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Langmuir 1997, 13, 5362-5367
Comparative Study of Polyacryloylacetone Monolayers at Dichloromethane-Water and Air-Water Interfaces K. Balashev,† A. Bois,‡ J. E. Proust,§ Tz. Ivanova,† I. Petkov,† S. Masuda,| and I. Panaiotov*,† Biophysical Chemistry Laboratory, University of Sofia, J. Bourchier 1 str., 1126 Sofia, Bulgaria; Laboratoire d’OptoElectronique, Universite´ de Toulon et de Var, 83130 La Garde, France; Pharmacie Gale´ nique et Biophysique Pharmaceutique, Faculte´ de Pharmacie, 16 Bd Daviers, 49100 Angers, France; and Department of Applied Chemistry, Technical College, The University of Tokoshima, Tokoshima 770, Japan Received July 17, 1996. In Final Form: February 27, 1997X The properties of polyacryloylacetone (PAA) monolayers adsorbed at dichloromethane-water (DCM/W) interfaces were investigated by using the dynamic pendant drop method and a new rheological approach. The results were compared with those obtained at the air-water (A/W) interface. The role of the two interfaces on the interfacial organization of enolic units and the formation of tridimensional structures was discussed. The adsorption kinetics of Cu2+ ions on PAA monolayers leading to metal-polymer complex formation were followed. The observed large difference in the kinetic rates at the two interfaces is probably related to the different interfacial organization of the enolic units.
Introduction Polymers containing a β-diketone group cause a serious interest concerning their various potential purposes.1-3 The proton transfer resulting from enol-keto lightinduced tautomerization has been used in photolithography, in chemical lasers, in energy storage devices at a molecular level, in high-energy radiation detectors, etc.1 This kind of polymer has also been proposed as a chelating agent to extract metal divalent ions due to their high ability to form polymer-metal ion complexes.2 The mechanisms of the photoisomerization and complex formation with metal ions depend on the mode of organization and orientation of the polymer chains in the studied systemssfrom simple solutions in various solvents to more complex structured solid films. For example, in dilute polymer solutions, the tautomeric transformation of polyacryloylacetone (PAA; see Figure 1) has been determined by measuring the adsorption spectra. The enolic fraction was evaluated to be approximately 96% in all studied solvents.3,4 The organization of the polymer chain in more complex multilayer systems is less known. It seems useful to investigate the structural organization of a PAA polymer multilayer by using the ideas and techniques developed in monolayer studies. Many polymers form insoluble or soluble monolayers at the air-water and oil-water interfaces.5,6 Some previous information about the behavior of a spread * To whom correspondence should be addressed. † University of Sofia. ‡ Universite ´ de Toulon et de Var. § Pharmacie Gale ´ nique et Biophysique Pharmaceutique. | Technical College. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobiol., A: Chem. 1993, 75, 1. (2) Tomida, T.; Tomida, M.; Nishihara, J.; Nakabayashi, I.; Okazaki, T.; Masuda, S. Polymer 1990, 31, 102. (3) Petkov, I.; Masuda, S.; Sertova, N.; Grigorov, L. J. Photochem. Photobiol. A: Chem. 1995, 85, 191; 1996, 95, 189. (4) Masuda, S.; Tanaka, M.; Minato, R.; Ota, T. Polym. J. 1986, 18, 967. (5) Mac Ritchie. Chemistry at interfaces; Academic Press: San Diego, CA, 1990. (6) Boury, F.; Ivanova, Tz.; Panaiotov, I.; Proust, J. E.; Bois, A.; Richou, J. J. Colloid Interface Sci. 1995, 169, 380.
