J. Phys. Chem. 1992,96, 7109-71 15 The rate constant data of Duplltre and Jonah will be reinterpreted elsewhere. References and Notes (1) Duplltre, G.; Jonah, C. D. J . Phys. Chem. 1991, 95, 897. (2) Atkins. P. W. Physicol Chemistry, 4th ed.; Freeman: New York, 1990; p 760-1 (note that B requires p). (3) Weast, R. C. Hondbook of Chemistry ond Physics, 70th ed.; CRC Press: Boca Raton, FL, 1989; p F-43. (4) Conway, B.E. Electrochemicol Doto; Elsevier: Amsterdam, 1952; pp
7109
155, 162. (5) Jones, G.; Fornwalt, H. J. J . Am. Chem. Soc. 1935, 57, 2041. (6) Honvath, A. L. Handbook of Aqueous Electrolyte Solutions; Wiley: New York, 1985; p 252. (7) (a) Milner, S.R. Philos. Mag. 1912, 23, 551; (b) 1913, 25, 742. (8) Debye, P.; Hiickel, E. Phys. Z . 1923, 24, 185, 305, 335. (9) Hasted, J. B.; Roderick, G . W. J . Chem. Phys. 1958, 29, 17. (10) Hill, N. E.; Vaughan, W. E.; Price, A. H.; Davies, M. Dielectric Properties ond Molecular Behouior; van Nostrand: New York, 1969; pp 348-354.
Structure of Reversed Micelles in the Cyclohexane/Polyoxyethylene(4)nonylphenol/Water System, As Studied by the Spin Probe Technique H. Caldaram,*qt Agneta CaragheorgheopoI,+M. Dimonie,* D. Donescu,* Ileana Dragutan," and N. Marinescut Romanian Academy, Institute of Physical Chemistry, Splaiul Independentei 202, 77208 Bucharest, Romania, Polytechnic Institute. Bucharest, Romania, Research Institute for Plastic Materials, Bucharest, Romania, and Romanian Academy, Center of Organic Chemistry, Bucharest, Romania (Received: July 31, 1991; In Final Form: April 2, 1992) The cyclohexane/polyoxyethylene(4)nonylphenol (E0(4)NP)/water system (2.8% EO(4)NP) with [H20]1[EO(4)NP] molar ratios between 0.1 and 6.9, has been investigated by electron spin resonance (ESR) spectroscopy using the following nitroxides: 3-carboxy-2,2,5,5-tetramethylpyrroIine-lsxyl (I), 2,5-dihydr~2,2,5,5-tetramethyl-3-[(triethylammonio)methyl]pyrrole1-oxy1 bromide (11), and 5-doxylstearic acid (IV) as spin probes. For radicals I and I1 the nitrogen hyperfine splitting, aN,and , been analyzed, yielding information about the polarity and viscosity of their locations. the rotational correlation time, T ~have At [H20]/[EO(4)NP] 6 0.3,the presence of "waterless" micellar aggregates in fast exchange (v, > 106 s-') with the monomer surfactant is evident. At [H20]/[EO(4)NP] = 0.6, the core viscoSity is at its maximum value and the exchange rate decreases below the limit of slow exchange (vex< lo6 s-I). Upon further increasing the water content, the core becomes more polar and less viscous, with these parameters approaching their values in bulk water. The effect of Cu(I1) ion (in an oil-soluble complex) on the spectra of radical II-confined to the water-pool-has been interpreted by using Leigh's theory, and the 'distance of closest approach" between these two paramagnetic species has been evaluated. This distance is considered to represent a measure of the penetrability of the micellar shell. Radical IV yielded anisotropic spectra in the aggregates with slow exchange. From their ESR parameters the order degree of the surfactant chains in the micellar shell has been evaluated. All results are consistent, indicating reduced penetrability and increased order with increasing dimensions of the water-pool.
Introduction Several surfactants are able to aggregate when dissolved in nonaqueous solvents to yield reversed micelles.'~2Formation of reversed micelles requires traces of water? with their polar core being able to solubilize significant amounts of water. In recent years considerable attention has been paid to these reversed systems both for their resemblance to biomembranes and for their peculiar behavior in catalysis of polar molecules.' The association mechanism of these surfactants in nonpolar solvents and, related to this, the mechanism of the uncommon highly catalytic activity of the system are poorly understood! Therefore, information on the association behavior of the surfactant, on the properties of formed aggregates, and on the water-pool entrapped therein is indispensable for the understanding of any of the two mechanisms. The ESR of spin probes in micellar systems has been proven as a powerful technique for studying the aggregation behavior of different surfactants, the properties (microviscosities and local polarities) of the environment around the probe in the micelles, the effect of different solubilizates on these properties, and the dynamics of the micellization processes.' However, only very few papers reported spin probe studies on reversed micellar systems, 'Romanian Academy, Institute of Physical Chemistry. 'Polytechnic Institute. f Research Institute for Plastic Materials. Romanian Academy, Center of Organic Chemistry.
