Langmuir 2000, 16, 8871-8878
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Polyelectrolytes I: Polyanion/Polycation Multilayers at the Air/Monolayer/Water Interface as Elements for Quantitative Polymer Adsorption Studies and Preparation of Hetero-superlattices on Solid Surfaces† J. Ruths,‡ F. Essler,‡,§ G. Decher,§,| and H. Riegler*,⊥ Institute for Physical Chemistry, University of Mainz, 55099 Mainz, Germany, C.N.R.S., Institut Charles Sadron (CRM), Strasbourg, France, Universite Louis Pasteur, Strasbourg, France, and MPI-KGF, Max-Planck-Campus, Am Mu¨ hlenberg 1, 14476 Golm/Potsdam, Germany Received February 23, 2000. In Final Form: May 11, 2000 Polyanion/polycation multilayers floating at the air/water interface were prepared by consecutive adsorption of polyelectrolyte layers onto a Langmuir monolayer from aqueous polyelectrolyte subphase solutions. With the positively charged Langmuir monolayer headgroups of the lipid dimethyldioctadecylammonium bromide, the layer sequence starts with the negatively charged polyelectrolyte polystyrene sulfonate. With the negatively charged monolayer dimyristoylphosphatidic acid, it starts with the positively charged polyallylamine. Equally charged monolayer headgroups and polyelectrolytes do not bind to each other. Consecutive subphase exchange cycles of polyelectrolyte solutions with alternating charges and pure solvent in between lead to the formation of floating multilayers consisting of an alternating polyelectrolyte sequence. Ellipsometric measurements show that the thickness (adsorbed amount per unit area) of the multilayer film at the air/water interface grows in proportion to the number of adsorbed polyelectrolyte layers. The thickness of individual layers increases with increasing polyelectrolyte and/or ion subphase concentration, respectively. The floating multilayers can be deposited as a sequence of layers of monomeric lipid and polyelectrolytes (hetero-superlattices) onto solid substrates via Langmuir-Blodgett transfer. UV-absorbance studies corroborate the quantitative interpretation of the ellipsometric data in terms of the polymer concentration in the layers, the individual layer thicknesses, and the adsorbed amounts per unit area.
1. Introduction The consecutive adsorption of polyanions and polycations has meanwhile become a widely used technique for the preparation of organic multilayers.1,2 It allows the preparation of well-defined multilayer architectures. One of the advantages of the technique is the use of electrostatic attraction as the driving force for the adsorption, which is sterically less demanding than the formation of covalent bonds. It also allows multicenter interactions and thus the incorporation of biological polyelectrolytes (proteins, DNA). It is obvious that it is important to understand the details of polymer adsorption to control and to assure the growth of multilayers in a reproducible way. Typically these multilayers are grown on solid substrates at solid/ fluid interfaces. However, for detailed investigations of the adsorption process, adsorption templates floating at gas/fluid interfaces have some advantages. If the template is, for example, a Langmuir monolayer, the layer formation can be monitored, controlled, and manipulated during the adsorption process by monolayer compression and expansion at the air/water interface. This configuration allows the online control of the density of the binding sites, surface charge, etc. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millenium”. ‡ University of Mainz. § C.N.R.S., Institut Charles Sadron (CRM). | Universite Louis Pasteur. ⊥ MPI-KGF, Max-Planck-Campus.
(1) Decher, G. Science 1997, 277, 1232. (2) Donath, E.; et al. Angew. Chem., Int. Ed. Engl. 1998, 37 (16), 2202.
Floating polyelectrolyte complexed monolayers and multilayers of proteins have been investigated before.3,4 But floating multilayers of self-assembled polyelectrolytes, as presented in the following, have, to our knowledge, not yet been described in the literature. In the following we will show that polyelectrolyte multilayers can be prepared in a controlled way at air/water interfaces and their formation can be monitored by ellipsometry. We obtain information on the thicknesses and refractive indices of the adsorbed polyelectrolyte films, which can be translated into adsorbed amounts. The floating films can be deposited onto solid substrates via the Langmuir-Blodgett technique. The resulting films consist of a defined sequence of layers of polyelectrolytes and lipid bilayers. 2. Materials and Methods The polymers polystyrene sulfonate sodium salt (PSS, MW ≈ 80000-100000) and polyallylamine hydrochloride (PAH, MW ≈ 50000-60000) were obtained from Aldrich. The PSS was further purified by dialysis (pore size: 14 000) against pure water and then freeze-dried. The PAH was used as received. The polyelectrolyte concentrations are listed as monomeric moles per liter (monomol/l). The lipids dimethyldioctadecylammonium bromide (DODAB, purity: >99%) and dimyristoylphosphatidic acid (DMPA, purity: >99%) were obtained from Sigma and used without further purification. The monolayers were spread from chloroform solutions (1 mg/mL) on water purified by a Millipore system (Milli-RO 35 and Milli-Q). The floating multilayers were prepared in a small Langmuir trough with a surface area of about 50 cm2 (“Biotrough”, Riegler (3) Paudler, M.; Ruths, J.; Alberti, B.; Riegler, H. Makromol. Chem., Macromol. Symp. 1991, 46, 401-407. (4) Herron, J. N.; et al. Langmuir 1992, 8, 1413-1416.
