Mixed micelle formation between alkyltrimethylammonium bromide and alkane-α,ω-bis(trimethylammonium) bromide in aqueous solution

Mixed Micelle Formation between Alkyltrimethylammonium Bromide and Alkane-o~,c0-bis(trimethylammonium) Bromide in Aqueous Solution R. ZANA,* Y. M U T O , t K. E S U M I , t AND K. M E G U R O t *Institut Charles Sadron, 6 rue Boussingault, 67000 Strasbourg, France, and "~Departmentof Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan

Received April 27, 1987; accepted July 16, 1987 Mixed micellization in mixtures of alkyltrimethylammonium bromides (C,Me3) and alkane-a,~obis(trimethylammonium)bromides (CreMe6)has been investigated by means of electrical conductivity [critical micellization concentration (CMC) determination, test for mixed micelle formation] and timeresolved fluorescenceprobing (determination of average micelle aggregation number). The results show that a low concentration, close to the CMC, mixed micelles form in the entire composition range in only two mixtures: C l o M e 3 - C l 6 M e 6 and C~4Me3-C22Met.Mixed micellesform at bolaformmole fractions at/or above 0.5 in C~oMe3-C~2Me6,C~2Me3-C22Met,and C16Me3-C22Me6mixtures. An increase in n of one unit requires an increase in m of about two units to retain mixed miceUization.The results clearly show that mixed micellization is enhanced by increasing the surfactant concentrations. The aggregation numbers of the mixed micellesas obtained from fluorescenceprobingdecrease upon increasingbolaform mole fraction, owing to the small size ofbolaform micelles. © 1988AcademicPress,Inc. INTRODUCTION A bolaform surfactant is composed of a polymethylene chain with one ionic or polar head group attached at each of its two ends (1, 2). Ionic bolaform surfactants show several important differences with respect to conventional surfactants having the same head group (2-6). First, their critical micellization concentrations (CMCs) are m u c h larger, their micelles are smaller, and the degree of ionization of their micelles is larger than that of a corresponding conventional surfactant having an alkyl chain with the same n u m b e r of carbon atoms (2-5). Second, the bolaform surfactants generally adopt a folded, wicket-like conformation at the air-water interface (2, 6). Third, various pieces of evidence indicate that the polymethylene chain ofdocosane-1,22-bis(trim e t h y l a m m o n i u m bromide) m a y adopt a folded conformation in aqueous solution, both in the molecularly dispersed and in the micellized state (5, 6). This last feature prompted us to investigate the possibility of mixed micellization between

bolaform and conventional surfactants with the same head group as a function of the chain length of the two surfactants. Indeed, as packing constraints play an important role in micellization, it was hoped that the planned study would reveal some interesting chain length requirements for mixed micelle formation in the mixtures of bolaform surfactants and homologous conventional surfactants as it did in the case of mixtures of conventional surfactants. As shown below, this expectation was borne out by the experiments. MATERIALS AND METHODS The surfactants selected for the present investigation were the alkyltrimethylammonium bromides CnH2,+IN+(CH3)3Br - (referred to as CnMe3) and the alkane-a,w-bis(trimethyla m m o n i u m ) bromides (CH3)3N+CmH2mN+(CH3)3Br~- (referred to as CmMe6). Samples of these surfactants (CloMe3, C12Me3, ClaMe3, C16Me3, C12Me6, C16Me6, and C22Me6) were available in our laboratory from previous studies in which their micellar behavior had been well characterized (3-5, 8, 9). 502

0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988

MIXED

The critical micelle concentrations were obtained by the electrical conductivity method. Conductivity K was measured as a function of surfactant concentration C using an automatic precision conductivity bridge (Wayne-Kerr B905). In Fig. 1 are illustrated the types of K vs C plots most often obtained in the present work. Curve 1 corresponds to the case in which mixed micelles do not form: the CMC corresponds to a break between two linear parts of the plot and is well defined. Curve 2 corresponds to a case where mixed micellization takes place. The ~ vs C plot is now composed of two linear parts connected by a curved part. The CMC is less well defined and an operational CMC was taken as corresponding to the intercept of the two linear parts. The CMCs of the CnMe3-CmMe6 surfactant mixtures have thus been determined systematically as a function of the mole fraction X = CB/(C +CB) of the bolaform, CB being the bolaform concentration expressed in moles ofbolaform per liter. For the sake of comparison we have also determined the CMC of C.Me3-KBr mixtures, as a function of the KBr mole fraction in the mixture. In order that the

