Tuning the Particle–Surface Interactions in Aqueous Solutions by Soft

Article pubs.acs.org/Langmuir

Tuning the Particle−Surface Interactions in Aqueous Solutions by Soft Microgel Particles Xiangjun Gong,†,‡ Li Hua,‡,§ Jingjing Wei,§ and To Ngai*,§ †

Faculty of Materials Science and Engineering, South China University of Technology, Guangzhou, China, 510640 Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong

§

S Supporting Information *

ABSTRACT: Due to the softness and deformability, interaction between colloidal surfaces induced by soft particles varies in a more complex way than for solid particles and thus has attracted much attention in recent years. In the present study, we use total internal reflection microscopy (TIRM) to directly measure the interaction between polystyrene (PS) microparticles and a flat glass surface in a poly(Nisopropylacrylamide) (PNIPAM) microgel dispersion with concentration varying from dilute (0.1 wt %) to highly concentrated regime (7.5 wt %). Our result shows that the PS particle−surface interactions mediated by the soft microgels are greatly affected by the particle concentration, the configuration of those microgels adsorbed on the surfaces, and the structure and packing of microgels in bulk solution. With increasing the microgel concentration (Cmicrogel), the interaction between the PS particle and surface turned from bridging attraction to steric repulsion, and then depletion attraction, which were mainly governed by the adsorption amount and configuration of microgels on the two surfaces. By further increasing Cmicrogel to condensed situation, structural force with oscillated energy wells was detected. The variation of interactions induced by the soft microgels was further confirmed by optical imaging. Crystallization of the PS microparticles appeared at moderate Cmicrogel; however, crystallization was hindered at higher Cmicrogel where the microgels are highly packed in the bulk solution. Furthermore, using TIRM, microgel packing with local energy well (0.1−1.0 kBT) at the highly condensed state (7.5 wt %) was resolved from the interaction profiles. Therefore, the shear force and modulus generated by such microgel packing can be determined as ∼0.2 pN and tens of mPa, respectively, which are much weaker than data given by conventional active methods.

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INTRODUCTION Binary or polynary colloidal suspensions composed of micronsized and nanosized colloids are often encountered in our daily life including foodstuffs,1 painting,2 and cosmetics.3 Among these components, nanoparticles in such colloidal suspensions play an important role in modulating the interactions among those micron colloids, and the resultant effect determines the dynamics, structures, and stabilities of the colloidal suspension.4 Based on this principle, different kinds of nanoparticles have been applied to mediate the interaction in colloidal systems including polymer chains,5,6 hard nanoparticles,7−10 soft particles,11,12 and core−shell particles.8 It was well demonstrated that the induced interactions were determined by the properties of the nanoparticles such as concentration of the nanoparticles, size, the adsorption ability of the nanoparticle onto the colloidal surfaces, surface charge, and their softness.8,10,12,13 Considering the interaction between colloids induced by polymer chains, either stabilization or flocculation can result depending on whether polymer adsorption occurs on the colloidal surfaces or not. When polymers are adsorbed but with a low surface coverage, polymer coils could bridge two colloidal surfaces, leading to an irreversible bridging attraction.5,14−16 © 2014 American Chemical Society

With increasing coverage of polymer chains on the colloidal surface, steric repulsion between the microcolloids might be found due to the entropy-driven configuration of the polymer chains.17,18 For interaction mediated by solid nanoparticles, similar to polymer chains, either repulsion or attraction might be induced depending on the adsorption configuration of nanoparticles on the colloidal surfaces. For nonadsorbing cases, the polymers and nanoparticles act as depletents, generating an osmotic pressure difference between the colloidal gap and the bulk and leading to a depletion attraction between the colloids.8,11,19 Charged particles and polyelectrolytes can further enhance the strength and change the range of the related interactions.20−22 Beyond the depletion regime, interaction force gets oscillatory fluctuations,9,23,24 resulting from the structures of nanoparticles formed at higher concentrations in the bulk. This oscillatory force has also been proven by Monte Carlo simulation and hypernetted chain-based theory.25 Received: September 7, 2014 Revised: October 13, 2014 Published: October 14, 2014 13182

