Isooctane

Langmuir 2003, 19, 5975-5983

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Articles Influence of Crown Ethers on the Electric Percolation of AOT/Isooctane/Water (w/o) Microemulsions J. Dasilva-Carvalhal,† L. Garcıá-Rıó,*,‡ D. Go´mez-Dıáz,§ J. C. Mejuto,† and P. Rodrı´guez-Dafonte† Departamento de Quı´mica Fı´sica and Departamento de Ingenierıá Quı´mica, Facultad de Quı´mica, Universidad de Santiago, 15782 Santiago, Spain, and Departamento de Quı´mica Fı´sica, Facultad de Ciencias de Ourense, Universidad de Vigo, 32004 Ourense, Spain Received November 15, 2002. In Final Form: May 11, 2003 A study was carried out on the influence of different crown ethers on the electric percolation of AOT/ isooctane/water microemulsions. The crown ethers used were chosen on the basis of two fundamental criteria: (a) the different sizes of the molecules, where variation is found in the external size as well as the size of the cavity, and (b) the different solubilities of the ethers in water. In all cases we observed a dual behavior of the crown ethers with regard to the percolative phenomenon. At low additive concentrations we observed how the presence of the crown ethers caused an increase in the percolation temperature of the microemulsions, whereas at high additive concentrations there was a reduction in the percolation temperature causing the percolation threshold of the system to move forward. This dual behavior allowed us to define the compensation concentration, which corresponds with the crown ether concentration at which there is no effect on the percolative phenomenon. We observed a correlation between the effect exerted by the crown ethers and the size of the cavity. This shows the importance of the capacity to complexate Na+ and solubilize it in the interface and the continuous medium on the electric percolation. We also observed a correlation between the effect of the crown ethers on the percolation temperature and their external size. This shows the importance of their inclusion in the interface on the percolative phenomenon. Such an inclusion modifies the properties of the AOT film, facilitating the exchange of matter between droplets. We also obtained a satisfactory multiparametric correlation between the logarithm of the compensation concentration, the logarithm of the distribution parameter of the crown ether between water and 1-octanol, and the number of oxygen atoms in the crown ether. This correlation shows that the effect of the crown ethers on the electric percolation is due to its size and capacity to sequester ions, as well as to its solubility in the interface of the microemulsion.

Introduction 1

Microemulsions are highly dynamic structures. Their components organize themselves in time and space by means of different interactions or collisions, giving rise to processes of coalescence and redispersion. Numerous studies have been carried out with the purpose of determining the structure, dimensions, and internal dynamics of these systems. Among these studies we can cite those involving ultrasedimentation,2 different dispersion techniques,3 fluorescence resolved in time,4 and nuclear magnetic resonance.5 The electric conductivity measurements constitute a very useful technique for obtaining information about micellar interactions.6 Under normal conditions, a microemulsion presents a very low specific conductivity (ca. 10-9 to 10-7 Ω-1‚cm-1). This conductivity is significantly †

Universidad de Vigo. Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago. § Departamento de Ingenierıá Quı´mica, Facultad de Quı´mica, Universidad de Santiago. ‡

(1) Eastoe, J.; Robinson, B. H.; Steytler, D. C.; Leeson, D. T. Adv. Colloid Sci. 1991, 36, 1. (2) Eicke, H. F.; Rehak, J. Helv. Chim. Acta 1976, 59, 2883. (3) Zana, R., Ed. Surfactant Solutions. New Methods of Investigations; Marcel Dekker: New York, 1987. (4) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3543. (5) Chachaty, C. Prog. NMR Spectrom. 1987, 19, 3543.

