Organization of Microgels at the AirâWater Interface under

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Organization of Microgels at the Air−Water Interface under Compression: Role of Electrostatics and Cross-Linking Density Christine Picard,† Patrick Garrigue,‡ Marie-Charlotte Tatry,†,‡ Véronique Lapeyre,‡ Serge Ravaine,† Véronique Schmitt,*,† and Valérie Ravaine*,‡ †

Université de Bordeaux, Centre de Recherche Paul Pascal, CNRS UPR 8641, 115 Avenue A. Schweitzer, 33600 Pessac, France Université de Bordeaux, ISM, CNRS UMR 5255, Bordeaux INP, Site ENSCBP, 16 Avenue Pey Berland, 33607 Pessac Cedex, France

‡

S Supporting Information *

ABSTRACT: Poly(N-isopropylacrylamide) (pNIPAM) microgels are soft and deformable particles, which can adsorb at liquid interfaces. In the present paper, we study the two-dimensional organization of charged and quasi-neutral microgels with different cross-linking densities, under compression at the air−water interface and the transfer of the microgel monolayer onto a solid substrate at different surface pressures. At low cross-linking densities, the microgels form highly ordered hexagonal lattices on the solid substrate over large areas, with a unique lattice parameter that decreases continuously as the surface pressure increases. We thus prove that the microgel conformation evolves at the air− water interface. The microgels undergo a continuous transition from a highly flattened state at low surface coverage, where the maximal polymer segments are adsorbed at the interface, to entangled flattened microgels, and finally the thickening of the layer up to a dense hydrogel layer of compacted microgels. Moreover, two batches of microgels, with and without charges, are compared. The contribution of electrostatic interactions is assessed via changing the charge density of the microgels or modulating the Debye length. In both cases, electrostatics does not change the lattice parameter, meaning that, despite the microgel different swelling ratio, charges do not affect neither interactions between particles at the interface nor microgels adsorption. Conversely, the cross-linking density has a strong impact on microgel packing at the interface: increasing the cross-linking density strongly decreases the extent of microgel flattening and promotes the occurrence of coexisting hexagonally ordered domains with different lattice parameters.

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INTRODUCTION Understanding the behavior of particles at liquid interfaces is a topic of interest for a wide field of applications ranging from the stabilization of Pickering emulsions1,2 and foams,3 the creation of patterned solid surfaces,4,5 or the understanding of the interactions of particles with biological membranes. Whereas solid particles have been widely investigated for years, soft and deformable particles have started to draw always more attention over the last years. Most of the studies have dealt with microgels, i.e., colloidal particles made of water-swollen lightly cross-linked polymers, which are considered as intermediates between hard-sphere colloids and linear polymers. Cross-linked micelles and polymersomes,6 as well as star polymeric structures,7,8 should probably enter the same category, although with varying degree of softness. Until now, poly(Nisopropylacrylamide) (pNIPAM) microgels have been chosen as the gold standard material in this area, but it has been shown recently that the concept could also be applied to protein microgels.9,10 The thermoresponsive microgels are waterswollen at temperatures below their volume phase transition temperature (VPTT = 33 °C) and expel water and shrink when heated above, due to hydrogen bond breakage.11 These microgels have attracted particular attention because they © 2017 American Chemical Society

could stabilize Pickering emulsions that were stable at temperatures below the VPTT and could be destabilize ondemand above.12−15 Since then, tremendous efforts of research have been devoted to understanding the role of the softness on the behavior of water-swollen microgels at liquid interfaces.16,17 Various studies report their organization at drop surfaces,13,15,18−25 at flat macroscopic liquid interfaces,26−37 or more recently in thin films.38 In contrast to hard colloidal particles, adsorption of soft microgels at liquid interfaces (air−water or oil−water) is not governed by their wetting contact angle but is driven by the surface activity of the polymeric chain.39−42 Upon adsorption, conformational changes are observed, which result from the balance between chain spreadingdriven by surface activity and microgel internal elasticity, promoted by cross-linking. The extent of spreading depends on various parameters such as the microgels internal structure, their concentration, or process parameters. At the scale of individual microgels, it is limited by the cross-linking density of the microgels, the more cross-linked Received: May 6, 2017 Revised: June 20, 2017 Published: July 18, 2017 7968

DOI: 10.1021/acs.langmuir.7b01538 Langmuir 2017, 33, 7968−7981

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undergoes several phase transitions. The organization of the microgels at the oil−water interface as a function of surface pressure was elucidated via two methods: (1) making the assumption that all the microgels were adsorbed, it was possible to determine the surface area per particle, which allowed calculating the lattice parameter of a 2D hexagonal array,26,31,45 (2) the organization at the liquid interface could be transferred on a solid substrate and further imaged by SEM33 or AFM.27,30,37 The microgels were found to explore various conformations starting from the most flattened unconnected microgels in the most dilute state, then reaching a hexagonal array of flattened microgels with lateral interpenetration of dangling chains. Upon compression, the array became denser, interpenetration increased, but the microgels were less flattened, meaning that the film became thicker. The importance of softness was investigated by comparing microgels with various cross-linker densities.31 This parameter affected the behavior of microgels only at high compression, whereas the lattice parameter remained unchanged at low compression but it was not discussed in details. The importance of softness was also investigated using composite microgels bearing a solid inorganic core (silica,46,47 gold33) and using hollow microgels with different cross-linking densities.46 Very recently, accurate structural studies were performed on small charged microgels. Isostructural solid−solid phase transition between two crystalline phases having the same hexagonal symmetry but different lattice parameters were observed.27 The role of electrostatics was also studied, but it gave rise to contradictory results: the isotherms were shifted to lower area per microgels for charged microgels, indicating that charged microgels could be compressed more easily than uncharged ones,29 though, studying the replica on a solid substrate after transfer indicated that the lattice parameter was the same.30 Owing to the diversity of investigated microgel compositions and sizes, it is difficult to draw a clear comparison and to establish the role of some important structural parameters. In the present paper, we intend to investigate more deeply the role of electrostatics and softness on the behavior of microgels at liquid interfaces and to establish relations between the microgel organization at given surface pressure and the properties of microgel-stabilized formulations. Our aim was to bring some answers to the following questions: (1) Is there any artifact in the transfer on solid substrate or is the organization at the liquid interface preserved during transfer? (2) If transfer is unaltered, what is the relation between microgels’ structure, their conformation at the interface, their packing and interfacial properties, and the stability of formulated systems? (3) Some microgels, when compressed, present phase coexistence between 2 hexagonal lattices.27 What is the controlling parameter leading to this observation? (4) Why is electrostatics important in some cases and negligible in other cases?29,48 To bring some insight, a series of microgels bearing different charges and different cross-linking ratios, prepared by batch precipitation polymerization, are studied. The structure of the microgels is already well-known and their behavior at the interface of oil drops in emulsion has been also well documented.19,48 We choose to work at the air−water interface, which discards the role of possible impurities at the interface coming from the oil. After transfer of the monolayer on a solid substrate, systematic AFM imaging of the replica gave insights into the structure of the monolayers, with a high reliability. We discuss the influence of the charges and the cross-linking density on the organization at the liquid interface.

