Influence of the nature of the polycation on the adsorption kinetics and on exchange processes in polyelectrolyte multilayer films

Journal of Colloid and Interface Science 366 (2012) 96–104

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Influence of the nature of the polycation on the adsorption kinetics and on exchange processes in polyelectrolyte multilayer films Vincent Ball ⇑, Abdelghani Laachachi, Valérie Toniazzo, David Ruch Advanced Materials and Structures, Centre de Recherche Public Henri Tudor, 66 rue de Luxembourg, L-4002 Esch-sur-Alzette, Luxembourg

a r t i c l e

i n f o

Article history: Received 15 July 2011 Accepted 19 September 2011 Available online 29 September 2011 Keywords: Polyelectrolyte multilayer films Exchange processes Deposition kinetics

a b s t r a c t The deposition of polyelectrolyte multilayer films (PEMs) appears more and more as a versatile tool to functionalize a broad range of materials with coatings having controlled thicknesses and properties. To increase the control over the properties of such coatings, a good knowledge of their deposition mechanism is required. Since Cohen Stuart et al. (Langmuir 18 (2002) 5607–5612) showed that the adsorption of one polyelectrolyte could induce desorption of polyelectrolyte complexes instead of regular deposition, more and more findings highlight peculiarities in the deposition of such films. Herein we demonstrate that the association of sodium polyphosphate (PSP) as the polyanion and either poly(-L-lysine hydrobromide) (PLL) or poly(allylamine chloride) (PAH) as the polycations may lead to non-monotonous film deposition as a function of time. Complementary, films containing PSP and PLL can be obtained from a (PLL–HA)n template films after the exchange of HA (hyaluronic acid) from the sacrificial template by PSP from the solution. This exchange is accompanied by pronounced film erosion. However, when starting from a (PAH–HA)n template, the film erosion and exchange due to the contact with PSP is by far less pronounced, nevertheless the film morphology changes. These findings show that the nature of the polycation used to deposit the PEM film may have a profound influence of the film’s response to a competing polyanion. Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction For a long-time, functionalization of solid–liquid interfaces was restricted to the adsorption or chemisorption of essentially monolayers with some specific deposition methods and techniques related to the chemistry of both the adsorbates and the substrates. The deposition of layers of amphiphiles by transfer from the liquid–gas interface to solid substrates, the more robust deposition of thiols on the surface of noble metals [1,2] or of alkyl silanes on the surfaces of oxides [3] belong to these specific methods. Electrostatic interactions are by far less specific since almost all surfaces carry surface charges. Hence, it is surprising to observe, a posteriori, how long the adsorption of polyelectrolytes at solid–liquid interfaces was restricted to the adsorption of ‘‘monolayers’’. It was certainly not realized that in certain conditions where the polyelectrolytes adopt coil conformations (i.e., at weak charge density or in the presence of strong electrostatic screening from the electrolyte solution) their adsorption leads to a charge overcompensation, i.e., the adsorption of the polyelectrolyte leads not only to an accurate compensation of the charges carried by the substrate to yield a perfectly neutral interface but to an excess of charges due to polyelectrolyte loops not in contact with the substrate. Hence, the ⇑ Corresponding author. E-mail address: [email protected] (V. Ball). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.09.045

adsorption of a polyelectrolyte in such conditions should allow for the adsorption of an oppositely charged polyelectrolyte or an oppositely charged colloidal particle. The feasibility of this idea was first demonstrated with rigid and oppositely charged silica particles by Iler in 1966 [4]. The lack of efficient surface characterization tools certainly precluded the extension of this concept to polyelectrolytes and it is only 20 years ago that Decher and coworkers demonstrated the possibility to deposit polyelectrolyte multilayer (PEM) films [5]. This first investigation showed that films made from at least 100 alternately deposited ‘‘layers’’ can be obtained. The occurrence of charge reversal after the deposition of each ‘‘layer’’ was also suggested in this article even if the first demonstration of surface potential reversal was only shown in 1996, to our knowledge [6]. Many other investigators have then shown that alternated reversal of the surface potential upon adsorption of oppositely charged polyelectrolytes is a signature of PEM deposition [7,8] yielding to the concept that ‘‘surface charge reversal is a driving force’’ for PEM deposition. However, 20 years of research in the field of layer-bylayer (LBL) deposition showed that the facts are by far much more complicated than predicted by the basic proposed model of slightly interpenetrating chains [9].

