Multiple structural transitions in Langmuir monolayers of charged soft

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Multiple structural transitions in Langmuir monolayers of charged soft-shell nanoparticles Ahmed El-Tawargy, David Stock, Markus Gallei, Wael Ramadan, Mamdoh Shams El-Din, Günter Reiter, and Renate Reiter Langmuir, Just Accepted Manuscript • DOI: 10.1021/ acs.langmuir.7b03656 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Multiple structural transitions in Langmuir monolayers of charged soft-shell nanoparticles A. S. El-Tawargya,b, D. Stockc†, M. Galleic, W. A. Ramadanb, M. A. Shams El-Dinb, G. Reiter a, R. Reiter a

a. Physikalisches Institut, Fakultät für Mathematik und Physik, Albert-Ludwigs-Universität, Freiburg, 79104, Germany. b. Department of Physics, Faculty of Science, Damietta University, Damietta, 34517, Egypt. c. Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Technische Universität Darmstadt, Darmstadt, 64287, Germany. †

in memoriam of David Stock

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Abstract We have investigated the morphologies of Langmuir layers of charged, polymeric hardcore/interlayer/soft-shell (CIS) nanoparticles spread at the air-water interface. Depending on various mutual interactions, which are correlated to the areal densities of the deposited nanoparticles, we observed ordered patterns of non-dense and closed-packed arrangements of CIS nanoparticle ordering. At low areal densities, we found an almost regular distribution of the charged CIS nanoparticles on the water surface, which resulted from long-range repulsive electrostatic interactions between them. At higher areal densities, domains of more closely packed and ordered nanoparticles were formed, coexisting with regions of randomly and sparsely distributed nanoparticles. We relate these domains to the interplay of electrostatic repulsion and capillary attraction caused by the dipolar character of like-charged particles at the interface, allowing for a characteristic separation distance between nanoparticles of about 3 - 4 times the nanoparticle diameter. At relatively high areal densities, attractive van der Waals forces were finally capable of getting nanoparticles into contact, leading to densely packed patches of hexagonally ordered nanoparticles coexisting with regions of rather well ordered nanoparticles separated by about 1 µm and regions of randomly and sparsely distributed nanoparticles. Intriguingly, upon re-expansion of the area available per nanoparticle, these densely-packed patches disappeared, indicating that steric repulsion due to presence of soft shells as well as longrange electrostatic repulsive forces were strong enough to assure reversibility of the morphological behavior.


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Introduction Natural opals are composed of three-dimensionally ordered, close-packed arrangements of sub-micrometer silica (SiO2) spheres [1]. Due to the crystalline arrangement of these spheres and their sizes, which are comparable to the wavelengths of the visible light, most opals can diffract such wavelengths in a way similar to the diffraction of x-rays by atoms [2]. That is the reason why opals appear in beautiful iridescent colors. Artificial SiO2 colloidal particles can mimic the properties of natural opals when they are in a crystalline arrangement [3-5]. Crystallization of SiO2 colloidal spheres can be achieved in quasi two-dimensional monolayers as well as in three-dimensional bulk structures where they are mostly packed in a face centered cubic (fcc) crystal lattice [6-10]. Synthetic opal structures can also be generated from polymeric spherical colloids composed of co-polymers as for example polystyrene/poly(methyl methacrylate) or polystyrene/ poly(acrylamide) [11]. Recently, polymeric colloids with a core/shell (CS) structure entered the field of opal research [12-19] as promising candidates due to the additional functions implemented in the properties of the shell. Tailoring the difference in refractive indices (∆n) between core and shell allowed to manipulate the color of films of ordered polymeric colloids. The contrast in the opalescence of the films is enhanced by increasing ∆n [12, 13]. Combining a hard-core with a soft-shell added a certain degree of deformability and the possibility to process these particles to free-standing bulk opal films while maintaining the spherical shape of the core [20].The rigid, non-deformable part (core) directs the assembly of the spheres to form ordered crystalline domains. Stimulated by an external trigger, the size and shape of the soft shell can be altered [16, 21]. Such a stimulus could be the addition of a solvent, exerting mechanical stress, changes in temperature, exposure to light sources or to electric or magnetic fields and changes in pH or ionic strength [13-15, 18, 22, 23]. Accordingly, the color of a film of ordered CS colloids can be changed in a controllable and reversible manner [18]. Therefore, these materials have been used as electronic inks for rewritable electronic paper technology and were used as active materials in mechano-, thermo-, and solvato-chromic sensors [13-15, 18, 24, 25]. Sometimes inorganic spheres are used instead of polymeric cores in order to increase the ∆n values [12, 16,26].


