Langmuir 1998, 14, 3351-3355
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Adsorption of Sodium Diisooctyl Sulfosuccinate onto Calcium Oxalate Crystals L. Tunik, H. Fu¨redi-Milhofer,* and N. Garti Casali Institute of Applied Chemistry, Graduate School of Applied Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel Received July 17, 1997. In Final Form: February 18, 1998 The adsorption of diisooctyl sulfosuccinate (AOT) from high ionic strength solution onto well-defined calcium oxalate monohydrate (COM) and dihydrate (COD) crystals was studied. Adsorption at the COM/ solution interface is characterized by a two-step (LS type) isotherm, starting at low equilibrium concentrations (5 mg dm-3). At ceq ) 15-30 mg dm-3 it reaches a plateau which is followed by a relatively steep inflection (ceq ) 30-50 mg dm-3) and a further slow increase of the adsorption as a function of increasing AOT concentration. In suspensions without surfactant the particles were negatively charged. Upon adsorption an initial slight decrease of the negative ζ-potential, coinciding with the first plateau, occurred which was followed by a sharp increase in concordance with the increasing surface concentration of the surfactant. In contrast, adsorption onto COD is characterized by a sigmoid isotherm. It commenced at 14 mg dm-3 of AOT and increased abruptly up to a plateau. The maximum adsorbed amount was about half the maximum amount adsorbed on COM, the corresponding adsorption densities in molecules per square nanometer being 7.45 for COD and 14.22 for COM, respectively. COD crystals suspended in electrolyte solution without surfactant were almost uncharged, and the negative ζ-potential increased in concordance with AOT adsorption. The results are discussed in accordance with literature data and by considering the ionic structure of the different crystal faces. We assume that the first adsorption step in the COM/ surfactant system is due to electrostatic interactions causing head-on adsorption of the surfactant molecules at high-energy sites, while in the second step a bilayer is formed. In the COD/surfactant system the hydration layers covering the COD crystal faces are shielding them from electrostatic interactions. Consequently AOT adsorption at the COD/solution interface proceeds only through surface aggregation, resulting in a bilayer of intertwined surfactant molecules.
Introduction In previous papers1-4 we reported our findings on the consequences of interactions of ionic surfactants with growing crystals of some sparingly soluble calcium oxalate hydrates, i.e., the thermodynamically stable monohydrate (COM; CaC2O4‚H2O) and the metastable dihydrate (COD; CaC2O4‚(2+x)H2O, where x e 0.5). It has been shown that cationic1 and anionic2 surfactants differently influence the growth morphology of COM crystals and that anionic surfactants at and above the critical micellar concentration (cmc) can control the type of the crystallizing phase.2-4 Thus, if crystallization proceeded from solutions supersaturated to several calcium oxalate hydrates, anionic surfactants induced the formation of COD, while in the control systems and in the presence of submicellar concentrations of surfactant COM would be the prevailing crystallizing phase. The observed effects were attributed to preferential adsorption of the respective surfactants at the COM/solution interface.2,4 Accordingly, growth of COD crystals would be enabled because of the strong inhibition of COM crystal growth by the adsorbed surfactant. To verify the above assumption, we decided to investigate the adsorption of ionic surfactants onto well-defined crystals of COM and COD. Adsorption of surfactants at polar surfaces has been extensively investigated because of the importance of (1) Skrtic, D.; Filipovic-Vincekovic, N.; Fu¨redi-Milhofer, H. J. Cryst. Growth 1991, 114, 118. (2) Tunik, L.; Addadi, L.; Garti, N.; Fu¨redi-Milhofer, H. J. Cryst. Growth 1996, 167, 748. (3) Skrtic, D.; Filipovic-Vincekovic, N. J. Cryst. Growth 1988, 88, 313. (4) Fu¨redi-Milhofer, H.; Tunik, L.; Filipovic-Vincekovic, N.; Skrtic, D.; Babic-Ivancic, V.; Garti, N. Scanning Microsc. Intern. 1995, 9, 1061.
