Colloids with Continuously Tunable Surface Charge - Langmuir (ACS

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Colloids with continuously tunable surface charge Bas G.P. van Ravensteijn, and Willem K. Kegel Langmuir, Just Accepted Manuscript • DOI: 10.1021/la501993c • Publication Date (Web): 15 Aug 2014 Downloaded from http://pubs.acs.org on August 16, 2014

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Colloids with continuously tunable surface charge Bas G.P. van Ravensteijn*, Willem K. Kegel* Van ’t Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for NanoMaterials Science, Utrecht University, Utrecht, The Netherlands Surface charge, zeta potential, charge reversal, self-assembly, colloidal clusters In this paper we present a robust way to tune the surface potential of polystyrene colloids without changing the pH, ionic strength etc. The colloids are composed of a cross-linked polystyrene core and a cross-linked vinylbenzyl chloride layer. Besides the chlorine groups, the particle surface contains sulfate/sulfonate groups (arising from the polymerization initiators) which provide a negative surface potential. Performing a Menschutkin reaction on the surface chlorine groups with tertiary amines enables us to introduce quaternary, positively charged amines. The overall charge on the particles is then determined by the ratio between the sulfate/sulfonate moieties and the quaternary amines. Using this process we were able to invert the charge in a continuous manner without losing colloidal stability upon passing the isoelectric point. The straightforward reaction mechanism together with the fact that the reaction could be quenched rapidly resulted in a colloidal system in which the zeta potential is tunable between -80 mV and +45 mV. As proof of principle, the positively charged particles were used in heterocoagulation experiments with nano-sized and micron-sized negatively charged silica particles to create geometrically well-defined colloidal (nano) clusters.

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1. Introduction Control over the surface charge of colloids is a priori a versatile way to tune interactions between particles.1,2 Examples of systems where this strategy has been used are capsules as delivery systems,3 colloidal clusters4 and colloidal ionic crystals.5 To achieve this, various methods have been described in the literature, e.g., the addition of pH-sensitive functionalities to the surface (carboxylic acid, amines and hydroxyl groups) usually in a covalent fashion by using functional monomers6-8 and initiators8-11 or adsorption of (poly)electrolytes.12-14 Here we present a new method to tune the surface potential of particles, namely via chemical reactions which covalently link charged species to the colloidal surface. We will focus on the charge reversal of particles which are initially negatively charged due to the presence of sulfate/sulfonate groups on the surface. Besides these negatively charged groups, the particles are decorated with chlorine groups which we can convert to permanently positively charged quaternary amines via the Menschutkin reaction using tertiary amines. The overall charge of the particle is then determined by the ratio between the sulfate/sulfonate groups and quaternary amines. The fact that the surface potential is determined by covalently attached moieties provides means to control the number of charges on the particles without changing other variables in the system, such as the ionic strength or the type of solvent. To show the synthetic utility of these particles, a few examples of electrostatic driven self-assembly leading to the formation of organic-inorganic hybrid colloidal clusters are shown.

2. Experimental Section 2.1. Materials. Styrene (St, 99%), divinylbenzene (DVB, 55% mixture of isomers, tech. grade), vinylbenzyl chloride (VBC, ≥ 90%, tech. grade), dimethyl formamide (DMF, > 99%), trimethylamine (TMA, ~45 wt% aqueous solution), tetra-ethoxysilane (TEOS, ≥ 99%, GC) and


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LUDOX® TMA colloidal silica (34 wt% aqueous suspension, particle diameter ≈ 17 nm (Sigma Aldrich data sheet)) were purchased from Sigma-Aldrich. Potassium persulfate (KPS, > 99% for analysis) and sodium bisulfite (reagent ACS) were obtained from Acros Organics. Triethylamine (TEA, Puriss. p.a.; ≥ 99.5%) was obtained from Fluka. Ethanol (Absolute, p.a.) was purchased from Merck. Ammonia (Analytical reagent grade, 25% aqueous solution) from Fischer was used. All the chemicals were used as received. The water used throughout all syntheses was purified using a Milli-Q water purification system. 2.2. Synthesis of chlorinated particles (CPs-Cl). The chlorinated cross-linked polystyrene particles were synthesized in a two-step method as described in reference 15. Firstly, crosslinked polystyrene (CPs) particles were synthesized using a standard emulsion polymerization. A 500 mL round bottom flask equipped with stir bar was placed in an oil bath of 80°C. Water (200 mL) was charged into the reactor and allowed to reach the bath temperature. Styrene (21.15 g), DVB (0.635 g) and SDS (0.25 g) dissolved in water (50 mL) were added. The complete mixture was allowed to heat up to the temperature of the bath. Finally, addition of KPS (0.39 g) dissolved in water (37.5 mL) initiated the polymerization. Reaction was allowed to continue for 24 hours at 80°C. The resulting latex had a solid content of 7% (measured gravimetrically). The resulting particles had a radius of 125 nm with a polydispersity of 3.8% as determined with TEM. The synthesized particles were used as seeds in the second step in which chlorine groups were introduced at the colloidal surface. The crude seed dispersion (25 mL) and water (10 mL) were introduced in a 50 mL round bottom flask equipped with a magnetic stir bar. The mixture was degassed with nitrogen for 30 minutes. VBC (1 mL) premixed with DVB (20 µL ) was added under inert atmosphere. The seeds were swollen for 1 hour at 30°C, after which the temperature was raised to 60°C. When this temperature was reached, KPS (0.04 g) and sodium bisulfite (0.03


