interaction between cationic, anionic, and non-ionic surfactants with aba block copolymer pluronic...

Journal of Colloid and Interface Science 301 (2006) 63–77www.elsevier.com/locate/jcis

Interaction between cationic, anionic, and non-ionic surfactants with ABAblock copolymer Pluronic PE6200 and with BAB reverse block copolymer

Pluronic 25R4

Ornella Ortona ∗, Gerardino D’Errico, Luigi Paduano, Vincenzo Vitagliano

Chemistry Department of Naples University “Federico II,” Via Cintia, Complesso di Montesantangelo, I-80126 Napoli, Italy

Received 16 February 2006; accepted 18 April 2006

Available online 13 June 2006

Abstract

The interaction in aqueous solution between either the normal block copolymer poly(ethylene oxide)–poly(propylene oxide)–poly(ethyleneoxide): Pluronic PE6200 [(EO)11–(PO)28–(EO)11], or the reverse block copolymer poly(propylene oxide)–poly(ethylene oxide)–poly(propyleneoxide): Pluronic 25R4 [(PO)19–(EO)33–(PO)19] and the surfactants sodium decylsulfate, C10OS, decyltrimethyl ammonium bromide, C10TAB,and pentaethylene glycol monodecyl ether, C10E5, was investigated and the aggregation behavior of these surfactants with Pluronics was com-pared. Surface tension measurements show that Pluronics in their non-aggregated state better interact with the anionic surfactant C10OS thanwith cationic and non-ionic ones. The presence of the two Pluronics induces the same lowering of the aggregation number of C10OS as shownby fluorescence quenching measurements. The number of polymer chains necessary to bind each C10OS aggregate has been estimated to be ∼6for PE6200 and ∼2 for 25R4. Furthermore, this surfactant also induces the same increment in the gyration radius of the polymers as revealed byviscosimetry. Calorimetric results have been reasonably reproduced by applying a simple equilibrium model to the aggregation processes.© 2006 Published by Elsevier Inc.

Keywords: Pluronic; Surfactant; Interaction; Surface tension; Fluorimetry; Calorimetry

1. Introduction

Water-soluble poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), (EO)n(PO)m(EO)n and poly(propyleneoxide)–poly(ethylene oxide)–poly(propylene oxide), (PO)n-(EO)m(PO)n, tri-block symmetric copolymers, produced byBASF, are known as Pluronics and Pluronics R. The samecompounds, produced by ICI, are called Synperonics. Themonomeric units EO and PO correspond to [–CH2–CH2–O–]and [–CH(CH3)–CH2–O–], respectively.

In water these molecules behave both as polymers and asnon-ionic surfactants. Their properties in solution are driven bytheir hydrophilic/lipophilic balance, HLB, that during the syn-thesis can be modulated by varying the number and the ratio ofethylene and propylene oxide units. Pluronics and Pluronics Rhave interesting technological applications as cleaning agents

* Corresponding author. Fax: +39 081 674090.E-mail address: [email protected] (O. Ortona).

0021-9797/$ – see front matter © 2006 Published by Elsevier Inc.doi:10.1016/j.jcis.2006.04.041

in cosmetic lotions and shampoos, as emulsifiers [1] for print-ing inks, paints and coatings, in pharmaceutical applicationsfor their solubilizing and controlled release properties [2–4], aswell as in corrosion protection [5].

In aqueous solution Pluronics R, indicated as BAB poly-mers, show distinct characteristics from the related Pluronicscopolymers, indicated as ABA polymers. Their different be-havior depends on the different solvent efficiency of water onpolypropylene, PPO, and polyethylene, PEO, chains; water ap-pears then to be a better solvent for PEO than for PPO. Thedifferent arrangement of their chains explains why Pluronic andPluronic R copolymers, having similar molecular weight andchemical composition, show quite different association behav-ior [6]. In general, from a technological point of view, Pluron-ics R, compared to normal Pluronics, show lower foaming andwetting properties.

In relation to the possible technological applications of bothkinds of Pluronics, the concomitant presence of surfactants isable to modify the copolymers physico-chemical properties.

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64 O. Ortona et al. / Journal of Colloid and Interface Science 301 (2006) 63–77

The formation of hydrophobic pools where hydrophobic solutesare solubilized better than in the binary Pluronic aqueous so-lution is then induced. Several studies are present on the in-teraction between Pluronics and surfactants: the more studiedsystems are those involving ABA polymers and sodium dode-cyl sulfate, SDS [7–14]; less abundant bibliography involvingnon-ionic [15] and cationic [16] surfactants can be found. Inparticular, De Lisi et al. [17–20] thoroughly investigated thisfield. In contrast, not much has been written on the interactionbetween BAB polymers and surfactants [21].

Surfactant aggregation, in water and in polymer aqueoussolution, can be treated either by mass action or phase sepa-ration model. The former approach appears to be more realis-tic. Assumptions such as critical micellar concentration (cmc),and critical polymer–surfactant aggregation concentration (cac)are set aside, since the entity of the aggregation is determinedthrough the equilibrium constants and the aggregation numbers.However, the relatively high number of these parameters makestheir evaluation by fitting procedures a quite tricky problem. Forexample, De Lisi et al. [18,19] quantitatively treated by a com-plex mass action model the aggregation properties of cationicand anionic surfactants with two normal Pluronics of a verydifferent molecular weight. The authors assumed that the mi-cellization, the formation of the monomer surfactant–polymeraggregate and the micelle–polymer mixed aggregate can takeplace simultaneously.

