Modeling Adsorption Processes of Poly-p-phenylenevinylene

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Langmuir 2009, 25, 2166-2171

Modeling Adsorption Processes of Poly-p-phenylenevinylene Precursor and Sodium Acid Dodecylbenzenesulfonate onto Layer-by-Layer Films Using a Langmuir-type Metastable Equilibrium Model Alexandre Marletta,*,† Raigna A. Silva,† Paulo A. Ribeiro,‡ Maria Raposo,‡ and De´bora Gonçalves§ Instituto de Fı´sica, UniVersidade Federal de Uberlaˆndia, CP 593, 38400-902 Uberlaˆndia-MG, Brazil, CEFITEC, Departamento de Fı´sica, Faculdade de Cieˆncias e Tecnologia, UniVersidade NoVa de Lisboa, 2829-516 Caparica, Lisboa, Portugal, and Instituto de Fı´sica de Saõ Carlos, UniVersidade de Saõ Paulo, CP 369, 13660-970 Saõ Carlos-SP, Brazil ReceiVed September 20, 2008. ReVised Manuscript ReceiVed NoVember 17, 2008 The adsorption kinetics curves of poly(xylylidene tetrahydrothiophenium chloride) (PTHT), a poly-p-phenylenevinylene (PPV) precursor, and the sodium salt of dodecylbenzene sulfonic acid (DBS), onto (PTHT/DBS)n layerby-layer (LBL) films were characterized by means of UV-vis spectroscopy. The amount of PTHT/DBS and PTHT adsorbed on each layer was shown to be practically independent of adsorption time. A Langmuir-type metastable equilibrium model was used to adjust the adsorption isotherms data and to estimate adsorption/desorption coefficients ratios, k ) kads/kdes, values of 2 × 105 and 4 × 106 for PTHT and PTHT/DBS layers, respectively. The desorption coefficient has been estimated, using literature values for poly(o-methoxyaniline) desorption coefficient, as was found to be in the range of 10-9 to 10-6 s-1, indicating that quasi equilibrium is rapidly attained.

1. Introduction The layer-by-layer (LBL) technique is being extensively addressed for the fabrication of molecular optoelectronics devices and biosensors at nanometer scale.1-5 Recently, the LBL technique has been used to fabricate organic light emitting diodes,6-9 mainly due to the possibility of obtaining functional multilayer macromolecular structures. In this case, the structure of the LBL device corresponds to a set of semiconductor/insulator junctions along the film growing direction, which will modify both optical and electrical properties of the emitting layers. These modifications include enhancement in emission efficiency and increase of potential barriers at the polymer/polymer interfaces. This last feature is not desirable in such electroluminescent LBL devices, and one should work toward the fabrication of emitting layers without the presence of barrier junctions between layers. This can be achieved via the LBL technique by assembling adequate polyelectrolytes. Poly(p-phenylenevinylene) (PPV) is one of the most studied light emitting polymers, particularly when prepared as a film by means of an alternative synthesis route that makes use of the water-soluble polyelectrolyte poly(xylylidene tetrahydrothiophe* Corresponding author. Tel.: +55-34-32394190. Fax: +55-34-32394106. E-mail: [email protected]. † Universidade Federal de Uberlaˆndia. ‡ Universidade Nova de Lisboa. § Universidade de Saõ Paulo. (1) Oliveira, O. N., Jr.; Raposo, M.; Dhanabalan, A. In Handbook of Surfaces and Interfaces of Materials; Nalwa, H. S., Ed.; Academic Press: New York, 2001; Vol. 4, p1. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 831, 210–211. (3) Lvov, Y.; Haas, H.; Decher, G.; Möhwald, H.; Kalachev, A. J. Phys. Chem. 1993, 97, 12835. (4) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481. (5) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. AdV. Mater. 2008, 9, 014109. (6) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (7) Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806. (8) Fou, A. C.; Rubner, M. F. J. Appl. Phys. 1996, 79, 7501. (9) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067.

