Different Types of Charged-Inverse Micelles in Nonpolar Media - Core

May 27, 2016 - Electronics and Information Systems and Center for Nano and Biophotonics (NB-Photonics), Ghent University, ... electrophoretic displays...
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Different Types of Charged-Inverse Micelles in Nonpolar Media Manoj Prasad,* Filip Strubbe, Filip Beunis, and Kristiaan Neyts Electronics and Information Systems and Center for Nano and Biophotonics (NB-Photonics), Ghent University, Technologiepark Zwijnaarde 15, 9052 Gent, Belgium ABSTRACT: Over the last few years, the electrodynamics of charged inverse micelles (CIMs) in nonpolar liquids and the generation mechanism and properties of newly generated CIMs have been studied extensively for the model system of polyisobutylene succinimide in dodecane. However, the newly generated CIMs, which accumulate at the electrodes when a continuous voltage is applied, behave differently compared to the regular CIMs present in equilibrium in the absence of a field. In this work, we use transient current measurements to investigate the behavior of the newly generated CIMs when the field is reduced to zero or reversed. We demonstrate that the newly generated CIMs do not participate in the diffuse double layer near the electrode formed by the regular CIMs but form an interface layer at the electrode surface. A fraction of the newly generated negative CIMs can be released from this interface layer when the field there becomes zero. The findings of this study provide a better understanding of fundamental processes in nonpolar liquids and are relevant for applications such as electronic ink displays and liquid toner printing.

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INTRODUCTION During the past decade, nonpolar liquids with added surfactant have been studied widely because of their relevance in applications such as motor oil,1,2 inks,3 liquid toner printing,4 electrophoretic displays,5−8 ceramic processing,9 and drug delivery systems.10 A number of measurement techniques based on conductivity,11−15 static/dynamic light scattering,15−17 small-angle X-ray/neutron scattering,16,18 optical tracking/trapping electrophoresis,13,19−21 pair interaction potentials,11,12 blinking optical tweezers,12,22 and transient currents23−26 have been used to study the mechanism of surfactant-mediated charging and charge stabilization in nonpolar media. From these studies, it has been concluded that charges in nonpolar surfactant systems exist in the form of inverse micelles that are formed above a certain surfactant concentration known as the critical micelle concentration.11−13,27,28 A small fraction (10−5 aerosol OT11,13 and 0.1 polyisobutylene succinimide24,29 (PIBS)) of the inverse micelles is charged as a result of a disproportionation/ comproportionation mechanism,11,12,14,15,24,28 in which two uncharged inverse micelles exchange a charge and become a positively and a negatively charged inverse micelle and vice versa. Charged inverse micelles (CIMs) play an important role in charging and charge stabilization in nonpolar media.11−15,30−32 Electrophoretic displays are based on the displacement of charged pigment particles between two electrodes in response to an applied voltage.5,7,25 In these systems, surfactant is often added to charge and stabilize the pigment particles.16,33,34 However, only a small fraction of surfactant molecules is adsorbed at the particle surfaces and other interfaces. Because of the presence of free CIMs, the nonpolar medium becomes more conductive, affecting the electric field and the switching characteristics of charged pigment particles.21 Controlling the © XXXX American Chemical Society

trajectory of particles in conductive media is challenging because of electrohydrodynamic flow,7,29 which causes particle clustering7 and pattern formation35−37 at the electrodes, resulting in image degradation, increased power consumption, and reduced lifetimes of electrophoretic devices.7 Many effects of charged inverse micelles can be studied without the presence of colloidal particles.29,33,38 The electrical behavior of these nonpolar systems with surfactant is governed by the drift, diffusion, and generation/recombination of CIMs.24,26,39 Measuring transient currents in response to a voltage step is a well-known sensitive technique for studying the surfactantmediated charging and charge dynamics in nonpolar systems.25,39,40 In the case of surfactant systems such as OLOA 11K, Span 80, and Solsperse 13940, when the voltage is reversed abruptly (V0 → −V1) after a sufficiently long polarizing voltage step (0 → V0, V0 > 1 V, t0 > 100 s), a second peak is observed in the transient current, which is not explained by drift and diffusion.29 Measurements have shown that the integral of this peak increases with increasing duration of the polarizing voltage step, with increasing device thickness, and with increasing mass fraction of surfactant in the mixture.38 This indicates that the peak is related to newly generated CIMs that accumulate at the electrodes during the long polarizing voltage step. By increasing the voltage stepwise to a voltage of the same polarity, the mobility of the newly generated CIMs has been measured to be the same as that of the regular CIMs present in equilibrium,38 so we can conclude that their charge (+e or −e) and their radius are also the same. If the newly generated CIMs were identical to the regular CIMs, then they would also end up in a diffuse double layer at Received: February 6, 2016 Revised: May 26, 2016

