pubs.acs.org/Langmuir © 2010 American Chemical Society
Particle Charging and Charge Screening in Nonpolar Dispersions with Nonionic Surfactants Carlos E. Espinosa, Qiong Guo, Virendra Singh, and Sven H. Behrens* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology 311 Ferst Drive NW, Atlanta, Georgia 30332, United States Received August 26, 2010. Revised Manuscript Received September 29, 2010 The electrostatic stabilization of colloidal dispersions is usually considered the domain of polar media only because of the high energetic cost associated with introducing electric charge in nonpolar environments. Nevertheless, some surfactants referred to as “charge control agents” are known to raise the conductivity of liquids with low electric permittivity and to mediate charge stabilization of nonpolar dispersions. Here we study an example of the particularly counterintuitive charging and electrostatic interaction of colloidal particles in a nonpolar solvent caused by nonionic surfactants. PMMA particles in hexane solutions of nonionic sorbitan oleate (Span) surfactants are found to exhibit a field-dependent electrophoretic mobility. Extrapolation to zero field strength yields evidence for large electrostatic surface potentials that decay with increasing surfactant concentration in a fashion reminiscent of electrostatic screening caused by salt in aqueous solutions. The amount of surface charge and screening ions in the nonpolar bulk is further characterized via measurements of the particles’ pair interaction energy. The latter is obtained by liquid structure analysis of quasi-2-dimensional equilibrium particle configurations studied with digital video microscopy. In contrast to the behavior reported for systems with ionic surfactants, we observe particle charging and a screened Coulomb type interaction both above and below the surfactant’s critical micelle concentration.
1. Introduction Electric surface charging of colloid particles is a common phenomenon in aqueous environments, and the resulting electrostatic particle interaction, mediated by small ions in solution, is the primary stabilizing mechanism for many colloidal dispersions.1 Apolar liquids, by contrast, are often considered charge-free because of the large energetic cost associated with the introduction of localized charge.2 The self-energy of a spherical ion of diameter dion and valency z in a medium of dielectric constant ε, for example, is given approximately by uB ¼
ðzeÞ2 4πεε0 dion
ð1Þ
where e is the elementary charge and ε0 the electric permittivity in vacuum. In water, with its dielectric constant of ε ≈ 80 at room temperature and a hydration layer effectively increasing the ion size, this electrostatic energy is comparable in magnitude to the available thermal energy. In nonpolar liquids such as alkanes (ε ≈ 2), by contrast, typically uB =kB T ¼ z2 λB =dion . 1
ð2Þ
even for monovalent species (z = 1), and thus the occurrence of small ions is largely suppressed. Here kBT is the thermal energy scale (the product of Boltzmann’s constant and absolute temperature), and λB is the Bjerrum length of the medium, around 28 nm for nonpolar liquids, as opposed to 0.7 nm for water. In spite of these energetic constraints, charging phenomena in nonpolar liquids do occur, and often, although not always, they *To whom correspondence should be addressed. (1) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, 1989. (2) Van der Hoeven, P. C.; Lyklema, J. Adv. Colloid Interface Sci. 1992, 42, 205– 277.
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are associated with the presence of surfactants.3 Micellar aggregates of these surfactants (“reverse micelles”) are believed to play a particularly important role in increasing the effective size of small ions, thus lowering their self-energy, by incorporating them in the polar micelle core. Two surfactant-mediated charging effects have often been observed: a dramatic increase in the electric conductivity of oils and the promotion of particle charging in nonpolar dispersions. These effects have been utilized in many applications, such as the prevention of flow electrification during petroleum transport,4,5 the stabilization of crude oil against asphaltene deposition,6 the formulation of liquid detergents,2 liquid electrostatic toners,7 and electrorheological fluids,8 as well as in the formulation of drugs carriers for inhalation,9 the development of electrophoretic displays,10-12 the assembly of new crystalline materials,13,14 and the functionalization of solid surfaces.15 Meanwhile, our understanding of the underlying mechanisms is still very incomplete. A review article by Morrison from 1993 provides an excellent overview of the confusing body of evidence gathered by that time.3 Later studies using advanced experimental tools such as (3) Morrison, I. D. Colloids Surf., A 1993, 71(1), 1–37. (4) Touchard, G. J. Electrost. 2001, 51, 440–447. (5) Tolpekin, V. A.; van den Ende, D.; Duits, M. H. G.; Mellema, J. Langmuir 2004, 20(20), 8460–8467. (6) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14(1), 6–10. (7) Pearlstine, K.; Page, L.; Elsayed, L. J. Imaging Sci. 1991, 35(1), 55–58. (8) Hao, T. Adv. Mater. 2001, 13(24), 1847–1857. (9) Jones, S. A.; Martin, G. P.; Brown, M. B. J. Pharm. Sci. 2006, 95(5), 1060– 1074. (10) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. Nature 1998, 394 (6690), 253–255. (11) Chen, Y.; Au, J.; Kazlas, P.; Ritenour, A.; Gates, H.; McCreary, M. Nature 2003, 423(6936), 136–136. (12) Yu, D.-G.; Kim, S.-H.; An, Y.-H. J. Ind. Eng. Chem. 2007, 13(3), 438–443. (13) Leunissen, M. E.; Christova, C. G.; Hynninen, A. P.; Royall, C. P.; Campbell, A. I.; Imhof, A.; Dijkstra, M.; van Roij, R.; van Blaaderen, A. Nature 2005, 437(7056), 235–240. (14) Bartlett, P.; Campbell, A. I. Phys. Rev. Lett. 2005, 95(12), 128302. (15) Tettey, K. E.; Yee, M. Q.; Lee, D. Langmuir 2010, 26(12), 9974–9980.