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by precipitation polymerization using N,N′-methylenebis(acrylamide) (BA) as cross-linker with the same conditions and monomer as well as cross-linker ratio.38 Therefore, the synthesized microgel samples are expected to have a nonuniform core−shell structure with a highly cross-linked core surrounded by polymer dangling chains due to much higher reactivity of the cross-linker than that of NIPAM monomers during the polymerization. The hydrodynamic diameter (dmicrogel) of microgel is 125 nm at 24 °C with polydispersity index (PDI) ∼ 0.164 as measured by dynamic light scattering (ALV5000e, Germany). The particles were purified by centrifugation three times, freeze-dried, and then redispersed into water for weeks to make sure they were well dispersed. For TIRM measurements, the microgel dispersions were prepared at 0.4 mM sodium chloride (NaCl) solution with concentrations (Cmicrogel) ranging from 0.01 to 7.5 wt %. PS sulfate latex microparticles (diameter = 6.0 μm, CV ∼ 4%, referred to as PS1), PS carboxyl latex microparticles (diameter = 3.5 μm, CV ∼ 4%, referred to as PS2), and silica beads (diameter = 5.0 μm, CV ∼ 10%) were purchased from Life Technologies and Polysciences Inc. respectively, and used without further purification. PS1 microparticles were first diluted and dispersed in microgel solutions with various concentrations for TIRM measurements of the particle−surface interaction induced by the microgel particles. PS2 microparticles were dispersed in the same microgel dispersions at a fixed solid concentration of 0.5 wt % and then viewed by optical microscopy. The glass slides (BK-7 glass, thickness = 1 mm) used in the TIRM and optical imaging experiments were obtained from Fisher Scientific Co. The cover glass slide (BK-7 glass, thickness = 0.13−0.17 mm, Sail Brand) was employed in confocal imaging. Both of them were cleaned by first immersion in 1 M NaOH ethanol solution for 1 h, then washed with tetrahydrofuran (THF) and deionized water, and finally dried before further experiments. Determination of the Effective Volume Fraction (ϕeff) of Microgel Solutions. First, ϕeff of a diluted microgel solution with concentration of 0.05 wt % was determined by obtaining the relative zero shear viscosity (η0/ηs) at 24 °C using an Ubbelohde viscometer. According to Bachelor’s expression which is valid in the diluted case,39−41 the relationship between η0/ηs and ϕeff can be expressed as η0 = 1 + 2.5ϕeff + 5.9ϕeff 2 ηs (1)

Interactions in colloidal systems have been extensively described by considering the nanoparticles as hard spheres. Actually, the hard sphere model has been proved to be successful to describe the structure or interactions in systems induced by hard particles or diluted soft particles.11 However, in many real cases, most particles and polymers are soft to some degree. Therefore, as density increases, the additional nanoparticles cannot be treated as hard spheres since they begin to deform or overlap. As a result, the packing structure of soft particles could be more diverse, resulting in complex interactions when the concentrations are varied. As mentioned above, oscillatory structure force was observed for concentrated nanoparticles, including PS particles, silica particles, and micelles.9,23,26 However, the volume fraction of those nanoparticles was still under 0.49, the value where hard colloids start to crystallize.27,28 In other words, interactions in the highly condensed and soft colloidal system are still poorly understood. One kind of soft particle that has received much attention is cross-linked poly(N-isopropylacrylamide) (PNIPAM)-based microgels. These microgels are sensitive to temperature, pH, and ionic strength,29,30 and it can be deformed to a degree that is determined by the cross-link density and polymer−solvent interaction.30,31 This unique property allows them to reach an effective volume fraction (ϕeff) far above the packing limit for hard sphere particles,32,33 making this soft particle a good model for studies in colloidal systems, especially in the studies of the structure and phase transition at dense concentrations.34,35 In order to directly explore the interaction variation of colloidal system induced by soft microgel particles from undisturbed particles to dense suspensions, an effective method to noninvasively probe the interactions involving the microgel particles needs to be developed. Among the force detection techniques, total internal reflective microscopey (TIRM) has been proved to be a sensitive technique to measure interaction between a micron-sized colloid under Brownian motion and a flat surface.36,37 It monitors the separation distance between the two surfaces. From the distribution of the separation distance, interaction between the two surfaces can be obtained. Compared with other similar techniques like atomic force microscopy (AFM) and surface force apparatus (SFA), TIRM is a noninvasive and passive approach much closer to real situations with better resolution. Therefore, in this study, we use TIRM to monitor the induced interactions between a negatively charged PS microparticle activated by thermal energy and a negatively charged glass surface in the PNIPAM microgel dispersions. The concentration of microgel was varied over a broad density range, spanning from the dilute region where they are separated to the concentrated region where they are in contact with each other. We showed that variation of the microgel concentration had a significant impact on the interactions between the two colloidal surfaces. Optical imaging of binary PS colloids and microgels at the same microgel concentrations confirms the interaction variation at higher microgel concentrations. Moreover, the TIRM measurements revealed a coincidence between the transition of the interaction curves and the transition of the confined microgel structure. In addition, the strength of the packed microgels was evaluated.

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Based on eq 1, the ϕeff at each microgel concentration dispersion can be deduced, and the results are summarized in Table 1.