modifying particle surfaces for many industrial applications.5-7 It has been shown7 that the basic features of adsorption isotherms can vary greatly because of different affinities between surface and surfactant. Consequently, among other factors, the nature of the adsorbent surface is of utmost importance. A great number of experimental and theoretical studies pertain to the adsorption of ionic surfactants at the oxide/water interface.8-14 Most experimental adsorption isotherms characterizing these systems are complex, exhibiting at least four characteristic regions with different slopes.9,13,14 The reason for this complexity is the interaction of multiple forces. At low surfactant concentrations direct electrostatic and/or specific interactions with the substrate prevail (region I) while a subsequent increase in the slope of the isotherm (region II) is due to the formation of surfactant aggregates in the adsorbed layer. At higher surfactant concentrations (region III) the slope of the isotherm decreases again (5) Sharma, R. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; ACS Symposium Series 615; American Chemical Society: Washington, DC, 1995; p 1. (6) Lyklema, J. Solid-Liquid Interfaces. Fundamentals of Interface and Colloid Science; Academic Press: London, 1995; Vol. II., Section 2.7.d. (7) Cases, J. M.; Villieras, F. Langmuir 1992, 8, 1251. (8) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 1594. (9) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2649. (10) Bo¨hmer, M. R.; Koopal, L. K. Langmuir 1992, 8, 2660. (11) Somasundaran, P.; Krishnakumar, S.; Kunjappu, J. T. In Surfactant Adsorption and Surface Solubilization; Sharma, R., Ed.; ACS Symposium Series 615; American Chemical Society: Washington, DC, 1995; p 104. (12) Scamehorn, J. F.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1982, 85, 463. (13) Hankins, N. P.; O’Haver, J. H.; Harwell, J. H. Ind. Eng. Chem. Res. 1996, 35, 2844. (14) Wa¨ngnerud, P.; Jo¨nsson, B. Langmuir 1994, 10, 3268.
S0743-7463(97)00804-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/20/1998
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probably because of repulsion forces between surfactant headgroups,9 while the following plateau region (IV), which begins at the critical micellar concentration, has been ascribed to a constant chemical potential sink caused by the micelles in the bulk solution.9,13 The ionic strength has a profound effect on surfactant adsorption which is particularly apparent at low surfactant concentrations.8-10 Thus, as predicted by theoretical calculation,8 an increase of the NaCl concentration by a factor of 10 causes a shift in the initial part of the adsorption isotherm by a decade in surfactant concentration.9 There is considerable disagreement about the nature of the adsorbent surface and the structure of the adsorbed layer at surfactant concentrations exceeding region I of the isotherm (for a review see ref 5). The adsorbing surface is treated as being homogeneous8,11,14 or as patches of different adsorption energies.12,13 Surface aggregates are viewed as hemimicelles11 (small aggregates of head-on adsorbed surfactant with their tails pointing into the surrounding solution), admicelles (local bilayered structures),12-14 or structures between them. Recent studies employing neutron specular reflection15 give additional evidence of the possible existence of small islands of surfactant bilayers at the solid/liquid interface. Although sodium diisooctyl sulfosuccinate (AOT) is an important dispersant and has been widely used in the preparations of microemulsions, only few studies of its adsorption from aqueous solutions to interfaces have so far been published.16-20 They include studies of AOT adsorption at the air/water interface,16,17 at hydrophobic surfaces such as self-assembled monolayers of octadecyltrichlorosilane18 and graphite,19 and on oxidized germanium surfaces.20 In this paper we report recent results on the adsorption of AOT onto preprepared, wellcharacterized crystals of COM and COD. We show significant differences in adsorption behavior of the two substrates originating in different ionic structures of the crystal surfaces. Materials and Methods Stock reactant solutions were prepared by dissolving analytically pure chemicals (Merck) in deionized water. AOT (99% pure, Sigma) was used without further purification. Before dissolution all chemicals were dried overnight in a desiccator over silica gel. The critical micellar concentration (cmc) of AOT was determined by surface tension measurements using a Lauda tensiometer fitted with a platinum-iridium ring. In a saturated solution of calcium oxalate, 0.3 M in sodium chloride, the value was 1.63 × 10-4 mol dm-3 (70.1 mg dm-3) as compared to 6.8 × 10-4 mol dm-3 in aqueous solution.21 Model COM Crystals were precipitated by parallel dropping of 250 mL each of 0.04 mol dm-3 calcium chloride and sodium oxalate solutions into a beaker containing 1500 mL of boiled deionized water. The suspension was kept at 75 °C and mechanically stirred at all times during and 2 h after completed synthesis and then slowly cooled to room temperature. The crystals were filtered through 0.2-µm Schleicher & Schuell filters, washed with deionized water, and dried in a vacuum desiccator. (15) McDermott, D. C.; McCartney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304. (16) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 1615. (17) Li, Z. X.; Lee, E. M.; Thomas, R. K.; Penfold, J. J. Colloid Interface Sci. 1997, 187, 492. (18) Fragneto, G.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1996, 178, 531. (19) Krishnakumar, S.; Somasundaran, P. Colloids Surf. A 1996, 117, 227. (20) McKeigne, K.; Gulari, E. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1271. (21) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley: New York, 1987; p 95.
