1195
Langmuir 1989,5, 1195-1199
light-scattering measurments on sodium oleate, that rodlike aggregates did not occur, at least in absence of added salt at 60 “C, the temperature of their measurements; they infer a hydrodynamic diameter of 46 A, in the range we find for the dimension of the total micelle. They speculate that the temperature difference might account for the disagreement with SAXS. However, our conclusions from SANS for 25-50 “C agree with theirs. In addition, Luzzati et al. did not fiid sharp differences between potassium and sodium oleates in liquid crystals.2 Although the double bonds must influence details of the packing of hydrocarbon in the micellar cores, the dependence of micellar size on length of extended chain, common with homologous series of surfactants, seems to be maintained. The fits obtained and the agreement of computed absolute intensities with experiment imply that the models are good approximations. However, the fits are not perfect, particularly for the low intensities at high angles, where oscillations in computed intensities are smeared out in the patterns. Some of the smearing arises from experiment-inhomogeneities of neutron wave length, finite size of sample, multiple scattering, etc. It is difficult (23)Stenius, P.; Palonen, P.; Stroem, G.; Oedberg, L. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; p 153.
to estimate these effects accurately. Polydispersity would have similar effects. In any case, the trends observed for unsaturation, concentration, and temperature are likely to be valid. Because the MSA version used to compute structure functions took no account of the finite size of the potassium counterions, the values of micellar charge reported are apparent but can be regarded as upper limits.u Acknowledgment. Research was sponsored by the office of Energy Utilization Research, Energy Conversion and Utilization Program (ECUT), U.S. Department of Energy under Contract No. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. We thank W. R. Busing for the computations of extended hydrocarbon chain lengths of the surfactants. Neutron-scattering measurements were carried out at facilities operated by the National Center for Small-Angle Scattering Research (NCSASR) and housed in the High Flux Isotope Reactor at Oak Ridge National Laboratory. Registry No. Potassium linolenate, 38660-45-6;potassium linoleate, 3414-89-9; potassium oleate, 143-18-0;potassium elaidate, 17378-39-1;potassium stearate, 593-29-3. (24) Naegele, G.; Klein, R.; Medina-Noyola, M. J.Chem. Phys. 1985, 83, 2560.
Geometry of Micelles of the Poly(oxyethylene) Nonionic Surfactants C16E6 and C1&8 in the Presence of Electrolyte P. G. Cummins* and E. Staples Unileuer Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, England
J . Penfold and R. K. Heenan Rutherford Applston Laboratory, Chilton, Didcot, Oxon, OX11 OQX England Receiued Nouember 28, 1988. In Final Form: April 13, 1989 Small-angle neutron scattering (SANS) has been used to investigate the geometry of micelles of the poly(oxyethy1ene) nonionic surfactants C16E.e y d ClaEa aligned in situ in a shear flow, in the presence of “salting in” or ‘salting out” electrolyte. With increasing temperature, a growth of rodlike micelles and a subsequent reduction in effective rod length are observed. This evolution of micelle rod length, accompanied by subtle changes in rod flexibility, is attributed to changes in intramicelle ethylene oxide (EO)-ethylene oxide interactions.
Introduction that small-angle It has been shown in recent neutron scattering from rod micelles aligned in a shear field can provide information on effective micelle length, polydispersity, and flexibility. In an earlier paper? we described the changes in micelle geometry that occur for the hexaethylene glycol monohexadecyl ether (C16E6)/D20system in the vicinity of the lower consolute boundary (-36 OC). In this system, the (1) Cummins, P. G.; Staples, E.;Hayter, J. B.; Penfold, J. J. Chem. SOC., Faraday Trans 1 1987,83, 2773. (2) Cummins, P. G.; Staples, E.;Hayter, J. B.; Penfold, J. J. Chem. Phys. Lett. 1987, 138, 436. (3)Herbot, L.;Hoffmann, H.; Kalw, J.; Thum, H. Neutron Scattering in the Nineties (MEA visnna); 1985, pp 501-606.
0743-7463/89/2405-1195$01.50/0
Kraft temperature (28 “C) limited the range of temperature and associated geometries that could be ~ t u d i e d . ~ Phase diagrams of poly(oxyethy1ene) (PEO) surfactants have, however, been extensively studied: and it has been established that the progression of mesophases that occur when a parameter of the system, such as the “solvent/ PEO” interaction, is changed can be rationalized with simple geometrical arguments. For example, an elevation in the temperature of such surfactants causes a dehydration-driven reduction in area/molecule (typically defined at the alkyl chain-water boundary) with the micelle in suitable systems evolving from spheres to rods (hexagonal) to disks (lamellar). Also, the addition of a ~~
(4)Kaheweit, M.; Strey, R.; Haase, D. J. Phys. Chem. 1985, 89,163.
