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J. Phys. Chem. C 2010, 114, 19452–19458
Forces within Single Pairs of Charged Colloids in Aqueous Solutions of Ionic Liquids as Studied by Optical Tweezers Mahdy M. Elmahdy,*,†,§ Christof Gutsche,† and Friedrich Kremer† Institute of Experimental Physics I, Leipzig UniVersity, Linne´strasse 5, 04103, Leipzig, Germany, and Department of Physics, Mansoura UniVersity, Mansoura 35516, Egypt ReceiVed: August 13, 2010; ReVised Manuscript ReceiVed: September 13, 2010
Forces of interaction within single pairs of negatively charged microsized blank colloids in aqueous solutions of water miscible room temperature ionic liquids (RTILs) have been measured at varying concentrations and pH by using optical tweezers (OT). Three different water miscible RTILs (1-butyl-3-methylimidazolium tetrafluoroborate [BMIM-BF4], 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIM-TfO], and 1-butyl-3-methylimidazolium chloride [BMIM-Cl]) having the same organic cation [BMIM]+ and different inorganic anions ([BF4]-, [TfO]-, and Cl-) are used and compared with the high temperature molten salt (KCl). The experimental data are well described by a size-corrected screened Coulomb interaction approach which originates from the linearized Poisson-Boltzmann (PB) equation. The effective surface charge density σ derived from the fitted force-separation data is found to be concentration and pH dependent. 1. Introduction Room-temperature ionic liquids (RTILs) are organic salts which are molten at room temperature.1-4 They are novel solvents with favorable environmental and technical features which make them distinct from conventional molecular liquids.1 They possess unique physicochemical properties, such as high ionic conductivity, nonflammability, negligible vapor pressure, and high thermal and chemical stability.5-9 The RTILs are used as novel inorganic and organic reaction media and have recently received much attention as solvents in the development of green chemistry to reduce environmental impact.10,11 The miscibility of RTILs with water12-17 or organic solvents18 varies with side chain lengths on the cation and with choice of anion. For example, the solubility of ionic liquids in water can be varied from completely miscibility to almost total immiscibility, by changing the anion from, for example, Cl- to hexafluorophosphate [PF6]-.1 The solubility of RTILs (especially imidazolium ILs) in water is of great importance for industrial extraction processes.19-21 Therefore, there have been several recent studies of mixtures of RTILs and water.22-26 The structure of solid-ionic liquid interfaces was first investigated for ethylammonium nitrate (EAN) by Horn et al. using the surface force apparatus (SFA).27 They were able to measure the interaction forces between smooth mica surfaces immersed in EAN ionic liquid and its mixture with water and found that EAN behaves as a typical 1:1 electrolyte at low salt concentrations and that the force between mica surfaces follows the predictions of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory.28-30 The same finding was observed by Atkin and co-workers using silica colloid probe atomic force microscopy (AFM).31 They observed that the addition of even small amounts of water reduces both the number and resilience of EAN solvation layers. * Corresponding author. Tel: +493419732566; fax: +493419732599; e-mail:
[email protected]. † Leipzig University. § Mansoura University.
