16611433

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Micellar properties of a mixed surfactant system constituted by

n-octyl-

␤

-

d-

thioglucopyranoside and sodium dodecyl sulphate

J.M. Hierrezuelo, J. Aguiar, C. Carnero Ruiz

∗

Grupo de Fluidos Estructurados y Sistemas Anfif´ılicos, Departamento de F´ısica Aplicada II, Escuela Universitaria Politécnica, Universidad de Málaga, Campus de El Ejido, 29013 Málaga, Spain

Received 12 January 2005; received in revised form 18 April 2005; accepted 30 April 2005 Available online 5 July 2005

Abstract

A fluorescence probe study on micellization of the binary surfactant system formed by n-octyl-␤-d-thioglucopyranoside (OTG) and sodium dodecyl sulphate (SDS) in 0.1 M NaCl aqueous solutions has been carried out. The CMC experimental data, as obtained by the pyrene 1:3 ratio method, have been analyzed in the context of the pseudophase separation model. The interaction parameter (β12) determined by using

the regular solution theory (RST) was found to be negative and independent on the solution composition. In addition, the experimental data on micellar composition showed a good agreement with those predicted by RST. Data reported by a treatment based on the Gibbs–Duhem equation showed a reasonable agreement with those obtained by RST, suggesting that the mixed system behaves as a regular solution. From the mixing thermodynamic function values can be inferred that the electrostatic repulsions between the sulphate groups of SDS control the stability of the mixed micelles. Size of micelles was examined through the micellar aggregation number as obtained by the static quenching method. It was found that the aggregation number initially increases, giving a maximum value at low content of the ionic component, remaining almost constant at larger content of this one. This behaviour was interpreted on the basis of the role played by the electrostatic repulsions between the headgroups of SDS in the stabilization of the mixed micelles. Finally, studies based on both pyrene 1:3 ratio index and intramolecular excimer forming of 1,3-dipyrenylpropane solubilized in the micellar phase, reported information on the microenvironmental properties of mixed micelles. The results obtained were rationalized on the basis of two effects: the electrostatic repulsions between the headgroups of the ionic component and the micellar hydration.

Keywords: Mixed micelles; Fluorescence probing study; n-Octyl-␤-d-thioglucopyranoside; sodium dodecyl sulphate

1. Introduction

The micellar properties of aqueous solutions containing mixed surfactant systems have been widely studied in recent years due to various reasons. On the one hand, technical grade surfactants are themselves mixtures and the purification pro-cess may be difficult or expro-cessively expensive; and on the other, these systems often behave better than a single sur-factant in determined technical applications [1–4]. Mixed surfactant systems have also been the object of investiga-tion from a theoretical point of view, and considerable efforts have been recently carried out in order to provide

appropri-∗_{Corresponding author. Tel.: +34 952132063; fax: +34 952132064.}

E-mail address: ccarnero@uma.es (C. Carnero Ruiz).

ate thermodynamic models capable not only of describing the behaviour of the mixed systems, but also of predicting their properties[5–18].

Alkylpolyglucosides (APG) are nonionic surfactants char-acterized by having a hydroxyl sugar group as hydrophilic moiety. The use of these surfactants has recently increased because they are biodegradable and considered dermatolog-ically safe [19]. Furthermore, APG surfactants present a number of advantageous properties when compared to the common alkyl polyglycol ether nonionics (CiEj). For

exam-ple, APG surfactants have both stronger lipophobicity and hydrophilicity, and its temperature dependence on the solu-tion properties is much less pronounced [20]. In addition, these surfactants present properties such as high solubilizing power, high critical micelle concentration, no denaturation

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Plate 1. Molecular structure of OTG.

of proteins, high solubility in water, and stability, and these properties are very suitable for the purpose of solubilization of membrane proteins, field in which these surfactants are usually applied[21].

