J. Phys. Chem. 1992,96,4667-4671
4661
Quaternary Water-in-Oll Microemuisions. 2. Effect of Carboxylic Acid Chain Length on Droplet Size and Exchange of Material between Droplets J. Lang,* N. Lalem, and R. Zana Institut Charles Sadron (CRM-EAHP), CNRS- ULP Strasbourg, 6,rue Boussingault, 67083 Strasbourg Cgdex, France (Received: August 20, 1991; In Final Form: January 28, 1992)
Water solubility, electrical conductivity, and time-resolved fluorescence quenching measurements have been performed in water-in-oil (w/o) microemulsions made of water, chlorobenzene, cationic surfactant, and n-carboxylic acid (used as cosurfactant) in order to investigate the effect of the n-carboxylic acid chain length on the size and dynamics of the w/o droplets. The results obtained in this study are similar to those obtained with the same w/o microemulsions systems, using n-alcohols as cosurfactants (Lang, J.; Lalem, N.; Zana, R. J. Phys. Chem. 1991, 95, 9533). Microemulsions with a short-chain acid showed a percolation of electrical conductivity upon increasing the [water]/ [surfactant] molar concentration ratio w. The variation of the water solubility in the microemulsion with the acid chain length suggested that the droplet size was limited by the intensity of attractive interdroplet interactions. At the percolation threshold the rate constant k, for interdroplet exchange of material via droplet collision was (1-2) X lo9 M-l s-l, as for all other percolating microemulsions investigated thus far. Microemulsions with a long-chain acid showed a monotonous decrease of the electrical conductivity upon increasing w, the water solubility in the microemulsion seemed to be limited by the natural curvature of the droplets and the rate constant k, remained below lo9 M-' SKI.The changes of droplet size and interdroplet interactions with the length of the carboxylic acid are in qualitative agreement with the trends obtained from theoretical predictions concerning the effect of the cosurfactant chain length.
Introduction Ternary water-in-oil (w/o) microemulsions have been studied in order to determine the effect of various parameters on the size and dynamics of water droplets, interdroplet attractive interactions, water solubility, and water self-diffusion coefficient in the microemulsions. More recently, the effect of the chain length of added alcohol (cosurfactant) on the same properties, but self-diffusion, of quaternary w/o microemulsions was investigated (part 1 in this series).6 These studies showed that droplet size and intensity of attractive interactions between droplets increase as the alkyl chain length of the alcohol decreases, in agreement with the theoretical prediction^.'-^ Also electrical percolation occurred only for those microemulsions characterized by a value of the rate constant k, for droplet collision with temporary merging (see Figure 1) at least equal to (1-2) X lo9 M-' s-l. Finally, the increase of droplet size was always accompanied by an increase of k,, irrespective of the parameter change responsible for the increase of droplet size: increase of temperature or water content, or decrease of surfactant or alcohol chain length. The purpose of this paper is to compare the effect of another series of cosurfactants, the n-alkanecarboxylic acids, on the size and dynamics of the water droplets in quaternary w/o microemulsions to that of n-alcohol cosurfactants.6 An additional interest in using carboxylic acids as cosurfactants stems from the use of such compounds as reactants in various chemical reactions performed in o/w microemulsions.'O All the results obtained in the present study show a striking similarity between the effects of alcohols and carboxylic acids on the investigated properties. (1) Lang, J.; Jada, A.; Malliaris, A. J . Phys. Chem. 1988, 92, 1946. (2) Jada, A.; Lang, J.; Zana, R. J . Phys. Chem. 1989,93, 10. (3) Jada, A,; Lang, J.; Zana, R. J . Phys. Chem. 1990, 94, 381. (4) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387. ( 5 ) Zana, R.; Lang, J.; Canet, D. J. Phys. Chem. 1991, 95, 3364. (6) Law, J.; Lalem, N.; Zana, R. J. Phys. Chem. 1991, 95,9533. (7) Mukherjee, S.;Miller, C. A.; Fort, T., Jr. J. Colloid Inrerface Sci. 1983, 91, 223. Jeng, J.-F.; Miller, C. A. In Surfacranrs in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 3, p 1829. (8) Leung, R.; Shah, D. 0. J . Colloid Interface Sci. 1987, J20, 320 and 330. (9) Hou, M.-J.; Shah, D. 0.Langmuir 1987, 3, 1086. (10) Walde, P. J. Am. Oil. Chem. SOC.1990,67, 110. Backmann, P. A,; Walde, P.; Luisi, P. L.; Lang, J. J . Am. Chem. SOC.1990, 112, 8200.