10.1021/la000257a CCC: $19.00 © 2000 American Chemical Society Published on Web 07/14/2000
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& Kirstein GmbH, Berlin), which allows the exchange of the subphase (subphase volume ≈ 35 mL) with a peristaltic pump. Suitable hardware and an optimized subphase exchange protocol allowed a >99% volume exchange with a minimum amount of flushing solution (≈70 mL). Typically, a complete subphase exchange took about 30 min. After flushing with a polyelectrolyte solution, before rinsing with a solution with oppositely charged polyelectrolyte, the trough was rinsed with polyelectrolyte-free solution (same ionic strength) to avoid precipitation of polyelectrolyte complexes. During the subphase exchange the monolayer area was kept fixed and the water level kept at constant height. The measurements were performed under constant surface pressure corresponding to densely packed, stable monolayers (DODAB, 35 mN/m; DMPA, 25 mN/m). In all cases the amount of polyelectrolyte in the subphase solutions was much more than the amount adsorbed to the interface. The multilayers were deposited onto solid substrates via the Langmuir-Blodgett technique with a home-build dipper at 2-3 mm/min. Quartz slides served as substrates (typical size: 1 × 4 cm2), which were hydrophobized with octadecyltrichlorosilane (OTS) as described elsewhere.5 The ellipsometric measurements were performed with a null ellipsometer.6 Ellipsometry is a nondestructive and sensitive method to investigate interfaces and thin films. It is based on the change of the polarization of a monochromatic light beam upon reflection from an interface. The change in the polarization is related to the optical and geometrical properties of the interface. In the case of a layer at the interface, the change in polarization is determined by the refractive indices of the interfacial layer and the adjacent bulk phases as well as the thickness of the interfacial layer. Typically the change in polarization due to the interface is measured in terms of two ellipsometric angles δ∆ and δΨ. For very thin interfacial layers only the change in δ∆ is significant. In this case the data may be analyzed with the so-called ultrathin film approximation7 as has been done in this report. We have assumed further that all the media are optically isotropic. Further details on the data analysis are given in section 3.1 and in the Discussion section. The films deposited on the quartz substrates were investigated via UV/vis spectroscopy (Perkin-Elmer-Lambda 17).
3. Results Figure 1 shows a schematic of the preparation of the floating multilayer architecture. By consecutive subphase exchange cycles (polyelectrolyte solution-pure solventoppositely charged polyelectrolyte solution, etc.), floating multilayers of up to four PSS/PAH layer pairs adsorbed to the monolayer template were prepared at the air/water interface. If not otherwise remarked (see section 3.4.), the polyelectrolyte solutions and the pure solutions had the same ion concentration in the exchange cycles. There appears no limit for the number of layer pairs. Only for practical reasons (experimental time) we stopped at four layer pairs. It is observed that only the oppositely charged polyelectrolyte layer adsorbs to the charged Langmuir monolayer (PSS onto DODAB or PAH onto DMPA). In the following only experiments with multilayers adsorbed onto positively charged DODAB monolayers will be presented and discussed (the adsorption behavior of polyelectrolyte multilayers onto negatively charged Langmuir monolayers is similar). The adsorption kinetics was faster than the time used for the subphase exchange; i.e., after completion of the subphase exchange no further polyelectrolyte adsorption was observed. Repeated rinsing and adsorption (5) The substrates were first kept for 5 min each in a sequence of methanol, methanol/chloroform (volume ratio 2:1), and chloroform solutions, respectively, under ultrasonication. Then it was kept for 30 min at 30-40 °C in a solution of Decalin, CCl4, chloroform, and OTS (volume ratio 70:20:10:1). Finally, washing with chloroform and methanol/chloroform (volume ratio 2:1) solutions was carried out. (6) Paudler, M.; et al., Makromol. Chem., Macromol. Symp. 1991, 46, 401. (7) Azzam, R. M. A.; Bashara, N. M. “Ellipsometry and Polarized Light”; Elsevier: Amsterdam, 1987.
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Figure 1. Schematic of the preparation of the floating multilayers by consecutive adsorption of polyelectrolyte layers to a Langmuir monolayer.