Among the APG surfactants n-octyl-␤-d-glucopyranoside (OG) has been probably the most widely investigated, and some studies on the mixed micellization involving to this sur-factant have recently been reported[22,23]. In addition, a few studies have been conducted on mixed micellization of binary surfactant systems where one component is an APG surfac-tant [24–33]. n-Octyl-␤-d-thioglucopyranoside is a related APG surfactant which differs only from OG in which the hydrophilic group is linked by a thio-ether to the hydropho-bic chain (Plate 1). This structural peculiarity provides OTG with solution behaviour substantially different from that of OG[21]. Furthermore, recent investigations have revealed the advantages of OTG against OG in the biomembrane field [34,35]. However, OTG has been much less studied, and as far as we know no study on mixed micellization involving OTG has been previously published. Following the work developed in our laboratory[33,36–38], we focus now our attention on a mixed surfactant system constituted by an APG nonionic surfactant, n-octyl-␤-d-thioglucopyranoside (OTG), and a typical anionic surfactant such as sodium dodecyl sulphate (SDS). In order to obtain information on the micellar prop-erties of the OTG–SDS mixed system, we have used the well established fluorescence probe technique. Recently, Maeda has published a study on the mixed micellization of sur-factants [39] where is established the convenience of dis-tinguishing three regimes depending on the ionic strength of medium. The second regime, corresponding to medium ionic strength (Csalt< 0.2 M), is characterized by the fact that the ionic strength is practically kept constant independent of the micelle concentration, whereas the long-range nature of the electric interaction is not longer negligible. For this rea-son we have carried out the present study in the presence of 0.1 M NaCl. The paper is presented in three sections. In the first, by using the pyrene 1:3 ratio method, we determine the critical micelle concentration (CMC) of the mixed sys-tems in the whole composition range, and then these data are analyzed on the light of various mixing thermodynamics models within the framework of the pseudophase separation approach. The second section concerns with the analysis of the size of the mixed micelles, for which the mean mixed

micelle aggregation number is obtained by applying the static quenching method. Finally, we examine possible alterations in the micellar microstructure through micropolarity and microviscosity changes upon variations in the mixed micelle composition.

2. Experimental

2.1. Materials

The nonionic surfactant OTG was acquired from Sigma, whereas the ionic surfactant sodium dodecyl sulphate (SDS) was purchased from Fluka. The fluorescence probe pyrene (Py) and the quencher cetylpyridinium chloride (CPyC) were also obtained from Sigma, and 1,3-dipyrenylpropane (P3P) from Molecular Probes. All these substances were used as received. Stock solutions of the pure surfactants and quencher were prepared by weight using doubly dis-tilled water, and that of the fluorescence probes in abso-lute ethanol. All these solutions were stored at 4◦C. All experiments were carried out with freshly prepared solu-tions. Working solutions were made in aqueous solutions 0.1 M NaCl.

2.2. Fluorescence measurements

Fluorescence measurements were recorded on a SPEX FluoroMax-2 steady-state spectrofluorimeter in the “S” mode with band-passes for excitation and emission of 1.05 nm. This apparatus is fitted with a 150 W xenon lamp, and equipped with a thermostated cell housing that allowed temperature control to ±0.1◦C. All fluorescence measurements were made at 25.0±0.1◦C.

2.3. Methods

In order to obtain the CMC values in each binary surfac-tant mixture, different solutions containing OTG and SDS in several proportions were prepared. The composition of the solutions was expressed in molar fraction (αj) of the

respec-tive surfactant, defined as:

αj= _[_S [Sj]

i]+[S_j] (1) where [Si] and [Sj] refer to the molar concentration of the

component surfactants. Working solutions of lower concen-tration were prepared by adding proper volumes of the pyrene ethanolic solution. The volume of this solution was small enough (0.1% of the total volume) so that the solvent did not have effect on the micellar system. From these solutions, fluorescence emission spectra were recorded using an exci-tation wavelength of 335 nm, and the fluorescence intensities at the wavelength corresponding to the first (I1) and third (I3) vibronic bands, located near 373 and 384 nm, were measured. The ratio I1/I3is the so-called pyrene 1:3 ratio.