0022-3654/92/2096-4667$03.00/0
Experimental Section 1. Materials. The surfactants dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromides (DTAB, TTAB, and HTAB, respectively) and the oil, chlorobenzene, were the same as previously to insure a valid comparison between the effect of alcohols and carboxylic acids? Propanoic, butanoic, and pentanoic acids (Fluka) and octanoic and nonanoic acids (Eastman Kodak) were of the best grade available and used as received. Hexanoic and heptanoic acids (Schuchardt) were purified by distillation. Water was deionized and distilled. 2. Methods. In the following the water and the acid present in the microemulsions are referred to the surfactant concentration C and expressed as the molar concentration ratios: w = [H,O]/C and z = [acid]/C (1) The experimental conditions adopted in this study were the same as in part l6 with C = 0.27M and z = [cosurfactant]/C = 2.5. The determination of the water solubility (taken as the value wlof w above which the system phase separates) has been described.6 The error on the wIvalue is estimated to be about 2% for 30 < ol< 70 and 4-10% for 7 < w, < 20. The mean surfactant aggregation number, N , per droplet, the pseudo-first-order rate constant for intradroplet fluorescence quenching, k,, and the rate constant, k,, for exchange of material between droplets have been determined using the time-resolved fluorescence quenching (TRFQ) method,'" and a single-photon counting apparatus" (excitation at 450 nm, emission monitored above 530 nm with a high-pass cutoff filter). Ruthenium tris(bipyridyl) ion (Ru(bpy), in the chloride salt form) was used as fluorescence probe and methylviologen ion (MV, in the chloride salt form) as quencher, as in other studies of w/o microemulsions based on cationic ~urfactants.~.~J*J~ The probe and the quencher are cations that are solubilized in the water droplets. The [Ru(bpy)]/[M] molar concentration ratio was between 0.01 and 0.05 ([MI is the droplet concentration) and the [MV]/[M] molar concentration ratio between 0.7 and 1.2. All solutions were thoroughly degassed prior to fluorescence decay measurements (1 1) Pfeffer, G.; Lami, H.; Laustriat, G.; Coche, A. C.R. Hebd. SCances Acad. Sci. 1963, 257, 434. (12) Atik, S. S.;Thomas, J. K. J . Am. Chem. SOC.1981, 103, 3543. (13) Lang, J.; Mascolo. G.; Zana, R.; Luisi, P. L. J. Phys. Chem. 1990, 94. 3069.
0 1992 American Chemical Society
4668 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Lang et al.
Figure 1. Exchange of material between droplets through collisions with temporary merging. The exchange is illustrated by the transfer of a quencher molecule (A)into a micelle which already contains a fluorescent probe (A) and by the transfer of a counterion (e) from one micelle to the other; surfactant (Ow); acid (Om).
