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Structural Micellar Transition for Fluorinated and Hydrogenated Sodium Carboxylates Induced by Solubilization of Benzyl Alcohol Alfredo Gonza´lez-Pe´rez, Juan M. Ruso, Gerardo Prieto, and Fe´lix Sarmiento* Group of Biophysics and Interfaces, Department of Applied Physics, Faculty of Physics, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain Received March 24, 2004. In Final Form: July 7, 2004 The solubility of benzyl alcohol in micellar solutions of sodium octanoate and sodium perfluorooctanoate was studied. From the isotherms of specific conductivity versus molality at different alcohol concentrations, the critical micelle concentration and the degree of ionization of the micelles were determined. The cmc linearly decreases upon increasing the amount of benzyl alcohol present in aqueous solutions with two distinct slopes. This phenomenon was interpreted as a clustering of alcohol molecules above a critical point, around 0.1 mol kg-1. Attending to the equivalent conductivity versus square root of molality, the presence of a second micellar structure for the fluorinated compound was assumed. The thermodynamic parameters associated with the process of micellization were estimated by applying Motomura’s model for binary surfactant mixtures, modified by Pe´rez-Villar et al. (Colloid Polym. Sci 1990, 268, 965) for the case of alcohol-surfactant solutions. A comparison of the hydrogenated and fluorinated compounds was carried out and discussed.
1. Introduction The behavior of alcohols in water is characterized by their low solubility. As expected, this solubility decreases with a rise in hydrophobicity, making them unable to form micelles because their solubility in water is lower than the critical micelle concentration (cmc). Despite this fact, alcohols can be used as “cosurfactants” to form microemulsions in combination with surfactants and oil. Studies of alcohol-surfactant interactions have become more important due to the potential use of these systems in different applications such as tertiary oil recovery,1 surfactant-base separation processes,2 and incorporating alcohols in biomolecular micellar-enhanced ionic reactions.3-5 Relevant contributions to alcohol-surfactant interactions have been made by different authors, starting with the studies initiated by Ward.6 A recent review was presented by Zana,7 and relevant monographic chapters on alcohol solubilization in surfactant aggregates are found in the literature.8 A short survey of the available literature has been reported in a previous paper.9 The introduction of alcohols into the micelles produces noticeable changes in micellar shape, given sphere-to-rod transitions.10-13 A distinct behavior has been found, depending on the length of the hydrocarbon alcohol chain * Corresponding author. Phone: +34 981 563 100. Fax: +34 981 520 676. E-mail:
[email protected]. (1) Garcı´a-Sa´nchez, F.; Eloisa-Jimenez, G.; Salas-Padro´n, A.; Herna´ndez-Garduzza, O.; Apam-Martı´nez, D. Chem. Eng. J. 2001, 84, 257. (2) In Surfactant-Based Separation Processes; Schameron, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989. (3) Bunton, C. A.; De Buzzaccarini, F. J. Phys. Chem. 1982, 86, 5010. (4) Bunton, C. A.; Nome, F.; Quina, F. M.; Romsted, L. S. Chem. Res. 1991, 24, 357. (5) Rubio, D. A.; Zanette, D.; Nome, F.; Bunton, C. A. Langmuir 1994, 10, 1155. (6) Ward, A. F. H. Proc. R. Soc. A 1940, 176, 412. (7) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (8) Solubilization in Surfactant Aggregates; Christian, S. D., Schamehorn, J. F., Eds.; Surfactant Science Series 55; Marcel Dekker: New York, 1995. (9) Gonza´lez-Pe´rez, A.; Czapkiewicz, J.; Del Castillo, J. L.; Rodrı´guez, J. R. J. Colloid Interface Sci. 2003, 262, 525.