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DOI: 10.1021/acs.langmuir.6b00468 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Transient current measurements (|I|) in response to voltage step 0 → ±V (V = 5 V). (a) The results of the experiment of type A show the effect of the relaxation time (ts) on the current peak. (b) The results of the experiment of type B show the effect of the polarizing voltage step duration (t0) on the duration of the plateau current between 0.03 and 1s for t0 ≥ 500 s. a slowly decreasing current is measured that is due to the generation of new CIMs in the bulk.23 For surfactant system OLOA 1200 in dodecane (which is similar to OLOA 11K used here) at room temperature, it has been shown that the CIMs are univalently charged.25,39 The mobility of the regular CIMs is obtained from μ = I(t0 = 0,0→V0)d/(2zneSV ̅ 0) using the initial current for which the system is still in a homogeneous situation and the electric field is everywhere V0/d. More accurate values of the mobility and the concentration are then obtained by matching measurements (0 → V0) with simulations on the basis of the Poisson−Nernst−Planck equations.23,29 The obtained values for the mobility and the concentration are 1320 μm2 V−1 s−1 and 12 μm−3. This value of the mobility corresponds to a hydrodynamic radius of 5 nm, which matches with previously reported values acquired by various methods.16,25,38,41 The same value of the mobility is used for the newly generated CIMs.38

the electrodes upon application of a voltage step. However, the analysis of the second peak during the reverse voltage (V0 → −V1) indicates that the newly generated CIMs are released later than the regular CIMs. This means that the electrical behavior at the interfaces and consequently also the release mechanism for the newly generated CIMs are different than for regular CIMs. In this work, we study this difference with transient current measurements. We find that the newly generated CIMs or SCIMs do not populate the diffuse double layer but are adsorbed at the electrode interfaces and contribute to the interface layer.

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MATERIALS AND METHODS

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Transient current measurements are performed using a custom-built setup based on a Keithley 428 current amplifier and devices consisting of two glass substrates coated with indium tin oxide (ITO) with an overlapping surface area of S = 1 cm2. Spherical glass beads mixed with ultraviolet curing glue (Norland) outside the overlap area define the distance d between the electrode, which is estimated to be 19.6 μm from the optical transmission spectrum. A mixture with a 0.003 mass fraction of OLOA11K (Chevron Oronite), consisting of the surfactant PIBS, in dodecane (Aldrich, εr = 2) is inserted between the electrodes. No particular measures were taken to eliminate traces of water from the mixture or to purify the mixture. After short circuiting the device for 5000 s, a polarizing voltage step of 0 → V0 (V0 = 5 V) is applied for t0 = 10 000 s. Then, two types of experiments are performed. In the first type of experiment (type A), the electrodes are short circuited again during a time ts after the polarizing voltage step. Subsequently, a voltage step of 0 → V is applied for 10 s, and the current is measured. The following cycle is repeated for different values of V and ts: 0 V (5000 s) → V0 (t0 = 10 000 s) → 0 V (ts) → V (10 s) → 0 V. In the second type of experiment (type B), the electrodes are short circuited for ts = 30 s after the polarizing voltage step and then disconnected. The space between the electrodes is replaced by air by sucking out the liquid with a suction pump and then refilling with pure dodecane. In a full cleaning procedure, the emptying/filling is repeated 10 times. Finally, the electrodes are reconnected to the measurement setup. Then a voltage step of 0 → V is applied, and the resulting current is measured. The following cycle of experiments is repeated for different values of V: 0 V (5000 s) → V0 (10 000 s) → 0 V (30 s) → cleaning procedure → V (300 s). The values of the mobility and concentration of regular CIMs are calculated from the current measured during a polarizing voltage step (0 → V0) with magnitude larger than 1 V, for which the bulk is depleted of CIMs. By integrating the transient current, the concentration of positively and negatively charged regular CIMs can be obtained: n̅ = n+ = n− = ∫ t0tr(I − Iqs) dt/(zedS) where z is the valency of CIMs, e is the elementary charge, d is the separation between electrodes, S is the overlapping area of the electrodes, ttr is the time at which all CIMs have reached the electrode of opposite polarity, and Iqs is the quasi-steady-state current. During the quasi-steady state,