by carrying out at least four freeze-pumpthaw cycles. The decay eq 2 was fitted to the decay data using the previously described p r o c e d ~ r e ' ~ ~ to ~ ' ~obtain - ' ~ the constants A2, A3, A4. Z ( t ) = Z(0) exp[-A2t
- A3(l - exp(-A4t))]
(2)
The probe decay rate constant, k,,, in absence of quencher, was determined in a separate experiment. The values of N, k,, and k, were then obtained from N=-
c
(A3A4
[MVI
+ A2 - koI2 A3A42
k, = A3A42/(A3A4 + A2 - ko)
(3)
(4)
k, = A4 - k ,
(5) Recall that no k, value below lo8 M-' s-' can be reasonably measured since the probe and quencher distribution then appear frozen on the probe fluorescence time ~ c a l e . l ~ ~In- this ~ J ~case, within the experimental error, A2 N ko and the long-time part of the decay curve in the presence of quencher shows the same slope as in the absence of quencher (Figure 2A). When the exchange is significant, A2 becomes larger than ko, and the long-time part of the decay shows a slope larger in the presence then in the absence of q ~ e n c h e r ' ~ J(Figure ~ J ~ 2B). If it is assumed that all the water and surfactant are in the form of water droplets that are spherical, monodisperse, and separated from the continuous oil phase by a monolayer of N surfactant ions, the radius of the water core, R,, and the spherical surface area, us, per surfactant ion at the water core surface are given by R, = [3N(wu, u, + q w u , ) / 4 ~ ] ' / ~ (6)
+
US
= 4rRWZ/N
TABLE I: Values of ko, kp, N, R', and R'i and ut and u," for Water/Chiorobenzene/DTAB/Acid Microemulsions for Various Values of w (C= 0.27 M z = 2.5: T = 20 O C )
(7)
with
= 18C'/(1000 - C'Ma/d) (8) In eqs 6-8, R, is in A and us in AZ. u, is the molecular volume of a water molecule (30 A3 at 20 "C),u, the molecular volume of the counterion Br- (49.3 A3),16ua the apparent molecular volume of the acid in water (1 12.7, 140.5, 166.9, and 192.6 A3 for propanoic, butanoic, pentanoic, and hexanoic acids, respectively, at 25 OC"), C'the acid concentration in the water droplets, and Ma the acid molecular weight. Since the partition coefficients of the acids between the three peudophases, namely the water pool, the interfacial layer, and the oil continuous phase, are difficult to determine,'*J9only the partition coefficient, P, between water and chlorobenzene has been measured. The values of P (ratio of the mole fractions of the acid in water and in chlorobenzene) have been found equal to 0.34,0.073,0.02, and 0.0032 for propanoic, butanoic, pentanoic, and hexanoic acids, respectively.20 These (14) Zana, R. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Dekker: New York, 1987; Chapter 5, p 241. (15 ) Lang, J. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1990; p 1. (16) Millero, F. Chem. Rev. 1971, 71, 147. (17) Hailand, H. Acta Chem. Scand. A 1974, 28,699. (18) Biais, J.; Odberg, L.; Stenius, P. J . Colloid Interface Sci. 1982,86, 350. (19) Biais, J.; Bodet, J. F.; Clin, B.; Lalanne, P.; Roux, D. J. Phys. Chem. 1986, 90, 5835.
9.0 14.0 20.0 25.2 30.0
1.31 1.38 1.43 1.49 1.50
2.8 1.9 1.6 1.3 1.2
Pentanoic Acid 328 29.2 661 42.0 995 53.6 1426 64.9 1667 72.2
29.3 42.1 53.7 65.1 72.5
32.7 33.5 36.3 37.1 39.3
32.8 33.7 36.5 37.3 39.6
7.1 15.1 20.1 25.1 30.1
1.31 1.42 1.47 1.49 1.50
3.2 3.9 3.1 2.5 1.9
Hexanoic Acid 174 22.2 348 34.6 530 43.5 767 52.7 1161 64.5
22.2 34.7 43.5 52.7 64.6
35.4 43.3 44.9 45.5 45.1
35.6 43.4 44.9 45.5 45.1
" Fluorescence decay rate constant of Ru(bpy) in the droplet (error +2%). *Rate constant for intermicellar quenching of Ru(bpy) by MV (error +lo%). CSurfactant aggregation number (error +lo%). "Water pool radius (error f 4 % ) . CSurfacearea per surfactant ion at the water pool surface (error f5%). TABLE II: Values of ko, kp, N, # , and R'! and u: and u:' for Water/Chlorobenzene/TIAB/Acid Microemulsions for Various Values of w (C = 0.27 M; y = 2.5; T = 20 "C)"
~~
5.2 10.0 14.9 20.0 25.0 30.0
1.28 1.31 1.37 1.41 1.43 1.47
2.3 3.1 2.2 1.8 1.4 0.9
Butanoic Acid 136 18.8 312 29.6 568 40.6 886 51.5 1235 61.7 2085 77.8
10.0 15.0 20.1 26.5 30.0
1.33 1.42 1.48 1.51 1.53
6.3 7.7 6.5 4.7 3.8
Hexanoic Acid 116 21.3 190 28.3 312 36.5 515 47.0 656 52.9
~~
18.9 29.8 41.0 51.9 62.2 78.5
32.6 35.2 36.5 37.7 38.7 36.5
33.0 35.8 37.1 38.3 39.4 37.2
21.3 28.3 36.5 47.0 53.0
49.2 52.9 53.6 53.8 53.7
49.2 53.0 53.6 53.9 53.7
"Same error margins as given in the footnotes of Table I.
