Relationship between the Growth Regime of Polyelectrolyte

J. Phys. Chem. B 2006, 110, 19443-19449

19443

Relationship between the Growth Regime of Polyelectrolyte Multilayers and the Polyanion/ Polycation Complexation Enthalpy Nicolas Laugel,† Cosette Betscha,‡,§ Mathias Winterhalter,⊥ Jean-Claude Voegel,‡,§ Pierre Schaaf,† and Vincent Ball*,‡,§ Centre National de la Recherche Scientifique et UniVersite´ Louis Pasteur, Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg Cedex, France, Institut National de la Sante´ et de la Recherche Me´ dicale. UniVersite´ Louis Pasteur, INSERM UMR 595, 11 rue Humann, 67085 Strasbourg Ce´ dex, France, Faculte´ de Chirurgie Dentaire, UniVersite´ Louis Pasteur, 1 Place de l’Hoˆ pital, 67000 Strasbourg, France, and School of Science and Engineering, International UniVersity of Bremen, Campus Ring, Bremen, Germany ReceiVed: April 12, 2006; In Final Form: June 26, 2006

The alternate deposition of polyanions and polycations leads to the formation of films called polyelectrolyte multilayer films (PEMs). Two types of growth processes are reported in the literature, leading to films that grow either linearly or exponentially with the number of deposition steps. In this article we try to establish a correlation between the nature of the growth process and the heat of complexation between the polyanions and the polycations constituting the PEM film. Isothermal titration microcalorimetry experiments performed on several polyanion/polycation systems seem to indicate that an endothermic complexation process is characteristic of an exponential film growth, whereas a strongly exothermic process corresponds to a linear growth regime. Finally, weakly exothermic processes seem to be associated with weakly exponentially growing films. These results thus show that exponentially growing processes are mainly driven by entropy. This explains why the exponential growth processes are more sensitive to temperature than the linear growing processes. This temperature sensitivity is shown on the poly-L-glutamic acid/poly(allylamine) system which grows either linearly or exponentially depending on the ionic strength of the polyelectrolyte solutions.

I. Introduction The concept of layer-by-layer deposition based on electrostatic interactions between anionic and cationic species was first introduced in 1966 by Iler for oppositely charged particles.1 This deposition method yields multilayered films and gained considerable interest in the scientific community after that. Decher and co-workers 2-4 demonstrated that it can also be applied to form nanometer-thick films by alternating the deposition of polyanions and polycations. The first investigated systems led to films whose mass and thickness increase linearly with the number of deposition steps.5 These films were shown to be nicely stratified, the polyanion layers alternating with the polycation ones with an inter-penetration that does not exceed a limited number of adjacent layers.6-8 A linear growth process thus takes place when the polyelectrolytes from the solution interact solely with the outer layers of the film without diffusing into its architecture. However, there is no physical reason that forbids the polyelectrolytes to diffuse into the multilayer architecture. If this takes place, the nature of the multilayer growth process changes entirely and it was shown that the film thickness and mass then increase exponentially with the number of deposition steps.9-11 The film is then much less structured than a linearly growing one, very hydrated and its thickness can reach several micrometers after only a few tens of deposition * To whom all correspondence should be addressed. Phone: +33 3 90 24 32 58. E-mail: [email protected]. † Centre National de la Recherche Scientifique et Universite ´ Louis Pasteur. ‡ Institut National de la Sante ´ et de la Recherche Me´dicale. Universite´ Louis Pasteur. § Faculte ´ de Chirurgie Dentaire, Universite´ Louis Pasteur. ⊥ International University of Bremen.

steps. Such films were mainly observed with polypeptides or polysaccharides and are called exponentially growing films. Besides, there have been some reports showing exponential growth with a combination of synthetic polyelectrolytes.12-14 The discovery of these two types of multilayer growth processes raises a number of fundamental questions, the central one concerning the prediction of the growth behavior for a given polyanion/polycation system. This question remains open today and our goal with this article is to report calorimetric data relative to polyanion/polycation complexation that allow us to propose a correlation between the polyanion/polycation complexation enthalpy and the corresponding multilayer growth process. Cohen Stuart and co-workers were the first to propose that the exponential growth process is a consequence of the weakness of the polyanion/polycation complexes.15 These authors also proposed that linearly growing films should correspond to multilayers in which the polyelectrolytes are more or less in a glassy state, whereas exponentially growing films rather correspond to systems that are above the glass transition. Exponentially growing films should thus more resemble gel-like films. This was indeed observed by comparing surface force apparatus compression experiments obtained on (polystyrene sulfonate/polyallylamine) (PSS/PAH) films that grow linearly with results obtained on the exponentially growing (poly-Lglutamic acid/poly-L-lysine) (PGA/PLL) multilayers.16,17 In the first case, after compression, the film keeps its final state when the stress is relaxed, whereas the exponentially growing film relaxes back to its initial state. These results should, however, be extended to other linearly growing films. Collin at al.18 also found, by using a piezo-rheometer, that slightly cross-linked (hyaluronic acid/poly-L-lysine) (HA/PLL) films behave elasti-

