9506
J. Phys. Chem. 1993,97, 9506-9512
Distinctive Behavior of the ESR Correlation Times of DOXYL Spin Probes in Mixed Micellar Solutions of Hydrocarbon and Fluorocarbon Surfactants Keiji Kamogawat and Kazuo Tajima’ Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444, Japan, and Department of Chemistry, Faculty of Engineering, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa 221, Japan Received: September 1, 1992; In Final Form: May 1 1 , 1993@
The ESR correlation time 7cof DOXYL stearic acid nitroxide spin probes was measured to clarify the micellar miscibility of perfluorocarbon and hydrocarbon surfactants in an aqueous solution. In one group, named type I micelles, 7cwas found to increase on mixing at first and then decrease at a particular composition. An invariant region separated such a change in 7c for another group, named type I1 micelles. The two types were consistent with the freely miscible, and the partially immiscible, micelles following Shinoda’s theory (J.Phys. Chem. 1980, 84, 365-369). Sodium perfluorooctanoate and sodium dodecyl sulfate at 45-100 mM concentration formed type I micelles. The 7c profile converted to type I1 behavior in the presence of 0.1 M NaCl. Ammonium perfluorononanoate and ammonium dodecyl sulfate also exhibited type 11behavior at 25 O C , accompanied by a discontinuous jump in T~ at XHC= 0.65. The invariant region disappeared at 50 O C leading to type I profile, as predicted theoretically. Prior to this conversion, the jump of 7cdisappeared at 40 O C . In an Arrhenius plot, the logarithm of 7c-1 for XHC= 0.7 and 1 both gave parallel, monotonic decreases with T1, though that for XHC= 0.5 coincided first with the line for XHC= 0.7 but appeared a transition at 40 OC to the XHC= 1 line. These characteristics were discussed in relation to standard 2-micelle models and a new 1-micelle model.
Introduction
of the micellar miscibility of perfluorocarbon and hydrocarbon surfactant solutions by ESR spin probe measurements.
Many reports have dealt with the mutual miscibility of perfluorocarbon and hydrocarbonsurfactants in aqueous micellar Experimental and Theoretical Procedures solutions. Analogous with the mixtures of hydrocarbon and perfluorocarbon liquids found to be partially miscible,’ these Materials. Surfactants of NaPFO, NH4PFN, NaDeS, and micelles may be separated into hydrocarbon-rich and fluorocarNH4DS were prepared and purified as described el~ewhere.~ bon-rich micelles under certain conditions. Such a possibility Methyl esters of perfluorononanoic and decanoic acids were was first pointed out for sodium perfluorooctanoate (NaPF0)synthesized to measure the miscibility in the bulk liquid phase. sodium decyl sulfate (NaDeS) and NaPFO-sodium dodecyl The spin probes of 1ZDOXYL, 2 4 1O-carboxydecyl)-4,4-dimethylsulfate (NaDS) from the stepwise decrease in the differential 2-hexyl-4,4-dimethyl-3-oxazolidinyloxyl, and 16DOXYL, 2-( 14conductance as a function of total surfactant concentration, carboxytetradecyl)-2-ethyl-4,4-dimethyl-3-oxazolidinyloxyl, radindicating two stages of micelle formation.2~~Later a similar icals (12NS and 16NS) purchased from Aldrich were employed result was also concluded for ammonium perfluooroctanoate to prepare the labeled micelles. (NHdPFN)-ammonium dodecyl sulfate (NH4DS) mixturesfrom Measurements. The critical micelle concentration was detera theoretical analysis of the partial and total critical micelle mined within f0.5% accuracy for an aqueous solution, as a concentrations ( c m c ’ ~ ) .The ~ possibility of two micelle structure function of XHC, the apparent