Langmuir 1995,11, 3351-3355
3351
Aqueous Solutions of Zwitterionic Surfactants with Varying Carbon Number of the Intercharge Group. 1. Micelle Aggregation Numbers N. Kamenka Laboratoire des Prockdks et Matkriaux Membranaires, UMR, CNRS, Universitk de Montpellier, Place E. Bataillon, 34095 Montpellier-Cedex, France
Y. Chevalier Laboratoire des Matkriaux Organiques 21 Propriktks Spkcifiques, CNRS, 69390, Vernaison, France
R. Zana* Institut C. Sadron (CRM-EAHP), CNRS-ULP, 6 rue Boussingault, 67000, Strasbourg, France Received April 3, 1995. In Final Form: June 2, 1995@ Time-resolvedfluorescence quenching has been used to determine the aggregationnumber, N, of a series of zwitterionic surfactants of the a-(dodecyldimethy1ammonio)alkane-o-carboxylatetype with the intercharge alkanediyl group being CH2, C3H6, C&O, and CloHzo (referred to as C12NlC02, C12N3C02, C12N5C02 and C12NlOC02, respectively), in order to investigate the dependence of N on the carbon number of the intercharge group. l-(Dodecyldimethylammonio)propane-3-sulfonate (C12N3S03)has also been investigated to establish the effect of the head group. The aggregation numbers have been found to depend very little on the surfactant concentration and salt content of the solution. An increase in temperature resulted in a decrease of N for C12NlC02, C12N3C02, C12N3S03, and C12N5C02, with the magnitude ofthe decreasebecoming smaller in this sequence. The aggregation number of C12NlOC02 slightly increased with temperature. Irrespective of the surfactant concentration and NaCl content, the values ofNat agiventemperaturefor thevarious surfactantswere in the sequence: C12NlC02 > C12N3S03 > C12N3C02 > C12N5C02 zz C12NlOC02. However, the values ofN indicated that the micelles of these surfactants were sphericalor spheroidal, under the experimental conditions used. The results are discussed on the basis of the variation of the conformation of the intercharge alkanediyl group with its carbon number.
Introduction Among water-soluble surfactants, zwitterionic surfactants have been less studied than ionic and nonionic ones but have recently attracted increasing attention owing to their wide applicability and their increased commercial uses, particularly in high value added formulations.1.2 Some types of zwitterionic surfactants show a good solubility in water, and are rather insensitive to the presence of salts, unlike ionic surfactants, and to temperature, unlike nonionic surfactants.lS2 Besides, they are compatible with a wide variety of ionic and nonionic surfactants, and these mixtures show interesting synergistic effect^.^ Fundamental studies have recently been published on the relationship between chemical structure and various physicochemical properties of the solutions of a series of homologous zwitterionic surfactants of increasing carbon number of the intercharge group a-(alkyldimethylammonio)alkane-o-carboxylates(also referred to as N-alkylN,N-dimethylammoni~alkanoates).~~~ Some results suggested that when the alkanediyl group separating the two charged groups contains 10carbon atoms, this group folds Abstract publishedinAdvanceACSAbstracts,August 15,1995. (1)Bluestein, B. R.; Hilton, C. L., Eds. Amphoteric Surfactants; Surfactant Sci. Ser. 12;Dekker; New York, 1982. (2)See the series of papers: Langmuir 1991, 7, 842-897. (3)Rosen, M. J. Langmuir 1991,7, 885 and references therein. (4)Chevalier, Y.; Storet, Y.; Pourchet, S.; Le Perchec, P. Langmuir 1991, 7,848. ( 5 ) Weers, J. G.; Rathman, J. F.; Axe, F. U.; Crichlow, C. A.; Foland, L. D.; Scheuing, D. R.; Wiersema, R. J.; Zielske, A. G. Langmuir 1991, 7, 854. @