and, hence, on the hydrophobicity of the alcohol. As previously shown, short- and medium-chain alcohols affect micellization through changes in the solvent. In fact, they act as cosolvents, affecting the chemical potential of the free surfactant. Short-chain alcohol-water systems are better solvents for surfactants than pure water, and the micelles are formed at higher concentrations of surfactants.7 Meanwhile, long-chain alcohols solubilize in the palisade layer of the surfactant micelle.14 The solubilization of alcohols at the micelle surface reduces the surface charge density. This effect is evident from the increase in the degree of ionization of the micelles.15,16 At higher additive concentrations, alcohol-based microstructures, stabilized by unassociated surfactant units, appear.17 The interactions of alcohols with fluorinated surfactants have received less attention than have alcohols with hydrogenated surfactants. According to the studies of Gerry et al.,18 the solubility of additives in perfluoro surfactants is lower than that in the corresponding hydrogenated ones. The solubilization of different alcohols in sodium perfluorooctanoate micelles has been investigated by Calfords and Stilbs using NMR.19 Preliminary data on the effect of hydrogenated and fluorinated alcohols on micellar systems has been investigated by Muto et al.20 Of special interest are the works of Milioto et al.21 on (10) Nguyen, D.; Bertrand, G. L. J. Phys. Chem. 1991, 96, 1994. (11) Staphany, S. M.; Kole, T. M.; Fisco, M. R. J. Phys. Chem. 1994, 98, 11126. (12) Forland, G. M.; Samseth, J.; Gjerde, M. I.; Høiland, H.; Jansen, A. O.; Mortensen, K. J. Colloid Interface Sci. 1998, 203, 328. (13) Preu, H.; Schirmer, C.; Tomsic, M.; Rogac, M. B.; Jamnik, A.; Belloni, L.; Kunz, W. J. Phys. Chem. B 2003, 107, 13862. (14) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1984, 101, 587. (15) Jain, A.; Singh, R. P. B. J. Colloid Interface Sci. 1981, 81, 536. (16) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (17) De Lisi, R.; Milioto, S.; Inglese, A. J. Phys. Chem. 1991, 95, 3322. (18) Gerry, H. E.; Jacobs, P. T.; Anacker, E. W. J. Colloid Interface Sci. 1977, 62, 556. (19) Carlfors, J.; Stilbs, P. J. Colloid Interface Sci. 1985, 103, 332. (20) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Meguro, K.; BinanaLimbele, W.; Zana, R. J. Colloid Interface Sci. 1989, 130, 165. (21) Milioto, S.; De Lisi, R. Langmuir 1994, 10, 1377.
10.1021/la049226w CCC: $27.50 © 2004 American Chemical Society Published on Web 08/26/2004
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the enthalpies of mixing some primary hydrogenated and fluorinated alcohols with sodium perfluorooctanoate. The apparent molar volumes of alcohols in sodium perfluorooctanoate and sodium octanoate have also been studied by Milioto el al.22 In previous papers we investigated the interaction of benzyl alcohol (BzOH) with cationic surfactants.9,23 Two distinct behaviors were found upon decreasing the critical micelle concentration (cmc). A characteristic break appeared, which is related to the alcohol dependence of the cmc, and suggests that the clustering of alcohol molecules is responsible for this unexpected behavior. The hydrophobicity of BzOH appears to be comparable to that of pentanol if one takes into account the data reported by Attwood et al.24 for the transfer of pentanol into the micelles of dodecyl-, tetradecyl-, and hexadecyl-trimethylammonium bromides. The hydrophobicity of the phenyl group thus corresponds to that of four methylene groups. To expand our knowledge of this special compound and continue