Surface activity and micellar behavior of dimethylamino- and trimethylammonium- tipped oxyethylene–oxybutylene diblock copolymers in aqueous media

Surface Activity and Micellar Behavior of Dimethylamino-and Trimethylammonium- Tipped Oxyethylene–Oxybutylene Diblock Copolymers in Aqueous Media

Abbas Khan, Mohammad Siddiq

Department of Chemistry, Quaid-I-Azam University, Islamabad, Pakistan

Received 4 May 2009; accepted 8 February 2010DOI 10.1002/app.32226Published online 14 July 2010 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Surface activity and micellar behavior inaqueous media in the temperature range 20–50�C of thetwo block copolymers, Me2N(CH2)2OE39B18, (DE40B18)and I�Me3N

þ(CH2)2OE39B18, (TE40B18) in the premicellarand postmicellar regions have been studied by surfacetensiometry, viscometry, and densitometry. Where E rep-resents an oxyethylene unit while B an oxybutyleneunit. Various fundamental parameters such as, surfaceexcess concentrations (Cm), area per molecule (aS1) at air/water interface and standard Gibbs free energy foradsorption, DG0

ads have been investigated for the premi-cellar region at several temperatures. The thermody-namic parameters of micellization such as, criticalmicelle concentrations, CMC, enthalpy of micellization,DH0

mic, standard free energy of micellization DG0mic, and

entropy of micellization DS0mic have also been calculatedfrom surface tension measurements. Dilute solution vis-cosities have been used to estimate the intrinsic viscos-ities, solute-solvent interaction parameter and hydrationof micelle. Partial specific volume and density of the mi-celle were obtained from the density measurements atvarious temperatures. The effect of modifying the endgroup of the hydrophilic block was investigated by com-paring the behavior of trimethylammonium- and dime-thylamino-tipped copolymers, designated TE40B18, andDE40B18, respectively. VC 2010 Wiley Periodicals, Inc. J ApplPolym Sci 118: 3324–3332, 2010

Key words: block copolymers; adsorption; micellization;surface tension; density; viscosity

INTRODUCTION

Block copolymers of hydrophilic poly(oxyethylene)and hydrophobic poly(oxybutylene) in dilute aque-ous solutions associate to form micelle, the hydro-phobic block form micellar cores and are mainlyresponsible for the formation of micelles. The hydro-philic block forms the micelle corona and its lengthmainly determines the micellar size and interac-tions.1–4 The micellar and associative propertiesof block copolymers incorporating hydrophilicblock of oxyethylene (E) units, A[CH2CH2O]A, andhydrophobic block of oxybutylene (B) units,A[CH(C2H5)CH2O]A in aqueous media have beenstudied in detail.1,5 To meet specific requirementsfor different applications, such as detergency, disper-sion stabilization, foaming, emulsifications, andpharmaceutical applications, a number of internalparameters, such as molecular weight, chemicalnature, block architectures, etc., and some external

parameters such as selection of solvent and condi-tions are adjusted.1,5,6 Among such parameters animportant one is the nature of the end group, whichmay have significant effect on the micellar propertiesof diblock copolymers. For example, Kelarakis et al.7

have studied a range of EmBn copolymers (m ¼ 18–184, n ¼ 9–18), and have reported that if the termi-nal hydroxy end of the B-block is methylated, theassociation number and thermodynamic radii arefound to increase while the hydrodynamic radius isdecreased. Maskos8 has also observed the effect ofPEO-sided end group on the morphology and char-acteristic dimension of PEO–PBO polymeric particlein selective solvent. He noticed prominent changesin various properties for his end-group modifieddiblock copolymers. The effect of block composition,block length and block architecture on the micellarproperties in aqueous media of block copoly(oxyal-kylene)s in which hydrophilic poly(ethylene oxide)is combined with hydrophobic poly(propylene ox-ide), poly(1,2-butylene oxide) or poly(styrene oxide)have recently been reviewed.9 At a given tempera-ture micellization of block copolymers can beachieved by increasing the concentration of blockcopolymers. The concentration at or above which themicellization start is called CMC (critical micelleconcentration). CMC of block copolymers can be

Correspondence to: M. Siddiq ([email protected]).Contract grant sponsor: Higher Education Commission

(H.E.C), Pakistan.

Journal ofAppliedPolymerScience,Vol. 118, 3324–3332 (2010)VC 2010 Wiley Periodicals, Inc.

judged by many concentration dependent physicalmethods.10 Recently, Mansur and coworkers11 haveevaluated and compared the critical micelle concen-tration (CMC) for aqueous solutions of linear blockcopolymers, determined by using three differenttechniques: such as tensiometry, fluorescence, andlight scattering, and the values of CMC found werein good agreements with each others. For the pres-ent work, we will use surface tension method,because the surface tension method is versatile as itcan estimate not only CMC values but also providevital information about adsorption characteristics ofsolutes at the air/water interface.10

The purpose of the work is to investigate the possi-ble effects on the micelle properties by substituting theterminal part of the hydrophilic group by dimethyla-mino- and trimethylammonium-groups. The presentwork concerns on the micelle properties in aqueous so-lution of a trimethylammonium-tipped copolymer,I�Me3N

þ(CH2)2OE39B18 (denoted TE40B18), and thedimethylamino-tipped, Me2N(CH2)2OE39B18 (denotedDE40B18). Under the experimental conditions, TE40B18

carries a charged tip, whilst the DE40B18 is apparentlyneutral.12 As far as we are aware, there are no literaturereports on the temperature dependence of CMC andother surface active properties for our copolymers.Moreover, we were interested to study micellar andsurface activity at air/water interface using very basic,fundamental, and easily available techniques like sur-face tension, viscosity, and density.

