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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.52 n.1 Concepción mar. 2007

http://dx.doi.org/10.4067/S0717-97072007000100006 

J. Chil. Chem. Soc, 52, N° 1 (2007)

EFFECT OF THE ORGANIC SOLVENT ON THE INTERFACIAL MICROPOLARITY OF AOT –WATER REVERSE MICELLES

 

E. ABUIN*, E. LISSI AND P. JARA

Universidad de Santiago de Chile - Facultad de Química y Biología, Casilla 40 - Correo 33 - Santiago – Chile, e-mail:eabuin@lauca.usach.cl


ABSTRACT

A study has been made on the effect of the organic solvent upon the properties of the interface of reverse micelles formed by sodium bis (2-ethylhexyl) sulfosuccinate (AOT) – water. The study has been performed using fluorescence and absorption spectroscopies. The probes considered were 8-anilinonaphtha lene-1-sulfonate (ANS-) and ruthenium tris [(bipyridine)]2+, [Ru (bpy)3]2+, in fluorescence spectroscopy, and ET(30) in absorption spectroscopy. These probes were selected since its spectroscopic behaviour depends on the micro polarity of the medium. Furthermore, they have equal charge (ANS-), opposite charge [Ru(bpy)3]2+ than that of the micelle, or its nature is zwitterionic (ET (30)), which allows for the sensing of the interfacial properties at different depths. The study has been performed at different W = [water]/[AOT] values ranging from ca. 1-2 up to saturation in the external solvent considered. Organic solvents employed were: n-heptane, n-hexane, isooctane, dodecane cyclohexane, benzene, toluene, cumene, chlorobenzene, dichlormethane (DCM), 1,2-dicholoroethane (1,2-DCE) and tetrachloroethylene (TCE), and the results are discussed taking n-heptane as reference solvent. For the three probes considered, n-hexane, isooctane, dodecane and cyclohexane behaves similarly to n-heptane over all the range of W values considered, irrespective of the probe employed. ANS- behaviour in benzene, toluene and TCE is similar to that observed in aliphatic hydrocarbons. Cumene, chlorobenzene, DCM and 1,2 – DCE show a microenvironment more polar than that of aliphatic hydrocarbons. The behaviour of dichloromethane and 1,2 - DCE is similar for the three probes and very different than that observed in aliphatic hydrocarbons. The microenvironment that they offer in front of [Ru(bpy)3]2+ is considerably more polar than that corresponding to n-heptane, while in front of ET(30) is less polar. The results obtained using [Ru(bpy)3]2+ are the more difficult to explain, since displacement towards more polar (in 1,2-DCE and DCM) or less polar (in benzene, cumene, TCE) media were observed.

Key words: AOT reverse micelles, ANS-, ET (30), [Ru(bpy)3]2+.


 

INTRODUCTION

Molecules residing at the interface between the organic solvent and the water pool water in reverse micelles experience, in general, a micropolarity that is intermediate between water and the organic solvent.1-4 In a recent study,5 it has been reported that the properties of the interface of reverse micelles formed by sodium 1,4-bis (2-ethylhexyl) sulfosuccinate /water / organic solvent, such as its capacity to act as solubilization site for an amphiphilic solute (pyrene methanol), as well as the micropolarity sensed by this solute, are dependent on the nature of the organic solvent.

In spite of the role that the organic solvent would play in determining the characteristics of the interface microenvironment, most previous studies have been performed in closely related saturated hydrocarbons, with very few exceptions.3,5,6

It is the purpose of this work to investigate on the effect of the organic solvent upon the interfacial microenvironment of AOT/water/organic solvent, sensed by different probes and covering external solvents of different chemical nature and /or geometry. The study has been performed by spectroscopic probing either in emission or absorption. Probe molecules considered were : 8-anilinonphthalene -1- sulfonate (ANS-), and tris (2,2´-bipyridine) ruthenium [Ru (bpy)3]2+ in emission spectroscopy and pyridinium phenol betaine (ET (30) in absorption spectroscopy. The reasoning for the selection of these probes was based on the ground to try to sense the interfacial micropolarity from molecules with the same kind of charge (ANS-), different kind of charge [Ru (bpy)3]2+ than that of the surfactant, as well as a zwitterionic one, ET(30). The study has been performed covering a wide range of W = [H2O]/[AOT] values ranging from ca. 1-2 up to saturation in the corresponding organic solvent.