0 1989 American Chemical Society
1196 Langmuir, Vol. 5, No. 5, 1989
"salting in or out" electrolyte has an effect on all regions of the phase diagram consistent with the ethylene oxide (EO) chain length being effectively increased (salting in) or decreased (salting In such materials, the Kraft temperature is dominated by the alkyl chain and direct EO-water interactions to such an extent that salting infout electrolytes have little effect, while the subtle modification of EOfEO interactions that can occur in the presence of electrolyte can modify the area per molecule and hence micelle geometry. In micellar solutions with long alkyl chains (compared to the EO length), this typically results in an areafmolecule such that rod micelles form, Le., 1/3 C V f A l , C lf2,where V is the volume per alkyl chain, A is the area per headgroup, and 1, is the length of the fully extended alkyl chain.5 The addition of a salting in electrolyte to C1&6/D20 therefore provides a method whereby the cloud curve can be elevated to an extent that the evolution of micelle size can be followed over a wider temperature change. By selection of suitable temperatures, it is therefore possible to compare rod micelles of identical equivalent rod length and concentration. A general observation from the scattering data of the systems studied previously is that the micelle rod length decreases with increasing temperature, for example, as reported for 0.6% C16E6.2'6 The addition of NaSCN to C1&6 and Na2S04to CI6E8results, however, in an initial growth of micelles and a subsequent reduction of effective micelle rod length with increasing solution temperature. The aim of this work is as follows: (1) To establish whether this evolution of micelle rod length is associated with a change in rod cross section (circular to elliptical, for example) or whether there is any evidence for the coexistence of disk and rod micelles on approaching the lower consolute boundary. (2) To comment on micelle polydispersity and flexibility.
Experimental Section A couette viscometer apparatus7 developed from a design reported by Oberthur and LindnerEwas used to maintain a constant shear flow over the area of the neutron beam. The total sample thickness was 1 mm, the sample temperature was controlled to 10.2 "C, and shear rates up to 25000 s-I were available. The SANS spectra were recorded on either the D l l / D 1 7 instruments a t the ILL, Grenoble, France,s or on the LOQ instrument associated with the spallation source, ISIS, Rutherford Appleton Laboratory, Didcot, UK.9 The neutron wavelengths used on D11 were A = 8 A (Ah/X = 10%)with sample to detector distances 4,6, and 18 m and X = 5 %, (Ah/X = 10%)at a sample to detector distance of 2 m. On D17 a t a sample to detector distance of 3.46 m, X = 15 %, (Ah/X = 10%). The D11 and D17 scans combine to give a Q range of 0.002-0.06 A-', whereas in contrast LOQ gives in a single scan the Q range 0.005-0.25 A-'. The actual Q range utilized for the various samples studied was within this broad range. Measurements have been made on different concentrations of the poly(oxyethy1ene)surfactants Cl& and c1& in both salting out (Na$04, NaC1) and salting (NaSCN) electrolytes, respectively. T h e poly(oxyethy1ene) surfactants (C&6, C&s) were obtained from Nikko Chemical Co. Ltd, Tokyo, Japan, and were used without further purification. In the neutron-scattering measurements presented here, the samples were subjected to a shear of 5000 s-'. This shear produces (5) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostmk, T.; McDonald, M. P. J. Chem. Sac., Faraday Trans 1 1983 79, 975. (6)Rendall, K.;Tiddy, G. J. T.; Lal, M.; Baxendall, L. G. 6th International Conference on Surfactants, New Delhi, India, August 1986. (7) Staples, E.;Cummins, P. G.; Penfold, J., to be published. (8) Oberthur, R. C.; Lindner, P. Reu. Phys. A p p . 1984, 19, 759. (9) Neutron beam facilities at the high-flux reactor available for users (Institut Laue-Langevin, Grenoble, 1983). (10)Neutron-scattering instruments at the ISIS facility, user guide (Rutherford Appleton Laboratory, UK, February 1987).
