Influence of Anionic Surfactants on the Electric Percolation of AOT

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Influence of Anionic Surfactants on the Electric Percolation of AOT/Isooctane/Water Microemulsions L. Garcı´a-Rı´o,*,† J. C. Mejuto,‡ M. Pe´rez-Lorenzo,† A. Rodrı´guez-A Ä lvarez,‡ and † P. Rodrı´guez-Dafonte Departamento de Quı´mica Fı´sica, Facultad de Quı´mica, Universidad de Santiago, 15782 Santiago, Spain, and Departamento de Quı´mica Fı´sica, Facultad de Ciencias, Universidad de Vigo en Ourense, 34002 Ourense, Spain Received January 24, 2005. In Final Form: April 27, 2005 A study was carried out concerning the influence of sodium alkyl sulfonates on the electric percolation of AOT/isooctane/water microemulsions ([AOT] ) 0.5 M and W ) [H2O]/[AOT] ) 22.2). An important effect was observed with regard to the percolation temperature caused by the addition of small quantities of alkyl sulfonates (F ) [alkyl sulfonate]/[AOT] ) 0.01). The short chain alkyl sulfonates (C3-C5) cause an increase in the percolation temperature, which in turn is reduced as we increase the chain length of the additive until we obtain a percolation temperature which is lower than that which is observed in the absence of an additive (C6-C8). For hydrocarbon chains of a greater length we can observe a new increase in the percolation temperature (C10-C18). This behavior has been explained as a consequence of (i) the incorporation of the additives at the interphase of the microemulsion and (ii) the geometric parameters of the different surfactants added to the microemulsion.

Introduction Microemulsions are dynamic structures with components that are organized due to different interactions, collisions, coalescence, or redispersion. These systems are transparent and isotropic dispersions of a polar compound in an apolar mediumsor vice versasin the presence of a surfactant, and they have been described as spherical droplets of a disperse phase separated from a continuous phase by a surfactant film. In particular in this study we will concentrate on microemulsions known as water in oil (w/o), that is, those in which the continuous medium is an apolar compoundsin our case, isooctanesand the disperse medium is water.1 In this sense a w/o microemulsion presents a very low conductivity (0.001-0.1 µS cm-1). However, this conductivity is significantly high with regard to the conductivity of the alkanes (∼10-8 µS cm-1). This different behavior shown by the pure continuous medium and the microemulsion can be explained by the fact that the microemulsions are able to transport electric charges and in this way they are associated with the phenomenon known as electric percolation. This consists of a drastic increase in conductivity, at a determined value of the fraction in volume of the disperse phase, or at a determined temperature while the composition of the microemulsion is kept constant. This increase is associated with a greater flow of electric charge between the discrete droplets of the disperse phase. The relationship between this electric percolation phenomenon and the droplet model has been shown in the literature.2 In fact, the rate of the matter exchange processes increases with the percolation transitions for microemulsions, in those which are based on † ‡

Universidad de Santiago. Universidad de Vigo en Ourense.

(1) Structure and Reactivity in Reverse Micelles; Pileni, M. P., Ed.; Elsevier: Amsterdam, 1989. (2) (a) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10-12. (b) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387-395. (c) Lang, J.; Mascolo, G.; Zana, R.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3069-3074.

