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(33) Wehry, E. L.; Ed. Modern Fluorescence Spectroscopy; .. Heyden: . London, 1976 (34) Berlman, I. B. J . Phys. Chem. 1970,74, 3085. (35) Boulu. L. G.: Patterson. L. K.: Chauvet. J. P.: Kozak. J. J. J. Chem. Phys. 1987,86, 503. (36) Pope, M., Swenberg, C. E., Eds. Electronic Processes in Organic Crystals; Clarendon Press: Oxford, 1982. (37) Wolf, H. C. In Festkdrperprobleme 4; Sauter, F., Ed.; Vieweg: Braunschweig, 1965. (38) Salcedo. J. R.:Sieaman, - A. E.: Dlott. D. D.; Fayer, M. D. Phys. Rev. &ti. 1'978, 41, 131. (39) Fayer, M. D. In Spectroscopy and Excitation Dynamics of Condensed Molecular Systems; Agranovich, V. M., Hochstrasser, R. M., Eds.; North Holland: Amsterdam, 1983; p 233. (40) Levenson, M. D.; Kano, S. S. Introduction to Nonlinear Laser Spectroscopy, revised ed.; Academic Press: New York, 1988. (41) Butcher, P. N.; Cotter, D. The Elements of Nonlinear Optics; Cambridge University Press: Cambridge, 1990. ~~
Chemlcal Shift Changes on Mlcelllzatlon of Linear Alkyl Benzenesulfonate and Oleate S . Das,* R. G. Bhirud, N. Nayyar, K. S. Narayan, and V. V. Kumar Hindustan Lever Research Centre, ICT Link Road, Andheri East, Bombay 400 099, India (Received: April 27, 1992)
The proton NMR of linear alkyl benzenesulfonate (LABS) has been studied as a function of concentration. On micellization, aromatic protons meta to the sulfonate group show a large upfield shift (0.25 ppm) whereas the aromatic protons in ortho position remain unshifted. When water is solubilized in polar core of the LABS reverse micelles in CDC13,it is aromatic protons ortho to the sulfonate group that show a large upfield shift whereas the aromatic protons in meta position remain unshifted. This difference in behavior of meta/ortho protons in aqueous and nonaqueous environments has been utilized to determine the extent of water penetration in LABS micelle. These rsults show that ortho protons exist in a polar environment while the meta protons are in a hydrocarbon environment. On comicellizationof LABS and oleate, a-CH2 protons of oleate show an upfield shift due to the ring current of aromatic group of LABS. There is no change in the LABS spectrum due to comicellization.
Introduction Linear alkyl benzenesulfonate (LABS) is one of the commercially important surfactants finding a wide range of industrial applications.' It is the largest tonnage anionic after soap. In addition, the presence of a number of isomers and the aromatic ring in the hydrophobic part make the study of this molecule extremely interesting. In recent years, the micelle structure of LABS has been investigated using a variety of techniques such as S A N S Zfluorescence spectroscopy? and NMR.4 Some of the important structural features of organization of LABS in mic e l l e ~ and ~ - ~comicellizatiod have been well studied. However, the transition from the monomeric to the micellar state particularly with respect to chemical shifts in NMR have not been investigated.6 In an early study,' Inoue and Nakagawa have reported some interesting chain length dependent changes in chemical shifts of w-(phenylalky1)trimethylammonium bromides and ascribed these as arising out of the terminal aromatic ring. Recent work in our laboratory has also indicated conformation-dependent changes on micellization in the case of sodium ricinoleate.' In view of this, a detailed NMR study of LABS was undertaken. It was also felt desirable to examine whether NMR could be used to establish comicellization. For this purpose, sodium oleate which also has a special molecular feature (a double bond in 9-10position) was selected as a cosurfactant. Several interesting features have been obtained with respect to the chemical shifts of the ortho and meta protons of LABS on micellization and a-methylene protons of oleate on comicellization, and they are summarized in this report.
Experimental Section Two types of LABS, both greater than 99% purity, have been used in these studies. C11 LABS was a mixture of all isomers 0022-3654/92/2096-7454%03.00/0
TABLE I: 'H Chemical Shift Assignments (in ppm) of 6-PK12
LABS 1 2 (below cmc) (above cmc) (0.1 mM) (100 mM) ortho 7.8 7.75 meta 7.5 7.18 PhCH (C6) 2.7 2.47 a-methylenes (C5, C7) 1.67-1.6 1.5-1.54 1.19 other methylenes (C2-C4, 1.22 C8-Cll) 1.195 0.84 terminal methyls (Cl,C l 2 ) 0.8 0.77
diff between 2 and 1 +0.05 +0.32 +0.23 -0.03 +0.025 -0.04
+0.03
and C12 LABS was a pure 6-phenyl isomer (6-PhLABS). Both the materials were obtained as gifts from Unilever Research, Port Sunlight, U.K. Oleic acid was from Aldrich Gold label (purity 99%). Sodium oleate was prepared by neutralization in DzOwith sodium carbonate to minimize the water of neutralization. The NMR spectra were taken on Varian X L 300 and Bruker W H 270 at 37 OC. No internal standard was used, and the chemical shifts were determined with respect to HDO at 4.78 ppm. The water peak was suppressed. The NMR chemical shifts are quite reproducible since the same results have been obtained with two different samples on two spectrometers of different frequencies. Results A schematic of 6-PhLABS molecule is given in Figure 1 indicating the numbering of the various carbons. The NMR spectra for 6-PhLABS as a function of concentration are given in Figure 2. The concentration range spans 0.1-100 mM (0.05-50 X cmc). 0 1992 American Chemical Society
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The Journal of Physical Chemistry, Vol. 96, No. 18, I992 7455
Micellization of LABS and Oleate
0.25
S O No
I
0
m
-
E,
2
0.1
0.05
0
II V
f I I
I .
-0 0 5
1
0
1500
2000
2500
-1
vgure 3. IH NMR chemical shift differencesof 6-PhLABSas a function of inverse concentration: (a) meta protons, (b) PhCH protons, (c) protons on C5, (d) other methylenea, (e) C12 methyl, and (f) C1 methyl.
12
Figure 1. Structure of LABS giving the numbering of carbon atoms.
La n n b -
A
c -
AI 4 wm
16
1000
I
ICONCENTRATIONIldm3 moli' 1
11
wm8o
500
I
2s
15
05
7 2
Figure 2. IH NMR of 6-PhC12 LABS at different concentrationsa-i, 0.1, 0.5, 1, 2.5, 5, 10, 25, 50, and 100 mM, respectively.
The assignments of the various peaks have been made following Bratt et al.4 A summary of the assignments and shifts at the lowest and the highest concentration are given in Table I for the sake of convenience. The chemical shifts as a function of inverse concentration for the significant protons are given in Figure 3. The following postmicellization changes in the NMR spectra of 6-PhLABS are observed. 1. Aromatic protons ortho to -SO3- group at 7.8 ppm do not show any significant change in chemical shift, and there is no line broadening after micellization. 2. Aromatic protons meta to -SO
