Molecular Association of a Nonionic and an Ionic-Induced Surfactant

Jan 7, 2003 - In the present work, to obtain information on the aggregation ... 221D content, the system has been studied by small-angle neutron scatt...
0 downloads 0 Views 90KB Size
Download PDF
554

Langmuir 2003, 19, 554-558

Molecular Association of a Nonionic and an Ionic-Induced Surfactant: Cryptand (221D) NaCl in Water E. Caponetti,* D. Chillura Martino, and L. Pedone Dip. di Chimica Fisica, Universita` di Palermo, Viale delle Scienze - Parco D’Orleans II, I-90128 Palermo Received May 30, 2000. In Final Form: October 6, 2000 The cryptand 5-decyl-4,7,13,16,21-pentaoxa-1,10-diazabicyclo-[8.8.5] tricosane [221D] strongly resembles a surfactant in its characteristics: it contains a hydrophilic headgroup, the cryptand unit, and a hydrophobic unit, the decyl chain. It is insoluble in water, but in the presence of an appropriate amount of NaCl, as a consequence of the complex formation between 221D and Na+, it becomes soluble and forms aggregates. The aggregates, depending on the NaCl-221D molar ratio, can be considered as mixed ionic nonionic micelles or ionic micelles. The evolution of the aggregate dimensions and their shape has already been studied at two NaCl-221D molar ratios as a function of the 221D concentration. In the present work, to obtain information on the aggregation behavior of 221D in water on varying the NaCl concentration from one third up to four times the 221D content, the system has been studied by small-angle neutron scattering. Increasing the NaCl-221D molar ratio up to 1, the aggregation number rapidly falls, and then slowly increases at higher salt content; this behavior has been attributed to electrostatic interactions. After the micelle dimension behavior, the shape evolves continuously from oblate (axial ratio ≈ 0.67) to prolate (axial ratio ≈ 1.2) ellipsoid up to [NaCl]/[221D] ) 1, and tends to be spherical at higher ratios.

Introduction Macrocyclic compounds having different cavity sizes and ring substituents and containing donor atoms such as oxygen nitrogen, sulfur, phosphorus, or their combination have been synthesized and largely investigated.1 Macrocyclic compounds having more than one cyclic ring form stable complexes with alkali, alkaline earth, and some transition elements. They have been studied, from the point of view, of both thermodynamics and kinetics, in different solvents such as water, methanol, and others.1-3 The interest in these classes of compounds has been stimulated by their extensive and successful use in several fields where it is important to choose a convenient cation and maintain it in a precise position in an organic moiety.4 Cryptands are macrocyclic compounds having at least two basic bridgehead nitrogen atoms in the ring; they encapsulate ions by their cagelike structure and form metal complexes having 1:1 metal-ligand molar ratios. For alkali and alkaline earth the stability constant of the complex, called cryptate, is strongly related to the matching of the ionic crystal radius and the cavity radius. The cryptand 5-decyl-4,7,13,16,21-pentaoxa-1,10-diazabicyclo-[8.8.5] tricosane identified as 221D5,6 according to the macrobicyclic ligands nomenclature is a derivative of the cryptand 221 obtained introducing a decyl hydrocarbon chain in position 5 of the tricosane ring. Its structure is represented in Figure 1. Considering the 221 cavity radius6 and the alkali and alkaline earth ions radii,7 it is expected * Corresponding author. E-mail: [email protected]. (1) Izatt, R. A.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. Rev. 1991, 91, 1721. (2) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J. Chem. Rev. 1985, 85, 271; see also references therein. (3) Cox, B. G.; Schneider, H. Coordination and Transport Properties of Macrocyclic compound in Solution; Elsevier: Amsterdam, 1992; see also references therein. (4) Votgle, F., Ed. Topics in Current Chemistry; Springer-Verlag: New York, 1982 (5) Lehn, J. M. Struct. Bonding (Berlin) 1975, 16, 1. (6) Lehn, J. M.; Sauvage, J. P. Chem. Commun. 1971, 440.