Published on Web 10/13/2010
DOI: 10.1021/la1033965
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a variety of force probing techniques,16-20 electroacoustic21 and electrokinetic methods,22-25 and small-angle neutron scattering26 have provided further insights, but so far a comprehensive picture of surfactant-mediated charging in nonpolar liquids remains elusive. Different electrostatic regimes have been distinguished, depending on the aggregation state of the surfactant.26,27 Above some concentration threshold, called “critical micelle concentration” (cmc) although the transition it marks is usually not very sharp, the surfactant is present mostly in the form of reverse micelles. In this micellar regime, mobile charges are believed to arise primarily from statistical equilibrium fluctuations in the micelle charge around a zero mean. A charge fluctuation model originally developed for water-in-oil microemulsions28-30 can also be applied to the population balance of charged and uncharged species in micellar solutions.25,31 The charging presumably occurs through disproportionation,3,32 a reaction of two initially neutral micelles M into a pair of oppositely charged micelles Mþ and M-, as expressed by eq 3: 2M T Mþ þ M -
ð3Þ
The micellar ions generated this way account for a linear increase in the conductivity of alkanes with surfactant concentration.26,27,32 Colloidal particles suspended in such micellar solutions also acquire electric charges, as inferred e.g. from electrophoresis19,25,26,33 and from interaction measurements.19,20,27 The connection between micelle charging and particle charging is not clear: asymmetric adsorption of charged micelles to particle surfaces has been proposed as a mechanism of particle charging,3,25 but there is no shortage of alternative models,3 involving for instance charge transfer between the surface and neutral micelles3 or the dissociation of individually adsorbed surfactant molecules.26 Whatever the origin of surface charge, the counterions required for overall electroneutrality are expected to be stabilized by incorporation in reverse micelles.3 A different electrostatic behavior is expected below the cmc. In nonpolar solutions of the much-studied ionic surfactant AOT (Aerosol-OT or sodium di-2-ethylhexylsulfosuccinate) below the cmc, the conductivity was found to scale with the square root of the surfactant concentration, which was explained by the (16) Briscoe, W. H.; Horn, R. G. Langmuir 2002, 18(10), 3945–3956. (17) Briscoe, W. H.; Horn, R. G. Prog. Colloid Polym. Sci. 2004, 123, 147–151. (18) McNamee, C. E.; Tsujii, Y.; Matsumoto, M. Langmuir 2004, 20(5), 1791– 1798. (19) Hsu, M. F.; Dufresne, E. R.; Weitz, D. A. Langmuir 2005, 21(11), 4881– 4887. (20) Sainis, S. K.; Germain, V.; Mejean, C. O.; Dufresne, E. R. Langmuir 2008, 24(4), 1160–1164. (21) Dukhin, A. S.; Goetz, P. J. J. Electroanal. Chem. 2006, 588(1), 44–50. (22) Keir, R. I.; Suparno; Thomas, J. C. Langmuir 2002, 18(5), 1463–1465. (23) Smith, P. G.; Patel, M. N.; Kim, J.; Milner, T. E.; Johnston, K. P. J. Phys. Chem. C 2007, 111(2), 840–848. (24) Strubbe, F.; Beunis, F.; Marescaux, M.; Verboven, B.; Neyts, K. Appl. Phys. Lett. 2008, 93(25), 3. (25) Roberts, G. S.; Sanchez, R.; Kemp, R.; Wood, T.; Bartlett, P. Langmuir 2008, 24(13), 6530–6541. (26) Kemp, R.; Sanchez, R.; Mutch, K. J.; Bartlett, P. Langmuir 2010, 26(10), 6967–6976. (27) Sainis, S. K.; Merrill, J. W.; Dufresne, E. R. Langmuir 2008, 24(23), 13334– 13337. (28) Eicke, H. F.; Borkovec, M.; Dasgupta, B. J. Phys. Chem. 1989, 93(1), 314– 317. (29) Hall, D. G. J. Phys. Chem. 1990, 94(1), 429–430. (30) Kallay, N.; Chittofrati, A. J. Phys. Chem. 1990, 94(11), 4755–4756. (31) Guo, Q.; Singh, V.; Behrens, S. H. Langmuir 2010, 26(5), 3203–3207. (32) Strubbe, F.; Verschueren, A. R. M.; Schlangen, L. J. M.; Beunis, F.; Neyts, K. J. Colloid Interface Sci. 2006, 300(1), 396–403. (33) Patel, M. N.; Smith, P. G.; Kim, J.; Milner, T. E.; Johnston, K. P. J. Colloid Interface Sci. 2010, 345(2), 194–199.
