Langmuir 1995,11, 2464-2470
2464
Effect of Crown Ether 1,4,7,10,13,16-Hexaoxacyclooctadecaneon the Structure of Sodium Dodecyl Sulfate and Dodecyltrimethylammonium Bromide Aqueous Micellar Solutions E. Caponetti,*?+D. Chillura Martino,? M. A. Floriano,? R. Triolo,*?§and G. D. Wignall* Dipartimento di Chimica Fisica, Universita di Palermo, Via Archirafi 26, 90123 Palermo, Italy, and W. C. Koheler Center for Small Angle Scattering Research, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37836 Received January 23, 1995. I n Final Form: April 18, 1995@ The effects of the addition of crown ether 1,4,7,10,13,16-hexaoxacyclooctadecane on the structure of aqueous solutions of surfactants sodium dodecyl sulfate and dodecyltrimethylammonium bromide have been studied by small angle neutron scattering. By modeling the scattering intensities, it was possible t o derive, simultaneously, both structural properties and information on the distribution of crown ether between the micellar and aqueous phases. In the case of sodium dodecyl sulfate, an appreciable amount of crown ether was found to be localized in the micellar phase, though it was not possible to establish whether it was in the core or in the shell; there was no evidence of crown ether localization in dodecyltrimethylammonium bromide micelles. The above observations indicate that although, at least in principle, the crown ether methylene groups could interact hydrophobically in the micellar core with the surfactant alkyl chains, the ability of crown ethers to form metal complexesmight also lead, in sodium dodecyl sulfate solutions, to electrostatic interactions in the external palisade between the crown ethersodium ion complex and the surfactant head groups. The absence of crown ethe: from dodecyltrimethylammoniumbromide micelles indicates that the macrocyclic moleculesinteract with sodium dodecyl sulfate micelles via the formation of a complex between the sodium ion and the crown ether and rules out the possibility of hydrophobic interaction in the micellar core.
Introduction Sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAJ3) are amphiphilic molecules (surfactants),i.e., molecules containing both hydrophilic and hydrophobic portions. In aqueous solutions, hydrophobic interactions between the hydrocarbon portion of the molecule and water hinder the dissolution of the surfactant; a t the same time, energetically favorable ionwater interactions or dipole-water interactions tend to stabilize the surfactant molecule in solution. With a n increase in their concentration, surfactants show a sudden change in most of their solution properties a t a critical micellar concentration (cmc). This change is related to the formation of micelles in which the contact between the hydrocarbon portion of the molecules and water is minimized, while, a t the same time, the hydrophilic interaction is maximized. A vast body of information on the thermodynamic properties and on the structure of surfactant solutions is available.l-16 It has been shown that the shape and size of micelles may vary by changing the amphiphile coni
Universita di Palermo.
* Oak Ridge National Laboratory.
8 On leave of absence from University of Palermo. Permanent address: Dipartimento di Chimica Fisica, Universith di Palermo, Via Archirafi 26,90123 Palermo, Italy. Abstract published in Advance A C S Abstracts, J u n e 1, 1995. (1)Triolo, R.; Caponetti, E. J . Solution Chem. 1986,15, 377. (2)Triolo, R.; Butler, P.; Caponetti, E.; Daus, K. A.; Ho, P. C.; Johnson, J . S.; Magid, L. J. J . Phys. Chem. 1987,116, 200. (3)Triolo, R.; Caponetti, E.; Graziano, V. J . Phys. Chem. 1985,89, @
5743. (4)Ikeda, S.;Hayashi, S.; Imae, T. J . Phys. Chem. 1981,85, 106. (5) Missel, P.J.;Mazer, N. A,; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1983,87,1264. (6)Caponetti, E.; Floriano, M. A.; Varisco, M.; Triolo, R. Instructure
and Dynamics of Supramolecular Aggregates and Strongly Interacting Colloids;Chen, S. H., Huang, J. S., Tartaglia, P., Eds.; Kluwer Academic Publisher: Dordrecht, Netherlands, 1992;p 535-555.
