Hollow and Core–Shell Microgels at Oil–Water Interfaces: Spreading

Nov 17, 2015 - A comparison of the core–shell and the corresponding hollow microgels gives valuable information about the mechanical properties of t...
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Hollow and Core−Shell Microgels at Oil−Water Interfaces: Spreading of Soft Particles Reduces the Compressibility of the Monolayer Karen Geisel,† Andrey A. Rudov,‡,§ Igor I. Potemkin,*,‡,§ and Walter Richtering*,† †

Institute of Physical Chemistry, RWTH Aachen University, Aachen, Germany Physics Department, Lomonosov Moscow State University, Moscow 119991, Russian Federation § DWI−Leibniz Institute for Interactive Materials e.V., Aachen, Germany ‡

S Supporting Information *

ABSTRACT: We investigate the influence of a solid core and of the cross-link density on the compression of microgel particles at oil−water interfaces by means of compression isotherms and computer simulations. We investigate particles with different morphology, namely core−shell particles containing a solid silica core surrounded by a cross-linked polymer shell of poly(N-isopropylacrylamide), and the corresponding hollow microgels where the core was dissolved. The polymer shell contains different amounts of cross-linker. The compression isotherms show that the removal of the core leads to an increase of the surface pressure at low compression, and the same effect can be observed when the polymer cross-link density is decreased. Low cross-link density and a missing core thus facilitate spreading of the polymer chains at the interface and, at high compression, hinder the transition to close hexagonal packing. Furthermore, the compression modulus only depends on the cross-link density at low compression, and no difference can be observed between the core−shell particles and the corresponding hollow microgels. It is especially remarkable that a low cross-link density leads to a high compression modulus at low compression, while this behavior is reversed at high compression. Thus, the core does not influence the particle behavior until the polymer shell is highly compressed and the core is directly exposed to the pressure. This is related to an enhanced spreading of polymer chains at the interface and thus high adsorption energy. These conclusions are fully supported by computer simulations which show that the cross-link density of the polymer shell defines the degree of deformation at the interface. Additionally, the core restricts the spreading of polymer chains at the interface. These results illustrate the special behavior of soft microgels at liquid interfaces.

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deformability.19 In contrast to solid particles that are used to stabilize conventional Pickering emulsions,20,21 microgels deform once adsorbed to liquid interfaces. They adopt a core−corona structure irrespective of their original morphology or the presence of charges.22,23 The properties of a polymer-coated solid particle are thus not only determined by the solid core but also by the softness of the polymer shell, and this generates differences to the behavior of hard particles. It was shown that a polymer shell facilitates the adsorption to liquid interfaces,24−26 determines the particle separation,27−29 and influences the particle arrangement at different compression states.30,31 Theoretical approaches underline these experimental results and report that the polymer shell stretches at the interface to cover a large area, resulting in a high affinity to interfaces.32−34 Removal of the solid core leads to hollow microgels or microgel capsules that present a new class of temperatureresponsive nanoparticles. Their cavity makes them ideal carriers for applications like drug delivery and targeted release.35−38

INTRODUCTION Microgels are soft, cross-linked polymer particles that show unique behaviors at liquid interfaces. They are investigated intensively, and several reviews provide excellent insight into recent results.1−5 Microgels show temperature responsiveness if they contain N-isopropylacrylamide (NiPAm) as main monomer and possess a volume phase transition temperature (VPTT) of 32 °C in water. The particles collapse above this temperature and reduce their size in bulk drastically.6−9 For instance, microgel-stabilized emulsions possess special properties when stimuli-sensitive microgels are used. Then emulsions can be broken on purpose and specific phase separation can be achieved.10−13 The behavior of microgels at liquid interfaces was investigated with respect to, for example, the adsorption energy14 and the influence of charges,15 and it was shown that charged microgels do not repel each other but behave similar to their uncharged counterparts.16,17 The special behavior of charged microgels is also shown by the fact that they adsorb very strongly to droplet surfaces and droplets covered with oppositely charged microgels do not coalesce. Instead, the droplets can be squeezed and deformed without rupture of the interfacial microgel layer.18 Another important aspect that influences the interfacial behavior of microgels is their © 2015 American Chemical Society

Received: September 23, 2015 Revised: November 17, 2015 Published: November 17, 2015 13145