the less deformable. Such behavior has been illustrated with pNIPAM microgels prepared by batch precipitation polymerization.15 Due to the higher reactivity of the cross-linker monomer compared to NIPAM monomer, such microgels have a “core−shell” structure,43,44 with a higher cross-linker concentration in the core than in the shell. When flattened at the liquid interface, microgels exhibit a fried egg-like structure.15 The more cross-linked microgels have a less deformed core than the less cross-linked ones. The extent of spreading has consequences on the capacity of adjacent microgels to entangle via their peripheral dangling chains, therefore making a two-dimensional elastic network of connecting microgels.15 Consequently, microgels with low deformability barely form entanglements and fail in stabilizing emulsions against mechanical disturbances, whereas highly deformed microgels gave rise to highly stable emulsions, where the drops were protected against coalescence by the elasticity of the interfacial network. The extent of microgel spreading also depends on the microgel concentration at the liquid interface. Indeed, the flattened state is reached only when microgel concentration at the interface is low, i.e., either at early stages of adsorption kinetics or at very low concentration in the bulk, when the surface area is large compared to the number of microgels. When the concentration at the interface is high or at late stages of adsorption, microgels were observed to be in a “compressed ” state, i.e., their center-to-center distance was lower than their hydrodynamic diameter in solution.31 In metastable systems like emulsions and foams, the microgels can remain trapped in a given conformational state, which might be out of equilibrium but depends on the pathway.20,21 In such cases, the adsorption kinetics might be of crucial importance. In emulsions16 or in thin foam films,38 it was found that the conformation depended on the concentration of microgels: at low concentration - when the adsorption kinetics was slow - the microgels were flattened whereas they were compressed at high concentration. The conformation had detrimental consequences on the film properties. It was shown that interfaces covered with flattened microgels were adhesive, owing to bridging between adjacent interfaces, because the thin polymeric separating adjacent microgels could be more easily ruptured.21,38 This thin film was all the more fragile since bridging between interfaces were high and since the entanglements between adjacent microgels on the same interface were poor. Thus, the less deformable microgels lead to more bridging in the case of flattened conformation.20 When the interfaces were covered with compressed microgels, the films were no more adhesive because the higher polymer thickness prevented interfaces from bridging.20,21,38 The relation between the conformation at the liquid interface and the bridging properties was found to be more general than a concentration effect. Indeed, other parameters such as the stirring energy21 or the size of the microgels22 could also change the extent of spreading and thus the polymeric thickness at the interface leading to similar consequences on bridging. In fact, “adhesion is promoted by a low polymer density and the existence of polymer density fluctuations parallel to the surface”.16 In order to shed more light on the relation between microgel conformation and surface properties, investigations were carried out on model flat interfaces, in particular using the Langmuir technique. In this case, the microgels are selectively deposited at the oil−water interface using a bad solvent of the polymer. After solvent evaporation, the monolayer is compressed. The surface pressure increases upon compression and 7969


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II.3. Particle Characterization. Particle sizes and polydispersity index were determined by dynamic light scattering (DLS) with a Zetasizer Nano S90 Malvern Instruments equipped with a HeNe laser at 90°. Hydrodynamic diameters were calculated from diffusion coefficient using the Stokes−Einstein equation. All correlogram analyses were performed with software supplied by the manufacturer. The polydispersity index is given by the cumulant analysis method. II.4. Langmuir Film. Compression isotherms were recorded at 22 °C using a homemade Langmuir-trough (trough dimensions: 145 mm × 690 mm, compressible area: 1000 cm2). The surface pressure was recorded by a balance equipped with a paper Wilhelmy-plate. The microgels were dispersed in a mixture of water and isopropanol (5:1). Prior to spreading, it was verified by DLS that the dispersion did not contain any aggregates. 0.5 × 0.5 cm2 pieces of silicon wafer were cleaned by a 30 min UV-ozone treatment and installed onto a horizontal hydrophilic glass slide with a tilt angle of 20°. The slide was immersed into the water subphase prior spreading of the microgels. The suspension of microgels was then deposited using a Hamilton syringe at the air−water interface. After a stabilization time of 1 h, the barriers were displaced symmetrically at a constant speed of 10 mm.mn−1. When the target desired surface pressure was reached, the glass slide was drawn up through the interface, while the trough feedback system adjusted the barrier position to maintain a constant surface pressure. The substrates were then left to dry at least 2 h before imaging. A schematic view of the experiment is represented in Figure S1. II.5. AFM Imaging. After transfer of the microgel monolayer from the liquid air−water interface to the solid silicon substrate, the latter was systematically analyzed by AFM (Veeco Multimode 8). Topographic images were obtained in Scanasyst-in Air mode using a silicon tip on nitride lever (Scanasyst-Air, Bruker) with a spring constant of 0.4 N·m−1 and a resonance frequency 70 kHz. 10 ×10 μm2, or sometimes larger, images were taken at different areas of the samples, distant by at least 3 mm, the maximum distance allowed by the screw of the setup. The spatial organization remained the same over the full explored range. As a usual treatment in AFM, the height images were flattened to remove slight tilt of the sample with respect to the AFM tip. After conversion to 8-bit gray scale, the images were further analyzed using ImageJ software. In particular, the radial distribution function between the microgel centers, defined as the maximum height, was established. II.6. Dynamic Surface Tension. The dynamic surface tension of the air−water interface was determined using the pendant-drop technique (Teclis). An aqueous drop containing a known concentration of microgels is suspended in air. The tension is deduced from the axisymmetric drop shape by fitting with the Laplace equation. A constant drop volume is maintained over 10 000 s, and the tension is recorded as a function of time. In this method, microgels adsorb spontaneously at the interface.

II. EXPERIMENTAL SECTION II.1. Chemicals. All reagents were purchased from Sigma-Aldrich. N-isopropylacrylamide (NIPAM) was recrystallized from hexane (ICS) and dried under vacuum overnight prior to use. N,N′methylenebis(acrylamide) (BIS), acrylic acid (AA), potassium persulfate (KPS), and isopropanol were used as received. Milli-Q water was used for all synthesis reactions, purification, and solution preparation. II.2. Particle Synthesis and Purification. pNIPAM microgels bearing different charges were synthesized. “Quasi-neutral” pNIPAM microgels were obtained by copolymerization of NIPAM and BIS. “Charged” microgels, referred to as pNIPAM-AA, resulted from the copolymerization of NIPAM, BIS, and acrylic acid (10 mol %). The presence of carboxylic groups introduces additional charges when the pH is above the pKa of the acrylic acid (pKa = 4.5). The cross-linker concentration was kept constant and equal to 2.5 mol % with respect to NIPAM. According to previous work by Pelton et al.,49,50 pNIPAMAA exhibits relatively low block content and a relatively homogeneous radial distribution of the charges in the gel matrix. The microgels were obtained by an aqueous free-radical precipitation polymerization classically employed for the synthesis of thermoresponsive microgels and especially pNIPAM microgels.11,44,51 Polymerization was performed in a 500 mL three-neck round-bottom flask, equipped with a magnetic stir bar, a reflux condenser, thermometer, and argon inlet. NIPAM and BIS were dissolved in 98 mL of water. The solutions were purified through a 0.2 μm membrane filter to remove residual particulate matter. The solutions were then heated up to 70 °C with argon thoroughly bubbling during at least 1 h prior to initiation. An appropriate amount of carboxylic acid comonomer AA was introduced. The initial total monomer concentration was held constant at 70 mM. The content of BIS was usually equal to 2.5 mol % compared to NIPAM, unless otherwise noted. Free radical polymerization was then initiated with KPS (2.5 mM) dissolved in 2 mL of water. The initially transparent solutions became progressively turbid as a consequence of the polymerization and precipitation process. The solutions were allowed to react for a period of 6 h in the presence of argon under stirring. To eliminate possible chemical residues, the microgels were purified by centrifugation-redispersion cycles at least five times (21 000g for 1 h, where g is 9.81 m·s−2). For each cycle, the supernatant was removed, and its surface tension was measured by the pendant drop method. The purification was repeated until the surface tension of the supernatant reached a value close to the one of pure water, i.e., above 70 mN/m, showing that the microgel dispersions were free of surface active impurities. The polymer content cpolymer (in g·cm−3) in aqueous dispersions was determined by the drying method. A known amount of the dispersion is first dried at a temperature higher than 50 °C and then weighted to determine the mass of polymer mpolymer. Following the work published by Lele et al.,52 we considered that a particle is composed of 71 wt % of polymer and 29 wt % of bound water at 50 °C. From the hydrodynamic particle diameter, d50 °C, measured by DLS at 50 °C, the particle number nparticles was estimated as

