Stable in Bulk and Aggregating at the Interface ... - ACS Publications

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Stable in Bulk and Aggregating at the Interface: Comparing Core− Shell Nanoparticles in Suspension and at Fluid Interfaces Siddarth A. Vasudevan,† Astrid Rauh,‡,§ Lorenzo Barbera,† Matthias Karg,§ and Lucio Isa*,† †

Langmuir 2018.34:886-895. Downloaded from pubs.acs.org by AUSTRALIAN NATL UNIV on 08/14/18. For personal use only.

Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zürich, Vladimir-Prelog-Weg 5, 8093 Zürich, Switzerland ‡ Physical Chemistry I, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth, Germany § Physical Chemistry I, Heinrich-Heine-University, Universitätsstr. 1, 40204 Düsseldorf, Germany S Supporting Information *

ABSTRACT: Colloidal particles are extensively used to assemble materials from bulk suspensions or after adsorption and confinement at fluid interfaces (e.g., oil-water interfaces). Interestingly, and often underestimated, optimizing interactions for bulk assembly may not lead to the same behavior at fluid interfaces. In this work, we compare model composite nanoparticles with a silica core coated with a poly-N-isopropylacrylamide hydrogel shell in bulk aqueous suspensions and after adsorption at an oil-water interface. Bulk properties are analyzed by confocal differential dynamic microscopy, a recently developed technique that allows one to simultaneously obtain structural and dynamical information up to high volume fractions. The results demonstrate excellent colloidal stability and the absence of aggregation in all cases. The behavior at the interface, investigated by a range of complementary approaches, is instead different. The same hydrogel shells that stabilize the particles in the bulk deform at the interface and induce attractive capillary interactions that lead to aggregation even at very low area fractions (surface coverage). Upon further compression of a particle-laden interface, a structural transition is observed where closely packed particle aggregates form. These findings emphasize the manifestation of different, and possibly unexpected, responses for sterically stabilized nanoparticles in the bulk and upon interfacial confinement. also find use in nanopatterning applications.13,14 Conversely, in other situations, electrostatic forces may not be effective enough to prevent aggregation caused by attractive capillary forces generated by contact line roughness. These may be due to surface chemical heterogeneities15 and can lead to the formation of two-dimensional particle aggregates,10,15−17 often irreversibly bound. The impact that adsorption at a fluid interface has on sterically stabilized particles is even more complex and less well studied and understood in spite of their promise for materials design. Among different ways to achieve bulk steric stabilization, the encapsulation of nanoparticles in hydrogels (i.e., highly swollen and cross-linked polymer matrices) to produce core-shell nanoparticles (CSNPs) holds significant potential in tailoring their structural and dynamical properties.18 The presence of deformable, solvated shells with a thickness that can be controlled independently of the size of the hard nanoparticle core offers in fact many opportunities.19−22

1. INTRODUCTION Colloidal stability against aggregation is at the core of engineering the surfaces of microparticles and nanoparticles. Modifying the surface of colloids to introduce additional repulsive interactions is a necessity motivated by the requirement to curb attractive forces, including ubiquitous Londonvan der Waals forces.1 The two most commonly employed methods to provide colloidal stability in a suspension are charge and steric stabilization.2 These additional interactions subsequently play a role in defining the structure and dynamics of the system. Crucially though, and often underestimated, interactions at fluid interfaces (e.g., oil-water or air-water) may be very different from the ones in the bulk.3,4 Hence, colloids optimized for bulk stability and assembly may lead to different results upon adsorption and confinement at a fluid interface.5 First, additional interactions exist at interfaces that are not present in the bulk (e.g., attractive capillary forces6). Second, the presence of the interface may modify the magnitude and effect of interactions also existing in the bulk (e.g., van der Waals7 or electrostatic8). For the common case of charge-stabilized colloids (e.g., silica or latex particles), electrostatic repulsion at a fluid interface may be sufficient to screen capillary attraction and enable a broad range of stable two-dimensional particle assemblies,9−12 which © 2017 American Chemical Society

Special Issue: Early Career Authors in Fundamental Colloid and Interface Science Received: June 14, 2017 Revised: July 28, 2017 Published: July 28, 2017 886

DOI: 10.1021/acs.langmuir.7b02015 Langmuir 2018, 34, 886−895

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function of concentration, similar to our discussion of the bulk data. Our results show that hydrogel shells designed to provide colloidal stability in the bulk are actually responsible for longranged attractive capillary forces at the interface causing aggregation. This systematic comparison of bulk and interface microstructure for a well-defined model system of CSNPs affords a clean analysis of the different requirements for particle stability, and hence assembly, under different conditions.