EXPERIMENTAL

Sodium 1,4-bis (2-ethylhexyl) sulfosuccinate, AOT (Sigma),was dried overnight under vacuum and kept under vacuum over P2O5. Pyridinium phenol betaine (PNPB), ET(30) (Aldrich), 8-anilinonaphthalene-1-sulfonate, ANS-(ammonium salt), (Fluka), and tris (2,2´-bipyridine) ruthenium (II) dichloride hexahydrate, [Ru(bipy)3]2+(Aldrich), were used as received. Water employed was conductivity grade purified by a Modulab Type II water purification system. Spectroscopic grade solvents, n-heptane, n-hexane. Isooctane, dodecane, cyclohexane, benzene, toluene, cumene, chlorobenzene, dichloromethane (DCM), 1,2-dicholoroethane (1,2-DCE) and tetrachloroethylene (TCE), all from Sigma were used as received.

Absorption spectra were recorded in a Hewlett Packard 8453 spectrophotometer. Emission spectra were registered in a Aminco Bowman Series 2 spectrofluorometer. All experiments were carried out a 20 ± 1 °C.

ANS- and [Ru(bipy)3]2+ were incorporated to the [AOT]/organic solvent mixture by adding an aliquot of a concentrated stock solution in water, as to obtain the minimum W value considered. To introduce ET(30), a concentrated solution was prepared in methanol, an aliquot of this solution was transferred to a volumetric flask and the methanol was removed by bubbling N2. The AOT-organic solvent solution was then added to the residue. In order to observe the solvatochromic absorption band of the conjugate form of ET(30) (pKa in water is 8,67), it was necessary to use water at pH=11 to prepare the water pools.

The maximum amount of water solubilized in the micellar solutions was expressed in terms of the parameter Wmax = [H2O]max / [AOT]. Aliquots of 10 µL of water were added successively to 3 mL of the micellar solution contained in a cuvette. After the addition of each aliquot, the cuvette was shaken for ca. 1 minute and visually observed for turbidity or transparency. In the neighbourhood of the saturation point, the volume of the aliquots was decreased and the shaking time was increased. The values of Wmax were established as those corresponding to the minimum amount of water leading to permanent turbidity.

RESULTS

(A) Water solubility in AOT reverse micelles in different solvents The capacity of the micellar solutions to solubilize water was found to be strongly dependent on the characteristics of the organic solvent. The value of Wmax = [H2O]max/ [AOT] obtained are summarized in Table 1.


As a practical guide for the selection of the organic solvent in thepreparation of AOT reverse micellar solutions we note that the Wmax values correlate with the index of polarity/polarizability of the organic solvent, π*,8 as shown in Figure 1.


 

(B) Results obtained using ANS-ANS- is a fluorescence probe displaying fluorescence intensities and wavelengths at the maximum of the spectrum, λmax, which are strongly dependent on the properties of the medium.9-12 The ANS- fluorescence properties have been exploited to obtain information about the microenvironment of the interface of reverse micelles,4,13 as polarity indicators in biological systems, in proteins4 and other organized assemblies.12

Figure 2 shows results obtained in solutions 0.1M of AOT in cyclohexane or heptane. The arrows indicate λmax in homogeneous solvents. Similar behaviours were observed in: isooctane, cumene (up to W=8), hexane, benzene (up to W = 10), toluene (up to W = 10), dodecane (up to W = 15), and TCE (up to W = 10).


 Figure 3 shows results obtained when the organic solvents were, 1,2 – DCE, chlorobenzene and DCM, taking heptane as organic solvent of reference.


The results of Figures 2 and 3 indicate that in all the solvents considered, the λmax of ANS- fluorescence shifts toward larger values when W increases, mainly up to W = 8-10, afterward reaching a constant value in those solvents that permit to dissolve larger amounts of water (eg.,heptane; the same occurs in hexane and isooctane). These results are interpreted in terms of an increase in the micropolarity of the interface associated with the hydration of the surfactant heads up to values of W @ 10.4 Nevertheless, the limiting value, λmax ≅ 485 nm is considerably lower than that corresponding to water (Table 2).