mole fraction, with conductance types for these solutions has also been suggested from C ~ C , and ~ ~surface tension measurements. The consolute temperature density,l*J and NMR12 measurements and a theoretical calwas determined for the liquid mixtures of methyl esters of culation.13 It was also proposed from gel filtration studies of perfluorononanoate and decanoate by the usual method at a lithium perfluorooctanoate (LiF0S)-lithium tetradecyl sulfate heating rate of 0.2 OC min-l with frequent stirring. and LiFOS-lithium dodecyl sulfate mixed s01utions~~J~ without The micelles were labeled with the spin probe as follows: 1 mL referring to NaPFO-NaDeS and NaPFO-NaDS solutions. of an ethereal solution containing 0.2 mM of the spin probe was These micelles undergo a dynamic equilibrium16J7with the transfered by pipette to a vial. The solvent was removed in vacuo singly dispersed ions and each others, which is much faster than at room temperature. One mL of a micellar solution of the the observationwindow of the experimental techniques described surfactants was added to the vial and kept at 40 OC for 2 h. The above. Correspondingly,it is difficult to distinguish the micelles solution was occasionally shaken to promote solubilizationof the as discrete particles. The micellar miscibilityshould be analyzed spin probe in the micelles. The labeled solution at 2 X 1V M directly from measurements of microscopic properties, at a rate probe concentration was sealed in a glass capillary (Hematocrit, faster than the exchange kinetics of monomer-micelle equilibrium. plain) of 1.65-mm diameter. The sealed capillaries were left The electron spin resonance (ESR) method is very useful to probe standing for 60-90 min at a given temperature in order to attain the dynamic properties of molecular association, such as those equilibriumbefore an ESR measurement. When phase separation of micelles, liquid crystals, and vesicles. In particular, the occurred for the liquid-liquid mixture, a given amount of the correlation time 7,for the rotational motion of the nitrosyl radical labeled solution was taken from each phase and was sealed in a used as a spin probe, has been used to characterize such organized glass capillary to obtain the ESR spectrum. solutions.1*-22This investigationstudies the microscopic aspect The ESR measurements were made at the same temperature as that for the sample preparation. The ESR spectra were Towhomthecorrespondenceshouldbea d d r d a t KanagawaUniversity. recorded with a JES PE 3X spectrometer (Nippon Denshi CO.). Institute for Molecular Science. Present address: Ele. & Sec. Bureau, Evaluation of the CorrelationTime. From the pioneeringwork Ministryof Education, Scienceand Culture, Kasumigaseki, Tokyo 100, Japan. *Abstract published in Aduunce ACS Absfrucfs,August 15, 1993. of McConnell’s group on spin ESR spectral broadening
0022-3654/93/2097-9506$04.00/0 0 1993 American Chemical Society
Intramicellar Miscibility of Mixed Surfactants
The Journal of Physical Chemistry, Vol. 97,No. 37, 1993 9507 The present results, using submillimolar probe concentrations, show the regular behavior of a narrow triplet profile ESR spectrum, as well as the regular time scale of T ~ . Theoretical Prediction of a Cmc Curve. Cmctcal US X,d" and X F ~A . cmc curve can be predicted for a mixed surfactant solution as a function of compositionfrom regular solution theory adopted to a micellar system+
J..-J
m=l
1 m=O
m -1
____z
H
Figure 1. Typical ESR spectrum of 16NSprobe in the 20 m M aqueous solution of NH4DS (XHC= 1) at 35 O C . Applied field was 3269-3319 G.
