Reversible Modification of Structure and Properties of Cellulose

Article pubs.acs.org/Langmuir

Reversible Modification of Structure and Properties of Cellulose Nanofibril-Based Multilayered Thin Films Induced by Postassembly Acid Treatment Firas Azzam,*,†,§ Céline Moreau,† Fabrice Cousin,‡ Alain Menelle,‡ Hervé Bizot,† and Bernard Cathala*,† †

INRA, UR1268 Biopolymères Interactions Assemblages, 44316 Nantes, France IMN, UMR 6502 CNRS-Université de Nantes, 44322 Nantes, France ‡ Laboratoire Léon Brillouin, CEA-CNRS Saclay, 91191 Gif sur Yvette, France §

S Supporting Information *

ABSTRACT: A postassembly acid-treatment consisting of an immersion in 5 mM HCl solution was applied to carboxylated cellulose nanofibrils (CNF)−poly(allylamine) hydrochloride (PAH) multilayered thin films. Our results show that the treatment did not affect the overall thickness of the films without any loss of the components. However, a modification of the surface morphology was observed, as well as the swelling behavior. The process was perfectly reversible since the original structure was recovered when the thin films were rinsed by ultrapure water. Moreover, a more pronounced antireflective character was detected for the treated films. The origin of these reversible modifications was discussed. Notably, the scattering length density (SLD) profiles of the films before and after treatment support the idea of a structural reorganization of the components within the film driven by the change of their charge densities induced by the acid treatment.

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INTRODUCTION

swelling behavior. However, no information about the effect of the immersion on the film properties after drying was provided. In the case of synthetic polyelectrolyte multilayered thin films, the postassembly treatment has been more frequently reported. For example, Rubner et al.40,41 have demonstrated that microporous thin films can be readily fabricated from LbLassembled multilayers of the weak polyelectrolytes poly(acrylic acid) (PAA) and poly(allylamine) (PAH) after a brief exposure to acidic aqueous solutions at pH 2.3−2.5. The same transition has also been reported for weak polyelectrolyte thin films when the final assembly has been exposed to either low42,43 or highpH solution.44−46 Fery et al.47 demonstrated the formation of nanoporous films through salt induced structural changes by exposing PAH/PAA multilayers prepared from salt-containing polyelectrolyte solutions to pure water. McAloney et al.48 and Schlenoff et al.49,50 observed a significant decrease of the surface roughness of PSS/PDADMAC multilayered films upon annealing with salts. In this work, we study one particular case of postassembly treatment on carboxylated cellulose nanofibril (CNF)−PAH thin films. We report the elaboration of thin coating with tunable optical properties. Antireflective character can be changed by a brief exposure of the films to a low-pH solution (pH 2.3) after assembly. Special attention is drawn to the

Multilayered thin films, elaborated using the so-called “layer-bylayer” (LbL) method, constitute a promising class of materials that possess high performance and advanced functionalities.1−5 In this framework, thanks to their biobased origin, low toxicity and different interesting properties,6−8 nanocelluloses have been used for the elaboration of thin coatings targeting various applications.9 Numerous studies have already been reported concerning the LbL assembly of nanocellulose/polymer10−12 by the incorporation of either cellulose nanocrystals or cellulose nanofibrils driven by different type of interactions (electrostatic13−27 or nonelectrostatic28−32). Variations in the growth pattern and architecture were studied as a function of the relevant experimental parameters such as the deposition procedure,11 concentration, 29,33,34 adsorption time, 14,24 and ionic strength.13,33,35 However, in all these studies, the experimental conditions were mainly varied during the buildup of the multilayers, while the effects of a postassembly treatment on the film properties were investigated more scarcely. Precisely, in the works of Cranston et al.36 and Olszewska et al.,37 cellulosebased multilayered films were immersed in solutions with different pH and direct surface forces were measured in the liquid state. Ultrathin films made by anionic or cationic cellulose nanofibrils were studied by Kontturi et al.38 and Olszewska et al.39 They identified the effect of the pH, the adsorption of cationic polyelectrolytes and the annealing on the © XXXX American Chemical Society

Received: January 19, 2015 Revised: February 20, 2015

A

DOI: 10.1021/acs.langmuir.5b00211 Langmuir XXXX, XXX, XXX−XXX

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Langmuir change of the structure of the film investigated by neutron reflectivity (NR) in relation with surface morphology and optical properties.

