J. Phys. Chem. 1994, 98, 12995-13000
12995
Solubilization of Benzene, Naphthalene, Anthracene, and Pyrene in Dodecylammonium Trifluoroacetate Micelles Tom0 Morisue, Yoshikiyo Moroi,* and Osamu Shibatat Department of Chemistry, Faculty of Science, and Department of Pharmaceutical Science, Faculty of Pharmaceutical Sciences, Kyushu University, Higashi-ku, Fukuoka 812, Japan Received: May 27, 1994; In Final Form: September 27, 1994@
The solubilization of benzene, naphthalene, anthracene, and pyrene in aqueous micellar solution of dodecylammonium trifluoroacetate (DAPA) was measured. The maximum additive concentrations (MACs) of naphthalene, anthracene, and pyrene and benzene concentrations in equilibrium were determined spectrophotometrically at 298.2, 303.2, and 308.2 K. The first stepwise association constants (E1) between solubilizate monomer and vacant micelle were evaluated from the MAC and the equilibrium concentration and found to increase with hydrophobicity of solubilizate molecules. Standard enthalpy and entropy changes of the solubilization were obtained from the temperature dependence of the K1 values, and the locus of these solubilizates in the micelle was thermodynamically discussed, where spectra of the solubilizates in solvents of varying dielectric constants were also used to compare with the ones in their solubilized state.
Introduction
Experimental Section
Properties of ionic surfactants are decided by both surfactant ions and counterions, and in particular, solution properties are govemed by the latter rather than the former, because counterions contact with the bulk aqueous phase. In recent years, counterions studied with interest have shifted from metallic and halogen ions to organic and the effect of hydrophobicity of counterions on micelle formation has been examined. Decyltrimethylammonium micelles with straight-chain carboxylate anions as counterions were studied,' which indicated that the alkyl group of the counterions and their hydrophobic interaction with inner micelles have a strong effect on micelle formation. Properties of micelles whose surfactant ion is hydrocarbon-likewhile the counterion is fluorocarbon-likewere found to be quite different from those of micelles whose counterion and surfactant ion are both hydrocarbon-like ion^.^,^ Interesting findings are expected for solubilization by using surfactants consisting of hydrocarbon-like surfactant ions and fluorocarbon-like counterions. In addition, it has been already found that micelles with fluorocarbon-likecounterions are larger in size than those with metallic counterions or hydrocarbonlike counterions.1° The aqueous solubility of organic substances increases by their incorporation into micelles in surfactant solutions. The phenomenon is called solubilization.' 1,12 Solubilization is very important industrially as well as bi~logically'~ and very interesting chemically. Indeed, solubilized polycyclic aromatic compounds have often been used as photosensitive probe molecules in recent phot~chemistry,'~~'~ but there are very few thermodynamic studies on their solubilization, and the first stepwise association constants16between a polycyclic aromatic compound and a vacant micelle have not been determined in most cases. In this article, the thermodynamic parameters of solubilization of polycyclic aromatic compounds and their locations in micelles were studied to know how differently surfactants composed of fluorocarbon counterions and hydrocarbon surfactant ions influence their solubilization into micelles.
Materials. Dodecylamine(99+%) was from Aldrich Chemical Co. Inc., and trifluoroacetic acid was of reagent grade from Kanto Chemical Co. Inc. Dodecylammonium trifluoroacetate (DAPA) was synthesized as described in the previous paper.* The purity was checked by elemental analysis, and the observed and calculated values were in satisfactory agreement: C 56.19 (56.17), H 9.46 (9.43), and N 4.69 (4.68), where the values in parentheses were the calculated ones. The benzene, naphthalene, anthracene, and pyrene used were the same as in the previous experiment.16 The water used was distilled twice from alkaline permanganate. Solubilization. Surfactant solution (4 mL) and small amounts of powdered solubilizate (naphthalene, anthracene, and pyrene) were put together into a 10 mL injector tube and were agitated for about 24 h until equilibrium was reached at 298.2, 303.2, and 308.2 K, where the temperature was controlled within fO.O1 K.17 A filtration of the suspended solution was performed through a filter of 0.2 pm pore size (Millipore FGLPO1300) by applying pressure upon the injector. The absorbance of the filtrate was measured by ultraviolet spectrophotometer (HITACHI MODEL100-50), and the maximum additive concentration (MAC) was determined using the molecular extinction coefficient given in the Results and Discussion section. A very simple glass vessel devised as an apparatus for the solubilization of volatile or gaseous substances was used for benzene18 because benzene as liquid solubilizate was easily emulsified through direct contact with the surfactant solution. Molecular Weight of Micelle. Light scattering was measured by the laser light scattering photometer DLS-600 (Otsuka Electronics, Inc.). The light source was a 5 mW He-Ne laser, and the wavelength was 633 nm. Temperature was controlled within f 0 . 3 K. The photometer and the cell were calibrated with purified benzene. The surfactant solutions were filtered four times through a membrane filter (Millipore MILLEX-VV) having a pore size of 0.1 pm. The refractive index increment of solutions was measured by the differential refractometer, DRM-1020 (Otsuka Electronics, Inc.). The light source was a 20 W iodine lamp of wavelength 632.8 nm. The refractometer and the cell were calibrated by means of aqueous KCl solutions, employing the literature values of refractive i n d e ~ . ' ~ . ~ ~
* To whom correspondence should be addressed at the Department of Chemistry. t Department of Phannaceutical Science. Abstract published in Advance ACS Abstracts, November 1, 1994. @
0022-365419412098-12995$04.5010
0 1994 American Chemical Society
12996 J. Phys. Chem., Vol. 98, No. 49, 1994
Morisue et al.