S0743-7463(96)00705-6 CCC: $14.00
Figure 1. (a) Formula of the nonionized enolic form of polyacryloylacetone (PAA). (b) Assumed structure of the PAACu2+ complex.
insoluble PAA monolayer at the air-water interface was presented in ref 7. The purpose of this paper is to study the role of dichloromethane-water (DCM/W) and air-water (A/W) interfaces on the organization and dilatational rheological properties of PAA monolayers. Special attention will be paid to the process of formation of PAA-Cu2+ ion complexes. Materials and Methods Polymers and Solvents. Polyacryloylacetone (PAA) was obtained from the Department of Applied Chemistry, Technical College, University of Tokoshima (Japan). The molecular weight of PAA, determined by gel permeation chromatography is larger than 100 000.2 Chromatographically pure chloroform and dichloromethane used for dissolving PAA were supplied by Prolabo (France). Ultrapure water was obtained from a Milli-Q Plus system (Millipore, France). CuSO4‚5H2O was supplied by Prolabo (Paris, France). Measurements at the DCM/W Interface. The interfacial tension (σ) at the DCM/W interface was measured in real time by using a previously described8 dynamic pendant drop method. The time evolution of the interfacial tension at a constant area was measured (i) during the polymer adsorption and formation of a saturated adsorption layer and (ii) during and after a fast small compression (or expansion) of the saturated adsorption layer. The relative rate of deformation was 6.4 min-1. The surface area (A) versus time (t) at a constant surface tension (σ) was measured during the adsorption of Cu2+ ions on the saturated adsorption layer by using barostatic regulation. (7) Balashev, K.; Panaiotov, I.; Ivanova, M.; Proust, J. E.; Boury, F.; Petkov, I.; Masuda, S. J. Dispersion Sci. Technol., in press. (8) Boury, F.; Ivanova, Tz.; Panaiotov, I.; Proust, J.; Bois, A.; Richou, J. Langmuir 1995, 11, 1636.
© 1997 American Chemical Society
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Figure 2. Schematic representation of the “zero order” barostat surface balance9 composed of a reaction compartment and a reservoir compartment communicating by means of a narrow surface canal. Measurements at the Air-Water Interface. In order to avoid any doubt about the accuracy of the obtained results, two methods were applied for the isotherm measurements. The first one was by using a Langmuir film balance (Lauda, Germany). The second one was by using the Wilhelmy method with a Pt plate and a Beckman electromicrobalance LM600 connected with a personal computer provided with user software for real time data measurements. The accuracy of measurement was better than 0.01 mN/m. The quickest rate of acquisition was about 17 values per second. Two kinds of experiments were performed: (i) The surface pressure (π)-surface area (A) isotherms were obtained after spreading of PAA from chloroform solutions by using an Exmire microsyringe on an aqueous subphase with maximum available area (927 cm2). The values of surface pressure after spreading were less than 0.1 mN/m. Monolayers were left for about 15 min and then compressed with constant velocity Ub ) 150 cm2/min. (ii) The surface pressure (π) versus time (t) was measured during the fast small compressions followed by a relaxation process at a constant area. The distance between the barrier and the platinum plate was equal to about 7 cm, and the relative rate of compression was 1.33 min-1. The surface area (A) versus time (t) at a cosntant surface pressure (π) was measured according to ref 9 during the adsorption of Cu2+ ions by using a barostat automatic surface balance KSV (Finland) with a “zero order”, composed of a reaction compartment (volume, 50 cm3; surface, 50 cm2) and a reservoir compartment (surface, 290 cm2) communicating by means of a narrow surface canal (Figure 2). To homogenize the water subphase, the bulk of the reaction compartment was stirred continuously with a magnetic rod 250 min-1 after injecting the CuSO4. The temperature of the subphase was 20 ( 0.5 °C. Rheological Measurements. The dynamic response of the polymer monolayer to a dilatational (or compression) mechanical stress was studied by using a theoretical approach based on twodimensional rheology.6,10,11 We consider the rheological response of the monolayer as a whole, neglecting the surface pressure distribution along the monolayer. To describe the surface pressure change ∆π ) π(t) - πi (Figure 3), during the time T of the compression (c) with a constant velocity Ub followed by a relaxation (r), we suppose that, at any moment, the total surface pressure change ∆π ) π(t) - πi can be expressed as a sum of one equilibrium ∆πe and one nonequilibrium ∆πne contribution.