our research on its interactions with anionic surfactants, we studied the effect of alcohol addition on the micellization of fluorinated and hydrogenated anionic surfactants for a range of alcohol molalities below 0.25 mol kg-1 at 25 °C. The results obtained from conductivity measurements have been used to apply Motomura’s theory to obtain the additional thermodynamic parameters related to the solubilization of benzyl alcohols into micelles. For the last 20 years the interaction of benzyl alcohol has attracted the attention of scientists due to its special behavior in its interactions with surfactants. BzOH is an uncharged local anesthetic which has much higher surface activity then the charged compound, due to the disappearance of the electrostatic interactions among the polar headgroups, characteristic of the charged ones.25 The interaction of aromatic alcohols with cetyltrimethylammonium bromide micelles was studied by AbuHamdiyyah et al.26 and Lissi et al.27 Relevant contributions have been made by Treiner et al.28-31 on the partition coefficients of different alcohols in surfactant solutions. The distribution coefficients of solutes in micellar systems was determined by Gao et al. using NMR.32 From NMR studies Shih et al.33 concluded that benzyl alcohol is probably not located at the double-layer palisade. In papers by Pons et al.34 and Bury et al.35 it is suggested that solubilization of BzOH in cationic surfactant solutions is specifically favored by intramolecular interactions between alcohol molecules and cationic micelles. (22) Milioto, S.; Crisantino, R.; De Lisi, R.; Inglese, A. Langmuir 1995, 11, 718. (23) Gonza´lez-Pe´rez, A.; Gala´n, J. J.; Rodrı´guez, J. R. J. Therm. Anal. Cal. 2003, 72, 471. (24) Attwood, D.; Mosquera, V.; Rodrı´guez, J.; Garcı´a, M.; Suarez, M. J. Colloid Polym. Sci. 1994, 272, 584. (25) Matsuki, H.; Shimada, K.; Kanashina, S.; Kamaya, H.; Ueda, I. Colloids Surf., B Biointerfaces 1998, 11, 287. (26) Abu-hamdiyyah, M. J. Phys. Chem. 1986, 90, 1345. (27) Lissi, E.; Abuin, E.; Rocha, A. M. J. Phys. Chem. 1980, 84, 2406. (28) Treiner, C.; Mannebach, M.-H. J. Colloid Interface Sci. 1987, 118, 243. (29) Treiner, C.; Chattopadhyay, A. K. Bury, R. J. Colloid Interface Sci. 1985, 104, 569. (30) Treiner, C.; Chattopadhyay, A. K. J. Colloid Interface Sci. 1984, 98, 447. (31) Treiner, C.; Chattopadhyay, A. K. J. Colloid Interface Sci. 1986, 109, 101. (32) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Phys. Chem. 1989, 93, 2190. (33) Shih, L. B.; Williams, R. W. J. Phys. Chem. 1986, 90, 1615. (34) Pons, R.; Bury, R.; Erra, P.; Treiner, C. Colloid Polym. Sci. 1991, 269, 62. (35) Bury, R.; Shouhalia, E.; Treiner, C. J. Phys. Chem. 1991, 95, 3824.
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2. Experimental Section 2.1. Materials. The surfactants sodium perfluorooctanoate (SPFO) and sodium octanoate (SO) were obtained from Lancaster, both with a purity of >97%. Benzyl alcohol was from Fluka with a purity of >99%. Distilled water with conductivity below 3 µS cm-1 at 25 °C was used in all samples. Surfactant and alcohol solutions were prepared by weight. 