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thickness values were obtained from the measurement of at least six spots per film. Atomic Force Microscopy. Surface observation of films deposited onto silicon wafers was carried out by atomic force microscopy (AFM) by means of an Innova AFM (Bruker). The images were collected in tapping mode under ambient air conditions (temperature and relative humidity) using a monolithic silicon tip (TESPA, Bruker; spring constant k = 42 N/m; frequency f 0 = 320 kHz). The two multilayered thin films examined had almost the same thickness of approximately 45 nm. Image processing was performed with WSxM 5.0 software. Root-mean-square (RMS) surface roughness and surface skewness (Ssk) were determined from a 10 × 10 μm2 scan area. Neutron Reflectivity. Specular neutron reflectivity experiments were carried out at the Orphée reactor on the EROS time-of-flight reflectometer at the Laboratoire Léon Brillouin (CEA Saclay, France). A broad momentum transfer (Q) range from 0.008 to 0.1 Å−1 was obtained by collecting data at two fixed angles with a neutron white beam covering wavelengths from 3 to 25 Å. For the air/solid geometry, the angles were 0.93 and 2°. For measurements in solution, silicon wafers coated with multilayered films were placed in a liquid− solid cell. For this geometry, the angles chosen were 1.34 and 2.5° for the D2O/solid interface. In air/solid geometry, the incident neutron beam comes through the film side to the interface, whereas in liquid/ solid geometry, it travels through the silicon side. A standard treatment was applied to the raw data to subtract background and to obtain reflectivity curves on an absolute scale, following a procedure described in Cousin et al.52 for both geometries. The data were then analyzed using a “box” model, which consisted of dividing the thin film into a series of layers. Each layer was characterized by a finite thickness (d), scattering length density (SLD), and interfacial roughness (σ) with the neighboring layer. Theoretical curves were calculated using the optical matrix method. Details about the fitting procedure can be found in the Supporting Information. UV−Visible Light Transmittance Measurements. Light transmittance measurements at normal incidence for the characterization of the thin films were performed in the 200−1000 nm range with a SPECORD S600 UV−visible spectrophotometer. The films were coated on one side of a quartz substrate over an area of 2.5 × 2.5 cm2.