centration and other parameters such a s the ionic Counterion charge and dimension are known to affect the structural and the thermodynamic properties of ionic m i c e l l e ~ . ~These * ~ J ~findings suggest that micellar shape and size depend on a delicate balance between hydrophobic, hydrophilic, and electrostatic forces.l1,I2As a consequence, any variation in one of these properties is expected to alter the micellar s t r ~ c t u r e . ' ~ - ' ~ Micellar solutions are used in many industrial applications; however, systems of technological interest are usually constituted by several components rather than being simple aqueous solutions of a single surfactant. For this reason, in the last few years, the effect of various additives such as a l ~ o h o l s , ~ ~ Jurea,17 5 , ~ 5 and others on micellar properties has been investigated.l8 Recently, the effect of polyethers and polyamines on micellar properties has been ~ t u d i e d . l ~ - ~ ~ (7)Caponetti, E.;Triolo, R. IN Industrial and Technological Application ofNeutrons; Rustichelli, F., Fontana, M., Coppola, R., Eds.; North Holland: Amsterdam, Netherlands, 1992;pp 403-424. (8)Caponetti, E.; Triolo, R.Adv. Colloid Interface Sci. 1990,32,235. (9)Beer, S.S.;Colemann, M. J.; Marriot Jones, R. R.; Johnson, J . J., Jr. J . Phys. Chem. 1986,90,6492. (10)Ben,$. S.;Jones, R. M. Ph.D. Thesis, Wake Forest University, Winston-Salem, NC, 1986;pp 174-202. (11)Tanford, C. In The Hydrophobic Effect; Wiley: New York, 1980. (12)Ishraechvili, J.N.; Mitchell, B. J.; Ninham, B. W. J . Chem. Soc., Faraday Trans. 2 1978,72,1525. (13)Jones, R. R. M.; Maldonado, R.; Szajdzinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986,90,1126. (14)Baglioni, P.; Kevan, L. J . Phys. Chem. 1987,91,1516. (15)Baglioni, P.;Kevan, L. J . Phys. Chem. 1987,91,2106. (16)Izatt, R. M.;Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J.; Sen, D. Chem. Rev. 1986,85, 271. (17)Caponetti, E.;Causi, S.; De Lisi, R.; Floriano, M. A.; Milioto, S.; Triolo, R. J . Phys. Chem. 1992,96,4950. (18)Almgren, M.;Swarup, S. J . Phys. Chem. 1983,87,876. (19)Baglioni, P.;Kevan, L. J . Chem. Soc., Faraday Trans. I 1988, 84,467. (20)Evans, D. F.;Sen,R.; Warr, G. G. J . Phys. Chem. 1986,90,5500. (21)Evans, D. F.; Evans, J. B.; Sen, R.; Warr, G. G. J . Phys. Chem. 1988,92,784.
0743-746319512411-2464$09.00/00 1995 American Chemical Society
Effect of 18C6 on SDS and DTAB Micellar Solutions The selectivity shown by cyclic polyethers and polyamines toward univalent and bivalent cations is well documented and constitutes one of the interesting features which distinguish these compoundsfrom noncyclic ligands. This cation selectivity is important in many areas such as biological transport mechanisms, solubilization of salts in solvents of low polarity, and development of carriermembrane system.16 In some applications macrocyclic compounds are used along with surfactants and, since they contain hydrophilic and hydrophobic portions, they can interact with micelles either by hydrophobic or by hydrophilic interaction and, in the case of anionic micelles, also by counterion complexation. We have focused our attention on the effect of 1,4,7,10,13,16-hexaoxacyclooctadecaneon SDS and DTAB aqueous solutions. This crown ether, labeled 18C6 according to the nomenclature proposed by Izatt et a1.,16 forms stable (K = 6.6) 1:l complexes with Na+ ionsz7 resulting from strong ion-dipole interactions. Upon addition of crown ethers to SDS micellar solutions, pronounced changes in thermodynamic and structural properties have been o b ~ e r v e d as ~ ~a- consequence ~~ of the sodium-crown complex formation. The complex draws the counterion away from the sulfate head group, increasing the repulsion between head groups, thereby inducing a decrease in micellar size and a corresponding greater surface curvature.20r21The decrease of the surfactant cmc as the crown ether concentration inc r e a s e ~ has ~ ~ been , ~ ~explained ,~~ by suggesting that 18C6 is distributed between the aqueous and micellar phases.24 Since the dissociation degree increases with crown ether concentration, it has been concludedthat the crown ethersodium complex is partially associated to the micelle.24 An ESR study has suggested that the macrocyclic compound is localized a t the micellar i n t e r f a ~ e Due . ~ ~ to ~~~ the complexationin aqueous solution of anionic surfactants containing sodium counterions, it has been observed that the solubilization of 12C4, 15C5, and 18C6 crown ethers increases with increasing ring d i m e n s i ~ n in ; ~addition, ~,~~ since the interaction of the complexes with micelles is essentially electrostatic, it has been supposed that their localization in the micellar core is p r e ~ e n t e d . ~ ~ , ~ ~ The formation of complexesbetween the polyether cavity and the counterions is expected to lead to significant alterations of the micellar structural properties; however, in the literature there is scarce information on micellar shape and size in the presence of polyethers; only Evans et a1.20,21 have obtained information on the aggregation number for the SDS-Dz0-18C6 system. In order to investigate the structural effect of 18C6 on SDS micellar solutions, we have used the technique of small angle neutron scattering (SANS). Similar measurements on micellar solutions of DTAB, whose counterion cannot be complexed by macrocyclic cavities, have been used to test the hypothesis made in the SDS-D2018C6 S A N S data analysis.
