DOI 10.1140/epje/i2014-14038-9

Regular Article

Eur. Phys. J. E (2014) 37: 38 THE EUROPEANPHYSICAL JOURNAL E

Interaction, solubilization and location of p-hydroxybenzoicacid and its sodium salt in micelles of moderately hydrophilicPEO-PPO-PEO triblock copolymers

Mehul Khimani1,a, Paresh Parekh1, Vinod K. Aswal2, and Pratap Bahadur1

1 Department of Chemistry, Veer Narmad South Gujarat University, Surat 395 007, India2 Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

Received 19 December 2013 and Received in final form 15 April 2014Published online: 26 May 2014 – c© EDP Sciences / Societa Italiana di Fisica / Springer-Verlag 2014

Abstract. Micelles of ABA type triblock copolymers (where A is polyethylene oxide PEO and B ispolypropylene oxide PPO) viz. PluronicR© P103, P104 and P105 (each containing almost the same PPOmol wt. ∼ 3250 g/mol and 30, 40 and 50 wt.% of PEO, respectively) in the presence of p-hydroxybenzoicacid (PHBA) and its sodium salt (Na-PHBA) were examined by viscosity, dynamic light scattering (DLS),small angle neutron scattering (SANS) and NMR. Spherical polymeric micelles (apparent hydrodynamicdiameter ∼ 20 nm) in water at 30 ◦C grow in the presence of PHBA and transform into prolate-ellipsoidalshape with an increased aggregation number. The micellar transition was favored at higher PHBA concen-tration, temperature and for copolymers with more hydrophobicity. The PHBA salt, however, increasedcloud point and showed only a marginal decrease in aggregation number even at much higher concentra-tions. The location of PHBA in micelle was elucidated by nuclear Overhauser enhancement spectroscopy(NOESY).

1 Introduction

Pluronics R© are amphiphilic block copolymers made ofa central hydrophobic polypropylene oxide (PPO) blockflanked by two same sizes of hydrophilic polyethylene ox-ide (PEO) end blocks. At temperature < 20 ◦C and lowconcentration, these block copolymers dissolve molecu-larly and exist as unimers but at higher temperatures,their molecules self-assemble to nanosized micelles withPPO core and heavily hydrated PEO shell. The dehy-dration of PPO blocks drives micellization above a cer-tain concentration (critical micelle concentration, CMC)or temperature (critical micelle temperature, CMT),while the alteration in size and geometry of micelleswith temperature depends on the gradual dehydrationof PEO blocks. Thus, temperature is often used as acontrol parameter for tuning the structure, size andshape of Pluronic R© micelles [1–4]. At high concentra-tion/temperature, Pluronics R© show the reversible thermo-rheological behavior [5] forming gel and several liquid crys-talline phases [6–8]. Sometimes time-dependent micellargrowth [9,10] has been observed in their aqueous copoly-mer solutions. Another interesting feature of these copoly-mer solutions is that their surface activity depends on so-

a e-mail: khimani mehul@yahoo.com;khimanimehul@gmail.com

lution conditions and often results in two break points inthe surface tension versus log concentration plots [11].

Pluronics R© find numerous industrial applications asdetergents, dispersants, stabilizers, emulsifiers, solubiliz-ers and foam regulator [12,13]. Micellization and sol-ubilization in aqueous micellar solutions of Pluronics R©

are important features which make these surfactants use-ful in fabricating mesoporous materials [14,15], nanopar-ticles [16,17] and as vehicles in drug delivery sys-tems [18,19]. Pluronics R© like other nonionic surfactantsare potentially useful for extraction of hydrophobic mate-rials [20,21]. The presence of additives such as inorganicsalts (with “salting out” and “salting in” effects) and neu-tral organic molecules like alcohols [22], phenols [23,24],amides [22,25], carboxylic acids [26] affects micellizationand micellar characteristics. Hydrophobic solutes favormicelle formation and promote growth by reducing theCMC and CMT while hydrophilic solutes lead to thedestabilization of micelles.

There are several studies on the solubilization of someweakly polar compounds. Donbrow et al. [27] using UV-Vis and NMR spectroscopy showed that solubilized ben-zoic acid resides in palisade layer. Mukerjee [28] studiedsolubilization and locus of benzoic acid derivatives andphenolic preservatives in nonionic micelles. From ESR andNMR experiments, the solubilization sites of phenolic an-tioxidants in micellar solutions were determined by Heins

Page 2 of 10 Eur. Phys. J. E (2014) 37: 38

Table 1. Characteristics of the copolymers used.

mol wt.wt. % CP, ◦C CMT, ◦C

PluronicsR© PPOPEO

PPO/PEO(1%)

HLB(1%)

block

P103 3465 30 1.76 86 7–12 19.5

P104 3540 40 1.13 81 12–18 21.5

P105 3250 50 0.76 92 12–18 21.7

et al. [29]. Recently, Parmar et al. [26] studied interactionand solubilization of phenolic antioxidants in Pluronic R©

micelles and showed that some of the extremely hydropho-bic solubilizates reside near the core whereas others re-main in palisade layer. Our group has recently shown theparabens induced sphere-to-rod growth and inter-micellarinteraction of Pluronic R© micelles [30].

