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Langmuir 1998, 14, 5046-5050
Micelar Flocculation of Anionic Surfactants F. I. Talens,* P. Pato´n, and S. Gaya TALENCO Chemical Engineering Consulting, P.O. Box 1035, Hospitalet de Llobregat 08902, Spain Received October 14, 1997. In Final Form: June 19, 1998 Cations such as Ca2+ or La3+ show a strong tendency to bind themselves to the surface of anionic surfactant micelles. If the cation concentration is raised until micelles are saturated, the excess cations may precipitate with the surfactant monomer if the solubility product is reached. This paper describes the lauryl sulfate (DS)-Al3+ system. In this case, Al3+ reduces the ζ-potential to 0. The subsequent aggregation of micelles differs from micellar enlargement as known to take place generally in the transition from micellar solutions to liquid crystal. This aggregation is the mechanism of insolubilization of DS in the presence of Al3+, whereas in other cases precipitation of the monomer would be the mechanism. This effect deserves interest both because it provides additional information on the cation-micelle binding process and because the resulting aggregate has properties as an adsorbent of organic compounds.
Introduction The precipitation of anionic surfactants in the presence of polyvalent cations, and especially Ca2+, has been studied1-6 in connection with its effect on enhanced oil recovery (EOR), recovery of surfactants from surfactantbased separation processes, and the formulation of detergents with high resistance to water hardness. In all these cases, precipitation of surfactants with polyvalent cations has both an effect on process efficiency and an economic impact. Cations such as Ba2+, Ca2+, or La3+ have a strong tendency to bind themselves to the surface of micelles. Thus the free cation concentration in solution is lowered, and because of that a higher surfactant concentration demands a higher cation concentration to begin precipitation. When precipitation begins, micelles are disrupted to maintain the critical micelle concentration (cmc), releasing more cations and accelerating the precipitation process. Workers on hardness tolerance of anionic surfactants to Ca2+ 4,7-9 consider that the effect of NaCl is to increase the resistance to precipitation of the surfactant solution by lowering the cmc. This causes an increase in the concentration of micelles, further reducing the free cation concentration. This is assumed to be a consequence of a low affinity of Na+ for the micellar surfaces. A mathematical model9 for precipitation of lauryl sulfate (DS) in the presence of Ca2+ included the hypothesis of a constant maximum value for the fraction of micellar charges that are neutralized by adsorbed cations. This factor had been found10 to be 0.65 for the * To whom correspondence should be addressed. (1) Somasundaran, P.; Ananthapadmanabhan, K. P.; Celik, M. S. Langmuir 1988, 4, 1061-1063. (2) Celik, M. S.; Somasundaran, P. J. Chem. Biotechnol. 1987, 40, 151-166. (3) Peacock, J. M.; Matijevic, E. J. Colloid Interface Sci. 1980, 77 (2), 548-554. (4) Chou, S. I.; Bae, J. I. J. Colloid Interface Sci. 1983, 96 (1), 192203. (5) Brant, L. L.; Stellner, K. L.; Scamehorn, J. F. In Surfactant Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989; p 323. (6) Rodrı´guez, C. H.; Scamehorn, J. F. Proc. CESIO Conf., 4th Barcelona, 1996 (1). (7) Bozic, J.; Krznaric, I.; Kallay, N. Colloid Polymer Sci. 1979, 257, 201-205. (8) Matheson, K. L. J. Am. Oil. Chem. Soc. 1985, 62, 1269. (9) Stellner, K. L.; Scamehorn, J. F. Langmuir 1989, 5, 70-77. (10) Rathman, J. F.; Scamehorn, J. F. Langmuir 1987, 3, 372.