and becomes part of the micelle hydrophobic core.4 The two charged groups are then present at a close distance on a single curved surface separating water from the micelle hydrophobic core. A similar situation is thought to occur in solutions of zwitterionic surfactants where the carboxylate group is replaced by a phenylphosphinate gr0up6J and also for the so-called gemini or dimeric surfactants for nearly the same carbon number of the polymethylene chain connecting the two alkyldimethylammonium bromide moieties constituting these surfactants.8 Arecent study of the same zwitterionic surfactants and other ones concluded that the polymethylene intercharge group remains in a somewhat extended conformation when containing six carbon atoms.5 There have been studies dealing with the size (or aggregation number) of zwitterionic surfactant mic e l l e ~ , ~but ~@ no- systematic ~~ investigation of the effect of (6) Chevalier, Y.; Germanaud, L.; Le Perchec, P. Colloid Polym. Sci. 1988,266,441. (7)Chevalier, Y.; MBlis, F.; Dalbiez, J.-P. J.Phrs. Chem. 1992.96, 8614. (8)Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991,7, 1072. (9)Herrmann, K. W.J.Colloid Interface Sci. 1966,22,352. (10)Kamenka, N.;Haouche, G.; Faucompr6, B.; Lindman, B.; Brun, B.J. Colloid Interface Sci. 1985,108,4517. (11)Zana, R.; Mackay, R. A. Langmuir 1986,2,109. (12)Marignan, J.; Gauthier-Fournier, F.; Appell, J.; Akoum,F.; Lang, J. J.Phys. Chem. 1988,92,445. (13)SBderman, 0.; Ginley, M.; Henriksson, U.; Malmvik, A. C.; Johansson, L.J. Chem. Soc., Faraday Trans. 1990,86,1555. (14)Hodge, D. J.; Laughlin, R. G.; Ottewill, R. H.; Rennie, A. R. Langmuir 1991,7,874. (15)Da Silva Baptista, M.; Cuccovia, I.; Chaimovich, H.; Politi, M. J.; Reed, W.F. J.Phys. Chem. 1992,96,6442.
0743-746319512411-3351$09.0010 0 1995 American Chemical Society
3352 Langmuir, Vol. 11, No. 9, 1995 the intercharge group carbon number, surfactant concentration, temperature, and salt content on the micellar properties. The present work fills this gap. In the first part in this series, time-resolved fluorescence quenching has been used to determine the micelle aggregation number of a-(alkyldimethy1ammonio)alkane-w-carboxylates, with CHZ,C3He, C5H10, and CloHzo alkanediyl groups (referred to as C12NlC02, C12N3C02, C12N5C02, and C12NlOC02), as a function of surfactant concentration, salt content, and temperature. For the sake ofcomparison, we have also investigated the aggregation behavior of l-(dodecyldimethylanio)propane-3-sulfonate (referred to as C12N3S03) to examine the effect of the nature of the head group. The results show remarkably little dependence of the aggregation numbers on these various parameters. The second part of this work will mainly report on counterion binding (Na+and C1-) by the micelles of these zwitterionic surfactants. The third part deals with self-diffusion and elastic and quasi-elastic light scatteringmeasurements for micellar solutions of the same surfactants. The interpretation of the results uses the micelle aggregation numbers reported in this paper and provides information on micelle size and hydration and on intermicellar interactions.