nium chloride) (PTHT). In this case, the LBL technique can be used to produce ultrathin PTHT + polyanion films in a nonself-limited process. Subsequently, the LBL PPV films are obtained using the thermal conversion reaction at above 200 °C under primary vacuum environment during 6 h. As a remark, the polymeric precursor route is the principal advantage of PPV when compared to soluble conjugated polymers, such as poly[2methoxy-5-(2′-ethylhexyloxy)-1,4-p-phenylenevinylene] (MEHPPV), because the PPV precursor allows it to be processed in LBL films. A new approach based on this feature has been addressed for the fabrication of PPV LBL films using PTHT and sodium salt of dodecylbenzenesulfonate (DBS) acid.10 This procedure has an additional advantage over traditional PPV films: the reduction of PTHT precursor thermal annealing temperature and increase in emission efficiency. This behavior has been observed by Morgado et al.11 when analyzing the role of conversion temperature on the physical properties of a PPVbased copolymer to which poly(ethylene oxide) (PEO) and lithium triflate were added. It was shown that solvation and complexation of lithium triflate by the PEO reduces the quenching of photoluminescence efficiency by the ionic charge and the effectiveness of formation of highly doped, low-barrier, polymer/ electrode interfaces. As the adsorption processes are dependent on substrate initial roughness,12-15 the amount in the first layer adsorbed onto the solid substrate is different from that in already adsorbed layers. The adsorption kinetic curve of an adsorbed layer onto an already (10) Marletta, A.; Castro, F. A.; Borges, C. A. M., Jr.; Faria, R. M.; Guimara˜es, F. E. G. Macromolecules 2002, 35, 9105. (11) Morgado, J.; Friend, R. H.; Cacialli, F.; Chuah, B. S.; Moratti, S. C.; Holmes, A. B. J. Appl. Phys. 1999, 85, 1784. (12) de Souza, N. C.; Silva, J. R.; Pereira-da-Silva, M. A.; Raposo, M.; Faria, R. M.; Giacometti, J. A.; Oliveira, O. N., Jr J. Nanosci. Nanotechnol. 2004, 4, 548. (13) Ribeiro, P. A.; Steitz, R.; Estrela-Lopis, I.; Haas, H.; Souza, N. C., Jr.; Raposo, M. J. Nanosci. Nanotechnol. 2006, 6, 1396. (14) Ferreira, Q.; Gomes, P. J.; Nunes, Y.; Maneira, M. J. P.; Ribeiro, P. A.; Raposo, M. Microelectron. Eng. J. 2007, 84, 506. (15) Ferreira, Q.; Gomes, P. J.; Maneira, M. J. P.; Ribeiro, P. A.; Raposo, M. Sens. Actuators B: Chem. 2007, 126, 311.

10.1021/la803093n CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

Modeling Adsorption Processes of PPV Precursor

Langmuir, Vol. 25, No. 4, 2009 2167

Figure 1. Chemical structures of the used electrolytes: poly(xylyliden tetrahydrothiophenium chloride) (PTHT), the poly-p-phenylenevinylene (PPV) precursor, and the sodium salt of dodecylbenzenesulfonic acid (DBS).

adsorbed one can be determined by preparing LBL films with different adsorption periods of time as shown in ref 16. The adsorption parameters then can be calculated from the adsorption kinetics curves obtained from solutions having different polyelectrolyte concentrations. This work contributes to the adsorption parameters calculation of PTHT and PTHT/DBS molecules adsorbed onto N bilayers of PTHT/DBS, (PTHT/DBS)N, LBL films already deposited onto quartz substrates. It should be remarked that for the PPV films investigated here to be used as light emitting diodes, they should be adsorbed onto transparent electrodes as indium-tin-oxide, although for convenience quartz has been used here as substrate. The substrate is not relevant in what concerns the growth of a polymer layer onto already adsorbed layers. The UV-vis spectra of (PTHT/DBS)N LBL films prepared with a different number of layers allowed the determination of the adsorbed amounts on each layer and one to obtain both adsorption kinetics and isotherms curves. The Langmuir-type metastable equilibrium model was used to adjust the experimental adsorption isotherms data and to obtain the adsorption parameter k ) kads/kdes, which gives information on the interactions magnitude of both PTHT and PTHT/DBS onto (PTHT/DBS)N LBL films.