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g) dissolved in water (2.5 mL) were added. A combination of KPS and sodium bisulfite was used as initiator system in order to shift the mechanism for radical formation from a purely thermal process (decomposition of persulfate) to a redox process in which the bisulfite ion acts as a reductor.16 In this way, the polymerization could be carried out at a lower temperature, suppressing the undesired hydrolysis of the relatively labile chlorine functionality of VBC. The reaction was allowed to run for 4 hours. The particles were washed by centrifugation and redispersion in water. The solid content of the resulting dispersion was adjusted to 5%. A particle radius of 155 nm and polydispersity of 4.4% was measured using TEM. The presence of the chlorine groups was confirmed using FT-IR (1266 cm-1) and XPS (200 and 270 eV (Cl)). 2.3. Surface reaction of CPs-Cl with TEA. CPs-Cl dispersion (0.5 mL) was diluted with water to a solid content of 1%. To this mixture triethylamine (35 µL) was added. The reaction was carried out at 20, 25, 35, 50 and 80°C in a thermostated oil bath. During the reaction samples (100 µL) were withdrawn from the mixture. The Menschutkin reaction was quenched by rapid centrifugation (several seconds at 11000g) and replacing the TEA containing supernatant with water. Additional washing steps with water (2 times) and ethanol (2 times) by centrifugation and redispersion were carried out. After the last washing step the particles were transferred back to water and the particles were characterized in terms of hydrodynamic size (and its distribution) and their zeta potential. The particle morphology was probed with transmission electron microscopy (TEM). 2.4. Synthesis of micron-sized silica colloids. For the synthesis of large silica spheres, the method of Zhang et al.17 was used. This method is based on the standard Stöber synthesis18 and on the seeded growth method developed by Philipse et al.19 To a mixture of ethanol (48.4 mL) and ammonia (11.6 mL), a mixture of TEOS (0.5 mL) in ethanol (2.0 mL) was added and stirred


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magnetically for 2 hours. Within a few minutes the dispersion turned opaque indicating the formation of the core particles. These particles were grown by adding TEOS (5 mL) diluted with ethanol (20 mL), in about 2h. The dispersion was stirred overnight and subsequently washed repeatedly by centrifugation and redispersion in ethanol. A particle radius of 465 nm and polydispersity of 2.6% was measured using TEM. 2.5. Heterocoagulation between positively charged polystyrene colloids and negatively charged silica nanoparticles (LUDOX®). LUDOX® dispersion (25 µL) was diluted to 0.5 mL and sonicated 10 minutes before use. The diluted silica dispersion was slowly dropped into a flask containing the positive charged polystyrene dispersion (25 µL, particles prepared as described in Section 2.3 at 20°C) and water (0.5 mL) while sonicating the mixture. The mixture was sonicated 10 more minutes after which the aggregates were washed by centrifugation and redispersion cycles in water. The washed aggregates were analyzed using TEM. 2.6. Heterocoagulation between positively charged polystyrene colloids and negatively charged micron sized silica colloids. The crude silica dispersion (0.3 mL, particles prepared as described in Section 2.4) was transferred from EtOH (in which the particles were prepared) to water (0.3 mL) by two centrifugation and redispersion cycles. The dispersion containing the positively charged polystyrene particles (2 µL) was diluted with water to 1 mL. The silica dispersion was added drop-wise while sonicating the mixture. The resulting dispersion was analyzed using light microscopy without any further purification. 2.7. Measurements. Transmission electron microscopy pictures were taken with a Philips Technai10 electron microscopy typically operating at 100 kV. Bright field images were recorded using a SIS Megaview II CCD camera. The samples were prepared by drying a drop of diluted, aqueous particle dispersion on top of polymer coated copper grids.