∆π ) ∆πe + ∆πne
(1)
The equilibrium part ∆πe is related to the equilibrium surface dilatational elasticity Ee. Thus (9) Verger, R.; deHaas, G. Chem. Phys. Lipids 1973, 10, 127. (10) Edwards, D.; Brener, H.; Wasan, D. T. Interfacial transport processes and rheology (in Chemical Engineering); ButterworthHeineman: Oxford, 1991. (11) Dukhin, S. S.; Kretzschmar, G.; Miller, R. Dynamics of Adsorption at Liquid Interfaces; Elsevier: Amsterdam, 1995.
Figure 3. Rheological model of monolayer. Deformation (∆A/Ai) and change of the surface pressure (∆π) with time (t) during a fast compression (c) with constant velocity (Ub), followed by a relaxation (r). T is the time of compression. For the other notations see the text.
∆πe ) Ee
Ubt Ai
(2)
where Ai is the initial surface area before the compression and Ubt/Ai ) ∆A/Ai is the corresponding strain (see Figure 3). This elastic behavior is represented by the upper branch of the mechanical model in Figure 3. The nonequilibrium part of the total surface pressure change ∆πne is associated with the accumulation of an elastic energy during the compression. The dissipation of this accumulated energy, through expulsion of segments in the adjacent phase(s), occurs during compression as well as relaxation. This viscoelastic behavior can be modeled using Maxwell’s equation
d∆πne ∆πne Ub + ) Ene dt τ Ai
(3)
where ∆πne is the applied stress, Ene is the nonequilibrium surface dilatational elasticity, and τ is the specific time of relaxation. This viscoelastic behavior is represented by the lower branch of the used mechanical model (Figure 3). The two branches of the mechanical model are coupled in parallel according to eq 1, which corrsponds to the addition of stresses. The general solution of eqs 1-3 is obtained in ref 6. When the time of compression (T) is much smaller than the time of relaxation (τ) (rapid compression or expansion), the following useful expressions allow us to determine experimental values of total elasticity (Ee + Ene) during the compression
UbT Ai
∆πt)T ) (Ee + Ene)
(4)
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Balashev et al.
Figure 5. Surface pressure (π)-mean area per enol unit (A) isotherm of a PAA monolayer spread at the A/W interface (pH ) 6.6 of the aqueous subphase).
Figure 4. (A, top) Evolution of the surface tension (σ) with time (t) during the adsorption of PAA at the DCM/W interface (pH ) 6.6 of the aqueous phase; bulk concentration c ) 0.01 g/L). (B, bottom) Equilibrium values of surface pressure as a function of the concentration. and the relaxation time (τ) during the relaxation process
∆π(t) - ∆π∞ π(t) - π∞ t ) e-t/τ ∆π0 - ∆π∞ π0 - π∞
(5)
If the relaxation process with two relaxation times τ1 and τ2 associated with two distinctive molecular mechanisms occurs, eq 5 can be extended as follows:
∆π(t) ) a + b exp(-t/τ1) + c exp(-t/τ2)
(6)
Atomic Force Microscopy (AFM) on Langmuir-Blodgett (LB) Films. The LB transfer was carried out on a freshly cleaved mica plate. The mica plate was first immersed into an aqueous subphase, and then the monolayer was spread and the film was transferred at a constant surface pressure on the mica. The speed of the transfer was 5 mm/min. We carried out AFM imaging in the noncontact mode with a commercial instrument (Autoprobe CP, Park Scientific Instruments).