2. Experimental Details The PPV films were prepared by the layer-by-layer (LBL) technique using the polyelectrolyte, poly(xylyliden tetrahydrothiophenium chloride) (PTHT), and the electrolyte, sodium salt of dodecylbenzenesulfonic acid (DBS). Figure 1 displays the chemical structures of the used electrolytes in the preparation of LBL films. The substrates were hydrophilized quartz plates/slides cleaned following some of the RCA protocol steps. Initially, the substrates were immersed in a H2O:H2SO4 solution at 7:3 v/v, and then in a H2O:H2O2:NH4OH solution at 5:1:1 v/v, both at 80 °C, for 30 min. After this, the substrates were exhaustively washed with ultra pure water. The deposition of the PTHT and DBS layers was carried out as a function of substrate immersion time, respectively, into the PTHT and DBS solutions, and also solution concentration. The aqueous PTHT and DBS solutions were prepared with concentrations of 2.6 × 10-4 mol L-1 (C0PTHT) and 10-1 mol L-1 (C0DBS), respectively. As a remark, the DBS solutions become saturated above 10-1 mol L-1 concentration. Lower concentrations were attained by dilution. Ultrapure water with a resistivity of 18.2 MΩ cm was supplied by a Millipore system (Milli-Q, Millipore GmbH). The substrates immersion times into the PTHT and DBS solutions, defined as t1 and t2, respectively, were varied from 2 s, 5 s, 10 s, 1 min, 3 min, and 10 min. After adsorption of each layer, the films were washed with ultra pure water and dried with a nitrogen flux. Both PTHT and DBS adsorbed amounts per unit of area onto already adsorbed layers were obtained by measuring the absorbance (16) Raposo, M.; Oliveira, O. N., Jr. Langmuir 2002, 18, 6866.

Figure 2. Absorbance spectra of PTHT/DBS LBL films as a function of the number of bilayers, N. The inset displays the absorbance intensity at 200 nm (-0-) and 325 nm (-O-) as a function of the number of bilayers, N. For comparison, the absorbance spectrum of a spin-coated film is also present.

UV-vis spectra with a Hitachi U-2001 spectrophotometer. The process of thermal conversion of PTHT into PPV was also accompanied by measuring the UV-vis spectra.

3. Results and Discussion 3.1. Buildup of Layer-by-Layer Films. Figure 2 shows the UV-vis spectra of LBL PTHT/DBS films with different number of bilayers (N). The immersion times of the substrates in the PTHT and DBS solutions, t1 and t2, were initially chosen as 3 min. Three main absorption bands are seen in the spectra, at about 200, 235, and 325 nm, which can be associated with localized electronic transitions within sulfonate groups and aromatic rings in DBS and PTHT (ca. 200-230 nm) and nonlocalized electronic transitions of a stilbene-like PPV structure (ca. 325 nm). The absorbance spectrum of a conventional spincoated PTHT film on quartz is also presented in Figure 2, where a similar line shape, by comparing to those obtained for LBL PTHT/DBS films, can be observed. The major differences are the better peak definition in the spectra of LBL films below 270 nm and the relative intensity decrease for the band in the 270-400 nm region, which is assigned to stilbenes aggregation.17 Possibly, in accordance with ref 10, the presence of DBS in the LBL film decrease the stilbene aggregation, and this facilitates the PTHT conversion as it has already been observed in ref 10. The inset of Figure 2 shows the maximum absorbance as a function of number of bilayers, which reveals a linear increase with number of bilayers. This behavior indicates that it can be possible to build up thin LBL films by using the polyelectrolyte, PTHT, and the electrolyte, DBS, with accurate thickness control. Previous results on LBL PTHT/DBS indicated a value of 3 nm of thickness per layer.10 Figure 3 shows the UV-vis spectra at the nonlocal transition range (πfπ* transition) of PPV for 10-bilayer PPV/DBS LBL films thermally converted at 110 and 230 °C, together with the spectrum of PTHT/DBS LBL film before the thermal conversion to PPV. After the thermal treatment, the average conjugation (17) Aguiar, M.; Akcelrud, L.; Pinto, M. R.; Atvars, T. D. Z.; Karasz, F. E.; Saltiel, J. J. Photosci. 2003, 10, 149.

