J . Phys. Chem. 1984,88, 5391-5397 explained by these spurious adsorption kinetic effects. So far all models which include kinetic effects assume linear isotherms, an assumption we have proven to be rather unrealistic. The convolution of all these phenomena makes the problem extremely difficult to solve, however. Work is in progress in these different areas and results will be reported later.
Acknowledgment. The technical assistance of Guy Preau is greatly appreciated. We thank Francesco Dondi (Ferrara) for his help in performing the calculations for the step and pulse method. Glossary U peak area (eq 35 of preceding article) total area of adsorbent surface in the column A Two-dimensional second virial coefficient (eq 10) B2D C concentration (pmol L-I) of solute at z and t (eq 3) average concentration of carrier gas (eq 16 of preceding CV article) solute concentration at peak maximum (eq 43 of preceding CM article) height of the sample injection pulse (eq 29a of preceding CO article) c,,c2, c, coefficients of the virial model of the adsorption isotherm (eq 8)
D
global or apparent diffusion coefficient in the gas phase (eq 1 of preceding article) auxiliary variable: De/(K1 + e) (eq 3b) outlet carrier-gas flow rate (eq 1) average plate height (eq 46 of preceding article)
D' FO R
5391
James and Martin factor (eq 2) first coefficient of the two-term expansion of the isotherm (es 6) second coefficient of the two-term expansion of the isotherm (eq 4) Limit capacity factor for zero sample size (eq 6 ) column length width of the sample pulse (eq 29a); sample size, CoLo number of moles of solute in the gas phase at equilibrium (eq 4) number of moles of solute in the stationary phase at equilibrium (eq 4) average column pressure solute partial pressure inlet to outlet pressure ratio molar heat of adsorption (kJ mol-') ideal gas constant (eq 7 ) column absolute temperature (eq 7 ) time; time origin at the injection of solute (eq 3) retention time, time of the peak maximum (eq 5 ) retention time of a nonretained compound (eq 6) limit retention time, obtained for a zero sample size (eq 5) auxiliary variable: uo/(l + K , / E )(eq 3a) outlet carrier gas velocity, under steady-state conditions (eq 34 column volume available to the gas phase (eq 1) abscissa along the column (eq 1 of preceding article) V o / Adefined in eq 19 of preceding article auxiliary variable: z - Ut (eq 3c) auxiliary variable: 2[(K2cv - Kl)/(Cv(e - K,))] (eq 3c) auxiliary variable: (XU/D?LoCo (eq 3d)
Mixed Micelles of Nonionic and Ionic Surfactants. A Nuclear Magnetic Resonance Self-Diffusion and Proton Relaxation Study Per-Gunnar Nilsson and Bjorn Lindman* Physical Chemistry I , Chemical Center, Lund University, S-220 07 Lund, Sweden (Received: February 6, 1984; In Final Form: April 25, 1984) The surfactant self-diffusion coefficient of mixed micellar solutions of ionic and nonionic surfactants has been measured under various conditions by the NMR pulsed field gradient technique. In addition, the line widths of the proton NMR signals have been monitored. The systems investigated are C12H25(OCH2CH2)50H (C12E5)/C12H25S04-Nat(SDS)/D,O, C,zE5/ClzH25N(CH3)3+C1(DTAC)/D20, and C,2H25(OCH2CH2)80H (C12E8)/SDS/D20. In the experimental series, the molar ratio D 2 0 to surfactant (ionic + nonionic) was kept constant while the surfactant mixing ratio was varied. For the C12E5 systems, the surfactant self-diffusion coefficient goes through a minimum when the surfactant mixing ratio is varied between 0 and 100% ionic surfactant. The observed decrease in self-diffusion coefficient as one starts to replace the nonionic surfactant by ionic surfactant is interpreted to mainly be due to an increased micelle-micelle repulsion. Then a diffusion mechanism in which monomers can be exchanged between different aggregates is partly inhibited. Such a mechanism is important for pure nonionic micellar solutions at temperatures close to the cloudpoint temperature, because then attractive interactions between the aggregates are present. The increase in self-diffusion coefficient occurring at higher fractions of ionic surfactant is shown to be due to a decrease in micelle size. For the C12E8system, the effect of the surfactant mixing ratio is much weaker which can be understood by considering the molecular geometry (large headgroup area) and the fact that the experimental temperature is far below the cloud-point temperature. Therefore repulsive interactions between the micelles are present also in the absence of ionic surfactant. The proton NMR line widths correlate well with the self-diffusion coefficients and broadening of the alkyl chain methylene signals is found when the self-diffusion coefficient is low. The broadening is interpreted to mainly be due to a partial inhibition of motions existing in the pure nonionic micellar solution which average the proton-proton dipolar couplings. The effect of various parameters, such as temperature and total surfactant concentration, on the self-diffusion coefficients and proton NMR line widths has also been investigated and interpreted according to the model described above. The dependence of the self-diffusion coefficient upon critical fluctuations is also briefly discussed. Small additions of ionic surfactant dramatically increase the cloud-point temperature but negligibly affect the observed self-diffusion coefficients. It is suggested that the micellar growth observed with increasing temperature for certain nonionic surfactant systems is determined by the absolute temperature rather than the distance from the phase separation limit.