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In order to attach the shell forming polymers permanently to the chains of the hard core, one can introduce an interlayer, which allows for a dense grafting of these shell-forming polymer chains [12, 13, 21]. Such colloidal nanoparticles are referred to as core/interlayer/shell (CIS) nanoparticles. Interactions of CIS nanoparticles during assembly processes are expected to be mediated and controlled by the soft shells of flexible polymer chains whereas the hard cores should essentially keep their properties. The most convenient routes to produce crystalline structures from colloidal particles are drying processes starting from dispersions of hard spheres deposited on flat substrates. Hydrodynamic pressure during slow drying and capillary forces of the volatilizing dispersion medium provide control parameters for colloidal crystallization [2, 11,16, 17, 21, 27]. For instance, Colvin et al. developed the method of vertical deposition, wherein the fluid medium of a colloidal dispersion is slowly evaporated inside a narrow vessel [28]. Besides such deposition techniques, spin coating can be used for the convenient formation of colloidal crystal structures. Nevertheless, high quality and scalable structures featuring almost perfect three-dimensional particle order still remain a significant challenge [29-37]. Crystallization in the course of sedimentation is a time consuming process since it may take days up to months to form crystalline domains of macroscopic size [11]. Alternatively, colloid crystallization can be induced by compression of nanospheric systems in the molten state, which, however, requires elevated temperatures. This may cause some deformation of the cores of the nanospheres. For example, it was witnessed that after heating above the glass transition temperature, PS nanoparticles exhibited an elongated shape [21]. Neither sedimentation nor melt compression allow to control the thickness of the crystal (number of layers of ordered colloids). A more promising way is based on the formation of monolayers of ordered colloidal nanoparticles, which serve as building units for the construction of three-dimensional photonic crystals. A controlled number of layers can be deposited, mostly via a bottom-up process [34-36]. The Langmuir technique is a useful and effective method for preparing monolayers of nanoparticles at a fluidfluid interface, with the possibility to control the areal density and thus the lateral packing of these nanoparticles [2,16, 17, 27, 34, 37]. Such a monolayer can be transferred to a solid substrate and multilayers can be obtained through multiple repetition of the transfer process.


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Uncharged colloidal particles show a strong tendency to form agglomerates because attractive van der Waals (vdW) forces prevail. One way to stabilize the particles is to introduce electrostatic repulsion by charging all particles with similar charges (i.e. positive or negative surfactant molecules) or by covalently attaching soft capping ligands. This strategy of stabilizing colloidal particles in dispersions can also be employed for particles at interfaces [16, 17, 38-44]. However, the stability of morphological arrangements of colloidal particles at interfaces may deviate from observations in (three-dimensional) bulk dispersions [45] where stability is governed by the interplay between repulsive (e.g., electrostatic) and attractive (e.g., vdW forces). At interfaces, two additional forces may act between the particles; capillary forces which might cause lateral attraction [46-48] and steric forces usually causing repulsion [49]. When a colloidal particle is deposited at an interface between a nonpolar and a polar fluid medium (e.g., air/water interface), the difference in dielectric constants of the two interfacing media induces a dipole, which is oriented normal to the interface [41-44]. This dipole causes a distortion of the initially planar interface. Interactions between approaching neighboring particles will now involve the overlap of locally distorted interfacial regions. In the present study, we report on morphological transitions in monolayers of polymeric CIS nanoparticles induced by changes of their areal density through compression / expansion of a Langmuir film. Brewster angle microscopy (BAM), bright field optical microscopy (OM) and atomic force microscopy (AFM) were used to probe the resulting structures on different length scales. Different regimes of particle packing and organization were identified as a function of mean inter-particle distance. At low areal densities, the CIS nanoparticles were distributed homogeneously and wellordered everywhere, revealing inter-particle distances 10 times larger than the diameter of the nanospheres. At intermediate areal densities, ordered aggregates formed, which showed reduced inter-particle distances but were not yet close packed. Further increase of the areal density produced colored domains (due to interference of light) consisting of closely packed nanoparticles.