both ionic and nonionic tensioactives,3 although in all cases this rate constant remains below the rate of diffusion control.4 It is well-known that moderately low concentrations of additives have a significant effect on the percolation threshold.5 The additives that increase the “rigidity” of the membranes increase the value of the percolation threshold of a microemulsion,6 while those that make them more flexible favor the process.6-7 In this sense the addition of cationic surfactants to AOT-based microemulsions causes an increase in the percolation temperature6 while the addition of nonconjugated bile hydroxy salts has the opposite effect.6 Previous studies in our laboratory have examined the influence of n-alkylaminesswhich like the sodium alkyl sulfonates give rise to micellar aggregates8son the electrical percolation. Our previous results showed a linear dependence of the percolation temperature with the amine chain length, and therefore with the hydrophobocity of the amines. In this study we examine the influence of sodium alkyl sulfonates on the electrical percolation of AOT-based microemulsions. Additives that increase the rigidity of the interface, like cholesterol and some surfactants, can convert AOT-based microemulsions in nonpercolative systems.6-7,9 In addition there are findings in (3) Mays, H.; Pochert, J.; Ilgenfritz, G. Langmuir 1995, 11, 43474354. (4) Feldman, Y.; Kozlovich, N.; Nir, I.; Garti, N.; Archipov, V.; Idiyatullin, Z.; Zuev, Y.; Fedotov, V. J. Phys. Chem. 1996, 100, 37453748. (5) (a) Garcia-Rio, L.; Leis, J. R.; Mejuto, J. C.; Pen˜a, M. E.; Iglesias, E. Langmuir 1994, 10, 1676-1683. (b) Garcı´a-Rı´o, L.; Herve´s, P.; Leis, J. R.; Mejuto, J. C. Langmuir 1997, 13, 6083-6088. (c) DasilvaCarvalhal, J.; Garcia-Rio, L.; Gomez-Diaz, D.; Mejuto, J. C.; RodriguezDafonte, P. Langmuir 2003, 19, 5975-5983. (6) Ray, S.; Bisal, S. R.; Moulik, S. P. J. Chem. Soc., Faraday Trans. 1993, 89, 3277-3282. (7) Mathew, C.; Patanjali, P. K.; Nabi, A.; Maitra, A. Colloid Surf. 1988, 30, 253-263. (8) Garcia-Rio, L.; Herve´s, P.; Mejuto, J. C.; Perez-Juste, J.; RodriguezDafonte, P. J. Colloid Interface Sci. 2000, 225, 259-264. (9) (a) Ray, S.; Paul, S.; Moulik, S. P. J. Colloid Interface Sci. 1996, 183, 6-12. (b) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Colloid Surf. 1990, 50, 295-308.

10.1021/la0501987 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/10/2005

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Figure 1. Typical determination of the percolation threshold for a microemulsion of AOT/isooctane/water.

the literature concerning the effects of other surface agents, such as Aracel,10 Brij,11 Igepal12 or different long chain alcohols.13 Because sodium alkyl sulfonates can be considered as surfactants and hence included in the surfactant film of the microemulsion, their influence on the electrical percolation should be investigated. Experimental Methods AOT (Aldrich, 99%) was desiccated in a vacuum and used without further purification. The rest of the additives were supplied by Aldrich at the maximum purity available commercially, and were used without further purification. The microemulsions were prepared by direct mixing, and the composition remained constant and equal to [AOT] ) 0.5 M (referred to the total volume of the microemulsion) and W ) [H2O]/[AOT] ) 22.2. Conductance (κ) measurements (with (0.1% accuracy) were taken at a frequency of 3.8 kHz using a Crison microCM 2202 conductivimeter equipped with a cell supplied by Crison (cell constant of 0.997 cm-1). The conductivimeter was calibrated using two standard conductivity solutions supplied by Crison ([KCl] ) 0.0100 M, κ ) 1413 µS cm-1 at 25 °C and [KCl] ) 0.100 M, κ ) 12.88 mS cm-1 at 25 °C). The microemulsions were introduced in a receptacle of 50 mL of volume, conveniently encased for its thermostatization, which could be stirred using a Teflon bar with a magnetic nucleus, and with a hermetically sealed lid with two openings through which the electrode was inserted to determine the specific conductivity and a thermometer to determine the temperature at which the sample was found. The temperature was determined at the same time as the conductivity. The water used for the preparation of the microemulsions was distilled and deionized with a conductivity of κ ) 0.10-0.50 µS cm-1. While measurements were being made, the temperature was kept constant by means of a cryostat thermostat Teche TE-8D RB-5, with a precision of (0.1 °C. Traditionally, the percolation temperature (Tp) was obtained from the conductivity/temperature data. As described in the literature, Tp was considered as the maximum of the plots (1/κ)(δκ/δT) vs T, as shown in Figure 1.14 An alternative analysis of conductivity/temperature data can be carried out in terms of the sigmoidal Boltzmann equation (SBE) proposed by Moulik et al.15 (eq 1):