Introduction Nonionic surfactants of the poly(ethy1ene oxide) variety are known to form mixed micelles with ionic surfactants in aqueous solution.'-6 Depending on the particular surfactants chosen as (1)
Ruriyama, K.; Inoue, H.; Nakagawa, T. Kolloid Z.Z.Polym.
1962,
183, 68.
0022-3654/84/2088-5391$01 S O / O
well as their mixing ratio, the total surfactant concentration, temperature, etc. one expects the micelle size and shape as well (2) Corkill, J. M.; Goodman, J. F.; Tate, J. R. Trans. Faraday SOC.1964, 60,986. ( 3 ) Schick, M. J.; Manning, D. F. J . Am. Oil Chem. SOC.1965, 43, 133. (4) Tokiwa, F.; Moriyama, N. J . Colloid Interface Sci. 1969, 30, 338.
0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
Nilsson and Lindman
100 v-
'In F(
E
-0
n 7
60
50
20
40
A
N
10
I
v
5
s
3
Q
2
20
1 ,
0
l
20
,
I
40
,
O
60
I
I
80
"
1
100
I
I
I
I
I
40
20
0
I
I
I
% SDS Figure 1. Surfactant self-diffusion coefficients vs. surfactant mixing ratio,
SDS/(SDS + Cl2E5) (weight percent), for the system C12E5/SDS/D20. The molar ratio of D20 to surfactant (ionic + nonionic) was 140. The experimental temperatures are given in the figure. as intermicellar interactions to vary. The information in the literature about the size and shape of the mixed micelles under various conditions is, however, very sparse. In order to provide such information we have therefore in the present work investigated mixed micellar solutions of nonionic and ionic surfactants using the N M R self-diffusion and proton N M R line-width methods. Experimental Section Pentaethylene glycol dodecyl ether (Ci2E5) and octaethylene glycol dodecyl ether (Ci2Es)of high quality were obtained from Nikko Chemical Co Ltd., Tokyo. Sodium dodecyl sulfate (SDS) ("fur biochemische Zwecke") was obtained from Merck and dodecyltrimethylammonium chloride (DTAC) was obtained from Eastman. The solvent used was DzO (>99.7 atom % isotopic purity) obtained from Ciba-Geigy or Norsk Hydro. All components were used without further purification. Solutions were prepared by weighing the components. The nuclear magnetic resonance self-diffusion studies were performed on a Bruker 3228 pulsed N M R spectrometer using 'H N M R at 60 M H z for the surfactant diffusion. The pulsed magnetic field gradient technique developed by Stejskal and Tanner' was used. For all samples, a single-exponential decay of the echo amplitude was observed. The fact that the surfactant self-diffusion can be described by a single self-diffusion coefficient is a strong evidence for mixed micelle formation. The proton N M R spectra for the line-width studies were recorded on a Jeol MH-IO0 spectrometer operating in the continuous-wave (CW) mode at 100 MHz. The experimental details of the N M R self-diffusion and linewidth studies have been described more fully in a previous paper.* Results The investigated systems consist of a nonionic surfactant, an ionic surfactant, and water (D20). The following systems were investigated ClzE5/SDS/D20,C,2E5/DTAC/D20,and Cl2E*/ SDS/D20. In each experimental series, the molar ratio of D 2 0 to surfactant (ionic + nonionic) was kept constant while the surfactant mixing ratio, Le., ionic surfactant to nonionic surfactant, was varied. The surfactant mixing ratio is given in weight percent ionic surfactant, unless otherwise indicated. The experimental temperature is 25 OC and the molar ratio of D 2 0 to surfactant is 140 unless otherwise indicated. Figures 1-3 show experimental results for the system Ci2ES/SDS/D2O. Figure 1 shows the surfactant self-diffusion ( 5 ) Tokiwa, F.; Aigami, K. Kolloid Z . Z . Polym. 1970, 239, 687.
(6) Scamehorn. J. F.: Schechter, R. S; Wade. W. H. J . Dispersion Sci. Technol. 1982, 3; 26 1. (7) Stejskal, E. 0.;Tanner, J. E. J . Chem. Phys. 1965, 42, 288. (8) Nilsson, P.-G.; Wennerstrom, H.; Lindman, B. J. Phys. Chem. 1983, 87, 1377.
I
I
80
60
100
% SDS Figure 2. Line widths at half-height of the main proton NMR signal of the methylene groups for the system C12E5/SDS/D20 plotted vs. sur-
factant mixing ratio, SDS/(SDS + C12E5) (weight percent). The molar ratio of D20 to surfactant (ionic + nonionic) was 140. The experimental temperatures are given in the figure. 100
50 v-
20
'In c4.
10
E5 -0
7 n
2 1 ..J
I
I
0
10
5
15
I
I
r
20
25
30
- - I
35
% SDS Figure 3. Surfactant self-diffusion coefficients vs. surfactant mixing ratio, SDS/(SDS + C12E5) (weight percent), for the system C12E5/SDS/D20. The molar ratios of D,O to surfactant (ionic + nonionic) are given in the figure. The experimental temperature was 25 "C.