case of Na+ bound to DS micelles, and it was assumed to have the same value for the case of Ca2+ bound to DS micelles. This hypothesis is qualitatively confirmed in the present work by ζ-potentiometric measures. In addition to the above cation-micelle interactions, the existence of aqua complexes in conditions of excess metal concentration has been described.11 These submicellar aggregates were postulated7,12 to be polymers of the form [(M)x(S)y]z, where M is the cation and S the surfactant. Their effect is to strongly increase the solubility of the surfactant. This paper is centered on the study of the DS-Al3+ system. It is shown that DS insolubilizes in the presence of Al3+, leading to an amorphous, nonstoichiometric aggregate. The aggregate forms quickly and integrates all the micellar surfactant into one single floc, which may float on top of the solution and is easily filtered. The behavior of the DS-Al3+ system differs from that of other systems involving Al3+, like dodecylbenzensulfonate (DDBS)-Al3+ and dodecansulfonate (DDS)-Al3+ or p-(1methylnonyl)benzenesulfonate-Al3+, which have been reported to precipitate at low pH like divalent cationsulfonate systems.1,3 The results of the present work suggest that the cation binding results in micellar flocculation. The aggregate resulting from flocculation has been found13,14 to be able to adsorb organic compounds from the solutions it forms in. This suggests that this aggregation phenomenon may be of interest for the development of a new surfactant-based separation process. This and the additional insight to be gained on the mechanisms of interaction between cations and anionic surfactant micelles justify the interest of this work. Additionally, the cmc of Al(DS)3 has been determined. It must be noticed that this value is absent from the exhaustive compilation by Mukerjee and Mysels.15 (11) Tezak, D.; Strajnar, F.; Milat, O.; Stubicar, M. Prog. Colloid Polym. Sci. 1984, 69, 100-105. (12) Fisher, L. R.; Oakenfull, D. G. Micelles in aqueous solutions. Chem. Soc. Rev. 1977, 6, 25-42. (13) Talens, F. I.; Porras, M.; Paton, P. Proc. Jorn. Com. Esp. Deterg., 26th 1995, 267-274. (14) Porras, M.; Talens, F. I. Proc. CESIO Conf., 4th 1996, 133-138. (15) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems (NSRDS-NBS 36); U.S. Government Printing Office: Washington, DC, 1971.
S0743-7463(97)01130-X CCC: $15.00 © 1998 American Chemical Society Published on Web 08/01/1998
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Figure 2. Surfactant concentration remaining in solution. AOS and DS redissolve at high [Al3+]. Figure 1. Evolution of pH with addition of Al2(SO4)3 to a 0.05 M solution of each of the surfactants. Within the flocculation interval, pH is below 4 and Al3+ is the only significant aluminum species in solution.
Experimental Section The surfactant employed was sodium lauryl sulfate (NaDS) kindly provided by KAO Corp. For some comparative experiments, both dodecylbenzenesulfonic acid (DDBS) and R-olefinsulfonate (AOS) also from KAO Corp. where employed. The NaDS organic matrix had 98% active matter (surfactant) and 2% insulfonated. Salt content (Na2SO4) was also about 2% of the active matter. The material was used as received, after analyzing the insulfated content by extraction with ether. This was done so because the behavior of this surfactant quality is the case of practical interest for the purpose of water-treatment separation processes. It will be shown latter that these impurities have little impact on the results. Cmc was estimated on this material as well. Other reagents were ZnSO4, NaCl, and n-decane from Panreac, all HP grade; AR Hyamine 1622 from Carlo Erba; AR Disulfine Blue V150 from Merck Schuchardt; and HP Diimidium Bromide, HP Al2(SO4)3, HP CaCl2, and HP CHCl3 from Probus. Water was Milli-Q grade. Stock solutions of the various reagents were kept at 25 ( 0.1 °C. These were mixed also at 25 ( 0.1 °C in 100 mL volumetric flasks, and the solutions were allowed so settle for 1 h before further handling. The pH was that naturally occurring in each solution. After the solutions had settled and flocculation had been completed, samples were filtered with 45 µm cellulose nitrate filters. Surfactant content in the filtrates was analyzed by twophase titration, with Hyamine 1622 as standard and Blue 1 acid and Diimidium Bromide as mixed indicator. Aluminum was determined by induction plasma, with a Jovyn-Ivon JI-38 apparatus. Wavelength was set to 308.215 nm. The detection limit was 45 ppb. ζ-potentials of NaDS solutions and filtrates of solutions with varying [Al3+] and [Ca2+] were also measured. The measures were performed in a Malvern ZetaMasterS ζ-potentiometer. A Malvern Instruments 4700 with a 488 nm Argon laser was used to measure the sizes of micelles. The cmc of Al(DS)3 was determined by the surface tension method. The apparatus was a Kruss K12 tensiometer, with a 20 mm width Wilhelmy platinum plate.
Results and Discussion System pH and Aluminum Species Present in the Solution. Figures 1 and 2 show that for the experimental conditions of this work the pH was below 4 in the region of insolubilization, and therefore Al(OH)3 could not be precipitated. According to Stumm and Morgan,16 Al3+ would be the only species significantly present under those pH conditions. As a buffer system may mask the true behavior of the system, the pH was not controlled. In Figure 1 some regions are signaled where the immediate result of the flocculation was not a white, opaque material (16) Stumm, W.; Morgan, J. J. Aquatic Chemistry; John Wiley and Sons: New York, 1981.
