J, Phys. Chem. 1983, 87, 1289-1291
1289
Micellar Properties of Alkyltrimethylammonium Hydroxides in Aqueous Solution Panaglotls Llanost and Raoul Zana" Centre de Recherches sur les Macromol6cules, CNRS 67083 Strasbourg-Cedex, France (Received: November 29, 1982; In Final Form: February 8, 1983)
Alkyltrimethylammonium hydroxide surfactants (alkyl = dodecyl, tetradecyl, and hexadecyl) have been prepared, and their micellar properties investigated by means of electrical conductivity, pH measurements, and fluorescence probing. With respect to the corresponding alkyltrimethylammonium bromide surfactants, the hydroxides form micelles at surfactant concentrations about twofold larger, and their micelles have an ionization degree which is about three times larger, whereas their aggregation number is about twofold smaller. The relevance of these results for the understanding of the effect of cationic micelles on the rate of chemical reactions involving hydroxide ions is examined.
Introduction Alkyltrimethylammonium halide (RTA+X-) surfactants have been extensively used to study the effect of the presence of cationic micelles on the rate of reaction of various compounds with hydroxide In such studies the concentration of hydroxide ions is often much larger than that of the surfactant. Owing to the ion exchange taking place in the systems$5 the Stern layer of the micelles will contain both X- and OH-. As the affinities of the micelles for these two ions are expected and indeed have been ~ h 0 ~ nto~differ, 9 ~ , the ~ properties of the initial RTA+X- micelles (critical micellization concentration, ionization, aggregation number) are expected to be modified by the exchange.' This in turn will affect the rate of the process under investigation. It is to be noted that the possibility that the properties of RTA+OH- and RTA+X- micelles differ strongly has been recently put forward.* In view of this fact, and for the purpose of contributing to the understanding of the effect of cationic micelles on the rate of reactions involving OH- ions, we felt it worthwhile t o synthesize three alkyltrimethylammonium hydroxide surfactants and to investigate their micellar properties.
Materials and Methods The investigated surfactants are the dodecyl-, tetradecyl-, and hexadecyltrimethylammonium hydroxides (DTA+OH-, TTA'OH-, and CTA+OH-, respectively). These surfactants were obtained from their bromide homologues by ion exchange with a strong cationic ion-exchange resin (Merck Ionenaustauscher 111). In a typical experiment 25 mL of a 0.25 M aqueous solution of RTA+Br- was slowly filtered through a column of the ion-exchange resin in the OH- form and a volume of about 50 mL of 0.12 M RTA+OH- solution was obtained. Contact with air was minimized during this operation by bubbling argon through the hydroxide surfactant solution. The solution concentration was determined by potentiometric titration with hydrochloric acid. The stock solution was stored a t 5 "C under argon. The critical micellization concentration (cmc) and micelle ionization degree were obtained from electrical conductivity and p H measurement^.^ In conductivity measurements the cmc was obtained both from the specific conductivity K vs. surfactant concentration C curve and from the equivalent conductivity A vs. C1I2curve, whereas the ionization degree was taken as the ratio of the values 'On leave of absence from the Department of Physics, University of Crete, Heraklion, Greece. 0022-365418312087-1289$0 1.5010
TABLE I: Cmc and Micelle Ionization Degree of RTA'OH- Surfactants at 25 "C 103cmc,M CY surfactant a b a b DTA'OH30.5 f 3 29.5 f 2 0.74 f 0.06 0.63 ? 0.06 TTA'OH7.2 f 0.6 5 i: 2 0.66 i: 0.06 0.69 f 0.06 CTA+OH-C 1.8 f 0.2 0.70 * 0.06 a From conductivity measurements (average of the cmc values obtained from the K vs. C and A vs. C"' curves). From pH measurements. The log C scale used for the plots, as well as problems associated with the reaction with CO,, resulted in a large error in the cmc (see ref 9). The cmc value in this table coincides with the value of the concentration for which Bunton et a1.6 reported a departure from linearity in the K vs. C plot. These authors, however, found a much smaller cmc value by means of surface tension. We believe that the conductivity value should be favored because it is obtained in easier experimental conditions than the surface tension value. Moreover, as noted in the text, the cmc value from conductivity obeys the logarithmic law usually found for the change of cmc with the number of carbon atoms of the surfactant chain, contrary to the surface tension value. of dK/dC above and below cmc.l0 The pH vs. log C curve showed a break corresponding to the cmc. This curve was analyzed to yield a,as in emf studies with specific electrodes.l0 The micelle aggregation number n was obtained, as in previous studies, from the analysis of the decay of the (1)Fendler, J. H.; Fendler, E. J. 'Catalysis in Micellar and Macromolecular Systems"; Academic Press: New York, 1975. (2)Almgren, M.; Rydholm, R. J.Phys. Chem. 1979,83,360, and references therein. (3) Al-Lohedan, H.; Bunton, C.; Romsted, L. J.Org. Chem. 1982,47, 3528. ( 4 ) Quina, F.; Chaimovich, H. J. Phys. Chem. 1979,83,1844. (5)Chaimovich, H.; Bonilha, J.; Politi, M.; Quina, F. J.Phys. Chem. 1979,83,1851. (6) Bunton, C.;Gan, L.; Moffatt, J.; Romsted, L.; Savelli, G. J.Phys. Chem. 1981,85,4118. (7)Notice that the changes that we refer to are not those associated with the salt effect (change of ionic strength) due to the addition of the hydroxide, but those due to the exchange X--OH-, in the absence of any salt effect.
