Induced Attraction between Polystyrene Colloidal Particles in a Binary

May 3, 2017 - We investigate the phase behaviors of binary mixtures of polystyrene (PS) hard-sphere and poly(N-isopropylacrylamide-co-acrylic acid) (P...
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Induced Attraction between Polystyrene Colloidal Particles in a Binary Mixture with PNIPAM Colloidal Microgels Isaiah E. Igwe,†,‡ Yiwu Zong,§ Xiunan Yang,§ Zhongcan Ouyang,*,† and Ke Chen*,§ †

Institute of Theoretical Physics, Chinese Academy of Sciences, Zhong Guan Cun East Street 55 #, P.O. Box 2735, Beijing 100190, P. R. China ‡ School of Physical Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, P. R. China § Beijing National Laboratory for Condensed Matter Physics and Key Laboratory of Soft Matter Physics, Institute of Physics, Chinese of Academy of Sciences, Beijing 100190, P. R. China ABSTRACT: We investigate the phase behaviors of binary mixtures of polystyrene (PS) hard-sphere and poly(Nisopropylacrylamide-co-acrylic acid) (PNIPAM) soft-sphere colloidal particles as a function of temperature. As the temperature increases, apparent attractions arise between the PS particles, to the point of clustering at the highest temperature. This attractive force is found to be due to the change in pH caused by the acrylic acid co-polymerized with the temperature-sensitive PNIPAM particles, which in turn collapses the sterical stabilizing surface layers on the PS particles. Our experiments provide a new way to tune colloidal interactions with temperature for particles that are otherwise insensitive to temperature variations.

1. INTRODUCTION Colloidal dispersions consisting of nanometer- to micrometersized particles exhibit a wide range of phase behaviors, which have attracted attention in various fields, ranging from physics to materials sciences.1−3 From the point of view of physics, colloidal systems serve as ideal model systems for studying fundamental statistical phenomena;4−8 and in materials sciences,9−12 the rich phase behaviors of the colloids provide the flexibility needed for designing smart materials that can adjust to external stimuli. At room temperature, the phase behaviors of colloidal dispersions are largely determined by the particle concentration and pair interactions between the particles, including van der Waals attractions, Coulombic interactions, steric repulsions, and hydrogen bonding. It is thus critical to control the microscopic interactions in colloidal suspensions to design or manipulate the macroscopic properties of colloidal systems.11−13 For micron-sized colloidal particles, the attractive van der Waals forces at small distances are strong enough to cause permanent aggregation in colloidal suspensions. Two main mechanisms have been developed to maintain the stability of colloidal dispersions. Electrostatic double layers induced by the ionization of molecules decorated on the surface of colloidal particles can repel particles with the same charge. In nonpolar solutions, wherein free ions are scarce, steric stabilization is often employed. Polymer chains attached to the surface of colloidal particles extend into the solvent. A repulsive force is generated when the chains from two particles come in contact, as the entanglement of the polymer chains is entropically unfavorable. Both electrostatic and steric stabilizations depend © 2017 American Chemical Society

on the chemical composition of the particles and solvent and are thus difficult to tune once a sample is prepared.13−20 Stimuli-sensitive polymers offer a limited means to adjust the interaction between colloidal particles. For example, small thermosensitive poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM) particles have been employed to tune the depletion attractions between hard-sphere colloids.21−23 pH values and ionic strength are two other environmental factors that colloidal particles may be responsive to,24−27 although reversible control of pH or ionic strength in colloidal suspensions is less convenient compared to that of temperature.25−28,37 To improve the control of interactions between colloidal particles, one possible path is to design a system in which particle responses to an external stimulus (e.g., temperature) can affect other environmental factors (e.g., pH values or ionic strength), hence influencing the interactions between particles that would otherwise be insensitive to the applied external stimulus. Such multicomponent mixtures of colloidal particles provide an interesting platform to study systems with complex interactions and may exhibit rich phase behaviors that are absent in singlecomponent colloidal systems. In this article, we experimentally investigate the influence of temperature-sensitive PNIPAM colloidal microgels on the clustering behavior of sterically stabilized polystyrene (PS) particles. We observe the development of apparent attractive potentials between PS particles as the sample temperature Received: December 27, 2016 Revised: May 1, 2017 Published: May 3, 2017 5391