100
1
20 10 5 2
I
0
20
40
60
80 Yo
100 DTAC
Figure 4. Surfactant self-diffusion coefficients vs. surfactant mixing ratio, DTAC/(DTAC + CI2E5)(weight percent), for the system CI2E5/ DTAC/D20. The molar ratio of D 2 0 to surfactant (ionic nonionic) was 140 and the experimental temperature 25 "C.
+
coefficientsat three different temperatures. In Figure 2, the proton N M R line widths (at half-signal height) for the main methylene signal of the alkyl chain are given at four different temperatures. In Figure 3 the surfactant self-diffusion coefficients as a function of SDS content are presented for five different molar ratios of D 2 0 to surfactant. Figures 4 and 5 show the surfactant selfdiffusion coefficients and proton N M R line widths, respectively, for the system ClzE5/DTAC/D20. In Figure 6 the corresponding
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5393
Mixed Micelles of Nonionic and Ionic Surfactants 25 20
N
+
15
I I
P 2
10 5 y
r
0
I
I
20
I
,
40
I
I
60
,
I
,
80
I
100
% DTAC Figure 5. Line widths at half-height of the main proton NMR signal of the methylene groups for the system C12E5/DTAC/D20plotted vs.
surfactant mixing ratio, DTAC/(DTAC + CI2ES)(weight percent). The molar ratio of D 2 0 to surfactant (ionic + nonionic) was 140 and the experimental temperature 25 OC.
54
;
n
centrations intermicellar interactions also give an important contribution.) Other things being equal, a bulky headgroup will favor small spherical aggregates while a small headgroup will favor larger rodlike or disklike aggregates. In the present investigation all surfactants have a hydrocarbon chain with 12 carbons and, at a constant molar ratio D 2 0 to surfactant (ionic nonionic), changes in the surfactant mixing ratio can be seen as essentially only a replacement of one type of headgroup with another. In a micelle consisting of a nonionic surfactant with a rather bulky headgroup, the steric repulsion in the headgroup region is reduced on incorporation of the less bulky ionic headgroups. At a higher fraction of ionic surfactant, when several ionic amphiphile molecules are incorporated into each micelle, the electrostatic repulsion between the ionic headgroups becomes important and the effective headgroup area starts to increase with increasing incorporation of ionic surfactant. From this simple reasoning one would expect the micelle size to pass through a maximum when the surfactant mixing ratio is varied. In some cases, the reduction in effective headgroup area when an ionic surfactant replaces a nonionic surfactant may not be large enough to alter the micellar size and shapc significantly before the electrostatic repulsions between the ionic headgroups become important. In such cases one expects only an insignificant growth of the micelles. If the nonionic surfactant micelles are long and rodlike the micelles may become more extended and less flexible with the introduction of charges because of the repulsive interaction between different parts of the aggregate. The situation is similar to the uncoiling of a polyelectrolyte occurring when the degree of ionization is increased (for example, due to a change in pH). The intermicellar interactions will, of course, also be affected by a change in the surfactant mixing ratio and an increased repulsion is expected when the fraction of ionic surfactant is increased. It will be seen below that intermicellar interactions are important to consider when the experimental data are interpreted. Self-Diffusion Data. In a previous study8 we investigated the concentration and temperature dependences of the surfactant self-diffusion coefficients for aqueous solutions of some nonionic surfactants. The self-diffusion coefficients were found to decrease rapidly with increasing surfactant concentration until a minimum is reached after which there is a slow increase in the self-diffusion coefficient toward the value for the pure liquid surfactant (possibly interupted by the formation of liquid crystaline phase(s)). The location of the minimum is at a lower surfactant concentration the higher the temperature. Our interpretation of these findings is that the observed self-diffusion coefficient is dominated by aggregate diffusion, Le., diffusion of closed surfactant aggregates in a continuous (aqueous) medium, at low surfactant concentrations, while at high surfactant concentrations the observed self-diffusion coefficient is dominated by molecular diffusion, Le., diffusion of surfactant molecules in surfactant continuous domains. The explanation of the occurrence of a minimum in the selfdiffusion coefficient is that there is a gradual changeover in the diffusion mechanism. In an intermediate region the situation is complex but one can imagine that monomer exchange between mutually attracting aggregates close in space becomes more and more important the higher the surfactant concentration causing the different aggregates to gradually merge into a surfactant continuous domain. (As regards the contribution from intermicellar exchange to the observed diffusion behavior it should be observed that the diffusion time in our work is 52 ms corresponding to displacements of the order of 10" m. Therefore, intramicellar motions cannot contribute to the self-diffusion coefficients in the absence of fusion and fission processes.) It is often ~ u g g e s t e d ' ~ , ' ~ that the resulting interaction when two nonionic surfactant aggregates approach each other will become more and more attractive the higher the temperature. When the attraction becomes sufficiently strong phase separation occurs (the clouding phe-
0 0
20
40
60
80
100
% SDS Figure 6. Surfactant self-diffusion coefficients and line widths at halfheight of the main proton NMR signal of the methylene groups for the system C,,E,/SDS/D,O plotted vs. surfactant mixing ratio SDS/(SDS C12E,)(weight percent). The molar ratio of D20to surfactant (ionic nonionic) was 140 and the experimental temperature 25 OC.
