periodicity of oscillations found in surface force data,8 suggesting a similar internal structure of complexes formed in bulk and at the solid-liquid interface. The adsorption of polyelectrolytes to surfaces, followed by surfactant association, is an important process that has yet to be investigated fully, though some studies have been conducted.7,9 Of particular interest is the work of Shubin et al.,9 where they follow polyelectrolyte-surfac(5) Claesson, P. M.; Fielden, M. L.; Dedinaite, A. J. Phys. Chem. B 1998, 102, 1270-1278. (6) Claesson, P. M.; Dedinaite, A.; Fielden, M.; Kjellin, M.; Audebert, R. Prog. Colloid Polym. Sci. 1997, 106, 24-33. (7) Claesson, P.; Dedinaite, A.; Blomberg, E.; Sergeyev, V. G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 1008-1013. (8) Claesson, P. M.; Bergstro¨m, M.; Dedinaite, A.; Kjellin, M.; Legrand, J. F.; Grillo, I. J. Phys. Chem. B 2000, 104, 11689-11694. (9) Shubin, V.; Petrov, P.; Lindman, B Colloid Polym. Sci. 1994, 272, 1590-1601.
10.1021/la015573j CCC: $22.00 © 2002 American Chemical Society Published on Web 01/23/2002
Adsorption and Swelling of Polyelectrolytes
Figure 1. Line bond (or Kekule´) structures of AM (left) and MAPTAC (right) monomers.
tant association at the solid-liquid interface, using both the surface forces technique and null ellipsometry. In this case they monitor the adsorbed amount and find that there is association of surfactant to preadsorbed polyelectrolyte layers. Above the critical micelle concentration (cmc) they find that the polyelectrolyte is desorbed from the surface by the surfactant. The quartz crystal microbalance with dissipation (QCMD)10 is a technique that allows submonolayer adsorption to be quantified.11 Additionally, with the simultaneous measurement of dissipation rates, the technique is able to give information not only on the adsorbed amount but also on the structure and viscoelastic nature of the layer. Since incorporation of surfactant into a preadsorbed polyelectrolyte layer changes not only the mass of the adsorbed layer but equally importantly the structure, the QCM-D technique is seen as an important technique for studying adsorption of polymers and subsequent swelling and deswelling process induced by changes in solvent composition or temperature or by additives. The latter aspect is considered in the present paper where an anionic surfactant is introduced in the solution contacting a gold surface precoated with polyelectrolyte. It should be noted that since the quartz crystal microbalance (QCM) technique in general does not distinguish between adsorbed species and trapped solvent, it is important to determine the effect of trapped solvent on the QCM response. Materials and Methods Materials. The polyelectrolyte used in this study was a random copolymer of approximately 90% uncharged acrylamide (AM) and 10% positively charged [3-(2-methylpropionamide)propyl]trimethylammonium chloride (MAPTAC), henceforth referred to as AM-MAPTAC-10 (Figure 1). It was obtained as a gift from Laboratoire de Physico-Chemie Macromoleculaire, Universite´ Pierre et Marie Curie, Paris. The analysis of the AM-MAPTAC10 sample with various techniques has given the following values for the MAPTAC fraction: NMR measurements gave 8.8 and 9.5%, elemental analysis 10%, and potentiometry 9%. The molecular weight of the polymer was determined to be on the order of 106 g/mol, giving a degree of polymerization of 11 600 (approximately 1160 charges per molecule). The surfactant used was sodium dodecyl sulfate (SDS, BDH, 99%), and the salt used to control the electrolyte concentration (10) Rodahl, M.; Hook, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Rev. Sci. Instrum. 1995, 66, 3924-3930. (11) Caruso, F.; Serizawa, T.; Furlong, D. N.; Okahata, Y. Langmuir 1995, 11, 1546.