On the other hand in the phase separation model thesurfactant–polymer aggregation, is described in terms of somecharacteristic concentrations, such as the cac, T ∗

2 and T2. Cac,named also T1, is an indication of the beginning of the coopera-tive surfactant aggregation to the polymer [11,22]. This value islower than cmc, that is the critical aggregation concentration ofsurfactants in neat water. The greater the difference cmc − cacis, the higher is the affinity of the surfactant for the polymer. Itis worth noting that T ∗

2 corresponds to polymer saturation, andT2 is related to the onset of free micelles in the bulk [11,22].The phase separation model, with respect to the mass actionapproach, is rougher, particularly in the case of surfactants witha short aliphatic chain. Nevertheless it is simpler, and allows afast and sufficiently reliable comparison between systems con-taining different surfactants and/or polymers. In this paper weuse the phase separation model to discuss the properties of thesystems under consideration. At the end of Section 3 we showhow some of our data could be also interpreted within the massaction model.

This paper deals with the interaction of two Pluronics,PE6200 and 25R4, with some surfactants having the samealiphatic chain and different hydrophilic heads. PE6200[(EO)11–(PO)28–(EO)11] is a normal Pluronic while 25R4,[(PO)19–(EO)33–(PO)19] is a reverse one. At the concentrationsused in this study and at the temperature of 25 ◦C, PE6200and 25R4 are in the monomeric state as reported in the lit-erature [23–25] and also tested by some preliminary DSCmeasurements which show that the cmt (critical micellizationtemperature) is 44.5 ◦C for PE6200 and 49.4 ◦C for 25R4 at0.5 wt%.

The choice of the selected surfactants C10E5, C10OS, C10-TAB was suggested because they have the same hydropho-bic tail, allowing evaluation of the influence of the polarhead on their aggregation properties both in neat water andin the presence of Pluronics. The C10 tail causes their hy-drophilic/lipophilic balance to be high enough to allow thestudy of their behavior also in the pre-micellar compositionrange.

Surface tension and fluorescence measurements are well-suited techniques to highlight surfactant aggregation, althoughwith a different sensitivity. Some preliminary results by surfacetension measurements on surfactant–Pluronic–water ternarysystems showed that the more interacting are those with C10OS.This is the reason why the systems C10OS–PE6200–waterand C10OS–25R4–water have been further investigated bycalorimetry and viscosimetry methods. Furthermore, the flu-orescence quenching method was used to evaluate the C10OSaggregation number with and without Pluronic.

The results of our investigation could give also some in-sight about the effect of chain architecture on the Pluronic–surfactant interaction. In principle, the more correct comparisonwould be that in which the normal and the reverse Pluronicpresent the same numbers of EO and PO units. Unfortunatelysuch products are usually not available from the producers. Inthis condition, the choice of the reverse Pluronic to be com-pared with the a given normal one is not univocal, since variousmolecular parameters are important in determining the copoly-mer behavior in water. In this paper we have chosen PE6200and 25R4, which have similar HLB as defined by Griffin [26]:HLB = 20[(17y + 44x)/M], where y stays for the number of–OH, x stays for the number of ethoxylic groups, and M is themolecular weight. It results that HLB (PE6200) = 7.4 and HLB(25R4) = 8.0, so that the reverse Pluronic is only slightly morehydrophilic than the normal one. However, in interpreting theexperimental results one has to be aware of the higher length ofPO and EO blocks in 25R4.

2. Experimental

2.1. Materials

Pluronic PE6200, declared molecular weight 2660, and25R4, declared molecular weight 3690, were gifts of BASF.These chemicals are prepared by polymerization reactions thatnecessarily result in a molecular mass distribution. The prob-lem of Pluronic polydispersity has been seldom studied [27].However, intradiffusion measurements on aqueous 25R4 put inevidence a low degree of polydispersity [24]. We do not have di-rect information on that one of PE6200; however, in the case ofPluronic L64 [(EO)13–(PO)28–(EO)13], very similar to PE6200,it is very low [28].

Sodium decylsulfate, C10OS, purity >99%, decyltrimethylammonium bromide, C10TAB, purity >99%, pentaethyleneglycol monodecyl ether, C10E5, purity >99.8%, pyrene, pu-rity >99%, were purchased from Sigma–Aldrich and used asreceived. All solutions were prepared by weight using doublydistilled water. All the measurements were performed at 25 ◦C.

O. Ortona et al. / Journal of Colloid and Interface Science 301 (2006) 63–77 65

2.2. Choice of transition points

In surfactant and surfactant–polymer solutions, transitionscorresponding to characteristic points (cmc, T1, T ∗

2 , T2) aremainly second-order transitions. They are recognized by theslope changes on the experimental observed properties. Thisrequires a criterion to define these points that, in the actualsystems, correspond to a less or more wide range of compo-sitions.

In the following we define the transition points at the compo-sition corresponding to the largest curvature of the experimentalproperties graphs.

2.3. Surface tension measurements

Measurements were performed by using the De Nouy ringmethod with a KSV Sigma 70 digital tensiometer. An automaticdevice was used to select the rising velocity of the platinumring and to set the time between two consecutive measure-ments. Thorough attention has to be paid in using the De Nouyring method to deduce bulk properties, because the surfactantadsorption kinetics can influence the results. In our experi-ments, we set the ring rising velocity low enough to reach theequilibrium between the air–solution interface and the solutionbulk.