Figure 3. Residual [DS] in solution vs mixing time.
with texture of floc but was a translucent, highly viscid liquid crystal. Figure 2 shows the differences in behavior between DDBS, which is known to precipitate in the presence of Al3+, and two other surfactants (AOS and NaDS), which aggregate forming a single floc with sorbing properties.13,14 Precipitation of Al(DDBS)3 begins at lower Al3+ concentration than in the case of AOS or NaDS. The precipitation is almost complete, and there is no redissolution of the surfactant at high Al3+ concentrations. Both DS and AOS redissolve when the concentration of Al3+ is raised. By means of a Polivar 2 microscope it was seen that the floc consisted in fragments of liquid crystals embedded in an amorphous matrix. This feature explains why the floc can retain solutes within its structure, unlike crystalline Ca(DDBS)2 or Ca(DS)2. It must be stressed that, according to Figures 1 and 2, in the range of high [Al2(SO4)3], Al3+ or its complexes with DS or AOS are the only possible spccies at pH about 3 and that any other aqueous Al species are negligible. Solubility Equilibrium. Figure 3 shows the dissolved DS concentration after addition of Al3+. Time scale refers to time between mixing and filtration of the solutions. There is no variation with time, for a time range between 2 min and more than 12 h. In fact, for up to 48 h the value remained constant. Equilibrium between insolubilized and dissolved DS is reached quickly. As the measures shown in this paper were taken after settling for 1 h, the values can be considered as equilibrium data. This result does not mean that the insolubilized surfactant is forming a thermodinamically stable phase, only that the concentration of DS in solution is in equilibrium. Solubility of DS in the DS-Al3+ System. Figures 4 and 5 show residual [DS] in solution. Figure 4 shows the effect of overall [NaDS], with the [Al2(SO4)3] as a parameter. The behavior is consistent with the StellnerScamehorn model. For a given [Al2(SO4)3], at a sufficiently high [DS] there is no precipitation. At high [Al3+] Somasundaran et al.1 observed that almost all DDBS
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Figure 4. Residual [DS] vs overall [DS], with [Al3+] as parameter. The plots are similar to those obtained by Somasundaran and co-workers with DDS and DDBS. Figure 6. Evolution of the size distribution of colloidal particles in the filtered solutions. [DS] ) 0.05 M.
Figure 5. Residual [DS] vs [Al3+], with overall [DS] as parameter. The effect of overall [DS] is small, although apparently increasing [DS] by a factor of 2 or 3 should cause an effect. However, if we consider the ratio between [Al3+] leading to flocculation and [DS] in each case, we see that there is a proportionality. Table 1. CmcDS for Various [Al2(SO4)3] [Al2(SO4)3] (mol/L)
cmc (mol/L)
0.002 0.005 0.010 0.040 0.080
5.1 × 10-4 5.9 × 10-4 5.9 × 10-4 3.9 × 10-4 1.4 × 10-4
would precipitate as Al(DDBS)3. It is remarkable that in the case of DS, at high [Al3+] barely 10% of all DS precipitates at [DS] between 0.05 and 0.1 M. Figure 5 shows the residual [DS] versus overall [Al2(SO4)3]. A minimum [DS] is reached about [Al2(SO4)3] 0.01 M. And at [Al2(SO4)3] 0.07 M all the DS is dissolved again. This may be explained by the presence of DS-Al3+ aqua complexes at high [Al3+]. These complexes have been described by Tezak et al.11 The residual [DS] value in Figure 5 at the minimum is in the order of magnitude of 10-4 M DS. It is coincident with the values shown in Table 1, which gives the values of cmc of DS for various [Al2(SO4)3]. These values were obtained at 25 °C by the surface tension method, as described in the procedures. This suggest that monomer DS remains in solution. Size and ζ-Potential of Al3+-Bound DS Micelles. It is known17 that increasing [NaCl] to 0.4M only increases the aggregation number of a DS micelle from 60 to 90. Figure 6 shows the effect of [Al2(SO4)3] on the size of micelles. A DS solution free of Al2(SO4)3 has micelles about 10 nm. A strong increase in size is observed as [Al3+] raises to the verge of insolubilization (0.004 M Al2(SO4)3). This might be due to micellar growth or to the aggregation of micelles by flocculation, as proposed for the case studied. At [Al2(SO4)3] up to 0.1 M, where DS is completely (17) Clint, J. H. Surfactant Aggregation; Blackie: Glasgow and London, 1992.