DOI: 10.1021/acs.jpcb.6b12999 J. Phys. Chem. B 2017, 121, 5391−5395

Article

The Journal of Physical Chemistry B increases and eventually the formation of clusters of PS particles. This attractive interaction between PS particles can be attributed to the effect of lower pH values induced by the response of PNIPAM at higher temperatures. Our experiments demonstrate that in complex colloidal systems the effect of tuning environmental factors may be amplified by the interactions between the particles, and it is possible to design or control the responsive particles via such a path.

aqueous solution, the particles appear darker than the surrounding liquid, as shown in Figure 1a. The PNIPAM particles are slightly less dark compared with the PS particles, as dye molecules can diffuse into PNIPAM microgels but not into the PS spheres, thus allowing the identification of particle species on the basis of their relative brightnesses. The acquired images are digitally inverted before being analyzed by particle tracking software.29−32

2. EXPERIMENTAL SECTION The samples consist of a mixture of PS and PNIPAM particles, sealed between two cover glasses, forming a quasi-twodimensional (2D) monolayer at the glass surface.28 The number ratio between the PS and PNIPAM particles is about 30:70. The PS particles are synthesized using the dispersion polymerization method.38 These particles are sterically stabilized with poly(4-vinyl pyridine) (PVP) chains decorated on the particle surface; thus, the interaction between the PS particles can be considered to be hard-sphere-like. The diameter of the PS particles is measured to be 1.0 μm by dynamic light scattering. The PNIPAM particles are synthesized using the no soap emulsion polymerization method.39 The interactions between the PNIPAM particles are softer than those in PS, due to the high water content of the microgel particles. The diameter of the PNIPAM particles is measured to be ∼1.0 μm at 25 °C. Of the two types of particles in our sample, PNIPAM particles, whose diameter decreases at higher temperatures, are responsive to temperature, whereas PS particles are not obviously temperature responsive. A small amount of fluorescein dye (∼0.3% w/v) is added to the colloidal mixture to improve imaging contrast. The samples are imaged by fluorescence video microscopy at 30 frames/s, at temperatures ranging from 24.0 to 40.0 °C. The sample temperature is controlled by thermal coupling to a resistively heated microscope objective (EHEM Professional 3), and the sample is allowed to equilibrate for 30 min at each temperature before data acquisition. Some snapshots of the mixture are shown in Figure 1. As the fluorescent dye is dispersed in the

3. RESULTS AND DISCUSSION We focus on the interaction potential between PS particles, which are not intrinsically temperature sensitive, in the mixtures at different temperatures. At low temperatures, it is observed that the PS particles diffuse freely in the mixture. As the temperature increases, the distribution of PS particles becomes less uniform, suggesting the development of attractive forces. At the same time, the diffusion of the PS particles becomes much slower. At the highest temperature, clear clusters of aggregated PS particles are observed, as shown in Figure 1d, indicating strong attractions between them. To quantify the interaction between the PS particles in the mixtures, we measured the pair correlation function g(r)32−35 and mean square displacements (MSDs) of the PS spheres in the mixtures at different temperatures. The obtained g(r) is averaged over 10 000 frames to reduce statistical noise at each temperature. In the dilute limit, the effective pairwise interaction potential, U(r), between colloidal particles in an equilibrium suspension is related to g(r) through the Boltzmann distribution as ⎛ U (r ) ⎞ g (r ) = exp⎜ − ⎟ ⎝ kBT ⎠