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Figure 3. Absorbance spectra of 10 bilayers of PPV/DBS LBL film thermally converted at 110 °C (-) and 230 °C (- - -), and of 10 bilayers of PTHT/DBS LBL film.

length of polymer chain is seen to increase, an effect that can be evidenced by the evolution of the nonlocalized band of conjugated PPV (at ca. 450 nm). As previously reported,10 chlorine contra-ions, primarily in PTHT, are replaced by DBS contraions during the LBL process. In this case, the thermal conversion of PTHT to PPV takes place at 110 °C instead of 230 °C, as verified in conventional PPV films prepared from PTHT without the presence of a long-chain counterion such as DBS.10 The red shift in the UV-vis spectrum of PPV film converted at 230 °C indicates that the major part of the tetrahydrothiophenium lateral groups of PTHT is eliminated. 3.2. Kinetics of Adsorption. The linear dependence of the absorbance maximum at the wavelengths of 200 and 325 nm, shown in the inset of Figure 2, indicates that the adsorbed amount per unit of area also increases linearly with the number of bilayers. As previously shown, the adsorption kinetics of a given layer in a multilayer structure can be established from the slope of maximum absorbance versus number of bilayers.14,15 This method was chosen because the LBL procedure departs from common adsorption studies where the polyelectrolyte is injected in the solvent that contains the adsorbent. On the contrary, during the production of LBL films, the substrate is immersed in the polymer solution in such a way that the substrate/adsorbent is instantaneously in contact with the polyelectrolyte. The LBL procedure is then giving rise to lower the adsorption characteristic times. To obtain the PTHT/DBS and PTHT adsorption kinetics curves, PTHT/DBS LBL films were prepared using different adsorption times in each polyelectrolyte and electrolyte solutions and for different solutions concentrations. In Figure 4a and b, the maximum absorbance for PTHT/DBS LBL film is plotted as a function of the number of bilayers, N. Figure 4a displays the values of absorbance at 200 nm for immersion times t1 ) 2 s, 10 s, 3 min, and 10 min, and t2 ) 2 s, while Figure 4b shows the absorbance at 325 nm for t1 ) 2 s, and t2 ) 2 s, 10 s, 3 min, and 10 min, with C0PTHT ) 2.6 × 10-4 M and C0DBS ) 10-1 M. It can be seen from both figures that the absorbance is directly proportional to the adsorbed amounts of PTHT/DBS and PTHT, respectively. The linear dependence of the absorbance at a fixed wavelength, A(t, C), versus the number of bilayers reveals the possibility of depositing layers of a polyelectrolyte and an electrolyte in a non-self-limited process, with the adsorbed amount per layer expressed by the absorbance per layer and corresponding to the slope of the straight lines after fitting the experimental data, that is, to the rate ∂A(t, C)/∂N, from now on called Al(t, C).

Marletta et al.

Figure 4. (a) Absorbance intensity at 200 nm for PTHT/DBS LBL films as a function of the number of bilayers N and t1 ) 2 s, 10 s, 3 min, and 10 min, and t2 ) 2 s. (b) Absorbance intensity at 325 nm for PTHT/DBS LBL films as a function of the number of bilayers N and t1 ) 2 s and t2 ) 2 s, 10 s, 3 min, and 10 min.

The adsorption kinetics curves of PTHT/DBS and PTHT onto (PTHT/DBS) LBL films are shown in Figure 5a and b. In these figures, the adsorbed amount per unit of area is represented by the absorbance per layer, Al(t, C), at 200 and 325 nm, as a function of immersion time, t1, for different PTHT concentrations (CPTHT). It should be noted here that the kinetic curves data present some dispersion, which in certain cases is of the order of 20% for small adsorbed amounts and of the order of 5-10% for higher adsorbed amounts. For the present case and taking into account the adsorption measurements carried out, one expects that the error bars should be of the order of 20%, which essentially arises from the fact that each film is prepared onto a different substrate.18,19 These results were obtained for a DBS concentration of 10-1 mol L-1, and an immersion time in the DBS solution of t2 ) 2 s. The curves of Figure 5 show that the adsorbed amounts of PTHT/ DBS and PTHT are practically independent of adsorption time after a few seconds of adsorption. These curves can be fitted with a kinetics curve represented by eq 1.20

