J. Phys. Chem. 1083, 87, 3537-3541
than in the gas-phase reaction due to solvation with solvent and, consequently, the chance of dimerization is lowered. Further speculation regarding the liquid-phase reaction is not the purpose of the present study. When TiOz single crystal is used as a photoanode of a PEC cell, the photocurrent (i.e., reaction rate) is usually proportional to light intensity.16 The rate of the gas-phase photo-Kolbe reaction over Pt/TiOz was, however, not proportional to light intensity as shown in Figure 5. A similar nonlinear relationship between reaction rate (current) and light intensity in a PEC cell system has been observed by Davidson et al.," who employed a Pt electrode coated with TiOz powder as a photoanode. They ascribed this result to an insufficient charge separation of a holeelectron pair at the surface of powdered TiOz. The light (16) Rajeshwar, K.; Singh, P.; DuBow, J. Electrochim. Acta 1978,23, 1117. (17)Davidson, R. S.;Slater, R. M.; Meek, R. R. J. Chem. SOC.,Faraday Trans. 1 1979,75,2507.
3537
intensity dependence of Pt/TiOz catalyst may be interpreted in the same way. In the present experiment we found that the reaction rate was linearly dependent upon light intensity when the intensity was less than 3% of full illumination. This result indicates that under weak illumination the band bending at the space charge layer of powdered TiOz is enough to separate all hole-electron pairs. With increasing light intensity beyond a critical intensity at which a chemical process cannot afford to follow a photoprocess, some of the electrons and holes produced are not consumed by the reaction and the band bending tends to be flat. As a result, the charge separation becomes insufficient, leading to an increase in the recombination of hole-electron pairs.
Acknowledgment. I thank Mr. T. Kadowaki for his measurement of BET surface area and Mrs. A. Yamazaki for her help in the ESR measurements. Registry No. Acetic acid, 64-19-7; formic acid, 64-18-6; platinum, 7440-06-4.
Micelle Size in Ethylammonium Nitrate As Determined by Classical and Quasi-Elastic Light Scattering D. Fennel1 Evans,* A. Yamauchl, Department of Chemical Engineering and Materiels Science, University of Minnesota, Mlnneapolls, Minnesota 55455
G. Jason Wel, and Vlctor A. Bloomfield Department of Biochemistry, University of Minnesota, St. Paul, Minnesota 55108 (Received: October 14, 1982; I n Final Form: February 2, 1983)
Aggregation of surfactants to form micelles in ethylammonium nitrate, a low melting fused salt, was investigated by classical and quasi-elasticlight scattering. For tetradecylpyridiniumbromide and hexadecylpyridinium bromide the following data were obtained: critical micelle concentrations, 8.0 X and 2.0 x mol kg-'; micellar aggregation numbers, 17 and 26; second virial coefficients, 1.64 X and 1.30 x mol cm3 g-2; and hydrodynamic radii, 14 and 22 A, respectively. The results are consistent with either a small classical spherical micelle containing only surfactant or a spherical mixed micelle containing surfactant and ethylammonium ions as a cosurfactant. The measured second virial coefficients are almost equal to those calculated for hard spheres and reflect highly screened electrostatic interactions in the totally ionized solvent.
Introduction Ethylammonium nitrate (EAN) is a low melting fused salt possessing a number of similarities to water. In particular, the transfer of rare gases and hydrocarbons from cyclohexane to this fused salt at 25 "C is accompanied by negative enthalpy and entropy changes.' These observations suggest that formation of structured solvent around nonpolar groups, similar to the phenomena observed in aqueous solution. However, ethylammonium nitrate can form only three hydrogen bonds per solvent molecule, and the tetrahedral structure of the cation and the planar structure of the anion in addition to the Coulombic forces must impose a very different type of solvent ordering than that in water. These differences are reflected in the low heat capacities observed in ethylammonium nitrate solutions compared to those in water.2
Similarities and differences between ethylammonium nitrate and water are also demonstrated by micelle3 and liquid crystal f ~ r m a t i o n . ~The critical micelle concentrations (cmcs) of surfactants are typically 7-10 times higher than those observed in aqueous solution. From the change of cmc with surfactant chain length, the free energy of transferring a methylene group from the fused salt into the interior of a micelle is -400 ~ a l / m o l .This ~ is considerably smaller than the corresponding value of -680 cal/mol observed in water.5 Thus, while the polar nature of EAN causes surfactants to cluster with their hydrocarbon chains away from the solvent, the ethylammonium cation appears to be a slightly better solvent than water for hydrocarbons. A t present there is no information on the size and structure of micelles formed in solvents other than water.
