Ind. Eng. Chem. Res. 2000, 39, 2677-2681
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ζ Potential at an Air-Water Surface Related to the Critical Micelle Concentration of Aqueous Mixed Surfactant Systems Alain Graciaa,† Patrice Creux,† Jean Lachaise,*,† and Jean-Louis Salager‡ Laboratoire des Fluides Complexes, Universite´ de Pau, BP 1155, 64013 Pau Cedex, France, and Laboratorio FIRP, Universidad de los Andes, Me´ rida 5101, Venezuela
The spinning bubble tensiometer technique provides a handy tool to measure the ζ potential at the air-water surface. The method is used to estimate the variation of the ζ potential at the surface of aqueous solutions containing binary mixtures of surfactants (anionic, nonionic, and fluorocarbon anionic). The evidence on molecular interactions at the air-water surface is compared with the information deduced from the critical micelle concentration experimental data. Introduction
Experimental Procedure
Foams are encountered in industrial processes as distinct as ore flotation, recycled paper deinking, fermentation and in the conditioning of miscellaneous products such as foodstuff, drilling fluids, and extinguisher liquids. Foams can be inopportune as well, such as, for instance, in gas-liquid absorption or distillation, and they must be destabilized.1,2 Foams are stabilized by a foaming agent, which is, in general, a surfactant mixture in an amount close to its critical micelle concentration (cmc), where a maximum foaminess is often found. The adsorbed surfactant is able to stabilize the foam by reducing the interbubble film drainage rate according to several mechanisms and phenomena.3,4 One of them is the electrical repulsion between approaching surfaces. This repulsion depends on the air-water surface charge, which is generally estimated through the measurement of the ζ potential. The ζ potential is routinely measured in the case of liquid droplets or solid particles, and many electrophoresis equipments are commercially available. However, this is not the case with gas bubbles, for at least two reasons. First, the density difference is much higher than that in the liquid-liquid case. Second, while it is relatively easy to make a very small droplet, say 10 µm in diameter, it is difficult to make a bubble smaller than 100 µm, in most cases because the surface tension of a surfactant solution is much higher than its interfacial tension with an oil phase. Both features result in a strong Archimedes pull that sweeps the bubble away quickly and rules out the use of conventional ζ-meters. Experiences in zero gravity would offset this drawback but are extremely expensive and cannot be routinely carried out. The situation was thus deadlocked until the spinning bubble ζ-meter was developed.5 This recently disclosed technique is used here to estimate the ζ potential attained with different kinds of surfactant mixtures that exhibit positive and negative interactions.
Spinning Bubble ζ-meter. In the mid-1970s the oil embargo triggered a lot of fundamental research on enhanced petroleum recovery, and one of the most attractive processes was the so-called low-tension flooding in which a surfactant solution was injected to untrap the oil globules that remained in the reservoir after water flooding. The interfacial tension to be attained was quite low, e.g., below microNewtons per meter, and could not be estimated easily by the available experimental methods. In a few years the so-called spinning drop tensiometer technique was improved in R. S. Schechter and W. H. Wade’s laboratory at The University of Texas at Austin, Austin, TX,6 and soon became the standard experimental method to measure low tension.7 This was a milestone in the enhanced oil recovery research because it made it much easier to test formulation effects. The spinning drop equipment consists of a rotating capillary tube filled with the densest liquid, in which a small drop of the less dense liquid is introduced. The centrifugal gravity pulls the drop to the spinning axis and results in drop elongation, while the interfacial tension opposes it. This allows one to calculate the tension value. If the tension is too high, the drop cannot be elongated significantly and the method fails, which is often the case when a gas bubble is introduced instead of a drop. However, the bubble remains suspended on the rotation axis thanks to the centrifugal gravity effect, which is a way to obliterate the annoying Archimedes pull. The natural gravity effect only produces a slight motion along the tube axis, depending on the slant with respect to the horizontal. However, a fine-tuning leveling device can easily adjust the axis to horizontal and avoid any lateral shift of the bubble. The spinning bubble ζ-meter is no more than a spinning tensiometer fitted with two electrodes in order to induce the electrophoretic motion of a bubble along the axis. Figure 1 indicates a schematic of the basic equipment, which is described in detail elsewhere.5 The glass tube is rotated at 1000 rpm, which is high enough to overwhelm the natural gravity effect without producing a significant elongation of the bubble. It is worth noting that in a typical spinning drop tensiometer measurement the tension is a hundred or a thousand
* Corresponding author. † Universite ´ de Pau. ‡ Universidad de los Andes.