TABLE 111: Values of ko, kp, N, R', and R'! and u: and u:' for Water/Chlorobenzene/HTAB/Acid Microemulsions for Various Values of w ( C = 0.27 M: Y = 2.5: T = 20 O C ) "
5.0 10.0 15.1 20.1 25.1
1.20 1.32 1.33 1.36 1.40
1.1 3.1 2.0 1.4 1.2
10.0 15.0 20.3 25.0 30.1
1.34 1.36 1.47 1.49 1.54
12.6 10.4 8.6 6.2 4.6
Propanoic 142 304 657 1129 1498
Acid 18.9 29.4 42.8 56.0 65.9
19.4 30.2 44.0 57.6 67.1
31.7 35.7 35.1 34.9 36.4
33.3 37.6 37.0 36.8 38.4
Hexanoic Acid 67 17.7 148 26.0 251 34.0 406 42.6 598 51.3
17.8 26.0 34.0 42.6 51.4
59.0 57.5 57.9 56.1 55.4
59.0 57.6 57.9 56.2 55.5
"Same error margins as given in the footnotes of Table I.
The Journal of Physical Chemistry. Vol. 96, No. 11, 1992 4669
Quaternary Water-in-Oil Microemulsions
Figure 2. Fluorescence decay curves for the systems water/chlorobenzene/HTAB/carboxylic acids with C = 0.27 M; z = 2.5, and T = 20 'C: (A) hexanoic acid at w = 25 (system with no exchange on the fluorescencetime scale); (B) propanoic acid at w = 10 (system with strong exchange). Curves 1 are without quencher and cumes 2 with quencher in the microemulsions. The water droplets in the two systems have nearly the same size (see Table
111).
60
tul
/rt
1
"C O:' 4 " 6 ' 8I " 10 Figure 3. Variation of water solubility (3 with the n-carboxylic acid for water/chlorobenzene/DTAB/carboxylic acid (A), chain length (k) water/chlorobenzene/TTAB/carboxylic acid (+), and water/chlorobenzene/HTAB/carboxylic acid ( 0 )microemulsions with z = 2.5, C = 0.27 M; T = 20 'C.
Figure 4. Variations of the electrical conductivity K, of the surfactant aggregation number N, and of the exchange rate constant k, with w for water/chlorobenzene/DTAB/carboxylic acid microemulsions with 1propanoic acid (O), 1-butanoic acid (A),1-pentanoic acid (+), l-hexanoic acid (O), I-heptanoic acid (a), I-octanoic acid (0),and 1-nonanoic acid (A). C = 0.27 M; z = 2.5; T = 20 'C. The polyphasic range is indicated by the gap in the K vs w plots.
000
values have been used for the calculation of the acid concentration C, in the water droplets. Since some acid is also solubilized in the interfacial layer, Cprepresents an upper limit of C! Of course, the exact values of R, and us fall between those calculated with C' = 0 (RL, ui) and C' = Cp (Rtf,ut). Tables 1-111 show differences of less than 3% between RL and Rtf, and less than 5% between u' and u". Therefore, the distinction between RL and R t , and usI and us', ? is no longer made in the following. The electrical conductivities were measured using an autobalanced conductivity bridge (Wayne-Kerr type B 905), operating at a frequency of 1 kHz. All measurements were made at 20 "C.
400
Figure 5. Variations of the electrical conductivity K,of the surfactant aggregation number N , and of the exchange rate constant &, with w for
water/chlorobenzene/TTAB/carboxylicacid microemulsions with 1propanoic acid ( O ) , 1-butanoic acid (A),1-pentanoicacid (+), l-hexanoic acid (O),1-heptanoicacid (a), 1-octanoic acid (0),and I-nonanoic acid (A). C = 0.27 M; z = 2.5; T = 20 'C.