(1)D. F.Evans, S.-H. Chen, G. W. Schriver, and E. M. Arnett, J.Am. Chem. SOC.,103,481(1981). (2) D. Mirejovsky and E. M. Arnett, J . Am. Chem. SOC.,105, 1112 (1983).
(3)D. F.Evans, A. Yamauchi, R. Roman and E. Z. Casassa, J . Colloid Interface Sci., 88,89 (1982). (4)D. F.Evans, E. W. Kaler, and W. Benton, J. Phys. Chem., 87,533 (1983). (5)P.Mukherjee, J. Phys. Chem., 66, 1375 (1962).
0022-365418312087-3537$0 1.50lO
0 1983 American Chemical Society
3538
The Journal of Physical Chemistry, Vol. 87, No. 18, 1983
Evans et ai.
1 C,,Pyr Br in EtNH, NO,
TABLE I : Physical Properties of Tetradecylpyridinium Bromide and Hexadecylpyridinium Bromide in Ethylammonium Nitrate C,,PBr ( 2 5 "C)
dnldc, cm3g-' 4 . 3 9 X
8.0 x ( 5 . 9 3 I0.51) X Nasg 1711 B, mol cm' g - * (1.64 i 0.08)X D, cm s - ' 5.85 X l o - * R, A 14i 3 KrT,O "C 14
cmc, mol kg-' M , g mol-'
-
lo3
"
P
C,,PBr ( 3 0 "C) 5.66 X 10.' 2.0 x l o - 2 (1.00 i 0.07) X l o 4 26 + 2 (1.30 + 0.14) X l o T 3 4.47 x l o - * 22+ 3 281 1
Krafft temperature; the Kraft temperature for C,,PBr coincides with melting point of EAN.
Such information is of considerable interest because it provides the opportunity to test the generality of proposed relationships between micellar size and structure, the polarity and other properties of the solvent, and the properties of the surfactant, such as chain length, hydrocarbon volume, and effective area of the head group. We have therefore measured the molecular properties of micelles in ethylammonium nitrate using classical and quasi-elastic light-scattering techniques. Experimental Section Ethylammonium nitrate (EAN) was prepared by adding 3 M nitric acid to a cooled 25% aqueous solution of ethylamine so that a slight excess of amine remained.3 Most of the water was removed with a rotary evaporator connected to a water aspirator; the EAN solution was heated to 60 OC in a water bath. The final amounts of water were removed with a lyophilizer; the rate of drying was increased by stirring the fused salt with a tefloncovered magnetic stirring bar. Upon storage, the fused salt sometimes turned a light yellow color and gave an unacceptably high level of fluorescence. The fused salt used in the light-scattering experiments was therefore purified by dissolving it in acetonitrile, mixing with activated charcoal, filtering, recrystallizing several times from acetonitrile, and redrying. The fused salt was filtered through Millex-GS Millipore membranes with a pore size of 0.22 pm and stored in a closed flask in a desiccator. Tetradecylpyridinium bromide (C14PBr)was prepared by refluxing stoichiometric amounts of 1-bromotetradecane and pyridine in absolute ethanol overnight. Hexadecylpyridinium bromide (CI6PBr)was purchased from Eastman Kodak Co. Both compounds were recrystallized from acetone three times, extracted with distilled ether for 10 h, and dried in a vacuum oven. Preparation of Surfactant Solutions The surfactant-fused salt solutions were prepared by weight and directly filtered into the light-scattering cells through 0.22-pm Millipore filters. The light-scattering cells previously had been thoroughly rinsed with filtered water and dried in a vacuum oven. We found that filtration of the surfactant solution changed the surfactant concentration, presumably due to adsorption of surfactant on the filter. We therefore measured the refractive index of all the unfiltered solutions with a Brice-Phoenix differential refractometer and constructed a calibration curve of refractive index vs. concentration. These were linear up to the highest concentrations studied, with slopes dn/dc given in Table I. At the end of the light-scatteringmeasurements, the refractive indices of the filtered solutions were determined and the calibration curves were used to obtain the actual concentration of surfactant in the light-scattering cells.