nparticles =

⎞ 6mpolymer ⎛ 1 0.29 ⎟ ⎜ + 3⎜ 0.71ρwater ⎟⎠ π(d50 ° C) ⎝ ρpolymer

III. RESULTS III.1. Properties of the Investigated Microgels. In the present paper, our intention is to better understand the role of the microgel structure on its packing at a liquid interface. We therefore chose to work with two reference microgels with a given cross-linking density, whose structure is well-documented, and the behavior at the surface of emulsion drops has been studied a lot:15,20,21,48 pNIPAM and pNIPAM-AA. In the latter, acrylic acid has been copolymerized with NIPAM in order to introduce some carboxylic groups in the network. Those carboxylic groups are deprotonated at neutral pH, i.e., above their pKA, and bring charges to the microgels. In the pH range between 4 and 8, their charge density is very high compared to that of pNIPAM microgels, bearing a few sulfate groups originating from the initiation step.48 The dispersion properties of the microgels are given in Table 1. It is wellknown that the presence of charges has a strong impact on the properties of microgels in solution. Indeed, pNIPAM-AA

(1)

where ρpolymer = 1.269 g·cm−3 and ρwater = 0.988 g·cm−3 are respectively the polymer and water densities. The good agreement between the calculation and the real number of microgels was also checked experimentally in confocal microscopy, using dilute fluorescent microgels.38 The microgels were very similar to those synthesized and used in a previous study.48 The electrophoretic mobility was then measured. For purely pNIPAM microgel, it was (− 0.08 ± 0.03) 10 −4 cm2 V−1 s−1. This very low value validates the use of the term “neutral” throughout the paper. For pNIPAM-AA, as expected from the presence of the comonomer, the electrophoretic mobility was a function of pH, it was equal to (−1.49 ± 0.04) 10 −4 cm2 V−1 s−1 at pH 6. 7970


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second regime (II) corresponds to a continuous increase of the surface pressure. This regime is analogous to an expanded liquid. In a third regime (III), the pressure increases slowly and tends to become a plateau while decreasing the interfacial area. Then, a second sharp increase (IV) followed by a plateau (V) are observed. The plateau is attributed to the collapse of the film. This quasi plateau between regime II and regime IV has been previously described as the coexistence between expanded and condensed liquids. III.3. Film Transfer on Solid Substrate. In the previous work on pNIPAM microgels at the oil−water interface,31 the organization of the film was interpreted making the assumption that the microgels were ordered as a hexagonal 2D packing. Taking into account the number of microgels, it was possible to deduce an area per particle that was further easily converted to the center-to-center distance between microgels. This method allowed capturing, in a blind study, the general features about the organization of microgels at a flat liquid interface. In particular, the flattening of microgels in the very dilute state, at zero pressure, was confirmed. However, very little was known about the organization of microgels in the domains III and IV, where the compression was high. In order to clarify unambiguously the organization of the monolayer, the liquid substrate is transferred onto a solid substrate. This method has been used previously by Isa et al. and applied to smaller microgels using special equipment at the oil−water interface.27,30 Here, the substrate is chosen to be small (0.5 × 0.5 cm2) compared to the film area (15 × 66, down to 15 × 4 cm2). The film is compressed up to a given surface pressure value, which is maintained by the barriers while the dipper-attached substrate is leveled off horizontally through the film (Figure S1). The substrate is chosen as a highly hydrophilic silicon wafer. After drying, the deposited microgels are observed by AFM. This method is applied to both pNIPAM and pNIPAM-AA microgels. Microgel monolayers at different surface pressures are transferred on silicon substrate. As shown in Figure S2, the different isotherms recorded during the different sample collections are highly reproducible. In general, the monolayers were spread in the gaseous state, i.e., at zero surface pressure. However, in order to reach highly compressed state, it was sometimes necessary to start at nonzero surface pressure. Such isotherms were perfectly superimposed with the others. Therefore, the samples obtained from various isotherms could be compared. For each surface pressure, two substrates were prepared. For each sample, several AFM images were recorded from different areas of the substrate. In any case, the organization was remarkably homogeneous all over the substrate, as the images were very similar at different areas of the substrate. Figure 2 shows a set of typical images for pNIPAM-AA collected at different surface pressures, but similar features were obtained for pNIPAM microgels (Figure S3). For all the surface pressures comprised between 0.5 and 30 mN/m, AFM images showed a regular hexagonal array of spherical dried microgels, which extended over large areas (Figure S4). The lattice parameters evolve between 1.7 and 0.6 μm. At higher surface pressures, typically 31−33 mN/m, the organization was still hexagonal but a few defects were visible. The lattice parameter of the hexagonal array, i.e., the center-tocenter distance between particles called dCC decreases with increasing surface pressure and will be discussed in a later section.

Table 1. Properties of the Microgels Dispersed in an Aqueous Phase

pNIPAM pNIPAMAA pNIPAM pNIPAM

BIS content (mol %)

dH (nm) T = 25 °C, pH 6

dH (nm) T = 50 °C, pH 6

dH (nm) T = 25 °C, pH 3

electrophoretic mobility at pH 6 (10−4 cm2·V−1· s−1)

2.5 2.5

630 1070

290 340

630 660

−0.08 ± 0.03 −1.49 ± 0.04

1 5

850 680

260 310

-

-

microgels are highly swollen in water when the pH is above their pKa, due to the osmotic pressure exerted by counterions in the microgels (swelling ratio of 31 for pNIPAM-AA microgels to be compared to 10 for pNIPAM microgels). They are less swollen in water at pH 3 (swelling ratio of 7), where they are almost neutral and behave like pNIPAM microgels. In the collapsed state (T = 50 °C), the diameters of the two batches of microgels are in the same range. For the sake of comparison, two batches of pNIPAM with different cross-linking densities were also prepared. Their diameter is also reported in Table 1. III.2. Compression Isotherms. In the present study, we focus on the behavior of microgels at the air−water interface while previous studies focused on an oil−water interface, which required specific Langmuir trough equipment. Using a trough with a high compression ratio, except for some cases reported later, we were able to record at once the full isotherm starting in the dilute state, i.e., zero surface pressure, where the microgels do not interact with each other, and reaching the collapsed state at high compression. Figure 1 shows the isotherms obtained for