Tunik et al. Microscopically platelike crystals with sizes 3-10 µm in length were observed. The sample had an average surface area of 1.612 ( 0.028 m2 g-1. The supernatant solution was stored in a closed beaker and used in adsorption and electrophoretic mobility measurements. Model COD crystals were precipitated by the following procedure, modified after ref 22: 1.85 mL of a 4.0 mol dm-3 solution of calcium chloride was added to 500 mL of a buffer solution containing 0.00963 mol dm-3 sodium citrate, 0.01155 mol dm-3 magnesium sulfate, and 0.0637 mol dm-3 potassium chloride. Sodium oxalate (7.65 mL of a 0.25 mol dm-3 solution) was added to the above gently swirled solution at room temperature, and the solution was left standing for 1 h. The suspension was then filtered, washed with deionized water and ethanol, and dried in a vacuum desiccator. The resulting crystals were octahedral bipyramids with sizes of 1-2 µm and an average surface area of 6.2 ( 0.15 m2 g-1. The composition of the model crystals was ascertained by X-ray powder diffraction measurements (Philips PW 1820 diffractometer with Cu KR radiation), and their average surface area was determined by BET analysis with N2 adsorption. For this purpose degassing of the powders was performed at room temperature. Saturated Solution for Adsorption and Electrophoretic Mobility Measurements. The supernatant of the model COM crystals was stored for a few days at room temperature. After it was ascertained that no additional crystallization had occurred sodium chloride was added to obtain a saturated solution of calcium oxalate, 0.3 M in NaCl. For adsorption and electrophoretic mobility experiments 0.100 g of COM or COD crystals was weighed into 50-mL beakers to which 25 mL of the above saturated solution of calcium oxalate containing different concentrations of AOT (0-200 mg dm-3) was added. The beakers where then stoppered and equilibrated for 24 h at 37 °C in a water bath shaker. Adsorption Experiments. The amount adsorbed was determined by the depletion method. Ten milliliters of the equilibrated suspension (see above) was centrifuged, and the equilibrium concentration of the surfactant in the supernatant ceq was determined. The equilibrium concentration of the surfactant in the solution after adsorption was determined by a variation of the Methylene blue method described in ref 23. The method is based on the extraction of the complex formed by Methylene blue with the sulfonated surfactant into chloroform and its subsequent colorimetric measurement at 256 nm. To account for the salting out of Methylene blue into the organic phase, the measurement was performed against a blank containing the extract of the working concentration of methylene blue from a 0.3 M NaCl aqueous solution into chloroform. In the range of working concentrations of AOT the calibration line followed the equation y ) 43.938x - 4.2513, R2 ) 0.9964. Electrophoretic mobility was measured using a Malvern Zeta Master. Fifteen milliliter of the equilibrated suspension (see above) was left to settle, and the supernatant suspension was separated by decantation and injected into the detector cell. Because of the high ionic strength of the suspension, low voltage (50 V) was used for the measurement which resulted in relatively low electrophoretic mobility. However, the instrument is equipped to handle such systems. Every data point is the mean value of five measurements performed automatically. The electrophoretic mobility of the particles was recalculated into a ζ-potential by the Malvern application software using the Smoluchowski equation.
Results The isotherm characterizing adsorption of AOT onto preprepared COM crystals and the results of the corre(22) Brown, P.; Ackerman, D.; Finlayson, B. In Urolithiasis; Sutton, R. A. L., Cameron, E. C., Walker, V., Robertson, B., Pak, C. Y. C., Eds; Plenum Press: New York, 1989; p 75. (23) . Greenberg, A. E., Clesceri, L. S., Eaton, A. D., Franson, M. A. H., Eds. Standard Methods for the Examination of Water and Wastewater, 18th ed.; American Public Health Association, American Water Works Association, Water Environment Federation: Washington, DC, 1992; p 5.