-
0 1st Measurements
1
300
100
2nd Measurements
1
I
500
700
m c 1 4 Pyr Br x l o 3
Flgure 1. Reduced intensity of scattered light vs. concentration for tetradecylpyridinium bromide (C,,PBr) in ethylammonium nitrate at 25 OC.
10
50
100
150
200
c18Pyr Br(m x lo3) Flgure 2. Reduced intensity of scattered light vs. concentration for hexadecyipyridinium bromide (C,BPBr) in ethylammonium nitrate at 30 OC.
Light Scattering The apparatus and methods of data analysis for total intensity and quasi-elastic light scattering have been described in detail el~ewhere.~~' Briefly, measurements were made with an argon ion laser operated at 488 nm, an ITT FW 130 photon-counting photomultiplier, and a Langley-Ford correlator. The autocorrelation functions were analyzed by a cumulant fitting procedure8 to obtain zaveraged diffusion coefficients, which were then converted to hydrodynamic radii by the Stokes-Einstein equation. Intensity measurements were analyzed as described in Results. Results The reduced intensity of scattered light vs. concentration for tetradecylpyridinium bromide (C14PBr)in the fused salt at 25 "C is shown in Figure 1. Similar data for hexadecylpyridinium bromide (C16PBr)at 30 OC are shown in Figure 2. The Krafft temperature for this compound in EAN is 28 "C. Since EAN is 11.1M in nitrate ion, this ion will be the dominant counterion. Both curves show (6) V. A. Bloomfield and T. K. Lm,Methods Enzymol., 48,415(1978). (7)C.H.Pletcher, R. M. Resnick, G. J. Wei, V. A. Bloomfield, and G . L. Nelsestuen, J. B i d . Chem., 256, 7433 (1980). (8) D.E. Koppel, J. Chem. Phys., 57,4814 (1972).
Micelle Size in Ethylammonium Nitrate
The Journal of Physical Chemistry, Vol. 87,No.
m I
50
t
0 I
XIO~
20 I
I
18, 1983 3539
40 I
I
80
60 I
I
I
C,4PBr
I
i'
40 30
%-.
50-
o\ a
z&
9
20
, I
I
I
I
I
I
1
I
C16PBr c16 Pyr
M -oi x lo4
I
I
N
-
F
l6 $
Br
N=X
----e---
40
1.0Y 1 50
100
1 150
1 200
AC/(mg/ml)
Figure 3. Debye plot for tetradecylpyridinium bromide and hexadecylpyridinium bromlde in ethylammonium nitrate.