10.1021/jp062264z CCC: $33.50 © 2006 American Chemical Society Published on Web 09/08/2006

19444 J. Phys. Chem. B, Vol. 110, No. 39, 2006 cally over the 1-103 Hz frequency range and that the storage and loss shear moduli (G′ and G′′) do not depend on the frequency over this frequency domain. This is characteristic of a gel behavior. They also found that the Young modulus, which is of the order of 90 kPa, is much smaller than the values usually reported for linearly growing films, which lie in the mega to giga Pascal range depending upon the investigated system.19-22 Kankare and co-workers also did a very nice series of experiments on (polystyrene sulfonate/poly(diallyldimethylammonium)) (PSS/PDADMA) films where they investigated the influence of the nature of the salt on the buildup process.23,24 They found an exponential growth when the film is constructed in the presence of chaotropic ions, which decrease the hydrogen bonding of water, such as Br- or NO3-, whereas the mass increases linearly with the number of deposition steps in the presence of chosmotropic ions, which increase the hydrogen bonding of water, such as F- and HCOO-. They explained this behavior by assuming that chaotropic anions bind more strongly to the cationic amine groups of PDADMA thereby reducing the net positive charge of the PDADMA chains and changing their conformation in a more coiled one. Such strong binding should also reduce the net interactions between the anion and cation groups worn, respectively, by the polyanion and polycation chains and could lead to an exponential growth regime. These authors did their experiments by using the quartz crystal microbalance technique that allows also one to get information on the storage shear modulus G′ in the mega Hertz frequency range. They found that in the presence of chosmotropic anions, when the film grows linearly, G′ is of the order of a few MPa, whereas when it grows exponentially in the presence of chaotropic anions, G′ reaches values of the order of 100 Mpa. This result seems in contradiction with those discussed above for linearly growing films that behave like glasses. However, great care must be taken to interpret these data because of the high frequency at which they were performed. Totally different modes could be excited at such frequencies compared to classical rheology experiments. Richert et al. investigated the effect of the NaCl concentration of the polyelectrolyte solutions on the buildup of a (hyaluronic acid/chitosan) (HA/CHI) films.25 Electrostatic interactions are known to become weaker upon increasing the ionic strength of the polyelectrolyte solutions used during the dipping. They found that at 0.15 M in NaCl at pH 7.4 the film grows exponentially whereas it becomes linear at 10-4M in NaCl. However, at this low ionic strength, even if the mass deposited on the substrate increases linearly with the number of deposition steps, no real film forms on the surface which is rather covered by small islands.25 All these results thus support the idea that exponentially growing films are observed when the polyanion/polycation interactions are weak. These results do not, however, constitute a proof and need to be supported by further experimental data. Moreover, the idea of weak polyanion/polycation interactions is rather vague and needs to be quantified. This raises the question of the precise origin of the interactions between polyanions and polycations in polyelectrolyte complexes. These interactions are mainly of electrostatic origin even if non covalent interactions such as van der Waals and hydrophobic ones also play a role. Polyanion/polycation complexation takes place spontaneously when the associated Gibbs free energy change, ∆G, is negative. ∆G contains an enthalpic, ∆H, and an entropic, -T∆S, part. Polyelectrolyte chains being charged, they are surrounded by counterions. When a polyanion and a polycation interact, some of these counterions are released into