Table 1. Summary of the Properties of Microgel Dispersions and the Equilibrium Distance between the Probe PS1 Microparticle and Glass Surface, he,a at Different Cmicrogel As Measured by TIRM Cmicrogel (wt %)

ϕeffb

number density (μm−3)

0.01 0.1 0.5 1.0 3.0 7.5

0.005 0.05 0.25 0.50 1.50 3.75

4.9 49 245 490 1470 3680

he (nm) 168 336 256 246 239 247

± ± ± ± ± ±

19 35 97 41 29 26

(1.3 (2.7 (2.0 (2.0 (1.9 (2.0

dmicrogel)c dmicrogel) dmicrogel) dmicrogel) dmicrogel) dmicrogel)

a

he: distance from the bottom at which the minimum of the potential curve is found as defined by eq 6. bEffective volume fraction. cdmicrogel: hydrodynamic diameter of the microgel particle.

Zeta Potential Measurements. The average mobility (μE) of the microgel-coated PS1 microparticles and silica beads under an electric field in an aqueous solution was determined using a commercial zeta potential spectrometer (ZetaPlus, Brookhaven). Each data point was averaged over 5 times. As a result, the zeta potential (ξpotential) was deduced from μE under the Smoluchowski limits (κR ≫ 1) as

EXPERIMENTAL SECTION

Sample Preparation and Characterization. A nanosized PNIPAM-based microgel was used in this work. It was synthesized 13183

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ημE ε

solution, the sample container was washed by deionized H2O to remove all the free microgels and dried with an air stream. During all the measurements, the probe PS bead was also monitored by CCD camera simultaneously to ensure that it stayed in the illumination area. All the measurements were performed at 24 °C. For optical imaging, the binary mixtures consisting of PS2 microparticles and microgels were first sealed in a cover glass made chamber (thickness of 0.17 mm). Then the PS2 microparticles were imaged under bright field when settled on bottom surface of the chamber. The whole bottom surface of chamber (∼1.25 cm2) was observed and checked during this experiment.

(2)

where κ−1 is the Debye length, R is the radius of the particle, and η and ε are the viscosity and permittivity of water. All the zeta potential measurements were performed at 24 °C, similar conditions to the TIRM experiments. Total Internal Reflection Microscopy (TIRM) and Optical Imaging. The principle of TIRM can be found elsewhere.42−44 Generally, it is a technique to measure the interaction potentials between a free-moving micron sphere (i.e., PS microparticle) and a flat glass slide. The evanescent wave is produced by reflecting a laser beam (λ = 632 nm, power = 25 mW) off the glass−water interface at a critical angle such that total internal reflection occurs. The resultant amplitude of the evanescent wave decays exponentially with the distance from the interface. Therefore, a micron-sized probe bead will scatter the evanescent wave as it moves into the evanescent wave illumination area, and the corresponding intensity reflects its distance from the interface. It has been proven that the scattering intensity also decays exponentially as42,45

I(h) = I0 exp(− βh)

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RESULTS AND DISCUSSION Adsorption of PNIPAM Microgels on the Silica and PS Microparticle Surfaces. In this section, characterization of surface properties of both PS particles and glass slide under different conditions was performed. Previous studies have shown that PNIPAM chains can be irreversibly adsorbed to the PS and glass surfaces through the hydrophobic interaction or hydrogen bonding.46−49 Confocal images from Figure S1A−E in the Supporting Information prove that the PNIPAM microgels can be irreversibly adsorbed on a glass surface with heterogeneous coverage. Also PS particles can be coated with a dense layer of microgels (Figure S1F, Supporting Information). In addition, ξpotential of the PS1 microparticle and silica surface (similar to glass surface) after adsorption of the microgels dispersed in 0.4 mM NaCl in Figure 1 gives a better

(3)

where h is the bead−surface separation, I0 is the scattered light intensity at h = 0, and β is the decay index expressed as β=

4π (n1 sin θ )2 − n2 2 λ

(4)

where θ is the incident angle; n1 and n2 are the refractive index of the incident and reflected medium, namely, the glass slide and the microgel solution; and β−1 is the penetration depth of the evanescent wave intensity. From eq 4, the refractive index n2 in each microgel solution with various Cmicrogel was determined by Abbe refractometer and then β−1 was calibrated at each Cmicrogel. The histogram of the separation p(h) over a long period leads to the potential profile between the probe bead and the surface wall through the Boltzmann distribution,

⎡ ϕ(h) ⎤ p(h) = A exp⎢ − ⎥ ⎣ kBT ⎦

(5)

where A is a constant normalizing the integration to unity. Furthermore, a so-called equilibrium distance he was defined as the separation where the maximum of p(h) locates in eq 5, corresponding to the local equilibrium position. The absolute value of he can be obtained by eq 3 as

he =

1 ⎡ I0 ⎤ ln⎢ ⎥ β ⎣ I(he) ⎦

Figure 1. Concentration dependence of ξpotential of microgel-adsorbed PS1 microparticle (diameter = 6.0 μm) and microgel-adsorbed silica microparticles (diameter = 5.0 μm). The dashed line at ξpotential = −10.0 mV is marked to empirically distinguish the charged and neutral states of PS and silica particles after microgel adsorption at their surfaces.