[ (

log κ ) log κf 1 +

)

log κi - log κf {1 + e((T-Tp)/∆T}-1 log κf

]

(1)

(10) Giammona, G.; Goffredi, F.; Turco Liveri, V.; Vassallo, G. J. Colloid Interface Sci. 1992, 154, 411-415. (11) Nazario, L. M. M.; Hatton, T. A.; Crespo, J. P. S. G. Langmuir 1996, 12, 6326-6335. (12) Schuebel, D.; Ilgenfritz, G. Langmuir 1997, 13, 4246-4250. (13) Ray, S.; Moulik, S. P. J. Colloid Interface Sci. 1995, 173, 28-33. (14) Kim, M. W.; Huang, J. S. Phys. Rev. A 1986, 34, 719-722. (15) Hait, S. K.; Moulik, S. P.; Palepu, R. Langmuir 2002, 18, 24712476.

Figure 2. Variation of the electric percolation threshold for microemulsions of AOT/isooctane/water in the presence of alkyl sulfonates. W ) 22.2, F ) 0.01, [AOT] ) 0.5 M. 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.

Results The percolation temperature (Tp) was determined on the basis of the data of the variation of the electric conductivity of the microemulsions with the temperature. The percolation threshold was assigned as a maximum in the graphic representation (∆κ/κ∆t) versus the temperature (see Figure 1 for a typical example). Likewise, Tp was determined on the basis of the fit of the experimental data to eq 1 (SBE). The influence of sodium alkyl sulfonates on the percolation temperature was determined for different chain lengths (Cn), ranging between 3 and 18. The concentration of sodium alkyl sulfonate remained constant throughout the series of experiments, keeping constant the relationship [Cn]/[AOT] ) 0.01. Table 1 shows the values of the electric percolation temperature for the different alkyl sulfonates studied. The behavior observed (see Figure 2) shows that a group of alkyl sulfonates increase the percolation temperature (C3-C5), to subsequently decrease the value of Tp (C6-C18). The absence of a linear dependency has also been found in the literature when analyzing the influence of alkyl sulfonates on the percolation of nonionic microemulsions.16 In the literature there are contradictory results concerning the addition of surfactants to microemulsions of AOT/isooctane/water. When cationic surfactants are added to AOT-based microemulsions, increases are observed in the percolation temperature.6 By contrast, the addition of anionic surfactants (SDS)17 causes a drastic decrease in the percolation temperature. This decrease in the presence of SDS could be predicted by taking into account the fact that the exchange of material between droplets is about 1.7 times greater in microemulsions of AOT/SDS/heptane/ water with F ) 0.1 than in microemulsions of AOT/heptane/ water (F ) 0, ke ) 13 × 10+6 M-1 s-1; F ) 0.1, ke ) 22 × 10+6 M-1 s-1).18 In recent studies17 it has been noted that the effect on the percolation temperature increases as the relationship F ) [Cn]/[AOT] increases. Specifically, the addition of SDS (16) (a) Eicke, H. F.; Meier, W. Colloid Polym. Sci. 2001, 279, 301304. (b) Eicke, H. F. J. Phys. Chem. B 2001, 105, 2753-2756. (17) Garcia-Rio, L.; Herve´s, P.; Mejuto, J. C.; Perez-Juste, J.; Rodriguez-Dafonte, P. Langmuir 2000, 16, 9716-9721. (18) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 35433545.

