the influence of a hydrotropic agent in the properties of aqueous solutions containing poly(ethylene...

Colloids and SurfacesA: Physicochemical and Engineering Aspects 149 (1999) 291–300

The influence of a hydrotropic agent in the properties of aqueoussolutions containing poly(ethylene oxide)–poly(propylene oxide)

surfactants

Claudia R.E. Mansur a, Luciana S. Spinelli a, Elizabete F. Lucas a,*, Gaspar Gonzalez ba Instituto de Macromoleculas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro (IMA/UFRJ),

P.O. Box 68525, 21945-970, Rio de Janeiro, Brazilb Petrobras Research Center, Ilha do Fundao, Q.7, Rio de Janeiro, Brazil

Received 10 August 1997; accepted 8 April 1998

Abstract

The aqueous solution behavior of diblock poly(ethylene oxide)–poly(propylene oxide) (PEO–PPO) copolymerscoupled with hydrocarbon groups was studied in the presence of the hydrotropic agent sodium p-toluenesulfonate(NaPTS). The change in phase of the aqueous systems was evaluated by building up temperature–concentrationphase diagrams. The critical micelle concentrations (CMC) of the copolymers and the aggregation points of NaPTSand NaPTS/copolymer mixtures were obtained by surface tension measurements, viscometry data and dye solubiliza-tion. The copolymers and NaPTS adsorb and reduce the surface tension of the solution until the surface becomessaturated: the CMC values are related to the solubility of the copolymers. Solutions containing NaPTS/copolymermixtures exhibit the opposite behavior: at constant copolymer concentrations and with increasing NaPTS concentration,the surface tension remains constant until the aggregates of NaPTS start to form. Above this concentration, thesurface tension increases. The surface tension and the aggregation points of the NaPTS solutions are dependent onthe structure of the copolymer. The influence of the length of the hydrocarbon groups and the PPO position segmentin the structure of the copolymers were also studied. From viscometric data, a pronounced increase in solutionviscosity was observed as aggregates began to form. The results obtained from dye solubilization are in goodagreement with the surface tension and viscometric measurements. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Critical micelle concentration; Hydrotrope; Surface tension; Surfactants; Viscometry

1. Introduction these systems in several applications [1,2]. Thesecompounds exhibit phase separation with increas-ing temperature, such temperatures being knownPoly(ethylene oxide)–poly(propylene oxide)as ‘‘cloud points’’ [3]. Before the change in phase(PEO–PPO) block copolymers are known toof aqueous solutions, PEO–PPO copolymers formbe non-ionic surfactants [1]. The behavior ofaggregates, called micelles, and are able to solubi-PEO–PPO di-functional triblock copolymerslize water-insoluble compounds and adsorb at(HO–PEO–PPO–PEO–OH) in aqueous solutionsinterfaces [3–5]. The critical micelle concentrationhas been widely studied due to the importance of(CMC) is the concentration at which the micellesstart to form [4].* Corresponding author. Tel. : +55 21 2701035; fax: +55 21

2701317. Other compounds are also able to solubilize a

0927-7757/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII S0927-7757 ( 98 ) 00417-8

292 C.R.E. Mansur et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 291–300

variety of hydrophobic compounds in water, hydrotropes are observed to be surface-active,albeit somewhat less so than classical surfactants.and these are classified as hydrotropes or hydro-

In this work, the aqueous-solution behavior oftropic agents: this phenomenon is known asPEO–PPO diblock copolymers coupled with‘‘hydrotropy’’ [6 ]. Some examples of hydrotropeshydrocarbon groups (R–PEO–PPO–OH andare anionic organic salts like sodium benzoate,R–PPO–PEO–OH) is studied in the presence ofsodium salicylate, and sodium benzene-, toluene-,the hydrotropic agent sodium p-toluenesulfonatexylene- and naphtalenesulfonate [7]. Cationic(NaPTS). The influence of the length of the hydro-molecules like p-aminobenzoic acid hydrochloride,carbon groups, the position of the PPO in theand non-ionic molecules like pyrogallol, cathecolPEO–PPO chains and the interaction between theand resorcinol, also exhibit hydrotropic proper-copolymers and NaPTS are analyzed. The studyties [8].is conducted using phase diagrams, surface tensionAlthough hydrotropy is used extensively inmeasurements, viscometric data and dye solubiliza-industry due to the possibilty of controlling thetion of the copolymer solutions.solubility of solutions, the molecular mechanism

of hydrotropic solubilization has not been yetelucidated completely. Moreover, the problem of