10.1021/ie9907977 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/24/2000
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Figure 1. Schematic representation of the spinning ζ-meter.
times smaller and the rotational speed is often in the 10 000 rpm range. The bubble is typically 1 mm in diameter, which is relatively large compared with the usual size of a spinning drop, though small enough to avoid a significant deformation from spherical geometry. The applied potential is typically 1000 V/m. In such experimental conditions the ζ potential may be estimated from electrophoretic mobility data according to the Sherwood relation,8 which is the one used in the present paper. Because of the presence of the double layer near the tube wall, the applied electrical field produces an electroosmosis phenomenon, i.e., a neat flow in the diffuse layer near the wall, with a returning flow taking place in the center of the tube. The motion of the bubble is thus due to the combination of the electrophoretic and electroosmosis effects. Early researchers have disregarded the latter,9 while subsequent investigators tried to evaluate it and to make the corresponding correction.10 More recently, the electrophoretic motion was evaluated at the zero velocity location, where the electroosmosis direct and return currents just compensate each other.11 In some cases a large-volume cell is used so that the returning flow is essentially negligible with respect to the electrophoretic motion.12 In the present case the electroosmosis is eliminated by keeping the fluid still over the electrical diffuse layer region. This effect is attained by depositing a thin crosslinked polymer layer at the wall surface, so that the shear plane is outside the diffuse layer.5 For each of the experiments the anti-electroosmosis surface treatment is tested by observing the motion of a polystyrene microsphere latex suspension. If there is no electroosmosis effect, the sphere velocity is the same in all regions of the tube because there is no secondary motion. Contrarily, an electroosmosis returning flow would exhibit a typical parabolic pattern outside the double layer, according to Poiseuille’s law. Products. A 18.2 MΩ conductivity water coming from a Millipore milli-Q 185E system is used in all experiments. Carbon dioxide dissolution is avoided by bubbling nitrogen into the water storage tank. Between two successive experiments, the tube is cleaned with a 10 wt % nitric acid solution and then profusely rinsed with conductivity water. The surfactants are used as received from the manufacturer, but they are believed to be highly pure reagent products. The nonionic species that we have used is hexaethylene glycol dodecanol purchased from Nikko; it is symbolized by NI. The cationic surfactant, referred to as CAT, is a n-decyltrimethylammonium chloride provided by Tokyo Kasei. We have used two hydrocarbon anionic surfactants, symbolized as AI1 and AI2, and a fluorocarbon anionic surfactant, symbolized as FAI. For AI-NI mixtures and AI-FAI mixtures, AI1 is a hexadecylbenzene-sodium sulfonate synthesized in Prof. Wade’s laboratory at The University of Texas at Austin; this molecule is the isomer with the benzene ring
Figure 2. Critical micelle concentration and ζ potential at an airwater surface for anionic-nonionic mixtures. The dashed line represents the overall surfactant concentration used for ζ potential measurements.
attached on the eighth carbon of the linear alkyl chain. For AI-CAT mixtures, AI2 is a sodium decyl sulfonate provided by Aldrich, and FAI is a branched unsaturated perfluorononylbenzene-sodium sulfonate sold as Ftergent by Neos, whose formula has been reported elsewhere.13 cmc and ζ Potential Measurements. The cmc has been mainly determined by surface tension measurements using the Wilhelmy plate method (Kru¨ss digital tensiometer). When the precision of this method was judged inadequate, we used conductivity measurements for the anionic-nonionic mixtures (Consort K610 conductometer) or the spectrophotometric method for some fluorocarbon-hydrocarbon mixtures (Beckman DU 640 spectrophotometer). ζ potential measurements are carried out at a total concentration that is below the lowest cmc exhibited by any binary mixture of the selected surfactants. Experimental details on the ζ potential estimation procedure are discussed elsewhere.14 Results and Discussion Anionic-Nonionic Surfactant Mixtures. Nonionic species have a much lower cmc than ionic ones and are thus driven out of the solution more easily. This is why the cmc of the NI surfactant with only 10 carbon atoms in the alkyl chain is lower than the cmc of the AI1, whose alkyl chain contains 16 carbon atoms, as is seen in Figure 2 (top). This figure shows that the cmc variation with the binary mixture composition is quite nonlinear, an indication of a considerable deviation from ideal solution behavior. If the surfactants were both anionic or both nonionic ones, their dissimilarity would not result in such a departure from ideal behavior.15 These mixtures can form micelles more easily because there is an interaction between the polyether chain of the nonionic surfactant and the sulfonate group of the ionic one.
