Influence of Crown Ethers and Macrocyclic Kryptands upon the

Langmuir 1997, 13, 6083-6088

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Influence of Crown Ethers and Macrocyclic Kryptands upon the Percolation Phenomena in AOT/Isooctane/H2O Microemulsions L. Garcıá-Rıó,† P. Herve´s,‡ J. R. Leis,*,† and J. C. Mejuto‡ Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago de Compostela, Santiago de Compostela, Spain, and Departamento de Quı´mica Fı´sica y Quı´mica Orga´ nica, Facultad de Ciencias, Universidad de Vigo, Vigo, Spain Received March 19, 1997. In Final Form: August 29, 1997X The influence of several additives upon the properties of the system AOT/isooctane/water has been investigated. The presence of crown ethers (18-crown-6) and kryptand complexes (kryptand 222, kryptand 221 and kryptand 211) have an important effect upon the electrical percolation phenomena. Low macrocyclic concentrations hinder the electrical percolation phenomenon. This effect seems to be due to the ability of macrocycles to complex ions and transfer them to the interface. Medium and high macrocyclic concentrations favor the electrical percolation, and this effect is related to their interfacial association to the AOT head groups. The observed behavior has been explained in terms of critical packing parameter, and it is compared with the influence of small additives as electrolytes or small organic molecules.

Introduction Of special significance among studies on the internal dynamics of microemulsions are those concerned with the intriguing phenomenon of electrical percolation.1 This phenomenon involves an abrupt increase in electrical conductivity with increase in the temperature or in the fraction of disperse phase in the microemulsion; the conductivity rises abruptly (in response to very slight temperature or composition changes) from very low values typical of an array of disperse droplets in an insulating medium to fairly high values. Zana et al. have demonstrated a relationship between electrical percolation and the rate constants of mass transfer among droplets.2 In fact, the rate of exchange process on the long time scale becomes larger with the percolation transition for microemulsions, both ionic and nonionic surfactants, but remains slower than the diffusion-controlled rate.2d Recent studies about the mechanism of transport of charge in AOT microemulsions near the percolation transition confirm all of these results.3 This process is crucial to the determination of the chemical reactivity in water in oil (w/o) microemulsions. Moderately low concentrations of some additives are known to affect the percolation threshold. Also, Matthew et al. and Moulik et al.4,5 found that those additives which †

Universidad de Santiago de Compostela. Universidad de Vigo. X Abstract published in Advance ACS Abstracts, October 15, 1997. ‡

(1) (a) Feldman, I.; Kozlovich, N.; Nir, I.; Garti, N. J. Phys. Rev. E 1995, 51, 478. (b) Eicke, H. F.; Bercovec, M.; Das-Gupta, B. J. Phys. Chem. 1989, 93, 314. (c) Cametti, C.; Codastefano, P.; Tartaglia, P.; Chen, S.; Rouch, J. J. Phys. Rev. A 1992, 45, R5358. (d) Clerc, J. P.; Giraud, G.; Laugier, J.; Luck, J. J. Adv. Phys. 1990, 39, 191. (e) Ponton, A.; Bose, T. K. J. Chem. Phys. 1991, 94, 6879. (f) Dijk, M. A.; Casteleijn, G.; Joosten, J. G. H.; Levine I. K. J. Chem. Phys. 1986, 85, 626. (g) Boned, C.; Peyrelasse, J.; Daidi, Z. Phys. Rev. E 1993, 47, 468. (h) Bug, A. L. R.; Safran, S. A.; Grest, G. S.; Webman, I. Phys. Rev. Lett. 1985, 55, 1896. (i) Garcıá-Rıó, L.; Mejuto, J. C. ; Leis, J. R.; Penã, M. E.; Iglesias, E. Langmuir 1994, 10, 1676. (2) (a) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10. (b) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau S. J. J. Phys. Chem 1990, 94, 387. (c) Lang, J.; Marcolo, G.; Zana, R.; Levisi P. L. J. Phys. Chem. 1990, 94, 3069. (d) Mays, H.; Pochert, J.; Ilgentirtz, G. Langmuir 1995, 11, 4347. (3) Feldman, I.; Kozlovich, N.; Nir, I.; Garti, N.; Archipov, V.; Idiyatullin, Z.; Zuev, I.; Fedotov, V. J. Phys. Chem. 1996, 100, 3745. (4) Mathew, C.; Patanjali, P. K.; Nabi, A.; Mayton A. Colloid Surf. Sci. 1988, 30, 253.

