J. Phys. Chem. 1987, 91, 383-389
383
Self-Association of Zwltterionic and Nonlonic Surfactants. NMR Self-Diffusion Studies Bernard Faucomprdt and Bjorn Lindman* Physical Chemistry 1, Chemical Center, Lund University, S-221 00 Lund, Sweden (Received: May 9, 1986; In Final Form: August 1 1 , 1986)
Basic aspects of the self-association of different types of zwitterionic and nonionic surfactants are investigated through Fourier transform pulsed-gradient spin-echo ‘H NMR self-diffusion studies. From the self-diffusion coefficients of surfactant, water, and solubilizate is obtained information on free surfactant concentration, micelle size, micelle hydration and intermicellar interactions. This is compared with corresponding data for ionic systems and with theoretical predictions. In contrast to ionic systems, the free surfactant concentration is constant or slowly increasing in a broad concentration range above the cmc. Micelle size and micelle hydration correspond to a maximal segregation between hydrophobic and hydrophilic parts in agreement with current models of surfactant self-association. The micelle self-diffusioncoefficient decreases proportionally to the micelle volume fraction up to high concentrations. The rate of decrease agrees with that predicted for hard spheres with hydrodynamic interactions. These observations do not apply to nonionic surfactants of the oligo(ethy1ene oxide) variety, where the micelles have a strong tendency to grow with increasing concentration and display attractive interactions growing in importance with increasing temperature.
Introduction Surfactant self-association is governed by the balance between attractive and repulsive intermolecular and intermicellar interactions. These interactions depend on surfactant molecular structure, for example, bulkiness of the nonpolar part and of the presence of charged groups, but also on other factors such as the presence of electrolyte, temperature, and the addition of nonpolar or weakly polar compo~nds.l-~Many of these effects have been documented in studies of micellization and phase behavior and theories have been worked out that account well for observations. For example, the interactions involving charged head groups can be quite well understood in Poisson-Boltzmann and Monte Carlo calculations on simple models.69 Less is understood for head groups with no net charge although much interest has been focused recently on one class of nonionic surfactants, Le., that with oligo(ethy1ene oxide) head groups.1s13 The class of zwitterionic surfactants with important biological and applied aspects gives another intriguing current research areae4,14-16 During some years we have systematically investigated the self-association of ionic surfactants by using self-diffusion techniques and have been able to characterize the counterion distribution, micelle hydration, mperativity, et^."-^ It was considered to be of interest to characterize in a similar way also other classes of surfactants. In the present article we present self-diffusion studies on some nonionic and zwitterionic surfactant systems using the recently developed Fourier transform pulsed-gradient spin-echo N M R technique:l which allows a rapid and precise determination of self-diffusion coefficients. In view of current work on the origin of hydration forces in zwitterionic lipid it was considered to be of special interest to study zwitterionic surfactants. Experimental Section Materials. Dodecylpropiobetaine, C I 2 H z 5 N + (CH3)2CH2CH2C099%), came from Calbiochem-Behring Corp. (It was noted by one of the reviewers that “Ammonio propionates have been shown to decompose reversibly to the ionic ammonium acrylate. While it seems improbable that this is true of this sample, based on the data ...”. While we saw no indications of time-dependent properties of the solutions, we have not in-
’
Present address: Lab. des SystEmes Polyphas&, Facults des Sciences, Montpellier, France.