cycles on the same multilayer film with polyelectrolyte and polyelectrolyte-free solutions of the same ionic strength did not change the ellipsometric signal attributed to the multilayer film. This means that the amount of adsorbed polyelectrolyte and the layer thickness are not changed by this procedure. At least for high ionic strengths and thick multilayers this could be proven unambiguously because in this case it was possible to independently determine the thickness and the refractive index of the film. For thinner films only the constancy of δ∆ is observed (changes in δΨ would have been too small). It might be conceivable that certain combined changes of the refractive indices and the layer thicknesses are in agreement with the observed unchanged δ∆. However, such specific combinations of changes in the thicknesses and refractive indices are very unlikely because constant δ∆ values were always observed under these flushing experiments with various different multilayer thicknesses and different ionic concentrations. The situation is different if the ionic strength has been changed during the flushing cycle. Then changes in δ∆ and δΨ are observed (cf. swelling experiments described in section 3.4.)! 3.1. Refractive Indices of the Polyelectrolyte Layers. The thicknesses of the adsorbed polyelectrolyte multilayer films are between a few angstroms (molecularly thin) and more than 100 Å. This wide range of thicknesses is important for the ellipsometric measurement. If the organic films at the air/water interface are only molecularly thin, then ellipsometric measurements produce only one significant data value: the change in the ellipsometric
Polyanion/Polycation Multilayers
angle, δ∆.8 In this case, for the data interpretation in terms of layer thicknesses d, the refractive indices n of the layers must be known from other measurements. For thicker films (typically >50 Å), the refractive indices and the film thicknesses can be determined independently from the ellipsometric data owing to significant changes in both ellipsometric angles ∆ and Ψ.8 In the case of multilayers this refractive index is an average of the indices of the individual layers weighted with their individual thicknesses. However, in the context of this report it is important to notice that, if the multilayer is already thick enough, both ellipsometric angles change even upon adsorption of only one additional layer. This holds even if the thickness increase/decrease due to this additional layer is only a few nanometers. Therefore, from the change of both ellipsometric angles in the course of the adsorption of single PSS and PAH layers onto already thick multilayer films, the thicknesses and refractive indices of individual PSS and PAH layers could be derived. We obtained refractive indices of nPSS ) 1.484 and nPAH ) 1.468. These refractive index values have also been used to quantify the growth of the molecularly thin films. We took the experimentally observed changes δ∆ in the course of the multilayer growth from molecularly thin to the thick multilayer films and calculated the expected thickness growth with the measured δ∆ and with the refractive indices derived from the measurements on thick films. We find that the total multilayer thicknesses of thick films resulting from the addition of the calculated individual layer thicknesses agrees excellently with the ones directly measured. Therefore it can be concluded that the refractive indices derived from the thick films represent the values of the individual PSS and PAH layers and that these values are independent of their distance to the Langmuir monolayer. That also means that the packing density of the polyelectrolytes is independent from their location. It should be mentioned that the comparison between the calculated and the measured thicknesses of the (thick) multilayers and the resulting estimation on the refractive indices and thicknesses of individual PAH and PSS layers was only possible for subphase ionic concentrations of cNaCl ) 0.1 M and above. At lower salt concentrations the refractive indices and polyelectrolyte concentrations could not be determined independently by ellipsometry. However, for cNaCl ) 0.01 M the proposed refractive indices match the very small δΨ conjectured from measurements with seven-layer films so that there is some vindication to use the values obtained for the high ionic strength also for the calculation of thicknesses and adsorbed amounts at low ion concentration. From the refractive indices, the polymer concentration in the layers can be estimated if the relation between refractive indices and concentration can be determined independently. This relation was obtained from measurements of the change of the refractive indices of polyelectrolyte bulk solutions as function of the polyelectrolyte concentration.9 The refractive index increments were determined at 20 °C up to polyelectrolyte concentrations of 0.12 g/mL. It is observed that up to this concentration the refractive indices increase strictly linearly proportional to the polyelectrolyte concentration (There are no indications for a deviation from the linear increase even at the maximum concentrations investigated). For pure water and salt solutions we obtained the values in Table 1. (8) McCrackin, M. L.; et al. J. Res. Natl. Bur. Stand. U.S.A. 1963, 67A (4), 363. (9) DeFeijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759.
Langmuir, Vol. 16, No. 23, 2000 8873 Table 1. Bulk Refractive Index Increments of PSS and PAH for Different Ionic Strengths solvent
n(PSS)
n(PAH)
H2O 0.5 M NaCl 1 M NaCl
1.3326 + 0.192 mL/g‚cPSS 1.3379 + 0.184 mL/g‚cPSS 1.3425 + 0.182 mL/g‚cPSS
1.3326 + 0.213 mL/g‚cPAH 1.3380 + 0.208 mL/g‚cPAH 1.3424 + 0.205 mL/g‚cPAH