Figure 1. Molecular structure of cryptand 5-decyl-4,7,13,16,21-pentaoxa-1,10-diazabicyclo-[8.8.5] tricosane (221D).

that the more stable complexes must be the ones with sodium and calcium ions. This is confirmed by the values of the stability constant, both in water and in nonaqueous media.1 The alkyl substitution, as shown by Cox et al. in methanol,8 is expected to reduce the cryptates stability and to increase their hydrophobicity. Whereas the 221 is very soluble in water, the 221D is practically insoluble, but the presence of a certain amount of salt makes it soluble because of the complex formation between the ligand and the cation. Increasing the concentration, the complex behaves like a cationic surfactant and form micelles.9 Similar behavior has been observed for other long-chain alkyl substitute macrocyclic compounds.10-12 Surfactants are amphiphilic molecules, that is, molecules made with a hydrophobic portion such as a hydrocarbon chain, and hydrophilic portions such as ionic (7) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (8) Cox, B. G.; Firman, P.; Schneider, I.; Schneider, H. Inorg. Chim. Acta 1981, 49, 153. (9) Caponetti, E.; Chillura Martino, D.; Floriano, M. A.; Triolo, R. J. Phys. IV Colloq. C8 1993, 3, 173.

10.1021/la000735h CCC: $25.00 © 2003 American Chemical Society Published on Web 01/07/2003

Cryptand (221D) NaCl in Water

or polar groups, therefore they display peculiar properties when dissolved in water. On the one hand, the interaction between the hydrophobic portion of the molecule and the water molecules hinders the dissolution of the surfactant in water, whereas on the other hand, the energetically favorable ion-water or dipole-water interactions tend to stabilize the surfactant molecule in solution. By increasing their concentration, surfactant molecules show a sudden change in most of their solution properties at a peculiar concentration called critical micelles concentration (cmc). This change is related to the formation of aggregates in which the contact between the hydrophobic portion of the molecules and water is minimized; at the same time, hydrophilic interaction is maximized. Micelles of different shapes can be obtained depending on the surfactant concentration, the length of the hydrocarbon chain, and the nature of the polar head. In the ionic surfactants, some of the counterions are bound to the surface of the micelle forming what is called “Stern layer”, whereas the remaining counterions are located at a greater distance from the surface of the micelle, in what is called “GouyChapmann electric double layer”. These aggregates are clearly not static but rather highly dynamic entities, although stable enough to be detected with different techniques. So far only a few studies exist on the aggregation behavior of macrocyclic alkyl substitutes in aqueous solution.10-12 The aggregation behavior of the cryptand 221D/NaCl/water system as a function of cryptand concentration was investigated previously by some of us using small angle neutron scattering technique at two constant NaCl-221D molar ratios.9 The two molar ratios were chosen to obtain, in one case, aggregates entirely made up of ionic monomers, and in the other, mixed micelles constituted by both ionic and nonionic units. It was found that smaller aggregates are formed at higher saltsurfactant molar ratio because of repulsions among the charged heads. These repulsions are weakened at the lower salt-surfactant molar ratio because of the insertion of neutral molecules between the charged ones; consequently, bigger aggregates are formed. In both cases the aggregation number increases with concentration. When the entire ionic surfactant is present it was also observed that, on increasing the concentration, the formation of new smaller aggregates is the favorite mechanism in solution rather than increasing the aggregates dimension of the aggregates. To rationalize the behavior of the system going from the mixture of an ionic and a nonionic surfactant at the lower salt-surfactant molar ratio up to the solution containing only the ionic surfactant, and to investigate the effect of NaCl excess on the ionic micelles, in this article we present the results of a study on the cryptand 221D/NaCl/water system as a function of the salt concentration at constant cryptand content. The sodium salt has been chosen because, among the alkali metals, the sodium ion is of appropriate size to fit the 221 cavity13 and to form the more stable complex with the 221 ligand. Assuming that the equilibrium constant for the Na+ complexation by 221D is not substantially different from that of cryptand 221: (10) Turro, N. J.; Kuo, P. L. J. Phys. Chem. 1986, 90, 837. (11) Baglioni, P.; Gambi, C. M. C.; Giordano, R.; Lo Nostro, P.; Teixeira, J. Physica B 1997, 234-236, 300. (12) Ozeki, S.; Kojima, A.; Harada, S. J. Phys. Chem. 1996, 100, 19446; see also reference therein. (13) Gokel, G. W., Ed. Crown Ethers and Cryptands Royal Society Chemistry: Cambridge, 1991; p 116