EXPERIMENTAL

Preparation and characterization of copolymers

The block copolymers were synthesized by ourresearch group at University of Manchester. Thepreparation of Me2N(CH2)2OE39B18 is described herefrom the previously published work.13,14 DAE, 2-dimethylaminoethanol (0.56 g, 6.28 mmol) was addedby syringe to freshly distilled THF (25 cm3) in an am-poule under N2. Potassium metal (ca. 0.09 g, 37 mol% DAE) was added piecewise to the stirred mixtureover 10 min. The mixture was subjected to twofreeze-pump-thaw cycles, and then stirred overnightat ambient temperature to allow all of the potassiumto react, giving a clear solution. EO (16.73 g, 61.5 molequivalent DAE) was transferred into the ampouleunder vacuum. The temperature was graduallyincreased from ambient to 40�C over 13 days, afterwhich time a thick, white precipitate was observed. Asample was removed for analysis. THF was trans-ferred out of the ampoule and BO (13.55 g, 32.3 molequivalent DAE) transferred in, under vacuum. Thetemperature was increased from 48 to 58�C over 7days (yield 28.90 g). The average composition fromnuclear magnetic resonance (NMR) end-group analy-

sis was Me2N(CH2)2OE39B18. The molar mass distri-bution of the copolymer was shown by gel permeationchromatography (GPC) to be narrow (M/Mn ¼ 1.06).The trimethylammonium-tipped copolymer TE40B18

was formed by quaternization with iodomethane inmethanol at ambient temperature in the dark.More detail for preparation and characterization of thecopolymers have been fully described previously.13,14

Surface tension measurements

Surface tension measurement was used for estimationof CMC and surface active parameters. The stock sol-utions (2 g dm�3) for each copolymer were preparedin deionized and doubled distilled water, and othercopolymer solutions were obtained by diluting thestock solutions. The surface tension, (c) of dilute aque-ous copolymer solutions were measured at tempera-ture in the range of 20–50�C, by detachment of plati-num ring (4 cm circumference), using a torsionbalance (White Elec. Inst. Co. Ltd., Model OS). TheTorsion Balance (White Elec. Inst. Co. Ltd., Model OS)for surface tension measurement was supplied byTorsion Balance Supplies Co. (Warwickshire, Eng-land, United Kingdom). And the Stabinger Viscome-ter G2 (SVM 3000/G2) supplied by Anton Paar. Theinstrument was well protected from vibration anddraught. A new solution was first equilibrated at low-est temperature for 1 h and then c was measured afterevery 30 min until consistent readings were obtained.Thereafter, the temperature was raised, the solutionswere re-equilibrated for 1 h, and the measurementprocedure was repeated.15 Before using a new solu-tion the ring was washed successively with diluteHCl and deionized water. To remove the possibleeffect of adsorption onto platinum ring, it was keptout of solution during temperature/concentrationequilibration. Moreover to see the possible adsorptionat the ring, each measurement was repeated threetimes with the same values of surface tension. Due tosmall surface area of the ring, however, no detectableadsorption was noticed, thus the change in surfacetension was assigned to the adsorption of copolymersat air/water interface. The accuracy of instrumentwas checked by frequent determination of the surfacetension of deionized water. The critical micelle con-centration, CMC, were obtained from the plots of sur-face tension (c) versus log C (where C is the concen-tration of the copolymers in g dm�3) in the aqueoussolutions. The CMC was assigned to the concentrationabove which the surface tension remained constant.16

Viscosity and density measurements

Solutions of different concentrations for density andviscosity studies were prepared in deionized anddoubled distilled water. Density and viscosity of

PROPERTIES OF COPOLYMERS IN AQUEOUS MEDIA 3325

Journal of Applied Polymer Science DOI 10.1002/app

dilute aqueous solutions of block copolymers weremeasured in the temperature ranging from 20 to50�C, with the help of Stabinger Viscometer G2(SVM 3000/G2) supplied by Anton Paar. This vis-cometer combines the accuracy of conventional cap-illary viscometers with the speed and ease of use ofAnton Paar’s world-leading digital density meters.The instrument works in different measuring modeseach having its own importance. We used M3 modefor our work, in which viscosity can be measuredover wide temperature range (with intervals of10�C) for a single solution. First the sample cell wascleaned and dried, and then sample (about 3 mL)was injected with the help of disposable syringethrough the filling inlet and by pressing enter key ofthe instrument, it automatically starts measuring vis-cosity and density over the given range of tempera-tures. After 10 min, the results were displayed onthe instrument screen and automatically printedwith the help of a printer connected to the instru-ment. After each measurement, the sample cell wascleaned and dried with an air pump. To measuredensity and viscosity over a wide concentrationrange, we diluted the stock solution, and resultswere obtained by repeating the above procedure.The instrument directly gave the values of density,dynamic viscosity, and kinematics viscosity.