0022-3654/87/2091-0383$01.50/0
vestigated this problem further.) Octyldimethylamine oxide (C,DAO), C8HI,N(CH3),O, was synthesized by Dr. Daniel HBtu, Department of Chemistry, University of Sherbrooke, Canada, and kindly given to us. Tetraoxyethylene glycol monooctyl ether, C8E4 (>99%), was a kind gift from Dr. Fred Schambil, Henckel, Diisseldorf, West Germany. Dodecyldimethylamine oxide (C12DAO),C12H25N(CH3)20, came from Serva Feinbiochemica, Heidelberg, West Germany. C8E4, C8DA0, C12DA0, and DDAPS were all used as received. All solutions were prepared by weight. Self-Diffusion Measurements. The self-diffusion coefficients were obtained with the Fourier transform pulsed-gradient spinecho N M R technique developed by Prof. Peter Stilbs and implanted in our laboratory by him on a Jeol FX-60 N M R spectrometer.21 We used ‘H N M R throughout and experimental conditions, data evaluation, etc., were as recommended by Stilbszl (cf. also previous papers from our l a b ~ r a t o r y ) . ~Error ~ - ~ ~limits (1) Shinoda, K.; Nakagawa, T.; Tamamushi, B. I.; Isemura, T. Colloidal Surfactants, Some Physicochemical Properties; Academic: New York, 1963. (2) Wennerstrom, H.; Lindman, B. Phys. Rep. 1979, 52, 1. (3) Lindman, B.; Wennerstrom, H. Top. Curr. Chem. 1980, 87, 1. (4) Israelachvili, J. N. Intermolecular and Surface Forces; Academic: London, 1985. (5) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1980, 77, 601. (6) Gunnarsson, G.; Jonsson,B.; Wennerstrom, H. J. Phys. Chem. 1980, 84, 31 14. (7) Jonsson,B.; Wennerstrom, H. J. Colloid Interface Sci. 1981, 80,482. (8) Jonsson, B.; Wennerstrom, H. J. Phys. Chem., in press. (9) Guldbrand, L.; Jonsson, B.; Wennerstrom, H.; Linse, P. J. Chem. Phys. 1984, 80, 2221. (10) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 915. (11) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (12) Kjellander, R. J. Chem. SOC.,Faraday Trans. 2 1982, 78, 2025. (13) Nilsson, P. G.; Wennerstrom, H.; Lindman, B. Chem. Scr. 1985, 25, 61. (14) Laughlin, R. G. Adu. Liquid Crysr. 1978, 3, 41, 99. (15) Nilsson, P. G.; Lindman, B.; Laughlin, R. G. J. Phys. Chem. 1984, 88, 6357. (16) Jonsson, B.; Wennerstrom, H. Chem. Scr. 1985, 25, 117. (17) Lindman, B.; Puyal, M. C.; Kamenka, N.; Brun, B.; Gunnarsson, G. J . Phys. Chem. 1982.86. 1702. (18) Lindman, B.; Kamenka, N.; Puyal, M. C.; Brun, B.; Jonsson, B. J. Phys. Chem. 1984, 88, 53. (19) Lindman, B.; Puyal, M. C.; Kamenka, N.; Rymden, R.; Stilbs, P. J. Phvs. Chem. 1984. 88. 5048. 120) Lindstrom; B.;’Khan, A,; Soderman, 0.;Kamenka, N.; Lindman, B. J . Phys. Chem. 1985,89, 5313. (21) Stilbs, P. Prog. Nucl. M a g i Reson. Spectrosc., in press. (22) Jansson, B., Ed. Hydration Forces and Molecular Aspects of Soluation; University Press: Cambridge, 1985. (23) Gusring, P.; Lindman, B. Langmuir 1985, I , 464. (24) Carnali, J. 0.;Ceglie, A,; Lindman, B.; Shinoda. K. Langmuir 1986, 2, 417.
0 1987 American Chemical Society
384
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
in reported self-diffusion coefficients correspond to 90% statistical confidence intervals, regarding random errors only. All measurements were performed at 25 OC with DzO as solvent. For use in the analysis of the self-diffusion coefficients, viscosities and densities were measured with standard techniques. From the densities, the partial molar volumes of the surfactants were obtained.
