Langmuir 1992,8, 460-463
460
Nuclear Magnetic Resonance Study of Polymer-Surfactant Interaction. 13C NMR Study of Polymer-Induced Non-Newtonian to Newtonian Transition in a Viscoelastic Micellar System Tuck C. Wong,’ ChangSheng Liu, and Chi-Duen Poon Department of Chemistry, University of Missouri, Columbia, Missouri 65211
David Kwoh Unilever Research Laboratory, Edgewater, New Jersey 07020 Received October 1, 1991. In Final Form: November 13, 1991 13C NMR relaxation of the surfactant molecules was used to study the effect and the resulting microstructural changes caused by the addition of a nonionic polymer poly(viny1methyl ether) [PVME] to the viscoelastic micellar system of hexadecyltrimethylammonium bromide (CTAB)/sodiumsalicylate/ water. The l3C relaxation shows that the drastic reduction of viscosity and the transition from a nonNewtonian to a Newtonian fluid are caused by the breaking down of the long rod-shaped micelles in the viscoelastic system to small and spherical micelles surrounded by the polymer PVME. Furthermore the association of the micelles with polymer is found to be rather loose. Proton chemical shifts of the salicylate ions in these systems show that the salicylate ions stay embedded in the micelles upon the addition of PVME. No difference is found in the dynamics of the polymer in the presence and absence of CTAB.
Introduction Surfactant-polymer systems have wide applications in detergent, cosmetic, pharmaceutical, and paint industries. A large number of studies of surfactant-polymer systems have been carried out in recent years.’?* The interaction between surfactant and polymer and the resulting association structure significantly affect many properties of the system, the rheological properties in particular. Recently, a rather remarkable polymer-induced transition from non-Newtonian to Newtonian behavior was observed3 in the well-known viscoelastic micellar system of hexadecyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal)/~ater.~ The polymers found to be effective are weakly hydrophobic but water-soluble polymers such as poly(viny1 methyl ether) (PVME) and poly(propy1ene oxide) (PPO). The difference between the viscosities (at low shearing rate) of the polymer-free and polymer-added systems is over a factor of lo3. This striking effect has been interpreted as being due to the breaking up of the long rodlike micelles in the viscoelastic solution upon the addition of the polymer into small spherical mi~elles.~ The spherical micelles, being “wrapped around- by the polymer molecules, become energetically favorable, because the unfavorable core-water contact is reduced. In order to investigate this effect further, particularly to investigate it from the microstructure level, we have undertaken a study of the CTAB/NaSal/water and CTAB/ NaSal/water/PVME systems by 13C nuclear magnetic relaxation. The changes in the structure between the vis(1) Robb, I. D. In Anionic Surfactants in Physical Chemistry of Surfactant Action; Lucassen-Reynders, Ed.; Dekker: New York, 1981. (2) Goddard, E. D. Colloids and Surf. 1986,19, 255; J. SOC.C o m e t . Chem. 1990,41, 23. (3) Brackman, J. C.; Engberta, J. B. F. N. J.Am. Chem. SOC.1990,112, 872. (4) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. Angel, M.; Hoffman, H.; Lobl, M.; Reizlein, K.; Thurn, H.; Wunderlich, I. Prog. Colloid Polym. Sci. 1984, 69, 12. (5) WennerstrBm, H.; Lindman, B.; Sijderman, 0.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. SOC.1979, 101, 6860. Halle, B.; WennerstrBm, H. J. Chem. Phys. 1981, 75, 1928.
coelastic and nonviscoelastic (with PVME) systems have been inferred directly from the present relaxation study.
Theory The theory and experimental approach of this work follow that of our previous work6p7closely. Basically, the motional spectral densities for the surfactant and, in some cases, polymer segments are determined by using the spinlattice relaxation time (TI) and nuclear Overhauser enhancement (NOE)( 7 ) measured at more than one magnetic field and differential line broadening (DLB).6 The detailed description of the theoretical background is given in refs 5-7. The spectral density functions thus obtained are then interpreted in terms of the two-step: and where necessary, the three-step’~~ models. All symbols have the same meanings as those given in ref. 7. Experimental Section Materials and Samples. CTAB was obtained from Aldrich Chemicals, Nasal was from Mallinkrodt, PVME in the form of 50% (by weight) aqueous solution was obtained from Aldrich Chemicals, and DzO (99.9%) was obtained from Isotec, Inc. All
chemicalswas used as received without further purification.The averagemolecular weight of PVME obtained from the same source has been determined to be -27 000 by viscosity measurement of its solution in b~tanone.~ Samples of CTAB/NaSal/water and CTAB/NaSal/PVME/waterwere prepared by adding appropriate amountsof the constituentsintoa NMR tube. The compositions of the samples are given in Table I. The water used in all solutions corresponds to a mixture with 50%:50% HzO-DzO,the latter being required for providing the field-frequency lock for NMR experiments. Samples were equilibrated for several days before being used for NMR measurements. The viscosity of the samples (6) Hwang, L. P.; Wang, P. L.; Wong, T. C. J. Phys. Chem. 1988,92, 4753. Wong, T. C.; Wang, P. L.; Duh, D. M.; Hwang, L. P. J.Phys. Chem. 1989,93, 1295. (7) Wong, T. C.; Thalberg, K.; Lindman, B.; Gracz, H. J . Phys. Chem. 1991. 95. 8850. (8) Nery, H.; Sbderman, 0,; Canet, D.; Walderhaug, H.; Lindman, B. J . Phys. Chem. 1986,90, 5802. (9) Brackman, J. C.; Engberts, J. B. F. N. J. Colloid Interface Sci. 1989, 132, 250.
