J. Phys. Chem. 1992, 96, 10068-10074
10068
size (over greater time), the polymer is more exposed to termiin the f w e show that C2/Cl nation by the inhibitor. The cu~es is quite sensitive to the tunable size. For example, at Z = 1.0 x 10-9 M, C,/C, exhibits approximately a 4-fold decrease when the tunable size increases by only a factor of 2 (from 20 to 40). Thus,if one could perform a perfect experiment in which 2 and all the rate constants as well as the diffusion coefficient were known, the tunable size could be determined with reasonable accuracy.
Acknowledgment. The authors are grateful to Dr.Michael Ellerby for his valuable discussion on the diffusion coefficient. This work was supported by NSF Grant No. CHE90-22215.
References a d Notes
Z
(XlO-"M)
Figure 1. Dependence of C2/C, on Z = [Z,] for (1) j * = 20, (2) j* = 30,and (3) j* 0 40. [Z,] = 0, K = 0.01 s-l, 2L = 2 0 cm, D = 0.1 cmz s-l, and k, = 1.0 X 106 M-I s-l.
results in a dramatic reduction in the concentration of polymers having siza larger than the tunable size,and therefow the number of observed drops. The rapid decrease of C2/C, for larger j * reflects the fact that, while attempting to propagate to a larger
( 1 ) Rabeony, H.; Reiss, H. Macromolecules 1988, 21, 912. (2) Arfken, G.Mathematical Methods for Physicists; Academic: New York, 1986; p 511. ( 3 ) Reiss, H. Science 1987, 238, 1368. (4) Odian, G. Principles ofPolymerization, 2nd ed.; John Wiley & Sons, New York, 1981; p 246. ( 5 ) El-Shall, M. S.;Bahta, A.; Rabeony, H.; Reiss, H. J. Chem. Phys. 1987, 87, 1329. (6) Eastmond, C. H.Comprehensive Chemical Kinetics; Bamford, C. H.; Tipper, C. F. H., Eds.; American Elsevier: New York, 1976; Vol. 14A, Chapter 3. (7) Reiss, H.; Rabeony, H.; El-Shall, M. S.; Bahta, A. J . Chem. Phys. 1987, 87, 1315.
Exchange of Hybrid Surfactant Molecules between Monomers and Mlcelles Wen GuoJ B. M. Fung,*lt and Edgar A. O'Rear* Departments of Chemistry and Chemical Engineering and Institute of Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma 73019 (Received: June 15, 1992; In Final Form.-August 17, 1992)
The synthesis and some of the physical properties of hybrid surfactants with separate hydrocarbon and fluorocarbon chains attached to the same head group have been reported earlier. The present work focuses on a study of the chain conformation and molecular exchange of these hybrid surfactant systems. Because of the unique molecular structureof these doubletailed surfactants, their NMR spectra show some unusual characteristics, from which interesting information can be obtained. Within the micelle, the rotation of the fluorocarbon chain along the C-C bonds is restricted, causing the two fluorine nuclei in alternating CF2groups to have inequivalent chemical shifts. The exchange rate for this series of hybrid surfactants between the monomer and micellar states is significantly lower than that for common ionic surfactants. The residence time of the s for compounds with short chains to 7 X 10-4 s for those with CF3 group in the micelle at the cmc ranges from 3 X longer chains, compared with 8 X lo-' s for single-chain fluorinated surfactants. For each surfactant, the residence time of the CF3 group in the micelle increases with the total surfactant concentration, probably due to changes in the micelle size and shape. The average residence time for the a-CF2group in the micelle is about 3 X lo-' s and is not very sensitive to the change in surfactant concentration.
Introduction Fluorocarbon surfactants exhibit characteristic properties such as large surface activity, high chemical stability, resistance to heat, In many cam, the and ability to emulsify fluor~chemicals.~-~ combination of a fluorocarbon (FC) surfactant and a suitable hydrocarbon (HC) surfactant may form mixtures which have special properties. Normally, in such a combination, it is the fluorocarbon surfactant which reduces the surface tension, while the hydrocarbon surfactant aids in the reduction of the interfacial tension' The net c~~ be a system that wets and spreads on otherwise hard to wet surfaces.4 However, because Of the phobicity between fluorocarbon and hydrocarbon moieties, mixed systems of a FC surfactant with a HC surfactant
with the same head group charge always give a positive nonideal deviation.4 Above the cmc,they tmd to form mixed mkdles whase diffmnt from that in the phase or cvcn two types of micelles, ollc rich in Fc and other in HC.CB Such demixing effects limit the application of HC/FC mixed surfactants. To overcome this problem, effomhave been made to synthesize surfactants with both FC and HC pofiiom in the same molstructure, mainly thoee with Fc and HC oxyeen linlrages,l~ segment bonded either dirsctly9 oT through In a previous paper, We have the synthtsLof two homologous series of novel double-tail hybrid surfactants (I) (ab= 6-8, = 1-9) as well as of their breviation: FmHn, physical propefiies.ll These surfactants contain a FC chain and
'Department of Chemistry and Institute of Applied Surfactant Research. *Departmentof Chemical Engineering and Institute of Applied Surfactant Research.