Langmuir, Vol. 18, No. 4, 2002 1275 was potassium bromide (KBr, Merck, pro analysis). All solutions were used within 48 h of being prepared. The QCM-D. The instrument used was a quartz crystal microbalance with built-in dissipation measurement (QCM-D), which is fully described elsewhere.10 This instrument uses a diskshaped 25 mm AT-cut quartz crystal with a fundamental frequency around 5 MHz (third overtone around 15 MHz). The substrate surface was an evaporated gold electrode (around 100 nm thick) attached to the quartz oscillator via a 5 nm thick chromium adherent layer. Prior to experiments, the gold layer is cleaned with a UV/ozone treatment12 to remove contamination. Additional in situ cleaning prior to conducting experiments was achieved by first passing an SDS-rich (above the critical micelle concentration (cmc)) solution across the crystal and then pure water. During operation, the crystal (plus adsorbed layers) is excited to oscillate in the thickness shear mode (approximately 1 nm amplitude13). The resonance frequencies are related to the total oscillating mass, and hence adsorption leads to a decrease in the resonance frequency. In this way, monitoring the resonance frequencies gives an apparent adsorbed mass. When the Sauerbrey relation14 holds, the change in frequency is directly proportional to the adsorbed mass and given by
∆f ) -
2f 2 f m)m ) -Cm Fqνq Fqtq
(1)
where f is the frequency, Fq and νq are the specific density and the shear wave velocity in quartz, respectively, tq is the thickness of the quartz plate, m is the adsorbed mass per unit area, and C is the calibration constant for the crystal. In the case of the fundamental frequency, the calibration factor is C ) 57 cm2 µg-1 s-1. Note that there is a factor of 3 between the fundamental and third overtone calibration factors (since there is a factor of 3 between the frequencies), and thus to compare the raw frequency shifts at the two resonances, one has to simply divide the observed third overtone frequency shift by 3. Any differences between these values is then either due to the viscoelastic properties of the adsorbed layer or related to the film thickness since different shear waves penetrate into the solution to different degrees. The decay length of the shear wave, δ (the distance from the substrate where the amplitude of the shear wave has fallen by a factor of e), is given by
δ ) (2ηf/ωFf)1/2
(2)
where Ff and ηf are the density and viscosity of the fluid and ω is the shear rate. Thus for a 5 MHz oscillator in water, the thickness of the water layer that influences the QCM frequency and dissipation responses is about 0.3 µm15. In liquid, eq 1 is not necessarily true, even for thin adsorbed layers. The reason is that the interaction with the bulk liquid, due to changes in density or viscosity, may change as its composition is varied. In the case of the current work however, bulk effects are minimal since the changes to the density and viscosity are minimal. It should be noted that solvent associated with the adsorbed layer is included in the measured mass. Hence, the adsorbed amount registered by the QCM-D device is expected to be larger than that registered by other means, for example, ellipsometry. For surfactant layers, the amount of water included in the measured mass is relatively low,16 whereas this may not be true for more complex adsorbed layers such as polymers.17 Finally, any viscoelastic response from the adsorbed layer may result in less than 100% of the mass being registered, which invalidates the Saurbrey equation.14 With measurement of the dissipation factor (D), information on the layer viscoelastic (12) Krozer, A.; Rodah,l M. J. Vac. Sci. Technol., A 1997, 15, 17041709. (13) Borovsky, B.; Mason, B. L.; Krim, J. J. Appl. Phys. 2000, 88, 4017-4021. (14) Saurbrey Z. Phys. 1959, 155, 206-222. (15) Rodahl, M.; Kasemo, B Sens. Actuators B 1996, 37, 111-116. (16) Furlong, Neil In Modern characterization of surfactant systems; Schick, M. J., Ed.; Marcel Dekker: New York, Basil, 1999; Vol. 83, pp 481-519. (17) Ho¨o¨k, F.; Rodahl, M.; Brzezinski, P.; Kasemo, B Submitted for publication in Langmuir.