2.2. Methods. The molality dependence of conductivity was obtained by continuous dilution of a concentrated solution of the surfactant studied. Alcohol solutions used for measurements were previously prepared by dissolving the appropriate amount of alcohol in water. Conductivities were measured by using a model CM-117 Kyoto Electronics Conductometer with a model K-121 cell. The cell constant was determined with KCl solutions according to the procedure of Monk.36 The estimation of the distribution of benzyl alcohol between the aqueous and micellar phases was determined by applying the thermodynamic treatment of binary micellar systems proposed by Motomura et al.37 This theory was used to determine the distribution characteristics of a solubilizate between micellar and aqueous phases from the variation of the critical micelle concentration on the butanol/sodium dodecyl sulfate/water system by Perez-Villar et al.38 Attwood et al.24 used the same procedure to study the homologous alkyltrimethylammonium bromides for different linear alcohols, and Castedo et al.39 applied the procedure for the temperature dependence of the system tetradecyltrimethylammonium bromide-butanol-water. More recently, Gonza´lez-Pe´rez et al.9,23 studied the application of the Motomura’s model for different cationic/alcohol/water systems. 2.3. Theoretical Background. The phase-separation model considers that above the cmc there are two well-defined phases, the micellar phase and the aqueous phase. Considering that the difference in the extensive thermodynamic magnitudes of the aqueous solution before and after the cmc is due exclusively to micelle formation, the Gibbs-Duhem equation, used to describe the thermodynamic behavior of the mixed micelle, can be obtained
RT (R X M + R2X2M) dm1 ) m1 1 1
(
∆SM dT + ∆VM dP R1
)
X1M X2M + R2 dX2 (1) X1 X2
where the subscripts 1 and 2 represent the two kinds of surfactants, XiM is the molar fraction of the surfactant i in the mixed micelle, and Xi is the molar fraction of surfactant i in the total surfactant content. Ri is the number of ions in which the surfactant dissociates, ∆SM is the molar entropy of micellization, and ∆VM is the molar volume of micellization. In the case of one surfactant and one alcohol, we can change the nomenclature and rewrite the equation, expressed now in molality
{ [ ]( ) } [ ( )] ( )
Rs ∂m1 RsRa 1 ms µ ∂ma T,P µ XaM ) Rs ∂ms Ra Ra - Rs ∂ms Ra + Rs + µ ∂ma T,P ms ∂ma T,P ma Rs
{
}
(2)
where Ra ) 1 and Rs ) 2 for the alcohol and the ionic surfactant, respectively. ma and ms are the molalities of the alcohol and surfactant correspondingly, and µ ) Rsms + Rama + (1000/Mw), Mw being the molecular mass of water. To obtain more information about the stability of our system when the alcohol is solubilized into the micelle, the standard (36) Monk, C. B. Electrolytic Dissociation; Academic Press: London, 1961. (37) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984, 262, 948. (38) Pe´rez-Villar, V.; Mosquera, V.; Garcia, M.; Rey, C.; Attwood, D. Colloid Polym. Sci. 1990, 268, 965. (39) Castedo, A.; Del Castillo, J. L.; Sua´rez-Filloy, M. J.; Rodrı´guez, J. R. J. Colloid Interface Sci. 1997, 196, 148.
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Figure 2. Gradients of slopes below, S1 (squares), and above, S2 (circles), the cmc for (a) sodium perfluorooctanoate and (b) sodium octanoate as a function of molality of added benzyl alcohol at 25 °C.
Figure 1. (a) Specific conductivity, κ, versus molality, m, and (b) equivalent conductivity, Λ, vs square root of molality of sodium perfluorooctanoate solutions at 25 °C: (9) in water, (O) in 0.0638 mol kg-1 benzyl alcohol solution. free energy of solubilization, ∆GS°, can be calculated from the relationship
∆GS° ) -RT ln
XaM Xa
(3)
where Xa is the molar fraction of the alcohol in the system and XaM is the mole fraction of the alcohol into the micelle. A more detailed explanation of Motomura’s theory and the modification reported by Pe´rez-Villar et al. with some examples of their application can be seen in refs 9, 23, 24, and 37-39.