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Presumably, the nonionic groups get inserted between ionic ones and thus reduce the electrical repulsion between the latter. A similar arrangement would take place at the air-water surface below the cmc. This phenomenon would reduce the electrical potential near the air-water surface or near the micelle. To estimate the electrical potential in the Stern layer at the airwater surface, Rathman and Scamehorn proposed two models.16 The moving adsorption model assumes a uniform potential in the Stern layer and supposes that the counterion binding is the same everywhere in it. On the contrary, the local adsorption model assumes that the counterions are bound only to charged hydrophilic groups. This second model does not take into account a possible interaction between the nonionic group and the ionic species, i.e., counterions or ionic hydrophiles. When the surfactant mixture contains a large proportion of anionic surfactant, both models forecast a slow decrease of the potential with the anion proportion, whereas the opposite occurs at low anionic content. Figure 2 (bottom) indicates the variation of the ζ potential at the air-water surface as a function of the proportion of anionic surfactant in anionic-nonionic mixtures. The constant overall surfactant concentration (0.05 mmol/L) is lower than the cmc of any surfactant mixture according to Figure 2 (top) data. As the ionic surfactant proportion is increased, the ζ potential decreases from -84 mV (pure nonionic) to -102 mV (pure anionic). First it is worth noting that this measurement corroborates that the ζ potential is substantially different from zero with pure nonionic surfactant, as reported previously by other researchers for oilwater interfaces.17 At low anionic content, the variation of the ζ potential seems to be less than that expected from the Scamehorn theory. It is not the purpose of this paper to deal with this discrepancy, but it may be conjectured that it could be due to the protonation of some ether groups, thus turning the polyether group slightly cationic, as in the case of the binding of Na+ ions.18,19 The spinning bubble ζ-meter may be a handy tool to settle this matter. Anionic-Cationic Surfactant Mixtures. It is wellknown that the cmc of anionic-cationic mixtures is considerably lower than the cmc’s of the single species,20,21 as illustrated in Figure 3 (top). This drastic reduction is due to the strong electrical interactions between the two surfactant species. If the micelles are supposed to follow a regular solution behavior, it is known22 that the interaction parameter β, deduced from the cmc variation as a function of the surfactant mixture composition, gives information on the nature of the interactions. If negative, it means that the interactions are attractive; if positive, it means that they are repulsive. Its modulus gets higher as the interactions get stronger. Furthermore, the global surfactant concentrations considered here are sufficiently close to the cmc for considering that the deduced parameter is representative of the parameter corresponding to the surface.22 The interaction parameter corresponding to Figure 3 (top) is β ) -18.4. It is quite larger in magnitude than the β ) -6.3 value corresponding to Figure 2 (top) data for the anionic-nonionic mixture. We do not have the interaction parameter at the air-water surface calculated, but it is likely to be of the same magnitude.23 The composition of the surface film has been calculated by Nagarajan with a similar value of the interaction
Figure 3. Critical micelle concentration and ζ potential at an airwater surface for anionic-cationic mixtures. The dashed line represents the overall surfactant concentration used for ζ potential measurements.