By using sum-frequency vibrational spectroscopy (SFVS), the effect of alkyl chain length and anion composition on the 1-alkyl-3-methylimidazolium structure and orientation at the room-temperature ionic liquid (RTIL)/SiO2 interface has been studied and indicates that the length of the alkyl chain dictates to a large degree the orientation of the imidazolium cation at the surface, regardless of anion composition. Furthermore, they found that as the SiO2 surface charge density becomes more negative the tilt angle lies more parallel to the surface.15,26 Mezger and co-workers32 have used high-energy X-rays to investigate the interfacial layering properties of RTILs (tris(pentafluoroethyl)trifluorophosphate) on a charged sapphire surface and found that the RTIL ions stack in alternately charged layers, starting with a cation layer at the substrate and decaying exponentially into the bulk liquid. In a separate study by the same group33 on the 1-butyl-3-methylimidazolium [BMIM]+ cation in combination with [PF6]- and [BF4]- anions, they found that the one having a larger anion, [PF6]-, exhibited a strong, alternate-charge layering at the sapphire interface while the one having a smaller anion, [BF4]-, showed only a single dense layer at the sapphire interface but no further layering. In recent studies using optical tweezers, the forces of interaction within single pairs of poly(acrylic acid) (PAA) and poly(2-vinylpyridine) (P2VP) grafted colloids were measured in dependence on the concentration and valency of the counterions of the surrounding medium as well as its pH.34,35 The data were quantitatiVely described by the Jusufi model36 for spherical polyelectrolyte brushes, which takes into account the entropic effect of the counterions. The transition from an osmotic to salted brush on varying the ionic strength and the valency of the surrounding medium was observed. The brush height increased with increasing pH for PAA-grafted colloids while the P2VP-grafted colloids showed a transformation in the conformation of the brushes. Recently, Min et al.37 combined dielectric spectroscopy and SFA techniques to measure the dielectric properties and the interaction forces across three different types of RTILs (1-butyl3-methylimidazolium trifluoromethanesulfonate [BMIM-TfO], 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM-BF4],
10.1021/jp107673f 2010 American Chemical Society Published on Web 10/25/2010
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TABLE 1: Molecular Structure, Molecular Weight (Mw), and Calculated Molecular Diameter (D) of the Investigated Salts
and trihexyltetradecylphosphonium bromide). They found repulsive forces in all three RTILs, decaying exponentially with distance, and the effective Debye screening lengths were found to be around 1-4 nm, much longer than expected from traditional theories and in the order of molecular size and strongly dependent on the size and molecular structure of the anion/cation. They also observed oscillatory forces at small separations. In the present contribution, we report on measurements of the forces of interactions within single pairs of charged colloids in aqueous solutions of RTILs using optical tweezers. For the purpose of this study, three different types of water miscible RTILs having the same cation (1-butyl-3-methylimidazolium [BMIM]+) and different anions ([BF4]-, [TfO]-, and Cl-) have been chosen. The high temperature molten salt Potassium Chloride (KCl) is used to compare with the BMIM-Cl. Two different types of colloids are investigated in this study (polystyrene (PS) and melamine (MF)). Moreover, our data are analyzed by using a size-corrected screened Coulomb interaction approach recently published by Gutsche et al.38 2. Materials and Methods Materials. Monodisperse spherical polystyrene (PS) and melamine (MF) beads with diameters of 2.23 ( 0.02 and 2.31 ( 0.05 µm, respectively, are purchased in dry state from Microparticles GmbH (Berlin, Germany). The PS microparticles and MF possess a negative charge due to surface sulfate groups (SO4) and carboxyl groups (COOH), respectively. Highest grade 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM-BF4], 1-butyl-3-methylimidazolium trifluoromethanesulfonate [BMIMTfO], and 1-butyl-3-methylimidazolium chloride [BMIM-Cl]) with 99% purity were purchased from Ionic Liquids Technologies GmbH, Germany. Potassium chloride (KCl) with 99.5% purity was