RESULTS AND DISCUSSIONS

Behavior of the hydrophilic end group

The two copolymers used in this work have the sameblock length and similar composition, the only differ-ence occur at their hydrophilic end group. One hasdimethylamino- group (denoted DE40B18), while theother posses trimethylammonium group (denotedTE40B18) at their modified ends. The possible localcharge near the hydrophilic end and its effect on dif-

ferent parameters calculated for the two copolymers,we have to consider the nature of the end group. In thecase of a trimethylammonium-tipped (T) copolymer,there is a positive charge and associated counterion,which contributes to a greater extent to excluded vol-ume for the hydrophilic block, as compared to a purelynonionic polymer. Under the present experimentalconditions, the solutions for dimethylamino-tipped (D)copolymer is expected and were found to be of neutralpH,12 but this does not necessarily mean that the endgroups are neutral, since there may be a different localpH in the immediate environment of the end group.Furthermore, because the amine is attached to an Echain, its local environment is effectively a mixed sol-vent of water and poly(ethylene glycol). For a lowmolar mass tertiary amine, such as triethylamine, theconjugate acid has a pKa of about 11 at ambient tem-perature.17 If the D-tip had a pKa of this magnitude, itcan be calculated that there would be at least 80%degree of protonation under the conditions of theexperiments. However, for tertiary amines incorpo-rated into surfactants, pKa values appear to be lower.18

On balance, we can expect that DE40B18 copolymerswill partially be protonated under the conditions of theexperiments. A protonated DE40B18 copolymer, like aTE40B18 copolymer, will carry a positive charge. How-ever, a significant difference is that a DHþ tip can forma hydrogen bond, either with an ether oxygen of thepoly(ethylene glycol) block or with a nonprotonated Dgroup. Thus intermolecular hydrogen bonding wouldstabilize micelle to a greater extent with higher aggre-gation number and size.12

Thermodynamic parameters of micellizationand adsorption

Plots of surface tension (c) versus logarithmic ofcopolymer concentration (log C) for aqueous solu-tions of DE40B18 and TE40B18 in the temperaturerange 20–50�C, were plotted for determination ofCMC. Only typical plots for DE40B18 at various tem-peratures are shown here in Figure 1. Following theprevious practice,16,19,20 the CMC was assigned tothe concentration at which the surface tensionreached a steady value. Moreover, the CMC definesthe point at which the complete Gibbs monolayer isformed and is used as an indirect indication of theconcentration at which molecules first form associ-ates in appreciable concentration, the argumentsbeing that adsorption at the air/water interface isfavored over micellization until full monolayer isformed. It may be noted that the effect is not itselfindicative of the formation of large micelle, as it ispossible for the surface tension curve to become con-stant when association is limited to dimers and nottrimers.15 The values of CMC along with cCMC forboth the copolymers are listed in Table I. It is clear

Figure 1 Typical plots of surface tension (c) as a functionof logarithmic of concentration (log C) for aqueous solutionsof DE40B18 at (n) 20

�C, (l) 30�C, (~) 40�C, and (!) 50�C.

3326 KHAN AND SIDDIQ


from the data that the values of CMC for DE40B18

are much lower than that of TE40B18, the effect isattributed to the difference in the end groups. TheT-tipped copolymer has greater affinity for water ascompared to D-tipped; hence the former copolymerneeds more amount of its unimers to micellise. TheCMC values for hydroxyl-ended group copolymers,1

for examples for E40B8 and E49B18, are 0.33 and 0.30g/L at 30�C, respectively. While some other diblockcopolymers of the same class,9 E96B18 and E18B10

show these values 0.01 and 0.063 g/L as CMC. Inour case, however, both the copolymers have theCMC values much higher than those in Refs. 1 and9. This effect can be attributed to greater interac-tions/solubility of these copolymers with water dueto dimethylamino- and trimethylamonium- groupsat the hydrophilic ends. Our previous laser lightscattering studies12 on D/T-tipped modified copoly-mers and their comparisons with conventional EmBn

copolymer showed prominent influence of end-group modification on various micellar parameterssuch as micelle size and aggregation number, etc.