RESULTS Figure 1 shows the transient current |I| for voltage step 0 → V for experiments of type A (for V = 5 V or −5 V and for ts = 3 or 30 s) and type B (for t0 = 500 and 10 000 s). The first part of the type A currents (t < 0.1 s) is roughly the same and corresponds to the well-known drift and diffusion of positive and negative CIMs that are homogeneously distributed in the bulk.23 The current peak, between 0.1 and 1 s, has a different nature. This peak is apparently linked to the generation of new charges because its amplitude increases with duration t0 of the polarizing voltage step (it is absent for t0 < 100 s and saturates for t0 > 8000 s) and the thickness of the device (not shown). The current peak in Figure 1a also depends on the duration of the relaxation time ts and the sign of applied voltage: for short values of the relaxation time (ts = 3 s), the peak appears only for V = −5 V (Q = 14 nC), i.e., when the polarity of V is opposite to that of V0. The currents in the type B experiment in Figure 1 are different from those of the type A experiment. The initial current is much lower, and instead of a peak, we observe a more or less constant current between 0.03 and 0.5 s with a magnitude of about 10 nA (much higher than the 1 pA current in pure dodecane). The drift and diffusion current observed for regular CIMs has disappeared, and we claim that this is because the cleaning procedure has taken away all of the regular CIMs in the bulk of the device. The current in the type B experiment is therefore ascribed solely to the newly generated special CIMs or SCIMs. Space−Charge-Limited Currents. To study the behavior of the SCIMs in more detail, we repeat the type B experiment for different applied voltages. The results are shown in Figure 2. For voltage V between 0.5 and 15 V, after a short initial phase B

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Polarity of Released SCIMs. To find out if the SCIMs that contribute to the current in Figure 2 and for the current peak in Figure 1a are positive or negative, we have taken the cleaning procedure of the type B experiments one step further. After the extraction/refilling procedure of the type B experiment, the substrate with the ground electrode is replaced by a clean substrate in the type C experiment. (In this case, the two substrates have not been glued together.) Figure 3 shows the measured currents |I| when voltage step 0 → V with V = +5 V or V = −5 V is applied for 5 s across the electrodes for the type C experiment.

Figure 2. Comparison between transient current measurements (colored dots) and the currents calculated from the Mott−Gurney equation (black lines) for the type B experiment in response to voltage step 0 → V. The measured currents are in agreement with the Mott− Gurney equation up to time t* and then decrease rapidly.

the current remains approximately constant for some time, after which it quickly drops to zero. The level of the constant current fits well with the Mott−Gurney equation,42,43 I = 9ε0εrμSV2/ (8d3), with I(A) being the current, ε0 = 8.85 × 10−11 F/m being the vacuum permittivity, εr being the relative dielectric constant of the liquid, V(V) being the applied voltage, S (m2) being the surface area of the electrodes, d(m) being the separation between electrodes, and μ(m2 V−1 s−1) being the mobility of the charge carriers. For the mobility, the same value is used as for regular CIMs of OLOA, namely, μ = 1320 μm2/(V s). In a previous paper, we showed that the mobility of generated CIMs is equal to that of regular CIMs.38 The initial phase of the current, before reaching the constant Mott−Gurney level, results from electrodynamics related to the release of SCIMs from the interface into the bulk. The Mott−Gurney formula indicates a space−charge-limited current42,43 in which charge carriers start from one interface where the field is clamped to zero. Field E increases away from the interface (E ≈ x1/2, with x being the distance from that interface) as a result of the concentration n of drifting charge carriers (n ≈ x−1/2). The good fit indicates that only one type of charge carrier (positive or negative) is contributing and that the SCIMs are present in the interface layer at the electrode (and not in the bulk) before the voltage step is applied. The total amount of charge that is transferred during voltage step 0 → Vor 0 → −V (the integral of the current) is more or less the same for all of the applied voltages, namely, Q* = 8 nC (or a charge density of 8 × 10−5 C/m2 over S = 10−4 m2). The total charge (16 nC on both electrodes) is a fraction of about 5% of the integral of the generation current during polarization voltage Qgen ≈ 300 nC. From the Mott−Gurney expression for the current and the total charge, we find the time t* after which the current drops to zero: t* = 8d3Q*/(9ε0εμSV2) (indicated by the vertical lines in Figure 2). To study the behavior of the SCIM in more detail, we applied reverse step V → −V after the 0 → V step in the type B experiment. The measured current was again a Mott−Gurney current as in Figure 2, but the time t* and the total charge transfer were a factor of 2 larger. This indicates that the SCIMs, which contribute to the current in the type B experiment, are again participating in the interface layer and contribute to the current during the next step.