Results and Discussion PAA Monolayers at DCM/W and A/W Interfaces. A typical kinetic curve of the surface tension (σ) versus time (t) for C ) 0.01 g/L at the DCM/W interface is shown in Figure 4A. An equilibrium value of interfacial pressure π t σ0 - σ ) 7 mN/m is reached. The equilibrium values of surface pressure as a function of the concentration are presented in Figure 4B. The state of the adsorbed PAA layer at π ∼ 6-7 mN/m corresponds to the adsorption saturation. We can consider that during the kinetics and the saturation of the adsorption layer some of the segments
of the adsorbed PAA polymer molecule are attached at the interface, forming trains, while others are immersed in the adjacent DCM phase, forming loops and tails. The way of determining the number of segments attached at the interface at saturation, from the data in Figure 4B by using modified Gibbs equations is still a matter of controversy.12,13 To compare the state of the PAA monolayer at DCM/W and A/W interfaces, the surface pressure (π)-surface area (A) isotherm at the A/W interface7 is presented in Figure 5. At pH 6.6 of the aqueous subphase the nonionized enolic form (Figure 1a) of PAA predominates. The inflection point at about 14 Å2/monomer corresponds to closely packed monomers according to the area occupied by an enolic acryloylacetate unit. The flat plateau occurring at π ) 18-19 mN/m is interpreted as a phase transition and formation into the air adjacent phase of tridimensional structures that could be constituted of both expelled portions (loops) and whole molecules. We assume that the association of enolic units in 3D-structures is related to the formation of hydrogen bonds between monomer units. The AFM imaging confirms the conclusions based on the π(A) isotherm. Below the plateau at π ) 6 mN/m (Figure 6A) no microdomains were observed and the average roughness was 0.25 Å. After the plateau at π ) 21.8 mN/m (Figure 6B), the appearance of objects with a typical overheight of 16 Å confirms the expulsion of polymer segments or whole molecules into the 3rd dimension and the formation of tridimensional structures. Additional information about the state of PAA monolayers at DCM/W and A/W interfaces can be obtained by measuring their rheological properties. Some typical results are presented in Figure 7. Small fast compressions, followed by relaxations, were produced at the saturation at the DCM/W interface (Figure 7A) and before (Figure 7B) and after (Figure 7C) the plateau at the A/W interface. The values of the total elasticity (Ee + Ene) are obtained by using eq 4, and the nonequilibrium part of the total surface pressure fne ) (∆πne/∆π) 100 % is estimated. The values of Ee + Ene ) 33 mN/m and fne > 95% are obtained for the saturated adsorption layer at the DCM/W interface. At the A/W interface the values Ee + Ene ) 49 mN/m and fne < 5% before and Ee + Ene ) 17 mN/m and fne ∼ 70% after the formation of 3D-structures are calculated. According to general ideas, the surface pressure measured during the monolayer compression is due exclusively to the train segments.5 Thus, the equilibrium elasticity (12) Zankveld, J. M. J.; Lyklema, J. J. Colloid Interface Sci. 1972, 41, 454. (13) Zankveld, J. M. J. Thesis, University of Wageningen, 1970.
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Figure 7. Surface pressure excess (∆π) as a function of time (t) during a fast compression followed by relaxation of PAA monolayers: saturated adsorption layer at the DCM/W interface (c ) 0.01 g/L; Ub/Ai ) 6.4 min-1) (A); spread monolayer at the A/W interface before (B) and after (C) the plateau (Ub/Ai ) 1.4 min-1).
Figure 6. AFM imaging of LB PAA films sampled: (A, top) below (π ) 6 mN/m) and (B, bottom) after (π ) 21 mN/m) the plateau of the phase transition (pH ) 6.6 of the aqueous subphase).