The observed increments are typical for proteins and highly charged polyelectrolytes.10 As an approximation we assume that these refractive index increments are also valid for the high polyelectrolyte concentrations within the adsorbed multilayers. With the refractive indices (nPSS ) 1.484, nPAH ) 1.468) and the increments of Table 1, the polymer concentration within the layers can be estimated as cPSS ) cPAH ≈ 0.7 g/mL for both PSS and PAH. If we assume a typical (dry) density for polymers between F ) 0.9 g/mL and F ) 1.4 g/mL, a water content of 20% and 50% can be estimated. With the estimated polymer concentration the layer thicknesses can be translated into adsorbed amounts per unit area and thickness of Γ/d ≈ 7 × 10-5 g/m2 Å for both polymers. This can be translated further into monomers per unit area and per thickness for PSS (Γ/d(PSS) ≈ 3.8 × 10-7 monomol/m2 Å) and for PAH (Γ/d(PAH) ≈ 1.1 × 10-6 monomol/m2 Å). These estimations are based on molecular weights (MW) per monomer unit without counterion (MWmonomol(PSS) ) 184 g/monomol, MWmonomol(PAH) ) 61 g/monomol). 3.2. Multilayer Growth as a Function of the Polyelectrolyte Concentration. Figure 2 shows the multilayer growth as a function of the polyelectrolyte concentration for low ion concentration (cNaCl ) 0.01 M). The change of the ellipsometric angle ∆ relative to the water value is presented in detail in Figure 2a. Figure 2b displays the corresponding ellipsometric angle increment δ∆ for each layer. The total layer thicknesses d and the adsorbed amounts Γ are plotted in Figure 2c. Both d as well as Γ are derived from the ellipsometric data with the refractive indices presented in the preceding paragraph. In the case of low polyelectrolyte concentration, the layer thickness of the first polyelectrolyte layer directly adsorbed to the monolayer is different from that of the following layers (see especially Figure 2b). The adsorbed amount of the first layer is nearly independent from the polyelectrolyte concentration. For all concentrations its ellipsometer angle increments δ∆ vary only between ≈-1.0 and -1.3. This corresponds to layer thicknesses of ≈4-6 Å, which is equivalent to adsorbed amounts of Γ(PSS) ≈(2.8-4.2) × 10-4 g/m2 (≈(1.5-2.3) × 10-6 monomol/m2). Beyond the first layer the ellipsometric angles (total film thicknesses) increase nearly linearly in proportion with the number of adsorption cycles. Depending on the polyelectrolyte concentration, two distinctly different regimes can be discerned. For low polyelectrolyte concentrations (5 × 10-4 M) the ellipsometric angle increments per layer are typically only δ∆ ≈ -0.3° (equivalent to δd ≈ 1 Å per layer or δΓ(per layer) ≈ 7 × 10-5 g/m2). For higher polyelectrolyte concentrations the layer growth is approximately independent of the polyelectrolyte concentrations. The growth increments are typically between -1.0° and -1.5° ()layer thicknesses of ≈4-6 Å or δΓ(per layer) ≈ (2.8-4.2) × 10-4 g/m2). 3.3. Multilayer Growth as a Function of the Ionic Strength. Figure 3 presents the ellipsometry, thickness, and adsorption data from experiments with five different (10) Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1989.
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Figure 2. Ellipsometric data of floating multilayers for four different polyelectrolyte concentrations (DODAB monolayer, π ) 35 mN/m, T ) 20 °C; the polyelectrolyte concentrations are given in molar concentrations of monomeric units). Figure 2a (top) shows the change of the ellipsometric angle 4. Figure 2b (middle) shows the corresponding ellipsometric angle increment for consecutive layers. Figure 2c (bottom) shows the total layer thickness growth d (with nPSS ) 1.484 and nPAH ) 1.468) as well as the total adsorbed amount (cPSS,PAH ) 0.7 g/mL) as a function of the number of layers (adsorption cycles).
ionic concentrations (NaCl) at constant, medium polyelectrolyte concentration (cPSS,PAH ) 0.01 monomol/L). In contrast to the adsorption behavior at very low polyelectrolyte concentration (see Figure 2b), the behaviors of the ellipsometric angle increments δ∆ of the first and of the following polyelectrolyte layers are not very different. However, for the pure water and the low ionic concentration (0.01 M NaCl), the total film thicknesses grows roughly in proportion to the number of adsorption cycles (δ∆ ≈ const for low ionic strengths, see Figure 3b), whereas at higher ionic strength a more than linearly proportional layer growth is observed. The thickness growth increments are between δd ≈ 4 Å per layer (on H2O) and δd ≈ 40 Å per layer (PSS layer 4 at 1 M NaCl), which translates into δΓ ≈ 2.8 × 10-4 g/m2 and δΓ ≈ 2.8 × 10-3 g/m2, respectively.
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Figure 3. Ellipsometric data of floating multilayers for five different NaCl concentrations (DODAB monolayer, π ) 35 mN/ m, cPSS,PAH ) 0.01 M, T ) 20 °C). The polyelectrolyte concentrations are given in molar concentrations of monomeric units. Figure 3a (top) shows the change of the ellipsometric angle 4. Figure 3b (middle) shows the corresponding ellipsometric angle increment for consecutive layers. Figure 3c (bottom) shows the total layer thickness growth d (with nPSS ) 1.484 and nPAH ) 1.468) as well as the total adsorbed amount (cPSS,PAH ) 0.7 g/mL) as a function of the number of layers (adsorption cycles).
In Figure 4 the data of the thicknesses and adsorbed amounts from Figure 3c are plotted as a function of the square root of the subphase ionic strength. The plot shows a linear increase of individual layer thicknesses (adsorbed amounts per layer). As a consequence also the total film thicknesses grow with the square root of the ion concentration except for some offset for ion-free solutions owing to the layer growth even for the pure H2O subphases. 3.4. Multilayer Composition as a Function of the Subphase Conditions. A multilayer architecture of two layer pairs was adsorbed onto DODAB at the high subphase ionic strength of 0.5 M NaCl. The experimental conditions were identical to the experiments described in section 3.3. (cPSS,PAH ) 0.01 monomol/L, T ) 20 °C, πDODAB
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Figure 4. Change of the layer thickness and adsorbed amount as a function of the square root of the ionic strength for individual layers.
