J. Phys. Chem. 1987, 91, 113-116
113
Organic Counterion Binding to Micelles. Effects of Counterion Structure on Micellar Aggregation and Counterion Binding and Location Mikael Jamson* and Peter Stilbs Institute of Physical Chemistry, Uppsala University, S-751 21 Uppsala, Sweden (Received: April 18, 1986; In Final Form: July 7, 1986)
‘H NMR self-diffusion measurements on D 2 0solutions of decylammonium surfactants with organic counterions are reported. The degree of counterion binding within a two-site bound-free model was demonstrated to depend on the hydrophobicity and polarizability of the counterion. A correlation was found between the cmc and the degree of counterion binding. In order to investigate the detailed location of the alkyl chains of the counterions, T I excess relaxation rate measurements on systems containing the micellary solubilized paramagnetic relaxation-agent Tempo were performed. These measurements indicate that the orientation of the micellarly bound counterions with respect to the micellar surface folds over at a particular counterion alkyl chain length.
Introduction The presence of counterions in the vicinity of the micellar surface contributes to the stabilization of micelles of ionic surfactants. Counterions reduce the repulsive forces between the hydrophilic parts of the amphiphiles, stabilizing the micelle configuration with reference to a solution of free surfactants. With regard to experimental studies of counterion “binding”, most studies so far have been performed on inorganic ions. There is presently an increasing interest also in the solution properties of purely “organic” surfactant^.'-^ The association behavior of hydrophobic organic counterions can be anticipated to be different from that of inorganic counterions but has been little investigated, partly due to the lack of suitable techniques. The N M R based Fourier transform pulsed-gradient spin echo (FT-PGSE) technique has been demonstrated to be a powerful tool in the investigation of counterion binding in polyelectrolyte It has been shown specifically that the technique gives direct and easy access to information also on organic counterion binding to micelle^.^ With this approach, one can simultaneously monitor the self-diffusion coefficients of the counterions and the amphiphile and, through a comparative procedure, conveniently quantify aggregation and ion binding processes in solution (see Experimental Section). In a previous paper,g we reported N M R self-diffusion measurements on D 2 0 solutions of decylammonium acetate, chloroacetate, and dichloroacetate. The association behavior of the three surfactants (quantified in terms of the degree of counterion binding, the cmc, and the nonmicellar amphiphile concentration) was found to be distinctly different and correlated to the hydrophobicity of the counterion. The present communication reports a detailed and extended study, using the FT-PGSE N M R technique, on micellar binding of counterions with pronounced hydrophobic character. The object of the study is to investigate how the counterion characteristics, especially the hydrophobicity, affect the degree of counterion binding and the association behavior of the amphiphile. The orientation of the counterion, when bound to the micelle, can be expected to vary between different counterions. Dependent (1) Broxton, T. J. Aust. J . Chem. 1981, 34, 2313. (2) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1984,100, 128. (3) Underwood, A. L.; Anacker, E. W. J. Phys. Chem. 1984,88, 2390. (4) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1984,106, 86. (5) Lindman, B.; Puyal, M-C.; Kamenka, N.; RymdEn, R.; Stilbs, P. J. Phys. Chem. 1984, 88, 5048. (6) Lindman, B.; Kamenka, N.; Puyal, M-C.; Brun, B.; Jbnsson, B. J. Phys. Chem. 1984, 88, 53. (7) Stilbs, P.; Lindman, B. J. Phys. Chem. 1982, 85, 2587. (8) Stilbs, P.; Lindman, B. J. Magn. Reson. 1982, 48, 132. (9) Jansson, M.; Stilbs, P. J. Phys. Chem. 1985, 89, 4868.