(

( τt ))

Al(t, C) ) Amax 1 - exp -

(1)

with Amax as the maximum absorbance per layer at fixed wavelength and τ as the adsorption characteristic time. This kinetics curve behavior is in accordance with what is expected for the adsorption of common polyelectrolytes as demonstrated by Lvov et al.19 The kinetic adsorption parameters were obtained for each kinetics curve displayed in Figure 5a and b. In Figure 5c, the maximum absorbance at 200 nm per layer and the adsorption characteristic times, calculated from kinetics curves data of Figure 5a, are represented for different PTHT concentrations. From the guidelines of Figure 5c, one can observe a fast decrease of the adsorption characteristic time and an increase of maximum adsorbed amount of PTHT/DBS per layer with the PTHT concentration. Consequently, one assumes that the kinetics of adsorption of PTHT and PTHT/DBS onto already deposited layers is under a quasi-stationary regime, with the term Al(t, C) practically independent of t1 and t2. This result indicates that the maximum adsorbed amount on the surface is rapidly achieved, a behavior that is usually observed in LBL polymer films such (18) Raposo, M.; Oliveira, O. N., Jr Langmuir 2000, 16, 2839. (19) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146, 337. (20) Raposo, M.; Pontes, R. S.; Mattoso, L. H. C.; Oliveira, O. N., Jr Macromolecules 1997, 30, 6095.


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Figure 6. (a) PTHT/DBS adsorption kinetics curves obtained from absorbance values at 200 nm per layer versus the adsorption time in DBS solutions with different concentrations. The solid lines represent the results from fitting (eq 1). (b) PTHT adsorption kinetics curves obtained from absorbance values at 325 nm per layer versus the adsorption time in DBS solutions with different concentrations. The solid lines represent the results from fitting (eq 1). The concentration of PTHT was maintained constant at 2.6 × 10-4 M (C0PTHT), and the adsorption time in this solution was 1 s (t1).

Figure 5. (a) PTHT/DBS adsorption kinetics curves obtained from absorbance values at 200 nm per layer versus the adsorption time in PTHT solutions with different concentrations. The solid lines represent the fitting using eq 1. (b) PTHT adsorption kinetics curves obtained from absorbance values at 325 nm per layer versus the adsorption time in PTHT solutions with different concentrations. The solid lines represent the fitting using eq 1. (c) Calculated parameters from obtained PTHT/ DBS kinetics curves, that is, maximum absorbance at 200 nm per layer and respective adsorption characteristic times. The solid lines are guidelines. The concentration of DBS was maintained constant at 1 × 10-1 M (C0DBS), and the adsorption time in this concentration was 2 s (t2).

as poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO).21 In this way, short immersion times, likely 2 s, can be used to study the adsorption isotherms of PTHT and DBS layers onto already deposited layers. (21) Ferreira, Q.; Gomes, P. J.; Raposo, M.; Giacometti, J. A.; Oliveira, O. N., Jr.; Ribeiro, P. A. J. Nanosci. Nanotechnol. 2007, 7, 2659.

The kinetics of adsorption of PTHT and PTHT/DBS onto (PTHT/DBS)N LBL films was similarly investigated by keeping constant the PTHT concentration at 2.6 × 10-4 mol L-1 and t1 ) 1 s. Figure 6a and b displays, respectively, the PTHT/DBS and PTHT kinetics curves by plotting the absorbance intensity per layer at 200 and 325 nm, respectively, for adsorption times, t2, varying from 2 s to 10 min and for different DBS solution concentrations, C0DBS. These kinetics curves were also found to be fitted with eq 1, allowing the adsorption parameters to be obtained. The plot of these parameters as a function of the DBS concentration is similar to those of of Figure 5c. The maximum absorbance per layer is rapidly attained, with the adsorption characteristic time independent of DBS concentration. 3.3. Adsorption Isotherms. The adsorption of a polyelectrolyte on a surface cannot be truly associated with an ideal equilibrium state, which makes difficult the evaluation of adsorption energies and equilibrium constants. However, considering the metastable equilibrium concept,22-24 information about the adsorption process can be achieved. The equivalent Langmuir analytic model, based on the metastable equilibrium concept, considers that a macromolecule is randomly deposited (22) Pan, G.; Liss, P. S. J. Colloids Interface Sci. 1998, 201, 71. (23) Shaughnessy, B. O.; Vavylonys, D. J. Phys.: Condens. Matter 2005, 17, R63. (24) Viot, P. Eur. J. Phys. 2005, 26, S39.