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The XPS measurements were carried out with a Thermo Scientific K-Alpha, equipped with a monochromatic small-spot X-ray source and a 180° double focusing hemispherical analyzer with a 128-channel detector. Spectra were obtained using an aluminium anode (Al Kα = 1486.6 eV) operating at 72W and a spot size of 400 µm. Survey scans were measured at a constant pass energy of 200 eV and region scans at 50 eV. The background pressure was 2·10-9 mbar and during measurement 3·10-7 mbar argon because of the charge compensation dual beam source. IR spectra were obtained using a Perkin Elmer FT-IR/FIR Frontier Spectrometer in attenuated total reflectance (ATR) mode. Measurements were carried out on powders obtained by drying the particle dispersion. Dynamic light scattering (DLS) and zeta potential (ZP) measurements were performed using a Malvern Zetasizer Nano using highly diluted aqueous dispersions at a temperature of 25°C. The DLS measurements were carried out in 7 runs of 15 individual measurements in backscatter mode (173°). The size of the colloids or aggregates are reported as a Z-average diameter and the corresponding polydispersity index (PDI). These values were obtained by using the cumulant method as described in reference 20. The absolute values obtained for the Z-average diameter and the PDI for the aggregated dispersions are less reliable. Nevertheless, these values can still be used to verify if a colloidal dispersion is randomly aggregating, which results in higher Z-average diameters and PDI values. For the ZP measurements, 7 runs of at least 50 individual measurements were carried out to obtain a statistically reliable potential. Electrophoretic mobilities were measured in Milli-Q water (ionic strength ≈ 10-5). Given this ionic strength and the size of our particles we used an analytical expression developed by Ohshima to calculate the value of the Henry function (f(κR)), which turned out to be 1.05.21 For simplicity, the Hückel limit (f(κR) = 1) of the Henry formula was used to convert the electrophoretic mobilities to zeta potentials.22


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Light microscopy was performed using a Zeiss Axioplan microscope equipped with an oil immersion lens (NA = 1.4, 100x magnification). The pictures were captures with a Basler scout camera. Samples were prepared by placing a drop of diluted dispersion on a microscopy slide.

3. Results and Discussion 3.1. Synthesis of chlorinated colloids and their charge inversion using the Menschutkin reaction. We synthesized polystyrene particles which contained a chlorinated outer layer via a previously published method.15 The presence of the benzyl chloride functionalities was confirmed using both IR and XPS measurements. The particles are charge stabilized by the presence of sulfate/sulfonate groups at the colloidal surface originating from potassium persulfate and sodium bisulfite which acted as an initiator system. The chlorine groups on the surface provide a chemical handle which can be easily converted to a variety of functionalities.23 The introduction of charged moieties, and therefore regulation of the surface potential, is also possible. In order to investigate this we make use of the Menschutkin reaction (Scheme 1).24

Scheme 1. The Menschutkin reaction between surface benzyl chloride groups and triethylamine leading to the formation of a permanently positively charged quaternary amine at the colloidal surface.


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Figure 1. a) The evolution of the zeta potential in time of the chlorinated particles during the reaction with triethylamine (TEA) at 80°C. b) The evolution of the Z-average diameter and the polydisperisty index (PDI) of the chlorinated particles during the reaction with TEA at 80°C. c) The zeta potential evolution of the chlorinated particles during the reaction with TEA at 35°C. d) The Z-average diameter and the PDI of the chlorinated particles during the reaction with TEA at


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35°C. e) Zeta potential evolution at a reaction temperature of 20°C. f) Z-average diameter and PDI in time at a reaction temperature of 20°C.

This choice was based on previous observations where chlorinated particles reversed their charge without losing colloidal stability upon a surface reaction with dimethyl formamide (See Supporting Information S1). The Menschutkin reaction converts the chlorine functionalities of the particles to quaternary amines with permanent positive charges in an one-step reaction with tertiary amines, in this case triethylamine. The final charge of the colloids is expected to be determined by the ratio between the negative sulfate/sulfonate groups and the newly introduced quaternary amines. The surface reaction was carried out at temperatures of 20, 25, 35, 50 and 80°C. The zeta potential as well as the Z-average diameter and its polydispersity index as a function of time for the reactions carried out at 20, 35 and 80°C are plotted in Figure 1. The plots for the other reaction temperatures can be found in the Supporting Information (Figure S3). In all cases a final potential of approximately +45 mV was found (Figure 1a, c and e). The reaction rate increases significantly with increasing temperature as reflected in the reaction times required to pass the point of zero charge: < 1 minute for 80°C and 180 minutes for 20°C. For the highest reaction temperatures the particles remain aggregated and colloidal stability is not restored as can be seen from the macroscopic appearance of the dispersions shown in Figure 2a and the large values measured for both the Z-average diameter and the PDI (Figure 1a and b). Interestingly, upon lowering the reaction temperature we found that colloidal stability could be restored (Figure 1c-f). The return of colloidal stability becomes more significant with decreasing temperature. At a reaction temperature of 20°C the PDI’s and size distributions were identical to


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those obtained for the chlorinated particles, indicating fully restored stability. The fact that the Zaverage diameter versus time and the PDI versus time curves have a common maximum indicates that the observed clusters are formed due to random aggregation. The restored stability is also reflected in the macroscopic picture of the dispersion (Figure 2b). Compared to Figure 2a it is immediately evident that the particles are much more stable. The samples reacted at 20°C are turbid due to the scattering of the freely moving particles suspended in the solution, while the macroscopic picture in Figure 2a shows large aggregates which sediment quickly to the bottom of the sample vial causing the clear appearance of these samples. Detailed analysis of the shapes of the zeta potential versus time curves and the mechanism for regaining colloidal stability will be discussed in Section 3.2 and 3.3.