The aim of the present study was to investigate theinteraction between PHBA and its sodium salt withPluronic R© (P103, P104 and P105) triblock copolymers.We observed that with the addition of PHBA, micelles un-dergo dehydration and hydrodynamic diameter increasesdrastically leading micelle growth/ transition. This couldbe an interesting study to solubilize the antioxidant in theblock copolymer micelles.

2 Materials and methods

2.1 Materials

Pluronics R© P103, P104 and P105 (table 1) were obtainedas a gift sample from BASF Corp. Parsippany, NJ, USA. p-Hydroxybenzoic acid (PHBA) and sodium p-hydroxyben-zoate were purchased from Sigma-Aldrich and used as re-ceived. Triply distilled water was always used. D2O wasused in SANS and NMR measurements.

2.2 Methods

2.2.1 Cloud point

Cloud points (CPs) of the solutions in the presence ofPHBA and Na-PHBA solutions were determined by slowlyheating the solution in 20ml thin glass tube immersedin a clean glass beaker containing water with continuedstirring. The first appearance of turbidity was taken as theCP. The obtained results were reproducible up to ±0.5 ◦C.

2.2.2 Viscosity

Ubbelohde capillary viscometer was used to measure vis-cosity of the solution. The viscometer was placed verti-cally in a thermostated water bath at 30 ± 0.1 ◦C. Theviscometer was cleaned and dried before and after eachmeasurement and the flow time of the solvent system andthe solutions were measured with a calibrated stopwatch.All the solutions showed Newtonian flow.

2.2.3 Dynamic light scattering (DLS)

DLS measurements were performed by Zetasizer 4800(Malvern Instruments, UK) equipped with a 192 chan-nel with a 7132 multibit digital correlator was used formeasurements. The average diffusion coefficient and thehydrodynamic size were obtained by the method of cumu-lants and using Stokes-Einstein equation. Each measure-ment was repeated at least three times.

2.2.4 Nuclear Overhauser enhancement spectroscopy(NOESY)

2D-NMR spectra were recorded on a Bruker AVANCE-IINMR spectrometer working frequency at 400.13MHz. Themixing and the delay times for experiments were estimatedfrom the spin-lattice relaxation times (T1 values). In allcases, an acquisition delay and mixing time ≈ 3 × T1 and1 × T1 were used to obtain the NOESY spectra. For allacquisitions, 256 transients of either 2 or 4 scans over 512complex data points were acquired.

2.2.5 Small-angle neutron scattering (SANS)

Small-angle neutron scattering (SANS) experiments werecarried out at Dhruva reactor, BARC, Trombay, India [31].The copolymer solutions were equilibrated 24 h and wereheld in teflon sealed 5mm thick quartz tube and neutronscattering cross sections were measured in the scatteringvector (Q) range from 0.017 to 0.35 A−1. The scatteringdata were corrected for the background.

In SANS experiments, one measures the coherent dif-ferential scattering cross-section per unit volume (dΣ/dΩ)as a function of Q. For monodispersed particles in amedium, it can be written as [32–37]

dΣ

dΩ= I(Q) = NV 2(Δρ)2Fmic(Q)S(Q) + BKG, (1)

where N is the number density of the micelles of volumeV , Δρ is the scattering length density difference betweenmicelle and solvent (D2O), Fmic(Q) is the form factorand S(Q) is the inter-particle structure factor. BKG is aconstant term that represents the incoherent backgroundscattering mainly from the hydrogen atoms present in thesample.

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For dilute system, S(Q) ∼ 1. Therefore, eq. (1) simpli-fies to

dΣ

dΩ= I(Q) = NV 2(Δρ)2Fmic(Q) + BKG. (2)

The Pluronic R© micelles consist of a hydrophobic core ofPPO surrounded by the hydrated shell of PEO. However,a part of PEO from the side of the PPO core becomeshydrophobic due to dehydration when the temperature isincreased or inorganic salt/hydrophobic solute is added.Hence, the micellar core not only consists with PPO butalso includes a part of PEO. For the experiment, thePluronic R© solutions are prepared in D2O and producedvery good contrast between the hydrophobic core and thesolvent, while there is a very poor contrast between the hy-drated shell and solvent. As a result, Fmic(Q) depends onlyon the hydrophobic core radius. The Fmic(Q) for sphericalmicelles can be written as

Fmic(Q) =[3(sin(QRc)) − QRc cos(QRc)

(QRc)3

]2

, (3)

where Rc, is the hydrophobic core radius which is at-tributed to the size of the micellar core.

The aggregation number is calculated by knowing thevalue of core sizes

Nagg =4π(Rc)3PPO

3nVPPO, (4)

where n is the number of propylene oxide monomers in thePPO block, (Rc)PPO is the radius of the core due to PPOonly, and VPPO is the volume of propylene oxide monomer.