Figure 3. Measured currents (|I|) when the substrate with the grounded electrode has been replaced (type C experiment). There is a significant current only when the applied voltage step is negative.

When a positive voltage step (0 → 5 V) is applied over the device (with the electrode at the replaced substrate grounded), no significant current is measured in Figure 3. Even when the electrode is biased to a very large positive voltage (100 V), the current remains low. This indicates that there are no positive SCIMs at the electrode of the unchanged substrate and, as expected, no negative SCIMs at the electrode of the replaced substrate. When a negative voltage V is applied (0 → −5 V) to the original electrode, an important current is observed. The current in Figure 3 is slightly lower than the current in Figure 2 for the same voltage and does not have a constant level. We ascribe this difference to the release of some SCIMs from the unchanged substrate during the replacement of the grounded substrate. Figure 3 therefore proves that only the negative SCIMs in the interface layer are responsible for the Mott− Gurney space−charge-limited current observed in Figure 2. The type B experiment has been repeated, but with the application of a negative voltage step (0 → −V) instead of a positive voltage step (currents shown in Figure 2) after the cleaning procedure. The measured currents have the opposite polarity but are otherwise the same as in Figure 2. This indicates that after the cleaning procedure the two electrodes can release the same number of negatively charged SCIMs. This is in agreement with the fact that the current peaks in Figure 1 (a) (0 → V and 0 → −V) have the same amplitude when the short circuit time is 30 s.

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DISCUSSION Figure 4 illustrates how the different types of CIMs contribute to the current. Initially, there are only uncharged inverse micelles (not shown) and regular CIMs (Figure 4.1) in the mixture. During the 10 000 s generation step (Figure 4.2), SCIMs are generated and travel to the electrode with the opposite polarity. Most of them (300 nC positive CIMs and −284 nC negative CIMs) join the interface layer and are never C

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Figure 4. Illustration of the location and motion of regular CIMs (blue and red circles) and SCIMs (green and light-blue circles) during the different experiments. (1) Initial situation (only regular CIMs), (2) polarizing step, (3) after a short circuit of ts = 3 s, (4) after a short circuit of ts = 30 s, (5) after removing the regular and neutral CIMs (type B), (6) voltage step after cleaning, and (7) after replacing the bottom substrate. The regular CIMs are depicted by red and blue circles; the generated SCIMs are depicted by green circles.

electrostatic attraction with the mirror image in the electrode can be stronger and incorporation into the interface layer is more likely. This phenomenon is also observed in the case of small micelles, such as AOT in dodecane (r = 1.6 nm11,13), that always form an interface layer.33 We can assume that all newly generated CIMs have their charge near the surface of the micelle. For most of them, the attractive force is so high that they are never released, but for a small fraction of negatively charged SCIMs, the charge is between the edge and the center so that these SCIMs can be released when the sign of the field is reversed. The view emerges that different types of charged micelles can be generated, with their charge at various positions in the interior or on the outside of the micelle. However, in the absence of a field, CIMs with a charge on the outside either quickly recombine or the charge gets stabilized internally so that they become regular CIMs. Therefore, in equilibrium, to good approximation only regular CIMs are present. In the presence of a field, the situation is different. When positive and negative CIMs are formed in the presence of a field, they are immediately separated, prohibiting recombination, and their charges can remain at the periphery of the micelle because of the continuous force acting on the charge that prohibits stabilization inside the micelle. The experiments indicate that most of the newly generated CIMs (all of the positive ones and 95% of the negative ones) travel to the electrode with opposite polarity and their charges are never released again. It is not clear whether they remain charged at the interfaces or transfer their charge to the electrode and become neutral. From the experimental results and discussion, we can distinguish six different types of inverse micelles in the system: uncharged inverse micelles, regular CIMs (positive and negative) that form a diffuse double layer when a field is present, SCIMs that are negative, participate in the interface layer, and are released when the field changes sign, and finally positive and negative CIMs that are created by generation and contribute to the interface layer but are never released from the electrodes.