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response provides additional information on the layer structure, and it also provides a means for judging the applicability of the Saurbrey equation. The dissipation factor is found from the rate of decrease in the magnitude of the sinusoidal oscillations once the driving voltage applied to the crystal has been turned off. The dissipation factor is defined as a modified ratio between the energy dissipated and the energy stored during a single-crystal oscillation 10 as shown below in eq 3
D ) Edissipated/2πEstored
(3)
Experimental Procedure. The experiment was initiated by exposing the gold-coated quartz oscillator to an aqueous salt solution, thus establishing a baseline for the frequency and energy dissipation. Next, a polyelectrolyte solution, containing the same type and concentration of inorganic electrolyte, was introduced in the measurement chamber. The adsorption process was followed by monitoring the change in frequency and dissipation as a function of time. After the system reached equilibrium, residual bulk polyelectrolyte was removed from the system by flushing with the original salt solution. In the next step, surfactant solutions of different concentrations were introduced and the association of surfactant to the substrate and adsorbed polyelectrolyte was monitored. Finally, the bulk solution was replaced by the initial electrolyte solution to record any changes to the adsorbed layer. It should be emphasized that these measurements were carried out in a cell where the liquid is introduced parallel to the surface. We noted that cells where the liquid instead was introduced normal to the surface were unsuitable for the kinetic investigation presented in this report. The reason is that the stagnant point in such a cell is smaller than the sensing area, which results in a rapid hydrodynamic transport of polyelectrolyte to part of the active surface area. XPS. X-ray photoelectron spectroscopy (XPS), also known as ESCA (electron spectroscopy for chemical analysis), is a surfacesensitive spectroscopic technique that can be employed for the chemical analysis of the top few nanometers of solid surfaces. The technique relies on the photoelectric effect whereby incident X-rays ionize inner shell electrons. Analysis of the energy of the emitted inner shell electrons identifies the surface elements and shifts in this energy can also be used to determine the chemical environment of the host atom. In the case of the current work, we have used the method of Rojas et al.18 to determine the equilibrium adsorbed amount of our polyelectrolyte both on gold (the current substrate) and on mica. XPS measurements were made using a Kratos (AXIS-HS) spectrometer equipped with a monochomator, Al X-ray source, and slotM lens. Adsorption was carried out on freshly prepared and cleaned substrates immersed in 20 ppm AM-MAPTAC-10 solution for 2 h. The samples were then left in Milli-Q water for 2 h before being blow dried under a filtered nitrogen gas source and then inserted into the XPS instrument within 30 min. We note that the strong affinity between the polyelectrolyte and the surface allows the removal of the substrate from the solution without appreciable loss of material as discussed in a previous report.19 The adsorption study on mica was used as a reference for calculating the absolute adsorbed amount on gold. Comparison between the adsorbed amount on gold and mica was achieved via the intensity of the nitrogen 1s peak, while the total adsorbed amount on mica was found from comparison between the nitrogen 1s peak and the potassium 2p peak in the mica sample. Since the number of exchangeable potassium ions on the mica surface is known, it is straightforward to calculate the total polyelectrolyte adsorbed amount on mica and thus also on gold. For further details, readers are referred to the work of Rojas et al.18
Results QCM. After the baseline in 10-4 M KBr salt solution was obtained, a 20 ppm AM-MAPTAC-10 solution in 10-4 M KBr was introduced into the measuring cell. The (18) Rojas, O. J.; Erstsson, M.; Neuman, R. D.; Claesson, P. M. J. Phys. Chem. B 2000, 104, 10032-10042. (19) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951.
Figure 2. Adsorption curves for AM-MAPTAC-10. The dashed lines indicate changes in frequency (mass) while the solid lines indicate changes in dissipation. The darker, heavier lines are from the third overtone (approximately 15 MHz) while the lighter and thinner curves are for the fundamental frequency (approximately 5 MHz).
Figure 3. Frequency versus dissipation curves for the adsorption of AM-MAPTAC-10. The three different regions labeled as 1, 2, and 3 are discussed in the text.