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Marletta et al. Table 1. Parameters Calculated by Fitting the Experimental Results with Langmuir-type Metastable Equilibrium Isotherm isotherm variable PTHT DBS

Figure 7. Adsorption isotherms of PTHT/DBS and PTHT onto (PTHT/ DBS)N LBL films. The solid lines represent the fittings to Langmuirtype metastable equilibrium isotherm. The concentration of DBS was maintained constant at 1 × 10-1 M (C0DBS), and the adsorption time in this concentration was 2 s (t2).

onto a surface, with the number of available adsorption sites decreasing as the number of adsorbed molecules increases. The dynamics of adsorption/desorption of molecules on a substrate can be represented by the following rate equation for the adsorbed amount per unit of area, Γ(t, x).

∂Γ(t, x) ) kads[Γmax - Γ(t, x)]x - kdesΓ(t, x) ∂t

(2)

where Γmax is the maximum adsorbed amount, t is the time, and x is the molar fraction of the solute, polyelectrolyte or anion. Denote that the molar fraction x is replacing the concentration C to obtain the nondimensional expression later. The adsorption and desorption constants, kads and kdes, respectively, are expected to consider the effect of mass transport on the surface. As the adsorbed amount in a layer is represented by the absorbance, eq 2 can be rewritten in terms of Al(t, x) as:

∂Γ(t, x) ∂Al(t, x) ∝ ∂t ∂t

(3)

Taking into account eqs 2 and 3, a new equation can be written as representing the dynamics of the adsorption/desorption process, eq 4.

∂Al(t, x) ) kads[Amax - Al(t, x)]x - kdesAl(t, x) ∂t

k at 200 nm

k at 325 nm

4.0 × 106 3.7 × 106 1.7 × 105 2.1 × 105

o o ∆Gads at ∆Gads at 200 nm (kJ/mol) 325 nm (kJ/mol)

-37.9 ( 0.8 -30.0 ( 0.3

-37.7( 0.5 -30.6( 0.9

The adsorption isotherms of PTHT/DBS and PTHT onto (PTHT/DBS)N LBL films, obtained by varying the PTHT concentration and maintaining the DBS concentration equal to 10-1 M, are plotted in Figure 7. These curves were obtained by plotting the maximum absorbance per layer at 200 and 325 nm, calculated from fitting to eq 1, as a function of PTHT molar fraction. The Langmuir-type metastable equilibrium isotherm, eq 5, was used to fit these data. The parameters resultant from fitting are displayed in Table 1. Mean values of 4.0 × 106 and 3.7 × 106 were obtained for both isotherms, which are close to the ones found in previous adsorption studies of poly(omethoxyaniline) onto glass substrates.21 This is a relatively high value of k, indicating that the adsorption constant is much higher than the desorption constant and evidencing that the adsorbed molecules are strongly bound to the previously adsorbed ones, because the estimated free energy is of -38 kJ/mol, a value slightly higher than the obtained for POMA.16 The free energy values for the adsorption of PTHT/DBS and PTHT are also listed in Table 1. A small value of k for the adsorption of PTHT indicates a higher affinity with the surface than the DBS. Figure 8 shows adsorption isotherms of PTHT/DBS and PTHT onto (PTHT/DBS)N LBL films, which is the plot of maximum absorbance per layer at 200 and 325 nm versus the DBS concentration, maintaining the PTHT concentration equal to 2.6 × 10-4 M, obtained from data of Figure 6. The fittings performed using eq 5 yield the following values for k: 1.7 × 105 at 200 nm and 2.1 × 105 at 325 nm, which are smaller than those obtained for adsorption of PTHT/DBS and PTHT when the DBS concentration was kept constant. This reveals smaller binding energies accounted for the different number of electrical charges, because DBS has only a charged group on its molecule, while PTHT has a large number of electrical charged groups. The free energy value of -30 kJ/mol found for DBS adsorption is also displayed in Table 1. Although no thermally stimulated desorption experiments were performed and consequently the adsorption and desorption coefficients were not calculated, one can estimate values for the