DOI: 10.1021/acs.langmuir.5b03530 Langmuir 2015, 31, 13145−13154

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characterization, and Millipore water was used as subphase in the Langmuir trough. Synthesis and Characterization of the Core−Shell and the Hollow Microgels. The core−shell and hollow-sphere particles were synthesized as published previously.41,42 In brief, the silica core particles were synthesized following the well-known Stoeber process of a condensation of TEOS in ammonia and ethanol.46 The surface of the particles was afterward modified with MPS. The shell was polymerized around the cores by a reaction of NiPAm, BIS, and the stabilizer PVP and was initiated with KPS, resulting in the core−shell (CS) particles, which were cleaned by centrifugation and redispersion. The polymer shell contains different amounts of the cross-linker BIS: 5 mol % (CS5), 10 mol % (CS-10), and 15 mol % (CS-15). Exact amounts are given in Table S1. The hollow (HS) microgels were prepared by stirring lyophilized CS particles in highly diluted hydrofluoric acid as reported previously.39−41 The particles were stirred overnight, dialyzed against bidistilled water, and lyophilized. The hydrodynamic radius of the core−shell and the hollow microgels was determined with dynamic light scattering (DLS) on an ALV setup with ALV 5000 goniometer, He−Ne laser (λ = 633 nm), and external temperature control. Measurements were performed at three angles at each temperature, and a linear fit of the decay rate Γ as a function of the squared scattering vector q2 gave the diffusion coefficient D. Rh is calculated from D using the Stokes−Einstein equation (Table S2). Thermogravimetric analysis (TGA) was performed on a PerkinElmer TGA STA6000 Simultaneous thermal analyzer equipped with a TL 8000 balanced flow FT-IR evolved gas analysis system with FTIR spectrometer (Frontier). The weight loss was monitored from 30 to 900 °C with a heating rate of 10.00 °C/min in a nitrogen atmosphere (20 mL of N2/min). All core−shell particles contained residual water of 7.4 ± 0.6 wt %. It was assumed that the hollow microgels in the dried state contain the same amount of water as the core−shell particles because both types were freeze-dried following the same procedure. Thus, the mass of microgel that was added to the interface (Table S3) was not corrected for residual water. The mass fraction of the core was calculated from the onset points of the pyrolysis around 350 °C and the complete combustion of organic material around 700 °C. Exemplary TGA curves are given in the Supporting Information (Figure S1). Compression Isotherms. The setup and the measurement procedure were described in detail previously.44 The compression isotherms were measured on a Langmuir trough for liquid−liquid interfaces with a compressible area of 398 cm2. The trough is made of Delrin and equipped with two movable Delrin barriers. The surface pressure is measured with a platinum Wilhelmy plate that is placed in between and parallel to the barriers. The trough is filled with Millipore water, and the surface is cleaned with a suction pump. Then, the plate is lowered to the surface, and n-decane was carefully poured onto the water surface. The n-decane was cleaned from polar contaminants by filtering it three times over a column of basic Al2O3 prior to use. The balance is set to zero, and the microgel dispersion is added dropwise to the interface with a Hamilton syringe. The microgel dispersion was prepared by mixing a 0.5 wt % dispersion in water with methanol in the ratio 5:1 (v:v). Methanol was used to facilitate spreading of the microgels at the interface. Isopropyl alcohol, which is usually used as spreading agent, could not be used because it induced flocculation of the microgels. Different volumes of the microgel dispersion, i.e., different amounts of microgels at the interface, were used to cover the whole range of compressible area. Compression was performed at 20 °C with a barrier speed of 10 mm/min. Computer Simulations. Molecular dynamics simulations of microgels in solutions with implicit solvent molecules were performed using a standard coarse-grained model. The monomer units of the microgels were modeled as Lennard-Jones particles (beads) of the diameter σ. The microgels were designed as follows. Fully stretched subchains of an ideal microgel (all subchains have equal length) were connected through tetrafunctional cross-links and repeated a unit cell of the diamond crystal lattice.47,48 Then, a cubic frame consisting of 9 × 9 × 9 unit cells of the diamond crystal lattice was constructed. The

Different types of hollow microgels have been prepared by using solid particles as templates and subsequent dissolution of the template yields pitted, hollow particles.39−42 Recently, small-angle neutron scattering (SANS) experiments revealed that the size of the void after removal of the core particle is smaller than the initial template. The polymer shell swells toward the interior, thus reducing the size of the void.41 The effect of the cross-link density was also shown to be of great importance for the size of the void and the resulting stability of the hollow microgels.41,43 It is anticipated that the interfacial behavior of solid core− polymeric shell microgels and their corresponding hollow counterparts is significantly different from that of homogeneous microgels or solid particles due to the complex interplay of soft and hard moieties. A Langmuir trough offers the possibility to investigate the compression of particle monolayers by monitoring the surface pressure during compression of the monolayer. Compression isotherms of conventional microgels showed that considerable deformation takes place during the compression.16,44,45 Additionally, the isotherms of core−shell particles with solid core and soft polymer shell already revealed that the shell defines the particle’s behavior and that distinct differences to hard particles exist.30 These authors already claimed that the particle interaction changes from soft to hard sphere repulsion during the compression. Similar results were obtained for particles containing a gold core, where Langmuir− Blodgett experiments clearly revealed that the particle arrangement at the interface is governed by the polymer shell.31 The influence of soft polymers and solid cores on the compression of microgel systems is discussed in more detail in the present publication. We investigate the influence of a solid core and varying cross-link density on the interfacial behavior of microgels of different morphology, namely core−shell (CS) and hollow (HS) particles. For this purpose, compression isotherms of microgel monolayers at oil−water interfaces are measured with a Langmuir trough. Core−shell microgels with solid silica core and soft, deformable microgel shell consisting of PNiPAm with varying cross-link density are used. The core− shell microgels were then transformed into hollow microgels by dissolving the core. A comparison of the core−shell and the corresponding hollow microgels gives valuable information about the mechanical properties of these soft particles with regard to the cross-link density in the polymer shell and the influence of the solid core. The experimental findings are compared to and agree with computer simulations of core−shell and hollow microgel particles in bulk and at oil−water interfaces that describe the particle deformation at different conditions. The results show that different particle properties govern the behavior at low and high compression. The compressibility of core−shell and hollow-sphere microgels is thus clearly different from that of classic microgels or hard spheres.

