Journal of Colloid and Interface Science 300 (2006) 713–723www.elsevier.com/locate/jcis

Micellar solubilization of tributylphosphate in aqueous solutionsof Pluronic block copolymers

Part I. Effect of the copolymer structure and temperatureon the phase behavior

J. Causse a, S. Lagerge a,∗, L.C. de Menorval a, S. Faure b

a Laboratoire des Agrégats Moléculaires et Matériaux Inorganiques, C.N.R.S. UMR-5072, Université de Montpellier II, CC 015, Place E. Bataillon,34095 Montpellier Cedex 05, France

b Laboratoire des Procédés Avancés de Décontamination, CEA Valrhô Marcoule, Bat 17171, 30207 Bagnols Sur Cèze, France

Received 2 November 2005; accepted 15 April 2006

Available online 25 April 2006

Abstract

Solubilization of tributylphosphate (TBP), a polar oil, in various micellar solutions of Pluronic has been investigated by turbidimetry empha-sizing the effect of temperature and the role of the PPO and PEO blocks on the phase behavior of the three components systems (Pluronic–TBP–water). [Temperature–composition] diagrams allow monophasic and diphasic domains to be delimited. Two temperatures are shown tohave a determining effect on the phase behavior (TBP solubilization); the well known cloud point temperature (CPT, here defined for the threecomponents system) and the solubilization minimum temperature (SMT) which is defined as the lowest temperature allowing solubilization ofTBP in the system. Both temperature depend on the copolymer structure and, interestingly, are directly related to the TBP concentration in themedium. Monophasic microemulsions are observed when the temperature ranges between the SMT and the CPT. When T < SMT, the phaseseparation occurs and is related to the formation of TBP in water emulsion droplets. When T > CPT the system separates in two phase due tothe co-precipitation of TBP and Pluronic. Moreover an unexpected evolution of the CPT with the TBP content clearly indicates the occurrenceof a structural change of the microemulsions which allows higher quantities of TBP to be solubilized. But the structural change does not allowalone higher quantities of TBP to be solubilized. A well compromise between the SMT and the CPT must be also observed so as to obtain a largeextent of monophasic domain after the restructuration. The best compromise is obtained with Pluronics with intermediate hydrophobic character.Reversely, hydrophobic and hydrophilic Pluronics exhibit a very small extent of monophasic domain after the restructuration which does not allowbenefit by the structural change.© 2006 Elsevier Inc. All rights reserved.

1. Introduction

Surfactants have been widely used so as to greatly increasethe solubilization of hydrophobic molecules in aqueous so-lution, due to the formation of micelles or microemulsions[1–8]. The ability of micellar solutions to incorporate solubi-lized materials is one of the fundamentally important proper-ties of micelles and provides the basis for the widespread useof surfactants and micellar solutions in various industrial, bi-

* Corresponding author. Fax: +33 (0) 4 67 14 33 04.E-mail address: slagerge@univ-montp2.fr (S. Lagerge).

0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.04.042

ological, pharmaceutical, chemical, and catalytic applications[1–3]. It is well known that a special kind of amphiphilic tri-block copolymers ((EO)n–(PO)m–(EO)n) consisting of two hy-drophilic side-blocks (poly(ethyleneoxide) ≡PEO or (EO)n)and an hydrophobic core block (poly(propyleneoxide) ≡PPOor (PO)m) can form micelles in water above a given concen-tration (critical micelle concentration, cmc) and temperature(critical micelle temperature, cmt). These so-called Pluronics(or Synperonics, or Poloxamers) triblock copolymers can beconsidered as supramolecular non-ionic surfactants exhibitingvery similar properties which are capable of greatly solubilizinghydrophobic molecules in aqueous solution. As a result their

714 J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723

aggregation properties have been widely investigated in thepast decade [9–18]. Both their low toxicity and physicochemi-cal properties gave rise to a wide industrial interest. Especiallythese are used throughout various applications such as deter-gency, cosmetics, pharmaceutics, medicine, controlled releasedor dye solubilization [19]. Regarding the oil solubilization, theefficiency depends on both the chemical structure of the solu-bilizing molecule and the copolymer block lengths. Hurter andHatton [20] studied the naphthalene solubilization in high mole-cular weight Pluronics. They concluded that the naphthalenesolubilization in a 7 wt% P123 solution reached 150 times theconcentration in pure water while Collett and Tobin showed thatan optimal PEO/PPO ratio was necessary to maximize the ac-etanilide solubilization [21]. Most of the investigations on thistopic deal with solubilization of benzene derivatives [22–24],but the literature concerning the solubilization of polar oils isstill scarce. It is now well established that the temperature playsa key role on the aggregation properties of Pluronics and onthe solubilization behavior of the resulting aggregates. Increas-ing the temperature shifts the cmc value to lower copolymerconcentrations [25,26]. Alexandridis et al. [9] explain this phe-nomenon by the decrease of PEO and PPO polarity as the tem-perature rises up leading to dehydration of the PEO chains. Thesolubility of these copolymers is driven by the PEO–H2O in-teractions and particularly the system tends to form aggregatesat lower copolymer concentrations when the PEO–H2O affinitydecreases. At a given temperature, the cloud point temperature(CPT), all the PEO chains are dehydrated resulting in the col-lapse of the polymer chains and consequently a diphasic systemappears with both a water-rich and a copolymer-rich phase. TheCPT is a common characteristic of non-ionic surfactants. Kjel-lander and Florin [27] suggested that the structuring water sur-rounding the PEO chains is entropically unfavorable. When thetemperature rises up, the entropic effect dominates and the sys-tem will tend to a copolymer precipitation so as to reduce theseunfavorable interactions. Another model based on conforma-tional change of the PEO chain at high temperatures is proposedto describe this phenomenon [28]. Two conformations coex-ist in a PEO chain; (i) a gauche conformation around the C–Cbound, and (ii) a trans-conformation around the C–O bound.The later polar conformation favorably interacts with water.When the system is heated, the thermal agitation damages thechain conformation and thereby significantly reduces the favor-able PEO–H2O interactions leading to a phase separation. NMRresults have confirmed these conformational changes [29,30].The presence of polyethylene(oxide) chains is mainly respon-sible for the solubility of non-ionic surfactants and Pluronicsin water. Indeed, surfactant solubility increases with increasingthe PEO chains length [31]. On this basis, the evolution of thecloud point temperature informs us about the PEO hydrationand thereby on the amphiphile solubility in aqueous solution.Several authors studied the influence of various additives onthe CPT of different non-ionic surfactants [32–35]. As a generaltrend, it appears that the presence of added inorganic salts de-press the cloud point temperature by a “salting-out” effect. Thesalt surrounds itself with water molecules and acts like a solventpump to dehydrate the PEO chains and consequently decrease