3. Results and Discussion The effect of benzyl alcohol on micellar behavior in both sodium octanoate (SO) and sodium perfluorooctanoate (SPFO) was studied by measuring the molality dependence of conductivity at 25 °C. Different concentrations of benzyl alcohol in water were prepared to study the variation of the critical micelle concentration. The alcohol molalities studied varied from 0 to 0.24 mol kg-1 at 25 °C. Figure 1 shows a sample of the molality dependence of (a) specific conductivity and (b) equivalent conductivity of SPFO in water and 0.0638 mol kg-1 of BzOH at 25 °C. The cmc values were estimated from the inflections in the plots of the specific conductivity, κ, against surfactant molality, m, of aqueous solutions of the surfactants in the presence of added BzOH of molality, ma, at constant temperature. From the break seen in Figure 1a, two linear fragments with well-defined slopes, S1, and S2, in the premicellar and postmicellar range, respectively, were obtained. The limit of applicability of this method for estimating the
cmc is characterized by the alcohol molality at which the two slopes are equal, and hence, there is no intersection point for both straight lines. Figure 2 shows characteristic slopes above, S2, and below, S1, the cmc for SPFO and SO at 25 °C as a function of the molality of BzOH. A linear dependence of the slopes against alcohol molality was found. Taking different sets of values on the conductivity versus molality plots, the uncertainties of both S1 and S2 were approximately (1. Both slopes, above and below the cmc, intercept at an estimated value of 0.5604 mol kg-1 for SO and 0.5482 mol kg-1 for SPFO. This result suggests the ability of the micelles to solubilize alcohol into their structures is comparable for both surfactants. In our case, a surfactant range up to 0.24 mol kg-1 and a narrow alcohol molality range were used to minimize possible errors in the cmc determination as a consequence of two very similar slopes S1 and S2. Moreover, it was necessary to work in the limit of solubility of BzOH in water.25 The fluorinated surfactant showed the highest slopes. The ratio of increase in the slope by changing the fluorinated by the hydrogenated chain on the surfactant did not seem to be affected by the amount of alcohol added. The increment in S1 and S2 by the substitution of the fluorine by hydrogen in the alkyl chain gave values of ∼19.06 and 11.78 mS cm-1 mol-1 kg for SPFO and SO, respectively. This increase was only dependent on the nature of the surfactant but also did not seem to be affected by the addition of alcohol. This suggests that the intrinsic peculiarities of the surfactant remain constant even with the addition of alcohol and the differences are only due to the nature of the surfactant. Figure 3 illustrates the cmc/cmco ratio for the surfactants studied as a function of molality of BzOH at 25 °C, cmco being the value of the estimated critical micelle concentration in pure water. The values of cmco in pure water at 25 °C were 0.36083 and 0.03067 mol kg-1 for SO and SPFO, respectively. Both values are in good agreement with previously reported results.40 The breaks observed in Figure 3, around 0.11 mol kg-1, define two regions with distinct trends. In the first region (I) the cmc decreases with the rise in alcohol concentration, suggesting that the BzOH appears to influence the hydrogenated surfactant slightly more. This ability to decrease the cmc was also observed for linear alcohols with longer hydrocarbon (40) Gonza´lez-Pe´rez, A.; Prieto, G.; Ruso, J. M.; Sarmiento, F. Mol. Phys. 2003, 101, 3185.
Fluorinated and Hydrogenated Sodium Carboxylates
Figure 3. Ratio of critical micelle concentration in benzyl alcohol to critical micelle concentration in water, cmc/cmco, as a function of molal concentration of alcohol, ma, at 25 °C for (b) sodium perfluorooctanoate and (9) sodium octanoate (dashed line indicates the two regions defined by the break).
chains as a consequence of the incorporation of alcohol molecules in the micellar structure. The alcohol acts as a cosurfactant, facilitating the formation of micelles. In our case this effect seemed to be more favorable for the hydrogenated compound. In region II a dramatic change in the slope, more pronounced in the case of the hydrogenated compound, is observed. This behavior was previously reported for cationic surfactants by Treiner and coworkers31,35 and more recently by Gonza´lez-Pe´rez et al.9,23 The effect on the cmc was explained by assuming the aggregation of benzyl alcohol in the aqueous phase on the basis of the findings of Zana and postulation of other authors.7 These authors confirm the presence of aggregates in propanol and ethanol water solutions at certain alcohol concentrations. In the second region the cmc depression line of the fluorinated and hydrogenated surfactants gives an intersection point (indicated by an arrow). This point suggests that at ∼0.175 mol kg-1 the same cmc/cmco value is reached by the two surfactants. As previously found, for the same chain length of surfactant and different headgroups the trend of the cmc/cmco against the concentration of alcohol differs only in a constant. In fact, we found parallel lines depending on the headgroup. The present unexpected result gives different slopes and will be discussed in the next section. Using the data of the slopes above and below the cmc, the degree of ionization of the micelles can be estimated. If the contribution of the monomers and the micelles to conductivity is small compared with the contribution of the counterions, the method suggested by Hoffman41 can be applied. The degree of ionization of the micelles, β, is then the ratio between the slopes above and below the cmc. Figure 4 shows the degree of ionization of the micelles as a function of alcohol molality for SO and SPFO at 25 °C. An increase in the degree of ionization of the micelles from ∼0.50 to 0.75 is observed, suggesting that the micelle becomes less neutral and, therefore, a decrease in the number of attached counterions is promoted by the incorporation of the BzOH molecules to the micellar structure. The increase in the degree of ionization of the micelles upon addition of alcohols is widely reported.42 It (41) Hoffmann, H.; Ulbright, W. Z. Phys. Chem. N. F. 1977, 106, 167. (42) Del Castillo, J. L.; Sua´rez-Filloy, M. J.; Castedo, A.; Svitova, T.; Rodrı´guez, J. R. J. Phys. Chem. B 1997, 101, 2782.
