water nonionic microemulsions

Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 49–54

Spectrophotometric study of metal–ligand reactions in isooctane/Brij30/water nonionic microemulsions C. Cabaleiro-Lago a , L. Garcia-R´ıo b , P. Herv´es a,∗ , J. P´erez-Juste a a

b

Departamento de Qu´ımica F´ısica, Facultad de Qu´ımica, Universidade de Vigo, 36310 Vigo, Spain Department of Physical Chemistry, Faculty of Chemistry, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain Received 5 May 2006; received in revised form 21 July 2006; accepted 10 August 2006 Available online 22 August 2006

Abstract The complexation between Ni2+ and pyridine-2-azo-p-dimethylaniline (PADA) was studied in isooctane/polyoxyethylenglycol dodecyl ether (Brij30)/water microemulsions at 25 ◦ C. The apparent complexation constant depends on microemulsion composition. The proposed model, which takes into account the heterogeneity of the system at the microscopic scale, allows us to determine the distribution constants of both reactants and the complexation constant at the interface. The complex is less stable in microemulsion than in water due to a more efficient hydration of the nickel hexahydrate coordination complex that arises from the interaction between the polar head group of the surfactant and the interfacial water. © 2006 Elsevier B.V. All rights reserved. Keywords: Microemulsion; Metal ligand reaction

1. Introduction A microemulsion is a thermodynamically stable dispersion of two immiscible liquids, usually water and oil, that at microscopic level coexist and form individual domains that are separated by a film of surfactant. The existence of three environments, a continuous organic medium, a disperse aqueous phase and an interface, the surface-active film, provides microemulsions with the ability to modulate chemical reactivity. The effect of the microemulsions on reactivity relies on the compartmentalization of reactants of very different nature, for instance, organic compounds and inorganic salts in different microenvironments and to allow the contact between such reactants in an intermediate interfacial region. Microemulsions have been used as reaction media for a number of reactions such as organic synthesis, polymerization, synthesis of metallic nanoparticles, or enzymatic reactions [1–3]. Another important field of application is the extraction of metals from a liquid phase using an environmentally friendly method [4,5]. The bidentanted ligand pyridine-2-azo-p-dimethylaniline (PADA) forms complexes with different metallic ions such as

∗

Corresponding author. Tel.: +34 986 812297; fax: +34 986 812556. E-mail address: [email protected] (P. Herv´es).

0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.08.029

Co2+ , Zn2+ , Mn2+ , Ni2+ , etc. [6,7] (Scheme 1). The formation of metal–ligand complexes between divalent metallic ions and the bidentanted ligand PADA has been studied in a variety of solvents and also in microheterogeneous media as micelles, [8–16] ionic microemulsions [17–19] and vesicles [20]. This reaction has been proposed in vesicles and micellar systems as a method of extraction of metallic ions from aqueous systems. The method has the advantage that the aggregate induced by the reaction is uniformly distributed throughout the medium [20,21]. In this paper we have focused on the spectrophotometric study of the metal–ligand reaction in microemulsions formed by the nonionic surfactant polyoxyethylenglycol dodecyl ether (Brij30). The nonionic surfactants based on fatty acids are ecologically suitable since they are biodegradable and show low water toxicity. Additionally, nonionic surfactants are good emulsifiers, and therefore they constitute an excellent choice for metal removal. Phase behaviour of polyoxyethylene glycol surfactant as C12 EO4 has been reported in several works [22–25] but unfortunately detail phase behaviour for Brij30 is not available in literature. Nevertheless several authors report the formation of microemulsions and reverse micelles in an extense water range for this particular surfactant [26–29]. Study of the structure of the polar core in reverse micelles show similar results for the monodisperse surfactant C12 EO4 and the commercial analogue Brij30, reporting the formation of reverse micelles at very low

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C. Cabaleiro-Lago et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 49–54

Scheme 1.

