Langmuir 2008, 24, 15-18
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Solubilization of n-Alkylbenzenes into Decanoyl-N-methylglucamide (Mega-10) Solution Shohei Nakamura,*,† Lisa Kobayashi,† Ryo Tanaka,† Teruyo Isoda-Yamashita,† Konomi Motomura,‡ and Yoshikiyo Moroi*,†,‡ Daiichi College of Pharmaceutical Sciences, 22-1 Tamagawamachi, Minami-ku, Fukuoka 815-8511, Japan and Department of Chemistry, Faculty of Science, Kyushu UniVersity, 4-2-1 Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan ReceiVed September 11, 2007. In Final Form: NoVember 14, 2007 Solubilization of benzene, toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, n-pentylbenzene, and n-hexylbenzene into micelles of decanoyl-N-methylglucamide (Mega-10) was studied, where equilibrium concentrations of the above solubilizates were determined spectrophotometrically at 303.2 K. The concentration of the above solubilizates remained constant below the critical micelle concentration (cmc) and increased linearly with an increase in Mega-10 concentration above the cmc. The Gibbs free energy change of the solubilizates from the aqueous bulk to the liquid solubilizate phase was evaluated from the dependence of their aqueous solubility on the alkyl chain length of the 0 solubilizates, which leads to -3.46 kJ mol-1 for ∆GCH , the energy change per CH2 group of the alkyl chain. The 2 first stepwise solubilization constant (K1) was evaluated from the slope of the change of solubilizate concentration versus Mega-10 concentration. The Gibbs free energy change (∆G0,s) for the solubilization decreased linearly with 0,s the carbon number of the alkyl chain of the solubilizates, from which ∆GCH was evaluated to be -2.71 kJ mol-1. 2 The similar values above clearly indicate that the location of the alkyl chain is a hydrophobic micellar core, which is also supported by the absorption spectrum of the solubilized molecules.
Introduction The aqueous solubility of sparingly soluble organic substances can be increased by the addition of an appropriate amphiphile into the system, resulting in a thermodynamically stable isotropic solution. The phenomenon is called solubilization.1-4 Indeed, solubilization has been applied to various scientific and chemical engineering fields such as the dissolution of drugs into aqueous solution and their transport through a living body, the dissolution of aromatic compounds into cosmetics, the preparation of agricultural chemical solutions, the recovery of oil, and so on. However, the study of solubilization based upon the phase diagram for three-component systems for nonionic surfactants is quite common.3 Unfortunately, there have been few solubilization studies that regard a micelle as a chemical species, although there are many based upon the partitioning of solubilizate molecules between micelles and the intermicellar bulk. Nonionic surfactants have no charge, which brings about such interesting properties as low critical micelle concentration (cmc) and large molecular aggregates compared with those of ionic surfactants.5 One of the well-known groups of nonionic surfactants is polyoxyethylene alkyl ethers (CnEm),6-8 which have been * Corresponding authors. E-mail:
[email protected],
[email protected]. † Daiichi College of Pharmaceutical Sciences. ‡ Kyushu University. (1) McBain, M. L. E.; Hutchinson, E. Solubilization and Related Phenomena; Academic Press: New York, 1955. (2) Elworthy, P. H.; Florence, A. T.; Macfarlane, C. B. Solubilization by Surface ActiVe Agents and Its Application in Chemistry and Biological Sciences; Chapman & Hall: London, 1968. (3) Mackay, R. A. In Nonionic Surfactants, Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; Chapter 6, p 297. (4) Moroi, Y. Micelles: Theoretical and Applied Aspects; Plenum Press: New York, 1992; Chapter 9. (5) Muhammad, R.; Moroi, Y.; Hlaing, T.; Matsuoka, K. Bull. Chem. Soc. Jpn. 2005, 78, 604. (6) Kahlweit, M.; Busse, G. J. Chem. Phys. 1993, 99, 5605. (7) Tamura, T.; Takeuchi, Y.; Kaneko, Y. J. Colloid Interface Sci. 1998, 206, 112. (8) Yeh, M. C.; Chen, L. J. J. Chem. Phys. 2001, 115, 8575.