0022-3654/92/2096-10068$03.00/0
0 1992 American Chemical Society
Exchange of Molecules between Monomers and Micelles
I
II
111
IV
V
VI
Figure 1. Staggered Newman projections of FmHn along the C * C bonds of the chiral center.
a HC chain attached to the same hydrophilic head group. The surface tensions of these hybrid surfactants in aqueous solutions are comparable to or slightly lower than that of common single chain fluorocarbon surfactants. Values of the critical micelle concentration (cmc) are relatively low, and the dependence of the cmc on the length of both chains obeys Kleven’s equation.” The introduction of each CH2group to the hydrocarbon chain reduces the cmc by about 3596, and the introduction of each CF2group to the fluorocarbon chain reduces the cmc by about 75%. Study of both the F-19 and H-1 chemical shifts indicates that for n 1 3, both FC and HC chains of the FmHn molecules are incorporated inside the micelle and that the interior of the micelle becomes more FC-rich as m increases or as n decreases. The aggregation number of the micelles as estimated from the NMR data is about 25. In this work, we present the results of more detailed NMR study on the relation between the molecular structure and micellization and the exchange rate between monomers and micelles.
Experimental Section Surfactants of FmHn were prepared as reported previously.” The H-1 NMR spectra were measured on a Varian VXR-500 spectrometer operating at 500 MHz. The F-19 NMR measurements were performed on both the Varian VXR-500 spectrometer operating at 470.3 MHz and Varian XL-300 spectrometer operating at 282.3 MHz. The mean lifetime calculations for the CF3 group were based on line width measurements on the XL-300. All solutions for NMR measurements were made with 99.9% D 2 0 as solvent. The H-1 chemical shift is referred to the internal HOD peak (6 = 4.63 ppm), and the F-19 chemical shift is referred to CF3COOH (6 = -79.45 ppm) as external reference.
Results md Discussion CenenL With separate HC and FC cham attached to the same head group, the hybrid surfactant FmHn has a special molecular structure. The carbon atom C* which connects the anionic head group, the FC chain, and the HC chain is a chiral center. Because of the size of fluorine atom, the FC chain is bulkier and more rigid than the HC chain. Thus, the carbon atom directly attached to the head group in these hybrid surfactants has a surrounding much more crowded than that for either single chain HC and FC surfactants or for double-tailed HC surfactants. This creates hindrance to the free rotation of the chains, especially the more rigid FC chain. Several possible staggered Newman projections along each of the two C-C* bonds of the chiral center are shown in Figure 1. The compounds studied are racemic mixtures, but the projections of only one optical isomer are shown. The situation of the a‘-CF2 group is determined by rotamers I-III. In rotamers I and II, the FC and HC segments are gauche staggered to each other, while in the rotamer III, they are anti staggered. Below the cmc, it is likely that the FC chain is free to rotate along all the C-C bonds. The apparent conformation is a weighted average due to such rotations, among which rotamer III may have a larger weighting factor because there is lets steric hindrance and mutual phobicity
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 10069
Figure 2. The most stable molecular conformation of F7H7 determined by molecular mechanics calculation.
between the FC and HC fragments. Above the cmc, the rotation of the FC chain would be severely restricted for molecules incorporated inside the micelles, and rotamer 111would become the most preferred conformer. Similarly, the apparent conformation of the u-CH2 group is a weighted average of rotamers IV-VI, with IV probably more preferred. Because of the chiral structure, in any one of these rotamers, P and Fb or Haand Hb are chemically and magnetically inequivalent. Further analysis of conformational equilibria was performed by using a modeling program called PCMODEL on a Macintosh IIci computer. This model uses molecular mechanics approach to minimize the energy level multing from bond length distortions, bond angle strain, van der Waals repulsion, and dipole-dipole interactionsi2 The most stable conformation thus obtained is shown in Figure 2. According to this model, the highest energy barrier for a rotation of 360’ around the HC*-CH2(a() bond is approximately 92 kJ mol-’ and that for the HC*-CH2(u) bond is approximately 13 kJ mol-’ (the rotational energy for the C-C bond in C2H6is 12 kJ moI-l).l2 Such energy barriers will not be high enough to hinder the free rotation of either chain in the monomer. However, when the molecule is incorporated inside a micelle, hindrance to rotation increases tremendously due to spatial limitation and lyophobic interaction of the FC chains. This effect can be readily observed from the characteristic NMR spectra of FmHn before and after the formation of micelles, as discussed in the following. H-1NMR spectra The FmHn compounds we synthesized were racemic mixtures. In an achiral environment (e.g., the D20solvent in the present case), the two enantiomers exhibit identical NMR spectral characteri~tics.’~ The H-1 spectra for F7H4 are shown in Figure 3. Spectra with longer HC chain lengths have similar characteristics, except that the signal for the y C H 2 group superimposes with those of the CH2 groups in the positions of 6, c, A, etc. Below the cmc, the multiplet splittings for each H-1 resonance is well resolved. The proton at the C* position is strongly coupled with neighboring nuclei in both FC and HC chains, and the overall signal is a rather complicated multiplet (Figure 3, insertA). Since Ha and Hb at the u-CH2 position are magnetically inequivalent, they exhibit an AB pattern, which is further split by couplings with other neighboring protons and fluorine nuclei. Thus, the overall signal for the protons in the a-CH2 group is a complicated multiplet for each possible rotamer, and the observed spectrum is the weighted average of these rotamers. Although the overall pattern is very complex (Figure 3, insert B), the chemical shift inequivalence of ca. 0.08 ppm seems to be quite clear for the two protons in the a-CH2group. Similar chemical shift inequivalence between the two protons at the u position next to a chiral center has been commonly observed in achiral solvents and should not be mistaken for enantiomeric resonances which depend on the composition of the mixture.”J4 A similar situation has been reported for the double-tailed HC surfactant, sodium, 1,2-bis[((2-ethylhexyl)oxy)carbonyl]ethenesulfonate (Aerosol OT).I4 Because the effect of the chiral center on the H-1 resonance diminishes along the HC chain, only a very small inequivalence
10070 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Guo et al.