S0743-7463(97)00297-7 CCC: $14.00

make surfactant membranes more rigid (e.g., cholesterol4,5) hinder electrical percolation (a similar effect was found when cationic surfactants were added to AOT microemulsions5), whereas those which make the membranes more flexible (e.g., gramicidin4 or nonconjugated hydroxy bile salts5) favor it. These results and those relating electrical percolation with mass transfer among droplets2 suggest that percolation is not associated with the formation of two-continuum structures in the microemulsion but rather that discrete droplets prevail. Probably, the number of collisions is much higher in the vicinity of the percolation temperature and leads to the formation of droplet clusters through which matter flows. In AOT microemulsions droplets are stable above the percolation threshold, while in the nonionic Igepal microemulsions the droplets collapse with the percolation transition.2d This different behavior was interpreted in the literature2d in terms of activation energy for the exchange process. The high activation energy for the exchange process in the nonionic microemulsion is due to this breakdown of structures. Preliminary tests carried out in our laboratory1i revealed the effect of small organic molecules such as ureas, thioureas, and amines on the percolation temperature. The temperature at which the phenomenon takes place was found to be decreased as the primary result of the addition of organic additives. This effect is related to their interfacial association to the surfactant film, thereby favoring the formation of positive curvature and hence mass transfer among droplets. The effect of inorganic salts on percolation was also studied1i and found to be of the opposite sign to that of organic substances. Percolation was delayed in the form of a higher percolation temperature. This can be ascribed to a change in the screening status of head groups that favors the formation of negative curvature and hinders mass transfer among droplets. In this work, we studied the effect of crown ethers and kryptand complexes on the conductivity of water/AOT/ isooctane microemulsions. These macrocycles have the (5) Ray, S.; Bisal, S. R.; Moulik, S. P. J. Chem. Soc., Faraday Trans. 1993, 89, 3277. (6) (a) Pendersen, C. J. J. Am. Chem. Soc. 1967, 89, 2495. (b) Pendersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017. (c) Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Tetrahedron Lett. 1969, 2885. (d) Dietrich, B.; Lehn, J. M.; Sauvage, J. P. Tetrahedron Lett. 1969, 2889. (e) Cram, D. J. Science 1988, 240, 760. (f) Sauvage, J. P.; Lehn, J. M. J. Am. Chem. Soc. 1975, 97, 6700.

© 1997 American Chemical Society

6084 Langmuir, Vol. 13, No. 23, 1997

ability to complex ions.6 A comparative study of crown ethers and kryptands should allow us to determine the influence of the complexing agent size; also, the kryptands studied allowed us to investigate the effect of their complexing capacity on the percolation phenomenon.7 These macrocycles (crown ethers and kryptand complexes) are also of a high interest on account of their ability to sequester ions8 and transfer them to the organic pseudophase or the interface.9 These compounds can also be used as models for ion transport10 across membranes. These complexing agents cause significant changes in micelle volume and geometry11 as well as in the fraction of bound counterions (β)12 aggregation number,12,13a and critical micelle concentration.9,13a These changes have been ascribed to the ability of the macrocycles to sequester ions and insert them into the micelle structure.9,13 Also in microemulsions their ability to decrease the mean droplet size was reported.14 Our results enable a better understanding of the factors that influence mass transfer in microemulsions and reveal the significance of small amounts of midsized molecules on the behavior and properties of w/o microemulsions. In this respect, our results underline the need to carefully interpret chemical reactivity data in microemulsions because the reactants themselves or their products may introduce significant changes into the system (particularly in “fast” reactions, where mass transfer can fully or partly determine the reaction rate).