Langmuir, Vol. 19, No. 3, 2003 555

221D + Na+ a 221DNa+

K ≈ 1012

the species present in each solution, at different stoichiometric compositions, are: (i) when [NaCl]/[221D] < 1, [221DNaCl] ≈ Csalt, and [221D] ≈ Cligand - Csalt (e.g., when Cligand ) 0.149 mol/L and Csalt ) 0.048 mol/L, [NaCl]uncomplexed ) 5 × 10-13 mol/L). In this case both the cationic surfactant 221DNaCl and the nonionic surfactant 221D are present in solution and hence mixed micelles are formed. (ii) when [NaCl]/[221D] ) 1, [221DNaCl] ≈ Csalt ≈ Cligand, and [221D] ≈ 0, hence aggregates, constituted by the cationic surfactant monomers only, are present in solution (e.g., when Cligand ) Csalt ) 0.149 mol/L, [NaCl]uncomplexed ) 3.9 × 10-7 mol/L). (iii) when [NaCl]/ [221D] > 1, [221DNaCl] ≈ Cligand, [221D] ≈ 0 and [NaCl] ≈ Csalt - Cligand (e.g., when Cligand ) 0.149 mol/L and Csalt ) 4.16 mol/L, [221D]uncomplexed ) 4 × 10-14 mol/L). In this case, the salt effect on the ionic surfactant micellar solution is observed. Small-angle neutron scattering (SANS) technique has been used because the size of the aggregates formed in solution is rather large and is within the dimensional range investigable by the technique. Experimental Section Materials. 5-Decyl-4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5] tricosane (Kryptofix 221D) Merck, 97% purity, was used as received. D2O was an Aldrich product (99.8 atom % D). NaCl was a Sigma ACS reagent. SANS Measurements. Scattering experiments were performed on the 30-m ORNL SANS camera of the National Center for Small Angle Scattering Research (NCSASR) at the High Flux Isotope Reactor (HFIR) in Oak Ridge, Tennessee (USA). Neutrons of 4.75 Å wavelength (∆λ/λ ) 6%) were used. A 64 × 64 cm bidimensional detector was placed 601 and 164 cm from the sample. With the above geometries above, the accessible ranges of momentum transfer Q (Q ) 4π sin θ/λ, 2θ being the scattering angle) were 0.05-0.35 Å-1 and 0.01-0.10 Å-1, respectively. The samples were contained in quartz photometric cells having 0.1and 0.2-cm path lengths. The temperature of the specially built cell holder was kept constant at 25.00 ( 0.05 °C by circulating fluid from an external bath. Scattering intensities from samples and solvent were corrected for detector background and sensitivity, empty cell scattering, computed incoherent scattering and sample transmission. These differences were converted to radial average intensities vs Q, and absolute elastic coherent cross-section (dΣ(Q)/dΩ) were computed from calibration based on the known scattering from pure water. All data correction was performed using software provided by NCSASR. Composition. It has been said that the nonionic surfactant cryptand 221D is practically insoluble in water, but its solubility increases with salt concentration. Given the high value of complexation constant, any amount of added salt forms the soluble ionic surfactant 221DNa+Cl-. It is able to bring in solution a certain amount of the nonionic surfactant. At 25 °C, it was necessary to add at least 0.045 mol/L of NaCl to solubilize 0.149 mol/L of 221D. In the range of surfactant concentration examined (0.03-0.15 mol/L), it seems that the minimum [NaCl]/[221D] molar ratio, roughly equal to 0.3, does not change appreciably. SANS intensities have been collected for 15 solutions containing different amount of NaCl at a constant 221D concentration, say 0.149 mol/L. For salt concentration ranges between 0.048 and 0.62 mol/L, in particular the following [NaCl]/[221D] molar ratios were obtained: 0.32, 0.42, 0.43, 0.46, 0.49, 0.55, 0.65, 0.77, 0.94, 1.14, 1.34, 1.66, 2.01, 2.94, and 4.16.

Results and Discussion Some of the experimental SANS data related to the system at [NaCl]/[221D] e 1 are reported in Figure 2; this choice was made for the sake of clarity, even if all data

556

Langmuir, Vol. 19, No. 3, 2003

Caponetti et al.

Figure 2. Experimental (symbols) and calculated (lines) SANS differential cross-section, dΣ(Q)/dΩ, as functions of momentum transfer, Q, for D2O + 221D + NaCl system. Symbols represent 221D-NaCl molar ratios as follows: O, 0.32; b, 0.42; 4, 0.46; 2, 0.65; 0, 0.77; 9, 0.94.