Figure 2. a) Macroscopic picture of the colloidal stability during the surface reaction with TEA at 80°C. Large aggregates of colloids are formed after crossing the iso-electric point (< 5 min). Fast sedimentation causes the clear appearance of the samples. b) Macroscopic picture of the colloidal stability at a reaction temperature of 20°C. The numbers in the bottom of the figure correspond to the reaction time given in minutes for both Figure 2a and 2b.


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The presented method provides a straightforward and reproducible way to control the zeta potential of the polystyrene colloids. In order to verify the robustness and the permanent character of the obtained potentials, we took 4 samples (45, 90, 240 and 360 minutes) during the charge reversal reaction carried out at 20°C. These samples were washed and the zeta potentials were measured directly afterwards. After 3 days the potentials were measured again. The results are shown in Figure 3. It is immediately clear that we obtain the same zeta potential as directly after the reaction within the experimental error. Storing the particles as a highly diluted (red dots) or concentrated dispersion (blue triangles) does not influence the measured potential, which is an

Figure 3. Change in the zeta potential of the chlorinated particles reacted with TEA after 3 days storing in water.

indication that the species that are causing the positive charge are strongly bound to the colloids which is in line with the mechanism of the Menschutkin reaction (Scheme 1). Quenching the reaction at a certain moment in time therefore provides an easy method to tune the overall surface charge of the particles.


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3.2. Theoretical considerations for regaining stability after charge inversion. To gain more insight in the process of charge reversal, attempts were made to explain the observations depicted in Figure 1 and 2. Restoring colloidal stability implies that the particles are not trapped in a deep attractive (Van der Waals) potential well close to the isoelectric point, as would be expected in the case of polystyrene particles in water according to DLVO theory. Or in other words, there must exist a cut-off distance which provides a limit to the minimal inter-particle distance, and therefore to the magnitude of the attractive Van der Waals force. This can be caused by the presence of a ‘hairy’ soft (polyelectrolyte) layer on the colloidal surface or by a minimal possible inter-particle distance. Vigil et al.25 described a similar observation for the interaction potentials between silica surfaces. At small separations (< 20Å) a repulsive force was measured instead of a strong attraction predicted by DLVO theory. This repulsion was assigned to protruding silanol and silicilic acid groups causing a strong steric repulsion. If we extrapolate these findings to our system, it is possible that the tertiary amines introduced to the surface of the polystyrene colloids fulfill the same steric stabilization as the silanol and silicilic groups. However, direct experimental evidence for this is very hard to obtain. TEM analysis revealed that the particles possess a rough surface with a typical length scale on the order of a few nanometers (Figure 4). As a crude approximation this rough surface can be modeled as a smooth surface where the minimal possible inter-particle distance is set by the magnitude of the roughness. The steric bulk of the introduced quaternary amines is much smaller than the roughness, so we assume that the steric layer on the particles does not influence the interaction potential between the colloids and conventional DLVO theory can be applied. The roughness was observed regardless of the reaction temperature used for the charge reversal reaction.


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Figure 4. Transmission electron microscope images of the positively charged particles. Using Eq. 11 we calculate the DLVO potential in units of kBT for smooth spheres (UDLVO/ kBT) by

[1]

where AH represents the dimensionless Hamaker constant of two polystyrene surfaces across water, R the radius of the particles, d the inter-particle distance, e the elemental charge, ζ the zeta potential, kB the Boltzmann constant, T the temperature, λB the Bjerrum length and κ the inverse screening length. κ was estimated based on the concentration of TEA. TEA is a weak base and via the base dissociation constant (pKb = 3.25 at 25°C)26 we could calculate the pH and therefore the ionic strength of the continuous phase. We assumed a constant ionic strength during the whole charge inversion, because TEA is present in a large excess compared to surface chlorine


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groups. On top of this we generate chlorine ions during the amination reaction which partially compensate for the loss of ions due to covalent coupling of TEA. Calculation of the DLVO potentials (see Supporting Information S3) assuming smooth surfaces showed that inter-particle distances below 1 nm should result in irreversible aggregation, while distance larger than this critical value results in restoring colloidal stability upon charging the colloids via introduction of the quaternary amines. The roughness observed on the particles (Figure 3) is clearly larger than this critical value, suggesting that based on this crude approximation, stability of the dispersion should indeed be regained. This finding explains the results obtained at reaction temperatures of 20°C. However, it does not explain the complete lack of colloidal stability after charge reversal at higher reaction temperatures (Figures 1 and 2). Chemically the surface of the colloids should be the same regardless of the temperature used to introduce the quaternary amines. The final zeta potential is of comparable value at all reaction temperatures, so the electrostatic repulsion after the charge reversal between the particles must also be comparable. Calculation of DLVO barriers with Debye screening lengths corrected for the temperature and the temperature dependence of the pKb of TEA (See Supporting Information S4) cannot explain the observed huge differences in colloidal stability. In fact the screening lengths increase with increasing temperature, suggesting more efficient charge stabilization at higher reaction temperatures which contradicts our results. From experiments carried out with a larger excess of TEA and trimethylamine (TMA), which is more reactive in these types of reactions,27 we conclude that the final stability is determined by the rate of charge reversal and is not directly related to the temperature (See Supporting Information S5). In both reactions fast rates were achieved at 20°C. Despite this low temperature, colloidal stability could not be regained.