reflects the density of the segments anchored at the interface by the train segments and their ability to stay in this conformation. The nonequilibrium elasticity can be related to the desorption of segments through the interface as loop shape. The data show that, at a monomer closed packed conformation of PAA at the A/W interface (π0 ) 6 mN/m), the high density of segments anchored at the interface is reflected by a high value of the equilibrium elasticity and their ability to stay in the train conformationsby a negligible nonequilibrium contribution. In contrast to such behavior, during the formation of 3Dstructures (π0 ) 19 mN/m), the total elasticity Ee + Ene decreases and the nonequilibrium contribution increases. The nonequilibrium effects when a saturated adsorption layer is compressed at the DCM/W interface are of the same order of magnitude. The relaxation times, corresponding to the dissipation of the energy accumulated during compression, are calculated using eqs 5 and 6. As an example, the graphical expression of the calculation, corresponding to the data from Figure 7, is shown in Figure 8. The experimental results (the triangles) are in good agreement with theoretical predictions (the curves) for one relaxation time (eq 5, Figure 8B) or two relaxation times (eq 6, Figure 8A and C). Analyzing the experimental curves for each case,
the following mean values of the relaxation times were obtained. At the saturation at the DCM/W interface two relaxation times τ1 ) 5 s and τ2 ) 49 s are obtained (see the example in Figure 8A). A relaxation process with only one relaxation time (τ ) 40 s) is detected before the plateau (see the example in Figure 8B), while the theoretical model with two relaxation times (τ1 ) 9 s, τ2 ) 102 s) describes well the data obtained during and after the plateau (see the example in Figure 8C) at the A/W interface. It can be noted that the molecular mechanisms corresponding to these relaxation times are unknown. We assume that the fast times (510 s) and the slow times (40-100 s) are related to the formation of loops and a consecutive reorganization of the monolayer as a whole. In terms of crossing segments through the interface, the faster times mean a facilitated desorption of segments. Some general conclusions based on these experimental results can be made. Some similarity (fast and slow relaxation times, large nonequilibrium effects) of the rheological properties of the PAA-saturated adsorption layer at the DCM/W interface and at the A/W interface after the formation of loops is observed. The PAA monolayers form the loops into the adjacent DCM phase at the DCM/W interface or into the air after the plateau at the A/W interface. The observed fast relaxation time and the large nonequilibrium effect at the two interfaces can be associated with the mechanism of formation of loops. In contrast to that behavior, the nonequilibrium effects in the PAA monolayer, constituted
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Balashev et al. Table 1. Rheological Properties of Saturated Adsorption Layers of PAA and PVA at a DCM/W interface (pH 6.6) τ (s) PAA PVA
Figure 8. Determination of the characteric time(s) of relaxation from the relaxation curves (∆π/t) in Figure 7: (s) experimental data; (4) theoretical predictions for one (eq 5) (B) or two (eq 6) (A and C) relaxation times.
Figure 9. Surface pressure excess versus time measured for a monolayer of PAA adsorbed at the DCM/water interface from a 0.01 g/L aqueous solution. The relative velocity of the compression (expansion) was 6.4 min-1.
of monomers packed at the A/W interface at π0 ) 6 mN/m, are quasi nonexistent. The rheological behavior of the PAA-saturated adsorption layer at the DCM/W interface was also studied. Figure 9 shows a typical result of the variation of the surface pressure of the PAA film versus time. This was obtained by slight compression or expansion followed by relaxation after the compression (expansion) was stopped. The values of Ee + Ene ) 11 mN/m and the two relaxations times τ1 ) 14 s and τ2 ) 102 s are obtained. The reason for such a difference between the rheological parameters
Ee + Ene (mN/m)
comp
exp
comp
exp
5 3.5
14 8.5
33 11.5
11 2.8
at expansion and compression (Ee + Ene ) 33 mN/m; τ1 ) 5 s and τ2 ) 49 s) is the asymmetry of the involved processes. The main question concerning the interpretation of the rheological data is whether after compression (or expansion) a true thermodynamic equilibrium between the interface and the bulk phase and also a segmental equilibrium between loops and trains at the interface are reached. It is clear that the simple application of the idea of adsorption equilibruim of small molecules should not be adequate to be polymer adsorption.5 Our experimental data (Figure 9) show that the nonequilibrium process after compression (or expansion) occurs relatively slowly and that a true equilibrium is not reached. That is why we prefer to interpret the reorganization of the polymer units in terms of crossing segments through the interface. The nonequilibrium