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Materials and Methods Properties of the CIS nanoparticles Core/interlayer/shell (CIS) nanoparticles were prepared by emulsion polymerization as described in [15,17]. The resulting CIS nanospheres were composed of a cross-linked hard core (co-polymer of styrene and butanediol diacrylate (BDDA)) and an outer soft shell formed by a co-polymer of hydroxyethyl methacrylate (HEMA), isobutyl methacrylate (IBMA) and ethyl acrylate (EA)). The interlayer between the hard core and the soft shell was a copolymer of EA and allyl methacrylate (ALMA). ALMA provided anchor sites for grafting. Thus, the chains of the soft shell were partially grafted to the cross-linked hard core. SDS (sodium dodecyl sulfate) and Dowfax 21 were used. The surfactant molecules acted as stabilizing agents by introducing negative charges during the synthesis process of core, interlayer and shell [15,17]. The monodisperse CIS nanoparticles with a diameter of ca. 250 nm (measured by dynamic light scattering (DLS) [15,17]) were dispersed in chloroform (CHCl3) and exhibited a Zeta-potential of ca. - 50 mV for both washed and non-washed nanoparticles. This means that the negatively charged surfactant molecules were strongly attached to the nanoparticles and resisted extended periods of washing. Accordingly, due to strong electrostatic repulsion, the dispersion was very stable against aggregation by van der Waals forces [50]. This is consistent with the observation of a phase of hexagonally ordered nanoparticles, separated by several micrometers, right after spreading them on the water-air interface. Furthermore, this ordered phase was stable over many hours, corroborating the strong attachment of the surfactant molecules to the nanoparticles.

Formation of monolayers and their transfer to solid substrates Measurements were performed on a Langmuir trough purchased from Riegler & Kirstein (R&K), Germany, having a maximal surface area of 192 cm2. The temperature was controlled by circulating thermostated water through the base plate of the trough. Measurements were performed at 10o C to reduce thermal noise, relevant especially for BAM measurements. The trough was mounted on a granite bench and kept inside a closed Plexiglas box to protect the device from vibrations and contaminations. The CHCl3-dispersion at a concentration of 1 % by weight was exposed to an ultrasonic bath at room temperature for 15 minutes to break down eventually existing agglomerates. The surface pressure Π was measured with the Wilhelmy plate 6 ACS Paragon Plus Environment

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technique. A small piece of completely wetted filter paper was used as a Wilhelmy plate (WP). The WP was positioned in the center of the trough. Covering planar water surfaces with surfaceactive species of lower surface tension reduces the surface tension. However, on curved surfaces, an additional Laplace pressure component has to be taken into account. Moreover, there is also an electrostatic contribution to the surface tension [43, 44]. Presently, there is still some debate about the significance and interpretation of the various contributions to the values measured with the Wilhelmy plate technique. To check for the cleanliness of the water surface, the barriers were fully compressed before adding CIS nanospheres. When the surface pressure stayed below 0.1 mN/m, which is within the range of accuracy of the device, the experiment was started. The CIS dispersion was deposited on the surface of ultra-pure water using a Hamilton micro-syringe. CHCl3 has a low boiling point and a low surface tension and should therefore facilitate the spreading of nanoparticles. To assure complete evaporation of CHCl3, the measurements were started 20 minutes after the deposition of the nanoparticles. The two parallel barriers were moved at a constant compression rate of 0.053 µm2/(nanoparticle∗min) and the (surface pressure-area) isotherm was recorded. At selected points of the isotherm, nanoparticles were transferred to silicon wafers (Silchem Handelsgesellschaft, Germany), which have been sonicated for 15 minutes in ethanol. The transfer process was always performed at the center of the trough, close to the WP. The orientation of the substrate with respect to the plane of the air-water interface was perpendicular for the Langmuir-Blodgett (LB) method and parallel for the LangmuirSchaefer (LS) method [2,16,17,27,51]. Both transfer geometries yielded films of very similar morphologies.