+ +
information for the system C12E8/SDS/D20is presented. Figure 7 shows selected proton NMR spectra for the different systems investigated. For an assignment of the different signals see ref 9 and 10. Figure 8 shows the surfactant self-diffusion coefficients for the systems C I 2 E 8 / D 2 0(from ref 8) and C,2E8/SDS/D20 as a function of surfactant concentration at two different temperatures. For the latter system the surfactant mixing ratio is 1.0 wt % SDS for all samples.
Discussion Mixed Micelles of Ionic and Nonionic Surfactants. General Considerations. For ionic surfactants, micelle formation is governed by the balance between the tendency of the alkyl chains to avoid contact with water and electrostatic repulsions between the headgroups, whereas for nonionic surfactants of the poly(ethylene oxide) variety the tendency for the alkyl chains to avoid contact with water is balanced by the hydration and space requirement of the poly(ethy1ene oxide) chains. The preferred size and shape of the micelles are mainly determined by the relative contributions to this b a l a n ~ e . ~ . ~(For ~ - ' ~high surfactant con(9) Christenson, H.; Friberg, s. J . Colloid Interface Sci. 1980, 75, 276. (10) Ulmius, J.; Wennerstrom, H. J . Magn. Reson. 1977, 28, 309. (11) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. SOC., Faraday Trans. 2 1976, 72, 1525. (12) Mitchell, D. J.; Ninham, B. W. J . Chem. SOC., Faraday Trans. 2 1981, 77, 601. (13) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P.J . Chem. Soc., Faraday Trans. 1 1983, 79, 975.
(14) Kjellander, R.; Florin, E. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 2053. Kjellander, R. J . Chem. Sot., Faraday Trans. 2 1982, 78, 2035. (15) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982, 260, 1023.
5394 The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
i
1
4 C12E5 +DTAC
Figure 7. Selected proton NMR spectra for the investigated systems. Spectra 1-3 refer to the system CI2E5/SDS/D2O. The surfactant mixing ratios are 9.8%SDS, 39.2% SDS, and 78.5%SDS, respectively. Spectra 4-6 refer to the system C,,E5/DTAC/D20. The surfactant mixing ratios are 10.5% DTAC, 39.5% DTAC, and 79.6% DTAC, respectively. Spectra 7-9 refer to the system CI2E8/SDS/D20.The surfactant mixing ratios are 9.9%SDS, 38.7% SDS, and 79.5% SDS, respectively. The molar ratio D,O to surfactant was 140 for all spectra and the experimental temperature 25 OC. The scale is the same for all spectra. I vu
50 v'u)
20
"E 10 (u-
v-g 5 0
2
1 .5 20
0
40
60
80
100
1
1
0
5
I
10 15 % surfactant
% surfactant Figure 8. Self-diffusion coefficients for the systems C12Es/D20 and Cl,Es/SDS/D20 vs. weight percent surfactant (ionic + nonionic). For the latter system the surfactant mixing ratio SDS/(SDS + C&) was 1.0 wt. % SDS for all samples. The experimental temperatures are given in the figure.
nomenon). Because of the stronger intermicellar attraction at higher temperatures, different aggregates can approach each other closer making the molecular diffusion mechanism more important and thus causing the minimum in self-diffusioncoefficient to occur at a lower surfactant concentration. Support for the idea that an increase in temperature results in an increasing attractive interaction between the aggregates is given in Figure 9 where the self-diffusion data of Figure 3 are plotted vs. weight-percent surfactant (ionic nonionic). It is seen that as the fraction of ionic surfactant is increased the minimum in self-diffusion coefficient is moved toward higher surfactant concentration. If Figure 9 is compared with Figures 2 and 7 of ref 8 it may be inferred that a decrease in temperature for the pure nonionic surfactant system produces qualitatively the same effect as an increase in fraction of ionic surfactant in the mixed system.
+
Figure 9. Surfactant self-diffusion coefficients for the system C,,E,/ SDS/D20 vs. weight percent surfactant (ionic + nonionic). The surfactant mixing ratios SDS/(SDS C12E3)(weight percent) are given in the figure. The experimental temperature was 25 "C.