P
I
5.0
4.0
.
.
.
I
,
3.0
.
Figure 4. The F-19 spectra of the CF3 group in FmHn at 282.3 MHz and 298 K (A) F7H7 klow cmc,plotting width = 80 Hz; (B) F6H6, 2 X cmc,plotting width = 80 Hz; (C) F7H4, 2 X cmc,plotting width = 80 Hz; and (D) F7H7, 2 X cmc,plotting width = 400 Hz.
. . .
- . , .
2.0
I"
1.0
ppm
Figure 3. The H-1 spectra of F7H4 at 500 MHz and 298 K. The inserts A and B are below cmc: the inserts C and D are at 7 X cmc.
is observed for the protons at the 4 position, and no obvious AB splitting is observed for the protons at the 7 position (Figure 3, insert B). Above the cmc, the observed H-1 signal is the weighted average of monomer and micelle peaks. The observed chemical shifts all move to lower frequencies (Figure 3, insert C and D),indicating that the protons in the HC chain are more shielded in the micelle due to the reduced polarizability of the FC environment." In the meantime, the line widths of the proton peaks also increase. This is likely a result of the exchange of the surfactant molecules between monomer and micellar states. The latter has a shorter T2due to a decrease in motional freedom.15 This effect is most obvious for the peak due to the C*H group (Figure 3, insert D), which has very restricted rotation in the micellar state. F-19NMR !Speclm The F-19 spectra of FmHn below the cmc has a pattern similar to that of the single chain FC surfactants.I6 For clarity, the resonance for the CF3 group and the resonance for the CF2 groups are shown separately in Figures 4 and 5, respectively. The peak assignment in Figure 5 was determined from a COSY experiment. For the hybrid surfactants FmHn, the shielding increases monotonically along the FC chain in the order CY < B < 7 < d < e < A, but the CF3 group is least shielded because of its terminal position. It is noted that this order is slightly different from that of common anionic single chain FC surfactants with the same FC chain length, such as sodium because of the difference in the size and perfl~orooctanoate,'~J' charge of the head groups. Below the cmc, although the two fluorine nuclei at the u position are inequivalent due to the chiral structure, no obvious AB pattern is observed. This is probably because the chemical shift difference caused by the configuration is small compared to the line broadening due to spin-spin couplings between the d-CF2and other CF, group in the FC chain. Below the cmc, the overall line width of the a'-CF2 peak is ca. 45 Hz, compared with individual line widths of ca. 4 Hz for the multiplet of d-CH2 group. Above the cmc, there are two substantial changes in the F-19 spectra. First, the line width of CF, group is significantly broader than those in common ionic micellar solutions undergoing fast exchange16 (Figure 4B-D). Second, clear AB patterns for the CFz groups emerge upon the formation of micelles and become more significant as the concentration increases (Figure 5). Determinrtion of tbe Micellar Residence Time from the Line B r o a h b g of the CF3Resonmce. Below the cmc, the signal of the CF3 group is well resolved into a triplet of triplets due to the splittings by the neighboring CF2groups (Figure 4A). The individual line width of this multiplet is about 2 Hz obtained by fitting the experimental spectra to the sum of nine Lorenztian functions.I6 Above the cmc, significant line broadening occurs
due to the exchange between monomers and micelles. As shown in Figure 4B-D, the triplet of triplets for the CF3group merges into a broad triplet (for F6H6 and MH3) or just a single broad peak (for MHn, n 1 4). Since the observed signal is a weighted average of the surfactant molecules in two environments, the dependence of the line width on m and n indicates that the exchange rate between the monomer and micelle decreases with the increase of the chain length. For a weighted average peak of two species x (monomer) and y (micelle) in exchange, the spin-spin relaxation rate is dependent on the mean lifetime T for the exchange process:18
1/T20bs = PX/T2, + Py/T2y + 4T2P#y7(Vx T,
=T
/ P ~and T~
=~ / p ,
- vy)z
(1) (2)