of paramagnetic nitroxide radicals has been used to deduce kinetic information in biological and nonbiological assemblies. The 14N center with nearly axial symmetry for the hyperfine coupling constant A and gvalues undergoes fast isotropic tumbling motion in low-viscosity media. This yields a Lorentzian-shaped, narrow three-line ESR spectrum, in which the line width A Wis determined by the spin-spin relaxation time T2. This broadening was not much modified by the substitution of long alkyl chain^.^^.^^ In such a model system, the rotational correlation time T~ was determined from the dependence of the Lorentzian line width AW(m) on the nuclear spin quantum number m as follows:26
= ~A~(m=l)[(Z(m=l)/Z(m=-1))"2- 11 (1) where D = 6.6 X 10-10s-1 G-1; AWis the peak-to-valley line width at the m = 1 peak (the lowest magnetic field line); Z(m=l) and Z(m=-1) are the peak-to-valley heights at m, as shown in Figure 1. There are no proximate protons in 12NS and 16NS. Ultrahyperfine coupling between the radical center and other distant protons gives a small but similar contribution to the broadening of each component of the triplet spectrum and therefore the contribution is regarded as cancelled in eq 1. Using eq 1, the change in T~ with temperature was found to be Arrhenius-type for several 2,2,6,6-tetramethyl nitroxide derivatives solubilized in aqueous, dodecane, and NaDS micellar s0lutions2~and ranged from 2 X 10-11to 2 X lO-9 s. The fast isotropic tumbling motion picture23 seems to be satisfactory at these time scales. In the spin-probe method, the partition of probe between aqueous and micellar phases needs to beconsidered. The solubility in water of the tetramethyl nitroxide derivatives with an octanoate tail is 4 X M and that with a dodecanoate tail was estimated to be more than 1 order smaller.24 16NS and another dimethyl nitroxide derivatives with carboxypropyl and tridecyl groups are all insoluble in 10-4 M solution without the presence of 10-3 M NaOH.25 Correspondingly, 16NS and the more hydrophobic 12NS, which were used at 1:10&500 molar ratio relative to the surfactants in the present study, were supposed to be almost completely trapped in the micelle phase. A narrow triplet profile ESR spectrum is an indication of the fast tumbling motion of the spin probe dispersed in a single medium, with negligible spin-spin exchange interaction. Ditert-butyl nitroxidederivativesgave two overlapping triplet spectra corresponding to bulk and micelle phases in NaDS solutions near the cmc.20 On the contrary, the tetramethyl nitroxide derivatives gave a narrow triplet spectrum in a solution of 0.1 mM of the radical concentrations and 1&200 mM of NaDS.24 Enrichment of the radicals in micelle phase also gave broadening effect as shown for the solution of NaDS saturated with the probe.27In that case, solubility of the probe to micelle was evaluated to be 0.46 1 g/g, i.e., 0.64 mol/mol at 35-1 50 mM NaDS concentration. From the data, more than tens of millimolar concentrations of nitroxide probes seem necessary to yield a broad singlet spectrum. T~
where (Y = eXp[Wm(xFcm)2/kT]and = eXp[Ww(XHcm)2/kT], and cmqw' denotes the total calculated cmc; cmc& and cmcHCo denote the cmc's for the pure perfluorocarbon and hydrocarbon surfactants, respectively. XFcm and XHcmdenote the mole fraction of the perfluorocarbon and the hydrocarbon surfactants, respectively, in the micelle. K, is the inverse power of the cmc's dependence on counterion concentration,2*corresponding to the degree of dissociation of the counterions at the micellar surface. In addition, wm/kTis the miscibility parameter of regular solution theory,29in which WFH/kT < 2 is the critical condition for mutual miscibility. CmcHc and Cmcpc us X,d" and X F ~ The . cmc for each component is given by
(4)
where C,, the concentration of counterion, is equivalent to cmc, in eq 2 in the absence of salt. Each cmc is expressed as a function of X H Cand ~ X F Cas~ well as cmc,Cal. Cmctca'us XH?. In contrast to the gas-liquid equilibrium of regular solutions, the surfactant concentrations in the bulk and micelle phases are comparable. Therefore,XHC~, the mole fraction of monomeric hydrocarbon surfactant, may be substituted for X H Cin~ eq 2 for practical purposes. For a given set of cmcpl, cmcFC, and cmcHc, X H Cis~ defined by cmcHc/cmcpl. CmcrCaiand Cmctobsus XHC. When determining cmqob, X H C ~ is taken as equivalent to XHC, the apparent mole fraction determined by weight. Entire profile of cmc,=l is very sensitive to KBand WFH/kT. The two parameters K, and ww/kT can be evaluated experimentally from a plot of cmqob vs XHCcurve using a computer simulation to find the best fit between c m p l and cmqob. Additional details of this procedure can be found in ref 3.