water content and the segregation of water into the water core at water content lower that the concentration needed to hydrate the EO groups of the surfactant head group [27]. The obtained results allowed us to compare the effect upon the reaction of a nonionic head group with the effect previously reported for ionic bis(2-ethylhexyl)sulfosuccinate (AOT) microemulsions [18]. 2. Experimental Piridine-2-azo-p-dimethylaniline, nickel nitrate Ni(NO2 )3 and polyoxyethylenglycol dodecyl ether (Brij30) were supplied by Aldrich and used without further purification. Microemulsions were prepared by mixing a stock solution of Brij30 in isooctane 2 M, isooctane and aqueous solutions of nickel salt (water was distilled and deionized) of different Ni2+ concentration, in the appropriate proportions. The PADA ligand was added to the samples by addition of a small volume of a stock solution of PADA in isooctane. The concentration of PADA was 2.5 × 10−5 M for all samples. The equilibrium constants for the formation of the complex Ni–PADA were determined by analyzing the UV–vis spectra of samples with a constant concentration of PADA and different nickel concentrations. Absorption spectra were registered by using a UV–vis spectrophotometer Hewlett Packard 8453 between 300 and 700 nm at 25 ◦ C. 3. Results and discussion 3.1. Effect of microemulsion composition on the complexation constant

Fig. 1. Variation of absorption spectra of PADA in isooctane/Brij30/water microemulsion at 25 ◦ C upon the addition of increasing amounts of Ni2+ (Ni2+ concentration ranges from 0 to 0.003 M). [Brij30] = 0.3 M, ω = 3 and [PADA] = 2.5 × 10−5 M. The absorption band at 420 nm corresponds to PADA whereas the band at 540 nm corresponds to Ni–PADA complex.

Z = [isooctane]/[Brij30], is varied. Several values of ω, ranging from ω = 3 to ω = 11, were used. From the variation of the absorbance at 540 nm the apparent complexation constant, Kap , can be obtained for a particular microemulsion composition. The absorbance at 540 nm can be expressed by the following equation ap

ap

A = εPADA [PADA] + εNi−PADA [Ni − PADA]

(1)

If the mass balance for PADA is considered [PADA]T = [PADA] + [Ni − PADA]

(2)

Eq. (1) can be transformed into ap

ap

ap

A = εPADA [PADA]T + (εNi−PADA − εPADA )[Ni − PADA] ap

(3)

ap

where εPADA and εNi−PADA are the apparent molar absorbances, which depend on the properties of the medium. To fit the absorbance values according to Eq. (3), the concentration of the complex [Ni–PADA] for each total concentration of Ni2+ must be determined. The apparent complexation constant can be defined as Kap =

[Ni − PADA] ([PADA]T − [Ni − PADA])([Ni2+ ]T − [Ni − PADA]) (4)

Thus, Eq. (4) can be rewritten as Fig. 1 shows an example of the evolution of the absorption spectra upon increase of Ni2+ concentration keeping constant the concentration of PADA. The band at 410 nm that corresponds to PADA, decreases as the concentration of Ni2+ increases whilst the band at 540 nm that corresponds to the Ni–PADA complex progressively increases. Experiments for various system compositions were performed to analyze the effect of microemulsion composition on complexation reaction. For each series, the ratio between the concentrations of water and surfactant, ω = [water]/[Brij30] is kept constant and the ratio between isooctane and surfactant,

Kap [Ni − PADA]2 − (Kap ([PADA]T + [Ni2+ ]T ) + 1) × [Ni − PADA] + Kap [PADA]T [Ni2+ ]T = 0

(5)

The solution of this equation yields the value of [Ni–PADA] for each pair of values [PADA]T and [Ni2+ ]T . To calculate the value for Kap , it is necessary to carry out a trial and error process. An initial guessed value for Kap is introduced into Eq. (5) and [Ni–PADA] is calculated for each [Ni2+ ] concentration. The resulting values are introduced into Eq. (3) and the process is repeated until a value of Kap is reached that provides the best fit

C. Cabaleiro-Lago et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 49–54

Fig. 2. Linear variation of Absorbance at 540 nm with the calculated concentration of Ni–PADA for the best value of Kap . [Brij30] = 0.7 M and ω (䊉) 3; () 4.5; and () 6.