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An explanation for this rate dependence might be the formation of an inhomogeneous surface charge distribution bearing positively charged ‘patches’ at high reaction rates. This idea is inspired by the formation of charged patches due to the adsorption of an oppositely charged polyelectrolyte on a colloid.28 These ‘patches’ strongly interact with the originally negatively charged surface of the colloids. The extra electrostatic attraction forces the particles in a deeper potential well causing irreversible aggregation. The formation of patches might be dominant at faster reaction rates because there is simply no time for the amines to explore the colloidal surface and probe the energetically most favorable reaction sites leading to situations in which positive charges are placed in close proximity on the particles. At lower temperatures, the reaction is slower and the TEA molecules now have time to sample the colloidal surface and find the most favorable reaction sites. This can result in a more uniform charge distribution. Although this is a hypothesis, experimental techniques based on atomic force microscopy (AFM) could be used to probe the charge distribution as function of time and position. At this point these experiments are beyond the scope of this work.

3.3. Explaining the zeta potential versus time curves. We will now focus on the shape of the zeta potential versus reaction time curves in more detail. All these curves possess a significant lag time before there is a measureable change in the zeta potential (Figure 1). This is probably caused by ion condensation to the surface of the colloid.29,30 Ion condensation is caused by a high charge density on the colloids resulting in ions that are strongly bound to the surface. When the zeta potential of such a system is measured one measures an effective charge of the colloid due to shielding by the condensed ions. The presence of a lag time does therefore not imply that there is no reaction occurring, but rather that the effect of the reaction is not reflected in a change in


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the zeta potential. At a certain point the surface charge becomes low enough so that ion condensation does not occur anymore, with the consequence that we can actually measure an increasing potential as shown in Figure 1. It is not likely that the reaction mechanism itself is causing the observed lag time, because it is a straightforward one-step reaction and not for example a two-step mechanism involving a slow initial reaction step or uncharged intermediates. In order to check this hypothesis we set up the following experiment:

Figure 5. Curve of the zeta potential versus reaction time for a two-step reaction to investigate if the observed lag time was intrinsically linked to the reaction mechanism.

TEA was allowed to react with the chlorinated particles at 20°C for 90 minutes after which the reaction is quenched by quickly washing the particles. The 90 minute period corresponds to the length of the lag time (Figure 1e). In the second step, TEA was again added to the resulting particles and the change of zeta potential in time was monitored (Figure 5). We observed that the potential immediately started to increase, without any lag time. From this we conclude that the lag time is indeed not caused by the surface reaction itself, because if this was the case we should also observe a lag time the second time we add TEA. The rate at which the point of zero


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potential is crossed is higher than in the one-step reaction carried out at 20°C (Figure 1e). This can be attributed to the larger excess of TEA which was present after the second addition.

Figure 6. a) Overlay of the zeta potential versus time curves for reactions carried out at 20, 25 and 35°C and extrapolation of the tangents around the point of zero potential to determine reaction rates and bare surface charges. b) Overlay of the zeta potential versus time-1 curves for reactions carried out at 20, 25 and 35°C and extrapolation of the tangents to obtain the full positive potential of the colloids after charge reversal. c) Master curve obtained after scaling the zeta potential versus time plots (Figure 6a) according to their reaction rates. d) Arrhenius plot of the rates obtained from the scaling factors of the zeta potential versus time curves (Figure 6c).


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As mentioned before, increasing the reaction temperature causes a significant increase in the reaction rate. The slope of the zeta potential versus time around zero potential is a measure for the reaction rate k. In Figure 6a the rates for the lower reaction temperatures are estimated by fitting a tangent to the linear part of the curve. This procedure was experimentally not possible for the higher reaction temperatures due to the high reaction rates. Extrapolation of the tangents revealed that the lines cross at one single point located on the y-axis at approximately -175 mV. The physical meaning of this extrapolation is rather straightforward: at t = 0, no TEA has been added to the colloids which must imply that the colloids bear their full negative charge in this limit. The extrapolation to zero time is phenomenological, and based on the observation that potential varies linearly with time over a small time interval. The constant extrapolated values point to an internally consistent procedure which provides a plausible estimate of the bare charge densities. As discussed before, the bare charge is not measureable due to ion condensation. Eq. 2 was used to check if these potentials were physically reasonable.30,31