where p , and p y are the mole fractions of species s and y, respectively, v, and uu are the corresponding resonance frequencies in the absence of exchange, and T2is the spin-pin relaxation time. T, and T~ are the residence times of a particular molecular fragment in the x and y states, respectively. If inhomogeneous broadening is neglected, Tz is equal to I / T Y ~ /where ~, is the line width at high-height of the NMR signal. A nonlinear least-squares algorithm known as SPIRAL19was used to fit the experimental spectra, which simulates each spectrum as the sum of nine Lorenztian functions for the signals with three resolvable peaks. The line widths of the extremely broad and unsolved peaks were obtained by fitting the spectrum to a single Lorenztian function. In this way, the line width of each component in the CF, group was obtained to give the values of T* Values of Tk and the monomer frequency v, were directly measured from the experimental data below the cmc. vy.was obtained by extrapolating the observed frequency vobsto infixute concentration ( l / c = O):20 vobs
= P A + PyVy
p , = cmc/c;
py= 1 -px
(3) (4)
where the monomer concentration is assumed to be constant (= cmc) in the whole concentration range (from the cmc to 10 X cmc), and the effect of the change in ionic strength with concentration is assumed to be negligible. Since the value of (v, v y ) and therefore the observed line widths are dependent on the spectrometer frequency, we found that the data obtained at 282.3 MHz gave better results than those obtained at 470.3 MHz for the purpose of least-squares analysis. Since the relaxation of the CF3 group is mainly determined by its internal rotation, which would not be affected by the formation of micelles, we assume that TZy T2,. Then, the mean lifetime T and the micelle residence time T~ can be calculated. Values of rYfor the CF3 group thus obtained are plotted against the total concentration of FmHn (in unit of cmc) in Figure 6.
-
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 10071
Exchange of Molecules between Monomers and Micelles
Fb. 5
1.1 n c.w
. A
5. Left: simulated F-19 spectra for the d - C F 2 group in F7H4. Right: experimental F-19 spectra of all the CF2 groups in F7H4 at different surfactant concentrations. The spectra were taken at 470.3 MHz and 298 K.
lod
n, m Figure 7. The residence time of the CF3 group in the micelle (TJ plotted against the lengths of HC chain (0)and the FC chain (a). 0
2
4
dcmc
8
8
10
Flgm 6. The residence time of the CF3 group in the micelle (ry)plotted against total surfactant concentration expressed in a unit of (c/cmc).
Under ideal circumstances, the size, shape, and aggregation number of the micelles are independent of the micellar concentration, and the micelle residence time T~ should also be independent of the total concentration. However, in actual surfactant solutions, the average aggregation number often increases with the total surfactant concentration.21 Such an increase might cause a surfactant molecule to stay longer inside the micelle. Our data show that T indeed always increases with total concentration (Figure 6). h e slope of the increase is smallest for F7H3, which has a very short HC chain, and the micellar size might be least affected by the total concentration. The slopg are larger for F7H4
to F7H7, indicating that the effect of total surfactant concentration on fY(and possibly the micellar size) is larger for intermediate HC chain Imgths. For even longer HC chains (F7H8and F7H9), f Ybecomes less sensitive to the total concentration again. This may be related to the increased contribution of the HC chain to the hydrophobic interaction in the formation of micelles. The actual dependence of the micelle size and shape on the total surfactant concentration has not been investigated for the hybrid surfactants and needs to be studied in more detail. By extrapolation of the dependence of fY on total concentration, the micelle lifetime at the cmc is obtained. The effect of the chain length on these values of T,(cmc) is shown in Figure7, which clearly indicates that the exchange rate between monomers and mi& in the FmHn systun decreasessisnificantly as either chain length increases. The linear relationship between log[~,(cmc)] and the length of either chain is reminiscent of the linear rela-
10072 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
'L
P -120E
-2
-121
'3 Q1
a u
QI
7
-122
.
-
I
0.5
0.0
.
1.0
.
1.5
.
I
2.0
llc, mM-'
Figure 8. The chemical shifts ?iA and 6~ for the a'-CH2 group in F7H4 plotted against the reciprocal of the total surfactant concentration.