Results and Deduced Quantities General Aspects of the Approach Used. As the self-diffusion approach to study surfactant self-association has been fully described in previous p ~ b l i c a t i o n s , ' ~we ~ ' ~only recall some basic aspects. The approach is based on the great difference in translational mobility (over macroscopic distances) between nonassociated ("free") molecules in the bulk solution and molecules associated with a large aggregate like a micelle. The observed self-diffusion coefficient of a species is a weighted average of the different environments the molecules sample as a function of time, and, in a simple two-site free vs. micellar model generally used, the concentrations of free and micellar molecules are directly given provided the D values of the free and micellar molecules are known. The latter is given by the self-diffusion coefficient of the micelles (provided the lifetime in a micelle is not extremely low) and can be determined in studies of the diffusion of solubilized molecules entirely confined to the micelles. The precision of the micellar D values needed for evaluation of the free micellar distribution is low. However, as these values were also used to provide information on micelle size, considerable efforts were made to obtain good micellar D values by comparing different added probes, etc. From the effect of the added probes (one to four molecules per 100 molecules of surfactant, i.e., one or a few molecules per micelle) on surfactant diffusion it could be deduced that these additions have quite negligible effects on the self-association. Dfreevalues can be obtained from measurement at sub-cmc concentrations; depending on the species studied a small correction due to an obstruction effectz6 from the micelles may be required. For each system we measured the self-diffusion coefficients of water (D") and solubilizate (Ds) as a function surfactant (P), of surfactant concentration (mat)(as well as viscosities at the cmc and densities). The Ds values generally equal the micelle diffusion coefficients (D,) with good precision; see further below. The P values at low concentrations gave the self-diffusion coefficient of free surfactant, Daf.Dwfwas obtained from the self-diffusion in neat water after a correction for the micelle obstruction effect based on the volume fraction of micelles, 4. The correction used involved multiplication by the factor (1 + (4/2))-l; i.e., spherical micelles were assumed; there is little difference for small or moderately sized prolates.z6 From the P values we calculated the free, maf,and micellar, ma,, surfactant concentrations. From the Dw values the hydration number, nh, was calculated; nh is defined as the (average) number of water molecules per surfactant molecule diffusing with the micelle as a kinetic entity. nh values are inaccurate at lower concentrations and these data are not given. The concentration dependences of the D , values are fitted to simple polynoms. The infinite dilution values, Dmo,are used to calculate a hydrodynamic radius from the Stokes-Einstein equation assuming spherical micelle shape. From the micelle volume and information on the surfactant partial molar volume, &, and the hydration number is obtained the micellar aggregation number. Dodecylpropiobetaine (DPB). Self-diffusion coefficients of surfactant, water, and solubilized hexamethylsiloxane are presented in Figure 1. The molar ratio of solubilizate-to-surfactant was varied between 0.02 and 0.04 without any significant effect. The cmc was obtained to be 3.5 X mol-kg-'. Qualitatively the self-diffusion behavior of DPB is quite similar to that of ionic surfactants with monotonic decreases in 08,D , ( 2 5 ) Olsson, U.; Shinoda, K.;Lindman, B. J . Phys. Chem. 1986,90,4083.
( 2 6 ) Jonsson, B.; Wennerstrom, H.; Nilsson, P. G.; Linse, P. Colloid Polym. Sci. 1986, 264, ll.
