2372
J. Phys. Chem. 1995, 99, 2372-2376
Solubilization of Benzene, Naphthalene, Anthracene, and Pyrene in 1-Dodecanesulfonic Acid Micelle Yoshikiyo Moroi,*>*Kayoko Mitsunobu,+ Tom0 Morisue,' Yousai Kadobayashi,' and Masashi Sakait Department of Chemistry, Faculty of Science, Kyushu University 33, Higashi-ku, Fukuoka 812, Japan, and Department of Industrial Chemistv, Faculty of Engineering, Kyushu Sangyo University, Higashi-ku, Fukuoka 813, Japan Received: April I , 1994; In Final Form: October 19, 1994@
The solubilization of benzene, naphthalene, anthracene, and pyrene in 1-dodecanesulfonic acid micelle was measured. The maximum additive concentrations (MAC'S) of these polycyclic compounds except benzene in aqueous solution were determined spgctroscopically at 25, 30, and 35 O C with surfactant concentration. The first stepwise association constants (K1) between solubilizate monomer and vacant micelle were evaluated from the association equilibria between solubilizates and micelles: 1.16 x lo3, 2.23 x lo4, 2.53 x lo5,and 1.09 x lo6 mol-' dm3 at 30 "C for benzene, naphthalene, anthracene, and pyrene, respectively. The increase in values with hydrophobicity of solubilizate molecules indicated that the solubilization was controlled mainly by hydrophobic interaction between the solubilizates and the micelles. Standard enthalpy and entropy changes of solubilization were calculated from the temperature change of the El values, and the solubilization is thermodynamically discussed. Spectral microenvironments of the solubilizates in micelle were found to be much more polar than expected from the thermodynamical parameters.
Experimental Section
Introduction An enhanced aqueous solubility of otherwise slightly soluble organic substances brought about by the presence of surfactant This solubility micelles is well-known as enhancement can be ascribed to an incorporation of hydrophobic organic substances into micelles in surfactant solution. Indeed, solubilization plays a very important role not only in industrial processes but also in biological processes such as absorption and transfer of material^,^ but its interpretation in the thermodynamical sense is rather qualitative. In other words, solubilization has been treated as partioning of solubilizate molecules between a micellar phase and intermicellar bulk phase,j-* although micelles are chemical species from the standpoint of the Gibbs phase r ~ l e . ~The . ' ~ authors have investigated solubilizations from the stepwise association equilibria between solubilizates and micelles using many kinds of solubilizate molecules. I l - l5 The site of solubilizates in micelles is another point of discussion, for which NMR is a very powerful method.16-'* This is also useful to measure a self-diffusion coefficient of s o l ~ b i l i z a t e . In ~ ~ addition, since micelles were found to be effective media for photochemistry, a number of papers and review articles on photochemical and photophysical processes have appeared,20-22 where polycyclic materials were offen e m p l ~ y e d . Photochemistry ~~.~~ is also quite powerful to estimate a rate of exchange of solubilizates between a bulk phase and a micellar domain.25 On the other hand, precise determination of the first stepwise association constant between polycyclic compounds and vacant micelles is quite useful for their application in micellar systems. This paper, then, aims to study the solubilization of polycyclic aromatic compounds in micelles in order to know the effect of the number of benzene rings or the number of carbon atoms in arene molecules on the thermodynamical parameters of solubilization, paying attention to the location of the solubilizates in the micelles.
@
Kyushu University. Kyushu Sangyo University. Abstract published in Advance ACS Abstracts, February 1. 1995.