Guo et al. exchange in the FmHn system is not due to phase separation. Solutions having concentrations of 8 X cmc for each FmHn sample was centrifuged at 14 kG for 15 min. If there exists a separate phase in the system, the centrifugal force would cause the larger and dense (due to fluorination) aggregate to sediment,I6J7and the intensity of the F-19 resonance would be reduced compared with the spectra before centrifugation.16J7 In the present case, both the F-19 signal intensity and line width were essentially the same before and after centrifugation. This suggests that the slower exchange rate in the FmHn is not caused by phase separation. The molecular structure of the hybrid surfactants is different from common FC surfactants by the presence of its double tail and the chiral center. By examining the conformation shown in Figure 2, it is obvious that hybrid surfactants are less favorable for aggregating into small spherical micelles, which have a large surface/volume ratio, than single-chain surfactants. Although we are not sure about the exact size and shape of the FmHn micelles, this argument should be valid for any small micellar systems. The difficulty for the monomeric FmHn molecules to aggregate to form micelles docs not seem to have a significant thermodynamic effect: the dependence of their cmc's on the chain lengths is similar to those of the single-chain HC and FC surfactants.]' However, it seems to change the kinetics of the aggregation of the FmHn monomers into micelles considerably, as indicated by the relatively long residence times. The Excbange Rate and the AB Patterns for the CF2Groups. As shown in Figure 5, the F-19 peab of the d- and 7'-CF2 groups in F7H4 are clearly split into AB patterns above the cmc. Those for other homologous compounds are of similar pattern. Obviously, this significant change is due to a greatly increased hindrance in the rotation of the FC chain. Rotamer I11 (Figure 1) is now much more favored or may even be the exclusive conformer, when the FmHn molecules aggregate to form micelles. The change in the peak positions with concentration (Figure 5 ) is due to the different weighting factors of the monomer and micellar states, not due to changes in the micellar chemical shifts, SAm and SBm. The observed peaks are weighted averages, and the chemical shifts SA and bB at each concentration can be calculated from the positions of the four individual peaks in the AB This has been done for the d-CF2group but not for the y-CF2 group, because some peaks of the latter overlap with other CF2 peaks. As it is shown in Figure 8, the dependence of SA or SB on the total concentration also obeys eq 3, and they merge into one monomer peak at the cmc. Since the a'-CF2 group is close to the head group, its change in chemical shift caused by micellization is rather different from that of the terminal CF3group. Upon the formation of micelles, one of the fluorine in the a'-CF2 group becomes more shielded than the monomer while the other one becomes less shielded than the monomer (Figure 8). This can be explained by considering the configuration shown in Figure 1. In the preferred rotamer 111, Fa is at a gauche staggered position to the sulfate group and thus gets substantial shielding enhancement from the electron-rich head group. Fb, being at the anti staggered position, is less affected by the sulfate group but more affected by the neighboring HC groups. In fact, the increase of SB in the micellar state indicates that Fb is located in a more HC-rich region.29 By extrapolation of SA and SB to 1 / c = 0, the values of the J coupling constant and the micellar chemical shifts SAm and SBm can be obtained. For FmHn with different n and m values, Jm is 284 f 2 Hz and (SAm - SBm) is -2.1 f 0.1 ppm. There is no systematic correlation observed between the chain length and (SAm - SB"), indicathg that the motional restriction of the a'-CF2 group is not very sensitive to the chain length. With the increase of total concentration, splittings of the peaks for the y'- and d-CF2 groups into AB patterns become more apparent (Figure 5 ) . A very small splitting of the X'-CF2 group at higher surfactant concentration is also evident, but no appreciable splitting for the i3'- and 6'-CF2 groups could be observed. By inspection of the most stable conformation shown in Figure 2, the a'-,y'-, and d-CF2 groups are located on one side of the FC chain closer to the HC chain. This may explain the larger inequivalence of the two fluorine atoms on the same
2'-4L d h
1.00e-4
P
I * 0
2
4
8
8 1 0 1 2
cJcmc Figure 9. The residence time in the micelle ( T J of the d - C F 2 group (0) and the CF3 group ( 0 )in F7H4 plotted against the reciprocal of the concentration expressed in a unit of (c/cmc).
