Solubilization of polycyclic aromatic hydrocarbons in C16E7 nonionic surfactant solutions

Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 133–139

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Solubilization of polycyclic aromatic hydrocarbons in C16 E7 nonionic surfactant solutions Emi Takeuchi a , Keisuke Matsuoka b,∗ , Shigeaki Ishii a , Sho Ishikawa a , Chikako Honda a , Kazutoyo Endo a a b

Department of Physical Chemistry, Showa Pharmaceutical University, Higashi-Tamagawagakuen 3-3165, Machida City, Tokyo 194-8543, Japan Faculty of Education, Laboratory of Chemistry, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama City, Saitama 338-8570, Japan

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Nonionic surfactant plays an important role in solubilizing PAHs.

• The string-like micelles grew with increasing incorporation of PAHs.

• The size of micelles depended on the concentration of three kinds of PAHs.

• The micelle remained string-like

structure even when they were saturated with PAHs. • The naphthalene in the micelles was mainly solubilized in palisade region.

a r t i c l e

i n f o

Article history: Received 11 July 2013 Received in revised form 30 August 2013 Accepted 4 September 2013 Available online 14 September 2013 Keywords: Polycyclic aromatic hydrocarbon Micelle Nonionic surfactant Solubilization

a b s t r a c t Polycyclic aromatic hydrocarbons (PAHs), namely, naphthalene, phenanthrene, and pyrene, were solubilized in nonionic surfactant micelles formed from heptaoxyethylene monohexadecyl ether (C16 E7 ). The sizes of the PAH-incorporated micelles, and the location of the solubilized molecules, were studied using dynamic light scattering, transmission electron microscopy (TEM), and 1 H nuclear magnetic resonance (NMR) spectroscopy. The solubilization of the PAHs increased significantly above a C16 E7 concentration of 1 mM, which corresponded to the point at which the morphology of the micelles changed from globular to string-like, as demonstrated by TEM imaging. The string-like micelles then grew with increasing incorporation of PAHs (naphthalene, phenanthrene, and pyrene). The 1 H NMR chemical shifts of the discrete groups of the C16 E7 micelles shifted upfield with increasing naphthalene concentration as a result of the ring current and/or local anisotropic effects of the PAH. The 1 H peak of the oxyethylene segment at 3.7 ppm clearly split with increasing naphthalene concentration in the C16 E7 solution. The rotating frame nuclear Overhauser and exchange spectroscopy of naphthalene solubilized in C16 E7 micelles showed a cross peak between the oxyethylene segment and the 1 H naphthalene peaks. The NMR spectral measurements showed that the solubilized naphthalene molecules were located in the palisade region of the micelles rather than in the core. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The solubilization of chemicals in surfactant micelles is widely researched across a variety of fields, including pharmaceuticals,

∗ Corresponding author. Tel.: +81 48 858 3220; fax: +81 48 858 3220. E-mail address: [email protected] (K. Matsuoka). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.09.011

cosmetics, and agriculture [1–4]. The solubilization of polycyclic aromatic hydrocarbons (PAHs) in surfactant micelles is of particular interest because increasing contamination by these compounds has become an area of great environmental concern [5–9]. The solubilization of PAHs or their competitive solubilization in surfactant solutions affects the physicochemical properties of the micelles [9]. 1 H nuclear magnetic resonance (NMR) spectroscopy has been widely used to investigate the properties of micelles in surfactant

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prepared solutions were diluted using the same concentrated stock surfactant solution. The absorbances of naphthalene (273 nm), phenanthrene (253 nm), and pyrene (273 nm) in the micellar solutions were measured using visible-ultraviolet spectroscopy (Hitachi, U-4100), and the concentrations were calculated using the molar absorption coefficients, i.e., 4.92 × 103 L mol−1 cm−1 for naphthalene and 4.02 × 104 L mol−1 cm−1 for pyrene [23]. The molar absorption coefficient of phenanthrene was determined to be 5.09 × 104 L mol−1 cm−1 from the optical absorbance at several concentrations of micellar solutions (C16 E7 ). Fig. 1. Chemical structures of naphthalene, phenanthrene, pyrene, and C16 E7 surfactant.