We note that, sensed by ANS- the local micropolarity of the micellar interface at the accessible W values is higher in the chlorinated solvents (Figure 3).

(C) Results obtained using [Ru(bipy)3]2+

The results are shown as plots of the position of the maximum of the emission spectrum, λmax, as a function of W, taking heptane as organic solvent of reference. Figure 4 show the results obtained in 0.1 M solutions of AOT in cyclohexane. The arrows indicate the organic solvents in which were observed the λmax indicated. Behaviours similar to those shown by this probe in AOT 0.1M in heptane were obtained when the organic solvents were : benzene (up to W =10), isooctane hexane dodecane (up to W = 12), toluene (up to W = 12), cumene (up to W = 8) and chlorobenzene (up to W = 8).


 

Figure 5 shows the results obtained in solutions 0.1M of AOT when the external solvents were: 1,2-DCM, TCE and DCM, respectively.


[Ru(bipy)3]2+ shows a solvent effect in the position of the emission spectra in such a way that it take place a shift towards larger wavelengths when the polarity decreases .15,16 Although the reasons of this effect are not clear, the probe has been used in the study of micellar interfaces.16,17 Some values of the position of the emission spectra of [Ru(bipy)3]2+ in homogeneous solvents are given in Table 3.


The results of Figures 4 and 5 indicate that, in all the solvents considered the λmax of [Ru(bipy)3]2+ shifts towards larger values when W increases, up to values of W @ 8-10, afterwards reaching a constant value in those solvents that permit to dissolve large amounts of water ( eg, heptane; the same occurs in hexane and isooctane.

The shifts of λmax towards larger values when W increases indicates a less polar microenvironment (Table 3) as sensed by this probe.

It is important to note that, sensed by [Ru(bipy)3]2+ the microenvironment at the W values accessible, indicate a medium more polar when the solvents were 1,2-DCE and DCM (Figures 5 (A) and 5 (C)), but less polar when the organic solvent is TCE (Figure 5 (B)).

(D) Results obtained using ET (30)

The results are shown as plots of the position of the maximum in the absorption band in the visible region, λmax, as a function of W, taking heptane as organic solvent of reference.

Figure 6 shows the results obtained in 0.1 M solutions of AOT in cyclohexane. Similar results to those corresponding to heptane were obtained when the external solvents were: isooctane, hexane and dodecane (up to W = 10).


Figure 7 show the results obtained in 0.1 M solutions of AOT when the external solvents were: benzene, chlorobenzene, DCM, TCE and 1,2 -DCE, respectively.


ET(30) is a parameter used to establish the polarity of a given solvent18 and is defined as the energy of the transition of the absorption band of larger wavelength, λmax, according with Eq. (1)

ET(30) = 2.85 10-3 / λ (kcal mol-1 with λ in cm)                  (1)

Table 4 shows some representative values of the λmax and the ET (30) parameter in homogeneous solvents. It has also been used to sense the microenvironment of the interfacial region in water-in-oil microemulsions1 and reverse micelles.19


DISCUSSION

(A) Results obtained in n-heptane as external solvent

The λmax of emission of ANS increases with the water content, reaching a plateau at W values of ca. 10-15. A low values of W the microenvironment is similar to that of a mixture octanol:hexane (1:5 v/v), while at high values of W it is intermediate between water and ethanol.

The value of ET (30) also increases when W increases, passing from a value similar to that corresponding to ethanol to one similar to methanol. These results are compatibles with a displacement towards the interior of the micelles with an increase in the water content. The larger change observed using ANS-is compatible with a higher expulsion of the probe from the surface towards the micellar interior, explained in terms of its negative charge. Nevertheless, it is important to note that, even at W = 30 the probe experience a medium of micropolarity smaller than that of water. This, together with the constancy of the value observed, would indicate that the probe still remain associated to the micellar interface.