Adsorption of Sodium Diisooctyl Sulfosuccinate
Figure 1. Interaction of AOT with COM crystals in saturated calcium oxalate solution containing 0.3 M NaCl. (a) Adsorption isotherm with 10% error bars. The marked area of low surfactant concentrations is enlarged in the inset. (b) ζ-potential of COM crystals vs equilibrium surfactant concentration. The error bars show the standard deviation of five measurements as calculated by the Zeta Master software. The equilibrium surfactant concentration corresponding to the cmc is marked by a dashed line.
sponding ζ-potential measurements of suspended particles are shown in Figure 1. Adsorption started at low surfactant concentrations (approximately 5 mg dm-3 or 0.07 × cmc). The first plateau was reached at ceq ) 15-30 mg dm-3, which amounts to (0.2-0.4) × cmc. This plateau is followed by a relatively steep inflection (ceq) 30-50 mg dm-3) and a further slow increase of the adsorption as a function of increasing AOT concentration. COM particles had an overall negative charge (ζ ) -5 mV), and the initial increase in AOT adsorption caused a small decrease in the ζ-potential (Figure 1b). However the rapid increase in AOT adsorption characterized by the inflection in the isotherm (Figure 1a) induced a parallel increase in the negative ζ-potential (up to -50 mV) while at higher concentrations the ζ-potential increased only slightly, following the slight increase of AOT adsorption. The isotherm showing adsorption of AOT onto COD crystals and the corresponding changes in the ζ-potential of the particles are represented in Figure 2. It is seen that adsorption onto COD commenced at the surfactant concentration 0.2 × cmc (ceq ) 14 mg dm-3). At lower surfactant concentrations the difference between the initial and the corresponding equilibrium concentrations was below the detection limit of the analytical method, but above this concentration adsorption rapidly increased up to a plateau commencing at concentrations equal to the cmc (Figure 2a). The results of the ζ-potential measurements (Figure 2b) show that COD crystals in the suspension without surfactant were almost uncharged, and their ζ-potential did not change until the commencement of AOT adsorption at ceq ) 14 mg dm-3. After that an increase in the negative ζ-potential up to a plateau at ζ ) -30 mV followed the corresponding increase in AOT adsorption shown in Figure 2a.
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Figure 2. Interaction of AOT with COD crystals in a saturated calcium oxalate solution containing 0.3 M NaCl. (a) Adsorption isotherm with 10% error bars. (b) ζ-potential of COD crystals vs equilibrium surfactant concentration. The error bars show the standard deviation of five measurements as calculated by the Zeta Master software. The marked areas of low surfactant concentrations are enlarged in the insets. The equilibrium surfactant concentration equal to the cmc is marked by a dashed line.
Discussion A comparison of Figures 1a and 2a reveals some similarities but also significant differences between the isotherms characterizing the adsorption of AOT onto COM and COD crystal surfaces, respectively. The adsorption of AOT onto COM crystals can be described by a two-step isotherm (LS type, Figure 1a) while in the case of COD the first step is not evident, and the result is a sigmoid adsorption isotherm (Figure 2a). Adsorption onto COM starts at much lower initial surfactant concentration (approximately 0.07 × cmc as compared to 0.2 × cmc for COD) and finally reaches twice the surface coverage of that for adsorption onto COD. As expected, both isotherms level off when the equilibrium surfactant concentration has reached the cmc, i.e., at a point where equilibrium between free micelles in solution and surfactant aggregates at the crystal/solution interface has been established. From the amounts of surfactant adsorbed in the plateau regions and the specific surface area of the respective model crystals we recalculated the plateau adsorption densities and compared them with literature data on the densities of adsorption of different anionic surfactants onto oxide surfaces (Table 1). It is apparent that the adsorption densities significantly depend on the substratesnote for instance the difference between sodium p-3-nonylbenzenesulfonate on rutile10 and sodium 4-nonylbenzenesulfonate on Al2O3.12 The maximum surface coverages for both calcium oxalate hydrates (second plateau’s in Table 1) are relatively high in comparison with adsorption densities at oxide surfaces. It therefore seems reasonable to assume a bilayer arrangement of the adsorbed surfactant molecules. We thus propose that the second plateaus of the adsorption isotherms (Figures 1a and 2a) indicate the formation of bilayers consisting of intertwined surfactant aggregates with one molecule
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Tunik et al.