The Fmic(Q) for ellipsoidal micellar system can bewritten as

Fmic(Q) =∫ 1

0

F 2(Q,μ)dμ, (5)

F (Q,μ) =3(sin x − x cos x)

x3, (6)

x = Q[a2μ2 + b2(1 − μ2)]1/2, (7)

where a and b = c are semi-major and semi-minor axesof micelles. The variable μ is the cosine of the angle be-tween the directions of a and Q. The aggregation numberis calculated by

Nagg =4πab2

3nVPPO, (8)

where n is the number of propylene oxide monomers inthe PPO block and VPPO is the volume of propylene oxidemonomer.

Throughout the data analysis, corrections were madefor instrumental smearing. The fitted parameters in theanalysis are micellar dimensions and aggregation number.The parameters in the analysis were optimized using anonlinear least-square fitting program.

Fig. 1. Cloud point of 1% P103, P104, P105 as a function ofvarying concentration of PHBA (filled symbols) and Na-PHBA(empty symbols). The continuous line is a guide for the eye.

3 Results and discussion

3.1 Cloud point

The cloud point (CP) of nonionic surfactants and somewater soluble neutral polymers can be tuned in the pres-ence of additives which alter surfactant-water interac-tions. Ascending solution temperature increases aggre-gation number/growth of micelles like rod, vesicle, etc.or inter-micellar interaction due to dehydration of watermolecule from PPO core and PEO shell leading to vis-ibly turbid solution at CP [30,38]. The CP is a man-ifestation of solvation/desolvation equilibrium in micel-lar solutions. Thus, the effect of species depends onhow they alter equilibrium. Effect of water soluble ad-ditives such as inorganic and organic salts (hydrotropes)can be explained by their influence on water structure.However, organic hydrophobic solute influences on thepacking properties of micelles i.e. stabilize or destabi-lize aggregates depending on the chemical nature and/orconcentration. Chaotropes (disorder-maker) disrupt thehydrogen-bonded network and favor demicellization whilekosmotropes (order-maker) reduce the aqueous solubil-ity of copolymer and thus favor micellization or micellargrowth [39,40].

Figure 1 shows that the CP decreases for P105 toP104 and increases significantly for P103. The reasonbehind this behavior is a micelle restructuring processthat slows down with increased hydrophobicity of copoly-mers [41,42]. Therefore, phase separation occurs at ahigher temperature than the actual CP. Ganguly et al. [9]and Kadam et al. [10] observed a similar kind of behaviorin the case of P123 copolymer, whose hydrophobicity iscomparable to P103 and also showed similar behavior dueto slow dehydration and restructuring of micelles.

The CP of 1% copolymers solutions as a function of[PHBA] is shown in fig. 1. As expected, increasing [PHBA]gradually depresses the CP to room temperature. This

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is due to dehydration of copolymer chain. However, thedecrease in CP was larger for P103 than P104 and P105due to its more hydrophobicity (less % PEO) which geteasily dehydrated by PHBA [43].

Interestingly, a contrast behavior was observed whenNa-PHBA is added to the solution. For this behavior, thetwo mechanisms can be proposed i) influences of addi-tive on the solvent property (indirect mechanism): ad-ditives change water structure and accelerate solvationof hydrophobic species and ii) replacement of solventmolecule by additive (direct mechanism): swapping of wa-ter molecules from hydration of shell by additive. The indi-rect mechanism is extensively accepted and experimentalstudies also support the hypothesis that Na-PHBA actsas a “water structure breaker”. Na-PHBA increases CPby making more water molecules available to micelles in-dicating that its behavior is like hydrotropes in aqueoussolution [38,44]. Hence, the CP of Pluronic increases andthe order of CP is P104 < P103 < P105 in presence ofNa-PHBA. The reason for this trend has been discussedearlier. Sharma et al. [45] observed a CP increasing effectof sodium arenesulfonate type hydrotropes on Pluronic R©

L64. Mahajan and coworkers [46] studied the effect of var-ious types of additives on the clouding behavior of triblockcopolymers in aqueous solutions.

In general, we can say that solubilization of PHBA inblock copolymer solution decreases the hydrophilic char-acter of micelles and therefore, decreases the cloud pointwhereas its sodium salt increases the cloud point.

3.2 Viscosity

Micellar characteristics of copolymers in aqueous solutionwere examined by viscosity method. Relative viscosity of1% solution at different [PHBA] and at different temper-ature is shown in fig. 2. It is evident from fig. 2a that therelative viscosity of P104 solution at 30 ◦C moderately in-creases above 25mM PHBA and at higher temperatureswhich indicates the possibility of micellar growth and tran-sition [24,30].

It is due to the presence of polar groups (-OH and-COOH) in PHBA which establish H-bond with highlyordered water molecules in a PEO environment of P104which indicates that the solubilization of PHBA at EO-POinterface of the micelle (further proved by NOESY) resultsin an increase in relative viscosity [47,48]. At higher tem-perature and PHBA concentration, PEO block also getsdehydrated and becomes less soluble in water. The inter-face curvature of micelle shrinks either due to loss of watermolecules or penetration of PHBA towards core-corona in-terface resulting in observed morphological changes [49].From fig. 2b, we also noticed the effect of PHBA on threedifferent copolymers. It can be inferred that P103 showedsignificant increase in viscosity at low PHBA concentra-tion whereas the others showed at high concentration.This is because of the difference in hydrophilic-lipophilicvalue of copolymers. However, Na-PHBA did not increasethe viscosity (fig. 2b) even at much higher concentrationsthan PHBA.