Results and Discussion 1. Water Solubility. Figure 3 shows that the water solubility goes through a maximum as the number nc of carbon atoms of the acid is increased. A similar variation was reported with n-alcohols as oos~rfactants."~~'The theory for the water solubility in w/o microemulsions as a function of the cosurfactant chain length has been worked outgand provides us with an explanation for the results in Figure 3. A maximum is indeed expected to characterize the variation of wlwith k.It has been showed that for the systems at the right-hand side of the maximum (here, systems with long-chain acids) water solubility and in turn droplet size are limited by the spontaneous radius of curvature, Ro, of the surfactant + cosurfactant layer separating oil and water. The systems at the left-hand side of the maximum (i.e., with short-chain (20) Lalem, N. Ph.D. Thesis, UniversitB Louis Pasteur, Strasbourg, France, in preparation. (21) Bansal, V. K.; Shah, D.0.;OConnell, J. P. J . Colloid Interface Sci.
1980, 75, 462.
Figure 6. Variations of the electrical conductivity K, of the surfactant aggregation number N, and of the exchange rate constant &, with w for water/chlorobenzene/HTAB/carboxylic acid microemulsions with 1propanoic acid ( O ) , 1-butanoic acid (A),1-pentanoic acid (+), l-hexanoic acid (O), I-heptanoic acid (a), I-octanoic acid (0),and 1-nonanoic acid (A). C = 0.27 M; z = 2.5; T = 20 OC.
acids) are those where water solubility and droplet size are limited by the intensity of the attractive interdroplet interactions, which allow only a maximum droplet size RC before the system phase-separates into two w/o microemulsions. Notice that in this case RC is smaller than R". The variation of RO with the acid chain length can be understood by considering the packing ratio u/la, where u is the volume of the hydrophobic moieties (surfactant and acid) per surfactant,
4670 The Journal of Physical Chemistry, Vol. 96, No. 11, 195'2
I the length of the surfactant chain in the interfacial layer, and a the optimal cross-sectional area per head group at the surface of the water pool.22 Assuming that the molar ratio of acid to surfactant in the droplet interfacial layer is independent of the acid chain u and therefore u/la decrease with nc. This corresponds for w/o microemulsions to an increase of Thus the droplet size should increase; that is, at a given surfactant concentration more water can be incorporated into a droplet, as nc decreases. The results on the Ro branch in Figure 3 do show an increase of wIupon decreasing nc. 2. Electrical Conductivity. The variations of the electrical conductivity, K , as a function of w represented in Figures 4-6 show the following: (i) at constant w, K increases as nc decreases; (ii) two types of variations of K with w can be distinguished-the short-chain acids which lie on the fl branch in Figure 3 give rise to an electrical percolation, whereas the long-chain acids, which lie on the RO branch show a monotonous decrease of K , sometimes followed by a small increase, as w increases; (iii) for a given short-chain acid the percolation threshold, up,value of w above which K increases steeply, and also w1 increases with the surfactant chain length (a clearcut example is provided by butanoic acid with DTAB- and HTAB-based microemulsions). These changes of upand q indicate a decrease of attractive interdroplet interactions upon increasing surfactant chain length as for ternary w/o microem~lsions.~ These results indicate that the acid-containing microemulsions behave similarly to alcohol-containing ones as far as attractive interdroplet interactions are concerned.6 Therefore, the results must be given a similar interpretation, based on the assumption of an interpenetration of the droplet interfacial layers upon droplet c o l l i ~ i o n . ~The ~ * ~interfacial ~ layer can be assumed to be made of a mixture of surfactant and acid molecules with the carboxylic group of each acid in contact with water and its alkyl chain in the oil phase. The theoryz4predicts an increase of the magnitude of interdroplet interactions with the extent of interfacial layer interpenetration. This, in turn, is determined by the difference m - nc24v25( m is the number of carbon atoms of the surfactant alkyl chain) and also, since m > n,,by the coiling of the surfactant hai in.