purchased from Carl Roth GmbH, Germany. Millipore deionized (DI) water (Purelab Plus, ELGA Lab Water) is used to prepare aqueous solutions of the different RTILs as well as KCl at different salt concentrations ranging from ∼10-5 M up to 0.01 M at pH ) 5.8 (pH value of the Millipore DI water ∼5.8). The pH is regulated by adding hydrochloric acid (HCl) (Titrisol, Merck). Table 1 compiles the three RTILs as well as the KCl salt investigated in the present study and lists their abbreviations, molecular structures, molecular weight (Mw), and calculated molecular diameters (D).37,39,40 Methods. The results presented here are obtained by the OT apparatus. These instruments are capable of manipulating
nanometer- and micrometer-sized dielectric particles by exerting extremely small forces via a highly focused laser beam.41-43 The experimental setup of the OT technique used in the present study is described in more details in ref 35. Briefly, an inverted microscope (Axiovert S 100 TV, Carl Zeiss, Jena, Germany) accomplished with a stabilized diode-pumped Nd:YAG laser (1064 nm, 1 W, LCS-DTL 322; Laser 2000, Wessling, Germany) is used. The beam is expanded and coupled into the back aperture of the microscope objective (Plan-Neofluor 100 1.30 Oil, Carl Zeiss, Jena, Germany). Video imaging and optical position detection are accomplished at 20 frames per second by a digital camera (1M60CL, DALSA, Gro¨benzell, Germany). The optical stage is positioned in three dimensions with nanometer resolution using piezoactuators (P-5173CD, Physik Instrumente, Karlsruhe, Germany). The sample cell with volume ∼300 µL consists of a stainless steel corpus which is covered at the top and the bottom by glass coverslips that allows the flushing of solutions by a syringe pump. In order to avoid effects of the walls of the sample cell or other neighboring particles, the focal position of the objective is set sufficiently apart from the cell bottom (∼40 µm); the concentration of the microparticles is below 10-18 M. The stainless steel corpus as well as the cover glass parts of the liquid cell is washed for one time in a solution of 1 mL of Hellmanex in 100 mL of Millipore DI water followed by four times in fresh Millipore DI water by using an ultrasonic bath at 60 °C for 30 min each time. As the next step, the cell bodies are immersed for 1 min in pure ethanol (HPLC grad) and then purged of the fluid by clean pressure nitrogen. Finally the cells are assembled in a dust-reduced surrounding. A custom-made micropipet with an inner tip diameter of ∼0.5 µm is inserted into the chamber to hold one colloid by capillary action (Figure 1a). The whole experimental setup is located in a temperaturecontrolled (298 ( 1 K) room. The calibration of the optical trap to determine the trap stiffness (ktrap) in Millipore DI water is based on Stokes law FStokes ) 6πηrV where η is the viscosity of the medium (η ) 0.001 Pa · s for Millipore DI water), r the radius of the bead, and V its velocity relative to the surrounding solution.44 Varying the velocity V between 50 and 1300 µm/s enables one to calibrate the optical forces with an accuracy of (10%. A typical force constant for the trap is 0.058 ( 0.006 and 0.06 ( 0.006 pN/nm for PS and MF beads, respectively (Figure 1b). The experiments are performed with one colloid fixed at the tip of a micropipet and the second one in an optical trap (Figure
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Figure 1. (a) Microscope image of a single pair of blank PS colloids (diameter ∼ 2.23 ( 0.02 µm) in an aqueous solution of the BMIMBF4 at a concentration of 2.3 × 10-5 M and pH 5.8. As illustrated in the image, one colloid is fixed at the tip of a micropipet by capillary action and the other in the optical trap. The force of the interaction F at separation D between the two blank surfaces is determined from the displacement of the trapped colloid out of its equilibrium position with an extraordinary resolution of (0.5 pN. (b) Calibration of the optical trap stiffness (ktrap) is made in Millipore DI water based on Stokes law for PS (red full squares) and MF (navy open triangles) colloids with diameters of 2.23 ( 0.02 µm and 2.31 ( 0.05 µm respectively, at a laser power of 270 mW. ∆x represents the displacement of a colloid in the optical trap at different velocities relative to the surrounding medium. The experimental uncertainty is smaller than the size of the symbols.