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EXPERIMENTAL SECTION

Materials. Ammonia solution (28%−30%, Merck), N,N′methylenebis(acrylamide) (BIS, Applichem), ethanol (Merck), methanol (p.a., Merck), N-isopropylacrylamide (NiPAm, Acros Organics), potassium peroxodisulfate (KPS, Acros Organics), sodium dodecyl sulfate (SDS, Merck), hydrofluoric acid (40%, Merck), tetraethyl orthosilicate (TEOS, Merck), 3-(trimethoxysilyl)propyl methacrylate (MPS, Sigma-Aldrich), and polyvinylpyrrolidone (PVP, Merck) were used as received. n-Decane (for synthesis, Merck) was filtered three times over basic Al2O3 prior to use to remove any polar contaminants. Doubly distilled Millipore water was used for particle synthesis and 13146

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Langmuir fraction of the cross-links is equal to 5%, so that the number of beads in each subchain was 12 (2 cross-link beads and 10 beads between them). Taking into account diamond crystal lattice, the fraction of the tetrafunctional cross-links in the frame (the number of cross-links divided by the total number of the beads) is calculated as 1/ (4 × 5 + 1) ≈ 0.05, where we selected 1 cross-link and 4 halves of the subchains jointed by the cross-link. The constructed frame is used for making the shell of the particles. The solid core of the core−shell microgel was also obtained on the basis of the diamond crystal lattice. However, the subchains of the core consisted of only one bead, and all the beads form a closely packed system with attractive interactions between the beads. The spherical shape of the core is provided via inscribing a sphere into the frame and cropping all the beads which are outside the sphere. Then two concentric spheres were inscribed into the frame with 5% of cross-links. The inner sphere has the same radius as the solid core, and the radius of the outer sphere is chosen in such a way to get desirable ratio of the core and the shell sizes. All the beads, which are outside the bigger sphere and inside the smaller one, are cropped (HS particle). Then the solid core is inserted into the hole of the shell with subsequent grafting to the shell via dangling chains (CS particle). The fractions of the beads in the core and the shell were fixed and equal to 28% and 72%, respectively, which are close to the experimental system. In the experiments, the radii of the core and the whole core−shell particle in the collapsed state (at 50 °C) are 82 and 148 nm, respectively (see below). We assumed that in collapsed state the shell contains some fraction of water which was taken 0.5 as measured in ref 49. Then the ratio of the numbers of the MD beads in the shell and the core is calculated as 0.5 × (1483 − 823)/823 = 2.44 ≈ 72/28. The interactions between any pair of the beads were described through the truncated-shifted Lennard-Jones potential (see refs 50 and 51 for details). The value of the dimensionless Lennard-Jones interaction parameter ε describing monomer−monomer interactions was varied between 0.01 and 1, corresponding to good and poor solvents, respectively. Attraction of the beads in the solid core of the CS microgel was described by the interaction parameter ε0 = 0.33, which means that the solvent is poor for the particles and they would aggregate without the polymer shell which is a characteristic feature of many neutral nanoparticles. Adsorption and spreading of the microgels on liquid−liquid interface were modeled using dissipative particle dynamics (DPD).52,53 Details of the simulation technique can be found in refs 54−56 The core−shell and hollow microgels of different cross-link fraction (5% and 10% referred with indexes −5 and −10, respectively) were designed as described above. For comparison, we also designed equivalent homogeneous microgels (M-5 and M-10). The simulations were performed in a cubic box of linear sizes Lx = Ly = Lz = 60 measured in units of the bead diameter with imposed periodic boundary conditions in all directions. There are four different types of particles in the system: water (W), oil (O), the beads forming polymer shell (P), and solid core (C). The conservative forces, which act between nonbonded particles, are quantified by the interaction parameters aij which can be related to the conventional Flory− Huggins parameters.57 Full set of the parameters and initial structure of the system before annealing are presented in Scheme 1. The oil (yellow) and water (blue) molecules occupy upper and bottom parts of the box, respectively. They are strongly immiscible (aOW = 70), so that negligible fraction of the molecules penetrates into the “foreign” phase after annealing. The microgel is placed at the interface. The oil is always poor solvent for the polymer (aOP = 38), and water can penetrate into the shell (aWP = 28) or collapse it (aWP = 40). Both oil and water are poor solvents for the core of the microgel (aOC = aWC = 32), and the core is more compatible with the polymer than with liquids (aPC = 28). In the case of the HS microgels, they were first annealed in a good solvent (water) and then placed at the interface for further annealing.