(i) First of all, Kotov [10] proposed some numerical estimates of the free energy changes upon LBL deposition of oppositely

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charged polyelectrolytes and arrived to the conclusion that other weak interactions than pure electrostatics in a medium of high dielectric constant, like water, should play a major role in the energy balance. (ii) Isothermal titration microcalorimetry experiments aimed to measure the enthalpy changes upon the interaction between oppositely charged polyelectrolytes [11] and the correlation of the obtained values with the film growth regime of the films made from the same polyelectrolytes [12] highlighted the importance of entropy changes in the deposition of PEM films. The finding that in many situations the film deposition can only occur through an increase in entropy is in perfect agreement with the early assessment of Michaels in the field of polyelectrolyte complexation [13]. The entropy increase may well be due to a release of counter-ions and from water of hydration from the polyelectrolyte chains upon their complexation. The adsorption of the chains decreases their degrees of freedom and has hence a negative contribution on the entropy change. The entropy balance during the deposition of PEM films is hence complicated because it contains terms that can mutually compensate. (iii)The observation of supralinear growth regimes [14], in which the amount of polyelectrolytes per unit area of the substrate increases much rapidly than linearly as expected for a regular growth, showed that the inter-diffusion of polyelectrolytes through a large part, but not necessarily through the whole thickness of the film, is possible. In this case, regular charge overcompensation was also observed even after the first deposition steps where the substrate was not yet homogenously covered with a film but with islands instead [15]. The existence of non-compensated charges on the surface of such films allows to explain why diffusion of the polyelectrolytes out of the film and back in the solution is most of the time not quantitative, otherwise the film would decompose. A reservoir of mobile molecules in such films is certainly also at the origin of their Donnan potential [16], meaning that there is not only an excess of charges at the film–solution interface (compensated by the counterions from the double layer in solution) but also in the film. (iv) Some recent investigations showed that some combinations of polyelectrolytes [17] or some peculiar physicochemical conditions [18] allow to deposit films that grow monotonously with the number of deposition steps but without alternated inversion of the surface potential. Indeed after about 100 alternated deposition steps of poly (sodium phosphate) (PSP) and poly(allylamine hydrochloride) (PAH), the deposit, made from non-coalescent islands, has a constant negative f potential, when the deposit is made with polyelectrolytes dissolved at 10 4 M in monomer units and in the presence of 0.15 M NaCl. This means that PSP adsorbs on the already deposited islands or in vacancies in between and in conditions of repulsive electrostatic interactions. Note that these PSP and PAH containing deposits were prepared through alternated spray deposition. These findings point to the need to further investigate about the origin of the driving forces leading to such self-assembled films. (v) Finally, it has been found that in certain circumstances, the alternated deposition of polyelectrolytes leads to very small deposition owing to the competition between adsorption and desorption phenomena [19,20]. This finding has been rationalized by the phase diagrams of polycation–polyanion mixtures in which there exists a glassy state and a soluble state depending on the physicochemical conditions (salt concentration, temperature). The occurrence of regular film deposition