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dissociation (NaAOT T Naþ þ AOT-) of (very few) surfactant molecules.26,27 Little or no surface charging of suspended poly(methyl methacrylate) (PMMA) particles was observed in this regime. As the surfactant concentration is raised above the cmc, the surface charge on dispersion particles gradually builds up as the solution conductivity transitions from the submicellar regime (σ C1/2) to the micelle-dominated regime (σ C).26,27 Forces measured between particles in the weakly charged state and between mica surfaces, interacting across nonpolar AOT solutions in this regime, are consistent with the power-law decay of an “unscreened” electrostatic interaction16,17,27 described by a “counterion-only” model.34 The dissociation of ionic surfactants plays a central role in many recent discussions of charge control in nonpolar liquids, but not all charge control surfactants can dissociate. Poly(isobutylene succinimide)s (PIBS), for instance, are usually considered nonionic and are commonly used to impart charge onto carbon black particles,35-37 presumably as a result of acid-base interactions.38,39 Sorbitan esters of the “Span” family, hydrophobic surfactants without ionic groups, have also been identified as powerful charge control agents.21,31,40,41 To test the hypothesis of acid-base interactions as the origin of particle charges, Poovarodom and Berg compared the effect of the Span 80 (sorbitan monooleate) and of PIBS in nonpolar dispersions of silica particles functionalized with either acidic or basic surface groups.41 No particle charging was detected at low surfactant concentrations, but around and above the cmc, Span 80 led to positive particle charging regardless of the surface characteristics, whereas the presence of PIBS invariably resulted in negatively charged particles; this behavior was attributed to the acidic (Span) and basic (PIBS) nature of the surfactants. Particle charging in these systems was accompanied by a linear scaling of conductivity with surfactant concentration. To our knowledge, no systematic study relating the solution conductivity and particle charge with the resulting particle interaction above and below the cmc has yet been reported for nonpolar systems with nondissociable surfactants as charge control agents. Charging at the particle-solution interface and the impact of surface and bulk charges on the particle-particle interaction is the topic of the present investigation. We focus our study on the electrostatic effects of the nonionic, very hydrophobic (HLB: 1.8) surfactant Span 85 (sorbitan trioleate), which lends itself for comparison with the ionic AOT, since both surfactants form spherical micelles of comparable size in alkane solutions. In a preceding publication, we presented cmc measurements in hexane and reported a linear increase in conductivity with Span 85 concentration both above and below the cmc as well as a remarkable indifference to added ionizable contaminants.31 We hypothesized that the detected ionic species originate from a disproportionation of micelles or premicellar complexes, presumably containing ionizable impurities, and that the energetic cost of disproportionation rather than the availability of impurities limits the charging process. Here, we present electrophoretic data for the charging of poly(methyl methacrylate) (PMMA) (34) Briscoe, W. H.; Attard, P. J. Chem. Phys. 2002, 117(11), 5452–5464. (35) Pugh, R. J.; Matsunaga, T.; Fowkes, F. M. Colloids Surf. 1983, 7(3), 183– 207. (36) Kornbrekke, R. E.; Morrison, I. D.; Oja, T. Langmuir 1992, 8(4), 1211– 1217. (37) Kim, J. Y.; Garoff, S.; Anderson, J. L.; Schlangen, L. J. M. Langmuir 2005, 21(24), 10941–10947. (38) Pugh, R. J.; Fowkes, F. M. Colloids Surf. 1984, 11(3-4), 423–427. (39) Fowkes, F. M. J. Adhesion Sci. Technol. 1990, 4, 669–691. (40) Wang, Y. T.; Zhao, X. P.; Wang, D. W. J. Microencapsulation 2006, 23(7), 762–768. (41) Poovarodom, S.; Berg, J. C. J. Colloid Interface Sci. 2010, 346(2), 370–377.