The shells can be used to provide control of the interparticle spacing and thus guarantee the stability up to very high solid loading, even beyond the point of shell-shell contacts.23 Accurate tuning of interparticle distances is also of interest in sensing and optics applications.24−27 Temperature- or pHresponsive shells can further be used to control the effective particle size by external stimuli or to encapsulate active ingredients that are transported by the nanoparticle and that can be released using the nanoparticle core as an antenna to trigger temperature changes.28 These features also make CSNPs interesting to realize two-dimensional materials upon confinement at a fluid interface. Here, the core-shell nature of the particles has been exploited in applications such as oil recovery29 and photonics and sensing.30 Finally, theory and numerical simulations also predict a very rich phase diagram for two-dimensional core-shell systems.31,32 The adsorption of CSNPs at an interface also implies an additional complication compared to the case of charged colloids described above. Adsorption may in fact also alter the shape of the particles themselves, giving rise to further differences relative to the bulk. For a large range of systems, from small nanoparticles stabilized by molecular layers33−36 to microgels37 and CSNPs particles similar to the ones studied here,38 it has indeed been shown that preferential wettability for one of the two fluids determines an asymmetry of the particle shape upon adsorption at the interface. If the shell material is also surface-active, then adsorption is additionally associated with an increase of the in-plane diameter at the interface, with significant implications in terms of the effective particle size at the interface.26,37−41 Ultimately, all of these factors affect the dynamics and interactions and hence colloidal stability at the interface. Some nanoparticles that are sterically stabilized in the bulk in fact form 2D aggregates upon adsorption at fluid interfaces.42−44 This fact may be frequently overlooked if dense particle monolayers are prepared, with surface coverage corresponding to more or less close-packed monolayers. In this article, we systematically examine the differences between the bulk and interface structural behavior of CSNPs comprising a 351 nm diameter silica core embedded in a 339 nm thick poly-N-isopropylacrylamide (PNIPAM) shell. These particles are significantly larger than colloids used in previous studies38 but present a similar shell-to-core size ratio of λ = 2.9, defined as the ratio between the total particle diameter and the core diameter at 20 °C. While existing work focused on the effect of shell thickness on interfacial assembly via indirect imaging methods, here, by studying almost micrometer-sized CSNPs, we have both direct in situ access to the interface microstructure by confocal microscopy and a comparison across different size ranges. We first study the stability of the nanoparticles in bulk water suspensions by employing a recently developed microscopy technique, namely, confocal differential dynamic microscopy (ConDDM), which, akin to light scattering, is able to provide structural and dynamical information on the particle suspension.45 We then compare these results with the investigation of the particle behavior upon spontaneous adsorption at an oil-water interface. Here, we combine in situ investigations in a confocal microscope with the compression and transfer of a particle monolayer onto a silicon wafer by means of a Langmuir-Blodgett (LB) trough. The latter technique allows us to investigate the structure of the monolayer as a function of surface coverage and compression and thus to comment on the interparticle interactions as a