the type of behavior generally associated with micelle formation in water. The cmc's given in Table I were determined from the abrupt increase in the intensity of scattered light and agree with those determined from surface tension measurement^.^ A general equation for the relation between the intensity of the scattered light, I, and the apparent molecular weight, Mapp, is I - IBol = p(Ie,,,)(an/ac)'cP(e)M,,, (1) where I1, and Ihnzare the scattering intensity of the pure solvent and the benzene standard, ,6 is the instrument constant, anlac is the refractive index increment in cm3/g, c is the mass concentration of the sample in g/cm3, and P(6) is the scattering form factor. p was determined from aqueous solutions of proteins of known molecular weights to be 0.591 mol/cm3. For the present study (1) a value of 0.643 mol/cm3 was used for 0,since p is proportional to the square of the solvent refractive index (1.45 for EAN, 1.33 for H,O); (2) Isol was replaced by I,,,, the scattering intensity of the sample just below the cmc, where only nonmicellar surfactant molecules contribute, because we are interested in the excess contribution from the micelles; (3) c was replaced by Ac = c - cmc; (4) P(6) = 1 since the size of the micelle is much smaller than the wavelength of the laser light; and (5) l/Mapp = 1/M + 2BAc, where M is the molecular weight of the micelle and B the second virial coefficient. With these modifications, eq 1 becomes
The graph of the left-hand side of eq 2 is plotted vs. concentration for the two surfactants in Figure 3. Extrapolation to zero concentration gives an apparent molecular weight of 5.95 X lo3 and 1.0 X lo4 g/mol for the C14P and c16P micelles. These correspond to aggregation numbers of 17 for C14PBrand 26 for C16PBr,whose molecular weights are 356 and 384. The second virial coefficients obtained from the slopes of the lines in Figure 3 mol cm3 g-2 for are 1.64 X for C14P and 1.30 X
10
12
14
mx
16
18
io3
Figure 4. Diffusion coefflcients and corresponding hydrodynamic radii (eq 3) for tetradecyipyridinium and hexadecylpyridinium micelles in ethylammonium nitrate as determined by quasi-elastic light-scattering measurements.
they appear to increase with concentration. We believe that this behavior results from the z-averaged D obtained with QLS, since at very low surfactant concentrations the contribution from residual dust particles or heterodyning becomes more important. This interpretation is bolstered by the observation that the effect is greater with the C14P surfactant, which forms smaller micelles. These problems are considerably worse in EAN than in water. Not only are the micelles small, but also, since the fused salt is more polar and 20 times more viscous than water, dust is much harder to remove. Consequently, we believe that the diffusion coefficients of 5.85 X and 4.47 x cm2 s-l for the C14Pand C16Pmicelles obtained at the highest concentrations are the most reliable. Hydrodynamic radii for the C14P and c16P micelles were calculated from Stokes law
D
= kT/6*qR
(3)
and give radii of 14 and 22 A, respectively. The viscosity of EAN is 26.6 and 23.1 cP at 25 and 30 "C. We estimate that the Stokes radii are uncertain by k 3 A. Attempts to obtain aggregation numbers for other surfactants in EAN were unsuccessful. The refractive index of surfactants containing only a saturated hydrocarbon chain is so close to the refractive index of EAN that the excess intensity of scattered light associated with the micelle is too small to be accurately measured.
Discussion Micellar aggregation numbers in ethylammonium nitrate are considerably smaller than those observed in water for surfactants of comparable chain length. Light-scattering measurements on CI6PBr (hexadecylpyridinium bromide) micelles in 0.2 M aqueous solutions of sodium fluoride, chloride, bromide, chlorate, iodate, and nitrate have been reported by Anacker and G h ~ s e .Aggregation ~ numbers of 100 to 11 000 were obtained for the Br, NO3,and C103 anions while aggregation numbers ranging from 100 to 130 were obtained for the other anions. Diffusion coefficients have been measured with QLS by Porte and co-workers1°
c16p*
Diffusion coefficients for the C14P and C15P micelles were determined by quasi-elastic light-scattering (QLS) and are shown in Figure 4. The diffusion coefficients show an unusual dependence on surfactant concentration, since
(9)E. W.Anacker and H. M. Ghose, J . Am. Chem. Soc., 90,3161 (1968). (10)G. Porte and J. Appell, J.Phys. Chem., 85,2511(1981);3. Appell and G. Porte, J . Colloid Interface Sci., 81,85 (1981);G. Porte, J. Appell, and Y. Poggi, J . Phys. Chem., 84,3105 (1980).
