pubs.acs.org/Langmuir © 2009 American Chemical Society
Electric Charging in Nonpolar Liquids Because of Nonionizable Surfactants Qiong Guo, Virendra Singh, and Sven Holger Behrens* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332 Received August 26, 2009. Revised Manuscript Received October 14, 2009 Nonpolar liquids do not easily accommodate electric charges, but it is known that surfactant additives can raise the conductivity and lead to electric charging of immersed solid surfaces. Here, we study the rarely considered conductivity effects induced by surfactant molecules without ionizable groups. Precision conductometry, light scattering, and Karl Fischer titration of sorbitan oleate solutions in hexane reveal a distinctly electrostatic action of the nonionic surfactant. The conductivity in dilute hexane solutions of sorbitan trioleate (Span 85) exhibits two regimes of linear scaling with surfactant concentration and a transition around the critical micelle concentration (cmc). Both regimes can be described with a statistical model of equilibrium charge fluctuations. The behavior observed above the cmc has a direct analogy in systems of ionic surfactants and can be explained by charge disproportionation of reverse micelles. The observed conductivity of Span 85 solutions below the cmc, however, represents a qualitative departure from the behavior reported for ionic surfactants. In the studied surfactant systems, the availability of ionic species may stem from a complexation of the surfactant with ionizable impurities; nonetheless, the ionization process appears to be limited entirely by the surfactant and not by the level of impurity. We therefore propose that nonionizable surfactants can offer a new and robust way of controlling the conductivity in nonpolar liquids.
1. Introduction Mobile ions and electric surface charges are ubiquitous in water and other polar liquids but should play no role in nonpolar liquids at room temperature, where the Born energy of common ions far exceeds the available thermal energy. Defying this conventional wisdom, surfactant additives can induce strong charging effects in nonpolar solvents,1 with practical consequences for the prevention of flow electrification in petroleum handling2-4 and asphaltene deposition in crude oil processing5 as well as applications in the development of detergents,6 the design of electrophoretic displays,7-9 liquid toners,10 and drug delivery systems.11 Certain surfactants with ionizable groups have been established as powerful “charge control agents”1,12,13 that can raise the conductivity of nonpolar liquids by many orders of magnitude14 and promote *To whom correspondence should be addressed.
(1) Morrison, I. D. Colloids Surf., A 1993, 71(1), 1–37. (2) Klinkenberg, A.; van der Minne, J. L. Electrostatics in the Petroleum Industry; Elsevier: New York, 1958. (3) Touchard, G. J. Electrostat. 2001, 51, 440–447. (4) Tolpekin, V. A.; van den Ende, D.; Duits, M. H. G.; Mellema, J. Langmuir 2004, 20(20), 8460–8467. (5) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14(1), 6–10. (6) van der Hoeven, P. C.; Lyklema, J. Adv. Colloid Interface Sci. 1992, 42, 205– 277. (7) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. Nature 1998, 394 (6690), 253–255. (8) Chen, Y.; Au, J.; Kazlas, P.; Ritenour, A.; Gates, H.; McCreary, M. Nature 2003, 423(6936), 136–136. (9) Yu, D.-G.; Kim, S.-H.; An, Y.-H. J. Ind. Eng. Chem. 2007, 13(3), 438–443. (10) Pearlstine, K.; Page, L.; Elsayed, L. J. Imaging Sci. 1991, 35(1), 55–58. (11) Jones, S. A.; Martin, G. P.; Brown, M. B. J. Pharm. Sci. 2006, 95(5), 1060– 1074. (12) Baur, R.; Macholdt, H. T. J. Electrost. 1993, 30, 213–222. (13) Michel, E.; Baur, R.; Macholdt, H. T. J. Electrost. 2001, 51-52, 91–96. (14) Sainis, S. K.; Merrill, J. W.; Dufresne, E. R. Langmuir 2008, 24(23), 13334– 13337. (15) Kitahara, A.; Karasawa, S.; Yamada, H. J. Colloid Interface Sci. 1967, 25 (4), 490–495. (16) Kornbrekke, R. E.; Morrison, I. D.; Oja, T. Langmuir 1992, 8(4), 1211– 1217. (17) Keir, R. I.; Suparno; Thomas, J. C. Langmuir 2002, 18(5), 1463–1465. (18) Briscoe, W. H.; Horn, R. G. Langmuir 2002, 18(10), 3945–3956.