) 35 mN/m). After the formation of the multilayer, the NaCl subphase was replaced by pure water and then again by a subphase with the original NaCl concentration. During the subphase exchange cycle the area per molecule of the DODAB monolayer was kept constant. In the course of this cycle the ellipsometric angle ∆ changed reversibly between 340.02° (NaCl subphase) and 341.33° (pure water). Ψ also changed reversibly between 2.905° and 3.040°. This means the following: (1) The reversibility indicates that the polyelectrolytes do not desorb during the exchange cycle. (2) The observed changes of the ellipsometric angles are much larger (by a factor of 9 in the case of δ∆!) than what can be attributed only to the changes of the subphase refractive indices. Therefore a reversible swelling and shrinking of the polyelectrolyte multilayer together with a change in the film refractive indices has to be concluded. Simulations with the measured ellipsometric angles and the assumption of the conservation of the adsorbed amount of polymer indicate an increase of the film thickness (swelling) of ≈60% with the pure water subphase. This can be estimated independently and in good agreement from either the change of both ellipsometric angles alone or from calculations assuming a 60% swelling with constant amounts of adsorbed polyelectrolyte based on the relation between refractive index and polyelectrolyte concentration as measured on bulk polyelectrolyte solutions (see Table 1). 3.5. Deposition of Monomer/Polyelectrolyte Multilayers onto Solid Substrates via LB Transfer. The floating multilayers (cNaCl ) 0.5 M, cpolymer ) 10-2 M) can be deposited onto solid substrates by the LB technique (Figure 5). The deposited multilayers were investigated by UV/vis spectroscopy (Figure 6). The peaks at 196 and 226 nm can be attributed to the styrene sulfonate chromophores. The linear increase of the absorbance with the number of dipping cycles k indicates a quantitative Langmuir-Blodgett transfer of the floating multilayer film onto the solid substrate. From the observed extinction values E and the literature value of the extinction coefficient of the styrene sulfonate chromophore in solution11 (PSS(λ ) 226 nm) ) 1 × 103 m2/monomol), the adsorbed amount ΓLB of PSS can be calculated:
ΓLB ) E/
(1)
For four dipping cycles k, for instance, we observe E ≈ 0.18, which means ΓLB(spectroscopy) ≈ 1.8 × 10-4 mono(11) Vink, H. Makromol. Chem. 1980, 182, 279.
Figure 5. Schematic of the preparation of hetero-superlattices ()mixed layers of lipids and polyelectrolytes) by transfer of the floating multilayers onto hydrophobized solid substrates with the Langmuir-Blodgett technique.
Figure 6. UV/vis spectra of hetero-superlattices deposited onto quartz substrates (hydrophobized with OTS) via the LB technique. k indicates the number of dipping cycles (one cycle is one downstroke plus one upstroke). The floating multilayers consisted of the DODAB monolayer plus 5 polyelectrolyte layers (3 × PSS and 2 × PAH, πDODAB ) 35 mN/m, cpolyelectrolyte ) 0.01 M, cNaCl ) 0.5 M, T ) 20 °C, vup ) 3 mm, vdown ) 5 mm, waiting times, tup ) 15 min, tdown ) 1 min). The extinction E is already normalized to one substrate surface.
mol/m2. We can compare these UV/vis data with the ellipsometric results. We assume that the multilayer is completely transferred onto the solid substrate without lateral expansion/compression or other mechanisms of loss or gain of material. According to the data of Figure 3c, one multilayer film with three PSS (and two PAH) layers corresponds to ≈Γ(PSS) ≈ 40 × 10-4 g/m2. This can be translated into ≈2.2 × 10-5 monomol/m2. The LB transfer means the deposition of 4 × 2 ) 8 of these layers, as result of the number of dipping cycles (4) and 2 layers per cycle (upstroke, downstroke). Thus we expect for the LB film according to the ellipsometric data ΓLB(ellipsometry) ≈ 1.7 × 10-4 monomol/m2, which is in good agreement with the spectroscopic results (E226 nm,k)4 ≈ 1.9, i.e., ΓLB ) 1.9 × 10-4 monomol/m2). 4. Discussion This report presents a quantitative analysis of the buildup of floating polyelectrolyte multilayers via adsorption from bulk solutions (“self-assembly”) at air/monolayer/ solution interfaces. The data are based on ellipsometric measurements that are complemented and corroborated by refractive index increment measurements on bulk solutions and UV/vis spectroscopy data. This supplemen-
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tation of the ellipsometry data is crucial and improves decisively the significance of the data interpretation. The quantitative analysis of ellipsometric data from ultrathin organic layers in terms of layer thicknesses and adsorbed amounts can be disputed.12 As already mentioned, the problem arises from the ambiguity of the experimental data (one, at best two, independent ellipsometric angle) with respect to details of the adsorbed layer (layers with boxlike or even more complicated refractive index profiles normal to the interface). In our case, with a data interpretation based on the ellipsometric angle(s), the bulk refractive index increments, and the adsorbed amounts (from the UV/vis measurements on the transferred films), it seems reasonable to quantitavely describe the film formation in terms of layer thicknesses (i.e., a simple box model) and polymer concentrations within the layers. With the bulk refractive index increment data this can be translated into adsorbed amounts per unit area and thickness. Lateral corrugations and interfacial roughnesses normal to the interfaces will exist in the real films, but they cannot be incorporated into the film model because this would definitely overstress the significance of the data. This, some uncertainty with