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on the amphiphilic character of the arganic counterion, the counterion can either be located mainly at the micelle surface or form what would essentially be mixed micelles with the decylammonium ions. In order to investigate the surface-related orientation of some of the longer counterions, relaxation rate measurements were performed using the paramagnetic and hydrophobic relaxation agent Tempo as a solubilized micellar probe. Orientational information of the counterion is obtained by comparing the paramagnetic relaxation contribution at two different proton locations on the counterion. Experimental Section The examined surfactants were decylammonium propanoate, chloropropanoate, butanoate, isobutanoate, pentanoate, and trimethylacetate. Preparations of the decylammonium surfactants were made by neutralizing an aqueous solution of the corresponding carboxylic acid with decylamine, as described in ref 9. Decylamine and the carboxylic acids were all of puriss quality. The diffusion measurements were performed at a temperature of 38 f 0.5 O C (in order to make the results intercomparable with those in ref 9) on a standard JEOL FX-100 Fourier transform NMR spectrometer operating at 2.3 T, using procedures described earlier. The monitoring of the association process is based on the difference in the intrinsic self-diffusion coefficient for the amphiphile monomers, counterions, and micelles (the latter monitored by measuring the self-diffusion coefficient of an added micellar solubilizate, hexamethyldisiloxane). The effective translational mobility of an amphiphile or a counterion is considerably reduced when diffusing with a micelle and the binding process is reflected in a sensitive manner in its time-averaged self-diffusion coefficient. When applying a two-site model
to the self-diffusion data, one can calculate the amount of miand the amount of associated councellized amphiphile, Cmic, (see ref 9 for a more complete description of the terions, Cmic procedure). In order to quantify the degree of association of the counterion to the micelle within the two-site bound-free model, we introduce the degree of counterion binding, 8, defined as C m x / C+mic.
The critical micelle concentration, cmc, was determined from the self-diffusion data of the amphiphile. According to the phase separation model,12a plot of the self-diffusion coefficient of the amphiphile, D,, vs. Cto;l gives a straight line, intersecting with D,, = Dfr, at cmc. Dfreewas taken from the low concentration (10) Stilbs, P.; Moseley, M. E. Chem. Scr. 1980, 15, 176. (11) Stilbs, P.; Moseley, M. E. Chem. Scr. 1980, 25, 215. (12) Lindman, B.; Wennerstrom, H. Top. Currenr Chem. 1980, 87, 1.
0 1987 American Chemical Society
114 The Journal of Physical Chemistry, Vol. 91, No. 1 , 1987 TABLE I: Observed fl Values at Different Surfactant Concentrations counterion 0.10 M 0.126 M
CHqCHlCOO-
0.66
0.66
Jansson and Stilbs
0.142 M
0.154 M
0.175 M
0.20 M
0.67 0.76 0.86 0.65 0.7 6 0.74
0.65
0.64
0.67
060
I-
050
-
I
I
I
1
I
02
0
c
0.LO 0'50
i
0
8 0
8
1
0
0.10
.
0 00 00
B
O
b
0
0
.
0
0
0
0
c I
2
> I
030
0
-
0.30
[L
O.1°
0 8
oooO\
I
I
I
I
1
10
20
30
LO
50
00
60
-
I
1
I
I
1
10
20
30
LO
50
[Tempo] 105/M
Decylammonium pentonoate
:I
0
8 0 8
0
8
1
0
0
0.50
3'0
L'O
5'0
6'0
[Tempo] 1 0 5 h Figure 2. The excess relaxation rate of the methyl protons (0) and the and ) a-methyleneprotons (0)of isobutanoate, and the methyl protons (I the methylene protons ( 0 )(an average value of all CH2 protons) of the decylammonium ion, as a function of the Tempo concentration in a 0.12 M D 2 0 solution of decylammonium isobutanoate.