+
Considering the Cl,E,/SDS/D20 and CI2E5/DTAC/D2O systems at a fixed D 2 0 to surfactant ratio of 140 (Figures 1 and 4) it is seen that the self-diffusion coefficient first decreases when CI2E5is replaced by an ionic surfactant, then goes through a minimum at approximately 10-20% ionic surfactant, and thereafter increases toward the value of the ionic surfactant solution. It can be assumed that the contribution from the free monomers to the observed self-diffusion is essentially negligible because of the low cmc's. (It seems that it is only for surfactant mixing ratios above 90 mol % ionic surfactant that the cmc of the mixed micelles approaches that of the ionic s ~ r f a c t a n t . ~One ) can postulate the following contributions to the decreasing self-diffusion coefficient at low fraction ionic surfactant: (a) increased repulsion between different aggregates due to the charging of the micelles resulting in a lower aggregate self-diffusion coefficient; (b) micellar growth due to the incorporation of the less bulky ionic headgroups into
Mixed Micelles of Nonionic and Ionic Surfactants
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5395
the micellar surface; (c) since the Cl2E5micelles are large8 and flexible and probably close to rodlike, the micelles start to uncoil due to electrostatic repulsion; (d) the importance of the molecular diffusion mechanism is reduced because of the introduced repulsive interaction. From the self-diffusion data presented in ref 8 it is seen that the self-diffusion coefficient of pure C12E5 micelles at the present concentration and temperatures is in a region where the selfdiffusion coefficient increases with increasing surfactant concentration indicating that the molecular diffusion mechanism gives the most important contribution. This is not surprising considering that the cloud point of the C12E5/watersystem is approximately 30 O C . Since mechanisms a, b, and c give important contributions to the observed self-diffusion coefficient only when aggregate diffusion is important we conclude that these mechanisms are of minor importance in the present case and that mechanism d, Le., a partial elimination of the molecular diffusion mechanism, gives the most important contribution to the observed decrease in the self-diffusion coefficient at low fraction of ionic surfactant. As the fraction of ionic surfactant increases the molecular diffusion mechanism is successively replaced by the aggregate diffusion mechanism. The increase in the self-diffusioncoefficient at higher fraction of ionic surfactant can be mainly referred to a decreasing micellar size. This decrease in micelle size is attributed to an increased electrostatic repulsion between the ionic headgroup which makes the effective headgroup area larger. For the system C12Ea/SDS/Dz0,where the component surfactants form small micelles under the experimental conditions, the surfactant self-diffusion coefficient shows quite a weak dependence on the surfactant mixing ratio. At 25 OC, the surfactant self-diffusion coefficient in the CI2E8/D20system decreases with increasing surfactant concentration over the whole stability range of the isotropic solution phase suggesting that the surfactant diffusion monitors the motion of distinct aggregates. These considerations indicate that mechanisms c and d are insignificant. The small initial decrease in the self-diffusion coefficient on addition of SDS to C12Esmicelles is, therefore, referred to increasing intermicellar repulsions and/or a minor micellar growth. The small decrease in the self-diffusion coefficient tends to suggest that the reduced area per polar group which results on the introduction of SDS has a rather small effect on micelle size or that the electrostatic interactions take over and become the dominant factor in determining the micellar size already at low mixing ratios. The increase in self-diffusion coefficient at higher SDS fractions is mainly caused by a gradually decreasing micelle size. Also for the systems C l 2 E 9 / S D S / D 2 O and C I 2 E l 2 C (C12H25(OCH,CH,)l,0CH3)/SDS/D~0which have been in~estigated',~ by light scattering a rather small effect of the surfactant mixing ratio upon the micelle size was observed (for experimental temperatures far below the cloud-point temperature). NMR Relaxation Data. For a system consisting of small micelles narrow proton N M R signals are encountered since transverse N M R relaxation is slow as a result of short correlation t i m e ~ . ~ ~ , 'As ~ J the ' surfactant aggregates increase in size, it takes longer time for the aggregates to rotate as well as for a molecule to diffuse around the micelle surface. Therefore, the transverse relaxation rates are strongly increased as the micelles grow. Except for correlation times, also the local order a t the micelle surface determines relaxation. The observed proton N M R line widths show a close correlation with the self-diffusion coefficients. For both systems containing Cl2E5,a maximum in the proton N M R line width is obtained at approximately the same surfactant mixing ratio as a minimum is found in the self-diffusioncoefficient, while for the CIZEssystem both the line widths and the self-diffusion coefficients are only slightly dependent on the surfactant mixing ratio. For the C12E5/D20system, the line width is somewhat less than 20 Hz (25 OC, 140 mol of DzOper mol of surfactant), demonstrating that aggregates larger than the minimal spherical ones (16) Wennerstrom, H.;Lindblom, G . Q. Rev. Biophys. 1977, 10, 67. (17) Wennerstrom, H.;Ulmius, J. J. Magn. Reson. 1976, 23, 431.
20
0
20
40
60
80
100
"6 SDS Figure 10. Apparent radii deduced from the self-diffusion data presented in Figure 1 by the Stokes-Einstein equation.