Garcıá-Rıó et al.

Figure 1. Variation of conductivity with temperature in AOT/ isooctane/water microemulsions ([AOT] ) 0.5 M, W ) 22) in the presence of kryptand 222 (concentration referred to water droplet volume): (b) [222] ) 0 M; (O) [222] ) 1.22 × 10-4 M; (0) [222] ) 2.99 × 10-2 M; (9) [222] ) 4.48 × 10-2 M.

Experimental Section AOT (Aldrich, 99%) was dried in a vacuum desiccator and used without further purification. All other reagents were used as supplied by Aldrich. Electrical conductivity (κ) was measured with a Radiometer CDM3 conductometer with a 1.0 cm-1 cell constant thermostated to within (0.1 °C. The percolation temperature was determined from the variation of the electrical conductivity of the microemulsions with temperature in 0.2 °C steps in the proximity of the percolation threshold (typically (4 °C). From those conductivity data (κ/T) the percolation threshold was determined following the method illustrated in Figures 1 and 2, which show plots of (1/κ)(dκ/dT) against T with a peak at the percolation temperature.15 The typical accuracy of percolation temperature measured was (0.2 °C. Maximum water solubilization capacity of the microemulsions (with or without an additive) was studied by adding an appropriate volume of water (or water + additive) to samples containing known amounts of AOT and isooctane under continuous stirring until permanent turbidity was observed. Additive concentration is always referred to the aqueous volume of the microemulsion. The samples thus prepared were stored at 25 °C for several weeks. The phases resolved were studied by using (7) (a) Lindoy, L. F. The Chemistry of macrociclic lingand complexes, Cambridge University Press: Cadmbridge, 1989. (b) Lehn, J. M. Supramolecular Chemistry; VCH: Weinhein, 1995. (8) Dietrich, B.; Lehn, J. M.; Sauvage, J. P.; Blanzat, J. Tetrahedron 1973, 29, 1674. (9) Quintela, P. A.; Reno, R. C. S.; Kaifer, A. E. J. Phys. Chem. 1987, 94, 3528. (10) Lamb, J. D.; Christensen, J. J.; Oscarson, J. L.; Nielsen, B. L.; Asay, B. W.; Izatt, R. M. J. Am. Chem. Soc. 1980, 102, 3399. (11) (a) Ozeki, S.; Kojima, A.; Harada, S.; Inokuma, S.; Takahashi, H.; Kuwamura, T.; Uchiyama, H.; Abe, M.; Ogino, K. J. Phys. Chem. 1990, 94, 8207. (b) Ozeki, S.; Harada, S.; Kojima, A.; Abe, M.; Ogino, K.; Takahashi, H.; Inokuma, S.; Kuwamura, T. J. Phys. Chem. 1990, 94, 8213. (c) Ozeki, S.; Seki, H. J. Phys. Chem. 1992, 96, 10074. (12) Evans, D. F.; Evans, J. B.; Sen, R.; Warr, G. G. J. Phys. Chem. 1988, 92, 784. (13) (a) Ginley, M.; Heriksson, U.; Li, P. J. Phys. Chem. 1990, 94, 4644. (b) Evans, D. F.; Sen, R.; Warr, G. G. J. Phys. Chem. 1986, 90, 5500. (c) Payne, K. A.; Magid, L. J.; Evans, D. F. Prog. Colloid Polym. Sci. 1987, 73, 10. (14) Magid, L. J.; Weber, R.; Leser, M. E.; Farago, B. Prog. Colloid Polym. Sci. 1990, 81, 64. (15) Kim, M. W.; Huang, J. S. Phys. Rev. A 1986, 34, 719. (16) Hou, M.; Shah, D. O. Langmuir 1987, 3, 1086.