tionship between log(cmc) and the chain length." The exchange of molecules between the monomer state and the micellar state is usually rapid and often believed to be diffumean lifetime ( T ) of an ionic s i o n - c o n t r ~ l l e d .The ~ ~ ~typical ~~ surfactant molecule in a micellar solution is usually of the order of 10-6_10-7.'6 In our previous work, it was found that the mean lifetime for the CF3 group in sodium perfluorooctanoate is 4.2 X s, at the cmc,16 Slower exchange s, or T~ = 8.4 X phenomena have been observed in dilute perfluorinated fatty acids (I = io4 s) or nonionic fluorocarbon surfactants (T = 1W-10-' s) at concentrations several times above the cmc,16 However, such slow exchange is in fact not between monomers and normal micelles but between monomclg and large surfactant aggregates. Many phospholipids and some double tail surfactants also tend to form lamellar liquid crystalline phase in aqueous systems even at relatively low total surfactant/water ratios (e.g., 1 2 wt%).24-26 However, the aggregation number of the FmHn micelles has been estimated to be about 25,'I which suggests the presence of small spherical micelles, as in some other double tail HC micellar solution~,~' rather than large aggregates. Nevertheless, another experiment was performed to confii that the intermediately slow
Exchange of Molecules between Monomers and Micelles carbon in these positions. This inequivalence diminishes toward the terminal group because of increasing flexibility of the chain. Although the 7’-CH2 group is located on the same side as the @’and 6’-CF2 groups, it is connected with the terminal CF3 group, making its environment slightly more different compared with those of the 8’- and 6’-CF2 groups, so that a small inequivalence for the two fluorine atoms in the X’-CF2 group can be observed at high surfactant concentrations (Figure 5 ) . Similar splitting patterns are observed for other FmHn compounds. Unlike the H-1 spectra (Figure 3), the F-19 peaks for the 8’- and 6’-CFz groups do not show obvious broadening above the cmc. This is because the inherent line widths ( 4 M O Hz) for the monomeric state are determined by unresolved spin-spin splittings rather than by a change in T2. Since the four peaks for the d-CFz group are well resolved for all concentrations above the cmc, their line widths can be analyzed quantitatively. According to eq 1, the observed line width is in proportion to (uim ( i = AI, A2, B1, B2 for the four peaks in the micellar state, and v, is the monomer frequency). Therefore, the line shapes in the observed AB pattern are not symmetrical when the total concentration is not too much higher than the cmc. At high surfactant concentrations, the population in the micellar phase dominates, and p y >> pnin eq 1. Then, the difference in the line widths of the four peaks is less obvious (Figure 5). For each peak in the AB multiplet, the line width is also determined by the exchange rate between the monomer and the micelle. Based upon the values of JAB and (6*- - 6Bm), the relative intensity ratios of the four peaks in the micellar state were found to be 0.83:1.17:1.17:0.83. Considering that each component is in rapid exchange with the corresponding one in the monomer state, the observed AB patterns can be readily calculated. The results are shown on the lefthand side in Figure 5 . Except at the lowest concentration, for which the assumption in eq 4 is not very good,” the calculated AB multiplets agrees with the observed spectra reasonably well. By comparing the experimental and the simulated spectra, the residence time of the d-CFz group in the micelle can be calculated using eqs 1 and 2. For F7H4, the residence times calculated based on the line width of peak A1 in the AB multiplet for the d-CF2group (Figure 5 ) are plotted in Figure 9 together with the values obtained from . the a’-CF2 group the CF3 signal. The results show that T ~for docs not change significantly with increasing surfactant concentration and is about 2-6 times less than ry for the CF3group; the difference becomes larger at higher concentration. This indicates that the residence time of the a’-CF2 group in the micelle is not very sensitive to the possible change of the micellar aggregation number with the increase in the total surfactant concentration. A possible explanation for the difference in the residence times for the a’-CF2 group and the CF3 group is the following. The terminal CF3 group, which resides in the interior of the micelle, has a longer diffusion path than the a’-CF2 group to go outside the micelle into the aqueous phase in the exchange process. Furthermore, for any individual molecule half way out of the micelle, it has equal probabilities to leave or to reenter the micelle. Therefore, for each complete in-and-out movement of the CF3 group, the d-CF2group might experience more than one exchange process. This is consistent with results of ‘3C spin-lattice relaxation studied in micellar solutions of single-chain30or doublechain HC surfactants?’ which indicate that the diffusion rate of the nuclei near the head group is larger than those approaching the terminal group. However, according to the calculation of Anian~son,~’ if the hybrid surfactants form normal micelles, the residence times should be much shorter, and the differences between the residence times of the head group and the terminal group should be much larger, than those shown in Figure 9. To resolve this inconsistency and interpret the residence times quantitatively, further investigations are needed to provide information on the morphology of the hybrid surfactant micelles. Conclusion The NMR spectra of hybrid surfactants with separate FC and HC chains attached to the same head eroup have been studied
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 10073 in detail. The two protons in each of the a- and &CH2 groups have different chemical shifts due to their inequivalent chemical environments. This difference in proton chemical shifts does not increase above the cmc, indicating that there is no appreciable decrease in the segmental rotational freedom of the HC chain when micelles are formed. On the other hand, the fluorine atoms in the CF, groups do not show chemical shift inequivalence below the cmc, probably because the splitting is too small compared to the line broadening caused by scalar couplings. Above the cmc, the FC chain is packed inside the micelle, and its rotation is restricted because it is bulkier and more rigid than the HC chain. Because of the chiral nature of the bridging carbon atom and hindrance in the rotation of the FC chain, each of the two fluorine atoms in the a’-,7’-, and e’-CF2 groups show resolvable AB patterns above the cmc. By the use of a quantitative analysis of the F-19 NMR spectra, it was found that the residence time (T,) of the FC chain in the micelles is considerably larger than that of common single chain surfactants. It was also found that 7,. for the CF3 group is several times longer than that of the d-CF2 group. The former increases monotonically with the increase in total surfactant concentration, while the latter does not have an obvious concentration dependence. We believe that this is the first time that the residence times of different parts of a surfactant molecule in a micelle have been evaluated quantitatively, but a quantitative interpretation of the data cannot be made without knowing the morphology of the hybrid surfactant micelles.