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corresponding to the ‘‘glassy’’ phase of the phase diagram whereas the adsorption–desorption regime corresponds to the ‘‘soluble’’ phase. The findings of the Cohen Stuart’s group [19,20] are of major importance because they provide a general framework for the deposition of PEM films, as well as of all kinds of ‘‘LBL’’ films (whose relying on polymers interacting through hydrogen bonds [21–23] or through donor–acceptor interactions [24]). This framework relies on the assumption that the knowledge of the phase diagram of a polycation–polyanion mixture could be a predictive tool for the occurrence of film deposition (or its absence) depending on the occurrence (or not) of phase separation when PE are mixed in different ratios even different from 1:1 in monomer ratios [25,26]. In the case of the adsorption–desorption regime, a close inspection of the adsorption kinetics performed by means of stagnation point optical reflectometry showed that the surface concentration of deposited polyelectrolyte increases up to a certain point in time before desorption starts leading eventually to a complete film erosion [19,20]. This finding suggests that the film deposition may be possible by allowing adsorption to occur only for a very short time duration. This could make alternated deposition of polyelectrolytes through dipping in polyelectrolyte solutions a very time efficient deposition method, competitive with alternated spin coating [27] or alternated spraying [28,29] in which each deposition step lasts over only a few seconds. This may also change the classical idea that LBL deposition through alternated dipping in solutions containing oppositely charged polyelectrolytes is time consuming. Indeed, Kleinfeld and Ferguson showed that PEM films made from poly(diallyldimethyl ammonium chloride) and sodium montmorilonite reach 95% of their maximal thickness when each adsorption step was performed during only 5 s of dipping the substrate in the clay or polycation containing solution [30]. Of course the concentration of each polyelectrolyte solution is of major importance in the film growth regime and kinetics [17,31]. Another important point in the dynamic aspects of PEM films is the possible occurrence of exchange phenomena in which a deposited PEM film can undergo compositional and structural changes upon exposure to a solution containing a polyelectrolyte different from its constituent chains [32–35]. Up to now, the influence of adsorption time and the possibility to produce films with modified composition through an exchange process has never been demonstrated simultaneously. We will show in this article some peculiar aspects of LBL deposition: namely that for some polycation–polyanion combinations, thicker films can be obtained by using shorter deposition times and that films of similar composition can be obtained from sacrificial PEM templates in which one kind of polyelectrolyte already present in the template can be almost quantitatively replaced by the polyelectrolyte of interest. However, such an exchange process depends dramatically on the polyelectrolytes used to build up the sacrificial template: for instance, PSP–PLL containing films can be build from a (PLL–HA)n template but not from a (PAH–HA)n film. HA represents sodium hyaluronate. Our data hence complement some suggestions proposed in previous work [36] about the importance of the interpolyelectrolyte interaction strength on the dynamics of PEM films. The interaction strength between the chains can be changed by playing not only on the structure of one of the polyelectrolytes, as in this work, but also by playing on physicochemical parameters as the pH [37], ionic strength [38], and temperature [39]. The present investigation aims at stimulating the need to investigate not only the deposition kinetics of

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were solubilized in 0.15 M NaCl solutions whose pH was not adjusted. The pH of the solutions was checked with a pH meter (Mettler Toledo) and was found equal to 6.7, 6.5, 7.5, and 8.0 for the PSP, HA, PLL, and PAH solutions, respectively. When not specified, all polyelectrolyte solutions were prepared at a concentration of 1 mg mL 1.

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n Fig. 2. Influence of dipping time on the deposition of (PAH–PSP)n deposits: ( ) 30 s, (d) 1 min, and (s) 5 min per adsorption step. The inset represents the average film thickness determined by ellipsometry for films made form n = 8 deposition cycles. The error bars correspond to one standard deviation over five measurements performed on an individual coated silicon slide.

polyelectrolytes during the deposition of PEM films but also to show that exchange processes are an important issue for the stability of PEM films.

2. Materials and methods 2.1. Chemicals All solutions were prepared from double distilled water (Millipore Simplicity system, q = 18.2 MX cm). The polyelectrolytes