data. This method does not determine the cmc with the same accuracy as the traditional conductance or surface tension methods. The cmc values which were essentially obtained as a "byproduct" of the analysis of the diffusion data are rather approximative, with estimated error limits of 20%. The proton NMR relaxation rate measurements were made under the same conditions as the diffusion measurements. The stable free-radical 2,2,6,6-tetramethylpiperidinyl- 1 -oxy, Tempo (dissolved in a 0.05 mM solution of a 1 5 mixture of ethanol and D20),was added in increments of 3 to 15 WL to 0.5 mL of a 0.12 M surfactant solution, and the relaxation rate was monitored up to a final Tempo concentration of 0.06 mM. Measurements were performed only on decylammonium propionate, butanoate, isobutanoate, and pentanoate since at least two different proton
1 0
0.40 c v)
2
0.30
a
0.20 0.10
000 2'0
60
Figure 3. The excess relaxation rate of the methyl protons (0) and the a-methylene protons (0)of butanoate, and the methyl protons (m) and the methylene protons ( 0 )(an average value of all CH, protons) of the decylammonium ion, as a function of the Tempo concentration in a 0.12 M D 2 0 solution of decylammonium butanoate.
Decylammonium isobutanoate
Ib
0
0 0
o
t1 t
-
8
0
0
[Tempo] 105/M Figure 1. The excess relaxation rate of the methyl protons (0) and the methylene protons (0)of propanoate, and the methyl protons (m) and the methylene protons ( 0 )(an average value of all CH2 protons) of the decylammonium ion, as a function of the Tempo concentration in a 0.12 M D 2 0 solution of decylammonium propanoate.
0.50 0.40
8
0
OLO c
a
[L
-
o
L
00
I
I
I
I
I
10
20
30
LO
50
60
[Tempo] l 0 Y M Figure 4. The excess relaxation rate of the methyl protons (0)and the
a-methylene protons (0)of pentanoate, and the methyl protons (m) and the methylene protons ( 0 )(an average value of all CHI protons) of the decylammonium ion, as a function of the Tempo concentration in a 0.12 M D20solution of decylammonium pentanoate. locations on the counterion are required in order to obtain orientational information. The relaxation data is represented as excess relaxation rate, Re,, defined by Re, = 1 / T , - 1/TIo
(1)
where 1 / T I 0is the relaxation rate without Tempo.
Results The cmc and the degree of counterion association were determined by measuring the self-diffusion coefficients of the decylammonium ion, the counterion, and the solubilizate at 14-16 different surfactant concentrations. The p values in the upper concentration range (>0.10 M) are listed in Table I (the p values
The Journal of Physical Chemistry, Vol. 91, No. 1, 1987 115
Organic Counterion Binding to Micelles
r----l
"O
0.9
1 -
.4 0.8 -
=
I 0.7 06
1
20
30
I
2A
I
-
1.
I
a
A2 2 0 30
0.7 .
lA&f 2
2.
10
-
m
I
I
I
I
0.6 . 0.5
1
0.00
1.
I
0.02
I
0.04
I
0.06
1
0.08
CMC/M
Figure 6. 6 vs. cmc. R groups in counterions RCOO- are indicated by numbers and symbol: 1 0 , CH,; 2 0 , CH3CH2; 3 0 , CH3CH2CHz;4 0 , CH3CHzCHZCHz; 1 A, (CH3)ZCH; 2 A, (CH3)jC; 1 0,ClCHZ; 2 0, C1CH2CHz;0,C12CH. The data for 1 0 , 1 0, and 0 are taken from ref 9.
