1493
Langmuir 1995,11, 1493-1499
A Structural Study of Mixed Micelles Containing cl6TA.B and C1& Surfactants Julie A. McDonald and Adrian R. Rennie" Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, U.K. Received September 23, 1994. I n Final Form: January 25, 1995@ Static and dynamic light scattering have been used to investigate the structure of micelles formed from mixed surfactant solutions. Aqueous micellar solutions containinghexadecyltrimethylammonium bromide (CISTAB)and hexaethylene glycol monododecyl ether (C12E6)have been investigated for this purpose at CISTAB mole fraction = 0.55. The results indicate, that as with some pure cationic surfactant systems, adding salt (KBr) to the mixed surfactant solutions initiates unidirectional growth to form elongated micelles. These micellar aggregates are not completely rigid but appear to develop some flexibility at higher salt concentrations. The growth of the micelles is primarily attributed to the screening effect that KBr exerts on the cationic surfactant headgroups. Micelle growth also occurs on increasing the surfactant concentration where salt is present. At the so-called crossover surfactant concentration, indicated by a decrease in scattering intensity and an increase in the apparent diffusion coefficient, overlap of micelle chains to form an entangled structure is possible. The onset of micelle growth is discussed in terms of scaling behavior and micelle dynamics.
Introduction There is a growing interest in the structure of mixed surfactant systems from both a n academic and industrial Mixtures are usually preferred in commercial applications not only due to the increased expense associated with pure surfactant preparations but also because mixed systems often exhibit enhanced properties through synergism. The objective of this paper is to investigate the structural properties of micelles formed from a two-component surfactant mixture. Aqueous solutions containing the cationic surfactant hexadecyltrimethylammonium bromide (cl6TAB) and the nonionic surfactant hexaethylene glycol monododecyl ether (C12E6) have been selected as the mixed surfactant system for this study. It is widely documented that micelles of cationic surfactants often undergo a sphere-to-rod transition in the presence of salt to form flexible wormlike chain^.^-^ This unidirectional growth process has been attributed to screening of the charged headgroups by the salt ions which is believed to facilitate cylindrical packing of the surfactant molecules into rods or extended aggregates. In the case of ethoxylated nonionic surfactants such as C12E6 there has been some debate on whether these surfactants form rodlike micelles. Some workers have attributed observed increases in scattering intensity with increasing temperature to the emergence of strongly attractive interactions between micelles leading eventually to a critical-type phase separation.1° Others have advocated the onset of unidirectional micellar growth into rodlike aggregates. 11-13 ~~~~~
Abstract published in Advance ACS Abstracts, April 15,1995. (1)Holland, P. M.,Rubingh, D. N., Eds. Mixed Surfactant Systems; ACS Symposium Series; American Chemical Society: Washington,DC, 1992. (2)Hoffmann, H.; Possnecker, G. Langmuir 1994,10,381. (3) Douglas, C. B.; Kaler, E. W. Langmuir 1994,10, 1075. (4)Candau, S. J.;Hirsch, E.; Zana, R. J . Phys. (Paris) 1984,45,1263. ( 5 ) Candau, S. J.;Hirsch, E.; Zana, R. J . Colloid Interface Sci. 1986, 105,521. (6)Imae, T.;Kamiya, R.; Ikeda,S. J . ColloidInterface Sci. 1986,108, @
21 5 .
(7)Imae, T.;Ikeda, S. J . Phys. Chem. 1986,90,5216. (8)Cates, M.E.; Candau, S. J. J . Phys.: Condens. Matter 1990,2, 6869. (9)Herzog, B.;Huber, K.; Rennie, A. R. J . Colloid Interface Sci. 1994, 164,370.
0743-746319512411-1493$09.00/0
In this study both static and dynamic light scattering measurements have been employed to investigate the structural properties of the CXTAB/C~ZE~ mixed surfactant system. Using a combination of light scattering techniques, it has been possible to gain information on the overall dimensions and shape of the mixed micelles which form from these surfactants. In particular, the effects of salt and overall surfactant concentration on the aggregation behavior of the micelles has been examined.