Figure 1. Compression isotherms of pNIPAM and pNIPAM-AA microgels (both with 2.5 mol % BIS) on pure water (pH 6), at 25 °C.

the two microgels with similar cross-linker content, pNIPAM and pNIPAM-AA, spread on pure water (subphase). Strikingly, the two isotherms are quasi-superimposed, showing minimal influence of the presence of comonomer. The general shape of the isotherm is very similar to that obtained for the same pNIPAM microgels at the oil−water interface,31 except that the surface pressure was slightly higher. At the oil−water interface, the compression isotherm was separated into different regimes labeled I to V. Since similar transitions are observed in the case of air−water interface, the domains are also represented in Figure 1. In the first regime (I) at zero surface pressure, the monolayer behaves like a gas of noninteracting particles. The 7971


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Figure 2. AFM images (a−f) and height profiles (g) of pNIPAM-AA microgels after transfer on silicon substrate at different surface pressures on pure water: (a) 0.5 mN/m, (b) 23 mN/m, (c) 27 mN/m, (d) 29 mN/m, (e) 30 mN/m, (f) 33 mN/m. The images are 10 × 10 μm2. Since the bare substrate cannot be detected, the height profile for the deposition at 33 mN/m is set at a maximum of 60 nm arbitrarily to ensure the film thickness remains at least constant.

polymer layer surrounding each microgel spot (Figure S5). This is in agreement with the “fried-egg”15 or “sombrero-like”54 structure that was previously observed for such microgels after their adsorption at liquid or solid interfaces, due to the uneven distribution of the cross-linker within the microgels. Obviously, the microgels form a 2D-hexagonal compact crystal of flattened particles, with lateral contact between the shells. This very thin pellicle of shell was also too thin to be grasped by the height profiles. These profiles could be fitted by a Gaussian with reasonable agreement (Figure S6). The height profiles show the link between the height of the dried microgels and their packing. The closer the microgels, the larger the height of the transferred microgels. If the microgels are highly diluted on the substrate, i.e., at very low surface pressures, they can spread and dry in a highly flattened conformation. The profile even goes back to the baseline between two microgels over a distance of about 400 nm, which

From the AFM images, the apparent diameter of the transferred microgels, i.e., the protrusion diameter, also decreases as a function of surface pressure. At this stage, the notion of microgel size has to be discussed with care. Indeed, the plot of the AFM images may be misleading, since there seems to be no contact between microgels in either of the images in Figure 2a−f. The apparent lack of contact arises from the color translation of the very smooth and flat profile of the microgels. Indeed, the height ranges from about 25 nm up to 60 nm for pNIPAM-AA (Figure 2g) and from 25 to 42 nm for pNIPAM (Figure S3), this is very flat compared to the width of the microgels which is in the micrometer range. In fact, very thin layers of polymers cannot be detected by the height mode. Phase imaging brings information about the mechanical properties of the sample53 and the difference can be made between the substrate and a very thin polymeric layer.27,54 Here, the phase images revealed the existence of a very thin 7972


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Langmuir corresponds to the thin shell. At surface pressures higher than 23 mN/m, the relief increases. The height profiles decrease to the baseline and increase immediately on the next microgel, showing a close proximity of adjacent microgel cores. Therefore, it can be concluded that the compression state of the microgel is responsible for its conformation. After reaching a maximum at 30 mN/m, the relief decreased when the surface pressure increases. We suppose that the relief is only that of the top layer, overlying on a continuous layer of polymer. It was thus impossible to know the exact height of the dried microgels. We provide an example of a profile where we made the assumption that the thickness remained constant at high surface pressure (Figure 2g). In this case, the continuous layer of polymer had a thickness of 20 nm at least, resulting from microgel lateral interpenetration. To further analyze microgel packing on the solid substrate, we extracted the positions of the maximal heights of the dried microgels and calculated their radial distribution functions (RDFs) (Figure S7). At low surface pressure, only a few microgels could be observed on a 10 × 10 μm2 image due to their relative large center-to-center distance. The RDF exhibits only one peak. However, decreasing the center-to-center distance and thus increasing the number of analyzed objects, higher orders became visible. Below 30 mN/m, the peaks appear at the hexagonal nearest neighbors’ locations, 1, √3, 2, √7, 3, therefore, proving the remarkable regularity of the array. From these analyses, we could extract the center-to-center distance dCC. The same method was applied to the two kinds of microgels pNIPAM and pNIPAM-AA. III.4. Influence of the Subphase Composition on Film Organization. Since “quasi-neutral” pNIPAM and “charged” pNIPAM-AA exhibit similar packings on the substrates and similar compression isotherms despite very different swelling states in solution, it was necessary to further investigate the role of charges on the film organization. In this section, we changed the composition of the subphase to modify electrostatic interactions, while using the same batch of microgels. This was achieved via two strategies: (1) salt was added in the subphase to modify the Debye length and decrease the impact of electrostatic repulsions, (2) the pH of the subphase was decreased to pH 3, lower than the pKa of the COOH groups in order to protonate the pNIPAM-AA microgels and change the charge density of the microgels.48 The resulting isotherms are presented in Figures 3 and S8. In all these conditions, the isotherms were nicely superimposed, meaning the absence of electrostatic contribution on the film organization. This conclusion was also verified by transferring the films formed either with brine or with water at pH 3 onto solid substrates at various surface pressures (Figures S9 and S10). The collected samples were again highly organized in a hexagonal packing. No significant difference could be observed compared to water at pH 6 (Table S1). In the presence of salt, the films imaged by AFM presented some additional dots which are due to salt crystals (Figure S9). From these data, we measured the dCC as a function of surface pressure, which will be presented in the following discussion. III.5. Influence of the Microgel Cross-Linking Density on Film Organization. To complete the study, the effect of cross-linking density was investigated, with three pNIPAM batches having different BIS contents. This parameter was found to have a significant influence on the isotherm (Figure 4). The surface pressure started to rise at higher surface areas for the lowest cross-linking ratio, thus indicating that the

Figure 3. Influence of salt concentration (a) and of pH (b) of the subphase on the compression isotherms of pNIPAM-AA microgels.

microgels could extend more on the air−water interface. This was consistent with the decrease of rigidity of the microgels. The width of the intermediate plateau (domain III) was even more pronounced as the microgels were more deformable. The highest compression state (domain IV) reached for each microgel occurred at the same normalized area. However, the surface pressure reached at film collapse was higher as the cross-linking density increased. Accordingly, at a given pressure, the center-to-center distance after transfer was higher for the less cross-linked microgels, and the height of the dried microgels was thinner (Figures 5 and S11). At 0.5 mN/m, the microgels had an ultraflat pancake shape, with a thickness below 5 nm. They exhibited impressive spreading, their interdistance being around 2.7 μm. In height images, the microgels look like very loose networks, and the profiles appear very rough (Figure S11). However, the phase image revealed that the substrate is fully covered by a very thin polymer layer (Figure S12). This interdistance decreases as the pressure increases, reaching 1.2 μm at the highest compression state of 30 mN/m while their height increases to about 10 nm; this value is still low compared to other samples (Figure S11a). For the highest cross-linking density (5 mol % in BIS) and at low surface pressures (domain I), the behavior was very similar to 2.5 mol % in BIS, but with lower center-to-center distances 7973