released. A small fraction of the negative charges (−16 nC) are SCIMs that join the interface layer and are released when the field near the negative electrode changes sign. When the device is short circuited (Figure 4.3), the regular CIMs move immediately into the bulk, but the negative SCIMs initially remain (ts = 3 s) in the interface layer. After 30 s of short circuiting, the negative SCIMs have redistributed in such a way that the same number is present at both electrodes (Figure 4.4). This explains why the current peak (Q = 7 nC (0 → ±5 V) when ts = 30 s) observed in Figure 1a between 0.1 and 1 s, for which the SCIMs are responsible, depends on the shortcircuit time. After the cleaning procedure is carried out, all regular CIMs and uncharged inverse micelles have disappeared (Figure 4.5). When a positive voltage step is applied as in Figure 2, the SCIMs move to the other side (Figure 4.6). When the bottom surface is replaced, only one surface contains SCIMs (Figure 4.7), which explains the asymmetry in Figure 3. The experiments indicate that the newly generated CIMs stick at the electrodes in the interface layer whereas the regular CIMs form a diffuse double layer. Let us discuss the origin of this different behavior at the interfaces. It is already known that both kinds of CIMs have the same mobility38 and (because their charge is +e or −e) are therefore equal in size. So we assume that newly generated CIMs consist of a similar spherical shell of surfactant molecules such that the micelle size can be ruled out as an explanation for the different behavior. We also assume that the polar interior of the CIMs is identical for all types of CIMs and does not affect the behavior of the micelles at the interfaces. It is also known that the presence of water, ions, and impurities in the polar core of the inverse micelles has no significant effect on the equilibrium concentration of CIMs.15,32 However, the difference in behavior might be explained by the location of the charge in the charged inverse micelle. If the charge is positioned in the center of the inverse micelle (with r = 5.5 nm for PIBS25,40,41), then the electrostatic force between the charged inverse micelle and its mirror charge in the electrode is low, which could explain why the inverse micelle does not stick electrostatically to the surface. If, on the other hand, the electrical charge is positioned closer to the edge of the inverse micelle as depicted in Figure 4.5, then the D