1000

Br- concentration be the same in CnMe3-KBr mixtures and CnMe3-CmMe6 mixtures of the same mole fraction, the mole fraction of KBr was taken as X = Cv,/(2C + CK) where CK is the KBr concentration. In addition to the shape of the r vs C plots, extensive use was made of the values of the apparent degree of miceUe ionization OLapobtained as (8-11)

~ap --

500

103 C ( H / f ) I

I 3

I 4-

I 5

I 6

FIG. 1. Electrical c o n d u c t i v i t y versus C14Me3 concentration for the m i x t u r e s C~4Me3-C12Mer, X = 0.5 (curve 1); CLaMe3-C22Me6, X = 0.5 (curve 2); a n d Cx4Me3-KBr,

X = 0.33 (curve 3).

(dK/dC)c>cMc (dK/dC)c
[1]

aap values for the CnMe3-CmMe6 mixtures were systematically compared with those for the CnMe3-KBr mixtures at constant X. Where no mixed micelles formed, aao increased very rapidly with X. Indeed, the second surfactant then behaved like an electrolyte and increased both the numerator and denominator of Eq. [1] by nearly equal amounts which increased with X and which became larger than (dK/ dC)ccMc when X >~ 0.5 (this is so because CmMe6 and KBr solutions have larger K's than CnMe3 solutions at a given C). Figure 1 also illustrates the above: O~apis clearly much larger for the systems corresponding to curves 1 and 3 (no mixed micellization) than for the system corresponding to curve 2 where mixed micelles form. The conductivity results have also been used to construct the equivalent conductivity A = (K Ko)/Cvs C 1/2plots, K0being the conductivity of the solvent. In cases where no mixed micelles formed, the A vs C 1/2 curves usually showed a single sharp break, whatever the X value. Similar behavior was observed in those cases where mixed micelles formed at large X, say above 0.75. The CMC obtained from the A vs C 1/2plots was generally close to, but nevertheless systematically smaller than, that obtained from the r vs C plots. In mixtures where mixed micelles formed, the A vs C ~/2 curves showed a considerable curvature after a break (11). This break generally occurred at a concentration well below that corresponding to the operational CMC obtained from the K vs C curves. In the next section only the latter values are listed. -

i 2

503

MICELLE FORMATION

-

Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988

504

ZANA ET AL.

At this stage it is worth pointing out that the conductivity data reveal only whether mixed micelles do or do not form at the C M C of the mixture. The behavior may be different at higher concentration, where intermicellar interactions become significant. It will be seen below that this is indeed the case. Micelle aggregation numbers were determined using the time-resolved fluorescence probing method described in our previous studies (13-15). Briefly, the fluorescence decay curves of micelle-solubilized pyrene, in the presence of an efficient micelle-solubilized quencher, cetylpyridinium chloride (CP+C1-) (14a), were determined using a single-photon counting apparatus and analyzed according to the equation (16) I(t) = I(O)exp[-A2t - A3[1 - exp(-A4t)]] [2]

to yield the constants A2, A3, and A4. I(t) and I(0) are the fluorescence intensities at times t and t = 0. The experimental conditions were such that [pyrene]/[micelle] ~ 0.01 and [CP]/ [micelle] - 0.6-1. Separate experiments performed in the absence of quencher permitted measurement of the pyrene fluorescence lifetime r0 in the micellar environment. For a given system it was found that A2 = r0-1, within experimental error (___2%). This result indicated that the distribution of pyrene and CP ÷ among the micelles was frozen on the fluorescence time scale (13). In such a case A3 =

[CP+]/[micelle]

and

A 4 = kQ

[3]

where k~ is the first-order rate constant for intramicellar quenching. The micelle aggregation number N was then obtained from N - C - CMC A ~ff~] 3.