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The Journal of Physical Chemistty, Vol. 87,
TABLE 11: Surface Area per Surfactant Head Group, Volumes, and Radii for Micelle Hydrocarbon Core N A' r, A A ( N - ' ) ,A '
v,
C,,P
17
c,,p
26
6 868 11902
11.8 14.2
Evans et al.
No. 18, 1983
TABLE 111: Surface Area per Charged Head Group, Volumes, and Radii for Mixed Micelles Containing Surfactant and Ethylammonium Ions (Volume = 81.2 A ' )
103 91
for hexadecylpyridinium bromide as a function of added sodium bromide. They report increase in micellar size with increasing surfactant and salt concentration and decreasing temperature. Micelles in EAN are also smaller than one might expect using commonly accepted relationships between surfactant structure and micellar size and shape. An understanding of why micelles in ethylammonium nitrate are so small and an estimate of the differences observed in water and in ethylammonium nitrate are principal aims of this discussion. We begin by considering the volumes and surface areas associated with micelles containing 17 tetradecylpyridinium and 26 hexadecylpyridinium surfactant molecules. The methods developed by Tanford" and extended by Israelachivili, Mitchell, and Ninham12J3will be employed. The volume in A3 of the micellar hydrocarbon core containing N surfactant chains can be calculated by V = [27.4 + 26.9(n,')]N (4) where n,' represents the number of carbon atoms of the hydrocarbon chain that are contained in the hydrocarbon core and 27.4 and 26.9 are the volumes in A3 associated with methyl and methylene groups." For n,' = 14 and 16, eq 4 gives hydrocarbon volumes of 6 868 and 11 902 A3. The corresponding radii of the spherical cores are 11.8 and 14.2 A3 (Table 11). The areas of hydrocarbon chain at the surface of the hydrocarbon core are 103 and 97 A2 for the C14 and C16compounds. The maximum chain length, l, for a fully extended chain can be estimated from I, = 1.5 + 1.265n,' (5) and gives values of 19 and 22 8, for the C14 and c16 hydrocarbon chains. Comparison of these values with the radius calculated for the hydrocarbon core gives a ratio of rcore/lmar= 0.62-0.65. This requires that many of the chains be folded so that their terminal methyl groups are located close to or at the surface of the hydrocarbon core. To these core radii must be added the length of the pyridinium ring in order to obtain the micellar radii. The effective length of the pyridinium ring is estimated to be 5.6 8, by using 0.7 8, for one-half of the C-N bond, 2.7 A for the pyridinium ring, 1.0 8, for the C-H bond, and 1.2 8, for the van der Waals radius of the aromatic hydrogen. This gives radii equal to the sum of the hydrocarbon core and the head group lengths of 17.4 and 19.8 8,for the C14P and Cl6P micelles. Considering the experimental uncertainties, these values are in reasonable agreement with the measured hydrodynamic radii (Table I). The second virial coefficients are compared to those calculated for hard spheres, B' = 4uz/M2,using the molecular weights and the hydrodynamic radii in Table I. uz is the specific volume, taken as (4rR3N,/(3MZ)J.We obtained B' = (0.9 f 0.5) X and (1.3 f 0.5) X l W 3 mole cm3 g-2 for C14P and C16P,respectively. These results suggest that the interactions between micelles in EAN are described reasonably well by a hard-sphere potential. (11) C. Tanford, "The Hydrophobic Effect", Wiley, New York, 1973. (12) J. N. Israelachivili, D. J. Mitchell, and B. W. Ninham, J. Chem. SOC.,Faraday Trans., 72, 1525 (1976). (13) D. J. Mitchell and B. W. Ninham, J. Chem. SOC., Faraday Trans. 77, 601 (1981).