The binary systems water–surfactant and the ternary systemswater–surfactant–polymer were studied by a titration proce-dure. For binary solutions, this method consists in adding to aweighed mass of water initially present in the apparatus vessel,weighed amounts of a surfactant solution well above the cmc.To be sure of the purity of the used doubly distilled water, ineach set of measurements the surface tension of the neat waterinto the vessel was always higher than 71.0 mN m−1 (surfacetension of ultra-pure water at 25 ◦C is 71.97 mN m−1). The bi-nary experimental trends were kept as reference for the relatedternaries.

The same procedure for ternary solutions was followed;however, in both the titrated and titrating solutions the samepolymer concentration has been used.

2.4. Pyrene fluorescence measurements

Pyrene was used as a probe to carry out fluorescence mea-surements. It is known that its fluorescence spectrum showsstrong environmental dependence [29]; this effect can be use-fully employed to study surfactant aggregation processes usingvery diluted solutions of this probe as a solvent. In the pres-ence of surfactant aggregates, pyrene solubilizes preferentiallyin their hydrophobic core thus showing a fluorescence spec-trum different from that shown in water. In particular, whenpyrene dissolves into a micelle, a decrease of the I1/I3 ra-tio of the emission intensities relative to λ1 = 372.5 nm andλ3 = 383.0 nm is observed. The reason is that the peak atλ1 = 372.5 nm is more sensitive to the polarity of its environ-ment than the other one [30].

Measurements were performed by using stock solutions pre-pared as described for surface tension measurements using

aqueous pyrene, 2 µM, as a solvent. A titration procedure wasfollowed. Fluorescence spectra were obtained by a Jasco spec-trofluorimeter Model FP-750 at an excitation wave length of335 nm.

2.5. Pyrene fluorescence quenching measurements

The aggregation number of surfactants has been estimatedby pyrene static fluorescence quenching, using dodecyl pyri-dinium chloride as a quencher. This procedure is described inliterature [22] and leads to aggregation numbers in agreementwith those evaluated through other experimental techniques.

Measurements were performed by progressively adding asurfactant solution well above its cmc, containing 2 µM pyrene,the quencher, and Pluronic (in the case of ternary systems), toa solution with the same components, at the same compositionsbut with no quencher. Along the titration procedure, the condi-tions provided by the Tachiya model [31] were always respected

namely:Cquencher

Cmicelle< 1 and

Cpyrene

Cmicelle� 1.

2.6. Viscosity measurements

The kinematic viscosity, t/tp, the ratio of the shear timeof the solution and that of the solvent, was measured by anUbbelohde viscosimeter for the binary system water–C10SONa,and for the same system in the presence of PE6200 or 25R4.A weighed amount of the surfactant solution was diluted intothe viscosimeter with known volumes of the proper solvent.

2.7. Isothermal titration calorimetry

The isothermal micro-calorimeter used was a TAM (Ther-mal Activity Monitor) instrument from thermometric, equippedwith a microprocessor-controlled motor-driven Hamilton sy-ringe.

For the C10OS–water system, the procedure consisted inadding small volumes of a concentrated surfactant solution wellabove the cmc, to a weighed mass of water in the calorimetricvessel. The density of the titrating surfactant solution has beenpreviously measured.

For the C10OS–Pluronic–water system, a concentrated sur-factant solution without polymer was added to a Pluronic aque-ous solution. After each addition, the heat released or absorbedas a result of the various processes occurring in the solution wasmonitored. This is a slightly different procedure from that nor-mally applied in this work, where polymer concentration wasalways kept constant. For the calorimetric measurements wepreferred the above described procedure because it is the onlyone able [22] to detect the critical aggregation composition, T1.This kind of experimental approach, defined as asymmetrictitration, is the most generally used in literature [13,15,16,21,32–37]. The enthalpic effect ascribable to Pluronic dilution isnegligible, as tested preliminarily.


3. Results and discussion

3.1. Surface tension measurements

The surface tension of aqueous surfactants solutions plottedas a function of surfactant molality are shown in the inserts ofFigs. 1 and 2 for C10OS, and in Fig. 3A (C10TAB) and Fig. 3B(C10E5). A semi-log scale was used; in the case of tensiome-try, this scale has a theoretical meaning, because, according tothe Gibbs’ approach, it allows the determination of the area permolecule on the air–solution interface. However, this scale isused also for other kinds of measurements since, in general, itallows a better detection of transition points [14].

Even if the surface tension is a characteristic of the liquid–airinterface and not of the bulk, if these measurements are per-formed slowly, a condition of equilibrium between the surfaceand the solution bulk is reached. Under these conditions the γ

trend provides insight on the bulk properties [14,37,38].The cmc can be detected from the abrupt change of the slope

of the experimental γ data that become almost constant. It canbe noted that γ of the ionic surfactants (C10TAB and C10OS) inthe presence of micelles is higher than that of non-ionic C10E5surfactant (γ ∼ 40 mN m−1 as compared to γ ∼ 28 mN m−1

for C10E5).In Figs. 1 and 2 the surface tension, γ , for the aqueous

systems C10OS–PE6200 and C10OS–25R4 is also plotted as a

Fig. 1. Surface tension of C10OS in PE6200 (0.5 wt%) at 25 ◦C. In the insert, the surface tension of C10OS in water.

Fig. 2. Surface tension of C10OS in 25R4 (0.5 wt%) at 25 ◦C. In the insert, the surface tension of C10OS in water.