Experimental Section
Kamenka et al. Table 1. Values of the Pyrene Polarity Ratio Zl/Zs for Various Systems system 10°C 25°C 40°C pure water 1.96 1.88 1.80 ethanol 1.32 1.27 1.22 0.194 M C12NlC02 + 0.194 M NaCl 1.458 1.400 1.342 0.467 M C12NlC02 + 0.465 M NaCl 1.398 0.451 M C12N3C02 + 0.451 M NaCl 1.557 1.499 1.435 1.487 0.200 M C12N5C02 0.200 M C12N5C02 0.200 M NaCl 1.486 0.197 M C12NlOC02 + 0.200 M NaCl 1.365 0.486 M C12NlOC02 1.343 0.193 M C12N3S03 + 0.193 M NaCl 1.527 1.475 1.424 0.452 M C12N3S03 + 0.452 M NaCl 1.480
+
Table 2. Values of the Pyrene Fluorescence Lifetime at 25 "C and of the Activation Energy for the Decay Rate of Pyrene Solubilized in Various Systems system 0.194 M C12NlC02 0.194 M NaCl 0.467 M C12NlC02 0.465 M NaCl 0.203 M C12N3C02 0.451 M C12N3C02 0.451 M NaCl 0.159 M C12N5C02 0.204 M C12NlOC02 0.193 M C12N3S03 0.193 NaCl
+ + +
+
(ns) 350 346 337 327 331 353 335
t
E l (kJ/mol) 3.13 i 0.25 3.22 0.25 3.34 i 0.25 3.47 i 0.25 2.88 i 0.25 3.01 i 0.25 3.22 =k 0.25
and temperatures. The results show practically no
The surfactants used in this workwere prepared as previously dependence of ZllZ3 on surfactant concentration and NaCl de~cribed.~ content, within the experimental accuracy of the experiThe sample of pyrene, used as fluorescence probe, and of ments (&la). tetradecylpyridinium chloride, used as quencher of the pyrene The values of Z1IZs decrease upon increasing temperafluorescence, were the same as in previous studies.17J8 The micelle aggregation numbers were determined using the ture. However, similar decreases are seen in Table 1with well-described time-resolvedfluorescencequenching m e t h ~ d . l ~ - ~ ~pure water and ethanol and appear to be a general feature The fluorescence decay curves were recorded using the single of the 11/18ratio and do not reflect some change of polarity photon counting apparatus in the absence of quencher at a very with temperature. low probe concentration, with a [probe]/[micellelmolar concenThe values of ZJZ3 fall in the sequence tration ratio below 0.01, and in the presence of quencher at a still low [probe]and a molar concentration ratio [quencherl/[micellel C12NlOC02 < C12NlC02 C12N5C02 x close to 1. The decay curves were analyzed using the usual leastC12N3S03 x C12N3C02 squares weighted procedure and the equations for intramicellar fluorescence quenching. For all surfactants but C12NlOC02, the pyrene lifetime was found to be the same in the absence and The value found for C12NlC02 is only slightly larger presence of quencher, indicating no probe andor quencher than for C12NlOC02 and identical to that for dodecylintermicellar migration on the experiment time scale (immobile trimethylammonium bromide (DTABhZ5Thus, the solureactant^).'^-^^ The data then yielded the values ofthe lifetime, bilization sites of pyrene in the micelles of these three t, of micelle-solubilized pyrene, the rate constant, k,, for surfactants are likely to be identical, in the micelle core, intramicellar quenching, and the [quencher]/[micelle]concentraclose to the charged nitrogen atom.26The low value found tion ratio, from which were obtained the micelle concentration, for C12NlOC02 is readily understood if one accepts that [micelle],and the micelle aggregation number, N . For C12N10indeed the intercharge group in C12NlOC02 is folded C02 the pyrene lifetime was found to be shorter in the presence within the micelle core. Pyrene would reside in the core, than in the absence of quencher. The data then yielded the pseudo-first-order rate constant for probe andor quencher where it is then shielded from water to the same extent intermicellar migration, in addition to the values oft, N , and k,. as in C12NlC02 and DTAB. The fluorescence emission spectra of micelle-solubilizedpyrene, The larger values of 11/13 found for C12N3C02, recorded using a Hitachi F 4010 spectrofluorometer, were used C12N5C02, and C12N3S03 indicate that pyrene senses to obtain the ratio 11/13 of the fluorescence intensities of the first a more polar environment than in micelles of C12NlC02 and third vibronic emission peaks. This ratio gives a measure and C12NlOC02 because it now resides, at least in part, of the polarity sensed by pyrene a t its micellar solubilization in the fairly hydrated shell of thickness corresponding to site.Ig
Results (1)Polarity Sensed by Pyrene at Its Solubilization Site. Table 1lists the values of Z1/Z3 for various systems (16)Bhatia, A.;Qutubuddin, S.Colloids Surf. 1993,69,277. (17)Frindi, M.; Michels, B.; Zana, R. J . Phys. Chem. 1994,98,6607. (18)Alami, E.; Van Os, N. M.; Ruppert, L. A.; De Jong, B.; Kerkhof, F. J. M.; Zana, R. J. Colloid InterfaceSci. 1993,160,205. (19)Zana, R. InSurfuctuntSolutions. New Methods oflnvestzgution; Zana, R., Ed.; M. Dekker Inc.: New York, 1987; Chapter 5,p 241. (20)Infelta, P.Chem. Phys. Lett. 1979,61,88. (21)Tachiya, M.Chem. Phys. Lett. 1976,33,289. (22)Almgren, M. Adv. Colloid Interface Sci. 1992,41,9. (23)Gehlen, M.; De Schryver, F. C. Chem. Rev. 1993,93,199. (24)Binana-Limb&, W. Doctorate Thesis,University Louis Pasteur,
Strasbourg, 1991.