The slope from the linear part of surface tension(c) versus log C is related to the surface excess con-centrations of the copolymer in the surface layercompared to the bulk, through Gibbs adsorption iso-therm.10,19

Cm ¼ � 1

2:303RT

@c@ logC

8>>:

9>>;

T

(1)

Within the assumptions that the concentration ofsolute in the solutions is negligible compared to thatin the surface layer, by using the value of surfaceexcess concentration (Cm), the area per molecule inthe surface monolayer in square angstroms can becalculated from the relationship,

as1 ¼1016

NACm(2)

where R is 8.314 J mol�1 K�1, T is absolute tempera-ture in K, qc/q log C is the slope of linear portionsof the c versus log C plots, NA is the Avogadro’snumber and Cm is in mol cm�2.10,19

From the surface tension vs concentration profilewe can also calculate surface excess pressure or sur-face pressure at critical concentration, pCMC. WherepCMC ¼ c0 � cCMC, c0 and cCMC are the values ofsurface tension of pure water and of the solutions atCMC.16,21 The values of Cm, a

s1 and pCMC are given

in the Table II. It is clear from the table that for agiven polymer the values of as1 increased withincrease in temperature in the range 20–50�C. Soniet al.22 have also reported a similar increase in as1values with increase in temperature for nonionic sili-cone surfactants. This increase in as1 could be due tothe copolymers adsorbed at air/water interface withhydrophilic (-EO-) units probably oriented parallelto the surface, with one end anchored by the hydro-phobic (-BO-) chain projecting outward from the sur-face, and the other end tending to move away fromthe surface into the bulk.23 In our copolymers wehave dimethylamino- and trimethyl ammoniumgroups at the end of hydrophilic -EO- block and itcan be suggested that this dimethylamino- and tri-methylammonium groups probably show preferen-tial orientation toward the water surface because ofhydration, the same explanation has been reportedby Soni et al.22 for nonionic silicone surfactants. Wecan see that at a given temperature DE40B18 havehigher values of as1 than TE40B18, means that theunimers of second copolymer have stronger interac-tions with water, and hence close arrangement. Rela-tively, a larger surface area was expected for T-tipped copolymer (cationic-tip) due to electrostaticrepulsion among its unimers. But the opposite effectwas observed in this case. Smaller area per moleculefor this copolymer suggests stronger pull of watermolecule in the bulk for the T-tipped hydrophilicportion of the copolymer. Thus, more stretching and

TABLE ICritical Micelle Concentrations (CMC), Surface Tensions at CMC (cCMC), Free Energyof Micellization (DG0

mic), Enthaly (DH0mic), and Entropy of Micellization (DS0mic) for

DE39B18 and TE39B18 in Aqueous Solutions at Different Temperatures

PolymerT

(�C)CMC(g/L)

cCMC

(mN/m)DH0

mic

(kJ/mol)DS0mic

[kJ/(mol K)]DG0

mic

(KJ/mol)

DE39B18 20 0.49 42.7 11.087 0.1012 �18.56530 0.45 41.51 11.100 0.1007 �19.41340 0.34 40.49 11.080 0.1018 �20.78350 0.29 39.22 11.072 0.1020 �21.874

TE39B18 20 0.71 42.2 24.151 0.1427 �17.66030 0.66 41.3 24.151 0.1406 �18.45040 0.40 40.4 24.149 0.1422 �20.36050 0.30 39.1 24.130 0.1422 �21.800

Estimated uncertainities: 610% in CMC; 64% in cCMC; 62% in Cm; 63% in aS1 ; 64%in pCMC; 64% in DG0

ads, DG0mic, DH

0mic, and DS0mic.



hence closer packing at the surface may be obtaineddue to stronger hydrogen bonding. While in case ofD-tipped copolymer, less number of unimers aresufficient for surface coverage; hence, it shows moresurface activity. Moreover, both these copolymersshow higher surface activity and higher surface areaper molecule as compared to conventional EmBndi-block copolymers.21 The effect can further be attrib-uted to the end-group modification.

For a closed association to micelles with narrowdistribution of association number (N) the equilib-rium between copolymers molecules (A) andmicelles (AN) can be written (concentration in moledm�3). The association equilibrium can be writtenas,

A , ½1=N� AN

Kc ¼ ½AN�eq1=N=½A�eq (3)

Hall in his detailed study showed that when asso-ciation number Nw is very large, then the equationbecomes,9,24

Kc ¼ 1=½A�eqKc ¼ 1=CMC (4)

where [A]eq can be considered to be CMC. Thestandard Gibb’s energy of micellization expressedper mole of chains may be calculated from the CMCas a molar concentration using the relationship.24

DG0mic ¼ �RT ln Kc (5)

DG0mic ¼ RT ln CMC

DG0mic ¼ RT ln ðCMC=WÞ (6)

R is gas constant, T is absolute temperature and Wis the molarity of pure water.