the surfactant or copolymer solubility. Conversely, addition ofacids to the solution raises the CPT of non-ionic surfactants. Inthis case, the hydrogen ions cause a “salting-in” effect of typ-ical non-ionic surfactants. It was also proved that short chainalcohols and urea have a similar effect than acids on the cloudpoint temperature.

In this paper, we have studied the solubilization of TBP inmicellar solutions of various Pluronic with a special emphasison its effect on the CPT value and on the resulting phase behav-ior. Tributylphosphate (TBP) is a kind of neutral phosphorusextractor, which has been widely used in the development ofnuclear technology and the separation of non-ferrous metals,rare earth metals, and actinides [36]. This lipophilic compoundis used as a complexant in liquid–liquid extraction cycles of ra-dioactive metals such as, for example, uranium, plutonium, andrhodium [37,43,44]. The determination of the extent to whicha particular solubilizate (TBP) can be incorporated into a spe-cific micellar solution, while maintaining the integrity of thatsolution, is fundamental for both the study of solubilized sys-tems and applications of micellar solutions. The solubilizationcapacity of various aqueous solutions of Pluronics for TBP hasbeen measured. Especially, the phase behavior of the three com-ponents systems (Pluronic–TBP–water) have been examined indetail.

2. Experimental

2.1. Material and sample preparation

The ternary systems are made of aqueous solutions ofamphiphilic triblock copolymers including TBP (a polar oil).Copolymers of ethylene(oxide) and propylene(oxide)(poly(ethyleneoxide)–poly(propyleneoxide)–poly(ethyleneox-ide)), (EO)n–(PO)m–(EO)n)) referred to as Pluronic L44, L64,P84, P94, P104, and P123 were obtained as a gift from BASFCorporation (France). Their structural description and sometypical properties at 298 and 308 K are presented in Table 1.L44, L64, P84, P94, P104 exhibit the same EO/PO composi-tion ratio (40 wt% of EO) while the same hydrophilic blocksize (PEO chain) is kept between P84 and P123. Pluronics con-taining the same EO/PO composition ratio in their molecularstructures, but with different molecular weights, differ in thelength of both their PEO and PPO blocks. As a result, for the

Table 1Structures and some physicochemical properties of the investigated Pluronics

Pluronic EOx–POy–EOxa Mw

(g mol−1)

PEO(wt%)

cmcb (%, w/w) CPTb

(K)298 K 308 K

L44 x = 10; y = 23 2200 40 – 0.78 (310.5 K) 65L64 x = 13; y = 30 2900 40 7.5 0.4 58P84 x = 19; y = 43 4200 40 2.6 0.15 74P94 x = 21; y = 47 4600 40 0.31 – 76P104 x = 27; y = 61 5900 40 0.3 0.008 81P123 x = 19; y = 69 5750 30 0.03 0.001 90

a Average number.b From Refs. [25,52,53].

J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723 715

investigated Pluronics all exhibiting 40 wt% of EO, the sizesof the PEO and PPO blocks are in the order P104 > P94 >

P84 > L64 > L44 (see Table 1). According to the cmc valuesreported in Table 1, the hydrophobic character is in the sameorder, i.e., increases with increasing the molecular weight orlength of PPO block. All these polymeric materials are knownto contain small contaminating quantities of hydrophobic im-purities such as diblock copolymers [38–41]. These impuritieswere removed following a purification procedure by dissolu-tion/precipitation using hexane (purity exceeding 99%) [42].Ten gram of Pluronic was incorporated in 25 ml of n-hexaneand stirred at room temperature for 2 h. The separated hexanephase containing solubilized hydrophobic impurities was dis-carded, and we repeated this operation four times. After the lastoperation, the remaining hexane phase was evaporated fromthe purified copolymer under moderate vacuum (10 mm Hg) at298 K for 3 h. The resulting copolymer was finally dried for24 h on a vacuum pump to remove any residual solvent. Thewater used in these experiments was distilled and deionizedwith a Millipore “Super Q” system (18 M� cm). Tributylphos-phate (TBP, (O=P(–O–(CH2)3–CH3)3) with purity ≈99% wassupplied by Merck (France). TBP is a complexing agent whichis widely used in the selective extraction of U and Pu from or-ganic solution [43,44].

Polymeric solutions were prepared by dissolving a givenmass of Pluronic in deionized water previously filtered througha MiniSart 0.22 µm cellulose acetate filter. Once the desiredconcentration of amphiphilic polymer was obtained, the wholewas maintained under gentle agitation for one night at constanttemperature. Then the solutions were kept under relaxation forone day before performing all the analysis.