kinetics of adsorption of the polyelectrolyte to the gold surface on the quartz crystal is shown in Figure 2. This figure contains both the frequency and dissipation changes at both the fundamental (5 MHz) and the third overtone (15 MHz). The frequency changes can be related, by eq 1, to the change in adsorbed mass. This gives an apparent adsorbed mass of 0.63 µg cm-2 for the fundamental and 0.53 µg cm-2 for the third overtone. The dissipation increase is relatively small ((2.2-2.5) × 10-6 units) indicating that the use of eq 1 does not introduce a significant error. The change in dissipation is slightly larger for the fundamental frequency than for the overtone that is most likely related to its slower decay. Plotting the frequency against the dissipation, which removes time as a variable, gives further insight into the adsorption process. This is shown for the case of the third overtone in Figure 3. The same behavior is observed for the fundamental frequency. The figure can be interpreted as a change in dissipation per unit of adsorbed mass (shown in terms of frequency change). In this case, three separate regions can be distinguished, which are labeled as 1, 2, and 3 and separated by vertical lines in the figure. The first region ranges between 0 and -25 Hz, the second between -25 and -90 Hz, and the last between -90 and -95 Hz. Approximate slopes for regions two and three (dissipation (units × 106)/frequency (Hz)) are -0.026, and -0.10, respectively, while the slope in region 1 varies from 0 to -0.02615 (the slope of region 2). It is noted that adsorption had not fully reached its plateau value (Figure 2) when the bulk polyelectrolyte was washed away, though
Adsorption and Swelling of Polyelectrolytes
Langmuir, Vol. 18, No. 4, 2002 1277 Table 1 mica
gold
intensity sensitivity factor intensity sensitivity factor (I) (S) (I) (S) N 1s K 2p K 2s Si 2p
1690 2906 1154 2494
0.42 1.24 0.39 0.27
1436
0.42
Constants for K 2p with Al X-ray Source with Monochromator and Detector at 90° f 0.18 d/λΚ sin θ 0.23766 (1 + R) 1.06 EN (eV) 1084.6 f(1 + R) 0.191 EK (eV) 1192.1
Figure 4. Plot showing the swelling of a preadsorbed layer of AM-MAPTAC-10 with anionic surfactant (SDS). The dashed line represents the frequency change while the dashed line represents the dissipation change. Both curves are for the fundamental frequency (approximately 5 MHz).
the adsorption kinetics were very slow and the adsorbed amount was approaching its equilibrium value. Once the frequency and dissipation signals had stabilized from the adsorption process, the bulk solution in contact with the crystal was exchanged for solutions containing the anionic surfactant SDS at various concentrations and 10-4 M KBr. The changes in frequency and dissipation factor for the fundamental resonance are shown in Figure 4, where the changes are measured relative to the values prior to adsorption of the polyelectrolyte. The third overtone response is similar, though the frequency changes are approximately 2.7 times larger, slightly less than the expected factor of 3. At points A and B, 10-4 M KBr was injected. This resulted in no change in frequency or dissipation, indicating that the adsorbed layer was unaffected. This is consistent with the generally slow desorption of high molecular weight polymers and is related to the many attachment points and the low driving force for transport away from the surface for polymers with a high affinity for the surface.20 We note that the spikes shown at the injection points are due to temperature and pressure changes associated with the exchange of liquids in the cell, and they do not contain any information on the adsorbed layers. The SDS concentration injected at point C was 0.17 mM, which corresponds to 0.02 cmc (cmc ) critical micelle concentration), point D to 0.34 mM (0.04 cmc). At these SDS concentrations no effect on the adsorbed layer was detected. However, increasing the SDS concentration to 1.7 mM (point E, 0.2 cmc) resulted in a small but significant increase in dissipation whereas no change in frequency was noticeable. This indicates that some SDS is incorporated in the adsorbed layer and swells it somewhat. However, the additional mass is very small and below the detection limit. A further increase in SDS concentration to 5 mM (point F, 0.6 cmc) results in further swelling without any observed change in the mass associated with the adsorbed layer. However, when the SDS concentration is increased further to 16.6 mM (point G, 2 cmc), a rapid and large increase in swelling and concomitant decrease in frequency is observed. Clearly, the swelling of the layer is now very significant and as a result the associated mass is now increased significantly to 9.5 µg cm-2. The increase in mass is unreasonably high had it been only caused by incorporation of SDS in the layer, and thus this result suggests that the amount of water associated with the (20) Fleer, G. J.; Cohen-Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at interfaces; Chapman and Hall: London, 1993.
