Vis Spectroscopic Monitoring of Polyelectrolyte Adsorption onto

Poly(2,5-bis(1-hexyltriethylammonium)-1,4-phenylene) iodide 2 (Pn about 38) ... as described previously.15 Poly(allylamine) hydrochloride 3 (Mw about ...
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Langmuir 2000, 16, 3407-3413

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UV/Vis Spectroscopic Monitoring of Polyelectrolyte Adsorption onto Monolayers of Azobenzene Amphiphiles J. Engelking,† M. Wittemann,‡ M. Rehahn,‡ and H. Menzel*,† Institut fu¨ r Makromolekulare Chemie der Universita¨ t Hannover, Am Kleinen Felde 30, D-30167 Hannover, Germany, and Polymer-Institut, Universita¨ t Karlsruhe, Kaiserstrasse 12, 76128 Karlsruhe, Germany Received July 9, 1999. In Final Form: November 16, 1999 The complexation of an azobenzene-containing carboxylic acid amphiphile with a cationic poly(pphenylene) polyelectrolyte at the air/water interface results in changes of the monolayer structure. Compared to the monolayer of the amphiphile on pure water, a more expanded liquidlike structure can be suggested for the complex monolayer according to the increased area per amphiphilic molecule and Brewster angle microscopy. Furthermore, aggregation of the azobenzene moieties is suppressed in the complex. The differences in the spectral properties of the pure amphiphile monolayer and the polyelectrolyte complex monolayer due to their different structural order can be employed for investigation of the adsorption process: The adsorption of the polyelectrolyte to an already compressed amphiphile monolayer results in structural rearrangements and can, therefore, be monitored in situ as a change in the monolayer UV/vis spectra.

Introduction Polyelectrolyte adsorption onto solid substrates plays an important role in many applications such as modification of surfaces, e.g., preparation of antistatic and protective coatings or biocompatibilization of artificial organs. But also many technical processes such as wastewater treatment and paper production are closely connected with polyelectrolyte adsorption.1 Although there is a great interest in the adsorption process, only a few methods are available for in situ investigation of the polyelectrolyte adsorption onto charged surfaces such as ellipsometry2 or surface plasmon resonance spectroscopy.3 Our attempt is to use monolayers of ionic amphiphiles at the air/water interface as a model surface for studying the interaction of a polyelectrolyte with an oppositely charged surface. Polyelectrolyte-amphiphile complexes are formed spontaneously when an amphiphile solution is spread on a subphase containing the polyelectrolyte.4,5 Complex formation may result in stabilization of monolayers and LB films of ionic amphiphiles and a change in their structure4,5 and morphology.6 Studying the monolayer behavior of the complexes allows one to obtain information about the interaction between polyelectrolyte and amphiphile and about the new type of structure that has been formed. In addition the adsorption process can be monitored if preformed monolayers are brought into contact with a polyelectrolyte-containing subphase.7,8 * Corresponding author. Phone: + 49 511 762 5971. Fax: +49 511 762 4996. E-mail: [email protected]. † Institut fu ¨ r Makromolekulare Chemie der Universita¨t Hannover. ‡ Universita ¨ t Karlsruhe. (1) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, Ch.; Stscherbina D. PolyelectrolytessFormation, Characterization and Application; Carl Hanser Verlag: Munich, 1994. (2) Walter, H.; Harrats, C.; Mu¨ller-Buschbaum, P.; Je´roˆme, R.; Stamm, M. Langmuir 1999, 15, 1260. (3) Kotov N. A.; De´ka´ny, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. (4) Erdelen, C.; Laschewsky, A.; Ringsdorf, H.; Schneider, J.; Schuster, A. Thin Solid Films 1989, 153, 180. (5) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (6) Engelking, J.; Menzel, H. Thin Solid Films 1998, 327-329, 90. (7) Miyano, K.; Asano, K.; Shimomura, M. Langmuir 1991, 7, 444.