(A)

(B)

Fig. 3. (A) Surface tension of C10TAB in pure water (!), 25R4 (0.5 wt%, F), and PE6200 (0.5 wt%, 1). (B) Surface tension of C10E5 in pure water (!), 25R4
(0.5 wt%, F), and PE6200 (0.5 wt%, 1).
function of surfactant molality at constant polymer composition(0.5 wt%). The presence of Pluronics causes a drastic decreaseof the water surface tension (from ∼71 to ∼37–40 mN m−1).

Figs. 1 and 2 show the three typical compositions re-lated to the surfactant–polymer interaction for C10OS–PE6200(0.5 wt%)–H2O and C10OS–25R4 (0.5 wt%)–H2O, respec-tively. Significant data are collected in Table 1. For concen-trations below T1, γ smoothly decreases because all the addedsurfactant partitions between the solution bulk as a monomerand the water–air interface, where the copolymer is, obviously,also present.

A slight dependence of the critical aggregation compositionon polymer content is observed. Fig. 4 shows the descending

trends of T1 with polymer concentration for C10OS–PE6200and C10OS–25R4 systems. Also De Lisi et al. observed thisslight decrease of T1 in a sodium decanoate–L64 system [17].In our opinion, this behavior could be due to an equilibriumbetween the monomeric surfactant and the Pluronic. In the com-position range T1 < m < T ∗

2 γ is constant. This is an indicationthat all the added surfactant aggregates on the polymer whilethe composition at the air–solution interface does not vary.The width of the T ∗

2 − T1 plateau depends essentially on thePluronic concentration, see Table 1 and Fig. 5.

In relation to the surfactant/copolymer interaction two op-tions are possible: (i) the saturation of the copolymer by surfac-tant molecules is followed by the formation of pure micelles


Fig. 4. Trend of the cooperative aggregation composition, T1, as a function of polymer molality at 25.0 ◦C. (") C10OS in 25R4 (0.5 wt%), (2) C10OS in PE6200(0.5 wt%).

Table 1Outstanding aggregation concentrations detected by surface tension measure-ments

mPlu cmc T1 T ∗2 T2 T ∗

2 − T1 cmcPlu

C10OS25R4 (wt%)0 0.0280.028 7.59 × 10−5 0.012 0.015 0.028 0.003 0.0270.068 1.84 × 10−4 0.010 0.019 0.051 0.009 0.0430.10 2.71 × 10−4 0.011 0.028 0.052 0.017 0.0350.50 1.36 × 10−3 0.008 0.039 0.053 0.031 0.0221.0 2.73 × 10−3 0.006 0.074 0.098 0.068 0.028

PE6200 (wt%)0 0.0280.10 3.76 × 10−4 0.011 0.017 0.020 0.006 0.0140.50 1.89 × 10−3 0.009 0.018 0.032 0.010 0.023

C10TAB25R4 (wt%)0 0.0610.50 1.36 × 10−3 ∼0.060

PE6200 (wt%)0.50 1.89 × 10−3 ∼0.060

C10E525R4 (wt%)0 0.00070.50 1.36 × 10−3 ∼6.7 × 10−4

PE6200 (wt%)0.50 1.89 × 10−3 ∼6.8 × 10−4

Note. All surfactant compositions are expressed in mol kg−1.

in the bulk, (ii) the surfactant aggregation to the copolymerproduces mixed micelles formed by surfactant molecules andpolymer chains. In the systems hereby studied, the presence of avery well defined plateau in the γ data, is more compatible withthe copolymer saturation mechanism than with the formation

of mixed micelles. In this second case, a copolymer saturationcomposition would be meaningless, since the addition of thesurfactant would only progressively change the composition ofthe mixed micelles.

In the range T ∗2 < m < T2 the surface tension decreases

again because the C10OS concentration increases as monomerin the bulk as well as at the air–water interface.

Above T2, γ reaches again a constant value because theadded surfactant increases only the concentration of the surfac-tant aggregates in the bulk.

C10OS–PE6200 (Fig. 1) behaves almost similarly to C10OS–25R4 (Fig. 2); the two systems show almost the same T1 valuesat the polymer composition 0.5 wt%. However, in the PE6200system the polymer saturation is reached earlier than in the25R4 system, as indicated by the ratio (T ∗

2 − T1)/mPlu. Thisratio in the normal Pluronic is in fact one third of that in the re-verse one at the same polymer concentration, mPlu. See Fig. 5.This different behavior is meaningful also taking into accountthe fact that the number of PO and EO units of these polymersis not exactly the same.

Finally, remembering that T1 + (T2 − T ∗2 ) is the molality

to which free surfactant in the bulk begins to aggregate, thiscomposition can be interpreted as the cmc in the presence ofPluronic, i.e., cmcPlu. The data in Table 1 show that the cmcPlu

values are quite scattered though not much different from thecmc of surfactant in the absence of Pluronics.

The above discussed complex γ trend is not observed in C10-TAB–PE6200–H2O, C10TAB–25R4–H2O, C10E5–PE6200–H2O, and C10E5–25R4–H2O ternary systems, see Figs. 3Aand 3B. In fact, in these four systems the γ data show a criti-cal aggregation concentration almost equal to the cmc in neatwater.

The surface tension measurements results allow us to statethat:


Fig. 5. Concentration of C10OS aggregated to Pluronics at the saturation as a function of polymer molality: (") C10OS in 25R4 (0.5 wt%), (2) C10OS in PE6200(0.5 wt%).