layer also increases dramatically. It should also be noted that with the large increase in dissipation, the use of eq 1 is no longer accurate and the calculated mass should be viewed as a course estimate, lower than the real value. After the initial increase in dissipation and decrease in frequency occurring after introducing the 2 cmc SDS solution, a much slower process occurs that results in not only a further increase in dissipation but also an increase in frequency. Hence, even though the associated mass decreases, the dissipation continues to increase. This indicates that some polyelectrolytes with associated SDS and water now desorb from the surface. The increase in dissipation suggests that the remaining polyelectrolytes adopt a conformation that extends further into the solution and increases the viscoelastic properties of the layer. The desorption process is halted at point H where a surfactant-free 10-4 M KBr solution is injected into the measuring cell. When this occurs the dissipation drops rapidly to a value that is even slightly lower than that for the preadsorbed polyelectrolyte layer prior to any SDS injection. Further, the frequency also decreases below the value found prior to the first SDS injection. Hence, it appears that all SDS is removed from the adsorbed layer and the remaining polyelectrolyte collapses back to the surface. Making the simplified assumption that the amount of mass measured with the QCM (including trapped water) is proportional to the adsorbed amount of polyelectrolyte results in an estimate that about 10-12% of the polyelectrolyte was desorbed. XPS. To calculate the adsorbed amount of polyelectrolyte on mica from the XPS results, we use eq 12 in Rojas et al.,18 which is given below as eq 4
NN ) A
(
) (
)
d d exp - K (2.1 × 1014) λK sin θ λ sin θ (4) EN -0.7 d hIKSN(1 + R)f 1 - exp - K λ sin θ EK
INSK
[
(
( ) )]
The intensities (I) (raw areas) measured and tabulated sensitivity factors (S) for the XPS instrument are shown in Table 1, along with the appropriate constants f and (1 + R) and the calculated reduced thickness d/λΚ sin θ.18 The factor f is defined as the ratio of the potassium 2p intensity that originates from the potassium ions located at the surface to that of those inside the mica crystal. (1 + R) is the constant used to account for the small amount of sodium ions that substitute for potassium ions in the mica lattice. Both these quantities were determined in separate experiments.18 Also included in Table 1 is the kinetic energy of the emitted photoelectrons from both nitrogen 1s (EN) and potassium 2p (EK). Thus, from eq 4 we can calculate the adsorbed amount on mica to be 0.19 µg cm-2. The adsorbed amount on gold
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weight molecules in the QCM case. This occurs since lower molecular weight molecules diffuse faster; in the case of the DLS however, larger molecules contribute more to the scattering intensity. Assuming a random coil model for the polyelectrolyte (which is not strictly correct due intrachain repulsions), the diffusion coefficients determine a hydrodynamic radius for the polyelectrolyte of 64 nm from the DLS measurements and 40 nm from the QCM-D results. Discussion
Figure 5. Plot of the adsorbed mass (µg/cm2) versus the square root of time (seconds). The upper straight line fit is to the QCM experimental data. The lower curve is based on the diffusion coefficient from the dynamic light scattering experiments. The middle curve is also from the dynamic light scattering experiments but includes the mass of bound water in the layer as calculated from XPS/QCM adsorbed amount comparisons.
is then calculated by comparing the N1s signal from the polyelectrolyte on mica and gold and found to be 0.16 µg cm-2. After obtaining information on the adsorbed amount of AM-MAPTAC-10 on gold from the XPS measurements, we can consider the dynamics of the adsorption process as evidenced by the QCM data illustrated in Figure 5. Initially there is a linear relationship between the adsorbed amount and the square root of time, as shown by the upper curve, which is a fit to the experimental data points. This is consistent with zero flow, diffusion controlled adsorption, where the adsorbed mass (M) can be expressed as in eq 521
M ) 2n0(Dt/π)1/2
(5)
In this case the adsorbed mass at time zero is zero, t is the time, n0 is the initial bulk concentration (mass/volume), and D is the diffusion coefficient. To apply eq 5, we need to know the diffusion coefficient and this was determined by dynamic light scattering measurements to be 3.8 × 10-12 m2/s. This value should be seen as an “average” value since the polyelectrolyte sample is polydisperse. The lower curve in Figure 5 is the expected result using the value of the diffusion coefficient found from the DLS measurements. Obviously there is a large discrepancy between the DLS-derived slope and the experimental data. However, by comparing the adsorbed amount determined by XPS and QCM-D, we find that the mass sensed with the QCM-D device is due to both polyelectrolytes (28%) and trapped water (72%), and hence when taking into account this extra adsorbed mass, the DLS measurements predict a different slope as depicted by the middle curve in Figure 5. The agreement between the upper two curves is still by no means perfect but nonetheless surprisingly good. The remaining discrepancy is most likely explained as being due to the preferred adsorption of lower molecular (21) Ward, A. F. H.; Tordai, L. J, Chem. Phys. 1946, 14, 453-461.