The PEM films were deposited on P-doped silicon wafers (Siltronix, Archamps, France) that were previously cut in the form of rectangles having a size of 4  1 cm2. These wafers were cleaned by successive immersion in hot (60 °C) Hellmanex solutions (2% v:v) (Hellma GmbH, Müllheim, Germany) during half an hour, distilled water (Millipore Simplicity System, q = 18.2 MX cm), hydrochloric acid at 0.1 M during 10 min, and finally with distilled water. Freshly cleaned silicon wafers were used for each new deposition experiment. A trapezoidal ZnSe crystal (Graseby-Specac, Orpington, UK) was used as a substrate for the infrared spectroscopy experiments in the totally attenuated reflection mode (ATR-FTIR spectroscopy). The crystal was cleaned with a methanol wiped optical paper before the deposition experiments. 2.3. Film deposition As long as we have not characterized the obtained coatings by means of atomic force microscopy, we will call them ‘‘deposits.’’ These ‘‘deposits’’ will be called films if they display continuous deposition, i.e., in the absence of channels going from the deposit–solution interface down to the substrate. To prepare the (PLL– PSP)n and (PAH–PSP)n PEM deposits on the freshly cleaned silicon wafers, the samples were hold with a cleaned tweezer and immersed manually in the polycation, in the sodium chloride rinsing solution, in the polyanion and again in the rinsing solution. Such a dipping cycle consists in the deposition of one polycation and one polyanion (defined as one layer pair in most articles in the ‘‘LBL’’ field). Each (polycation–polyanion)n deposit was prepared individually without intermediate drying and rehydration in order to avoid possible artifacts due to such a partial drying step which is necessary for the characterization of the deposit. Hence, each point in the figures corresponds to an individually prepared deposit. The immersion times in the polycation and polyanion solutions were the same and changed from one experiment to the other. The deposition experiments were performed at (23 ± 2) °C in a room fitted with an air conditioner. The importance of controlling the temperature in the deposition of PEM films is of major importance particularly for those displaying a supralinear increase in their thickness with the number of deposition steps [39]. 2.4. Characterization methods 2.4.1. Ellipsometry The average thickness of the deposits was measured by means of single wavelength ellipsometry at k = 632.8 nm (He–Ne laser) and at an angle of incidence of 70° (PZ 2000 Horiba, Longjumeau, France). To convert the ellipsometric angles W and D, into thickness values, we had to assume a value for the refractive index of the film. We choose a value of 1.465 as in our previous investigations on (PLL–HA)n and (PLL–PGA)n films which is reasonable for a film made from polymer and containing a certain volume fraction of water [40] and counterions. Since the structure and hydration of PSP containing films are not yet known and since phosphorous is a somewhat more polarizable element than carbon, nitrogen, and oxygen, the fact to fix the refractive index of the film at 1.465 could induce some systematic over or underestimation of the film

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Fig. 3. AFM surface topographies of (PAH–PSP)n deposits for different dipping times and for films prepared from n = 4 and n = 8 deposition cycles. Each sample has undergone some needle scratching to distinguish the deposit from the silicon substrate (right part of each image). The image sizes were of 20  20 lm2.

thickness. To that aim, we used atomic force microscopy (AFM) profiling across sections made in the films deposited on silicon to check the consistency of the thickness determination made by ellipsometry. In addition, ellipsometry allows only to determine the average film thickness over an area close to 1 mm2, corresponding to the size of the light spot shined on the substrate. One is hence not able to detect whether a true film (without channels reaching the substrate) or islands are deposited. Many LBL deposits grow in a continuous manner but without forming films but rather droplets that may [15] or not [17] coalesce upon an increase in the number of deposition steps. It is hence extremely dangerous to rely on ellipsometry or UV–vis spectroscopy alone to ascertain the deposition of a film [41]. One or both of these techniques have to be combined with local imaging by AFM. In all the figures, the thickness of the deposits will be denoted by ‘‘d’’.

2.4.3. Infrared spectroscopy in the attenuated total reflexion mode (ATR-FTIR) The ATR-FTIR spectra were acquired by the accumulation of 512 interferograms with a Equinox 55 spectrophotometer (Bruker, Germany) at a spectral resolution of 4 cm 1 and at wavenumbers between 700 and 4000 cm 1. The PEM films were deposited from NaCl solutions prepared in D2O. Additional details can be found in previous articles [35,36,42]. The IR spectrum of the PSP powder was also acquired in the transmission mode with a Tensor 27 spectrometer (Bruker, Germany).

3. Results and discussion 3.1. Deposition kinetics of the (PLL–PSP)n and (PAH–PSP)n films

2.4.2. Atomic force microscopy The AFM topographies of the dried films (maintained in a vacuum chamber during a few hours before image acquisition) were acquired in the tapping mode with a Pico SPM microscope (Molecular Imaging) at a frequency of 1 Hz. Each image was acquired with a new pyramidal silicon tip. The images were acquired on deposits that were needle scratched just before image acquisition in order to have access to the morphology of the deposits as well as their thickness. The line profiles of the obtained sections were averages over 30 line scans. The images were acquired over squares 20  20 or 30  30 lm in area.