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Micellar aggregation numbers were determined by the static quenching method. Fluorescence quenching studies were carried out using Py as a luminescence probe and CPyC as a quencher. Stock solutions containing Py and pure or mixed surfactant were prepared in aqueous 0.1 M NaCl solu-tions. Working solutions of lower concentration (1␮M in pyrene and 30 mM in surfactant) were prepared by adding appropriate volumes of quencher solutions. In these studies, the quencher concentrations employed were maintained low enough so as not to interfere with the assembly of the micel-lar system under study. From these solutions, fluorescence intensities were recorded by using excitation and emission wavelengths of 335 and 383 nm, respectively. For each mix-ture composition the quenching experiments were repeated three times.

Micropolarity and microviscosity studies of the mixed micelles were carried out using solutions with a total sur-factant concentration well above the corresponding CMC (30 mM). The pyrene 1:3 ratio index was also used to obtain information on the micropolarity of the mixed micelles. In this case, the fluorescence measurements were recorded under the same optical conditions than in the CMC deter-mination assays. Similarly, the emission spectra of P3P in micellar solutions were obtained between 350 and 550 nm by using the same recording mode and band-passes and an exci-tation wavelength of 346 nm. The intensities of the emission of the monomer (IM) was recorded at the wavelength corre-sponding to the first vibronic peak of the monomer, located near 378 nm, and that of the excimer (IE) at around 490 nm. The solutions for excimer forming studies were prepared by adding the appropriate volumes of a concentrated P3P ethano-lic solution to aqueous surfactant solutions. These solutions were then sonicated during 1 h and kept for equilibration on a bath at 25◦C for at least 12 h.

3. Theory

Mixed micellization of surfactants is usually considered in the context of the pseudophase separation model. Based on this approach, and in the case of ideal behaviour, Clint[40] proposed a relationship to obtain the CMC of the mixed sys-tem, C*, as a function of the molar fraction of each surfactant in the solution,αi:

1 C∗ =

i

αi

Ci (2)

where Ci refers to the CMC of the pure components.

Non-ideality is introduced with the inclusion of the activity coef-ficients, fi, in such a way that Eq.(2)is expressed in this case

as:

1 C∗ =

i

αi

fiCi (3)

According to the pseudophase separation model, when the CMC in a binary surfactant system is reached the following relations are fulfilled:

α1C∗=x1f1C1 (4)

(1−α1)C∗=(1−x1)f2C2 (5)

Starting from here, we can consider two different approaches. The one more commonly used, due to Rubingh [41], is based on the regular solution theory (RST). Accord-ing to this model the activity coefficients are related to an interaction parameter,β12, by:

f1=expβ12(1−x1)2 (6)

f2=expβ12x2₁ (7)

where x1 denotes the mole fraction of the component 1 in the mixed micelle. The β12 parameter characterizes the interaction between the two surfactants and accounts for the deviation from the ideality for the binary system. From a physical point of view, theβ12parameter can be interpreted in terms of an energetic parameter that represents the excess Gibbs free energy of mixing. This interpretation is correct if, according to the regular solution theory, the excess entropy of mixing equals to zero. Theβ12parameter can be obtained from experimental CMC values using:

β12= ln

α1C∗

x1C1

(1−x1)2 (8)

In addition, the micellar composition, x1, can be determined solving iteratively the equation:

x2 1ln

α1C∗

x1C1

(1−x1)2ln

(1−α1)C∗ (1−x1)C2

=1 (9)

It has been discussed that the error in the determination of theβ12parameter increases when one component is in large excess or when the CMC value of pure surfactants are very different [3]. This fact can represent an inconvenience for the Rubingh’s treatment. An alternative approach consists in using the Gibbs–Duhem equation to obtain the activity coeffi-cients. This approach has been discussed in detail by Rodenas et al. and applied to study a mixture of ionic–nonionic surfac-tants[42]. From this approach the micellar composition can be obtained, if the CMC of the system is known as a function of the bulk composition, by the expression:

x1=α1−α1(1−α1)d lnC ∗

dα1 (10)

In this manner, by using Eqs. (4) and (5), it is possible to obtain the activity coefficient values without the previous determination of theβ12parameter.

