Article pubs.acs.org/Langmuir
Interactions between Solid Surfaces Mediated by Polyethylene Oxide Polymers: Effect of Polymer Concentration Xiaoling Wei, Xiangjun Gong,* and To Ngai* Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong ABSTRACT: Using total internal reflection microscopy (TIRM), we have systematically measured the interactions between a microsphere and a flat hydrophilic surface in the presence of polyethylene oxide (PEO) polymer solution. Our results reveal that PEO significantly mediates the interaction forces between the two surfaces. At low polymer concentration, the interactions between two surfaces in the presence of PEO are mainly dominated by repulsive forces, originating from diffuse layer overlap. At intermediate polymer concentration, a long-range and weak attraction sets in. This force is likely attributed to the depletion attraction due to the presence of free PEO chains in bulk solution; however, a simple hardsphere AO model fails to precisely describe the attraction. At high polymer concentration where PEO chains overlap, the attraction disappears, and levitation of the microsphere probe is detected. We argue that at this overlapping region, the correlation length of PEO chains is much smaller than the size of single PEO molecule, leading to weakening and disappearing of the depletion attraction. Finally, at very high concentration, oscillatory structural force is obviously found, indicating the significant structural ordering of the PEO chains under confinement.
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particles,15−17 and polymers18−21 in good solvents, interact with each other to produce nonideal solution osmotic pressure that are not generally captured by a simple hard sphere AO model. To overcome these limitations, a number of theoretical methods have been developed to model different types of depletants.22−25 In addition, it also shows that, as the concentration of depletant increases beyond the dilute limit, the ordering of the depletants under confinement at a higher concentration can lead to an oscillatory force profile between the large colloidal particles.14,26−29 Polyethylene oxide (PEO) is a nonionic polymer that is highly soluble in both aqueous and organic solvents. This, combined with its relative biocompatibility, has made it a popular model polymer to mediate the interaction between the solid surfaces and found application in various fields, especially in biological applications such as drug encapsulation,30 antifouling applications,31−33 and biological sensors.34 Therefore, a systematic exploration of the interaction between colloids induced by PEO provides an essential insight into PEO-mediated biological systems. Many direct force measurements were performed to evaluate the interactions mediated by the presence of PEO chains, but the information available in the literature is rather conflicting. For instance, Ohshima et al.8 utilized laser radiation pressure to directly measure the interactions between a colloidal particle and a glass surface in a PEO solution. An attractive force, which was attributed to the depletion force, was found. The measured force could be well
INTRODUCTION The adsorption of polymers from solution to solid surfaces is used to control interfacial properties such as steric stabilization or flocculation, surface tension, and lubrication properties. This is especially important in many practical applications including cosmetics, water treatment, foodstuffs, paint, and inks, in which the application of solid particles often involves the use of polymers.1−3 Polymers may be physically adsorbed to the colloidal particles, chemically attached to their surfaces, or free in bulk solution. Whatever forms polymers take, their presence will significantly mediate the interactions between two surfaces. If polymers do not adsorb onto the particle surfaces, from a traditional view, polymers in aqueous solution can promote flocculation of stable colloidal suspensions through a depletion interaction. Asakura and Oosawa4,5 as well as Vrij6 theoretically (AO theory) described this force and its origins. The term “depletion” presents the exclusion of these nonadsorbed polymers from the gap region between two large colloidal particles that arise when the separation distance between the two particles is less than the diameter of the depleting polymers. The resulting concentration gradient between gap region and bulk solution leads to an effective attraction, whose magnitude depends on the polymer concentration, the surface charge, and the size ratio of particle to depletant. Although depletion interaction is weak, it has been directly measured by using atomic force microscope (AFM),7 laser radiation pressure,8 surface force apparatus (SFA),9 total internal reflection microscopy (TIRM),10 and magnetic chaining technique.11 In simple AO theory, the depletants are treated as hard sphere and do not interact with each other. However, many depletants, such as micelles,12−14 charged nano© 2013 American Chemical Society
Received: May 2, 2013 Revised: July 9, 2013 Published: August 5, 2013 11038
dx.doi.org/10.1021/la401671m | Langmuir 2013, 29, 11038−11045
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fitted by simple AO theory, but the magnitude was larger than the predicted one. Rudhardt et al.10 performed measurements on the interaction between a charged polystyrene (PS) particle and a glass surface in dilute PEO solutions with TIRM under similar conditions that were conducted by Ohshima. They also concluded the resulting interaction potentials to be strongly dependent on PEO polymer concentration and the occurrence of depletion force induced by free PEO chains in bulk solution. However, no such depletion interaction was observed in the same system by Kleschchanok and Lang using TIRM.35 Conversely, a steric repulsion due to the formation of a brushlike PEO layer on the PS particle and the glass surface was detected. Besides treating PEO as nonadsorbing polymer on PS particle and glass surfaces in depletion studies, some scientists also investigated the adsorption behavior of PEO polymers onto various surfaces, including mica,36 glass,37 silica,38,39 and PS.40 For example, Braithwaite et al.37 employed AFM to study the development of the PEO layer onto glass surface in an aqueous system and found a time-dependent PEO polymer layer coverage. At partial polymer coverage, a weak attraction was often detected, which was attributed to polymer bridging between the two surfaces; at full polymer coverage, repulsive interactions at all surface separations were observed. However, Chai and Klein41 argued that PEO polymer does not adsorb onto mica surfaces from water in the absence of added salts. Therefore, the interesting question here is what is the exact role of PEO polymer chains in mediating the interactions between the common surfaces including hydrophilic silica and hydrophobic polystyrene surfaces, and whether they cause bridging, steric repulsion, or depleted from interfaces to result in depletion attraction? In this Article, we try to answer this question by performing a systematic study on the interactions between a free-moving micrometer-sized polystyrene (PS) particle and a flat silica surface in the presence of long PEO chains in aqueous solutions. Particularly, we use TIRM to explore the interaction potential profiles of the polymer-mediated interaction in solution over a broad range of PEO concentrations, spanning the dilute region where individual chains are separated to the semidilute regime where they become entangled and overlapped. A better understanding of the effects of additional PEO in the colloidal systems has been shown in this study, although they had been extensively investigated before.