(6)

In our TIRM measurements, the sample container was made by sticking a glass ring onto a cleaned glass slide (∼1 mL) and then sealed with a cover glass slide to avoid evaporation. First, I0 was measured in saline solutions and used as the referenced intensity of the following TIRM experiments in the absence of microgel adsorption at the bottom glass surface. This value was obtained by measuring the averaged intensity of 6 different bare PS1 microparticles immobilized on the bottom glass surface. This is an appropriate method since the size distribution of the PS1 microparticles is narrow (CV = 4%) and the bottom glass slide is extremely flat (RMS < 1 nm measured by AFM). After that, interaction potential profiles of the bare PS1 microparticles in pure 0.4 mM NaCl solution were measured. Finally, the interaction potential profiles between the PS1 microparticles and the glass surface in the presence of microgel solutions with various Cmicrogel ranging from diluted (0.01 wt %) to concentrated (7.5 wt %) at 0.4 mM NaCl aqueous solutions were measured. In each measurement, an individual PS bead of average brightness was selected while other beads were sparsely dispersed in such a way that they were far away and would not interrupt the tracked one. The measurements were performed on 3 to 5 different beads at each microgel concentration. Besides, when replacing the next sample

understanding of how the adsorption coverage changes with the concentration of microgels. Before the microgel adsorption, PS1 microparticles (−90 mV) and silica substrates (−80 mV) are highly negatively charged, and the adsorption of neutral PNIPAM microgels has significantly reduced the surface charge. Figure 1 shows that ξpotential decreases with increasing concentration of microgel particles, and both surfaces become almost neutral at Cmicrogel > 0.5 wt % (ξpotential < −10 mV). Furthermore, the adsorption at both PS and silica surfaces is irreversible because repeated washing slightly changes ξpotential. Interaction Measurements between Microgel-Adsorbed Surfaces by TIRM in Microgel Solutions at ϕeff ≤ 0.5. After confirming the adsorption of PNIPAM microgels on both PS1 and silica surfaces, TIRM was applied to measure the interaction profiles between the PS1 microparticle and the glass surface immersed in microgel dispersions with varying 13184

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the presence of a PNIPAM microgel dispersion at 0.4 mM NaCl with concentrations varying from 0 to 7.5 wt % respectively, and potential profile in 100 mM NaCl solution with the same glass slide after the above measurements. It should be noted that each potential curve in Figure 2 was measured with different particles with the same size located at different places above the same glass slide. In pure 0.4 mM NaCl aqueous solution (Cmicrogel = 0 wt %), the PS particle undergoes moving with a particle−surface separation distance, he, ∼ 80 nm shown in Figure 2a. Both negatively charged surfaces result in electrostatic repulsion, avoiding the crush of two surfaces. This can be easily understood through Figure 3a. Based on DLVO theory, the corresponding potential, Φtol(h) is attributed to two parts, the double layer repulsion (Debye length κ−1) and the effective gravity (Geff), and thus can be expressed as43

concentrations at 0.4 mM NaCl, respectively. From Table 1, it can be seen that the effective volume of microgel ϕeff ranges from 0.005 to 3.75, corresponding to microgel concentration varied from 0.01 to 7.5 wt %. This makes our study obviously distinguished from most previous investigations of the same system, where the highest microgel concentration was limited in the range of ϕeff < 2. Our approach thus measures the interactions induced by the microgel dispersions across a structure transition from individual microgel particle to a highly packed state in order to explore the impact of softness as density increases. Figure 2 compares a series of potential profiles for the interactions between a PS1 particle and a flat glass surface in

Φtol (h) = B exp[−κh] + Geff h

(7)

where B is a constant which is determined by both the Stern potentials of the PS particle and the bottom glass surface. However, similar to previously reported results, κ−1 and Geff at the pure 0.4 mM NaCl curve show a large deviation from the theoretical value due to the fact that eq 7 fails to describe the interaction between the surfaces at high salt environment because of emergence of interactions like van der Waals force.50 As the sample is replaced by a mixture of microgels and PS1 particles, the potential profiles are altered. In the presence of 0.01 wt % microgels, the measured potential became narrower and moved further from the bottom surface (he ∼ 1.3dmicrogel). In addition, PS1 were located at a narrow distance, in the range 160 < h < ∼200 nm (1.3dmicrogel − 1.5dmicrogel) in Figure 2b, which is much higher than the case in pure NaCl solution (∼80 nm). This might be due to the fact that with the presence of microgels, the surfaces of both microparticle and glass surface are covered by some of them, which shifts the PS particle away. On the other hand, such a microgel-adsorbed surface likely facilitates polymer bridging between the two microgel bearing surfaces to immobilize the PS particle (Figure 3b).51,52 As the particle deposits and moves near the surface, due to the weak double layer repulsion in 0.4 mM NaCl, the particle with some adsorbed microgels contacts and builds polymer chain bridges with the bottom surface. However, this bridging effect will be weakened as Cmicrogel increases since bridging effect is preferred at low adsorption coverage.5,16,53,54