We first investigated the influence of the dipping time per deposition step on the evolution of the average thickness of the deposits calculated from ellipsometry measurements for two kinds of polyelectrolyte combinations having a common polyanion, PSP, but differing in the deposited polycation, PLL (Fig. 1) versus PAH (Fig. 2). In the presence of 0.15 M NaCl as the supporting electrolyte, it appears that for all number of deposition steps, the thickness of the (PLL–PSP)n deposits passes through a maximum when the deposition time is of 1 min. The inset of Fig. 1 exemplifies the situation in the case of deposits made from n = 8 deposition cycles. When the silicon wafers are immersed in the PLL or PSP

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Fig. 4. Profile scans (average over 30 scans) over the AFM images acquired in Fig. 3.

Table 1 Mean root squared roughness of the (PAH–PSP)n deposits imaged in Fig. 3.

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37.6 71.6

46.8 134

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solutions for 5 min, the average thickness of the deposits is only slightly higher than when the deposition is performed during 30 s. In these conditions of NaCl concentration, the PLL–PSP combination of polyelectrolytes behaves as the systems described by Cohen Stuart et al. [19,20], namely as a system in which desorption of polyelectrolyte complexes occurs in concurrence to polyelectrolyte adsorption and resulting in a situation in which maximal deposition occurs for an optimal adsorption time. We show here that the adsorption kinetics passes through an ‘‘overshoot.’’ For a deposition time corresponding to the maximal deposited thickness, a supralinear increase in the thickness is observed (Fig. 1). If we would not have investigated the influence of the adsorption time and chosen arbitrarily a long adsorption time (5 or 10 min) as is usual for step-by-step deposition of polyelectrolytes, we would have concluded that the deposition of (PLL–PSP)n yields to only very thin deposits. This is obviously not the case provided the adsorption time is adjusted to reach an optimal deposition. Deposition times of 2 and 3 min yield already deposits of smaller thickness than those obtained after 1 min (data not shown). These results are similar as those reported by Izumrudov et al. [25]. When replacing PLL by PAH for the alternated dipping process with PSP, the situation is totally different in the sense that the film

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thickness increases monotonously with the dipping time (Fig. 2). The inset in Fig. 2 shows the situation for deposits made from n = 8 dipping cycles. For this combination of polyelectrolytes displaying a supralinear increase in thickness with the number of

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1/λ (cm-1) Fig. 7. ATR-FTIR spectra of a (PLL–HA)9 film before ( ) and after (++++) being put in contact with PSP at 1 mg mL 1 during 25 min. The red line corresponds to the spectrum of the PSP powder. (For the interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

deposition cycles, it appears that the thicker the films are the more time is required to reach a steady state in the adsorbed amount. Indeed, if one restricts to the deposition of less than n = 4 deposition cycles the thickness reaches a constant value even after 30 s (Fig. 2). This may be in relationship with the mechanism of film growth: in the case of exponentially growing films, the growth occurs through the diffusion of one polyelectrolytes from the film to the solution in order to reach the oppositely charged polyelectrolyte in solution [43]. The polyelectrolyte complexation then occurs at the film solution interface. The time required for the polyelectrolytes in the film to reach the film–solution interface may depend

Fig. 8. Film thickness change of (PAH–HA)n films in contact with PSP solutions (at 1 mg mL 1 during 25 min) as a function of the number of deposited layer pairs. Each pair of open disks and black squares corresponds to an individual experiment. The black arrow indicates the thickness change of the as deposited (PAH–HA)n film (s) after 25 min of contact with the PSP solution (j).