Experimental Section SDS (Fluka)and DTAB (Sigma)were crystallized from ethanol and an ethanol-ethyl acetate mixture, respectively, and dried under vacuum a t 60 "C for 2 days; 18C6 ether (Sigma)and DzO (22) Baglioni, P.; Rivara-Minten, E.; Kevan, L. J.Phys. Chem. 1988, 92, 4726. (23) McManus, H. J. D.; Kang, Y. S.; Kevan, L. J.Phys. Chem. 1993, 97, 255. (24) Bakshi, M. S.; Crisantino, R.; De Lisi, R.; Milioto, S. Langmuir 1994, 10, 423. (25) Stilbs, P.J. Colloid Interface Sci. 1982,87,385. (26) Stilbs, P. J . Colloid Interface Sci. 1983, 94, 463. (27) Hoiland, H.; Ringseth, J. A.; Brun, T. J . Solution Chem. 1979, 8,779.
Langmuir, Vol. 11, No. 7, 1995 2465
4
I
A
n
2
h
?
Euo
v
C
0
0.1
QIA.'
0.2
0.3
Figure 1. Experimental (symbols)and calculated (lines) SANS differential scattering cross sections, dC(Q)ldQ, as functions of the scattering vector, Q, for SDS-Dz0-18C6 systems at different surfactant concentrations (M). Part A ([18C61 = 0): 0,0.02;0,0.03;0,0.04; e,0.09; A, 0.13;A, 0.23. Part B ([18C6] = 0.22 M): 0 , 0.02; 0 , 0.03; 0,0.04; e, 0.08; A, 0.13; A, 0.23. (Aldrich 99.8 atom % D) were used as received. Solutions were prepared by weight in approximately 2 mL quantities. The samples were sealed with a Teflon septum cap and stored a t room temperature until use. SANS data were collected on the W. C. Koheler 30 m SANS facilityz8at Oak Ridge National Laboratory (ORNL),using a 64 x 64 cm2 area detector and cell (element) size of -1 cm2. The data were corrected for instrumental backgropnds and detector efficiency. The neutron wavelength was 4.75 A(MlJ. 5%), and the sample-detector distances were 1.64 and 2.11 m, to give Q-ranges (Q 7 (437 sin 6)/J.,with 26 the scattering angle) of 0.040 < Q < 0.36 A-l and 0.095 < Q < 0.27 Awl, respectively. The coherent intensities from the sample were obtained by subtracting the corresponding solvent intensities, which formed only a minor correctionto the sample data. The net intensities were converted to absolute ( 3 5 % ) differential cross sections per unit sample volume (in units of cm-l) by comparison with precalibrated secondary standards based on the measurements of beam flux, vanadium incoherent cross section, the scattering from water, and other references materials.29 The efficiencycalibration was based on the scattering from light water, and this led to angleindependent scattering for vanadium, H-polymer blanks, and water samples of different thicknesses in the range 1-10 mm. SANS measurements were performed on SDS-Dz0-18C6 solutions a t two fixed macrocycle concentrations (0 and 0.22 M) as functions of surfactant concentration in the range 0.02-0.23 M. Measurements on DTAB-Dz0-18C6 solutions were performed a t two fixed macrocycle concentrations (0 and 0.21 M) and surfactant concentrations in the range 0.05-0.18 M. All measurements were performed a t 25.0 = 0.2i"C.
-
(28) Koehler, W. C. Physica (Utrecht) 1986, 137B,320. (29) Wigmall, G . D.; Bates, F. S. J.Appl. Crystallogr. 1986,20,28.
2466 Langmuir, Vol. 11, No. 7, 1995
6
I
I
Caponetti et al. 1
‘
A I
4
Table 1. Group Parameters Used in Modeling Experimental SANS Datad group -CH3 -CH2
1012Zbi,acm
-0.457 -0.0832 2.607 0.363 -0.435 0.677 2.483 2.846 1.915
V,b A3 10-lOe,cm-2 hydration no.
54.3 26.9 57.9 3.94 102.3 39.3 374.3 374.3 30.2
-0.842 -0.309 4.50 9.21 -0.425 1.72 0.663 0.760 6.34
-sod4 Na+ 6c 1 -N(CH3)aS Br4 18C6 Na18C6+ 1.3c DzO a Reference 34. Reference 35 and 36. Reference 27. bi, scattering length. V, volume. @, scattering length density.
2
8 . h
v
h
3 4
2
0 0
0.1
QIA.’
