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Specific ion effects and pH dependence on the interaction forces between polystyrene particles Francisco Javier Montes Ruiz-Cabello, Tamas Oncsik, Miguel A. Rodríguez-Valverde, Plinio Maroni, and Miguel A. Cabrerizo-Vílchez Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03316 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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Specific ion effects and pH dependence on the interaction forces between polystyrene particles F. Javier Montes Ruiz-Cabello1,*, T. Oncsik2, M.A. Rodríguez-Valverde1, P. Maroni2, M. Cabrerizo-Vilchez1 1Biocolloid

and Fluid Physics Group, Applied Physics Department, Faculty of Sciences,

University of Granada, Campus de Fuentenueva s/n, 18071, Granada, Spain 2Department

of Inorganic and Analytical Chemistry, University of Geneva, Sciences II, Quai Ernest-Ansermet 30, 1205 Geneva, Switzerland

KEYWORDS Ion specificity, Surface Forces, Colloidal Probe Technique, Polystyrene

1 ACS Paragon Plus Environment

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Graphical Abstract

ABSTRACT

Colloidal interactions have been extensively studied due to the wide number of applications where colloids are present. In general, the electric double layer force and the van der Waals interaction dominate the net force acting between two colloids at large separation distances. However, it is well accepted that some other phenomena, especially those acting at short separation distances, might be relevant and induce substantial changes in the force profiles. Within these phenomena, those related to the surface contact angle, the hydration degree of the ions or the pH, may dominate the force profiles features, not only at short distances. In this paper, we analyzed the effect of the pH and counterion type on the long-range as well as the short-range forces between polystyrene colloidal particles by using the colloidal probe technique based on AFM. Our results confirm that


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the features of the force profiles between polystyrene surfaces are strongly affected by the pH and hydration degree of the counterions in solution. Additionally, we performed a study of the role of the pH on the wettability properties of hydrated and non-hydrated polystyrene sheets to scan the wettability properties of this material with pH. Contact angle measurements confirmed that the polystyrene surface is hydrophobic in aqueous solutions over the entire range of pHs investigated. These results are in good agreement with the features observed in the force profiles at low pH. At high pH, a short range repulsion similar to the one observed for hydrophilic materials, is observed. This repulsion scales with the pH and it also depends on the hydration degree of the ions in solution. This way, the short range forces between polystyrene surfaces may be tunable with the pH and its origin does not seem to be related to the hydrophobicity of the material.

Introduction Direct force measurements between surfaces across liquids is a very useful tool for understanding many phenomena revealed in systems of strong interest such as colloidal suspensions1, drilling fluids2, ceramic green bodies and many diverse biological systems3. In aqueous systems involving electrically charged solids at low electrolyte concentrations, the interaction forces are strongly dominated by the electric properties of the surfaces and they are well described by the classical Poisson-Boltzmann equation, provided that effective charge rather than net-charge is considered 4, 5.

The surface potential of a given surface depends on the chemical environment: ions may adsorb

on the surfaces6, they correlate7 and they alter the activation degree of the surface functional groups8. For this reason, the actual value of the surface potential is hardly predictable. This issue is more pronounced for hydrophobic surfaces9 or in presence of multivalent ions10, where the sign of the surface charge may be even reversed. Nevertheless, once the effective surface potential is known, the ionic distribution around the surface and the forces between surfaces at sufficiently


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large separation distances (above 10nm) and low electrolytes concentrations, can be easily described5, 11. In contrast, at short distances and high salt levels, the picture is much more complicated and other forces might be relevant. First, a phenomenon which must be taken into account is the socalled crowding effect12, 13, which is particularly important at very high ionic strength and short separation distances: ions have finite size and this effect is not covered by the classical PoissonBoltzmann description14. Additionally, the classical DLVO theory assumes that the van der Waals interaction is responsible for the aggregation of colloidal particles at high salt levels. However, additional repulsive or attractive forces could be present, resulting to unexpected behaviors. For instance, some studies report the restabilization of colloidal suspensions upon salt addition15, not justified by charge reversal. In other studies16, 17, colloidal particles aggregate at salt concentrations lower than expected by the DLVO theory. These observations seem to point out the existence of non-DLVO interactions. Direct force measurements between colloidal particles enable to reveal short-ranged non-DLVO forces. One of the most common tool for measuring surface forces is the colloidal probe technique based on AFM18, 19. In this technique, a colloidal particle attached on a tip-less cantilever is used to measure particle-particle or particle-surface interactions. In most cases, silica particles are used as probes and recent studies have shown that the features of the force profiles between silica surfaces are extendable to other mineral oxides20, 21. For these materials and close to the isoelectric point, forces can be interpreted reasonably well with DLVO-based models above a certain separation distance if the model used eliminates the divergence introduced by the van der Waals force at short distances22. However, for high pH, the limit of applicability of the pure DLVO is reduced and a short-range repulsive force of unknown origin can be clearly observed20, 21. Another


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material which is widely used to fabricate colloidal probes is polystyrene4. Recent studies5, 16 revealed that the interaction forces between polystyrene surfaces may be attributed to its hydrophobicity. It is well accepted that some of the short-ranged forces are connected to the wettability properties of the surfaces23, because they are attractive between hydrophobic materials24, 25 and they turn to repulsive between hydrophilic ones15,

20.