Figure 2. Determination of the percolation temperature for AOT/isooctane/water microemulsions ([AOT] ) 0.5 M, W ) 22) in the presence of C6 at a 1.0 × 10-3 M concentration referred to water droplet volume. the dyes Sudan IV (soluble in the organic phase) and Methylene Blue (water-soluble) according to reported procedures.16 Liquid crystal phases were observed by using cross-polarizers.

Results We studied the effect of the presence of crown ethers and kryptand complexes on the percolation temperature of AOT/isooctane/H2O microemulsions. The kryptand complexes used as additives were 4,7,13,16,21,24-hexaoxa1,10-diazabicyclo[8.8.8]hexacosane (kryptand 222), 4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane (kryptand 221), and 4,7,13,18-tetraoxa-1,10-diazabicyclo[8.8.5]eicosane (kryptand 211). The crown ether studied was 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6, C6). The kryptands and crown ether are depicted in Chart 1 and the results obtained with them are shown in Tables 1-4 and Figure 3. As can be seen from Tables 1-4, the study of the influence of different concentrations of crown ether (C6) and kryptand complexes (222, 221, and 211) upon the percolation temperature revealed two opposing effects. At low additive concentrations, the percolation threshold was raised (by more than 2 °C in some cases, Figure 3). Nevertheless, further increasing of the additive concen-

Percolation Phenomena in Microemulsions

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Chart 1

Table 4. Variation of the Percolation Temperature of AOT/Isooctane/Water Microemulsions ([AOT] ) 0.5 M, W ) 22) in the Presence of Variable Concentrations of Kryptand 221 (Referred to Water Droplet Volume), δT ) Tp(with additive) - Tp(without additive) [211]/M

T/°C

δT/°C

0 1.00 × 10-5 7.00 × 10-5 1.40 × 10-4 7.00 × 10-4 1.40 × 10-3 6.98 × 10-3 1.39 × 10-2

36.5 36.7 37.4 36.8 36.0 35.5 34.5 33.1

0.0 0.2 0.9 0.2 -0.6 -1.1 -2.1 -3.5

Table 1. Variation of the Percolation Temperature of AOT/Isooctane/Water Microemulsions ([AOT] ) 0.5 M, W ) 22) in the Presence of Variable Concentrations of 18-Crown-6 (C6) (Referred to Water Droplet Volume), δT ) Tp(with additive) - Tp(without additive) [C6]/M

Tp/°C

δT/°C

0 1.00 × 10-4 5.00 × 10-4 1.00 × 10-3 4.30 × 10-3 5.00 × 10-3 8.50 × 10-3 1.42 × 10-2 5.70 × 10-2 0.114

36.5 36.6 36.8 38.7 36.1 36.4 36.4 35.3 29.1 21.1

0.0 0.1 0.3 2.2 -0.4 -0.1 -0.1 -1.2 -7.4 -15.4

Figure 3. Effect of the concentration of kryptand 222 on the percolation temperature of an AOT/isooctane/water microemulsion ([AOT] ) 0.5 M, W ) 22).


Tp/°C

δT/°C

0 4.00 × 10-5 1.20 × 10-4 4.90 × 10-4 9.80 × 10-4 2.99 × 10-2 4.48 × 10-2

36.5 37.4 37.9 37.0 36.5 33.6 31.6

0.0 0.9 1.4 0.4 0.0 -3.0 -5.0


Tp /°C

δT/°C

0.00000 4.00 × 10-5 8.00 × 10-5 4.20 × 10-4 8.40 × 10-4 4.21 × 10-3 8.41 × 10-3

36.6 36.7 36.8 36.4 35.7 35.4 32.7

0.0 0.1 0.2 -0.2 -0.9 -1.2 -3.8

tration lowered the threshold (by up to 15 °C for C6 at a concentration of 0.114 M relative to water droplets). The behavior at high additive concentrations was similar to that previously observed with other organic additives at moderate and high concentrations,1i which has been ascribed to the association of the organic molecules to the AOT film. On the other hand, the behavior at low additive concentrations is like that