2.2. Turbidity measurements

Turbidity measurements were made using a spectroscopictechnique (Metrohm 662 photometer at 600 nm) in order todetect the solubility limit of TBP in the polymeric solution.The solubilization procedure was carried out as follows. Smallaliquots (20 µl) of the pure TBP solution was injected step-wise, using an external syringe, into a baker containing 25 mlof an homogeneous solution of Pluronic above the aggregationconcentration. After each injection, the system was kept underagitation during 15 min before measuring the turbidity change.It is thus possible to detect the turbidity changes associated withsubsequent steps, i.e., occurring while introducing TBP in thepolymeric solutions and to follow the phase behavior of the sys-tems (solubilization process) step by step. The turbidity changewas followed by measuring the voltages output U of the spec-trode after each injection. This voltage output directly reflectsthe light transmission through the solution. Thus typical solubi-lization curves could be constructed by plotting the evolutionof voltage against the concentration of TBP in the medium,U = f (C). The temperature of the system was maintained atT ± 0.1 K during all the solubilization process.

Evolution of the cloud point temperature (CPT) and solu-bilization minimal temperature (SMT) of the Pluronics as afunction of the TBP content was determined as follows; a series

of Pluronic solutions with different TBP contents were preparedin the baker and thermostated at 20 ◦C for 30 min. Then, foreach three phase system (TBP–Pluronic–water) the temperaturewas stepwise increased. After each temperature increment, thesystem was maintained under gentle agitation during 15 min be-fore measuring the turbidity change. The highest and the lowesttemperature at which the system remained clear (monophasic)under stirring was reported as the CPT and SMT, respectively.

3. Results

TBP is a polar oil (ε = 8.09 and μ = 3.1 debyes [45])which is soluble in deionized water to a very small extent;its saturation concentration in pure deionized water, Csat, is1.6 × 10−3 mol kg−1 (=0.42 g l−1). TBP molecules in aque-ous solution at concentration close to the saturation value ex-hibit surface properties (γ = 42.1 mN m−1) own to their po-larity. Moreover these show a slight amphiphilic character inoil phase; for instance TBP nanosized micelles are formed indodecane in the presence of small amount of water in the sys-tem [46,47]. The solubility of TBP in aqueous solutions maybe drastically increased in the presence of amphiphilic copoly-mers which lead to self-assembly into various structures andmicroemulsions. In the present paper the solubilization behav-ior of TBP has been investigated in various amphiphilic sys-tems. The three components systems containing water, TBPand Pluronic molecules (described in Section 2) have been sys-tematically examined for the phase behavior essentially usingturbidity measurements. We aim at evidencing the role of thecopolymer structure (length of the PPO and PEO blocks and/orhydrophilic–hydrophobic character of the copolymer (relatedto EO/PO ratio)) on the phase behavior of the water–TBP–Pluronic systems. In this attempt a comparative study has beenundertaken allowing the effect of the temperature, of the natureof the copolymer, and of the Pluronic concentration on the sol-ubilization capacity to be evidenced. A part of the results havebeen previously reported and they are only rapidly presentedhere to aid understanding of the present results [48].

Figs. 1a–1c display the phase behaviors of various threecomponents systems containing Pluronics with similar EO/POratio (40 wt%). These plots reflect the evolution of the turbidityof the Pluronic solutions following addition of TBP with specialemphasis on the effect of the temperature. In abscissa we havereported the normalized TBP concentration, C/Csat, where C

and Csat are the solubility of TBP in the polymeric solutionand in pure deionized water, respectively. Figs. 1a–1c showthe typical variation of the normalized voltage U/Ur with TBPcomposition (C/Csat) of the 10 wt% P84, L64, and L44 solu-tions, respectively, at different temperatures. Ur is the referencevoltage obtained in the pure polymeric solution (without TBPaddition) and U is the voltage obtained for a Pluronic solutionat a given TBP content. The Pluronic concentration is expressedin mass ratio of Pluronic to Pluronic plus water. The maximalvoltage (U ) corresponding to the maximal transmittance of thesolution is obviously obtained with monophasic systems. Con-sequently the extent of the plateau region at U/Ur ≈ 1 gives

716 J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723

(a) (b)

(c)

Fig. 1. (a) Temperature effect on the TBP solubilization behavior of 10 wt% aqueous solutions of P84. (b) Temperature effect on the TBP solubilization behavior of10 wt% aqueous solutions of L64. (c) Temperature effect on the TBP solubilization behavior of 10 wt% aqueous solutions of L44.

direct information on the solubilization capacity of the Pluronicsolutions.

Pluronics containing the same EO/PO composition ratio intheir molecular structures, but with different molecular weights,differ in the length of both their PEO and PPO blocks. As aresult, for the investigated Pluronics all exhibiting 40 wt% ofEO, the sizes of the PEO and PPO blocks are in the orderP104 > P94 > P84 > L64 > L44 (see Table 1). According tothe cmc values reported in Table 1, the hydrophobic characteris in the same order, i.e., increases with increasing the mole-cular weight or length of PPO block. Figs. 2a–2e shows theeffect of the Pluronic concentration on the TBP solubilizationbehavior at 308 K for the L44, P94, P84, and L64 systems,respectively. Fig. 3 reports the effect of the block length onthe solubilization of TBP at 308 K by 10 wt% solutions ofPluronics with the same EO/PO ratio (L44, P94, P84, and L64).

Finally, the solubilization behaviors of P84 and P123 are com-pared in Fig. 4. The same hydrophilic block size (PEO chain)is kept between P84 and P123 (x = 19, see Table 1) but the re-lated phase behavior of the three components systems are quitedifferent.