effects during the compression of the layer could mainly be related to the desorption only of the molecules’ segments (formation of loops), and not to the desorption of the whole molecule.5 The expansion is probably connected with the adsorption of both segments and whole molecules. The relaxation times measured after compression are shorter than those measured after expansion. In terms of crossing segments through the interface, this means that the desorption of polymer units is facilitated compared to their adsorption. This suggests an asymmetrical adsorption-desorption barrier. It is interesting to compare the rheological behavior of the PAA-saturated adsorption layer at the DCM/W interface with that of the poly(vinyl alcohol) (PVA) monolayer from the previous study at the same interface.6 The results are summarized in Table 1. The total surface elasticity (Ee + Ene) and the relaxation times for PAA monolayers are higher by comparison with those for the PVA adsorption layer. Therefore, the number of PAA segments adsorbed at the saturation is larger and their desorption into loop configuration slower than those for PVA. Both adsorption layers show an asymmetric behavior at compression and expansion, i.e. an asymmetrical desorption-adsorption barrier. PAA-Cu2+ Ion Complex Formation at DCM/W and A/W Interfaces. As was shown in some previous studies, the presence of divalent ions, Cu2+, Cd2+, etc., leads to formation of a metal-polymer complex. To study the role of the interfacial organization of enolic units on the complex formation, the kinetics of adsorption of Cu2+ on the PAA monolayer at DCM/W and A/W interfaces are studied. The relative decrease of the surface area (∆A/∆Ai) with time (t) during the adsorption of Cu2+ ions on the PAA-saturated adsorption layer at the DCM/W interface after the injection of CuSO4 with a final concentration in the aqueous phase c ) 1.43 × 10-6 mol/ cm3 is presented in Figure 10. The kinetic curve shows a lag time and a sigmoidal shape. The observed decreasing of the area of about 10% is due to a condensation effect because of the formation of complexes between two enolic units and the Cu2+ ion (Figure 1b). The kinetic curves ∆A versus time at a constant π, during the adsorption of Cu2+ ions on the monolayer of closed packed enolic units at π ) 6 mN/m at the A/W interface, for three final concentrations in the aqueous phase (2, 4, and 5 × 10-5 mol/cm3)
Polyacryloylacetone Monolayers at Fluid Interfaces
Figure 10. Relative decrease of the surface area 100(∆A/Ai) with time (t) at constant surface pressure (π) during the adsorption of Cu2+ ions on a PAA-saturated adsorption layer at the DCM/W interface after injection (at t ) 0) of CuSO4 (c ) 1.43 m/cm3) in the reaction compartment.
are presented in Figure 11. The similar shapes of the curves and a condensation effect are observed, but the kinetics are more than one order of magnitude slower than those at the DCM/W interface. The estimations show that such a difference cannot be explained by a slower diffusion flow in the bulk of the reaction compartment, which was stirred continuously. The observed large difference in the kinetic rate is probably related to the completely different organization of enolic units at the two interfaces. Conclusion The comparison of the behavior of PAA monolayers at DCM/W and A/W interfaces shows that the saturated adsorption layer at the DCM/W interface does not correspond to a true monolayer of closely packed enolic monomer units. A certain number of segments are immersed in the DCM phase in loop configurations. Their rheological properties during and after compression (or expansion) are related to a desorption of segments (or adsorption both of segments and whole molecules). A certain similarity of the rheological properties at the
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Figure 11. Relative decrease of the surface area 100(∆A/Ai) with time (t) at constant surface pressure π ) 6 mN/m during the adsorption of Cu2+ ions on a PAA monolayer spread at the A/W interface after injection (at t ) 0) of CuSO4 in the reaction compartment (top) c ) 2 × 10-5 mol/cm3; (middle) c ) 4 × 10-5 mol/cm3; (bottom) c ) 5 × 10-5 mol/cm3.
DCM/W interface with those of the PAA monolayer forming at the A/W interface (π > 19 mN/m) loops toward the air phase. The kinetic rates of adsorption of Cu2+ and complex formation on the PAA-saturated adsorption layer at the DCM/W interface and on the PAA spread monolayer with closed packed enolic units at the A/W interface are very different. This finding is related to the completely different organization of the enolic units at the two interfaces. The obtained information about the state and mechanical properties of PAA monolayers will be useful in the study of the photoinduced keto-enolic tautomerization in molecular model systems. Acknowledgment. This work was partially supported financially by the Bulgarian National Foundation for Scientific Research. K.B. and I.P. were financially supported by Ministe`re de l’Education Nationale (France). LA9607058