2. Experimentalemulsification (which is normally encountered withsurfactant solutions) is not encountered with

2.1. Materialshydrotrope solutions [9]. Studies have shown thathydrotropy differs from the micellar solubilization

Mono-functional diblock copolymers of ethy-of hydrophobic substances in water by surfactantlene oxide (EO) and propylene oxide (PO)molecules and from the ‘‘salting-in’’ effect pro-(R–PEO–PPO–OH and R–PPO–PEO–OH; Rmoted by some inorganic salts [10,11].length=linear C4) and C12–(EO)6–OH wereSchott and Royce [12] studied the influence ofobtained from Grupo Ultra-Divisao Quımica. A

some hydrotropes on the phase behavior of aque-mono-functional diblock copolymer of EO–PO

ous, non-ionic octoxynol surfactants. A pro- (R–PEO–PPO–OH, R length=linear C12–14) wasnounced increase in the cloud point of this aqueous obtained from Henkel SA Industrias Quımicas.surfactant by sodium salicylate and sodium benzo- Sodium p-toluenesulfonate (NaPTS) and the dyeate was observed. In another work [13], it was 1,4-bis(isopropylamino)anthraquinone wereproposed that hydrotropes increased surfactant obtained from COEMA SA and ZENECA Brasilsolubility because they prevent the formation of SA, respectively.association structures and subsequent phase sepa-ration from aqueous solutions. However, this 2.2. Copolymer characterizationobservation seems to be another aspect of thehydrotropic effect rather than its molecular mecha- Copolymer characterization data are summa-nism, especially when it is considered that other rized in Table 1 [16,17].molecules which do not necessarily form crystalphases, like dyes or drugs, also are solubilized by 2.3. Methodshydrotropes [14].

By studying the solution-state properties in 2.3.1. Phase diagramswater of some hydrotropes [14,15], it was observed Cloud-point measurements were performed inthat they form aggregates at high concentrations an electrically thermostated bath, with two deter-in aqueous solutions. It appears that a minimal minations for each double sample. The cloud pointhydrotropic concentration (MHC) is essential was taken from the average of the temperaturesbefore hydrotropy occurs. Hydrotropy above the where the last visible sign of clouds disappearedMHC is not strictly analogous to the CMC dis- on cooling and the first visible sign of clouds

appeared on heating.played by surfactants. It was also shown that

293C.R.E. Mansur et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 149 (1999) 291–300

Table 1Characterization of block copolymers of poly(ethylene oxide) and poly(propylene oxide) [16,17]

Copolymer M9 na M9 w/M9 nb Monomer repeat unitsc EO/PO ratio O/C ratiod

EO PO

C4–(EO)4–(PO)11–OH 920 1.24 4 11 0.36 0.36C4–(PO)10–(EO)6–OH 900 1.23 6 10 0.60 0.37C12–14–(EO)6–(PO)5–OH 900 1.15 6 5 1.20 0.30

aDetermined by vapor pressure osmometry (VPO).bDetermined by gel permeation chromatography (GPC).cDetermined by 1H nuclear magnetic resonance spectroscopy (1H NMR).dO/C=oxygen/carbon atom ratio.