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Table 1. Percolation Temperature of Microemulsions of AOT/Isooctane/Water in the Presence of Alkyl Sulfonatesa

a

sodium alkyl sulfonate

symbol

no. of carbons

Tp/°C

∆TNa p

sodium propyl sulfonate sodium butyl sulfonate sodium pentyl sulfonate sodium hexyl sulfonate sodium octyl sulfonate sodium decyl sulfonate sodium lauryl sulfonate sodium tetradecyl sulfonate sodium cetyl sulfonate sodium octadecyl sulfonate

C3 C4 C5 C6 C8 C10 C12 C14 C16 C18

3 4 5 6 8 10 12 14 16 18

50.5 44.5 34.5 25.5 19.0 20.5 21.5 22.5 26.5 27.0

2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6 2.6

+

∆TAS p

b

14.3 8.3 -1.7 -10.7 -17.2 -15.7 -14.7 -13.7 -9.7 -9.2

Tp/°C (SBE) 50.1 ( 0.5 44.9 ( 0.4 33.9 ( 0.3 24.9 ( 0.2 19.2 ( 0.2 20.7 ( 0.2 21.2 ( 0.2 22.3 ( 0.2 26.6 ( 0.3 26.9 ( 0.3

sulfonate W ) 22.2, F ) 0.01, [AOT] ) 0.5 M. b ∆Talkyl . p

gives rise to a decrease in the percolation temperature of more than 30 °C as F increases. However, this increase in the parameter F also gives rise to a decrease in the stability of the microemulsions, with the consequent reduction in its maximum capacity of solubilization of water. Given that the objective of the present study is to investigate the length of the hydrocarbon chain of sodium alkyl sulfonates (Cn) on the percolation temperature, we decided to use a relationship F ) [Cn]/[AOT] in which there is a significant effect on the percolation temperature, and which also has stable systems for all the alkyl sulfonates studied (chain lengths between 3 and 18).

Chart 1

Discussion The behavior observed can be attributed to the anionic surfactant properties of the alkyl sulfonates: on one hand they are sodium salts, and on the other hand they have a hydrocarbon chain of varying length. In this sense, for short chains (less than six carbon atoms) the dominating effect to observe with the addition of these substrates would be that which corresponds to an increase in the concentration of Na+ ions inside the aqueous microdroplet. Its effect on the electric percolation is justified by the reduction of the effective area of the polar head of the tensioactive, due to the screening of the electrostatic repulsion between the charged headgroups of the tensioactives.5a In this way, it can be considered that for short chains, the main role of these additives is as an extra source of Na+ for the aqueous microdroplet, whereby its behavior is typical of a sodium salt.5a,19 For long chain surfactants, the predominant effect would be that of their inclusion in the interface, modifying the packing of the interface (see Chart 1). This modification of the packing allows the existence of regions inside the interface where the formation of positive curves is more favored. It is clear that this situation would favor the formation of channels for the material exchange between droplets, and therefore it would favor the percolation process. It is necessary to approach this study isolating each of the factors that can intervene in this dual behavior of the alkyl sulfonates: Effect of the Counterion. As indicated previously, the alkyl sulfonates are sodium salts, whereby their addition to the microemulsion would imply an increase in the concentration of Na+ ions in the interior of the aqueous microdroplet. Therefore the addition of alkyl sulfonates would give rise to an increase of the percolation temperature. This behavior is justified by the reduction of the effective area of the polar head of the tensioactive, due to the screening of the electrostatic repulsions between the