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tributed between the different phases according to their nature. Hydrophilic Ni2+ ions will be distributed between aqueous phase Ni . The analysis and interface with a distribution constant Kwi of absorption spectra of PADA in AOT microemulsions indicates that PADA is distributed between the oil phase and the interface. Taking into account the results obtained in AOT [18] and with regard to the red shift of PADA spectra observed in Brij30 microemulsion, we consider that PADA is also distributed between the oil phase and the interface with a distribution conPADA . stant, Koi From a qualitative point of view, it can be seen in Table 1 that Kap increases as Z increases. That is, the lower is the concentration of Brij30 in the system, the higher is the apparent association constant. Taking into account the low concentration of PADA in the system and the most likely high value of the distribution constant of PADA, for the lowest concentration of Brij30 presumably all PADA is associated to the interface. Further addition of surfactant to the system increases the volume of the interface leading to the consequent dilution of PADA at the interface with the consequent reduction of Kap . On the other hand, for Z constant, Kap slightly decreases as ω increases. An increase on ω implies an increase on the size of water microdomains and this gives rise in turn to a dilution of Ni2+ ions in the aqueous core shifting the partition equilibrium to the water side and causing a desorption of Ni2+ at the interface and consequently a decrease of the complexation constant. For all cases, Kap is lower than the complexation constant in water, [21], i.e. the formation of Ni–PADA complexes is more favorable in water than in this kind of microemulsions. 3.2. Determination of the real equilibrium constants

Scheme 2.

to a straight line according to Eq. (3). Fig. 2 shows, as an example, the linear variation of absorbance with the concentration of Ni–PADA. The concentrations of complex for each total concentration of Ni have been obtained supposing the best value for Kap for several microemulsions composition. Values for Kap were obtained for different compositions, varying Z for constant values of ω (Table 1). To interpret theses results it is necessary to propose a complexation scheme in the microemulsion (Scheme 2). Considering the pseudophase model, the microemulsion is considered to be divided into three different environments, aqueous phase, oil phase and interface. The reactants will be dis-

To carry out a quantitative analysis, the complexation constant at the interface must be defined. K=

[Ni − PADA]

(6)

i

[PADA]ii [Ni2+ ]i

The concentrations at the interface are referred to the volume of the interface. The concentrations of any species referred to the interface [X]ii can be expressed in terms of the concentration related to the total volume of the system [X]i taking into account the molar volume of the surfactant, V ; [X]ii =

[X]i V [Brij30]

(7)

Table 1 Variation of Kap for different microemulsion compositions ω

Z

Kap (M−1 )

3 3 3 3 3 3

28 21 12 9.4 7.1 5.5

6000 5200 3200 2980 2700 2180

± ± ± ± ± ±

200 325 100 75 100 75

ω

Z

Kap (M−1 )

4.5 4.5 4.5 4.5 4.5 4.5

26 21 17 14 9.4 5.3

5400 4500 4300 3400 2340 1960

± ± ± ± ± ±

Complexation constant in water [21] Kw = 1.2 × 104 M−1 .