are present. We have earlier suggesteds that, with increasing temperature, the C12E5 aggregates both grow and become flexible and interact strongly with each other in a way which gives rise to an averaging of the proton-proton dipolar couplings and hence to more narrow N M R signals. To explain the increase in line width when a small fraction of C12E5 is replaced by ionic surfactant we consider the following contributions: (a) the aggregates become larger because of lower effective headgroup repulsions; (b) mechanisms for partial averaging of the proton-proton dipolar couplings which exist in the nonionic micellar solution are eliminated to significant extent; (c) the aggregates become less flexible because of larger lateral interactions in the micellar surface. Mechanisms a and b can be considered to mainly increase the correlation time while mechanism c increases the order parameter. Considering the close correlation with the self-diffusion data, and the situation in the pure nonionic micellar system, we suggest that the dominant contributions to the increasing line width come from fnechanisms b and c. At still higher fraction of ionic surfactant the line width starts to decrease with increasing fraction of ionic surfactant because of a decreasing size of the mixed micelles. For the C1zEa/SDS/D20 system, the micelles seem to be rather small in the whole surfactant mixing range. The observed selfdiffusion coefficients correspond to apparent micellar radii between 50 and 100 A (calculated from the Stokes-Einstein equation). Since intermicellar interactions tend to decrease the self-diffusion coefficients these figures are upper limits of the true radii. For micelle sizes in this range one can estimate that line broadening should be quite srnall.l0 In accordance with expectation, narrow proton N M R signals are found in this system (Figure 6). Influence of Temperature. For the system C I 2 E 5 / S D S / D 2 0 the self-diffusion coefficients have been measured at three different temperatures, 25,45, and 65 OC (Figure l ) , and qualitatively the behavior is the same at all three temperatures. The comparison of micellar self-diffusion data obtained at different temperatures is facilitated if the apparent radius of an equivalent sphere is calculated from the Stokes-Einstein equationa thus eliminating the nonspecifictemperature dependence (medium viscosity effect.) Since intermicellar interactions are not taken into account in this procedure and (which is more important) since at low fractions of SDS the observed self-diffusion coefficients do not correspond to aggregate diffusion, the obtained radii are not true micellar radii. However, for mixed micelles with a large amount of SDS, the temperature dependence of the apparent radii can be assumed to parallel that of the true micellar radii. The apparent radii calculated from the self-diffusion data given in Figure 1 are presented in Figure 10. SDS micelles are known to decrease in size with increasing t e m p e r a t ~ r e ' J and ~ ~ ' ~this is confirmed by (18) Young, C.Y.;Missel, P. J.; Mazer, N. A,; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1978, 82, 1375. (19) Corti, M.; Degiorgio, V. J . Phys. Chem. 1981, 85, 71 1.
5396
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
I
o i 0
C12E5+DTAc!
/
I
I
5
10
, 15
I
I
I
20
25
mol -% ionic surfactant Figure 11. Surfactant self-diffusion coefficients (25 "C) vs. surfactant mixing ratio, ionic surfactant/(ionicsurfactant + CI2E5)(mol percent), for the systems CI2E5/SDS/D20and CI2E5/DTAC/D20. the same trend in the apparent radius of the pure SDS micelles. The same weak temperature dependence as for the SDS micelles is also obtained over a wide surfactant composition range for the mixed micelles. The observed temperature dependence is stronger for the micelles with much C12E5. Since the diffusion mechanism is not dominated by aggregate diffusion for these samples there is no reason for the temperature dependence to follow that given by the Stokes-Einstein equation. The fact that the apparent radius is much lower at high temperature than at low temperature for mixed micelles with a high fraction of CI2E5,however, indicates that the molecular diffusion mechanism is more important at higher temperatures. In line with this is that, at the two highest temperatures, phase separation occurs at the lowest SDS fractions showing that the attractive interactions are strongest for these samples. The temperature dependence of the proton N M R line widths presented in Figure 2 shows no unexpected features. One usually expects (without major changes in solution structure) a decreasing line width with increasing temperature as a result of accelerated molecular and aggregate motion. This is also found. However, for samples with high fraction of nonionic surfactant the temperature dependence is stronger than expected without assuming changes in the solution structure. It is because the averaging mechanisms for the dipolar interactions (mechanism b in the previous section) and micellar flexibility (mechanism c in the previous section) become more important at high temperatures. Influence of Ionic Surfactant. If the surfactant self-diffusion coefficients (Figures 1 and 4) and proton N M R line widths (Figures 2 and 5) of the systems C I 2 E 5 / S D S / D 2 0and CI2E5/ DTAC/D20 are compared it is seen that qualitatively the behavior is the same for both systems. This indicates that the observed behavior is not caused by specific interactions between the ionic and nonionic headgroups but is a general geometric/electrostatic phenomenon. To better compare the two systems, the surfactant self-diffusion coefficients at 25 OC are plotted in Figure 11 vs. the mole percentage ionic surfactant (elsewhere the self-diffusion coefficients have been plotted vs. the weight percentage ionic surfactant), It is seen that below 10 mol % ionic surfactant there is no significant difference between the two ionic surfactants. If the decrease in self-diffusion coefficient at low fraction ionic surfactant is mainly caused by an elimination of the molecular diffusion mechanism one would indeed expect essentially no difference between different ionic surfactants. It has been suggested that nonionic surfactants of the poly(ethy1ene oxide) variety have a weakly cationic character resulting from oxonium ion formation with protons from the watersz0 There is no indication of such an effect in the present data. At approximately 15 mol % DTAC, the surfactant self-diffusion coefficient begins to increase rapidly with increasing surfactant mixing ratio while the self-diffusion coefficient for the SDS system continues to decrease (20) Becher, P.J . Colloid Sci. 1962, 17, 325.