EXPERIMENTAL SECTION

Materials. A commercial softwood bleached Kraft pulp was provided by Zellstof Stendal GmbH (Ameburg, Germany) in the form of never-dried wet fibers with 70% water content. The pulp was purified with 0.3% NaClO2 in acetate buffer at pH 4.8 and 60 °C for 2 h until the pulp became completely white. The pulp was then washed with water by filtration and stored at 4 °C. Poly(allylamine) hydrochloride (PAH; average Mw = 120 000−200 000 g mol−1) was purchased from Polysciences, and 4-Acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl (AcNH-TEMPO), sodium chloride (NaCl), sodium chlorite (NaClO2), sodium hypochlorite (NaClO), and other chemicals were purchased from Sigma-Aldrich and used without further purification. Deionized water (18.2 MΩ, Millipore Milli-Q purification system) was used for all experiments. Preparation of Cellulose Nanofibril Suspensions. CNFs were prepared following the protocol described by Saito et al.51 with minor modifications. Briefly, cellulose (1 g) was suspended in 0.1 M sodium acetate buffer (500 mL, pH 4.8) dissolving AcNH-TEMPO (0.1 g, 0.5 mmol) and NaClO2 (80%, 5.6 g, 50 mmol). The 2 M NaClO solution (3 mL, 5.0 mmol) was diluted to 0.1 M with the same 0.1 M buffer used as the oxidation medium and was added in one step to the mixture. The suspension was stirred at 500 rpm and 40 °C for 48h. After cooling the suspension to room temperature, the oxidized pulp was thoroughly washed with water by filtration. The washed pulp was suspended in water again and treated mechanically with a blender (Waring Commercial Blender, USA) for 10 min at 12 000 rpm, and then with an ULTRA-TURRAX homogenizer (Hudolph Instruments, Germany) for two cycles (4 min, 20 000 rpm) in 100 mL batches. Batches of 250 mL were then sonicated for 4 min using an ultrasonic homogenizer (Qsonica sonicators, Delta Labo, Avignon France) with an output power of 300 W (12.7 mm probe tip diameter). Finally, the suspension was centrifuged for 30 min at 14 000 rpm. The supernatant was dialyzed for 10 days and filtered on 4 μm Whatman filters. The CNF obtained had a cross-section of 3-4 nm and a length of approximately 0.5−1μm length, with a carboxylic acid surface charge of 1.1 mmol·g−1.35 Multilayered Films Preparation and Treatment. Multilayered films were deposited on silicon wafers as solid substrate, cleaned beforehand for 30 min in a mixture of H2O2/H2SO4 (70%/30% v/v), thoroughly rinsed in Millipore water, and finally dried under a nitrogen stream. PAH−CNF films were built on the basis of the conventional LbL dipping method. The silicon wafers were alternately immersed in respectively a PAH solution (4 g·L−1, pH 5.0, containing 1 M NaCl) and a CNF suspension (0.8 g·L−1, pH 5.0) for 1 min. After each immersion step, the substrates were rinsed in pure water and, the surfaces were then dried under a nitrogen stream. The dipping sequence was repeated until an n-bilayer film was formed, with one bilayer defined as a single deposit of a PAH and CNF layer. The corresponding film is denoted (PAH−CNF)n. The acid treatment of the films consisted on immersion of the film in a 5 mM HCL solution (pH = 2.3) for 15 min and then dried under nitrogen stream. The immersion of the acid-treated film in ultrapure water (pH 7) was also for 15 min, and the film was then dried under nitrogen stream. Ellipsometry. The thickness of multilayered films was evaluated using a variable-angle spectroscopic ellipsometer (M-2000U; J. A. Woollam, Lincoln, NE, USA). The ellipsometric angles, Δ and Ψ, were acquired over the spectroscopic range of 250−1000 nm at three angles of incidence 65, 70, and 75°. Optical modeling and data analysis were performed using the CompleteEASE software package (J. A. Woollam Co., Inc.) with a three-layer model consisting of the Si(100) substrate layer, a thin SiO2 layer, and the single Cauchy layer that describes the multilayered (CNF−PAH) film. The Cauchy parameters for each sample were used to model the ellipsometry data from the multilayered films to obtain their respective thicknesses. Average

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RESULTS AND DISCUSSION

In this work, we studied the influence of the acid-treatment on the different properties of cellulose-based multilayered films. The latter were built by the dipping-assisted LbL technique and using cellulose nanofibrils as anionic colloidal nanoparticles and poly(allylamine) hydrochloride as cationic polyelectrolytes. One M of NaCl was added to the PAH dipping-solution. In this case, the polymer chains adopt a Gaussian conformation induced by the screening of the charges by the salt. After their construction, the films were treated with acid by immersion in 5 mM HCl solution (pH = 2.3) for 15 min. Film thicknesses, surface topography, and optical properties were determined before and after treatment. Film Thicknesses. The thickness of the films was evaluated by ellipsometry and plotted as a function of the number of bilayers (Figure 1). With the building conditions described below, the PAH−CNF films present a linear growth. The Gaussian conformation of PAH chains induced by the high ionic strength induces the deposition of thick CNF layers as described elsewhere.14,33,35 The film growth from two bilayers and up to 16 bilayers is presented in Figure 1 and the thickness increment per bilayer is roughly about 20 nm. The thicknesses of each film were determined before and after the acidic treatment, and no significant variation of the film thickness was noted, i.e., the thickness of (PAH−CNF)16 film before and after immersion in 5 mM of HCl solution was approximately 300 and 290 nm, respectively, measured by ellipsometry. B