Results and Discussion Miscibility Parameters. Figure 2a shows the observed and the simulated cmc's vs X H C and ~ vs XHC~, the mole fraction of monomeric hydrocarbon composition for the mixed solutions of NaPFO/NaDeS. The abscissa, XHCI,is equivalent to XHC,the apparent mole fraction of the hydrocarbon surfactant. Hereafter, XHCwill be used in a cmc, diagram for the comparison with other quantities. The solid line and two broken lines are respectively cmc,=I and the partial cmc's as a function of XHC' as predicted from eqs 2-4. The dotted line shows cmcpl as a function of X H ~ThesolidlinegaveK,andum/kTequal . to0.65 and 1.82, respectively. The latter reveals that this micelle is freely miscible over the whole range of the composition. In Figure 2b, the adjusted parameters were 0.73 and 2.18 for NHflFNINH4DS. Therefore, this micelle seems partially immiscible throughXHcm= 0.25 and0.75. Both results essentially agree with earlier conclusions.4 Criterionof Phase Mixing by the ESR Method. Liquid-Liquid Mixture. We first investigatedthe correspondence of T~ with the miscibilityfor the liquid-liquid mixtures. Attention must be paid to the functional groups of the molecules as well as the size of
9508 The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 I
Kamogawa and Tajima
~
I
15
I
o ' , 0
,' 0
c
I
'
0.5
I
3
I 1
1.0
0
~
~
0
"
'
~
I
'
'
I
'
" " " " ' 0.5
x:, Gc
x,:
I
8
1.0
x,",
Figure 2. Cmc vs mole fraction curves for (a, left) NaPFO-NaDeS at 25 OC and for (b, right) NWFN-NH4DS at 25 O C . Open circles are the observed cmc. The solid line shows the predicted cmc curve from eq 2. The dotted line denotes the plot of cmGd vs mole fraction of the hydrocarbon surfactants in the micelles. The broken lines show the partial cmc's for the perfluorocarbon and hydrocarbon surfactants.
30t
I
',
3.5
-
\
I
I
I 0
I I I I I I 8
I
L
30n '
-
P
I I I
I
I
I
b
I
c
r"
2.5 10
-0
-
-1 0 FC
a5
1:
1.0
HC Figure 3. Phase diagram for methyl perfluorononanoate and methyl decanoate mixture. The consolute temperaturewas 26.5 OC at 0.63 mol fraction. Xnc
the alkyl chain, since alkanes without specific interaction may force the polar nitroxide radicals to associate with each other. Enhancement in the spin-spin exchange interaction in such aggregates would convert the narrow triplet ESR spectrum to a broad singlet 011.2.27 Figure 3 reveals the phase diagram for the mixture of methyl perfluorononanoate and methyl decanoate under atmospheric pressure. The upper consolute temperature was almost 26.5 O C at XHC= 0.63, where XHCdenotes the apparent mole fraction of the hydrocarbon component. Therefore, ESR samples with the 12NS probe were prepared at 22 O C for the two phase system and a t 30 'C for the uniform one. Figure 4 shows the r, as a function of XHC. The rc values at (1.7-3.7) X 10-10 s were comparably as small as those for another nitroxide radicals in d o d e ~ a nand e ~ ~SDS micelle^.^^.^^ The r, for the uniform phase
-
2.0-
1.5
0 FC
0.5 XnC
1 HC
Figure 4. 7, vs mole fraction curve for methyl perfluorononanoate and methyl decanoate mixture labeled with 12 NS at 22 OC (0)and at 30 OC (0). The two dotted lines represent the boundary of the immiscible region. gave a smooth curve as a function of XHC.On the contrary, the ?,at 22 OCwasinvariantover0.4