Acknowledgment. This work was supported by the National Science Foundation under Grant No. CTS-9812806 and the Oklahoma Center for the Advancement of Science & Technology (OCAST) Award No. ARO-075. The authors gratefully acknowledge the assistance of industrial sponsors of the Institute for Applied Surfactant Research, including Kerr-McGee Corporation, Sandoz Chemicals Corp., E. I. Du Pont de Nemours & Co, and Union Carbide Corp. Helpful discussions with Dr. S.D. Christian and J.-P. Bayle are also acknowledged. References and Notes (1) La Mesa, C.; Sesta, B. J . Phys. Chem. 1987, 91, 1450. (2) Tadros, T. J. Colloid Interface Sci. 1980, 74, 196. (3) Gangoda, M.; Fung, B. M.; ORear, E. A. J. Colloid Interface Sci. 1987, 116, 230. (4) Funasaki, N. In Mixed Surfacranr Sysrems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, in press. (5) Mukerjee, P.; Mysels, K. J. In Colloidal Dispersion and Micellar Behavior; Kerker, M., Ed..; The ACS Symposium; 1975; Part 17, p 239, Mukerjee, P.; Yang, A. Y. S.J. Phys. Chem. 1976.80. 1388. (6) Zhu, B. Y.; Zhao, G. X.; Cui, J. G. In Phenomena in Mixed Surfociant Systems; Scamehorn, J. F., Ed.;American Chemical Society: Washington, DC, 1986; Vol. 311, pp 173 and 185. (7) Sugihara, G.; Nakamura, D.; Okwauchi, M.; Sakai, S.; Kuriyama, K.; Tanaka, M.; Ikawa, Y. Fukuka. Uniu. Sci. Reports 1987, 17, 31. (8) Guo, W.; Fung, B. M.; Guzman, E. K.; Christian, S.D. Mixed Surfactant Systems, ACS Symp. Ser., in pres. (9) Shafrin, E. G.; Zisman, W. A. J . Phys. Chem. 1962,66, 740. (10) Abenin, P. S.;Szdnyi, F.; Cambon, A. J. Fluorine Chem. 1991,55, 1. (11) Guo, W.; Li, Z.; Fung, B. M.; ORcar, E. A.; Hanuell, J. H. J. Phys. Chem. 1992, 96, 6738. (12) Carey, F. A.; Sundberg, R. J. Aduanced Organic Chemistry; 3rd ed.; Plenum Press: New York, 1990; p 150. (13) Rinaldi, P. L. Prog. Nuclear Magn. Reson. Specrrosc. 1983, IS, 291. (14) Ueno, M.; Kishimoto, H.; Kyogoku, Y. Chem. Lrtt. 1977, 599. (15) Zhao, J. Master Thesis, The University of Oklahoma, 1992. (16) Guo, W.; Brown, T. A.; Fung, B. M. J . Phys. Chem. 1991,95,1829. (17) Guzman, E. K. Master Thesis, The University of Oklahoma, 1989. (18) Martin, M. L.; Delpuech, J. J.; Martin, G. J. Practical NMR Spectroscopy; Heyden & Son: Chicheater, 1980; pp 3 W 3 0 3 . (19) Jones, A. J. Comput. Phys. 1970, 13, 201. (20) Muller, N.; Timothy, W. J. J . Phys. Chem. 1969, 73, 2024. (21) Rosen, M. J. Surfactants and Interfacial Phenomena; Wiley & Sons, Inc.: 1978; Chapter 3. (22) Lang, J.; Tondre, C.; Zana, R.;Bauer, R.; Hoffmann, H.; Ulbricht, W. J . Phys. Chem. 1975, 79, 276. (23) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, - J.; Tondre, C. J . Phys. Chem. 1976, 80, 905. (24) Frames, E. I.; Davis. H. T.: Miller. W. G.; Scriven. L. E. Am. Chem. SOC.Symp. Ser. 1979, 91, 35. (25) Frames, E. I.; Talmon. Y.; Scriven, L. E.; Davis, H. T.; Miller, W. G. J . Colloid Interface Sci. 1982, 82, 449.
J. Phys. Chem. 1992, 96, 10074-10080
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(26) Franaes, E. I.; Miller, W. G. J. Colloid Interface Sci. 1984,101, 500. (27) Ueno, M.; Kishimoto, H.; Kyogoku, Y. J . Colloid Inrerface Sci. 1978, 63, 113. (28) Bccker, E. D. High Resolution NMR, Theory and Chemical Appli-
cations, 2nd edition; Academic Press: New York, 1980; Chapter 7. (29) Guo, W.; Fung, B. M.; Christian, S. D. bngmuir 1992, 8, 446. (30) Wennerstrh, H.; Lindman, B.; SWennan, 0.;Drakenberg, T.; R e senholm, J. B. J. Am. Chem. Soc. 1979,101,6860.