Fig. 2. (a) Viscosity of 1% P104 as a function of temperature(◦C) in the presence of PHBA. (b) Viscosity of 1% PluronicsR©

in the presence of PHBA at 30 ◦C. The continuous line is aguide for the eye.

3.3 Dynamic light scattering (DLS)

DLS provides information about both dynamics of micel-lar systems and its diffusion in solution. It is a widely ap-plied technique to know the hydrodynamic radius of blockcopolymer micelles in dilute solution using the Stokes-Einstein equation.

DLS was employed to characterize micelles in the pres-ence and absence of PHBA/Na-PHBA at different concen-tration and also as a function of temperature. Normally,with an increase in temperature, the micelle size grows.This may be due to two mechanisms: i) Dehydration ofPPO blocks from the core of the micelles and ii) the as-sociation of micelles (inter-micellar interaction or micellargrowth). The hydrodynamic diameter (Dh) of 1% P104micelles was not much affected with an increase in tem-perature as shown in fig. 3(a). For low concentration, ithas been observed that the second mechanism was lessimportant because of the low number density of micellesin solution [50]. Alexandridis et al. [51] observed the con-stant value of the hydrodynamic radii with respect to tem-perature at 0.5% and 2.5% of P104 and F108. Therefore,at high temperatures, 1% P104 had an almost constant

Eur. Phys. J. E (2014) 37: 38 Page 5 of 10

Fig. 3. Hydrodynamic diameter of (a) 1% P104, (b) 5% P104, (C) 5% P105 at 30 ◦C and 50 ◦C.

micelle size. But as expected, in 5% P104 micelle sizegrows with temperature. This may be due to second mech-anism. Thus, Dh increases (fig. 3b). Figure 3(c) shows thesize distribution profile of 5% P105 at two different tem-peratures. A polydisperse solution exhibits a small peakat ∼ 7 nm (unimers) which merges in a single micellarpeak (ca 20 nm). The unimers get solubilized in micellesat 50 ◦C showing single micellar peak with low polydisper-sity. This can be ascribed through two effects: i) increasethe hydrophobicity of both blocks, which makes the struc-ture more compact and ii) swelling of micelles.

Figure 4 shows the correlation function at differ-ent concentrations and temperatures. The DLS correla-tion functions are usually characterized by two dynamicmodes. At lower temperature and copolymeric concentra-tion both type of structure i.e. unimers and micelles arepresent. For 1% P104 and 5% P105 at 30 ◦C, the correla-tion function indicated a diffusive mode with shorter timewhile it displayed lesser decay time at 50 ◦C. Once, ther-mal energy is applied to polymeric solution, the structurebecomes compact due to dehydration as discussed ear-lier. Therefore, Brownian motion of particle in solutionbecomes more random. Hence, the diffusion coefficient of1% P104 increases with temperature (31.2 × 10−12 m2/sto 45.9 × 10−12 m2/s). However, the diffusion coefficientfor 5% P104 is 38.2 × 10−12 m2/s at 30 ◦C and decreasesat 50 ◦C (32 × 10−12 m2/s). This is either due to inter-micellar interaction and/or micellar growth, which resistthe motion in solution. Therefore, the correlation func-tions showed a fast decay followed by a slower relaxationwith a longer decay time.

The dramatic increase in Dh of 1% P104 was seen withthe addition of 20mM PHBA as a function of tempera-ture shown in fig. 5. The core of micelles becomes moreand more compact owing to the increase of hydrophobicitywith the temperature. Under the same effect, the coronaalso dehydrated and minimize contact with solvent, thusthe size of the micelle was reduced. Above some temper-ature, size of micelles increased. The micellar transition

Fig. 4. Correlation function diagram of (a) (◦) 1% P104 at30 ◦C (�) 1% P104 at 50 ◦C (♦) 5% P104 at 30 ◦C (�) 5%P104 at 50 ◦C and (b) (◦) 5% P105 at 30 ◦C (�) 5% P105 at50 ◦C (♦) 5% P104 at 30 ◦C (�) 5% P104 at 50 ◦C. The solidline represented fitted data.

Page 6 of 10 Eur. Phys. J. E (2014) 37: 38

Fig. 5. Dh vs. Temperature: (◦) 1% P104, (�) 1% P104 +20 mM PHBA, (�) 1% P104 + 150 mM Na-PHBA. The con-tinuous line is a guide for the eye.

has also been proved by small angle neutron scattering(SANS) in a later section. Our DLS results in the studiedtemperature range are in good agreement with the viscos-ity results. However, the Dh of 1% P104 micelle remainedalmost constant by the addition of 150mM Na-PHBA atvarious temperatures due to its hydrotropic nature.