Adsorption of Polyelectrolytes to Gold and Mineral Surfaces. The adsorption of AM-MAPTAC-10 on glass surfaces,22 and of a similar polymer on mica,23 has been studied with surface force techniques. The surface force data for mica6 was obtained for a 10% charged polyelectrolyte having CMA ((2-acryloxyethyl)trimethylammonium chloride) as the charged unit rather than MAPTAC. This latter study also addressed swelling of the preadsorbed polyelectrolyte layer by SDS. We will use results from these earlier investigations to interpret the data presented in this paper, but first we must consider how the adsorption of the polyelectrolyte to the different surfaces may differ. Mica, glass, and gold are all negatively charged in water24 at ambient pH. Hence, in all cases electrostatic forces facilitate adsorption of cationic polyelectrolytes up to the charge neutralization point. On mica and glass a slight recharging of the surfaces occurs when the plateau adsorbed amount is reached, but the degree of overcompensation is rather small. This indicates that electrostatic forces are decisive for the adsorption process. In this work, we employed a gold substrate, and the driving force for adsorption may be different. In addition to the attraction between permanent negative surface charges present on the gold surface, as evidenced by direct force measurements,24 one may expect image charge effects to facilitate the adsorption. The mobility of electrons in the gold is also high, which means that mobile negative charges may come very close to the charged groups of the polyelectrolyte that contact the surface. Indeed, the detailed spectrum of the N 1s photoelectron peak emanating from the adsorbed polyelectrolyte appears different on gold and mica substrates (Figure 7). On mica substrates one can clearly distinguish between two types of nitrogen, the charged nitrogen with a binding energy of just above 402 eV and the uncharged one with a binding energy of just below 400 eV. The ratio between these peaks shows that 13.3% of the nitrogen atoms in the adsorbed layer is charged. This is a slightly higher value than that expected from the average structure of the polyelectrolyte, but consistent with a recent XPS investigation.18 This result can be understood by considering that the polyelectrolyte is polydisperse not only in molecular weight but also in composition, with a higher affinity to the surface for the more highly charged ones. On the other hand, the N 1s peak from the adsorbed polyelectrolyte on gold shows no evidence for any charged nitrogen. This can only be rationalized by considering that mobile electrons in the gold surface are able to come very close to the adsorbed charged quaternary ammonium groups making the binding energy of the photoelectron similar to that of an uncharged amine. With this result (22) Poptoshev, E.; Claesson, P. M. Submitted for publication. (23) Kjellin, M. U. R.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1997, 190, 476-484. (24) Ederth, T. In Novel surfaces for force measurements; Kungl Tekniska Ho¨gskolen: Stockholm, 1999.
Adsorption and Swelling of Polyelectrolytes
Figure 6. Frequency versus dissipation curves for the swelling of a preadsorbed layer of AM-MAPTAC-10 with anionic surfactant (SDS). The arrows on the plot indicate the flow of time during the swelling, thus indicating not only the swelling but also desorption. The final arrow indicates the collapse of the swelled layer upon removal of the surfactant.
Figure 7. XPS, nitrogen 1S peaks for layers of adsorbed AMMAPTAC-10, on gold (the upper figure), and on mica (lower figure). Of special interest is the peak at around 402 eV that is present only in the mica case, this is discussed in the text.