The temperature versus concentration phase dia- 2.3.3. Dye solubilization methodThe dye solubilization method [18] was used tograms of each copolymer solution were built up

verify the CMC values of the aqueous copolymerusing the following concentration ranges. Forsolutions. The hydrophobic dye used wasaqueous solutions, the copolymer concentrations1,4-bis(isopropylamino)anthraquinone. The dyewere 0.1–10.0 wt.%. For aqueous solutions con-solution in methanol was analyzed usingtaining sodium p-toluenesulfonate (NaPTS), theUV–visible equipment (CAMSPEC spectropho-copolymer concentration was constant and equaltometer, model M330) to determine the wave-to 1 wt.%. NaPTS concentrations werelength at which dye absorption was at its0.13–2.10 M. The C4–(PO)10–(EO)6–OH copoly-maximum. Methanol was used to prepare the dyemer at 2 wt.% concentration was also analyzed insolution because some studies have shown that atthese NaPTS solutions.low concentrations, this solvent has little influenceon the CMC values of non-ionic aqueous surfac-2.3.2. Surface tension measurementstant solutions [19].Surface tension was measured at 30°C using a

The aqueous copolymer solutions were preparedKruss tensiometer equipped with a platinum ringat concentrations in the range 0.1–4.5 wt.%. The( Kruss instruments, K-10 model, digital tensiome-concentration of the dye solution in methanol waster) controlled by a circulating-water thermostatic0.4 mM, and 2.5 ml of this dye solution was added

bath (Ophterm, ECV model, serial number to 2.5 ml of copolymer solution, so that the final088179) which was connected to the surface copolymer solution contained 1% v/v methanoltension cell compartment. For each copolymer and 0.004 mM dye. The solutions were left tosolution, a surface tension (mM m−1) versus copol- equilibrate at the measurement temperature for atymer concentration (wt.%) plot was built up in the least 3 h (temperature range 16–40°C ). The UVrange 3–0.0001 wt.% in order to determine CMC absorption of copolymers/hydrophobic dye invalues. methanol/water samples was recorded at 640 nm

NaPTS measurements were obtained by the (the maximum absorption of the dye) using amethod described above. First, surface tension colorimeter. The formation of aggregates ofmeasurements of aqueous solutions as a function NaPTS containing the C4-(PO)10–(EO)6–OHof the NaPTS concentration (0.05–2.10 M) were copolymer was also determined via this method,analyzed. After this, surface tension measurements using the same procedure as described above.of copolymer solutions (1 wt.%) containingNaPTS concentrations in the range 0.05–2.10 M 2.3.4. Viscometrywere analyzed. All analyses were performed at Viscometric data were obtained using a Mettler30°C. The reliability of the results was Toledo Low Shear 40 rheometer operating with

coaxial cylinders. The apparent viscosities (g) of±0.5 dynes cm−1.


samples were measured in the shear rate range (the coacervate phase) and another containing alow surfactant concentration (the dilute phase).0.1–100 s−1 in order to determine the zone of

Newtonian behavior. The specific viscosities gsp The aqueous NaPTS solutions did not exhibit achange in phase in the concentration range studiedwere obtained using the equation [19]:(0.05–2.10 M).

gsp=grel−1 (1)Fig. 2 shows the cloud points of copolymer

solutions (1 wt.%) as a function of the NaPTSwhere grel is the relative viscosity, obtained by thecorrelation between the apparent viscosity of the concentration. The C4–(PO)10–(EO)6–OH copoly-

mer was also analyzed at a concentration ofsample and the apparent viscosity of the solvent(gsolution/gsolvent). 2 wt.%. The hydrotropy effect is observed for all

copolymer solutions, i.e. the cloud points increaseAqueous copolymer concentrations were ana-lyzed in the range 0.1–10 wt.%, aqueous NaPTS with increasing NaPTS concentration, and the

shape of the curves depends on the copolymerconcentrations were analyzed in the range0.05–2.10 M and NaPTS solutions, in the same added to the solution.range, were analysed containing 1 wt.% of copoly-mer at 30°C. 3.2. Surface tension measurements of aqueous

polymer solutions

In order to study the CMC of EO–PO block3. Resultscopolymers, surface tension measurements wereperformed as a function of aqueous copolymer3.1. Phase diagramssolution concentration. In general, surface tensionversus concentration plots exhibit a discontinuityThe phase diagrams of the aqueous copolymer

solutions are shown in Fig. 1. At temperatures in their slopes. The first portion of the curve (atlower concentrations) is related to surfactantabove each curve the solutions separate into two

phases, one which is concentrated in surfactant adsorption at the interface, and the second portionof the curve (at higher concentrations) is in general

Fig. 2. Phase diagrams of aqueous solutions of EO–PO blockFig. 1. Phase diagrams of aqueous solutions of ethylene oxide copolymers (1 and 2 wt.%) as a function of sodium p-toluenesul-

fonate (NaPTS) concentration.and propylene oxide block copolymers.


a straight line parallel to the concentration axis,indicating that in this step, adsorption does nottake place. The extension of both portions leadsto the discontinuity average, which is related tothe aggregation of the copolymer in solution, i.e.the CMC [3,4,20]. Fig. 3 shows surface tensionversus concentration plots and the CMC valuesobtained from the discontinuity of these curves.