[2] In this equation, Z is the apparent number of charges. Dividing Z by the surface area of the particles leads to a charge density of around 0.22 negative charges per nm2 (1 charge per 4.6 nm2), being reasonable keeping in mind that the negative charges are caused by relatively small sulfate/sulfonate entities. Regardless of the reaction temperature, a limiting value around +45 mV was measured. Naïvely one might explain this by crowding of the colloidal surface with the rather bulky reaction product of TEA. In order to verify if this is the case we applied the same strategy as


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used to estimate the number of negative charges on the colloid, however in this case the zeta potential was plotted against the reciprocal time. Constructing a tangent to the linear part of the curves and preforming the same phenomenological extrapolation to t = 0 leads to the potential obtained after infinite reaction time or the true value of the surface potential without ion condensation. Also in this limit the three curves show a common intersect at the y-axis at a potential of approximately +178 mV (Figure 6b). Overcharging to a potential of +178 mV requires a total of 352 mV worth of positive charges. Plugging this potential in Eq. 2 and dividing by the surface of a particle, we find 0.43 positive charges per nm2 charges (or 1 charge per 2.3 nm2). The area covered by the reaction product of TEA (equivalent to one positive charge) was estimated to be 2.1 nm2 (see Supporting Information S6). Comparing both numbers shows that it is possible to accommodate all the charges on the surface and that the surface is indeed quite packed with quaternary amines. The remaining surface area is probably occupied by the negative charges yielding a fully covered surface. The fact that the tangents drawn in Figure 6a intersect at one particular point suggest that the obtained curves are scalable to a master curve which describes the kinetics and the influence of temperature on these reactions. The length of the lag time scales inversely proportional with the reaction rate, because it is solely caused by ion condensation as discussed before. To check if a master curve could be generated, an attempt was made to collapse the raw experimental data by multiplying the x-axis with a certain factor. Overlaying factors of 1, 0.81 and 0.3 were required for the reaction temperatures of 20, 25 and 35°C respectively (Figure 6c). The inverse of these factors were indeed in good agreement with the values of the tangents of each corresponding curve and therefore to the reaction rate. Constructing an Arrhenius plot based on the inverse of the overlaying factors shows that plotting ln(k) against T-1 yields a straight line (Figure 6d). This


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indicates that the surface reaction is following classical behavior of a single-step thermally activated reaction which is in line with the hypothesis that the lag time is caused by ion condensation, and is not an intrinsic property of the reaction mechanism.

3.4. Preparation of hybrid colloidal clusters via heterocoagulation. With the method to tune the charge at hand we briefly explored the potential of this system in electrostatic mediated assembly of colloids. As proof of principle we prepared organic-inorganic hybrid clusters by the addition of negatively charged silica particles to the colloids after the charge reversal reaction. The first example deals with negatively charged nano-sized silica particles (LUDOX®). A large excess of these particles was mixed with the positively charged polystyrene particles in order to prevent bridging of the positive polystyrene particles by the negative silica leading to macroscopic aggregation. The resulting, stable dispersions were washed by centrifugation to remove the free silica particles. TEM analysis yielded the pictures depicted in Figure 7a and b. As can clearly be seen, the polystyrene particles are fully covered with the silica particles yielding raspberry-like colloidal aggregates.32-35 The uniform distribution of the silica over the polystyrene core is a good indication for a uniform charge distribution of the particles. Measuring the zeta potential of these particles yielded a value of -41 mV, indicating that the surface is now overcharged with negative species resulting in the observed colloidal stability. This phenomenon is also observed in the overcharging with polymeric polyelectrolytes which adsorb on an oppositely charged surface or colloid.28,36,37 To ensure that the assembly of the negatively charged particles is electrostatically driven, the same silica particles were also added to the original negatively charged chlorinated colloids (Figure 7c and d). In this case hardly any silica particles adsorbed onto the polystyrene surface.


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The incidental aggregation of the two particles can be attributed to a drying effect during sample preparation.

Figure 7. a & b) TEM images of the clusters formed after electrostatically driven self-assembly of negatively nano-sized silica particles and positively charged polystyrene particles obtained after amination with TEA. c & d) TEM pictures of a blanco experiment in which negatively charged silica particles were added to negatively charged chlorinated polystyrene colloids.

Besides using nano-sized silica, the other extreme in which micron-sized negatively charged silica is added to the positive polystyrene particles was also explored.38,39 Again, a large excess of negatively charged silica particles was added. The resulting unwashed dispersions were analyzed using optical microscopy. Representative images are shown in Figure 8a and b and


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Figure S7 in the Supporting Information. In the obtained images only the micron-sized silica particles are visible. The smaller polystyrene particles (Figure 8e) appear as dots in the picture.

Figure 8. a & b) Light microscopy images of clusters formed after mixing of the positively charged polystyrene particles and micron-sized negatively charged silica colloids. c) Schematic representation of the tetramers (left) and trimers (right) of silica colloids (blue) formed after heterocoagulation with the positive charged up polystyrene colloids (red). d) Light microscopy image of solely the silica colloids. e) Light microscopy image of solely the polystyrene particles. All pictures were taken at a magnification of 100x.