Table 1. Adsorption Densities of Anionic Surfactants on Calcium Oxalate Hydrate and Oxide Surfaces sorbent
sorbate
molecules/nm2 (1st plateau)
molecules/nm2 (2nd plateau)
ref
COM COD Al2O3 Al2O3 Al2O3 rutile rutile
AOT AOT sodium 4-nonylbenzenesulfonate sodium dodecyl sulfate C11 paraxylenesulfonate sodium p-3-nonylbenzenesulfonate sodium p-3-dodecylbenzenesulfonate
2.7 none none none none none none
14.22 7.45 4.8 4.2 3.74 1.8 1.2
Figure 1a, this paper Figure 2a, this paper 12 11 13 10 10
adsorbed with its headgroup toward the surface, while the headgroup of the second molecule points to the solution. Such an arrangement is consistent with the data showing a parallel increase of the negative ζ-potential with increasing surface concentration of the surfactant (Figures 1b and 2b). The adsorption of surfactants in the form of bilayers has been previously assumed by several authors.12,13 Recently evidence to that effect has been presented by the neutron reflection technique.15 Given all other experimental factors identical, the differences between the adsorption isotherms shown in Figures 1a and 2a can be attributed to differences in the substrate surfaces which can be understood by considering the corresponding crystal structures24,25 and the molecular structures of the crystal faces affected by adsorption.2 COD crystals are tetragonal and crystallize in the form of octahedral bipyramids with eight faces equivalent to (011 h ) and four faces equivalent to (010).24 The crystal structure includes “zeolitic” water channels. The molecular structure of the (010) and (011 h ) faces is shown in Figure 3. It is seen that the developed (011 h ) faces of COD include water molecules in high concentration which alternate with pure water layers (Figure 3a). A similar situation is found for the (010) faces where oxalate and calcium ions alternate with water molecules at the approximate molar ratio 1:3 (Figure 3). There is a high probability that in crystals grown and exposed to aqueous solution the water layers will be exposed most of the real time, thus significantly reducing the surface charge of the crystals. The only regions of high charge density in COD crystals (because of protruding oxalate ions) are the tips of the bipyramids, which, however, comprise a very small part of the total crystal surface area (Figure 3a). The relatively low surface charge of COD crystals has been confirmed by ζ-potential measurements, which show that crystals suspended in electrolyte solution without surfactant are almost uncharged (Figure 2b). Because of the presence of the hydrated layer the COD/electrolyte solution interface exhibits a marked similarity with the oxide (hydroxide)/solution interface, and consequently it stands to reason that similar adsorption behavior can be expected of ionic surfactants. Indeed, the isotherm characterizing adsorption of AOT onto COD crystals (Figure 2a) shows the main features of isotherms characterizing adsorption of ionic surfactants from high ionic strength solutions to oxide/electrolyte solution interfaces;8,9 i.e., region I, typical of surfactant adsorption through Coulombic interactions, is negligible because of the screening effect of the neutral electrolyte, while regions II-IV are expressed in the system. Assuming bilayer formation, the plateau adsorption density for COD (Table 1) corresponds to an average of 27 Å2 per bimolecular unit which seems a reasonable value. COM crystals are monoclinic (1 h 01) plates.24,25 On the basis of the crystal structure given in ref 25 we show that (24) Tazzoli, V.; Domeneghetti, C. Am. Miner. 1980, 65, 327 (for COM see also: Cocco, G. Atti Accad. Naz. Lincei 1961, 31, 292). (25) Deganello, S. Acta Crystallogr., Sect. B 1981, 37, 826.