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IV. DISCUSSION IV.1. Discussion about the Method Used for the Measurement of Distances at the Air−Water Interface: Accuracy of the Replica. Although the evolution of the distance between microgels on the solid substrate is in qualitative agreement with our expectations, we first intended to probe the quantitative agreement between the organization at the air−water interface and the replica after transfer and drying. We first examined the drying effect by comparing the volumes of the transferred microgels to their initial volumes. We could roughly estimate the volume of the dried microgels and compare it to their volume in solution. The calculation could be made via three approximations: (1) the shape of the transferred microgels is that of a cone of height h corresponding to the maximum height of the microgels as measured by AFM profile and a radius corresponding to half of the distance between the centers of two neighboring microgels, (2) the shape of the microgels is a spherical cap with the same base radius and the height being the same (Figure S13), (3) the shape is a revolution Gaussian. Table S2 reports the result of the calculation for various surface pressures and for the different microgels used in the study. For the two first methods, the calculated volume remained quasi-constant, showing volume preservation for the various compression states. The Gaussian fit gave the same order of magnitude, with values intermediate between the two first models, but the values were more scattered. It is believed that the Gaussian fit could not take into account the fried-egg structure and thus underestimate the volume of the peripheral shell at low surface pressures. By contrast, the models of spherical cap and cone-like structure use the distance between the microgels to estimate the extension of the microgels. Therefore, they take the shell into account. Although more sophisticated, the Gaussian fit was considered to be less relevant. Making the approximation of spherical caplike microgels, the volume of the pNIPAM-AA microgels was about 0.025 μm3. It was about 0.015 μm3 with the cone-like approximation. Those volumes could be compared to that of the spherical microgel in solution. In the swollen state, it is about 0.64 μm3, more than 1 order of magnitude higher, whereas in the collapsed state, it is 0.02 μm3. Knowing that the collapsed microgels contain about 30% of water,52 the calculated volumes are clearly consistent with the volume of the microgel in the dried state. The same calculation was proposed for the pNIPAM microgels bearing different crosslinking densities. For each series, the calculated volumes were in the same order of magnitude than the volumes in the collapsed state, although the estimations from AFM gave slightly higher values. This overestimation certainly arises from the difficulty to properly estimate the position of the baseline and to integrate the exact shape. The preservation of the microgel volume in the dried state means that the distance between microgels was fixed by the compression, while the height of the transferred microgels is a consequence of the volume preservation. It indicates that microgels expel the water while drying at given positions. They adapt their shape while maintaining a constant dried volume whatever the initial compression state. Therefore, the interdistance between microgels is fixed by the compression and microgels stayed immobilized after transfer just losing their water content. The 2D organization of the microgels at the solid surface is thus the replica of their organization at the air− water interface.

Figure 4. Influence of the microgel cross-linking density on the compression isotherms of pNIPAM microgels (the subphase is pure water at pH 6). (a) The isotherm is plotted as a function of normalized area; (b) the isotherm is plotted as a function of distance between microgels, as calculated from eq 2 (see Discussion for details).

and higher thicknesses: 1.2 μm with a height of 50 nm at 0.5 mN/m (Figure S11b). The height profiles could be fitted by a Gaussian curve (data not shown). However, at higher surface pressures (domain III), the perfect hexagonal order was lost. Some clusters of dense hexagonal phase were present among another hexagonal phase, showing the coexistence of two lattice parameters: 1.15 and 0.55 μm. It is worth emphasizing that such a coexistence has not been observed for less cross-linked microgels (see Figure 5a−d). The distance between microgels in the clusters remained constant until the final compression state is reached (domain IV). However, the size of the clusters increased as the surface pressure increased (Figure 5f,g). For this sample, domain III corresponds to a phase coexistence between the hexagonal packing that is reached at the end of domain II and the hexagonal packing corresponding to the second phase transition. To give a better overview of the different situations, the collected data from AFM images are summarized in Table S1. 7974


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Figure 5. AFM images of pNIPAM microgels with 1 (a−d) and 5 mol % BIS (e−h), after transfer on silicon substrate at different surface pressures on pure water subphase (pH 6): (a,e) 0.5 mN/m, (b,f) 23 mN/m, (c,g) 27 mN/m, (d,h) 30 mN/m. The images are 10 × 10 μm2, except image a, which is 20 × 20 μm2.

clearly showing that the presence of charges in the microgels does not influence their packing at the air−water interface. The high reproducibility from batch to batch and the unique curve describing the influence of the surface pressure also show that the organization of the film on the solid substrate is an accurate replica of the organization at the air−water interface. The very good correlation between the measured distance and the calculated one is also shown in Figure S14. According to eq 1, it requires a precise determination of the dried mass of microgels and of their hydrodynamic diameter in the collapsed state. We could also suspect that the microgel sample could contain some molecular surface-active species. In this case, the deposited amount of microgels would be lower than expected and their surface area per particle would be higher. For pNIPAM microgels prepared by precipitation polymerization, it is well-known that water-soluble polymers and oligomers are created during the polymerization step. Those species may adsorb at a liquid interface. It was shown by Richtering14 and by us22 that water-soluble oligomers or polymers created during polymerization and isolated by centrifugation could themselves stabilize an emulsion. In the present work, great care was given to the purification of the microgels, which were centrifuged and washed with water 5 times. We therefore analyzed the supernatant of a suspension of microgels that was centrifuged for a sixth time, and we measured the dried mass of matter in the supernatant. The amount of water-soluble compounds compared to the microgels was approximately 1 wt %, which rules out the contribution of adsorbed molecular species in our study. It should be underlined that the very accurate purification of our materials is certainly an important key for the reproducibility of our data and the quite good agreement between the calculations and the experimental measurements. Furthermore, choosing to work at the air−water interface might also be a clue for the success of these experiments, as we avoid any impurity coming from the oil phase. The very good agreement shows: (1) that the calculation of the number of microgels is based on correct assumptions, (2) that all the microgels adsorb and they do not redisperse in the subphase, (3) that no other species than microgels are adsorbed, meaning that they are highly pure, (4) that the transfer on the solid substrate does not induce any deformation of the center-to-

From all the investigated conditions regarding microgels with 2.5 mol % BIS, it was possible to directly measure the center-tocenter distance between dried microgels on solid substrates. Those values could be compared to the estimated distances, making the assumption that all the microgels were adsorbed. Knowing their number, which was calculated using eq 1, we could deduce the interfacial area per particle. Then the area per particle could be transformed into a center-to-center distance dCC assuming a 2D hexagonal packing using: ⎛ 2a ⎞1/2 dcc = ⎜ ⎟ ⎝ 3⎠