Characterization of CIS Langmuir layers Brewster angle microscopy Brewster angle micrographs were recorded with a Multiskop purchased from Optrel, Germany, which was equipped with a Langmuir trough from R&K, identical to the trough described above. For BAM imaging, the films were prepared under the same experimental conditions as the monolayers, which were subsequently transferred to silicon wafers for AFM and OM analysis. Laser light of a wavelength of 632.8 nm, polarized in the direction parallel to 7 ACS Paragon Plus Environment

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the plane of incidence, was reflected from the water surface at the Brewster angle of water (53.1°). Polarizer and analyzer were set to 0 ° to suppress reflected light from a pure water surface. The Brewster angle micrographs were focused on a CCD camera for visualization. An objective with tenfold magnification (Mitutoyo, numerical aperture N.A. = 0.28) was mounted on a piezo translation stage, which performed a continuous movement to bring consecutive stripes of the image into focus. A streaming video sequence was recorded to construct an overall sharp image. In-situ monitoring of the CIS films at different stages of compression or expansion was performed with a lateral resolution of several micrometers. The obtained micrographs were processed with the software tools provided by ImageJ [52].

Bright field optical microscopy After transferring the nanoparticle films onto silicon (100) native oxide surfaces, the resulting films were characterized with an optical microscope (Olympus BX51M), with a 100x objective lens, N.A. = 0.9. At low areal densities, the favorable optical contrast between nanoparticles and the substrate allowed to resolve the ca. 250 nm sized nanoparticles individually. The obtained micrographs were processed with the software tools provided by ImageJ [52].

Atomic force microscopy AFM images were recorded with Nano-Wizard II purchased from JPK, Germany. The surface morphology of the transferred films was scanned in (intermittent contact) tapping mode. Cantilevers with a resonance frequency of 150 - 160 kHz, holding tips with a radius of curvature of ca. 10 nm, were used. All images were measured at a scan frequency of 0.4 - 1.2 Hz and at a high set point ratio (representing a low tapping force) in order to avoid both tip-induced structural changes and artefacts due to dragging of material with the tip. Each sample was scanned several times and at several regions of the sample to ensure representative images. The chosen regions were always far from the edges of the substrate to avoid regions containing artefacts introduced during transfer. The obtained images were analyzed with the image


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processing software provided by JPK. All presented images have been flattened but were not processed any further.


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Measurements of isotherms and characterization of the resulting morphologies Figure (1) shows the surface pressure (Π) - area (A) isotherm of a compression/expansion cycle after depositing 2 µL of nanoparticle dispersion on the water surface. The BAM images shown in figure (2) were captured at the positions marked by the blue arrows in the isotherm of figure (1). On pure water, BAM images were fully black due to the condition that p-polarized light, impinging on the water surface under the Brewster angle, is not reflected. However, due to the presence of nanoparticles, having different optical properties than water, the Brewster angle condition was no longer fulfilled for the entire surface and some light was reflected. Accordingly, at large values of the mean area (A) available per CIS nanoparticle on the surface (low areal density of nanoparticles), BAM images of figure (2) revealed that the reflectivity has changed, which can roughly be quantified by the average gray value displayed in figure (1). With the lateral resolution of BAM, which is of the order of a few micrometers, no aggregation of CIS nanoparticles was detectable at position (i) of figure (1). However, with compression (i.e., reduction of A), nanoparticles started to form clusters as indicated by patches of higher brightness with respect to the black background (see e.g. image (ii) in figure 2). While BAM provided information only with low resolution, the images of figure (2) supported the formation and coalescence of clusters upon compression. Interestingly, BAM images (iv) and (v) of figure (2), taken upon expansion, indicated that these aggregation processes were reversible, in line with the observed reversibility of the isotherms of figure (1). These observations suggested that the formed aggregates could be broken-up, due to repulsive steric and electrostatic forces that were regaining dominance upon expansion, especially at low average areal densities [49, 53-55]. The soft ligand brushes forming the shell of the nanoparticles are of polymeric nature, introducing an entropic force due to conformational changes. In addition to electrostatic repulsion controlling the re-distribution of CIS nanoparticles on the water surface over distances much larger than the size of the nanoparticles, this entropic force acting at much shorter distances contributes to the re-dispersion of the aggregates upon expansion, assuring that the aggregation process became reversible. A similar observation was made earlier for the forced desorption of nanoparticles from an oil-water interface [55]. Intriguingly, as shown in figure (1) (and later on, in figure (4)) through the values of A/A0 of about 10 - 20, we observed the 10 ACS Paragon Plus Environment