extensively studied because of the advantage of their practical use.9,10 In fact, fundamental studies on the physicochemical properties of surfactants of this type are ongoing from several physicochemical aspects.11,12 However, recent reports on solubilization into CnEm micelles are rare,13 whereas that into other nonionic micelles is common.13,14 Therefore, the systematic solubilization study was carried out for a C14E8 solution, where the alkylbenzenes were used for solubilizates.15 What was made clear in the study was that benzene molecules were not concentrated or solubilized in the C14E8 micelles whereas other alkylbenzenes were solubilized into the micelles with the alkyl chain located in the hydrophobic inner micelle. This study aimed to determine whether other nonionic micelles can solubilize benzene molecule and to find out why the C14E8 micelle cannot solubilize it, using Mega-10 with definite micellar properties as nonionic surfactant and the same alkylbenzenes as the solubilizate. Experimental Section Materials. Decanoyl-N-methylglucamide (Mega-10) was purchased from Kishida Chemical Co. Mega-10 is a surfactant that desolubilizes protein from a living membrane, and the cmc is reported to be 7 mmol dm-3 on the label. The cmc of the original Mega-10 was determined to be 5.5 mmol dm-3 by surface tension measurement, and no minimum was found for surface tension versus concentration plots. Mega-10 was used in this study without further purification. All alkylbenzenes are the same as used in the previous study.15 The water used was distilled once after deionization. Solubilization. The solubilization apparatus and the solubilization method are the same as used in the previous study.15 Nine surfactant (9) Bruillete, D.; Perron, G.; Desnoyers, J. E. J. Solution Chem. 1998, 27, 151. (10) Berthod, A.; Tomer, S.; Dorsey, J. G. Talanta 2001, 55, 69. (11) Grell, E.; Lewitzki, E.; von Raumer, M.; Hormann, A. J. Therm. Anal. Calorim. 1999, 57, 371. (12) Islam, M. N.; Kato, T. Langmuir 2003, 19, 7201. (13) Xiarchos, I.; Doulia, D. J. Hazard. Mater. 2006, 136, 882. (14) Mitra, S.; Dungan, S. R. J. Agric. Food Chem. 2001, 49, 384. (15) Sato, Y.; Nakahara, H.; Moroi, Y.; Shibata, O. Langmuir 2007, 23, 7505.
10.1021/la702820h CCC: $40.75 © 2008 American Chemical Society Published on Web 12/06/2007
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Figure 1. Solubilization apparatus for volatile solubilizates. solutions, five solutions below the cmc and four solutions above the cmc, were poured separately into nine photocells, which were set in the solubilization apparatus (Figure 1). A certain volume of benzene and toluene was placed on the bottom of a small vessel in the middle of the glass apparatus in order for absorbance to be less than 1.5. As for other alkylbenzenes, their liquid needed for more than the maximum additive concentration (MAC) was placed inside the middle vessel. The whole glass apparatus with its cover was kept inside the air thermostat controlled at 303.2 ( 0.1 K to reach complete solubilization equilibrium, where the chemical potential of the solubilizate molecule becomes equal throughout whole system via the gas phase. After equilibration, the solubilizate concentrations in the surfactant solutions were determined spectrophotometrically by the optical density of the solutions (Shimadzu UV-2100) and the molar extinction coefficient (); the values at 260 nm were 232, 216, 201, 211, 222, and 177 mol-1 dm3 cm-1 for toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, n-pentylbenzene, and n-hexylbenzene, respectively,16,17 and the value at 254 nm was 188 mol-1 dm3 cm-1 for benzene.18
Results and Discussion The chemical potential of the monomeric solubilizate in the aqueous bulk is equal to that in its liquid phase. The Gibbs energy change (∆G0) of the monomeric solubilizate from the aqueous bulk to the liquid phase can be obtained from the aqueous solubility as 0 ∆G0 ) µl,0 R - µR ) RT ln C
(1)
Figure 2. Gibbs energy change for the transfer of solubilizate molecules from an aqueous environment to the corresponding liquid solubilizate at 303.2 K, where the solubility of n-hexylbenzene is extrapolated from the solubilities of other n-alkylbenzenes.