1a). The approach speed is 0.25 ( 0.05 µm/s. The interaction force F is obtained from the displacement of the optical trapped colloid out of its equilibrium position. In other words, optical tweezer force can generally be described by F ) -ktrapx, where ktrap is the trap stiffness and x the displacement of the colloid out of its equilibrium position. From the digital images, the displacement of the colloid in the optical trap out of the equilibrium position (x) and the separation between the centers of the colloids are determined using a custom-made LabVIEW image analysis routine.38 By that the separation between the two colloids could be determined with an accuracy of (3 nm and the interacting forces with a resolution of (0.5 pN.42 After the measurements are finalized, the reproducibility of the experiment as a whole is ensured by remeasuring the forceseparation dependence in the initial salt concentration. At the end of a measurement, the average diameter of the two colloids is determined by measuring the force-separation dependence in a 3 M KCl solution, where the force is well described by a hard-sphere potential. 3. Results and Discussion Optical tweezers are experimental tools with extraordinary resolution in positioning ((3 nm) a micrometer-sized colloid and in the measurement of forces ((0.5 pN) acting on it, without any mechanical contact. Herein, the experiments are carried out with a single pair of colloids for which the solvent is exchanged to eliminate possible variations between different colloids (variation in diameter, surface roughness, the charge per colloid, etc.). The force-separation dependence profiles as measured within a single pair of PS colloids across the aqueous solutions of three different types of RTILs (BMIM-BF4, BMIM-TfO, and BMIMCl) are displayed in Figure 2. These RTILs have the same cation [BMIM]+ and different anions ([BF4]-, [TfO]-, and Cl-).The latter have the same valency (monovalent) and different molecular sizes (Table 1). As seen in Figure 2, the force is monotonically repulsive and decaying roughly exponentially with the separation distance for all RTILs. At sufficiently large separations, the force levels to zero, while as the salt concentration of the surrounding medium is increased, the potential
Figure 2. Force F vs separation D for a single pair of blank polystyrene (PS) colloids (diameter∼2.23 ( 0.02 µm) in aqueous solution of three different types of ionic liquids having the same cation [BMIM]+ and different anions ([BF4]-, [TfO]-, and Cl-). Symbols represent the experimental data at different concentrations of the ionic liquids at pH 5.8 while the solid lines represent the fits according to eq 4 for sphere-sphere interaction. (a) BMIM-BF4 at 2.3 × 10-5 M (black full squares), 5 × 10-5 M (red open up-triangles), 1 × 10-4 M (blue open down-triangles), 5 × 10-4 M (cyan open diamond), 1 × 10-3 M (orange open hexagon), and 0.01 M (olive open right-triangles). In order to ensure the reproducibility of the exchange of the medium and to exclude hysteresis effects due to possible adsorption effects on the colloids, the sample cell is flushed again at the end of each measurement cycle with the initial concentration 2.3 × 10-5 M BMIM-BF4 (black open squares). (b) BMIM-TfO at 1.4 × 10-5 M (black full pentagon), 5 × 10-5 M (red open up-triangles), 1 × 10-4 M (blue open downtriangles), 5 × 10-4 M (cyan open diamond), 1 × 10-3 M (orange open hexagon), 0.01 M (olive open right-triangles), and the reproducibility at 1.4 × 10-5 M (black open pentagon). (c) BMIM-Cl at 1 × 10-5 M (black full circles), 5 × 10-5 M (red open up-triangles), 1 × 10-4 M (blue open down- triangles), 5 × 10-4 M (cyan open diamond), 1 × 10-3 M (orange open hexagon), 0.01 M (olive open right-triangles), and reproducibility check at 1 × 10-5 M (black open circles).
becomes steeper and the interaction extends to shorter distances. In order to gain a deeper understanding of the effect of the salt type and concentration on the interaction forces between the blank PS colloids, the interaction length λ between the solid surfaces of the colloids at a given force (at 2 pN) is deduced (Figure 3). As seen in Figure 3, the interaction length decreases with increasing salt concentration from about 380 nm up to about 50 nm, all three RTILs behave as a simple electrolyte, and the interaction length is slightly dependent on the type of ionic liquid. These results are consistent with previously published data using SFA27 and silica probe AFM31 across water-EAN mixtures. The equation proposed by DLVO28-30 is only applicable for colloids, with constant surface charge density, in symmetric electrolytes; hence, it cannot be used here. Recently, Gutsche et al.38 have used a size-corrected screened Coulomb interaction approach which originates from the linearized PB equation in spherical geometry. This approach is often
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J. Phys. Chem. C, Vol. 114, No. 45, 2010 19455
Figure 3. Interaction length at force of 2 pN (λF)2 pN) (obtained from the force-separation dependencies presented in Figure 2) vs concentration for a single pair of blank PS colloids across the three different types of water miscible ionic liquids: BMIM-BF4 (blue full up-triangles), BMIM-TfO (red full circles), and BMIM-Cl (black full squares) at pH 5.8. The inset includes the concentration dependence of the effective surface charge density σ of PS colloids in the aqueous solutions of BMIM-BF4 (blue full up-triangles), BMIM-TfO (red full circles), and BMIM-Cl (black full squares). The values of σ are obtained from fitting the experimental data given in Figure 2 with eq 4.