all of them concerning ionic surfactants, either sodium bis( 2ethylhexyl) sulfosuccinate (AOT)5-8or dodecylammonium propi~nate.~ The present work reports the detailed information obtained by the spin probe technique on the aggregation behavior of a nonionic surfactant (polyoxyethylene(4)nonylphenol) in a nonaqueous solvent (cyclohexane) in the presence of increasing amounts of water. This system has been formerly studied by Kitahara,lowho found by the light scattering method that the micellar weight increases with the added amount of water up to a [H20]/[surfactant] molar ratio of 1.65. In the present work hydrophilic spin probes have been chosen with the aim of focusing the investigation on the aqueous part of the aggregate. The location of the spin probe has been established using appropriate paramagnetic Cu(I1) compounds which preferentially dissolved in hydrophilic or in the hydrophobic part of the system. In the latter case of a Cu(I1) compound soluble only in cyclohexane, one observes that the ESR signal of the spin probe-confined to the water-pool-loses its intensity without apparent broadening. This experimental fact has been treated within the frame of Leigh's theoretical approach and led to information about the level of cyclohexane penetration through the micellar shell as a function of the amount of water in the aggregate. The use of 5-doxylstearic acid as a spin probe provided valuable information both on the structural order in the shell of the studied reversed micelles and on its dependence on the added amount of water.
0022-3654/92/2096-7109$03.00/00 1992 American Chemical Society
7110 The Journal of Physical Chemistry, Vol. 96, No. 17, 19192
Experimental Part Materials. Polyoxyethylene(4)nonylphenol (EO(4)NP) (commercial product) has been purified by repeated treatments with deionized water for removing the water-soluble ethylene glycols. The final product has been dried in vacuum. Liquid chromatography analysis indicated a distribution of polyoxyethylene chain lengths which closely approximated a Poisson distribution with an average number of four ethylene oxide units per molecule. Carbowax 200 (molecular weight = 200) (Loba Chemie) has been used without any further purification. Cyclohexane has been dried on molecular sieves and then distilled. Freshly prepared deionized water has been used. The following nitroxide radicals have been used as spin probes: 3-carboxy-2,2,5,5-tetramethylpyrroline-l-oxyl (I) COOH
6 1
has been prepared according to Rozantsev." 2,5-Dihydro-2,2,5,5-tetramethyl-3-[ (triethy1ammonio)methyllpyrrole-1-oxy1 bromide (11)
0
II
has been prepared as follows: To 0.2 g (0.858 mmol) of 3(bromomethyl)-2,2,5,5-tetramethyl-2,5-dihydropyrrole1-oxy1a 5-fold molar excess of dry triethylamine was added and the red solution was heated for 2 h at 80 "C. The reaction mass has been then triturated in ethyl ether and the resulting precipitate quickly filtered; purification by crystallization from e t h a n o l d e r afforded 0.13 g of the spin probe, light-brown crystals. Mp 208-21 1 OC (dec). IR spectrum (KBr pellet): ,Y 1465,2930,2975,and 3050 cm-I. Anal. Calcd for C,,HNN20Br: C, 53.88; H, 9.04, N, 8.38; Br, 23.90. Found: C, 53.61; H, 8.78; N, 8.43; Br, 24.11. The spin-labeled surfactant I11 CH3- (CH218
W O I C H 2 CH20kOC
i 0
111
The cyclohexane-soluble Cu(I1) complex, CuX2, where X = 1-methyl-2-carbethoxyglyoxal-1-ptolylanil)-2-(m-tolylhydrazone) has the following formula:
i L
B I
cH3
.-CH3J
2
V
,Samples. A solution of surfactant (5.5 X M) in cyclohexane has been prepared. Various amounts of water were added to this solution and the resulting samples, with [H20]/[surfactant] molar ratios between 0.1 and 6.9, were mechanically stirred. Addition of more water led to phase separation. The concentration of water was measured by the Karl Fischer method. The resulting transparent solutions preserved their properties for months. The spin probes (nitroxides) were added to the samples by the following procedure: the required amount of nitroxide in absolute ethanol was thoroughly evaporated in a volumetric flask and the surfactant solution sample was then added to give a concentration of the spin probe of 1 X lC4M. Other nitroxide concentrations were also used. Dilute samples refer to concentrations such that a unique spectrum is observed. Concentrated samples were also prepared to follow nitroxide-nitroxide interaction in the same micelle. The samples were shaken to ensure the solubilization of the spin probe. Precautions have been taken for keeping both solution sample flasks and the ESR sample tubes tightly closed for preventing the constituent evaporation. ESR Meuwemeats. The ESR spectra were recorded at 25 "C on a JES3B (JEOL) spectrometer with 100-kHz field modulation using X-band frequency. Variable temperature measurements have been carried out with the spectrometer attachments. Precautions on the microwave power and field modulation amplitude have been taken to avoid line broadening artifacts. The individual components of the nitrogen triplet were recorded with about 1 G/min scan rate, each sample being measured 3-5 times for reproducibility. The parameters of the ESR spectra were measured in comparison with those of Fremy's salt (aN = 13.0 G). The rotational correlation time T~ has been calculated according to the formulai3 TC
=
10-io)AH(0)([h(O)/h(-l)]i/2+ [h(0)/h(l)]1/2 - 2) s where AH(0) is the line width in gauss and h(-l), h(O), and h( 1) are the peak heights of the M = -1,O, and +1 lines, respectively. (6.51
X
Results and Discussion
has been obtained from 3-(chlorocarbonyl)-2,2,5,5-tetramethylpyrroline-1-oxy1 and surfactant itself under essentially the same conditions as those describedI2for PEG samples with molecular weights between 600 and 20000. After reaction the mixture was filtered with suction and the benzene filtrate washed several times with a 10% sodium hydroxide aqueous solution and water. Removal of the solvent followed by column chromatography of the residue on alumina (ethyl ether as eluent) afforded a red, viscous oil (yield 85%). An IR spectrum (CC14)showed no free hydroxyl groups; vco 1710 cm-I. 5-Doxylstearic acid (IV) CH3-ICH2Ip-
Caldararu et al.