solutions [10–19]. There are many reports regarding the solubilization of chemicals with poor water solubilities in ionic and nonionic surfactant micelles. Ribeiro and Dennis determined the structure of alkyl polyoxyethylene nonionic surfactant micelles from their 13 C spin–lattice relaxation times and 1 H NMR spectra [11]. Gao et al. investigated nonionic–ionic surfactant mixed micelles using 1 H NMR chemical shifts, spin–lattice relaxations, spin–spin relaxations, self-diffusion coefficients, and two-dimensional 1 H NMR spectroscopy [14]. To the best of our knowledge, there are few reports on the changes in micellar properties with the amount of solubilizate in nonionic and ionic surfactant systems [18–21]. Therefore, the main purpose of this study was to investigate the morphological changes in micelles as a result of solubilization of PAHs in nonionic surfactant systems. Heptaoxyethylene monohexadecyl ether (C16 E7 ) was chosen as a typical nonionic surfactant, and three PAHs (naphthalene, phenanthrene, and pyrene) were chosen as chemicals with poor water solubilities. The maximum solubilities of these chemicals were determined as a function of C16 E7 concentration. The sizes and shapes of the micelles with increasing amounts of solubilizates were investigated using dynamic light scattering (DLS) and transmission electron microscopy (TEM). The solubilization sites in the micelles were investigated using a variety of 1 H NMR methods. The results obtained from this study will be helpful for furthering our understanding of the solubilization phenomena.

2.3. TEM measurements TEM was performed using JEOL-1200EX and JEM-200FX (JEOL) instruments with accelerating voltages of 100 kV and 200 kV, respectively. A drop of the sample solution was placed on a carbonfilm grid for 2 min. The excess solvent on the grid was blotted off by touching the grid with one end of a filter paper. After the grid had been partially dried, a drop of the staining solution, which consisted of 2% uranyl acetate aqueous solution, was added to the grid, and kept there for 2 min. The excess staining solution was then blotted off with a filter paper, and the grid was dried at room temperature. 2.4. DLS measurements The hydrodynamic radii of the micelles and of the PAHsolubilized micelles were measured using a light-scattering photometer. DLS was performed using an ALV-5000 instrument with an Nd-YAG semiconductor laser operating at 532 nm and a scattering angle of 90◦ . The solutions containing solubilized PAHs were optically purified through a Teflon filter with a nominal pore size of 0.22 ␮m. The autocorrelation functions of the electric field of the scattered light were measured, and were then analyzed using the cumulant expansion method [24]. The diffusion coefficients were determined from the average decay rate, and the apparent hydrodynamic radii were calculated using the Stokes–Einstein equation, under the assumption of spherical micelles. Therefore, the micellar size indicates approximate size in case of non-spherical micelle.

2. Experimental 2.5. NMR spectroscopy 2.1. Materials A nonionic surfactant, i.e., heptaoxyethylene monohexadecyl ether (C16 E7 , Lot No. 8011) was purchased from Nikko Chemicals (Tokyo, Japan). The surfactant purity was confirmed by chromatography (99%). This sample was used without further purification. The PAHs used in this study, i.e., naphthalene, phenanthrene, and pyrene, were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan); phenanthrene and pyrene were recrystallized from ethanol. The water solubilities of naphthalene, phenanthrene, and pyrene are 2.5 × 10−4 , 7.2 × 10−6 , and 6.7 × 10−7 M (M: mol L−1 ), respectively, at 298.2 K [22]. Deuterium oxide (99%, CIL) and 3(trimethylsilyl)propionic acid-d4 sodium salt (98%, Merck) were used as the solvent and internal standard, respectively, in the 1 H NMR spectral measurements. Fig. 1 shows the structures of the PAHs and the C16 E7 surfactant. 2.2. Solubilization The concentration of C16 E7 surfactant used in the solubilization experiments ranged from 1 × 10−7 M to 3 × 10−2 M. The surfactant solutions of different concentrations were poured into glass syringes that had Millipore filters (pore size: 0.22 ␮m), and excess PAH was added to the syringes. The solutions were stirred in a thermo-bath, and kept at 298.2 K for 48 h. The