parameter. As in the mixed micelle, the equimolar composition is favored because it provides the formation of a pseudononionic surfactant that is insensitive to electrolytes24 and exhibits a cloud-point temperature.25 This equimolar association surfactant, sometimes called catanionic, is much less hydrophilic than its components.26,27 Figure 3 (bottom) data indicate that the ζ potential is close to zero, though not exactly zero, on the wide midrange composition where the cmc corresponds to the catanionic species. The minor increase in the ζ potential from 20% to 80% CAT in the mixture indicates, however, that the surface charge is slightly sensitive to the bulk phase composition, whereas the cmc remains thoroughly constant over an even more extended range. It is also worth noting that the isoelectric point does not take place exactly at a 50% molar proportion, but at a higher CAT amount, e.g., 60%, in accordance with the value reported by researchers dealing with detergency phenomena.28 This is to be related to the fact that the surface exhibits a negative ζ potential in the absence of charged surfactant, though the negative contribution, presumably due to the segregation of water ions, is much lower here than in Figure 2 (bottom), because the ζ potential at 50% CAT is barely -5 mV. At 100% AI or CAT the ζ potential is about 100 mV with the corresponding sign, and it may be conjectured that the adsorption density is probably very similar in both cases, not a surprising result because the packing is probably limited by the electrostatic repulsion between neighboring adsorbed molecules. Hydrocarbon-Fluorocarbon Surfactant Mixtures. The variation of the surface tension of AI and FAI surfactant mixtures versus the overall surfactant concentration exhibits two changes of trend. Mukerjee and Mysels suggested that the first change of trend, at the lower concentration, corresponds to the formation of the first mixed micelle, while the second change of
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Conclusions The measurement of the ζ potential at a bubble surface, with an aqueous phase containing a surfactant mixture below the cmc, provides information on the molecular interactions of the surfactant species adsorbed at the surface. This is consistent with the evidence attained from cmc measurement. The match is probably due to the similarity between the micelle palisade and the air-water surface near the cmc. However, the air-water surface ζ potential offers direct information on the surface packing condition and the related situation at the foam thin film surface. The exhibited data corroborate that the ζ potential at the air nonionic surfactant solution surface is definitely negative at about -80 mV and that the variation of the ζ potential is a handy indication of the molecular interactions at the surface. AI-NI mixtures exhibit a much weaker interaction than AI-CAT mixtures, in which case half the charge seems to have vanished. In the case of the AI-FAI mixture, the lipophobic characteristic of the fluorocarbon hydrophobic part, which is incompatible with the hydrocarbon hydrophobic part, results in a much sparser packing at the air-water surface. Figure 4. Critical micelle concentration and ζ potential at an airwater surface for hydrocarbon and fluorocarbon anionic surfactant mixtures. The dashed line represents the overall surfactant concentration used for ζ potential measurements.
trend could correspond to the segregation of two different micelle types, one containing mostly hydrocarbon surfactant and the other one mostly fluorocarbon surfactant.29,30 The cmc is taken as the concentration at which the first trend change takes place. Figure 4 (top) indicates that the mixing of the two surfactant species produces a positive interaction, i.e., some repulsive effect that inhibits the formation of micelles. This is due to the fact that the perfluorinated hydrophobic group is also lipophobic, i.e., noncompatible with the hydrocarbon surfactant alkyl chain. This is consistent with the fact that the worst situation, i.e., the highest cmc, is attained at the equimolecular mixture. The experimental data may be represented by a regular solution model with a positive interaction coefficient β ) +8, though the fit is not as good as that with other surfactant mixtures. Figure 4 (bottom) indicates the variation of the ζ potential for the same surfactant binary at a total concentration 0.3 mmol/L, slightly below the AI surfactant cmc. According to previous data, both sulfonate surfactants in the pure state exhibit a ζ potential of about -110 mV, with a slightly lower absolute value for the FAI surfactant whose branched hydrophobic tail could tend to reduce the packing. If the mixtures were ideal, such a ζ potential value would be attained for the whole range of mixture composition. The experimental data show that the ζ potential undergoes a maximum, which is a minimum in absolute value, that indicates a maximum of incompatibility located near the equimolar mixture at which the electrostatic repulsion is enhanced by the lipophobic character of the fluorocarbon hydrophobic group. Because the ζ potential in the equimolar case is almost half the value attained with the pure surfactants, it may be conjectured that the adsorption density is considerably reduced by the extra repulsion provided by the lipophobic effect of the fluorocarbon group.