Figure 2. Potential energy profiles for interaction between PS1 particle (diameter: 6 μm, negatively charged) and a flat glass surface in the presence of microgels at 0.4 mM NaCl solution. The microgel concentration in this series of measurements varies: Cmicrogel = 0 (a), 0.01 (b), 0.1 (c), 0.5 (d), 1.0 (e), 3.0 (f), 7.5 (g) wt %. The potential energy profile in panel h with green curve was measured in pure 100 mM NaCl solution after microgel suspension was washed off. The order of these profiles is in accordance with the order of measurements. Solid black curve in panel h is eq 7 with Geff = 54 fN and κ−1 = 26 nm.

Figure 3. Schematics illustrate how the particle−surface interaction between the PS1 particle and the glass surface changes while the concentration of microgels is varied, going through electrostatic repulsion (a), bridging attraction (b), steric repulsion (c), depletion attraction (d), and oscillatory structure force (e). Finally, a polymer brush like repulsive interaction (f) was observed after the microgel suspension was washed out and replaced by 100 mM NaCl solution. 13185

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Figure 4. Optical images of the PS2 particles (diameter = 3.5 μm) dispersed in a series of binary colloidal suspensions containing both microgel nanoparticles (invisible here due to the size and refractive index) and PS2 microparticles under bright field. The scale bar in each figure is 10 μm. All the samples contain the same concentration of PS2 particles, 0.5% w/v. Cmicrogel varies in system: 1.0 (a), 3.0 (b), 7.5 (c) wt %.

oscillatory phenomenon was first observed in micelle solution by P. Richetti;23,24 and later a similar phenomenon was reported in hard nanoparticle suspension by Dinsmore.9 Simulation by Dickman showed that this oscillatory interaction is entropy-driven and resulted from a structure formed by nanoparticles in the bulk solution.25 However, the amplitude of the fluctuations induced by microgels given in Figure 2 is much larger than those in previously reported works. In our case, both surfaces are covered and surrounded by microgels, and the interaction between the PS particle and the bottom surface thus was determined by the packing structure of microgels nearby. Due to the deformation of microgels, as Cmicrogel increases, the condensed microgel suspension was expected to consist of more condensed and heterogeneous packing structures than the hard particle dispersion (Figure 3e), leading to an enhanced oscillatory interaction. To further confirm the results from TIRM measurements at higher Cmicrogel, in particular few previous published studies could achieve this regime, optical images of the binary suspension of micron-sized PS2 particles and PNIPAM microgels at concentrations of 1.0, 3.0, and 7.5 wt % are presented in Figure 4. To avoid the influence of electrostatic repulsions and fasten the colloidal kinetics at higher Cmicrogel, PS latex particles which were nearly neutral with smaller size (3.5 μm, 0.5% w/v, with carboxyl group at the surfaces, ξpotential ∼ −21 mV in 0.4 mM NaCl) were dispersed in microgel suspension with Cmicrogel from 1.0 to 7.5 wt % and then imaged under bright field. At this moment, both the glass surface and PS particles were saturated by microgels and the surfaces were neutral. In this case, observed interactions between microgeladsorbed PS−PS surfaces in the binary suspension were similar to interactions between microgel-adsorbed PS−glass surfaces in TIRM experiments. As shown in Figure 4a, crystals and tiny nuclei of PS particles were found in the sample with Cmicrogel = 1.0 wt %, where depletion attraction likely was the dominant force to force the particles together, and this is consistent with the TIRM result as shown in Figure 2d. However, as shown in Figure 4b, the size of PS crystals found in Cmicrogel = 3.0 wt % is smaller than in Cmicrogel = 1.0 wt %. Furthermore, in the most condensed sample, Cmicrogel = 7.5 wt %, no PS cluster was found throughout the available observation region (1.25 cm2) shown in Figure 4c. All these phenomena agreed well with the TIRM results in condensed conditions as shown in Figures 2f and 2g, where the PS particle could move again, and depletion interaction faded away and was replaced by structure force. One might argue that this weakened crystallization process in condensed conditions might be due to the increase of apparent