greater than it would be if we consider the alkane, which constitutes the continuous medium and is the main component of a microemulsion of water in oil (w/o microemulsions) (∼10-14 Ω-1‚cm-1). This increase in the electrical conductivity of the microemulsions by comparison with that of the pure continuous medium is due to the fact that microemulsions are able to transport charges. We should not forget the high ionic strength of water microdroplets in which the stabilizing headgroup counterions of the surfactant are concentrated. When we reach a certain volume of the disperse phase, the conductivity is abruptly increased to give values of up to 4 orders of magnitude, which are greater than the typical conductivity of the w/o microemulsions. This increase remains invariable after reaching a maximum value, which is much higher than that for the microemulsions present before this transition occurs. Similar behavior can be observed when, for a fixed composition of microemulsion, the temperature increases. This phenomenon is known as electric percolation, and the moment at which an abrupt transition occurs from a poor electric conductor (6) (a) Maitra, A.; Mathew, C.; Varshney, M. J. Phys. Chem. 1990, 94, 5290. (b) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Colloids Surf. 1990, 50, 295. (c) Hamilton, R. T.; Billman, J. F.; Kaler, E. W. Langmuir 1990, 6, 1696. (d) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Phys. Chem. 1995, 99, 8222. (e) Ray, S.; Bisal, S. R.; Moulik, S. P. J. Chem. Soc., Faraday Tans. 1993, 89, 3277. (f) Hurugen, J. P.; Authier, M.; Greffe, J. L.; Pileni, M. P. Langmuir 1991, 7, 243.

10.1021/la026857m CCC: $25.00 © 2003 American Chemical Society Published on Web 06/26/2003

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Dasilva-Carvalhal et al.

Scheme 1

system (∼10-7 Ω-1‚cm-1) to a system with a fluid electric charge circulation (∼10-3 Ω-1‚cm-1) is termed the percolation threshold. When this phenomenon is induced by varying the temperature at which the system is found, this temperature is known as the percolation temperature. The mechanism proposed to explain the electric percolation phenomenon is based on the formation of channels exchanging matter between the disperse water droplets in the continuous phase, as shown in Scheme 1. It is then necessary to have an effective collision between two water droplets of the microemulsion, causing the droplets to fuse together. Subsequently, an exchange of matter between the water droplets would take place (allowing the charge conduction), which would produce their separation by means of a process of fission. Over the past decade a number of studies have been carried out to analyze the effect of crown ethers on the internal dynamics of the microemulsions.7 Moulik et al. (see above) showed that, at high concentrations of crown ethers, the ethers are inserted into the surfactant film, favoring the formation of channels between droplets. This process facilitates the exchange of electric charge, acting as a bridge between the surfactant films that stabilize two droplets and allowing an easier fusion of both monolayers of the surfactant.8 Our research group studied the effect of cryptands on the electric percolation, as well as that of the crown ether 18-crown-6,9 finding a behavior analogous to that described in the literature for high concentrations of macrocycle. At low additive concentrations the opposite behavior was observed; the presence of the cryptand had a negative effect on the process of electric percolation. In this study we present an exhaustive analysis of the effect of different crown ethers on the electric percolation in the AOT/ isooctane/water system, with the purpose of shedding light on their behavior depending on their concentration in the microemulsion. Different crown ethers were selected on the basis of two fundamental criteria: (a) the different sizes of the molecules and hence the different sizes of the cavities of the ethers and (b) the different solubilities of the ethers in water. Three crown ethers were studied: 12-crown-4 (C4), 15-crown-5 (C5), and 18-crown-6 (C6). Three benzo crown ethers were studied: benzo-12-crown-4 (BC4), benzo-15-crown-5 (BC5), and benzo-18-crown-6 (BC6). Five dibenzo crown ethers were studied: dibenzo15-crown-5 (DBC5), dibenzo-18-crown-6 (DBC6), dibenzo21-crown-7 (DBC7), dibenzo-24-crown-8 (DBC8), and dibenzo-30-crown-10 (DBC10). Their structures are shown in Chart 1. (7) (a) Schuebel, D. Colloid Polym. Sci. 1998, 276, 743. (b) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Indian J. Chem. A 1993, 32A, 485. (c) Paul, B. K.; Moulik, S. P. Indian J. Biochem. Biophys. 1991, 28, 174. (d) Paul, B. K.; Das, M. L.; Mukherjee, D. C.; Moulik, S. P. Indian J. Chem. A 1991, 30A, 328. (e) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Colloids Surf. 1990, 50, 29. (8) (a) Moulik, S. P.; De, G. C.; Bhowmik, B. B.; Panda, A. K. J. Phys. Chem. B 1999, 103, 7122. (b) Hait, S. K.; Sanyal, A.; Moulik, S. P. J. Phys. Chem. B 2002, 106, 12642. (c) Hait, S. K.; Moulik, S. P.; Rodgers, M. P.; Burke, S. E.; Palepu, R. J. Phys. Chem. B 2001, 105, 7145. (9) (a) Garcia-Rio, L.; Herves, P.; Leis, J. R.; Mejuto, J. C. Langmuir 1997, 13, 6083. (b) Alvarez, E.; Garcia-Rio, L.; Gomez-Diaz, D.; Mejuto, J. C.; Navaza, J. M. J. Chem. Eng. Data 2001, 46, 526. (c) Alvarez, E.; Garcia-Rio, L.; Gomez-Diaz, D.; Mejuto, J. C.; Navaza, J. M.; PerezJuste, J. J. Chem. Eng. Data 2000, 45, 428.