Scheme 1. Snapshot of the Initial Structure of CS Particle at Water−Oil Interface before Annealinga

a All values of the interaction parameters are reflected on the right scheme. Letters O, W, C, and P in the circles denote oil, water, core and polymer, respectively. The numbers near the links between two neighbor circles are the corresponding values of the interaction parameters.

scattering. At 20 °C, the hydrodynamic radius of the silica core in water is 82 ± 1 nm. The radii of the other particles are depicted in Figure 1 and Table S2. The temperature-dependent

Figure 1. Hydrodynamic radii of the core−shell and the hollow microgels in water as a function of the temperature. All particles collapse at temperatures above the volume phase transition temperature of 32 °C.

collapse of the PNiPAm-based microgels is clearly visible. Both the core−shell and the hollow microgels collapse at a volume phase transition temperature (VPTT) around 32 °C. The influence of the cross-linker content on the swelling properties of the microgel can be represented by the swelling ratio α. It is defined as the ratio of the particle volumes in the swollen and the collapsed state α = Vswollen/Vcollapsed. The swelling ratio decreases with the cross-link density. Thus, high cross-link density prevents swelling and leads to smaller particle size in the swollen state as compared to particles with low cross-link density. This behavior can be observed for all particle types in Figure 1. Additionally, the particles do not change their size significantly after dissolution of the core, which has also been described for these kind of particles previously.42 The fact that the core−shell particles and hollow microgels have nearly the same size in the collapsed state at high temperatures indicates that the hole survives and does not shrink despite of attraction between the polymer segments and an excess surface energy at the inner microgel/solvent interface. The stability of the hole under poor solvent conditions can be explained by elasticity of

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RESULTS Microgels in Solution. The size of the core, the core− shell, and hollow microgels was measured with dynamic light 13147

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Figure 2. Snapshots on the left represent a cross section through the center of mass of the core−shell (CS-5) and hollow (HS-5) microgels in good (upper row) and poor (bottom row) solvents obtained with MD simulations. The corresponding concentration profiles of the particles are shown on the right side. Orange and black lines depict the core and shell concentration profiles of CS-5 microgel, and blue line corresponds to the HS-5 particle.

agreement with the density profiles calculated from neutron scattering profiles of core−shell and hollow microgels41 and thus confirm the experimental results of these authors. Microgels at Oil−Water Interfaces. The compression isotherms of the core−shell and the hollow microgels are shown in Figure 3. The isotherms are normalized to the mass of

the shell which increases upon the collapse of the microgel (the shell becomes denser). Any shrinkage of the hole has to be accompanied by microgel deformation (deviation from the spherical shape) and penalty in the elastic bending energy of the shell. Thus, a cross-linker content of 5 mol % is sufficient to keep the hollow structure of the microgel in the collapsed state. On the other hand, one can expect that a proper decrease of the fraction of the cross-linker will result in homogenization of the internal structure of HS microgel at high temperature. Using SANS, it has been shown recently that the hollow structure is stable in the swollen and the collapsed state, despite polymer chains moving toward the space that was previously occupied by the core.41 The stability of the hollow structure of the microgel with 5% of cross-linker in poor solvent and qualitative agreement with the experimental results is demonstrated in computer simulations (Figure 2). One can see that the sizes of the core−shell and hollow microgels in the good solvent are practically the same (upper images). The concentration of monomer units in the shell is also the same. A slight elevation of polymer concentration is observed in the vicinity of the core (upper left image in Figure 2). This effect is related to a high surface energy of core-forming solid particles. The solvent was considered to be poor for the core and good for the shell. Therefore, the increased concentration of monomer units near the core is a result of the screening of unfavorable contacts of the core with the solvent molecules. This effect has also been observed experimentally in a SANS study on core−shell microgels: the polymer density increased in close proximity to the core.41 The decrease of the solvent quality for the shell induces its collapse (Figure 2). The radius of the hollow microgel is slightly smaller than of the core−shell particle. The cavity decreases in comparison with the swollen state and becomes smaller than the size of the solid core of the core−shell particle. Such transformations are driven by the decrease of the surface energy of the microgel. However, the hole does not shrink completely due to elasticity of the shell. Our results are in good