0.2
0.3
Figure 2. Experimental (symbols)andcalculated (lines)SANS differential scattering cross sections, dE(Q)/dQ,as functions of
the scattering vector, Q,for DTAB-D20-18C6 systems at different surfactant concentrations (M) Part A ([ 18C]= 0): 0, 0.047; 0 , 0.065; 0,0.096; 0.132; A, 0.183. Part B ([18C6] = 0.22 M): 0, 0.047; 0 , 0.065; 0 , 0.096; 0.132; A, 0.183.
*,
*,
Experimental SANS differential cross sections, dE(Q)/dQ,as functionsof Q for all differentcompositions of the systemsSDSDzO and SDS-Dz0-18C6 are reported in parts A and B, respectively,of Figure 1; the corresponding data for the systems DTAB-D20 and DTAB-Dz0-18C6 are reported in Figure 2.
Data Analysis The computation of the SANS differential cross section, on the basis of a physical model describing the system, is the basic step in the process of extracting information from SANS experimental data.’,* In the present work, micellar solutions can be modeled a s a two-phase system in which charged interacting particles (micelles) are dispersed in a medium constituted by the solvent molecules, a fraction of surfactant counterions, and the molecules of surfactant which are not associated with micelles. In such hypothesis the differential scattering cross section for the system, once the decoupling approximation has been applied, can be calculated by the following expression,6-s
where N p is the particle number density, P(Q) = (F(Q))2 is the scattering function of a single particle, SCQ) is the structure function, A(Q) = (F(QY)- (F(Q))2isa term which takes into account deviations from sphericity and/or from monodispersity where for rigorously monodisperse spheres
A(Q) = 0, and the term Cincludes the incoherent scattering and the machine background. P(Q) depends on the particle shape and dimension and on the resulting distribution of the atomic scattering length densities within the aggregate; hence, it varies from system to system. S(Q) is related to interparticle interactions and depends on the volume fraction of the micellar phase, on the radius, and on the net charge of the single particle; it was computed by means of the rescaled mean spherical approximation (RMSA) using a screened Coulombic potential plus hard sphere r e p u l ~ i o n . ~In~the ,~~ one-component macrofluid model (OCM)the counterions and solvent molecules are treated as a continuous neutralizing background which determines the screening in the system; the value of the micellar charge must be considered, to some extent, a n apparent charge. SDS-D20 System. The SDS-D20 system has been extensively studied as a model for anionic mice1le~;~J detailed models have been proposed such as that considering aggregates a s being constituted by a core plus two shells;32 for our purposes the simpler “core plus shell” model was deemed sufficient to compute P(Q). In this model the core contains the entire alkyl chains and the shell contains the charged head groups, a fraction of counterions, and hydration water molecules. It is known that, a t low surfactant concentrations, SDS micelles are usually spherical, while, on increasing the surfactant concentration, observed changes in several thermodynamic properties suggest a growth mechanism leading to elongated micelles.33 For this reason, in the present study it was assumed that micelles could deviate from spherical symmetry and, as a consequence,the (F(Q))2 and (F(Q12)terms were computed for a prolate ellipsoid by averaging the scattering amplitude @o(x) over all orientations of the aggregate with respect to the direction of the scattering vector:
where VI and el are the volume and scattering length density of the core, subscript 2 indicates the same quantities for the shell, and subscript s denotes the solvent; the scattering length densities for the various groups were computed from atomic scattering lengths34 and from volumes reported in the l i t e r a t ~ r e both : ~ ~ quantities ?~~ are summarized in Table 1 together with the hydration (30)Ashcroft, N. W.; Lekner, J. Phys. Reu. 1966,145, 83. (31)Hayter, J. B.; Penfold, J. Mol. Phys. 1981,42, 109;J. Chem. Soc., Faraday Trans. 1 1981,77,1851. (32) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983,269,1022. (33)Mazer, N. A.;Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1976, 80, 1075. (34) Bacon, G.E. InNeutronDiffraction, 3rded.; Clarendon: Oxford, 1975; p 38. (35)Immirzi, A.; Perini, B. Acta Crystallogr., Sect. A 1972,33,216.
Effect of 18C6 on SDS and DTAB Micellar Solutions
Langmuir, Vol. 11, No. 7, 1995 2467
Table 2. Parameters from Least-Squares Fits to SANS Data of SDS and of SDS + 18C6 in DzO Using the Models Described in the Texta [SDSI, mol L-’ 0.019 0.029 0.043 0.085 0.134 0.226
[18C61mol L-l
0.019 0.029 0.043 0.081 0.134 0.226
0.22 0.22 0.22 0.22 0.22 0.22
0.019 0.029 0.043 0.081 0.134 0.226
0.22 0.22 0.22 0.22 0.22 0.22
Y
monomers
€1 V, nm3 9(1) 1.2 48.2 10(1) 1.3 50.3 13.3(9) 1.3 50.6 17.2(7) 1.4 54.4 20.6(9) 1.5 56.7 17.2(4) 1.6 61.2 18C6 in the Shell 13(2) 30.9 14(1) 34.3 15W 36.4 16.2(8) 43.0 18.6(8) 46.2 16.5(6) 1.2 50.3 18C6 in the Core 13(2) 35.4 142) 39.2 15(U 41.3 16(1) 46.7 19(1) 49.3 17(1) 52.6
2,e.u.