(13) Zana, R. J. J . Colloid Interface Sci. 1980, 78, 330. (14) Khan, A.; SMerman, 0.;Lindbolm, G. J. Colloid Interface Sci. 1980, 90
chlorine-containing counterions. This demonstrates that @ is correlated to the hydrophobicity of the counterion, and that a large contribution to the micelle-counterion attraction forces is hydrophobic in nature. The branched counterions exhibit lower /3 values than their unbranched counterparts. The difference in counterion binding for isobutanoate (/3 = 0.67) and butanoate (@= 0.75) is striking. This may partly be explained with the lower hydrophobicity of the branched counterion. An interesting parallel is found in the studies of micelle solubilization; it has previously been found that branched-chain solubilizates have a lower degree of solubilization than their n-alkyl analogues.'* The @ values found for the chlorine-containing counterions are high as compared to those of acetate and propanoate. As for the branched counterions, this observation cannot be explained by hydrophobic interactions alone. It is reasonable to assume that the larger polarizability of the chlorine-atom contributes to the ion-micelle attraction, leading to high @ values. Relation between @ and the Overall Amphiphile Aggregation. There is a distinct correlation between the amount of bound counterions and the aggregation behavior of the amphiphile. In a previous paper9 we demonstrated that the rate of reduction of the free amphiphile concentration with increasing surfactant concentration is dependent on the @ value of the counterion, in line with other theoretical and experimental investigation^.^^^^'^ Furthermore, Underwood et aL2 have demonstrated that the aggregation number of the micelle is influenced by the nature of the counterion. Figure 6 illustrates another correlation between the degree of counterion binding and the amphiphile aggregation. In Figure 6 the degree of counterion association is displayed as a function of the cmc for the corresponding surfactant (the cmc values are approximative as pointed out in the Experimental Section). As seen from the figure, there is a correlation between a high @ and a low cmc that exceeds any experimental errors. One could attribute this to the higher extent of reduction of the repulsive forces between the hydrophilic parts of the amphiphiles when /3 is high. The micelle configuration then becomes stabilized relative to a solution of free surfactants, and micelle formations will be promoted at lower concentrations as compared to a surfactant with a low 0. Tempo Excess Relaxation Rate Measurements. The four examined counterions propanoate, isobutanoate, butanoate, and pentanoate have separable peaks in the 100-MHz proton NMR spectra along the alkyl chain. By comparing the influence on the
- 3 7
l o , LI I .
(15) Vikingstad, E. J. Colloid Interface Sci. 1980, 73, 260. (16) Leo, A.; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 521. (17) Hansch, C.; Leo,A. Substituent Constants for Correlation Analysis in Chemistry and Biology;Wiley: New York, 1979.
(18) Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385. (19) Hartley, G. S. Aqueous Solutions of Paraffin Chain Salts; Hermann: Paris, 1936.
J . Phys. Chem. 1987, 91, 116-120
116
relaxation rate at two different counterion proton sites upon addition of a relaxation agent, one can obtain orientational information of the counterion, related to the nitroxyl-proton distance. The use of nitroxides as spectroscopic probes in hydrocarbon assemblies and the interpretation of any results has been a matter of considerable controversy in the past. In particular, the actual location of the nitroxide group in microheterogenous systems has been much debated. Mukerjee et a1.2@zzhave presented arguments for an exclusive location of the nitroxide group in the water-micelle region as based on arguments about microenvironmental polarities and band shifts in the UV and visible region. On the other hand, ESR data have been interpreted to be consistent with a location of the nitroxide group in the micellar hydrocarbon core.23 Symons and Pena-Nuiiez in a very recent paper summarize much of the earlier literature and present a study on di-tert-butyl nitroxide (DTBN) solvation in various solvent systems,24where they suggest that nitroxides in micelles and membranes are monohydrated. In any case, one must expect the location of the solubilized nitroxides to differ with nitroxide type. A bulky or aromatic molecule may be difficult to physically incorporate into the hydrocarbon chain environment and such nitroxides may be forced to reside at the micellar surface. In the case of polyfunctional molecules one must also consider any specific interactions between the nitroxyls and the functional groups. Figures 1-4 demonstrates that the excess relaxation rate is higher for the amphiphile part of the surfactant than for the counterion, both for the methylene and the methyl protons. This is partly due to the partial dissociation of the counterions from the micelle, but even for pentanoate for which /3 = 0.87 there is a significant difference between the counterion and amphiphile excess relaxation rates. This observation would indicate that the nitroxide group is not completely located in the water-micelle region. (20) Mukerjee, P.; Ramachandran, C.; Pyter, R. A. J . Phys. Chem. 1982, 86, 3189. (21) Ramachandran, C.; Pyter, R. A,; Mukerjee, P. J . Phys. Chem. 1982, 86, 3198. (22) Pyter, R. A,; Ramachandran, C.; Mukerjee, P. J . Phys. Chem. 1982, 86, 3206. (23) Yoshioka, H. J . Am. Chem. SOC.1979, 101, 28. (24) Symons, M.; Pena-Nuiiez, A. J . Chem. Soc., Faraday Trans. 1 1985, 85, 242 1.