Fig. 5 reports the evolution of both the CPT and SMT of the10 wt% P84, L44, and L64 systems as a function of the TBPcontent in the solution (C/Csat). The CPT is the well-knowncloud point temperature. It is commonly defined for the bi-nary systems of water–non-ionic amphiphile. Here the so-calledCPT is defined for the three components system meaning thatFig. 5 reflects the dependence of the CPT with the TBP content.The SMT is defined as a solubilization minimal temperature,i.e., the minimum temperature allowing solubilization of TBPinto the Pluronic solution at a given TBP content. This is thelowest temperature required so as to obtain the suitable hy-

J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723 717

(a) (b)

(c) (d)

(e)

Fig. 2. Effect of the Pluronic concentration (wt%) on the TBP solubilization behavior at 308 K for the L44 (a), P94 (b), P84 (c), and L64 (d and e) systems.

drophobicity of Pluronic aggregates (micelles) allowing TBPsolubilization at a given concentration. The three componentssystem is monophasic (transparent microemulsion) when thetemperature ranges between the SMT and the CPT. For temper-atures lower than the SMT or upper than the CPT, the systemseparates in two phases. Under stirring the three componentssystem is cloudy which is evidenced by very low U value orU/Ur index tending to zero. In the former case (below theSMT), the phase separation is related to the emulsification ofthe system (non-stable TBP in water emulsion droplets). Atrest this system is characterized by an upper TBP-rich phaseand an homogeneous and stable lower water-rich phase con-taining both copolymer and solubilized TBP (C > Csat), i.e.,a TBP/water/Pluronic microemulsion. In the later case (abovethe CPT), the phase separation is due to the precipitation of the

copolymer. At rest one observes an upper water-rich phase con-taining TBP molecules at a concentration lower than Csat and alower phase containing most of the copolymer co-precipitatedwith the TBP. This co-precipitation phenomenon is describedin the literature and is used as an extraction process of organicsolvents known as cloud point extraction [49]. In each case, theCPT curve gradually decreases following the addition of TBPup to a minimal value, then the CPT value stabilizes in an inter-mediate region and finally slightly increases again, especially inthe case of the L64 system. The initial declining part is logicallymuch more marked with the most hydrophobic copolymers (inthe order P84 > L64 > L44). The SMT increases with the ad-dition of TBP in the system and, as expected, the highest SMTvalues are obtained with the most hydrophilic copolymers (inthe order L44 > L64 > P84).

718 J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723

Fig. 3. Effect of the block length on the solubilization of TBP at 308 K by10 wt% solutions of Pluronics with the same EO/PO ration: L44 (1), P94 (×),P84 (P), and L64 (!).

Fig. 4. Solubilization of TBP at 308 K by 10 wt% solutions of Pluronics withthe same length of PPO block, i.e., y = 19 for both P84 (P) and P123 (!).

4. Discussion

4.1. Effect of the temperature on the TBP solubilization in thecopolymer solutions

All solubilization curves in Figs. 1a–1c exhibit a quasi-horizontal initial part in the range of low TBP equilibrium con-centrations, up to (C/Csat)

T1 , where the TBP molecules well

solubilize in the polymeric solution. In this first plateau regionthe three components systems are transparent but slightly bluishdue to the light scattering by swelled aggregates (micelles) inaqueous solution (microemulsion). At the end of the plateauregion (at (C/Csat)

T1 ), the solubilization curves exhibit a drop

in transmittance indicating that the three components systems(Pluronic–TBP–water) separate in two phases. Consequently

(a)

(b)

Fig. 5. Evolution of both the CPT (black symbols) and SMT (open symbols)against the normalized TBP concentration (C/Csat) for 10 wt% aqueous solu-tions of (a) P84 and (b) L44 and L64.

(C/Csat)T1 refers to as the maximal solubilization capacity of

TBP at the given temperature (T ) and was estimated as the in-tersection of the linear (first horizontal part of the curve) and thesquare function (subsequent declining part of the curve). When(C/Csat)

T > (C/Csat)T1 , the TBP molecules are no more sol-

ubilized into the micellar solution of amphiphilic copolymer.Figs. 1a–1c indicate that the extent of the initial part dependson both the nature of the amphiphilic molecule and the temper-ature. As a general trend it is evident that highest solubilizationcapacities are observed with the Pluronics exhibiting the largestPPO chains (in the order P84 > L64 > L44). It is also clearthat the solubilization capacity increases with increasing thetemperature. Solubilization experiments with the L44 systemwere performed for temperature upper than 308 K due to thehigh cmt value of L44 (cmc = 0.78% at 310.5 K [50]). Evenat 308 K the solubilization capacity is not very well defineddue to the very small quantity of L44 copolymer micelles insolution at this temperature. As a general trend, Figs. 1a–1calso show that the solubilization capacity of the Pluronic so-lutions increases with the temperature. This result was alreadyobserved by other authors [51] and is explained by an increase

J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723 719

of the hydrophobic character of the PEO and PPO chains atelevated temperature. In the case of P84 (Fig. 1a), this temper-ature effect is observed by increasing the temperature from 298to 303 K. At 298 K we can note that the maximal solubiliza-tion capacity (end of plateau region) is not very well definedprobably due to the polydisperse character of the micelles atthis concentration and temperature. At 303 K, the cmc of P84is lower (0.6 wt% against 2.6 wt% at 298 K), consequently theaggregates are much more monodisperse resulting in a betterdefined plateau region. However two particular points have tobe noted; (i) In the case of P84 (Fig. 1a), the increase of thetemperature between 303 and 308 K provokes the reduction ofthe initial solubilization plateau. At 308 K the TBP solubiliza-tion capacity decreases and is even lower than that observedat 298 K. (ii) The 10 wt% L64 solution (Fig. 1b) shows anunexpected solubilization behavior at 308 K with the occur-rence of the following three regions. The quasi-horizontal initialpart I up to (C/Csat)