Samples for NMR spectroscopy were prepared by deuterium oxide solutions. The NMR experiments were performed at 298.2 K ± 0.5 K using a JEOL AL-300 spectrometer to determine the chemical shifts. Rotating frame nuclear Overhauser and exchange spectroscopy (ROESY) was performed using an AV-300 (Bruker) spectrometer at a mixing time of 50 ms with standard three-pulse sequences; the 50 ms mixing time was selected on the basis of experiments performed with various mixing times. 3. Micellar morphological changes with solubilization of PAHs 3.1. Maximum PAH solubilization quantity The typical micellar properties of C16 E7 have been reported in our previous publications [20,21]. Of particular note is that the critical micelle concentration (cmc) of an aqueous solution of C16 E7 is quite low at 3 × 10−6 M. In addition, the surfactant has a relatively tractable cloud temperature (326 K) over the experimental concentration range. According to previous reports, the shape of C16 E7 micelles change from globular to string-like at a concentration of ca. 1 mM [20]. In this study, we found changes in the micellar structure to be greatly influenced by PAH solubilization.

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maximum solubilities of phenanthrene and pyrene below the cmc were slightly higher than their solubilities in water. This indicates that their solubilities were increased slightly in the presence of the C16 E7 surfactant molecules. The maximum PAH solubilities increased slightly above the cmc, and the solubilities increased significantly at C16 E7 concentrations above ca. 1 mM. The point at which the solubilities began to increase corresponded with changes in the micellar structure, with TEM observations showing that the shape of the micelles changed from globular to string-like at this concentration (Fig. 3). The quantities of solubilized PAHs increased almost linearly with surfactant concentration above the cmc. The approximate slopes in Fig. 2 correspond to molar solubilization ratios (MSRs). Chaiko et al. defined the MSR as the ratio of the maximum number of molecules incorporated into the micelles to the total number of surfactant molecules [25]. In other words, the MSR value indicates the intrinsic solubilization ability. This parameter can therefore be used to describe the solubilizations in different categories of surfactants. The MSRs for the three PAHs in C16 E7 surfactant solutions were determined as follows: MSR = Fig. 2. Concentration of solubilized PAHs with increasing surfactant (C16 E7 ) concentration. The horizontal axis is shown for logarithm expression, whereas the inset is ordinary one.

Fig. 2 shows the maximum solubilities of the PAHs as a function of C16 E7 surfactant concentration. The maximum solubility of naphthalene below the cmc was 2.25 × 10−4 M, which is almost in accordance with the aqueous solubility [22]. In contrast, the

SPAH − SPAH, cmc , Csurf − cmc

(1)

where SPAH is the maximum solubility of the PAH; SPAH, cmc is the apparent solubility of the PAH at the cmc, with the concentration of PAH in the bulk taken to be the nearly the same as the solubility in pure water [25]; and Csurf is the surfactant concentration. The average MSR values for the respective PAHs in the concentration range 1–10 mM are shown in Table 1, where it can be seen that the values decreased with increasing hydrophobicity of the PAH. These values

Fig. 3. TEM images of C16 E7 micelles at (a) 1 mM and (b) 10 mM, and of 10 mM C16 E7 with maximum solubilized amounts of (c) naphthalene, (d) phenanthrene, and (e) pyrene.