(2)

where a is the area per microgel. The pressure can hence be plotted as a function of dCC, and the experimental dCC values obtained by AFM were superimposed (Figure 6). All the data collapse in a unique curve for all the investigated conditions,

Figure 6. Surface pressure as a function of distance for different microgels: the continuous line shows the isotherm obtained for pNIPAM-AA microgels (the surface area is converted into dCC via eq 2 assuming a hexagonal packing). The discrete symbols represent the distances measured after transfer at a given pressure on a solid substrate. 7975


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the middle of domain II, at a surface pressure comprised between 10 and 15 mN/m. In a previous paper, we showed that this evolution of the compressibility was very similar to the dilatational elastic modulus measured after spontaneous adsorption of microgels, using the oscillatory drop method using a pendant drop apparatus.31 The maximum of elasticity arises from the density of adsorbed polymer segments, lying flat at the interface. At some point, when packing density increases due to area reduction, the chains run out of space at the interface, and part of them desorb and form loops in the subphase. The elastic response of the interface to compression−dilatation may be lower due to the possibility to exchange polymer segments between the surface (proximal zone) and the underlying zone (distal zone), thereby easily relaxing surface stresses. Thus, the maximum of elasticity appears as the point at which some segments desorb. At this surface pressure, the distance between microgels is respectively 1700, 1550, and 1200 nm for the 1, 2.5, and 5 mol % in BIS. The microgels are thus flattened, their shells being interpenetrated. It is likely that desorption of segments in the shell occurs first, until the end of domain II. It is interesting to note that the maximum of compressibility depends on the cross-linking density. It is respectively, 90, 70, and 58 mN/m for the 1, 2.5, and 5 mol % in BIS. This means that the density of stretched and interpenetrated chains at the interface increases as the softness increases. The link between microgel deformability, elasticity, and the stability of formulated systems, in particular emulsions, has already been pointed out by several authors.15,55 For microgels with low cross-linking densities, the transition from domain II to III is not a phase transition, but only a change in the slope of the isotherm. It happens at around 1300 nm for the 2.5% BIS and 2400 nm for the 1% BIS. In another work obtained with smaller microgels and at the oil−water interface, domain III of the isotherm corresponded to a coexistence between two solid hexagonal crystalline phases having different lattice constants. It was attributed to the coexistence between domains of core−core contacts and shell− shell contacts.27 In our case, no such coexistence is observed for the microgels with 1 and 2.5 mol % BIS. The microgels become more and more compressed, but the hexagonal order with a unique lattice parameter is kept, translating that interactions are purely repulsive. The origin of those interactions might be steric. This is in agreement with previous observations by Mugele et al.32 The weak evolution of the surface pressure over large compressions shows that the density of polymeric segments increases only slightly. Domain III could correspond to the desorption of polymeric segments, either in the core of the microgels or in the shell. Indeed, at high compressions, it might be energetically favorable for the dangling polymer chains at the periphery of the particles to desorb rather than to entangle. For the microgels having the highest cross-linking density, a phase coexistence between two hexagonal phases is observed (Figure 5f,g). The rigidity of the microgels is obviously at the origin of this phase coexistence. As a second hexagonal phase appears with a lower lattice parameter, attractive interactions arise when particles become less deformable or possess less peripheral dangling chains. The origin of such attractive interaction is still not clear. It can be thought that, in the presence of a more rigid core, capillary attractive interactions between the particles exists, as reported for non-deformable spheres,47 which cannot be compensated by the steric repulsion of peripheral chains. The two hexagonal phases coexisting are those with lattice parameters fixed at the

center distance (microgel displacement), meaning that no capillary effect interferes with the replica. We could therefore consider that the present study has established a reliable relation between the packing of microgels at a liquid interface and the surface pressure. In the next section, we discuss the organization of microgels at the interface as a function of surface pressure. IV.2. Phase Transitions at the Air−Water Interface under Compression. For all the microgels with a given crosslinking density, whatever the conditions and thus whatever their swelling ratio, the evolution of packing at the interface upon compression is the same. However, when the crosslinking density changes, i.e., the rigidity of the particles, some discrepancies appear that will be commented on further. Figure 4b reports the three typical isotherms showing the evolution of surface pressure as a function of lattice parameter for the 3 cross-linking densities. We start discussing the behavior of the microgels having the lowest cross-linking densities (2.5% and 1% BIS). At zero surface pressure (domain I), the microgels do not interact with each other. As soon as the surface pressure starts rising (domain II), they form a hexagonal array with an interdistance that is much higher than their initial diameter in solution. Therefore, the microgels are highly flattened when the surface pressure is low. The lower the cross-linking density, the more the microgels flatten. In fact, microgel spreading is the consequence of pNIPAM surface activity. The maximum number of segments adsorbs at the interface, but this number is restricted by the 3D network structure. The transition from domain I to II is marked by an increase of the surface pressure. It is due to shell interpenetration. Upon further compression, the microgels undergo continuous decrease of the interparticle distance, while preserving a hexagonal array, whatever the compression, until the end of domain III. The remarkable homogeneity of their organization over large areas (Figure S3) probably comes from their capacity to adapt their shape to the lattice parameter. To better understand the conformation of microgels in domains I−III, we discuss the evolution of the surface elasticity. It can be assessed by determining the compression modulus, EG = −dπ/dLnA, also called κ, which represents the compressibility of particle layer.31 As shown in Figures 7 and S15, a maximum is found in

Figure 7. Surface elasticity, EG = −dπ/dLnA, as a function of surface pressure for microgels with different cross-linking densities. 7976


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contribution of molecular species, having pH-dependent interfacial activities. The observed invariance means that the electrostatic interactions do not play any role, neither in the interactions between particles at the interface, nor in their adsorption despite their different swelling ratio. The adsorption of microgels at the interface is governed by the amphiphilic character of the polymer, leading to the extension of chains at the interface and microgel flattening. The motor for adsorption relates more to the polymeric nature of the microgels rather than their particle nature. As stated above, particle−particle interactions are purely repulsive. Since electrostatics has only a minor contribution, it is believed that steric contributions dominate the interactions. The presence of loosely cross-linked dangling chain at the periphery of the microgels certainly explains the prevailing of steric interactions over others. Despite the absence of electrostatic effect on the packing parameter, electrostatics affects the swelling state of the microgels. Thus, at a given packing parameter, the preservation of the volume in the swollen state implies that the microgel layers should have a very different extension in the aqueous phase. The monolayer thickness should depend on the charged state of the microgels. IV.4. Comparison with Other Systems: Spontaneous Adsorption versus Compression. The present investigation of the packing at the air−water interface has shown the link between the packing of the microgels and the surface pressure. Qualitatively, the behavior of microgels follows the same trend as that at the oil−water interface.31 However, the isotherms are quantitatively different. For the same microgels, the transition from domain I−II was found at much lower interfacial area for the air−water interface, possibly due to a higher protrusion of microgels in the oil phase, which could reduce the extension of microgel flattening. The typical transitions occur here at surface pressures 3 to 4 mN/m higher than those at the air−water interface. Therefore, we will discuss only how microgels behave at the air−water interface, after different adsorption pathways. The present study has given a full description of the link between microgel conformation/packing and the surface pressure, where the packing was controlled by compression. In formulated systems, the properties depend on the packing, which is related to the spontaneous adsorption dynamics in the case of systems prepared under low stirring.21,38 Figure 8 shows the spontaneous adsorption of pNIPAM and pNIPAM-AA microgels at the air−water interface. It is out of the scope of the present paper to discuss the adsorption kinetics, so the concentrations were chosen to be relatively high in order to observe the equilibrium surface pressures. Again, the presence of charge has no effect on the behavior of microgels at equilibrium, since all the microgels tend to reach the same final state (at long times). The surface tension is close to 44−45 mN/m, in agreement with the literature.40−42 It is also interesting to note that the microgels spontaneously adsorb to reach a surface pressure of 28−29 mN/m in spontaneous adsorption. According to the compression experiments, it corresponds to a center-to-center distance of 800−900 nm. We can once more notice the difference between air−water and oil−water interface, where the equilibrium interfacial pressure is close to 35 mN/m.31 These values can be compared to experimental values obtained in thin foam films prepared with thin-film pressure balance apparatus, using similar pNIPAM microgels but slightly different cross-linking densities.38 In this case, the microgels adsorbed spontaneously at the air−water interface. The effect