The wavelength at the maximum of the emission spectra of [Ru(bipy)3]2+ also increases with the water content at W values lower than ca. 10, afterwards remaining constant up to W = 30 (λ = 638 nm). The microenvironment changes from one similar to an octanol: heptane (1:2v/v) mixture towards a medium considerably less polar. This behaviour, qualitatively different than that shown by the other probes, can be explained in terms of the positive charge of [Ru(bipy)3]2+, which permit that the probe be incorporated to the micellar interface (negative) for all the W values. An explanation of the changes observed corresponding to a medium less polar can be advanced in terms of the formation of ion-pairs between the probe and the surfactant heads. The addition of water would act as spacer of the ion-pairs, so decreasing the electrostatic interaction between them.

These results show that the type of probe employed not only modifies the estimation of the microenvironment of the micellar surface at low W values, but also can influence the type of response obtained when the water content increases until levels were a water pool can be defined. These differences emphasises the difficulties associated with the interpretation of results obtained in micelles on the ground of calibrations employing pure solvents or solvent mixtures. These differences are due to the micro - heterogeneity of the micellar media, even at low values of W were a water pool is not defined. This micro-heterogeneity have two consequences:

(i) Different probes can locate in different regions (at the same W value) and move in a different manner when W is modified, and

(ii) It can modify anomalously the emission and /or absorption spectra. In particular, it is important take into account that at the micellar interface there exist a strong electric field that is not present in the homogeneous solvents. The effect of the presence of this field on the spectra has not been investigated.

Regarding point (ii) it is important to note that the use of ET(30) or ANS- as probes of polarity is based on the changes in the dipole moment associated to the electronic transition. It is then evident that the orientation of the dipoles in the electric field can modify the energy of the transition and hence, the wavelength of the emission spectra.

(B) Effects of the external solvent

The characteristics of the external solvent notably affect the amount of water that the micelles can take up to saturation and the properties of the interface.

Regarding the Wmax, the data of Table 1 indicate that the there is a variation from a value of ca.60 (in hexane and isooctane) up to values lower than 10 (in chlorobenzene, DCM and cumene).

The different hydrocarbons employed in this work, behave similarly to n-heptane in all the range of accessible W values and for the three probes considered. It is important to note that the differences observed in the capacity of the micelles to dissolve water (Table 1) is not reflected in differences in the microenvironments that they offer for the different probes.

The behaviour of the micelles in the other solvents is dependent on the probe employed and the larger differences are observed at low values of W.

For ANS-, benzene, toluene and TCE show a behaviour similar to the aliphatic solvents. On the other hand, considerably differences are observed when cumene, chlorobenzene, DCM and 1,2-DCE are used. In particular, in these solvents the microenvironment appears as considerably less polar than those corresponding to the aliphatic solvents. The behaviour of dichloromethane and 1,2 - DCE is similar for the three probes and very different than that observed in the aliphatic hydrocarbons. The microenvironment that they offer for [Ru(bpy)3]2+ is considerably more polar, while for ET(30) it is less polar. These tendencies are the same than that observed when W increases in heptane. In particular, the less polar medium that is sensed by ET(30) can be explained in terms of the capacity of these solvents to interact with the charged surfactant heads of the surfactant, so decreasing the interaction of it with the probe. It is interesting to note hat dichloromethane and 1,2-DCE are the solvents that have the larger values for the π* parameter (Table 1). Similar arguments can explain the lower values of the ET(30) parameter observed in other solvents of relatively high polarizability, such as benzene, chlorobenzene and TCE.

The results obtained employing [Ru(bpy)3]2+ as probe are the more difficult to explain, since when the organic solvent is changed shifts towards more polar (1,2- DCE and DCM) or less polar (benzene, cumene, TCE and toluene) are observed. Furthermore, in some solvents (toluene and TCE) the differences with respect to the aliphatic solvents seems to disappear at high values of W.

The whole of the results obtained in this work show the difficulties that can be encountered when solvent-sensitive spectroscopic probes are used to sense the microproperties of micellar aggregates.

ACKNOWLEDGMENTS

Thanks are given to Dicyt and Fondecyt (Grant # 1050058) for financial support.

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(Received: November 10, 2006 - Accepted: December 6, 2006)

 

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