3081 , above described, where the TBP mole-

cules well solubilize in the polymeric solution (microemulsion).Then, at (C/Csat)

3081 , the solubility curve exhibits a first sharp

drop which announces the beginning of a second intermedi-ate region II in which (C/Csat)

3081 < (C/Csat) < (C/Csat)

3082

and where the system separates in two phases. At (C/Csat)3082 ,

one observes a steeply rising part indicating that the wholethree components system tends to form again a single phaseas in the first region I (transparent monophasic system withU/Ur → 1). Then the third region III above (C/Csat)

3082 starts

where the added TBP molecules still solubilize in the aque-ous polymeric solution up to (C/Csat)

3083 . In this final domain

(III) the system is more bluish than in region I due to the lightscattering by presumably larger aggregates in solution. Finallyat (C/Csat)

3083 , the solubilization curve shows a second drop

characterizing again the phase separation of the system. In sucha case (C/Csat)

T3 corresponds to the saturation concentration

of TBP in the 10% L64 solution, i.e., the maximal solubiliza-tion of TBP. At 308 K, the maximal solubilization of TBP forthe 10 wt% L64 solution occurs at a (C/Csat)3 value which ismore than ten times upper than the value obtained at 298 K((C/Csat)

2981 ).

At this stage we can make essential remarks concerning theorigin of the phase separation occurring at (C/Csat)

T1 . In Fig. 1a

it is clear that two kind of drop in the transmittance are observedwhich are related to two different type of phase separation of thesystem. At 308 K, one observes a sharp drop in transmittanceat (C/Csat)

3081 while at 298 and 303 K, the transmittance more

gradually decreases from (C/Csat)2981 and (C/Csat)

3031 , respec-

tively. To understand the origin of the phase separation, thesolubilization curves (Figs. 1a–1c) have been correlated withthe evolution of both the CPT and SMT with the TBP content(Fig. 5).

4.2. Dependence of the CPT and SMT of the Pluronicsolutions with the TBP content

In Fig. 5, it is thus evident that the phase separation ob-served with the P84 system at 308 K ((C/Csat)

3081 = 90) occurs

when the CPT becomes lower than the experimental solubiliza-

tion temperature (arrow 1 in Fig. 5a). In this case the phaseseparation (T > CPT) is therefore correlated with the precip-itation of the copolymer and is characterized by a sharp dropin transmittance (Fig. 1a). At 308 K the solubilization capac-ity of the P84 system is therefore limited by the CPT. At 298and 303 K, the decreases in transmittance at (C/Csat)

2981 = 110

and (C/Csat)3031 = 155 are less marked and are clearly observed

when the SMT becomes higher than the experimental temper-ature (arrows 2 and 3 in Fig. 5a). This second type of phaseseparation therefore corresponds to the formation of TBP inwater emulsion droplets. Consequently the TBP solubilizationis limited by the SMT (when T < SMT). On the same basiswe note that the phase separations observed with L44 at 308,313 and 323 K (Fig. 1c) all originate from the emulsification ofthe system when T < SMT (arrows 1, 2, and 3 in Fig. 5b). Thesolubilization capacity is only limited by the SMT because theCPT never falls down to a value lower than the experimentaltemperature due to the very hydrophilic character of L44 (shortPPO chain) [50]. Own to its highest hydrophilicity, L44 is thecopolymer exhibiting the highest cmc (see Table 1) and therebythe lowest solubilization capacity at 308 K ((C/Csat)

3081 ≈ 30).

The very low amount of solubilized TBP never allows the CPTvalue to be reached. The situation is quite different with the L64solution (Figs. 1b and 5b). First, the L64 solubilization capacityat 298 K is much lower than that observed with P84 at the sametemperature due to the less important hydrophobic character ofL64 ((C/Csat)

2981 = 20 with L64 against 110 with P84). Due

to its lower PPO block, L64 is less hydrophobic that P84. Asa result the micellar concentration is lower in the 10 wt% L64solution at 298 K (cmc of L64 is equal to 7.5 wt% at 298 K)which gives rise to a lower solubilization capacity. The lesshydrophobic character of L64 is also evidenced by the higherSMT values compared to P84. In both cases the solubilizationis limited by the SMT and the phase separation is characterizedby the formation TBP in water emulsion droplets. The sametrend is observed at 308 K. At this temperature the phase sep-aration originates from the copolymer precipitation (T > CPT)and occurs at a TBP concentration which is lower for L64 thanfor P84 ((C/Csat)

3081 = 67 with L64 against 90 with P84) due

to the longer PPO chains of P84 (higher hydrophobic charac-ter). More interesting, at 308 K, Figs. 1b and 5b indicate thatthe L64 system exhibits a first phase separation at (C/Csat)

3081

due to the precipitation of the copolymer and a second onedue to the emulsification of the system at (C/Csat)

3083 . The

trend of the CPT curve in Fig. 5b clearly indicates that thefirst phase separation at (C/Csat)

3081 occurs when the CPT be-

comes lower than the solubilization temperature (arrow 1′ inFig. 5b). In the intermediate region II (two-phase system), theCPT remains constant and lower than or equal to the solubi-lization temperature. Finally Figs. 1b and 5b also evidence thatthe second steeply rising part ((C/Csat)

3082 = 137), indicating

that the whole system turns again monophasic, is due to thereturn of the CPT above the solubilization temperature (arrow2′ in Fig. 5b). In the intermediate region II the added TBP isincorporated in the lower precipitated copolymer structure. At308 K, in regions I and III, the L64 solution remains transpar-ent and slightly bluish which indicates the occurrence of mi-

720 J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723

croemulsions with large aggregates. It is therefore probable thatwhile introducing TBP in the two component system (water–L64), one crosses a phase in the diagram whose structure isdifferent (lyotropic (cubic) phase). The return to a monopha-sic system at (C/Csat)