Laugel et al. the solution leading to an increase of the global entropy. The entropy also changes because of the restructuring of water molecules around the charges during the polyanion/polycation interaction as well as to changes in the polyelectrolyte conformation. Due to the complexity of the water structure and the subtlety of the processes involved, it is difficult to estimate at first sight the total entropic contribution. Nevertheless, experiments performed on peptide/DNA or protein/DNA interactions reveal that polyanion/polycation interactions are usually accompanied by an entropy increase.26,27 This contribution seems even to be the major driving force for the complexation of many polyanion/polycation complexes as was already recognized by Michaels in 1965.28 As far as the enthalpic contribution is concerned, it is also difficult to discuss it in a general way due to the complexity of the involved processes: charge/charge, charge/water, water structure, etc. To illustrate this complexity, one can mention that bringing a chloride anion and a sodium cation together in water is an endothermic and not an exothermic process, as one could assume at first sight.29 Nevertheless, it is expected that when the polyanion/polycation complexation process is exothermic (negative ∆H) both the enthalpy and entropy changes favor the complexation and the complexes are expected to be “strong”. This is expected to be the case for linearly growing films. On the other hand, when the polyanion/ polycation complexation process is endothermic (positive ∆H), enthalpy and entropy can play opposite roles in the complexation process so that the complexes should be much weaker. This is expected to be the case for exponentially growing films. In this article we will present isothermal titration calorimetry (ITC) experiments of several polyanion/polycation systems to support this view. Our arguments will solely be based on the complexation enthalpy. In principle ITC experiments performed at different temperatures should also allow us to get access to the entropy change during the process.30 However, this information can only be obtained by using a model for the polyanion/ polycation binding and requires that one reaches thermodynamic equilibrium after complexation. This, however, is far for being sure. Indeed, it is the same complexation process that drives the multilayer buildup process and it is known that once the multilayer is formed, it can be brought in contact with the buffer solution from which it has been built without dissolving. This is in clear contradiction with the assumption of thermodynamic equilibrium. Thus, the only thermodynamic parameter that can be reliably measured, without resorting to a model, is the enthalpy change. The enthalpy is a state function of the system and it is equal to the heat exchanged by the system with its surroundings is held at constant pressure. This heat exchanged can be directly measured by means of ITC experiments. As we will see, and as anticipated, there seems to exist a correlation between an endothermic (respectively exothermic) polyanion/ polycation complexation process and the buildup of an exponentially (respectively linearly) growing multilayer film. II. Material and Methods Chemicals. All the polyelectrolytes were used as received and dissolved in Milli Q water (F ) 18.2 MΩ cm). The polyanions used were poly-L-glutamic acid (PGA, Sigma, P-4761, viscosimetric molecular mass: 17 000 g mol-1), polyL-aspartic acid (Pasp, Sigma, P-6762, lot 10K5907, viscosimetric molecular mass: 33 400 g mol-1) polystyrene sulfonate (PSS, Aldrich, catalog no. 24,305-1, weight average molecular mass: 70000 g mol-1), hyaluronic acid (HA, Bioiberica, weight average molecular mass: 400 000 g mol-1) and were all used without any further purification.

Polyelectrolyte Multilayer Films

J. Phys. Chem. B, Vol. 110, No. 39, 2006 19445

TABLE 1: Reaction Enthalpies for the Investigated Polyanion/Polycation Pairs polyelectrolyte combinationa PSS(2)-PAH(2) PSS(2)-PAH(2) PSS(2)-PAH(2) PGA(1)-PAH(1) PGA(2)-PAH(2) PGA(1)-PAH(1) PGA(1)-PAH (1) PGA(2)-PAH(2) PGA(1)-PAH(1) PGA(1)-PAH(1) PGA(2)-PAH(2) PGA(1)-PLL(3) PSS (2)-PDADMAC (2) HA (3)-PAH (2) HA(3)-PLL (3) HA(3)-PDADMAC(2)

temperature and ionic strengthb

global enthalpy J.mol-1

dilution enthalpy J/mol

reaction enthalpy J/mol

25 °C, Tris 10 mM, NaCl 150 mM, pH ) 7.4 40 °C, Tris 10 mM , NaCl 150 mM, pH ) 7.4 25 °C, Tris 10 mM, NaCl 2 M, pH ) 7.4 25 °C, NaCl 160 mM, pH ) 7.4 25 °C, NaCl 1 mM, pH ) 7.4 35 °C, NaCl 160 mM, pH ) 7.4 35 °C, NaCl 10 mM, pH ) 7.4 35 °C, NaCl 1 mM, pH ) 7.4 45 °C, NaCl 160 mM, pH ) 7.4 45 °C, NaCl 10 mM, pH ) 7.4 45 °C, NaCl 1 mM, pH ) 7.4 25 °C, Tris 10 mM, NaCl 150 mM, pH ) 7.4 25 °C, Tris 10 mM, NaCl 150 mM, pH ) 7.4 25 °C, Tris 10 mM., NaCl 150 mM, pH ) 7.4 25 °C, Tris 10 mM, NaCl 150 mM, pH ) 7.4 25 °C, Tris 10 mM., NaCl 150 mM, pH ) 7.4