Figure 3. Compression isotherms of the core−shell (CS) and hollow (HS) microgels where the surface pressure is shown as a function of the relative area. Full lines: core−shell particles (CS). Dashed lines: hollow microgels (HS). Black: CS/HS-5; orange: CS/HS-10; blue: CS/HS-15. The vertical dashed lines indicate different regions in the isotherms (using sample HS-5 as an example) and are explained in the text. The amount of microgels used for the different compression isotherms is given in Table S3.

the microgels at the interface to show that the curves of different amounts of microgels at the interface overlap and form a mastercurve.44 The overlap of isotherms with different initial microgel concentration supports the assumption that the microgels stay at the interface and do not migrate into the bulk water phase, which would lead to different concentrations at the interface and thus less overlap of the isotherms.44 However, when comparing the isotherms, it is important to note that the mass of the core−shell particles is higher than the mass of the hollow microgels due to the solid silica core. That 13148

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Figure 4. Compression isotherms as a function of the relative area. The isotherms of the core−shell (CS) particles were shifted toward the isotherms of the hollow particles (HS) by the respective mass of the core: CS-5:18.7%; CS-10:20.3%; CS-15:19.4%. Full lines: CS particles. Dashed lines: HS particles.

First, the onset of the increase of the surface pressure, which corresponds to the transition from region I to region II, is discussed. A comparison of the core−shell particles with the corresponding hollow microgels full and corresponding dashed lines) shows that the surface pressure starts to increase at higher relative areas in the case of the hollow microgels. This means that the hollow microgels start to interact already at lower compression than their corresponding core−shell counterparts. Thus, the effective area covered by a hollow microgel is larger than the area covered by the corresponding core−shell particle, even though the chemical nature of the polymer shell is identical. Additionally, the cross-link density influences the behavior under compression in the transition region from I to II. To achieve a valid comparison of the three core−shell and hollow microgels in dependence of the cross-link density, a similar calculation as in the previous paragraph was performed. Based on the mass of the core in the three core−shell microgels that was determined with TGA, the isotherms of Figure 3 were normalized to the number of core particles. Thus, even though the absolute particle number at the interface is unknown, the normalization assures that the comparison of the isotherms of the different systems is valid. The lower cross-link density in the case of the CS-5 and CS-10 particles leads to an increase in surface pressure at higher relative areas (meaning lower compression) as compared to the CS-15 particles. The same order is observed for the hollow microgels (Figure S2). The influence of the core and the cross-link density on the first increase of the surface pressure (region I → region II) can be explained by different spreading of polymer networks at the interface. On the one hand, the core restricts the spreading of the polymer shell, resulting in a lower area per particle at the interface. On the other hand, high cross-link density reduces the thickness of the fuzzy layer around the core−shell and the hollow microgels in bulk.41 Thus, the size of the fuzzy corona of the particles decreases with increasing cross-link density, also leading to less spreading at the interface and lower area per particle. Consequently, a hollow microgel with low cross-link density spreads the most at the oil−water interface whereas a solid core and high cross-link density hinder the spreading at the interface. We will show below that the experimental results are fully supported by computer simulations. The influence of the core on the compression can be discussed when comparing the core−shell and hollow microgels. Figure 5 shows that when the isotherms of the core−shell microgels are shifted, they match the isotherms of the hollow microgels in regions I to III. The isotherms of the core−shell and of the hollow microgels overlap completely at low compression, showing that the core does not influence the surface pressure but the shell dominates. The shift factor

issue will be discussed in detail in the Discussion section with regard to the mass fraction of the core in the core−shell microgels. The shape of the compression isotherms of the core−shell and of the hollow microgels is similar to what has been observed before for microgels without solid core.30,44,45 The isotherms follow a two-step increase of the surface pressure and can be divided into distinct regions. These can be attributed to different interactions and arrangements of the particles at the interface. First, the surface pressure is very low at high relative area, indicating almost no interaction of the microgels at the interface and a gas-like arrangement of the particles (region I). Further compression leads to an increasing surface pressure, meaning that the fuzzy coronas of the microgels get in contact. With ongoing compression, the surface pressure increases drastically (region II), and the particles form a hexagonal array with particle distances larger than the particle diameter.16 When the pseudoplateau in region III is reached, the microgels are strongly compressed. In this region, the transition to close hexagonal arrangement takes place. After a second increase of the surface pressure in region IV, a final plateau is reached at high surface pressure (region V); no further compression of the microgels is possible, and the monolayer collapses. However, it is challenging to analyze the collapse of the monolayer in more detail, but we have shown previously that the collapse leads to a buckling of the interfacial microgel layer.16