[41

Additional experiments were performed in the absence of quencher, at higher pyrene concentration, such that [pyrene]/[micelle] - 0 . 5 - 1 . Pyrene excimer can then form in those micelles containing more than one solubilized pyrene molecule. The fluorescence decay still obeys Eqs. [2] and [3], but now Journal of Colloid and Interface Science,

Vol. 123,No. 2, June 1988

A3 =

[pyrene]/[micelle]

and

A4 = kE [5]

where kE is the rate constant of intramicellar excimer formation. The micelle aggregation number is calculated using Eq. [4]. These measurements permitted comparison of the two most widely used time-resolved fluorescence methods for the determination of micelle aggregation number. For reasons discussed elsewhere recall that in cationic micelles kE is generally smaller than kQ (17). The N values were determined by keeping the CnMe3 concentration C constant (usually much larger than the CMC of pure CnMe3 so that the change in the CMC induced by the additive has a small or negligible effect on the calculated N value) and increasing the CreMe6 concentration CB. For the sake of comparison and a better understanding of the results, similar measurements were performed in which CmMe6was replaced by KBr. All investigations were performed at 25°C. RESULTS AND DISCUSSION

1. Critical Micellization Concentration and Mixed Micellization at the C M C

CMC and aap data are plotted against X in Figs. 2 to 5. Note that the CMC values are expressed in terms of the CnMe3 concentration in the mixtures, except when stated otherwise in the figure captions. 1.I. CloMe3-containing mixtures. C~0Me3 does not comicellize with C22Me6. This is clearly indicated by the aap vs X curve for this mixture which is nearly coincident with that for the addition of KBr (see Fig. 2). At each mole fraction of C22Me6, the K vs C curve showed two breaks separated by a factor of about 10. The first break was associated with the micellization of C22Me6, CloMe3 then playing the role of an electrolyte. This was clearly demonstrated by studying the effect of KBr on the CMC of pure C22Me6: the CMC values thus obtained were equal, within experimental error, to those found for the C22Me6-CloMe3 mixture, at the same mole fraction of KBr and CloMe3. Mixed mieelli-

MIXED MICELLE FORMATION

-lo'chc(M/~l

i

i

Q

×

d-ap

'

I

0

0 O.

o:2s ols

o175

X

)

o

o25

o15

0!75

X

FIG. 2. C~oMe3-containing systems. Variation of CMC and of apparent degree of ionization with the mole fraction X of additive: (O) KBr; (X) Cl2Me6; (0) Ca6Mer; (A) CnMer. The e symbols refer to the CMC in C,oMe3-

C~6Me6mixtures expressedin total concentrationof surfactant.

zation is dearly taking place with C16Me6, and probably also with C]2Me6, at X >~ 0.5 where the aap vs X curve clearly departs from those for KBr and C22Me6 additions. 1.2. C12Me3-containing mixtures. Mixed micellization takes place only in the C~2Me3C22Me6 mixtures at X > 0.25. Indeed, the CI2Me3-C]2Me6 and CI2Me3-C]6Me6 mixtures are characterized by CMC vs X curves coincident with that of the C~2Me3-KBr mixtures and by a~p vs X curves clearly different from that for the C12Me3-C22Me6 mixtures (see Fig. 3). When mixed micelles do not form, the bolaform-containing mixtures are always characterized by aap values smaller than those for the KBr-containing systems, at a given X

i03 C'M C(Mi.[)

'

I

~op'

'

505

(see Figs. 2 and 3, as well as Figs. 4 and 5 below). The difference is due to the fact that the conductivity of K ÷ is larger than that of the bolaform ion. Therefore, at a given X, the apparent increase in aap due to the additive will be larger for KBr than for the bolaform surfactant. 1.3. C14Mes-containing mixtures. The behavior of these mixtures is fairly close to that of the C]2Me3-containing mixtures. Mixed micellization appears to take place only in the C14Me3-C22Me6 mixtures (see Fig. 4). For CI4Me3 the effect of KBr on the CMC was known and allowed us to give quantitative support to this conclusion. Thus, at a total concentration Ci in counterion (Br-) the CMC is given by (18) log CMC = - 0 . 6 9 6 log Ci - 4.111.