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4. Results and discussion

4.1. CMC, interaction, composition, and mixing thermodynamic function

CMC values for pure surfactants and their respective binary mixtures were determined by using the pyrene 1:3 ratio method[43].Fig. 1shows representative plots of pyrene 1:3 ratio versus the total surfactant concentration for the OTG–SDS system. These plots present a typical sigmoidal decrease as the surfactant concentration increases. Below the CMC the pyrene 1:3 ratio index has a value corresponding to a polar environment; this value decreases abruptly as the sur-factant concentration increases, suggesting that the probe is sensing a more hydrophobic environment. Above the CMC, the pyrene 1:3 ratio index reaches a roughly constant value due to the complete association of the probe to the micelles. From plots in Fig. 1, the corresponding CMC values were obtained by using the data treatment previously described [44].Fig. 2a shows the CMC values obtained experimentally as a function of the mole fraction of SDS in the solution (αSDS). In this figure it can be seen that mixed CMC experi-mental values are lower than those obtained assuming an ideal behaviour, indicating a certain attractive interaction between the two components in the mixed micelle. It is to be noted that ideal and non-ideal mixing have been found in other systems similar to ours[45–47]. By using RST we have found that the interaction parameter for our system is essentially constant

Fig. 1. Plots of pyrene 1:3 ratio vs. total concentration of surfactant at dif-ferent solution composition values for the OTG–SDS system.

Fig. 2. (a) Variation of the CMC with the mole fraction of SDS in the solu-tion,αSDS, and (b) micellar composition, xSDS, as a function ofαSDS. Open circles represent experimental values, the dashed line represents the phase separation model prediction for an ideal behaviour, and the solid line is for a regular solution (β12=−1.52).

through the total composition range. The solid line inFig. 2a represents the best fit of the experimental data using RST for a fittedβ12 parameter of −1.52. This attractive interaction between anionic and nonionic surfactants is due to various effects. First of all, it must be taken into account that the intercalation of the nonionic component shields the repul-sive interaction between the negatively charged headgroups of SDS, improving the electrostatic stabilization of the mixed micelle. Moreover, it is also possible the contribution of an attractive interaction ion-dipole, which could be significant in our system due to the high charge density of the sulphate group of the ionic component. Another interesting aspect in Fig. 2a is that the addition of a small amount of ionic sur-factant reduces considerably the CMC of the mixture. This effect has been previously observed[28,30], and explained by the reduction in the steric hindrance between the glycoside headgroups of the nonionic micelle as a result of the incor-poration of the ionic component to form the mixed micelle [28].

Fig. 2b shows the variation of the mole fraction of SDS in the mixed micelle, xSDS, as obtained from Eq.(9), with the composition of the solution,αSDS. From this figure it can be observed, on the one hand, that there is a good agreement between the experimental data and those predicted by RST;

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Fig. 3. Variation of the critical micelle concentration of the mixed system with the solution composition: (䊉) experimental results, the solid line is the best fit of data to an exponential decay of second order.

and on the other, that small amounts of SDS origins mixed micelles with a considerable content in the ionic component. On the other hand, we have treated our experimental data by using the approach based on the Gibbs–Duhem rela-tion (GD), as described in the theoretical secrela-tion. First of all, we have plotted our experimental CMC values as a function of the solution composition (α1), from which the function ln C*= f(α1) has been obtained (Fig. 3), then we have determined the micellar composition by using the Eq. (10) and, lastly, we have obtained the activity coefficients from Eqs. (4) and (5). The corresponding results are pre-sented in Table 1 together to those obtained by RST. It is observed an acceptable agreement between the results obtained by both treatments (particularly as high concentra-tion of the nonionic component), suggesting that the mixed system behaves as a regular solution [42]. With the aim of obtaining additional information on our mixed surfactant system, we have determined the mixing thermodynamic func-tions assuming the quantitative description given by RST. This approach considers that the excess entropy of mixing (SE) is zero, that is, the mixing entropy equals to that of the ideal behaviour ( SM= Sideal). Therefore, the follow-ing relationship between the excess free energy (GE), the excess of enthalpy (HE), and the enthalpy of mixing ( HM) must be considered:

GE₌_HE₌_HM₌_RT

i

xilnf_i (11)

Table 2

Variation of the mixing thermodynamic functions with the molar fraction of the nonionic component in the bulk, obtained by using the treatment based on the regular solution theory for the OTG–SDS system

αOTG xOTG GM(kJ mol−1) HM(kJ mol−1) T SM(kJ mol−1)

0.2 0.15 −1.50 −0.47 1.03

0.4 0.24 −2.04 −0.68 1.36

0.6 0.33 −2.39 −0.83 1.56

0.8 0.47 −2.65 −0.94 1.71

0.9 0.58 −2.60 −0.92 1.68

The excess free energy of mixing (GE) represents the devia-tion from the ideal behaviour (GE= GM− Gideal_M ). In fact, according to RST, this magnitude is related to the interaction parameter β12, for a two-component system, by:

GE₌_RTβ12x1(1₋_x1) ₍₁₂₎

On the other hand, as the ideal mixing free energy ( Gideal_M ) is given by:

Gideal

M =RT

i

xilnx_i (13)

the mixing free energy ( GM) is given by: GM=RT

i

xilnx_if_i (14)

finally, the corresponding entropic contribution (T SM) can be obtained by:

T SM= HM− GM (15)

In Table 2 are listed the mixing thermodynamic func-tions as obtained by the above treatment. Firstly, it must be noted that the values obtained for the mixing thermodynamic functions are of the same magnitude order than those cal-culated by other authors[42]and by ourselves [38]. From data in Table 2, it is observed that the tendency followed by GM indicates that the formation of mixed micelles is more favourable as the nonionic content increase, accord-ing to the electrostatic stabilization concept. As previously commented, the intercalation of nonionic monomers into the ionic micelles prevents the electrostatic repulsions between the charged headgroups, leading to the electrostatic stabi-lization of the mixed system. It also observed that the change

Table 1

CMC of pure and mixed systems together with comparative results of micellar composition and activity coefficients (fOTGand fSDS) for the OTG–SDS system as obtained by the treatment based on the Gibbs–Duhem equation (GD) and on the regular solution theory (RST)

αOTG CMC (mM) xOTG(GD) xOTG(RST) fSDS(GD) fSDS(RST) fOTG(GD) fOTG(RST)

0 1.47 0 0 – – – –

0.2 1.51 0.13 0.15 0.947 0.968 0.300 0.330

0.4 1.70 0.24 0.24 0.913 0.918 0.373 0.413

0.6 2.12 0.34 0.33 0.868 0.851 0.498 0.501

0.8 2.74 0.43 0.47 0.650 0.720 0.675 0.647

0.9 3.88 0.51 0.58 0.533 0.598 0.908 0.767

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Fig. 4. Excess free energy of mixing (solid line) and free energy for an ideal mixture (dashed line) for the OTG–SDS system according to RST.

of enthalpy or its equivalent the excess free energy presents a minimum. On the other hand, from Eqs.(11)–(15)it can be concluded that T SM= − Gideal_M . InFig. 4 we have plotted both the GE and Gideal_M versus the micellar com-position, expressed as the ionic component content. These plots show that both magnitudes are symmetrical functions with respect to the micellar composition, according to the RST prediction. Finally, from data inTable 2it can be seen that the entropic contribution increases with the nonionic component content. This behaviour is probably due to the change in the counterion condensation in the mixed micelle upon the addition of the nonionic surfactant. Indeed, if a non-ionic surfactant is added to the system, the mixed micelles resulting will have a lower surface charge density. In this manner, a smaller number of counterions will be bound to the micelle, and this effect will produce an entropy gain in form-ing the mixed micelles in comparison to micelles formed by SDS[30].