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electrical charging of immersed solid surfaces and suspended particles.15-18 According to the emerging picture, these dissociable surfactants can introduce mobile charges in nonpolar liquids by two different mechanisms.14 Above the critical micelle concentration (cmc), charging of ionic surfactants appears to be dominated by reverse micelles, which can undergo charge disproportionation, a process in which two electrically neutral micelles exchange a charge, thus forming a pair of oppositely charged micelles.19,20 It has been hypothesized that this process involves a single intermediate aggregate, which then splits up into two charged micelles,21 although such details have not yet been verified experimentally. Below the cmc, each surfactant molecule is dissolved individually and can dissociate, with some exceedingly low probability, into a charged molecule and its countercharge.6,14 From the mass action law, it follows that the concentration of dissociated ions is proportional to the square root of the concentration of undissociated surfactant; hence, the electric conductivity varies with the square root of the added amount of surfactant, σ C1/2, as observed in recent experimental studies.14,22 In 2006, Dukhin and Goetz reported the counterintuitive introduction of mobile ions and particle charge in a range of nonpolar liquids using different nonionizable surfactants.23 Differences between the micellar and submicellar concentration regime, however, were not investigated in this study, nor were changes in the micelle morphology as a function of surfactant concentration taken into account. With the exception of the above-mentioned study and a very recent study on the effect of external electric (19) Hsu, M. F.; Dufresne, E. R.; Weitz, D. A. Langmuir 2005, 21(11), 4881– 4887. (20) Roberts, G. S.; Sanchez, R.; Kemp, R.; Wood, T.; Bartlett, P. Langmuir 2008, 24(13), 6530–6541. (21) Strubbe, F.; Verschueren, A. R. M.; Schlangen, L. J. M.; Beunis, F.; Neyts, K. J. Colloid Interface Sci. 2006, 300(1), 396–403. (22) Bartlett, P. Keynote presentation at the 2009 ACS Colloid and Surface Science Symposium. (23) Dukhin, A. S.; Goetz, P. J. J. Electroanal. Chem. 2006, 588(1), 44–50.
Published on Web 11/18/2009
DOI: 10.1021/la903182e
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fields,32 the remarkable charge-mediating properties of nonionizable surfactants have been largely ignored. On the basis of the previously described, most recent understanding of ionic surfactants, one might not expect any electrostatic significance of nonionizable surfactant below their cmc. In this paper we present evidence for ionization processes in hexane triggered by nonionizable sorbitan oleate surfactants, both above and below the cmc. A linear increase in conductivity with added surfactant in the submicellar regime, σ C, marks a clear distinction from the case of ionic surfactants.