the intercharge distance. The values of the lifetime, z, of the excited state of the micelle-solubilize pyrene, listed in Table 2, lead to the same conclusion,even though the changes are not as large as for the Z1IZ3 ratio. It is seen that the values o f t for C12NlC02 and C12NlOC02 are very close but are larger than those for the other three surfactants, indicating a lesser polarity for the former. (2) MicelleAggregation Numbers. Table 3 lists the values of the aggregation number, N , of the micelles of the surfactant investigated under various conditions of concentration, NaCl content, and temperature. Figure 1 (26)Zana, R.; Levy, H. J . Colloid Interface Sci. 1995,170,128. (26)Lianos, P.;Viriot,M.-L.; Zana, R. J.Phys.Chem. 1984,88,1098.
Langmuir, Vol. 11, No. 9, 1995 3353
Aqueous Solutions of Zwitterionic Surfactants Table 3. Values of the Aggregation Numbers of the Micelles of the Investigated Surfactants surfactant system T ("C) N 0.197 M C12NlC02 0.194 M C12NlC02
+ 0.194 M NaCl
0.467 M C12NlC02
+ 0.465 M NaCl
0.203 M C12N3C02
10 25 40 9.8 25 40 10 25 40 6.5 9 18.4 25.2 35 40
0.498 M C12N3C02
0.451 M C12N3C02
+ 0.451 M NaCl
0,199 M C12N3S03 0.193 M C12N3S03
+ 0.193 M NaCl
0.452 M C12N3S03
+ 0.452 M NaCl
0.159 M C12N5C02
0.205 M C12N5C02
+ 0.200 M NaCl
0.520 M C12N5C02 0.204 M C12NlOC02
0.486 M C12NlOC02 0.197 M C12NlOC02
+ 0.200 M NaCl
44.3 7.8 16.6 25 33.5 43.2 9.7 25 40 12 25 40.8 10 25 40 10 25 40 9.8 15.7 25.2 41 10 25 40 10 25 40 12 15.7 25 41 17 25 40 17.8 25 40
87 80 75 89 85 80 86 82 77 59 58 58 56 53.5 52 51 56 57 55 53 51.5 58 56 52 66 59 59 68 63 60 70 67 63 40 40 39 38 44 43 41
44 43 40 38 42 40.5 42 44 46 49 40 41 43
shows the variation ofNwith temperature for the systems investigated. The following comments can be made. (i) For C 1 2 N l C 0 2 , C12N3C02, C12N5C02, and C 12N3S03, N is independent of the surfactant concentration and salt content, within the experimental error of the measurements (f5%),even at the fairly large concentration values used in the present study. Thus the zwitterionic surfactants investigated display very little tendencyto micelle growth. A similar conclusion had been previously reached for the phenylphosphinate betaine m i ~ e l l e s .The ~ very small change ofNwith concentration leads one to expect the zwitterionic surfactant micelles to be fairly monodisperse and close to s p h e r i d Z 7Note the small increase of N with concentration for C12NlOC02 at 40 "C, the highest temperature investigated. (27) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. SOC.,Faraday Trans. 1 1978,72, 1525.