Figure 4. Effect of variable concentrations of 18-crown-6 (C6), NaOH, and urea (referred to water droplet volume) on the percolation threshold for AOT/isooctane/water microemulsions ([AOT] ) 0.5 M, W ) 22): (b) C6; (O) NaOH; (0) Urea.

reported for inorganic salts, which delay percolation through a change in the screening of charged head groups in AOT. Figure 4 compares the variations of the percolation temperature due to addition of NaOH and urea with those caused by variable amounts of C6. In order to confirm the occurrence of these two types of behavior, we determined the maximum water solubilization capacity in the presence of these additives and compared it with the maximum water solubilization capacity of the microemulsions in the presence of electrolytes and organic substances.

6086 Langmuir, Vol. 13, No. 23, 1997


Table 5. Solubilizing Capacity of Microemulsions Containing Additives: Breakage W and Types of Phases Resolveda additive

[additive]/M

Wbreakage

phases

without additive tetramethylthiourea methylthiourea 1,3-dimethylthiourea thiourea 1,3-dimethylurea Na2SO4 NH4Cl NaBr NaClO4 C6 kryptand 222 kryptand 221 kryptand 211

0 0.2 0.2 0.2 0.2 0.2 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08

55.5 44.4 47.2 44.4 47.2 41.6 30.5 36.1 38.9 41.6 48.5 47.2 47.7 47.1

LC LC LC LC LC LC mm + (H2O)exc. mm + (H2O)exc. mm + (H2O)exc. mm + (H2O)exc. LC LC LC LC

a

Additive concentrations are referred to water droplet volume.

In the absence of additives the maximum water solubilization capacity was found at a mole ratio ([water]/ [AOT] ) W) of W ) 56, consistent with reported results (ca. W ) 60 at 20 °C).17 However, the presence in the aqueous phase of a moderate concentration of organic additives (0.2 M relative to water volume) decreased the solubilization capacity and led to phase separation at a W value of W ) 47 with thiourea (TU) and methylthiourea (MTU), W ) 44 with 1,3-dimethylthiourea (DMTU) and tetramethylthiourea (TTMTU), and W ) 42 with 1,3-dimethylurea (DMU). Breakage of the microemulsion led to the formation of a liquid crystal (LC) phase in every case (see Table 5). Consistent with previous results,18 the presence of electrolytes (0.08 M relative to water volume) significantly reduced the maximum water solubilization capacity of the AOT/isooctane/water system: from W ) 56 in the absence of additive to W ) 42 with NaClO4, W ) 36 with NH4Cl, W ) 34 with NaBr, and W ) 30.5 with Na2SO4. In all cases, breakage of the microemulsion and its resolution into different phases led to a heavier aqueous phase containing excess salt, in equilibrium with the w/o microemulsion. Unlikely with organic substances (e.g., ureas and thioureas), a liquid crystal phase was formed (Table 5). The presence of moderate concentrations (0.08 M relative to water volume) of the crown ether and/or kryptand complexes further decreased the maximum water solubilization capacity of these systems: from W ) 56 in the absence of additive to W ) 48.5 with C6 and to W ) 47.2, W ) 47.7, and W ) 47.1 with the three kryptands studied (222, 221, and 211, respectively). The effect was much more pronounced than that observed with the organic substances, which required higher concentrations for a similar effect. By contrast, the effect was much weaker than that observed in the presence of electrolytes, which led to a much greater decrease in maximum water solubilization capacity at the same concentrations as the crown ether and kryptands (Table 5). Beyond the maximum water solubilization capacity, the transition to the liquid crystal phase was observed (similarly as with organic additives). This suggests that the organic nature of the additives is the prevailing factor in the two opposing effects observed (see Figure 3 and Tables 1-4). (17) Huruguen, J. P.; Authier, M.; Greffe, J. L.; Pileńi, M. P. Langmuir 1991, 7, 243. (18) (a) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977, 99, 4730. (b) Maitra, A. J. Phys. Chem. 1984, 88, 5122. (19) Cabos, C.; Delord, P. J. Phys. Lett. 1980, 41, L-455. (20) Lang, J.; Jada, A.; Milliaris, A. J. Phys. Chem. 1988, 92, 1946.