on the film thickness and can explain the present finding (Fig. 2) that the thicker the films are, the more time is required to reach an optimal film thickness. In the case where the film grows linearly, with little interpenetration of the different layers, an increase in the dipping time is not expected to have a major influence of the thickness of the deposit. In addition, in both systems, (PLL–PSP)n and (PAH–PSP)n, a continuous film is formed after the deposition of n = 4 layer pairs (Figs. 3 and 4). For these AFM characterizations, the (PLL–PSP)n and (PAH–PSP)n deposits were produced by changing the dipping time: 30 s, 1 min, and 5 min. The (PAH–PSP)n films start to form a continuous film after four deposition cycles (Fig. 3). Our findings are not in contradiction with those described for films made from the same polyelectrolytes sprayed on silicon in the presence of the same supporting electrolyte (NaCl at 0.15 M) [17]. Indeed, the spray coating experiments were performed at polyelectrolyte concentrations of 10 4 M in monomer units whereas the experiments described herein were performed at a concentration of 1 mg mL 1, namely close to 10 2 M in monomer units. For the spray deposition method at polyelectrolyte concentrations of 10 4 M in monomer units, the deposits never coalesced even after 75 deposition cycles [17]. The mean squared roughness of the (PAH–PSP)n films is given in Table 1. Its value increases with the number of deposition steps at constant immersion time as well as with the immersion time for a deposit made from a constant number of deposition steps. The (PLL–PSP)n deposits also form continuous films after the deposition of at least four layer pairs as exemplified from the profile scans over the scratched regions in the imaged regions (data not shown). The main message of this first part of the present article is that the optimal adsorption time to reach the thickest possible deposit has to be optimized for each particular film and that some combinations of polyelectrolytes may lead to a maximal thickness for very short adsorption times before the occurrence of film erosion. One other conclusion from this investigation is that the affirmation that step-by-step deposition by alternately dipping the substrates in oppositely charged polyelectrolyte solutions is a long and cumbersome process, may not always be true. Indeed for the (PLL–PSP)n combination, the thickest films are obtained after only

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Fig. 9. Film morphology (20  20 lm) of a (PAH–HA)10 film before (upper row) and after (middle row) exposure to a PSP solution at 1 mg mL corresponds to images (30  30 lm) of (PAH–HA)15 film exposed during 25 min to a 1 mg mL 1 PSP containing solution.

1 min of immersion of the substrate, whatever the number of deposition steps. 3.2. Influence of PSP on (HA–PLL)n and on (HA–PAH)n films In the present part, we ask whether (PLL–PSP)n and (PAH–PSP)n films whose deposition kinetics are markedly different could not be obtained through an exchange process in which a common polyanion, hyaluronic acid (HA), from (PLL–HA)n and from (PAH–HA)n films could be exchanged by the polyanion, PSP, put in contact with the film. Exchange process in PEM films have been widely described in the literature [32–34], but we aim here to go a bit further in this direction to highlight that the nature of the already deposited film plays a major role on the issue of the exchange process. By

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during 25 min. The lower row

changing the nature of the polycation, PLL or PAH, of the initially deposited and exponentially growing films, the issue of the exchange process is totally different. In the case of (PLL–HA)n films, the average thickness of the deposit decreases rapidly when the films are put in contact with a PSP solution at 1 mg mL 1 and reaches a steady state after about 5 min (Fig. 5). Most interestingly for (PLL–HA)n deposits with n smaller than 10, the thicker the initially deposited film, the higher is the resulting film thickness (Fig. 6). However, for n larger than 10, the obtained deposit has an almost constant thickness and is a continuous film (inset of Fig. 6). This result is comparable to the one obtained when (PAH–HA)n films are put in contact with potassium hexacyanoferrate: such films undergo an important erosion but after the transition from the exponential to the linear growth regime, occurring at