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(16) Parent, M. E.; Yang, J.; Jeon, Y.; Toney, M. F.; Zhou, Z.-L.; Henze, D. Influence of Surfactant Structure on Reverse Micelle Size and Charge for Nonpolar Electrophoretic Inks. Langmuir 2011, 27 (19), 11845−11851. (17) Lee, J.; Zhou, Z.-L.; Alas, G.; Behrens, S. H. Mechanisms of Particle Charging by Surfactants in Nonpolar Dispersions. Langmuir 2015, 31 (44), 11989−11999. (18) Kemp, R.; Sanchez, R.; Mutch, K. J.; Bartlett, P. Nanoparticle Charge Control in Nonpolar Liquids: Insights from Small-Angle Neutron Scattering and Microelectrophoresis. Langmuir 2010, 26 (10), 6967−6976. (19) Seth Roberts, G.; Wood, T. a; Frith, W. J.; Bartlett, P. Direct Measurement of the Effective Charge in Nonpolar Suspensions by Optical Tracking of Single Particles. J. Chem. Phys. 2007, 126, 194503. (20) Strubbe, F.; Beunis, F.; Brans, T.; Karvar, M.; Woestenborghs, W.; Neyts, K. Electrophoretic Retardation of Colloidal Particles in Nonpolar Liquids. Phys. Rev. X 2013, 3.10.1103/PhysRevX.3.021001 (21) Strubbe, F.; Beunis, F.; Marescaux, M.; Verboven, B.; Neyts, K. Electrokinetics of Colloidal Particles in Nonpolar Media Containing Charged Inverse Micelles. Appl. Phys. Lett. 2008, 93, 254106. (22) Sainis, S. K.; Germain, V.; Dufresne, E. R. Statistics of Particle Trajectories at Short Time Intervals Reveal fN-Scale Colloidal Forces. Phys. Rev. Lett. 2007, 99, doi: 10.1103/PhysRevLett.99.018303. (23) Beunis, F.; Strubbe, F.; Karvar, M.; Drobchak, O.; Brans, T.; Neyts, K. Inverse Micelles as Charge Carriers in Nonpolar Liquids: Characterization with Current Measurements. Curr. Opin. Colloid Interface Sci. 2013, 18, 129−136. (24) Strubbe, F.; Verschueren, A. R. M.; Schlangen, L. J. M.; Beunis, F.; Neyts, K. Generation Current of Charged Micelles in Nonaqueous Liquids: Measurements and Simulations. J. Colloid Interface Sci. 2006, 300, 396−403. (25) Verschueren, A. R. M.; Notten, P. H. L.; Schlangen, L. J. M.; Strubbe, F.; Beunis, F.; Neyts, K. Screening and Separation of Charges in Microscale Devices: Complete Planar Solution of the Poisson Boltzmann Equation. J. Phys. Chem. B 2008, 112, 13038−13050. (26) Kornilovitch, P.; Jeon, Y. Transient Electrophoretic Current in a Nonpolar Solvent. J. Appl. Phys. 2011, 109 (6), 064509. (27) Micelles; Topics in Current Chemistry; Springer-Verlag: Berlin, 1980; Vol. 87. (28) Morrison, I. D. Electrical Charges in Nonaqueous Media. Colloids Surf., A 1993, 71 (1), 1−37. (29) Prasad, M.; Beunis, F.; Neyts, K.; Strubbe, F. Switching of Charged Inverse Micelles in Non-Polar Liquids. J. Colloid Interface Sci. 2015, 458, 39−44. (30) Espinosa, C. E.; Guo, Q.; Singh, V.; Behrens, S. H. Particle Charging and Charge Screening in Nonpolar Dispersions with Nonionic Surfactants. Langmuir 2010, 26 (22), 16941−16948. (31) Smith, G. N.; Hallett, J. E.; Eastoe, J. Celebrating Soft Matter’s 10th Anniversary: Influencing the Charge of Poly(methyl Methacrylate) Latexes in Nonpolar Solvents. Soft Matter 2015, 11, 8029−8041. (32) Dukhin, A. S.; Goetz, P. J. How Non-Ionic “electrically Neutral” Surfactants Enhance Electrical Conductivity and Ion Stability in NonPolar Liquids. J. Electroanal. Chem. 2006, 588 (1), 44−50. (33) Karvar, M.; Strubbe, F.; Beunis, F.; Kemp, R.; Smith, A.; Goulding, M.; Neyts, K. Transport of Charged Aerosol OT Inverse Micelles in Nonpolar Liquids. Langmuir 2011, 27, 10386−10391. (34) Lin, T.; Kodger, T. E.; Weitz, D. A. Transport of Charged Colloids in a Nonpolar Solvent. Soft Matter 2013, 9 (21), 5173. (35) Trau, M.; Sankaran, S.; Saville, D. A.; Aksay, I. A. Pattern Formation in Nonaqueous Colloidal Dispersions via Electrohydrodynamic Flow. Langmuir 1995, 11 (12), 4665−4672. (36) Trau, M.; Sankaran, S.; Saville, D. A.; Aksay, I. A. Electric-FieldInduced Pattern Formation in Colloidal Dispersions. Nature 1995, 374 (6521), 437−439. (37) Lin, T.; Rubinstein, S. M.; Korchev, A.; Weitz, D. A. Pattern Formation of Charged Particles in an Electric Field. Langmuir 2014, 30 (41), 12119−12123.

CONCLUSIONS Newly generated charged inverse micelles (CIMs) are physically different from regular CIMs because they form an interface layer instead of a diffuse double layer. Only a small fraction of the negatively charged, generated inverse micelles are released from the interface layer when the field there changes sign. During the release of these special CIMs (SCIMs), the field is clamped to zero and the currents match the Mott−Gurney equation. From the analysis of the differences in the electrical behavior of CIMs at the interfaces, we have identified six different types of inverse micelles.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by Research Foundation-Flanders (FWO-Vlaanderen), IWT, IAP project photon@be funded by the Belgian Science Policy program, and the Hercules Foundation.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b00468 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b00468 Langmuir XXXX, XXX, XXX−XXX