Rigid rodlike polyelectrolytes such as poly(2,5-bis(1hexyltriethylammonium)-1,4-phenylene) iodide 2 (Scheme 1) are favorable model polymers for investigation of the adsorption process because the conformation of the polymer backbone is not affected by solution properties (concentration, salt) or the interaction with the charged surface. Therefore, the distance and distribution of the ionic sites is more defined than in polymers with a flexible backbone. Furthermore, a considerable degree of order is necessary to achieve a high density of polyelectrolyte rods at the air/water interface. The complexes of the cationic poly(p-phenylene) 2 with an azobenzene-containing carboxylic acid amphiphile 1 are compared to complexes of cationic polyelectrolytes with a flexible backbone, i.e., poly(allylamine) hydrochloride 3 and poly(diallyldimethylammonium) chloride 4. Depending on the substitution of the phenylene units in the backbone, poly(p-phenylene)s show two typical bands in their solution UV/vis spectra.9,10 The longwavelength absorption band (A band, λmax > 300 nm) is due to electronic transitions between orbitals delocalized over conjugated phenylene rings.10,11 However, the alkyl residues at the phenylene units present in polymer 2 result in a strong twist of the phenylene units along the polymer backbone.10,12 The conjugation length in polymer 2, therefore, is considerably decreased, and no band around 300 nm can be observed for the aqueous solution.13 The absence of an overlapping polymer band allows the use of azobenzene-containing carboxylic acid 1 as an amphiphilic probe. Since the UV/vis spectroscopic properties of the azobenzene moieties strongly depend on their organization,14 the structural changes of the amphiphile (8) Asano K.; Miyano K.; Ui H.; Shimomura M.; Ohta, Y. Langmuir 1993, 9, 3587. (9) Cimrova´, V.; Remmers, M.; Neher, D.; Wegner, G. Adv. Mater. 1996, 8, 146. (10) Vahlenkamp, T.; Wegner, G. Macromol. Chem. Phys. 1994, 195, 1933. (11) Suzuki, H. Bull. Chem. Soc. Jpn. 1959, 32, 1340. (12) Rehahn, M. Dissertation, Universita¨t Mainz, Germany, 1990. (13) Rehahn, M.; Schlu¨ter, A.-D.; Wegner, G. Makromol. Chem. 1990, 191, 1991. (14) Kimizuka, N.; Kunitake, T. Colloids Surf. 1989, 38, 79.

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Scheme 1

monolayer due to the complexation and the compression can be assessed by UV/vis spectroscopy as will be shown here. Experimental Section Poly(2,5-bis(1-hexyltriethylammonium)-1,4-phenylene) iodide 2 (Pn about 38) was synthesized and characterized as described previously.15 Poly(allylamine) hydrochloride 3 (Mw about 60 000 g/mol, Aldrich) was used as received. Poly(diallyldimethylammonium) chloride 4 (Mw about 90 000 g/mol) was kindly donated by W. Jaeger (Fraunhofer-Institute for Applied Polymer Research, Teltow, Germany). The azobenzene amphiphile was synthesized via azo coupling of hexylaniline and phenol and etherification of the product with 8-bromooctanoic acid ethylester. The product was saponified with potassium hydroxide in an ethanol-water mixture. The acid was precipitated in an excess of hydrochloric acid in water and purified by repeated recrystallization from ethanol. The purity and structure were checked by TLC and 1H NMR spectroscopy, respectively. Monolayers were investigated using a Nima 611MC (300 cm2) two-compartment trough equipped with a Wilhelmy plate pressure sensor Nima PS4. The barrier speed was set to 10 cm2/ min. Furthermore, for some experiments a Lauda FW1 trough was used, which is equipped with a torsion balance. Monolayers were prepared under red light conditions by spreading a solution of the amphiphile in chloroform on a subphase of ultrapure water (>18 MΩ‚cm, Barnstaedt NanopureIII, 20 °C) containing the polyelectrolyte (concentration 5 × 10-5 mol/L with respect to the cationic groups). After spreading, evaporation of chloroform and complex formation were allowed to take place for 15 min before the recording of the isotherms was started. Brewster angle microscopy was performed using a MiniBAM (NFT, Go¨ttingen). The setup and procedure for recording the monolayer UV/vis spectra have been reported in ref 16. The adsorption experiments were performed by using the two-compartment trough described above. The amphiphile solution was spread on pure water in one compartment (about 90 mL). After 15 min the monolayer was compressed and transferred onto the polyelectrolyte-containing subphase in the second compartment (about 120 mL). After completion of the transfer (2 min), a reference spectrum was taken and the time-dependent UV/vis spectroscopic measurements were started.