solubilizate should become the same throughout the phases under equilibrium or the concentration of the monomeric solubilizate can be identical in the nine solutions. This is quite useful for developing thermodynamics for solubilization into micelles. The stepwise association or solubilization equilibria between micelles (M) and solubilizates (R) can be represented schematically as follows K1
0 where µl,0 R is the chemical potential of liquid solubilizate R, µR is the standard chemical potential at infinite dilution, and the activity is replaced by the aqueous solubility C because of very low solubility. Figure 2 illustrates the change in ∆G0 with the number of carbon atoms in the alkyl chain for the solubilizates, which gives 0 good linearity between them. The ∆GCH value thus obtained 2 from the slope was found to be -3.46 kJ mol-1, which clearly indicates the transfer of the CH2 group from the aqueous bulk to a hydrophobic environment.19 This linearity was employed to evaluate the monomeric solubility of n-hexylbenzene for the succeeding solubilization study. Inside the apparatus, the volatile solubilizates (n-alkylbenzenes) can easily evaporate because of their high volatility, and thus the chemical potential of the gaseous solubilizate molecule for each
M + R 798 MR1 K2
MR1 + R 798 MR2 ..................................... Km
MRm - 1 + R798 MRm
where MRi represents the micelles associated with i molecules of solubilizate, Ki is the stepwise association constant between MRi - 1 and a monomer molecule of solubilizate, and m is the maximum number of solubilizate molecules per micelles. Hence, the average number of solubilizate molecules per micelle (R h ) is given by
R h) (16) Take’uchi, M.; Moroi, Y. Langmuir 1995, 11, 4719. (17) Take’uchi, M.: Moroi, Y. J. Colloid Interface Sci. 1998, 197, 230. (18) Eastman, J. W.; Rehfeld, S. J. J. Phys. Chem. 1970, 74, 1438. (19) Mukerjee, P. In Micellization, Solubilization, and Microemulsion; Mittal, K. L., Ed.; Plenum Press: New York, 1997; Vol. 1, p 171
(2)
[Rt] - [R] [Mt]
(3)
where [Rt] is the total solubilizate concentration, [R] is the monomeric solubilizate concentration, and [Mt] is the total
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Langmuir, Vol. 24, No. 1, 2008 17
Figure 3. Concentration changes of the alkylbenzenes with changes in surfactant concentration at 303.2 K.
micellar concentration. When R h is small compared to the micellar aggregation number (N), the mother micelles keep their original properties and the solubilizate molecules are solubilized into micelles obeying the Poisson distribution among the micelles.20,21 Then, the following useful expression can be derived for solubilization4,22
[Rt] - [R] [R]
)
K1(C - cmc) N
(4)
where C is the total surfactant concentration and N is the micellar aggregation number. Hence, when the right-hand side of eq 4 is plotted against the total surfactant concentration, K1/N is obtained from the slope of the plots and the K1 value can be evaluated from the aggregation number N. Solubilizate concentration changes with Mega-10 concentration are given in Figure 3. The solubilizate concentrations ([R]) below the cmc remain almost constant for all of the solubilizates regardless of the surfactant concentrations, which indicates the constancy of the chemical potential of the solubilizate molecule throughout the phases inside the apparatus as mentioned above. In addition, for alkylbenzenes except for benzene and toluene, their aqueous solubilities are consistent with the reference ones.23,24 A steep increase in solubilizate concentration above the cmc is brought about by the incorporation of the solubilizates into micelles. Each concentration is the maximum additive (20) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (21) Infelta, P. Chem. Phys. Lett. 1979, 61, 88. (22) Moroi, Y. J. Phys. Chem. 1980, 84, 2186. (23) Sanemasa, I. Bull. Chem. Soc. Jpn. 1982, 55, 1054. (24) Tewari, Y. B.; Miller, M. M.; Wasik, S. P.; Martire, D. E. J. Chem. Eng. Data 1982, 27, 451.
Figure 4. Plots of ([Rt] - [R])/[R] against Ct - cmc for the solubilizates. The lines are based upon the linear regression analysis of the plots.
concentration (MAC, [Rt]) except for benzene and toluene because the solubilizate liquid phase coexists in the system. Now we can determine the first stepwise association constant (K1) from the values in Figure 3 on the basis of eq 4. That is, the slope of the line obtained by plots of ([Rt] - [R])/[R] against C - cmc gives K1/N. The relations are illustrated in Figure 4. Fortunately, N was precisely determined to be 77 at 303.2 K.25 The K1 values thus determined became larger for longer alkyl chains of the solubilizates (Supporting Information, Table S1). In addition, the R h value was evaluated to be 9.4, 13, 19, 8.6, 8.8, 7.3, and 5.9 for benzene, toluene, ethylbenzene, n-propylbenzene, nbutylbenzene, n-pentylbenzene, and n-hexylbenzene, respectively. The R h values above support the Poisson distribution.26 The Gibbs energy change of solubilization can be expressed in the following form:
∆G0,s ) -RT lnK1
(5)
Here ∆G0,s is made up of three contributions 0 - µ0M - µ0R ∆G0,s ) µMR 1
(6)
where µ0i is the standard chemical potential of species i at infinite dilution. The absolute value of ∆G0,s increases as the number of carbons in the alkyl chain increases, which corresponds to an increase in the K1 value. The Gibbs energy changes in the solubilization of n-alkylbenzenes are plotted against the carbon number in the alkyl chain in Figure 5. The plots show good (25) Okawauchi, M.; Hagio, M.; Ikawa, Y.; Sugihara, G.; Murata, Y.; Tanaka, M. Bull. Chem. Soc. Jpn. 1987, 60, 2718. (26) Morisue, T.; Moroi, Y.; Shibata, O. J. Phys. Chem. 1994, 98, 12995.