^^^ Briefly, the main prediction is that the intensity of attractive interactions should increase as n,or m decreases.s39 This is indeed what has been found experimentally with the short-chain acids. Finally, the monotonous decrease of K upon increasing w for the long-chain acid systems results from the adopted experimental procedure where w is increased at constant surfactant concentration. This procedure results in an increase of droplet size and, in turn, a reduction of the number of charge carriers @e., droplets) and of their mobility as w is increased. 3. Droplet Size. For each of the three surfactants investigated the values of N , R,, and us have been obtained in the presence of two acids selected for their contrasting effect on the change of the conductivity with w . A percolation characterizes the system with the selected short-chain acid contrary to the system with the selected long chain acid. The variations of N with w are given in Figures 4-6 and the numerical values of N , R,, and US listed in Tables 1-111. For each system N a n d R , increase with w, a result expected from geometric considerations. As for n-alcohol-containing microemulsions,6 N and R, increase as the acid chain length decreases at a given w . This result confirms the prediction of a statistical thermodynamic treatment of w/o microemulsions? Le., that RO increases as n,decreases, on the assumption that the value of RO determines the droplet size, irrespective of the factor which limits the water solubility in the microemulsion. This result also ~~
~~
(22) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W . J . Chem. SOC., Faraday Trons. 2 1976, 72, 1525. Mitchell, D. J.; Ninham, B. W. J . Chem. SOC.,Faraday Trons. 2 1981, 77, 601. (23) Only a slight decrease has been found as the alcohol chain length increases.6 (24) Lemaire, E.;Bothorel, P.; Roux, D. J . Phys. Chem. 1983, 87, 1023. (25) Brunetti, S.; Roux, D.; Bellocq, A. M.; Fourche, G.; Bothorel, P. J . Phys. Chem. 1983,87, 1028.
Lang et al. agrees with packing ratio considerations which predict an increase of IP upon decreasing nc (see above). The results in Figures 4-6 and Tables 1-111 also show that, for a given acid (hexanoic acid, for instance), at a given 0, the droplet size increases as the surfactant chain length decreases. A similar result was found for ternary4 and quaternary w/o microemulsions.6 The us values listed in Tables 1-111 call for the following remarks: (i) for a given acid (hexanoic acid, for instance) us increases, at a given w, with the surfactant alkyl chain (m), owing probably to the increased coiling of the surfactant chain;6 (ii) for a given surfactant at a given w , us increases with nc, because a larger area is needed to accommodate the acid in the interfacial layer as nc increases; (iii) in view of the surface area per head group values reported for alkyltrimethylammonium bromides at the air-water interface, 50 f 6 A2,26327 the us values with the shorter acids appear to be much too small, whichever the surfactant used. These acids are precisely those giving rise to electrical percolation (Figures 4-6). Microemulsions with short-chain n-alcohols were also characterized by abnormally low us values6 which were assigned to some roughness of the droplet surface not taken into account when calculating us. The same explanation is likely to hold for the microemulsions with short-chain acids. Besides, the assumption of surface roughness is in agreement with the fact that these systems show a percolation of electrical conductivity. Indeed this roughness reveals that the droplet interfacial layer is easily deformed under the effect of thermal and must have a low rigidity m o d ~ l u s . Such ~ deformable interfacial layers are expected to "open" easily upon droplet collisions. 4. Exchange Rate Constant k,. As for n-alcohol-containing microemulsions a clear correlation exists between the values of k, and the behavior of the electrical conductivity (Figures 4-6). For systems where no electrical percolation occurs (DTAB, "TAB, and HTAB with hexanoic acid) the values of k, remain below 8 X los M-' S-I in the whole w range, whereas the percolating systems (DTAB with pentanoic acid, TTAB with butanoic acid, and HTAB with propanoic acid) show a large increase of k, with w to values above lo9 M-I s-l. The onset of electrical percolation in the K versus w plots is not clearly apparent in the case of acid-containing systems (Figures 4-6). Nevertheless, for the three percolating systems investigated, k, = (1-2) X lo9 M-I s-' a t w = 10, i.e., close to the electrical percolation threshold. This narrow range of k, value has been found to correspond to the electrical percolation threshold for all w/o microemulsions where a comparison between the variations of k, and K with either w2*4or the temperature2J3 could be made. This experimental observation thus holds for a wide range of systems. 