the extrapolation of the bulk refractive index increment data to the high polymer concentrations in the layers, as well as the extrapolation of the ellipsometric data from high ion concentrations (with changes in δ∆ and δΨ) to those from low ionic strengths (with only changes in δ∆), has to be kept in mind. Still, the measurable changes of both ellipsometric angles in the case of thicker films render the interpretation in terms of density and thickness ()equivalent to adsorbed amounts) reasonable. The measurements from the thicker films are the starting point for the detailed quantitative interpretation. With thicker films we consistently obtain ellipsometric data indicating refractive indices of 1.484 for the PSS layers and 1.468 for the PAH layers. It is remarkable and an indication for the validity of the numbers that we can calculate with these indices also individual thicknesses of the thinner films, which in sum make up a multilayer film whose total thickness matches quite well the ones measured directly. The refractive indices have been used together with bulk refractive index increment data to determine the polymer concentration in the layers. The resulting density of 0.7 g/mL is plausible. With an estimated density of the dry polymers between 0.9 and 1.4 g/mL this means a water content of about 20-50%. It is well worth noticing that the polyelectrolyte density deduced from the measurements is within a reasonable range. It is below the upper limit of ≈1.4 g/mL but still high enough to represent a layer of concentrated polymers. To better understand the details on the layer growth, it is helpful to first roughly estimate a few characteristic values for the polyelectrolytes in solution. The PSS molecules consists of typically NPSS ≈ 500 monomeric units. With a length of l ≈ 3 Å per monomeric unit, the contour length for the PSS molecules is LPSS ≈ 1500 Å. The radius of gyration Rg dependssinter aliason the persistence length, which in turn is strongly dependent on the ion concentration. If we assume that the polyelectrolytes are completely uncharged (in the case of complete shielding at very high ion concentration), we can estimate very crudely a minimum radius of gyration for PSS of Rg,PSS,min ≈ 30 Å (with Rg2 ) 1/6l2N13 and l ) 3 Å). Of course, it is well-known that Rg can be defined in various different (12) Teppner, R.; et al. Langmuir 1999, 15 (20), 7002. (13) Polyelectrolytes; Masanori, Ed.; Marcel Dekker: New York, 1993.
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ways.13,14 Also, the assumed persistence length is only a crude estimation. Nevertheless, Rg,PSS,min ≈ 30 Å is good enough for some rough estimations. For PAH the numbers are NPAH ≈ 900, LPAH ≈ 2700 Å, and Rg,PAH,min ≈ 36 Å. The free volume per macromolecule is vPoly ≈ 1.7 × 10-24[m]N/CP[M] with CP ) concentration of the monomolecular units. With a maximum polymer concentration of CP ) 1 × 10-2 M investigated in this article, we obtain vPSS,min ≈ 8.3 × 107 Å3 (vPAH,min ≈ 1.5 × 108 Å3), which is equivalent to a minimum average distance between the molecules of dmin ≈ 500 Å. This is about 1 order of magnitude larger than the minimum radius of gyration but still in the range of the contour length. The ion concentration CS at which the polyelectrolyte solution approaches that of the behavior of conventional, uncharged polymers has been estimated as roughly CS,conv ≈ 0.5ZCPN-1 ) 0.5CP with Z ) average number of elementary charges per macromolecule n.14 The polymer concentrations that were investigated ranged from CP ) 5 × 10-4 M to CP ) 10-2 M, which corresponds to CS,conv between 2.5 × 10-4 M and 0.5 × 10-2 M. As the ion concentrations were varied from 0 to 1 M, the conformation of the polyelectrolytes will have changed from roughly rodlike to more globular. If we assume perfectly rodlike macromolecules (low ion concentration), the average distance between the molecules equals the contour length at concentrations of roughly CPSS,L ≈ 2.5 × 10-4 M and CPAH,L ≈ 7.8 × 10-5 M. From the ellipsometric data we have estimated that the polyelectrolyte density is roughly the same for both polymers and for all ion concentrations. Taking into account the shielding effect of the ions, which leads to drastical conformation changes of the molecules, this may seem unreasonable. One has to take into account, however, that in the case of low ionic strengths the molecules lie flat at the interface as rods (approximately). At high ionic strengths they adsorb as globular blobs. The volume per molecule of a flat adsorbed rod Vrod,ads is
Vrod,ads ≈ lNd(d + x)
(1)
with d as the diameter of the rod and x the additional distance due to the electrostatic repulsion between the rods. For eq 1 it is assumed that the layer thickness d is the diameter of the rod; i.e., there is no electrostatic repulsion between the molecule and the interface(s). The volume per blob is
Vblob,ads ≈ (2Rg)3 ≈ (2l(N/6)0.5)3
(2)
Both volumes (and thus also the densities) are equal for distances xequ:
xequ ≈ l2N0.5/(2d) - d
(3)
With typical numbers (NPSS ) 500, l ) 3 Å, d ) 4 Å (See Figure 3c)), one obtains xequ ≈ 17 Å. For PAH one obtains xequ ≈ 26 Å. These are reasonable numbers taking into account the attractive electrostatic interaction between molecule and interface, which counteracts the electrostatic repulsion that will maximize the intermolecular distance x. This crude calculation shows that the polyelectrolyte density can be similar at low and high ionic strength despite significant differences in the intermolecular repulsion. Of course there are big differences in the layer (14) Polymers at Interfaces; Fleer, G. J., Cohen Stuart, M. A., Scheutjens, J. M. H. M., Cosgrove, T., Vincent, B., Eds.; Chapman & Hill: London, 1993.