2. EXPERIMENTAL SECTION 2.1. Chemicals. Ammonium hydroxide solutions (NH3(aq), Sigma-Aldrich, 30−33%), tetraethylorthosilicate (TEOS, SigmaAldrich, 98%), rhodamine b isothiocyanate (RITC, Sigma-Aldrich, mixed isomers), (3-aminopropyl)trimethoxysilane (APS, SigmaAldrich, 97%), ethanol (EtOH, Sigma-Aldrich, ≥99.8%), 3-(trimethoxysilyl)propyl methacrylate (MPS, Sigma-Aldrich, 98%), N-isopropylacrylamide (NIPAM, Sigma-Aldrich, 97%), N,N″-methylenbis(acrylamide) (BIS, Sigma-Aldrich, 99%), sodium dodecyl sulfate (SDS, Merck, Ph. Eur.), and potassium peroxodisulfate (PPS, Fluka, ≥99%) were used as received. Hexadecane as obtained from the supplier (Acros Organics, 99%) was further purified to remove surface-active contaminants by passing it through a column containing alumina powder (EcoChrom, MP Alumina B Act.1) and silica gel 60 (Merck). Water was purified using a Milli-Q system, resulting in a final resistivity of 18 MΩ cm. 2.2. Synthesis and Functionalization of the Silica Particles. Silica particles labeled with a fluorescent dye were prepared on the basis of a recently reported protocol.38 Prior to silica synthesis, an ethanolic solution of dye RITC (10 mM) was functionalized with APS, which was added in 10-fold excess to guarantee covalent binding to the RITC molecules. After stirring in the dark for at least 2 h, 500 μL of the solution was diluted with ethanol (1:5). The silica particles were synthesized according to the well-known Stöber method.46 An ammonium solution (27.5 mL, 30−33%) and 16.1 mL of water were added to 56.4 mL of ethanol in a three-necked round-bottomed flask and heated to 50 °C. After equilibration for 20 min, a mixture of 40.2 mL of ethanol and 10.1 mL of TEOS was heated to 50 °C and quickly added to the mixture. As soon as the clear, colorless solution became turbid, the dye solution was added dropwise within 10 min. The reaction was allowed to proceed for 24 h at 50 °C. After cooling to room temperature, the reaction mixture was purified twice by centrifugation at a relative centrifugal force (rcf) of 1664 for 20 min. Redispersion occurred in ethanol. A detailed description of the functionalization of the silica particles is reported in ref 38. The final particle number concentration of the functionalized silica seed stock dispersion was 0.0184 μM. The average diameter of the silica core (later on referred to as the core diameter) was determined to be 351 ± 16 nm by measuring the size of 170 particles from SEM images using the software ImageJ. (See the Supporting Information for additional details.) 2.3. Synthesis of the SiO2-PNIPAM Particles. The silica particles were encapsulated using free radical seeded precipitation polymerization in a semibatch fashion. The reaction setup consisted of a three-necked round-bottomed flask equipped with a reflux condenser, a magnetic stirrer, and a syringe pump. NIPAM (113 mg), BIS (7.7 mg), and SDS (1.2 mg) were dissolved in 20 mL of water. The mixture was heated to 70 °C and purged with nitrogen. After the mixture was equilibrated for 30 min, 1018 μL of the silica seed stock dispersion was added dropwise. After further equilibration for 10 min, the reaction was initiated by 2 mg of PPS dissolved in 1 mL of water. Four sequential additions of the monomers, SDS and PPS, were necessary to obtain large PNIPAM shells. These additions were performed in the following way: 45 min after the initiation, the respective mass of SDS dissolved in 2 mL of water was dropped into the reaction mixture (Table 1). Then, the respective amount of NIPAM in 4 mL of water was added continuously using a syringe pump over a time period of 30 min. The appropriate mass of BIS was dissolved in 2 mL of water and added dropwise after the NIPAM addition was started. Subsequently, PPS dissolved in 1 mL of water 887


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DDM is its applicability to dense turbid suspensions without the effect of multiple scattering. Details of the DDM analysis applied to our system are described in Section 3.1. Briefly, in a typical DDM analysis, a time series of real-space images of the CSNP suspensions, I(x, y; t), are Fourier transformed to obtain I(̃ q⃗, t), where q⃗ is the wave vector. Subsequently, time-averaged and azimuthally averaged operations are performed on I(̃ q⃗, t) to extract the dynamic image structure function (DISF) as a function of the lag time, Δt: D(q, Δt) = ⟨|I(̃ q⃗, t + Δt) − I(̃ q⃗, t)|2⟩ for the different q values. The algorithm for performing the ConDDM analysis was implemented in MATLAB, wherein the timeaverage operations for each lag time Δt were parallelized using a Tesla K40c graphical processing unit. 2.7. Confocal Measurements. All of our measurements were performed using a Nipkow spinning disk confocal microscope (CSUW1, 3i), which includes a 561 nm diode-pumped solid state laser with a power of 200 mW, a Hamamatsu Orca-flash 4.0 V2 camera, and a 60× oil-immersion objective with a numerical aperture of 1.4. To characterize the suspension behavior using ConDDM, time-lapse movies containing 10 000 images, 300 × 300 pixels2 each, were acquired at 66.5 frames per second. Prior to the acquisition of timelapse movies, which were eventually used for ConDDM analysis, the particles in the field of view were forcefully photobleached until the average image intensity with time remained nearly constant. For the confocal measurements performed at the water−hexadecane interface, a 60× water-immersion objective with a numerical aperture of 1.2 and a working distance of 0.32 mm was used to acquire images at 7 frames per second. 2.8. Monolayer Compression, Deposition, and Analysis. CSNP monolayers were prepared, compressed, and deposited from a water-hexane interface in a Langmuir-Blodgett (LB) trough (KSV 5000) equipped with a customized deposition well. The particles were deposited on 1 × 2 cm2 silicon wafers cleaned by 15 min of ultrasonication in toluene (Fluka Analytical, 99.7%), isopropanol (Fisher Chemical, 99.97%), and Milli-Q water, followed by drying under a N2 stream and UV-ozone cleaning for 30 min. The area of the trough was 197.5 cm2 and it could be compressed down to 57.5 cm2 using two movable Delrin barriers. The surface pressure was measured with a roughened platinum Wilhelmy plate. The silicon substrate was screwed onto a Teflon dipper arm/sample holder at an angle of 30° with respect to a plane parallel to the interface. Before each experiment, all of the components of the LB trough (trough, barriers, Wilhelmy plate and dipper) were thoroughly rinsed with ethanol (Fluka Analytical, 99.8%) and Milli-Q water and dried with a N2 gun. After the lower part of the trough was filled with Milli-Q water, the barriers were closed once to check for any surfaceactive contamination. If the surface pressure was lower than 0.20 mN/ m, the interface was considered clean. If this criterion was not met, then the surface was aspirated using a 1 mL pipet tip (Tip One) attached to a vacuum pump (Vacuum Brand PC3000) and checked again. At this point, the barriers were fully opened and the dipper arm was positioned such that the specimen was just below the water surface. Hexane (100 mL) was then slowly added to create the oil− water interface. The surface pressure was zeroed, and CSNP solution was added to the interface with a 100 μL glass syringe (Hamilton 710N 100 μL) that had been rinsed with ethanol and Milli-Q water. For the spreading at the interface, different volumes of 1.37 wt % 80:20 water−isopropyl alcohol were used. After a few minutes of equilibration, the compression/deposition experiments were initiated. The barrier compression speed was set to 2.3 mm/min, and the dipper moved up at a constant speed of 0.3 mm/min. These velocities were chosen so that the specimen would be completely out of the water phase as the interface reached full compression. As a consequence of the simultaneous compression and extraction, different positions on the silicon wafer corresponded to different values of surface pressure and coverage, allowing for smoothly screening the microstructural evolution of the monolayer under compression. The overall deposition procedure lasted approximately 45 min, after which the water and hexane could be removed through aspiration. Once the trough was empty, the sample could be carefully removed, dried under N2, and transferred to the SEM for imaging. Before imaging, the specimen was