5. Comparison of the Effect of a-Carboxylic Acids and aAlcohols on Microemulsion Properties. The similarities existing between the effect of n-carboxylic acids and n-alcohols on the size and dynamics of w/o microemulsion droplets have been emphasized above. However, there exist also differences. A first one is in the effect of these two cosurfactants on the variations of wl with nc (Figure 3). For alcohols a maximum of wIis found at nc = 5 independently of the surfactant chain length (see Figure 2 in ref 6). For carboxylic acids, wIis maximum around nc = 5 with HTAB, nc = 6-7 with TTAB, while DTAB-based microemulsions show a peculiar behavior indicated by gaps in the K versus w plots in Figure 4. Indeed as w increases, the water/chlorobenzene/DTAB/acid systems show a miscibility gap in the w range between w = 7 and 9 or 12 and 15 depending on the acid which was not observed with the alcohol-containing DTAB-based systemse6 Notice that the variations of K,N , and k, with w show no discontinuity in going from below to above the _ _ _ _ ~ ~
_ _ _ _ _ ~
~
~
(26) Verrall, R.; Milioto, S.; Zana, R. J . Phys. Chem. 1988, 92, 3939. (27) Miyajima, K.; Yoshida, H.; Maetani, J.; Nakagaki, M. Bull. Chem. SOC.Jpn. 1980, 53, 1523. (28) Langevin, D. Adu. Colloid Inrerfoce Sd. 1991.34, 583 and references therein. (29) De Gennes, P. G.;Taupin, C. J . Phys. Chem. 1982, 86, 2294. (30) Safran, S.In Modern Amphiphilic Physics; Ben-Shaul, A., Gelbart, W., Roux, D., Eds.; Springer-Verlag, in press.
Quaternary Water-in-Oil Microemulsions
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4671
gap (Figure 4). Thus, there is probably no structural change other then involves collisions between discrete droplets. It is realized than droplet growth in going from below to above this gap. Since that even in this case the rate-limiting step remains the "opening" phase separation occurs at low w (around 10) and a t high w of the surfactant (plus cosurfactant if any) double interfacial layer separating two water pools, which is characterized by a first-order (around 30) the wIversus nc curve for DTAB-based systems is rate constant k,, = k,[M]. At the percolation threshold we have uncertain. The experimental error on wI does not permit an accurate determination of the nc value which corresponds to the observed that the values of k,, for the systems investigated thus far'-3*4-6 vary within a narrow range between 4 X lo5 and lo6 s-l. maximum of wIfor these systems. The comparison between the values of N and k, obtained for Above percolation threshold k,, varies little when, for instance, the systems containing carboxylic acids and alcohols reveals some w or Tare increased. Thus, our observation that percolation occurs interesting features. Thus, the N vs w plots obtained for systems only when the rate constant for efficient collisions becomes larger with hexanoic acid and pentanol are coincident, within the exthan a critical value of about lo9 M-' s-l remains valid when perimental error, for the three surfactants used in both studies. considering openings of surfactant layers instead of collisions, the The values of k, are fairly small for the two cosurfactants in critical value of the first-order rate constant then being of (4-10) systems with HTAB and TTAB. They are larger for the systems X lo5 This result merits theoretical investigation. with DTAB, those for pentanol being about 2.5 times larger than Recently, Lianos et have analyzed TRFQ data for mifor hexanoic acid. For the former, the k, values increase above croemulsions using an approach based on fractals. This looks lo9 M-l s-l upon increasing w , whereas they remain below 6 X promising particularly for systems above percolation threshold lo8 M-l s-I for hexanoic acid. This difference may look unimeven though the derived parameters still lack a clear physicoportant. However, it has a dramatic bearing on the change of chemical meaning. Below percolation