The data of Figures 1-4 show that the orientation of butanoate and pentanoate differ from that of propanoate and isobutanoate. The methyl excess relaxation rate is higher than the a-methylene excess relaxation rate for butanoate and pentanote, but the reverse is observed for propanoate and isobutanoate. If we assume that the paramagnetic relaxation contribution is higher inside the micelle, as the amphiphile relaxation rate excess measurements indicate, this would lead to the conclusion that it is more likely to find the methyl group inside the micelle than the methylene group for butanoate and pentanoate. This means that the hydrocarbon tails of butanoate and pentanoate would be located inside the micelle to a greater extent than those of isobutanoate and propanoate, corresponding to more of a mixed micelle situation. The still unresolved question of the actual location of the probe in the micelle makes it hazardous to draw firm conclusions from the measurements, however. Nevertheless, a comparison of the results found for the examined counterions demonstrate that there is a relative orientation difference between butanoate and pentanoate on one hand, and the less hydrophobic counterions on the other.
Conclusions It is demonstrated that the degree of counterion association is affected by several counterion characteristics. Hydrophobic interactions between the micelle and the counterion are demonstrated to be important contributions to the ion-micelle attraction and a correlation is found between the degree of counterion association and the aggregation behavior of the amphiphile, even though the longer unbranched counterions seem to be less efficient in promoting a low cmc than the smaller and less hydropobic counterions. The excess relaxation measurements indicate that there are differences in the orientation of the micellarly bound counterions with regard to the micellar surface. The alkyl chains of butanoate and pentanoate most probably aggregate with the decylammonium surfactant part so as to form a mixed-micelle-like situation. Registry No. Tempo, 2564-83-2; decylammonium propanoate, 39108-01-5; decylammonium chloropropanoate, 104875-20-9;decylammonium butanoate, 73702-94-0; decylammonium isobutanoate, 104875-21-0; decylammonium pentanoate, 104875-22-1; decylammonium trimethylacetate, 104875-23-2;hexamethyldisiloxane, 10746-0.
Nuclear Magnetic Relaxation in Micellar Systems. Influence of Micellar Size 0. Sodeman,*+ U. Henriksson,* and U. OIssont Division of Physical Chemistry 1. University of Lund, P.O.B. 124, S-221 00 Lund, Sweden, and Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, Sweden (Received: April 30, 1986; In Final Form: August 13, 1986)
An extensive 2H NMR relaxation study for hexadecyltrimethylammonium chloride micelles, with spin-lattice relaxation rates measured at nine magnetic field strengths and spin-spin relaxation rates at three magnetic field strengths, is presented. The measured relaxation rates show a strong dependence upon the magnetic field strength, and the data are analyzed with the "two-step" model of relaxation. A comparison is made with previously reported relaxation data for dodecyltrimethylammonium chloride micelles and the two-step model proves to be an excellent description of NMR relaxation in spherical micellar systems. Moreover, the motion causing the frequency dependence in the NMR relaxation can be described with an exponential correlation function with a correlation time that increases with the size of the spherical micelles.
Introduction Nuclear magnetic resonance ( N M R ) ~relaxation studies have been used extensively in the study of isotropic surfactant systems, e.g. micellar systems. In particular, I3C TI and nuclear Overhauser University of Lund. 'The Royal Institute of Technology.
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enhancements have proven very valuable in this regard.2-5 A general result of these studies is that several different types Of (1) Abbreviations used: NMR, nuclear magnetic resonance; CI,TAC1, dodecyltrimethylammonium chloride; C,,TACl, hexadecyltrimethylammonium chloride; CI,TAF, hexadecyltrimethylammonium fluoride. (2) London, R. E.; Avitabile, J. J . Am. Chem. SOC.1977, 99, 7765.
0 1987 American Chemical Society