2. Materials and Methods 2.1. Samples. We investigate electric properties of nonpolar surfactant solutions. Hexane (BDH, ACS grade) was chosen as the solvent in all of the reported experiment because of its very low dielectric constant (ε = 1.89). Compared to longer chain alkanes, it also offers the advantage of better optical contrast with the surfactants used in this study, but it has the slight disadvantage of being more volatile. Small material losses observed gravimetrically correspond to an increase in concentrations of up to 1% over the course of our experiments; the reported concentrations are averages of initial and final values. Sorbitan monooleate (Span 80), sorbitan trioleate (Span 85), or sodium bis(2-ethylhexyl)sulfosuccinate (AOT), all purchased from Sigma-Aldrich, were dissolved in the alkane without further purification. For one measurement series described below, the resulting solutions were deliberately “contaminated” with oleic acid (SigmaAldrich). All surfactant solutions were allowed to equilibrate for 1 day prior to use. Ultrapure water with a resistivity of at least 18.2 MΩ 3 cm (Barnstead) was used for swelling inverse micelles and for interfacial tension measurements. All experiments were performed in a thermostated environment at 22 ( 0.5 C. 2.2. Karl Fischer Titration. The water content of our solutions was determined using Karl Fischer titration (TitroLine KF, SCHOTT). We confirmed a water content below 0.003 wt % for the pure solvent and found that very little water is introduced with the nonionic surfactant, with a molar water-to-surfactant ratio in their micellar solutions below 0.05. 2.3. Conductivity Measurements. Measurements were taken with a high-precision conductivity meter (model 1154, Emcee Electronics). Prior to measurements, the sample cell was carefully cleaned in a multistep procedure with hot water, DI water, acetone, and toluene/isopropyl alcohol mixture, dried with nitrogen, and rinsed with sample solution. Conductivity values below 0.5 pS/m were consistently found for the pure solvent (hexane). 2.4. Static and Dynamic Light Scattering. Reverse micelles of Span 85 were characterized by static (SLS) and dynamic light scattering (DLS) using a precision goniometer setup (ALV/DLS/SLS-5022F). DLS at a scattering angle of 45 and 90 was used to determine the onset of micellization and the hydrodynamic micelle radius; the latter was obtained from a secondorder cumulant fit to the intensity autocorrelation function. The SLS signal above the cmc allowed a determination of the micelles’ mass and, by extension, their aggregation number. In the interpretation of the static data, we used a value of 0.1565 mL/g for the refractive index increment, determined with an Abbe refractometer in the concentration range of 10-50 mg/mL, and an independently determined value of 10 mM for the cmc discussed in the Results and Discussion. 2.5. Interfacial Tensiometry. The interfacial tension of drops of pure water in hexane solutions of Span 85 were measured with a drop volume tensiometer (TVT-2, Lauda).
addition of small amounts of this nonionic surfactants causes a dramatic increase in the number of mobile ions: a surfactant concentration around 1 mM, for instance, increases the conductivity by 4 orders of magnitude. The observed effect is likely caused by the Span 80 micelles, which are present in the hexane solution at all nonzero concentrations explored here.24 Since the micelle size and shape varies with surfactant concentration, as suggested by light scattering data (not shown), we do not attempt a detailed interpretation of Figure 1. Instead, we devote the remainder of our study to a surfactant with a smaller effect on conductivity, but the advantage of being more amenable to a quantitative description. Span 85 (sorbitan trioleate), the three-legged analogue of Span 80, is more suitable in this regard. It forms small, spherical reverse micelles with a roughly constant size, much like the widely studied ionic surfactant AOT.25,26 A value around 2.8 nm for the hydrodynamic diameter of Span 85 micelles can be inferred from the plateau in the dynamic light scattering data seen in Figure 2. Below the critical micelle concentration of 10 mM, the apparent hydrodynamic diameter as well as the scattering intensity (not shown) drops sharply, making the experimental uncertainty prohibitively large for the lowest concentrations shown (open symbols). An independent confirmation of the cmc is obtained by interfacial tension measurements on water drops in Span 85/hexane solutions; their results are plotted in the inset of Figure 2. The conductivity of Span 85 solutions in hexane as a function of the surfactant concentration is shown in Figure 3. Experimental data shown represent an average over three measurements and have error bars below or comparable to the size of the markers. Both above and below the cmc, we observe a linear increase in conductivity with added surfactant, although with different slopes (insets). A correlation coefficient R near 1 attests to the near-perfect linearity of the concentration dependence in these two regimes. Even though the transition is not sharp, but extends roughly from 5 to 25 mM, experimental data in the entire concentration range are approximated reasonably well by a double linear fit (Figure 3, solid line): σ = σ0 þ C (dσ/dC)Ccmc
e2 χ dC0 e2 χ 1 ¼ ξ dC 6πηRH N
ð4Þ
where N is the average number of surfactant molecules in a micelle, and we have made the common assumption that all surfactant in excess of the cmc aggregates into micelles. Equation 4 neglects the concentration dependence of the viscosity, which we have verified to be legitimate for the entire concentration range shown in Figure 3. Using a value of 1.42 nm for the hydrodynamic micelle radius determined by light scattering (Figure 2) and an approximate value of N = 3.5 obtained from static light scattering, we can use eq 4 with the experimentally determined conductivity slope to solve for χ. We find that a tiny fraction χ = 4.86 10-7 of charged micelles accounts for the observed conductivity increase above the cmc. From this charge fraction we can infer a Born radius of a = 0.98 nm, which does not seem an entirely unreasonable magnitude for the hydrophilic micelle core. 3.2. Role of Impurities. The arguments presented above show that the charge disproportionation of neutral Span 85 micelles is viable from a statistical mechanics point of view but do not offer any explanation regarding the reaction pathway or the actual source of electric charges in the absence of ionizable surfactant head groups. Given the small fraction of charged micelles created, the presence of trace amounts of ionizable impurities incorporated in the micelle core would suffice to DOI: 10.1021/la903182e
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Figure 4. Conductivity of hexane solutions containing either 30 mM Span 85 or 20 mM AOT as a function of solubilized DI water (solid markers) or solubilized 0.1 M NaCl solution (open markers). The molar concentration of water was determined by Karl Fischer titration.