The standard enthalpy of micellization is givenapproximately by;24

DH0mic ¼

�Rd½lnKc�dð1=TÞ (7)

As we cannot directly estimate Kc, so using eq. (4),Kc can be replaced by CMC: thus eq. (7) can be writ-ten as,

DH0mic ¼

Rd½lnCMC�dð1=TÞ (8)

The entropy of formation of micelles is given by,

DS0mic ¼ ðDH0mic � DG0

micÞ=T (9)

The standard enthalpy of micellization, i.e., DH0mic

was obtained from the slope of the plots of ln CMCversus inverse of temperature;12 by using eq. (9) andFigure 2.The standard Gibbs free energy of adsorption,

DG0ads, in water for these copolymers can also be cal-

culated from the standard Gibbs free energy ofmicellization using eq. (9) and the surface tensiondata using following equation.25

DG0ads ¼ DG0

mic �pCMC

Cm(10)

where pCMC is again the surface pressure at criticalconcentration and calculated as: pCMC ¼ c0 � cCMC,c0 and cCMC are the values surface tension of purewater and in the solutions at CMC. The standardGibbs free energy for adsorption at air/water inter-face in the temperature range 20–50�C, was calcu-lated by using the above eq. (10), and the values arelisted in Table II. It is clear that all the values ofDG0

ads are negative, indicating that the migration of

TABLE IISurface Excess Concentrations (Cm), Area Per Copolymer Molecules (aS1), Surface

Pressure (pCMC), Free Energy of Adsorption (DG0ads), Enthaly (DH0

ads), and Entropy ofAdsorption (DS0ads) for Dimethylamino- and Trimethylamonium-Tipped Oxyethylene–Oxybutylene Diblock Copolymers in Aqueous Solutions at Different Temperatures

PolymerT

(�C) Cm� 1010/mol cm�2aS1(A2)

pCMC

(mN/m)DG0

ads(kJ/mol)

DH0ads

(kJ/mol)DS0ads

[kJ/(mol K)]

DE39B18 20 0.75 220 30.05 �58.742 376.06 1.08330 0.66 252 29.68 �64.504 392.65 1.08340 0.44 378 29.46 �87.890 426.90 1.08350 0.43 381 28.70 �87.054 436.90 1.083

TE39B18 20 0.84 198 30.55 �54.160 277.80 0.76330 0.77 216 29.88 �57.306 288.50 0.76340 0.56 296 29.55 �73.033 311.85 0.76350 0.55 302 28.82 �74.345 321.00 0.763

Estimated uncertainities 63% in Cm; 64% in aS1 ; 63% in pCMC; 65% in DG0ads, DH

0ads,

and DS0ads.



copolymers molecules in the monomer state to theair/water interface is a spontaneous process andfavored by hydrophobic-hydrophilic balance (HHB)effect. At a given temperature, the higher value(more negative) values of DG0

ads for DE40B18 reflectmore surface activity. It is also clear that for a givencopolymer the value of DG0

ads becomes more nega-tive with temperature, indicates rapid adsorptionwith temperature and hence early completion ofGibbs monolayer.

The calculated value of enthalpies of micellization,DH0

mic, for both copolymers are positive and thispoints to the well known fact that micellization isdriven by positive entropy change associated withhydrophobic effect.25,26 This also shows that themicellization process is spontaneous but endother-mic in nature. The very low DHmic per B unit foundfor copolymers with the larger B blocks is attribut-able to those blocks being tightly coiled in the dis-persed molecular state so that the interaction of Bunit with water (hydrophobic interaction) is muchreduced in comparison with the enthalpy of theunits of short blocks, which are relatively extendedin the molecular state. An unassociated copolymerwith its hydrophobic block in such a tightly coiledstate is often called a monomolecular micelle. Mejiroet al.27 have reported that DHmic decreases withincreasing number of B units. Figure 3 shows thecomparison of standard enthalpy of micelization perB unit of our tipped-copolymers with hydroxyl- andethoxy-ended copolymers from earlier work.9 It canbe seen that enthalpy per B unit decrease withincrease in B-block length and hence with hydropho-bicity. Both DE40B18 and TE40B18 follow the patternshown by hydroxyl- and ethoxy ended copolymerswith some deviation, which is due to the change intipp. The lower value for DE40B18 reflects its morehydrophobic behavior as compared to TE40B18. Inother words we can say that TE40B18 shows more

hydrophilicity and stronger interactions with waterdue to trimethylammonium group. This means thatmore bonds between E-chain and water have to bebroken to form micelle.The values of DG0

mic for each copolymer are nega-tive and become more negative with increase in tem-perature which means that the rate of micellizationincreases with elevating temperature. At a giventemperature the value of DG0

mic for DE40B18 issmaller than TE40B18; this can be attributed to morepositive charge at the tip of the trimethylamonium-tipped copolymer. The effect of E-block length, onCMC and hence other thermodynamic parameters isnot prominent. The change in entropy of micelliza-tion, DS0mic for both copolymers is positive andnearly constant at all temperatures. The rapidincrease in association number at higher temperaturemay make the micelle more compact and hence lessfreedom for B-block within the core of micelle.During the process of micellization, the entropy

change is always positive, it is because of two rea-sons: (i) structure of water molecules is affected anddestroyed, as hydrophobic blocks are removed fromthe aqueous bulk to the interior of micelle at theinterface and (ii) it also suggests that freedom ofhydrophobic block in the interior of micelle isincreased.10

The standard entropy of adsorption (DS0ads) wasobtained from the slope of DG0

ads versus temperature(in Kelvin) plot, while the enthalpy (DH0

ads) wasdeduced from the well known thermodynamic equa-tion, i.e., DH0

ads ¼ DG0ads þ TDS0ads.