Discussion and Conclusions As noted earlier, the effect of crown ethers or kryptand complexes on the percolation temperature depended on their concentration and is consistent with those of other additives, previously studied.1i At moderate concentrations, they behave similarly to electrolytes; i.e., they increase the percolation temperature. In fact, this effect of electrolytes is one of the most widely documented19-21 and it is consistent with our results. Salts are known to diminish the effective polar area of surfactants by screening electrostatic repulsions.22 This increases the surfactant’s curvature parameter, called the “critical packing parameter” (cpp).23 The decrease in the surfactant polar head area induces a more markedly wedge-shaped structure in AOT and increases the natural negative curvature (i.e., the tendency to produce inverted structures) of the surfactant. This is consistent with the fact that salts diminish the maximum water solubilizing capacity of microemulsions since the solubilization processsthrough an increased droplet sizesreduces the negative curvature of the interface, which counters the natural tendency of the surfactant. A tentative explanation based on the hypothesis of Rouviere et al.21 could be that salts may increase droplet sphericity. Although we assumed our droplets to be initially spherical, they may in fact be slightly distorted with local zones of decreased negative curvature. The addition of salts may increase droplet sphericity in such a way that the same volume of disperse phase could be accommodated in a more spherical structure, which would make a lower aggregation number compatible with a decreased area per polar head. Our experimental results suggest that electrical conductivity of microemulsions is due to the passage of cations through transient channels formed between droplets that have collided. As noted by Robinson et al.,24 the opening of surfactant films (to form transient channels) involves large activation energies related to the creation of local regions of positive curvature. The less naturally prone the surfactant is to adopt positive curvature, the more difficult will be the mass transfer process and hence electrical conduction. In this respect, the efficiency with which cations hinder channel opening increases for the Hofmeister series and similar series for the affinity of cations for “normal” anionic micelles in water.25 Our results suggest a similar behavior for microemulsions: the association of cations to the polar heads of AOT appears to involve the hydrated cation (at least at W ) 22). This retention of the cation’s hydration layer appears to be typical of association due to weak electrostatic fields. On the other hand, the presence of salts in the microemulsion decreases attractive forces between droplets, which is seemingly related to droplet interpenetration. In fact, theoretical calculations26 suggest that the most important contribution to attractive interactions is that of the overlapping zone. A phenomenological parameter is often used to characterize the penetrable length of the interfacial region during droplet interpenetration; (21) Rouviere, J.; Couret, J. M.; Lindheimer, A.; Lindheimer, M.; Brun, B. J. Chim. Phys.-Chim. Biol. 1979, 76, 297. (22) Finer, E. G.; Franks, F.; Tait, M. J. J. Am. Chem. Soc. 1972, 94, 4424. (23) Evans, D. F.; Michell, D. J.; Ninham, D. W. J. Phys. Chem. 1986, 90, 2817. (24) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985. (25) (a) Bravo, C.; Herve´s, P.; Leis, J. R.; Penã, M. E. J. Phys. Chem. 1990, 94, 8816. (b) He, Z.; O’Connor, P. J.; Romsted, L. S.; Zanette, D. J. J. Phys. Chem. 1989, 93, 4219. (26) (a) Lemaire, B.; Bothorel, P.; Roux, D. J. Phys. Chem. 1983, 87, 1023. (b) Brunetti, S.; Roux, D.; Bellocq, A. M.; Fourche, G.; Bothorel, P. J. Phys. Chem. 1983, 87, 1028. (27) Hou, M.; Shau, D. O. Langmuir 1987, 3, 1089.