Literature Cited (1) Kouloheris, A. P. SurfactantssFriend or Foe? Chem. Eng. 1987, 26, 88. (2) Defoaming: Theory and Industrial Applications; Garret, P. R., Ed.; Dekker: New York, 1993. (3) Thin Liquid Films: Fundamentals and Applications; Ivanov, I. B., Ed.; Dekker: New York, 1988. (4) Foams; Exerowa, D., Kruglyakov, P. M., Eds.; Elsevier: Amsterdam, The Netherlands, 1998. (5) Graciaa, A.; Morel, G.; Saulnier, P.; Lachaise, J.; Schechter, R. S. The zeta potential of gas bubbles. J. Colloid Interface Sci. 1995, 172, 131. (6) Cayias, J. L.; Schechter, R. S.; Wade, W. Measurement of low interfacial tension via the spinning drop technique. ACS Symposium Series; American Chemical Society: Washington, DC, 1975; Vol. 8, p 234. (7) Improved Oil Recovery by Surfactant and Polymer Flooding; Shah, D. O., Schechter, R. S., Eds.; Academic Press: New York, 1977. (8) Sherwood, J. D. Electrophoresis of gas bubbles in a rotating fluid. J. Fluid. Mech. 1986, 162, 129. (9) Alty, T. Cataphoresis of gas bubbles in water. Proc. R. Soc. London 1924, A106, 315. (10) Bach, N. A.; Gilman, A. Electrokinetic potential at gasliquid interfaces. IIsCataphoresis of gas bubbles in solution of capillary-active organic electrolytes. Acta Physicochim. URSS 1938, 9 (1), 1. (11) Collins, G. L.; Motarjemi, M.; Jameson, G. J. A method for measuring the charge on small gas bubbles. J. Colloid Interface Sci. 1978, 63, 69. (12) Kelsall, G. H.; Tang, S.; Yurdakul, S.; Smith, A. L. Electrophoretic behaviour of bubbles in aqueous electrolytes. J. Chem. Soc., Faraday Trans. 1996, 92, 3887. (13) Ben Ghoulam, M.; Moatadid, N.; Graciaa, A.; Marion, G.; Lachaise, J. Hydrocarbon/Fluorocarbon mixed micelle diagram from surface tensiometry. Langmuir 1996, 12, 5048. (14) Saulnier, P.; Lachaise, J.; Morel, G.; Graciaa, A. ζ-potential of air bubbles in Surfactant solutions. J. Colloid Interface Sci. 1996, 182, 395. (15) Nagarajan, R. Micellization of Binary Surfactant Mixtures. In ACS Symposium Series; Holland, P. M., Rubingh, D. N., Eds.; American Chemical Society: Washington, DC, 1992; Vol. 501, p 54. (16) Rathman, J. F.; Scamehorn, J. F. Counterion binding on mixed micelles. J. Phys. Chem. 1984, 88, 5807. (17) Florence, A. T.; Rogers, J. A. Emulsion Stabilization. J. Colloid Interface Sci. 1971, 35, 23.
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2681 (18) Rosen, M. J.; Hua, X. Y. Synergism in binary mixtures of surfactants. IIsSome experimental data. J. Am. Oil Chem. Soc. 1982, 59, 582. (19) Rosen, M. J.; Hua, X. Y. Binary mixtures of surfactants. J. Colloid Interface Sci. 1983, 95, 443. (20) Malliaris, A.; Binana-Limbele, W.; Zana, R. Fluorescence probing studies of Surfactant aggregation in aqueous solutions of mixed ionic micelles. J. Colloid Interface Sci. 1986, 114, 110. (21) Stellner, K. L.; Amante, J. C.; Scamehorn, J. F.; Harwell, J. H. Precipitation phenomena in mixtures of anionic and cationic surfactants in aqueous solutions. J. Colloid Interface Sci. 1988, 186, 123. (22) Holland, P. M.; Rubingh, D. N. Nonideal multicomponent mixed micelle model. J. Phys. Chem. 1983, 87, 1984. (23) Rosen, M. J. Predicting synergism in binary mixtures of surfactants. Prog. Colloid Polym. Sci. 1994, 95, 39. (24) Corkill, J. M.; Goodman, J. F.; Harrold, S. P.; Tate, J. R. Monolayers formed by mixtures of anionic and cationic surfaceactive agents. Trans. Faraday Soc. 1967, 63, 247. (25) Mehreteab, A.; Loprest, F. J. Formation of pseudo nonionic complexes of anionic and cationic surfactants. J. Colloid Interface Sci. 1988, 125, 602. (26) Bourrel, M.; Graciaa, A.; Bernard, B. Properties of binary
mixtures of anionic and cationic surfactants: micellization and microemulsions. Tenside Deterg. 1984, 21, 311. (27) Anto´n, R. E.; Gomez, D.; Graciaa, A.; Lachaise, J.; Salager, J. L. Surfactant-oil-water systems near the affinity inversion. IXsOptimum formulation and phase behavior of mixed anioniccationic systems. J. Dispers. Sci. Technol. 1993, 14, 401. (28) Rubingh, D. N. Surface-active cationic compounds in detergency. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series 37; Dekker: New York, 1990; p 489. (29) Mukerjee, P.; Mysels, K. J. Anomalies of partially fluorinated surfactant micelles. In ACS Symposium Series; Mittal, K. L., Ed.; American Chemical Society: Washington, DC, 1975; Vol. 311, p 239. (30) Funasaki, N. Coexistence of two kinds of mixed micelles of fluorocarbon and hydrocarbon surfactant. In Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; Dekker: New York, 1992.
Received for review November 4, 1999 Revised manuscript received February 29, 2000 Accepted March 3, 2000 IE9907977