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Figure 4. Degree of ionization of the micelles, β, as a function of molal concentration of added benzyl at 25 °C for (b) sodium perfluorooctanoate and (9) sodium octanoate.
was shown previously by Gao et al.32 and Lissi et al.27 that the phenyl groups appear to be located at the micellar interface. This finding agrees with previous results reported by Gonza´lez-Pe´rez et al.9 for tetradecyltrimethylammonium bromide and tetradecyldimethylbenzylammonium chloride in benzyl alcohol-water solutions. Their results show a stronger effect on the increase in the degree of ionization of the micelles in the case of benzyl alcohol than for butyl alcohol. They found that to reach the same degree of ionization reported with benzyl alcohol, approximately twice as much butyl alcohol was needed. In our case the small discrepancies in the degree of ionization of the micelles can be attributed to the special characteristics of the fluorinated surfactant. Also, it was observed that the linear behavior of the hydrogenated surfactant cannot be found in the fluorinated one. In fact, it appears that the linearity of β versus µa vanishes at alcohol concentrations above 0.16 mol kg-1. Above this concentration the degree of ionization of the micelles has a polynomial growth and decreases in comparison with the linear behavior expected. To explore the second region II and try to explain the unexpected behavior observed in Figures 3 and 4, the isotherms of conductivity in this region were studied. Figure 5 represents the equivalent concentration versus the square root of molality of SPFO and SO at 25 °C for different alcohol concentrations in region II. Linear behavior below the cmc and the classical decrease above the cmc can be observed. In the case of SO, the decrease is similar for the alcohol concentration range studied. In the SPFO compound a new break in the decreasing region was observed. This behavior suggests the presence of a second structure of the micelle and could indicate the presence of a micellar transition in the SPFO-BzOHwater system. This hypothesis is in accordance with the previous results observed in Figures 3 and 4. The alcohol concentration at which the cmc/cmco relationship is the same for both surfactants represents the point at which the second structure of the micelles starts to be more relevant. This can explain the different slopes observed in the case of the fluorinated surfactant. This behavior is accompanied by a deviation from the degree of ionization of the micelles, when the value is lower than the linear one expected, as seen in Figure 4. The decrease in the degree of ionization of the micelles is typically observed at the second micelle concentration.
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Figure 5. Equivalent conductivity, Λ, as a function of square root of molal concentration for (a) sodium perfluorooctanoate and (b) sodium octanoate at 25 °C in benzyl alcohol solutions in region (II) (see text).