3082 related to a final increase of the CPT

probably suggests a structural change of the polymeric solu-tion in the intermediate region II (between (C/Csat)

3081 = 67

and (C/Csat)3082 = 137) and particularly a structural or confor-

mational change of the L64–TBP aggregates. This structuralchange and the subsequent return to a monophasic system isspontaneous and gives rise to a final microemulsion (regionIII) which allows greatest quantities of TBP to be solubilized.Finally at (C/Csat)

3083 , the hydrophobic character of the aggre-

gates is not sufficient to allow TBP to be solubilized and/orstabilized in the aqueous solution. Consequently, subsequentaddition of TBP leads to a phase separation of the system char-acterized by TBP in water emulsion droplets. This system isobviously unstable from a thermodynamic point of view andthereby gives rise to a diphasic system at rest. We note that, asexpected, the emulsification occurs when the experimental tem-perature is lower to the SMT (arrow 3′ in Fig. 5b). Howeverthe normalized voltage recorded at the second solubilizationplateau (between (C/Csat)

3082 and (C/Csat)

3083 ) decrease as the

TBP concentration increases. This means that the light scatter-ing due to the aggregates in solution is more marked in thesecond plateau region. If we now consider the solubilizationcurve at 298 K (Fig. 1b), the phase separation is due to the for-mation TBP in water emulsion droplets which occurs when theSMT becomes higher than the experimental temperature fol-lowing addition of TBP (arrow 4′ in Fig. 5b). However at thistemperature, if the addition of TBP is maintained, we note a par-tial final increase of the normalized voltage at (C/Csat) ≈ 140.This partial increase is also correlated with the final increaseof the CPT. That means that a critical TBP concentration maybe defined that provokes change in the copolymer aggregateconformation so as to increase the amount of TBP solubilized.At (C/Csat) = 140, the aggregate structural change starts (fi-nal increase of CPT) and allows additional amount of TBP tobe solubilized (U/Ur starts to increase in Fig. 1b). However si-multaneously, the hydrophobic character of the copolymer hasto increase (SMT increases) to allow such quantities of TBPto be solubilized. The requested hydrophobic character can-not be obtained at 298 K resulting in an only partial return toa monophasic system. Consequently the analysis of both theCPT and SMT curves gives direct information on the solu-bilization capacity of a given system. Indeed, concerning the10 wt% L64 solution, from Fig. 5b, the optimal solubilizationtemperature is found around 316 K (arrow 5′) which corre-sponds to a value of (C/Csat)3 = 312 (0.499 mol kg−1 or 1.328g TBP/g L64). This optimal temperature allows to obtain theoptimal hydrophobic character of the L64 Pluronic required forthe maximal solubilization of TBP in water solution withoutprovoking precipitation of the copolymer. The evolution of theCPT (Fig. 5b) clearly evidences the occurrence of an aggregatestructural change from a given TBP content characterized bythe final increase of CPT. This structural change is also clearon the solubilization curve at 308 K (Fig. 1b) and evidenced by

the intermediate region II. The solubilization curves obtainedat temperature lower than 308 K do not allow the structuralchange to be observed because, according to Fig. 5b, these tem-peratures range between the SMT and the CPT and thereby donot allow an intermediate diphasic region to be observed. How-ever, even at T < 308 K, the structural change occurs at a TBPconcentration where the final increase of CPT is observed andallows great quantities of TBP to be solubilized. With the L64system, it is very easy to observe the effect of the structuralchange because after the final increase of CPT, the final mi-croemulsion (monophasic domain between the SMT and theCPT) persists on a large domain of temperature and TBP con-centration (hachured part). The final increase of the CPT valuealso exists with the L44 system (gray zone in Fig. 5b) but ismuch less marked compared to the L64 system and do not per-sist on a large domain of temperature and TBP concentration.This similar evolution of the CPT observed with the L44 andL64 systems indicates that their solubilization behaviors aresimilar even if it is not clear according to their solubilizationcurves (Figs. 1b and 1c). Indeed, according to Fig. 5b, it is evi-dent that it is very difficult to find a temperature allowing the fi-nal microemulsion formation to be observed (after the structuralchange). After the final increase of CPT, the final microemul-sion (monophasic domain between CPT and SMT) persists ona very small range of temperature (less than 2 ◦C) and C/Csat(see gray part). It is probable that a solubilization curve show-ing an intermediate diphasic domain and a final microemulsionshould be obtained with the 10% L44 system at a temperaturearound 325 K. The same conclusions may be drawn with the10% P84 system where the final increase of CPT (gray zonein Fig. 5a) is even much lower compared to the L44 and L64systems. The final increase of CPT starts at C/Csat ≈ 215. Af-ter this structural change, the monophasic domain persists on avery small range of temperature (≈1 ◦C, gray part). After thestructural change, the CPT and SMT curves are very similarmeaning that a very small addition of TBP provoke the emul-sification of the system (T < SMT) without formation of thefinal monophasic microemulsion. However, even if the struc-tural change phenomenon is much less marked, the behavior ofthe L44 and P84 systems is similar to that of L64. To summa-rize, the occurrence of a structural change occurs from a givenTBP concentration and allows very high solubilization capacityto be obtained. But this structural change is not enough. A wellcompromise between the SMT and the CPT must be also ob-served so as to obtain a large extent of monophasic domainafter the restructuration (hachured or gray part). This compro-mise depends on the hydrophobic character (especially lengthof the PPO block) of the copolymer. According to Fig. 5, thebest compromise giving the largest monophasic domain afterthe restructuration is obtained with Pluronics with intermedi-ate hydrophobic character (L64). As a result L64 shows a veryhigh and the best TBP solubilization capacity. Reversely, hy-drophobic (P84, long PPO block) and hydrophilic (L44, shortPPO block) Pluronics exhibit a very small extent of monophasicdomain after the restructuration which does not allow benefit bythe structural change. In these cases, the extent of the final mi-croemulsion is limited by the both the CPT and the SMT. In