Results and Discussion Monolayer Structure of the in-Situ-Formed Polyelectrolyte Complexes. Isotherms. Polyelectrolytes present in the subphase form a complex with oppositely charged amphiphiles spread at the air/water interface.5,6 The complex formation results in a stabilization of the monolayer. Furthermore, the structure of the complex may be different from the structure of the pure amphiphile monolayer. Changes in the π/A isotherms give a first insight into the structural differences. The isotherms for (15) Brodowsky, G.; Horvath, A.; Ballauff, M.; Rehahn, M. Macromolecules 1996, 29, 6962. (16) Menzel, H. Macromol. Chem. Phys. 1994, 195, 3747.

Figure 1. Pressure-area isotherms of amphiphile 1 on (a) pure water and on a subphase containing (b) polymer 2, (c) polymer 3, and (d) polymer 4 (concentration of the polymers 5 × 10-5 mol/L with respect to the ionic groups; subphase temperature 20 °C).

the azobenzene amphiphile on pure water and subphases containing the polyelectrolytes poly(2,5-bis(1-hexyltriethylammonium)-1,4-phenylene) iodide (2), poly(allylamine) hydrochloride (3), or poly(diallyldimethylammonium) chloride (4) are depicted in Figure 1. The isotherms for the amphiphile on pure water and on subphases containing polymers 3 or 4 differ mainly in the pressure at which the monolayer collapses. As expected the collapse pressure is higher for the complexes (1/3 44 mN/m, 1/4 49 mN/m) than for the monolayer on pure water (35 mN/m). However, the area per amphiphilic molecule remains nearly unchanged for the complexes with polymers 3 and 4 (0.30-0.33 nm2), indicating that the amphiphile determines the area requirements. The shape of the isotherms is also very similar. The steep curves indicate closepacked, solid analogous phases for the monolayers of the free amphiphile 1 and its complexes with polymers 3 and 4. While this solid analogous phase is present for the complexes even at very low surface pressures, for monolayers of the amphiphile 1 on pure water a phase transition at approximately 5 mN/m can be observed. So the complexation with polymers 3 and 4 supports the formation of a close-packed arrangement of the amphiphiles and stabilizes the monolayers. The situation is significantly different when polymer 2 is present in the subphase. The area per amphiphile molecule at zero surface pressure A0 is increased to about 0.51 nm2 for the complex. Furthermore, the less steep isotherm indicates a higher compressibility for the complex with 2. Both the higher compressibility and the higher area per amphiphile indicate a less densely packed and more liquidlike structure for the 1/2 complex. This less dense packing can be a consequence of the distance between the ionic sites, which can be rather large for polymer 2 since they are located at the end of spacers with six methylene units. In addition the triethylammonium groups are rather bulky and, therefore, may increase the area per amphiphile. Comparable effects on the area per amphiphile molecule and the monolayer compressibility were observed for polyelectrolyte complexes from amphiphiles with triethylammonium headgroups.17 Furthermore, an insertion of the hydrophobic parts of the polymer into the monolayer has to be considered as a reason for the increased area requirements and compressibility. Both mechanisms, increasing the distance (17) Engelking, J. Dissertation, Universita¨t Hannover, 2000.