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n = 10–12, the resulting film has an almost constant thickness [36]. Such phenomena may well be associated with a change in the cohesion of the films after the exponential to linear growth transition. This point remains to be explored in future investigations. The inset represents an AFM topography of a (PLL–HA)20 film after exposure to a PSP solution during 25 min. The film has been needle scratched before image acquisition. The resulting film not only contains PSP as shown by means of ATR-FTIR spectroscopy (peaks at 875, 1080, and 1270 cm 1) but almost all HA in the initial (PLL–HA)9 template has been displaced during the PSP–HA exchange process (Fig. 7). It is highly surprising that such an exchange process is so rapid, being achieved in less than 10 min. This highlights again the dynamic nature of the (PLL–HA)n films in which a high chain mobility has been found by means of fluorescence recovery after photobleaching [44]. However, when one considers the (PAH–HA)n films, it appears that the thickness reduction is much less pronounced (Fig. 8) after 25 min of contact with PSP solutions (at 1 mg mL 1) than for the (PLL–HA)n films (Fig. 6). As another marked difference with the (PLL–HA)n deposits, the erosion of the (PAH–HA)n deposits increases continuously when n increases: there is almost no film erosion for the (PAH–HA)5 films but about 33% decrease in average thickness of the deposit for the (PAH–HA)15 films. In Fig. 9, we show that the root mean square roughness of the (PAH–HA)10 films increases from 91 nm to 159 nm when put in contact with PSP at 1 mg mL 1. Clearly the deposits are not yet films after n = 10 deposition cycles but uncoalesced islands. The state of a continuous film is reached only after 15 deposition cycles (Fig. 9). The ATR-FTIR spectra (data not shown) of the (PAH–HA)9 films after contact with the PSP solution show the incorporation of some PSP in the films but without a complete disappearance of the peaks assigned to HA (at 1610 cm 1) as was observed for the (HA–PLL)9 films (Fig. 7). This shows that the amount of exchangeable HA is by far much lower in (PAH–HA)n than is the (PLL–HA)n films. We make the assumption that the pronounced difference in the deposition kinetics of (PLL–PSP)n and (PAH–PSP)n coatings originates from much stronger interactions between PSP and PAH than between PSP and PLL during the deposition. On the other side, we attribute the strong film erosion of (PLL–HA)n in the presence of PSP versus the weak erosion of (PAH–HA)n films to the stronger cohesion of the last kind of films with respect to the former ones. This assumption is in line with Isothermal Titration Calorimetry experiments which showed that in the conditions of the present experiments, the reaction enthalpy between HA and PAH amounts to +640 J mol 1 (of monomer units) whereas the reaction enthalpy of HA and PLL amounts to +535 J mol 1 (of monomer units) [12].

4. Conclusions In this investigation, we showed that the deposition kinetics leading to (PLL–PSP)n and to (PAH–PSP)n deposits are markedly different when the experiments are performed in identical conditions. In the case where PAH is the polycation, the film thickness increases monotonously when the adsorption time of each polyelectrolyte increases. The influence of the adsorption time becomes significant only when the number of deposition cycles, n, is higher than 6 in probable relationship with the supralinear (exponential) growth of the deposit. However, in the case where PLL is the polycation, the film thickness is maximal, whatever the number of deposition cycles, when adsorption is allowed for only 1 min meaning that the deposition occurs through the interplay of competitive adsorption–desorption phenomena. Hence, the change of the nature of the polycation has a major influence on the deposi-