10,25 The surfaceactive and thermodynamic parameters are listed inTable II. The standard entropy of adsorption DS0ads ispositive; in all cases this reflects the greater freedom

Figure 3 Standard enthalpy of micellization per B unitfor aqueous solutions of hydroxyl-ended EmBn(for data seeRef. 9 and references therein), and values obtained for ourtipped-copolymers, TE40B18 and DE40B18.

Figure 2 Logarithm of CMC versus inverse of tempera-ture for aqueous solutions of DE40B18.



of copolymer molecule at air/water interface. Thestandard change in entropy of adsorption is greaterthan that of micellization for the same copolymer.This implies that there may be greater freedom of B-chain on the planner air/water interface as com-pared to the interior (core) of micelle. Moreover thedimethylamino-tipped copolymer has greater DS0adsvalue as compared to the trimethylammonium-tipped copolymer, the effect can be attributed tomore interactions of the latter’s charged hydrophilicend with surface and hence hydrated monomersmay be in more ordered form as compared to thefirst polymer. Likewise the enthalpy of adsorptionDH0

ads is also positive in all cases which indicates theendothermic nature of adsorption process andbecome more endothermic at elevated temperature.As the DH0

mic values are less positive than the corre-sponding values of DH0

ads. This behavior shows thatless number of bonds between E-chain oxygen andwater are broken in micellization than in adsorption.The DH0

ads value for the dimethylamino-tipped co-

polymer is greater than the trimethylammonium-tipped, this again shows more adsorption for thesecond copolymer as compared to the first. Thesame is clear from surface excess concentration atair/water interface. From surface tension results, itis clear that the dimethylamino-tipped copolymerhas lower adsorption but enhanced micellar proper-ties than the trimethyamonium-tipped copolymerand this can be attributed to intermicellar chargeeffects. The possible local charge near the hydro-philic end and its effect on the copolymer solution isdiscussed in the start of discussion part.

Density and viscosity results

Density, intrinsic viscosity, and hydrationsof micellar solutions

The partial specific volume of the micelles (mmic) wasdetermined from the measured solution density(qsoln) values using relation as follow;28

qsoln ¼ qsolv þ ð1þ mmicqsolvÞðC� CMCÞ (11)

Thus plotting (qsoln) versus C-CMC the interceptgives solvent density (qsolv) and from the slope thecorresponding partial specific volume of the micelle(mmic) was obtained, the typical plots for DE40B18 areshown in Figure 4. By taking inverse of mmic, we gotthe micellar density (qmic). The results are given inTable III. It is clear that the value of mmic show littleincrease with increase in temperature. This behavioris due to normal expansion hydrophobic block andthus the aggregation increase. As qmic is inverse ofmmic so the effect of temperature is also opposite formicellar density than micellar partial specific vol-ume. At a given temperature, qmic for TE40B18 isgreater than DE40B18, this little difference could beattributed to the greater interactions of cationiccharged-tipped with water. More water may be

TABLE IIIMicellar Density, qmic, Partial Specific Volume, mmic, Intrinsic Viscosity, [g], andInteraction Parameter, KH, for DE40B18 and TE40B18 in Aqueous Solutions at

Different Temperatures

PolymerT

(�C)qsolv

(g/mL)qmic

(g/mL)

mmic

(mL/g)[g]

(mL/g)KH

(mL/g)

DE40B18 20 0.9981 1.0970 0.9117 21.70 �23.0430 0.9957 1.0822 0.9240 17.40 �23.0040 0.9923 1.0787 0.9270 13.00 �23.0050 0.9880 1.0741 0.9310 14.00 �21.40

TE40B18 20 0.9978 1.1086 0.9020 40.20 �14.9230 0.9962 1.0946 0.935 40.40 �17.3240 0.9925 1.0907 0.9168 40.90 �17.1150 0.9884 1.0743 0.9308 39.20 �15.30

Estimated uncertainities: 62% in qmic; 63% mmic; 67% in [g]; 69% in KH; 68% in.

Figure 4 Plots of solution density versus C-CMC foraqueous solutions of DE39B18 at temperatures (^) 20, (n)30, (~) 40, and (�) 50�C.



attached to its unimers/micelle and thus increase itsoverall mass.

Viscosity measurements provide very useful infor-mation on the hydrodynamic volume of micellaraggregates. The hydrodynamic volume is propor-tional to intrinsic viscosities, [g], of micellar solu-tions.22 To obtain intrinsic viscosity, the dynamicviscosity (obtained directly from instrument) wasconverted to relative viscosity, gr which was thenconverted to specific viscosity (gs ¼ gr � 1) andfinally to reduced viscosity. The intrinsic viscosity[g] and intermicellar parameter (KH) for each poly-mer at different temperature was obtained from theextrapolation of reduced viscosities to zero concen-tration following modified Huggins’s relation asgiven below;28

gsp=C�CMC ¼ ½g� þ KH½g�C� CMC (12)