-1545

-29

-1516

linear 35

-1593

-225

-1368

linear 36

-1649

-1544

-105

+2333

+479.6

+1853

growth regime of the polyelectrolyte multilayer filmc

exponential37 1/i0 ) (5.59 ( 0.54) × 10-2 exponential11 1/i0 ) (0.39 ( 0.01) linear (this study) 1/i0 ) (1.6 ( 25) × 10-3 exponential (this study)

-372

+5693

-6065

+2303

+446

+1857

+1024

+1529

-505

+1176

+3428

-2252

exponential (this study) 1/i0 ) (0.18 ( 0.04) linear (this study)

+2266

+857

+1409

not yet investigated d

+1411

+1462

-51

not yet investigatedd

+767

+2730

-1963

not yet investigatedd

+2908

-13

+2921

exponential38

-1264

-225

-1039

linear8

+608

-32

+640

+503

-32

+535

+1571

-32

+1603

exponential39 1/i0 ) (0.19 ( 0.02) exponential 9,10 1/iO )(0.175 ( 0.005) exponential (this study, data not shown) 1/iO ) (0.20 ( 0.02)

a P1(x ) - P2(x ) means that the experiment was performed by injecting 10 µL of polyanion P1 solution at the concentration x mg.mL-1 into 1 2 1 1.44 mL of the solution of the polycation P2 at the concentration of x2 mg.mL-1. b Characteristics of the polyelectrolyte solutions used during the experiment. c 1/i0 corresponds to the main fitting parameter of eq 1 to the experimental - (∆fν/ν)values obtained from QCM-D measurements.d The QCM-D experiments at 45 °C are very difficult to perform because of temperature stabilization difficulties. Nevertheless, it is anticipated, that the growth regime of PEI-(PGA-PAH)n films is exponential at 160 and 10 mM in NaCl and remains linear at 1 mM in NaCl.

The polycations used in this study were polyethyleneimine (PEI, Sigma, P-3143, lot 093K0098, molecular mass of about 750 000 g mol-1), polyallyamine (PAH, Aldrich, catalog no. 28,322-3, lot 06503TB-015, molecular mass of about 65 000 g mol-1), polydimethylammonium chloride (PDADMA, Aldrich, 20% w/v solution, weight average molecular mass: 100 000-200 000 g mol-1), and poly-L-lysine (PLL, Fluka Biochemika, ref: 81338, weight average molecular mass: 30 000-70 000 g mol-1) All these polyelectrolytes were dissolved at a final concentration of 1, 2, or 3 mg.mL-1 (the detailed concentrations will be listed in the first column of Table 1). The pH of these solutions was adjusted to 7.4 after polyelectrolyte dissolution by adding either concentrated HCl or NaOH. In some cases, the polyelectrolyte solutions were buffered with 10 mM Tris(hydroxymethyl amino methane) (Tris, from Sigma, T-1503). Before each experiment, fresh polyelectrolyte solutions were prepared. In all cases the pH difference between the polyanion and electrolyte solution in which it was dissolved was lower than 0.1 pH units. The pH of all solutions was checked shortly before each ITC experiments in order to ensure that the measured enthalpy do not contain a significant contribution due to protonation/ deprotonation of the weak polyelectrolytes. Isothermal Titration Microcalorimetry. We will describe the detailed experimental procedure only in the case of the PGA-PAH combination of polyelectrolytes. The same procedure was used for the other combinations of polyelectrolytes.

The reaction heat between PGA and PAH was measured by means of isothermal titration microcalorimetry (VP-ITC microcalorimeter from Microcal) at three different temperatures, namely 25, 35, and 45 °C. This microcalorimeter is fitted with two thermostated cells: one reference cell and a sample cuvette containing the buffer or the polyelectrolyte solution to be titrated. Both cells have a volume of 1.44 mL. The polyelectrolyte of opposite sign is injected in successive injections steps by means of an automated microsyringue. For all these experiments, PGA and PAH were dissolved at a concentration of 1 mg mL-1 in NaCl solutions whose salt concentration was equal to 150 mM, 10 mM, or 1 mM. The pH of these solutions was adjusted to 7.4 ( 0.1 with HCL or NaOH before each ITC experiment. The polyanion solution dissolved at a given salt concentration of NaCl was injected in the polycation solution by means of the calibrated microsyringue. 10 µL of the polyanion solution was injected at each step in the polycation solution by means of a microsiryngue. The injection needle was ended with a propeller which allowed to stir the solution. In our experiments the propeller was rotated at 300 rpm. Two consecutive injections were separated by a resting period of 600 s to allow the microcalorimeter trace to come back to a baseline corresponding to the absence of any heat flow between the cell in which PGAPAH interactions took place and the reference cell which was filled with distilled water. The polyanion injection was repeated four times, meaning that the “mixing ratio”, the number of PGA monomers provided to the number of initially available PAH