Materials. 1-Dodecanesulfonic acid was synthesized as described in a previous paper,26and the solution concentration was determined by acid-base titration. Benzene of guaranteed reagent grade from Nacalai Tesque was used intact. Naphthalene and anthracene of the same grade from Nacalai Tesque were purified by recrystallizations 3 times from the methanol solution. Pyrene of the same grade from Tokyo Kasei Kogyo Co., Ltd., was also purified by recrystallizations 4 times from the methanol solution. Methanol, used as the solvent, was removed from the solubilizates by exhaustion for a long period of time. The purity of these compounds was checked by elemental analysis, and the observed and calculated values were in satisfactory agreement; C 93.718(93.77%) and H 6.29%(6.32%) for naphthalene, C 94.41%(94.34%) and H 5.62%(5.66%) for anthracene, and C 94.73%(95.02%) and H 4.98%(4.98%) for pyrene, where the values in parentheses were the calculated ones. The solvent, methanol of guaranteed reagent grade, was used without further purification. The water used was distilled twice from alkaline permanganate. Solubilization. A suspension of solubilizate powder in surfactant solution was stirred initially at room temperature by a disk rotor in a 10-mL injector tube. The injector was then dipped into a thermostat until equilibrium was reached, and the temperature was kept constant at 25, 30, and 35 "C and controlled within fO.O1 "C." A period of time required for solubilization equilibrium was less than 10 h, but a filtration of the suspended solution was carried out after 1 day for the complete equilibriation, where a filler of 0.2-pm pore size (Millipore; FGLPOl300) was used. The MAC concentration was determined spectrophotometrically for naphthalene, anthracene, and pyrene using their molecular extinction coefficients given in the Results and Discussion section. A newly devised apparatus was used for benzene sol~bilization~~ in order to avoid a microemulsion formation resulting from direct contact of benzene with surfactant solution (Figure 1).
0022-365419512099-2372$09.00/0 0 1995 American Chemical Society
Solubilization in 1-Dodecanesulfonic Acid Micelle
J. Phys. Chem., Vol. 99, No. 8, 1995 2373
n
equations for the total micelle concentration ([M,]), the total equivalent concentration of solubilizate ([R,]) or MAC, and the average number of solubilizate molecules per micelles (E): m
m
[RJ = [R]
C Figure 1. Solubilization apparatus for volatile solubilizates; a = liquid solubilizate, benzene, b = surfactant solution, c = disk rotor, and d = magnetic stirrer.
Microenvironment. Aromatic molecules show solventinduced bands in their spectrum which are absent in the vapor phase. If a, and avare the absorbances of the peak and of the adjacent valley, respectively, R,, can be represented as the ratio of ada,. The Rpv value is a linear function of the solvent p ~ l a r i t y . ~ Though * , ~ ~ the wavelengths of the peak and valley change with solvent, the following wavelengths (nm) were employed for the peaktvalley to evaluate R,,; 2601257 for benzene, 2831280 for naphthalene, 3751365 for anthracene, and 3331325 for pyrene. Brief Survey of the Theory Micelle formation can be expressed by the following association equilibrium between surfactant monomers ( S ) and micelles (M):'O K"
nS+M
(1)
where K,, is the equilibrium constant of micelle formation and the monodispersity of micelles in aggregation number n is assumed in the absence of solubilizate in order to remove the difficulty arising from their polydispersity. Even in the case of the polydispersity, the present discussion remains essentially the same." The stepwise association equilibria between micelles and solubilizates (R) are presented by E, M+R=+MR, K2
MR,+R*MR,
..........
where MRi designates the micelles associated with i molecules of solubilizate, Ki is the stepwise association constant between MRi-I and a monomer molecule of solubilizate, and m is an arbitrary number. From eqs 1 and 2, we have the following
i
i
+K,[S]nci(nq)[R]i
(4)
In the case where excess solid solubilizate phase is present, the concentration of monomeric solubilizate ([R]) is automatically determined as the solubility of the solubilizate at a specified temperature and pressure. As for benzene, the gaseous benzene phase is a substitute of solid solubilizate phase.27 A normalized parameter to estimate the magnitude of interaction between solubilizate and micelle is the value of K1, which is the first stepwise association constant between a monomeric solubilizate and a vacant micelle. When the concentration of solubilizate is less than a few times the micellar concentration, the incorporation of the solubilizates into micelles can be assumed to be so small that it does no change the intrinsic properties of the micelles. In this case, we have the-folloying equation for the stepwise association constants:30 Kj = K10. Then, the probability ( P ) that a micelle is associated with i solubilizate molecules can be given by the Poisson distribution as
~ ( i=) R' exp(-k)/i!
(6)
and eqs 3, 4, and 5 become the following simplified equations with infinite m: [Mtl = [MI exp(K,[RI)
[&I = [RI + K,[RI[Ml
exp(K,[RI)
R =