Chart 1

Experimental Section The AOT was supplied by Aldrich (purity 98%). Given its highly hygroscopic nature, it was vacuum-dried and used without any further purification. The crown ethers were supplied by Fluka, being of the maximum commercially available purity (within the range 97-99%). The microemulsions were prepared by weight. The composition of the microemulsions remained constant and equal to [AOT] ) 0.5 mol‚dm-3 (referred to the total volume of the microemulsion) and W ) [H2O]/[AOT] ) 22.2. The conductivity was measured using a Crison GPL 32 conductivimeter with a conductivity cell constant equal to 0.0109 cm-1. The conductivimeter was calibrated using standard solutions of KCl supplied by the manufacturer with the following specific concentration and conductivity: [KCl] ) 0.0100 mol‚dm-3, κ ) 1.413 × 10-3 Ω-1‚cm-1 (κ ) 1413 µS‚cm-1) at 25 °C and [KCl] ) 0.100 mol‚dm-3, κ ) 1.288 × 10-2 Ω-1‚cm-1 (κ ) 12.88 mS‚cm-1) at 25 °C. During the measurements the temperature was controlled using a Teche TE-8D RB-5 thermostat-cryostat, which kept the temperature constant with a precision of (0.1 °C. The microemulsions were introduced in a recipient of 50 mL, conveniently encased for its thermostatization. It was stirred using a Teflon bar with a magnetic nucleus and also was hermetically sealed with a lid with two openings through which an electrode was inserted to determine the specific conductivity and also a thermometer to determine the temperature at which the sample was found, this temperature being determined at the same time as the conductivity. The percolation temperature (tp) was obtained from the conductivity/temperature data. As described in the literature,10a tp was considered as the maximum of the plots 1/κ(δκ/δt) versus t, as shown in Figure 1. An alternative analysis of conductivity/temperature data can be carried out in terms of the Sigmoidal-Boltzmann equation (SBE) porposed by Moulik et al.10b (eq 1):

[ (

log κ ) log κf 1 +

)

]

log κi - log κf {1 + e(t-tp)/∆t}-1 log κf

(1)

where κ and t represent conductivity and temperature, respectively; ∆t is the constant interval of t; and the i, f, and p subscripts stand for initial, final, and percolation stages, respectively. (10) (a) Kim, M. W.; Huang, J. S. Phys. Rev. A 1986, 34, 719. (b) Hait, S. K.; Moulik, S. P.; Palepu, R. Langmuir 2002, 18, 2471.

AOT/Isooctane/Water (w/o) Microemulsions

Figure 1. Determination of the percolation temperature in accordance with the variation of the electric conductivity with the temperature for a microemulsion of AOT/isooctane/water (W ) 22.2, [AOT] ) 0.5 M) in the presence of C5 ([C5] ) 0.001 M, referred to the volume of water).

Langmuir, Vol. 19, No. 15, 2003 5977

Figure 3. Average value of n (scaling equation) for different CEs: (O) crown ethers; (b) benzo crown ethers; (4) dibenzo crown ethers. The dashed line represents the global average value.

the surfactant film, as shown in studies carried out on “normal” micelles.14 1. Performance of the Scaling Equation. A typical treatment of the experimental data κ/T was carried out by means of the following scaling equation:8

Figure 2. Influence of the concentration of crown ether on the percolation temperature for a microemulsion of AOT/isooctane/ water (W ) 22.2, [AOT] ) 0.5 M) in the presence of C4 (b) and BC4 (O) (concentrations of crown ether referred to the volume of water). The dashed line corresponds with the value observed for the percolation temperature in the absence of EC. The geometry of the crown ether, together with the estimation of the radius of its cavity, was optimized by means of MM2 calculations11 using a commercial program (CS ChemBats3D Pro 4.0 supplied by Cambridge Soft Corporation) based on QCPE 395.