3.3. Surface tension measurements of aqueousNaPTS solutions containing 1 wt.% copolymer

Study of surface tension as a function of theconcentration of aqueous NaPTS solutions showedthat NaPTS is surface-active (Fig. 4). NaPTSreduces the surface tension of water and, at aconcentration of around 0.50 M (~8.9 wt.%),surface tension becomes constant at around

Fig. 4. Surface tension as a function of aqueous NaPTS solution51 dynes cm−1.concentrations at 30°C. Agg=aggregation point of the aqueousIn order to study the influence of copolymerNaPTS solution.structure on aqueous NaPTS solutions, surface

tension curves were plotted as a function of NaPTSsolutions containing 1 wt.% of copolymer (Fig. 5).The opposite behavior to that observed for NaPTSsolutions was found for NaPTS solutions contain-ing 1 wt.% copolymer: with increasing NaPTS

Fig. 5. Surface tension as a function of aqueous NaPTS solutionconcentrations containing 1 wt.% of copolymer at 30°C. Agg=aggregation point of NaPTS in solution.

concentration, the surface tension remains con-stant until aggregates of NaPTS start to form.Beyond this concentration, the surface tensionFig. 3. Surface tension as a function of aqueous copolymer solu-

tion concentrations at 30°C. increases. From these analyses, it can be observed


that the aggregation concentration of NaPTSdepends on the structure and composition of thecopolymer.

3.4. Dye solubilization method

For dye solubilization, 1,4-bis(isopropyl-amino)anthraquinone was used as the hydro-phobic dye. The absorption intensities of aqueouscopolymer solutions containing the hydrophobicdye were measured at 640 nm (the wavelength atwhich dye absorption was at a maximum). Fig. 6shows the dye absorption intensities as a functionof the concentration of aqueous C4–(PO)10–(EO)6–OH solutions at different temperatures.CMC values were obtained from these curves.

Dye absorption was not observed at low concen-trations due to the hydrophobic character of thedye, which did not disperse in the solution. At

Fig. 7. Absorption intensities of hydrophobic dye at 640 nm ashigh concentrations, the micelles of the EO–PO a function of NaPTS concentration of aqueous NaPTS solu-block copolymer start to form: the dye can be tions with and without 1 wt.% of C4–(PO)10–(EO)6–OH copol-

ymer. Temperature=30°C, Agg=aggregation point of NaPTSsolubilized into the micelles and the solutionin solution.becomes coloured, exhibiting absorption at

640 nm.Fig. 7 shows the aggregation results obtained by dye solubilization for aqueous NaPTS solutions

containing 1 wt.% of the C4–(PO)10–(EO)6–OHcopolymer.

3.5. Viscometry

Plots of the apparent viscosity as a function ofshear rate for the C4–(PO)10–(EO)6–OH copoly-mer (at 1 wt.%) and NaPTS (at 0.13, 0.25, 0.50,0.75 and 1.50 M) with and without copolymer areshown in Fig. 8. The solutions exhibit Newtonianbehavior above 18 s−1.

Fig. 9Fig. 10 show specific viscosities as a func-tion of solute concentration. The curves exhibit aninflection around the concentration at which aggre-gates start to form.