Molecular Interactions in ((Dodecyioxy)methyi)-I 8-crown-6 Micelles Sumio Ozeki* Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Chiba 260, Japan
and Hiroko Seki Analysis Center, Chiba University, 1-33 Yayoi-cho, Chiba 260, Japan (Received: April 15, 1992; In Final Form: September 15, 1992)
The structural changes of ((dodecyloxy)methyl)-18-crown-6(C12-OM-crown)micelles were studied as a function of the degree of K+ chelation and temperature by dynamic light scattering and proton nuclear magnetic resonance methods. In water, the interaction between the tails and heads in uncharged micelles appeared along with a temperature-induced sphertrod transition, suggesting that the rugose surfaces of the spherical micelles should change into smooth surfaces of rodlike micelles. In the presence of KCl, the cores of spherical, charged micelles comprising C12-OM-crownand K+-chelated molecules seem to be a pseudointerdigitated structure at low temperature, as implied by the strong interactions between the crown heads and the methyl groups in tails evident in the NOESY spectra. The trend of the NMR chemical shift with temperature was interpreted in terms of a counter ion penetration into the rugose surfaces of the charged micelles. The structural transition of molecular assemblies, e.g., among interdigitated micelles (premicelles),pseudointerdigitatedmicelles, and normal micelles, is discussed.
Introduction A crown surfactant ((dodecyloxy)methyl)-18-crown-6 has a
high chelating ability and a selectivity for cations which result from the head group, 18crown-6. The crown ring as a head group is either planar and disklike or asymmetric. We can thus expect some cyclization effects on the micellar states.l In preceding we inferred that C,2-OM-crown forms some molecular assemblies in different states, unlike classical linear nonionic surfactants, oligo(oxyethy1cne)giycol " a l k y l ether: premicelles and a few micelles having different aggregation modes and shapes. The micelles in water depend on the surfactant concentration and are rodlike at 25 "C. Those in KCl solutions comprise C12-OMcrown (D) and C12-OMcrown*K+(DK+), and are thus charged and spherical. The formation of mixed micelles is also one of the cyclization effects. The crown head groups at the micelle surfaces seem to have a laser chelating ability for K+ than the intrinsic ability of 18crown-6 molecules in the bulk phase: e.g., in 0.1 mol/dm3 KCl the chelated fraction is ca. 40% at the micellar surfaces and 90% in a bulk solution.2 The extraordinary small {-potential, which could not be explained by the low charge density of the micelle surfaces, suggested that the mixed micelles have rugose (rough) surfaces at which counterions (Cl-) are distributed among the crown head groups.3 Since it is not plausible that the several changes of the micelle properties arise from changes in the micellar shape alone, the complex behavior of the physical properties should be ascribed to molecular interactions in a micelle: the structure of the micelle core, the surface structure, and the distribution of the head groups of D and DK+. In this study, in order to elucidate the complex transition of the micellar state we examined by means of 'H NMR spectroscopy, especially N O S Y , what interactions are possible between tails, tail and heads, and heads in micelles. Here, we 'Author to whom correspondence should be addressed.
discuss the (pseudo)interdigitated structure of hydrophobic cores which have antiparallel side-by-side contact between the hydrocarbon tails, as well as the rugosity (roughness) of the micellar surfaces. Experimental Sectien
C12-OM-crownwas prepared as previously r e p ~ r t e d . ~ The mutual diffusion coefficient (0)was measured using an electrophoretic light scattering spectrophotometer (Otsuka Electronits Co, ELS-800).3The cell temperature was controlled by circulating temperature-controlled water (within 10.1 "C). The sample solutions were circulated in advance through a Millipore membrane filter (0.2pm pore); the dust-free portion of the sample was supplied into the cell. We then irradiated 632.8-nm light from a H e N e laser onto the cell at incident angles of 5-20°. The apparent hydrodynamic radius (Rh)as a spherical micelle was calculated from D using the Einstein-Stokes equation. The aggregation number (N,)of the micelles was determined by an ultracentrifugal method (using a Beckman ultracentrifuge Spinco L8-80). Micellar solutions contained Sudan Red B (1[4'-(3''-tolylazo)-3'-tolylazo]-2-naphtho1) as a probe for UV detestion.2*5 The 'H NMR spectra were measured using a JEOL GSX400 Fourier-transform spectrometer (400 MHz for the proton resonance) at 0-70°C. The resolution was 0.180 Hz,or 0.00045 ppm. CI2-OM-crownwas dissolved in D20 (>99.75% Wako Jyunyah Co.,Ltd., Japan) containing KCI; the solution was then transferred to a 5 mm diameter NMR cell. A reference peak was obtained from sodium 3-(trimcthylsylyl)~esulfonate(Merck) in a glass capillary tube. ReSultS The apparent hydrodynamic radii and aggregation number (N,) of C12-OM-crownmicelles are summarized in Table I, along with the previous m ~ l t s . ' - ~ Figure 9~ 1 shows the variations of Rh and N, as a function of the temperature. In water, small micelles are
0022-365419212096-10074$03.00/0 0 1992 American Chemical Society