J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723 721

(a)

(b)

Fig. 6. Evolution of the CPT and the SMT versus the TBP concentration for a15 wt% L44 solution (a)—correspondence to the TBP solubilization behaviorof a 15 wt% L44 solution at 328.7 K (b).

order to support our conclusions we aimed at evidencing the in-termediate two phase region and the subsequent microemulsionformation with another system. In this attempt we performedsimilar experiments with a 15 wt% L44 system. Fig. 6a displaysthe evolution of both the CPT and SMT with the TBP content.This graph indicates that, in these conditions, a solubilizationcurve at 328.7 K should evidence both the intermediate dipha-sic domain and the final microemulsion formation (gray part).As expected, the solubilization curve at 328.7 K (Fig. 6b) showsa first phase separations around (C/Csat)

328.71 = 205, the sub-

sequent microemulsion formation around (C/Csat)328.72 = 260

and the final phase separation around (C/Csat)328.73 = 385.

These values fully correlate with T > CPT, T < CPT, andT < SMT, respectively. However, the high concentration andtemperature at which the monophasic system is observed maylet to assume that the monophasic system originates from aphase transition. As above mentioned, the structural changemay also be detected on the solubilization curve by the finalincrease of the normalized voltage.

4.3. Effect of the copolymer concentration on the TBPsolubilization

The evolution of the solubilization curves with the Pluronicconcentration at 308 K is reported in Figs. 2a–2e for the L44,P94, P84, and L64 systems, respectively. The solubilization ca-pacity of the 10 wt% L44 system (Fig. 2a) is very low dueto the very small quantity of copolymer micelles in the so-lution (cmc = 0.78% at 310.5 K). The solubilization plateauincreases up to (C/Csat)

3081 = 106 by increasing the concentra-

tion to 15 wt%. In Fig. 2b it is clear that the maximal TBPsolubilization capacity proportionally increases with the P94concentration. The maximal amount of TBP solubilized in the10 wt% P94 solution ((C/Csat)

3081 = 115) is twice higher as

that solubilized in the 5 wt% P94 system ((C/Csat)3081 = 60).

This means that there is no concentration effect on the micel-lar structures capable of solubilizing TBP. An increase in thecopolymer concentration only increases the micelle concentra-tion without modifying their structure. In Fig. 2c, on the onehand, we can note a quite similar behavior as for P94, i.e.,that the maximal TBP solubilization capacity proportionally in-creases with the P84 concentration. The (C/Csat)

3081 values are

found to be 25, 44, 66, and 94 for P84 concentrations of 3, 5,7.5, and 10 wt%, respectively. On the other hand, if the ad-dition of TBP is maintained after the phase separation (after(C/Csat)

3081 ), one observes a final slight increase of the nor-

malized voltage at high TBP contents. This suggests that theP84 system tends to behave as the L64 system, with the occur-rence of an intermediate diphasic domain. However for the P84system the final slight increase of U/Ur is much less markedcompared to the L64 system indicating only a partial return toa monophasic system. This partial process is also evidenced bythe absence of both the second solubilization plateau (region IIIbetween (C/Csat)

3082 and (C/Csat)

3083 for L64, Fig. 1b) and the

well defined subsequent drop in transmittance (at (C/Csat)3083

for L64 system, Fig. 1b). This means that, with P84, the threecomponents systems start to return monophasic but still remaindiphasic and cannot completely return to a monophasic do-main. Finally, we note that the smaller the P84 concentrationthe higher the final slight increase of U/Ur and the smaller theintermediate domain. If we now consider the L64 systems inFigs. 2d and 2e, the two step solubilization behavior with theoccurrence of the above described intermediate diphasic regionfor 10 wt% L64 system is maintained when the L64 concen-tration is decreased up to 7.5 wt% Indeed for concentrations of7.5, 10, and 15 wt%, one observes a well defined second solubi-lization plateau (region III) and the subsequent drop in transmit-tance ((C/Csat)

3083 ) indicating, in these cases, a return to a fully

monophasic system (microemulsion). However the normalizedvoltages recorded at the second solubilization plateau (between(C/Csat)

3082 and (C/Csat)

3083 ) decreases as the copolymer con-

centration decreases. This means that the light scattering due tothe aggregates in solution is more marked at lower copolymerconcentrations. For the 5% L64 system, the curve evidencesonly a partial return to a single phase (at (C/Csat)

3082 = 120)

similarly to the behavior observed with P84 in Fig. 2c; a fi-nal declining regime is evidenced without the occurrence of

722 J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723

both the final monophasic region III and the second sharp drop((C/Csat)

3083 ). Finally the solubilization behavior of the system

at 3 wt% is similar to that observed with P94 and L44.Figs. 2c, 2d, and 2e which depict the effect of the Pluronic

concentration (P84 and L64) are consistent with the fact that thestructural change that occurs from a given TBP content allowsgreatest quantities of TBP to be solubilized for a given Pluronicconcentration. Indeed the structural change occurs when theamount of Pluronic is to low to allow the given TBP amountto be stabilized. On this basis it is evident that this structuralchange occurs at TBP content as lower as the Pluronic con-centration decreases as observed in Figs. 2c, 2d, and 2e. Thatmeans that the structural change allows to decrease the amountof Pluronic which is necessary to stabilize a given TBP con-tent. However in each case the final return to a monophasicsystem is only partial probably because limited by the SMT orbecause there is not enough copolymer in the medium to sol-ubilize the amount of TBP. For example with the 10 wt% P84system the final increase of the normalized voltage (arrows inFig. 2c) starts simultaneously with the final increase of CPT(structural rearrangement around C/Csat = 210 in Fig. 5) butalso quite simultaneously the SMT is reached (C/Csat = 225,emulsification). The same trend is observed with the L64 sys-tem where only the 10, 7.5, and 5 wt% concentrations allow thefinal microemulsion to be formed.