However, the definition of

hydrophobic/hydrophilic materials are currently being a subject of discussion and controversy and to include a given material within any of these groups might be a difficult task26, 27. Similarly, recent studies28 on simulated interaction forces between surfaces with different contact angle values has revealed that the features of the force profiles between surfaces, traditionally considered as hydrophilic (contact angle lower than 90º), are comparable to those observed between hydrophobic surfaces (contact angle higher than 90º). However, it is commonly assumed that, when dealing with clearly hydrophobic surfaces, one may observe the so-called hydrophobic interaction29, 30. The hydrophobic force is strong and its range depends on the contact angle of the surface, according to how the water is structured around the surface28. Some of us performed two studies5, 16 where this short-range force was analyzed in detail and we concluded that its strength depends on the type of counterion and its concentration. A previous study31 showed that the forces between polystyrene surfaces are attractive at short distances for low pHs, but they are repulsive for high pHs. It was also pointed out that this change was not correlated to the contact angle of the polystyrene, which is not expected considering that the surface became more charged upon addition of hydroxide ions to solution. This is in contrast with other materials with ionizable surface groups, that showed a clear correlation between the pH and their wettability properties32.


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Recent studies using the colloidal probe technique or other techniques16, 33, 34, 35 have contributed to a better understanding of the Hofmeister effect. This effect is related to the hydration state of the ions which governs their adsorption degree on solid surfaces. Adsorption of well hydrated ions is more pronounced on hydrophilic surfaces than on hydrophobic ones, and the effect is reversed in case of poorly hydrated ions. Although contact angle seems to play a role, other parameters, such as pH, may be also relevant for the ion-specificity. Force measurements between silica surfaces33 revealed a reversal of the natural order within the Hoffmeister series for hydrophilic surfaces by increasing the pH. The pH-induced orientation of the water molecules with respect to the silanol groups onto the surface was identified as the origin of this inversion. In this paper, we conducted direct force measurements with AFM between polystyrene sulfate functionalized particles. We studied the wettability properties (advancing and receding contact angles) of non-hydrated and hydrated polystyrene sheets (under water conditions) These results were compared to the long- and short-ranged features observed in the force profiles between polystyrene particles at pH4 and pH10 and in presence of a wide range of concentrations of CsCl and LiCl. These salts were chosen because Cs+ is an example of poorly hydrated counterion, while Li+ is a well-hydrated one36. We also analyzed by electrophoresis the electric properties of the polystyrene particles in similar conditions, for comparison. Finally, in order to contribute to a better understanding of the role of the pH in the short-ranged forces and its ion specificity, we conducted force measurements between polystyrene particles at very high salt concentration (1M) of CsCl and LiCl over a wide range of pHs (pH 4 up to pH 11). Under these conditions, the surface charge is fully screened by the counterions and the force profiles are only dominated by van der Waals forces and short range non-DLVO forces.


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Material and Methods Colloidal particles, chemicals and sheets Sulfate terminated colloidal latex particles supplied as aqueous dispersions by Invitrogen (Life Technologies Corporation) were used in this study. The particle diameter was 3.0 µm with a polydispersity of 4.1%. These values were provided by the manufacturer. The stock suspension was dialyzed against Milli-Q water for one week using a cellulose ester membrane (Spectrum) with a molecular mass cut-off of 300 kg/mol. The colloidal suspensions were prepared in Milli-Q water. The different ionic strengths of the suspensions were adjusted by adding proper amount of more concentrated LiCl (Acros Organics) or CsCl (Sigma Aldrich) solutions, accordingly. The pH was adjusted with HCl (Sigma Aldrich) for acidic solutions and LiOH (Alfa Aesar) or CsOH (Acros Organics) for basic solutions containing LiCl or CsCl, respectively. Polystyrene sheets of size (30x30) cm2 and thickness 2 mm were supplied by Goodfellow. These samples were then cut into pieces of (2x2) cm2 to measure their wettability properties. Contact angle measurements were performed using Milli-Q water with pH adjusted with HCl (ScharlauChemie S.A.) or KOH (PanreacQuímica S.A.), accordingly. Imaging Roughness of the polystyrene particles and sheets was analyzed by AFM operating in intermitted contact mode, scanning an area of (0.5x0.5) µm2. Particles roughness was measured in an aqueous solution, while the sheets roughness was analyzed in air and liquid at the very same condition as the particles (100mM KCl, pH4). Note that for these measurements, KCl was added to the pH4 water to screen completely the electrostatic interaction between the tip and the surface when operating in tapping mode. We assume that this salt has no influence in the roughness parameters estimated for polystyrene in liquid37, 38.