Results and Discussion We determined the percolation temperature of AOT/ isooctane/water microemulsions ([AOT] ) 0.5 mol‚dm-3 and W ) 22.2) in the presence of different crown ether concentrations. The obtained results are shown in Table 1. In all cases, we observed an increase in the percolation temperature of the system as the crown ether concentration increases, until a maximum value is reached, from which the percolation temperature decreases. Thus, at high crown ether concentrations the percolation temperature is significantly lower than that observed for the microemulsions without additive. For example, Figure 2 shows the behavior observed in the presence of 12-crown-4 and benzo-12-crown-4. As is the case for other macrocycles,9 the observed biphasic behavior in the presence of these additives can be rationalized by taking into account the double nature of these substrates: (i) their capacity to sequester ions,12 transport them through the interface,13 and solubilize them in apolar phases and (ii) their ability to associate at (11) Allinger, N. L. Molecular Mechanics; American Chemical Society: Washinton, DC, 1982.

κ ) P(T - Tp)n

(2)

ln(κ) ) ln(P) + n ln(T - Tp)

(3)

In all cases, we observed a satisfactory fit of the scaling equation to the experimental data. The obtained values of n and ln P are shown in Table 1. We did not observe clear trends between the values obtained for each additive. The obtained n values are not dependent on the additive concentration, giving a mean value of n ) (1.3 ( 0.2). This value is compatible with those reported in the literature for different microemulsions in the presence and absence of additives. Values of n ) (0.34-1.56) were reported for microemulsions of AOT/n-heptane/water,8a,b n ) (0.811.39) for microemulsions of AOT/n-octane/water,8b n ) (0.53-1.66) for microemulsions of AOT/n-decane/water,8b,c and n ) (0.61-2.04) for AOT/isooctane/water microemulsions.8c Moulik et al.8c found a value of n ) (1.49 ( 0.06) for microemulsions with similar composition to those of our systems (in the absence of crown ether): [AOT] ) 0.415 M and W ) 22.5. This value is compatible with our value of n ) (1.3 ( 0.2). The average values for each crown ether are shown in Table 1 (Figure 3). The obtained ln(P) values presented a high dispersion, and we did not observe correlations with the substrate concentration. It increases with the presence of aromatic rings in the crown ether (see Table 1 and Figure 4). 2. Influence of a Low Macrocycle Concentration. Addition of low crown ether concentrations causes an increase in the percolation temperature of AOT-based microemulsions. This result is consistent with those previously obtained in our laboratory in the study of the influence of other additives15 on the percolation temperature of AOT/isooctane/water microemulsions. The increase of the percolation temperature observed at low (12) (a) Chirstensen, 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. (13) Lamb, J. D.; Chirstensen, J. J.; Oscarson, J. L.; Nielsen, B. L.; Asay, B. W.; Izatt, R. M. J. Am. Chem. Soc. 1980, 102, 3399. (14) (a) Holland, P. M.; Rubingh, D. N. J. Phys. Chem. 1983, 87, 1984. (b) Ginley, M.; Henriksson, U.; Li, P. J. Phys. Chem. 1990, 94, 4644. (c) Diedirch, B.; Lehn, J. M.; Sauvage, L. P.; Blanzat, J. Tetrahedron 1973, 29, 1647.

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Table 1. Adjustment Parameters for the Scale Equation (Eq 3) and Percolation Temperature Values for AOT/Isooctane/ Water Microemulsions in the Presence of Different Concentrations of Crown Ethers (DF, Differential Method; SBE, Sigmoidal Boltzmann Equation)


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Table 1. (Continued)

concentrations is similar to the behavior found by the addition of electrolytes, one of the most widely documented percolative behaviors.16

Influence of Electrolytes on the Curvature Parameter of the Surfactant. The addition of salts to the microemulsions produces variations in the effective polar area of the

(15) (a) Garcia-Rio, L.; Leis, J. R.; Mejuto, J. C.; Pena, M. E.; Iglesias, E. Langmuir 1994, 10, 1676. (b) Garcia-Rio, L.; Herves, P.; Mejuto, J. C.; Perez-Juste, J.; Rodriguez-Dafonte, P. J. Colloid Interface Sci. 2000, 225, 259.