Polyanion/Polycation Multilayers
thickness at different ion concentrations (2Rg,PSS ≈ 50 Å, 2Rg,PAH ≈ 70 Å, compared to 3 Å)! The results presented in Figure 2 represent a regime with comparatively low shielding due to the low ion concentration. Hence the conformation of the polyelectrolytes can be assumed to be more rodlike than globular. This is confirmed by the layer thicknesses. To a first approximation, if the adsorbed molecules do not change their conformation after adsorption, the layer thickness is indicative of the smallest dimension of the polyelectrolyte conformation in solution, i.e., the diameter d of the polymer rods. To a first approximation the results for polymer concentrations of 10-3 M and above indeed confirm this adsorption scenario. In this regime the layer thickness is of the dimension of the molecular diameter and largely independent from the polyelectrolyte bulk concentration as can be expected from this simple “hitand-stick” adsorption mechanism. For the lowest polymer concentration, however, we observe something different. The amount adsorbed to the monolayer is about the same as the one adsorbed at higher concentration, whereas the second and following layers are significantly thinner than those for the higher concentrations. The thicker first layer proves that the thinner layers are not a consequence of not enough polyelectrolyte available from the subphase bulk solution or because there was not enough time available for approaching the “equilibrium” layer thickness (equilibrium in the sense that the thickness does not change any more within any reasonable experimental time). It also shows that in this concentration regime the layer thickness is not a function of the bulk polyelectrolyte conformation alone. It also depends on the specific interface properties. The lowest concentration is close to the concentration of the crossover between the dilute (vP > NL) and semidilute (vP < NL) regime for perfectly rodlike molecules (CPoly,L ≈ 1 × 10-4 M, see above). This means that at the lowest concentration the molecules adsorb to the interface individually. Then they have some time to adjust to the new environment before they “feel” neighboring adsorbed molecules. At high concentrations they adsorb to the interface while they are constantly “in touch” with neighboring molecules already in the bulk phase. Hence there is no time for independent conformational arrangements. The difference between the bare monolayer surface and the surface already covered with a polyelectrolyte layer of opposite charge is presumably the higher surface charge of the monolayer. In this case the interface/ polymer interactions are stronger, the adsorbed polymer sticks tighter to the interface, and it takes much longer to change its conformation. Therefore the conformation of the molecules in the layer directly adsorbed to the monolayer is more reminiscent of that of the bulk solution. This scenario is in agreement with the fact that at higher polyelectrolyte concentrations the layer thicknesses on both types of interfaces, the bare monolayer and the one already coated with polymer, are comparable. The data of Figures 3 and 4 prove the transition from a rodlike to a more globular conformation and the concommitant expected increase with layer thickness with increasing ionic strength. The maximum individual layer thickness of ≈40 Å (last layer for CNaCl ) 1.0 M) indeed comes within the range of the estimated diameter of a completely globular conformation. The thickness increase of individual layers follows remarkably well the theoretically predicted dependency on the square root of the ionic strength (Figure 4). However, why the adsorbed amount also increases with increasing layer numbers is not so well understood. It might be due to roughness effects that lead to an increasing effective interfacial area with
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increasing total film thickness. That there is some interfacial roughness seems plausible. Each layer consists of at most one polyelectrolyte monolayer. The increased thickness compared to the layers at low ionic strength can only be attributed to the more globular polyelectrolyte conformation. This, of course, increases the interfacial corrugations. The consequence is an increased effective interface area that leads to a blossoming, fractallike growth. The nonlinear film thickness growth is presumably not caused by the grossly different surface charge conditions because the growth on the bare monolayer surface is similar to the one on the already polyelectrolytecoated interfaces. Presumably the transition between the linear growth for concentrations up to CNaCl ) 10-2 M and the more than proportional growth at higher ionic strengths is not related to the transition between the diluted and the semidiluted regime. If the interpretation of the data of Figure 2 is correct, then a concentration of 10-2 M for both the polymers and the salt represents already the semidiluted range. Altogether, the adsorbed amounts are in remarkably good agreement with values derived from lightscattering measurements for the adsorption of polyelectrolyte multilayers in colloidal systems.15 The reversibility of the swelling as described in section 3.4 shows that the polyelectrolytes do not desorb during the flushing cycle. Quite remarkable, that the adsorbed amount of polyelectrolyte remains unchanged can independently be deduced from the analysis of the change of both ellipsometric angles in combination with the observed refractive index increment! The results of the swelling experiment mandates two conclusions. First, its reversibility verifies the nonequilibrium character of the polyelectrolyte adsorption. Second, the ≈ 60% thickness increase upon reducing the ion concentration from 0.5 M NaCl to pure water seems at first sight to contradict the results presented in Figure 3 where we showed that increasing salt concentration means increasing layer thicknesses (and increasing adsorbed amounts). It also seems to contradict the assumption that the polyelectrolyte density in the layers is roughly independent of the ion concentration as discussed above, on which the analysis of the results presented in Figures 2 and 3 is based. However, as the reversibility already indicates, the swelling experiment cannot be compared to the other adsorption experiments. At high ionic strengths the polyelectrolyte molecules adsorb in a partially globular conformation forming a layer whose thickness is somehow given by the smallest dimension of the polymer blob. If the ion concentration is then lowered, the shielding of the polyelectrolyte charges is reduced and the repulsive interaction within different parts of the same molecule and between different molecules increases. If the molecules had the choice, then they would try to achieve a more rodlike conformation and to adsorb flat to the interface. Owing to the intermolecular repulsion they in any case form at most one monolayer. Of course, a monolayer formed by blobs is thicker than a monolayer of flat adsorbed rodlike molecules. Therefore some of the polyelectrolyte would have to desorb. This, however, is barely possible. First, each individual molecule is bound to the adjacent interface of opposite charge. Second, even if these bindings would break, the molecules would have problems detaching into the subphase because the swelling experiments were performed with multilayers; i.e., the molecules were sandwiched within the film. The only choice for the molecules to adjust to the increased repulsion is to stretch (15) Sukhorukov, G. B.; et al. Colloids Surf., A 1998, 137, 253.