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DISCUSSION Ideally, the number of particles at the interface is considered when compression isotherms of different particle types are compared. However, in the case of complex microgels, it is difficult to determine the molecular mass (and thus the number of particles) with sufficient accuracy. For this reason, the mass fraction of the core in the core−shell particles is determined, and the mass of the core is excluded from the relative area of the compression isotherms, as explained in the following paragraph. Consequently, the normalization to the total polymer mass at the interface corresponds to the normalization to the number of particles, and a valid comparison of the core− shell and the hollow microgels is thus achieved. The comparison of the isotherms of core−shell and hollow microgels was performed by shifting the isotherms of the core− shell particles to higher relative areas corresponding to the mass fractions of the core as calculated from thermogravimetric analysis (TGA). The mass fraction of the core of the core−shell particles (see Experimental Section) is 18.7 ± 0.2% (CS-5), 20.3 ± 0.9% (CS-10), and 19.4 ± 1.4% (CS-15). A direct comparison of core−shell and hollow microgels is thus possible and the shifted isotherms are depicted in Figure 4. 13149

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pronounced for particles with high cross-linker content (Figure S3). This observation supports the finding that the transition to close hexagonal packing is more favorable for rigid microgels, whether the rigidity may be caused by core−shell structures or high cross-link densities. Furthermore, the particles’ behavior under compression is further evaluated by calculating the compression modulus C−1. The compression modulus is a measure of the surface elasticity of the film4559 and is defined as C−1 = −A(dπ/dA), where C (sometimes also called κ) represents the compressibility of a particle layer. A high elasticity C −1 thus means low compressibility of the particle layer. The compression modulus of the core−shell and hollow microgel layers in dependence of the surface pressure is depicted in Figure 6.

Figure 5. Compression isotherms as a function of the relative area. The isotherms of the core−shell (CS) particles were shifted to match the isotherms of the hollow microgels (HS) in region III. The apparent mass of the core is 54% (CS-5), 65% (CS-10), and 49% (CS15). Full lines: CS particles. Dashed lines: hollow microgels. Black: CS/HS-5; orange: CS/HS-10; blue: CS/HS-15.

however is not identical to the mass ratio of core and shell and does not have a physical meaning. It was used to compare the shape of the isotherms and the plateau region at high compression. The apparent mass fraction of the core was calculated from the shifted isotherms and equals 54% (CS-5), 65% (CS-10), and 49% (CS-15), which is 3 times larger than the values calculated from TGA measurements, as it was done for the isotherms in Figure 4. This again shows that the hollow microgels occupy a larger area at the interface compared to the core−shell particles. Furthermore, Figure 5 allows an evaluation of the influence of the core at high compression. There is a deviation between the isotherms of the core−shell and hollow microgels. The region III between surface pressures of 25 and 30 mN/m is more pronounced in the case of the hollow microgels, and the slope of the surface pressure is smaller. The core−shell particles thus resemble solid particles in a way that they are less compressible and deformable at the interface than their hollow counterparts. It is well-known that the compression of layers of rigid particles leads to a very steep increase of the surface pressure.30,58 Thus, it is not until high compression that the influence of the core is visible and the isotherms of the core− shell, and the hollow microgels do not overlap anymore. The plateau in region III is thus attributed to a deformation and considerable compression of the microgels.45 This is accompanied by restructuring of the microgels and a significant decrease in the particle distance of the hexagonal array.16 A short region III thus indicates not only solid-like behavior but also that the structural transition does not require high pressure. Instead, the microgels can easily overcome the threshold to the close-packed structure. In contrast to that the hollow microgels are softer than the core−shell microgels and deform at high compression due to the missing core. It has been shown before that the particle distance in self-assembled monolayers of core−shell particles is determined by the thickness of the polymer layer around a solid core.27−29 It is thus very reasonable that the resistance against a close-packed structure is increased when a soft and deformed polymer shell is present and that more energy is required for the structural transition to take place. High cross-link density induces similar effects as a solid core. The length of region III is directly correlated to the cross-link density for the core−shell and the hollow microgels and is less

Figure 6. Compression modulus as a function of the surface pressure as derived from the compression isotherms shown in Figure 4. Full symbols: core−shell particles. Open symbols: hollow microgels. Black: CS/HS-5; orange: CS/HS-10; blue: CS/HS-15.

The compression modulus of all particles shows two maxima that correspond to the two-step increase (regions II and IV) of the compression isotherms. The values of the maxima are given in Table 1. The modulus in region II is independent of the Table 1. Surface Pressure π and Compression Modulus C−1 of the First and the Second Maximum of the Surface Elasticity As Depicted in Figure 6a πmax,1 [10−3 N/m] CSHSCSHSCSHS-