(16) (a) Cabos, C.; Delord, P. J. Phys. Lett. 1980, 41, L-455. (b) Lang, J.; Jada, A.; Milliaris, A. J. Phys. Chem. 1988, 92, 1946. (c) Rouviere, J.; Couret, J. M.; Lindheimer, A.; Lindheimer, M.; Brun, B. J. Chim. Phys.-Chim. Biol. 1979, 76, 297.

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Figure 4. Average value of ln(P) (scaling equation) for different CEs: (O) crown ethers; (b) benzo crown ethers; (4) dibenzo crown ethers.

surfactant. These variations are due to an increase in the screening of the electrostatic charges of the headgroups and, therefore, to a decrease in the electrostatic repulsions.17 This effect has a direct repercussion on the increase in the curvature parameter of the surfactant.18 This parameter is related with the effective area of the headgroup of the surfactant. Decreasing this geometric parameter, we increase the negative natural curvature of the surfactant and its tendency to produce reverse structures. The described behavior is consistent with the fact that the presence of electrolytes causes a decrease in the maximum capacity of water solubilization by the microemulsion. The size of the water droplet is directly proportional to the composition parameter W.19 The process of water solubilization gives rise to an increase in the size of the droplet; likewise the increase of the water content of the microemulsion corresponds with a greater volume in the disperse phase. This aqueous microdroplet is surrounded by a surfactant film for which negative curvature decreases as the water content of the system increases. This reduction of the negative curvature of the interface associated with the increase of water content in the microemulsion would be opposite to the natural tendency of the surfactant. It is known that the electrical conductivity in AOTbased microemulsions is due to the transport of cations by means of “instant” channels formed between droplets. The channel aperture at the interface20 involves a high activation energy due to the creation of local positive curvature regions within the surfactant film. The slight natural tendency of AOT to adopt positive curvatures would make this channel opening difficult. Therefore, the greater the tendency of the AOT to form structures with a negative curvature, the more difficult it will be for the percolative process to take place. In this sense, addition of cations to the aqueous microdroplet makes the channels’ opening difficult and delays the percolation threshold. Small variations of the cation concentration in the vicinity of the charged headgroups would be compatible with the experimental behavior observed at low macrocycle concentrations. Influence of Electrolyte Concentrations on Interdroplet Interactions. The presence of electrolytes in the micro(17) Finer, E. G.; Franks, F.; Tait, M. J. J. Am. Chem. Soc. 1972, 94, 4424. (18) Evans, D. F.; Michell, D. J.; Ninham, D.W. J. Phys. Chem. 1986, 90, 2817. (19) Pileni, M. P. Structure and reactivity in reverse micelles; Elsevier: 1989. (20) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985.


emulsions causes a decrease in the attracting forces between droplets, which is related with droplets’ interpenetration.21,22 This decrease in the attracting forces would reduce the number of effective collisions which could give rise to the channels opening. The reduction of the number of effective collisions would contribute to a greater difficulty in reaching the percolation threshold. The penetrability in the interfacial region during the droplet overlapping processes is determined by a phenomenological parameter,22 whose value decreases as the rigidity of the interface increases. The increase of electrolyte concentration at the aqueous microdroplet implies an increase in the negative curvature of the surfactant as a consequence of an increase in the rigidity of the surfactant film. That increase also makes the surfactant film much more difficult to be distorted, decreasing the interaction between droplets. The crown ethers are characterized by their capacity to capture cations.23 The Na+ ions are found in the water droplets of the microemulsion as counterions of the AOT headgroups. The presence of these additives is then able to alter the screening status of the AOT headgroups, decreasing their effective area, as observed with the addition of electrolyte to the microemulsion (see above). This decrease in the effective area would imply more trapezoidal geometry of the AOT molecule and would increase its natural tendency toward the formation of structures with negative curvature (its natural tendency to form reverse micelles). This would make the channel opening more difficult, and the emergence of electric percolation would be delayed. In summary, the addition of crown ethers varies the charge density at the aqueous microdroplet, giving rise to a modification of the headgroups’ screening and consequently a reduction of the effective area of the headgroup of the surfactant. Such a reduction would cause an increase in the curvature parameter of the tensioactive18 which impedes the channels’ opening that allows the charge transfer between droplets. The capacity of the crown ethers to associate Na+ ions and include them in the interface would cause a “local” increase of Na+ concentration, improving the screening role of the counterions24 on the headgroups, as shown in Scheme 2. 3. Influence of a Moderate and High Macrocycle Concentration. Increasing the crown ether concentration causes the percolation temperature to decrease. Under those conditions, the regions of high conductivity can be reached at surprisingly low temperatures. This effect also takes place when we study the electric percolation at 25 °C, varying the fraction of the disperse phase. It is then observed that the percolation threshold is reached at abnormally low volume fractions of the disperse phase. The presence of moderate concentrations of crown ether means that the electric percolation will take place at (21) (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. (22) Hou, M.; Shau, D. O. Langmuir 1987, 3, 1089. (23) Magid, L. J.; Weber, R.; Leser, M. E.; Farago, B. Prog. Colloid Polym. Sci. 1990, 81, 64. (24) As one of the reviewers suggests, the formation of negative and positive radii of curvature in the presence of salt in relation to the actions of crowns is difficult to reconcile. However, it is well documented in the literature that added cations increase the screening between the AOT headgroups and hence allow the headgroups to be closer in the aggregate. Such a modification of the aggregate structure implies an increase in the natural tendency of the surfactant for negative curvatures and increases the percolation temperature. When cations are complexed by crown ethers, they can penetrate deeply in the interface so that the screening effect should be larger than that in the absence of crown ethers.