4.2. Micellar aggregation numbers

The effect of the micellar composition on the structure of mixed micelles was studied through the variation of the mean micellar aggregation number in the whole range of composition. With this purpose we use the static quenching method originally proposed by Turro and Yekta[48]on the basis of the previous analysis carried out by Tachiya [49] on the kinetics of the fluorescence quenching in micellar solutions. This method can be applied when the following conditions are fulfilled: (i) the probe and quencher must be solubilized in the micelle and be immobile, remaining within the micelle during the lifetime of the probe; (ii) the quenching rate must be faster than the emission lifetime of the probe, so that fluorescence is observed only from micelles with probe but without quencher; (iii) the probe and quencher must be distributed among micelles following a Poisson distribution. If the above conditions occur, the ratio between the fluorescence intensities in the presence (I) and in the absence (I0) of the quencher is related to the quencher

Fig. 5. Quenching plots of pyrene in OTG–SDS micellar solutions for dif-ferent system compositions. Solid lines are the best fit of data to (18). [Q] and micelle [M] concentrations, by:

I I0 =exp

−[Q] [M]

(16)

here the micelle concentration [M] is given by:

[M]= [S]−CMC

Nagg (17)

being [S] the total surfactant concentration, and Nagg the mean micelle aggregation number. Finally, from Eqs.(16) and (17)one obtains:

lnI0 I =

Nagg

[S]−CMC[Q] (18)

We have carried out quenching experiments by using Py as a probe and CPyC as a quencher, as this donor–quencher pair has been found to obey the requirements described above and, therefore, it has been used for a large range of micellar media [50].Fig. 5shows some results of our quenching experiments plotted according to Eq.(18). In all cases, a good linearly (r > 0.99) was obtained. Values of Naggdetermined from the slopes of the linear fits of experimental data in Fig. 5are reported inTable 3. It is interesting to point out that the Nagg value that we have obtained for pure SDS micelles agrees rather well with that previously measured by light scatter-ing technique[51]. According to geometrical considerations, data fromTable 3suggest that the micelles present a globular structure in the whole range of composition. It is observed

Table 3

Effect of the micellar composition on the micellar aggregation numbers of OTG–SDS micelles, Nagg, in aqueous solutions 0.1 M NaCl at 25◦C, together with the corresponding individual contributions of both components.

αSDS xSDS Nagga NSDS NOTG

0 0 92±1 0 92

0.2 0.54 128±1 69 59

0.4 0.67 102±3 68 34

0.6 0.76 97±1 74 23

0.8 0.85 106±4 70 16

1 1 99±1 99 0

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Fig. 6. Variation of pyrene 1:3 ratio (䊉) and of the monomer to excimer intensity ratio of P3P () with the solution composition,αSDS.

that in the presence of NaCl 0.1 M the two pure surfactants have similar aggregation numbers. Data inTable 3indicate that the aggregation number initially increases with solution composition, up to a maximum value atαSDS= 0.2, remain-ing roughly constant for mixed systems with higher content of SDS. A similar behaviour has been observed by Shiloach and Blankschtein[15]for the SDS–C12E6system in aque-ous solutions 0.1 M NaCl. According to the aforementioned authors, this behaviour can be rationalized as follows: at low SDS content, the electrostatic repulsions between the charged SDS heads are not yet important; however, the incorporation of SDS monomers, with smaller SDS heads than those of OTG, reduces the steric interactions between the larger OTG heads. This produces a smaller steric free energy, and hence a reduction of the area required per surfactant head, allowing the mixed micelle to adopt a structure with lower curvature and resulting in the growth of the mixed micelle. Neverthe-less, when the participation of the ionic component reaches a certain value (αSDS> 0.2), the electrostatic repulsions between the ionic headgroups produce an increase in electro-static free energy that dominates the decrease in steric free energy. This effect results in an increase in the area per head-group required, which forces to the micelle to adopt a higher curvature and, consequently, a lower aggregation number.

4.3. Microenvironmental properties

Micropolarity and microviscosity are two relevant proper-ties usually employed to obtain information on the possible changes in the microstructure of micellar systems. The pyrene 1:3 ratio index is well known to be a sensitive indicator of the polarity in the probe microenvironment[52]. Pyrene is pref-erentially solubilized close to the surface of micelles, in the so-called palisade layer[50,52], therefore the polarity sensed by the probe could give information on the degree of solvation in this micellar region. We have monitored the micropolarity in the OTG–SDS system by measuring the pyrene 1:3 ratio in micellar solutions well above the CMC (30 mM) and at differ-ent compositions.Fig. 6shows the variation of the pyrene 1:3 ratio as a function of the mole fraction of SDS in the solution.