100
,
O'
1-1
1
I
I
I
I
I
I
I
I
I
i
m
t r ,
1
lo 01
I
I
I
I
1
I
I
I
I
0
5
10
15
20
25
30
35
40
45
50
Figure 1. Variation of the micelle aggregation ,number with temperature for various surfactant solutions. The empty and filledsymbols correspond to solutions in water and to solutions in the presence of NaCl with a molar concentration ratio [surfactantl/[saltl= 1. The symbols 0,0,A,and A correspond to a surfactant concentration close to 0.2 M, and the symbols 0, W, and v are for a surfactant concentration around 0.5 M: (-0) 0.197 M C12NlC02, (-0) 0.194 M C12NlC02 _+ 0.194 M NaC1, (-W) 0,467 M C12NlC02 + 0.465 M NaC1, (0)0.203 M C12N3C02, (0)0.498 M C12N3C02, (W) 0.451 M C12N3C02 0.451 M NaCl, (0-1 0.199 M C12N3S03, (0-) 0.193 M C12N3S03 + 0.193 M NaC1, (W-10.452 M C12N3S03 0.452 M NaC1, (0)0.159 M C12N5C02, (0)0.202 M C12N5C02 0.202 M NaC1, ( 0 ) 0.500 M C12N5C02, (A) 0.204 M C12NlOC02, (A)0.197 M C12NlOC02 + 0.200 M NaCl, (v) 0.500 M C12NlOC02. For the sake of clarity, the data for C12NlOC02 have been shifted downward along the N-axisby 15 units.
+
+
+
(ii) The comparison of the values for C12N3C02 and C12N3S03 reveals that the substitution ofthe carboxylate group by a sulfonate group results in a small increase of
N. (iii) The values of N for C12NlC02, C12N3C02, C12N5C02, and C12N3S03 decrease upon increasing temperature, as for ionic surfactants,28 but with a somewhat smaller temperature coefficient: the larger the aggregation number, the larger the decrease of N in the range 10-40 "C. In contradistinction, C12NlOC02 shows a small but definite increase ofNwith T and, thus, behaves somewhat like a nonionic polyethoxylated s u r f a ~ t a n t . ~ ~ The effect responsible for this increase of N with T for C12NlOC02 has not yet been identified. (iv) At a given T,the values of N show significant differences, but always decrease in the sequence C12NlC02 > C12N3S03 > C12N3C02 > C12N5C02 FZ C12NlOC02. This sequence is the same as that of head group sizes, if one disregards C12N3S03, which has a different head group. The same sequence has also been found for the aggregation numbers at the critical micelle concentration (cmc)ofthe series of zwitterionic surfactants where the carboxylate is replaced by a phenylphosphinate group.' All N values in Figure 1, except those for C12NlC02, are not very different from those expected for the spherical micelles formed by surfactants with a dodecyl chain, i.e., about 55-65 (when using the oil drop model and the equations reported for the length and volume of the alkyl chain, assuming a small protrusion of about 0.1 nm of the surfactant chain from the surface (28)Malliaris, A.; Le Moigne, J.;Sturm, J.;Zana, R. J.Phys. Chem. 1965,89,2709. (29)Zana, R.; Weill, C. J.Phys. Lett. 1988,46, L953. (30)Aniansson,E. A. G. J . Phys. Chem. 1978,82, 2805.
Kamenka et al.