It is well known that the addition of electrolytes44-46 or some nonelectrolytes, especially alcohols,29,47-49 can induce structural transitions in micellar solutions of surfactants at concentrations higher than about 2 orders of magnitude of the cmc. However, micellar structural transitions can be obtained also by increasing the surfactant concentrations in the absence of any additional additive.50-53 This transition is called second critical micelle concentration and can be detected by a variety of physical techniques. Recently, it was found that the logarithm of this second cmc followed a linear relationship with the alkyl chain in alkyltrimethylammonium bromide homologues,53 giving a similar behavior to the well-known Stauff-Klevens rule for the critical micelle concentration.54,55 Just as the cmc decreases by the addition of alcohols with long and medium alkyl chain, a comparable effect on the second cmc is expected. This effect must be more evident with the increase in the long-chain surfactants due to their sharper break shown by different techniques in the cmc and also in the second cmc. This fact can explain the presence in this transition of the fluorinated compound and not of the hydrogenated one. The more hydrophobic moiety of the fluorinated surfactant that is comparable with a sodium dodecanoate can induce the transition more easily. The presence of two kinds of micelles can explain the different slopes, comparing with the hydrogenated surfactant, as shown in region II in Figure 3. For more information on the characteristics of the micellar composition, the thermodynamic treatment suggested by Perez-Villar38 was used. To apply eq 2 the derivative, (∂ms/∂ma)T,P must be known. To determine (43) Swanson-Vethamuthu, M.; Feitosa, E.; Brown, W. Langmuir 1998, 14, 1590. (44) Pisa´rcik, M.; Devı´nsky, F.; Svajdlenka, E. Colloids Surf., A: Physicochem. Eng. Asp. 1996, 119, 115. (45) Zielinski, R. Pol. J. Chem. 1998, 72, 127. (46) Miyagishi, S. Bull. Chem. Soc. Jpn. 1976, 49, 34. (47) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1984, 101, 587. (48) Quirion, F.; Desnoyers, J. E. J. Colloid Interface Sci. 1987, 115, 176. (49) Treiner, C.; Makayssi, A. Langmuir 1992, 8, 794. (50) Chung, M.,III; Tak, I. J.; Lee, K. E. Taehan Hwahak Hoechi 1975, 19, 398. (51) Hoffmann, H.; Rehage, H.; Plantz, G.; Schorr, W.; Thurn, H.; Ulbricht, W. Colloid Polym. Sci. 1982, 260, 1042. (52) De Lisi, R.; Fisicaro, E.; Milioto, S. J. Sol. Chem. 1988, 17, 1015. (53) Gonza´lez-Pe´rez, A.; Czapkiewicz, J.; Prieto, G.; Rodrı´guez, J. R. Colloid Polym. Sci. 2003, 281, 1191. (54) Stauff, J. Z. Phys. Chem., A 1938, 183, 55. (55) Klevens, H. B J. Am. Chem. Soc. 1953, 30, 74.
Gonza´ lez-Pe´ rez et al.
Figure 6. Mole fraction, XaM, of benzyl alcohol in micelles as a function of molality of added alcohol ma, at 25 °C for (b) sodium perfluorooctanoate and (0) sodium octanoate.
this derivative the fitted values of both linear fragments of the cmc versus alcohol concentration dependence were used. The molar fraction of alcohol in the micelle against the molality of alcohol in the aqueous phase for SPFO and SO systems is shown in Figure 6. A maximum is observed in both curves. This behavior has been previously detected with other surfactants and suggests that this effect can be treated as a salting-out process.9 In our case a small shift was found in the dependence of both systems, indicating that the solubilization behavior is similar and has little dependence on the hydro- or fluorocarbon chain. The depression observed after the maximum is attributed to the reduction in the number of available alcohol molecules in the solution as a consequence of the clustering of these. The observed final linear increase is a result of the increase in the amount of alcohol molecules in the solution. The higher amount of alcohol molecules found in the fluorinated surfactant is consistent with the suggestion that a large type of micelles is present in the solution, and therefore, additional space is available to dissolve more alcohol molecules. We are now interested in obtaining more information about the stability of our systems when the alcohol is solubilized into the micelle. This information can be found by determining the standard free energy of solubilization by applying eq 3. The results are shown in Figure 7 for SO and SPFO. Two differentiated regions were detected. In the first region I, when the alcohol induces a decrease in the cmc, the expected increase in free energy of solubilization was found. This result indicates a small difference between the hydro- and fluorosurfactant. This fact suggests that the solubilization of alcohols into the micelles is slightly dependent on the type of surfactant and the change of the hydrogenated counterpart by a fluorinated one induces only a small facility to dissolve the alcohol into the micelle. The estimated difference in part a of Figure 7 is -0.291 kJ mol-1. In region II the difference in the free energy of solubilization between the hydro- and fluorocompound is notable. In this case the solubility is promoted by the use of the fluorinated surfactant and the free energy of solubilization decreases by -1.989 kJ mol-1, as seen in zone b. This effect is due to the presence of different types of micelles in region II with fluorinated surfactant. The hydrophobic nature of the fluorocompound, which is comparable to a hydrogenated one with 12 carbon atoms in the chain, induces changes in the structure of the micelles. These structural
Fluorinated and Hydrogenated Sodium Carboxylates
Figure 7. Standard Gibbs free energy of solubilization of benzyl alcohol in the micelle as a function of alcohol molality in the system: (b) sodium perfluorooctanoate and (O) sodium octanoate.