(i) The interaction of PE6200 and 25R4 with the anionic sur-factant is more effective compared to cationic and non-ionic ones with the same hydrocarbon chain. It is notcompletely clear the reason of the higher affinity of thesepolymers for anionic surfactants: it seems to be related tothe different hydrophobic hydration around the methylenegroup adjacent to the hydrophilic head of anionic surfac-tants in comparison with that of cationic and non-ionicones [39,40].

(ii) The beginning of cooperative aggregation of C10OS to thepolymer backbone depends only slightly on the concentra-tion of this surfactant. The decrease of T1 with an increas-ing polymer concentration can be qualitatively interpretedon the basis of a simple equilibrium; i.e., the increas-ing concentration of a component promotes the bindingprocess of the other.

(iii) As regard to C10TAB and C10E5, their behavior in thepresence of Pluronics is an evidence of a poor or absentpolymer–surfactant interaction, although the formation ofmixed micelles cannot be excluded at the surfactant cmc.In literature, evidences of interactions between Pluronicsor polypropylene glycol (PPO) [34,41] and cationic sur-factants, and between Pluronics and non-ionic ethoxylatedsurfactants [42] are present. In particular the formation ofmixed aggregates of Pluronics and ethoxylated surfactantshas been put in evidence.

(iv) At the same concentration, 25R4 is able to bind more an-ionic surfactant than PE6200: (T ∗

2 − T1)/m25R4 > (T ∗2 −

T1)/mPE6200.

3.2. Pyrene fluorescence measurements

Also pyrene fluorescence can be used to detect surfactant mi-cellization in binary systems and surfactant–polymer aggrega-tion in ternary ones. In Fig. 6A the I1/I3 trends for the aqueous

solutions of the studied surfactants are shown as a function oftheir concentration on a semi-log scale.

The environment experienced by pyrene in micellar aggre-gates is very different depending on the chosen surfactant. Infact the I1/I3 values of the three surfactants at the cmc is in theorder: C10TAB > C10OS > C10E5, namely: ∼1.1, 1.0, 0.9, re-spectively. Furthermore, in the case of C10TAB and C10OS aslight descending trend of I1/I3 is observed, while for C10E5 itremains constant in a wide composition range above the cmc.From the first evidence it can be argued that C10E5 micellesare more compact and hydrophobic than those of the anionicand especially of the cationic surfactants, while the descendingtrends of I1/I3 for both the ionic surfactants put in evidencethe partitioning of the probe (pyrene) between the micellarphase and the bulk. This is not the case for C10E5 micelleswhose compact and hydrophobic core adsorb the pyrene com-pletely.

Fluorescence measurements are rather sensitive to the inter-action of C10OS with Pluronics as shown by comparison of theC10OS data in Figs. 6A–6C. The T1 values correspond to thosemeasured by surface tension as can be seen for both ternary sys-tems C10OS–25R4–H2O and C10OS–PE6200–H2O in Figs. 6Band 6C (see also Table 2) where a large decrease of I1/I3 be-fore T1 is also observed. This is an indication of the growth ofponds where pyrene can be well located even before the surfac-tant aggregation at T1. This may reasonably be attributed to theincreased hydrophobicity of the polymeric network due to thesparse binding of the surfactant molecules along the Pluronicchains. As it will be discussed in the following, calorimetry ex-periments support this interpretation.

As in the case of surface tension, no particular effects areshown by C10TAB and C10E5 in the presence of Pluronics.In all the curves only one transition point can be detected, asshown in Fig. 7. In the case of C10E5 it corresponds to the cmcin the binary system.


(A)

Fig. 6. (A) Relative fluorescence of pyrene in C10E5, C10OS, C10TAB–water systems at 25 ◦C. The lines are a guide for the eye. (B) Relative fluorescence of pyrenein C10OS–25R4 (0.5 wt%)–H2O at 25 ◦C. (C) Relative fluorescence of pyrene in C10OS–PE6200 (0.5 wt%)–H2O at 25 ◦C.

Table 2Outstanding aggregation concentration detected by fluorescence measurements

cmc T1 T ∗2 T2

C10OS25R4 (wt%)0 0.040.10 ∼0.011 ∼0.024 ∼0.0350.50 ∼0.008 0.040 ∼0.08

PE6200 (wt%)0 ∼0.040.10 ∼0.0130.50 ∼0.008 ∼0.07

C10TAB25R4 (wt%)0 ∼0.070.50 ∼0.048PE6200 (wt%)0.50 ∼0.0461.00 ∼0.028

C10E525R4 (wt%)0 ∼0.0010.50 ∼0.0004PE6200 (wt%)0.50 ∼0.0005


C10TAB in the presence of Pluronics shows a slightly lowercmc, signed as T1 in Figs. 7A and 7B, that suggest a mild poly-mer–surfactant interaction.

3.3. Pyrene fluorescence quenching measurements

The aggregation number of the C10OS micelles, both in bi-nary and ternary systems, has been measured by fluorescence

quenching through the general method proposed by Turro andYekta [43] in 1978. We determined the aggregation number forC10OS–PE6200 0.5 wt% and C10OS–25R4 0.5 wt% systemsbecause they are the more interacting ones. On the assumptionsbased on the Tachiya [31] model, the following equation holds:

(1)lnI0

I= n

C0 − cmc[Q].

In the presence of a polymer the cmc in Eq. (1) must besubstituted by T1.