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Table 1 Solubilization parameters in a 10 mM solution of C16 E7 . G◦ refers to the solubilization. Solubilizate

MSR

G◦ (kJ mol−1 )

Naphthalene Phenanthrene Pyrene

0.36 0.16 0.067

−29.1 −36.3 −41.4

were slightly larger than those for hexadecyltrimethylammmonium bromide or hexadecylbenzyldimethylammonium chloride cationic micelles, both containing a C16 alkyl chain, in aqueous solution [7]. The magnitudes of the MSR values were similar to those reported for PAHs in nonionic Brij56 micelles [10]. The solubilization ability of the C16 E7 surfactant was therefore almost equivalent to these other surfactants, which demonstrates that the nonionic surfactant C16 E7 has normal solubilization power. Estimating changes in the Gibbs free energy (G◦ ) during PAH solubilization enables comparisons to be made with other solubilization systems. G◦ indicates the stabilization energy required for the transfer of a solubilizate molecule from the aqueous phase to the aggregate phase. Each G◦ was calculated for a 10 mM C16 E7 solution using a series of definitions [26]. The mole fractions of solubilizate in the micelle phase and the aqueous phase are given by Eqs. (2) and (3), respectively: XRM =

[Rt ] − [R] (C − cmc) + ([Rt ] − [R])

(2)

XRW =

[R] 55.5 + [R] + cmc

(3)

where [Rt ] denotes the total equivalent concentration of the solubilizate, [R] is the aqueous solubility of the solubilizate [22], and C denotes the total surfactant concentration. The mole fractions can be calculated from Fig. 2. The chemical potential of each phase at temperature T and pressure P is expressed as follows: 0,M M + RT lnXRM R = R

(4)

0,W W + RT lnXRW R = R

(5)

where the subscript R refers to the solubilizate, and the superscripts M and W refer to the micelle phase and the aqueous phase, respectively. The superscript 0 implies the standard chemical potential at infinite dilution. These results yield the following relationship for the solubilizate molecule under equilibrium:



RT ln

XRM XRW



= −(0,M − 0,W ) = −G0 R R

(6)

From this equation, we can determine the G◦ required for the transfer of a solubilizate molecule from the aqueous phase to the aggregate phase. The G◦ changes in the respective systems are summarized in Table 1. A good linear relationship between G◦ and number of aromatic rings in the PAH can be seen. The G◦ linearly decreased with increasing number of aromatic rings, with the average slope calculated to be 6 kJ mol−1 , which corresponds to the G◦ gaps for the PAHs in a system. To date, only a few studies have reported the G◦ values for the solubilization of naphthalene or pyrene [27,28]. Moroi et al. reported that the solubilization of pyrene in 1-dodecanesulfonic acid micelles was more energetically favored than that of naphthalene by −10.4 kJ mol−1 [27]. A reduction in the G◦ indicates that the solubilizate has a greater affinity for the micelle phase in solution. A comparison of the three systems in Table 1 shows that the most hydrophobic compound, i.e., pyrene, was stabilized in the C16 E7 solution as well as in conventional ionic surfactant systems.

Fig. 4. Changes in Rhapp of PAHs solubilized in 10 mM C16 E7 micelles with changing PAH concentration.

3.2. Effect of PAH solubilization on micellar morphology As shown in Fig. 2, the maximum solubilities of PAHs increased significantly at surfactant concentrations above 1 mM. This change corresponded to a transformation in morphology from globular to string-like micelles. Fig. 3(a) and (b) shows TEM images of C16 E7 micelles at concentrations of 1 mM and 10 mM, respectively. It is apparent from the images that the micelles were almost globular at a concentration of 1 mM, whereas at 10 mM, the micelles assumed a string-like form as a result of intermicellar aggregation [20]. The micellar properties of a pure surfactant system were reported in our previous paper [20]. Here, we discuss the effect of PAH solubilization on micellar size and structure, based on TEM images and measured hydrodynamic radii. The surfactant concentration was fixed at 10 mM, meaning that the micelles would be in a string-like form. Fig. 3(c)–(e) shows TEM images of 10 mM C16 E7 solutions at the maximum solubilities of naphthalene, phenanthrene, and pyrene, respectively. The cross-section diameters of the PAH-solubilized micelles can be seen to be ca. 12–14 nm. It is evident from Fig. 3 that were no significant differences between the pure micelles and those with solubilized PAHs, with the micellar shape remaining string-like even after PAH solubilization. Small micellar changes resulting from the incorporation of PAHs were subsequently analyzed using DLS. Fig. 4 shows the changes in the apparent hydrodynamic radii (Rhapp ) of the micelles (10 mM) containing solubilized PAHs. These data were then used to determine the small changes in the micelles as a result of PAH solubilization. The Rhapp of pure C16 E7 micelles at a concentration of 10 mM was 14.8 nm, as shown by the arrow in Fig. 4. In contrast, the Rhapp of micelles with solubilized naphthalene (3 mM) was approximately 27 nm, which was around twice the size of the pure C16 E7 micelles. The increase in the Rhapp value was found to depend on the quantity of solubilized PAH in the micelles, but was independent of the type of PAH (naphthalene, phenanthrene, and pyrene). Goldenberg et al. reported that the hydrodynamic radii of C8 PhEO10 and C11–15 EO9 micelles increased as a result of solubilization of frathiocab [29]. In the present study, the solubilization of PAH in C16 E7 micelles affected the Rhapp value in a way similar to that of frathiocab in C8 PhEO10 micelles. It was clear that the string-like micelles of C16 E7 gradually grew with increasing PAH incorporation, regardless of the type of PAH. In contrast,