4. Discussion

By comparing the results obtained from thephase diagrams of the mono-functional (RFig. 6. Absorption intensities of hydrophobic dye at 640 nm aslength=C4) diblock copolymers, it can be seena function of aqueous C4–(PO)10–(EO)6–OH copolymer con-

centrations at different temperatures. that both exhibit a similar curve, as shown in


Fig. 8. Shear rate as a function of the apparent viscosity of Fig. 9. Specific viscosity as a function of aqueousC4–(PO)10–(EO)6–OH copolymer (1 wt.%) and NaPTS solu- C4–(PO)10–(EO)6–OH copolymer concentration at differenttions with and without copolymer. shear rates (30°C).

Fig. 1. As the concentration of the copolymerincreases, the cloud points decrease until they ing different affinities to the solvent, plays an

important role in the solubility of the copolymerbecome almost constant, starting from ~2 wt.%.C4–(EO)4–(PO)11–OH exhibits higher cloud in water.

The results of the plots of the surface tensionpoints than C4–(PO)10–(EO)6–OH: however, con-sidering that the former copolymer has a lower versus the concentration of the aqueous copoly-

mers solutions (Fig. 3) show that the copolymerEO/PO ratio (Table 1), it would be expected tohave lower cloud points. This discrepancy can be with an R length of C12–14 exhibits a much lower

CMC value than those found for copolymers withascribed to the position of the hydrophilic (EOand OH) and hydrophobic (R and PO) segments R lengths of C4. Analysing the CMC result for

C12–(EO)6–OH solutions, it was observed that thisof these copolymers. Such behavior has beenattributed to the easy molecular association value is similar to that exhibited by the

C12–14–(EO)6–(PO)5–OH copolymer. This beha-induced by the structure of the copolymer, inwhich hydrophilic and hydrophobic segments are vior confirms that the length of the hydrophobic

segment has a greater influence on the CMC andadjacent. We have already proposed a model formolecular association in both cases [17]. cloud-point values of these copolymers than does

the EO/PO ratio (Table 1).A different curve behavior can be seen for themono-functional (R length=C12–14) diblock Comparing the CMC results (Fig. 3) with the

phase diagrams (Fig. 1), it can be seen copolymercopolymer solutions, which present cloud pointswhich are nearly constant over the concentration solutions with R lengths of C4 have CMC values

which are close to the concentration at which therange analyzed (i.e. 0.1–10 wt.%). This curve out-line can be ascribed to the length of the alkyl slope of the phase diagram changes. This suggests

that the change in slope is related to molecularchain (=C12–14) of this copolymer, which gives amore hydrophobic character to the molecule. The aggregation. On the other hand, the CMC value

of the copolymer solution with an R length ofresults obtained here suggest that the structure, interms of its overall hydrophilic/hydrophobic bal- C12–14 is below the concentration range analyzed

(0.1–10 wt.%). This explains the almost constantance and the relative position of the groups exhibit-


micelles. This process introduces an electrostaticrepulsion between the micelles which, in combina-tion with the existing steric forces, hinders theformation of a coacervate phase.

In order to study the influence of the structureof the copolymer on aqueous NaPTS solutions,surface tension curves were plotted as a functionof NaPTS solutions containing 1 wt.% of thecopolymer (Fig. 5). Analysis of these plots showedthat the aggregation concentration of NaPTSdepends on the structure and composition of thecopolymer.

The copolymer concentrations at 1 wt.% areabove the CMC values for the copolymer with anR length of C12–14 (Fig. 3), and it can be seen thatthe micelles of these copolymers did not alter theaggregation concentration of NaPTS in aqueoussolution to a great extent. The surface tension

Fig. 10. Specific viscosity as a function of aqueous NaPTS solu-values at 0.05 M of NaPTS shown in Fig. 5 aretion concentrations with and without C4–(PO)10–(EO)6–OHsimilar to the surface tension values obtained atcopolymer (1 wt.%). Shear rate=20 s−1, temperature=30°C.above the surface saturation concentration(Fig. 3). Therefore, it can be concluded that thefirst portions of the curves shown in Fig. 5 arevalues of the phase-diagram curve obtained for

this copolymer. related to copolymer adsorption at the air/waterinterface and that NaPTS molecules in solutionFig. 2 shows the cloud points of polymer solu-

tions (1 wt.%) as a function of the NaPTS concen- can be associated with the micelles of the copoly-mer because the interface is saturated. Aftertration. The C12–14–(EO)6–(PO)5–OH copolymer

is sensitive to small amounts of NaPTS, and thus NaPTS aggregation in solution, an increase in thesurface tension is observed (second part of thethe change in the cloud points with concentration

is decreased. curves). This effect may indicate a displacement ofcopolymer molecules from the interface to theAn unusual effect is observed for