Materials and Methods The surfactants hexadecyltrimethylammoniumbromide (>99%) and hexaethyleneglycol monododecyl ether (198%)were obtained from Fluka and were used without further purification. Potassium bromide (299%)was purchased from East Anglian Chemicals. All solutions were prepared using Elgastat UHQ water. Surface TensionMeasurements. A Kriiss K12 tensiometer was used to perform the surface tension measurements. Temperature was controlled at 30.0 =! 0.1 "C using a Haake K20 water circulator. To ensure that contaminants were absent, all glassware was soaked in nitric acid, thoroughly rinsed with distilled water, and then steamed before use. Surface tension curves for the mixed surfactant solutions are shown in Figure 1. As expected the critical micelle concentration (cmc)was found to decrease as a function of increasingsalt concentration;values determined for the cmc in surfactant solutionscontaining0, 0.1, and 0.5 M KBr are summarized in Table 1. Preparation of Solutions for Light Scattering Experiments. Cylindrical quartz cells of 10 mm diameter (Hellma,
U.K.) were used in all the light scattering experiments. The cells were soaked in nitric acid, rinsed with distilled water, and finallyrinsed with freshly distilledacetone before use. Surfactant solutionswere prepared containing0, 0.1, and 0.5M KBr. The mole fraction of C16TAB in the surfactant mixture was 0.55 in all measurements. Solutions were filtered through a 0.22-pm Millipore filter directly into the light scattering cells. Light ScatteringMeasurements. Static and dynamic light scatteringexperimentswere performedon aMalvern 4700 photon correlation spectroscopy(PCS)system using a 25-mWHeNe laser emittingverticallypolarized light at a wavelength of 633 nm. All (10)Zulauf, M.;Weckstrom, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J . Phys. Chem. 1986,89,3411. (11)Lum Wan, J. A.; Wan-, G. G.; White, L. R.; Grieser, F. Colloid Polym. Sci. 1987,265,265. (12)Cebula, D. J.; Ottewill, R. H. Colloid Polym. Sci. 1982,260, 1118. ~~~. (13) Cummins, P. G.; Staples, E.; Penfold, J.;Heenan, R. K.Langmuir
1989,5, 1195.
0 1995 American Chemical Society
1494 Langmuir, Vol. 11, No. 5, 1995
McDonald and Rennie factor, and Bz is the second vinal coefficient. The parameter A& is the Rayleigh ratio of the micelles which is obtained from the difference between the Rayleighratios of the surfactant solution and the solvent medium:
60 0
(3)
= ROsoln - RBsolv
where Rosohand are the Rayleigh ratios of the solution and solvent, respectively. The constant K for light scattered without a change of polarization is
h ?
'E 50
z
E
v
K = 4n?n,2(dn/d~)2/NAA:
C
.-0
(4)
v)
c
where no is the solvent refractive index, dn/dc is the refractive index increment of the sample solution, and ;lo is the wavelength of incident light in a vacuum. The form factor term P(q) essentially describes the angular dependence of the scattering intensity and is usually expressed in terms of the scattering wave vector q given by
0
. I -
8
40
(5)
q = 47~n&~sin(8/2)
30
IO-^
IO-'
10-~
lo4
lo3
I(
1
Surfactant concentration (mol dnT3) Figure 1. Surface tension of mixed surfactant solutions at 30 "C: no Kl3r (0); 0.1 M Kl3r (+); 0.5 M KBr (0).The solid lines
are drawn to guide the eye.
Table 1. Parameters Determined for Static Light Scattering Experiments KBr concentration dnldc cmc (mol dm-3)
(cm-3 g-1)
(mol dm-3)
0 0.1
0.136 0.148 0.150
1.1 10-4 3.2 10-5 1.9 10-5
0.5
where ZOandZbl are the scatteringintensityofthe sample solution and the toluene, respectively, and Rbl is the Rayleigh ratio of toluene. For Rtolat 1 = 633 nm a value of 1.36 x cm-l was assumed.14 Refractive Index Measurements. Refractive indices of sample solutions were measured using an Abbe refractometer thermostated at 30.0 =! 0.1 "C. Values of dnldc were slightly dependent on salt content increasing from 0.136 cm3g-l with no salt to 0.150 cm3 8-l in the presence of 0.5 M KBr (Table 1).
Theory Static Light Scattering. In the Rayleigh-GansDebye limit the scattered intensity of light from a dilute dispersion of weakly interacting particles may be approximated by
+
- eo)
where (R:), is the z-average of the squared radius of gyration. Substituting eq 6 into eq 1and assuming that interactions between micelles are negligible (i.e., Bz ,.-0), then the radius of gyration can be determined from the relationship
K(c - c0)/AR, = [l + ((R,2),q2/3)1/M
measurements were carried out at 30.0 k 0.1 "C. By use of this PCS apparatus the angular dependence of the scattering could be measured over the range 30-150". In the static light scattering experiments the Rayleigh ratio of the sample solutions was determined using toluene as a standard according to the relationship
K(c - c,)/hR, = l/(MP(q)) 2B,(c
where 8 is the scattering angle. Where qR,