in mind we expect a stronger binding of the polyelectrolyte to gold than on mica, and as will be discussed below this finding is consistent with the difference in removal of the polyelectrolyte from mica and gold substrates by addition of SDS. Let us now return to previous surface force measurements using mica surfaces and preadsorbed 10% charged cationic polyelectrolytes.23 The surface force measurements show that the layer thickness is about 4 nm and that the layer only can be moderately compressed by increasing the applied force. Hence, the adsorbed layers are rather compact. We note that the XPS results demonstrate that the adsorbed amount on mica and gold is very similar, and that the affinity to the gold surface may be higher than that to mica. Hence, we expect that
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the adsorbed AM-MAPTAC-10 layers on gold also are compact. Indeed, the small change in dissipation values seen in this study when adsorbing the polyelectrolyte onto gold surfaces is indicative of a compact structure. Water Content of the Adsorbed Layer. With the Sauerbrey relation, the adsorbed mass on gold can be approximated to be 0.63 µg cm-2 using the fundamental and 0.53 µg cm-2 by the third overtone. The fact that both the fundamental and third overtone result in similar adsorbed masses suggests that the layer is equally well sensed by both these frequencies. Since the difference between the two frequencies is mainly in the range of detection from the surface (assuming no difference in viscoelastic behavior over this frequency range), this suggests that the adsorbed layer is thin. The small difference between the two measured adsorbed masses may however still be a result of the different decay lengths of the two resonant frequencies (eq 2). For convenience the average of the value obtained from the two frequencies is used as the adsorbed amount (0.58 µg cm2) in the following discussion. The fact that the dissipation increases slightly upon polyelectrolyte adsorption suggests that the layer is composed of small loops and tails in addition to trains in direct contact with the surface. This type of layer conformation thus suggests some degree of solvation, which increases the deduced adsorbed mass. In previous experiments on protein adsorption, comparison of results from ellipsometry, optical waveguide lightmode spectroscopy (OWLS) and QCM17 show that for certain relatively flat conformations of the proteins (hemoglobin and human serum albumin) there is a 30-50% overestimate in the mass found by QCM. This overestimate is reported to be mainly due to water, which is directly bound to the protein molecules. The addition of antibodies (polyclonal IgG) however, causes the proteins to adopt more extended conformations and leads to an overestimate of the adsorbed amount by a factor of around 3.5, suggested to be due to trapped solvent within the extended protein network attached to the surface.17 An estimate of the amount of water sensed by the QCM-D in the present case can be obtained by comparing the XPS and QCM-D results. From these data we estimate the relative amount of solvent in the layer as approximately 70% of the total mass. The fact that the dissipation change is small even with such a high solvent concentration within the layer is consistent with surface force data23 that shows a relatively incompressible adsorbed layer. Additionally, calculation of the volume of the adsorbed layer on mica (from the SFA results) and assuming a desity of 1 g/cm3 gives a mass of the layer (polyelectrolyte + solvent) of 0.5 µg/cm2. Since the XPS calculated polyelectrolyte mass on mica was 0.19 µg/cm2, this predicts the solvent concentration in the layer to be 62%. This is in reasonable agreement with the QCM-D/ XPS derived value of about 70% solvent in the AMMAPTAC-10 layer on gold. Build-Up of the Adsorbed Polyelectrolyte Layer. The dissipation/frequency, or DF, plot shown in Figure 3 gives additional information on the adsorption process. By removing time from the representation of the data, it can be seen that the dissipation factor of the adsorbed layer is not linear with adsorbed mass. It is apparent that the process involves three distinct regions. The initial adsorption gives rise to a very limited increase in dissipation, indicating that the adsorbing polymers adopt very flat and rigid conformations. In this region the slope slowly increases until it reaches a constant value (region 2) that compromises most of the adsorption process. Finally the last region, which amounts to around 5% of the total