L
o
I
I
1
I
a05
aio
015
020
I,,
"
I
I
I
I
05
10
15
20
m,, mollkg
Figure 1. Self-diffusionbehavior of dodecylpropiobetaine solutions (in D20at 25 " C ) . Water (A),surfactant (O),and solubilizate (0)diffusion coefficients (Din m2 s-l) are shown as a function of surfactant concen-
tration. 2 x lo-a 108
water Dodecyldlmethylammoniopropaneaultonate
5 10"
I
n
5 x lo-"
3 d 2
mn 10." 0
rolublllsate (micelle)
I
I
0.5
1.0
1
1.5
2.0
2.5
m, , mollkg
Figure 2. Self-diffusionbehavior of dodecyldimethylammoniopropanesulfonate (D20, 25 "C). Water (A),surfactant (O), and solubilizate (0) diffusion coefficients (Din m2s-') are shown as a function of surfactant
concentration. and Dw with increasing concentration. In this case we put D = D, without any observable error; Da is higher throughout than Ds. At higher concentrations DB and D, become approximately equal since most of the surfactant molecules are micellized. The mz SKI, infinite dilution values are Dao = (5.45 f 0.35) X Dwo = (21.9 f 0.35) X m2 s-l, and Dm0= (1.007 f 0.027) x IO-'' m2 s-1. The hydrodynamic radius (24.1 A) obtained from Dm0gives with the partial specific volume (0.97 mL-g-') a micelle aggregation number of 62 f 6. The hydrodynamic radius is close to the length of the extended surfactant molecule (ca. 24.6 A). The concentration dependence of D , can be fitted to D , = 1.019(f0.009)(1 - 1.59(f0.05)$) X mz s-l. The concentration of the nonmicellar surfactant, which can be determined with good precision only at lower concentrations, was found to be roughly constant at (4.4 0.5) X mol-kg-' in the range 1 cmc-10 cmc. The hydration number, which can be obtained only at higher concentrations, was determined to be constant at 16 f 3 in the range 0.1-1.0 molqkg-'. Dodecyldimethylammoniopropanesulfonate(DDAPS). Selfdiffusion coefficients of surfactant, water, and solubilized p-xylene or tetramethylsilane are presented in Figure 2 and Table I. The molar ratio of solubilizate to surfactant was varied between 0.015
*
The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 385
Self-Association of Surfactants TABLE I: DodecyldimethylammoniopropanesulfonateSystem: Observed Self-Diffusion Coefficients (1O-Io m* s-l) of Surfactant (D'), Water (D"),and Solubilizate (D') as a Function of Concentration in
zX1oQ
10'
D20'
5, lo-n
4.71 X lo4 9.88 X 10' 2.01 x 10-3 3.92 x 10-3 6.06 x 10-3 7.962 X lo-' 1.00 x 10-2 1.966 X 4.00 X 5.97 x 10-2 7.92 X 0.100 0.100 0.150 0.203 0.281 0.300 0.383 0.487 0.600 0.648 0.846 0.846 0.999 1.187 1.187 1.406 1.553 1.787 2.063 2.498
5.94 (8.7) 5.36 (3.3) 4.57 (1.6) 3.87 (10) 2.84 (2.9) 2.47 (2.1) 2.24 (2.4) 1.65 (2.6) 1.23 (2.8) 1.32 (1.9) 1.20 (5.2) 0.975 (1.3) 1.015 0.841 (2.4) 0.797 (2.9) 0.709 (1.9) 0.750 0.677 0.586 (2.2) 0.496 (3.2) 0.477 (2.4) 0.361 (3.6)
22.00 21.69 21.43 21.38 21.54 21.27 21.12
0.339 (2.7) 0.241 (5.7)
15.13 (0.8) 13.60 (1.4)
0.194 0.175 0.129 0.098 0.062
12.25 (0.7) 11.65 (2.7) 11.53 (1.5) 10.35 (1.1) 9.50 (0.7)
(3.1) (2.8) (4.7) (6.7) (14)
(0.3) (0.7) (1.1) (0.7) (0.6) (0.7) (2.1)
3.10-
5
2.10a
E 0
5.10'' 3.10''
20.65 (0.4) 21.10 (2.9) 20.14 (0.6) 19.94 (3.3) 19.28 (0.6) 18.00 (2.8) 18.40 (1.1) 17.22 (1.0) 15.81 (1.0)
I
0.81 (2.1) 0.829 (0.7) 0.795 (6.2) 0.726 (3.9) 0.681 0.599 0.516 0.463 0.446 0.397 0.342 0.291 0.282 0.260 0.183 0.168
I
0
0.05
I
I
I
0.10
015
020
I,, "
I
I
I
1
0.5 1.0 1.5 2.0
m, , mollkg
Figure 3. Self-diffusion behavior of dodecyldimethylammonioethanesulfonate (in D20at 25 "C). Water (A),surfactant (D),and solubilizate (0)diffusion coefficients ( D in m2 s-') are shown as a function of surfactant concentration.
(3.2) (2.5) (3.1) (2.5) (5.8) (3.0) (7.3) (9.6) (5.7) (1.9) 5.16"
"Random errors in percent in parentheses.
and 0.06 with only small effects (