2
17 17
C,J 8 248 9 629
1 2
26 26
16P 1 4 013 16 124
1
c
12.5 13.2
58 43
15.0 15.6
54 39
Since EAN is 11.1 M in salt and completely ionized, the positive charge associated with the surfactant head groups at the surface of the micelle must be almost completely neutralized by counterions contained within the region occupied by the pyridinium ions. Since there is no separation of charge, there is no double layer in the usual sense, consequently, Coulombic interactions are diminished. The analysis given so far parallels that for micelles in aqueous solution. However, water and ethylammonium nitrate differ in a number of significant ways. One main difference, in terms of the interaction with surfadants and micelles, can be illustrated by an interesting observation involving a freshly cleaved mica surface. When a small drop of ethylammonium nitrate is placed on the mica surface it forms a small drop rather than spreading over the surface. If a drop of water is then brought into contact with the mica-fused salt-air interface, the water drop spontaneously moves over the mica surface but does not cross the path it has already traversed. The most likely explanation is that the initial contact between the two drops creates a concentration gradient of ethylammonium ion in the water drop. The ethylammonium ions then replace the potassium ions on the mica surface with the ethyl groups oriented out from the surface. Thus, ion exchange causes the mica surfaces to become hydrophobic and the water to move away from this region of the surface. This demonstrates that, in addition to being ionic and extensively hydrogen bonded, this fused salt also contains a hydrophobic moiety. It is likely that the orientation of ethyl groups at the air-fused salt interface is also responsible for the surprisingly low surface tension of 50 dyn/cm at 25 Oca3 The hydrocarbon portion of the solvent might also be expected to stabilize surfactant monomers in the ethylammonium nitrate system. This would account at least in part for the higher cmc's observed in EAN. In addition, the hydrocarbon portion of the ethylammonium ions could promote the incorporation of this cation into the micelle in the same way that alcohols are incorporated into micelles in water. In this case, the ethylammonium ion would serve both as a solvent and a cosurfactant. Coulombic repulsion between the positively charged surfactant and the positively charged cosurfactant would not be an important consideration because of the ubiquitous presence of counterions. In order to investigate this possibility further, we have repeated the calculations given in Table I1 for micelles containing a 1:l and 1:2 molar ratio of surfactant and ethylammonium ions (Table 111). The micelle hydrocarbon volume increases only slightly upon inclusion of the ethyl groups, but the area per charged head group including the cosurfactant decreases by almost a factor of two for the 1:l molar ratio and becomes comparable to those surface areas observed in aqueous solution. Whether micelles in EAN have solvent incorporated into them is a question which cannot be resolved by the information presently available. In aqueous solution, the
J. Phys. Chem. 1983, 87. 3541-3550
relationship between micelle size and shape and surfactant structure focuses on the balance between minimizing hydrocarbon interaction which favors large micelles and minimizing head group repulsion which favors small micelles. The relationship between these quantities is often discussed in terms of the dimensionless ratio v/ (aolc) (6) where V is the volume per hydrocarbon chain, 1, its effective length, and a. the effective area occupied by the surfactant head groups. The prediction in water is that, if V/(aolJis less than 1/3, spherical micelles are obtained. For values larger than this, cylindrical micelles, bilayers, or inverted structures are predicted. For aqueous surfactant solution Vis calculated from eq 4,Zc 0.81, from eq 5, and a. is typically 60-70 A2 for ionic surfactants in the absence of added salt. For fused salt surfactant solution it is difficult to test the generality of eq 6. The quantities given in Table I give ratios of V/(l,ao) of 0.30 as indeed they must, given our assumption of spherical micelles. The free energy of transferring a -CH2- group from EAN to the micelle is -400 cal mol-’ compared to -680 cal mol-l for water. Preliminary measurements on oil-fused salt interfacial tension give a value of 20 dyn/cm compared to a value of
-
3541
50 dyn/cm for the oil-water interfa~e.’~Both of these results suggest that EAN is a more hospitable environment for hydrophobic moieties than is water. Thus small micelles with appreciable portions of the surfactant hydrocarbon chain exposed to the fused salt are not unrealistic. The idnic nature and large size of the solvent will result in very different electrostatic interactions, and consequently different values of ao, than those seen in water. For the mixed micelles containing surfactant and ethylammonium ion as cosurfactant, the assignment of effective head group areas becomes even more nebulous. Neutron scattering experiments could provide information on the distribution of surfactants, terminal methyl groups, and ethylammonium ions within the micelles in this fused salt system.