3354 Langmuir, Vol. 11, No. 9, 1995 Table 4. Lifetime of the Pyrene Excited State in Various Systems in the Absence ( t )and in the Presence (YAz)of Quencher and Pseudo-First Order Migration Rate Constant of Probe and/or Quencher in C12NlOC02 Micelles system 0.200 M C12NlC02 0.500 M C12NlC02 0.5 M NaCl 0.500 M C12N3C02 0.199 M C12NlOC02 0.200 M NaCl
+
0.504 M C12NlOC02
+
40 40
325 324.6
330 318.3
40 17.8 25 40 17 25 40
306 366 353 331.5 352 339.5 312
301 326 296 223 231 184 111.5
0.32 0.62 1.60 1.10 1.85 4.30
of the core30). Even for C12NlC02, the departure from a spherical shape is small. Indeed, assuming a spherocylindrical shape and using the aggregation number value, the overall micelle lengtwdiameter ratio is calculated to be about 1.5, a very slight elongation. Notice that a discussion of the size of C12NlOC02 micelles must take into account the fact that the decanediyl spacer is partly or wholly located in the micelle core. If one assumes that half of the spacer, i.e. (CH& is located in the core, the volume ofthe hydrophobic core ofthe C12NlOC02micelles would be equal to that of 56 dodecyl chains, a value fairly close to those for the other surfactants, whereas the aggregation number is only 35-40. (3)Intermicellar Exchanges in Solutions of C12N10C02. The pyrene lifetime in solutions of C12NlOC02 was found to be significantly shorter in the presence than in the absence of quencher, contrary to the other surfactants (see Table 4). This indicates that a redistribution of probe andor quencher between C12NlOC02 micelles occurs on the fluorescence time scale. The full equations for intramicellar fluorescence q u e n ~ h i n g ' ~have - ~ ~ been used to calculate the values of the pseudo-first-order migration rate constant (k,) listed in Table 4. k, is relatively small at 17 "C but increases rapidly with temperature. It also increases almost linearly with the surfactant concentration, i.e., with the micelle concentration, since the micelle aggregation number depends only very little on C. This result indicates that, most likely, the intermicellar exchanges take place through micellar collisionswith temporary merging of the collided micelles, during a time sufficient to allow exchange of material (probe and quencher and also surfactant) between these micelles. R e c d that such a process also occurs in solutions of nonionic ethoxylated surfactants at temperatures ~ , ~in' some sufficiently close to the cloud t e m p e r a t ~ r e ~and water-in-oil micro emulsion^,^^ where strong attractive interactions exist between particles. Strongly attractive intermicellar interactions have been evidenced in solutions of C12NlOPPh,' a zwitterionic surfactant identical to C12NlOC02 except that a phenylphosphinategroup (PPh) replaces the carboxylate group, and are therefore also likelyto exist between C12NlOC02 micelles. In C12N10PPh the strength of these attractions is large enough as to induce phase separation at temperature above 30 'C.' Accepting that the intermicellar exchanges take place via micelle collisions, the bimolecular rate constant for efficient collisions (k,) (that is, accompanied by exchange of material) is then given by the ratio k,J[micellel with the molar micelle concentration, [micelle],equal to (Ccmc)/N. Using the values of N and k, leads to k, values (31) Alami, E.; Kamenka, N.; Raharihamina, A.; Zana, R. J.Colloid Interface Sci. 1993,158, 342 and references therein. (32) Lang, J.; Lalem, N.; Zana, R. Colloids Surf.1992,68, 199 and references therein.
Table 5. Values of the QuantityNk, at 26 "C and of Its Activation Energy for Various Systems Nkq (9-l) E i k (kJ/mol)
system 0.197 M C12NlC02 0.194 M C12NlC02 0.467 M C12NlC02 0.203 M C12N3C02 0.498 M C12N3C02 0.451 M C12N3C02 0.159 M C12N5C02 0.204 M C12NlOC02 0.193 M C12N3S03 0.452 M C12N3S03
+ 0.194 M NaCl
1
+ 0.465 M NaCl
26.3 f 2
+ 0.451 M NaCl
23 f 2
+ 0.193 NaCl + 0.452 NaCl 1.8 and 4 x lo8 M-l
9.6 5.2
27.2 f 2 28 f 2 24.2 f 2
of about s-l at 25 and 40 "C, respectively. These values are nearly 2 orders of magnitude lower than for a fully diffision-controlled process. (4) Pyrene Fluorescence Decay Rates. Small differences exist between the values measured for the pyrene lifetime, t,at a given T, for the different surfactants. These differences reflect small variations in the polarity sensed by the probe in its micellar solubilization site, as discussed above. The activation energy, E l , for the pyrene decay rate, k = l/z, in its micellar environment have been obtained from the linear variations of In t with 1/T (not shown), and are listed in Table 2. Within the experimental error, the results show no dependence on the nature of the surfactant. The values are somewhat smaller than those reported for ionic surfactants (=4.2 kJ/mo1),28but rather close to those for nonionic ethoxylated surfactant^.