Figure 3. Calcium oxalate dihydrate crystallographic projections of the 100 (a) and 001 (b) planes calculated from the crystal structure given in ref 24.
the (1h 01) crystal faces are characterized by oxalate ions emerging oblique to the faces with a dense pattern of complexed calcium ions exposed (Figure 4a), while the (010) faces are characterized by oxalate ions lying perpendicular to the face alternating with those parallel to the face (Figure 4b and ref 26). The results of morphological studies suggest2 that AOT adsorbs on both of these crystal faces, which together comprise about 90% of the total available surface area. The adsorption of the anionic surfactant is facilitated by charge-determining calcium ions (under the experimental conditions employed, the solution concentrations of both oxalate and calcium ions, as calculated from the respective solubility product,27 are c(C2O42-) ) c(Ca2+) ) 3 × 10-4 mol dm-3). The first plateau shown in Figure 1a is thus a consequence of Coulombic interactions at the crystal/solution interface. We assume that the surfactant molecules are adsorbed head-on, forming monolayers at high charge density sites (26) A similar analysis was made in ref 2, where the crystal structure given in ref 24 was used as a model for calculations. However, by using the crystal structure given in ref 25, a much clearer picture of the molecular structure of the (1 h 01) faces (Figure 4b) is obtained. (27) Fu¨redi-Milhofer, H.; Markovic, M.; Uzelac, M. J. Cryst. Growth 1987, 80, 60.
Adsorption of Sodium Diisooctyl Sulfosuccinate
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micelles for small water pools was estimated to be 10 Å2 (see ref 28). The above results show that, with all other parameters constant, adsorption of surfactants at crystal/solution interfaces strongly depends on the molecular and/or ionic structure of the substrate surfaces. A significant influence of the hydration layer at crystal faces, reducing the surface charge and thus inhibiting electrostatic interactions, is also apparent. The finding that COM and COD crystals have significantly different affinities for AOT (Table 1) seems to confirm our previous assumption2,4 that preferential adsorption of the surfactant at the surfaces of nascent COM crystals is the main reason that AOT at c gcmc induces crystallization of COD on account of COM. Conclusions
Figure 4. Calcium oxalate monohydrate crystallographic projections of the 010 (a) and 001 (b) planes calculated from the crystal structure given in ref 25.
(from the adsorption densities given in Table 1, assuming a monolayer, about 40% of the total available adsorption sites are occupied). This assumption is in accordance with the slight decrease of the negative ζ-potential in the region of the first plateau (Figure 1b) and with the previously observed2 formation of oriented crystal aggregates when COM crystals are grown in the presence of low concentrations of AOT. We furthermore assume that the upsurge in AOT adsorption between the first and second plateaus is caused by association of surfactant molecules while the second plateau of the COM adsorption isotherm (Figure 1a) indicates full bilayer coverage. This would correspond to 14 A2 as the average surface area occupied by a bimolecular unit. This result is plausible, since the AOT headgroup density at the AOT/water/oil interface in reversed
Adsorption of AOT from a 0.3 M NaCl solution onto calcium oxalate monohydrate (COM) and dihydrate (COD) crystals has been studied. It is shown that the different ionic structures of the substrate surfaces result in significantly different substrate/surfactant affinities and consequently in different adsorption behavior. COD crystal faces, being covered by a hydration layer, showed adsorption behavior similar to the oxide (hydroxide)/ solution interface at high salt concentrations. At low surfactant concentrations adsorption was insignificant because of the screening effect of the electrolyte; when it commenced, it rapidly increased up to a plateau which was reached at the cmc of the surfactant. Assuming bilayer coverage, the plateau adsorption density corresponded to 27 Å2 per bimolecular unit. Adsorption at the highly charged COM/solution interface commenced at 0.07 × cmc, reaching a first plateau at (0.2-0.4) × cmc. In this region, assuming head to surface adsorption due to Coulombic interactions, the adsorption density corresponded to 37 Å2 per molecule. The second plateau was reached at the cmc of the surfactant and, assuming the formation of bimolecular, intertwined aggregates, corresponded to an adsorption density of 14 Å2 per bimolecular unit. The above findings confirm our previous assumption that preferential adsorption of the surfactant at the surfaces of nascent COM crystals causes AOT at c g cmc to induce crystallization of COD on account of COM. Acknowledgment. It is a pleasure to thank Prof. Lia Addadi from the Weizmann Institute of Science, Rehovot, Israel, for her help and advice concerning the modeling of ionic structures of crystal faces. LA9708041 (28) Bakale, G.; Bak, G.; Thomas, J. K. J. Phys. Chem. 1992, 96, 2328.