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formation of ordered aggregates already at an effective radius of the nanoparticles, which was about 3-4 times larger than the geometric size of these nanoparticles. We attribute these large area occupied per nanoparticle compared to its cross-sectional area to the balance of electrostatic repulsion and capillary attraction caused by the dipolar character of the nanoparticles and the corresponding local reorganization of water molecules at the interface between the nanoparticles and water. As a quantitative measure for analyzing BAM images, we have determined the mean gray values (the summation of gray values of all pixels of the image divided by the total number of the pixels) [52] of the BAM images. Intriguingly, these values, shown in figure (1) together with the Π-A isotherms, varied with A in a similar fashion as the surface pressure Π, suggesting that an increase/decrease of Π was caused by the formation/disaggregation of (coalescing) clusters with decreasing/increasing A. Inverse packing density 20 40 60

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Figure (1): Compression/expansion cycle (black and red solid lines, respectively) and mean gray values estimated from some BAM images for the deposited volume of 2 µL at different areal densities during compression (black solid squares) and expansion (red open circles). The top axis represents the inverse packing density, i.e., the mean area A available per nanoparticle divided by the cross-sectional area A0 of one CIS nanoparticle. The estimated values of the error bars are 5 %. Blue arrows refer to the values of A where the BAM images of figure (2) were recorded. 11 ACS Paragon Plus Environment

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Figure (2): BAM images of layers of CIS nanoparticles at the air-water interface prepared from a dispersion volume of 2 µL. Images were taken during compression at A: (i) 4.92 µm2, (ii) 1.39 µm2, (iii) 0.65 µm2 and during expansion at A: (iv) 1.39 µm2, (v) 4.92 µm2, respectively. The changes in gray values within the sequence of images are relevant in spite of poorly resolved details within individual images.

In order to gain further and more detailed insight into the distribution of the nanoparticles on the water surface, we transferred the films of nanoparticles onto a solid substrate and analyzed these layers by OM. The resulting images are presented in figure (3) for films transferred at the same values of A where the BAM images shown in figure (2) were taken. Right after deposition (figure (3i)), CIS nanoparticles were uniformly distributed over the total surface area, indicating the dominance of electrostatic repulsion at this low areal density (i.e., large area per nanoparticle). Confirming the conclusions drawn from the BAM images, OM images showed that upon compression CIS nanoparticles started to form clusters, which disaggregated again upon expansion. Most intriguingly, however, images (i) and (v) of figure 3 demonstrate that at low areal densities the nanoparticles were not distributed randomly but were rather equally spaced with a mean separation distance about 10 times the nanoparticle diameter due to the longrange electrostatic repulsive force


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In figure (1), the initial surface pressure was 0 mN/m. There, the nanoparticles were far enough from each other so that they did not experience any attractive capillary forces, which would have been capable of inducing a surface pressure. At the experimentally realizable lowest area per nanoparticle of 0.5 µm2, a maximum surface pressure value of 27 mN/m was achieved. As shown in the microscopy images of figures (2) and (3), the increase of the surface pressure upon decreasing the area per nanoparticle allowed for capillary attraction between nanoparticles and the formation of clusters of ordered but non-densely packed nanoparticles. However, the shape of the isotherm did not indicate a clear phase transitions, as additionally confirmed by the corresponding static elastic modulus, which showed only one peak at the onset of the region of the pseudo-plateau (data not shown). Interestingly, this peak in the static elastic modulus was in close vicinity of the position where the curve of the isotherm measured upon re-expansion was re-joining the curve of the isotherm measured upon initial compression. According to our microscopy observations upon re-expansion, at this position of the isotherm, all densely packed aggregates have been disassembled and disappeared. Based on above-mentioned observations, 13 ACS Paragon Plus Environment