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Compared to the image of solely the silica colloids (Figure 8d), the images of the mixed system shows the presence of a relative large number of small clusters (Figure 8a and b). Evidently, the number of single silica particles is high, due to the large excess of these colloids which was added. When all the clusters (excluding the single particles) of a large number of pictures were counted, we found that 43% of all clusters are tetramers, consisting of one polystyrene center particle and three silica spheres. 34% of all the clusters were trimers (one polystyrene particles with two silica particles) while the remaining clusters were built up out of 5 or more colloids. Geometrically, no more than three large silica spheres can fit around the central positive colloid explaining the observed preference for tetramers and trimers (Figure 8c. (The ratio of the radii of the red and blue circles is equivalent to that of the polystyrene and silica colloids used in these experiments). A short movie of the clusters recorded with the optical microscope is included in the Supporting Information to show that the clusters remain intact and move as single entities through the solution. The fact that the silica particles seem to be in contact in the light microscope images can be attributed to the limited resolution of this technique. In principle, the type of cluster that is most dominant can be tuned by the changing the sizeand concentration ratio between both species.38,39 These results will be published elsewhere.

4. Conclusions We have developed a straightforward method to control and even reverse the overall surface charge of polystyrene colloids. The colloidal surface is initially covered with sulfate/sulfonate moieties providing negative charge for stabilization and benzyl chloride groups as chemical handle for further modification. Introduction of positive charges proceeds via the Menschutkin reaction between the surface chlorine groups and tertiary amines. The overall charge is then


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determined by the ratio between positive quaternary amines and negative sulfate/sulfonate groups on the surface. Despite the fact that the charge reversal is a continuous process, and the colloids pass through the point of zero overall charge, colloidal stability could be maintained under appropriate conditions. Crucial for maintaining the stability was to carry out the charge reversal reaction at sufficiently low reaction rates. Increasing the temperature, and therefore the reaction rate, resulted in irreversible aggregated dispersions. This behavior is possibly related to the formation of positively charged ‘patches’ on the surface of the particles which cause an extra electrostatic contribution to the attractive forces between the colloids. The time evolution of the zeta potential can be explained in terms of ion condensation and classical Arrhenius type reaction mechanism. As an example of an application of an adjustable zeta potential of the colloids, we demonstrated the formation of organic-inorganic hybrid clusters via heterocoagulation with both nano-sized and micron-sized silica particles yielding raspberry-like particles and colloidal tetramers and trimers respectively.


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AUTHOR INFORMATION Corresponding Authors * Prof. Dr. Willem K. Kegel, Ir. Bas G.P. van Ravensteijn Van ’t Hoff Laboratory for Physical and Colloid Chemistry Debye Institute for NanoMaterials Science, Utrecht University Padualaan 8, 3584 CH, Utrecht (The Netherlands) E-mail: [email protected], [email protected] Funding Sources We would like to thank the Netherlands Organization for Scientific Research (NWO) for financial support. ACKNOWLEDGMENT Jan Groenewold is thanked for useful discussions regarding the physical understanding of the charge reversal reactions, Sonja Castillo for the idea to use nano-sized silica for the heterocoagulation experiments and Dominique Thies-Weesie for kindly providing the micronsized silica colloids. ASSOCIATED CONTENT Supporting Information. Description of the charge regulation of the chlorinated colloids using DMF, plots of the evolution of the zeta potential, PDI and Z-average size for the reactions carried out at reaction temperatures of 50°C and 25°C, DLVO and Debye screening length calculations, details of the Menschutkin reactions carried out with a larger excess of TEA or TMA, estimation of the surface area occupied by a quaternary amine and additional microscopy


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pictures of the heterocoagulation experiments with micron-sized silica colloids. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Verwey, E.J.W.; Overbeek, J.T.G. In Theory of the Stability of Lyophobic Colloids. 2nd ed.; Dover Publications, Mineola, New York, 1999. (2)

Israelachvili, J.N. In Intermolecular & Surface Forces. 3th ed.; Academic Press,

Amsterdam, The Netherlands, 2011. (3) Wang, Y.; Angelatos, A.S.; Caruso, F. Template synthesis of nanostructured materials via layer-by-layer assembly. Chem. Mater., 2008, 20, 848-858. (4) Spruijt E.; Bakker, H.E.; Kodger, T.E.; Sprakel, J.; Cohen Stuart, M.A.; Van der Gucht, J. Reversible assembly of oppositely charged hairy colloids in water, Soft Matter, 2011, 7, 82818290. (5) Leunissen, M.E.; Christove, C.G.; Hynninen, A.-P.; Royall, C.P.; Campbell, A.I.; Imhof, A.; Dijkstra, M.; Van Roij, R.; Van Blaaderen, A. Ionic colloidal crystals of oppositely charged particles. Nature 2005, 437, 235-240. (6) Harding, I.H. Amphoteric polystyrene latex colloids: polymerization pathway and the control of particle size and potential. Colloid Polym. Sci. 1985, 263, 58-66. (7) Harding, I.H.; Healy, T.W. Electrical double layer properties of amphoteric polymer latex colloids. J. Colloid Interface Sci. 1985, 107, 382-297.