Percolation Phenomena in Microemulsions

as noted by Hou and Shah,27 such a parameter should decrease with increasing rigidity of the interface, which increases with increasing salinity. In summary, salts increase the natural negative curvature of the surfactant by increasing the rigidity of its film and hindering its distortion, thereby decreasing interactions among droplets. Both crown ethers and kryptand complexes can capture Na+ ions6f,14 present in aqueous droplets as counterions for AOT charged heads and dissolve them in the AOT film. It seems then obvious that the presence of these substances should alter the screening status of AOT head groups and decrease the effective area a. As noted earlier, the decrease in the area leads to a more markedly trapezoidal AOT shape and increases the natural negative curvature of the surfactant (i.e., its tendency to give inverted structures), thus hindering the channel opening required for mass transfer among droplets and hence electrical percolation. In the results obtained at moderate and high concentrations of the macrocycles studied, the percolation temperature is lowered to such a degree that the highconductivity region can be reached at unusually low temperatures. This effect also results in electrical percolation at room temperature taking place in anomalously low volume fractions of the disperse phase. In fact the presence of moderate concentrations of these complexing agents results in electrical percolation occurring in a volume fraction of the disperse phase (φ) well below the “theoretical” fraction predicted by geometric percolation models such as the effective medium theory,28 which predicts percolation at φ > 0.33. Even if droplet interactions are considered, the effective medium theory modified for dipole-dipole interactions29 predicts percolation at φ > 0.15. Similarly to these small organic additives, kryptand complexes and crown ethers are very sparsely soluble in isooctane, so they must associate to the surfactant film. Then it is possible to follow a similar reasoning to that used to explain the behavior of other organic substances.1i If additives associate to the AOT film, they increase the effective area of the heads and decrease the surfactant’s curvature parameter, v/al. This decrease would favor droplet fusion as a result of the decreased natural tendency of AOT to form inverted structures. In addition, the association of additives to the surfactant film could increase disorder in the interfacial zone, thereby decreasing the film rigidity and increasing its deformability. In this respect, it is interesting to ascertain whether the role of these additives in the interfacial region is to replace water molecules. The studies by Kevan et al.30 on the effect of various organic molecules on frozen vesicles suggest that the addition of moderate concentrations of such molecules “opens” the interface and facilitates water penetration into the vesicle structure. However, at high concentrations the additive replaces water molecules at the interface, thereby playing a direct role in the solvation of head groups. Recent experiments that support our reasoning are the light scattering measurements by Politi et al.31 They observed an increase in the hydrodynamic radius of water droplets in the presence of high concentrations of urea which means that interfacial area is not changing with urea. This indicates that the presence of urea in the AOT film produces only disorganization of the interface, (28) Granqvist, C. G.; Hunderi, O. Phys. Rev. B 1978, 18, 1554. (29) Fang, J.; Venable, R. L. J. Colloid. Interface. Sci. 1987, 116, 269. (30) Kang, Y. S.; McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1992, 96, 10055. (31) Costa-Amaral, C. L.; Itri, R.; Politi, M. J. Langmuir 1996, 12, 4638.