103N,mol ples. L-’ 0.160 0.295 0.477 0.965 1.499 2.424
a
0.65(2) 1.13(3) 1.88(3) 3.31(4) 5.62(6) 9.61(9)
0.338 0.549 0.856 1.279 2.021 3.171
3.2 5.8 9.9 10.4 16 22
4 4 5 6 8 5
0.69(2) 1.22(3) 2.03(4) 3.45(6) 5.83(8) 9.60(9)
0.324 0.516 0.817 1.227 1.963 3.036
2.8 5.2 9.1 9.1 15 14
4 4 5 7 10 7
102v 0.47 0.89 1.45 3.16 5.11 8.92
Y
1 2 3 4 6 4
The fit parameters are identified by the errors in parentheses. Y , aggregation number = number of monomers/micelle. 2,Total net charge of one micelle. €1, micellar core axial ratio. V, micellar total volume. 1,volume fraction of the dispersed phase. N p ,particle number density. a,crown ether content (percent) in micellar phase. x, standard deviation of the fit.
numbers used in building up the micelle structural model. The scattering amplitude is given by the relation O&i) = 3(sin ui - ui cos ui)/ui3,where ui = Q[(€a)?p2 a; (1 - p2)10.5, p being the cosine of the angle between the direction of the long dimension and the scattering vector, ai the length of the minor semiaxis, and ci the axial ratio. In order to reduce the number of fitting parameters, the minor semiaxis of the micelle was fixed to the length of the fully extended alkyl chain; this assumption has been successfully used in interpreting SANS data of surfactant solutions whose micelles are weakly e l ~ n g a t e d . ~ The core axial ratio €1 was computed once the volume of the aggregates was known. The assumption was a constant thickness of the shell (the total minor semiaxis is defined by a2 = a1 thickness and the total major semiaxis by € 2 ~ 2= €la1 thickness); hence, €2 * €1. The adjustable parameters in the fit procedure were the total net charge of one micelle, 2, and the aggregation number, i.e, the number of monomers in one micelle, v. The total volume of the aggregate was computed from v and from the volume of the micelle constituents. N p was derived from v and from the surfacant stoichiometric concentration corrected for the cmc. The C term of equation (1)was evaluated a t high Q in the Porod region of the scattering curve from the slope of QUCQ) vs Q4.37 By use of this fitting procedure, the calculated intensities were in good agreement with the experimental data (see Figure 1A). The fitting parameters, along with derived quantities, are reported in Table 2 for all surfactant concentrations; the picture arising from these results is in agreement with literature data.32s38 SDS-Dz0-18C6 System. The ternary system with nearly equimolar (0.22M) content of surfactant and crown ether was preliminarily analyzed by assuming that the macrocyclic compound was completely localized in the aqueous phase. The results of the fitting procedure are compared with the experimental data in Figure 3Awhere the corresponding P(Q) and S ( Q )terms of eq 1 are also reported. The serious disagreement, particularly pro-
+
1
0.5
0 in h
0 v
+ +
(36) Millero, F.J.In Water andAqueous Solution; Home, R. A,, Ed.; Wiley Interscience: New York, 1982; Chapter 13, p 519. (37) Porod, G. Kolloid 2.1951,124, 831. (38)Payne, K.A.;Magid, L. J.;Evans, D. F. Prog. Colloid Polym. Sci. 1987,73,10.
1
0.5
0.1
Qli.
‘
.
0 2
0 0.3
Figure 3. Experimental (A)and calculated (solid line) SANS differential scattering cross sections, dZ(Q)ldQ, as functions of the scattering vector, Q , for [SDS] x [18C61 0.22M solution: (A) the calculated line was obtained by “core plus shell” model localizing entirely the crown ether in the aqueous phase; (B) the calculated intensities obtained by the “coreplus shell” model with a variable 7 (see text) and localizing the crown ether in the micellar core are indistinguishable from those obtained by localizing the crown ether in the shell. In both (A) and (B) the terms P(Q), multiplied by N,,, and S(Q) (see eq 1) are also reported.
nounced in the interaction peak position, indicates that the volume fraction derived from the model is lower than the experimental one. Because the volume fraction ofthe dispersed phase depends on the surfactant concentration and on the crown ether content in the micellar phase, this
Cuponetti et al.