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reaches the initial value. Figure 4 shows the variation of the transmittance of (PAH−CNF)16 film during five successive immersion cycles, with no significant modification of the film thickness after each cycle. Probing the Structure by Neutron Reflectivity. We used NR to probe the internal structure of our PAH−CNF thin films before and after the post assembly acid treatment. Experiments were carried out in the dry state and the swollen state, where the swelling liquid is heavy water (D2O). Reflectivity curves from the two films in each states (Figure 5) were fitted using a “box” model, which consisted in dividing the thin film into a series of layers.33,35 The corresponding profiles are plotted in Figure 6. Two important parameters were extracted from such profiles: the thickness of the film (d) and the average SLD. Results are summarized in Table 1. First, we noticed that in the dry state, thickness values (ddry film, Table 1) deduced from fits were in good agreement with those calculated from ellipsometry measurements. No significant decrease of the film thickness was detected after the acid treatment. The average SLD mean_dry were also very close, showing that there is no loss of materials due to the post acid treatment. Indeed, the amount of adsorbed materials Γ is ddry film × SLD mean_dry since it is the integral of the profile and is almost constant (see Table 1). Such conservation of mass is consistent with the perfect reversibility of transmittance observed during several cycles of acid treatment/rinsing shown in Figure 4. The slight difference of SLD mean_dry observed in the untreated and acid treated sample may originate from the modification of the SLD of CNF and PAH with the pH 7. A deeper study focusing on the effect of pH on the SLD would be necessary to clarify this point. In summary, measurements in dry state demonstrates the conservation of thickness and materials with the post-treatment but point out a very important reorganization of species within the films since the profiles are very different. Second, it is possible to compare the swelling behavior of the two films. After immersion in heavy water, we observed a decrease of the amplitudes of spectra fringes and their shift toward low Q. This indicates both an overall increase of the layer thickness upon swelling and of the roughness at the interface (the film air interface in the dry state and the film− D2O interface in water, respectively). The thicknesses of the swollen films (86 and 98 nm) deduced from the fits are much larger than those in the dry state (52.3 and 51.7 nm). A swelling ratio of the film in heavy water, Φswelling, can be calculated (Table 1) as follows:

Figure 1. Thickness of untreated and acid-treated thin films measured by ellipsometry versus number of bilayers.

Surface topography. Surfaces of the films before and after acid treatment were imaged by AFM. Figure 2a shows the 10 × 10 μm2 three-dimensional AFM images (PAH−CNF)4 film. The surface appears as a “sharp needle-like” morphology, indicating the somewhat porous structure of the film. RMS roughness was evaluated for this scan area and was 6 ± 1.2 nm. After acid treatment, the surface of the film shows coarse texture and displays larger “hills” and “valleys”, with an increase of the roughness to 9 ± 0.8 nm (Figure 2b). The corresponding two-dimensional AFM images can be found in the Supporting Information (Figure S1). The surface skewness (Ssk), another surface roughness parameter, was evaluated. It describes the asymmetry of the height distribution. A positive Ssk value (0.61) was obtained for the untreated PAH−CNF film, meaning that the local summits dominate on the surface. In contrast, after treatment Ssk becomes negative (−0.24), indicating that valleys dominate over the peak regime.53 The acid treated film was immersed in pure water (pH 7) for 15 min, and the topography of the film seems to be similar to that of the untreated film, with comparable “sharp needle-like” morphology and positive surface skewness, indicating a reversible character of the treatment. These findings are expected to be related to the modification of the surface charge density of the carboxylated CNF as a function of the immersion solution pH and will be discussed later. Optical Properties. With the same conditions described below, the films were deposited onto single side quartz substrates, and the transmittance was measured using UV− visible spectroscopy (Figure 3). (PAH−CNF)n films display antireflective properties: the percentage of transmitted light is higher compared to the bare quartz substrate. The origin of this behavior was elucidated in our previous paper35 and was attributed to the porous structure of the films, which makes their refractive index sufficiently low to have such a property. After immersing films in 5 mM HCl solution for 15 min, the transmission spectra show an increase by about 2 to 5%, depending on the film thickness, indicating a more pronounced antireflective character. For instance, after acid treatment, (PAH−CNF)4 film transmittance at 500 nm shifts from 97 to 99% (Figure 3a), while (PAH−CNF)16 film transmittance increases from 94 to 99% approximately (Figure 3b). The variation of the transmittance was reversible since, after the immersion of the acid-treated film in ultrapure water (pH =7) for 15 min, the transmission at 500 nm decreases and

Φswelling =

dswollen film − ddry film dswolle nfilm

(1)

where ddry film and dswollen film are the thickness of the film in the dry and swollen state, respectively. We noticed that acid-treated (PAH−CNF) films present a more pronounced swelling behavior, with a swelling ratio of 0.47, compared to the untreated film (Φswelling = 0.39). This reveals a modification of the film structure upon immersion in 5 mM HCl solution. The stronger swelling for the acid-treated films is supported by the higher SLD obtained in D2O (5.25), which indicates that a higher amount of heavy water molecules diffused into the film.

