Langmuir 1992,8, 31-35
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A Study of the Structure of Mixed Cationic/Nonionic Micelles by Small-Angle Neutron Scattering Spectrometry P. G. Cummins,* J. Penfold,? and E. Staples'J ISZS Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K., and Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, U.K. Received April 26, 1991. Zn Final Form: August 7, 1991 Small-angle neutron scattering spectrometry has been used to study the geometry of mixed cationic/ nonionic micelles of hexadecyltrimethylammonium bromide (C16TAB) and hexaethylene glycol monododecyl ether (C16E6). The anisotropic interacting micelles show a temperature dependence of the micelle geometry similar to that previously observed for C16E6 alone. The results demonstrate the existance of the theoretically predicted local ordering in strongly interacting anisotropic systems.
Introduction It has been demonstrated by small-angle neutron scattering (SANS) spectrometry that many rodlike micellar systems can be readily aligned by laminar shear Despite the inherent complexity of systems whose particles do not possess integrity, the scattering data from many different systems can be interpreted using a very simple model for the micelle^.^ The picture that emerges is, however, frequently in conflict with the conclusions drawn from traditional characterization techniques, such as light scattering4~~ and viscosity,6 and at variance with recent theories of micelle asso~iation.'~~ In earlier work we have characterized several ionic micellar systemsg in the presence of excess electrolyte, and nonionic systems,'O under the influence of shear and have found that the measured scattering profiles at high shear are consistent with the existence of fairly rigid, relatively monodisperse (compared to theory) noninteracting mic e l l e ~ .The ~ lack of interactions under high shear, at concentrations where micelle overlap is expected, can be attributed to the low collision probability of aligned micelles. However, the small polydispersity and lack of flexibility as evidenced by an absence of scatter in the 811 direction and the overall shape of the 2D scattering patterngJ1 are, even if unexpected, an unambiguous interpretation of the experimental data. The rodlike nonionic system hexaethylene glycol monododecyl ether (C16E6) has a lower consolute boundary and thus permits a controlled modification of the intermicellar interactions by selection of a suitable temperature. In earlier work, we described the temperature-dependent changes in micelle geometry that result as the phase boundary is approached. In other work,ll changes in miScience Division. Unilever Research. (1) Cummins, P. G.; Staples, E.; Hayter, J. B.; Penfold, J. Chem. Phys. Lett. 1988, 149, 191. (2) Hoffmann, H.; Kalus, J.;Reizlein, K.; Ulbrickt, W.; Ibel, K. Colloid + ISIS
Polym. Sci. 1982, 260,435. (3) Hayter, J. B.; Penfold, J. J. Phys. Chem. 1984, 88, 4589. (4) Candau, S. J.; Hirsch, E.; Zana, R. J. Phys. (Paris)1984,45,1263. (5) Candau, S. J.;Hirsch, E.; Zana, R. Colloid Interface Sci. 1985,105,
-".
DSl.
(6) Imae, I.; Sasaki, M,; Ikem, S. J. Colloid Interface Sci. 1989, 127,
511. (7) Mukerjee, P.
J. Phys. Chem. 1972, 76,565. (8)Israelachvili, J. N.; Mitchell, D.; Ninham, B. J. Chem. SOC.,Faraday Trans. 2 1976, 72,1525. (9) Cummins, P. G.; Staples, E.; Hayter, J. B.; Penfold, J. J. Chem. SOC., Faraday Trans. 1 1987,83,2773. (10) Cummins, P. G.; Hayter, J. B.; Penfold, J.; Staples, E. Chem. Phys. Lett. 1987, 138, 436. (11) Cummins, P. G.;Staples, E.; Penfold, J.; Heenan, R. K. Langmuir 1989,5, 1195.
0743-7463/92/2408-0031$03.00/0
celle geometry were investigated when the micelle interactions, as evidenced by cloud point changes, were further modified by the addition of salting-inlsalting-out electrolyte. The changes in micelle geometry that arise were found to be very similar on a reduced temperature scale and could be rationalized by consideration of the surfactant headgroup area and changes in flexibility as mediated by the adsorption (positive/negative) of the ionic species. By suitable addition of a charged surfactant (e.g., hexadecylmethylammonium bromide (Cl6TAB))to C16E6 micelles, a long-range repulsive interaction can be introduced to the system while an accessiblelower consulate boundary is preserved. In this paper we describe observations on mixed micelles of C ~ ~ T A B / C Iunder ~ E ~ ,the influence of shear, as the lower consolute boundary is approached.