At Cmicrogel = 0.1 wt % (ϕeff = 0.05), the two surfaces adsorbed more microgels. The profile in Figure 2c becomes broad, revealing that the PS particle can move again. However, eq 6 failed to fit the measured curve well, similar to the interactions between microgel particles and silica surface.55 The fitting yields a smaller Geff and larger κ−1 compared with the theoretical values, indicating that other interactions like steric repulsion may be generated by the adsorbed microgels and become dominant instead of the bridging effect, preventing the two surfaces from further compression (Figure 3c). Upon further increase of Cmicrogel to 0.5 wt % (ϕeff = 0.25), the particle shifted closer and the potential in Figure 2d became narrow again compared with the curve in Cmicrogel = 0.1 wt %. We suspect that this variation of induced interaction from immobilized particle (Cmicrogel = 0.01 wt %) to movable (Cmicrogel = 0.1 wt %) and then to immobilized situation again (Cmicrogel = 0.5 wt %) might indicate that the particle−surface interaction changes from bridging to steric repulsion and then to depletion attraction, as the microgel adsorption tends to be saturated as Cmicrogel increases in this region (Figure 3d). Similar phenomena were reported by Zhao et al. in similar PS− PNIPAM microgel mixtures with variable microgel amounts.51 This is also supported by the zeta potential measurements as shown in Figure 1, in which the ξpotential of the two surfaces kept slight change as Cmicrogel ≥ 0.5 wt %, indicating that at Cmicrogel ≥ 0.5 wt %, the adsorption on both of the surfaces tends to be saturated. Therefore, in this region, depletion attraction induced by the free microgels in bulk might be involved at Cmicrogel ≥ 0.5 wt % where the number density of microgel particles in bulk is high enough (>49 μm−3, shown in Table 1) to produce a measurable depletion attraction as previously described.51,56,57 When Cmicrogel was increased to 1.0 wt %, (ϕeff = 0.5), a concentration very near phase transition of random packing of hard spheres,58 weakly immobilized beads were still found in Figure 2e, indicating that the depletion attraction is still dominant which is induced by the nonadsorbed microgel particles in the bulk. As schematically shown in Figure 3d, when the gap between the two microgel coated surfaces is smaller than the size of microgel, the depletion attraction appears. Upon further increase of Cmicrogel to ϕeff > 0.5, the microgel solution enters the region of dense packing, where random close packing for hard spheres tends to happen. As can be seen from Figures 2f and 2g, the potential profiles showed that the PS particle can move again but in an unusual way in such a condensed condition, reflecting that the depletion effect had disappeared. At Cmicrogel = 3.0 wt % (ϕeff = 1.5), an oscillated energy profile appears through a long-range distance. As Cmicrogel further increases to 7.5 wt % (ϕeff = 3.75), the fluctuation of the potential profile is enhanced. This kind of 13186

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viscosity as Cmicrogel increases. Admittedly, condensed microgel suspensions present quite big apparent viscosities, hundreds or thousands of times bigger than the diluted ones, which may cause a delay of PS particles’ crystallization process. The sedimentation of PS particles in Figure 4a was observed in no more than 1 h; while particles in Figure 4c took 1 week for full sedimentation and were observed one more week later after the sedimentation. We thus estimate the time for the emergence of the PS nucleation based on diffusion in bulk. Our estimation shows (see the Supporting Information) that in dilute sample Cmicrogel = 0.1 wt %, the diffusion coefficient (D) of PS particles is about 1150 times bigger than in the sample Cmicrogel = 7.5 wt %. Considering that the nucleation time in the sample Cmicrogel = 0.1 wt % is about 10 min (Figure S2, Supporting Information), the nucleation should start after ∼14 days for Cmicrogel = 7.5 wt %. In this way, 2 weeks should be long enough for observation. Therefore, the reductive kinetics may not be mainly responsible for the failure of nucleation at higher Cmicrogel. Furthermore, the structure of PS2 particles might give us information about the surrounding microgel structures. In our case, the crystallization process near the surface was a twodimensional (2D) case. As shown in the Supporting Information, the variation of the Gibbs free energy (ΔG) in the crystallization of PS particles was composed of two parts: the entropic loss contributed by PS259,60 and the entropic gain of the PNIPAM microgel particles. Assuming that no interactions exist between microgel particles, the increase of the microgel concentration should lead to an enhanced crystallization process. However, the crystallization became weaker and disappeared as Cmicrogel increased, meaning that the energy contribution from microgel particles changed and some structures (like aggregators) were formed among the microgel particles. The results from TIRM measurement luckily detected this structural variation in microgel suspensions as schematically shown in Figure 3e, although invisible from optical imaging. Brush-like Interactions between Microgel-Adsorbed Glass and Bare PS Particle. After finishing the measurements in all microgel solutions, the microgels in solution were removed and the sample solution was replaced by bare PS bead dispersed in pure 100 mM NaCl so that the same microgel bearing bottom glass slide was kept. The resultant potential profile is shown in Figure 2h. Surprisingly, the probe bead still moved in such a high salty environment, in which the double layer repulsion was significantly screened. Hence, we argue that the repulsion part to prevent the sticking of the PS bead should be the steric interaction of the brush-like PNIPAM chains on the shell of the adsorbed microgels at the bottom glass. The interaction potential by simplification of Alexander de Gennes model on the steric repulsion between polymer brushes was given as16,17,54,61 ⎡ h − he ⎤ Φtol (h − he) Geff = A exp⎢ − (h − he) ⎥+ ⎣ ⎦ kBT t kBT