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Fig. 5. NMR spectrum of naphthalene solubilized in C16 E7 micelles. Inset shows spectral changes in the (CH2 CH2 O)7 peak with increasing naphthalene concentration.

we previously reported that C16 E7 micelles were transformed from string-like to spherical micelles on solubilization of indomethacin [21]. Indomethacin has a relatively large molecular volume and a carboxylic acid group in its structure. Therefore, it is clear that micellar growth strongly depends on the molecular structure of the solubilizate, or on its concentration. 4. Naphthalene solubilization sites in C16 E7 micelles 4.1. 1D-1 H NMR spectroscopy NMR spectroscopy is an extremely useful technique for acquiring information about molecular environments, structures, and interactions. In the present study, we investigated the solubilization sites of PAHs within C16 E7 micelles using 1 H NMR spectroscopy. Fig. 5 shows the NMR spectrum of C16 E7 micelles with solubilized naphthalene in D2 O solution. NMR peaks of the C16 E7 surfactant were assigned according to Ribeiro et al. [11], Gao et al. [14], and Sulthana et al. [30] as follows: 0.904 ppm, terminal CH3 ((e) in Fig. 5); 1.327 ppm, (CH2 )13 ((d) in Fig. 5); 1.585 ppm, CH2 ((c) in Fig. 5); 3.459 ppm, CH2 ((b) in Fig. 5); and 3.703 ppm, (CH2 CH2 O)7 ((a) in Fig. 5). Several peaks at 1.327 ppm and in the 3.703 ppm region merged into a single peak and could not be resolved in the case of pure C16 E7 micelles. A slightly higher magnetic field proton shift was observed for discrete groups in the C16 E7 surfactant with increasing solubilizate concentration. The ring-current-induced effect on the chemical shift was dependent on the solubilizate concentration. The chemical shifts of the three PAHs, each at a concentration of 0.5 mM, were then compared. The order of the ring-current-induced effect on a discrete group of the C16 E7 micelles was found to be pyrene > phenanthrene > naphthalene. The pyrene ring current had a major influence on the NMR chemical shifts of all segments of the C16 E7 micelles [31–34]. It was extremely difficult to detect the NMR signals of the solubilizates because of their low concentrations. Furó reported that the low NMR susceptibility was a consequence of the low interaction energies of nuclear spins with the magnetic field [16]. The maximum solubilizate concentrations were approximately 0.7 mM for pyrene, 1.3 mM for phenanthrene, and 4 mM for naphthalene in a 10 mM C16 E7 aqueous solution. We chose naphthalene to evaluate the solubilizate concentration-dependence of the NMR spectra. The inset in Fig. 5 shows the chemical shifts and splittings of C16 E7 oxyethylene 1 H peaks (a) with increasing naphthalene