Cummins et al. Table I. Effect of Electrolyte on the Cloud Point of 1% Cis& in DsO" salt ionic strength. M cloud Doint. Oc 38 NaSCN 0.5 51 NaCl 0.5 34
" G = 5000 s-l. Effective rod length
(dl
0
8
1' +0
0
2000
t
,0001 0
I
I
2
4
I
6 (Tc-TT)
I
S ("C)
I
I
10
12
Figure 1. Effective rod length as a function of reduced tem1%C & & ,( 0 )3% Cl&6, and (+) 1% perature (T, - 2') for (0) C&6 in 0.5 M NaC1. significant orientation of rigid rod micelles greater than 100 nm in length. As a result, the probability that the micelles will collide (shear induced) is reduced, and our assumption that the rods are interacting only with the simple shear field is sufficiently justified.'*" Higher shear rates are avoided, as they can result in reduced orientation reflecting the onset of partial turbulence or micelle degradation. The general method of analysis of the scattering profiles has been described previously,' where the rod dimension is obtained from a fit to the Q1, Qll intensity c w e s (see, for example, Figure 4) and as described by eq 1-7 in ref 12. The rod lengths displayed in the figures represent a n effective average rod length, Le., the scattering is assumed to arise from monodisperse, rigid, noninteracting rods whose orientational probability can be described by using the expression derived in Hayter and Penfold."
Results and Discussion The effect of different electrolytes on the cloud point of CI6E6as measured by the onset of turbidity at a shear rate of 5000 sV1is shown in Table I. Figure 1 shows the variation of the effective rod length with reduced temperature at two concentrations, 1% and 3%, for C16E6. While the Kraft point restricts the range over which comparison can be made, it can be seen that the micelle dimensions are very similar on this reduced temperature scale. The effect of electrolyte on the cloud point can be regarded as a change in the hydration of the ethylene oxide groups resulting from the adsorption (positive or negative) of electrolyte at the interface. The invariance in the effective lengths on a reduced temperature scale of the 1%CI6E6data in the presence of elec(11)Penfold, J. J . AppE. Crystallogr. 1988,21, 770. (12) Hayter, J. B.;Penfold, J. J. Phys. Chem. 1984, 88,4589.
Langmuir, Vol. 5, No. 5, 1989 1197
Geometry of Poly(oxyethy1ene) Micelles
I (
Effecttvekmath ( % I I
I
I
4000
3000
0
0
0
0
0 0
00
2000
1
l0OC
I
0
I
10 (T,-T)
I
I
I
PI1
20 ("C)
Figure 2. Effective rod length as a function of temperature for ( 0 ) 1% C&& in 0.5 M NaSCN and (0) 3% C&6 in 0.5 M NaSCN. trolyte represented in Figure 1indicates that these electrolyte-induced changes in hydration are equivalent to those induced by temperature alone. For large micelles, it has been proposed that dispersion forces acting between the micelle cores could play a significant role in micelle-micelle intera~ti0ns.l~Since the micelle geometry scales with the reduced temperature, the packing of the ethylene oxide chains must be very similar at any given reduced temperature. This means that the lateral EO forces, the intermicelle EO interactions, and the dispersion forces (if significant) scale (or fortuitously cancel) with reduced temperature. I t is clear that these measurements provide no insight as to the significance of dispersion forces. The scaling observed does, however, confirm that the electrolyte adsorption associated with salting in/salting out effects cannot involve a well-defined interface (e.g., alkyl chain/water). Such a process would result in a greater effect on micelle geometry with intermicelle interactions affected to a lesser extent. This conclusion is, of course, not surprising in that the density of ions is very low. The addition of 0.5 M NaSCN elevates the cloud point to 51 "C,and we can now observe both the growth and decline of the effective micelle length with temperature, as shown in Figure 2. A t the lower temperature, there is an indication that for C16E6in 0.5 M NaSCN the micelle rod length increases with concentration (see Figure 2). It is, however, possible that this concentration dependence reflects the enhanced alignment induced by micelle ballistic collisions; certainly the reduced alignment and increased number density of micelles that are associated with reduced rod length will favor such processes. In support of this argument, the effective rod length reaches the same maximum value in both concentrations and involves a very high degree of orientation. Inspection of the contour plot associated with this maximum rod length (concentration 1 % ) indicates that the mass of surfactant involved in unaligned micelle rods < 100 nm is negligible (see Figure 3). This is inconsistent (13) Israelachvilli, J. N.; Mitchell,D. J.; Ninham, B.W. J.Chem. SOC., Faraday Tram 2 1976, 72, 1515.
at 28 O C . Figure 3. Intensity contour plot for (a) 1% Calculated curves for (b) monodisperse rods, 21,1600 A, 2a = 60 A, (c) with polydispersity, u = 0.4, and (d) IJ = 0.95, for a lognormal distribution.