Thus by plotting the graph of gsp/ C-CMC versusC-CMC as given in Figure 5, [g] was obtained fromintercept and KH from the slope. When the solutionconcentration approaches zero, the intrinsic viscosityis defined by the following formula:29

½g� ¼ limc!0 ¼ gsp=C�CMC ¼ limc!0 ¼ lngr=C�CMC

(13)

However for the whole concentration region Hug-gin’s relation is not obeyed by the data. The depend-ence of reduced viscosities versus concentrationsprofiles in very dilute region (mostly in the concen-tration below 3 g dm�3) showed a nonlinear curva-ture and the values shoot up with the decrease inconcentration for both the copolymers. Such devia-tion from the straight line is indicated by typicalplots as in Figure 5. Soni et al.21 have also reporteda similar complex dependence of reduced viscositieson concentration for diblock and triblock of poly-(oxethylene)-poly(oxybutylenes) copolymers. The

authors have reported that this trend occurs due toadsorption of the copolymers molecules on the capil-lary wall of viscometer, which in turn wouldincrease the flow time and hence the measured gsp

/C values at low concentrations are apparent. Theseauthors21,30 and especially Ohrn et al.31 have consid-ered this effect to relate the true and apparent valuesof gsp /C by,

ðgsp=CÞ� ¼ gsp=Cþ D and D � ðgr=CÞð4alay=rÞ (14)

where * indicates the apparent value. The terms alayand r in the D relation are the thickness of theadsorbed layer and the radius of the capillary of theviscometer used. In our case, we have used rota-tional viscometer which gives results in good agree-ment with capillary viscometer of capillary size,0.300 mm. As for dilute solutions gr close to unity,so gsp/C values can be estimated by changing thechosen values for adsorbed layer thickness. In ourcase, the thickness of adsorbed layer was found tobe nearly, 0.00022, 0.00026, 0.00030, and 0.00034 at20, 30, 40, and 50�C, respectively for both thecopolymers. This shows that adsorption effectincreased with increase in temperature.The values of intrinsic viscosities were obtained

from the linear portion on the plots of reduced vis-cosities versus concentration, as shown in Figure 6.It is clear from Table III that the values of intrinsicviscosity for DE40B18 decrease with increase in tem-perature up to 40�C, which is a normal trend, but at50�C there is a little increase in intrinsic viscositythis may be due to change in micellar shape at ele-vated temperature. For TE40B18, the values of intrin-sic viscosity show a little increase with temperaturebut at 50�C there is a little decrease, again this maybe due to change in shape, the change in viscosityvalue are not prominent for this copolymer withtemperature as compared to DE40B18. At a given

Figure 5 Typical plots of concentration dependence ofreduced viscosity for aqueous solutions of TE40B18 at tem-peratures (^) 20, (n) 30, (~) 40, and (�) 50�C.

Figure 6 Typical plots of reduced versus C for aqueoussolutions of TE40B18 at temperatures (^) 20, (n) 30, (~)40, and (�) 50�C.



temperature, the values of intrinsic viscosity forDE40B18 are smaller than TE40B18; this is due tostrong interaction with solvent due to charged-tipped, this is also supported by solvent-solute inter-action (Huggin’s parameter) parameter. Moreover,Kelarakis et al.30 have also observed a prominentchange in intrinsic viscosity with changing thehydrophilic end from hydroxyl to methoxy group.Their hydroxyl-ended copolymer (E18B10H) has avalue of 3.8 cm3 g�1 while that with methoxy-ended(E18B10M) got a value of 13.1 cm3 g�1. The copoly-mer with smaller value was supposed to formspherical micelle while worm like micelle wasassigned to the one (E18B10M) has higher intrinsicviscosities value. Similarly lower values (in therange of 3–6 cm3 g�1) of intrinsic viscosities werereported for PEO/PBO diblock copolymers by Soniet al.21 In our case (see Table III), however, the veryhigh values of intrinsic viscosities for both copoly-mers reflect the effect of end-group modification.These higher values point toward the formation ofnon-spherical aggregates under present experimen-tal conditions.

CONCLUSIONS

Surface activity and micelle properties for aqueoussolutions of both the copolymers, in the pre- andpost-micellar-regions, are found to be temperaturedependent. Both the adsorption and micellizationprocess are spontaneous and endothermic in nature.Our results shows that both the copolymers tend tooccupy more surface area at elevated temperature inthe temperature range 20–50�C. The increase in theaS1 with increase in temperature can be attributed tothe orientation and hydration of dimethlyamino-and trimethylamonium- groups at the tip of hydro-philic block. The effect of temperature was found toenhance the process of micellization and hencedecrease CMC. The values of DG0

ads are more nega-tive than DG0

mic and become more negative withincreasing temperature in all cases, this show themore spontaneous nature of adsorption than micelli-zation. DS0ads > DS0mic, this reflects the greater free-dom of B-chain at planner air/water interface ascompared to the interior of micelle. We can concludethat both the adsorptions as well the micellizationare entropy driven. Moreover D-tipped copolymerhas enhanced micellar properties while T-tipped hasgreater surface activity. Viscosities and densitiesdata also support the temperature dependent micel-lization/aggregation and change in the shape ofaggregates with temperature. These higher valuesof intrinsic viscosities point toward the formation ofnon-spherical aggregates under present experimentalconditions.