I0 and I are the fluorescence intensity at a fixed wave lengthwithout and with the quencher, respectively. C0 is the stoi-chiometric surfactant composition, [Q] is the quencher molarconcentration, and n is the aggregation number. According toEq. (1) the slope of the straight line obtained reporting ln(I0/I)

versus [Q] allows to determine the aggregation number.In Fig. 8 the aggregation numbers of C10OS in the presence

of PE6200 and 25R4 0.5 wt% are reported. All measurementshave been carried out at compositions always exceeding the T1.In the T1–T ∗

2 range we make the reasonable assumption thatthe surfactant in the bulk is present exclusively as a monomer,so that the measured aggregation number, n, is representativeonly of the aggregates to the polymer; on the other hand at ahigher concentration (more than T2) n depends also on the freemicelles present in the solution. For concentrations well aboveT2, n will tend to its estimated value in water.

As reported in the figure, the surfactant aggregation numberin the presence of Pluronics is lower than that in water (63 ±5).This datum is in agreement with the reported literature [44].The lower value of the aggregation number in the presence of apolymer is a general behavior observed for surfactants [22,45].The average values of the aggregation numbers, at T ∗

2 concen-tration, are 28 for PE6200 and 50 for 25R4. Using 2660 and3690 as molecular weights of PE6200 and 25R4, respectively,


(B)

(C)

Fig. 6. (continued)

it is possible to calculate the number of polymer chains nec-essary to bind each surfactant aggregate: ∼6 for PE6200, ∼2for 25R4. This shows the much higher ability of 25R4 to bindC10OS.

3.4. Viscosimetry

Measurements of the kinematic viscosity, namely the ratio ofthe solution and of the solvent shear time, are able to show T ∗

2 ,and T2, but not T1. In Fig. 9 the relative kinematic viscosity forthe systems C10OS–water and C10OS–25R4 (0.5 wt%)–wateris reported; the outstanding compositions are collected in Ta-ble 3. The binary system shows the classical trend expectedfor aqueous surfactants, while for the ternary system a clearplateau in the range T ∗–T2 is observed; in this concentration
2
range t/tp is constant because the added surfactant increasesonly the monomer concentration that scarcely affects the mea-sured shear time.

An estimate of the increment of the gyration radius, Rg,of the polymer due to the presence of surfactant aggregates ispossible through the relation based on the classical Fox–Florymodel [46]:

(2)(Rg)S

(Rg)0=

[ (t−tsolvtsolv

)S=S( tp−tH2O

tH2O

)S=0

]1/3

.

In Eq. (2) t is the shear time of the surfactant–polymer–water mixtures, tsolv that of the solvent in which the com-plex surfactant–polymer moves, tp, is the shear time of water–polymer solution and tH2O that of the water. This equation holds


Fig. 7. Relative fluorescence of pyrene in (A) C10TAB in PE6200; (B) C10TAB in 25R4; (C) C10E5 in PE6200; (D) C10E5 in 25R4. The polymer composition is0.5 wt%. Full circles, ("), refer to the related surfactant binary systems.

Fig. 8. Aggregation number of C10OS in PE6200 (0.5 wt%, 2) and C10OS in 25R4 (0.5 wt%, "), as a function of surfactant molality at 25.0 ◦C. The line is a guidefor the eye.


Fig. 9. Kinematic viscosity of C OS in 25R4 (0.5 wt%, !) and of C OS in H O (") at 25.0 ◦C.
10 10 2
Table 3Outstanding aggregation concentrations detected by viscosity measurements

cmc T ∗2 T2 Gyr. radius incr.

C10OS0.03

25R4 (0.5 wt%) 0.041 0.057 15%PE6200 (0.5 wt%) 0.017 0.028 14%


in the hypothesis of the equivalence between intrinsic and re-duced viscosity [47]. We found a 15% increment of the gyrationradius at the saturation composition, T ∗

2 , in both polymers. Thisincrement can be ascribed to the volumetric hindrance of thesurfactant aggregates to the polymer backbone.

3.5. Isothermal calorimetric titration

In Fig. 10 the results of the calorimetric titrations of 0.5 wt%solution of 25R4 and of PE6200, with 0.192 mol kg−1 of C10OSaqueous solution are reported. The parameter on the verticalaxis of the graph is

(3)�Hobs = qobs

n0,

where qobs is the measured heat effect due to the injection of n0moles of surfactant in the calorimetric vessel. The �Hobs trendsgive information on the system association behavior even if itsdriving force is the Gibbs energy variation.

The calorimetric titrations shown in Fig. 10 are similar tothat obtained by Olofsson and Wang [32] for the poly(ethyleneoxide)–sodium dodecyl sulfate (SDS) system. However, whilethe SDS micellization is exothermic, that of C10OS is endother-mic. This fact has already been observed by Blandamer etal. [48].

The calorimetric titrations of C10OS in the presence of 25R4and PE6200 respectively, put in evidence four surfactant com-

position regions: in region 1, by adding the concentrated sur-factant solution to a Pluronic solution in the calorimetric vessel,�Hobs, initially negative, increases thus becoming positive upto a sharp maximum which corresponds approximately to theT1 composition for both Pluronics. In region 2, increasing thesurfactant concentration, �Hobs decreases, thus turning nega-tive again and reaching a broad minimum. In the case of 25R4,a slope change can be detected. It corresponds to the T ∗

2 com-position. In region 3, a further increase of C10OS concentrationcauses a decrease of the absolute �Hobs values that, finally,merge to the data of the binary system (region 4). The out-standing concentration detected by this technique are collectedin Table 4.