2468 Langmuir, Vol. 11, No. 7, 1995 result clearly indicates that a certain amount of crown ether must be present in the micelles. Another model entirely localizing the macrocyclic compound in the micellar phase was tried. The disagreement between the computed intensities and the experimental data was rather serious; clearly not all the crown ether is localized in the micelles. Incidentally, it might be noted that these results show that the fitting procedure is indeed quite sensitive to the amount of crown ether in the micellar phase and, therefore, the volume fraction of the dispersed phase, q, was used as an additional adjustable parameter in the fit procedure. Due to the presence of both hydrophobic and hydrophilic moieties in the 18C6 molecules, in principle, the macrocyclic compound can be present either in the micellar shell or in the core or it can be shared between core and shell. In the hypothesis that the crown ether is localized in the micellar core, the previous assumption that micelles could assume a n ellipsoidal shape is no longer necessary and a “core plus shell” spherical model is adequate to describe the system; therefore, F(Q) was computed using the following expression:
where V, and e, have the same meaning as in eq 2, R1 and R2 are the radii of the core and of the shell, respectively, and @o(QR,)= (sin QR, - QR, cos QR,)/(QR,)3. In the hypothesis that the crown ether is localized in the shell, it was supposed to complex the sodium ion. In this case the value of 1.3 for the hydration number of Na18C6+ complexz7was used, being plausible that the cation, in order to enter the crown ether hole, must lose part of its hydration sheath. For the composition [SDS] % [18C6] % 0.22 M, a comparison between the experimental and calculated scattering cross sections under the assumption that the fraction of crown ether localized in the micellar phase was either entirely present in the shell or in the core is shown in Figure 3B. The two computed intensities are indistinguishable and in good agreement with the experimental data. Results of the fit procedures under the two assumptions are reported in Table 2. The fit with the model considering the 18C6 in the shell gave a slight deviation from sphericity (€1 % 1.2), but the values of the parameters obtained with the two models were the same within the experimental errors. Due to the indistinguishability of the results obtained under the two extreme conditions of crown ether localization, no intermediate cases were tested. Once the fitting procedure was positively established, the data a t lower SDS concentrations were analyzed. The spherical model was used also in the case of 18C6 localization in the shell because deviations from sphericity become less important in more diluted compositions. For all compositions examined the agreement between the experimental and the calculated intensities (see Figure 1B) was satisfactory. The fitting and the derived parameters obtained using the two submodels (18C6 in the core, 18C6 in the shell) are reported in Table 2. Once again, the two submodels gave values for the parameters that were the same within the experimental errors. Payne et al.3s have performed a SANS study on the system SDS-D20 in the presence of a macrobicyclic polyamine 4,7,13,16,2 1,24-hexaoxa-l,l0-diazabicyclo[8.8.8lhexacosane that, following the nomenclature proposed by Izatt et a1.,16is labeled 222. The two rings in the 222 polyamine have the same number of atoms as the 18C6 crown ether. The model used by Payne et al. in interpreting SANS data is different, but the main findings
are qualitatively in agreement with our results. A quantitative comparison can be made between the results for [SDSI % [18C61 % 0.22M and [SDSI = [2221= 0.25 M compositions since their study was performed for equimolar concentrations of surfactant and polyamine while in our study the crown ether concentration is maintained constant. The fraction of 222 macrocyclicpolyamine that has been found by Payne et al. in the SDS micellar phase (0.60)is greater than that of 18C6 found by us (0.22).The difference can be attributed to the very different Na+ complexation constants (5 x lo3for 222 and 6.6 for 18C6Y6 in agreement with the conclusion by S t i l b ~ ,using ~~?~~ Fourier transform NMR, that the distribution coefficients are related to the complexation constants. DTAB-DsO System. DTAB belongs to a class of surfactants used as a model for cationic amphiphiles. Its aqueous solutions have been studied to infer thermodyn a m i and ~ ~ structural40 ~ information. At low surfactant concentrations, DTAB micelles are spherical, but on increasing concentration ([DTAB] % 0.3 m) deviations from sphericity become evident.39 In the present work SANS data were modeled using the same ellipsoidal “core plus shell model” used for the SDS-D20 system; other models40could be used, but the general conclusions would hardly change. The quantities used for the computation of the volumes and scattering length densities are listed in Table 1. Comparisons between calculated and experimental data are reported in Figure 2A, the best-fit and derived parameters are reported in Table 3. DTAB-D20-1SC6 System. In analogy to the SDSDz0-18C6 system, the DTAB ternary system was modeled by using q, the volume fraction of the dispersed phase, a s a n adjustable parameter in the fit procedure. Again, the crown ether could be either in the core or in the shell of the aggregate. The calculated intensities for all the compositions as functions of surfactant concentration, shown in Figure 2B, agree very well with the experimental data. Corresponding values for the fitting and derived parameters are reported in Table 3.