18 Langmuir, Vol. 24, No. 1, 2008
Figure 5. Gibbs energy changes for the first stepwise association constant versus the number of carbon atoms of the solubilizates at 303.2 K. 0,s linearity, from the slope of which the ∆GCH value was evaluated 2 -1 to be -2.71 kJ mol . This value is a little less than -3.46 kJ mol-1 in magnitude for the dissolution above, but the similar values above clearly indicate that the location of the alkyl chain is a hydrophobic micellar core. These values can be compared with the transfer free energy change, -3.51 kJ mol-1, of the CH2 group from the aqueous bulk to liquid alkyl chains.19 This is also supported by the absorption spectrum of the solubilizate molecules in the Mega-10 solution above the cmc, whose highest absorption band is located at 262 nm,15 which will be discussed in more detail in the succeeding paper. Now it becomes evident that benzene is solubilized in Mega10 micelles, although benzene was not concentrated in the C14E8 micellar region.15 The hydrophobic alkyl chain length of Mega10 is much shorter than that of C14E8, which means that the latter has a much larger hydrophobic micellar core volume than does the former. Nevertheless, the latter cannot concentrate benzene molecules into the micelle. Therefore, it is highly possible that outer part of micelle or the volume occupied by E8 groups plays a more important role in the solubilization. The outer part of the C14E8 micelle whose volume is ca. 19 times larger than that of the inner hydrophobic core of a spherical micelle, when the E8 groups are fully extended, contains many water molecules around ethylene oxide chains. Therefore, the interaction of alkylbenzene molecules with the outer micelles should be relatively small when they are solubilized into the outer part of the micelle. In fact, their solubilization site is quite hydrophilic as supported by
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the UV spectra.15 Actually, the ∆H0 values for solubilization into C14E8 micelles range from 0 for benzene to -15 ( 5 kJ mol-1 for toluene to butylbenzene without any directivity. However, for the anionic 1-dodecanesulfonic acid micelle, the ∆H0 values monotonously decrease from -15.8 kJ mol-1 for benzene down to -28.5 kJ mol-1 at 303.2 K for butylbenzene,16 which means that the alkyl chain is incorporated into the inner hydrophobic core of the micelle together with the benzene ring. Judging from the above difference in solubilization, the present solubilization is very similar to that for the anionic micelle, as is also clear from the relatively large negative ∆G0,s value. For example, the ∆G0,s values for n-pentylbenzene are -31.2 and -32.1 kJ mol-1 at 303.2 K for Mega-10 and 1-dodecanesulfonic acid, respectively. In addition, compared to micelle aggregation number 64 for sodium dodecylsulfate (SDS) with carbon number 12,27 present aggregation number 77 for Mega-10 with carbon number 9 strongly suggests that C9H19 chains are highly condensed in the micellar core. That is, the core of the Mega-10 micelle is more hydrophobic than that of the SDS micelle. These are the two main reasons that benzene molecules are concentrated into the nonionic Mega-10 micelles, whereas they are not concentrated in the micelles of nonionic C14E8. Finally, the reason that C14E8 micelles are not able to accommodate benzene molecules is also available from another point of view. According to the Davies method of evaluating the HLB value for an amphiphile,28 the HLB value of C14E8 is ca. 5. This is consistent with the observation that benzene is not solubilized into micelles of a surfactant whose HLB value is less than 13.29 The relatively small HLB value can be another reason that benzene was not solubilized into C14E8 micelles in the preceding study.15 Other thermodynamic parameters such as enthalpy and entropy changes will be reported in the succeeding paper together with the spectrum change. Supporting Information Available: Change in the K1 value with the carbon number of the alkyl chain of the solubilizates. This material is available free of charge via the Internet at http://pubs.acs.org. LA702820H (27) Moroi, Y.; Humphry-Baker, R.; Graetzel, M. J. Colloid Interface Sci. 1987, 119, 588. (28) Davies, J. T. In Proceedings of the 2nd International Congress of Surface ActiVity; Butterworths: London, 1957; Vol. 1, p 426. (29) Diallo, M. S.; Abiola, L. M.; Weber, Jr, W. J. EnViron. Sci. Technol. 1994, 28, 1829.