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DISCUSSION The primary aim of this work was to investigate the effect of postassembly acid treatment on carboxylated CNF−PAH C

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Figure 2. 3D tapping-mode AFM height images (at left) over an area of 10 × 10 μm2 of (PAH−CNF)4 film before treatment (a), after pH 2.3 acid treatment (b) and after immersion in pH7 pure water (c). The corresponding profiles are presented at right.

multilayers properties. Please first note that the two components involved in such architectures, CNF and PAH, are weak polyelectrolytes with different pKa. Hence, their surface charge densities depend on pH. The TEMPO-oxidized CNF, having surface carboxylic acid groups, possesses an apparent pKa around 3.5,54 while PAH has a pKa around 8.5.55 Consequently, when the film is immersed in pH 2.3 HCl

solution, i.e., a pH lower than the pKa of CNF, more than the half of the carboxylic acid groups will be protonated COOH, and the charge density of the nanofibrils will decrease considerably.56 On the contrary, for PAH chains, exposure to a 2.3 pH-solution may slightly increase the proportion of the protonated monomers and consequently the charge density. Indeed, at pH 7 the degree of ionization is near of 85% and D

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Figure 3. UV−vis light transmittance spectra of an uncoated quartz slide (black curves) and coated slide with untreated (PAH−CNF) thin films (red) and acid-treated (PAH−CNF) thin films with a number of bilayers of 4 (a) and 16 (b).

assembly process via the tuning of the chains conformations. For instance, Shiratori et al.57 reported a significant difference of the thickness per layer between pH 2.5 (1 nm) and pH 7 (3 nm). On the other hand, postassembly modification of the pH can induce a charge density variation that can in turn induce a breaking and reforming of ionic cross-links, and may possibly create nanoporosity in the weak polyelectrolyte films. In our case, the postacid treatment induces the protonation of a significant part of CNF carboxylic acid groups. This may lead to a slight aggregation of the cellulose nanofibrils. The surface topography with “hills and valleys” revealed by the 3D AFM image after treatment (Figure 2) supports this hypothesis. Under such conditions, i.e., the decrease of CNF charge density, the PAH chains are weakly bound to the CNF and consequently have more mobility, which leads to their reorganization within the film. Neutron reflectivity profiles in the dry state (Figure 6a) seem to corroborate such a reorganization. Indeed, the untreated film profile shows a plateau between 5 and 40 nm from the surface, thus suggesting an homogeneous organization of PAH and CNF within the film, with a short tail at larger distance with a small SLD, i.e., a low content in material. By contrast, for the acid-treated film, there is an excess of SLD close to the surface, and the plateau is hardly identified between 30 and 40 nm. The SLD decreases much more continuously from the wafer to the outer edge of the film, compared to the untreated film. Since the SLD of PAH (0.59) is considerably lower than the one of cellulose nanofibrils (2.1), this suggests a densification of CNF close to the wafer surface and an increase of PAH content close to the surface. It is likely that after acid-treatment, PAH chains have a much more extended spatial conformation driven by the increase of their surface charge density which forces to swell at the outer edge of the film. Such hypothesis is reinforced by the larger swelling of the acid-treated film observed in NR solid/ liquid measurements. It is also in accordance with the more heterogeneous lateral structure of the film with “hills” and “valleys” depicted from the AFM. The mechanism of this reorganization remains tricky, however, since it involves a complex interplay between the changes in CNF charge densities and their aggregation, the possible slight increase of PAH charge density, and the formation of new hydrogen bonds between CNF and PAH due to the protonation of CNF at pH 2.3. Many studies have reported that polyelectrolytes may diffuse within the film, and this motion is highly dependent on pH, ionic strength, charge density, swelling solvent, and temperature.36,58−60 In the case of CNF−PAH films, it is unlikely that cellulose nanofibrils will be

Figure 4. Evolution of the transmittance (●) at 500 nm and the thickness (■) of (PAH−CNF)16 film during successive postassembly treatment. The even cycle numbers correspond to an immersion in a 5 mM HCL solution (pH 2.3), and the odd cycle numbers correspond to an immersion in ultrapure water (pH 7).