Figure 3. Experimental (symbols) and calculated (lines) SANS differential cross section, dΣ(Q)/dΩ, as functions of momentum transfer, Q, for D2O + 221D + NaCl system. Symbols represent 221D-NaCl molar ratios as follows: O, 0.94; b, 1.34; 4, 2.01; 2, 2.94; 9, 4.16.

Table 1. Parameters from Least-Squares Fits to SANS Data of 221D + NaCl in D2O Using the “Core + Shell” Elliptical Model Described in the Texta

dΣ(Q)/dΩ ) NpP(Q)S(Q) - Np∆(Q)

[221D]/ [NaCl]

ν (molecules)

Z (e.u.)

0.32 0.42 0.43 0.46 0.49 0.55 0.65 0.77 0.94 1.14 1.34 1.66 2.01 2.94 4.16

90(2) 68(1) 62.3(6) 57.5(7) 52.9(7) 43.4(5) 37.3(4) 37.9(6) 27.4(4) 27.7(2) 29.3(3) 29.7(3) 31.6(3) 34(7) 33.4(3)

10.9(7) 14.1(7) 14.4(6) 14.3(5) 14.2(5) 12.1(4) 13.4(4) 10.7(4) 13.2(2) 12.5(3) 11.7(3) 12.1(3) 11.5(5) 9(1) 10



A (Å)

H (molecules)

χ

0.67 0.77 0.80 0.84 0.87 0.96 1.04 1.03 1.21 1.20 1.17 1.16 1.13 1.09 1.09

21.2 18.5 17.7 16.9 16.3 14.7 13.7 13.8 11.7 11.8 12.1 12.2 12.6 13.0 13.0

7.4(5) 6.0(4) 10.0(3) 10.2(4) 10.0(4) 13.1(4) 10.9(4) 8.3(5) 7.0(5) 6.3(2) 6.6(2) 5.9(2) 5.6(2) 2.3(4) 6.4(2)

9.7 8.2 9.2 8.4 8.0 3.7 5.2 7.2 4.6 4.2 3.8 3.7 4.8 7.6 5.3

a ν is the aggregation number, the number of surfactant monomers/micelle, Z is the total net charge of one micelle,  is the micellar core axial ratio, A is the micellar core semiaxes, H is the hydration number of the headgroup and χ is the standard deviation of the fit. The rotation micellar core semiaxis has been set equal to the length of the fully extended alkyl chain. Fit parameters are identified by the error on the last digit.

had been analyzed, and the results reported in Table 1. The well-defined peak indicates the presence of aggregates in the system; this is due to the interference among neutrons scattered from equal or different nuclei placed in different aggregates. On increasing the salt content up to [NaCl]/[221D] ) 1, a decrease in the intensity and a shift of the interaction peak position toward higher Q values is observed; this trend indicates a decrease in particle dimensions. Experimental SANS data collected for the system at [NaCl]/[221D] > 1 are reported in Figure 3. On increasing the salt content, the intensity remains approximately constant, the peak does not change its position, significantly but tends to disappear because of the increasing compressibility; the total curve appears as one of a noninteracting particles system. For two-phase systems, such as the one considered here, consisting of a dispersed phase and a continuous phase, within the decoupling approximation,14 the SANS differential cross-section can be calculated by the following expression:

(1)

where Np is the particle number density, P(Q) ) 〈F(Q)〉2 is the scattering caused by a single particle, S(Q) is the structure function related to interparticle interactions, and ∆(Q) ) 〈F(Q)〉2 - 〈F(Q)2〉. The latter quantity is equal to zero in the spherical monodisperse particles and different from zero in the cases in which deviation from spherical symmetry or from a strictly monodisperse system is present. P(Q) depends on micelle dimension and on the scattering length densities of the different regions of the aggregate. The structure function S(Q), which depends on the volume fraction of the dispersed phase and on the net charge and dimension of the micelles, can be obtained by the rescaled mean spherical approximation (RMSA), by assuming that the interaction potential is a sum of Coulombic and hard-sphere contributions.15 Full details of the calculation procedure can be found elsewhere.14,16,17 Essentially, the calculation procedure consists of fitting the full experimental SANS curve using eq 1. To calculate both P(Q) and S(Q) a physical model describing the structure of aggregates and the interaction between them is hypothesized. In the fitting procedure the adjustable parameters are quantities of physical interest, like dimension, aggregation number, charge, and so on. To reproduce the experimental curves, in the present work, several models were tried. Good results were obtained when the particle shape deviates from a spherical symmetry. In particular, allowing the axial ratio to vary in the fitting procedure, small aggregates become prolate ellipsoids and big aggregates become oblate ellipsoids. Although different models can give better results in the two regions of the NaCl-221D molar ratio (mixed ionic and nonionic surfactant and ionic surfactant in the presence of electrolyte excess), we prefer to report results related to a model that can reproduce the experimental intensities in all ranges of the NaCl-221D molar ratio. This model is very simple; it considers the aggregates (14) Caponetti, E.; Floriano, M. A.; Varisco, M.; Triolo, R. Structure and Dynamics of Supramolecular Aggregates and Strongly Interacting Colloids; Chen, S. H.; Huang, J. S.; Tartaglia, P., Eds.; Kluwer Academic: Dordrecht, 1992; pp 535-555. (15) Hayter, J. B.; Penfold, J. J. Chem. Soc., Faraday Trans. I 1981, 77, 1851; Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 269, 1022. (16) Caponetti, E.; Triolo, R. Industrial and Technological Application of Neutrons; Rustichelli, F., Fontana, M., Coppola, R., Eds.; NorthHolland: Amsterdam, 1992; pp 403-424. (17) Caponetti, E.; Triolo, R. Adv. Colloid Interface Sci. 1990, 32, 235.