(CH213-CCOH qpy-0
't IV
has been purchased from Aldrich Chemical Co. CuC12.H20(Analar) has been dehydrated at 150 "C for 3 h and kept in a desiccator.
Radical (I). All ESR spectra of this spin probe (lo4 M)in the studied series (except samples 0 and 1) have asymmetric or even split lines, corresponding to overlapped spectra (Figure la). So,the nitroxide is distributed between at least two locations, the exchange rate being slow on the ESR time scale (vex< IO6 s-l). Addition of Cu2+ions, as a water-insoluble organic complex CuX2 (Figure lb) or as water-soluble CuC1, (Figure IC),leads to the selective broadening of one or the other of the superimposed spectra, leaving a unique spectrum with symmetric lines. The "quenching" with Cu2+ions clearly demonstrates the presence of the nitroxide (I) in two phases, i.e. a water-like and an oil-like phase. The ESR parameters of these spectra correspond to the above assignments, as the spectrum 'quenched" by CuX2 has an aNvalue close to that in water, whereas the other one "quenched" by CuC12 has a value close to that in cyclohexane. Due to the very small proportion of water in our samples, the amounts of spin probe in the two phases are comparable, in spite of its low distribution ~oefficient.'~Reducing the overall nitroxide concentration leads to the preferential reduction of the oil-like spectrum intensity, since in the first situation the water
Structure of Reversed Micelles
n
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 7111 TABLE II: ESR Parameters of Spin Probe I1 in the Micellar Solutions, Measured at 25 OC
1 2 3 4 5 6 7
0.3 0.6 0.9 1.a
2.6 5.8 6.9
water
14.5’ 15.1 15.3 15.4 15.5 15.7 15.7 15.9
a
5.4 4.8 3.4 2.7 1.5 1.4 0.5
‘Very weak spectrum. values are believed to correspond to rapid exchange (at ESR time scale) of the spin probe between cyclohexane and “waterless” micellar aggregates. The same situation has been encountered in the case of spin probe IV, the effect on the ESR parameters being more noticeable, and will be discussed in more detail in that case. These data indicate that in all samples the spin probe has been incorported into micellar aggregates. As radical IV in samples 4-7 presents anisotropic spectra (see later), contribution of the micelle rotation and of the surfactant diffusion to the observed T, is excluded in those cases. It is known that when the hydrodynamic radius of the micelles exceeds 20 A, its tumbling rate becomes too slow ( u < lo8 s-l) to contribute to the rotational correlation time.” This should be the case for “well-organized” EO(4)NP micelles, when the surfactant molecules have their chains at least partly extended (the length of the fully extended chain is 30 A). Thus, the measured T, refers to the intramicellar motion of the spin probe. As T, has its maximum value in sample 2 and gradually decreases with increasing water content toward sample 7, one can assume that in samples 2 and 3 T, also refers to the intramicellar motion; otherwise a different variation of T, with the water content would be expected. The aggregates in samples 0 and 1 are probably smaller and a contribution of the micellar motion to T, cannot be excluded.I6 However, due to the presence of the rapid exchange effect, the T, values in the aggregates could not be evaluated. Radical (II). This charged radical is practically insoluble in cyclohexane, so that we could not obtain its ESR spectrum in the pure solvent. In samples 0 and 1 the spectra were so weak that only uN but not T, could be determined. In samples 2-7 low concentrations of the spin probe yielded ESR spectra with three symmetrical lines of a unique species, which corresponds to the nitroxide in the polar phase. This location assignment results from the uN values (Table 11), as well as from the total “quenching” of the spectrum with CuC12. A concentrated solution of the spin probe in sample 2 yielded a single broad line spectrum, which is known to result from the exchange narrowing effect caused by the high nitroxide concentration within the micelle aqueous c ~ r e . ~ ’ Upon , ’ ~ nitroxide dilution this spectrum gradually changes into the usual triplet spectrum of the dilute sample; both the latter spectrum and the single line spectrum are “quenched” by CuC12. The cyclohexane-soluble CuX2 complex also affects these spectra, but in a different way, reducing their intensity without apparent broadening. This effect has been used to evaluate the “distance of closest approach” between the spin probe and Cu(1I) ion in micelles with different water content, the results being discussed later. The following discussion will be focused on the uN and T , parameters for the dilute solutions of the spin probe in samples 2-7. These parameters obtained at 25 OC are given in Table 11. The same variation of uN and T , is observed as for the aqueous spectra of radical I. As in the case of radical I (see above), the measured T , refer to the intramicellar motion of the spin probe. It is well-known that T, can be identified with the reorientational correlation time given by the Debye-Stokes-Einstein theory
-
Figure 1. ESR spectra of radical I in sample 5 ([HzO]/[EO(4)NP] = 2.6): (a) overlapped spectra; (b) with CuXz,the water-like spectrum;
(c) with CuClZ,the oil-like spectrum.