Table 1. Amounts of Added Chemicals during the Sequential Addition Steps for the Growth of the PNIPAM Shell addition step

mSDS (mg)

mNIPAM (mg)

mBIS (mg)

mPPS (mg)

1 2 3 4

1.7 2.3 2.9 3.5

170 226 283 339

11.6 15.4 19.3 23.1

3 4 5 6

was added quickly, and the reaction proceeded for 45 min until the next addition was performed. The respective masses of monomer, surfactant, and initiator for these additions are listed in Table 1. After the last addition, the reaction was allowed to proceed for 2 h. After cooling, the dispersion was centrifuged four times (1664 rcf, 30 min) to remove residues from the reaction. After each centrifugation step, the supernatant was discarded and the particles were redispersed in water. 2.4. Dynamic Light Scattering. Dynamic light scattering measurements on dilute suspensions were performed to determine the hydrodynamic diameter of the CSNPs as a function of temperature. The measurements were performed using a Zetasizer Nano ZS device (Malvern) equipped with a 633 nm laser. The measurements were conducted at a scattering angle of 173° in a temperature range of 20 to 60 °C in steps of 5 °C. At each temperature, the sample was equilibrated for 10 min prior to the conduction of five individual measurements. Intensity time autocorrelation functions were recorded using acquisition times of 60 s. The data were analyzed by the standard cumulants method provided by the instrument software. The average hydrodynamic radius at 20 °C was 514 ± 5 nm. The standard deviation was determined from the zaverage values of the five individual measurements. More details on the particle size at different temperatures are given in the Supporting Information. 2.5. Preparation of Sample Cells and Suspensions. The suspensions were prepared by the addition of appropriate amounts of freeze-dried CSNPs to Milli-Q water in an Eppendorf tube. (See the Supporting Information for more details.) The resulting suspension was ultrasonicated for 10 min and subsequently mixed at 20 °C for a time period of 12 h using an Eppendorf ThermoMixer. The samples for the ConDDM measurements were prepared by filling hollow rectangular glass capillary tubes (VitroCom) with dimensions of 100 μm × 50 mm × 2 mm (thickness, length, and width, respectively) with CSNP suspensions. Before being filled, the capillary tubes were cleaned with ethanol and isopropyl alcohol and profusely rinsed with Milli-Q water. After filling, the ends of the capillaries were sealed and attached to a rectangular glass cover slide by using a UV-curable glue (Norland Optical Adhesive NOA 81). In order to image the CSNPs at the water-hexadecane interface, we used custom-made sample cells by sticking a thin (0.08−0.12 mm, Thermo Scientific) cover glass with a 10 mm diameter hole (machined with a Hunst laser cutter equipped with a CO2 laser with a wavelength of 10600 nm) and an aluminum ring with an inner diameter of 20 mm to a cover glass of 40 mm diameter. This design enabled us to obtain interfaces 180 to 230 μm away from the bottom cover glass, thus allowing the use of a high-numerical-aperture 60× water-immersion objective for imaging. We prepared Gibbs monolayers using an aqueous CSNP suspension with a concentration of 3 × 10−3 wt %, which was used to fill the inner cavity of the sample cells. The oil phase was subsequently added on top of the water phase to create a nearly flat liquid-liquid interface. The particles present in the water phase spontaneously adsorbed to the liquid-liquid interface to create a monolayer. The sample cell was further sealed from the top using a glass cover slide to avoid convection. 2.6. Confocal Differential Dynamic Microscopy. We used confocal differential dynamic microscopy (ConDDM) as the technique to characterize the dynamics and static structure properties of CSNP suspensions in bulk Milli-Q water. ConDDM is an extension of the originally developed bright-field DDM to a confocal microscope.45,47 The main advantage of ConDDM over bright-field 888