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Figure 2. Brewster angle micrographs of a monolayer of amphiphile 1 at surface pressure zero (a) on pure water and on a subphase containing (b) 3, (c) 4, and (d) 2 (concentration of the polymers 5 × 10-5 mol/L with respect to the ionic groups; subphase temperature 20 °C).

between the amphiphile headgroups as well as insertion of hydrophobic parts of the polymer, would result in a less dense packing. In contrast to the area per amphiphile at zero pressure, the area at the collapse and the collapse pressure for the complex with polymer 2 are not significantly different from those found for complexes with polymers 3 or 4. It can be conjectured that upon applying a surface pressure the monolayers become more densely packed, because either the hydrophobic parts of the polymer are expelled from the monolayer or the ionic sites with the flexible spacers are moved closer. Brewster Angle Microscopy. The morphology of the monolayer can be examined employing Brewster angle microscopy (BAM).6,18 In the case of the complex monolayers under consideration, the morphology at zero surface pressure is of particular interest. The images as depicted in Figure 2 reveal significant differences. The monolayers of amphiphile on pure water give BAM images, which show neither a clear domain structure nor a homogeneous monolayer without any features. This might be a consequence of very small domains with a size at the resolution limit of the BAM used here. The BAM images for complexes of the amphiphile 1 with polymers 3 and 4 show a distinct domain structure with large domains (several millimeters in diameter), which is in accordance with the steep isotherms. On the other hand, the BAM images for the 1/2 complex show a very homogeneous monolayer without any features as would be expected for a liquidlike monolayer. So Brewster angle microscopy confirms the conclusions drawn from the isotherms. The complexes with polymers 3 and 4 are solidlike close-packed, while the (18) Ho¨nig, D.; Overbeck, G. A.; Mo¨bius, D. Adv. Mater. 1992, 4, 419.

monolayer of the complex with polymer 2 is less densely packed liquidlike. UV/Vis Spectroscopy. Further information about the structure of the monolayers can be obtained employing UV/vis spectroscopy. It is well-known that azobenzene chromophores in a closely packed arrangement show electronic interactions. This leads to a change in the excitation energy for the electronic transition19 and thereby to a band shift in the UV/vis spectra. This phenomenon has been used to investigate the type of organization of azobenzene moieties and other chromophores in selfassembled systems, monolayers, and LB films.5,14,20-28 According to the molecular exciton theory,19 the angle between the transition dipoles and the line connecting them (θ) is one of the parameters defining the direction and size of the band shift (see Figure 3). Since the amphiphiles are anchored to the water surface, θ is directly correlated with the tilt of the amphiphiles. As long as the (19) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. Pure Appl. Chem. 1965, 11, 371. (20) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsenges. Phys. Chem. 1983, 87, 1134. (21) Kunitake, T. Colloids Surf. 1986, 19, 225. (22) Shimomura, M.; Aiba, S.; Tagima, N.; Inoue, N.; Okuyama, K. Langmuir 1995, 11, 969. (23) Kimizuka, N.; Kunitake, T. Colloids Surf. 1989, 38, 79. (24) Everaars, M. D.; Marcelis, A. T. M.; Kuijpers, A. J.; Laverdure, E.; Koronova, J.; Koudijs, A.; Sudho¨lter, E. J. R. Langmuir 1995, 11, 3705. (25) Everaars, M. D.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Langmuir 1996, 12, 3964. (26) Nieuwkerk, A. C.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Langmuir 1997, 13, 3325. (27) Menzel, H.; Weichart, B.; Schmidt, A.; Paul, S.; Knoll, W.; Stumpe, J.; Fischer, T. Langmuir 1994, 10, 1926. (28) Menzel, H.; McBride, J. S.; Weichart, B.; Ru¨ther, M. Thin Solid Films 1996, 284-285, 640.

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Figure 3. (a) Exciton band energy diagram for a molecular dimer with coplanar transition dipoles inclined to the interconnected axis by the angle θ.19 (b) Organization of the transition dipoles with θ > 54.7° resulting in a blue shift. Figure 5. λmax of the azobenzene π-π* band as function of the area per amphiphile molecule and isotherm for amphiphile 1 on a subphase containing polymer 2 (5 × 10-5 mol/L with respect to the ionic groups; subphase temperature 20 °C).