TABLE I: ESR P”eten of Spin Probe (I) in the Micellar Mutiom, Measmed at 25 O C sample [WI/~EO(4)NPl ON, G fc(10-’0), s 0 0.1 14.1 0.6‘ 1 0.3 14.2 0.7 2 0.6 15.0 5.4 3 0.9 15.4 4.8 1 .a 15.5 4.5 4 2.6 15.6 3.5 5 5.8 15.7 2.2 6 I 6.9 15.8 1.3 water 16.1 0.3 cyclohexane 14.1 0.2‘ a
Spectra measured after deoxygenation.
phase was already saturated with the nitroxide. So, at a lower nitroxide concentration-the actual value depending on the water content of the examined samplerone can reach the situation when only the water-like spectrum is observed. In Table I the more precise ESR parameters measured on these diluted samples at 25 OC are given. The uN parameter for sample 2 indicates a value clearly influenced by water (15.0 G vs 14.1 G in cyclohexane and 16.1 G in water), which further increases with increasing water amount, approaching the value in bulk water. The T, value has a maximum for sample 2, which corresponds to a significant immobilization of the spin probe as compared to its value either in water or in cyclohexane. As the amount of water increases, T, decreases but remains significantly higher than in bulk water. In samples 0 and 1 a unique spectrum is observed, with the same spectrum coexisting in samples 2-7 with the water-like spectrum. Its ESR parameters have been measured in samples 2-7 after “quenching” the water-like spectrum with CuCI2. In sample 0 the parameters have been measured after its thorough deoxygenation (by bubbling argon saturated with cyclohexane), when the lines become narrower and T, can be determined more precisely (Table I). In this case the uN = 14.1 G and T , = 0.6 X 1O-Io s
T,
= 4rR37/3kT
7112 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 I
Caldararu et al.
-1
I
11 I
\
31‘
0
I--i_L_il 1
2
3
L
5
6
7
[H201/IEOlILl NP
Figure 2. Dependence of the rotational correlation time, T ~of, radical I1 on the [H20]/[EO(4)NP] molar ratio of the micellar samples at
various temperatures.
0-4
TABLE III: ESR P8lrWterS of Spin probe II in Pdy(oxyetbykw) ( c N b O W 8 X W)/Water Mixtures, M c r e d at 25 O C sample [H201/[Carbowaxl ON, G Tc(lo-’o), S M-3 M-4 M-5 M-7
1’
1 .o 1.8 2.6 6.9
14.9 15.0 15.1 15.2
6.5 5.3 4.6 2.8
where R is the hydrodynamic radius of the spin probe, q is the viscosity, and the other parameters have their usual meanings. In this expression it is assumed that the solvent is a structureless continuum, a condition which is quite approximate in spin probe studies. The temperature dependence of T, is presented in Figure 2. At each temperature the same trend is observed, T~ increases with decreasing water content more rapidly toward the lower water content. The increase in T, is steepest at the lowest temperature. The aNvalues did not vary over the investigated temperature range, a precious indication that the micellar structure remains unchanged within this temperature range, and the observed variation of r , reflects the viscosity variations of the micellar core. The following considerationscan be made concerning the description of the micellar core in a m d with the expeiimental data. At maximum water content the aN and T , values approach the values in bulk water. The same situation is found in AOT reversed 1nicelles~3~ when proper spin probes (not in fast exchange with the shell) were used; in this case a large water-pool is reportedS5On the other hand, for (EO), alkyl surfactants literature data19v20 indicate that the maximum hydration corresponds to two to three water molecules per ethylene oxide unit. In our system the largest accepted water amount is close to this value. At this point the following question arises: is the observed variation due to a continuous hydration of the ethylene oxide chains by all available water, or is it due to a partial one, accompanied by the formation of a “water-pool”? In order to distinguish between these two possibilities, polyoxyethylene (Carbowax 200)/water mixtures have been prepared, which reproduce the micellar core compositions. The U N and T , values of radical I1 in these mixtures, measured at 25 OC are presented in Table 111, while the T, values at various temperatures are given in Figure 3. On comparison of the curves in Figures 2 and 3 at the same temperature, as well as the ( I N values in Tables I1 and 111, it is obvious that in the micellar core the spin probe senses a more polar and less viscous environment than in the poly(oxyethylene)/water mixture with the corresponding overall composition. Thus, it seems
[ ti20I/ Ico r bowax 2001 Figure 3. Dependence of the rotational correlation time, T,, of radical I1 on the [H20]/[Carbowax]molar ratio of H20/Carbowaxmixtures
at various temperatures.