3.4 Small Angle Neutron Scattering (SANS)

Small angle neutron scattering (SANS) is a powerful toolfor characterizing soft condensed matter and has beenextensively used to examine self-assembly in surfactantsand block copolymers. Hydrophilic block copolymers usu-ally do not exhibit micellar growth or transitions evenat high temperature or close to CP whereas highly hy-drophobic block copolymers show rapid micellar growthor sometimes vesicle formation immediately before phaseseparation [52,53]. It can be concluded that micellar trans-formation is the special features shown by only moderatelyhydrophobic-hydrophilic class of block copolymers [9].Thus, we used SANS technique to see changes in the mor-phology of micelle for P103, P104 and P105 in water andin the presence of Na-PHBA and PHBA at different tem-peratures and concentration. Some representative resultsare discussed below. These are shown in table 2.

The presence of PHBA up to 20mM concentration didnot change the shape of the micelles for P104 and P105 ex-cept Nagg increased from 90 to 105 (for P104) and 70 to 94(for P105), respectively. The shape change was, however,noticed for more hydrophobic P103. At same compositionand temperature, Pluronic R© P103 micelles show scatter-ing in low Q region as displayed in fig. 6a. This meansthat they have a higher aggregation number than P104and P105 micelles due to its more hydrophobic character-istic. Foster et al. [54] reported similar behavior for thesePluronics R© at 20 ◦C. They showed that 5% P103 formsmicelles with Nagg = 34.8 whereas P104 and P105 show

only a few unimers. This is also in agreement with our re-sults shown in table 2 that aggregation number (Nagg) of1% Pluronic R© at 30◦ decreases from 109 to 72 as the hy-drophilicity increases. The characteristics of PEO or PPOblocks in its molecule do not alter with copolymer con-centration. Hence, Rc and Nagg values can be expectedconstant for any copolymer concentration [53]. The coreradius of Pluronics R© is ∼ 53 A, ∼ 50 A and ∼ 44 A forP103, P104 and P105, respectively at 30 ◦C as shown intable 2. However, a little difference observed in Nagg dueto different batches of commercial Pluronic R© samples.

Unlike conventional nonionic surfactants, EO/POblock copolymeric surfactants show remarkable changesin micellar morphology in the presence of a small amountof additive or increase in temperature [3,11]. The addi-tion of hydrophobic solute or salt in very low concen-tration polydisperse micellar solution turns to monodis-perse system [53,54]. The increase in temperature or ad-ditive concentration increase only the number density ofblock copolymers micelles and aggregation number [55].Upon addition of 20mM PHBA at 30 ◦C, all three sys-tems showed dramatic changes (fig. 6a) with similar trendsobserved for the different Pluronic R© samples in D2O. Inall cases, core radius and aggregation number increased(table 2). For all the systems, except P103 with 20mMPHBA at 30 ◦C spherical form factor was used for fittingexperimental data. However, for P103 with 20mM PHBAellipsoidal core-shell form factor was used. From the fit-ting function, it was observed that micelles are sphericalexcept for P103 at 20mM PHBA.

Solubilization of solute increases/decreases the micel-lar core radius while the corona dimension is not verymuch affected. In general, the increases/decreases in coreradius depend on the aggregate morphology [56]. Parekhet al. [57] observed that solubilized aromatic hydrocarbonsin P104 do not affect the micelle shape; spherical micelleswith increase in core radius and aggregation number wereseen similar to the behavior observed by us at lower con-centration (10 and 20mM at 30 ◦C) of PHBA in P104(fig. 6b).

The shifting of Q value at lower region in the form fac-tor displayed increases in micellar size, whereas the struc-ture factor is related to inter-micelles distance. Here, wehave taken 1% Pluronic R©, therefore the value of structurefactor is ∼ 1. Guo et al. [49] described a change in shapeof P105 micelles upon the addition of 1-phenylethanol(spherical-cylindrical-disklike micelles). Shift at lower Qregion was observed in form factor for Pluronic R© P103with 20mM (fig. 6a) and for P104 with 30mM PHBA(fig. 6b) at 30 ◦C in our study. This indicates micellar mor-phological transition i.e. spherical to prolate-ellipsoidal. Itcan be seen from table 2 that the core radius of micelleslightly decreases but aggregation number increases.

The temperature is also a key factor in the aggregationcharacteristics of Pluronics R©. The aggregation behaviorof Pluronic R© P104 with 20mM PHBA was investigatedin a series of temperatures shown in fig. 6c. Upon increas-ing the temperature further, similar trends were observed:an increase in the aggregation number and decrease incore radius (table 2). Scattering intensity at low Q region

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Table 2. Core radius and aggregation number of 1% PluronicR© as a function of temperature and PHBA concentration in D2O.

PluronicsR© [PHBA] Temperature Ra Rb Axial ratio Nagg(mM) ( ◦C) (nm) (nm)

(a)P103 0 30 5.3 ± 0.1 109 ± 6

P103 20 30 4.7 ± 0.1 17.4 3.70 279 ± 17(a)P104 0 30 4.9 ± 0.1 85 ± 5(a)P104 10 30 5.2 ± 0.1 101 ± 6(a)P104 20 30 5.3 ± 0.1 105 ± 6

P104 30 30 4.6 ± 0.1 14.1 3.03 216 ± 14

P104 20 40 4.8 ± 0.1 15.1 3.18 243 ± 15

P104 20 50 4.8 ± 0.1 14.5 3.02 239 ± 15(a)P105 0 30 4.5 ± 0.1 72 ± 5(a)P105 20 30 4.9 ± 0.1 94 ± 6

(a)Rc = core radius fitted from spherical form factor.