In the absence of salt, Ci = CMC and Eq. [6] yields CMC = 3.8 × 10 -3 M, in good agreement with the value CMC = 3.7 × 10 -3 M obtained from the K vs C plot. In the present experiment it can be shown that Ci

=

CMC(1 + X)/(1 - X).

log CMC = -0.4104 log[(1 + X)/ (1 - X)] - 2.424.

[8]

The CMC values calculated as a function of X are listed in Table I, together with the

'

,o3c~c(M#)

' 1

0.8

10

[7]

Inserting Eq. [7] into Eq. [6] yields

14. 12

[6]

~ctP~

'

,

I 0.5

0.6

8 6 4.

E2

2 0

0)25

0.5

[1.75

0

~ 0.25

) 0.5

i X 0.75

FIG. 3. C~2Me3-containing systems. Variation of CMC and of aap with X. Same symbols as in Fig. 2. The • symbols refer to the CMC in Ca2M%-Cz2Me6 mixtures expressed in total concentration of surfactant.

0

I

I

I

0.25

115

0.75

X

0

0.25

) 0.75

X

FIG. 4. C~4Me3-containing systems. Variation of CMC and ofaap with X. Same symbols as in Fig. 2. The • symbols refer to the CMC of the C14Me3-C22Me6 mixtures expressed in total concentration of surfactant. Journal of Colloid and Interface Science,

Vol. 123,No. 2, June 1988

506

Z A N A ET AL. TABLE I

TABLE II

Experimental and Calculated CMCs (moles/liter) of C~4Me3-Containing Mixed Systems

Mixed Micellization in CnMe3-CmMe6 Mixtures at 25°C CreMe6

KBr CI2M% CI6M% C22Me6

X = 0.25

X = 0.5

X = 0.75

3.05

103CMC(calc) 2.4

1.7

2.9 2.9 2.9 2.9

103CMC(exp) 2.2 2.3 2.3 2.0

1.6 1.7 1.7 0.9

experimental values of the CMC. It can be seen that experimental and calculated values agree quite well except for the C14Me6-Cz2Me6 mixtures, owing to mixed micelle formation in the latter. 1.4. C16MeTeontaining mixtures. Comparison of the C16Me3-KBr and C16Mea-CmMe6 mixtures reveals that mixed micellization is probably taking place only in the C16Me3C22Me6 systems at X > 0.5 (see Fig. 5). Calculations similar to those described for the C14Me3-containing mixture have been performed, using Eq. [6] modified to take into account the lower CMC of C16Me3with respect to C14Me3. The calculated CMC values were in very good agreement with the experimental ones except for the C16Me3-CzzMe6 mixture at X > 0.5 where mixed micelles form. Table II summarizes the above results. It is seen that mixed micellization in the mixtures

I

I

i

10 - 104CMC{M/~)

0.9

a~ I

i

,/

!

8\ 0.5

0.3

2 0

X I I I 0.25 0.5 0.75

0.1 0

v t i i i ^ 0.25 0.5 0.75

1

FIG. 5. C~6Mes-containing systems. Variation of C M C and of aap with X. Same symbols as in Fig, 2.

Journal of Colloid and Interface Science. Vol. 123,No. 2, June 1988

CriMe3

Ct2Me6

Cx6Me6

C22Me~

CloMe3

Yes

No

C~2Me3

Yes, only at X>0.5 No

No

C,4Mes C16Me3

No No

No No

Yes, only at X > 0.25 Yes Yes, only at X>0.5

investigated takes place only when the chain lengths of the two surfactants match each other, probably as a result of packing constraints (19). m is seen to increase about twice as fast as n. Indeed, for Cl0Me3 mixed micellization takes place with Cl2Me6 (only at X > 0.5), whereas for C16Me3 it occurs with C22Me6 (only at X > 0.5). Such a factor 2 is easily understood if one pictures the bolaform surfactant with a folded conformation in the mixed micelle. It may be considered another piece of evidence for such a folded conformation.