explain the availability of separable charges. Good reproducibility of the experimental results with different batches of surfactant and hexane, however, suggests that the charging reaction is not limited by the amount of impurities present in the micelle core, but instead by the size and number of micelles. This is confirmed by solubilization experiments shown in Figure 4. Small aliquots of either pure water (solid squares) or 0.1 M NaCl solution (open triangles) were mixed and equilibrated with a 30 mM Span 85 solution. For each case, the amount of water incorporated in the micelle cores was determined by Karl Fischer titration and translated, for better comparison with literature data, into the molar ratio W0 of water to surfactant molecules. As one might expect from eqs 2 and 3, swelling the reverse micelles with aqueous solution increases the conductivity, which depends sensitively on the core radius a. According to the results of Figure 4, it makes no difference for the conductivity whether deionized water or a salt solution is used to swell the micelles. This leads us to the central conclusion that the size of the micelle core, not its precise ion content, determines the micelles’ tendency of acquiring a net charge. It would be wrong to infer from this that impurities play no role at all; their presence in the micelle core may well be crucial for the generation of charged micelles. It rather appears that ionizable impurities always provide enough ions to accommodate the extent of net micelle charging consistent with the charged micelles’ Born energy U = kBT(λB/2a). This energetic constraint for a given core size (eq 3) cannot be removed by raising the concentration of ionizable molecules in the core even further, hence the insensitivity of micelle charging to added salt (Figure 4). It is not surprising, considering the above arguments, that the micelles of ionizable and nonionizable surfactants show the same qualitative behavior and are well described by the same statistical model for charge fluctuations (eqs 1-3, refs 27-29). Although this well established model does not explicitly account for the ionic character of the surfactant, its application has practically been reserved to micelles of ionic surfactants. The authors of one conductivity study on mixed surfactant microemulsions acknowledged with surprise the model’s good agreement with experiments (29) Kallay, N.; Tomic, M.; Chittofrati, A. Colloid Polym. Sci. 1992, 270, 194– 196.