C4–(EO)4–(PO)11–OH and C4–(PO)10–(EO)6– solution phase due to their enhanced solubilitycaused by the hydrotropic effect of NaPTS.OH, which are made up of long, hydrophobic PPO

segments counterbalacing the short R-length By analyzing the results obtained for copolymerswith an R length of C4, i.e. a concentration of(C4). The similar behavior of the curves, in spite

of different copolymer structures, could be caused 1 wt.% in aqueous solutions is below the CMC(according to Fig. 3), it can be seen that theseby the short R-length of the aliphatic chain, which

does not contribute markedly to the solubility copolymers reduce the aggregation concentrationof NaPTS in aqueous solutions. This effect maydifference as a function of the polymer structure

in systems containing NaPTS. It was also observed be ascribed to the presence of surfactant moleculeswhich do not associate, and thus decrease thethat at a concentration of the C4–(PO)10–

(EO)6–OH copolymer of 2 wt.%, the profile of the concentration of free solvent molecules which caninteract with the solute. Therefore, the first portioncurve is similar to that exhibited by the 1 wt.%

solution, indicating that, in this case, the behavior of these curves is related to the adsorption of thecopolymer and NaPTS at the air/water interfaceof the solution does not depend on micelle forma-

tion. The unusual behavior of the latter two copol- because, in agreement with Fig. 3, this interface isnot yet saturated by copolymer molecules andymers can be ascribed to the incorporation of

NaPTS into non-ionic micelles to form mixed NaPTS molecules which adsorb at this interface


have little influence on the surface tension. The the aggregation point. The apparent viscosities ofthe copolymer/NaPTS mixtures were higher thansecond portion of the curves exhibits the same

behavior as observed for copolymers with an R the pure NaPTS solutions, comparing solutionsat the same NaPTS concentrations, due to thelength of C12–14.

Analyzing the results obtained by dye solubiliza- increase in solute concentration. The differencebetween the viscosities of the copolymer/NaPTStion (Fig. 6), we can see that the CMC decreases

gradually as the temperature of the copolymer and NaPTS solutions became more pronouncedwith increasing NaPTS concentration (i.e. assolution increases. The CMC values obtained from

surface tension measurements and the dye solu- aggregates formed). We can suggest two explana-tions for this: (1) the interaction between thebilization method are in good agreement: for

a C4–(PO)10–(EO)6–OH solution at 30°C they solutes and the solvent increases, and/or (2) thesize of the aggregates in solution increases. Thisare 1.4 wt.% (Fig. 3) and 1.1 wt.% (Fig. 6),

respectively. second explanation is in agreement with the discus-sion of the dye solubilization method, where theFig. 7 shows the aggregation results obtained by

dye solubilization for aqueous copolymer solutions formation of mixed aggregates in solution wassuggested.containing NaPTS. These results are in agreement

with those obtained from surface tension measure- Fig. 9 shows the results of specific viscosity as afunction of aqueous C4–(PO)10–(EO)6–OH copol-ments. From Fig. 7, it is clear that NaPTS was

able to solubilize the hydrophobic dye and also ymer concentration at two shear rates (10 and20 s−1). The change in slope is related to thethat the C4–(PO)10–(EO)6–OH copolymer reduced

the aggregation concentration of NaPTS. The aggregation point (1.5 wt.%) and the results aremore shapely and reliable at 20 s−1, a shear rateinteraction between the copolymer and NaPTS

aggregates causes a decrease in the solution absorp- at which the behavior is Newtonian.The relationship between the change in slopetion intensity: it is supposed that the NaPTS

aggregates associate with the copolymer, resulting and the aggregation point is confirmed in Fig. 10.The aggregation point of NaPTS is ~0.50 M,in a reduction in the volume available to dissolve

the organic dye. If this true, the NaPTS aggregates while that of NaPTS/C4–(PO)10–(EO)6–OH wasat lower concentrations due to the influence of thesolubilize the dye 1,4-bis(isopropylamino)an-

thraquinone better than mixed aggregates which copolymer, as outlined above. At high NaPTSconcentrations, the viscosities of NaPTS andconsist of NaPTS and the copolymer.