dissolved, DS micelles exist and have a size of 10 nm, the typical size in the absence of salts. The conclusion is that there is not salt-induced micellar growth as such in the case of Al3+. If micelles are not enlarged by effect of a high [Al3+], then micellar enlargement and transition to liquid crystal cannot be the explanation of the growth of colloidal particles at low [Al3+] and the appearance of an amorphous aggregate containing liquid crystal fragments. The increase in size observed at low [Al2(SO4)3] should be caused by the flocculation of the micelles. This is supported by Figure 7, which shows ζ-potential distributions for a 0.05 M SDS reference solution and four other solutions: two containing Al2(SO4)3 and two containing CaCl2. The results show that there is a displacement toward zero ζ-potential in the case of Al2(SO4)3, and afterward a recession toward negative values at higher Al3+ concentration. On the other hand, in the presence of different Ca2+ concentrations the ζ-potential becomes constant. The increase in ζ-potential of the Ca2+-bound DS micelles from -80 to -40 mV is in good qualitative agreement with the constant electroneutralization fraction of 0.65 predicted by Stellner and Scamehorn. The results strongly suggest that Al3+-bound micelles undergo flocculation due to electroneutralization of their surfaces. It must be noticed that flocculation of precipitated crystals leading to turbid colloids with near zero ζ-potential has been reported in the literature, for instance, for the Ca2+laurate system.18 But in these systems the surfactant concentration was below the cmc, and the particles appeared by nucleation of small precipitate particles in a region where no micelles existed. In this case, it is well above the cmc that the existing colloids (micelles) become almost neutral in charge. If the micelles coexisted with unstable solid particles caused by nucleation, both kinds of particles would appear in the light-scattering and ζ-potential scans. On the other hand, phenomena like coacervation as in alkaline earth-DDBS systems19 take place in a region bound by surfactant/cation precipitation curves which follow the pattern of the Stellner and Scamehorn model and are therefore related to monomercation precipitation. An additional result of interest is that above 0.08 M Al2(SO4)3 no signals from colloids are detected in the ZetaMasterS analyzer but are recorded in the 4700 argon light scattering meter. This is due to the fact that the actual concentration of colloids is between the sensitivity (18) Young, S. L.; Matijevic, E. J. Colloid Interface Sci. 1977, 61, 287-301. (19) Tezak, D.; Strajnar, F.; Sarcevic, D.; Milat, O.; Stubicar, M. Croat. Chem. Acta 1984, 57 (1), 93-107.
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Figure 7. ζ-potential distributions for a 0.05 M SDS concentration without any added salts and for 0.05 M SDS solutions with different Ca2+ and Al3+ concentrations. The two calcium concentrations lead to precipitation. Even though the second is five times higher, the ζ-potential value is the same. The values shown here are a representative selection of the trends observed over more exhaustive series for both cations.
Figure 8. Effect of [NaCl]. Very high [NaCl] allows the competition between Na+ and Al3+. In this concentration range, [NaCl] affects both cmcDS and aggregation number of DS micelles. [DS] ) 0.05 M.