like layer of microgels resting on the glass slide, and seems to be slightly deformed by the bottom surface, as schematically shown in Figure 3f. This speculation has been proved by Scheffold et al.,62 who confirmed a brush-like model to successfully describe the interaction between core−shell PNIPAM microgel particles. Structure and Elasticity of Microgel Solution at Cmicrogel = 7.5 wt %. The fluctuating potential profile at Cmicrogel = 7.5 wt % (ϕeff = 3.75) was enlarged in Figure 5,

Figure 5. (a) Potential energy profiles after subtraction of the gravity repetitively measured between microgel-adsorbed PS and glass surfaces in the same microgel solutions at Cmicrogel = 7.5 wt % with 0.4 mM NaCl at 24 °C. (b) Normalized force profiles for comparison with other force techniques which were deduced by differentiation of the corresponding potential curves (F = −dΦ(h)/dh) in Figure 3a. R is the radius (3.0 μm) of the probe PS particle. (c) The shear modulus G(h) = dΦ(h)/dV exerted by the microgel bulk.

where the gravity effect was removed. The curve was drawn as a function of both h and the ratio of the distance to the diameter of the PNIPAM microgel (h/dmicrogel). Potential profiles taken at Cmicrogel = 7.5 wt % showed fluctuating curves with several potential wells from h ∼ 1.4dmicrogel to 2.6dmicrogel, corresponding to the locally favorite position of the PS particle entrapped in the condensed microgels, while the peak corresponds to the disfavored position. The energy wells were found nearly located at close distance from the glass surface even using different PS probes, indicating the uniformity of this energy profile over a large landscape. Although the energy curves vary with time and probes, the oscillation period and the depth of the energy well deviate slightly; Figure 5a shows that the PS particle is limited within a spatial range of ∼dmicrogel, with local energy wells ∼0.1 to 1.0 kBT. Similar energy values were reported in PS− PNIPAM core−shell particles and micelle condensed solution

(8)

where L = 2πt is the brush length and A is a constant. It should be noted that eq 8 is valid at intermediate surface separations: 0.2L ≤ h ≤ 0.9L. Fitting eq 8 yielded G = 54 fN, which was close to the theoretical value 50 fN for 6 μm PS particles, while the left part gave t = 26 nm, corresponding to a brush length of L = 165.2 nm in the brush model, which is comparable to the measured he ∼ 160 nm (1.2dmicrogel). This result shows a brush13187

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dΦ dV −1 dΦ ⎛⎜ dV ⎞⎟ =− · dh ⎝ dh ⎠

(0.1 to 10 kBT) measured with bulk rheometer by Koumakis et al.,63 while condensed hard PMMA spheres showed a much stiffer structure with elastic energy ∼100 kBT. However, no fluctuating profiles were given in their works because comparable size of the probes and the bulk were used, and also the limitation of the instrument sensitivity. Furthermore, we found that several coincident positions of the energy well measured on different PS beads existed, i.e., at h ∼ 210, 233, 255, 278, 300 nm, corresponding to h/dmicrogel ∼ 1.70, 1.85, 2.05, 2.20, 2.40. These positions represented a characteristic structural length at Cmicrogel = 7.5 wt % in the confined case as ∼25 nm (0.20dmicrogel), corresponds to a local length scale of geometry of the packed microgel particles in bulk. Therefore, the averaged potential oscillatory period and respective energy well could be obtained, sketching the microstructure of the microgels between walls at such condensed states. The averaged width of the energy well (W, the distance between the neighboring energy peaks) equals 38 and 14 nm, and the depth of the energy well of all the oscillatory potential curves (E, the energy difference between the local peak and the neighboring valley) equals 0.35 and 0.67 kBT at 3.0 and 7.5 wt % respectively. As the concentration increased, W became narrower while E slightly increased, both indicating that the local microgel piles packed more densely. As a result, the tension T = E/W generated by the microgels on the probe particle could be deduced as 0.037 pN and 0.192 pN at Cmicrogel of 3.0 and 7.5 wt %, which was ∼1 and 5 times of the particle’s gravity (0.042 pN), respectively. Figure 5b gives the profile of the force along the h direction calculated by the differentiation of the corresponding potential curves and normalized by the PS probe’s radius. As can be seen, the local force fluctuated mostly between ±0.1 μN/m from 1.6 to 2.2dmicrogel. Unlike from those ordered structure oscillatory interactions found in ionic micelle,23 polyelectrolyte solutions,64 and the steric resistance between microgels and silica surface,55,65 the structural strength here was 2−3-fold smaller. This might be due to the absence of the strong electrostatic effects and the passive nature of the measurements compared to techniques like AFM. Shear modulus G(h) of the microgel dispersion in mPa unit defined by G(h) = −dΦ(h)/dV can be further obtained and are shown in Figure 5c from the potential profile in Figure 5a and force in Figure 5b, where dV is defined as excluded volume of surrounding microgel as a particle at h moving to h + dh, and dV can be deduced considering a small shift dh for a microgel covered PS sphere with radius of (R + dmicrogel/(ϕeff)1/3) as