Discussion We are now in a position to discuss the results shown in Tables 2 and 3. The volume fraction of the micellar phase obtained for the two binary systems, shown in Figure 4, increases almost linearly with increasing micellized surfactant concentration. The rate of increase of the volume fraction for the SDS system is greater because the volume of the hydration molecules compensates for the smaller SDS head group plus the fraction of counterion volume with respect to the volume of the corresponding head group and counterion fraction of the DTAB system. The addition of 18C6 causes no variations in the q trends in the DTAB-DzO system, but it causes a n increase of q in the SDS-D20 system. These results indicate that the crown ether in the DTAB-D20- 18C6system is completely localized in the aqueous phase, while in the SDS-D2018C6 system it is distributed between the micellar and the aqueous phase. The content of macrocyclic compound in the SDS micellar phase computed from the variation of the micellar phase volume fraction, reported in Figure 5 , increases on increasing the surfactant concentration. Within the experimental errors no 18C6 is localized in the micellar phase of the DTAB-Dz0-18C6 system, indicating that the interactions between the 18C6 and DTAB molecules (39)De Lisi, R.;Milioto, S. J . Solution Chem. 1987, 16, 767. (40) Beer, S. S. J . Phys. Chem. 1987, 91, 4760.
Langmuir, Vol. 11, No. 7, 1995 2469
Effect of 18C6 on SDS and DTAB Micellar Solutions
Table 3. Parameters from Least-Squares Fits to SANS Data of DTAB and of DTAB Described in the Text" [DTAB] mol L-' 0.047 0.065 0.095 0.132 0.183
[18C6] mol L-l
0.046 0.064 0.093 0.130 0.183
0.21 0.21 0.21 0.21 0.21
63.9(8) 67.4(8) 65.4(7)
0.046 0.064 0.093 0.130 0.183
0.21 0.21 0.21 0.21 0.21
60(2) 63(3) 641) 67(2) 6X1)
a
Y
monomers 69.6(8) 71.6(8) 72.3(6) 73.1(6) 73.1(6) 59.7(8)
Z e.u. el V nm3 1.26 44.4 13(2) 1.29 45.5 14(1) 1.30 45.5 16.8(8) 1.31 45.6 18.4(8) 1.32 45.5 19.6(8) 18C6 in the Shell 12(1) 1.08 37.9 13.7(2) 1.14 39.6 15.3(9) 1.15 39.9 16.6(8) 1.21 41.0 17.4(8) 1.18 40.6 18C6 in the Core 37.9 12(1) 1.08 1.10 38.8 14.6(7) 1.14 39.6 15.5(9) 1.13 40.4 16.7(9) 17.3(8) 1.18 40.6
+ 18C6 in D2O Using the Models x
102q 1.16 1.83 2.95 4.35 6.06
103Np,mol ples. L-' 0.433 0.668 1.078 1.582 2.214
a
1.03(1) 1.69(1) 2.78(1) 4.15(4) 5.95(3)
0.452 0.710 1.158 1.677 2.431
0.1 -0.5 -0.4 -3 -0.2
4 4 5 7 10
1.03(1) 1.68(1) 2.78(2) 4.09(4) 5.96(6)
0.454 0.719 1.165 1.689 2.437
0.1 -0.4 -0.3 -3 1
4 5 5 8 10
6 6 6 8 8
The fit parameters are identified by the errors in parentheses. Symbols are the same as those of Table 2. I
100
I
80 60 F N
s:
40 20
0 L
2
(C-cmc)/M Figure 4. Volume fraction of the dispersed phase, 11, vs micellized surfactant concentration(C - cmc) for the systems: 0 , SDS-DzO; 0, SDS-Dz0-18C6; . , DTAB-D20; 0,DTAB-
'
3 061$
Dz0-18C6.
50 40
I
I/'
-
/.
+
p
40
30
20
1
/
0 ' 0
I
I
0.1 (C
0.2
- cmc)/M
Figure 6. Particle number density, Np(circles),and aggrega-
tion number, Y (squares),vs micellized surfactantconcentration (C - cmc) for the two surfactants: open symbols [18C61 = 0; filled symbols [18C6] = 0.22 M. I
I d v
-
0
0.1 (C -cmc)/M
0.2
Figure 5. Amount of 18C6Naf vs micellized surfactant concentration (C - cmc) for the SDS-Dz0-18C6 system: W, computed by assuming that the all sodium ions are complexed; 0, amount present in the SDS micellar phase computed from the variation of the micellar phase volume fraction.
are not sufficient to draw the crown ether inside the aggregates. Since the core of SDS micelles is not different from the core of DTAB micelles, it can be inferred that in the case of the SDS-18C6 system, the macrocycle molecules should be localized in the shell.