Experimental Section A Couette apparatus12developed from a design reported by
Oberthur and Lindner was used to maintain a constant shear flow over the area of the neutron beam. The total sample thicknesswas 1mm. Sample temperaturewas controlled to f0.2 "C, and shear rates (G)up to 25 OOO s-l were possible. The SANS spectra were recorded on either the D11 or D17 instruments at the ILL, Grenoble, France, or on the LOQ instrument at the spallation neutron source, ISIS, Rutherford Appleton Laboratory, Didcot, U.K. The neutron wavelengths used on the D11 were X = 8 8, and AXIX = 10% with sample to detector distances 4,6, and 18 m and X = 5 A, AX//X = 10% at a sample to detector distance of 2 m and on the D17 at a sample to detector distance of 3.46 m with X = 15 8, and AXIX = 10%. The D11 and D17 scans combine to give a Q range of 0.002-0.06 A-l, whereasthe LOQ (usingthe white beam time of flight method) gives in a single scan the Q range 0.005-0.25 8,-1. Hexaethylene glycol monododecyl ether (c16&)was obtained from Nikkol Chemical Ltd., Tokyo, Japan, and hexadecyltrimethylammonium bromide (C16TAB) and DzO were obtained from BDH. All were used without further purification.
Results and Discussion Before the effects of added cationic surfactant to the rodlike micelles are discussed, the earlier published results of Cl6E6 alonelo and in the presence of added electrolyte" are briefly reviewed. The reader should refer to those earlier papers for a more complete discussion and interpretation. Figure 1shows the intensity contour plot obtained from a 1% Cl6& solution in D2O at a temperature of 31 O C and at a shear gradient (G)of 5000 s-l. The intensity profiles obtained with the scattering vector in a plane parallel (811) (12) Cummins, P. G.; Staples, E.; Millen, B.; Penfold, J. Meas. Sci. Technol. 1990, I , 179.
0 1992 American Chemical Society
Cummins et al.
32 Langmuir, Vol. 8, No. 1, 1992 Effective r o d length
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I I i! I
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2000c
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10000
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Figure 2. Effective rod length as a function of reduced temperature (T,- )'2 for 1% C16Es/Dz0/0.5M NaSCN at G = 5000
- 301 0
5-1.
20
3 0 Momentum tronrfer. 0 ( 1 - l I
Figure 1. (a, top) Intensity contour plot for 1% Cl&&/DzO at 31 "C and G = 5000 5-1. (b, bottom) Scattered intensity I ( Q ) versus momentum transfer Q for 1% C1&6/D& at 31 "C and G = 5000 5-1: (a) Q1 and (0) Qil. The solid lines are calculated curves for 21 = 3400 A and 2a = 60 A. to and perpendicular (81) to the flow direction are shown in Figure lb; the solid lines are calculated profiles obtained using a previously described analysis m e t h ~ d . The ~ approach assumes monodispersed, rigid, noninteracting rods whose orientational probability can be described by the expression derived by Hayter and Penfold3 and results in the determination of an effective rod length, 21 (and width, 2a); the justification for such an approach is described in detail elsewhere.+ll It is only possible to monitor the effectivemicellar length of in water over a small temperature range as the lower consolute boundary a t low surfactant concentrations (37 "C) is close to the Kraft point (27 "C). The addition of salting in electrolyte, (0.5 M sodium thiocyanate) elevates the clouding temperature (50 "C), and it is then possible to follow the evolution of micelle geometryll over a wider range, as indicated in Figure 2. With increasing temperature, an increase in "effective rod length" is initially observed and associated intensity contour plots are consistent with rigid, rodlike micelles. Furthermore, as reported previously, there is a complete absence of small micelles. Closer to the lower consolute boundary a reduction in effective rod length is observed, and the changes in the intensity contours indicate a subtle increase in rod flexibility. The increase in rod length may arise from the dehydration-driven reduction in area per molecule; whereas changes in flexibility have been attributed to the changes from circular to elliptical cross section,13which are thought
* * * * * * *\
44 100
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0
0 005
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Figure 3. Scattered intensity Z(Q) versus momentum transfer Q as 3% C&e at 28 "C and G = 5000 5% (a) Q1and (0)1 parallel. The solid lines are calculated curves for 21 = 3300 ,
and 2a = 60 A.