5 10 15

10 10 13 11 15 15

± ± ± ± ± ±

1 1 1 1 1 1

Cmax,1−1 [10−3 N/m] 47 46 33 33 25 27

± ± ± ± ± ±

2 2 2 4 2 1

πmax,2 [10−3 N/m] 34.0 33.0 32.4 31.5 32.8 31.5

± ± ± ± ± ±

0.4 0.3 0.4 0.3 0.4 0.7

Cmax,2−1 [10−3 N/m] 19.7 10.5 25.8 14.6 35.3 19.7

± ± ± ± ± ±

0.4 0.5 0.8 1.0 1.5 1.0

a

Maximum 1 can be defined less clearly than maximum 2 due to the broad scattering of data points. The errors were estimated from the standard deviation of the compression modulus at the maximum, which was determined manually.

presence of the core and the morphology in bulk. Instead, the cross-link density defines the compressibility. The modulus decreases with increasing cross-link density, meaning that highly cross-linked microgels show high compressibility. This behavior was already observed before when the compression of uniform PNiPAm microgels at oil−water interfaces was investigated. Microgels with 5 mol % cross-linker had a compression modulus of C−1 = 32 mN/m while 2.5 mol % cross-linker yielded C−1 = 40 mN/m.45 This behavior is very 13150

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Figure 7. Snapshots of core−shell (CS), homogeneous (M), and hollow (HS) microgels adsorbed on water (blue)/oil (yellow) interface. They depict cross section through the center of mass of the particles. Upper and lower rows correspond to the microgels with 5% and 10% of cross-linkers, respectively. The oil phase is always considered as a poor solvent, and the solvent quality of water is variable.

the cross-linker content (5% and 10%) were adsorbed on the water/oil interface from the water phase, which was considered as a good solvent for the polymeric components of all particles. The driving force for the adsorption is a minimization of the water/oil surface energy via adsorption of polymer chains (subchains) which decrease the area of water−oil contacts. The oil is a poor solvent and the penetration of the oil molecules into the microgels of all sorts is negligible in comparison with the water molecules. As a result, the bigger part of the microgels is located in the water phase, and the narrow polymer/oil interface is slightly bent (Figure 7). Such a bent interface was recently observed by means of X-ray microscopy with oil-inwater emulsions stabilized by large microgels.60 Distinct differences in the shapes of the core−shell and hollow microgels at the interface are observed. The shell of the core−shell particle is weakly perturbed by different interactions with water, oil, and water/oil interface. One can distinguish a dense polymer layer in-between the solid core and the oil phase and a swollen region exposed to the water phase. A tendency to shield the water/oil area leads to deviation of the cross section of the particle from the spherical one (Figure 7). However, the presence of the solid core does not allow strong shape variation. In contrast, the hollow microgel essentially spreads on the interface. The ultimate shape of the particle is controlled by a balance between a gain in the interfacial energy due to the spreading and a penalty in the elastic free energy of the deformed microgel. The spreading of homogeneous (M) microgel with 5% of cross-links is similar to the spreading of the HS microgel (Figure 7). The effect of the cross-link density is also demonstrated in Figure 7. The CS-5, HS-5, and M-5 microgels reveal higher extent of spreading in comparison with denser cross-linked counterparts CS-10, HS-10, and M-10. Such difference is due to the different elasticity of the shells. Quantitative information about dimensions of the particles is presented in Figures 8 and 9. The highest spreading (lateral radius) of HS-5 in Figure 8 and gradual decrease of the spreading in a sequence HS-5 → HS-10 → CS-5 → CS-10 fully correlate with Figure 3, where the rising of the surface pressure upon compression of the monolayer occurs in the same sequence. Normal dimensions of the microgels and their immersion into water and oil are also shown in Figure 9. In the case of the HS-5 microgel, the spreading at the interface flattens out the cavity completely whereas the higher elasticity of the HS-10 shell retains the deformed cavity (second

remarkable and shows again that soft particles behave very differently from hard particles. An explanation for this behavior is that the adsorption energy gives significant contribution to the modulus of the monolayer whereas the network elasticity contributes differently at different compression degrees. The flattened structure of the microgel is characterized by a gain in the interfacial (adsorption) energy and a penalty in the network elasticity (the subchains are laterally stretched): the lower the fraction of cross-links, the higher the gain in the energy (more subchains are at the interface). Compression of such flattened microgels is accompanied by desorption of monomer units and subchains from the interface which is energetically unfavorable: the stronger the adsorption, the lower the compressibility of the microgel. Therefore, the energy required to reduce the number of contacts during compression will be higher in the case of weakly cross-linked microgel in comparison to the highly crosslinked one because the latter has smaller adsorbed subchains. On the other hand, a contribution of the network elasticity to the modulus will be negative at low compression degree: laterally stretched subchains relax during compression. Therefore, the weakly cross-linked microgels are less compressible at low compression degree. At high compression degrees, when the microgels are laterally jammed, the network elasticity gives a positive contribution to the elastic modulus of the monolayer. Also, at the second maximum in region IV, the modulus does no longer only depend on the cross-link density but also on the presence of the solid core. The elasticity of the hollow microgels is lower than the elasticity of the corresponding core−shell particles, showing that microgels with solid core are less compressible. Furthermore, the dependence on the cross-link density is reversed as compared to region II. Particles with higher crosslink density have a higher modulus. This is a consequence of the dominant contribution of the network elasticity to the elastic modulus at high compression degree. The fact that the moduli of CS-5 and HS-15 are similar additionally indicates that the effect of the core may be replaced by high cross-link density. A high modulus can thus be caused by either high cross-link density or the presence of a solid core, or both. Computer Simulations of Single Microgels on Liquid Interfaces. The conclusions and assumptions from the compression isotherms about shape and spreading of individual particles are nicely supported by computer simulations. The CS and HS particles and homogeneous microgel (M) differing in 13151