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we claim that the emergence of surface pressure was correlated to the presence of clusters of ordered but non-densely packed nanoparticles and densely packed aggregates of nanoparticles. Phase transitions may be “masked” if the kinetics of ordering processes is slower than the compression process. Thus, there are many possible cases where the isotherms do not indicate any well-defined phase transitions. We anticipate that the equilibrium structure is a single but large ordered structure embedded in a coexisting phase of less or different order, as defined by the thermodynamic parameters of the phase diagram. However, the evolution from many small domains due to a high nucleation probability to a single large domain implies relaxation kinetics (e.g. of transport and reorganization) and thus may require extended periods of time. Accordingly, as we did not allow for the required equilibration time, most of the observed structures and patterns are not fully equilibrated. As the smallest experimentally achievable area between the two movable barriers of the Langmuir trough did not allow compressing the CIS nanoparticles from a deposited volume of 2 µL of the dispersion to mean areas per nanoparticle smaller than ca. 0.5 µm2 (i.e., equivalent to an area about 10 times the cross-sectional area of ca. 0.05 µm2 of the nanoparticles), we performed additional experiments with a larger amount of nanoparticles, i.e., a deposited volume of 5 µL of the dispersion. The number of nanoparticles can be controlled either by using different volumes of a fixed concentration or by using a fixed volume of different concentrations. In this work, we have chosen to keep the concentration of the nanoparticle dispersion constant. Accordingly, we have increased the number of nanoparticles by spreading a higher volume. In the current study, the total number of nanoparticles spread was 3.78×109 ± 0.47×109 and 9.44×109 ± 0.90×109 nanoparticles for the spread volumes of 2 µL and 5 µL, respectively. Nanoparticle dispersions of concentrations higher the 1% dispersion used in our study appeared to be milky and were not employed for this study. The corresponding compression/expansion cycle for a deposited volume of 5 µL of the dispersion is displayed in the isotherm shown in figure (4). In contrast to figure (1), the isotherm showed a pronounced hysteretic behavior upon expansion after having compressed the layer to areas per nanoparticle smaller than ca. 0.2 µm2. When depositing a larger number of nanoparticles, they tended to form clusters of ordered but non-densely packed nanoparticles already right after deposition, even without compression. We tentatively anticipate that the formation of large ordered domains from such clusters would


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The corresponding OM and AFM height images of CIS films, which were transferred at different values of A (marked by dashed lines in figure (4)), are shown in figure (5) and figure (6), respectively. Table (1) in the supplementary information summarizes the values of surface pressure and time (measured from the beginning of compression) required to reach the values A at which films were transferred from which the images shown in figures (5) and (6) were taken.

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Figure (5): OM images of films of CIS nanoparticles prepared from a dispersion volume of 5 µL, transferred during compression at A (i) 1.97 µm2, (ii) 1.39 µm2, (iii) 0.73 µm2 and (iv) 0.24 µm2 and during expansion at A (v) 0.73 µm2, (vi) 1.39 µm2 and (vii) 1.97 µm2.


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Figure (6): AFM height images recorded from the transferred films shown in figure (5). As a clear difference to the previous case (see figure 1), where the isotherm reached only areas as low as ca. 0.5 µm2 per CIS nanoparticle, the images of figures (5) and (6) show the formation of extended patches of densely packed nanoparticles when compressed to smaller areas per nanoparticle. While in OM images (iv) and (v) of figure (5) these patches appear as regions of bluish-green color (due to interference of light reflected at the substrate and at the surface of the domain), the corresponding AFM images clearly demonstrate that these patches consisted of densely packed nanoparticles, which were in close contact (i.e., these nanoparticles experienced steric repulsion). Within these patches, several domains of hexagonally packed CIS nanoparticles could be identified. Upon further compression, the areal fraction covered by such patches of closely packed CIS nanoparticles increased [16, 17, 42-44]. It is expected that at small interparticle distances vdW forces will cause attraction between nanoparticles, reducing the interparticle distance further until steric interactions between the flexible polymers forming the shells of touching nanoparticles will generate a repulsive force. Thus, at relatively high areal densities, close-packed patches are the result of attractive capillary and vdW forces, which overcome electrostatic repulsion and bring the nanoparticles to separation distances only limited by the size of the nanoparticles, i.e., steric forces [56]. The various patches of close-packed nanoparticles had rather irregular shapes and differed significantly in size (see figure 5 (iv) and 17 ACS Paragon Plus Environment