= c
g-
-
60-
5040-
0 30U
20 10
0 0
0.005
0010
0 020
0.030
0.1
(I-') Figure 4. Intensity I (in arbitrary units) versus momentum transfer Q (in the restricted Q range O.OO6-0.04 A-') for 1% C I A at 31 O C . Solid lines are fits to the data (for parameters see Table 11), as described in the text. Momentumtransfer, 9
250 0.
b 100 0 H
50 0
10 Momentum transfer, Q
(8-l)
Figure 5. As for Figure 4, but for 3% CI6E6at 31 "c. with the polydispersity prediction of most theories of micelle a~sociation~~J* but is in agreement with the Poisson (14) Mukerjee, J. J. Phys. Chem. 1972, 76,565.
1198 Langmuir, Vol. 5, No. 5, 1989
Cummins et al.
Table 11. Parameters for C& rod inner temp, length, radius, sample 1%CIeEB .. .
3% C ~ ~ E G
1% C18E8/0.5M NaSCN
oc
A
28 29 31 34 36 38 28 29 31 33 34 35 36 38 30 34 37 40
4000 3200 3400 2950 2700 1600 3700 3600 3400 3100 2850 2500 2300 2100 1800 3050 3700 3800 3050 2650 2400 3200 3000 2750 2500 2300 1800
42
3% C&6/0.5 M NaCl
47 48 28 29 30 31 32 34
outer radius,
scale factor 18.0 & 1 29 & 2 0.61 0.57 0.37 0.35 0.25 0.28 18.01 0.58 29.34 0.51 0.45 0.44 0.44 0.33 0.48 0.27 18.0 0.56 29.34 0.58 0.51 0.50 0.33 0.57 0.39 18.0 0.31 29.34 0.39 0.39 0.42 0.42 0.33
A
A
distribution of rod lengths suggested by Ikeda et al.15 Figure 4 shows the scattering vector dependence of the absolute intensity obtained at 31 OC from a 1% sample of in directions parallel and perpendicular to the flow direction. The quality of the associated theoretical fit is good over the entire Q range. This is in contrast with the same analysis of the 3% C16& data (see Figure 5). While good fits are obtained at high Q values, at low Q the absolute intensity is significantly suppressed. Very similar intensity profiles are obtained if the solvent H20/D20ratio is varied such that the relative scattering cross section is reduced by a factor of 10. This coupled with the invariance of the spectra at different wavelength bands (using LOQ) eliminates multiple scattering as a possible contribution and the possibility exists that it results from an anisotropic structure factor. This suggests the existence of long-range repulsive interactions between the micelles. At the higher concentrations, the effect has been observed in the presence of salting in and salting out electrolyte: it is mast significant for the salting in electrolyte and least significant, though still present, with the addition of salting out electrolyte. However, it is unlikely that the repulsion is due to the intermicellar EO interaction. In that case, micelle dimensions would be significantly reduced (or the effective rod length would indicate an unusual shear dependence), and the structure perhaps reflects an ordering of the micelle that minimizes collisions and is perhaps a function of micelle flexibility. The origin of the intermicellar interaction is not clear at this stage. Examination of the absolute intensity obtained at high Q over this temperature range is consistent with our earlier work in that to obtain a good theoretical fit a scaling factor (which represents the ratio between the predicted to measured absolute intensities) has to be employed. The scale factors and micelle dimensions at the various temperatures are listed in Table 11. It was found that for the micelle diameter no obvious temperature dependence existed consistent with the mass/unit length being constant (15) Ikeda, S. J. Phys. Chem. 1984,88, 2144.