Nilsson and Lindman and passes through a deeper minimum at a higher surfactant mixing ratio. The reason for this difference may be found in a larger tendency for DTAC than for SDS to form small micelles. The situation is, however, complex. Sterically, the headgroup is larger for DTAC than for SDS. The counterion bonding is, however, lower for SDS than for DTAC,21,22making the S D S micelles more charged than the DTAC micelles. For surfactant mixing ratios where the electrostatic repulsion between the charged headgroups is of relatively small importance (low fraction ionic surfactant) mixed micelles containing DTAC will have a larger tendency to form small spherical micelles than those containing SDS because of the larger headgroup of DTAC. However, also at higher fractions of ionic surfactant as well as for the pure ionic micelles, the micelles containing DTAC seem to have a stronger tendency to form small spherical micelles indicating that the difference in headgroup size between the two ionic surfactants may be more important than the (relatively small) difference in counterion bonding. It is known that DTAC forms a cubic phase composed of closed surfactant aggregates when the saturation limit is e x ~ e e d e d ,while ~ ~ . ~S~D S forms a hexagonal phase.25 This indicates that for the SDS system there is a growth in micelle size when the surfactant concentration is increased while the DTAC micelles remain small over the whole concentration range. The proton N M R line widths parallel closely the self-diffusion coefficients and give support for the interpretation given. As an example, the maximum in line width is both higher and extend to higher surfactant mixing ratios for the SDS system than for the DTAC system. Znfuence of Total Surfactant Concentration. Hitherto for all systems we have considered data pertaining to a molar ratio of D 2 0 to surfactant of 140. In Figure 3, the surfactant self-diffusion coefficients for the system C12E5/SDS/D20are presented for different total surfactant concentrations. The surfactant mixing ratio SDS/(SDS + C12E5) is varied between 0 and 30%. It is seen that the self-diffusion coefficients go through a minimum for all cases. The lower the surfactant concentration, the lower is the surfactant mixing ratio where the minimum occurs and the shallower the minimum. An effect of decreasing the surfactant concentration is that the molecular diffusion mechanism becomes less important or negligible while the aggregate diffusion mechanism becomes more important and dominates at low concentrations. We find then that for dilute CI2E5micelles, SDS addition produces at first a small micelle growth and thereafter a strong decrease in micelle size. The rather weak minimum in self-diffusion coefficient found for the low surfactant concentrations gives support for the interpretation that the elimination of the molecular diffusion mechanism gives the most important contribution to the observed change in self-diffusion coefficient for the higher surfactant concentrations. Self-Diffusion and Critical Fluctuations. In Figure 8 the surfactant self-diffusion coefficients are given for two different temperatures firstly for solutions of CI2EIand secondly for the same solutions where a small fraction (1%) of C12Eshas been replaced by SDS. As expected considering Figure 6, the presence of such a small amount of SDS has only an insignificant effect on the observed self-diffusion coefficient. The effect on the intraand intermicellar interactions is not large enough for the selfdiffusion coefficient to change significantly. The effect on the cloud point is, however, quite dramatic. For the system C12E8/SDS/D20,with a surfactant mixing ratio of 1% SDS, the isotropic solution phase persists to above 90 OC for all surfactant concentrations, while the lower consolute temperature for the binary system C I 2 E 8 / D 2 0is approximately 75 "C. This shows that the balance determining the cloud-point temperature is very delicate and a very small extra repulsive interaction may significantly raise the cloud-point temperature. (21) Lindman, B.; Wennerstrom, H. Top. Curr. Chem. 1980, 87, 1. (22) Wennerstrom, H.; Lindman, B. Phys. Rep. 1979, 52, 1. (23) Balmbra, R. R.; Clunie, J. S.;Goodman, J. F. Nature (London) 1969, 222, 1159. (24) Bull, T.; Lindman, B. Mol. Cryst. Liq. Cryst. 1973, 28, 155. (25) Fontell, K. Mol. Cryst. Liq. Crysl. 1981, 63, 59.
J. Phys. Chem. 1984, 88, 5397-5400 These data show that the changes in solution structure with increasing temperature and concentration observed with the self-diffusion method are not directly associated with the distance from the phase separation. This illustrates the independence of self-diffusion coefficients to critical effects.26 A combination of mutual diffusion and self-diffusion is, therefore, appropriate for the distinction between critical fluctuations and micelle gro~th.~,~~,~~ (26) Hamann, H.; Hoheisel, C.; Richtering, H. Ber. Bunsenges. Phys. Chem. 1972, 76, 249. (27) Brown, W.; Johnsen, R.; Stilbs, P.; Lindman, B. J . Phys. Chem. 1983, 87, 4548.
5397
The self-diffusion data also indicate that when micellar growth occurs with increasing temperature and concentration it is not directly associated with the distance from the phase separation.
Acknowledgment. We thank Dr. Bengt Jonsson and Prof. Hakan Wennerstrom for helpful discussions. Grants have been obtained from Stiftelsen Bengt Lundquists Minne and The Swedish Board of Technical Development. Registry No. C12ES,3055-95-6; C1,Es, 3055-98-9; SDS, 15 1-21-3; DTAC, 112-00-5. (28) Corti, M.; Minero, C.; Degiorgio, V. J . Phys. Chern. 1984,88, 309.