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Contact angle measurements In this work, we used two different methods: Sessile Drop (SD) and Captive Bubble (CB). The SD is a liquid drop placed on a solid surface in air, while the CB is an air bubble trapped against a solid substrate immersed in liquid. Advancing and receding contact angles (ACA, RCA) are obtained from the SD/CB profiles extracted from side images of growing/shrinking SD/CB. On the polystyrene samples, a hole of diameter 0.5 mm was drilled and the SD/CB was formed through it, once the sample was fixed horizontally. The injection/suction of water/air was controlled with a micro-injector (Hamilton PSD3). The maximum drop/bubble volume was 200 µL and it was changed at a rate of 1µL/s. Each second, a lateral SD/CB image was captured with a digital camera. The ACA and RCA values were averaged over those points of each experiment where the contact line was moving. Advancing contact lines are observed for growing SD /shrinking CB, and receding contact lines for shrinking SD/growing CB. Multiparticle Colloidal Probe Experiments Force measurements between colloidal particles were carried out with a closed-loop AFM (MFP3D, Asylum Research) mounted on an inverted optical microscope (Zeiss Axiovert 200). The glass located at the bottom of the fluid cell was cleaned using “piranha” solution for 1 hour, which consists of a mixture of H2SO4 (98%) and H2O2 (30%) in a volumetric ratio 3:1. After this process, it was rinsed with Milli-Q water, dried with nitrogen and air-plasma treated for 20 minutes. Subsequently, it was vapor-phase silanized using 20µL of 3-(ethoxydimethylsilyl)propylamine silane (Sigma-Aldrich). Cantilevers were silanized in the same fashion. The silanization was aimed to facilitate the attachment of particles to the glass and the cantilever. If the particles were not firmly attached, it was a signal of a poor salinization and the entire process had to be again conducted. The glass was inserted in the fluid cell and a colloidal suspension in Milli-Q water of


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concentration about 100mg/L was injected and it was allowed to settle down for 2 hours. Subsequently, the fluid cell was rinsed with a solution of desired pH and ionic strength. The particles which were attached to the glass stayed in the fluid cell, while the dispersed ones were removed. The cantilever was then immersed in the fluid cell and a colloidal particle was attached to the cantilever by pressing it against a particle fixed to the substrate. Afterwards, it was centered over a neighbor particle by observing the interference pattern in the optical microscope. Once both particles were aligned, about 200 approach-retraction cycles were recorded at 300nm/s and a frequency of 0.5 Hz. The data acquisition rate was 5 kHz and the trigger was fixed to 60 mV. The zero separation distance was obtained from the constant compliance region with an accuracy of 0.2 nm. The forces were obtained from the deflection and the spring constant of the cantilever which was determined by the thermal and the Sader methods39. The calculated values were oscillating between 0.1 and 0.3 N/m. The interaction forces, estimated from the approaching part of each force cycle were determined by averaging at least 120 cycles in time bin sizes of 3x10-4 s and subsequently down-sampled to a frequency of 0.2Hz, leading to a force resolution of around 1 pN. The same sequence was repeated with at least two additional pairs of particles and a good reproducibility was generally found, confirmed by the surface potential deviations of less than 15% in the fitted force profiles between different pairs. The retractive part was used to determine the adhesion between the particles by calculating the average of the maximum force, which takes place during the jump-out of the particles. Mobility measurements The electrophoresis measurements were carried out using a ZetaSizer Nano ZS (Malvern). The particle concentration of the colloidal suspension was 80 mg/L in each measurement. For each


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condition, the mobility was measured at least 6 times. The average value was used to estimate the ζ-potential using the Henry function40. Force profiles interpretation We used two different models for the interpretation of the force profiles depending on the range of separation distances explored, and the relevance of the electric double layer interaction. The first model was used for those cases where the electrostatic interaction is significant for separation distances above 3-4 nm. The second model was used only where the electric double layer contribution is negligible for separation distances above 1nm (salt concentration of 1 M). For the first case, we used a pure-DLVO model, since above 3-4 nm the presence of other non-DLVO forces is unlikely. For the second case, we used a non-DLVO model which includes a shifted van der Waals force and an additional short-ranged force. a) Pure DLVO model DLVO theory assumes that the total force is the sum of two different contributions, namely the van der Waals force and the electric double layer force: FT = FvdW + Fedl