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core−shell particles were transferred into the hollow counterparts by dissolving the core. Different particle properties govern the behavior at different compressions. At high compression, particles with high crosslink density and a solid core have the highest modulus and thus decrease the compressibility of the system. Furthermore, the structural transition to close hexagonal packing is facilitated when the rigidity of the particles is increased by high cross-link density or the presence of a core. However, at low compression, the modulus is independent of the presence of the core and high cross-link density leads to a low modulus. The deformation at low cross-link density and the resulting adsorption energy play an important role at low compression. Low cross-link density leads to microgel deformation and high surface coverage that is independent from the core. Less compression is thus needed to establish the first contact between these microgels at the interface, and the surface pressure rises already at high area per microgel. Computer simulation studies reveal the different deformation of the core−shell and hollow microgels at liquid interfaces and support the experimental finding that low cross-link density and a hollow structure lead to increased spreading of the polymer chains at the interface. Simulations also predict the internal structures of microgels in good and bad solvents and show that the dimension of the cavity in the interior of the hollow microgels varies with the quality of the solvent and the rigidity of the polymer shell. In conclusion, the behavior of core−shell and hollow microgels is clearly different from solid spheres. The core does not influence the microgel layer until the particles are highly compressed and low cross-link density of the shell results in low compressibility due to spreading polymer chains and high adsorption energies. Our findings contribute to the general understanding of the complex interplay of microgel-based materials at liquid interfaces. They also allow for the specific fabrication of microgel arrays and nanostructures.

Figure 8. Average lateral radius of the core−shell (CS), homogeneous (M), and hollow (HS) microgels adsorbed on oil/water interface for the case when the water is good (left) and poor (right) solvent for the microgel shells (the corresponding snapshots are shown in Figure 7). Oil is always poor solvent.

Figure 9. Height of the core−shell (CS), homogeneous (M), and hollow (HS) microgel parts immersed into oil (top) and water phase (bottom) when the water is good (left) and poor (right) solvent for the microgel shells (the corresponding snapshots are shown in Figure 7). Oil is always poor solvent.

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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.5b03530. Synthesis parameters of core−shell particles (Table S1), hydrodynamic radii and swelling ratios of core−shell and hollow microgels (Table S2), TGA curves of core−shell particles (Figure S1), amount of microgels used for measuring compression isotherms (Table S3), compression isotherms normalized to the amount of microgels at the interface (Figure S2), shifted compression isotherms to compare region III (Figure S3), calculation of the core mass fraction based on the thickness of the polymer shell (PDF)

images in Figure 7). Furthermore, computer simulations allow also studying the behavior when water is a poor solvent for the shells. This corresponds to experiments with PNiPAm microgels above their VPTT. However, such experiments are challenging due to the volatility of the oil. Computer simulations show that one can trap water in the cavity under poor solvent conditions (second images from the right-hand side in Figure 7). In this case, the shells become denser and of higher elastic moduli, and the cavity is restored both for HS-5 and HS-10 particles. Also, the particles immerse deeper into the oil phase (Figure 9). Therefore, one can conclude that the hollow structure can exist not only in bulk but also on the liquid interfaces as well.

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CONCLUSIONS We have investigated core−shell and hollow microgels at oil− water interfaces and studied their behavior using compression isotherms and computer simulations. A comparison of the compressibility of the different particles was achieved by the evaluation of the compression modulus. The core−shell microgels contained a solid silica core that was surrounded by a polymer shell with different cross-link density. These

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (I.I.P.). *E-mail [email protected] (W.R.). Notes

The authors declare no competing financial interest. 13152

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ACKNOWLEDGMENTS We thank Janine Dubbert and Matthias Karg for synthesis and characterization of the particles and Claudia Pörschke for the TGA measurements. The Deutsche Forschungsgemeinschaft (DFG) is acknowledged for financial support within the Sonderforschungsbereich SFB 985 “Functional Microgels and Microgel Systems”. A.A.R. and I.I.P. acknowledge financial support from the Russian Science Foundation, project # 15-1300124. Simulations were performed on Multi-Petaflop supercomputer “Lomonosov” at Moscow State University.

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