(4)

In the particular circumstance where the absorbance rate is independent of immersion time, one has ∂Al(t, x)/∂t ) 0 and Al(t, x) ) Al(x), a result that is attained when a metastable equilibrium is rapidly achieved. From this consideration, eq 4 can be solved and its solution written as:

Al(x) )

Amaxkx 1 + kx

(5)

where k ) kads/kdes. This equation states that the adsorbed amount depends only on the molar fraction parameter x, which represents the solute concentration and can be denominated as Langmuirtype metastable equilibrium isotherm equation. The value of k o of adsorption using enables one to estimate the free energy ∆Gads the following equation:

∆Goads ) -RT

ln k

(6)

where R is the gas constant and T is the absolute temperature.

Figure 8. Adsorption isotherms of PTHT/DBS and PTHT onto (PTHT/ DBS)N LBL films. The solid lines represent the fittings to Langmuirtype metastable equilibrium isotherm. The concentration of PTHT was maintained constant at 2.6 × 10-4 M (C0PTHT), and the adsorption time in this concentration was 1 s (t1).


adsorption and desorption coefficients to be in the ranges 10-3 to 100 and 10-9 to 10-6 s-1, respectively. These values were estimated making use of the desorption coefficient for poly(omethoxyaniline), of 10-6 s-1 order.16,18 This nonzero value is indicating the attainment of a rapid metastable equilibrium. Finally, the obtained values are consistent with the presence of physical interaction forces as hydrogen bonds and van der Waals forces, which accounts for the adsorption of a polyelectrolyte onto a electrolyte. Similar behavior has been observed for both poly(o-methoxyaniline)20 and the PPV precursor25 that are seen to adsorb onto themselves. For the present case, it is believed that the sodium acid dodecylbenzenesulfonate reduces the repulsive ionic forces, allowing more PPV precursor molecules to be adsorbed.

4. Conclusions The adsorption isotherms of PTHT/DBS films obtained by the LBL technique can be accounted for by the Langmuir metastable equilibrium model. The rate expression for the adsorbed amount has been established in terms of the absorbance as a function of time and solution concentration. From the linear dependence of the absorbance with the number of layers, the absorbance rate per layer can be replaced by an absorbance function. This result (25) Pontes, R. S.; Raposo, M.; Camilo, C. S.; Dhanabalan, A.; Oliveira, O. N., Jr. Phys. Status Solidi A 1999, 173, 41.

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is achieved when the adsorption takes place at short periods of time, as observed in PTHT/DBS LBL films, for which the PTHT or DBS adsorbed amounts are not substantially affected for immersions times in the range of 2 s to 10 min. In this case, the metastable thermodynamic equilibrium between adsorbed layer and solute is rapidly attained because DBS is a small molecule, which is in contrast with the habitual LBL procedure, where two distinct polyelectrolytes are used. By applying the metastable equilibrium Langmuir isotherm, the coefficients k were determined for both PTHT/DBS and PTHT, with obtained values of 4 × 106 and 2 × 105, respectively. The order of magnitude of these values indicates that the adsorption is governed by ionic interactions. In addition, the obtained results also reveal that the adsorption process for LBL-PTHT/DBS films is weakly dependent on the PTHT concentration. Finally, a simple model was found to be suitable to address adsorption processes in which a pseudoequilibrium is rapidly attained. Acknowledgment. We thank the GRICES/CAPES 2005/2008 action (Proc. no 4.1.3/CAPES/CPLP), the Brazilian financial support of CNPq, CAPES, FAPEMIG, and MCT/IMMP, and the Portuguese “Fundaçaõ para a Cieˆncia e Tecnologia” for both “Plurianval” and project POCTI/FAT/47529/2002 financial contributions. LA803093N

Modeling Adsorption Processes of Poly-p-phenylenevinylene

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