levels of both equipment. This means that micelles are fewer, even though they are not larger. If all the surfactant is dissolved, the monomer [DS] is low (as shown in Table 1) and the micellar [DS] is also low; the only option is that the rest of the surfactant is in the form of stoichiometric aqua complexes sufficiently small not to be detected as colloidal particles. Interference of Na+ and Zn2+. This section illustrates the effect of varying concentrations of other cations, providing information about intercation competition and changes in cmc on the behavior of the DS-Al3+ system. Figure 8 shows the effect of NaCl over a solution with [DS] ) 0.05 M. Even though the results are not plotted in the figure, it is important to note that the addition of
NaCl between 0.005 and 0.015 M did not change the flocculation curve. Therefore, small amounts of Na+ (like those present in a technical grade anionic surfactant) have no effect on the flocculation mechanism. An increase in [NaCl] reduces the range of [Al2(SO4)3] leading to flocculation and increases the [DS] remaining in solution. As [NaCl] increases to values in the range 0.125-0.5 M, the actual cmc raises from that of Al(DS)3 to that of NaDS, which is between 5 × 10-3 and 8 × 10-3 M. This results in an increase of dissolved [DS]. However, an increase in [NaCl] enhances the resistance to precipitation of anionic surfactants by Ca2+ by lowering the cmc.4,7 The peculiar behavior observed here may be explained if the micellar DS is involved in insolubilization by flocculation: monomer surfactant would then remain in solution. The narrowing of the flocculation region may be affected by two factors: an increase in the size of DS micelles and competition between Na+ and Al3+ for the micellar surface, both because of the high [NaCl]. The overall effect of both factors would be to lower the degree of electroneutralization of the micelles achieved by Al3+. Figure 9 shows the effect of [ZnSO4]. It causes an increase in the solubility of DS. At 0.094 M, ZnSO4 totally prevents the flocculation of DS. It seems a consequence of two effects. First, competition between Zn2+ and Al3+ for the micellar surface: the low charge/radius ratio of Zn2+ would prevent efficient flocculation of the micelles. This result suggests that Al(DS)3 is a very soluble salt: if the flocculation is disrupted, Al(DS)3 does not precipitate. Second, higher [Zn2+] might cause the appearance of ZnDS aqua complexes. This might explain that, while the
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tion is reduced, confirming the idea that increases in the micellar size reduce the extent of flocculation. The increase in [DS] remaining dissolved cannot be explained now by a change in the cmc. The strong shift of the upper boundaries toward lower [Al2(SO4)3] suggests that the reason for this increase of solubilized [DS] is the inability of Al3+ to cause the effective flocculation of all the micellar DS. For the higher decane/aqueous phase ratios the curves are identical. This suggests that micelles have reached the maximum swelling due to decane solubilization.
Figure 9. Effect of [ZnSO4]. The presence of Zn concentration strongly reduces the extent of micelle flocculation. At the higher [Zn] it may be possible that both Al and Zn aqua complexes prevent the occurrence of precipitation.
Figure 10. Effect of solubilization of hydrocarbons on micelle flocculation. The figures give the relationship in volume between decane phase and aqueous phase. 10% means that the decane phase has 1/10th of the volume of the aqueous phase. [DS] ) 0.05 M.
effect of NaCl in preventing insolubilization is smaller at higher concentrations, the effect of ZnSO4 is greater at higher concentrations. Effect of Micellar Swelling. Biphase decane/SDS/ Al2(SO4)3/water mixtures were prepared, with different proportions of decane (expressed as percentage in volume of the decane phase related to the volume of the aqueous phase). After strong mixing, settling was allowed for 1 h at 25 °C. As a result, varying amounts of decane became solubilized in the micelles, swelling them. The effect on micelle size, as shown in Figure 10 is similar to that of high [NaCl]. The range of [Al2(SO4)3] leading to floccula-
Conclusions In the case of the DS-Al3+ system, the following facts have been observed: DS insolubilizes forming a single disorganized aggregate that contains liquid crystal fragments; solubility of DS increases when cmc is raised due to salt concentration, in opposition to the generally observed trend for many other systems; the residual [DS] is very close to the cmc in the region of minimal solubility; ζ-potential shows a complete electroneutralization of DS micelles in the presence of Al3+ and a simultaneous increase in size of the colloidal particles present in solution; the adsorption of Ca2+ on DS micelles causes a reduction of the ζ-potential to a constant nonzero limit value, in agreement with the prediction by Stellner and Scamehorn; DS micelles at high [Al3+] have the same size as those in the absence of salt; at high [Al3+] DS redissolves and the number of micelles becomes low, hinting at Al3+-DS aqua complexes as the main DS form in solution; ζ-potentiometry can be used on DS micellar solutions to derive useful information on the phenomenon of cation adsorption. The conclusion derived from these facts is that, in this specific case, DS becomes insoluble due to flocculation of its micelles by binding Al3+. Acknowledgment. The authors wish to thank the Bureau of Clean Technologies of the Catalonian Regional Government and its head, Mr. Jacint Corderas, for funding this research, Drs. C. Solans, N. Azemar, and F. Comelles from CID-CSIC (Barcelona, Spain) for their cooperation in light scattering and cmc determinations, and Mr. Jesu´s Carlos Puebla, from OPTILAS Ibe´rica (Madrid, Spain), for kindly performing the ζ-potentiometric measures. Dr. Talens also wishes to acknowledge the involvement of Drs. C. Mans-Teixido´, J. Costa-Lo´pez, and S. EsplugasVidal in his redundancy from the Department of Chemical Engineering of the University of Barcelona. LA971130X