f
to be a necessary precursor to the formation of the lamellar phase. In the absence of shear, and a t temperatures at which the micelle rod length is a maximum, there is no evidence of intermicellar interactions: the intensity profiles are consistent with a rodlike particle scattering factor. This is to be expected, as in noninteracting dilute rodlike systems there is no orientational coupling and micelle centers of mass can be randomly located. In the presence of significant shear, an interaction peak is observed (as indicated by the suppression of scattered intensity at low q in the Q1 direction in Figure 3) despite an expectation that rod alignment reduces micellar" collisions. On the assumption that the rod centers are on a three-dimensional lattice, the number of micelles inferred from the position (13)Tiddy, G., private communication. Conroy, J. P.; Hall, C.; Leng, C. A,; Rendall, K.; Tiddy, G. J. T.; Walsh, J.; Lindblom, G. Prog. Colloid Polym. Sci. 1990, 82, 253.
Langmuir, Vol. 8, No. 1, 1992 33
SANS Study of Mixed CationiclNonionic Micelles
L 0
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Figure 4. Isotropic scattered intensity Z(Q) versus momentum transfer 8 for 3% (CleEd2O mol % C16TAB)/D& at (+) 32, (X) 40,and (0) 48 "C. of the interaction peak is consistent with a rod length of 4000 8,. This compares favorably with the estimate of 3700 8, based on the alignment of an isolated, rigid rod in the shear field (see the calculated lines in Figure 3) and with estimates of rigid rod length determined by lowshear rheological measurement9.'4 The existence of such an interaction peak requires a long-ranged intermicellar repulsion; however, the presence of electrolyte negates an electrostatic origin, and a shear-induced structure that minimizes intermicellar collisions must be invoked. Such a structure peak further requires that the micelles be relatively monodispersed; however, it is possible that the mechanism that is responsible for the formation of structure also produces a change in the intrinsic distribution of micellar sizes. The introduction of a long-range repulsion by the addition of cationic surfactant has a profound effect on the lower consolute boundary, the small-angle neutron scattering data, and the rheology. For compositions with greater than 10 mol 7% cations, a significant interaction peak is observed even in the absence of shear. In addition to the intermicellar effects, the presence of weakly screened (no supporting electrolyte) charges is expected to increase the area per molecule at the oil/water interface, resulting in smaller mi~e1les.l~The clouding temperature varies dramatically with composition; that is, as expected the electrostatic interactions are dominant over the ethylene oxide/ethylene oxide attractions and composition fluctuations are inhibited even at the lower consolute boundary. This has been verified by us with SANS measurements by using deuterated CleTAB with H2O as the solvent. The observation that the same scattering profiles are measured with this sample confirms the compositional integrity of the micelles. At relatively high ClsTAB concentrations (20 mol % ), the forms of the scattering profiles are insensitive to shear; that is, there is no shear-induced anisotropy in the scattering patterns. This is consistent with the existence of small micelles. The scattering pattern is dominated by the strong electrostatic interparticle interactions (see Figure 4). Although there are changes with increasing temperature that may be ascribed to alterations in the strength of the effective interaction, the most pronounced change is in the position of the interaction peak. This is to good approximation simply related to the number of micelles. Hence, the evolution of this peak with temperature is consistent with some modest micellar growth as temperature is increased: a situation which has been previously ascribed to the (141Rendal1, K.; Tiddy, G. J. T.; Lal, M.; Baxandall, L. G. 6th Institutional Conference on Surfactants, New Delhi, India, August 1986. (15) Mitchell, D. J.; Tiddy, G . J. T.;Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. SOC.,Faraday Trans. 1 1983, 79,975.