Langmuir, Vol. 19, No. 15, 2003 5981 Scheme 2

volume fractions (φ) of the disperse phase significantly lower than the theoretical value predicted by geometric percolation models such as the effective medium theory25 (which predicts φ values close to 0.33). The predicted value obtained considering the interactions between droplets, according to the effective medium theory modified to take into account dipole-dipole interactions26 (φ ∼ 0.15), is significantly greater than the observed one. This behavior is analogous to that found when organic compounds (like ureas or thioureas) are added to the microemulsions. Influence of Organic Molecules on the Curvature Parameter of the Surfactant. The effect exerted by the organic molecules is a consequence of its incorporation into the surfactant film, modifying its geometry. The water solubility of crown ethers, mainly benzoethers and dibenzoethers, is small, which causes its localization in the microemulsions to take place within the surfactant film. The influence of crown ethers on the percolation temperature will be similar to that exerted by organic molecules:15 (i) The additive acts as “spacers” within the surfactant monolayer, increasing the effective area of the polar headgroups of the AOT. This phenomenon would imply a decrease in the curvature parameter of the surfactant, favoring the formation of positive curvature regions, and hence the fusion between droplets, accelerating the percolative process. (ii) The association of additives to the surfactant film increases the degree of disorder in the interfacial region, which would imply a decrease in the rigidity of the AOT film and hence an increase of the deformability of the film. Other authors have drawn attention to the capacity of the crown ethers to act as an intermembrane bridge.8 This capacity favors the channels’ opening required for the electric percolation to take place. The role played by the crown ethers at high concentrations in the AOT/isooctane/ water microemulsions is illustrated in Chart 2. Influence of Additives on the Replacement of Water Molecules at the Interface. It is interesting to establish the role played by the organic molecules in the interfacial region with regards to their capacity to replace water molecules in the vicinity of the polar area of the surfactant film. In the literature27 it is suggested that moderate concentrations of different organic molecules “open” the interface and facilitate the penetrability of water. Kevan27 suggests that the addition of organic molecules to frozen vesicles facilitates the water penetration in the vesicular structure. The same studies show that high concentrations (25) Granqvist, C. G.; Hunderi, O. Phys. Rev. B 1978, 18, 1554. (26) Fang, J.; Venable, R. L. J. Colloid. Interface Sci. 1987, 116, 269. (27) Kang, Y. S.; McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1992, 96, 10055.