2. Micelle Aggregation Numbers in CnMe3C,,Me6 Mixtures The addition to a micellar solution of an electrolyte having a common ion with the surfactant always results in an increase in the micelle aggregation number (7). On the other hand, mixed mieellization in a binary surfacrant mixture results in micelle aggregation numbers or sizes varying continuously (7) from the aggregation number of a pure surfactant to the aggregation number of the other pure surfactant. A qualitatively similar effect, however, is expected if two types of micelles coexist in the solution. Indeed the measured aggregation number will then be a weighted average of the aggregation numbers of the two types of micelles. This average is expected to depend on the technique used for the measurement. For the small micelles investigated

507

MIXED MICELLE FORMATION

150 N

'~

100

N

40

50 ~

30

.

2O

10 0

0

i

0.25

c (M#) i

0.50

FIG. 6. Variation of the micellar aggregation number N with the surfactant concentration for Cl4Me3 (O, O) and C22Me6 ([], II) from pyrene excimer formation (solid symbols) and pyrene fluorescence quenching by CP ÷ (open symbols).

here, fluorescence probing yields a numberaverage aggregation number. This number is expected to vary monotonously with the composition of the mixture. Thus, difficulties can be expected in attempts to distinguish between mixed micellization and coexistence of two micellar species on the basis of measurements of aggregation number versus composition. 2.1. Pure surfactants. The N values for pure C14Me3 a n d C22Me6 are plotted in Fig. 6 as a function of the surfactant concentration C. The results show that the values obtained from

quenching and excimer formation experiments coincide within experimental error on N (+10%). Table III lists N values of some of the pure surfactants investigated. The values for C22Me6 are larger than that reported previously (5). The latter was obtained from intensity measurements upon static quenching of the pyrene fluorescence by dimethylaniline and the partitioning of dimethylaniline between micelles and bulk was not taken into account. This is known to result in N values that can be much lower than the real ones (13). 2.2. C,dkle3-CmMe6 mixtures. The results are shown in Figs. 7-9. The plotted N values have been calculated on the basis of the CnMe3 concentration only, using the values of the CMC in Figs. 2-5. (Note that correcting for the CMC in Eq. [4] is important only for Cl0Me3.)

As for the pure C14Me3 and C22Me6, the plotted data show that, within experimental error, the same aggregation numbers were obtained in experiments involving pyrene excimer formation and pyrene fluorescence quenching by CP ÷. Regular trends can be seen in the changes o fNwith the additive concentration when the

6O

40 30

Aggregation Numbers N of the Pure Surfactants in Water at 25°C a

Cl0M% C14Me3

concentration 0.25

I4

50

TABLE III

surfaetant

I I

N '

20

N

10 0

45

0.05 0.10 0.20 0.30 0.50

88 93 + 9 105 + 10 1 1 0 + 11 130 - 12

C16Me3

0.10

142 + 15

C22Me6

0.05 0.10 0.20

24 ___ 2 26 + 3 30 + 3

0.15 [

0.3 i I

o.1

X

0.4-5 i[

cBm/e) o.2

FIG. 7. Variation of the micellar aggregation number N of 0.25 M Cl0Mea with the concentration of added KBr (O, O), Cl2Me6 (A, A), CI6Me6 ([], II), and C22Me6 (V, ~')

from pyrene excimer formation (solid symbols) and pyrene fluorescence quenching (open symbols). To permit a direct comparison between the effects of KBr and bolaform sur-

factants the KBr concentration has been divided by 2. The mole fractions X have been calculated as indicated in the the text.