Figure 5. Intensity contour plot for 3% (C16E6/5 mol % C16TAB)/D20 at 32 "C and G = 1000 s?. dehydration of the ethylene oxide groups." Assuming a rod diameter of 60 A, the aspect ratio remains 2:l (at maximum) at the measured temperatures, and as the mean micellar spacing is greater than twice the inferred rod length, it is to be expected that despite the electrostatic interactions little orientation coupling will occur. I t has been shown that the MSA theory,16which has been used extensively on other systems, can be used to parameterize the interactions present here.17 However, an analysis in terms of micelle surface charge would be invalid as the micelles are anisotropic, and the approach has not been pursued further. At intermediate mole fractions of CxTAB an analysis of the scattering data is more complicated and no adequate theory exists. However, it is still possible to extract useful information from such data. It is anticipated that a reduction in the C16TAB concentration will result in an increase in the micellar length and some enhancement of the hindering of the rotational diffusion. It is also no longer valid to assume that the position of the interaction peak reflech the micelle number density. The structure peak may arise from the distribution of mass orthogonal to the rod axis and be insensitive to rod length. That is, beyond a certain degree of anisotropy the compressibility of a rodlike system with significant repulsive interactions is only weakly dependent on rod length. In the systems we have studied that can be readily aligned and have an interaction peak, the position of the scattered intensity maximum is invariant with shear. As will be described later, the ease with which the sample orients is a function of temperature; however, a shear rate and temperature can be selected at which alignment is almost complete. This behavior is observed over a large range of compositions and the data from 3 % (C16E6/5mol % C16TAB)/D20in Figure 5 are typical. In the 8 11direction the total absence of scatter is consistent with the absence of small micelles. However, as shown in Figure 6, the peak position is invariant with shear, indicating that interactions are dominated by terms orthogonal to the rod major axis rather than the rod dimensions being unaffected by shear. Furthermore, the shape of the interaction peak is unaffected by shear, suggesting that the local alignment of micelles is preserved even in the absence of shear. In Figure 7, the dependence of the peak position with concentration C is shown. The gradient of the log-log plot shows a C1I2dependence, further confirming that the ~~
~~
(16) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (17) Penfold, J., unpublished results.
Cummins et al.
34 Langmuir, Vol. 8, No. 1,1992
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Figure 6. Scattering intensity I(Q)versus momentum transfer Q for 3% (c16&/5 mol 5% ClsTAB)/D20at T = 30 "C in (0)Q1 (0)&I[ for (a) G = 50 s-l, (b) G = 100 s-l, and (c) G = 200 s-l.
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Figure 8. Scattering for 3% (C16&/10 mol 3' 6 CI~TAB)/D&at T = 40 "C and G = 7000 s-l presented as (from top to bottom):
:0.01
(a) isometricplot, (b) intensity contour plot, and (c) intensity in the ( 0 )Q1 and ( 0 )Qll directions. 0.5
1.0
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Figure 7. log-log plot of peak position versus surfactant concentration. The solid line has a slope of 0.5.
scattering entities are rodlike with very little mass in the form of micelles shorter than the mean spacing. The C1I2 dependence has been predicted theoretically for locally aligned rods,18 it is essentially a manifestation of the (18) Wayerich, B.; DAguanno, B.; Canessa, E.; Klein, R. Faraday Discuss. Chem. SOC.1990,90,245.
reduced dimensionality of the system and indicates that even in the absence of shear there are no small micelles. It is now, of course, not possible to derive a number density simply from the position of the interaction peak, as it is for truely three-dimensional "arrays". The extent of the local ordering can be seen in Figure 8, where in the direction perpendicular to the alignment the first and second peaks in the structure factor axis (81) can be clearly seen. The existence of local ordering has been observed in other systems of strongly interacting anisotropic-shaped particles3J9and has been predicted in
SANS Study of Mixed CationiclNonionic Micelles
3ool
Intensity maximum (in orbitmry units)
t
2ool \ loot L
i
Temperoture ( ‘C 1 Intensity maximum (in arbitrary units1
loot
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Figure 9. Intensity maximum in the ( 0 )QL and (0) 811 parallel directions as a function of temperature: (a, top) 3% (C&e/B mol % ratio CMTAB)/D~O and G = TOO0 s-’, (b,bottom) 3% (ClwlOmol % ratio CleTAB)/D20 and G = lo00 d.