Fig. 6. (a) Scattering stacks of various PluronicsR© at 30 ◦C. Symbol (◦) indicates 1% PluronicsR© and (�) for PluronicsR© in20 mM PHBA. (b) Scattering curves of 1% P104 in the presence of various concentrations of PHBA at 30 ◦C. (c) Scatteringcurves of 1% P104 in the presence of 20 mM PHBA at various temperatures. (d) Scattering curves of 1% P104 in the presenceof 150mM Na-PHBA* at 30 ◦C. The solid lines represent fitted data. (*Values of P104 + 150 mM Na-PHBA at 30 ◦C areRc = 4.73 nm, S = 0.26 and Nagg = 77).

begins to increase at 40 ◦C. This is the characteristics of achange in micelle shape i.e. spherical to ellipsoidal at hightemperatures with lower [PHBA]. This means that thepresence of higher [PHBA] actually influences the shapeof the micelles at room temperature or at higher tempera-ture with lower [PHBA]. However, an addition of 150mMNa-PHBA in 1% P104 (fig. 6d), the core radius and aggre-gation number both are decreased (47.3 A and 77). There-fore, we conclude that the micelle gets slightly extended.The contradictory behavior confirmed that Na-PHBA actsas hydrotropes. Our SANS data also support DLS and vis-cosity results.

3.5 1H NMR

1H NMR chemical shift is sensitive to the local electronicenvironment around the molecule. Chemical shift varia-tion is an excellent approach to provide evidence of mi-

cellization. The addition of an inorganic salt/hydrophobicsolute/ascending in temperature is usually attributed tothe dehydration of PPO core and favors the micelliza-tion [58,59]. Here, we studied the change in proton chem-ical shift of 1% P103 with PHBA concentration at a fixedtemperature (30 ◦C) as shown in fig. 7. Increasing con-centration of PHBA decreases CMT and leads to micelleformation at lower temperature.

Chemical shift of methyl proton of PPO and methy-lene proton of PEO block depends on the temperature oradditive concentration. With increasing [PHBA], the res-onance of methyl (-CH3) proton of PPO block exhibitedan upfield shift from 1.1 to 1 ppm while the resonanceof methylene (-CH2) proton slightly changed. A chemicalshift of methylene group proton in the PEO segment ofblock copolymer has an identical local chemical environ-ment. This is because the PEO blocks are in contact withthe solvent and segregated from the PPO blocks (fig. 7).The line width of NMR signal could be used to investigate

Page 8 of 10 Eur. Phys. J. E (2014) 37: 38

Fig. 7. 1H NMR spectra of (a) 1% P103, (b) 1% P103 + 10 mM PHBA and (c) 1% P103 + 20mM PHBA at 30 ◦C.

Fig. 8. Schematic diagram of (a) PHBA and (b) Na-PHBA.

morphological transitions in micelles [60]. However, signifi-cant change observed in peak width (fig. 7c). Interestingly,the signal intensity of 20mM PHBA in P103 system de-creases. It is caused mainly by conformational changes i.e.micellar shape transition. The transition of micelle fromspherical to prolate was already discussed by SANS. Athigher concentration, PHBA (30mM) gave a similar ef-fect in P104 whereas at lower concentration, no significantchange was observed in their chemical shift (figure notshown). Therefore, we concluded that the molecular tran-sition is not characteristic of all three copolymers at lowertemperature, concentration and lower concentrations ofadditives.

The resonance of the methyl group of the PPO blockis seen as a singlet at 1.1 ppm. The broad peak from∼ 3.65 to 3.45 ppm is assigned to the PPO -CH2- pro-tons, whereas the intense resonance observed at around3.7 ppm is due to the -CH2- protons of PEO block. Thus,the 1H NMR spectrum of EO-PO block copolymer hasmostly three peaks assigned (fig. 7). Figure 8 displays thestructure of PHBA and Na-PHBA. Its 1D NMR spectragive peak at ∼ 7.0 ppm of Ha and Hb at ∼ 8 ppm; how-

ever, aromatic phenolic -OH peak has a wide range ofchemical shift depending upon chemical environment inmolecule, temperature and concentration [61]. As a result,PHBA/Na-PHBA is dissolved in P104 block copolymersolution, -OH peak of PHBA/Na-PHBA overlaps/mergeswith -CH2 peak. Therefore, we cannot understand its be-havior in solution i.e. location in micelle/solution. Hence,NOESY spectra are essential for understanding locus ofsolute.

2D NMR provides more information about a moleculethan 1H NMR spectra; it is particularly useful for deter-mining the location of a molecule in the micelle whichis complicated to work with using one-dimensional NMR.NOESY was employed to obtain the distance between in-tra and inter molecular protons present in the systems,aggregation and specific localization of interacting frag-ments in solution. From the NOESY spectrum, it is pos-sible to trace out whole coupling network in the molecule.However, in colloidal systems, cross-peaks gave crucial in-formation concerning locus of solubilizates in micelle.