19446 J. Phys. Chem. B, Vol. 110, No. 39, 2006 monomers, never exceeded 0.02. This allows us to make the assumption that all the PGA chains must have reacted with the amino groups born by PAH, which are present in large excess. For each temperature and salt concentration, we also measured the reaction heat corresponding to the injection of the same amount of PGA in the NaCl solution without PAH. To obtain the binding enthalpy, the integrated heat obtained during the dilution of PGA in the NaCl solution was subtracted from the whole reaction heat obtained by injecting the same PGA solution in the PAH solution (at the same salt concentration and pH value). Finally, the resulting “heat of reaction” was divided by the number of provided PGA monomers to yield the enthalpy of the polyanion-polycation complexation process. The peak integration was performed with Origin 7.5 after establishment of a linear baseline made from 50 points. This baseline was drawn on the basis of the initial line corresponding to the time lasting between the beginning of the experiment and the first PGA injection as well as during the time intervals where the heat flow reached a steady value between two consecutive PGA injections. Identical microcalorimetry experiments were performed by injecting 10 µL of different polyanions P1 into 1.44 mL of buffer solution or into buffer containing a polycation P2. The reproducibility of these microcalorimetry experiments was of the order of 10%. It has to be noted that we systematically injected the polyanion-containing solution in either the electrolyte solution (for the determination of the dilution enthalpy) or in the polycation containing solution (for the determination of the whole enthalpy change) in order to reduce the influence of adsorption process in the measured enthalpy change. Indeed the walls of the injection syringue and of the cuvette are made of glass and asteloy respectively, both these materials are negatively charged at pH 7.4, and hence, the adsorption of polyanions onto their surface should be reduced. However, in the presence of a polycation in the cuvette, this polyelectrolyte can adsorb on the walls of the cuvette, and upon polyanion injection, some of this polyanion can bind not only to dissolved polycation but also on an adsorbed one. These two binding events are not necessarily identical as will be discussed again in the Results and Discussion section. Nevertheless, the contribution of the adsorbed polycations is negligible with respect to that of the dissolved ones because the surface over volume ratio of the cuvette is small and the concentration of the dissolved polycation pretty high (typically of 1 mg/mL). In addition, the amount of added polyanion is small (the ratio of the provided polyanion to the available polycation does not exceed 0.02 after four successive 10 µL injections) so that the injected polyanions will encounter a huge excess of polycation to which they will bind before having any chance to reach the polycation-coated wall. At the end of each dilution or binding experiment, the injection syringe and cuvette were cleaned with a 5% (v/v) containing detergent solution (Decon) during at least half an hour and intensively rinsed with Milli Q water. Quartz-Crystal Microbalance with Dissipation. The polyelectrolyte multilayers were built up in situ on the surface of a quartz crystal coated with 50 nm of silica (Q Sense AB, Go¨thenborg, Sweden). Each experiment was performed on a new quartz crystal, which was never put in contact with polyelectrolytes before. The frequency and dissipation changes were followed in situ with a quartz crystal microbalance (Q 300, Q Sense AB) by monitoring the resonance frequencies close to the fundamental frequency of the quartz crystal (close to 5 MHz) as well as close to the third, fifth, and seventh harmon-