Chart 2

of additive cause a replacement of water in the interface and modify the solvation of the headgroups. These results imply that molecules such as ureas or thioureas could play an important role in the solvation of the headgroups of the AOT and justify in part their capacity to modify the percolation threshold of the microemulsions when they are added as additives. Because of their peculiar structure, the crown ethers would be able to present similar behavior with regard to their capacity to substitute the water molecules within the solvation “sphere” of the AOT headgroups. Their solvation ability is shown, for example, when we study the catalysis exerted by these macrocycles on the esters’ aminolysis in apolar solvents.28 4. Compensation Concentration. In all cases we observed the existence of a crown ether concentration at which the percolation temperature was the same as that observed in the absence of additive. This concentration was termed the compensation concentration. The corresponding values are shown in Table 2. This concentration would correspond with a situation in which the role of charge stabilizer at the interface and the role of “intermembrane bridge” would be compensated. It would be necessary to verify that these concentrations correlate satisfactorily with the parameters that determine (i) the capacity of the crown ethers to associate and stabilize Na+ in its cavity and (ii) the geometric parameters describing its effect when it is inserted into the AOT film. A parameter defining both aspects is the internal radius of the crown ether, which schematically can be represented by a toroidal structure. With the purpose of obtaining the geometric parameters of the crown ethers studied, we carried out MM2 calculations to estimate the internal radii (28) Hogan, J. C.; Gandour, R. D. J. Am. Chem. Soc. 1980, 102, 2865.

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Figure 5. Correlation between the internal radius of the crown ethers and the compensation concentration of the system: (O) crown ethers; (b) benzo crown ethers; (9) dibenzo crown ethers.

Figure 6. Correlation between the number of oxygens in the crown ether and the composition concentration of the system: (O) crown ethers; (b) benzo crown ethers ([CE]c/10-1); and (0) dibenzo crown ethers.

Table 2. Values of Partition Coefficients for Crown Ethers between Water and 1-Octanol (log[P1-octanol]), Radius of the Cavity of the Crown Ethers (Ri), and Compensation Concentrations ([EC]c) additive

N of O

Ri/pm

log(Poctanol)

10-3[CE]c/M

C4 C5 C6 BC4 BC5 BC6 DBC5 DBC6 DBC7 DBC8 DBC10

4 5 6 4 5 6 5 6 7 8 10

332 438 570 338 443 568 450 568

-0.04 -0.48 -0.68 >1.78 0.91 0.58 2.31 2.20 2.17 2.11 1.80

89.80 57.90 27.45 10.43 6.59 2.85 17.87 13.20 4.21 2.73 0.75

of the ethers. This magnitude was obtained as half the mean of the distance between the opposing oxygen atoms in each of the crown ethers. Figure 5 shows the existence of a satisfactory correlation between the internal radius of the crown ether and the compensation concentration. In both cases, the linear squared correlation coefficients were r2 ) 0.9942 and r2 ) 0.9966, respectively, for the crown and benzo crown ethers. The larger crown ethers (dibenzo crown ethers) are not included in these correlations due to the deformation they undergo with regard to the ring-shaped conformation presented by the smaller crown ethers. Accordingly, increasing the internal radius will increase the Na+ complexation by the crown ether and the effect exerted by the crown ether as a solubilizing charge in the interface. The effect observed will be greater before the percolation temperature decreases again once the concentration of the crown ether increases. Thus, for the sequence 12-crown-4, 15-crown-5, and 18-crown-6, we observed that the maximum increase δTp (δTp ) Tpadditive - Tpwithout additive) reached is proportional to the internal radius. As the radius increases, the volume of the molecule favors its role as a bridge in the formation of channels. The greater the radius, the more marked the decrease observed in the percolation temperature and consequently the lower the value of the compensation concentration. This latter fact is shown in the inverse correlation of Figure 5. Na+ ion is stabilized in the interior of the cavity by its interaction with the solitary electronic pairs of the oxygen atoms in the crown ethers. Another linear correlation should then be shown between the number of oxygen atoms present in the crown ether molecule and the compensation concentration. Figure 6 shows the correlations obtained, which were satisfactory in all cases. The squared cor-

Figure 7. Correlation between the logarithm of the partition coefficient of the crown ethers between water and 1-octanol and the logarithm of the compensation concentration of the system: (O) crown ethers; (b) benzo crown ethers; (0) dibenzo crown ethers.