(1)

where g(r) is the pair correlation function, kBT is the thermal energy, U(r) is the pairwise interaction potential, and r is the interparticle distance. Equation 1 is applicable to dilute colloidal suspensions, wherein pairwise correlations are dominant. At finite concentrations, we measure the potential of mean force W(r) = kBT ln{g(r)}, which reduces to U(r) when the concentration approaches zero. Many-body correlations may become significant at high effective packing fractions or for samples under confinement, which will lead to peaks in g(r) even for purely repulsive spheres.29,35 In our experiments, the overall packing fraction of PS and PNIPAM spheres is ∼7%, with the packing fraction of PS spheres at ∼3%. Figure 2a plots the measured g(r) curves for PS particles in the mixture at different temperatures. Figure 2a plots the measured g(r) curves for PS particles in the mixture at different temperatures. As the temperature

Figure 1. Images of PS + PNIPAM colloidal suspensions in the quasi2D confinement. (a) Raw image of PS + PNIPAM at 24 °C. (b) Inverted image of (a). (c) Snapshot of the mixture at 30 °C. (d) Snapshot of the mixture at 40 °C. In all of these images, the PS particles appear slightly smaller, with sharper contrasts, whereas the PNIPAM spheres appear larger with fuzzy edges.

Figure 2. Temperature dependence of PS particles in the mixture. (a) g(r) as a function of distance r for PS particles. (b) MSD of PS particles. 5392

DOI: 10.1021/acs.jpcb.6b12999 J. Phys. Chem. B 2017, 121, 5391−5395

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The Journal of Physical Chemistry B increases, the first peak of the g(r) curves becomes more pronounced, as plotted in Figure 2a (inset), indicating the development of attractive potentials between PS particles in the mixtures with an increase in the sample temperature. The emergence of attractive interactions is further confirmed by MSD measurements, as shown in Figure 2b. At low temperatures, the MSD of the PS particles is close to a straight line, suggesting unimpeded free diffusions. As the temperature increases, the MSD curves start to bend. Because the overall concentration of the samples remains unchanged, the bending of the MSD curves is unlikely the result of caging effects commonly found in supercooled liquids or glasses and is consistent with the observed attractive interactions between PS spheres. Several factors may contribute to the apparent temperature sensitivity of sterically stabilized PS particles, including temperature responsiveness of the decorated polymer chains on the surface of PS particles or the change in solution pH values in the presence of PNIPAM spheres. We perform several control experiments to examine the effects of these factors. First, we measure the temperature dependence of PS particles in the absence of PNIPAM spheres. Figure 3a plots

fact that neither PS nor PVP is known to be temperature responsive in the temperature range of our experiment. Next, we test the hypothesis that the attractive interaction between PS particles is due to the change in the environment of the solution by the temperature-responsive PNIPAM microgels. As a weak acid, the ionization of acrylic acid (AAc) depends on temperature and may change the pH values and ionic strength of a sealed solution. Figure 4a plots the measured pH values in a suspension of PNIPAM particles. The concentration of the suspension is comparable to the PNIPAM concentration in the experimental mixture. The pH value decreases from 5 to below 4 when the temperature increases from 24 to 40 °C, probably due to the increased ionization of −AAc groups in PNIPAM particles, confirming that in response to temperature variations the PNIPAM particles also affect the ionic balance of the solution. The g(r) between PVP-decorated PS particles in the same pH ranges are measured by video microscopy and are plotted in Figure 4b, which clearly shows the development of attractive interactions between PS spheres as the pH value is lowered, as the intensity of the g(r) peaks increase from 1.7 to more than 4 (Figure 4b, inset). Thus far, we have demonstrated that temperature-responsive PNIPAM-co-AAc microgels are able to change the pH values of a solution in response to temperature variations, which in turn will induce attractive interactions between PS particles in the same solution. The change in the pH values in the environment is a result of ionization of the AAc copolymer in the PNIPAM spheres, and the pH responsiveness of the PS spheres may be due to the response of the PVP chains attached to the particle surfaces for the stabilization of the colloids. Previous studies have reported pH sensitivity of the PVP polymer chains.36 Using DLS, we measured the hydrodynamic diameter of free PVP polymer chains (m/w 30 000; Sinopharm Chemical Reagent Co. Ltd.) in solutions of different pH values. As plotted in Figure 4c, the hydrodynamic radius of free PVP chains decreases when the solution pH value is lowered, which suggest that the chains fold in more acidic environments, thus becoming less efficient as steric stabilizers for the PS particles. We can now propose a plausible mechanism for the apparent temperature sensitivity of PS spheres observed in our experiment. When the temperature increases, the pH value of the solution decreases and the concentration of ions increases. In response to the change in the solution environment, the PVP chains on the PS surface will fold, which leads to the destabilization of PS spheres, eventually causing the PS particles to form clusters at the highest temperature. In addition to the collapse of steric layers on the PS particles, the shift in the ionic balance of the solution may also have contributed to the