^^ (5) IntramicellarQuenchingRate Constants. The values ofk, for the different surfactants cannot be directly compared. Indeed, for spherical or nearly spherical micelles, as is the case here, k, is nearly proportional to N j 3 and the relevant quantity to be compared are the values of Nk,. These values reflect the microviscosity sensed by the probe and quencher in the diffusive motion which brings them together. Since k, is proportional to a diffision coefficient which, in turn, is proportional to the reciprocal of the viscosity, Nk, represents a sizecorrected measure of the reciprocal of the microviscosity, Le., the microfluidity of the medium in which probe and quencher diffise in the micelle. The values listed in Table 5 show a significant decrease, by a factor of about 3, of the micelle microfluidity upon increasing carbon number of the intercharge group, in going from C12NlC02 to C12NlOC02. Again, this effect is probably related to the folded conformation of the intercharge group and its location in the micelle hydrophobic core for C12NlOC02. A small change in the average location of pyrene in the micelle with the intercharge group carbon number may also contribute to the observed changes of microfluidity. Table 5 also lists the values of the activation energies, E&k,,for the size-corrected quenching rate constant. The activation energy is seen to be independent of the nature of the system and its value is close t o those found for a variety of other surfactants, both ionic and n o n i o n i ~ . ~ ~ , ~ ~
Discussion As pointed out above, the values of the aggregation number indicate that the micelles of the five zwitterionic surfactants investigated, including C12NlC02, which shows the highest N values, are spherical or close to spherical, even at the high concentration (0.5 M) at which some of the measurements were performed. Such a conclusion is consistent with the reported values of the (33)Van der Auweraer, M.; De Schryver, F. C. Ciaem. Phys. 1987, 111, 105.
Langmuir, Vol. 11, No. 9, 1995 3355
Aqueous Solutions of Zwitterionic Surfactants surface area per head which are such that the packing parameter ofthe surfactants investigated is below 1/3.27 Spherical micelles are expected to show little polydispersity, and this expectation is borne out by the very little dependence, if any at all, of the aggregation number on surfactant concentration (Figure 1). Contrary to ionic surfactants, the presence of salt (NaCl), even at the high concentration of 0.5 M, does not affect this behavior. The effect of temperature on N is small, becoming smaller as the intercharge carbon number is increased. Qualitatively, however, N decreases upon increasing T,as for ionic surfactants, for C12NlC02, C12N3C02, C12N5C02, and C12N3S03. Also, the cmc values of these surfactants (mM range), are between the cmc values for ionic (10 mM range) and nonionic (0.1 mM range) surfactants having a dodecyl group. Moreover, the N values of these zwitterionic surfactants are similar to those for ionic surfactants with a dodecyl chain. Finally, no intermicellar exchanges were detected in the solutions of these surfactants, as in the case of ionic surfactants. All these results indicate that these four zwitterionic surfactants behave somewhat like ionic surfactants, even though their overall charge is zero. Such a behavior is well in line with the conclusion reached for the series of zwitterionic phenylphosphinato betaine surfactants. There, it was shown that the zwitterionic group of the above surfactants is in an extended c~nfonnation.~ The situation appears to be different with C12NlOC02, which shows intermicellar migration via micellar collisions and a slight increase of N with T,as for ethoxylated nonionic surfactant^.^^,^^ Besides, some results in Table 4 suggest that the intercharge decanediyl groups are at least partly located in the micelle hydrophobic core. Indeed, although the C12N5C02 and C12NlOC02 micelles have about the same aggregation number, the value of the product Nk, for C12N10C02 is about 2 times smaller than for C12N5C02. Many factors influence the value of Nk,: diffusive motion rates, solubilization sites, etc. Our opinion is that the lesser quenching efficiency is due to the microviscosity ofthe C12NlOC02 surface being about twice as large. This is readily explained by the hindrance that the intercharge groups would bring to the diffusive motion of the probe and quencher in the outer layer of the micelle core if they are located in this region. The intercharge groups bring about a rigidification of the micellar outer layer in the same manner as the chemical links between head groups of amphiphilic repeat units in the hydrophobic microdomains formed in solutions of p~lyamphiphiles.~~~~~ Notice that the two charged groups of the C12NlOC02 head group cannot come into contact upon folding of the decanediyl spacer group, owing to the chemical nature of the quaternary ammonium group. The closest approach corresponds to a separation of the carboxylate and ammonium groups by a distance of about 0.35 nm. This (34)Aizawa, M.; Komatsu, T.; Nakagawa, T. Bull. Chem. Soc. Jpn. 1977,50,3107. (35)Chu, D.-Y.; Thomas, J. K. Macromolecules 1987,20,2133.