charged groups of the tensioactives (see above). It is evident that, for short chain lengths, the main role of these additives is as an extra source of Na+ to the pool, whereby its behavior is typical of a sodium salt.5a,19 It would seem that the short chain alkyl sulfonates are found inside the aqueous core, due to the electrostatic repulsion between the AOT headgroups and the negative charge that these organic anions carry. Previous studies carried out in our laboratory have shown that the effect of different sodium salts on the electric percolation is independent of the anion present for AOT-based microemulsions. The increase in the percolation temperature is independent of the nature of the anion present, as shown on comparing the series NaCl, NaBr, NaI, NaNO3 NaClO4 NaSCN, NaOH ([salt] ) 0.04 M referred to the volume of the aqueous microdroplet in an AOT/isooctane/water microemulsion of [AOT] ) 0.5 M and W ) 22.2). The following percolation temperatures are obtained: 38 °C, 39 °C, 39 °C, 37 °C, 38 °C, 37 °C, and 37 °C, respectively, for the aforesaid salts.19,20 Supposing that our alkyl sulfonates behave like strong electrolytes and therefore are totally dissociated, the quantity of Na+ contributed to the system (F ) 0.01) would be 0.025 M referred to the volume of the aqueous microdroplet. The influence of the Na+ concentration on the percolation threshold has a linear dependency on the concentration of electrolyte added. Thus, from the data of the influence of the concentration of NaOH (a source of Na+) on the percolation threshold20 (Figure 3) we can estimate the effect of the Na+ contributed by the different alkyl sulfonates, allowing us to estimate an increase in the percolation temperature 2.6 °C. The effect of the sodium alkyl sulfonates on the percolation temperature can be expressed by

(19) Alvarez, E.; Garcia-Rio, L.; Mejuto, J. C.; Navaza, J. M. J. Chem. Eng. Data 1998, 43, 519-523.

(20) Alvarez, E.; Garcia-Rio, L.; Mejuto, J. C.; Navaza, J. M.; PerezJuste, J. J. Chem. Eng. Data 1999, 44, 846-849.

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Garcı´a-Rı´o et al. Chart 2

Figure 3. Increase in the percolation temperature on microemulsions of AOT/isooctane/water (W ) 22.2, [AOT] ) 0.5 M) in the presence of different quantities of added Na+. Concentration of Na+ referred to the water pool. Tp, percolation temperature of the system. Tp° percolation temperature of the system in the absence of an electrolyte. (X) Extrapolation of the variation of the percolation temperature for a quantity of added sodium equivalent to a microemulsion F ) 0.01 ([Na+] ) 0.025 M referred to the water pool).

Figure 4. Contributions of anions and cations over the percolation temperature for microemulsions of AOT/isooctane/ water in the presence of sodium alkyl sulfonate. [AOT] ) 0.5 M, W ) 22.2, F ) 0.01.

the following equation: +

sulfonate Tp ) Tp° + ∆Talkyl + ∆TNa p p

(2)

where Tp is the percolation temperature, determined experimentally in the presence of the alkyl sulfonate; Tp° is the percolation temperature of an AOT-based microemulsion of W ) 22.2 and [AOT] ) 0.50 M in the absence sulfonate of additives; ∆Talkyl is the contribution of the anion p + is the contrito the percolation temperature; and ∆TNa p bution of the cation Na+. The effects, having been separated in this way, are shown in Table 1 and Figure 4. In view of Figure 4, it is necessary to comment on the isolated behavior of the anion of the alkyl sulfonates. Effect of the Short Chain Alkyl Sulfonates (C3C4). The short chain alkyl sulfonates have a very high solubility in water, so they are hardly integrated in the structure of the surfactant film that forms the microemulsion. This implies that the effect of the anions would be negligible, as shown in previous studies, where it has been noted that the effect on the percolation temperature by NaOH, NaCl, NaBr, and NaI is independent of the anion.5a,19,20 However, the surfactant nature of the alkyl sulfonate invalidates the supposition that all the anions would be found concentrated in the aqueous core and distant from the vicinity of the charged heads of the AOT. In this sense, the effect of the anions would have to be