Acknowledgment. This research was supported in part by NSF Grant CPE-8014567and U.S.Army Contract DAA G29-81-K-0099to D.F.E., and by NSF Grants PCM 7806777 and PCM 81-18107 to V.A.B. Registry No. Hexadecylpyridinium bromide, 140-72-7; tetradecylpyridinium bromide, 1155-74-4; ethylammonium nitrate, 22113-86-6. (14)S. Mukerjee and D. F. Evans, to be published.
Glant Micelles in Ideal Solutlons. Either Rods or Vesicles 0. Portet Laboratolre de Spectrom6trle RayMgh Brlllouln, Unlversitci des Sclences et Technlques du Langueobc, 34060 Montpelller Cedex?France (Received October 18, 1982; In Flnal Form: March 23, 1983)
The phenomenological description of the “sphere-to-rodtransition”in dilute micellar solution,which was proposed by authors such as Mukerjee (1972),Israelachvili et al. (1976),and Mazer et al. (1976),has found extensive experimental support in recent years. However, in its initial form it cannot account for some effects due to the flexibility of giant micelles. We thus reconsider this theory and modify the initial assumptions in the following way: (i) we a priori allow for the coexistence of micelles of different shapes (rods and toroids; disks and vesicles) in solution; (ii) we introduce into the asymptotic expression of the standard free energy of a N-micelle a term which accounts for the configurational entropy associated with the ensemble of bent conformations for the flexible giant micelles. Doing so we are able to explain why the spontaneous formation of closed rings does not inhibit the growth of large rodlike micelles. We also show that in dilute solution the formation of vesicles is the general occurrence when bilayered local structure is involved. We, by the way, point out the connections between the growth of giant micelles and other phenomena such as the reversible polymerization of proteins or the fluid-to-viscous phase transition in liquid sulfur.
Introduction Amphiphilic molecules in aqueous solutions are known to form various types of micelles depending on the particular experimental conditions. U s d y , ionic amphiphiles form small globular micelles in binary solutions at low concentration near the cmc. Deviations from these simple initial conditions sometimes result in a strong modification of the size and shape of the micelles in solution; this is indicated by spectacular changes in some macroscopic properties of the solution such as turbidity and viscosity. t Permanent address, where correspondence should be sent: Laboratoire de Mindralogie, Centre de Dynamique des Phases Condensdes, Universitd des Sciences et Techniques du Languedoc, 34060 Montpellier Cedex, France.
Actually this shape and size “transition” for the micelles can be achieved in various ways such as the following: (i) a large increase of the amphiphile concentration,’ (ii) and/or the use of some organic additives (short-chained alcohols,2benzene, salicylate salts? etc.), (iii) and/or the addition of large amounts of certain mineral salts.”12 The (1) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68,3904. (2)Staples, E.J.;Tiddy, G. T. J. Chem. SOC.,Faraday Trans. 1 1978, 74,2530.Larsen, J. W.; Magid, L. J.; Payton, V. Tetrahedron Lett. 1973, 29,2663. (3) Ulmius, J.; Wennerstrbm, H.; Johanson, L. B.; Lindblom, G.; Gravsholt, S. J.Phys. Chem. 1979,83, 2232. (4)Anacker, E.W.; Ghose, H. M. J. Phys. Chem. 1963, 67,1713;J. Am. Chem. SOC.1968, 90,3161. Debye, P.;Anacker, E. W. J. Phys. Colloid Chem. 1951, 55, 644.
0 1983 American Chemical Society 0022-3654/83/2087-3541$01.5Q/~