From data inFig. 6we can derive some conclusions. First of all, it is observed that micelles formed by pure surfactants present a low and similar polarity, suggesting that in both type of micelles the water penetration is very restricted in the pyrene solubilization site. Second, it is also observed that pyrene 1:3 ratio index initially decreases with the SDS con-tent, up to a minimum atαSDS= 0.2. This result is consistent with the tendency observed in the micellar aggregation num-ber. Note that as the micellar size increases, pyrene will be more effectively prevented from being influenced by water molecules. It is reasonable, therefore, that pyrene 1:3 ratio and micellar aggregation number follow a reverse behaviour. With the aim of examining the micellar microviscosity we have carried out experiments on the intramolecular excimer formation of P3P. This is a well-known viscosity-sensitive fluorescent probe[53]. The degree of intramolecular excimer formation and emission of P3P depends on the local viscos-ity of the probe imposed by its microenvironment. Therefore, the monomer to excimer intensity ratio, IM/IE, gives a quali-tative index of the microviscosity as sensed by the probe. The results obtained in our P3P intramolecular excimer formation experiments are also presented inFig. 6. From this figure it is observed that, in the case of micelles formed by pure surfac-tants, the microviscosity of OTG micelles is higher than that of SDS. Microviscosity in micelles is usually attributed to two different effects: hydration, which increases the microvis-cosity, and electrostatic repulsions between the headgroups, which reduces it[36]. In our case, it is clear that the electro-static repulsions between the sulphate groups of SDS play a dominant role in this context, at least in the low composition range as commented below. The tendency of IM/IEinFig. 6 reflects initially a considerable reduction of the microviscos-ity as the SDS content increases, up to minimum value around αSDS= 0.4, and then increases lightly for higher values of αSDS. The initial decrease of the micellar microviscosity is due to the increasing participation of the ionic component in the mixed micelle, which produces a rise of the electrostatic repulsions between the charged headgroups, originating the formation of micelles with a more loosened structure. Note that the abrupt decrease of the microviscosity in this compo-sition range (0 <αSDS< 0.4) is consistent with the behaviour shown inFig. 2b. In this figure it is observed, in this same composition range, a large increase in the participation of the ionic component withαSDS, to the point that the micellar composition becomes around 70% in SDS atαSDS= 0.4. For higher values ofαSDSthe microviscosity experiments a light increase, which could be justified by the rise of the micellar hydration reflected by the behaviour of the pyrene 1:3 ratio in this region.

5. Conclusions

The micellar properties of the OTG–SDS system in 0.1 M NaCl aqueous solutions have been investigated. The behaviour of the mixed system was analyzed by using two

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different approaches in the context of the pseudophase sep-aration model. Data reported by RST were compared with those obtained by a treatment based on the Gibbs–Duhem equation, and a good agreement was found between them. This fact demonstrates that our mixed system behaves as a regular solution. The variations in the micellar aggregation number with the solution composition were attributed to the effects produced by the charged headgroup of the ionic com-ponent when this is incorporated into the mixed micelle. In this sense, it was established that the micellar size is deter-mined by a delicate balance of two opposite contributions: the repulsive interactions between the headgroups of the ionic surfactant, which prevents the micellar increasing, and the steric interactions favoured by the size of the headgroup of SDS in comparison with that of OTG. The data of micel-lar micropomicel-larity show little variations and are in accordance with the trend observed in the aggregation number, whereas those of microviscosity suggest the importance of the role played by the above interactions.

Acknowledgment

The authors are thankful to the Spanish Science and Technology Ministry (Plan Nacional de Investigación, Desar-rollo e Innovación Tecnológica (I + D + I)) for supporting this investigation (Project MAT2001-1743).

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