The addition of PHBA into copolymer solutions in-creases the viscosity. This may due to some kind of in-

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Fig. 9. NOESY spectra of (a) 5% P104 in the presence of 50mM PHBA and (b) 5% P104 in the presence of 150mM Na-PHBA.

teraction between PHBA and block copolymer which wasproved by NOESY experiment. Selective NOESY spec-tra of PHBA/Na-PHBA in Pluronic R© 104 micellar sys-tems are displayed in fig. 9. Cross-peaks were observedbetween Ha proton of PHBA molecule and methyl protonof PPO and methylene proton of both the blocks. Sim-ilarly, methylene proton of P104 molecule interacts withHb proton of PHBA. This indicates that PHBA moleculeresides at the EO-PO interface of the micelle. Hence, theviscosity of solution and micellar size increases. A simi-lar kind of behaviour was observed by Parmar et al. [26].There were no intra/inter molecular interactions noticedin P104 and Na-PHBA by NOESY cross- peaks. The lackof cross-peak intensity indicates that Na-PHBA is solu-bilized in aqueous phase rather than micellar phase. Ourviscosity and scattering results also support NOESY.

4 Conclusion

Our work on the effect of PHBA, an antioxidant, and itssodium salt on the structure of Pluronics R© micelle hav-ing different hydrophobicity obtained through CP, viscom-etry, scattering techniques (DLS and SANS) and NMRshows that the acid and its salt behave oppositely in af-fecting the solution behavior of Pluronics R©. The pres-ence of PHBA favors micellization and promotes micellargrowth and transition whereas its salt decreases micellesize. Increase in viscosity at high PHBA concentration orwith increasing temperature inferred micellar transition.Spherical micelles formed at room temperature or at lowerconcentrations of PHBA transform into prolate spheroid(ellipsoidal shape) at higher concentration or an increasein temperature which was proved by DLS and SANS. Thelocation of PHBA in micelle was responsible for micellargrowth/transition which were proved by NOESY experi-ments.

The authors thank UGC-DAE CSR, BARC, Mumbai (ProjectNo: CRS-M-174) for financial support.

References

1. P. Alexandridis, B. Lindman, Amphiphilic Block Copoly-mers: Self-Assembly and Applications (Elsevier, 2000).

2. K. Devanand, J. Selser, Nature 343, 739 (1990).3. G. Wanka, H. Hoffmann, W. Ulbricht, Macromolecules 27,

4145 (1994).4. B. Chu, Z. Zhou, Physical Chemistry of Polyoxyalkylene

Block Copolymer Surfactants (CRC Press, 1996).5. G. Dumortier, J.L. Grossiord, F. Agnely, J.C. Chaumeil,

Pharm. Res. 23, 2709 (2006).6. R. Ivanova, B. Lindman, P. Alexandridis, Adv. Colloid In-

terface Sci. 89, 351 (2001).7. P. Alexandridis, U. Olsson, B. Lindman, Langmuir 14,

2627 (1998).8. J. Zipfel, J. Berghausen, G. Schmidt, P. Lindner, P.

Alexandridis, M. Tsianou, W. Richtering, Phys. Chem.Chem. Phys. 1, 3905 (1999).

9. R. Ganguly, M. Kumbhakar, V. Aswal, J. Phys. Chem. B113, 9441 (2009).

10. Y. Kadam, R. Ganguly, M. Kumbhakar, V.K. Aswal, P.A.Hassan, P. Bahadur, J. Phys. Chem. B 113, 16296 (2009).

11. P. Alexandridis, V. Athanassiou, S. Fukuda, T.A. Hatton,Langmuir 10, 2604 (1994).

12. G. Riess, J. Nervo, D. Rogez, Polym. Eng. Sci. 17, 634(1977).

13. J. Horsky, Z. Walterova, Colloid. Polym. Sci. 283, 1033(2005).

14. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D.Stucky, Nature 396, 152 (1998).

15. K. Flodstrom, V. Alfredsson, Microporous MesoporousMater. 59, 167 (2003).

16. P. Alexandridis, M. Tsianou, Eur. Polym. J. 47, 569(2011).

17. M.S. Bakshi, A. Kaura, P. Bhandari, G. Kaur, K. Torigoe,K. Esumi, J. Nanosci. Nanotechnol. 6, 1405 (2006).

Page 10 of 10 Eur. Phys. J. E (2014) 37: 38

18. A.V. Kabanov, E.V. Batrakova, V.Y. Alakhov, Adv. DrugDelivery Rev. 54, 759 (2002).

19. K. Kataoka, A. Harada, Y. Nagasaki, Adv. Drug DeliveryRev. 47, 113 (2001).

20. L. Heerema, D. Cakali, M. Roelands, E. Goetheer, D. Ver-does, J. Keurentjes, Sep. Purif. Technol. 74, 55 (2010).

21. L. Heerema, D. Cakali, M. Roelands, E. Goetheer, D. Ver-does, J. Keurentjes, Sep. Purif. Technol. 73, 319 (2010).

22. P. Alexandridis, L. Yang, Macromolecules 33, 5574 (2000).23. J. Mata, P. Majhi, O. Kubota, A. Khanal, K. Nakashima,

P. Bahadur, J. Colloid Interface Sci. 320, 275 (2008).24. R. Ganguly, K. Kuperkar, P. Parekh, V. Aswal, P. Ba-

hadur, J. Colloid Interface Sci. 378, 118 (2012).25. J. Armstrong, B. Chowdhry, J. Mitchell, A. Beezer, S.