taken into account and would justify the anomalous contribution values of the anion which are found for the molecules with the shortest chains (C3 and C4). This behavior could be explained according to the manner of insertion of these organic anions within the structure of the interface. If we suppose that the penetrability of the alkyl sulfonate in the interior of the interface is low (which is consistent with its high solubility in water), we can consider a structure like that which is shown in Chart 2. According to Chart 2 we can suppose that the two AOT molecules, together with a molecule of C3 or C4, would form a group which, in keeping with the geometric considerations of packing, would represent an increase in its tendency to form negative curves. This greater tendency to form inverse structures directly implies a greater difficulty in the formation of the interdroplet channels that are necessary for the matter exchange to occur between droplets. This fact translates directly to an increase in the percolation temperature. Effect of the Long Chain Alkyl Sulfonates (C6C18). For the alkyl sulfonates that are dominated by their amphiphilic nature (C6-C18), anomalous behavior can once again be observed. As mentioned already, our research group has evaluated the effect of substrates with a variable length alkylic chain (alkyl amines). In the case of the alkyl amines a linear dependency has been found for the effect on the electric percolation against the number of carbons present in the alkyl chain.8 In the case of the alkyl sulfonates that linear dependency cannot be observed. Neither is there a correlation between the value of the critical micellar concentration of these surface agents in an aqueous solution and the effect on the electric percolation (data not shown). We must take into account the fact that the critical micelle concentration is related to the hydrophobic-hydrophilic interactions between the surfactant and its environment. The experimental results show that the percolation temperature is lowered as the length of the hydrocarbon chain increases until a minimum is reached for C8. For the range of chain lengths between C10 and C18 the effect on the electric percolation is much smaller than that which is observed before the minimum is reached. In fact, between C3 and C8 there is a difference of 31.5 °C, while between C8 and C18 the variation in the percolation temperature is barely 8 °C. The difference observed between C16 and C18 is less than 1.0 °C. The obtained results are surprising if we limit ourselves to supposing that the effect of the inclusion of amphiphiles, such as cosurfactants, in the AOT film would imply a variation in the mean packing parameter of the system. The inclusion of surface agents with critical packing parameters below 0.5 (the case of the micelle-forming amphiphiles) in a surfactant film formed by molecules with a packing parameter greater than 1 (the case of the AOT) would imply that in certain regions of the mixed film the formation of positive curves is favored. This in turn would imply that there would be a decrease in the percolation temperature (as observed in the case of the alkyl sulfonates in the range C6-C18). This reasoning,

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Table 2. Chain Length of the Surfactant, Assuming It To Be Long, Obtained by Molecular Mechanical Calculations surfactant

l/Å

surfactant

l/Å

AOT C3 C4 C5 C6 C7

7.52 2.52 3.88 5.03 6.33 7.51

C8 C10 C12 C14 C16 C18

8.83 11.33 13.85 16.33 19.21 21.93

Table 3. Energetics of Clustering in the Presence of Different Sodium Alkyl Sulfonatesa surfactant ∆G°cl/kJ mol-1 ∆H° cl/kJ mol-1 ∆S° cl/J mol-1 K-1 C3 C4 C5 C6 C8 C10 C12 C14 C16 C18

-22.3 -21.9 -21.2 -20.5 -20.2 -20.3 -20.3 -20.4 -20.7 -20.7

23.8 21.7 20.9 20.3 19.9 20.1 20.0 20.0 20.4 20.4

142.2 134.5 129.6 125.8 123.3 124.3 124.1 124.5 126.8 126.8

a Standard deviations in ∆G° , ∆H° , and ∆S° are 6%, 6%, and cl cl cl 9%, respectively.