throughout this temperature range. Also, the analysis of the data collected at high Q shows that the inclusion of an elliptical cross section in the micelle geometry causes only a reduction in quality of the fit and that a cylindrical cross section can adequately model the data independent of temperature. Indeed, for the existence of rigid rods as suggested in this work, the micelle would be expected to have a highly uniform cylindrical cross section that would produce a characteristic minimum at very high Q (Q 0.15 A-1), From our data, we see no clear evidence for a well-defined minimum. The high Q data were of sufficient quality, however, to impose a more stringent limit on the values of inner (hydrocarbon) and outer (hydrocarbon EO sheath) radii of the micelle in the concentric cylinder approximation published previously.2 These values are listed in Table 11. The information available on both the long and short dimensions of the micelles indicates the value in collecting data over a wide Q range. From fits to this data, a single uniform cylinder is clearly inappropriate. The variations in scale factor from the above arguments cannot, within the model used, be attributed to changes in rod cross section, flexibility, or polydispersity and could reflect some inadequancy in the geometry of the short dimension used in the model (the rod length will be essentially unaltered, being determined from the anistropy of the scattering at any given shear). It is possible that the reduction in the hydration of the PEO results in an increased EO compressibility with a consequent increase in micelle flexibility and reduction in rod length. If this flexibility is subtle, the consequent effective rod length will be little affected while a characteristic modification of the scattering contour will be generated. (The consequences of extreme flexibility are difficult to establish as the effect of shear on internal configuration must be considered in addition to considerations of micelle lifetime and "strength".) Parts a and b of Figure 6 show the intensity contour plots obtained from 1% C16E6 in 0.5 M NaSCN at 34 and 42 "C, respectively. The effective rod length is similar in both cases, Le., 3050 and 3100 A. The enhanced scatter observed in the non Q perpendicular and non Q parallel directions in Figure 6b has been associated with micelle flexibi1ity.l As discussed earlier, the micelle polydispersity when the rod length is at a maximum is not large; however, the sensitivity of this technique to polydispersity is obviously reduced when the mean micelle size and hence the degree of orientation are smaller. Constrasting the contour plots at temperatures where similar effective mean rod lengths exist (see figure 6a,b) shows little evidence for changes in polydispersity. However, a change in polydispersity with temperature cannot be ruled out. When the effective rod length is of order 1600 A, the sensitivity to polydispersity is such that the distribution width (log normal) must be less than 0.4. It should be noted that the theoretically predicted polydispersity for such a rod approximates to a log normal distribution of width, u, of 0.65-0.95. Other observations of changes in flexibility have been made for the original 0.6% CI6E6data.2 An observable decrease in flexibility with temperature is evident for the 3% Cl6E6 data and will be discussed subsequently for the C16E8/0.5 M Na2S04data. The temperature dependence of the effective rod length of C16EB in the presence of 0.5 M Na2S04is reminiscent of the c,6&/0.5 M NaSCN data and is depicted in Figure 7. The reduced temperature at which the maximum rod length occurs is as found for C16E6/0.5M NaSCN, while the associated rod length is larger in the latter case. A close inspection of the intensity contour plots for the C@&/0.5
+
Langmuir, Vol. 5, No. 5, 1989 1199
Geometry of Poly(oayethy1ene) Micelles
Effective rod length
(A)
I a a
I
a
1000 20
30
40
Temperature ( ' C )
Figure 7. Effective rod length for CI6E8in 0.5 M Na2S04as a function of temperature (rod diameter 31 f 2 A, see comments on rod diameters of C&,).
M Na2S04data suggests an increased flexibility with increasing temperature, similar to that observed for C1&/0.5 M NaSCN.
phase behavior of CIGEnsurfactants and attribute the changes as equivalent to changes in the effective n of the (EO), chain. For all the systems investigated, consistent with earlier work3we observe a reduction in effective micelle rod length as the cloud point is approached. For C16E6/ NaSCN and Cl6E8/Na2SO4,we observe initially an increase in rod length and eventually a decrease with increasing temperature: this evolution of rod length is accompanied by increases in rod flexibility. The increase in rod length is attributed to the dehydration-driven reduction in area/molecule, whereas the subsequent decrease in length and changes in flexibility are associated with changes in the intramicelle EO-EO interaction. In the situation where rod lengths are at a maximum, our results are consistent with the existence of rigid rod micelles with a small (Poisson) polydispersity and consequently at variance with mechanistic approaches that assume isodesmic ass~ciation.'~J~
Summary We have observed the effect of added electrolyte on the
Registry No. Cl&, 5168-91-2; C&, 5698-39-5; NaCl, 7647-14-5; NaSCN, 540-72-7; Na2S04,7757-82-6.
Figure 6. Intensity contour plot for 1% Cl& in 0.5 M NaSCN (a) 34 OC; (b) 42 OC.