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Figure 1. Dynamic image structure function D(q, Δt) behavior for CSNP suspensions at three different wave vectors q as a function of lag time Δt. (a−e) ϕ = 0.015, 0.042, 0.260, 0.337, and 0.462, respectively, as shown in the corresponding confocal fluorescence images. The q values for the DISFs are chosen around the peak value of A(q) for all volume fractions. The empty symbols are the experimental data, and the solid black lines are fits of the experimental data to eq 1. The images are obtained using a 60× oil-immersion objective at an exposure time of 10 ms. sputter-coated with a 3 nm platinum layer. SEM micrographs (1024 × 700 pixels2) were taken every 1 mm along the compression direction and analyzed to count the deposited particles and extract their coordinates using a custom MATLAB code.

associated with the images. When β = 1, we recover the DISF for dilute suspensions, which exhibit simple diffusive behavior.47,48 Crucially, the fact that a single relaxation time can fit the DISFs at all volume fractions is a strong indication of the absence of aggregation. The presence of aggregates of different sizes would in fact impose a broad distribution of relaxation times for any given q.49,50 These conclusions are further supported by the absence of any visual detection of aggregates in the confocal images used for the DDM analysis. For the most dilute suspension with ϕ = 0.015 (Figure 1a), we observe that an increase in the q value results in a decrease in both the long-time limit of D(q, Δt) and the relaxation time τ(q). This implies that both the amplitude and the characteristic time scale of density fluctuations decrease with decreasing length scale, as one would expect for the dilute limit. Both of these trends are lost for denser suspensions, where structural effects come into play. In particular, the extent of deviations from the trends increases for higher volume fractions. To show this fact explicitly, DISF data are shown for different values of q for each suspension in Figure 1b−e. Here, we notice that the dependence of the long-time plateau value of D(q, Δt) and relaxation time τ(q) becomes nonmonotonic in q. This is a strong indicator for structuredependent dynamics of the system at higher volume fractions. These observations are further confirmed by examining the q dependence of A(q) for the studied volume fractions, which is a measure of the difference in the long-time and short-time limits of D(q, Δt). Consequently, for all volume fractions with the exception of ϕ = 0.015, non-monotonic behavior of A(q) can be observed in Figure 2a. A maximum, which becomes clearer at higher volume fractions, develops at a particular q value for a given volume fraction, which increases with ϕ, indicating the emergence of a decreasing structural length scale with increasing particle concentration. This feature is highlighted even more clearly in Figure 2b, which shows the form of A(q) for different ϕ values normalized by Adil(q), i.e., the values of A(q) for the most dilute suspension with ϕ = 0.015. This ratio, obtained directly from experimental measurements, is qualitatively equivalent to the static structure factor (i.e., the q position of the peak is the same for both functions; see the

3. RESULTS AND DISCUSSION 3.1. Dynamic Image Structure Functions (DISFs) in the Bulk. The starting point of this study is the investigation of the behavior of our CSNPs in aqueous suspensions for different values of the volume fraction. This will allow us to evaluate if they can be considered to be strongly stable against aggregation in the bulk, thus establishing a useful benchmark to compare with their behavior at the oil-water interface. The details of the determination of the suspension’s effective volume fraction ϕ are given in the Supporting Information. Starting from timelapse sequences of confocal images such as the ones shown in Figure 1, we calculated the dynamic image structure function (DISF), D(q, Δt), using ConDDM analysis. D(q, Δt) is a direct measure of intensity fluctuations in the images, which are connected to particle density fluctuations. This function is obtained from the ConDDM analysis as a function of the magnitude of the wave vector q⃗, i.e., the length scale of the density fluctuations, and of the lag time between images Δt, i.e., the time scale of the density fluctuations. The bottom row of Figure 1 shows the DISF obtained for different volume fractions ϕ for a given choice of representative q values. From the experimental data for the DISF (empty symbols in Figure 1), it can be observed that, for all volume fractions and wave vectors, the intensity (particle density) fluctuations relax to an equilibrium state via a single relaxation time. The experimental DISFs are well described by a single expression with the functional form shown below in eq 1 D(q , Δt ) = A(q)[1 − exp( −Δt /τ(q))β(q)] + B(q)