relation coefficients in each of the cases were r2 ) 0.9998 (crown ethers), r2 ) 1 (benzo crown ethers), and r2 ) 0.8256 (dibenzo crown ethers). The solubilities of the crown ethers, in the disperse phase as well as the continuous phase, will determine the different equilibria governing the distribution of the crown ether (free as well as complexated) throughout the different domains of the microemulsion. The behavior patterns of the crown ethers in the microemulsions would be based on the value of the partition coefficients between water and 1-octanol.29 We have found a linear correlation between the logarithm of the distribution coefficient between water and 1-octanol and the logarithm of the value of the compensation concentration of the system for all cases, as shown in Figure 7 (r2 ) 0.9423, r2 ) 0.8776, and r2 ) 0.9203 for crown, benzo crown, and dibenzo crown ethers, respectively). It is not possible to isolate the contribution of the solubility of the substrates in the different polarity domains of the microemulsion and their different complexation capacities of Na+ ions. The fact that we find poor correlations for some substrates studied (in particular for the case of the dibenzo crown ethers) can be justified by their “different” structure with regard to the crown ethers of a smaller size. The large size of the dibenzo crown ethers, in particular DBC8 and DBC10, would allow a greater flexibility of the molecular structure of these molecules and have repercussions on two fundamental (29) Stolwijk, T. B.; Vos, L. C.; Sudho¨lter, E. J. R.; Reinhoudt, D. N. Recl. Trav. Chim. Pays-Bas 1989, 108, 103.


Langmuir, Vol. 19, No. 15, 2003 5983

and 1-octanol, N is the number of oxygen atoms in the macrocycle, and R, β, and γ are the adjustable parameters in the multiparametric correlation. On the basis of the values obtained for R (-0.179) and β (-0.233) it is possible to evaluate the relative weight of the two effects set against those to which we have attributed the biphasic behavior of the variation of the percolation temperature of the microemulsions AOT/ isooctane/water. We could assume a contribution of 43.5% for the capacity of the crown ethers to capture and transport Na+ ions, and one of 56.5% for the effect exerted by the organic substrate inserted in the interface.

Figure 8. Multiparametric adjustment of the compensation concentration according to the number of carbons present in the structure of the crown ether and the logarithm of the partition coefficient between the water and the 1-octanol.

aspects: (i) the distortions produced in the interface of AOT in the moment of its insertion and (ii) the possibility of adopting a conformation to optimize the capacity for solvation of these molecules on the heads of AOT when displacing water molecules. The high solubility of these substrates in isooctane would cause deviations in the expected sequence, as occurs in the case of the ureas and the thioureas15a (since the effect of tetramethyl urea and tetramethyl thiourea on the electric percolation is significantly lower than that expected observing the sequence presented for other ureas and thioureas). With the purpose of combining both factors, it is possible to carry out a multiparametric analysis in which we will take into account the size of the cavity as well as the solubility of the macrocycle. It has been considered that the solubility of the crown ether is represented by its partition coefficient between water and 1-octanol. As a representative measurement of the size of the crown ether and its capacity to complexate cations, we have considered the number of oxygen atoms present in the macrocycle. As we can observe in Figure 8, we have found a satisfactory correlation between both parameters and the logarithm of the compensation concentration by means of the following expression:

lg [CE]c ) R log(Poctanol) + βN + γ

(4)

where [CE]c is the compensation concentration, Poctanol is the partition coefficient of the crown ether between water

Conclusions We can thus conclude that the net effect of the crown ethers studied is a combination of two effects: (i) the complexation of metallic cations and their incorporation into the interface that impedes the emergence of the percolative process and, (ii) likewise, their association to the surfactant film as organic compounds, increasing the head area of AOT and favoring the formation of positive curvatures and, consequently, facilitating the exchange of matter between droplets. In fact, these molecules are known to be able to “capture” and “sequester” ions and at the same time solubilize them preferentially at microemulsion interfaces. Thus, the increased percolation temperature observed at low concentrations of the crown ethers 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 crown ether used, we can assume that the surfactant layer volume was scarcely altered. Then their negative curvature would prevail over the positive curvature and percolation would be hindered as a result. The effect observed at high concentrations of these substances in the microemulsions is that expected for an organic substance that associates to the interface. The resulting perturbation of the surfactant film favored the formation of the positive curvature; this in turn favored the mass exchange among droplets and, hence, electric percolation. Acknowledgment. Financial support from the Xunta de Galicia (PGIDT00PXI20907PR) and Ministerio de Ciencia y Tecnologıá (Project BQU2002-01184) is gratefully acknowledged. D.G.-D. thanks the University of Santiago de Compostela for a predoctoral research grant. LA026857M

Isooctane

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