concentration. The oxyethylene 1 H signal became broader, with definite splitting, with increasing solubilizate concentration. This splitting became clearer with increasing naphthalene concentration, with the signal splitting into a maximum of 10 peaks. Grätzel et al. reported that splitting of the main methylene peak in the NMR spectrum was observed for pyrene solubilized in 0.1 M hexadecyltrimethylammmonium bromide micelles with increasing solubilizate content [31]. Suratkar and Mahapatra observed splitting of the methylene proton signal of sodium dodecyl sulfate (SDS) that incorporated an aromatic compound with a phenolic hydroxyl group, at various concentrations [32]. They attributed this observation to efficient shielding by the diamagnetic susceptibility of the aromatic ring of the solubilizate. Fig. 6 shows the differences in the proton chemical shifts for discrete groups in C16 E7 micelles without solubilizates and those for micelles with naphthalene. All the chemical shifts of the discrete groups of the C16 E7 micelles moved to a higher magnetic field with increasing naphthalene concentration in the micelles. The differences in these chemical shifts were seen to depend on the solubilizate concentration. The differences were small at low naphthalene concentrations; however, at high concentrations, the peaks shifted significantly. The methylene protons ((b) and (c) in Fig. 6)

Fig. 6. Chemical shift differences of discrete groups (ıx − ı1 ) between 10 mM pure C16 E7 solution (ı1 ) and solubilized naphthalene solution (ıx ).

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of an aqueous solvent and water-insoluble hydrocarbons. Surfactants therefore play an important role in facilitating the mixing of functional solutions. In the present study, we obtained new results regarding the solubilization of PAHs in a conventional nonionic surfactant. The formed micellar structures retained their stringlike morphologies even when saturated with PAH. The size of the micelles depended on the solubilizate concentration but was independent of the particular PAH solubilized. NMR spectroscopy showed that the solubilizate in the C16 E7 nonionic micelles was mainly distributed in the oxyethylene segment in the palisade region rather than in the core of the micelle. These results will be useful for further developments of surfactant systems and their applications. References

Fig. 7. ROESY spectrum of C16 E7 micelles with solubilized naphthalene.

were the most affected by naphthalene solubilization. Protons (b) and (c) were located at the junction between the polyoxyethylene segment and the hexadecyl hydrocarbon segment, which suggests that the chemical shift was influenced by the orientation and/or ring current of the solubilizate. This indicates that the naphthalene molecules were located near these discrete groups. Bernardez investigated the location of naphthalene and phenanthrene in several non-ionic surfactant systems using 1 H NMR spectroscopy. The solubilizates located preferentially in the shell region [10]. Ganesh et al. also reported ring current-induced changes in the NMR spectral signals of various cationic, anionic, and nonionic surfactant micelles with solubilized acridine, methylindole, and benzophenone as probes. The results showed that in Brij 58 micelles, the probe was located in the palisade region [35]. In the present study, naphthalene was not located at the micelle core, but was located in the palisade region instead. 4.2. 2D-1 H NMR spectroscopy (ROESY) Rotating frame nuclear Overhauser and exchange spectroscopy (ROESY) was performed on a 10 mM C16 E7 solution containing the maximum amount of solubilized naphthalene in order to identify the key protons involved in direct interactions between the micelles and the PAH. This technique has the advantage of enabling direct investigation of intramicellar and intermicellar interactions by measuring the cross-relaxation [36]. Fortunately, the NMR signals of naphthalene do not overlap with those of C16 E7 , as shown in Fig. 5. The ROESY spectrum is shown in Fig. 7, where the cross peak indicates an interaction between the oxyethylene segment and naphthalene in the micelles. This suggests that the naphthalene molecules were mainly located in the palisade region of the oxyethylene segment, which is consistent with the splitting of the (CH2 CH2 O)7 peak with increasing naphthalene concentration shown in Fig. 5. 5. Conclusion Determination of the solubilities of PAHs in surfactant systems along with identification of their sites of interaction is of extreme importance in the toiletries industry and medical fields because some of the solutions used in these applications consist

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