(1)

where A(q) is a qualitative measure of the long-time limit of the density fluctuations and, as detailed later, depends on the microscope transfer function, the particle form factor, and the static structure factor of the suspension. τ(q) is the characteristic relaxation time for a given value of q, β is a q-dependent stretching exponent, and B(q) is the background noise 889


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Figure 3. Stretching exponent β at different wave vectors q and volume fractions ϕ.

lowest volume fraction, corresponding to what we earlier defined as the dilute case, we see that, within the experimental errors coming from the DISF fitting, β(q) is equal to 1 for all q. An exponent of 1 reduces the expression for the DISF reported in eq 1 to the one for purely diffusive dynamics.45,47,48 Values of β(q) lower than 1 indicate that the dynamics is slower than simple diffusion, hinting toward a volume-fraction-dependent slowing of the dynamics to subdiffusive behavior. As the volume fraction increases from 0.260 to 0.462, we observe that density fluctuations become increasingly more subdiffusive, especially for larger length scales (i.e., low q). Additionally, the range of length scales associated with subdiffusive behavior broadens with increasing volume fraction due to crowding. The observations relative to the data in Figure 3 can be further confirmed by looking at the relaxation time τ at different q values for each volume fraction, as displayed in Figure 4. In line with what is reported above, the dilute CSNP suspension at ϕ = 0.015 gives τ(q) ≈ 1/q2 over the entire q range, following the expected behavior for simple diffusive dynamics. For higher volume fractions, we observe an additional increase in the relaxation time τ for q values around the position of qmax in A(q)/Adil(q) (Figure 2b). This additional increase in τ around

Figure 2. (a) A(q) vs q behavior for different volume fractions ϕ obtained by fitting experimental DISF data to eq 1. (b) Ratio A(q)/ Adil(q), which is a measure of the static structure factor for different volume fractions ϕ.

Supporting Information). The observation of a single peak in Figure 2b serves as an additional indication for the bulk stability of the CSNPs up to very high volume fractions. 3.2. Bulk Particle Dynamics. The conclusions drawn above from the behavior of the DISF can be further expanded by examining the particle dynamics in more detail. ConDDM has the significant advantage to be able to monitor dynamics in parallel with structure by looking at the following two parameters of the DISF reported in eq 1: the stretching exponent β and the relaxation time τ. Both quantities are in general q-dependent and connected to the nature of the collective particle motion. Starting with the stretching exponent, Figure 3 shows the values of β(q) for different volume fractions ϕ as a function of the length scale. Several observations can be made by examining the figure. The first point is that the values of β(q) have a different dependence on q for the various volume fractions. For the lowest ϕ (i.e., 0.015 and 0.042), the values of β(q) are basically constant over the whole accessible q range. For higher values of ϕ, the stretching exponent instead exhibits non-monotonic behavior, and it decreases significantly at low q. Another important point concerns the values of β(q). For the

Figure 4. Relaxation time τ for different wave vectors q and volume fractions ϕ. 890


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hard core, still have a core-shell morphology.58,59 In both cases, attraction has been ascribed to capillary forces caused by a local deformation of the interface as a consequence of the wetting of the deformable shell. Further compression of the interface compacts this open-particle network until all voids are filled and a homogeneous monolayer is obtained. At this stage, compressing the interface even further causes a steep rise in the surface pressure until a critical value is reached and a kink is observed in the curve. To investigate in closer detail the reason for these different responses, we report the nearest-neighbor interparticle distance d as a function of Ap in Figure 6. From these data we see that, at

qmax is a structure-induced effect and is commonly referred to as de Gennes narrowing.51−55 In addition to this, we can also note that for q < 4 μm−1 there is an increasingly larger deviation from a slope of −2 for increasing ϕ, confirming the subdiffusive relaxation of density fluctuations over these length scales. The data reported in Figure 4 above, taken together with the discussion of the results presented in Figure 1, unambiguously show that particles remain mobile, even at volume fractions for which crowding effects render their motion subdiffusive, emphasizing, as expected, stability and the absence of aggregation. 3.3. Structure at the Oil-Water Interface. Using the bulk characterization as a starting point, we now move to the characterization of monolayers of our CSNPs prepared at oilwater interfaces as described in Section 2. To this end, we employed two different approaches: compression and deposition from a LB trough38,56,57 and direct observation with a confocal microscope. Beginning from the first experiments, Figure 5 shows a compression curve of our CSNPs at a hexane-

Figure 6. Nearest-neighbor interparticle distance d versus Ap for our CSNPs at the hexane−water interface. The different symbols correspond to different initial amounts of spread particles as in Figure 5.