Figure 4. UV/vis spectra of monolayers of the amphiphile 1 recorded at zero surface pressure (a) on pure water and on subphases containing (b) 3, (c) 4, and (d) 2 and (e) UV/vis spectrum of the amphiphile 1 in CHCl3.

monolayer is not in the gaseous state, the chromophores are more or less perpendicular to the surface and θ can reach a value larger than 54.7° (see Figure 3b). In this case a blue shift of the electronic transition is expected. Besides θ, the distance and aggregation number of the chromophores influence the spectral change, in particular, the extent of the band shift, which, therefore, is a measure of the structural order within the monolayer. As can be seen from the UV/vis spectra (see Figure 4), the peak maximum is significantly blue shifted for the monolayer of the amphiphile on pure water (λmax ) 324 nm) compared to the solution (λmax ) 352 nm) even without any surface pressure applied. This blue shift is an indication for a close-packed arrangement of the chromophores, in which strong π-π interactions are possible. Compared to that of the monolayer on pure water, the blue shift is even stronger for the complexes with polymers 3 and 4 (λmax ) 320 nm and λmax ) 321 nm, respectively). So these polyelectrolytes support the arrangement of the amphiphiles and increase the order. This result is in accordance with the isotherms and the BAM images. The peak maximum is considerably less blue shifted for the 1/2 complex monolayer (λmax ) 339 nm), indicating a less densely packed structure in which the chromophores are more tilted or have a greater distance.19 Again this supports the interpretation of the isotherms and BAM images. The complexes with polymers 3 and 4 as well as the amphiphile 1 on pure water show only a small additional blue shift of the π-π* band (λmax,c ) 311-314 nm ) upon compression. However, the spectra of the 1/2 complex monolayer change significantly upon compression. With increasing surface pressure, there is a strong additional peak shift for the π-π* band from λmax,0 ) 339 nm to λmax,c

) 316 nm (see Figure 5). At a high surface pressure, the spectra, and with that the stacking of the chromophores for 1/2 complex monolayers, are not significantly different from those of the 1/3 or 1/4 complex monolayers. The additional peak shift can be ascribed to an increase in packing density of the chromophores, either by decreasing the distance, by decreasing the tilt of the chromophores, or both. Indeed the isotherms and the BAM images already indicate that there is either an increased spacing between the amphiphiles to a spread out conformation of the side chains bearing the ionic sites or an interaction of the hydrophobic polymer backbone with the hydrophobic region of the amphiphile monolayer. If the backbone and parts of the spacer are incorporated into the hydrophobic region of the monolayer (see Figure 6), a loose packing of the chromophores and a liquidlike character of the monolayer would result. It can be conjectured that the backbone and spacer are repelled from the monolayer upon compression resulting in a decreased chromophore distance. However, a pressure-induced reduction of the distance between the ionic sites (see Figure 6) would be in accordance with the spectroscopic behavior, too. Monolayers as Model Surfaces for Studying the Polyelectrolyte Adsorption. Monolayers of ionic amphiphiles at the air/water interface can be employed to study the polyelectrolyte adsorption onto an oppositely charged surface when the adsorption process results in changes of the monolayer properties that can be monitored.7,8 Among the polyelectrolytes studied here, polymers 3 and 4 do not change the monolayer properties very much. Therefore, monolayers of amphiphile 1 are not suited as model surfaces for these polyelectrolytes. However, complexation with polymer 2 alters the structure of the amphiphile 1 monolayer considerably and results in significantly different UV/vis spectra (see Figure 7). Therefore, we anticipate that a monolayer of the amphiphile 1, prepared on pure water and subsequently brought into contact with a solution containing polymer 2, will change its spectral properties and can be used to monitor the adsorption process. There are several ways to bring a preformed monolayer into contact with a polyelectrolyte solution. The polyelectrolyte can be added by injection of a concentrated solution into the subphase29,30 or the subphase can be exchanged from pure water to the polyelectrolyte solution.7,8 The (29) Taneva, S.; Voelker, D. R.; Keough, K. M. W. Biochemistry 1997, 36, 8173. (30) Ellison, E. H.; Castellino, F. J. Biochemistry 1998, 37, 7997.

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Figure 6. Schematic representation of the situation in a 1/2 complex monolayer at low surface pressure, where the polymer backbone and parts of the spacer are incorporated into the monolayer (left) or the large distance of the ionic sites result in a more tilted arrangement (right), and at high surface pressure (middle), where the backbone and spacer are repelled from the monolayer or the headgroups are pushed together, respectively.