(8) Romsted, L. Talk presented at the International Symposium on Surfactants in Solution, Lund, Sweden, 1982,to be published. (9)Argon was bubbled through the solutions during the conductivity meaeurements at a rate sufficiently low to prevent foaming, after saturating the conductivity cell with argon. Experimental constraints prevented argon bubbling during the pH measurements. The reaction of the hydroxyde surfactant with atmospheric COz did not matter for DTA+OHsolutions which were rather concentrated, but led to a large error in the cmc value for TTA+OH- and did not permit meaningful measurements on CTA+OH- solutions because of their very low concentrations. (10)Zana, R. J. Colloid Interface Sci. 1980,78,330.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Voi. 87, No. 8, 1983
Letters
TABLE 11: Micelle Aggregation Number n and Fluorescence Lifetime T~ and I , / I , for Micelle Solubilized Pyrene at 22 "C
series an increase of the alkyl chain length brings about a small decrease of 11/13and, thus, of the effective polarity. This probably reflects a tighter packing of the surfactant head groups, and thus a lesser penetration of water in the micelle palisade layer as the chain length is increased. The n values of RTA+OH- micelles are about twofold smaller than for the corresponding RTA'Br- micelles. These results explain in part the larger a values found for RTA+OH- with respect to RTA+Br- micelles. Recall that the n values for the RTA+Br- surfactants were shown to be close to those which can be calculated from the spherical oil drop model for micelles,16 assuming a micelle hydrophobic core with a radius equal to the length lc of the fully extended alkyl chain.12 The small n values of the RTA'OH- micelles therefore indicate that these micelles are spherical, with a core radius R smaller than lc. This suggests that the alkyl chains in the micelle core must be more entangled than in the minimum spherical micelles, in order to achieve maximum packing, as in the case of micelles of tetradecyltrimethylamonium bromides.12 In agreement with this model, the microviscosity of RTA+OH- micelles, measured by using the spectral properties of dipyrenylpr~pane,'~ was found to be two- to threefold larger than that of SDS or RTA+Br- and to increase with chain length.l8 The values of R have been calculated from the equations given by Tanfordlg for the volume per surfactant alkyl chain in the micelle. For comparison we have given the values of lC.l9 The surface charge density of the RTA+OH- micelles expressed as the ratio cyn/47rR2 was found to be nearly independent of the alkyl chain length and about 20 times larger than for RTA+Br- micelles (about 8 X charge/A2 against 4 X charge/A2). Relevance of the Above Results in Studies of the Rate of Reactions Involving OH- Ions in the Presence of Cationic Micelles. The difference of properties between RTA+OH- and RTA+Br- micelles may influence the rate of chemical reactions involving OH- ions at two different levels. First, in experiments performed at given concentrations of surfactant and water-insoluble reactants, the probability of finding micelles containing 2 or more than 2 reactants will be much lower with RTA+OH- than with RTA+Br- micelles because the micelle concentration of the former is about twofold that of the latter. As a result the rate of reactions involving proximity effects of reactants will be affected. Second, the large value of a for RTA+OH- micelles indicates that the OH- concentration of the micelle Stern layer will be well below that of Br- in the same region of RTA+Br- micelles. Thus, approximate calculations based on the values of CY and of the micelle radius yielded an OHconcentration in the Stern layer of RTA+OH- micelles about twofold lower than that of Br- in the Stern layer of RTA+Br- micelles. This result explains why, in solutions of cationic micelles containing both OH- and halide ions (Cl- or Br-), the (halide-)/(OH-) concentration ratio in the micelle Stern layer has been consistently found to be much larger than the ratio of the overall concentrations of these two ion^.^^^^^
1290
surfactant DTkOHTTA'OHCTA'OH
concn, M
11/13 R,, A
I,, A
11.8
16.6
16.2
19.2
17.4
21.7
na
1 0 9 r ,s
0.1 0.2
20 ( 5 7 ) 21
370 370