that at least part of the water molecules in the core tend to gather up in the region of polar heads, forming what is usually called a ‘water-pool”. Pemtdility of the MiceUar Aggregates. The addition of CuXz complex, soluble only in cyclohexane, to the micellar solutions produces a decrease in the signal intensity of the spin probe I1 (confined to the water-pool), without apparent change in the line shape.21 The magnitude of the effect is different in different samples, decreasing from sample 2 to sample 7. This effect, theoretically explained by has been encountered by Taylor et al.23324in the case of the interaction between a spin probe (nitroxide) covalently bound to a protein and a Mn2+ ion in a specifically fixed position and has been exploited for measuring the spin probparamagnetic ion radial distance. Sama et have also observed the same effect for the interaction between the paramagnetic metal ions (Mn2+,Cu2+,Gd3+,other lanthanide ions) and radicals in systems of biological interest. Using the Redfield theory, Leigh calculated the effect on the line shape of the A term of dipolar Hamiltonian for the paramagnetic ionradical interaction. Under certain conditions, for a single paramagnetic ion-radical pair at a fmed distance, he showed that the nitroxide radical line decreases in intensity with little apparent change in its shape. Sama et al.25suggested that the same theory can be applied for somewhat different circumstances: (a) more than one ion interacting with each radical, (b) no single radial distance, (c) no fmed orientation of the radial vector with respect to the nitroxide molecular coordinate system. Hyde and RaoZ6 using the different approach of van Vleck’s ‘method of moments” on the other terms (B, C, D, E, and F)of the dipolar Hamiltonian calculated the saturation recovery and progressive saturation parameters for the case of more than one paramagnetic ion interacting with the spin probe at varying radial distances. They concluded that the effects are dominated by the ‘distance of cloaest approach” between the radical and ion, extending this conclusion to the Leigh-type experiment^.^^.^^ Our experimentalobservations of apparent loss of intensity in the nitroxide radical spectra were treated theoretically within the frame of the above circumstances. These observations can be explained by assuming that the CuX2 complex approaches the spin probe confined to the water-pool penetrating between the surfactant chains of the aggregates.
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 7113
Structure of Reversed Micelles
TABLE I V P, the Calculated ”Distance of Closest Approach” between the Spin Probe (II) and Cu(II) Ion f,
sample
W 2 0 1 /[EO(ONPI
2 5 7
0.6 2.6 6.9
1” 13.6 22.5 33.4
A 26
20.2 33.4 49.6
“Calculated with Tlk = 2 x IPS. bCalculatedwith TIk = 2 x lo-* S.
Figure 4. ESR spectra of radical I1 in (A) sample 5 ( [HzO]/[E0(4)NP] = 2.6) and (B) sample 2 ([HzO]/[EO(4)NP] = 0.6): (a) without CuXz and (b) with CuX,.