Phase Transition Properties of 1,3-Dipalmitoylphosphatldylethanolamine B. Z. Chowdhry, A. W. Dalziel, G. Lipka, and J. M. Sturtevant* Department of Chemistry, Yale University, New Haven, Connecticut 0651 1 (Received: February 21, 1984; In Final Form: June 1 , 1984)
The phase transition properties of aqueous suspensions of 1,3-dipalmitoylphosphatidylethanolamine(1,3-DPPE) have been examined before and after heating to 10 "C above the simultaneous hydration and chain-melting transition (Thfmtransition) in H 2 0and D 2 0by high-sensitivity differential scanning calorimetry at a scan rate of 0.1 K min-I. In H 2 0 1,3-DPPEundergoes the Th+, transition at 79.5"C. If this scan is continued until 90 "C and the sample is cooled in the calorimeter to 20 OC and then rescanned, phase transitions are observed at 42.8 and 53.1 "C but the rh+,transition is absent. In D20the corresponding temperatures are 80.1, 42.8, and 53.5 "C, respectively. The sum of the calorimetric enthalpies represented by the curves of excess apparent specific heat vs. temperature, for fully hydrated phospholipids, is the same in D 2 0as in H20. Both transitions of fully hydrated 1,3-DPPE liposomes are reversible. Fully hydrated 1,3-DPPE incubated for 1-14 days at 0-25 "C shows curves of excess apparent specific heat vs. temperature identical with those of freshly prepared samples which have been heated to above the Th+, transition. For fully hydrated 1,3-DPPE, 31PNMR spectra are consistent with the existence of a lamellar structure throughout the 0-60 "C temperature range. In addition, the spectra suggest a substantial head-group conformational or motional change between 40 and 60 "C.
Introduction The physical properties of saturated 1,2- and 1,3-diacylphosphatidylcholines (PC's) have been extensively examined and compared' because of the importance of the former as constituents of model membranesZused in structural and functional studies of cell membrane and enzyme (e.g. phospholipase) system^.^ The thermotropic and lyotropic behavior of such compounds is also of interest in comparative studies of various classes of liquid crystal^.^ Although 1,2-diacylphosphatidylethanolamines(1,2PE's) and chemically modified 1,2-PE's have been studied by a variety of physical t e c h n i q ~ e s , no ~ . ~information is available concerning differences between the mesomorphic properties of 1,2and 1,3-PE's. Even though 1,3-phospholipids have not, to our knowledge, been reported to occur naturally in biomembranes, such comparative investigations are of importance for the insights they may give into the physical behavior of synthetic 1,2-PE's. Furthermore, since hydration plays a very important role in the properties of both 1,2- and 1,3-dia~ylphospholipids,~ substitution of HzO by DzO offers one way of investigating hydration phenomenas in these compounds. (1) Chowdhry, B. Z . ; Lipka, G.; Sturtevant, J. M., submitted for publi-
cation (2) (3) (4) (5)
in Biochemistry. Ostro, M. J., Ed. "Liposomes"; Marcel Dekker: New York, 1983. Roelofsen, B.; Zwaal, R. F. A. Methods Membr. Biol. 1976, 7 , 147. Brown, G.H.; Cooker, P. 0. Chem. Eng. News 1983, 61, 24. Vaughan, D. J.; Keough, K. M. FEBS Lett. 1974, 47, 158. (6) Casal, H. L.; Mantsch, H. H. Biochim. Biophys. Acta 1983, 735, 387. (7) Finean, J. B.; Michell, R. H. In "New Comprehensive Biochemistry"; Neuberger, A., Van Deenan, L. L. M., Eds.; Elsevier North Holland: New York, 1981; Vol. 1, Chapter 1, pp 23, 24.
0022-3654/84/2088-5397$01.50/0
We report here studies of the phase transition behavior of 1,3-DPPE in excess H 2 0 and D 2 0 employing high-sensitivity differential scanning calorimetry (DSC) and phosphorus-3 1 nuclear magnetic resonance spectroscopy (,IP N M R ) .
Experimental Section Chemicals. 1,2-DPPE (1,2-dipalmitoylphosphatidylethanolamine, Avanti Polar Lipids Inc., Birmingham, AL) and 1,3-DPPE (Calbiochem-Boehring, La Jolla CA, and Serdary Research Laboratories Inc., London, Ontario) were tested for purity by analytical TLC (Uniplate Silica Gel H , Analtech, Newark, N J ) using (a) CHCl,:MeOH:HAc:H,O (85:15:10:3, v/v/v/v), (b) CHCl,:MeOH:HzO (65:25:5, v/v/v), and (c) CHC13:MeOH:7 N N H , (230:90:15, v/v/v) as eluents and spotting at a concentration of 400 bg cm-' in chloroform. Eluent b gave the best resolution. With this solvent no impurities could be detected for 1,2-DPPE (Rf of 0.43). 1,3-DPPE, however, gave spots with Rf values of 0.06, 0.27, 0.36, 0.43, 0.6, 0.85, and 0.95. 1,3-DPPE was therefore purified by preparative TLC (20 X 20 cm2, 500 pm thick) using eluent b and phospholipid bands visualized by briefly placing the TLC plates, which had been covered with saran wrap (except for 2 cm on each side), in iodine vapor. The 1,3-DPPE band was scraped off and 0.5 mL of HzO, 2.5 mL of MeOH, and 2.5 mI, of CHCI, added in consecutivesteps; the sample was gently handshaken for 2 min and centrifuged and the supernatant collected. This elution process was repeated, the fractions were combined and washed with 3 mL of 1% NaC1, and the nonaqueous fraction was collected. This fraction was further purified by (8) Chen, C.-H. J . Phys. Chem. 1982, 86, 3559 and references therein.
0 1984 American Chemical Society