G(h) = −

2⎫ ⎧ ⎡ dmicrogel ⎤ ⎪ dΦ ⎪ ⎢ ⎥⎬ =− ·⎨π R + 1/3 dh ⎪ ⎢⎣ ⎥⎦ ⎪ ϕ ( ) eff ⎩ ⎭

−1

G(h) = −dΦ(h)/dV = dF/dh demonstrates the restored strength of the local microgel solution as the Brownian PS particle disturbs this system, which fluctuates with value ∼tens of mPa. Compared with the reports from active mechanics which showed elastic modulus of kPa inside a single microgel,66−68 our results indicate extremely soft physical contact existence between microgel particles. The fact that the apparent pressure is much smaller than the value reported on the active measurement of the elasticity of a single microgel suggests that the internal structure of this self-packing microgel pile is not packed as strongly as we commonly expected.

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CONCLUSION We measured the particle−surface interaction induced by PNIPAM microgel particles with concentration dependence by monitoring the energy profiles on a micron-sized PS particle immersed in the bulk of microgels using TIRM. Our result described a structure transition of the microgels from dilute dispersion to concentrated state. First, at very dilute Cmicrogel, a bridging effect was observed. As Cmicrogel increased, the interaction turned to steric repulsion as the soft microgel particles increasingly adsorbed on the two surfaces. As Cmicrogel reached the point where both two surfaces were saturated with soft microgels, a transition from steric repulsion to depletion attraction was found. By further increasing Cmicrogel, the microgels were closely packed between the two surfaces, and the dominant interaction turned out to be structural interaction which weakly oscillates. The optical image of PS particles immersed in the concentrated suspension of microgel particles agreed with the TIRM results: the crystallization of PS particles was weakened and finally disappeared as Cmicrogel increased. Analysis of the measured interaction potentials showed that the local energy well produced by these dense packed microgels to trap the PS probe is ∼0.1−1 kBT, with width ∼0.2dmicrogel. Decreased spatial period and increased depth of the energy well were found as Cmicrogel increased. The shear force and modulus of the condensed microgels with Cmicrogel = 7.5 wt % exerting on the probe was ∼0.2 pN and tens of mPa, indicating that the condensed microgels are not as densely packed as expected. Conclusively, this work provides a new insight into the soft colloid involved interactions, and proposes a noninvasive way to investigate the local energy profiles related with dense and fragile hydrogel particles.

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2 ⎧⎡ 2⎫ dmicrogel ⎤ ⎪ ⎪ h (d ) ⎥ − ⎬ dV = π dh⎨⎢R + 1/3 12 ⎪ ⎪⎢⎣ (ϕeff ) ⎥⎦ ⎭ ⎩ 2 ⎡ dmicrogel ⎤ ⎥ dh ≃ π ⎢R + ⎢⎣ (ϕeff )1/3 ⎥⎦

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ASSOCIATED CONTENT

S Supporting Information *

Additional details on adsorption of PNIPAM microgels on the glass and PS microparticle surfaces, estimation of the crystallization time in condensed microgel dispersion, and estimation of energy variation in 2D crystallization process in a binary system. Confocal micrographs showing the adsorption behavior of fluorescent PM2 microgels. Table of data for apparent viscosity of microgel solution at different Cmicrogel.

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As a result, 13188

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Figure depicting colloidal nuclei of PS2 particles in a binary suspension. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of this work by the Hong Kong Special Administration Region (HKSAR) General Research Fund (CUHK402712, 2130304) and the Direct Grant for Research 2013/2014 (4053057) of the Chinese University of Hong Kong is gratefully acknowledged.

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