however, cannot justify the fact that our system presents a minimum percolation temperature for C8, but indicates that the percolation temperature will continue to decrease as we increase the number of carbon atoms in the alkyl chain. The experimental results are contrary to this tendency and, in fact, for chain lengths greater than C10 we can observe a slight increase in the percolation temperature as Cn increases. The fact that a minimum is observed for C8 could be justified on the basis of the values corresponding to the length of the hydrocarbon chain. Table 2 shows the values of the chain length of the AOT and the different alkyl sulfonates obtained from molecular mechanical calculations, where the force field is based on a CHARMM parametrization.21 The data obtained show that the chain length of the AOT molecule (7.52 Å) is between a C7 and a C8 alkyl sulfonate (7.51 Å and 8.83 Å, respectively). This indicates that alkyl sulfonates with a hydrocarbon chain larger than C8 cannot be satisfactorily accommodated within the surfactant film. These problems of locating the simple chain surfactants within the AOT film could give rise to a process of looping in the surfactant chain. This looping of the chains is observed in normal micelles, where there is experimental evidence to suggest a certain degree of contact between the water and the terminal carbon atoms of the hydrocarbon tails.22 A study has been carried out in an aqueous medium on the difference between the chemical displacements of 1H NMR of surfactants (i.e. LTABr, TTABr, and CTABr, which have hydrocarbon chains of 12, 14, and 16 carbon atoms, respectively) in D2O and CDCl3. The results show that the most affected protons are those that are found in position R to the headgroup and those that are found in the terminal carbon.23 The ease with which this (21) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; States, D. J.; Swaminathan, S.; Karplus, M. J. Comput. Chem. 1983, 4, 187-217. (22) (a) Menger, F. M. Acc. Chem. Res. 1979, 12, 111-117. (b) Shobha, J.; Balasubramanian, D. J. Phys. Chem. 1986, 90, 2800-2802. (c) Goon, P.; Das, S.; Clemett, C. J.; Tiddy, G. J. T.; Kumar, V. V. Langmuir 1997, 13, 5577-5582. (d) Gruen, D. W. R. J. Phys. Chem. 1985, 89, 146. (e) Gruen, D. W. R. J. Phys. Chem. 1985, 89, 153. (23) Bravo, C.; Garcı´a-Rı´o, L.; Leis, J. R.; Pen˜a, M. E.; Iglesias, E. J. Colloid Interface Sci. 1994, 166, 316-320.

looping phenomenon occurs must increase together with the length of the hydrocarbon chain of the surfactant. In this way, if a looping phenomenon occurs in the surfactant chains, the considerations regarding the critical packing would not be entirely correct, since the effect of the inclusion of a single chain surfactant (alkyl sulfonates) between the surfactant molecules with a double chain (AOT) would be less if looping occurred, as shown in Chart 3. Thermodynamics. Eicke and co-workers have shown that the threshold of electrical percolation corresponds to the formation of the first open structure of infinite cluster.24 Beyond the percolation threshold, the associated microdroplets in the continuum medium were considered to be in a different phase with different physical properties such as conductance. This is comparable with the pseudophase concept of surfactant micelles. Dilution with oil of a clustered microemulsion system at constant W should lower the conductance rapidly until the structure fragments into individual droplets below the percolation threshold, the phenomenon being comparable with the process of demicellization below the cmc.6 Analogous considerations can be made on increasing and decreasing temperature. The increase of temperature implies an increase of collisions between droplets, and hence implies also an increase of efficient collisions that result in the formation of an open structure of infinite cluster (analogous to micellization). The decrease of temperature implies a lower percentage of efficient collisions and would be comparable with a demicellization process. This concept has been considered by Moulik and Ray,25 Ray et al.,9a Ajith and Rakshit,26 and Alexandradis et al.27 for the estimation of energetics of clustering of different w/o microemulsion systems. In light of this concept, the Gibbs energy of clustering of microdroplets in solution (∆G°cl) was calculated from the relation

∆G°cl ) RT ln xd

(3)

where xd is the mole fraction of the droplets28 and the other terms have their usual significance. Using the Gibbs-Helmholtz equation and Gibbs equation, enthalpy and entropy of clustering can be obtained. The thermodynamic parameters for the droplet clustering process are presented in Table 3. The process manifested absorption of heat and positive entropy change. Prior to the droplet association, the surrounding oil barrier needed to be removed. This endothermic event was followed by the exothermic process of clustering; energetically, the first exceeded the second, making the overall effect endothermic. This effect has been reported extensively in the literature.6,9a,25,28,29 The compensation of ∆H°cl and ∆S°cl is satisfactory. In the literature9a a large influence of microemulsion composition in the thermodynamics of clustering was found. The variation of W between 10 and 40 implies that ∆H°cl increases 6 times (from 16 to 93 kJ mol-1). An increase in W results in an (24) Borkovec, M.; Eicke, H. F.; Hammerich, H.; Dasgupta, B. J. Phys. Chem. 1988, 92, 206. (25) Moulik, S. P.; Ray, S. Pure Appl. Chem. 1994, 66, 521. (26) Ajith, S.; Rakshit, A. K. J. Phys. Chem. 1995, 99, 14778. (27) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Phys. Chem. 1995, 99, 8222. (28) (a) Hait, S. K.; Moulik, S. P.; Rodgers, M. P.; Burke, S. E.; Palepu, R. J. Phys. Chem. B 2001, 105, 7145. (b) Hait, S. K.; Sanyal, A.; Moulik, S. P. J. Phys. Chem. B 2002, 106, 12642. (29) (a) Mays, H. J. Phys. Chem. B 1997, 101, 10271. (b) Moulik, S. P.; De, G. C.; Bhowmik, B. B.; Panda, A. K. J. Phys. Chem. B 1999, 103, 7122.