Leharne, J. Phys. Chem. 100, 1738 (1996).26. A. Parmar, K. Singh, A. Bahadur, G. Marangoni, P. Ba-

hadur, Colloids Surf. B 86, 319 (2011).27. M. Donbrow, C. Rhodes, J. Pharm. Pharmacol. 18, 424

(1966).28. P. Mukerjee, J. Pharm. Sci. 60, 1528 (1971).29. A. Heins, D.B. McPhail, T. Sokolowski, H. Stockmann, K.

Schwarz, Lipids 42, 573 (2007).30. M. Khimani, R. Ganguly, V. Aswal, S. Nath, P. Bahadur,

J. Phys. Chem. B 116, 14943 (2012).31. V. Aswal, P. Goyal, Curr. Sci. 79, 947 (2000).32. J. Hayter, J. Penfold, Colloid Polym. Sci. 261, 1022 (1983).33. J.B. Hayter, J. Penfold, J. Chem. Soc., Faraday Trans. 1

77, 1851 (1981).34. J.B. Hayter, J. Penfold, Mol. Phys. 42, 109 (1981).35. J.-P. Hansen, J.B. Hayter, Mol. Phys. 46, 651 (1982).36. S.-H. Chen, Annu. Rev. Phys. Chem 37, 351 (1986).37. S.-H. Chen, T.-L. Lin, Methods Exp. Phys. 23, 489 (1987).38. R.C.d. Silva, W. Loh, J. Colloid Interface Sci. 202, 385

(1998).39. Y. Zhang, P.S. Cremer, Curr. Opin. Chem. Biol. 10, 658

(2006).40. Y. Zhang, S. Furyk, D.E. Bergbreiter, P.S. Cremer, J. Am.

Chem. Soc. 127, 14505 (2005).41. G. Waton, B. Michels, R. Zana, J. Colloid Interface Sci.

212, 593 (1999).

42. M.J. Kositza, C. Bohne, P. Alexandridis, T.A. Hatton, J.F.Holzwarth, Langmuir 15, 322 (1999).

43. M. Prasad, S. Moulik, D. Chisholm, R. Palepu, J. OleoSci. 52, 523 (2003).

44. A. Klaus, G.J. Tiddy, R. Rachel, A.P. Trinh, E. Maurer,D. Touraud, W. Kunz, Langmuir 27, 4403 (2011).

45. R. Sharma, P. Bahadur, J. Surfactants Deterg. 5, 263(2002).

46. A. Shaheen, N. Kaur, R.K. Mahajan, Colloid Polym. Sci.286, 319 (2008).

47. Y.P. Saraf, S.S. Bhagwat, Sep. Technol. 5, 207 (1995).48. W.A. Pryor, J.A. Cornicelli, L.J. Devall, B. Tait, B.

Trivedi, D. Witiak, M. Wu, J. Org. Chem. 58, 3521 (1993).49. L. Guo, R.H. Colby, P. Thiyagarajan, Physica B: Condens.

Matter 385, 685 (2006).50. I. Goldmints, F.K. von Gottberg, K.A. Smith, T.A. Hat-

ton, Langmuir 13, 3659 (1997).51. P. Alexandridis, T. Nivaggioli, T.A. Hatton, Langmuir 11,

1468 (1995).52. F. Li, L.H.J. de Haan, A.T.M. Marcelis, F.A.M. Leermak-

ers, M.A.C. Stuart, E.J.R. Sudholter, Soft Matter 5, 4042(2009).

53. N. Jain, V. Aswal, P. Goyal, P. Bahadur, Colloids Surf. A173, 85 (2000).

54. B. Foster, T. Cosgrove, B. Hammouda, Langmuir 25, 6760(2009).

55. G. Wanka, H. Hoffmann, W. Ulbricht, Colloid Polym. Sci.268, 101 (1990).

56. R. Nagarajan, Colloids Surf. B 16, 55 (1999).57. P. Parekh, K. Singh, D.G. Marangoni, V.K. Aswal, P. Ba-

hadur, J. Surfactants Deterg. 15, 1 (2012).58. J. Ma, C. Guo, Y. Tang, J. Wang, L. Zheng, X. Liang, S.

Chen, H. Liu, Langmuir 23, 3075 (2007).59. X. Liang, C. Guo, J. Ma, J. Wang, S. Chen, H. Liu, J.

Phys. Chem. B 111, 13217 (2007).60. J. Ma, C. Guo, Y. Tang, H. Liu, Langmuir 23, 9596 (2007).61. D.L. Pavia, G.M. Lampman, G.S. Kriz, R.G. Engel, A

Small-Scale Approach to Organic Laboratory Techniques(Cengage Learning, 2010).