To obtain the quantitative information about the “distance of closest approach” using Leigh’s theory,22our experiments were performed as follows: the samples containing the spin probe were treated with CuX2 complex until successive addition brought no decrease in the signal amplitude. In each case the broadened spectrum was compared with a blank spectrum of the same sample with no CuX2 in it. Typical examples of such spectra are given in Figure 4. Within the frame of Leigh’s theory, each line of Lorentzian shape which makes up the spectrum is broadened by dipolar interaction with the paramagnetic ion, its line shape function being
where and
cos e/, = sin 8, sin 8 cos (p, - (p)
+ cos Br cos 8
(3)
and (4) Here (8,cp) and (&,(p,) are the (azimuthal, polar) angles of the applied field H and of the radial vector (of r magnitude) respectively, tYr is the azimuthal angle between the radial vector and the H field direction, 6Hois the line width in the absence of dipolar broadening; p is the effective magnetic moment of the paramagnetic ion, T]kis its spin-lattice relaxation time. The broadened composite line shape is obtained by integrating over all possible orientations 8,cp in the expression
In our case of the isotropic radical motion within a relative confinedvolume (water-pool), characterized by no fmed orientation of the radial vector with respect to the nitroxide coordinate system and by no single radial distance, the definitions of 8, and cpr in eq 3 are arbitrary22and r in eq 4 becomes i, the “distance of closest approach”. A computer program for solving the combined equations (1-5) has been developed on a IBM PC-XT computer, yielding the absorption line shape and its derivative. The experimental spectra without CuX2and with CuX2complex were simulated with the computer program, using 6Ho and C as parameters. The P value depends on the Tlk value (eq 4) of the
CuX2 complex. Values for Tlk of 2 x and 2 X 10-~s were found in the l i t e r a t ~ r for e ~ Cu(I1) ~ ~ ~ ~ion coordinated with water and organic molecules, respectively. The f values calculated with these Tlk values are given in Table I v . The f values in Table IV are subjected to some errors chiefly due to the uncertainty in the value of Tlk. However, for the same Tlk,the P values for different samples are relative estimates of the level of CuX2 penetrability through the surfactant chains. Some differences between the level of cyclohexane penetrability and that of CuX2may be taken into account due to the difference in their molecular size. The f values for the almost nonbroadened spectrum of sample 7 ([H20]/[E0(4)NP] = 6.9) (Table IV) are an indication for the thickness of the shell for the aggregate with a well-formed “water-pool”. These values seem reasonably good if we consider the calculated value for the length of the surfactant chain from the usual bond lengths and bond angles as being 30 A. The results in Table IV show that as aggregates develop a larger water-pool, the shell becomes less penetrable. These data are also consistent with the order parameters obtained with 5doxylstearic acid (radical IV) as a spin probe in the same aggregates. Radical (m). This spin probe has been synthesized by labeling the surfactant actually used in this study, in order to follow its aggregation behavior. However, the radical does not behave as a surfactant either in pure cyclohexane or after water addition. In cyclohexane, in the 1 X l0“L to 7 X 10-I M concentration range, the ESR spectra were representative of the nitroxide solutions undergoing electron spin exchange interaction^.^^ The exchange-narrowed single line is observed only at concentrations higher than 2 X 10-1M. There were no signs of the simultaneous presence of exchange-narrowed spectrum of the aggregate and of the three-line spectrum of the m ~ n o m e r . ’ ~Addition ~ ’ ~ of small amounts of water to a cyclohexane solution of 111( 10-1M) yielded turbid solutions and no changes in the spectra. These results indicate that the hydrophilic/hydrophobic balance of the EO(4)NP surfactant is modified by the substitution of its OH group with the nitroxide moiety and points out the role of the OH groups in aggregation. Radical (IV). The addition of this spin probe (1 X M) to the studied samples yielded spectra whose patterns were characteristic either of an unique isotropic spectrum (samples 0 and 1) or of the overlapping of the isotropic spectrum and a highly anisotropicone (samples 4-7). In the latter case the addition of CuCl, completely broadened out the anisotropic spectrum, the remaining isotropic spectrum presenting for all samples the same ESR parameters: U N = 14.1 G, 7, = 2.2 X s. Comparing these parameters with those obtained in cyclohexane, uN = 13.8 G and 7, = 0.2 X lO-’Os, we came to the conclusion that the nitroxide has to be associated with some EO(4)NP aggregates, since its motion is significantly restricted. Taking into account the solubility of the nitroxide (IV) in cyclohexane, one would expect to find it in this medium too. So, the observed isotropic spectrum probably results from fast exchange of the spin probe between cyclohexane and “waterless” EO(4)NP aggregates (samples 0 and 1) or between cyclohexane and micellar shell (samples 4-7). In this case the parameters represent to a first approximation the weight average of the parameters corresponding to the spectra in the separate phases7 BN
= fchxachx -k f a g g u q g
= fchxTchn
+ faggTagg
--:
7114 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992