recent theoretical work.18 However, this is not always universally true: with systems exhibiting more complex rheological behavior Hoffman et a1.20 have associated changes in the interaction peak with the development of shear-induced structures. A contradiction is suggested from the observation that in the 8 11 direction there is a subtle movement of the interaction peak to lower values of Q as alignment is increased. This may be evidence for the existence of some small micelles; this is, the peak corresponds to a centercenter separation with the fraction of micellar material contributing to this scattering process decreasing with increasing shear. The dominance of interactions precludes the application of the simple micelle model used in the analysis of the anisotropic scattering data from C16E6 alone. However, the ease of alignment may reflect the basic micelle dimension if the kinetic units are domains of mutually (19) Kalus, J.; Hoffman, H.; Chen, S. H.; Lindner, P.J. Phys. Chem. 1989,93,4267. (20) Kalus, J.; Hoffmann, H.; Ibel, K. Colloid Polym. Sci. 1989,267, 818.
Langmuir, Vol. 8, No. 1, 1992 35 aligned rods, that is, if the dimension over which rod orientations are coupled scales with rod length. At low total concentrations (for example, 0.5% ( C l a d 5 mol 3’% C16TAB)/D20)where the interactions are smaller, the “structure” peak is both enhanced and moves to higher Q with shear. This situation is reminiscent of C16E6 alone and would indicate that there is no domain structure in the absence of shear. The possible picture that emerges is that as interactions are increased the intermicellar interactions initially dominate, imposing orientational correlations on the larger micelles while having little effect on small micelles. The application of shear not only aligns the large micellar domains but reduces the number of small micelles. For the higher total surfactant concentration the temperature dependence of the intensity anisotropy is shown for a fixed rate of shear in Figure 9a,b, where the intensity at a Q value .orresponding to the peak in the structure factor in both the 8 11 and QI directions are plotted as a function of temperature. In Figure 9 (3% (C16E6/5 mol % C16TAB)/D20),the anisotropy is seen to decrease as the lower consulate boundary is approached. By contrast the sample with 10 mol % C16TAB (Figure 8b) shows a marked tendency to increase in anisotropy over the same temperature range, tending to reach a constant value at the maximum temperature accessible in the shear cell. The rheology of the system at higher temperatures would indicate that this sample also has to become less aligned as the lower consolute boundary (>90 “C) is approached. The addition of ClsTAB results in a repulsive intermicellar potential such that higher temperatures (that is, further dehydration of the ethylene oxide groups) are required to achieve the attraction that results in micellar aggregation at the lower consolute boundaryS2l The reduction in area of c16E6 per headgroup that occurs when the temperature is elevated has been associated with the growth phase of rod micelles” of Cl6E6 in 0.5 M sodium thiocyanate. The existence of charge (C16TAB) within the micelle increases the mean curvature such that higher temperatures are required to achieve the mean curvature associated with the longest rods. Further elevation of the temperature will ultimately result in the formation of a lamellar phase driven by the continual reduction in area/molecule. The decrease in effective rod length on approaching the lower consolute boundary may be associated therefore with a change in the cross section of the short dimension of the micelle rods. Such an anistropic cross section, the precursor of a lamellar phase,I3 will allow greater micelle flexibility. I t should be noted that subtle changes in flexibility were indeed observed for C16E6 in added electrolyte.” The SANS results on the mixed cationic/ nonionic surfactants reported here do not indicate a change in cross section. However, there is the possibility that the flexibility may be mediated by only subtle changes in geometry. Although it is not possible to separate the inter- and intramicellar contributions to the scattering in such anisotropic interacting systems, the temperature-dependent changes in micelle geometry observed in pure C I ~ E ~ are also seen in the mixed C&6/C16TAB micelles despite the presence of strong intermicellar interactions. A generic behavior with temperature is thus postulated for the micelles of C&6 and its variants; a “growth phase” followed by a reduction in effective micelle dimension with increased micelle flexibility as the lower consolute boundary is approached. (21) Valaulikar, B. S.; Manohan, C . J.ColloidInterface Sci. 1985,108, 403.