Figure 5. Surface pressure Π versus specific area Ap compression curve of our CSNP at a hexane−water interface. The different symbols correspond to different initial amounts of spread particles from a 1.37 wt% particle dispersion (red ■, 20 μL; green ●, 50 μL; and blue ▲, 110 μL). The insets show corresponding SEM images at various surface pressures. Scale bars for the top images are 2 μm. Scale bars for the right images are 10 μm.

low surface coverage, d is independent of the number of particles per unit area (filled red squares). This is a consequence of aggregation, where neighboring particles within the aggregates are found at a well-defined distance of approximately 1.4 μm, corresponding to the point at which steric repulsion from the shell at the interface counterbalances the attractive forces. Note that this value is significantly larger than the hydrodynamic diameter in the bulk, confirming the deformation and stretching of the shells upon adsorption at the interface. Further compression only increases the surface coverage, but the interparticle separation remains unaltered. This continues until the full interface is covered by a uniform monolayer. Starting from this characteristic value of Ap, there is a very small region of coverage where the contacting shells of the NPs get compressed and d slightly decreases, before something rather drastic happens. This point corresponds to the clustering of some of the particles into close-packed aggregates. This intermediate region is much less pronounced compared to the corresponding region for core-shell microgels without a rigid core59,60 and for smaller CSNPs with similar values of λ,38 indicating the existence of nontrivial effects connected to the absolute core size and compressibility. From this point on, two distinct values of d are detected: one corresponding to the particles with extended shells in contact (filled green circles and blue triangles) and one corresponding to particles with compressed shells in contact (empty green

water interface. Because of the limited area of the LB trough used for the experiments, the whole compression curve cannot be accessed in a single experiment and it is instead acquired piecewise by merging different experiments with different initial amounts of CSNPs spread at the interface, as represented by the different symbols. SEM images of the CSNP monolayers deposited at varying compressions are reported. The morphology of the monolayers and the corresponding compression curves clearly highlight differences unexpected from the bulk behavior of the CSNPs. The first significant difference that we observe is the fact that at low surface coverage (i.e., high area per particle Ap), the particles do not uniformly occupy the whole available space at the interface, but they rather form a network of 2D percolating aggregates with large voids. The existence of these structures is directly connected to the presence of attractive forces between the CSNPs, which, albeit absent in the bulk, appear when the colloids adsorb at the oil-water interface. The presence of such attractive forces has been recently shown for smaller CSNPs of a similar type38 and also for microgels, which, even without a 891


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Figure 7. Time-lapse image sequence of a single CSNP adsorption event at a hexadecane−water interface. The adsorbing particle is highlighted in red, and it goes from freely diffusing in the water subphase below the interface (a) to adsorbing at the interface (b) and being attracted by an existing aggregate (c). After a small rearrangement (d), the particle remains permanently trapped in position (e).

circles and empty blue triangles). The latter value of approximately 700 nm is basically constant from this point on, whereas at a critical surface pressure the former one starts to decrease significantly. This can be attributed to the fact that, initially, compression causes only a growth of the number of particles in the close-packed clusters, but when a sufficient number of them is formed, further compression can proceed only if the clusters not only grow but also get closer to each other (i.e., below the initial interparticle separation at low surface coverage corresponding to extended shell contacts). The onset of this regime corresponds to the position of the kink in the compression curve of Figure 5. Correspondingly, the transitions between the other two regions are also highlighted by the vertical dashed lines in Figures 5 and 6. In addition to the investigations reported here on the monolayers after deposition on a silicon substrate, we have also performed in situ analysis in the low-surface-coverage region by means of confocal microscopy. Here, instead of spreading the particles at the interface, we examine their behavior upon spontaneous adsorption from the bulk aqueous subphase, as described in Section 2. Representative images of the CSNPladen oil-water interface are given in Figure 7. As a consequence of sample preparation, a given number of particles are already unavoidably present at the interface before imaging can start. What we always observe is that the interface is instantly covered by a percolating network of 2D particle clusters, analogous to the ones previously discussed. This confirms that the structures we observed on the substrates are not the result of artifacts and also confirms the existence of attractive forces driving the aggregation. The latter observation can be even further confirmed by examining the time-lapse image sequence reported in Figure 7, which depicts a single adsorption event. The red circle and arrow highlight one individual CSNP that at instant t = 0 s is freely diffusing below the interface plane. The particle briefly disappears from the field of view and 3.9 s later reappears in the field of view and adsorbs at the interface. Within less than a second, the particle is attracted toward an existing aggregate and, after a small rearrangement, remains permanently trapped in place. 3.4. Comparison of the Structure in the Bulk and at the Interface. We conclude the discussion by comparing the structural behavior of the CSNPs at a liquid-liquid interface and in the bulk. Figure 8 shows the bulk static structure factor extracted from ConDDM analysis (empty symbols) and the S(q) of the interfacial particle monolayer for the six different values of the surface coverage corresponding to the SEM images in Figure 5 (solid lines). The details of the S(q) calculation in the bulk and from 2D images are provided in the

Figure 8. Comparison of the static structure factors S(q) for CSNPs at the interface (solid lines) and in the bulk (empty symbols) as a function of the number density of particles ρn. The number density is expressed as particles/μm2 and particles/μm3 at the interface and in the bulk, respectively.