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tion kinetics. This finding highlights for the need to investigate the deposition kinetics for each considered pair of polycation/polyanion as long as a general ‘‘structure-build-up’’ relationship is not known for PEM films. Even more important is the finding that (PLL–HA)n and (PAH– HA)n films react in a different manner when exposed to a solution containing PSP: the first ones undergo a marked dissolution with an almost quantitative displacement of HA in favor of PSP whereas the second kind of films undergo only a very small thickness reduction with some morphological change, namely an increase in film roughness. Our investigation shows that PEM film can display fascinating dynamic behavior that need to be investigated with molecular spectroscopy techniques, like NMR methods. Acknowledgment The authors thank the FEDER ‘‘Compétitivité régionale et emploi’’ 2007-2013 for financial support of this research Chaptochem project N° 2009-02-039-35. References [1] R.G. Nuzzo, D.L. Allara, J. Am. Chem. Soc. 105 (1983) 4481–4483. [2] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103–1169. [3] I. Haller, J. Am. Chem. Soc. 100 (1978) 8050–8055. [4] R.K. Iler, J. Colloid Interf. Sci. 21 (1966) 569–594. [5] G. Decher, J.D. Hong, J. Schmitt, Thin Solid Films 210–211 (1992) 831–835. [6] N.G. Hoogeveen, M.A. Cohen Stuart, G.J. Fleer, M.R. Böhmer, Langmuir 12 (1996) 3675–3681. [7] F. Caruso, E. Donath, H. Möhwald, J. Phys. Chem. B 102 (1998) 2011– 2016. [8] G. Ladam, P. Schaad, J.-C. Voegel, P. Schaaf, G. Decher, F.J.G. Cuisinier, Langmuir 16 (2000) 1249–1255. [9] G. Decher, Science 277 (1997) 1232–1237. [10] N.A. Kotov, Nanostruct. Mater. 12 (1999) 789–796. [11] C.B. Bucur, Z. Sui, J.B. Schlenoff, J. Am. Chem. Soc. 128 (2006) 13690–13691. [12] N. Laugel, C. Betscha, M. Winterhalter, J.-C. Voegel, P. Schaaf, V. Ball, J. Phys. Chem. B 110 (2006) 19443–19449. [13] A.S. Michaels, Ind. Eng. Chem. 57 (1965) 32–36. [14] D.L. Elbert, C.B. Herbert, J.A. Hubbell, Langmuir 15 (1999) 5355–5362. [15] C. Picart, Ph. Lavalle, P. Hubert, J.F.G. Cuisinier, G. Decher, P. Schaaf, J.-C. Voegel, Langmuir 17 (2001) 7414–7424. [16] E. Hübsch, G. Fleith, J. Fatisson, P. Labbé, J.-C. Voegel, P. Schaaf, V. Ball, Langmuir 21 (2005) 3664–3669. [17] N. Cini, T. Tulun, G. Decher, V. Ball, J. Am. Chem. Soc. 132 (2010) 8264– 8265. [18] M. Adusumilli, M.L. Bruening, Langmuir 25 (2009) 7478–7485. [19] D. Kovacˇevic´, S. Van der Burgh, A. de Keizer, M.A. Cohen Stuart, Langmuir 18 (2002) 5607–5612. [20] D. Kovacˇevic´, S. Van der Burgh, A. de Keizer, M.A. Cohen Stuart, J. Phys. Chem. B 107 (2003) 7998–8002. [21] A. Laschewsky, E. Wischerhoff, E. Denzinger, H. Ringsdorf, A. Delcorte, P. Bertrand, Chem. Eur. J. 3 (1997) 34–38. [22] W.B. Stockton, M.F. Rubner, Macromolecules 30 (1997) 2717–2725. [23] S.A. Sukhishvili, S. Granick, J. Am. Chem. Soc. 122 (2000) 9550–9551. [24] Y. Shimazaki, R. Nakamura, S. Ito, M. Yamamoto, Langmuir 17 (2001) 953– 956. [25] S.A. Sukhishvili, E. Kharlampieva, V. Izumrudov, Macromolecules 39 (2006) 8873–8881. [26] H. Mjahed, J.-C. Voegel, A. Chassepot, B. Senger, P. Schaaf, F. Boulmedais, V. Ball, J. Colloid Int. Sci. 346 (2010) 163–171. [27] P.A. Chiarelli, M.S. Johal, J.L. Casson, J.B. Roberts, J.M. Robinson, H.-L. Wang, Adv. Mater. 13 (2001) 1167–1169. [28] J.B. Schlenoff, S.T. Dubas, T.R. Fahrat, Langmuir 16 (2000) 9968–9969. [29] A. Izquierdo, S.S. Ono, J.-C. Voegel, P. Schaaf, G. Decher, Langmuir 21 (2005) 7558–7567. [30] E.R. Kleinfeld, G.S. Ferguson, Science 265 (1994) 370–373. [31] L. Richert, Ph. lavalle, E. Payan, X.Z. Shu, G.D. Prestwich, J.-F. Stoltz, P. Schaaf, J.C. Voegel, C. Picart, Langmuir 20 (2004) 448–458. [32] H.W. Jomaa, J.B. Schlenoff, Langmuir 21 (2005) 8081–8084. [33] N.S. Zacharia, M. Modestino, P.T. Hammond, Macromolecules 40 (2007) 9523– 9528. [34] V. Ball, E. Hübsch, R. Schweiss, J.C. Voegel, P. Schaaf, W. Knoll, Langmuir 21 (2005) 8526–8531. [35] A.M. Pilbat, V. Ball, P. Schaaf, J.-C. Voegel, B. Szalontai, Langmuir 22 (2006) 5753–5759. [36] C. Betscha, V. Ball, Soft Matter 7 (2011) 1819–1829. [37] D. Yoo, S.S. Shiratori, M.F. Rubner, Macromolecules 31 (1998) 4309–4318.

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