Laugel et al. ics.31,32 Each polyelectrolyte adsorption step was performed by injecting 2 mL of polyelectrolyte solution during 1 min which was left in contact with the substrate in the absence of flow for 9 additional minutes. The buffer rinse step was performed in the same manner. These QCM-D experiments were performed at (25.0 ( 0.1) °C at different salt concentrations between 1 and 160 mM in NaCl. Some experiments were also performed at either 1 mM or 50 mM in NaCl at different temperatures, either (15.0 ( 0.2), (25.0 ( 0.2), or (35.0 ( 0.2) °C. To ensure that the polyelectrolyte solutions were equilibrated at the desired temperature, they were injected in the thermostated T-loop of the QCM-D apparatus at least three minutes before the injection over the sensing quartz crystal. In all figures representing the multilayer buildup, we will plot frequency changes as a function of the number of deposited layers, denoted by i. Odd and even values of i correspond to the deposition of a polycation and a polyanion, respectively. i ) 1 corresponds to the deposition of the PEI precursor layer. The frequency changes reported in the figures correspond to the frequency changes measured between the end of the buffer rinse step (after the adsorption of a polyelectrolyte) and the frequency measured for the bare crystal equilibrated with the buffer. Atomic Force Microscopy (Image Mode). To check the uniformity of the surface coverage due to the PEM film deposition, we performed atomic force microscopy experiments in both the dry and wet state for most of the investigated salt concentrations. The surfaces were imaged in the contact mode with a Nanoscope IV microscope (Veeco) using silicon nitride cantilevers and tips with a nominal spring constant of 0.05 N m-1. The PEI-(PGA-PAH)24 multilayer films were built up at a given salt concentration after an initial adsorption of a PEI layer with an automated dipping robot (Dipping Robot DR3, Kirstein and Riegler, GmbH, Germany) as described in detail elsewhere.25 These films were deposited on glass slides 12 mm in diameter (VWR, France) and which were cleaned by dipping them in a hot 10 mM sodium dodecyl sulfate solution, with distilled water, hot HCl (at 0.1 M), and finally with distilled water again before film deposition. Just before imaging, the film was rapidly rinsed with distilled water, blown dry with a stream of nitrogen, and scratched with an ethanol-cleaned syringe needle. The film was then imaged dry (or wet in the presence of an NaCl solution) in the contact mode in a scan direction perpendicular to the scratched line. Before image acquisition, the stability of the visualized structures was checked by allowing several scans over the investigated area. III. Results and Discussion We performed isothermal titration calorimetry experiments on various polyanion/polycation systems in different conditions. We checked that the dilution of the polycation solutions (present in the microcalorimeter cuvette) by 10 µL of the pure electrolyte solution was negligible in comparison to the reaction heat measured upon injecting the same volume of polyanion solution in either the buffer or the polycation solution. This result is in accordance with data from the literature.33 A typical experimental curve is given in Figure 1 for the PGA/PAH combination of polyelectrolytes. For some of the investigated polyelectrolyte combinations the nature of the buildup process (exponential or linear) is known from the literature, while for others we investigated it by quartz crystal microbalance. The results are summarized in Table 1. In all these experiments polyanion P1 was injected into a

Polyelectrolyte Multilayer Films

J. Phys. Chem. B, Vol. 110, No. 39, 2006 19447

Figure 1. Experimental heat flow obtained in a typical microcalorimetry experiment performed at 25 °C in the presence of a 160 mM sodium chloride aqueous solution at pH 7.4. In this case, a PGA solution at 1 mg mL-1 is injected in a PAH solution at 1 mg mL-1 (b) or in a buffer solution without polyelectrolyte (s).

Figure 2. Effect of temperature on the build up of a PEI-(PGA-PAH)n film at a salt concentration of 50 mM, black symbols, and at a salt concentration of 1 mM, white symbols. The temperatures at which the buildups were followed are indicated in the inset. For the sake of clarity of the representation, only the opposite of the normalized frequency changes for υ ) 3 are given.

solution of polycation P2 under conditions where the final concentration of P1 represents less than 2% of the concentration of P2 (in monomer units). Under these conditions, the chains P1 establish a maximum of interactions with the P2 chains. To get the reaction enthalpy, one must subtract from the measured heat, the dilution heat of the polyelectrolyte P1 in the solution. One can notice that this heat of dilution is far from being negligible. It appears even that the dilution heat of PGA appears endothermic which is very surprising at first glance. Indeed one expects that the dilution of a concentrated solution of charged polymers would lead to an exothermic process since the average spacing between the chains will increase upon dilution. Such an exothermic process is indeed measured for the dilution of PSS and HA (Table 1), and among the polyanions we investigated, PGA appears as an exception. However our findings are in accordance with published microcalorimetry data 34 where it was found that the dilution of PGA can be an endothermic process at intermediate degrees of ionization where the endothermic contribution associated with a conformational transition of PGA (characterized by circular dichroı¨sm spectroscopy) has a higher value than the exothermic effect associated with the dilution of the negatively charged chains. We will concentrate more on this effect in a forthcoming publication. The reaction enthalpies are given here per monomer unit of polyelectrolyte P1. They are always less than 1 kT (2500 J‚mol-1) in absolute value. However, it is expected that all the charges of P1 cannot participate in a direct anion/cation interaction. The energy per monomer of P1 effectively involved in these interactions is thus certainly much larger than the given value which represents a mean per monomer over the entire P1 chains. Our data clearly indicate that when the heat of reaction is positive (endothermic process) the P1/P2 multilayer buildup process is exponential. On the other hand, when the heat of reaction is strongly exothermic (more negative than -1000 J‚mol-1), the multilayer buildup process is linear. Finally when the reaction is weakly exothermic (reaction heat larger than -500 J‚mol-1), the film buildup process is also exponential but seems to be weakly exponential as will be shown later. These results seem to indicate that linearly growing processes are associated with strongly exothermic reactions, whereas exponentially growing ones are associated with endothermic complexation reactions. In this latter case the multilayer buildup