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mass, shows a much less rigidly attached layer. There are two possible suggestions for this relatively large dissipation increase caused by the last polyelectrolytes reaching the surface. The first possibility is that the last arriving chains have a lower percentage of units attached to the substrate and, thus, extend further into solution than the chains initially attached to the surface. The second possibility is that the layer as a whole reorganizes and becomes more extended. The surface force data by Kjellin et al.23 and Poptoshev et al.22 support the first view. In these studies, using mica and glass substrates, it is observed that a weak long-range electrosteric force develops at high adsorption densities, which is interpreted as being due to compression of a few extended tails. There is no reason to believe that the situation should be dramatically different on gold. It can also be seen from Figure 5 that the adsorption is consistent with a diffusion-controlled process, up until approximately 0.4 µg/cm2 (23 Hz for the third overtone), where the adsorption process slows down as binding sites on the gold substrate become less accessible. This is consistent with the adsorption process suggested by the DF plot in Figure 3. Swelling of Preadsorbed Polyelectrolyte Layers by SDS. Swelling of the adsorbed polyelectrolyte layer is achieved by the addition of SDS. In the surface force study using mica substrates,6 it was seen that at a concentration of 0.01 of the cmc there was no swelling. On the other hand, when the SDS concentration was increased to 0.02 of the cmc, the layer swelled extensively. From the dissipation change reported in Figure 4, it can be seen that to swell the polyelectrolyte adsorbed on gold, much higher concentrations of surfactant are needed. The difference between the two substrates is surprisingly large but has to be related to a higher affinity of the polyelectrolyte for gold than for mica. We suggest that the mobility of the electrons in the gold layer is one crucial factor determining the adsorption strength and find support for this view in the detail spectra of the nitrogen 1s signal, Figure 7. The swelling-desorption-layer collapse process of AM-MAPTAC-10 on gold is illustrated in Figure 6 as a DF plot. In this case both the fundamental and overtone are shown on the same plot, the axis and scales set in order to overlap the DF data. Following the time arrows on the plot shows that the apparent adsorbed mass initially increases along with the dissipation (surfactant-induced swelling) but that this trend reverses after a certain point and the sensed mass decreases with increasing dissipation. The observed increase in dissipation is clearly due to the swelling of the polyelctrolyte layer, which is caused by the association of surfactant to the preadsorbed polyelectrolyte, with possible contributions from competitive adsorption of SDS on gold that replace some polymer segment-surface contacts. The changes in the apparent adsorbed mass are due to a combination of effects. The initial increase is due to the incorporation of surfactant and solvent into the adsorbed layer. The magnitude of the change indicates that solvent incorporation is by far most
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important and overshadows the decreased sensitivity of the technique due to the higher dissipation value. At a later stage the decrease in apparent adsorbed mass could be the result of two factors, desorption and decreased sensitivity. We note that after removal of the surfactant the apparent adsorbed mass is lower as compared to prior to surfactant addition, demonstrating that surfactantinduced desorption of polyelectrolyte has occurred. Thus, it seems plausible to discount the second possibility since the first is sufficient to explain the observations. Conclusion The adsorption process can be easily monitored by the QCM-D technique, giving information on the adsorbed amount, adsorption kinetics, and changes in layer viscoelasticity. The major part of the adsorption process is diffusion controlled, and the rate of adsorption is in reasonable agreement with theoretical predictions provided that the solvent content in the layer is accounted for. This can be achieved by monitoring the sensed mass with the QCM-D technique in combination with an alternative technique, in our case XPS. From such a comparison the contribution from the solvent in the layer is shown to amount to about 70% of the total adsorbed mass sensed by the QCM-D device. This value is similar to that deduced using a similar 10% charged polyelectrolyte and mica substrates and surface force/XPS data, which gave a solvent content of the adsorbed layer of about 60%. An understanding of the build-up of the adsorbed layer is facilitated by simultaneous measurements of changes in frequency and dissipation. The conclusion reached is that the initial polymers adsorb very flat on the surface, followed by a region which constitutes the majority of the process where we observe a constant dissipation per adsorbed mass. The final 5% increase in adsorption leads too much higher relative dissipation increase, which indicates that the last chains that arrive to the surface adopt more extended conformations. The swelling of the preadsorbed polyelectrolyte layer by SDS is shown to occur at much higher concentrations than seen for similar polyelectrolytes on mica. This is due to the higher affinity of the polyelectrolyte for the gold substrate. Acknowledgment. The authors thank the Laboratoire de Physico-Chemie Macromoleculaire, Universite´ Pierre et Marie Curie, Paris, for supplying the polyelectrolyte used in these experiments. We also thank Fredrick Ho¨o¨k and Hans Elwing for supplying experimental and technical assistance in the initial QCM-D experiments and Marie Ernstsson for carrying out the XPS measurements. Finally Mark Plunkett thanks the Swedish foundation for Strategic Research, the Colloid and Interface Science Program, for funding this work. LA015573J