250 250 150 125 60 90

␻

Z

Kap (M−1 )

6 6 6 6 6 6

12 10 9.1 7.4 5.9 4.6

2690 2500 2300 1980 1700 1530

± ± ± ± ± ±

75 100 130 50 125 65

ω

Z

Kap (M−1 )

7 7 7 7 7 7

10.34 9.39 7.99 6.34 5.29 4.54

2225 2110 1780 1555 1445 1335

± ± ± ± ± ±

25 50 80 90 70 15

ω

Z

Kap (M−1 )

7.5 7.5 7.5 7.5 7.5 7.5

10 8 6.3 5.2 4.8 4

2020 1700 1500 1330 1300 1260

± ± ± ± ± ±

65 100 50 45 50 30

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so that Eq. (6) can be transformed into   [Ni − PADA]i Brij30 V¯ K= [PADA]i [Ni2+ ]i

(8)

Considering the proposed mechanism (Scheme 2) the apparent association constant (Eq. (4)) can be rewritten as Kap =

[Ni − PADA]i ([PADA]o + [PADA]i )([Ni2+ ]w + [Ni2+ ]i )

(9)

The expressions for all the species in the different pseudophases can be obtained taking into account the distribution constants defined as PADA Koi =

[PADA]i Z; [PADA]o

Ni Kwi =

[Ni2+ ]i ω [Ni2+ ]w

(10)

and the complexation constant at the interface, K. If it is assumed that, the total concentration of PADA is the sum of the concentration of PADA in the different phases plus the concentration of the complex at the interface, the following equations are obtained [PADA]i = [PADA]o =

PADA [PADA] Koi T PADA Koi

PADA K[Ni2+ ] /[Brij30]V ¯) + Z+(Koi i

Z[PADA]T PADA + Z+ (K PADA K[Ni2+ ] /[Brij30]V ¯) Koi i oi

(11) (12)

[Ni − PADA]i =

PADA [PADA] Koi K[Ni2+ ]i T   PADA + Z + (K PADA K[Ni2+ ] / Brij30 V ¯) [Brij30]V¯ Koi i oi (13)

By combination of Eqs. (9)–(13), an expression for the apparent complexation constant can be proposed Kap =

PADA KKoi [Ni2+ ] PADA ) [Brij30]V¯ (Z + Koi ([Ni2+ ]w + [Ni2+ ]i )

(14)

Fig. 3. Variation of the inverse of Kap and Z for ω (䊉) 3; () 4.5; () 6, in isooctane/Brij30/water microemulsion at 25 ◦ C.

trend have been observed for all the studied values of ω. The ratio between slope and intercept yields the value for the distribution constant for PADA. The values for each series are independent of microemulsion composition and the mean value of the PADA = 79.1 ± 7.0. The distribution distribution constant is Koi PADA = 100. constant in AOT microemulsion is slightly higher Koi Although the discrepancy is not significant, the difference in distribution constant could arise from the disparity of the polarity between an ionic and a nonionic interface. The data of polarity in terms of the empiric polarity scale ET (30) show that the polarity of the interface for the same ω value (ω = 3) for AOT microemulsions (ET (30) = 42.3) [30] is lower than the polarity of the interface for microemulsions of Brij30 (ET (30) = 47.5) [31]. Most probably the association of the hydrophobic PADA to the interface in Brij30 microemulsion would be weaker than the association to AOT microemulsion, and consequently the distribution constant would be lower, due to its higher polarity. From Eq. (17) the following expression can be obtained that points to a linear correlation between the intercept of the previous plots and the water content, ω 1 ω + Ni K KKwi

From the mass balance for Ni2+ , the following expression can be written

Intercept =

KNi [Ni2+ ]i = Ni wi 2+ Kwi + ω ([Ni ]w + [Ni ]i )

Fig. 4 confirms the expected linear correlation between the intercept of the previous plots and ω. The observed linear

2+

(15)

Eq. (14) can be simplified by using Eq. (15) to an expression in terms of the distribution constants and the parameters that define the microemulsion composition Ni Kwi KKPADA  oi Kap =  PADA Ni Brij30 V¯ (Z + Koi ) Kwi + ω

(16)

For constant ω, Eq. (16) can be rewritten showing a linear dependence between the inverse of Kap and Z. Ni + ω KNi + ω Kwi 1   = wi Ni + Z PADA K Ni Kap Brij30 V¯ KKwi KKoi wi

(17)

As can be observed in Fig. 3, the plot of the inverse of Kap versus Z follows the linear trend expected from Eq. (17). For clarity, only three values of ω have been showed in the plot but the same

Fig. 4. Variation of the intercept (according to Eq. (18)) and ω.