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for different ratios of ionic and nonionic species in the surfactant mixture;30 yet, we are not aware of any previous application of this model to micelles of nonionizable surfactant only. Conductivity measurements on swollen micelles of AOT are also included in Figure 4. Again, the conductivity appears completely insensitive to salt ions in the swelling solution. The ionic surfactant AOT is much more hygroscopic than Span 85; the lowest indicated water content of W0 ≈ 0.5 for this surfactant was determined before addition of any swelling solution. Since even in the unswollen state AOT micelles are a little larger25,26 than Span 85 micelles, it takes a slightly larger Span 85 concentration to obtain a comparable conductivity. Conversely, Span 85 micelles can only solubilize very small amounts of water and are so small, even in the swollen state, that their size variation upon swelling could not be resolved by light scattering. 3.3. Conductivity in the Submicellar Regime. Below the cmc, the observed linear scaling of conductivity with added Span 85 (Figure 3) represents a sharp contrast to the square root scaling that was reported for AOT solutions and explained by the dissociation of individual AOT molecules.14 For Span 85 the charging mechanism must obviously be different. One conceivable pathway again involves the complexation of ionizable impurities with surfactant molecules, followed by a charge disproportionation of the resulting complexes. Note, for instance, that a hypothetical complex involving two trioleate surfactant molecules would have six lipophilic arms and might assume conformations that resemble a “miniature micelle”. One might therefore imagine that such complexes would mimic the previously discussed charging behavior of Span 85 micelles. Moreover, Span 85 is known to contain not only pure sorbitan trioleate but also other (similarly nondissociable) sorbitan esters, such as tetraoleate.31 We do not currently know which of the nonionic surfactant species present in Span 85 participate in forming the charged entities. Whatever the detailed composition and architecture of these complexes may be, their charging tendency might again be captured by charge fluctuation theory. If, for example, all charged complexes had a hydrodynamic radius RH,c and a Born radius ac, then the conductivity slope in the lower part of Figure 3 should be given, in perfect analogy to eqs 3 and 4, by dσ e2 χ c 1 ¼ ð5Þ dC Ccmc observed in Figure 3 for the submicellar regime does not imply a larger charging tendency χc of the submicellar complexes compared to micelles but can be explained by the difference in the number of surfactant molecules contained in the charged species and the difference in hydrodynamic size. Since the proposed charging mechanism involves ionizable impurities, one might wonder whether the observed conductivity could be an “artifact” due to ionizable impurities introduced with the surfactant and not involve the surfactant molecules at all. In that case, however, no linear conductivity increase would be expected unless the impurity was completely dissociated in the (30) Bumajdad, A.; Eastoe, J. J. Colloid Interface Sci. 2004, 274(1), 268–276. (31) Kato, K.; et al. J. Dispersion Sci. Technol. 2006, 27(8), 1217–1222. (32) Park, J. K.; Ryu, J. C.; Kim, W. K.; Kang, K. H. J. Phys. Chem. B 2009, 113, 12271–12276.
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We conclude that, although ionogenic impurities, such as salts, oleic acid, or residual water, may play an important role for the conductivity of our solutions, it appears to be the surfactant, not the impurity, which limits the ionization process. Note that in ref 23 the same conclusion is reached in a very different approach that considers the “steric stabilization of ions”, rather than the Born energy associated with introducing an electric charge in the confinement of a hydrophilic region embedded in a nonpolar medium. The conclusion of surfactantlimited ionization has an important practical implication: precise control of solution conductivity by nonionizable surfactants should be possible without detailed knowledge or control of impurities.
4. Conclusions
Figure 5. Conductivity of Span 85/hexane solutions as a function of added oleic acid.
nonpolar oil, which seems rather unrealistic and still would not explain the transition between conductivity regimes around the cmc. We verified experimentally that unreacted oleic acid, the most likely ionizable impurity to be found in Span 85, is not responsible for the observed behavior, at least not in the sense of a simple contamination acting independently of the surfactant molecules. Increasing amounts of oleic acid were added to Span 85/hexane solutions of different surfactant concentrations; the resulting conductivities are plotted in Figure 5. The nonionic surfactant emerges as the determining factor for the conductivity: no raise in conductivity is observed at all upon addition of the acid; in fact, the conductivity of the surfactant solutions slightly decreases in response to a slight dilution of the surfactant by the acid.
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In summary, we find that nonionizable surfactants can raise the electric conductivity of nonpolar solvents both above and below the critical micelle concentration. For Span 85 solutions in hexane, a linear increase of conductivity with surfactant concentration is found in both concentration regimes. It is consistent with a statistical description of equilibrium charge fluctuations, which may arise from charge disproportionation of reverse micelles above the cmc or of surfactant complexes with ionizable impurities below the cmc. In both regimes, the ionization process appears to be limited by the surfactant but not by the ionizable species. Our findings suggest that the conductivity of nonpolar solutions can be controlled in a robust way via nonionizable surfactants. We expect that charge screening and conductivity control with nonionic surfactants will prove especially useful for oils in contact with a water phase, which would typically act as a drain for ionic surfactants. Acknowledgment. This work was supported by the Camille and Henry Dreyfus New Faculty Awards Program.
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