Journal of Colloid and Interface Science. Vol. 123,No. 2, June 1988

508

ZANA ET AL. I

I

I

I

I

I

Fig. 7), whereas, close to the CMC, this effect takes place only at X > 0.5 (see Fig. 2). Some trends are also apparent in the changes in N induced by C16Me6 and C22Me6 additions to the CnMe3 solutions: the N vs CB 100 curves which are nearly coincident with CloMe3 become increasingly separated in going to C14Me3 and C16Me3. However, in50 terpretation of the effect of addition of C16Me6 and C22Me6 in terms of mixed micellization is more difficult because these two surfactants 0.1 0.2 0.3 X 0.4are capable of forming micelles on their own, 0 a I ) ~ I , at concentrations at which they were used in 0 0.025 O.05CB(Mq, )/ 0.075 the present investigation. [Their CMCs have FIG. 8. Variation of the micellaraggregationnumber N been remeasured by electrical conductivity of 0.1 M Cl4Me3 with the concentration of added KBr, and found to be (4.5 ___0.3) X 10 -2 and (3.3 C12Mer, CI6Me6, and C22Me~. Same symbols and con_ 0.1) X 10 -3 M, respectively, in good agreevention as in Fig. 7. ment with our previously reported values (4, 5).] To clarify the situation additional electrical conductivity measurements were performed, chain length of C,Me3 is increased, at least for at 0.1 M C,Me3 and increasing CB concentrations. The results are shown in Figs. 10 and KBr and Cl2Me6 additions. The increase in N induced by the presence 1 1. The curves for the addition of C22Me6 to of KBr at a given concentration is the largest C14Me3 and C16Me3 are perfectly linear and for the longest CnMe3 surfactant, as is ex- run parallel to better than 1% in the whole CB pected. Thus, 0.025 M KBr leaves N nearly range. Moreover, the value of the slope is unchanged for CloMe3 and increases N b y 15% slightly lower than that measured for C22Me6 for C14Me3. Additions of C12Me6 to both C14Me3 and 200 , I , C16Me3 solutions induce an increase in N that N is quite close to that brought about by KBr addition in the case of C14Me3. This clearly indicates that C12Me6 behaves like an electro150 lyte and does not comicellize with the longalkyl-chaln CnMe3 surfactants. However, addition of Cl2Me6 to CloMe3 first leaves N un100 changed up to about X = 0.2, then decreases N. This decrease takes place at CB ranging between 0.1 and 0.2 M, that is, at concentrations well below the CMC of Ct2Me6 (0.6 M) (5). It 50 therefore indicates that mixed micellization is &l 0.2 X 0.13 30 i [ taking place between CloMe3 and C12Me6, as 0.025 Ce,(i,4/~,) 0.'05 was already inferred from the conductivity measurements discussed above. However, it is FIG. 9. Variation of the micellaraggregationnumber N seen that at the high surfactant concentrations of 0.1 M C~6Me3with the concentration of added KBr, used in fluorescence probing, mixed micelli- C~2Me6, ClrMe6, and C22Me6. Same symbols and conzation is already occurring at X > 0.15 (see vention as in Fig. 7. 150

/

Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988

MIXED MICELLE FORMATION alone at C > CMC. This indicates that C22Me6 comicellizes with C]4Me3 and Cl6Me3 in the whole CB range. Note that the conductivity data reported in the first part of the discussion indicated mixed micellization in the entire composition range for the C~4Me3-C22Me6 mixtures, and at X > 0.5 for the C16Me3C22Me6 mixtures at C close to CMC. Thus, as for the C~oMe3-Cl2Me6 mixture, an increase in total surfactant concentration is seen to enhance the formation of mixed micelles. The total aggregation numbers NT and the composition XM of the CnMe3-C22Me6 mixed micelles can easily be calculated from the data in Figs. 8 and 9. For instance, NT = N[(0.1 +

cB)/o, l].