(18)

C. Cabaleiro-Lago et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 49–54

Scheme 3.

trend supports the proposed model and the capability of the model to describe the system and to obtain the distribution and complexation constants of the process. From the relation between intercept and ω (Fig. 4), the distributions constant Ni = 1.86 ± 0.25 and the complexation constant, for Ni+2 , Kwi K = 2005 ± 200 M−1 , were calculated. 3.3. Evaluation and comparison of the complexation constant The value for K is much smaller than the value in water, Kw = 1.2 × 104 M−1 [21]. This low value can be explained if we consider that the interaction between the polar head group of the Brij30 and the interfacial water increases the negative charge on the oxygen atom of the water molecules, an effect that increases the stability of the nickel hexahydrate complex, (Ni(H2 O)6 )2+ (see Scheme 3). For AOT microemulsions, the complexation constant is, again for all microemulsion compositions, higher than the constant in nonionic microemulsion. For AOT two opposite effects are present: The polar head group of AOT increases the charge at the oxygen in a similar way that Brij30, but the counterion Na+ exerts an opposite effect. The balance between both interactions results in a lower hydration of Ni2+ and, accordingly, a higher equilibrium constant for the complex in AOT microemulsion than in Brij30 microemulsion. In nonionic microemulsions, no counterion is present and therefore the counterion contribution does not reduce the effect of head group interaction. The complexation constant K remains invariable presumably as a result of a compensation of effects. The decrease of ω enhances the hydration on Ni2+ due to the interaction between head group and water and, consequently, K decreases. The effect of surfactant head group has been studied previously showing that a decrease on the water content of the microemulsion give a higher disruption of the water structure [31] enhancing the ability to interact with the metal ion. On the other hand, as ω decreases, the concentration of water available to form the hydration shell decreases and therefore the complexation constant, K, increases. Despite of it has been reported that the complete hydration of the EO groups is not necessary to form a water pool, [27] at low water content, water molecules are predominantly hydrating the surfactant head group and are not free to form the Ni2+ hydration shell.

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Previous studies in ionic microemulsions, where the pseudophase model with ionic exchange can be applied, [18] showed that the complexation constant depends on the concentration of water. In the present system the model proposed does not allow us to analyze this kind of dependence so only a mean value for the complexation constant could be obtained. If we assume that limitation, the linear trend expected from Eq. (18), which it is observed in our results, should be a consequence of a covariance of the constant which finally results in a linear trend between intercept and ω (Fig. 4). However, most probably, the observed linear correlation confirms the validity of the model and suggests that if a dependence on ω does exist, it must be weak. Another justification would be a low sensitivity of this reaction towards the water content beyond the limited range of ω that can be studied. For systems where a wider range of ω can be explored, probably a significant effect could be observed. Taking into consideration the restricted range of compositions studied we can presume that the obtained result is a suitable value for the complexation constant of the Ni–PADA complex for the explored conditions. 4. Conclusions A thermodynamic study of the metal–ligand reaction for the formation of Ni–PADA complexes in nonionic microemulsions has been carried out. The analysis of the apparent parameters shows a clear dependence on microemulsion composition. The stability of the Ni–PADA complex is lower in microemulsions than in water though its stability increases as both the water content of the microemulsion and the concentration of surfactant decrease. The pseudophase model has been applied to this kind of reversible reactions and allowed us to determine the real association constant. The proposed model is not suitable to analyze the dependence of the reaction on microemulsion composition because the parameters that define the composition, ω and Z, are included in the derived expression. In spite of that, the results suggest that this dependence on composition must be negligible, in clear disagreement with the results obtained in AOT microemulsions. The complexation constant is lower in microemulsion than in water due to the more efficient solvation of the nickel hexahydrate complex because of the interaction between the head group of the surfactant with the interfacial water. Acknowledgments Financial support from the Ministerio de Educaci´on y Ciencia (Project MAT2004-02991 and CTQ2005-04779) and from Xunta de Galicia (PGIDIT04MT209003PR) is gratefully acknowledged. C.C.-L. thanks a FPU fellowship). References [1] R.A. Mackay, Chemical reactions in microemulsions, Adv. Colloid Interface Sci. 15 (1981) 131–156.