The Wang and Olofsson [34] interpretation of the titrationgraph seems suitable to our systems. This titration correspondsto the dilution of a concentrated micelles solution in water intothe solution containing the polymer. Initially, this transfer cor-responds to a dissociation of the micelles with heat develop-ment; in parallel surfactant adsorbs in a non-aggregate formto the polymer with heat absorption (region 1). This heat ab-sorption is probably due to the dehydration of the ethoxylicgroups of the chain [34] while interacting with the surfac-tant. This process continues up to the positive maximum ofthe �Hobs curve. The initial �Hobs difference between the bi-nary system (�H 0

obs ∼ −2060 J mol−1) and that in the presenceof 0.5% Pluronics (�H 0

obs ∼ −880 J mol−1 with PE6200 and�H 0

obs ∼ −1400 J mol−1 with 25R4) is a balance between theheat of the micelles dissociation and that of the surfactant bind-ing on the polymers. This behavior could be an indication of thefraction of the surfactant bound on the polymer backbone.

At a concentration higher than T1 the bound surfactantmonomers start to aggregate. The aggregation of the surfac-tant in the vicinity of the polymer favors the re-hydration of thepolymer ethoxylic groups with the decrease of �Hobs whichturns negative again (region 2).


Fig. 10. Calorimetric titration curves for addition of C10OS micellar aqueous solution to water ("), 0.5 wt% PE6200 (1), and 0.5 wt% 25R4 (!), respectively.

Table 4Outstanding aggregation concentrations detected by titration calorimetry

cmc T1 T ∗2 T2

C10OS25R4 (0.5 wt%)0 0.0320.50 0.009 0.040 0.050–0.065

PE6200 (0.5 wt%)0 0.0320.50 0.009 0.034–0.050


Finally, when the surfactant concentration increases overT ∗

2 , the negative calorimetric data are again an indication thatmicelles dissociate in free water (region 3). The �Hobs ab-solute value decreases as the surfactant concentration reachesT2 where �Hobs’s merge to the value obtained for the binarysystem surfactant–water. From hereon, �Hobs’s corresponds tothe dilution of micelles in free water (region 4).

Below, we present a simple semi-empirical model that repro-duces reasonably well the experimental calorimetric data shownin Fig. 10 for the titration of C10OS in 0.5 wt% 25R4. Since themodel does not account for the presence of counterions, it is,in principle, limited to non-ionic surfactant. However, we showthat it gives satisfactory results also for the system C10OSNa–25R4.

The following equilibrium reactions have been assumed:Binding of surfactant as monomer S to Pluronic chain

sites P ,

(4)S + PkD�kA

SP , K = kA

kD.

Fig. 11. Pictorial representation of surfactant aggregation on the polymer back-bone.

Binding of surfactant as micelle Sp to Pluronic chain sitesQ, where a Q site corresponds to sP sites:

(5)pS + Qk∗

D�k∗

A

SpQ, Kp = k∗A

k∗D

.

Formation of micelles of free surfactant Sq in the bulk,

(6)qSkM�kF

Sq, KL = kF

kM.

The binding of surfactant on Pluronic is treated as a Lang-muir isotherm assuming that the number of binding site re-quired for binding a micelle is s times that required for amonomer surfactant molecule, namely [P ] = s[Q]. Fig. 11 is apictorial representation of the surfactant–polymer aggregate.

According to this model, the following equations hold:

(7)m0 = m1 + m2 + mB + mM,

(8)m2 = m1([P ] − smB/p)

K + m1,

(9)mB = pmp1[P ]/s

Kp + mp1

,

(10)mM = mq1

KL,


Fig. 12. (1) Concentration of monomer free surfactant, m1. (2) Concentration of monomer surfactant bound to Pluronic, m2. (3) Concentration of surfactant asbound micelles, mB. (4) Concentration of micelles in the bulk solution, mM. Sites required for binding a micelle s = 10, p = 20, q = 60, K = 0.10 mol kg−1,KP = 2.00 × 10−36 mol20 kg−20, KL = 5.00 × 10−85 mol59 kg−59, [P ] = 0.014 mol kg−1. T1, T ∗

2 , and T2 are the experimental characteristic compositions givenin Table 1.

where the following symbols have been used: m0, total con-centration of surfactant; m1, concentration of free monomersurfactant in solution; m2, concentration of monomer surfactantbound to Pluronic; mB, concentration of micellized surfactantbound to Pluronic; mM, concentration of free micellized surfac-tant; [P ], concentration of binding sites on Pluronic chain forthe monomer surfactant; [Q] = [P ]/s, concentration of bindingsites on Pluronic chain for the micelles; p, aggregation numberof micelles bound to Pluronic; q , aggregation number of freemicelles in solution; s, number of P sites required to bind asurfactant micelle.

Equation (7) is the material balance equation.Equation (8) is the adsorption isotherm of monomer bound

to Pluronic, in this equation the sites available for the monomersurfactant are the total ones minus those occupied by the mi-celles, ([P ] − smB/p).

Equation (9) is the adsorption isotherm of the micelles boundto Pluronic, for them the available sites are the total ones,namely [P ]/s, because in this model micelles substitute s adja-cent monomer sites (or grow on s adjacent monomers).

Equation (10) is the micellization equilibrium of free surfac-tant.