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high chain overlapping. The concentration range of semidilute solution is not well-defined. Most of the investigations were rather arbitrarily limited to a volume fraction no more than 10% (w/v). The used deionized water was purified with an inverse osmosis filtration (Nano Pure, Barnstead) until its resistivity reached 18.2 MΩ cm (at 20 °C), and then filtered with a Millipore PTFE 0.45 μm hydrophilic filter. Sodium choloride (NaCl) was heated to 200 °C to get rid of organic impurities. Total Internal Reflection Microscopy (TIRM). TIRM is a sensitive noninvasive single molecule force microscopy to measure the interaction between a colloidal particle and a wall with femtonewton resolution. TIRM uses evanescent wave light scattered by a single micrometer-sized particle near a flat surface to determine the equilibrium distribution of particle−surface separations (h) and the associated interaction energy. TIRM has proven to be able to sensitively resolve colloid−surface interactions on the kT energy scale and nanometer length scale. In TIRM experiments, the evanescent wave, which decays exponentially with the distance from the interface, is generated at the solid/liquid interface by a total internally reflected within the glass slide. The scattered intensity by the colloidal probe depends on the h and can be determined to be:
I(h) = I0 exp(− βh)
(1)
where I0 is the “stuck” particle intensity, which can be obtained by salting out on the PS particle in 100 mM NaCl on the bottom silica surface after finishing all of the experiments and washing the system with large amount of water and salt solution. β−1 is the characteristic penetration length. Measuring the scattering intensity over time provides a histogram of separation distances. The probability of finding a particle at any given separation distance, p(h), is related to the total potential energy at that point according to the Boltzmann relationship: ⎛ Φ (h) ⎞ p(h) = A exp⎜− tol ⎟ ⎝ kT ⎠
(2)
where Φtol(h) is the total interaction potential and A is a constant normalizing the integrated distribution to unity. TIRM setup has been described in previous literature,43 while in our experiment, the characteristic penetration depth value β−1 is 100.4 nm according to the following equation: β −1 =
λ 4π (n1 sin θ)2 − n22
(3)
where n1 = 1.515 and n2 = 1.330 are the refractive indices of the glass slide and water, respectively. It should be noted that the influence of the free PEO polymers and electrolytes in the solution on β−1 can be neglected in our system, because n2 is estimated as 1.331 at the highest PEO concentration, based on the refractive index increments for NaCl (dn/dc = 1.70 × 10−4 L/g), NaOH (dn/dc = 2.78 × 10−4 L/g), and PEO (dn/dc = 1.35 × 10−5 L/g).35 As a matter of fact, we found that the background scattering intensity ratio (ratio of background scattering intensity to the particle stuck intensity) showed 0.1% deviation in the presence of highest concentration of PEO (6.40 g/L) polymer, which was in the range of fluctuation of the laser light. Despite this, a more significant effect might be expected on the variation of β−1 due to the adsorption of PEO molecules onto the bottom silica surface. However, it has been reported that PEO only weakly adsorbs onto mica and silica surfaces in the presence of sodium salt solution.44,45 Therefore, in our system, we believe that the attachment of PEO to the silica surfaces is weak, resulting in low surface coverage of the glass surface from the Na+-containing solution. As a result, β−1 is assumed to be varied slightly all through the TIRM experiments. All TIRM experiments were conducted at 23 °C. Typically, the experiments were first conducted in background solvent (0.1 mM NaOH + 0.1 mM NaCl), where NaOH was used to make both of the surfaces highly and negatively charged and NaCl was used to adjust the ionic strength. After that, the environmental medium was replaced by PEO solution prepared with the same background solvent, in which
EXPERIMENTAL SECTION
Materials. Polystyrene (PS) microspheres with sulfate groups (sulfate latex, diameter ∼5.6 μm, CV 4.1%, highly charged) were purchased from Interfacial Dynamics Co., U.S., and used without further treatment. Extremely smooth BK-7 silica slides were provided by Fischer Scientific Co. The hydrophilic glass slides were treated by ultrasonication in ethanol for 10 min, dried with nitrogen, and then dipped in 4% HF aqueous solution for a few seconds. After being washed with a large amount of deionized water, the treated hydrophilic slides were dried with nitrogen and further cleaned by UV-zone plasma cleaner (Harrick Sci. Co.) before use. Monodisperse polyethylene oxide (PEO) polymer (polydispersity