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[2] K. Holmberg, Organic and bioorganic reactions in microemulsions, Adv. Colloid Interface Sci. 51 (1994) 137–174. [3] M.P. Pileni, Reverse micelles as microreactors, J. Phys. Chem. 97 (1993) 6961–6973. [4] C. Tondre, M. Hebrant, M. Perdicakis, J. Bessiere, Removal of copper ions by micelle-based separation processes. Electrochemical behavior of copper ions trapped in micellar particles, Langmuir 13 (1997) 1446– 1450. [5] G.D. Smith, B.B. Garrett, S.L. Holt, R.E. Barden, Properties of metal complexes in the interphase of an oil continuous microemulsion. 2. Interaction of copper(II) with the side chains of lysine, glutamine, and methionine, Inorg. Chem. 16 (1977) 558–562. [6] M.A. Cobb, D.N. Hague, Role of metals in enzymic reactions. 4. Kinetics of complex formation in model systems involving cobalt(II) and pyridine-2-azo-p-dimethylaniline, Trans. Faraday Soc. 67 (1971) 3069– 3075. [7] G.R. Cayley, D.N. Hague, Role of metals in enzymic reactions. 3. Kinetics of complex formation in model systems involving zinc and p-(2pyridylazo)-N,N-dimethylaniline, Trans. Faraday Soc. 67 (1971) 786–793. [8] E.F. Caldin, R.C. Greenwood, Activation volumes for the ligandsubstitution reactions of nickel(II) and cobalt(II) cations with pyridine2-azo-4-dimethylaniline in various solvents, J. Chem. Soc. Faraday Trans. I 77 (1981) 773–782. [9] E.F. Caldin, P. Godfrey, Kinetics of the reaction of the nickel(II) ion with pyridine-2-azo-p-dimethylaniline in tert-butanol–water mixtures, J. Chem. Soc. Faraday Trans. I 70 (1974) 2260–2266. [10] B.H. Robinson, N.C. White, Kinetics and mechanism of fast metal–ligand substitution processes in aqueous and micellar solutions studied by means of a dye-laser photochemical relaxation technique, J. Chem. Soc. Faraday Trans. I 74 (1978) 2625–2636. [11] J.R. Hicks, V.C. Reinsborough, Rate enhancement of the nickel(II)-pada complex formation in sodium alkanesulfonate micellar solutions, Aust. J. Chem. 35 (1982) 15–19. [12] D.J. Jobe, V.C. Reinsborough, Surfactant structure and micellar rate enhancements: comparison within a related group of one-tailed, two-tailed and two-headed anionic surfactants, Aust. J. Chem. 37 (1984) 1593–1599. [13] A.I. Carbone, F.P. Cavasino, E. Di Dio, C. Sbriziolo, Effects of nonionic, cationic, and anionic micelles on the kinetics of the complexation reaction of nickel(II) with pyridine-2-azo-p-dimethylaniline, Int. J. Chem. Kinet. 18 (1986) 609–621. [14] V.C. Reinsborough, T.D.M. Stultz, X. Xiang, Rate enhancement of nickel(II)-pada complex formation in mixed sodium perfluorooctanoate/ octanesulfonate micellar solutions, Aust. J. Chem. 43 (1990) 11–19. [15] K.A. Berberich, V.C. Reinsborough, C.N. Shaw, Kinetic and solubility studies in zwitterionic surfactant solutions, J. Solution Chem. 29 (2000) 1017–1026.