Fig. 12 is a graphs of Eqs. (7)–(10) computed for s = 10.The constants used have been chosen to reasonably approachthe experimental results for the system C10OS in 25R4 0.5 wt%and are given on the figures comments.

Due to the approximate model the computed Ti valuesshown in Fig. 12 are slightly different from the experimentalones, however the trend of free surfactant molality, m1, cor-responds qualitatively to that of surface tension measurementsshown in Figs. 1 and 2. It is also interesting to see that thecomputed molality of monomer bound surfactant (m2) firstly

increases and then decreases because monomer surfactant issubstituted on the binding sites by the bound micelles.

The differential enthalpy of titration has been divided inthree contributions:

(1) Enthalpy of binding of monomer surfactant to Pluronicchain,

(11)�H1 = A1m1�m2/�m0;(2) Enthalpy of binding of surfactant to Pluronic as micelle,

(12)�H2 = A2m1�mB/�m0;(3) Enthalpy of dissociation of free micelles into the dilute so-

lution,

(13)�H3 = A3(m1,sat − m1)�m1/�m0,

where Ai are constants and m1,sat is the free monomer mo-lality in the presence of excess free micelles in solution(assumed constant and equal to the m1 molality at the high-est value of m0 (namely 0.1 m). The terms � indicate themolality difference of the corresponding species on eachaddition of C10OS solution.

Accordingly, the differential enthalpy of mixing is given by

(14)�Hmix = �H1 + �H2 + �H3.

The numerical values of the constants in Eqs. (11)–(14) aregiven in the comments of Fig. 13.

The term m1 in Eqs. (11) and (12) has been introducedto account for the experimental evidence that the differentialenthalpy of binding increases by increasing the surfactant mo-lality. For the same reason the term (m1,sat − m1) in Eq. (13)


Fig. 13. Data computed with s = 10 (Eqs. (8) and (9)). The full curve is a graph of computed �Hmix while the dotted line is a graph of �H3. Circlesare the experimental results reported as �Hobs for 0.5 wt% 25R4–C10OS aqueous system in Fig. 10. A1 = 9.15 × 106 J mol−2, A2 = 1.24 × 107 J mol−2,A3 = −9.06 × 104 J mol−2.

accounts for the experimental evidence that the enthalpy ofmicelles dissociation decreases by increasing m1 approachingzero at m1 saturation.

Accounting for the very simplified treatment of titrationmodel, the agreement between experimental and computed datais very satisfactory, and supports the interpretation suggested byWang and Olofsson [34] of the calorimetric behavior of theseternary systems.

4. Conclusions

This work has presented a comprehensive experimental in-vestigation on the interaction of Pluronics and Pluronics re-verse with ionic and non-ionic surfactants. Various experimen-tal techniques have been used, namely: tensiometry, fluores-cence, fluorescence quenching, viscosimetry, and calorimetry.

Despite small discrepancies, all techniques give the samegeneral picture of the investigated systems.

In particular, we found that, regarding the interaction withsurfactants, Pluronics are highly selective. In fact, both Pluron-ics and Pluronics R’s efficiently interact with the anionic sur-factant C10OS, while this interaction is very weak in the case ofcationic surfactant C10TAB and almost completely absent forthe non-ionic surfactant, C10E5.

This result suggests that the Pluronic–surfactants interactionis driven by a combinations of different factors among whichthe dipole–ion interaction between the polymer ether oxygenand the surfactant head-group is fundamental in stabilizing (i)the hydrophobic interaction between the polymer methylenegroups and the surfactant hydrophobic tail, and (ii) the coop-erative aggregation of the surfactant tails in forming aggregatesadsorbed to the polymer chain.

Our results support a mechanism of Pluronic–C10OS inter-action in which the surfactant initially adsorbs as monomer

to the polymer chain. The crowding of surfactants close tothe polymers induces, above a critical surfactant concentration(T1), the formation of surfactant aggregates bound to polymermolecules. These are formed by few polymer chains and a num-ber of C10OS molecules which increase with increasing con-centration but is always lower than the aggregation number ofpure micelles.

Once the polymer is saturated, further increasing the sur-factant concentration results in an increasing of monomers insolution, up to another critical concentration at which pure mi-celles form.

It is interesting to compare these results with those ob-tained by De Lisi et al. [18]. These authors, interpreting den-simetry and calorimetry data, found that Pluronics and sur-factants form mixed micelles whose composition graduallychanges with increasing surfactant concentration. In the sys-tems they investigated no critical concentration correspondingto the polymer saturation was observed. In this connection, wemay observe that the Pluronic–surfactant interaction mecha-nism could be strongly dependent on the Pluronics and sur-factants species. In fact, our evidence of polymer saturationand the absence of mixed polymer–surfactant micelles in thepresence of C10OS is clearly supported by the surface tensionand calorimetric measurements, in contrast with De Lisi et al.results obtained with different Pluronics and surfactant mix-tures.

From all the experimental techniques used, it can be inferredthat 25R4 presents a much stronger ability to interact with an-ionic surfactants than PE6200. In our opinion this evidence istoo strong to be attributed exclusively to the higher length ofPEO and PPO blocks of 25R4, suggesting that the differentchain architecture plays a role. In other words it seems that twoPPO blocks are more effective in promoting the interaction thana single PPO block with similar total length.


Finally, the calorimetric behavior has been interpreted by asimple equilibrium model of the aggregation processes obtain-ing a reasonable agreement with the experimental data.

Acknowledgment

This work has been carried on by FIRB N. RBAU01RBEHfinancial support.

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