Figure 3. Temperature dependence of PS particles without PNIPAM. (a) g(r) as a function of distance (r) for PS without PNIPAM. (b) MSD of PS without PNIPAM.

the measured g(r) curves for PS particles at different temperatures. The peaks of the g(r) curve are close to 1 and are much lower than those in Figure 2, and the peak intensity does not change systematically with a change in the sample temperature, as shown in Figure 3a (inset). The lack of temperature sensitivity is also confirmed by the MSD measurements (Figure 3b), which show straight lines for all experimental temperatures. This control experiment excludes the possibility of intrinsic temperature responsiveness of the PS particles in our experiment, which is further supported by the

Figure 4. (a) pH value of PNIPAM suspensions as a function of temperature. (b) g(r) of PVP-decorated PS over different pH ranges. (c) Hydrodynamic diameter of PVP measured by dynamic light scattering (DLS) as a function of solution pH values. The error bar stands for standard deviation. 5393

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emergence of attractive interactions between PS spheres, as the thickness of the charged double layers near the PS spheres will be reduced from the increased ionic concentration at lower pH values. The PS particles in our experiments are weakly charged and rely on the steric repulsion for stability; therefore, the effects from charged layers are expected to be minor. An interesting feature of the observed attractive interactions between PS particles is that the interaction appears to be long-ranged at the highest temperature, extending to several particle diameters away. The exact origin of this long reach of the attraction is not clear. We speculate that the observed longrange correlations may be due to the breakdown of system ergodicity as the PS particles start to form nearly permanent clusters at the highest temperature, which may eventually lead to phase separation between the PS and PNIPAM particles in the mixture.

4. CONCLUSIONS To summarize, we investigated a binary mixture of colloidal PS particles and temperature-sensitive PNIPAM particles and discovered that by tuning the temperature of the mixture attractive potentials between PS particles, which are otherwise temperature-insensitive, can be induced. By performing a series of control experiments, we show that the attractive potential is due to the change in the environment of the solution by the temperature-sensitive PNIPAM spheres, specifically the pH values. Our experiment demonstrates that by designing elemental interactions in colloidal mixtures one can take advantage of the solvent−particle interactions to control and manipulate the responses of the composite particles to external stimuli, thus providing the opportunity to create complex colloidal materials from simple ingredients.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.O.). *E-mail: [email protected] (K.C.). ORCID

Ke Chen: 0000-0002-1886-3715 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.E.I. gratefully acknowledges the CAS-TWAS President’s Fellowship. K.C. acknowledges the support from the MOST 973 Program (No. 2015CB856800) and the NSFC (No. 11474327). Z.O. acknowledges the National Basic Research Program of China (973 program, No. 2013CB932803).

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REFERENCES

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DOI: 10.1021/acs.jpcb.6b12999 J. Phys. Chem. B 2017, 121, 5391−5395