process should be mainly entropically driven and thus sensitive to temperature changes. We tried to verify this latter point on the PGA/PAH system which is linear at low ionic strength (1 mM of NaCl) and exponential at high ionic strength (above 10 mM, see below). PGA/PAH multilayers were built at three different temperatures (15, 25, and 35 °C) from 1 and 50 mM NaCl solutions. The buildup process was followed by QCM-D at four different frequencies. It appears that the reduced frequency shifts, -∆f/υ, where ν represents the overtone number, is almost independent of ν. This implies that -∆f/υ is, in first approximation, proportional to the mass of the film, including its water content, according to Sauerbrey’s relation.40 Figure 2 represents the evolution of the reduced frequency shift with the number of deposition steps, i, for the third overtone (15 MHz). The odd numbers correspond to the deposition of PAH. One observes that at all three investigated temperatures the mass of the film corresponding to 1 mM of NaCl increases linearly with i and that the mass increment is almost independent of temperature. In contrast, at 50 mM of NaCl, the films grow exponentially and the growth process is very sensitive to temperature. After 12 deposition steps (six PGA/PAH bilayers) a 450% increase in the film mass between 15 and 35 °C is observed at an ionic strength of 50 mM. A similar strong temperature dependence of exponentially growing film processes and a much smaller dependence of linearly growing ones was already observed by Saloma¨ki and Kankare who investigated the buildup of PSS/PDADMA and PSS/PAH multilayers.36 Tan at al.41 and Bu¨scher et al.42 also investigated the effect of temperature on the buildup of linearly growing films. Even if they observed an increase in the thickness increment per bilayer with temperature, it was much smaller than the changes observed here and by Saloma¨ki et al. for exponentially growing films. Finally, we also investigated the effect of ionic strength on the PGA/PAH buildup process and on the enthalpy of the PGA/ PAH interactions in solution. From Table 1, it comes out that, at a given temperature, the PGA/PAH complexation process is exothermic at low ionic strength, but becomes endothermic at high ionic strength. We thus wished to verify if the PGA/PAH buildup process changes from a linear to an exponential behavior by increasing the NaCl concentration. As already mentioned, this is indeed the case. These experiments were again carried

19448 J. Phys. Chem. B, Vol. 110, No. 39, 2006

Laugel et al. Indeed the adsorbed chains have a reduced amount of conformational freedom and the contribution of the entropy changes to the free energy changes will certainly be different. Hence, we plan to measure the interaction enthalpies between polyelectrolytes adsorbed on colloidal particles and polyelectrolytes of opposite charge in solution to correlate more finely these enthalpy changes to the multilayer growth regime. Conclusion

Figure 3. Evolution of the value of the fitting parameter 1/i0 in eq 1 as a function of the NaCl concentration used for the buildup experiment of PEI-(PGA-PAH)n multilayer films. All the corresponding experiments where performed at 25 °C.

out by QCM-D at 25 °C. We checked by means of atomic force microscopy that a continuous film is always formed whatever the ionic strength of the solution, provided a sufficient number of bilayers are deposited (Figure 1 of the Supporting Information). The following exponential function could well fit the experimental QCM-D data:

-

( ) ( )( ( ) )