The r vs CB curve for the addition of Cl6Me6 to C16Me3 shows a significant change of slope at about 3.0 × 10 -2 M, i.e., slightly lower than the CMC of pure C[6Mer. Moreover, the slope dK/dCB at concentrations below this break is only slightly larger than (dr/dCB)cB
I

I

10 3 K (S)

/

509

10 3 K ( S] 7

4-+/

eX~-'*f

,

/

3 2

102CB(M/~'1 I i 2 4 FIG. l I. Variation of the electrical conductivity of a0.1 M C~6Me3 solution upon addition of C~oMe6(×) and C22Me6(O). I

,

I

]

ing constraints are predominant and at Ca > 3.0 × 10 -2 M t w o types of micelles coexist: large C16Me3-rich micelles containing a small amount of C16Me6 and small C16Me6-rich micelles containing a small amount of C16Me3. In this case, fluorescence probing measures an average aggregation number 2V, which can be approximated as /V = N[CnMe3](1 - ~0a) + N[C16Me6]~PB

f

I / I j-/

/

.f

: /

~OB = m ( C B -- CMCB)/

//

[m(CB - CMCB) + nC]

Y

2

10 c B (H/~)

/J/ I

0

where N[CnMe3] and N[CI6Me6] are the aggregation numbers of the pure surfactants, assumed to be constant, and

2

i

I

4

t

is the volume fraction of the core of the bolaform micelles with respect to the total volume of micelle core where probe and quencher can solubilize. Using N[C16Me3] = 142, N[CI6Me6] = 13, l CMCB = 3 × 10-2M, and C = 0.1

FIG. 10. Variation of the electrical conductivity of a i N[C16Mer]was calculatedfrom the aggregationnumCI4M%solutionupon addition OfClrMe6(X) and C22Me6 (O). The concentration of Cj4Me3 remained equal to ber of C22Me6,assumingN[CI6Me6] = N[C22Me6] × (16/ 22)2. 0.1M. Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988

510

ZANA ET AL.

M, one obtains N = 127 as a compared with the experimental value I 15. The agreement is satisfactory in view of the fact that N[Ct6Me3] is likely to decrease upon incorporation of some bolaform surfactant. The r vs CB curve for the addition o f f l 6 M e 6 to C14Me3 (Fig. 10) is very complex but can now be rationalized in view of the results obtained for the C16Me3-C16Me6 mixture. Indeed, up to the CB value CB~ corresponding to the dotted line (1), the r vs CB curve for the C14Me3-Ci6Me6 mixture is very similar to that for the C16Me3-C16Me6 mixture. The break corresponding to the formation of C 16Me6 micelles takes place at about C = 0.02 Minstead of 0.03 M, revealing that these micelles probably incorporate some C~4Mea. However, as CBis increased the small C16Me6 micelles tend to incorporate more and more C14Me3. This results in an increase in size, and a decrease in ionization (Fig. 10 shows that indeed dr/ dCBdecreases as CB increases but remains below Cm). However, at the same time, the CI4Me3 micelles start incorporating C16Me6, with a resulting decrease in size and increase in ionization. This effect is expected to become predominant at higher CB and is thought to be responsible for the change in slope of the r vs CBplot at CB values above CB~.In this range the two micellar species present in the system, large C14Me3-rich micelles and the small C16Me6-rich micelles, tend to become closer and closer in composition and thus in size. The N vs CB curve for the C14Me3-CI6Me6 system (see Fig. 8) reflects the above. Thus, up to CB = CMCB "~ 0.02 M, N changes only little because the weak salt effect Of Cl6Me6 on the N values of C14Me3 micelles is compensated by the incorporation of a small amount of bolaform into these micelles. Then, at higher CB, a sharp decrease in N occurs because of the formation of an increasing number of small C16Me6 micelles and because the CIaMe3 micelle size is reduced by incorporation o f C16Me6. The fluorescence probing data do not permit us to obtain the aggregation numbers of the two coexisting miceUar species. Journal of Colloid andlnterface Science, Vol. 123, No. 2, June 1988