and fagg being the fractions of the spin probe associated with cyclohexane and aggregates, respectively. In order to check this supposition we have recorded the spectrum of the spin probe in sample 1 ([EO(4)NP] = 5.5 X M)after 1/10 and 1/50 dilution with cyclohexane ([E0(4)NP] = 5.5 X and 1 X M, respectively). As expected, a reduction of to 1.3 X 1O-Ioand 0.9 X s respectively T~ from 2.2 X was observed, since the number of aggregates, and hence fagg, decreases with the decrease of the surfactant concentration. The same behavior was noticed and mentioned in the case of the spin probe (I) in all studied samples, when T~ observed is 0.6 X 1O-Io s, different from its value (0.2 X 1O-Io) in pure cyclohexane. The different T~ value in this case as compared to spin probe IV could result from different distributions of the nitroxides between the two phases. The charged spin probe (11) does not present this behavior, as it associates only with aqueous part of the aggregates. The fast exchange of the nitroxide (IV) between “waterless” EO(4)NP aggregates and cyclohexane in contrast to its behavior in water-swollen micelles is in line with other results of this work, concerning the order degree and penetrability of the aggregates. It is worth mentioning that the exchange rate of radical IV between micellar shell and water/surfactant interface is slow, since in samples 4-7 separate spectra of these species are simultaneously observed. Order Degrees of the Aggregates. The isotropic spectrum could be completely broadened out by successive addition of the CuX2 complex. In the case of samples 4-7, the remaining anisotropic spectra with fairly well defined extrema presented in Figure 5 are characteristic of the motion of the spin probe in a system with order at molecular level. The distance between the extrema varies with the water content. Similar spectra were obtained by Hauser et a1.8 in AOT reversed micelles and by Ramachandran et al.31 in microemulsions. The anisotropy of the spectra is an undoubted indication that the tumbling rate of the micelle and the diffusion of the EO(4)NP surfactant molecules on the curved surface of the water-pool are too slow (v < lo8 s-I) to contribute to the averaging of the g factor and hyperfine splitting tens0rs.~J~9’~ The observed shape of the spectra was identified with that arising from axial symmetry of the nitroxide group motional features. The theory of the motion of the doxy1 fatty acid probes in oriented liquid crystals32and membranes3) is well documented. Spectra of Figure 5 can be treated within the theory developed for those systems, the more so as the micellar systems are regarded as the precursors of ordered liquid crystals.32 In our micellar system the long axes of the surfactant molecules tend to orient parallel to each other, their polar head groups being in the water-pool, while the nonpolar tails are oriented toward cyclohexane. The behavior of the incorporated spin probe-5-doxylstearic acid-resembles very much that of the surfactant, its chain tending to orient parallel to the surfactant one and its carboxylic group being strongly anchored at the water/shell interface of the swollen aggregates. The relevant parameter is the order parameter S which is defined in terms of observed spectral parameters as
Caldararu et al.
fchx
S=
A,, - A , A,, - ( 4 r x + A , ) / 2
(6)
The values A,,, A,,, and A, are the principal elements of the A tensor in the absence of molecular motion and All- A, are derived from the experimental spectra (Figure 5). The order parameters S is related to the mean angular fluctuation (cos2 e) by the relation
s = (3(c0s2 e) - 1)/2
(7)
where 0 = e(t) is the time-dependent angle between the nitrogen 2p, orbital ( 2 ) axis and the normal to the interface (the axis of the largest hyperfine (hf), splitting); the angular brackets denote time average over a time determined by the reciprocal of the anisotropy of the hf and &man interactions (in frequency units), i.e. in the range of 10-7-10-9s. The order parameter S in eq 7 has a physical significance if the correlation time for the spin probe
I
Figure 5. Anisotropic ESR spectra of radical IV (a) in sample 4 ([HzO]/[EO(4)NP] = 1.Q (b) in sample 5 ([H20]/[EO(4)NP] = 2.6), and (c) in sample 7 ([H20]/[EO(4)NP] = 6.9).
TABLE V ESR Panmeters of the Anisotropic Spectra of Spin Probe IV in Micellar Solutiona (at 25 “C) and S, the Order Parameter
samDle 4 5 6
7
~HZOl/ IEO(4)NPl 1.8 2.6
5.8 6.9
A , , ,G 21.1 21.6 22.6 22.9
A,,G 11.8 11.7 11.2 11.1
uN. G
S
14.9 15.0
0.35 0.37 0.42 0.44
15.0 15.0
motion is short s) compared to these values, a condition which is assumed to be fulfilled in our highly fluid system. Order parameters corrected both for differences in polarity32 and for the inaccuracy of the measured A, valueMwere calculated with the expression given in ref 34, using the following values: A,, = 33.5 G, A,, = 6.3 G, and Ayy = 5.8 G.34The ESR parameters as well as the order parameter S are given in Table V. The isotropic UN value calculated for all samples (15.0 G) (Table V) is almost identical to that measured (14.9 G)35in a poly(oxyethylene) (Carbowax 200)/water mixture, whose [H,O]/ [Carbowax] molar ratio = 1.02 and very far from that measured either in cyclohexane (13.8 G)35 or in water (15.6 G).33 This result indicates that the nitroxide NO group is far from the water/ surfactant interface and buried in the poly(oxyethy1ene) part of the shell. The identical uN (15.0 G) values in all samples are a reasonable indication that the nitroxide NO group encounters a surrounding with the same polarity in all studied aggregates. Also, these data indicate that,at the position where this spin probe senses the polarity, little penetration of H 2 0 molecules occurs with increasing water content. This result is in line with the image of the water-pool formation in this system, as resulted from the information obtained with the spin probe (11) (see above). Full