1.53
0.064 0.128
42b ( 6 8 ) 46b (70)
370 370
1.43
0.05 0.105
46 (90) 51
370 370'
1.40d
a The values in brackets are for the RTA'Br- surfactants (from ref 1 2 ) . The n values were found to be independent of the (pyrene)/(micelle)concentration ratio in the range 0.45-1.75. However, the rate constants for intramicellar excimer formation was found t o slightly increase with this ratio (by a factor of about 1.5 when the (pyrene)/ (micelle) ratio was increased by a factor of about 4 ) . The The value 1.4 was value 390 ns was found for CTA'C1-. found for CTA'CI- ,
fluorescence of micelle-solubilized pyrene.'lJ2 This analysis also yielded the fluorescence lifetime, rot of pyrene in the micelles, as well as the rate constant for the formation of pyrene excimer in the micelles. Finally, the value of the ratio 11/13of the intensities of the first to the third vibronic peaks of the fluorescence spectrum of micelle-solubilized monomeric pyrene was also determined, as this value provides an estimate of the polarity sensed by pyrene in its micellar solubilization site, that is, the micelle palisade layer.12-14
Results and Discussion Solubility of CTA+OH- in Water. A 0.09 M solution of CTA+OH- was found to remain perfectly clear when maintained a t 0 "C for several days. This indicates that the Krafft point of CTA+OH- is below 0 "C, that is, much lower than for CTA+Br- which precipitates out at 20-25 OC. Cmc and Micelle Ionization Degree. The cmc and cy values for the three surfactants are listed in Table I. As for other ionic surfactants, the cmc decreases by a factor very close to 2 per additional methylene group in the surfactant alkyl chain.15 However, at a given chain length the cmc of RTA+OH- is almost exactly twofold larger than that of RTA+Br-. The most interesting result, however, is probably the much larger a values found for the hydroxide surfactants (a N 0.7) with respect to the bromide surfactants ( a = 0.251°). Micelle Aggregation Number. The values of n are listed in Table 11, together with the values of the fluorescence lifetime T~ of micelle-solubilized pyrene, and of 1 1 / 1 3 . The value of T~ in RTA+OH- micelles is very close to that found in sodium dodecyl sulfate (SDS) and in CTA+Cl-, and about two times larger than in RTA+Br-.12 In the latter the quenching of the pyrene fluorescence by Br- is responsible for the observed shorter lifetime.I2 The local polarity sensed by pyrene in its solubilization values, site in RTA+OH- micelles, as expressed by the 11/13 is close to that sensed in CTA+Cl- or RTA+Br- micelles but still larger than in SDS. Finally, as for the RTA+Br(11)Atik, S.;Nam,M.; Singer, L. Chem. Phys. Lett. 1979,67, 75. (12)Lianos, P.;Zana, R. J.Phys. Chem. 1980,84,3339, Chem. Phys. Lett. 1980,76, 62,J. Colloid Interface Sci. 1981,84,100,1982,88,594. (13)Dorrance, R. C.; Hunter, T. F. J . Chem. Soc., Faraday Trans. 1 1972,68,1312,1974,70,1572. (14)Kalyanasundaram, K.; Thomas, J. K. J.Am. Chem. SOC.1977,99, 2039. (15)Shinoda, K. 'Colloidal Surfactants"; Hutchinson and Rysselberghe, Ed.; Academic Press: New York, 1963;Chapter I.
Conclusions The results presented in this paper show that the RTA+OH- surfactants form micelles which are about (16)Tartar, H. V. J. Colloid Interface Sci. 1959,14, 115. (17)Zachariasse, M.Chem. Phys. Lett. 1978,57, 428. (18)Part of the difference between the microviscosities found for RTA+OH- and SDS or RTA+Br- micelles may arise from the smaller size of the micelle core in the former than in the later. The difference, however, is too large to arise only from a difference of size. (19)Tanford, C. J. Phys. Chem. 1972,76, 3020.
Letters
twofold smaller (in aggregation number) and three times more ionized than the corresponding RTA+OH- surfactants. Also micellization occurs at a concentration which is about two times larger. The results suggest that the X--OH- exchange which occurs in R T A W micellar so-
The Journal of Physical Chemistry, Vol. 87, No. 8, 1983
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lutions upon addition of OH- may influence the rate of chemical reactions through a change in the distribution of water-insoluble reactants among the micelles and a lower than expected concentration of OH- in the micelle Stern layer.