∆f ∆f ) ν ν

+ -

PEI

∆f ν

exp

1

i -1 i0

(1)

where (-(∆f/ν))PEI is the reduced frequency shift corresponding to the precursor PEI layer, (-(∆f/ν))1 and i0 being two characteristic growth parameters. In particular, 1/i0 reflects the “strength” of the exponential character. Values of 1/i0 close to 0 correspond to linearly growing films, whereas larger values of the order of 0.4 as obtained at high ionic strength mean that the thickness of the film almost doubles every two deposition steps. Figure 3 represents the evolution of 1/i0 as a function of the NaCl concentration. One observes that the growth process changes from a linear to an exponential behavior between 3 and 10 mM of NaCl. The “exponentiality” saturates above 30 mM of NaCl with a value of 1/i0 of the order of 0.4. Notice that at 10 mM in NaCl, the PGA/PAH complexation enthalpy was slightly exothermic and that the multilayer is only weakly exponential. A similar behavior is observed for the system PSS/PAH at 2 M of NaCl, which is slightly exothermic and only weakly exponential (Table 1). The increase of the “exponentiality” (quantified by 1/i0) with salt concentration was already observed on the system chitosan/hyaluronic acid by Richert et al.25 It is interesting to notice that at 160 mM in NaCl the reaction enthalpy between PGA and PAH does not depend strongly on the temperature between 25 and 45 °C (Table 1). This observation, combined with the fact that the exponential growth process are very temperature sensitive, again confirms that the exponential growth process is mainly entropically driven. On the other hand, at 1 mM in NaCl, when the film grows linearly, the interaction enthalpy varies strongly with temperature, whereas the film growth is only slightly affected by temperature changes. This indicates that in this case it is the enthalpy change that mainly drives the growth process. However, it has to be noted that the interaction enthalpies measured in solution between “free” polyelectrolytes are not necessarily the same as the enthalpies between an adsorbing polyelectrolyte and an already adsorbed one of opposite charge.

Isothermal titration microcalorimetry experiments performed on several polyanion/polycation systems suggest a correlation between the complexation heat and the nature of the film growth process: an endothermic complexation process seems to corresponds to an exponential film growth, a strongly exothermic process corresponds to a linearly film growth, and weakly exothermic processes giving weakly exponentially growing films. These results show that the exponentially growing processes are mainly driven by entropy which explains why these processes are very temperature sensitive. Hence, it is tempting to anticipate that one could tune the buildup regime of every polyelectrolyte multilayer film from linear to exponential or vice versa by a weakening or a strengthening of the polyelectrolytepolyelectrolyte interactions by changing the ionic strength and/or the temperature of the dipping solutions. Acknowledgment. This work was supported by the ACI “Nanoscience” (NR204) from the Ministe`re Franc¸ ais de la Recherche. Supporting Information Available: AFM section obtained in the contact mode and in air of a PEI-(PGA-PAH)24 bilayer build up on a silica slide in the presence of 1 mM. NaCl at pH 7.4. This material is available free of charge via the Internet at http://pubs.acs.org References and Notes (1) Iller, R. K. J. Colloid Interface Sci. 1966, 21, 569-594. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831-835. (3) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321-327. (4) Decher, G. Science 1997, 277, 1232-1237. (5) Ramsden, J. J.; Lvov, Y. M.; Decher, G. Thin Solid Films 1995, 254, 246-251. (6) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058-7063. (7) Korneeva, D.; Lvov, Y.; Decher, G.; Schmitt, J.; Yaradaikin, S. Physica B 1995, 213-214, 954-956. (8) Jomaa, H. W.; Schlenoff, J. B. Macromolecules 2005, 38, 84738480. (9) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J.-C. Langmuir 2001, 17, 7414-7424. (10) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531-12535. (11) Boulmedais, F.; Ball, V.; Schwinte´, P.; Frisch, B.; Schaaf, P.; Voegel, J.-C. Langmuir 2003, 19, 440-445. (12) Poptoshev, E.; Schoeler, B.; Caruso, F. Langmuir 2004, 20, 829834. (13) Delongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem. Mater. 2003, 15, 1575-1586. (14) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655-6663. (15) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Langmuir 2002, 18, 5607-5612. (16) Kulcsar, A.; Lavalle, P.; Voegel, J. C.; Schaaf, P.; Ke´kicheff, P. Langmuir 2004, 20, 282-286. (17) Kulcsar, A.; Voegel, J.-C.; Schaaf, P.; Kekicheff, P. Langmuir 2005, 21, 1166-1170. (18) Collin, D.; Lavalle, P.; Garza, J. M.; Voegel, J.-C.; Schaaf, P.; Martinoty, P. Macromolecules 2004, 37, 10195-10198. (19) Heuvingh, J.; Zappa, M.; Fery, A. Langmuir 2005, 21, 3165-3171.

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