Cryptand (221D) NaCl in Water

Langmuir, Vol. 19, No. 3, 2003 557

Figure 4. Form factor, P(Q), as functions of momentum transfer, Q, for D2O + 221D + NaCl system. Lines represent 221D-NaCl molar ratios. (A) s, 0.32; s - s, 0.42; s s, 0.46; - - -, 0.65; ss ss, 0.94. (B) s, 0.94; s - s, 1.34; s s, 2.01; - - -, 2.94; ss ss, 4.16.

Figure 5. Structure factor, S(Q), as functions of momentum transfer, Q, for D2O + 221D + NaCl system. Lines represent 221D-NaCl molar ratios. (A) s, 0.32; s - s, 0.42; s s, 0.46; - - -, 0.65; s s, 0.94. (B) s, 0.94; s - s, 1.34; s s, 2.01; - -, 2.94; ss ss, 4.16.

constituted by a core containing all the hydrocarbon moieties and a shell containing the polar and/or charged headgroups, a fraction of counterions and hydration water molecules. To reduce the adjustable parameters in the fitting procedure, we have chosen to maintain constant the rotation core semiaxis that was set equal to the length of the fully extended hydrocarbon chain (14.15 Å). This assumption has been used successfully in interpreting SANS data of surfactant solutions whose micelles are only slightly elongated.16 The constancy of one of the core semiaxes implies that the aggregates are spherical when the number of monomers in one micelle, ν, is equal to 41; aggregates with higher ν values result in oblate ellipsoids, axial ratio,  < 1, and aggregates with lower ν values results in prolate ellipsoids,  > 1. Being  * 1 F(Q) was computed by averaging the scattering amplitude Φ0(x) on all orientations of the aggregate with respect to the direction of the scattering vector:

of the long dimension and the scattering vector, ai is the length of an ellipsoid semiaxis, and i is the axial ratio. The background was evaluated at high Q, in the Porod region of the scattering curve, from slope of Q4I(Q) versus Q.4 In the fitting procedure the adjustable parameters were ν, the total net charge of one micelle, Z, and the hydration number of the headgroup, H. Np was derived from the 221D concentration, cmc, and aggregation number. The length of the two remaining core semiaxes and the axial ratio were derived from the aggregation number and the volume of the hydrocarbon chain (269.4 Å3).18 To separate the effects of variations of dimension from the effects of variations of interactions, it is interesting to observe the two contribute to the total intensity P(Q) and S(Q), separately. On increasing the salt content to [NaCl]/[221D] ) 1 the form factor, shown in Figure 4A, registers strong variation: the position of first minimum moves toward higher Q values because of the rapid decrease of the micelle dimension, and it becomes more pronounced which indicates spherical aggregates at [NaCl]/[221D] ) 0.55. At higher salt content (Figure 4B) the less pronounced variations are due to the small increase of the micelle dimension. In Figure 5 S(Q) is reported at low salt content (Figure 5A) the main peak moves toward higher Q because of the decreasing particles volume, and surprisingly the sharpness of the peak does not change, which suggests that the decreasing volume is balanced by an increasing net charge. At higher salt

F(Q) )