The interaction between the solute and the NaPTS/copolymer solutions tends to becomeequal. With increasing NaPTS concentration, thesolvent can be analyzed by viscometry. Fig. 8

shows the apparent viscosity versus shear rate influence of 1 wt.% copolymer on the size of theaggregates in solution decreases.curves for C4–(PO)10–(EO)6–OH copolymer

(1 wt.%) and NaPTS solutions (0.13, 0.25, 0.50,0.75 and 1.50 M) with and without copolymer.Newtonian behavior is observed at around Acknowledgment18 s−1.

The C4–(PO)10–(EO)6–OH copolymer (1 wt.%) The authors would like to acknowledge financialand NaPTS solutions before the aggregation point support from CNPq/Rhae, CEPG-UFRJ and(0.13 and 0.25 M) present similar viscosities. This FAPERJ.suggests that the free molecules of NaPTS insolution have a higher interaction with water thanthe copolymer, since the molecular weight of ReferencesNaPTS is smaller than the molecular weight of thecopolymer. The viscosity of the NaPTS solutions [1] J.R. Schmolka, in; M.J. Schick, F.M. Fowkes (Eds.),increases as the NaPTS concentration increases, a Nonionic Surfactants, vol. 1, Marcel Dekker, New York,

1966, ch. 10.more pronounced increase being observed above


[2] G. Wanka, H. Hoffman, W. Ulbricht, Macromolecules 27 [12] H. Schott, A.E. Royce, J. Pharm. Sci. 73 (6) (1984) 793.[13] S.E. Friberg, S.B. Rananavare, D.W. Osborne, J. Colloid(1994) 4145.

[3] T.F. Tadros, in: R.A. Meyers (Ed.), Encyclopedia of Interface Sci. 109 (1986) 487.[14] D. Balasubramanian, V. Srinivas, V.G. Gaikar, M.M.Physical Science and Technology, vol. 13, Academic Press,

Orlando, FL, 1987. Sharma, J. Phys. Chem. 93 (1989) 3865.[15] B.A. Ali, M.B. Zugul, A.A. Badwan, J. Disp. Sci. Technol.[4] W.J. Moore, Physical Chemistry, Prentice-Hall,

Englewood Cliffs, NJ, 1972. 16 (6) (1995) 3865.[16 ] C.M.F. Oliveira, E.F. Lucas, Polym. Bull. 24 (1990) 363.[5] B. Chu, Langmuir 11 (1995) 414.

[6 ] C. Neuberg, Biochem. Z. 76 (1916) 107. [17] C.R.E. Mansur, C.M.F. Oliveira, G. Gonzalez, E.F.Lucas, J. Appl. Polym. Sci. 66 (1997) 1767.[7] R.H. McKee, Ind. Eng. Chem. Ind. Ed. 38 (1946) 382.

[8] A.M. Saleh, L.K. El-Khordagui, Int. J. Pharm. 24 [18] P. Alexandridis, J.F. Holwarth, T.A. Hatton,Macromolecules 27 (1994) 2414.(1985) 231.

[9] V.G. Gaikar, M.M. Sharma, Solvent Extr. Ion Exch. 4 [19] W.J. Freeman, in: H.M. Mark, N.M. Bikales, C.G.Overberger, G. Menges (Eds.), Encyclopedia of Polymer(1986) 839.

[10] A.M. Saleh, A.A. Badwan, L.K. El-Khordagui, Int. J. Science and Engineering, vol. 3, Wiley, New York, 1985,p. 758.Pharm. 17 (1983) 115.

[11] D. Balasubramanian, P. Mitra, J. Phys. Chem. 83 (1979) [20] J. O’Scik, Adsorption, Wiley, New York, 1982.2724.

the influence of a hydrotropic agent in the properties of aqueous solutions containing poly(ethylene...

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