is also about the distance which separates the two charged groups in C12NlC02, where obviously there is no folding of the intercharge segment. Nevertheless the properties of C12NlC02 and C12NlOC02 are quite different. This lends further support to the conclusion that the orientations of the surfactant dipoles in C12NlC02 and C12NlOC02micellesare Merent, the dipolein the former being perpendicular to the micelle surface and the later somewhat parallel to this surface. A similar conclusion was reached in a very recent small angle neutron scattering study of a similar compound, C20N5C02, with a much longer alkyl chain.36 As the above zwitterionic surfactants aggregate into small globular micelles without any variation of size with concentration, the first mesophase encountered as the surfactant concentration is increased is a periodic array of small globular micelles. Thus, a cubic mesophase of Pm3n symmetry, as described by FontelP7and predicted is found with many surfactants by Charvolin and Sad0c3~1~~ having a bulky head g r ~ u p . ~ OOn - ~ the ~ contrary, the zwitterionicsurfactant dodecyldimethylamine oxide, which has a smaller head group, forms larger micelles (N = 76),9p46,47 which organize into an hexagonal m e ~ o p h a s e , ~ ~ as the concentration is increased.
Conclusions The present investigation of a series of zwitterionic surfactanb of increasingintercharge group carbon number showed remarkably little dependence of the micelle aggregation number on surfactant concentration, temperature, and salt content of the solution. The values of the aggregation numbers are consistent with the formation of small spherical micelles, with little polydispersity. The results support the conclusion of previous studies of similar zwitterionic surfactants that when the intercharge group contains 10 carbon atoms it can fold, and be partially located in the micelle hydrophobic core. Acknowledgment. The authors acknowledge the financial support of the GDR No. 1082 “Systkmes Colloidaux Mixtes” of the CNRS, directed by Dr.T. Zemb. LA9502599 (36)Ottewill, R. H.; Rennie, A. R.; Laughlin, R. G.; Bunke, G. Langmuir 1994,10,3493. (37)Fontell, K.; Fox, K. K.; Hansson, E. Mol. Cryst. Liq. Cryst. 1986, 1, 9. (38)Charvolin, J.;Sadoc, J. F. J. Phys. Fr. 1988,49,521. (39)Charvolin, J.;Sadoc, J. F. Colloid Polym. Sci. 1990,268,190. (40)Laughlin, R. G. A d v . Liq. Cryst. 1978,3,99. (41)Eriksson, P.0.;Lindblom, G.; Amidson, G. J.Phys. Chem. 1985, 89,1050. (42)Faulkner, P.G.; Ward, A. J. I.; Osborne, D. W. Langmuir 1989, 5,924. (43)Vargas, R.;Mariani, P.; Gulik, A.; Luzzati, V. J . Mol. Biol. 1992, 225, 137. (44)Delacroix, H.; Gulik-Krzywicki, T.; Mariani, P.; Luzzati, V. J . Mol. B i d . 1993,229,526. (45)Amrhar, J.;Chevalier, Y.; Gallot, B.; Le Perchec, P.; Auvray, X.; Petipas, C. Langmuir 1994,10, 3435. (46)Corkill, J. M.; Herrmann, K. W. J . Phys. Chem. lgSg,67,934. (47)Ikeda, S.;Tsunoda, M. A,; Maeda, H. J . Colloid Interface Sci. 1979,70,448. (48)Lutton, E. S.J . Am. Oil Chem. SOC.1966,43,28.