beginning and at the end of domain III. The phase having the higher lattice parameter would correspond to the contact between partially deformed microgels with interpenetrated extended shells, whereas the other one would correspond to contact between incompressible microgels, as discussed below. It is worth underlying that the phase coexistence for the highest cross-linking density is a first order transition. It should then lead to a well-defined plateau. On the contrary, low cross-linked microgels present a continuous transition between phases which is obviously second-order. It is striking to note that the plateau is all the more pronounced since the transition is smooth. Domain III ends at about 30 mN/m, i.e., a distance of 600− 700 nm between the microgels for all the batches and conditions. Above this surface pressure, another transition is visible, leading to domain IV. In this region, the microgels are densely packed. Their hexagonal arrays present defects that look like the packing of hard particles (Figure 2f). It is likely that this phase transition occurs once microgels cannot be compressed anymore. This distance is about 600 nm, which is much above the size of the microgels in the collapsed state and close to the size of the swollen microgels in solution. It confirms that microgels do not dehydrate. This is in agreement with previous studies showing that microgels are incompressible.56 It is remarkable to note that this transition occurs at a given distance that is independent of the cross-linking ratio. At the end of domain III, microgels should make a dense layer of packed microgels. Thus, the transition from domain III to IV could be related to the soft-to-hard particle transition. In regime IV, particles behave as elastomers. Finally, the surface pressure at film collapse (transition from IV to V) increases with the microgel cross-linking ratio, reflecting the increase of the network rigidity. IV.3. Role of the Charges on the Film Organization. In the present paper, we have clearly demonstrated that the presence of charges on microgels does not have any impact on their packing at the interface. Not only do pNIPAM and pNIPAM-AA microgels with similar cross-linking densities have exactly the same behavior, but the absence of electrostatic role is confirmed by studying the effect of salt addition and variation of pH on pNIPAM-AA microgels. In the presence of salt, the Debye length decreases by 1 order of magnitude (above 10 nm in pure water and decreasing down to 1 nm at 0.1 mM in NaCl), and, thus, the range of interactions happens at a much smaller scale. At low pH, the microgels become quasi unchargedonly the moieties resulting from initiation were charged. Strikingly, all the isotherms were superimposed, and the lattice parameters were found to be only dependent on surface pressure. Electrostatics was found to be inefficient on microgel organization at the air−water interface. This effect is in agreement with previous results regarding the oil−water interface of drops. In microgel-stabilized emulsions, microgels pack in a hexagonal array, whose lattice parameter is independent of the number of charges and range of electrostatic interactions.48 At the flat oil−water interface, Isa et al. also indicated the absence of influence of electrostatics on microgel packing.30 However, in this case, this result seemed contradictory to a previous result in which the same authors showed that the isotherms had exactly the same shape but were shifted toward lower interfacial areas when increasing the charge density upon pH increase.29 We rather believe that this shift of the isotherms could be due to the additional 7977


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which corresponds to surface pressures around 30 mN/m, i.e., at the foot of the second phase transition or above. As the maximum surface pressure obtained in pendant drop is about 28−29 mN/m, it suggests that microgels reach this state. At low concentration, the surface coverage is low with an interdistance of 900 to 1200 nm. The film was composed of a monolayer of microgels, whose thickness was about 350 nm. The corresponding surface pressure is between 18 and 23 mN/ m, so probably at the end of domain II, but the error bars are important because the distance varies strongly with surface pressure in this region. It should be noted that the thickness does not refer to the same type of monolayer. When the microgels are flattened, they arranged in a monolayer, sandwiched between the two interfaces. With more compressed microgels, the films are constituted with bilayers. The polymer thickness is more uniform, which inhibits bridging.16,22 This is in agreement with the AFM images obtained for the corresponding distances, showing a strong lateral interpenetration with thick junctions between microgels (Figure 2f,g). It would be interesting to measure the thickness of a foam film using charged pNIPAM-AA microgels to investigate the impact on swelling ratio on the thickness since there is no impact on interfacial packing.

Figure 8. Spontaneous adsorption of microgels: surface pressures as a function of time for pNIPAM and pNIPAM-AA microgels. The concentration of pNIPAM is 0.1 and 0.5 wt % in pure water. The concentration of pNIPAM is 0.1 and 0.5 wt % in brine ([NaCl] = 10−2 M).

of microgel concentration was studied. Films with different organization were obtained, and the consequence of their organization on the adhesion properties of the film was measured. The film adhesion was probed by reducing the pressure and measuring the angle between the film and the meniscus. In spontaneous adsorption, microgel packing and the thickness of the films resulted from the adsorption dynamics, itself controlled by the cross-linking density and the concentration of microgels. This study provided the evolution of the distance between microgels, calculated from geometrical consideration of film area, volume and microgel number, and the thickness of the film, measured by interferometry. In adhesive films, the microgels were organized in a monolayer, where microgels were bridged between the two water−air interfaces, similarly to what was observed previously in emulsions.20 In nonadhesive films, microgels were organized in bilayers. Thanks to these data and the present ones, we can go one step further and estimate the corresponding surface pressure obtained in compression. Table 2 summarizes the data collected with pNIPAM microgels having different cross-linking densities. In their “compressed state”, the microgels formed a bilayer of about 1000 nm in thickness. The interdistance between microgels was estimated to be about 600−700 nm,

V. CONCLUSION In this work, we provided a full description of the behavior of soft microgels having different structures (charges, softness), under compression at the air−water interface. Thanks to the transfer of the monolayer on a solid substrate, the packing of the microgels at the air−water interface could be monitored. It was confirmed that the microgels could deform at the liquid interface, being flattened at low surface concentration and progressively compacting and thickening upon compression. As soon as they interact together, the surface pressure is nonzero, and they adopt a hexagonal array with characteristic distances that decrease as the compression increases. This transfer technique was found to provide a reliable replica of the organization at the air−water interface since the measured interdistances corroborated the estimation that could be made knowing the number of microgels. Better insights into the role of microgel structural parameters could be determined. In particular, it was proved that charges have no influence on the packing of microgels. By contrast, the cross-linking ratio was found to play a fundamental role. At low compression, microgels spread all the more since they were poorly cross-linked. Their spreading is ruled by their surface