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Langmuir, Vol. 21, No. 14, 2005

Garcı´a-Rı´o et al. Chart 3

increase in ∆H°cl. Analogous behavior was found for ∆S°cl; in this way an increase of 4 times in W (from 10 to 40) increases ∆S°cl 4 times (from 98 to 391 J mol-1 K-1). The variation of W between 10 and 40 implies a 16 °C decreases in the percolation temperature for AOT/isooctane/water microemulsion with an [AOT] ) 0.50 M. However, the presence of different amounts of additives implies a low influence upon both thermodynamic parameters. The presence of 0.1 M of sodium cholate29a implies changes of 6% in ∆G°cl, 33% in ∆H°cl, and 20% in ∆S°cl and a decrease of 22 °C in the percolation temperature. As can be observed, the decrease in percolation temperature is not reflected in the variation of thermodynamic parameters. When using 1 mM of sodium salicylate29a authors found a variation of 3% in ∆G°cl, 30% in ∆H°cl, and 15% in ∆S°cl. In this case the addition of 1 mM of sodium salicylate implies a 5 °C increase in the percolation temperature. When comparing the influence of sodium cholate and sodium salicylate on the thermodynamic parameters we can observe a similar variation of ∆G°cl, ∆H°cl, and ∆S°cl. However the concentration of sodium salicylate is 100 times smaller than sodium cholate. Equivalent percentages were found for other additives28 as 2-methoxyphenol, 4-methoxyphenol, 5-methoxyresorcinol, R-naphthol, β-naphthol, 3-methoxy catechol, catechol, resorcinol, urea, hydroquinone, or pyrogallol. The variation of clustering thermodynamic parameters obtained by us (Table 3) exhibits behavior similar to that of the additives previously reported although their influence on the electrical temperature is as large as 31.5 °C. The presence of small amounts of additives has no significant influence upon the clustering thermodynamics compared with their impact upon the percolation temperature. This fact would be justified because 1-2 molecules of additive per 200-500 molecules of AOT would not have a large influence in ∆G°cl, ∆H°cl, and ∆S°cl, but local changes in the structure of surfactant film can increase or decrease dramatically the threshold of percolation.

Chart 4

Conclusions It has been observed that the effect of the addition of sodium alkyl sulfonates to microemulsions of AOT/ isooctane/water presents a particular behavior according to which a group of alkyl sulfonates increases the percolation temperature (C3-C5), only to subsequently decrease it (C6-C18). This behavior can be justified by assuming the contribution of Na+ to the system and the insertion of the alkyl chain in the surfactant film as we show in Chart 4. In addition, it has been shown that the geometry of the added amphiphile as a cosurfactant plays a very important part in the behavior observed. Thus, the minimum percolation temperature observed for C8 corresponds with the value for which the chain length of the surfactant is greater than the chain length of the AOT. Acknowledgment. Financial support from Ministerio de Ciencia y Tecnologı´a (Project BQU2002-01184) and Xunta de Galicia (PGIDT03-PXIC20905PN and PGIDIT04TMT209003PR) is gratefully acknowledged. LA0501987