Supporting Information. In order to compare the systems, the values of area per particle and volume fraction have been converted to particle number density per unit area and per unit volume ρn, respectively. Several observations can be made. First, it appears clear that a larger q range is available from SEM images (due to the smaller physical pixel size), and thus the higher-order oscillations in S(q) are visible. We then observe the presence of two main peaks of S(q) at the interface: the first one at low q corresponds to the extended shells of the particles being in contact and the second one at larger q indicates closely packed particles. The former peak is present only at low number densities, and the latter one grows in amplitude for increasing surface coverage due to the growing number of particles belonging to close-packed clusters. The fact that the position of the first peak at the interface does not move as a function of ρn emphasizes again that the particles are arranged at a characteristic distance in the aggregates below full interface coverage. Finally, we see that the position of the low-q peak at the interface is shifted toward larger distances compared to the position of the bulk peak. This confirms once more that the particles deform upon adsorption at the interface and that their effective size at the interface is larger than in the bulk, as commonly found for a broad class of deformable surface-active objects described in Section 1. In particular, the bulk particle size falls in between the extended size at the interface and the collapsed state. 892


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4. CONCLUSIONS This work provides a direct comparison between the behavior of model CSNPs, having a silica core and a soft, cross-linked PNIPAM shell, in the bulk and after adsorption at oil−water interfaces. The bulk behavior was explored by confocal differential dynamic microscopy, and our work illustrates the potential of this relatively new experimental technique to provide comprehensive descriptions of dilute and dense suspensions of complex nanoparticles in an easily accessible way. In particular, our experiments demonstrate that, as expected, the PNIPAM shell gives excellent colloidal stability to the NPs even in concentrated suspensions. The measured dynamical and structural properties all support the absence of bulk aggregation over a broad volume fraction range. At odds with their bulk behavior, the particles show very different properties upon adsorption at a fluid interface and a very different response to increasing area fraction. More specifically, the presence of the soft shell, which acts as a steric stabilizer in the bulk, deforms upon adsorption at the interface and triggers attractive capillary forces, which cause the formation of twodimensional aggregates, even at very low surface coverage, and eliminate long-range particle diffusivity at the interface. The interparticle separation in the aggregates at low surface coverage is independent of the area fraction and corresponds to a distance where the attractive forces are balanced by the steric interactions between spread-out shells at the interface. In addition to this, area fractions beyond the formation of uniform monolayers with all particles in steric shell contacts can be easily achieved by interface compression in an LB trough. The shells respond highly nonlinearly to compression, with sudden localized collapses that lead to the nucleation and growth of 2D particle clusters in close contact. Interestingly, even if the qualitative behavior under compression is similar to that of smaller CSNPs with analogous relative core-to-shell sizes, the different absolute core size affects the mechanical response at the interface, imposing a much narrower region of monolayer compression before the collapse of local contacts. These results open numerous directions for further research. On one hand, the description of the interplay between particle architecture and interparticle interactions at the interface certainly deserves further work. On the other hand, relating these aspects to the microstructure of the interfaces has significant merit in harnessing the interactions between CSNPs to obtain monolayers with specific properties. But perhaps the most intriguing consequence of these findings is the emphasis on the fact that the same design rules used to tune the colloidal properties of particles in the bulk do not necessarily apply to particles adsorbed at fluid interfaces. These observations therefore appeal to the colloidal chemistry community involved in the synthesis of particles for the fabrication of 2D nanostructured materials in order to develop new systems where the same exquisite control achieved in the bulk can be transferred to fluid interfaces.

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static structure factor of hard-sphere suspensions. Structural features of CSNPs at interfaces. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Siddarth A. Vasudevan: 0000-0003-2471-2793 Matthias Karg: 0000-0002-6247-3976 Lucio Isa: 0000-0001-6731-9620 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors kindly acknowledge Roberto Cerbino, Fabio Giavazzi, Roberto Piazza, Stefano Buzzaccaro, and Daniele Vigolo for useful discussions on differential dynamic microscopy. L.I. and S.A.V. acknowledge financial support from Swiss National Science Founadtion grant PP00P2_144646/1 and ETH Research Grant ETH-13 14-1. M.K. acknowledges financial support from the German Research Foundation (DFG) through the Emmy Noether-Programm (KA 3880/1). André Studart and the ETH Scientific Center for Optical and Electron Microscopy (ScopeM) are acknowledged for access to instrumentation and technical support.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b02015. Core-shell nanoparticle and suspension characterization. Hard-sphere mapping. Percus-Yevick predictions for the 893


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