The amount of 18C6Na' computed in the approximation that all sodium ions are involved in the complexation equilibrium is also reported in Figure 5; obviously it increases with surfactant concentration but, since in the whole range it is always greater than the amount of 18C6 in micellar phase, it can be concluded that all the crown ether in the micellar phase is present as sodium complex. The aggregation number and the particle number density, for the systems containing SDS and DTAB,are reported in Figure 6; in all the systems examined on increasing the surfactants concentration, both quantities increase. In the case of the SDS solutions the faster increase ofN, indicates that, on increasing the surfactant concentration, the formation of new micelles is energetically favored with respect to the increase of the aggregate
2470 Langmuir, Vol. 11, No. 7, 1995
Caponetti et al.
I
I
1
80
60
-
e
z>
0.2
i
OS4 0 1
0
-
m-
40
SDS -DTAB
20
1
0.1 (C-cmc)/M
I
0.2
1
Figure 7. Relative aggregation number V/VO vs micellized and DTAB (H) surfactant concentration(C - cmc) for SDS (0)
systems.
dimension; this preference is more evident in the systems containing DTAB, where the aggregation number increases only slightly. The main effect due to the presence of 18C6 in both systems is the reduction of the aggregation number. It has been found by conductance measurements that in the presence of 18C6 the cmc of DTAB in D20 increases from 0.017 to 0.019 M, while that of SDS in DzO decreases from 0.0076 to 0.0045 M. A comparison between the aggregation number of the systems with the crown ether, v , normalized to the aggregation number of the systems without the crown ether, yo, is reported in Figure 7 as a function ofmicellized surfactant concentration. Trends of vlvo similar to that of the DTAB-DzO-18C6 system have been observed for solutes which are localized in the aqueous phase and cause a reduction of aggregation number by altering the structural properties of the so1vent.l' The lower values of vlvo observed in the SDS-Dz0-18C6 system cannot be explained by simply invoking a solvent effect, but direct interactions between the crown ether and the micelles must be involved; this is in agreement with the conclusion that the crown ether is localized in the shell of micelles. The decrease of the aggregation number found by Payne et al.38 in SDS-D20 system as a consequence of the addition of 222 polyamine is more pronounced than in our case; this can be explained in terms of the greater amount of macrocyclic compound in micellar phase caused by the higher complexation constant. To give an image of the micelle constituents, the SDS aggregation number, the number of sodium ions in one micelle, i.e., Y - 2, and the number of 18C6 molecules per micelle are reported in Figure 8. The behavior of the number of 18C6 molecules per micelle, showing a maximum a t a given micellized surfactant concentration, confirms the hypothesis that all the 18C6 molecules in micellar phase complex the sodium ions: a t low surfactant concentration most of the sodium ions in solution are complexed, being the amount of crown ether in excess
t 0
0.1 (C -cmc)/M
0.2
Figure 8. Image of the micelle constituents vs micellized surfactant concentration (C - cmc) for the SDS-Dz0-18C6 number of sodium ions in system: 0,aggregation number; I, micelle; 0, number of 18C6 molecules in micelle.
(0.22M). With an increase ofthe surfactant concentration the number of sodium ions increases and so does the number of 18C6 molecules in the micelle. After a certain value of surfactant concentration, the increasing number of micelles causes a reduction of the number of 18C6 molecules per micelle because the amount of 18C6 in the micellar phase tends to reach a plateau. Conclusions It can be concluded that the SANS technique and the proposed calculation procedure are adequate to determine the amount of crown ether in the micellar phase, but it is not able to discriminate the region of its location in the micelle; the presence of crown ether in the core or in the shell changes the relative volumes and scattering densities, but the amount of these changes is not sufficient to give an appreciable variation in P(Q) and hence in scattering intensity. Results from SANS data analysis have shown that crown ether 18C6influences only slightly the structure of DTAB micelles, probably altering the solvent properties, but strongly affects the structure of SDS micelles. The evidence that no crown ether is present inside the DTAB micelles implies the absence ofinteraction between the DTAB micelles and 18C6 and excludes that interactions take place between the crown ether molecules and the core of the surfactant micelles, thus leading to the conclusionthat the macrocyclicmolecules interact with SDS micelles only via complex formation between the sodium ion and the crown ether. Acknowledgment. The research a t Oak Ridge was supported by the Division of Material Sciences, U S . Department of Energy under Contract No. DE-ACO5840R21400 with Martin Marietta Energy Systems Inc. The authors are grateful to the Consiglio Nazionale delle Ricerche (Progetto Finalizzato Chimica Fine 11) and to the Minister0 dell'Universith e della Ricerca Scientifica e Tecnologica (MURST) for financial support. LA950047B