support for this result is given by the ESEM data obtained by Kevan et al.36 with the same spin probe (IV) in the aqueous micellar solutions of some similar surfactants (hexa- and octaethylene glycol monododecyl ethers). They have found that the nitroxide group is located in the micelle ethylene oxide region (at -6 A from the interface) and that little penetration of H 2 0 molecules into this region is present.36 Also, these results indicate that the behavior of the poly(oxyethy1ene) part of the shell toward water seems to be the same either in the aqueous micelle or in the reversed micelle. Unlike these findings, the investigation of the AOT reversed micelles in the AOT/isooctane/water system with the same spin probe (IV) has indicated a large increase in UN with increasing water content up to [H20]/[AOT] 20: when the uN value (15.5 G)reaches the value in water (15.6 G).33 This is due to the penetration of more H 2 0 molecules into the AOT shell of the reversed micelle: as the amount of water is increased. In the case of 5-doxylstearicacid the direction of the nitrogen 2p, axis coincides with that of the long molecular axis if the chain
-
Structure of Reversed Micelles has its fully extended all-trans conformation. Therefore the order parameters S and the mean angular fluctuation (cos2 0) yield information about the alignment of the molecular axis in the aggregate and about the flexibility of the hydrocarbon chain. In the studied samples this information refers to the specific order at a distance of 4 bonds from the water/shell interface, where the carboxylic group is considered to be anchored. The value of S increases with the increase of water amount in the aggregate. So, it appears that the structural order of the shell depends on the dimensions (and probably the structure) of the water-pool. However, at a molecular level the role of the water molecules in ordering (packing together the surfactant molecules) of the aggregates is not known. Taking into account the explanations given for the flexibility of the chain in the case of lyotropic liquid crystals by Seelig,32 one can infer that in our case too, increased contributions of the gauche conformationsof spin probe chain are expected as the order degree in the aggregate decreases. Studies with the same spin probe (IV) incorporated in AOT reversed micelles (AOT/isooctane/water system) have indicated a similar dependence of the order parameter S on the amount of water in the micelle as that found in our case, Le. the order parameter incream with the water content.s However, in the two micellar systems the same values of the order parameter S are found at very different [H20]/[surfactant] ratios. Thus, while in EO(4)NP reversed micelles, values for S of 0.37 and 0.44 are found for [H20]/[EO(4)NP] molar ratios of 2.6 and 6.9, respectively (Table v), in the case of AOT reversed micelles, the same values for S are found for [H20]/[AOT] molar ratios of about 10 and 15, respectively! For [H20]/[AOT] = 10, the same spin probe (IV) incorporated in the AOT/heptane/water micellar system gave a lower value ( -0.2)3s737for S than that found in the AOT/isooctane/water system (~0.38).~ Light scattering data of AOT reversed micelles have revealed that for such molar ratios rather large water-pools are formedS (for [H20]/[AOT] = 8.9 the water-pool has a radius of about 14 A’ and contains roughly 400 water molecules5). These results suggest that besides the water-pool dimension, some other factors, like the structure (linear or branched) of the surfactant chain, the nature of the interaction between the surfactant polar groups and water molecules, and the nature of the external organic phase, play an important role in the order degree of the micellar aggregates. Conclusion All reported results on the investigated system lead to a coherent image of the structure and dynamics of the reversed micelles as being highly dependent on the presence of water and its quantity. Even with traces of water, surfactant aggregates are observed, which are in fast exchange with the monomer. At [H20]/[surfactant] = 0.6 the exchange with the monomer becomes slow (on ESR time scale). The core vis(3osity is maximal, its polarity is higher, the shell is totally penetrable, and no ordering of the surfactant chains can be detected. With further increase of water content, the core becomes more polar and less viscous. Comparison of these properties in the micellar cores and in poly(oxyethylene)/water mixtures designed to reproduce the overall core compositions indicate that the water molecules gather up at the polar heads of the surfactant, forming a water-pool, and are not distributed uniformly along poly(ethy1ene oxide) chains, even though the hydration of these chains is not complete. With increasing water-pools the micellar shells become less penetrable and the order degree of the surfactant chains increases. The conclusion emerges that there is no point in discussing nonionic surfactants in apolar solvents as two-component systems. In reality trace amounts of water are always present and its
The Journal of Physical Chemistry, Vol. 96, NO. 17, 1992 7115 amount determines the specific aggregation properties (including the cmc), as well as the dynamics and structure of the aggregates formed.
Acknowledgment. We are indebted to Dr. A. Banciu for a generous gift of a CuX2 sample. R e t r y NO. I, 2154-67-8; 11, 136908-10-6; 111, 141903-95-9; V, 136853-77-5; NP-4, 9016-45-9; cyclohexane, 110-82-7; 5-doxylstearic acid, 29545-48-0; 3-(bromoethyl)-2,2,5,5-tetramethyl-2,5-dihydropyrrole-1-oxyl, 141903-94-8; 3-(chlorocarbonyl)-2,2,5,5-tetramethylpyrroline- 1-oxyl, 13810-21-4.
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