4.4. Effect of the blocks length on the TBP solubilization

Pluronics containing the same EO/PO composition ratio intheir molecular structures, but with different molecular weights,differ in the length of both their PEO and PPO blocks. As aresult, for the investigated Pluronics all exhibiting 40 wt% ofEO, the sizes of the PEO and PPO blocks are in the orderP104 > P94 > P84 > L64 > L44 (see Table 1). According tothe cmc values reported in Table 1, the hydrophobic characteris in the same order, i.e., increases with increasing the molec-ular weight or length of PPO block. The solubilization curvesrelated to these copolymers in 10 wt% aqueous solutions arecompared in Fig. 3. As a general trend the initial solubiliza-tion plateau is identical for all Pluronic systems. Two differentphase behavior are observed according to the PPO length ofthe Pluronics; the most hydrophobic Pluronics (P94 and P104)show a classical solubilization behavior with an initial solu-bilization plateau (micellar solubilization) and the subsequentphase separation (saturation resulting in the emulsification ofthe system at T < SMT). A similar behavior is observed withthe most hydrophilic copolymer (L44). Taking into accountthese three copolymers, the highest solubilization capacitiesare obtained with the Pluronics exhibiting the highest molec-ular weight, i.e., the highest PPO chains. If we now compareall the Pluronic in the whole series, L64 shows a very highand the best TBP solubilization capacity. As above mentioned,the best compromise between SMT and CPT is obtained withthe Pluronics exhibiting an intermediate hydrophobic character,i.e., intermediate chain length (especially L64), resulting in thelargest monophasic domain after the restructuration. As a result,at 308 K, the highest solubilization capacity is observed with

the L64 system. The most hydrophobic (P84, P94, and P104)and hydrophilic (L44) copolymers exhibit a very small extentof monophasic domain after the restructuration which does notallow benefit by the structural change. The final monophasic re-gion and thereby the TBP solubilization is limited by both theCPT and SMT that tend to be very similar after the restructura-tion. At 308 K, the behavior observed with P84, which is theless hydrophobic between P94 and P104, tends to resemble thatof the L64. Indeed the first phase separation alone (T > CPT)is observed. The subsequent effect of the restructuration is notvisible because of the very low extent of monophasic domainbetween SMT and CPT (Fig. 5). The hydrophilic character ofL44 does not allow high solubilization Therefore, at 308 K, itssolubilization is limited by the SMT. Finally, the solubilizationbehaviors of P84 and P123 are compared in Fig. 4. The samehydrophilic block size (PEO chain) is kept between P84 andP123 (x = 19, see Table 1) but the related phase behavior of thethree components systems are quite different. The phase sep-aration is due either to a precipitation of the copolymer whenT < CPT (sharp drop of U/Ur ) in the P84 system while itis related to the emulsification of the system when T < SMT(gradual decrease of U/Ur ) with P123. This suggests that thePEO block is not affected by the TBP solubilization, and partic-ularly that TBP is preferentially solubilized in the hydrophobiccore of the micelles.

5. Conclusions

Solubilization of tributylphosphate (TBP), a polar oil, in var-ious micellar solutions of Pluronic has been investigated byturbidimetry. Different Pluronics were chosen so as to estimatethe role of the PPO and PEO blocks on the phase behavior of thethree components systems Pluronic–TBP–water. The tempera-ture dependence on the solubilization capacity of the variousamphiphilic copolymers is evidenced and allows the role oftwo significant temperatures on the phase behavior to be em-phasized; (i) the solubilization minimum temperature (SMT)and the well known cloud point temperature (CPT, here de-fined for the three components system). The former is definedas the lowest temperature allowing solubilization of TBP inthe three components system. Both temperature depend on thecopolymer structure (length of the PPO and PEO blocks oron the hydrophilic–hydrophobic character of the copolymer),and, interestingly, are directly related to the TBP concentra-tion in the medium. [Temperature–composition] diagrams al-low monophasic and diphasic domains to be delimited. It isshown that the CPT and SMT values are the driving force ofthe phase behavior of the three components system which is amonophasic microemulsion when the experimental temperatureranges between the SMT and the CPT (CPT > T > SMT). Thesolubilization of TBP in the Pluronic solution is therefore lim-ited either by the SMT or by the CPT. In the former case whenT < SMT, the phase separation occurs and is related to the for-mation of TBP in water emulsion droplets. In the latter casewhen T > CPT the system separates in two phase due to theco-precipitation of TBP and Pluronic. Moreover an unexpectedevolution of the CPT with the TBP content is also observed

J. Causse et al. / Journal of Colloid and Interface Science 300 (2006) 713–723 723

and clearly indicates the occurrence of a structural change ofthe microemulsions which allows higher quantities of TBP tobe solubilized. However this structural change is not enough toallow higher quantities of TBP to be solubilized. A well com-promise between the SMT and the CPT must be also observedso as to obtain a large extent of monophasic domain after the re-structuration. The best compromise is obtained with Pluronicswith intermediate hydrophobic character. Reversely, hydropho-bic and hydrophilic Pluronics exhibit a very small extent ofmonophasic domain after the restructuration which does not al-low benefit by the structural change. In these cases, the extentof the final microemulsion is limited by the CPT and the SMT,respectively.

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