Table 2. Organization of pNIPAM Microgels in Thin Foam Films Measured in Ref 38 and Determination of the Corresponding Surface Pressure Using Present Dataa BIS contentb concentration of microgels in waterb morphology of the filmb thickness of a monolayerb dCC (calculated based on the assumption of microgel incompressibility)b corresponding surface pressure in the compression isothermd

1.5 mol %

1.5 mol %

5 mol %

5 mol %

0.1 wt % monolayer (promotes film adhesion) 340 nm 1270 nm

1 wt % bilayer

1 wt % bilayer

600 nmc 680 nm

0.1 wt % monolayer (promotes film adhesion) 400 nm 906 nm

450 nmc 600 nm

23e −30f mN/m

30e −31f mN/m

18 mN/m

30 mN/m

a

Since the microgels used in the present study did not have exactly the same cross-linking density, a comparison is drawn between the 1.5 mol % BIS used in reference 38 and the two closest cross-linking densities, 1 mol % and 2.5 mol %. bThe data were taken from ref 38. cThe thickness of the film is divided by two to get the thickness of a monolayer in case of a bilayer. dThe data were taken from the present study. eThe values are those obtained for the pNIPAM with 2.5% BIS in the present study. fThe values are those obtained for the pNIPAM with 1% BIS in the present study. 7978


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Langmuir activity of the polymeric chain but is restricted by the number of cross-links. Consequently, the surface elasticity increased as the cross-linking density decreased. At higher compression, poorly cross-linked microgels formed a regular hexagonal array, with a unique lattice parameter whatever the compression state. For the highest cross-linking ratio, the coexistence between two hexagonal phases having different lattice parameters was observed, at a quasi-constant surface pressure. The two phases corresponded to shell−shell and core−core contacts, respectively. It was deduced that the rigidity of the cores induced capillary attractive interactions between them. So, despite the similarity of their isotherm shapes, microgels with various rigidities undergo different packing. At very high compression, another transition was observed which was independent of the microgel cross-linking ratio. It corresponds to a soft-to-hard transition, where the rigid cores could not be compressed anymore and behave as hard particles. This transition occurs without any dehydration of the polymer chains. This work presents a full mapping of the organization of microgels at the air−water interface. It brings a complete description of the structural parameters affecting their packing, showing once more that their softness is a crucial parameter, which grants them some original properties. Working at the air−water interface allowed a simplification of the system, providing robust and reproducible results. Moreover, this method can be viewed as a very simple method to produce well-defined and tunable arrays over large areas on solid substrate. Some analogies with the oil−water interface were found, but differences were also pointed out, raising new questions about the similarity of the microgels at the two interfaces.

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pressures; volumes of the dried microgels on solid substrates, obtained with the Gaussian approximation profile; comparison of the dried microgel volumes in the films using the various approximations and in the collapsed state; scheme showing the simplified shapes of the microgels used for volume estimations; agreement between the calculated distance using eq 2 and the distance measured by AFM after transfer on the solid substrate; surface elasticity as a function of surface pressure for both charged and neutral microgels (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (V.S.). *E-mail: [email protected] (V.R.). ORCID

Serge Ravaine: 0000-0002-6343-8793 Valérie Ravaine: 0000-0002-1192-7974 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the “Ministère de lʼEnseignement supérieur, de la Recherche et de lʼInnovation” for funding the fellowship of M.C.T.

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REFERENCES

(1) Binks, B. P. Particles as surfactants - Similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7 (1−2), 21. (2) Leal-Calderon, F.; Schmitt, V. Solid-stabilized emulsions. Curr. Opin. Colloid Interface Sci. 2008, 13 (4), 217. (3) Cervantes Martinez, A.; Rio, E.; Delon, G.; Saint-Jalmes, A.; Langevin, D.; Binks, B. P. On the origin of the remarkable stability of aqueous foams stabilised by nanoparticles: link with microscopic surface properties. Soft Matter 2008, 4 (7), 1531−1535. (4) Rey, B. M.; Elnathan, R.; Ditcovski, R.; Geisel, K.; Zanini, M.; Fernandez-Rodriguez, M.-A.; Naik, V. V.; Frutiger, A.; Richtering, W.; Ellenbogen, T.; Voelcker, N. H.; Isa, L. Fully Tunable Silicon Nanowire Arrays Fabricated by Soft Nanoparticle Templating. Nano Lett. 2016, 16 (1), 157−163. (5) Honold, T.; Volk, K.; Rauh, A.; Fitzgerald, J. P. S.; Karg, M. Tunable plasmonic surfaces via colloid assembly. J. Mater. Chem. C 2015, 3 (43), 11449−11457. (6) Wang, Z.; Rutjes, F. P. J. T.; van Hest, J. C. M. pH responsive polymersome Pickering emulsion for simple and efficient Janus polymersome fabrication. Chem. Commun. 2014, 50 (93), 14550− 14553. (7) Qiu, Q.; Liu, G.; An, Z. Efficient and versatile synthesis of star polymers in water and their use as emulsifiers. Chem. Commun. 2011, 47 (47), 12685−12687. (8) Shi, X.; Miao, M.; An, Z. Core cross-linked star (CCS) polymers with tunable polarity: synthesis by RAFT dispersion polymerization, self-assembly and emulsification. Polym. Chem. 2013, 4 (6), 1950− 1959. (9) Destribats, M.; Rouvet, M.; Gehin-Delval, C.; Schmitt, C.; Binks, B. P. Emulsions stabilised by whey protein microgel particles: towards food-grade Pickering emulsions. Soft Matter 2014, 10 (36), 6941− 6954. (10) Murphy, R. W.; Farkas, B. E.; Jones, O. G. Dynamic and viscoelastic interfacial behavior of β-lactoglobulin microgels of varying sizes at fluid interfaces. J. Colloid Interface Sci. 2016, 466, 12−19. (11) Pelton, R. Temperature-sensitive aqueous microgels. Adv. Colloid Interface Sci. 2000, 85 (1), 1.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01538. Scheme representing the experimental setup for the preparation and transfer of the monolayer; compression isotherms of pNIPAM-AA and pNIPAM microgels on pure water; AFM images and height profiles of pNIPAM microgels after transfer on silicon substrate at different surface pressures on pure water; large view AFM image of pNIPAM-AA microgels; AFM height images and corresponding phase images obtained for pNIPAM microgels; summary of the lattice parameters from the AFM images of the transferred monolayers at different surface pressures; Gaussian fits of the AFM profiles for pNIPAM-AA and pNIPAM microgels; radial distribution functions obtained from AFM images; influence of salt concentration and pH of the subphase on the compression isotherms of pNIPAM microgels; AFM images of pNIPAM-AA microgels after transfer on silicon substrate at different surface pressures on [NaCl] = 0.01M, of pNIPAM-AA microgels after transfer on silicon substrate at different surface pressures on aqueous phase at pH 3; height profiles of the dried pNIPAM microgels with 1 mol % BIS and with 5 mol % BIS after transfer on silicon substrate; AFM height and phase images obtained for pNIPAM microgels with 1% BIS; summary of the lattice parameters from the AFM images of the transferred monolayers at different surface 7979


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