Figure 7. Comparison of the monolayer spectra for amphiphile 1 (a) on pure water and (b) on a subphase containing polymer 2 (5 × 10-5 mol/L with respect to the ionic groups; subphase temperature 20 °C) at an area of 0.46 nm2 per amphiphile molecule and (c) the difference spectrum.

method used in this study was originally described by Fromherz and developed for studying protein adsorption onto lipid layers at the air/water interface by the use of a multicompartment trough.31 The technique comprises spreading and compression of the amphiphile monolayer on a compartment filled with pure water and subsequent transfer to a separated compartment filled with a polyelectrolyte solution.32,33 Amphiphile 1 was spread on pure water, compressed to an area of 0.46 nm2 per amphiphile (zero surface pressure), and transferred to a compartment containing a solution (31) Fromherz, P. Biochim. Biophys. Acta 1971, 225, 382. (32) Kozarac, Z.; Dhathathreyan, A.; Mo¨bius, D. Colloids Surf. 1988, 33, 11. (33) Sundaram, S.; Ferri, J. K.; Vollhardt, D.; Stebe, K. J. Langmuir 1988, 14, 1208.

Figure 8. Absorbance change for an amphiphile monolayer transferred to a subphase containing polymer 2 (c ) 3.8 mg/L) as function of time.

of polymer 2. The UV/vis spectrum recorded directly after the transfer cannot be distinguished from a spectrum recorded on pure water and was taken as reference. Subsequently, the change in the monolayer UV/vis spectrum (∆A) was monitored. As can be seen from Figure 8, the difference in the absorbance compared to that of the pure amphiphile monolayer develops within minutes after the transfer. The shape of the ∆A vs wavelength curve is almost exactly as expected from the difference spectrum (see Figure 7). In particular the maximum of the ∆A vs wavelength curve is at 350 nm as is the case for the difference spectrum. To exclude any change in the spectrum due to the transfer or due to the irradiation with the probing light, a control experiment was carried out, in which the monolayer was transferred to a second compartment filled with pure water. No changes in the spectrum were detected in the control experiment. There-

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Figure 9. Change of the absorbance at 350 nm for a monolayer of amphiphile 1 transferred onto subphases containing the cationic polyelectrolyte 2 at concentration of (a) 13 and (b) 3.8 mg/L.

fore, it can be concluded that in fact adsorption of polymer 2 gives rise to structural changes in the monolayer that in turn result in spectral changes. The change in absorbance at 350 nm (the maximum of the ∆A vs wavelength curve) therefore can be used to describe the adsorption process (see Figure 9). The ∆A350 nm vs time curve has three different regions. There is an induction period in which no change of the monolayer spectrum can be observed. Subsequently, ∆A350 nm increases rapidly with time and finally reaches a plateau as the monolayer coverage saturates. The duration of the induction period strongly depends on the concentration of the polyelectrolyte. It is longer for the lower concentration. However, the extent of change (the height of the plateau) and the rate of the change (steepness of the curve) do not change significantly with the polyelectrolyte concentration. For polyelectrolytes with a flexible backbone, it has been reported that the adsorption of the polymer molecules is diffusion-limited,2,8 and no induction period has been observed. Two possible explanations can be discussed as the origin of the induction period observed in the adsorption of polymer 2. In contrast to previous works in which a chromophore-labeled polymer was used,8 we do not observe the polyelectrolyte adsorption directly, but the structural changes due to the adsorption. Therefore, it can be argued that the induction period may be a result of the time required for the structural changes to take place after the adsorption of the polyelectrolyte. It is reasonable to assume that the incorporation of the hydrophobic part of the polyelectrolyte into the hydrophobic region of the monolayer is slower than the adsorption itself, resulting in a delayed response of the monolayer. A very similar effect already has been observed for azobenzene-containing monolayers: The expansion of a monolayer upon irradiation is significantly slower than the photoisomerization of the azobenzene that causes the expansion.34 However, the delayed response of the monolayer upon adsorption, although it might be a significant contribution, cannot be solely accounted for by the induction period. Experiments in which the surface pressure change upon polyelectrolyte adsorption is monitored have been carried out with rigid and flexible anionic polymers. In these experiments the polyelectrolyte adsorption again is not monitored directly, but only the structural changes upon adsorption. The change in the surface pressure shows an induction period for rigid rodlike (34) Menzel, H. Macromol. Chem. Phys. 1994, 195, 3747.