The authors thank Dr. Carin Tattershall for synthesis of thesecopolymers at the Department of Chemistry, University ofManchester, U.K. They also thank Dr. Peter M. Budd for hishelpful advice.

References

1. Booth, C.; Attwood, D. Macromol Rapid Commun 2000, 21, 501.

2. Alexandridis, P. Curr Opin Colloid Interface Sci 1997, 2, 478.

3. Chu, B.; Zhou, Z.-K. In Nonionic Surfactants; Nance, V. M.,Ed.; Marcel Dekker: New York, 1996; Vol. 60; p67

4. Alemgren, M.; Brown, W.; Hvidt, S. Colloid Polym Sci 1995, 2,273.

5. Booth, C.; Yu, G. E.; Nace, V. M. In Amphiphilic BlockCopolymers: Self Assembly and Applications; Alexandridis,P., Lindman, B., Eds.; Elsevier: Amsterdam, 2000; p 57.

6. Luo, C.; Han, X.; Gao, Y.; Liu, H.; Hu, Y. J Appl Polym Sci2009, 113, 907.

7. Kelarakis, A.; Mai, S. M.; Havredaki, V.; Nace, V. M.; Booth,C. Phys Chem Chem Phys 2001, 3, 4037.

8. Maskos, M. Polymer 2006, 47, 1172.

9. Booth, C.; Attwood, D.; Price, C. Phys Chem Chem Phys 2006,8, 3612.

10. Rosen, M. J. Surfactants and Interfacial Phenomenon; WileyInter Science Publication: New York, 1978; p 83.

11. Silva, R. S.; Priscila, C. M.; Aparecida, R. E.; Mansur, C. JAppl Polym Sci 2009, 113, 392.

12. Siddiq, M.; Harrison, W.; Tattershall, C. E.; Budd, P. M. PhysChem Chem Phys 2003, 5, 3968.

13. Tattershall, C. E.; Aslam, S. J.; Budd, P. M. J Mater Chem2002, 12, 2286.

14. Tattershall, C. E.; Jerome, N. P.; Budd, P. M. J Mater Chem2001, 11, 2979.

15. Kelarakis, A.; Havredaki, V.; Rekatas, C. J.; Booth, C. PhysChem Chem Phys 2001, 3, 5550.

16. Yang, Z.; Pickard, S.; Deng, N. J.; Barlow, R. J.; Attwood, D.;Booth, C. Macromolecules 1994, 27, 2371.

17. James, A. M.; Lord, M. P. Macmillan’s Chemical and PhysicalData; Macmillan: London, 1992.

18. Diaz-Fernandez, Y.; Foti, F.; Mangano, C.; Pallavicini, P.;Patroni, S.; Perez-Gramatges, A.; Rodriguez-Calvo, S. ChemEur J 2006, 12, 921.

19. Zhang, H. B. Z.; Ying, S. K.; Hu, H. Q.; Xu, X. D. J ApplPolym Sci 2002, 83, 2625.

20. Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Haton, T. A.Langmuir 1994, 10, 2604.

21. Soni, S. S.; Sastry, N. V.; Patra, A. K.; Joshi, J. V.; Goyal, P. S. JPhys Chem B 2002, 106, 13069.

22. Soni, S. S.; Sastry, N. V.; Aswal, V. K.; Goyal, P. S. J PhysChem B 2002, 106, 2606.

23. Barry, B. W.; El Eini, D. I. D. J Colloid Interface Sci 1976, 54, 339.

24. Hall, D. G. In Nonionic Surfactants Physical Chemistry; Schick,M. J., Ed.; Marcel Dekker: New York, 1987; Vol. 23, p 247.

25. Rosen, M. J.; Cohen, A. W.; Dahanayeke, M.; Hua, X.-Y. JPhys Chem 1982, 86, 541.

26. Tanford, C. The Hydrophobic Effect; Willey: New York, 1980.

27. Kelarakis, A.; Yang, Z.; Pousia, E.; Nixon, S. K.; Price, C.; Booth, C.;Hamley, I. W.; Cartellet, V.; Fundin, J. Langmuir 2001, 17, 8085.

28. Abed, M. A.; Saxena, A.; Bohidar, H. B. Colloids Surf A Physi-cochem Eng Asp 2004, 233, 181.

29. Siddiq, M.; Wu, C.; Nawaz, M.; Baloch, M. K.; Mahmood, K.;Mohammad, B. J Chem Soc Pak 2001, 23, 200.

30. Kelarakis, A.; Havredaki, V.; Booth, C.; Nace, V. M. Macromo-lecules 2002, 35, 5591.

31. Ohrn, O. E. J Polym Sci 1955, 17, 137.



Documents

Surface activity and micellar behavior of dimethylamino- and trimethylammonium- tipped oxyethylene–oxybutylene diblock copolymers in aqueous media