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(v)). The various ordered domains of random shape are the consequence of the balance between attractive and repulsive interactions of various origins. For example, at the smallest areas per nanoparticle, attractive van der Waals forces are balanced by repulsive forces caused by the loss in entropy of the deformed soft shells [26]. Therefore, the shell-shell interaction plays a crucial role in the formation of ordered domains of close-packed nanoparticles [16, 17]. Control experiments and corresponding observations clearly indicated that the loss of charges (if existing at all) is an extremely slow process. Measuring the surface pressure over more than five hours at a constant mean area per nanoparticle did not show any detectable changes as long as the area per nanoparticle was larger than ca. 2 µm2. For smaller areas per nanoparticle (and at surface pressures larger than ca. 5 mN/m), the kinetics of structure formation, involving also a coarsening of the domains, typically led to a ca. 5% decay in surface pressure. Furthermore, if the electrostatic forces caused by strongly attached surfactant would not be constant at least for the duration of our experiments, we could not expect to observe a fully reversible behavior. Based on all these observations we are confident that our observations are not caused by impurities or irreversible changes in the surfactant concentration. We would like to emphasize that for most of our observations, the distance between the nanoparticles is (at least on the average) much longer than the contour length (i.e., the maximum length) of the polymers forming the shell. At an area per nanoparticle of about 1 µm2, the mean distance between nanoparticle is about four times the particle diameter. Thus, it is not possible that the shells of two nanoparticles are interacting in a steric way (no shell-shell contact possible). Consequently, we have attributed the observed formation of an ordered structure without contact between shells to the interplay of electrostatic repulsion and capillary attraction. However, at high compressions the deformability of the shells and possibly the redistribution of charge densities within the nanoparticles cannot be neglected. Interestingly, as shown in the images (v) – (vii) of figures (5) and (6), corresponding to the expansion trace of the isotherm of figure (4), these patches of closely packed nanoparticles largely disappeared when the average area per nanoparticle was re-increased. Thus, although having been in contact, nanoparticles repelled each other to larger interparticle distances of about 0.9 µm, allowing for a largely reversible behavior. However, images of figure 5 (i) and (vii) were not fully identical, probably due to the limited time allowed for the expansion (the barriers were 18 ACS Paragon Plus Environment

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separated at a fixed rate, defining the (possibly insufficient) time provided for equilibration). Some small patches, which did not exist at the beginning of the compression process, were visible in figure (5vii). This might explain the hysteresis in the compression/expansion curve, figure (4). A detailed analysis of AFM images, shown in figure (7), revealed that the individual nanoparticles had a width to height ratio larger than unity, suggesting that the nanoparticles were compressed in the direction normal to the substrate. Such flattening has two reasons; firstly, it might result already on the water surface due to the difference in solvent quality for the part of the nanoparticle exposed to air and the part immersed in water where soft chains were more extended inside water [49, 57]. In addition, on the solid substrate, the soft shells of nanoparticles might be flattened due to adsorption on the substrate. Results were similar for images taken at various points of the compression and expansion traces of the isotherm shown in figure (4). 200

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measured by dynamic light scattering. (b) Cross-sections for a dimer extracted from image (iii) in figure (6). (c) 3D representation of a part of image (iii) in figure (6).