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normal to the interface. This leads to the observed increase of the layer/film thickness and the concommitant decrease of the density. Reversible swelling of the polyelecrolyte films as a consequence of a change of the ionic strength has also been observed with other systems.16 Only at first sight it seems astonishing that the floating multilayers can be transferred onto solid substrates via the Langmuir-Blodgett method in view of bringing together layers of the same composition and charge at the meniscus. However, the first film deposited onto the hydrophobized surface via downstroke is straightforward. As soon as the substrate is then moved upward, the film has only a few choices. It could break at the meniscus, leading to a “transfer” of a thin water film or exposing the hydrophilic polyelectrolyte layer to the (hydrophobic) air. This is energetically very unfavorable if one takes into account a surface tension of about 72 mN/m for this case compared to some 30 mN/m less energy for the interface covered with the monolayer. Second, some of the polyelectrolyte layers sandwiched between the monolayer might come off and dissolve in the subphase. In view of the strong, virtually irreversible adhesion of polyelectrolytes to the interfaces, this is very unlikely. In any case, in view of optimizing electrostatic interactions it would be sufficient if only one of the most inner layers comes off and dissolves in the subphase. The UV/vis spectroscopic data show that there is a quantitive Langmuir-Blogdgett transfer.17 The spectrocopic data even strongly corroborate the quantitative interpretation of the ellipsometric data. 5. Summary and Conclusion It has been shown that multilayers floating at the air/ water interface can be prepared from polyelectrolytes via controlled adsorption from solutions. The range of polyelectrolyte and ion concentration that was investigated covers the regime between diluted and semidiluted bulk solutions. The polyelectrolyte conformation varies between more rodlike and more globular. The layer growth was investigated via ellipsometry, bulk solution refractive index measurements, and UV/vis studies. The data were (16) Donath, E.; Budde, A.; Knippel, E.; Ba¨umler, H. Langmuir 1996, 12, 4832. (17) Neutron and X-ray diffraction data (publication in preparation) from samples prepared as depicted in Figure 5 reveal many details on the internal structure of the deposited hetero-superlattices. The results corroborate the formation of hetero-superlattices as depicted in the figure.
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analyzed in terms of layer thicknesses and polyelectrolyte densities in the layers (adsorbed amounts) with a focus on coherence and cross-check between the data obtained by the different methods. Except for the lowest polyelectrolyte concentration, the adsorbed layer thickness (adsorbed amount of polymer per unit area) is largely independent of the polyelectrolyte concentration. This is in agreement with a scenario of a high-affinity isotherm. For the lowest concentration the adsorbed amount depends on the interface onto which the molecules adsorb. On the highly charged monolayer the layer thickness is comparable to the one observed at higher concentrations. For the adsorption onto interfaces already covered with polyelectrolyte of opposite charge, we observe much thinner layers, which we attribute to molecular conformation changes after adsorption. The layer thicknesses increase with the square root of the ion concentration in good agreement with theoretical predictions. We also observe a more than proportional growth with the layer number. This might be attributed to roughness effects. Swelling experiments where the same polyelectrolyte multilayer film has been exposed to a sequence of subphases of high and low ion concentration reveal that the adsorption is irreversible. At low ion concentration the layer is ≈60% thicker with a corresponding decrease of the polyelectrolyte density. The floating multilayers can be transferred quantitatively onto solid substrates with the Langmuir-Blodgett method. In the context here the transferred multilayers were predominantly used to (successfully) cross-check the validity of the analysis of the ellipsometric data with UV/ vis measurement performed on the transferred films. Altogether, the individual polyelectrolyte layer thicknesses varied between a few angstroms (i.e., a few 10-4 g/m2) for very low ionic strength (pure water) and several tens of angstroms (≈10-2 g/m2). Irrespective of the ion concentration, the refractive indices of the polyelectrolytes in the layers is ≈1.484 (PSS) and ≈1.468 (PAH). This can be translated into polyelectrolyte densities of about 0.7 g/mL within the layers, which corresponds to water contents of about 20-50%. Acknowledgment. Helpful discussions with Karlheinz Graf, Helmuth Mo¨hwald, and especially Edwin Donath are appreciated. LA000257A