0743-7463/92/2408-0460$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 461
NMR Study of Polymersurfactant Interaction Table I. Compositions and Viscosities of Samples Studied [CTAB], [Nasal], [PVMEI, sample mM mM mg/mL 7, cp I 25 15 2771b I1
I11
25
15
5
2.23 f 0.09 (23 "Qa 1.630 f 0.006 (25 "Ob
Table 11. Spin-Lattice Relaxation Time, TI(s), Nuclear Overhauser Enhancement, q, and Differential Line Broadening (Hz)for Selected Signals of CTAB/NaSal/ Water (I),CTAB/NaSal/Water/PVME (11),and PVME/ Water (111) at 35 "C I I1 I11
5
a This work. Reference 3. was measured accordingto the procedure of McGoury and Marklo using an Ostwald viscometer. NMR Experiments. All NMR experiments were performed on either a Nicolet NT-300 or Varian VXR 300 (BO= 7.05 T) spectrometer or aBruker AMX-500 (Bo= 11.75 T) spectrometer. On the NT spectrometer, a 20 mm probe was used. The 13Cd 2 pulse is 45 ps. On the other spectrometers, both a 5- and a 10mm probe were used. The 13Cu/2 pulse is 6 and 14 ps,respectively. Differential line broadening (DLB) was extracted from the coupled 13Cspectra which were obtained with the inverselygated decoupling scheme. The spin-lattice relaxation time, 2'1, was measured by using the fast inversion recovery (FIRFT)method," where the delay between acquisitions was set to 1-2 times the estimated Tl. NOE waa measured by using the dynamic NOE method.12 Broad-band noise decoupling was used on the 300MHz spectrometer and composite pulse decoupling GARPI3was used on the 500-MHz spectrometer. All measurements, except the temperature-dependent study where the temperature of the sample was varied between 35 and 45 "C, were done at 35 "C. Both the Tl and the NOE were calculated by using the threeparameter fit routine.14
Results and Discussion The 13C spectral features for the CTAB/NaSal/water samples with and without the addition of the polymer PVME (samples I and 11, respectively) are significantly different. In the N-methyl (of CTAB) region (- 52.5 ppm) of the proton-coupled 13Cspectra, DLB was very obvious for sample I (Table 11). The DLB results (Av(-1/2, 1/2) of -8 Hz a t Bo = 7 T and 13.5 Hz at Bo = 11.75T) (Figure 1) are consistent with those obtained from our previous work on the same viscoelastic system (at slightly different temperaturesh6 In contrast, for sample 11, the DLB was practically zero (Table 11). Since a DLB of 0.2 Hz would have been detectable, the difference in the DLB in the N-methyl group of I and I1 is thus at least 40-70-fold. Such degree of differences was also observed qualitatively in other parts of the spectra for signals arising from the surfactant CTAB. For example, as shown in Figure 2, the spectral region between 30 and 28 ppm in the decoupled 13Cspectrum for I1revealed well-resolved signals of many carbon segments of the CTAB hydrocarbon chain, typical of what is normally observed for surfactant hydrocarbon chains in small spherical micelles. On the other hand, the same spectral region for the viscoelastic sample I rendered rather broad and poorly resolved signals. Thus, the decoupled spectra also show qualitatively the vast difference between the line widths (l/aTz)of the two samples. The and NOE are also talongitudinal relaxation time, TI, bulated in Table 11. Contrary to the DLB and line width measurements, the differences in 21' and NOE for the two samples are relatively small, only amounting to less than 20-30% , despite the drastic difference in the apparent viscosities of the two samples. This observation is the same as that found in another surfactant-polymer system.' (10)McGoury, T.E.;Mark, H.In Techniques of Organic Chemistry; Weiaaberger, A., Ed.; Interscience: New York, 1949, Vol. I. (11) Canet, D.; Levy, G. C.; Peat, I. R. J. Magn. Reson. 1975,18,199. (12) Freeman, R.; Hill, H. D. W.; Kaptein, R. J. Magn. Reson. 1972, 7, 327. (13) Shaka, A. J.; Barker, P. B.; Freeman, R. J. Magn. Reson. 1985, 64, 547. (14) Levy, G. C.; Peat, I. R. J. Magn. Reson. 1975, 18, 500.
7.05T 11.75T 7.05T 11.75T 7.05T 11.75T
CTAB signals NCH3 c4-5a
PVME signals CH OCH3 CH2 38.6 ppm 37.4 ppm CTAB signals NCH3 c4-5a
PVME signals CH OCH3 CH2 38.6 ppm 37.4 ppm NCH3
Ti, 0.45 0.58
0.62 0.81
0.59 0.59
0.63 0.76
0.21 0.91 0.15 0.12
0.32
1.70 1.12
1.68 1.30
1.08
0.33 1.07
0.18 0.17
0.18 0.18
ll
1.77 1.26
1.76 1.36
0.83 0.75 0.67 0.73 b b 0.53 0.58 DLB, Av (-1/2,1/2),C HZ 8.2 13.5