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particles in nonpolar Span 85 solutions as well as profiles of the particle-particle interaction energy inferred from a statistical analysis of equilibrium particle configurations. The results suggest a screened Coulomb interaction down to the lowest surfactant concentration capable of stabilizing the particles against aggregation. Just like the solution conductivity, the particles’ surface potential and their interaction energy show no transition between qualitatively different regimes at the surfactant’s cmc ; quite in contrast to the reported conductivity, surface potential, and particle interaction caused by the ionic surfactant AOT.
2. Materials and Methods 2.1. Samples. Hexane (BDH, ACS grade) was chosen as the nonpolar solvent because of its very low dielectric constant (1.89) and because of a relatively large refractive index mismatch with surfactant solutes and dispersion particles of interest. Sorbitan trioleate (Span 85) and monooleate (Span 80) purchased from Sigma-Aldrich were dissolved in hexane without further purification. All surfactant solutions were allowed to equilibrate for 1 day prior to use. The PMMA particles (Bangs Laboratories; diameter 1.08 μm: product no. PP04N/6896; 0.52 μm: PP02N/8813; 0.11 μm: PP02N/5814) were originally delivered as a dispersion in deionized water; only the 0.52 μm particle dispersion contained an antimicrobial additive, 0.1% NaN3. Ultrapure water with a resistivity of 18.3 MΩ 3 cm (Barnstead) and isopropanol (SigmaAldrich, >99.5%) were used in the solvent swap described below. All experiments were performed in a thermostated environment at 22 ( 0.5 C, except for the electrophoresis experiments, which were carried out at 25 C.42 2.2. Solvent Swap. The PMMA particles were first transferred from their aqueous environment into isopropanol as the intermediate solvent and then further into solutions of Span 85 in hexane. In each transfer step, centrifugation, disposal of the supernatant, redispersion of particles in the target solvent, and sonication were repeated three times. Microscopy and light scattering confirm the colloidal stability of the final nonpolar dispersions containing a minimum of 0.5 mM Span 85; when transferred into hexane without added surfactant, however, particles aggregate immediately. 2.3. Electrophoretic Mobility and Particle Size. Phase analysis light scattering (PALS)43 was used (Malvern Zetasizer Nano ZS90) to determine the electrophoretic mobility of the dispersed particles. The external electric field is applied using a dip cell designed for measurement in nonaqueous environment. It consists of a glass cuvette with square cross-section as the sample cell and, dipped into it, a unit that introduces two planar palladium electrodes separated by 2 mm. Prior to measurements, the dip cell electrodes and glass cuvette were carefully cleaned and sonicated in methanol. Electrophoresis experiments in nonpolar liquids are complicated by the fact that the large electric fields required to observe particle motion can sometimes alter the electrophoretic mobility and lead to measurement artifacts.44 A systematic variation of the field strength indeed revealed fielddependent effects in many of our samples, as discussed in the Results section. Misinterpretations were avoided by extrapolating electrophoretic mobilities to zero field strength. We confirmed that the measured mobility was insensitive to the precise particle concentrations used. A dynamic light scattering mode for particle size measurements offered by the same instrument (Zetasizer) was used to assess the stability of dispersions prepared for electrophoresis. (42) Changes in the viscosity and electric permittivity of our samples due to this temperature difference have been accounted for. Slight shifts in equilibrium reaction constants may also occur but are not expected to devalidate our conclusions. (43) Miller, J. F.; Schatzel, K.; Vincent, B. J. Colloid Interface Sci. 1991, 143(2), 532–554. (44) Thomas, J. C.; Crosby, B. J.; Keir, R. I.; Hanton, K. L. Langmuir 2002, 18 (11), 4243–4247.
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2.4. Conductivity Measurements. The electric conductivity of the Span 85/hexane solutions used as medium for the investigated particles was measured with a high-precision conductivity meter (model 1154, Emcee Electronics). Prior to measurements, the sample cell was cleaned successively with hot water, DI water, acetone, and a toluene/isopropyl alcohol mixture, then dried with nitrogen, and finally rinsed with sample solution. Conductivity values measured for hexane without any surfactant were consistently below 0.5 pS/m. 2.5. Particle Interaction. The pair interaction of 1.08 μm PMMA particles in nonpolar dispersions was studied by analysis of quasi-2-dimensional equilibrium particle configurations. The procedure closely resembles the one described in ref 19. Dispersions are confined in a narrow gap between two almost parallel glass coverslips (18 mm 18 mm) spin-coated on their facing surfaces with a thin (0.1 μm) layer of PMMA. The confining gap is created by placing the coverslips together, with a 15 μm poly(ethylene terephthalate) spacer on one side, and sealing the two contacting sides with a UV-curable epoxy adhesive (NOA 71). The sealant slightly increases the overall spacing between the glass surfaces, which varies roughly between 5 and 22 μm over the full length of the coverslips, but shows insignificant variation (