TABLE 1: Dielectric Constants (0)and Wavelengths (nm) of Reference Solvents of Various Polarities at 303.2 K
solvents n-heptane 2-propanol ethanol
23.55
methanol 70% MeOH-H20 50% MeOH-H20
30.68 43.63 53.47
30% MeOH-H20 H20
62.71
D 1.9 17.38
76.73
apeallavallcy
260.51258.0 260.01258.0 260.0l257.5 260.01257.5 260.01257.5 259.51257.0 259.01256.5 25831256.5
m
Micellar Microenvironment. Aromatic molecules show solvent-induced bands in their spectra which are absent in the vapor phase. If up& and avdeyare the absorbance of the peak band and of the adjacent valley, respectively, R,, can be represented as the ratio of a* to aVdey.R,, is a linear function of the solvent polarity.21,22Though wavelengths of the peak and the valley change with solvent, we chose the following wavelengths for the peaklvalley to evaluate Rpv: 260/257 for benzene, 283/280 for naphthalene, 375/365 for anthracene, and 333/325 nm for pyrene. Table 1 gives a list of various polarities with corresponding dielectric constants at 303.2 K23and of the wavelengths at ape&and avdey for benzene. Partition. The solubilities of naphthalene, anthracene, and pyrene in n-heptane (>99.0%, Kanto Chemical Co. Inc.) at 303.2 K were measured spectrophotometrically by the same method as the solubilization, where the suspended solubilizates in n-heptane were agitated for about 36 h until equilibrium was reached. The aqueous solubilities of these compounds were determined by solubilization. The partition coefficient is the solubility ratio from the thermodynamic point of view. In the case of partition of benzene, a small amount of benzene was added into 3 mL of n-heptane in a test tube, and then water was introduced at the bottom of the solution, which was stirred for about 36 h. After equilibrium was reached, the concentrations of benzene in water and in n-heptane were measured spectrophotometrically. Theory Monodispersity of micellar aggregation (aggregation number n) is assumed to avoid the difficulties arising from polydispersity." Micelle formation is expressed by the following association equilibrium between surfactant monomers (S)and micelles (M) K"
n S e M
(1)
where K, is the equilibrium constant of micelle formation. The stepwise association equilibrium between micelles and solubilizates (R) is E,
M+R=MR, MR,
freedom is 3 by the Gibbs' phase rule.24 In addition, the chemical potential of benzene can be kept constant, so the value of benzene can be obtained in the same way as that for solid solubilizate. From eqs 1 and 2, we have the following equations for the total micelle concentration (@&I), the total equivalent concentration of solubilizate ([Rt]), and the average number of solubilizate molecules per micelle (i):
+ R E* MR,
where MRi is the micelle associated with i solubilizate molecules, ki is the stepwise association constant between MRi-1 and a solubilizate monomer, and m is the maximum number of solubilizate molecules per micelle. When an excess of pure solid phase coexists with surfactant solution phase, the MAC is fixed by specifying the total surfactant concentration at constant temperature and pressure, because the degree of
i
m
m
i
,
m
R =zi(n&)[R]'/[I
i
+ C(nk,)[R]']
(5)
When the equivalent concentration of solubilizate is less than a few times the micelle concentration, incorporation of the solubilizates into micelles is assumed not to change the intrinsic properties of the mother micelles. In this case, the stepwise association constants can be dealt with as follows: the probability of a solubilizate molecule escaping from a mother micelle containing j solubilizate molecules is j times the probability of the molecule escaping from a micelle containing just one solubilizate molecule. Moreover, the probability of solubilizatemolecules coming into the micelle remains the same. We, therefore, have the following equation as to the stepwise association or solubilization constants:25
4
4 = Kl/j Then, the probability that a micelle is associated with i solubilizate molecules can be expressed as ~ ( i =) ii' exp(-R)/i!
(7)
This expression is exactly the Poisson distribution. Thus, the following equations are derived from eqs 3-5 by summing m to infinity:"
[WI= [MI exp(K,[RI)
(8)
[&I = [RI + K,[Rl[Ml exp(K,[RI)
(9)
R = ([&I - [Rl)/[WI = KlBI
(10)
Equation 10 can be rearranged as