[16] M.L. Turco Liveri, R. Lombardo, C. Sbriziolo, G. Viscardi, P. Quagliotto, Kinetic evidence for the solubilization of pyridine-2-azo-p-dimethylaniline in alkanediyl-a,w-bis(dimethylcetylammonium nitrate) surfactants. Role of the spacer chain length, New J. Chem. 28 (2004) 793–799. [17] R. Schomaecker, Chemical reactions in microemulsions: probing the local dielectric number of the dispersed water, J. Phys. Chem. 95 (1991) 451–457. [18] E. Fernandez, L. Garcia-Rio, A. Knoerl, J.R. Leis, Metal–ligand complexation in water-in-oil microemulsions. I. Thermodynamic approach, Langmuir 19 (2003) 6611–6619. [19] N.Z. Atay, B.H. Robinson, Kinetic studies of metal ion complexation in glycerol-in-oil microemulsions, Langmuir 15 (1999) 5056–5064. [20] H.A. Gazzaz, B.H. Robinson, Kinetics involving divalent metal ions and ligands in surfactant self-assembly systems: applications to metal-ion extraction, Langmuir 16 (2000) 8685–8691. [21] G. Monteleone, L. Morroni, B.H. Robinson, M.R. Tin´e, M. Venturini, F. Secco, Metal ion extraction in surfactant solution: Ni2+(aq) and Cd2+(aq) with the ligands PADA and PAR in SDS micellar systems, Colloids Surf. A: Physicochem. Eng. Aspects 243 (2004) 23–31. [22] M. Kahlweit, How To prepare microemulsions at prescribed temperature, oil, and brine, J. Phys. Chem. 99 (1995) 1281–1284. [23] M. Kahlweit, Microemulsions, Annu. Rep. Prog. Chem. Sect. C: Phys. Chem. 95 (1999) 89–115. [24] K. Shinoda, H. Kunieda, Conditions to produce so-called microemulsions. Factors to increase the mutual solubility of oil and water by solubilizer, J. Colloid Interface Sci. 42 (1973) 381–387. [25] J.C. Ravey, M. Buzier, Composition dependence of the structure of waterswollen nonionic micelles, J. Colloid Interface Sci. 116 (1987) 30–36. [26] N.M. Correa, M.A. Biasutti, J.J. Silber, Micropolarity of reversed micelles: comparison between anionic, cationic, and nonionic reversed micelles, J. Colloid Interface Sci. 184 (1996) 570–578. [27] H. Caldararu, A. Caragheorgheopol, M. Vasilescu, I. Dragutan, H. Lemmetyinen, Structure of the polar core in reverse micelles of nonionic poly(oxyethylene) surfactants, as studied by spin-probe and fluorescence probe techniques, J. Phys. Chem. 98 (1994) 5320–5331. [28] A. Forgiarini, J. Esquena, C. Gonzalez, C. Solans, Formation of nanoemulsions by low-energy emulsification methods at constant temperature, Langmuir 17 (2001) 2076–2083. [29] A.M. Chinelatto, L.T. Okano, O.A. Elseoud, Effect of anionic and nonionic water-in-oil microemulsions on acid-base equilibria of hydrophilic indicators, Colloid Polym. Sci. 269 (1991) 264–271. [30] C.A.T. Laia, S.M.B. Costa, Probing the interface polarity of AOT reversed micelles using centro-symmetrical squaraine molecules, Phys. Chem. Chem. Phys. 1 (1999) 4409–4416. [31] C. Cabaleiro-Lago, L. Garcia-Rio, P. Herves, J. Perez-Juste, Reactivity of benzoyl chlorides in nonionic microemulsions: potential application as indicators of system properties, J. Phys. Chem. B 109 (2005) 22614–22622.