changes do not appear for the hydrogenated surfactant at this amount of alcohol. The temperature dependence of a second critical micelle concentration was recently reported for SO by Gonza´lez-Pe´rez et al.56 Probably the second cmc of the hydrogenated surfactant decreases also by the increase in the amount of alcohol in the system but outside of the range studied. This effect will be investigated in the future depending on the amount of alcohol and taking into account the temperature of the system. 4. Summary and Conclusions SPFO-BzOH and SO-BzOH systems have been studied at 25 °C over a range of alcohol concentrations from 0 to 0.24 mol kg-1. It was found that the cmc decreases with the alcohol concentrations and exhibits a characteristic break at alcohol concentrations close to 0.1 mol kg-1. This fact was previously explained as the possible clustering of alcohol molecules. This clustering reduces the effective amount of alcohol molecules available in the aqueous medium, and therefore, the cooperative interaction between the alcohol and the surfactant to form micelles is shifted to lower concentrations and the cmc is (56) Gonza´lez-Pe´rez, A.; Ruso, J. M.; Prieto, G.; Sarmiento, F. Langmuir 2004, 20, 2512.
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effectively higher than that expected. Additionally, a more detailed study of the conductivity isotherms on the form of equivalent conductivity against the square root of molality indicates the presence of two kinds of aggregates above the characteristic break observed at an alcohol concentration of 0.1 mol kg-1. This result seems to be clear for SPFO, but no evidence of such behavior was observed for SO. This is in concordance with the idea that the inclusion of alcohol into the micelles as cosurfactant reduces not only the cmc but also the second cmc. Attending to this fact, the anomalous behavior of the fluorinated surfactant system was explained. The different slopes in the cmc/cmc0 versus alcohol molality and the deviation from the usual linearity in the β versus alcohol molality was considered as a consequence of the cooperative effect of clustering of alcohol molecules and the presence of two kinds of molecules in the fluorinated surfactant system. The mole fraction of alcohol into the micelles was estimated by applying the thermodynamic model suggested by Motomura et al.37 and modified appropriately by Pe´rez-Villar et al.38 to study alcohol solubilization. The results show an increase in the amount of alcohol into micelles with the increase in alcohol in the system. A maximum corresponding to the inflection point reached at 0.1 mol kg-1 was observed. The formation of clusters of alcohol reduces the mole fraction of alcohol into micelle. Also, it was observed that raising the molality of the alcohol the amount of this into micelles increases once more. In this region the behavior is strongly differentiated depending on the nature of the surfactant, and the discrepancies have been explained by the presence of a second type of micelle in the system with the fluorinated surfactant. Attending to the free energy of solubilization of alcohol into micelles, we found that in the first region the solubilization is easier in the hydrogenated surfactant and becomes more difficult with the increase in the amount of alcohol in the system. Above the critical point, the second region, the solubilization is promoted by the presence of two kinds of micelles in the fluorinated system. Acknowledgment. This research was funded by the Spanish Ministry of Science and Technology (Project MAT2002-00608, European FEDER support included) and Xunta de Galicia (Project PGIDIT03PXIC20615PN). LA049226W