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rendering it more flexible and fluid and as a result leading to an increase in the interdroplets attractive interaction. The larger values of hydrodynamic radius were related by Politi et al.31 to interpenetration of the surfactant tails, due to attractive forces, which leads to a greater effective radius for light scattering observations. X-ray scattering results31 indicate that the structure of the discrete spherical droplets is kept after the percolation phenomenon. An interplay between attractive and repulsive interactions decreasing the attractive component at higher concentration explains the observed plateau in the conductivity measurements. This is also reflected in light scattering where the scattered intensity decreases as the volume fraction of the disperse phase increases. This effect was explained by Politi31 in terms of the fact that whereas the attractive term due to urea remains fixed, the hard sphere excluded volume increases steadily as the volume fraction of the disperse phase (φ) increases. The strong interactions among droplets observed in the presence of these additives could be consistent with an increase in the parameter used to characterize droplet interpenetration, which would increase with increasing disorder at the interface. This increase is also the likely origin of the decreased maximum water solubilizing capacity of the microemulsion, which is seemingly confirmed by the fact that phase separation takes place in order to produce a liquid crystal phase rather than to remove excess aqueous solution. A similar reasoning can be used to explain the effect of formamide and its methyl derivatives on the system, as well as that of ethylene glycol, which, like other alcohols,32 can associate to surfactant heads. Similar effects were found when long chain alcohols were used as cosurfactants. In fact, not many studies have been made using AOT and cosurfactant. Shah et al.,33 using dynamic light scattering to study droplet size and interactions, concluded that the radius increases linearly with water content and that the cosurfactants increased or decreased interactions according to their structure. They made the distinction between short and long alcohols: the former increase interactions, while the latter decrease them. A decrease in interactions was also noticed for Arlacel (a long chain nonionic surfactant).33 About the long chain alcohols,32B when decanol is added to the system the resulting microemulsion has a higher percolation temperature, and as the concentration of the alcohol is increased, the percolation temperature is again increased. If the cosurfactant added is a poly(oxyethylene) alkyl ether (C10E4), the opposite behavior is observed;32B there is a decrease in the percolation temperature that decreases further with an increase in concentration. In view of the percolating temperatures, long chain alcohols make the interface more rigid and hence make clustering, aggregation, and consequently percolation more difficult, while the poly(oxyethylene) alkyl ethers have the opposite effect. Other experiments that support our conclusions are the measurements of percolation temperature and viscosity of AOT/heptane/water microemulsions in the presence of nonconjugated hydroxy bile salts and cationic surfactants.5 In this work Moulik et al. prove that percolation phenomena imply association of microdroplets. We can thus conclude that the net effect of the kryptands and crown ether studied is a combination of the two above(32) (a) Giammona, G.; Goffredi, F.; Turco-Liveri, V.; Vassallo, G. J. Colloid Interface Sci. 1992, 154, 411. (b) Naza´rio, L. M. M.; Hatton, T. A.; Crespo, J. P. S. G. Langmuir 1996, 12, 6326. (33) Hou, M. J.; Kim, M.; Shah, D. O. Langmuir 1988, 123, 398. (34) (a) Christensen, J. J.; Eatough, D. J.; Izatt, R. M. Chem. Rev. 1974, 74, 351. (b) Lehn, J. M.; Sauvage, J. P. J. Am.Chem. Soc. 1975, 97, 6700. (c) Keuffmann, E.; Lehn, J. M.; Sauvage, J. P. Helv. Chim. Acta 1976, 59, 1099.

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described effects. In fact, these molecules are known to be able to “capture” and “sequester” ions34 and at the same time they solubilize preferentially at microemulsion interfaces, as shown by studies on “normal” micelles (vide supra). Thus, the increased percolation temperature observed at low concentrations of the crown ether and kryptands studied could be due to counterions sequestered by these molecules and transferred to the interface, thereby leading to an altered critical packing parameter. Because of the low concentrations of kryptands and crown ether used, we can assume that surfactant layer volume was scarcely altered and then negative curvature would prevail over positive curvature and percolation would be hindered as a result. On the other hand, the effect observed on addition of high concentrations of these substances to the microemulsions is that expected from


an organic substance that associates to the interface. The resulting perturbation of the surfactant film favored the formation of positive curvature; this in turn favored mass exchange among droplets and hence electrical percolation. We can thus conclude that the anomalous behavior observed is a combination of the phenomena that result from the addition of salts (altered screening of head groups) and organic molecules (altered surfactant film structure). Acknowledgment. Financial support from Xunta de Galicia (Project XUGA 20906B93) and from the Direccioń General de Investigacioń Cientı´fica y Tećnica of Spain (Project PB93-0524) is gratefully acknowledged. LA970297N

Influence of Crown Ethers and Macrocyclic Kryptands upon the

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