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(17) Zhang,; J.H.; Zhan, P.;Wang, Z.L.; Zhang, W.Y.; Ming, N.B. Preparation of monodisperse silica particles with controllable size and shape. J. Mater. Res. 2003, 18, 649-653. (18) Stöber, W.; Fink, A.; Bohn, E. Controlled growth of monodispersed spheres in the micron size range. J. Colloid Interface Sci. 1968, 26, 62-69. (19) Philipse, A.P.; Vrij, A. Polydispersity probed by light scattering of secondary particles in controlled growth experiments of silica spheres. J. Chem. Phys. 1987, 87, 5634-5643. (20) Thomas, J.C. The determination of log normal particle size distributions by dynamic light scattering. J. Colloid Interface Sci. 1987, 117, 187-192. (21) Ohshima, H. A simple expression for Henry’s function for the retardation effect in electrophoresis of spherical colloidal particles. J. Colloid Interface Sci. 1994, 168, 269-271 (22) Hunter, R.J.; In Foundations of Colloid Science 2nd ed.; Oxford University Press, New York, United States, 2009. (23) Monthéard, J.P.; Jegat, C.; Camps, M. Vinylbenzylchloride (Chloromethylstyrene), polymers, and copolymers. Recent reactions and applications. J. Macromol. Sci. Polymer Rev. 1999, 39, 135-174. (24) Menschutkin, N. Beiträgen zur Kenntnis der Affinitätskoeffizienten der Alkylhaloide und der organischen Amine. Z. Phys. Chem. 1890, 6, 41−57. (25) Vigil, G.; XU, Z.; Steinberg, S.; Israelachvili, J. Interaction of silica surfaces. J. Colloid Interface Sci. 1994, 165, 367-385.


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(26) Lide, D.R.; Frederikse, H.P.R. CRC Handbook of Chemistry and Physics. 76th ed.; CRC Press, 1995. (27) Evans, D.P. The influence of alkyl groups upon reaction velocities in solution. Part V. The formation of phenyltrialkylammonium iodides in methyl alcohol. J. Chem. Soc. 1944, 422-425. (28) Popa,I.; Gillies, G.; Papastavrou, G.; Borkovec, M. Attractive Electrostatic Forces between Identical Colloidal Particles Induced by Adsorbed Polyelectrolytes. Langmuir 2012, 28, 6211-6215. (29) Belloni, L. Ionic condensation and charge renormalization in colloidal suspensions. Colloids Surf., A 1998, 140, 227–243. (30) Alexander, S.; Chaikin, P.M.; Grant, P.; Morales, G.J.; Pincus, P. Charge renormalization, osmotic pressure, and bulk modulus of colloidal crystals: Theory. J. Chem. Phys. 1984, 80, 5776-5781. (31) Hsu, M.F.; Dufresne, E.R.; Weitz, D.A. Charge Stabilization in Nonpolar Solvents. Langmuir 2005, 21, 4881-4887. (32) Harley, S.; Thompson, D.W.; Vincent, B. The adsorption of small particles onto larger particles of opposite charge: Direct electron microscope studies. Colloids and Surfaces 1992, 62, 163-176. (33) Furusawa, K.; Anzai, C. Heterocoagulation behaviour of polymer latices with spherical silica. Colloids and Surfaces 1992, 63,103-111. (34) Islam, A.M.; Chowdhry, B.Z.; Snowden, M.J. Heteroaggregation in colloidal dispersions. Adv. Colloid Interface Sci. 1995, 62, 109-136.


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(35) Chen, M.; Zhou, S.; You, B.; Wu. L. Novel Preparation Method of Raspberry-like PMMA/SiO2 Hybrid Microspheres. Macromolecules 2005, 38, 6411-6417. (36) Szilagyi, I.; Sadeghpour, A.; Borkovec, M. Destabilization of colloidal suspensions by multivalent ions and polyelectrolytes: From screening to overcharging. Langmuir 2012, 28, 6211−6215. (37) Szilagyi, I.; Trefalt, G.; Tiraferri, A.; Maroni, P. Borkovec, M. Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation. Soft Matter 2014, 10, 2479-2502. (38) Phillips, C.L.; Jankowski, E.; Marval, M.; Glotzer, S.C. Self-Assembled clusters of spheres related to spherical codes. Phys. Rev. E. 2012, 86, 041124. (39) Schade, N.B.; Holmes-Cerfon, M.C.; Chen, E.R.; Aronzon, D.; Collins, J.W.; Fan, J.A.; Capasso, F.; Manoharan, V.N. Tetrahedral colloidal clusters from random parking of bidisperse spheres. Phys. Rev. Lett. 2013 110, 148303.


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TOC graphic (for Table of Contents only):


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