Figure (7-a) shows cross-sectional height profiles for individual nanoparticles visible in figures (6i) and (6vii) showing measurements from samples obtaiend upon compression and expansion. From these profiles, the diameter at the base, i.e. at the interface between substrate and nanoparticle, was about 319 ± 10 nm (this value represents an average over five individual measurements taken on isolated nanoparticles presented in 6i and 6vii). The measured height was about 196 ± 5 nm (average value of five individual measurements). The diameter of these CIS nanospheres measured at a distance of 100 nm above the substrate was found to be 264 ± 10 nm. These values have to be compared to the results from dynamic light scattering [13], yielding a mean diameter of the CIS nanospheres of 249 ± 22 nm, with a hard core diameter 158 ± 12 nm. When nanoparticles touch each other, the soft shells are compressed also in the direction parallel to the plane of the substrate. For the dimer shown in figure (7-b), the total width at the base, i.e. at the substrate interface was about 558 ± 10 nm. The height of this dimer was about 196 ± 5 nm. In contrast, soft microgels with soft cores were able to restructure themselves in order to conserve their volumes [17, 58-60]. For nanoparticles arranged in close packing, the mean lateral distance (from five individual measurements) between the centers of two nanoparticles was found to be 207 ± 5 nm (see Figure (7-c)) and a typical height of the nanoparticles still of the order of 200 nm. Thus, even in the compressed state, the hard cores of the nanoparticles with a diameter of about 158 nm [13] were still far from being in direct contact. At the largest examined mean areas per nanoparticle of about 4 µm2, we observed hexagonally ordered patterns caused by electrostatic repulsion. There, the mean distance between nanoparticles was about 2 µm. As we compressed such a layer of ordered nanoparticles, we observed ordered domains coexisting with disordered regions. Within the ordered domains, the nanoparticles were separated by about 1.0 µm to 0.8 µm (i.e., about 4 to 3 times the diameter of these nanoparticles) as we decreased the mean area per nanoparticle from about 2 to ca. 0.75 µm2. When compressed further, domains with a third type of ordered packing of nanoparticles were observed. There, the nanoparticles were actually in contact and thus separated by about their diameter. As discussed in the manuscript, some squeezing of the “soft” shell was observed. 20 ACS Paragon Plus Environment

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A more detailed and systematic study in order to identify precisely the physical parameters which control the distance between nanoparticles in the three types of ordered domains. Such systematic investigations include a variation of characteristic parameters of the nanoparticles (size, thickness of the “soft corona”, amount and type of charges on the surface or within the nanoparticles, …) as well as parameters controlling the change in mean area per nanoparticles (rate of compression, waiting or relaxation times, pH of subphase, …). Studies along these lines are in progress.


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Conclusions CIS nanoparticles with a diameter of ca. 250 nm were deposited on the water surface to prepare Langmuir layers. At relative areal densities (A0/A) less than about 0.017 (i.e., for each nanoparticle an area of about 60 times the cross-sectional area was available), repulsive interactions between the charged CIS nanoparticles led to an almost regular distribution on the water surface. Upon increasing the relative areal density above 0.017, an increase of the surface pressure was observed, which we relate to the formation of clusters of ordered but non-densely packed nanoparticles, coexisting with regions of randomly and sparsely distributed nanoparticles. We assume that such clusters were generated through capillary attraction caused by the dipolar character of like-charged nanoparticles at the interface. Due to the huge difference between the dielectric constants of water and air, attractive capillary forces started to compete with the longrange electrostatic repulsive forces. However, an effective repulsive force remained, which assured that the nanoparticles were still separated at distances about 3-4 times the nanoparticle diameter. At relative areal densities A0/A > 0.1, attractive van der Waals forces were finally capable of getting nanoparticles into contact. Due to such attractive forces, densely packed patches of hexagonally ordered nanoparticles were formed, coexisting with regions of rather well ordered nanoparticles separated by about 1 µm and with regions of randomly and sparsely distributed nanoparticles. Intriguingly, upon re-expansion of the area available par nanoparticle, these densely-packed patches disappeared, indicating that long-range repulsive forces dominated and assured reversibility of the morphological behavior.

Acknowledgement The authors would like to thank Tamara Winter for measurements of the zeta potential, as well as Sabrina Kraus and Stefanie Dold for their assistance with the Langmuir experiments. A. S. El-Tawargy would like to thank the Egyptian ministry of higher education for the financial support.

Supporting information Supplementary information for: Multiple structural transitions in Langmuir monolayers of charged soft-shell nanoparticles (PDF). 22 ACS Paragon Plus Environment

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A figure for the graphical abstract 30

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