threshold, this approach conductivity with w, with the Occurrence of percolation for pentanol looks less valuable to colloid chemists as it no longer uses the but not for hexanoic acid. This result brings further evidence that TRFQ data to obtain surfactant aggregation numbers per droplet the droplet size is not an important parameter in determining the or rate constants of droplets collision with merging even though intensity of interdroplet attractions and the droplet dynamics6 the TRFQ has showed its usefulness and validity for determining These properties are related to the details of the chemical structure such quantities.I4J5 In particular, we have checked for AOT-based of the components constituting the interfacial layer. microemulsions that the N values from TRFQ' are in quantitative 6. Rate Constants ko and kq. The values of the probe decay agreement with those from other techniques such as ultracenrate constant, ko, in the absence of quencher and of the pseudotrifugation.j6 first-order rate constant, k,, for intradroplet fluorescence Conclusions quenching are listed in Tables 1-111. The results reported above show that n-alcohols and n-carboxylic The value of ko increases whereas the value of k, shows a acids have very similar effects on the size and dynamics of water monotonous decrease or a maximum at small w upon increasing droplets in w/o microemulsions based on alkyltrimethylammonium w , as for the previously investigated system^.^*^," The variation bromide surfactants and chlorobenzene. In particular, the rate of ko has been attributed to the change in interactions between constant k, for the exchange of material between droplets increases Ru(bpy) and the surfactant counterions upon increasing The when the cosurfactant (alcohol or acid) chain length and/or the variations of kQ have been explained436in t e m of the antagonistic effects of the droplet size and of the water pool micr~viscosity.~~-~~surfactant chain length decrease and an electrical percolation takes place when k, reaches a value of (1-2) X lo9 M-I s-l . The It can also be seen that at a given R,, ko decreases and k variation of the water solubility in the microemulsion with the Le., as the amount of acid solubilizd increases upon increasing k, carbon number of the cosurfactant goes through a maximum for in the water pool decreases. This decrease may affect the water the microemulsions containing alcohol or carboxylic acids. A pool properties (viscosity, polarity) as to result in the observed percolation of conductivity is observed only for cosurfactantchanges of ko and k,. containing microemulsions falling on the part of the solubility curve 7. General Remarks Concerning the Analysis of TRFQ Data a t the left-hand side of the maximum (Figure 2). This part for MicroemuEpiomSystems. In a recent paper Johannsson et corresponds to systems where the microemulsion stability and have pointed out that the exchange of material between droplets water solubility are limited by attractive interactions between takes place within clusters of droplets and that the rate constant droplets. for the exchange should be a first-order rate constant rather than a second-order rate constant as used above. We agree with this R&try NO. DTAB, 1119-94-4; TTAB, 11 19-97-7; HTAB, 57-09-0; comment for systems above percolation threshold. For systems Ru(bpy), 15158-62-0; MV ion, 4685-14-7; RhCI, 108-90-7; H02CCHIGH,, 79-09-4; HO&(CH2)2CH,, 107-92-6; HOIC(CHz)$H,, 109below percolation threshold, however, a large fraction of droplets 52-4; H02C(CH2)6CHS, 124-07-2; H02C(CHZ)$H3, 112-05-0; HOZCmay not be under the form of clusters and the exchange of material 0 . ~ 2 ~
(CH2)dCHo, 142-62-1; H O ~ C ( C H ~ ) S C H1,1,1-14-8.
(31) Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1987, 11 I , 105. (32) Zinsli, P . J . Phys. Chem. 1979, 83, 3223. (33) Tsujii, K.; Sunamoto, J.; Fendler, J. Bull. Chem. Soc. Jpn. 1983, 56, 2889. (34) Johannsson, R.; Almgren, M.; Alsins, J. J . Phys. Chem. 1991, 95, 3819.
(35) Lianos, P.; Modes, S.J . Phys. Chem. 1987, 91, 6088. Lianos, P. J . Chem. Phys. 1988, 89, 5231. Lianos, P. In The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Us.Kluwer ; Academic Publishers: Dordrecht, The Netherlands, 1990; p 309. Modes, S.; Lianos, P.; Xenakis, A. J. Phys. Chem. 1990, 94, 3363. (36) Verbeeck, A.; De Schryver, F. C. fungmuir 1987,3,494 (see Figure 6).