Figure 5. NR curves of untreated (PAH−CNF) film in a dry (black ■) and swollen (blue Δ) state and acid-treated (PAH−CNF) film in a dry (red ●) and swollen (magenta ▼) state. The solid lines are best fits obtained from the model described in the text.

becomes total (100%) for a pH near 2.55 Many studies in the literature have reported the influence of pH on weak polyelectrolyte multilayer films. They showed that the pH mainly influences the growth of the films during the selfE

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Figure 6. NR profiles for two different films: (PAH−CNF) 4 and acid-treated (PAH−CNF)4 in the dry (a) and swollen (b) states.

moth’s eye.61 Rubner et al. showed such a graded refractive index on acid and salt-treated PAH−PAA antireflective films using AFM and calculated near normal reflection curves.41 Similar optical effects were also observed for the so-called “subwavelength structures” that are reminiscent of the features observed on the AFM images.62−64 Finally, it was noteworthy that, after the immersion of an untreated film in pH 7 ultrapure water for 15 min and drying, the optical properties of the film, particularly its transmittance spectra ,did not change (data not shown), indicating that its internal structure was not modified upon swelling. This means that the modifications that occurred in the film after the treatment are mainly due to the acid character of the solution and to the extent of the swelling in the aqueous media itself.

Table 1. Parameters Deduced Form Neutron Reflectivity Curves Fits in the Dry and Swollen State filma

(PAH− CNF)4

(PAH−CNF)4 (acid treated)

ddry film by NR/nm (ddry film by ellipsometry/nm) dswollen film by NR/nm Φswelling SLDmean dry film/ × 10−6 Å−2 SLDmean swollen film/ × 10−6 Å−2

52.3 (47 ± 2.3) 86.2 0.39 1.25 5.11

51.7 (44.6 ± 2.7) 97.6 0.47 1.204 5.25

a

d: film thickness; SLD: scattering length density (× 10−6 Å−2)

mobile because of steric hindrance due to entanglement; however the mobility of more flexible PAH chains cannot be ruled out. A significant modification of the optical properties after the treatment was observed. The acid-treated film has more pronounced antireflective character. In fact, to obtain antireflective properties, the refractive index of the coating should satisfy the relationship nc = (na.ns)1/2, where nc, na, and ns are the refractive indices of the coating, the air and the substrate, respectively. In the case of quartz, a coating should then have a refractive index of approximately 1.26. Since refractive indices of cellulose, PAH and air are 1.55, 1.42, and 1, respectively, it is necessary that the film contains a sufficient fraction of air/void (pores) to obtain such a low refractive index (1.26). In the case of untreated PAH−CNF films, it was probed that this condition is satisfied, which explains the antireflective character.35 In the case of acid-treated film, the more pronounced antireflective character (higher transmittance) supposes that its average refractive index is lower, which means that the treated film contains more air/void (more porous). Yet, an overall higher porosity is unexpected since the treatment did not modify the thickness of the film or its composition. Moreover, the same area was found under the two films’ SLD profiles, indicating that no loss of components occurred during the treatment. As a result, we would expect that the two films have the same average refractive index. The optical properties difference between both films arises likely from the polymer distribution observed in the neutron reflectivity profile of the PAH−CNF acid-treated film, which shows a gradual decrease of the scattering length density when approaching the surface. This results in a graded refractive index between air and quartz, i.e., n decreases regularly from the silicon to air. A similar structure occurs in nature and has been identified as an antireflective surface, such as the cornea of the

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CONCLUSIONS We described here the effect of the postassembly acidtreatment on the properties of CNF−PAH multilayered thin films. These were immersed in 5 mM HCl solution for 15 min and compared to untreated films. Results showed that no modification occurred on the films thickness and composition after the treatment. The major modifications were observed at the level of the surface topography, the swelling, and the optical properties. The treated films display “hills” and “valleys” surface morphology, accompanied by a higher swelling ratio and more pronounced antireflective character. These reversible modifications are explained by the change in the charge densities of the components, leading to their reorganization within the film, as shown by neutron reflectivity data. This treatment constitutes a simple way to control the properties of CNF-based thin films.

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ASSOCIATED CONTENT

S Supporting Information *

The box model used to fit neutron reflectivity curves and parameters of the best fits obtained from this model. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: fi[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS We are grateful for the financial support from the MATIERES project and the “Région Pays de la Loire”. The authors acknowledge the BIBS platform of INRA Angers-Nantes for access to its microscopy facilities. We would also like to thank Nadège Beury for her excellent technical support for the AFM images.

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DOI: 10.1021/acs.langmuir.5b00211 Langmuir XXXX, XXX, XXX−XXX