taken as the electrostatic contribution to the DLVO force acting between two charged colloids,45 but this expression generally differs from the original one given by DLVO.29,30 In this formalism, the electrostatic interaction energy U operating between two equal colloids possessing the charge Z and radius R at the center-to-center distance r is given by45
U(r) )
(eZ)2 e[-κ(r-2R)] 4πε0εr (1 + κR)2r
(1)
where ε0 is the permittivity of vacuum, εr (∼80) the relative permittivity of the solution, e is the elementary charge, and the inverse Debye screening length is
κ)
[
NAe2 n c z2 ε0εrkBT i)1 i i
∑
]
1/2
(2)
with NA being Avogadro’s number, kB the Boltzmann constant, T () 298 K) the temperature, zi the valence of species i (for our ionic liquids, zi ) ( 1), and ci the bulk concentration of species i. The interaction energy and the force at surface-tosurface separation D ) r - 2R become
U(D) )
e-κD 4πσ2R4 ε0εr (1 + κR)2(D + 2R)2
(3)
F(D) )
4πσ2R4 e-κD[1 + κ(D + 2R)] ε0εr (1 + κR)2(D + 2R)2
(4)
and
where σ() eZ/4πR2) is the effective surface charge density. Equation 4 is used to describe the experimental force-distance dependencies, assuming that the effective surface charge density σ and the radius R of the two colloids are equal. Also, the van der Waals attraction is neglected, since it contributes at high
Figure 4. Force F vs separation D for a single pair of blank PS colloids (diameter∼2.23 ( 0.02 µm) in aqueous solution of BMIM-Cl at fixed concentration of 1 × 10-4 M and different pH values of 2.1 (olive full pentagon), 2.6 (orange open hexagon), 3.1 (navy full diamond), 3.7 (magenta open down-triangles), 4.2 (blue full up-triangles), 4.8 (red open circles), and 5.8 (black full squares). The pH values were regulated by adding HCl. To ensure reproducibility of the exchange of the medium, the sample cell is flushed again with 1 × 10-4 M at pH 2.1 (black open squares) at the end of a measurement cycle. The solid lines represent the global fits of the effective surface charge density σ at different pH values with eq 4. Inset: Interaction length at force of 2 pN (λF)2 pN) versus pH at fixed BMIM-Cl concentration of 1 × 10-4 M (black full circles). The pH dependence of the effective surface charge density σ obtained from the fitting of the force separation curves at different concentrations with eq 4 is illustrated in the inset.
salt concentration and at surface separations smaller than 10 nm38,46 which is not the case in the present study. As readily seen from the fits (the solid lines in Figure 2), eq 4 successfully describes the double layer forces of the water-RTIL mixtures. The effective surface charge density σ is the outcome of this fitting (inset of Figure 3) and reveals that σ increases with increasing concentration and is slightly dependent on the type of ionic liquid. The increase of σ presumably arises either from dissociation of ionizable groups on the surface or from preferential adsorption of the cations [BMIM]+ with its charged imidazolium end next to the negatively charged PS surface, and the alkyl chain points to the liquid forming the Stern layer which would approximately neutralize the charge so that there would not be a strong tendency for the next layer to consist solely of anions. If there are only cations in the first layer, it will be thinner than subsequent layers. The molecular diameters of the investigated molecules (cations and anions) are small (see Table 1); therefore, any molecular structure effects would occur only at small separations (