∫01V1(F1 - F2)Φ0(u1) + V2(F2 - Fs)Φ0(u2)dµ

(2)

where Vi and Fi are the volume and scattering length density of the core (subscript 1), of the shell (subscript 2), and of the solvent (subscript s). The scattering length densities for the various groups were computed from atomic scattering lengths and from reported volumes. The scattering amplitude is given by the relation Φ0(ui) ) 3 (sin ui - ui cos ui)/ui3, and ui ) Q[(a)i2 + ai2(1 - µ)]0.5, where µ is the cosine of the angle between the direction

(18) Immirzi, A.; Perini, B. Acta Crystallogr., Sect. A 1972, 33, 216; Millero, F. J. Water and Aqueous Solution; Horne, R. A., Ed.; WileyInterscience: N.Y., 1982; p 519.

558

Langmuir, Vol. 19, No. 3, 2003

Caponetti et al.

sequence of the electrolyte addition is generally higher for micelles composed of conventional ionic surfactant. For example, the addition of NaCl to sodium dodecyl sulfate aqueous solutions in the concentration range up to four times the surfactant content (0.05 mol/L) causes a 60% increase of ν.19 On the contrary the addition of neutral electrolyte to a solution of a nonionic micelles slightly affects the aggregation number.20 The intermediate aggregation number increase observed for the micelles formed by 221DNa+Cl- can be explained by considering that the charge is dispersed over all the oxygen and nitrogen atoms in the cryptate ring so that its behavior results is intermediate between ionic and a nonionic micelles. Figure 6. Aggregation number (b), ν, and axial ratio (9), , as a function of 221D-NaCl molar ratios. Lines guides for the eye only.

content (Figure 5B) a shift and a broadening of the S(Q) peak is observed as a consequence of the screening in the system. The calculated intensities are in good agreement with the experimental ones; some of them are shown as continuous lines in Figures 2 and 3. Table 1 reports the fitting and derived parameters for all the compositions examined.  and ν, are reported in Figure 6 as a function of NaCl-221D molar ratio. Increasing the NaCl content up to [NaCl]/[221D] ) 1 the micelle shape evolves from an oblate ellipsoid toward a prolate ellipsoid, and the axial ratio reaches a maximum value. At the same time, the aggregation number rapidly falls reaching a minimum value; this ν behavior is a consequence of the increase of the charged headgroups fraction. In fact, as said previously, the added electrolyte is involved in the micellization process forming the ionic species 221DNa+Cl-; as a consequence the system evolves from a mixture of ionic and nonionic surfactant up to a system constituted by the ionic surfactant only. It is wellknown that the electrostatic repulsion between the charged headgroups reduces micelle dimension, but it is interesting how big this effect is in this case where the only difference between the two monomers is the charge. Starting from [NaCl]/[221D] ) 1 the NaCl concentration increase generates an increase of the micelle aggregation number that is about 20% when [NaCl]/[221D] ) 4. This is a consequence of the repulsive interaction screening between the charged headgroups and between the ionic aggregates. The aggregation number increase as a con-

Conclusion The evolution of the 221D-NaCl-D2O system at constant 221D amount and as a function of NaCl concentration has been described using one simple elliptical “core + shell” model. Because the rotation core semiaxis was kept constant and equal to the length of the fully extended alkyl chain, the axial ratio varies according to the micellar dimension. The model has been able to reproduce data of the ionic/non ionic mixed system as well as of the ionic system and to take into account of the salt effect on the ionic system. This is important even if details on the structure could be loosen. At low salt-surfactant molar ratios only a fraction of molecules are charged, and big oblate, elliptically shaped aggregates are formed because the insertion of neutral monomers between the charged headgroups reduces the electrostatic interactions. At higher salt-surfactant molar ratios, smaller prolate elliptically shaped aggregates are formed as a consequence of the strong repulsion among charged heads. Because the only difference between the two kinds of monomers that make up the aggregates is the charge, this system can be considered as “model” system for mixed micelles. Acknowledgment. The authors are grateful to the CNR (Consiglio Nazionale delle Ricerche) and to the MURST (Ministero dell’Universita` e della ricerca Scientifica e Tecnologica) for financial support. They also thank the Oak Ridge W.C. Koheler Center for Small Angle Scattering Research funded by the Department of Energy. LA000735H (19) Berr, S. S. Ph.D. Thesis, 1986. (20) Al-Saden, A. A.; Florence, A. T.; Whateley, T. L. J. Colloid Interface Sci. 1982, 86, 51.