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polyelectrolytes, but not for flexible polyelectrolytes.17,35 So the rigid rodlike structure of the polyelectrolyte might be the reason for the occurrence of an induction period. Upon adsorption, flexible polyelectrolytes can adjust their conformation in order to cover the whole surface; however, rigid-rod-like polyelectrolytes have a fixed conformation. Therefore, in this case ordering effects may play an important role. To cover the surface of the monolayer completely with rigid-rod-like polyelectrolytes, the polymer molecules have to be parallel to each other. It can be envisioned that in the beginning very few polyelectrolyte rods are adsorbed, resulting only in a minor change of the monolayer properties. These rods are arranged with random orientation, and the side chains with the ionic groups have a maximum distance in order to minimize their Coulombic interactions. In this arrangement the rods occupy a large area and block the surface. For a more complete coverage, the adsorbed polymer molecules have to rearrange themselves; they have to form domains with a parallel arrangement. This reorientation requires movements of the polymer rods, which should be much easier if the monolayer is in a fluid state. As indicated by the difference in the isotherms, the monolayer of the polyelectrolyte complex is in a more fluidlike state, while the pure amphiphile is in a condensed state. Therefore, it can be suggested that the rearrangement of the rods is slow as long as only few polymer molecules are adsorbed. However, as soon as some domains of parallel rods have formed and have induced some local fluidity in the monolayer, the rearrangement, and with that the adsorption of the polymer, becomes faster. The adsorption kinetics of polymer 2 therefore can be compared with an autocatalytic reaction. The exact reason for the induction period observed in the adsorption for polymer 2 onto a monolayer of amphiphile 1 still has to be elucidated, but it can be envisioned that the high degree of order necessary for a full coverage in the case of rodlike polyelectrolytes may play an important role. Although a delayed response of the monolayer upon the polyelectrolyte adsorption cannot account for all effects observedsin particular the fact the there is no induction period in similar experiments with flexible anionic polyelectrolytessit might be a contribution to the induction period observed for the adsorption of polymer 2. Further experiments, in particular employing suitable polyelectrolytes with a flexible backbone and direct observation of the adsorption, are necessary and will be part of future work. Conclusion Complexes of an anionic azo-amphiphile and cationic polyelectrolytes (poly(2,5-bis(1-hexyltriethylammonium)1,4-phenylene) iodide (2), poly(allylamine) hydrochloride (3), and poly(diallyldimethylammonium) chloride (4) are formed in situ at the air/water interface. Complexation of the amphiphile with polyelectrolytes having a flexible backbone (3 and 4) results in a very densely packed monolayer structure. However, monolayers of complexes with 2 are more expanded. Upon compression a change in the structure of the 1/2 complex monolayer takes place, which can be monitored employing UV/vis spectroscopy. A shift of the azobenzene electronic transition with increasing pressure is observed. The significant differences in the spectroscopic properties of the monolayer on pure water and for the 1/2 complex monolayer can be used to monitor the adsorption of polymer 2 to a preformed and already compressed monolayer as a model surface. This (35) Menzel, H.; Engelking, J. In preparation.

UV/Vis Monitoring of Polyelectrolyte Adsorption

has been demonstrated employing the Fromherz technique to transfer a monolayer prepared on pure water to a subphase containing the polyelectrolyte. The first results indicate that there is an induction period in the adsorption process of polymer 2, as monitored by the change in the UV/vis spectra, which might be caused by the rigid-rodlike structure of this polyelectrolyte.

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Acknowledgment. We acknowledge financial support by the Deutsche Forschungsgemeinschaft within the special program “Polyelectrolytes” and the Fonds der Chemischen Industrie. LA990898+