CONCLUSIONS

The electrical conductivity method has been shown to be a convenient tool to probe mixed micellization in mixtures of conventional surfactants C~Me3 and bolaform surfactants C,nMe6, both having the trimethylammonium bromide head group. The results show that at low concentration, close to the CMC, mixed micellization takes place in the whole range of composition only with some mixtures: CloMe3-Cl6Me6 and Cj4Me3-C22Me6. Mixed micelles form at higher content of bolaform in the Cl0Mea-Ct2Me6, C12Me3-C22Me6, and C16Me3-C22Me6 mixtures. An increase in n of one unit requires an increase in m of about two units to retain mixed micellization. Conductivity measurements have also permitted us to probe mixed micellization at concentrations much larger than the CMC. The results dearly show that mixed micellization is enhanced by increasing surfactant concentration. The aggregation numbers of the mixed micelles have been determined through time-resolved pyrene fluorescence quenching and pyrene excimer formation. Addition of CreMe6 to CnMe3 solutions always results in a decrease in micelle aggregation number when mixed micellization takes place. ACKNOWLEDGMENTS Part of this research benefited from the support of the PIRSEM (CNRS, AIP 1086). Y.M. thanks the CNRS for financial help during his stay in Strasbourg. REFERENCES 1. Brody, O. V., and Fuoss, R. M., J. Phys. Chem. 60, 156 (1956). 2. Menger, F. M., and Wrenn, S., J. Phys. Chem. 78, 1387 (1974). 3. Yiv, S., Kale, K. M., and Zana, R., J. Phys. Chem. 80, 2651 (1976), and references therein. 4. Yiv, S., and Zana, R., J. Colloid Interface Sci. 77, 449 (1980). 5. Zana, R., Yiv, S., and Kale, K. M., J. Colloid Interface Sci. 77, 456 (1980). 6. Meguro, K., Ikeda, K., Otsuji, A., Taya, M., Yasuda, M., and Esumi, K., J. Colloid Interface Sci. 118, 372 (1987).

MIXED MICELLE FORMATION 7. Shinoda, K., in "Colloidal Surfactants" (Hutchinson and Van Rysselberghe, Eds.), Chap. 1. Academic Press, New York, 1963. 8. Lianos, P., and Zana, R., J. Colloid Interface Sci. 84, 100 (1981). 9. Zana, R., J. Colloid Interface Sci. 78, 330 (1980). 10. Hoffmann, H., and Ulbricht, W., Z. Phys. Chem. N.F. 106, 167 (1977). 11. Lianos, P., and Lang, J., J. Colloid Interface Sci. 96, 222 (1983). 12. Mukerjee, P., and Yang, Y., J. Phys. Chem. 80, 1388 (1976). 13. Zana, R., in "Surfactant Solutions, New Methods of Investigation" (R. Zana, Ed.), Chap. 5 and references therein. Dekker, New York, 1987, 14. (a)Malliaris, A., Lang, J., and Zana, R., J. Chem. Soc. Faraday Trans. 1, 82, 109 (1986); (b) J. Phys.

15. 16.

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Chem. 90, 655 (1986); (c)J. Phys. Chem. 91, 1475 (1987). Malliaris, A., Le Moigne, J., Sturm, J., and Zana, R., J. Phys. Chem. 89, 2709 (1985). Tachiya, M., Chem. Phys. Lett. 33, 289 (1975); Infelta, P., Gratzel, M., and Thomas, J. K., J. Phys. Chem. 78, 190 (1974); Infelta, P., Chem. Phys. Lett. 61, 88 (1979); Dederen, J., Van der Auweraer, M., and De Schryver, F. C., Chem. Phys. Lett. 68, 451 (1979). Lianos, P., Viriot, M. -L., and Zana, R,, J. Phys. Chem. 88, 1098 (1984). Jones, M. N., and Reed, D., Kolloid Z. Z. Polym. 235, 1196 (1969). Israelachvili, J., Mitchell, J., and Ninham, B., J. Chem. Soc. Faraday Trans. 2 72, 1525 (1976).

Journal of Colloid and Interface Science, Vol. 123, No. 2, June 1988