1

where the van der Waals force, including the Derjaguin approximation, is defined in a shifted and non-retarded form as: FvdW ( Reff , H , δ , h) = − Reff

H 6(h + δ )

2

2

being h the separation distance, H the Hamaker constant of the particles in water, δ is the shift between the origin of the van der Waals force and the contact point and Reff is the effective radius, assuming the Derjaguin approximation, which for the symmetric sphere-sphere geometry for a nominal particle radius of R is given by:


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Reff = R

2

3

The shift parameter δ is crucial for the correct fitting of force profiles which are repulsive at short distances41, since it eliminates the divergence introduced by the van der Waals force, written in the standard form38 at short distances. The electric double layer force is written from the Poisson Boltzmann model with the constant regulation approximation, which details are given elsewhere16. In this scenario, the force profile is a function of the effective radius Reff, the surface potential ψ, the ion concentrations ci and the regulation parameter p: Fedl = Fedl ( Reff , ci , p,ψ )

4

where p and Ψ are the only fitting parameters, since Reff and ci may be easily calculated and then fixed throughout. b) Non-DLVO and short-ranged model In this model, where the electric contribution is completely screened, the net force is defined as the sum of two different terms, namely the shifted and non-retarded van der Waals force FvdW (Eq. 2) and the short range force FSR : FT = FvdW + FSR

5

assuming the Derjaguin approximation, the short range force is modeled with an exponential decay:

FSR = Reff Be− qh

6

where B the amplitude and q-1 the range of the interaction4, 16.


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This term is aimed to empirically capture the effect on the net force of additional forces such as the hydrophobic force and/or short term pH-induced repulsion. Results and discussion Contact angle measurements on polystyrene sheets at different pHs Contact angle measurements were carried with the Axisymmetric Drop Shape Analysis-Profile (ADSA-P) technique42. We studied the wetting properties of polystyrene sheets surfaces. The aim was to determine whether the polystyrene surface is hydrophobic or hydrophilic and whether the pH plays a role or not. We obtained the ACA and RCA of the sheets in air (SD method) and in water (CB method). In Figure 1, we show the pH dependence of the ACA and RCA values and we can confirm that the pH has no influence in contact angle, as found by Drechsler et al31, and our results with SD (ACA∼92º, RCA∼74º) agree reasonably well with their results. However, ACA and RCA values measured with the CB method are higher than the values measured with the SD method. This suggests that the hydrated polystyrene is more hydrophobic than the dry polystyrene (ACA∼100º, RCA∼82º). The discrepancy between both experimental methods on polymer surfaces can be explained by many different effects including surface mobility, swelling or liquid penetration43. Polystyrene rms roughness measured in air is 0.81±0.06 while the rms roughness in water is 0.93±0.11. However, the differences between the ACA and RCA values obtained by both methods cannot be justified by the differences in roughness. For smooth surfaces, an increase in the roughness parameter would lead to an increase in the hysteresis of the contact angle, i.e. an increase of the ACA and decrease of the RCA. In this case, we observe that both ACA and RCA values are higher when one uses the CB method, but however the hysteresis remains constant. This means that the possible different roughness has no significant influence on the contact angles. For


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this reason, we assume that the differences between both experimental methods are not of topographic origin.

Figure 1. Advancing (circles) and Receding (triangles) water contact angles measurements on polystyrene sheets as a function of the pH using the Sessile Drop (open symbols) and Captive Bubble (solid symbols) methods. Dashed lines are guides to the eye.

From the results of contact angle, we concluded that a hydrated polystyrene surface is intrinsically hydrophobic in the entire pH range explored; therefore, one would expect that the adsorption of Cs+ ions should be more pronounced than the one for Li+, and additional short-range attractions should be observed due to hydrophobic forces. The wettability results of the polymeric sheets must be carefully taken in consideration when comparing them with the wettability properties of the latex particles. First, because the roughness properties are not the same, though comparable: rms of the particles 0.73±0.05µm, while the rms


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of the sheets in liquid is 0.93±0.11. Besides, unlike the sheets the particles are functionalized with ionizable surface groups and this might affect their contact angle and its dependence with the pH. Ion specificity and pH dependence of the surface potentials Force curves measured at pH 4 with LiCl and CsCl concentration of 1 M were used to estimate the Hamaker constant (H) by assuming a zero shift parameter (δ = 0) and by choosing a range for fitting from 3 nm up to a value corresponding to the baseline origin, which considering the conditions and our force resolution was typically 10 nm. The value of the mean H is shown in Table 1. This value agrees reasonably well with other Hamaker constant values reported previously for similar systems4, 16, 38. This value was fixed throughout for the rest of the fittings in this first study, since it has been demonstrated that the value of the Hamaker constant depends only weakly on the salt level and no dependence on the pH and type of ions was found38. Force curves at low salt concentrations (

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